“listening” and “talking” to neurons: implications of immune activation for pain control and...

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Review

“Listening” and “talking” to neurons: Implications of immuneactivation for pain control and increasingthe efficacy of opioids

Linda R. Watkins⁎, Mark R. Hutchinson, Erin D. Milligan, Steven F. MaierDepartment of Psychology and Center for Neuroscience, University of Colorado at Boulder, Boulder, CO 80309-0345, USA

A R T I C L E I N F O

⁎ Corresponding author. Department of PsychBoulder, Boulder, CO 80309-0345, USA. Fax: +

E-mail address: [email protected]

0165-0173/$ – see front matter © 2007 Elsevidoi:10.1016/j.brainresrev.2007.06.006

A B S T R A C T

Article history:Accepted 26 June 2007Available online 13 July 2007

It is recently become clear that activated immune cells and immune-like glial cells candramatically alter neuronal function. By increasing neuronal excitability, these non-neuronalcells are now implicated in the creation and maintenance of pathological pain, such as occursin response to peripheral nerve injury. Such effects are exerted at multiple sites along the painpathway, including at peripheral nerves, dorsal root ganglia, and spinal cord. In addition,activated glial cells are now recognized as disrupting the pain suppressive effects of opioiddrugs and contributing to opioid tolerance and opioid dependence/withdrawal. While thisreview focuses on regulation of pain and opioid actions, such immune–neuronal interactionsare broad in their implications. Such changes in neuronal function would be expected to occurwherever immune-derived substances come in close contact with neurons.

© 2007 Elsevier B.V. All rights reserved.

Keywords:Neurophatic painMorphineToleranceDependence/withdrawalInterleukinCytokinesGliaMicrogliaAstrocytesDorsal root gangliaPeripheral nervesSpinal cord

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1492. Immune activation as a driving force for pathological pain states . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150

2.1. Localized immune activation associated with peripheral nerve injury and inflammation . . . . . . . . . . . . 1502.1.1. Immunology of peripheral nerves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1502.1.2. Peripheral nerve changes in response to trauma. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1502.1.3. Peripheral nerve changes in response to inflammation . . . . . . . . . . . . . . . . . . . . . . . . . . 1512.1.4. Effects of immune-derived substances on peripheral nerve. . . . . . . . . . . . . . . . . . . . . . . . 151

ology, Muenzinger Psychology Bldg, Rm D-244, Campus Box 345, University of Colorado at1 303 492 2967.du (L.R. Watkins).

er B.V. All rights reserved.

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http://dx.doi.org/10.1016/j.brainresrev.2007.06.006

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2.2. Localized immune and glial activation impacts the function of sensory neurons in dorsal root ganglia (DRG) 1522.2.1. Immunology of DRGs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1522.2.2. DRG changes in response to herniated discs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1522.2.3. DRG changes after peripheral nerve injury . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153

2.3. Peripheral nerve injury leads to glial activation in spinal cord . . . . . . . . . . . . . . . . . . . . . . . . . . 1542.3.1. Communication from neurons to glia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1542.3.2. Neuronal transmitters and modulators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1542.3.3. Neuronal chemokines. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1542.3.4. Signaling molecules released by damaged, dying, and dead neurons . . . . . . . . . . . . . . . . . . 155

2.4. Glial activation impacts the function of pain-responsive neurons in spinal cord . . . . . . . . . . . . . . . . 1552.4.1. Functions of glia under basal and activated states . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1552.4.2. Mechanisms whereby glial activation affects neuronal activity . . . . . . . . . . . . . . . . . . . . . 1562.4.3. Beyond glia: immunocompetent cells other than astrocytes and microglia warrant consideration. . 1572.4.4. Beyond spinal cord: glial regulation of pain in the brain . . . . . . . . . . . . . . . . . . . . . . . . . 158

3. Immune activation as a driving force for disrupting the efficacy of opioids. . . . . . . . . . . . . . . . . . . . . . . 1583.1. Opioid-induced glial activation undermines opioid-induced pain suppression . . . . . . . . . . . . . . . . . 1583.2. Opioid-induced glial activation contributes to opioid reward, dependence and withdrawal . . . . . . . . . . 1593.3. Opioid-induced glial activation opposes acute opioid analgesia: involvement of non-classical opioid receptors . 159

4. Evidence for immune/glial regulation of pain from studies of human chronic pain patients . . . . . . . . . . . . . 1595. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161

1. Introduction

Within the past 15 years, evidence has accrued that the immunesystem can dynamically and dramatically alter neuronal func-tion. This reviewexplores thedysregulation of pain byperipheralimmune cells and by immune-like glial cells. The focus is onimmune–neuronal interactions at three key sites: the peripheralnerve, the dorsal root ganglia, and the spinal cord. While thisreview uses pain modulation as the focal point, the immune–neuronal interactions that are described have far broaderimplications than just pain. These interactions would beexpected to occur wherever immune-derived substances comein close contact with neurons. Thus these data have wide-ranging implications for alterations in neural structure andfunction throughout theperipheral andcentral nervous systems.

Regarding pain, pain serves highly adaptive, survival func-tions under normal circumstances. It protects the individualfrom harm from dangers in the environment and encouragesrecuperative behaviors in response to pain arising from withinthe body itself. However, pain can go wrong, such that painbecomes chronic and serves no adaptive, physiologicallyrelevant function. Such pathological pain destroys lives, afflict-ing an estimated one-sixth of the world's population (Campbelland Meyer, 2006).

Neuropathic pain is a form of pathological pain that arisesfrom trauma, inflammation, and/or infection of peripheralnerves. Here, sensations from the affected body region aregrossly abnormal. Environmental stimuli that would never nor-mally be perceived as pain now, and environmental stimuli thatare normally perceived as painful now elicit amplified percep-tions of pain. In addition, environmental stimuli may evokeabnormal perceptions of electric tingling or shocks (paraesthe-sias) and/or sensations having unusually unpleasant qualities(dysesthesias). Lastly, spontaneous pain frequently occurs withvarying qualities and from varying perceived body locations.

Why neuropathic pain occurs is a dilemma, as is how tosuccessfully treat such pain. Over the past several decades,

animal models of inflammatory and traumatic neuropathyhave revealed a remarkable degree of plasticity in sensorynerves, sensory nerve somas, and spinal cord. For example,damaged peripheral nerve fibers may develop spontaneousactivity arising not only from its peripheral nerve terminalsbut also from the site of axonal damage as well as from theneuronal cell bodies far from the injury site. Damaged nervesmay also alter their expression of receptors along the axon,now becoming increasingly responsive to pain-inducingsubstances and even to substances to which sensory neuronsare normally unresponsive. In addition, neurons that nor-mally do not signal pain may alter their gene expressionsuch that they now produce “pain” neurotransmitters. Spinalcord “pain” responsive neurons show equally remarkableplasticity along similar lines. While space does not allowmorethan this brief introduction to neural plasticity associatedwith neuropathic pain, readers are referred to several excel-lent reviews (Bolay and Moskowitz, 2002; Campbell, 2001;Campbell and Meyer, 2006; Woolf et al., 2004; Zimmermann,2001).

Based on such documented changes in neuronal functionunder conditions of neuropathic pain, a variety of drugs havebeen prescribed in hopes of controlling such pain. By-and-large these drugs fail in the great majority of patients whenused to treat clinical pain, including anticonvulsants (McQuayet al., 1995), antidepressants (McQuay et al., 1996), opioids(Kingery, 1997), gabapentin (Sindrup and Jensen, 2000),pregabalin (Finnerup et al., 2005), and others. Some drugswork partially in some patients. But even when combinationsof drugs are administered that target different putative causesof neuropathic pain, they fail (Sindrup and Jensen, 1999). Thequestion naturally arises as to why current therapies fail tocontrol neuropathic pain, given that their development anduse were predicated on the results from animal models ofneuropathic pain, described above. Were the results obtainedfrom such animal models that documented neuropathy-induced changes in neuronal function simply wrong?

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Alternatively, might there be a critical additional mediator forthe creation and maintenance of neuropathic pain?

A potentially key, recent discovery is the role of theimmune system in pathological pain states, including neuro-pathic pain.Within the last decade, research has accrued at anever-accelerating rate that supports the idea that immunecells in and around peripheral nerves, and immune-like glialcells in spinal cord, are key players in both the creation andmaintenance of pathological pain states. Within just the past5 years, this concept has taken a second leap forward with therecognition that these same immune-like glial cells compro-mise the efficacy of opioids for pain control.

The purpose of this review is to explore these issues, with afocus on publications within the past 5 years.We first focus onsensory nerves and then on sensory nerve somas as thetargets of immune actions that enhance pain. The discussionthen moves centrally to explore the role of immune-like glialcells both in pathological pain as well as in dysregulating theactions of analgesic drugs such as opioids. The basicconclusion will be that immune and glial activation can haveprofound effects on neuronal responses to pain and opioids,such that pain signaling is amplified and opioid efficacy isdiminished as a consequence of inflammatory mediatorsreleased by these non-neuronal cells.

2. Immune activation as a driving force forpathological pain states

2.1. Localized immune activation associated withperipheral nerve injury and inflammation

2.1.1. Immunology of peripheral nervesIn healthy peripheral nerves, most of the ongoing immunesurveillance is accomplished by immune cells that residewithin the nerve itself (Moalem and Tracey, 2006; Myers et al.,2006). Resident immunocompetent cells include Schwanncells, fibroblasts, endothelial cells, dendritic cells, macro-phages, and mast cells. Here, immunocompetent refers tocells that can respond to inflammation, infection, and/ortrauma by the production and release of inflammatorymedia-tors classically thought of as immune-derived. In healthynerves, these cells are “resting” in the sense that they provideactive surveillance of the nerve's microenvironment but arenot releasing proinflammatory mediators as they do uponactivation. With the exception of circulating activated Tlymphocytes (Ho et al., 1998), blood-borne immune cells haverelatively limited access to peripheral nerves under normalcircumstances, due to the blood–nerve barrier (Olsson, 1990).

This scenario changes dramatically upon trauma to, andinflammation of, peripheral nerves. Upon activation by nervetraumaor inflammation, a number of these immune cells releasechemoattractant cytokines called “chemokines” (e.g., macrophageinflammatory protein-2 andmonocyte chemoattractant protein-1 [MCP-1]) that recruit neutrophils and macrophages from thecirculation into nerve and proinflammatory cytokines, as well asnitric oxide (NO) and reactive oxygen species (ROS) that killinvading microorganisms. In addition to these useful functions,these proinflammatory chemokines and cytokines, NO, and ROScan unfortunately also directly increase nerve excitability,

damagemyelin, and disrupt the blood–nerve barrier, thus furtherfacilitating the movement of immune products into damagednerve. In addition, some activated resident immune cells releasedegradative enzymes and acids in response to nerve trauma thatexposes peripheral nerve proteins (e.g., P0, P2). Nerve proteinssuch as P0 and P2 are responded to as “non-self” as they arenormally buried within the myelin sheath and not detected byimmune cells (Hughes et al., 2006). Once released, immune-derived enzymes and acids attack myelin and disrupt the blood–nerve barrier, again allowing increased access of the nerve toblood-borne immune cells (Olsson, 1990).

Clinically relevant peripheral nerve damage can occur as aresult of antibody attack, complement activation, and T-lym-phocyte attack, in addition to frank trauma and inflammation(for detailed review and clinical correlations see Watkins andMaier, 2002). However, given space constraints, discussion belowwill be limited to trauma-induced nerve damage and itsassociated inflammatory components. This is because thesehave been the focus of the most laboratory animal research todate. This focus also arises, in part, from the clinical findings thatnerve biopsies from both traumatic and inflammatory neurop-athies indicate that nerve biopsy levels of proinflammatorycytokines directly correlatewith both the degree of axonal degen-eration and neuropathic pain (Lindenlaub and Sommer, 2003).

2.1.2. Peripheral nerve changes in response to traumaThe possibility that immune cells are involved in the develop-ment of neuropathic pain arose in the mid-1990s (Frisen et al.,1993; Sommer et al., 1993). Research that followed providedconvincing evidence that immune cells activated as a result ofpartial nerve injury importantly contribute to the resultantexaggerated pain state as well as axonal hyperexcitability andWallerian degeneration (Moalem and Tracey, 2006; Myerset al., 2006). Nerve injury leads to an immediate activation ofcalpain, a calcium-activated protease in Schwann cells andassociated myelin (Watkins and Maier, 2002). Calpain, inaddition to destroying myelin, causes a rapid burst of proin-flammatory cytokine and chemokine release from injuredSchwann cells (Uceyler et al., 2007) that, in turn, leads to therecruitment of circulating neutrophils andmacrophages to theinjury site and further proinflammatory cytokine and chemo-kine release (Shamash et al., 2002; Sommer and Schafers, 1998;Sorkin et al., in press). Driven by TNF and IL-1, the extracellularprotease matrix metalloproteinase-9 (MMP-9) is induced inSchwann cells and macrophages, contributing to neuropathicpain, neurovascular permeability, immune cell recruitment,demyelination, and degeneration (Chattopadhyay et al., 2007).The anti-inflammatory cytokine IL-10 also rapidly falls afterinjury, further disrupting the balance between pro- and anti-inflammatory cytokine influences (George et al., 2004).

Both trauma-induced Wallerian degeneration (Beuche andFriede, 1984) and enhanced pain (Myers et al., 2006; Sorkin et al.,in press) have been associated with the activity of macrophagesrecruited to the site of injury. Simply delaying macrophagerecruitment to the site of nerve injury delays both thedevelopment of neuropathic pain and Wallerian degeneration(Liu et al., 2000b;Myers et al., 1996). Indeed, encasing a transectednerve in silastic tubing to reduce contact with recruited immunecells and their products reduces pain-like behavior (Okuda et al.,2006). Conversely, actively attracting macrophages to the injury


site enhances neuropathic pain (Maves et al., 1993). Notably,activated macrophages have been found to persist for yearswithin human painful neuromas, suggestive that they mayperseveratively influence pain from peripheral nerve damage(Durrenberger et al., 2006).

The proinflammatory cytokines (TNF, IL-1, IL-6) appear to bethe key immune-derived factors as they increase at the site oftrauma in parallel with pain enhancement (Cui et al., 2000;Okamoto et al., 2001), being produced bymacrophages aswell asother resident immune and recruited immune cells. Injection ofproinflammatory cytokines onto or into peripheral nervesenhances pain responsivity (Sorkin and Doom, 2000; Zelenkaet al., 2005). Blockade of proinflammatory cytokine actions at thelevel of the sciatic nerve reduces neuropathic pain, as well asreducing immune cell recruitment and demyelination (Ma andQuirion, 2006a; Shamash et al., 2002; Sommer et al., 1998, 1999).Treatments, such as thalidomide which concomitantlydecreases the proinflammatory cytokine TNF and increasesthe anti-inflammatory cytokine IL-10, prevent neuropathic pain(George et al., 2000). Furthermore, neuropathic pain is preventedin IL-6 knockout mice (Ramer et al., 1998) and the magnitude ofneuropathic pain correlates with both the number of activatedmacrophages and the number of IL-6-producing cells at the siteof injury (Cui et al., 2000). As (a) the effects of proinflammatorycytokines synergize, and (b) each increases the production andrelease of the others, it is to be expected that the family ofproinflammatory cytokines, rather than simply one member, isimportantly involved (Watkins and Maier, 2002).

Recent data have also provided evidence that, in addition tomacrophages, mast cells, T lymphocytes, neutrophils, andSchwann cells all contribute to neuropathic pain at the site ofnerve injury (Moalem and Tracey, 2006; Sorkin et al., in press).Also, products produced by immune cells beyond proinflamma-tory cytokines have now been implicated, including ROS, NO,growth factors, prostaglandins, activation of the complementcascade, and mast cell products (Liu et al., 2000a; Ma andEisenach, 2003; Takahashi et al., 2004; Theodosiou et al., 1999).As one example, mast cells release tryptase, which activatesprotease-activated receptor-2. This tryptase receptor modulatesthe function of transient receptor potential vanilloid-1 (TRPV1)channels, lowering their threshold for activation from 42 °C towell below body temperature (Dai et al., 2004; Moalem andTracey, 2006). As functional TRPV1 channels are expressedmid-axon, this provides a mechanism whereby mast cell degranu-lation could directly increase spontaneous activity and excit-ability of sensory neurons. This may have clinical relevance asmast cells remain elevated at sites of nerve trauma long afterinjury andmast cell degranulation/release has been proposed tocontribute to neuroma pain in humans (Sorkin et al., in press).

Further support for the importance of immune cells inneuropathic pain comes from the study of clonidine, analpha2-adrenoceptor agonist. While alpha2 adrenoceptorsare absent from normal nerves, levels elevate in injurednerve due to expression by recruited macrophages, lympho-cytes, and other immune cells. Peri-sciatic clonidine at the siteof nerve injury both prevents and reverses neuropathic pain(Lavand'homme et al., 2003; Romero-Sandoval and Eisenach,2006). This effect is associatedwith a reduction of IL-1 and TNFin sciatic nerve as well as with an elevation of the anti-inflammatory cytokineTGF-beta1 (Lavand'hommeet al., 2003).

2.1.3. Peripheral nerve changes in response to inflammationIt is important to distinguish the roles of inflammation vs.trauma in neuropathic pain. This is important because (a)nerve trauma produces both frank injury as well as inflam-mation, making the relative contributions of these 2 factorsentangled when studying trauma models, and (b) approxi-mately half of clinical neuropathies involve infection/inflam-mation in the absence of frank trauma (Said and Hontebeyrie-Joskowicz, 1992). Hence, inflammatory regulation of nervefunction in the absence of traumatic injury is of both theo-retical and clinical interest.

The first evidence from animal models that the manipula-tion of a nerve in the absence of frank trauma can producepain enhancement came from the demonstration that simplyplacing immunologically activated immune cells near healthyseaslug (Aplysia) sensory nerves increases their excitability(Clatworthy et al., 1994). Later studies in rats reported thatplacing gut suture (Maves et al., 1993), killed bacteria (Eliavet al., 1999), algae protein (carrageenan) (Eliav et al., 1999),yeast cell walls (zymosan) (Chacur et al., 2001), or viralcomponents (Herzberg and Sagen, 2001) near healthy sciaticnerves produces enhanced pain responses. Such changes inpain responsivity aremimicked by proinflammatory cytokines(Chacur et al., 2001). For example, TNF injected into the sciaticnerve enhances pain (Sorkin and Doom, 2000) and inducesectopic activity in single primary afferent “pain”-receptivefibers (Sorkin et al., 1997). Such effects are not restricted toproinflammatory cytokines as ATP, ROS, complement activa-tion, and phospholipase A2 similarly increase pain behaviorsand peripheral nerve excitability (Chacur et al., 2004; Irnichet al., 2002; Twining et al., 2004; van der Laan et al., 1998).

As observed in studies of nerve trauma (see above), peri-sciatic clonidine also reduced pain enhancement induced bysciatic inflammation. In addition, it prevented peri-sciaticzymosan-induced immune cell recruitment to the site andconsequent elevations in proinflammatory cytokines(Romero-Sandoval and Eisenach, 2006). Thus both inflam-matory as well as traumatic neuropathy models supportthat clonidine may be useful in controlling pain in suchcircumstances.

2.1.4. Effects of immune-derived substances on peripheralnerveIt is now clear that proinflammatory mediators can enhanceperipheral nerve excitability and consequent pain behaviors.For illustrative purposes, proinflammatory cytokines will beconsidered here.

While proinflammatory cytokine receptors are expressedon DRG cell bodies and peripheral terminals in skin, whetherthese receptors are expressed along the course of theperipheral nerve fibers has, with rare exception (Shubayevand Myers, 2001), never been investigated. As other receptors,such as those responsive to ATP and capsaicin, are nowrecognized to be functionally expressed along axons (Hayeset al., 1984; Irnich et al., 2002; Moalem et al., 2005), it is feasiblethat receptors for proinflammatory substances may be foundto exist along the length of axons as well. This would be inkeeping with the rapid increases in neuronal excitationobserved for TNF, suggestive of receptor-mediated, directeffects on axons (Sorkin et al., 1997).


Support for this idea comes from the observation that TNFis retrogradely transported from the site of nerve injury to DRGsomas of both injured as well as adjacent spared axons(Schafers et al., 2003a; Shubayev and Myers, 2001). Intriguingly,this retrogradely transported TNF is bound to TNF receptors.This suggests that TNF internalization and transport occurredfollowing TNF binding to its receptors expressed along the axon(Shubayev and Myers, 2001). Thus TNF may not only directlyenhance axonal excitability at the site of nerve injury but, inaddition, may produce distant changes in neuronal function aswell. Such receptor-mediated retrograde transport is thought toregulate gene expression and exert autocrine/paracrine actionswithin the DRG (see below). This provides one potentialmechanism for the increased neuronal excitability of sparedsensory neurons that is known to occur as a result of nearbyneuroinflammation (Gold, 2000).

In addition to receptor-mediated effects, studies of thestructure of TNF suggest that it may be able to trimerize andinsert itself into lipid membranes, forming cation-permeablepores (Kagan et al., 1992). This effect is enhanced underconditions of lowered pH, as occurs at sites of inflammation(Baldwin et al., 1996) and is a common theme by which theimmune system seeks to destroy cells under pathologicalconditions. In addition, TNF, as well as other proinflammatorycytokines, may alter the functioning of endogenous sodium andcalcium channels, thereby increasing membrane conductance(Qiu et al., 1998; van der Goot et al., 1999; Wilkinson et al., 1996).

Lastly, TNF causes demyelination of peripheral nerves, atleast in part via TNF-induced release of extracellular pro-teases, such as MMP-9 (Chattopadhyay et al., 2007). Suchdemyelination of injured sensory nerve fibers induces activeremodeling of the exposed axonal membrane. Remodelingleads to the insertion of sodium channels into exposedmembrane, a process normally inhibited by the presence ofmyelin. Such de novo expression of sodium channels indemyelinated nerve is associated with ectopic action poten-tials and neuropathic pain (Devor, 2006).

Thus, taken together, there are multiple mechanisms bywhich immune products can alter neuronal function, both byreceptor mediated and receptor independent events.

2.2. Localized immune and glial activation impacts thefunction of sensory neurons in dorsal root ganglia (DRG)

2.2.1. Immunology of DRGsUnder normal conditions, DRGs are comprised of sensoryneuronal cell bodies and their proximal processes (includingtheir enwrapping Schwann cells), clusters of glially derivedsatellite cells which closely envelop each neuronal cell body,dendritic cells, macrophages that are in close contact withneuron/satellite cell complexes, and dense networks ofendothelial cells that lack a blood–brain or blood–nerve barrier(Hanani, 2005; Olsson, 1990). Thus, other than neuronal cellbodies, all of the constituents of DRGs are immune orimmune-like cells in that each is known to be capable ofproducing proinflammatory cytokines and other substancesclassically considered as “immune-derived” (Watkins andMaier, 2002).

Of these cell types, perhaps the most mysterious are thesatellite cells. Satellite cells have only recently begun to be

investigated for their potential roles in regulating pain(Hanani, 2005). What is known is that satellite cells are similarto astrocytes in that they upregulate glial fibrillary acidicprotein (GFAP) upon activation and that activated satellitecells express and release an array of neuroexcitatory sub-stances, including proinflammatory cytokines (Takeda et al.,2007). The presence of satellite cells enhances the response ofDRG neurons to inflammatory mediators (Heblich et al., 2001)and activated satellite cells increase the neuroexcitability ofDRG neurons, at least in part, via the release of IL-1 (Takedaet al., 2007). In addition, satellite cell P2X7 ATP receptors areupregulated in DRGs of humans with chronic neuropathicpain (Chessell et al., 2005), suggestive that satellite cells haveenhanced responsivity to ATP released by either damagedsensory neurons or by resident or recruited immune cells.Satellite cells can also regulate extracellular levels of excit-atory amino acids, given their expression of the glial glu-tamate transporter GLAST (Berger and Hediger, 2000). Lastly,they appear to affect neuronal excitability via gap junctions,which form a rapid, non-synaptic means of cell-to-cell com-munication (Hanani, 2005). DRG gap junctions dramaticallyincrease both between adjoining satellite cells and betweensatellite and neuronal cells, in response to peripheral inflam-mation or peripheral neuropathy. These changes suggest that,after peripheral inflammation, satellite cells may now haveincreased regulation of neuronal excitability (Cherkas et al.,2004; Dublin and Hanani, 2007). Indeed, disruption of gapjunction communication, using a peripherally restricted gapjunction inhibitor, prevents pain enhancement from periph-eral inflammation (Dublin and Hanani, 2007).

2.2.2. DRG changes in response to herniated discsAlterations in DRG function are relevant to pathological painarising both from peripheral nerve inflammation/trauma (seebelow), as well as from inflammation of the DRG itself, such asoccurs in response to herniated discs. DRG changes induced byherniated discs have been a focus of study, given that backpain from herniated discs is a common source of severe pain.Acute mechanical compression of the DRG is sufficient toproduce spontaneous activity in sensory afferents and toupregulate DRG mRNA and protein levels of proinflammatorycytokines (e.g., TNF, IL-1, IL-6), proinflammatory chemokines(e.g., MCP-1) as well as other proinflammatory products(Kawakami et al., 1999; White et al., 2005; Xie et al., 2006). Inaddition to the direct effects of DRG mechanical compression,herniated discs are perceived by the immune system as“foreign” and so induce a localized inflammatory response(Doita et al., 1996). These herniated disc-stimulated, DRG-derived proinflammatory mediators are implicated in painenhancement (Homma et al., 2002; von Banchet et al., 2005).Indeed, proinflammatory cytokines, proinflammatory chemo-kines, and “inflammatory soup” (bradykinin, serotonin, PGE2,and histamine) each enhance excitability of DRG neurons innormal rats, an effect that is greatly increased in neurons fromrats with chronically compressed DRGs (Liu et al., 2002; Maet al., 2006; White et al., 2005; Zhang et al., 2002a). Correlatedwith this increase in neuronal excitability, TNF, IL-1, and IL-6applied to DRG neurons in vitro increase the release “pain”transmitters (e.g., substance P, CGRP) (Obreja et al., 2005, 2002;Opree and Kress, 2000), suggesting that proinflammatory


cytokine induction in DRG may enhance pain by enhancingsensory neuron release of “pain” transmitters in spinal cord.Behaviorally, exposure of the DRG to nucleus pulposus (theshock absorbing central part of vertebral discs subject toherniation) produces pain enhancement, which is greaterwhen combined with DRG compression (Kawakami et al.,2000a). Indeed, such pain enhancement directly correlateswith elevations in DRG-derived proinflammatory mediatorsnoted above (Kawakami et al., 1999).

In addition to DRG sources of proinflammatory mediators,there is growing evidence that immune factors derived fromthe herniated disc itself are involved, as well. Herniated discsare sources of a variety of neuroexcitatory substances,including NO, TNF, IL-1, IL-6, chemokines, prostaglandins,thromboxane, leukotrienes, and phospholipase A2. Thisprofile of proinflammatory products has led to the suggestionthat these herniated disc-derived substances may be anadditional major factor in the creation of back pain (Ahnet al., 2002; Burke et al., 2002). Indeed, enhanced pain inducedby exposure of DRG to nucleus pulposus is reduced byblockade of proinflammatory factors, including TNF, throm-boxane, leukotrienes, and phospholipase A2 (Kawakami et al.,2001, 1997). Furthermore, DRG exposure to nucleus pulposusor other inflammatory mediators (e.g., IL-1, TNF) leads tospontaneous electrical activity in sensory afferents and en-hanced pain responses, effects which are reduced by appro-priate inhibitors (Cuellar et al., 2004; Murata et al., 2006; Obrejaet al., 2002; Ozaktay et al., 2006; Xie et al., 2006).

2.2.3. DRG changes after peripheral nerve injuryAlternations in DRG anatomy and function occur in responseto peripheral nerve injury, as well. Responses of DRG neuronsare sensitized following nerve injury such that, in injured DRG,very low TNF doses produce faster, greater, and longer lastingactivity in DRG neurons than observed for DRG neurons withhealthy axons (Schafers et al., 2003b). In addition, peripheralnerve injury distant from the DRG causes immune cells (e.g.,macrophages, T-cells) to be recruited into the DRG (Hu et al.,2007; Hu andMcLachlan, 2002; Peters et al., 2006) and DRG glialsatellite cells to proliferate and become activated (Peters et al.,2006; Sorkin et al., in press; Xie et al., 2006) (see also Section2.2.1, above). Such changes in DRGs are observed not only aftertraumatic injury to peripheral nerves, but also after otherpain-enhancing manipulations, as well. These include che-motherapy-induced neuropathies (Jimenez-Andrade et al.,2006), bone cancer (Peters et al., 2005), exposure of DRGs tonucleus pulposus (Murata et al., 2004; Obata et al., 2002), spinaltransection (McKay and McLachlan, 2004), and ventral (motor)root transection which exposes intact sensory neurons toWallerian degeneration of inter-mingled transected motoraxons (Li et al., 2002). Thus satellite cell activation andimmune cell recruitment into DRG are responses common toa broad range of manipulations that enhance pain.

Invasion of the DRG by immune cells after nerve injury isthought to have functional consequences. One observation insupport of this is that most macrophages recruited into DRGsafter peripheral nerve injury concentrate around neuronswithdamaged axons (Hu et al., 2007). Macrophages recruited toDRG after nerve injury express at least PGE2, IL-6 and CGRP(Ma and Quirion, 2006b). These inflammatory mediators

facilitate spontaneous ectopic activity in neurons and amplifytheir responsivity to nociceptive stimuli (Ma and Quirion,2006b). Similarly, depletion of circulating immune cells so toprevent their recruitment into DRG inhibits the developmentof hyperalgesia in response to DRG exposure to nucleuspulposus (Kawakami et al., 2000b).

While it is clear that DRG anatomy and function are alteredin response to peripheral nerve injury, it is not as yet clear whysuch changes occur. One hypothesis is that DRG alters its geneexpression due to signals retrogradely transported from thesite of peripheral nerve injury (Myers et al., 2006) (also, seeabove). For example, following peripheral nerve transection,about one third of DRG neurons express IL-6 mRNA. Whileblocking retrograde transport by injection of colchicine into anotherwise healthy sciatic nerve did not induce IL-6 mRNA inDRG, injection of colchicine into the nerve stump aftertransection prevented DRG IL-6 mRNA induction (Murphyet al., 1999). This led to the conclusion that IL-6 is induced byan injury factor in the nerve stump. Immune productsreleased at the site of nerve injury are obvious candidates.For example, mast cell degranulation (which releases a host ofinflammatory mediators) in intact sciatic nerve was sufficientto stimulate IL-6 mRNA induction in DRG and, correspond-ingly, administration of mast cell stabilizing agents decreasedIL-6 mRNA induction in DRG after sciatic injury (Murphy et al.,1999). While the identity of such retrograde signals is thesubject of ongoing investigation, TNF, leukemia inhibitoryfactor (LIF), IL-6, and nerve growth factor (NGF) retrogradelytransported from the site of peripheral nerve injury to DRGhave each been posited be such signals (Myers et al., 2006).

Intriguingly, it would appear that retrograde transport occursin intact nerves as well as damaged ones as DRG neurons withhealthy axons show altered gene expression quite similar tothose of damaged neurons (Campbell andMeyer, 2006). As notedabove, retrograde transport of such signals and resultantalterations in DRG function may contribute to neuropathy-induced enhanced pain responses of healthy, in addition todamaged, sensory neurons (Gold, 2000).

An additional issue is how peripheral nerve injury leads tothe: (a) activation of DRG glia (satellite cells), (b) activation ofother non-neuronal DRG cells (e.g., dendritic cells, macro-phages), and (c) recruitment and activation of blood-borneimmune cells (e.g., macrophages, T-lymphocytes, neutrophils)(McLachlan et al., in press). Neuronally derived NO is onecandidate that has been proposed to account for such effects.DRG neuronal NO synthase is upregulated in response to bothperipheral nerve injury and inflammation, and NO can activatesatellite cells (Dublin and Hanani, 2007). In addition, injury-induced upregulation of proinflammatory cytokines in DRGsomas has been postulated as serving an autocrine/paracrinerole serving chemoattractant and activating signals for immuneand glial cells (Copray et al., 2001). Injury also upregulatesproinflammatory chemokines such as MCP-1, first in neuronsand later by satellite cells, which acts as a chemoattractant forimmune cell influx aswell as enhancedpain (McLachlan et al., inpress). Thus multiple immune-derived signals in DRG may wellenhance pain.

In summary, satellite glial cells, as well as resident andrecruited immune cells, are well-positioned to alter theexcitability of both damaged and healthy DRG neurons.


2.3. Peripheral nerve injury leads to glial activation inspinal cord

2.3.1. Communication from neurons to gliaFollowing inflammation or trauma of peripheral tissues orperipheral nerves, microglial and astrocytic activation occursat a distance; that is, within (at least) the spinal cord. As will bediscussed below, this glial activation drives pain amplifica-tion. This link between glial activation and pain enhancementwas first recognized in the early-to-mid-1990s (Garrison et al.,1994, 1991; Meller et al., 1994;Watkins et al., 1994). Studies thatfollowed substantiated, using diverse animal models andendpoints, that spinal cord glia are key players in the creationand maintenance of enhanced pain states, including neuro-pathic pain (Watkins and Maier., 2003).

However, the fact that glia within the spinal cord becomeactivated as a consequence of inflammation/damage in theperiphery raises a fundamental question. The central issueis simple: how these spinal cord glia “know” to become acti-vated following nerve injury that has occurred far from thespinal cord? While this phenomenon was discovered∼30 years ago in a context independent of pain (Kreutzbergand Barron, 1978), the underlying mechanisms are only nowbeing clarified.

The signal has to come from either neurons or humoralfactors. Indeed, a number of neuron-to-glia signals that“trigger” glial activation have recently been identified. Mostof this work has supported a key role of microglia as exquisitesensors of “not self” or “not normal”, leading them to be morehighly responsive to CNS challenges than other cell types(Kreutzberg, 1996). Microglia are also frequently the earliestglial cell type to be activated in response to peripheralinflammation and injury (Ledeboer et al., 2005; Raghavendraet al., 2003). While the current literature suggests that, by-and-large, microglia are activated first and their activation, in turn,leads to the activation of astrocytes (Tanga et al., 2004), it isalready clear that astrocytes can also be initiators of patho-logical pain under certain conditions (Sorkin et al., 2006).

The best documented of the neuron-to-glia signals arebriefly reviewed below.

2.3.2. Neuronal transmitters and modulatorsSensory neurons release an array of neurotransmitters inspinal cord upon their activation by noxious stimuli. Theseinclude substance P, excitatory amino acids, and ATP. Spinalcord astrocytes become activated by each of these transmit-ters via binding to NK-1 receptors, metabotropic glutamatereceptors, ionotropic non-NMDA receptors (AMPA and kai-nite), as well as NMDA receptors (Aicher et al., 1997; Besonget al., 2002). Indeed, glia may be maximally responsive underconditions where more than one activation signal isreceived. For example, spinal cord astrocyte release of ATPis far greater when stimulated by glutamate plus substance P,than by either transmitter alone (Werry et al., 2006). Receptoractivation, in turn, leads to the activation of microglia andastrocytes and their consequent release of neuroexcitatorysubstances, including prostaglandins, IL-1, IL-6, and NO(Svensson et al., 2005, 2003; Tikka and Koistinaho, 2001).

In addition, there is growing evidence that microglia canenhance pain following their activation by extracellular ATP.

Microglia are exquisitely responsive to extracellular ATP,whether released by cellular damage (see below), nearbyastrocytes, or neurons from either synaptic or non-synapticregions (Davalos et al., 2005; Di Virgilio, 2006).

Two disparate lines of evidence implicate ATP in painenhancement, but via quite different underlyingmechanisms.One line of evidence argues that microglial P2X4 receptoractivation by ATP is central to enhanced pain states. Thesestudies document that P2X4 receptors are strongly upregu-lated in microglia in spinal cord dorsal horn in response toperipheral nerve injury, an effect driven by fibronectin (Beggset al., in press; Inoue, 2006). Treatment with P2X4 antisensereduces neuropathic pain, thereby implicating P2X4 signalingin such pain changes (Beggs et al., in press). Furthermore,perispinal (intrathecal; into the cerebrospinal fluid surround-ing the spinal cord) injection of microglia activated by ATP invitro is sufficient to enhance pain (Tsuda et al., 2003) and doesso through the release of brain-derived neurotrophic factor(BDNF) (Trang et al., 2006). In dorsal spinal cord, microglialBDNF release causes neuroexcitation by decreasing GABA-ergic and glycinergic inhibition (Trang et al., 2006).

Beyond P2X4, additional lines of evidence implicate micro-glial P2X7 and P2Y receptors in pain enhancement. Activation ofthese P2X7 receptors leads, in turn, to activation of P38 MAPkinase and to rapid release of IL-1, TNF, and superoxide (Inoue,2006; Shieh et al., 2006; Zhang et al., 2005). Indeed, P2X7 acti-vation by ATP has been argued to be the most potent stimulusfor the release of IL-1 (Di Virgilio, 2006). P2X7 knockout miceshow suppressed inflammatory pain and neuropathic painwhile retaining normal responses to pain under basal conditions(Honore et al., 2006b). In addition, rats treated with selectiveP2X7 inhibitors show reduced inflammatory pain and neuro-pathic pain, effects that were consistent across multiple animalmodels (Honore et al., 2006b). Such effects were evident at bothbehavioral and spinal cord electrophysiological levels of analy-sis. Regarding P2Y, this receptor has been linked to microglialactivation and enhanced pain in response to peripheralinflammation (Haynes et al., 2006,Wu et al., 2004b).

In addition to the neurotransmitters noted above, neuronsalso upregulate their release of NO, prostaglandins, anddynorphin under conditions where pain facilitation occurs.Neuronal NO enhances spinal production and release of TNF,IL-1 and IL-6 (Holguin et al., 2004) and PGE2 is important in theinitiation of both glial activation and neuropathic pain(Takeda et al., 2005). Lastly, the pain enhancing effects ofneuronally derived spinal dynorphin have recently beenlinked to selective activation of microglial p38 MAP kinasewhich, in turn, leads to the release of PGE2 and IL-1 thatenhances pain (Laughlin et al., 2000; Svensson et al., 2005).

2.3.3. Neuronal chemokinesChemokines are chemoattractant cytokines, consisting of afamily of over 50 proteins. While initially thought to be solelyof immune origin, it is now clear that neurons can produceand release a number of chemokines as well (Stievano et al.,2004).

Fractalkine was the first chemokine discovered to be aneuron-to-glia signal. It is an unusual chemokine as it istethered to the extracellular surface of neurons and is releasedupon strong neuronal activation, forming a diffusible signal


(Chapman et al., 2000). Indeed, neuropathy induces a dramaticreduction in membrane bound fractalkine in DRG and spinalcord neurons, suggestive of release (Zhuang et al., in press). Inspinal cord, the receptor for neuronal fractalkine is expressedonly by microglia and its expression in microglia is upregu-lated under neuropathic pain conditions as well as by arthritis(Lindia et al., 2005; Shan et al., 2006; Verge et al., 2004; Zhuanget al., in press). Intrathecal administration of fractalkineenhances pain, via activation of microglial p38 MAP kinaseand the release of proinflammatory cytokines (Milligan et al.,2005; Milligan et al., 2004; Zhuang et al., in press). Importantly,endogenous fractalkine also enhances pain, as neutralizingantibodies directed against the fractalkine receptor both delayand reverse the development of neuropathic pain and arthritispain (Milligan et al., 2005, 2004; Shan et al., 2006). In agreementwith such behavioral results, electrophysiological studiesreveal that fractalkine causes hyper-responsivity of spinalneurons to brush and pinch, as well as increases in the num-bers of neurons exhibiting prolonged after-discharges indica-tive of spontaneous pain and central sensitization (Owolabiand Saab, 2006). Fractalkine-induced increases in neuronalexcitability occurred after a delay, a result supportive of anindirect action of fractalkine on spinal neurons (Zhuang et al.,in press).

While fractalkine was the first chemokine implicated as aneuron-to-glia signal, it is not alone. Growing evidenceimplicates chemokines such as interferon-inducible proteinof 10 kDa (IP-10) and monocyte chemoattractant protein-1(MCP-1) as signals that are rapidly induced in, and releasedfrom, damaged neurons (Biber et al., 2002; Rappert et al., 2004).Indeed, neurons also package and transport such chemokinesto distant presynaptic terminals. This allows for remoteactivation of glia which express receptors for these chemo-kines, as occurs in spinal cord following peripheral nerveinjury (Zhang and De Koninck, 2006). That such chemokinesare involved in pain facilitation is supported by the observa-tions that intrathecal injection of MCP-1 produces painfacilitation (Tanaka et al., 2004) and intrathecal administrationof neutralizing antibodies to MCP-1 suppresses neuropathicpain (Abbadie et al., 2003).

2.3.4. Signaling molecules released by damaged, dying, anddead neuronsNeurons release a variety of substance that signal their damageand death. Such signals are released in spinal cord followingtrauma to peripheral nerves, dorsal root ganglia, or spinal cord(Costigan et al., 1998). As the clearance of myelin and othercellular debris is a remarkably protracted process continuing formany years in the human CNS compared to the periphery(Vargas et al., 2006), substances released by injured neuronsunder such conditions may be able to provide ongoingstimulation to maintain equally protracted glial activation. Inaddition, there are reports of neurons in dorsal spinal cord dyingsecondary to peripheral nerve injury (Scholz et al., 2005). ATP isone major substance released by damaged and dying cells andhas been argued to serve as an intrinsic “danger signal”,triggering the activation of nearby microglia (Di Virgilio, 2006).Detection of extracellular ATP causes rapid convergence ofmicroglial processes toward its source (Davalos et al., 2005) andthe release of plasminogen, a protein which enhances NMDA

receptor function (Inoue et al., 1994). As ATP is also released as aneurotransmitter in response to painful stimuli, its involvementin glial activation has already been reviewed, above.

Other candidate neuron-to-glia signals include ligands forpattern recognition receptors, such as toll-like receptors (TLRs),that are expressed by glia and enable them to detect dangersignals (Tanga et al., 2005). Such danger signals includesubstances released by cellular injury (cell membrane compo-nents such as gangliosides and lysophosphatidic acid (LPA),nuclear components such as highmobility group box 1 (HMGB1),heat shock proteins) as well as more traditionally recognizeddanger signals such as conserved motifs expressed by bacteriaor viruses (Jou et al., 2006; Park et al., 2006). In response to nerveinjury, TLR1, TLR2, and TLR4 have each been reported to beupregulated in CNS, linked to the production of proinflamma-tory cytokines (e.g., TNF, IL-1) and chemokines (e.g., MCP-1)(Owens et al., 2005). To date, several of these damage/deathsignals have been examined behaviorally and found to enhancepain following intrathecal administration, including ATP,HMGB1, and LPA (Inoue et al., 2004; O'Connor et al., 2003).

From above it is clear that there are numerous ways forneurons to trigger the activation ofmicroglia and astrocytes inspinal cord. As will be reviewed below, this glial activation candrive pain amplification.

2.4. Glial activation impacts the function ofpain-responsive neurons in spinal cord

2.4.1. Functions of glia under basal and activated statesIn normal, healthy CNS, microglia and astrocytes serve avariety of important functions. For microglia, this is primarilyan active surveillance role, rapidly extending and retractingprocesses to constantly sample the extracellular microenvi-ronment (Raivich, 2005). Astrocytes, on the other hand, areprimarily involved in providing energy sources and neuro-transmitter precursors to neurons, cleaning up debris, regu-lating extracellular levels of ions and neurotransmitters, andactively modulating synaptic transmission (Haydon andCarmignoto, 2006).

Under basal conditions, microglia and astrocytes do notappear to be important regulators of pain transmission. Thisconclusion is based on the fact that various drugs thatsuppress glial function or block the actions of various glialproducts (such as proinflammatory cytokines) do not alterresponses to heat or mechanical stimuli in normal animals(Hashizumeet al., 2000; Ledeboer et al., 2005;Meller et al., 1994).

The roles of both microglia and astrocytes dramaticallychange when they are triggered to leave their basal conditionand enter an activated state. In response to inflammation ordamage to peripheral tissues, peripheral nerves, spinal nerves,or spinal cord, both microglia and astrocytes in spinal cordbecome activated (McMahon et al., 2005; Watkins and Maier.,2003). Their activation is inferred by upregulation of cell typespecific activation markers, such as GFAP in astrocytes andcomplement receptors for microglia. Their activation is alsoinferred by the fact that, as discussed below, intrathecaladministration of glial inhibitors resolves enhanced painresponses induced by all of the same models that induceglial activation (Tables 1 and 2). To date, fluorocitrate andminocycline have been the drugs most commonly tested in

Table 1 – Pain models associated with upregulation ofspinal cord microglial and/or astrocyte activation markers

Complete Freund's adjuvant, subcutaneousFormalin, subcutaneousPhospholipase A2, subcutaneousZymosan, subcutaneousSciatic nerve injury (chronic constriction injury)Inferior alveolar and mental nerve transectionPartial sciatic nerve ligationSciatic nerve inflammation with zymosanSciatic nerve inflammation with HIV-1 gp120Sciatic nerve inflammation with phospholipase A2Spinal nerve transectionSpinal nerve root injurySpinal cord injuryHind paw incisionBone cancerHIV-1 gp120, intrathecalLipopolysaccharide, intrathecalChronic opioids; opioid withdrawal-induced hyperalgesia

Modified and updated from Ledeboer et al. (2006).

Table 2 – Pain facilitation is suppressed or reversed byinhibition of spinal glial activation or proinflammatorycytokine actions

Model Intervention

Mustard oil, topical FluorocitrateCarrageenan, subcutaneous Minocycline, IL-1 knockoutComplete Freund's

adjuvant, subcutaneousIL-1ra, IL-1 knockout

Formalin, subcutaneous Fluorocitrate, IL-1ra, minocycline;IL-1 knockout

Phospholipase A2,subcutaneous

Fluorocitrate, IL-1ra, sTNFR

Zymosan, subcutaneous FluorocitrateHind paw incision FluorocitrateInferior alveolar and

mental nerve transectionMinocycline

Sciatic nerve injury(chronic constriction injury)

IL-1ra, IL-10; IL-1 knockout

Sciatic nerve inflammationwith zymosan

Fluorocitrate, minocycline IL-1ra,sTNFR, IL-6 antibody

Sciatic nerve inflammationwith phospholipase A2

IL-1ra, anti-IL-6, IL-10

Sciatic nervetetanic stimulation

Fluorocitrate

Spinal nerve transection Propentofylline, minocycline,IL-1ra, sTNFR, anti-IL-6,

Spinal nerve root injury Methotrexate; IL-1 knockoutSpinal cord injury IL-10, IL-1ra, minocyclineHIV-1 gp120, intrathecal Fluorocitrate, IL-1ra, sTNFR,

minocyclineLipopolysaccharide,

intrathecalIL-1ra

Dynorphin, intrathecal IL-1ra, IL-10Fractalkine, intrathecal Minocycline, IL-1ra, anti-IL-6

Abbreviations: IL-1ra, interleukin-1 receptor antagonist; sTNFR,soluble TNF receptor; IL-10, interleukin-10.Modified and updated from Ledeboer et al. (2006). For completereferences, see Clark et al. (2006), Hains and Waxman (2006),Honore et al. (2006a), Ledeboer et al. (2006), Obata et al. (2006), Piaoet al. (2006), Watkins et al. (2005), and Xie et al. (2007b).


this regard. Fluorocitrate inhibits aconitase, an enzyme in theKrebs energy cycle of astrocytes and microglia, but notneurons. Minocycline, on the other hand, inhibits microgliabut not astrocytes or neurons. This difference in the mecha-nism of action of these drugs, along with explorations of thetimecourse of expression ofmicroglial vs. astrocytic activationmarkers after amanipulation that enhances pain, has allowedthe relative roles of microglia and astrocytes to be explored.Typically, but not exclusively (Obata et al., 2006), microglia areobserved to become activated first, followed after a delay byastrocyte activation (Tanga et al., 2004). The importance ofmicroglia is generally thought to diminish, whereas theimportance of astrocytes is generally thought to increase,over time as, quite strikingly, minocycline prevents the devel-opment of pathological pain in response to diverse manipula-tions yet fails to reverse enhanced pain responses once theyhave developed (Ledeboer et al., 2005; Raghavendra et al.,2003). However, a perseverative role for microglia has recentlybeen reported (Hains and Waxman, 2006; Tawfik et al., 2007),suggestive that this issue warrants further study.

2.4.2. Mechanisms whereby glial activation affects neuronalactivityDiverse enhanced pain states are characterized by spinal cordglial activation and are suppressed by inhibitors that suppressastrocyte and/or microglial function (Watkins and Maier.,2003). Indeed, only one pain model to date fails to showevidence of glial involvement for as yet undefined reasons;that is, intramuscular acidic saline that induces bilateralallodynia (Ledeboer et al., 2006) (Tables 1 and 2). Studies aimedat identifying what glial products may modulate pain haveimplicated a host of neuroexcitatory substances that can bereleased by activated glia, including proinflammatory cyto-kines and chemokines, NO, ROS, ATP, prostaglandins, andexcitatory amino acids. Astrocytes, and recently microglia(Martineau et al., 2006), have also been documented as sourcesof D-serine, an endogenous ligand for the glycine modulatorysite on NMDA receptors. D-Serine is released in spinal cord inresponse to peripheral inflammation, causing enhanced C-

fiber mediated excitation of pain-responsive neurons (Guoet al., 2006). In addition, glia can increase neuronal excitabilityand pain via downregulation of glial glutamate transporters inspinal cord as a consequence of peripheral nerve injury (Binnset al., 2005, Tawfik and DeLeo, in press).

While the mechanisms by which most glially derivedsubstances enhance neuronal excitability are obvious, themechanisms underlying increased neuronal excitability byproinflammatory cytokines are only now becoming clear.What is evident is that their actions are not nearly as simpleas merely binding of proinflammatory cytokines to knownneuronal receptors (Dame and Juul, 2000; Holmes et al., 2004;Ohtori et al., 2004) so to induce rapid increases of neuronalexcitability (Oka et al., 1994; Reeve et al., 2000; Samad et al., 2004).Several of the effects of proinflammatory cytokines on neuronshave already been discussed above (see Section 2.1.4), sowill notbe repeated here. Beyond these, IL-1 has been shown, includingin spinal dorsal horn, to enhance neuroexcitability via indirectactions; namely, by inducing the release of substance P fromsensory afferents (Morioka et al., 2002) and increasing calcium


conductivity of neuronal NMDA receptors (Samad et al., 2004;Viviani et al., 2003). This latter effect occurs via intracellularsignaling pathways leading to the phosphorylation of NMDAreceptor subunits (Viviani et al., 2006). TNF increases theconductivity of glutamatergic AMPA receptors (De et al., 2003),increases the percentage of capsaicin-responsive cells in DRG(Nicol et al., 1997), and potentiates inward currents in neuronaltetrodotoxin-resistant sodium channels (Jin and Gereau, 2006).TNF also increases spontaneous and evoked neurotransmitterrelease from presynaptic terminals (Grassi et al., 1994; Zhuanget al., in press). TNF, and to a lesser extent, IL-1, upregulate theneuronal cell surface expression of both AMPA and NMDAreceptors while downregulating cell surface expression ofreceptors for the inhibitory neurotransmitter, GABA. Thispattern of changes would create an overall increase in neuronalexcitatory tone (Stellwagen et al., 2005). Intriguingly, the AMPAreceptors that are upregulated by TNF are quite unusual in thatthey are calcium permeable, suggesting that these AMPAreceptors likely contribute to the production of neuronal NOand prostaglandins, furthering neuronal excitability (Beattie andStellwagen, in press). Lastly, IL-1 is implicated in neuropathy-induced downregulation of G protein-coupled receptor kinase 2(GRK2) expression in dorsal horn neurons, an action predicted toincrease neuronal excitability by decreasing receptor desensiti-zation (Kleibeuker et al., 2007).

Beyond these actions, proinflammatory cytokines lead to therelease of a host of neuroexcitatory substances, including moreproinflammatory cytokines, NO, ATP, prostaglandins, nervegrowth factors, ROS, proinflammatory chemokines, excitatoryamino acids, and BDNF (Inoue, 2006; John et al., 2005; Sperlaghet al., 2004;Watkins et al., 1999). For example, TNF stimulates theover-production and release of glutamate from microglia byupregulation of microglial glutaminase (Takeuchi et al., 2006).Proinflammatory cytokines can also indirectly elevate extracel-lular glutamate levels via downregulation of glial and neuronalglutamate transporters that serve to keep extracellular gluta-mate levels low under normal conditions (Tawfik et al., 2006).Thus, taken together, proinflammatory cytokines exert multipleeffects with the end result being neuroexcitation.

A last point worth noting here is that the effects of glialproducts, such as proinflammatory cytokines, on spinal corddorsal horn neurons can depend on the presence or absence ofongoing neuropathic pain. This is exemplified by recent workdemonstrating that, in naive rats, intrathecal TNF has no effecton spinal long-term potentiation (LTP) (Liu et al., 2007). Incontrast, in neuropathic rats, TNF receptors are upregulated indorsal horn neurons and intrathecal TNF now induces spinalLTP. Also unlike spinal LTP in naive rats, induction of spinal LTPby TNF in neuropathic rats involves JNK, p38 MAP kinase, andNF-kappa B. Whether this alteration in the cascade reflectschanges in neurons vs. glia is as yet unknown (Liu et al., 2007).

2.4.3. Beyond glia: immunocompetent cells other thanastrocytes and microglia warrant considerationTo date, virtually every spinal cord study of immunedysregulation of pain has focused onmicroglia and astrocytes.However, it is highly unlikely that these cells will prove to bethe only non-neuronal cells involved in pain enhancement.Endothelial cells, fibroblasts, oligodendroglia, mast cells,dendritic cells, perivascular macrophages, recruited immune

cells, and other cell types in both the spinal cord and overlyingmeninges can also producemany of the same neuroexcitatorysubstances as do astrocytes and microglia. It is simply anaccident of evolution that astrocytes and microglia, unlikemany other immunocompetent cells, have been discovered toreadily upregulate cell-type specific mRNA and proteinproducts in a graded fashion upon activation. These so-calledactivation markers have allowed for their activation to beeasily quantified.

Recently, evidence has begun to accrue that cell types otherthan microglia and astrocytes are indeed responsive toconditions associated with pathological pain. For example,the meninges surrounding spinal cord have been demonstrat-ed to increase TNF, IL-1, IL-6, and iNOS gene expression and torelease TNF, IL-1, and IL-6 protein in response to immuneactivators such as HIV-1 gp120, administered intrathecally at adose previously documented to enhance pain (Wieseler et al.,2007). As these effects replicate in vitro in the absence ofcirculatingwhite blood cells, these proinflammatory responsesare being generated by cells intrinsic to the meninges ratherthan by immune cells recruited from the blood. Intriguingly,similar upregulation of proinflammatory products in themeninges occurs following peripheral nerve injury, implyingthat the meninges are responsive not only to inflammatorymediators within the CSF space but also to signals released bydamaged sensory afferents (Wieseler et al., 2006).

In addition, circulating immune cells can be recruited intospinal cord after peripheral nerve injury, suggesting thatimmune products released by these cells may modulate painas well. After lumbar spinal nerve lesion, marked leakage ofalbumin was observed by 2 weeks and persisted for over2 months, suggestive of a defect of the blood-spinal cord barrierfunction localized to the spinal level innervated by the injuredperipheral nerve (Gordh et al., 2006). L5 spinal nerve transectionhas also been reported to induce leukocyte trafficking into L5spinal cord (Sweitzer et al., 2002). Here, trafficking was detectedby identifying donor rat leukocytes thatmigrated into the spinalcord of host rats that had been previously bone-marrowirradiated,whichprevents rejection of the transplanted immunecells. This approach revealed recruitment of cells with macro-phage-like and T cell-like morphologies (Sweitzer et al., 2002).Using another approach, T-cell recruitment into the spinal cordwas likewise documented after peripheral nerve injury (chronicconstriction injury), but not sciatic nerve transection (Hu et al.,2007). There was a marked concentration of these cells in thedorsal horn ipsilateral to the site of sciatic injury, co-localizedwith sites of microglial activation (Hu et al., 2007). T-cellnumbers remained elevated in superficial dorsal horns even10 weeks after sciatic injury. That T-cells may contribute toneuropathic pain is supported by athymic nude rats, which lackT-lymphocytes, exhibits reduced neuropathic pain in responseto chronic constriction injury (Moalem et al., 2004). Based on theobservation that T-cell recruitment only occurred after sciaticnerve damage that spared sciatic axons (i.e., it occurred afterchronic constriction injury but not after sciatic transection), anas yet unidentified retrograde signal has been postulated to beinvolved in T-cell recruitment to spinal cord (Hu et al., 2007).

From above it is clear that glial activation, and potentiallyrecruitment and activation of other non-neuronal cells, candramatically increase neuronal excitability and enhance pain.


The effects of glially derived proinflammatory cytokines onneuronal excitability are only now becoming well understood,with diverse effects that extend well beyond simple increasesin ion permeability upon cytokine binding.

2.4.4. Beyond spinal cord: glial regulation of pain in the brainThe study of the involvement of brain glia in pain regulation isstill in its infancy. Clarifying what role(s) glia play in brainprocessing of pain information is a very important and virtuallyunexplored topic. Within just the past year, it has become clearthat glia in the medullary trigeminal nuclei regulate pain (Guoet al., 2007; Piao et al., 2006; Xie et al., 2007a). Beyond the first CNSsynapse for sensory information, it seems highly likely that gliain the brain will be found to play important regulatory roles inpain enhancement. Aswill be reviewed in sections that follow, itis already clear that glia in the brain regulate many importantfunctions; for example, responses to opioids such as morphine(Hutchinson et al., in press-a). Furthermore, it would beanticipated that activation of brain glia would be a naturalconsequence of prolonged glial activation in spinal cord ortrigeminal nuclei, such as occurs in response to neuropathy,spinal cord injury, multiple sclerosis, and other syndromesleading to chronic pain. This is based on prior reports that glialactivation in one CNS region can lead to glial activation inprojection regions (Holguin et al., 2007; Moumdjian et al., 1991).Glial activation in brain subsequent to peripheral inflammationhas been reported (Raghavendra et al., 2004b), but whether suchglial activation is causal to, vs. correlated with, inflammation-induced pain enhancement is as yet unknown. The possibilitythat peripheral inflammation-induced activation of brain gliamay regulate pain has strong precedence, given the largeliterature on immune-to-brain communication having profoundeffects on brain functioning as a result of glial activation andproinflammatory cytokines, such as induction of an array ofsickness responses including fever, alterations in food andwaterintake, activation of brain-to-spinal cord circuitry to enhancepain, induction of depressive-like behaviors, and so on (Maierand Watkins, 1998; Watkins and Maier, 2000). Regarding pain,glia may be involved beyond basic sensory processing. Forexample, glia in the anterior cingulate cortex are now impli-cated in the induction of pain-related aversion via glial release ofD-serine, the endogenous ligand of the glycine modulatory siteon NMDA receptors (Ren et al., 2006).

3. Immune activation as a driving force fordisrupting the efficacy of opioids

3.1. Opioid-induced glial activation underminesopioid-induced pain suppression

The mechanisms underlying neuropathic pain and morphinetolerance are strikingly similar (Mayer et al., 1999). Given thissimilarity, it was natural to also explore whether glia mightimpact the efficacy of opioids for pain control. As will bereviewed below, it is now clear from recent studies that glia doindeed compromise the ability of opioids to suppress pain(Watkins et al., 2005; Hutchinson et al., in press-a).

Since the initial report in 2001 of a link between glia andmorphine tolerance (Song and Zhao, 2001), evidence rapidly

accumulated that chronic morphine: (a) activates both astro-cytes and microglia (Cui et al., 2006; Raghavendra et al., 2002;Song and Zhao, 2001; Tai et al., 2006), (b) activates microglialp38MAP kinase (Liu et al., 2006) and stimulates the productionof spinal proinflammatory cytokines (Johnston et al., 2004;Raghavendra et al., 2002; Tai et al., 2006), both of which areassociated with the development of morphine tolerance (Liuet al., 2006; Raghavendra et al., 2004a). That glial activationwas causal to, rather than simply correlated with, morphinetolerance was supported by the finding that morphinetolerance was slowed or reversed by either inhibition of spinalproinflammatory cytokines (Johnston et al., 2004; Raghaven-dra et al., 2002; Shavit et al., 2005) or by knockout of IL-1signaling (Shavit et al., 2005). An additional role of glia inmorphine tolerance is suggested by the finding that morphinetolerance is associated with a downregulation of glial GLASTand GLT-1 glutamate transporters (the major transportersresponsible for regulating extracellular levels of excitatoryamino acids) in spinal cord dorsal horn, which concomitantlyleads to an upregulation of extracellular excitatory aminoacids (Mao et al., 2002; Tai et al., 2006). Such data suggest thattolerance may, in part, be created by an opposing increase inneuronal excitability due to glially induced elevations inglutamate and proinflammatory cytokines.

Importantly, a link back to neuropathic pain has also beenmade. What is known from clinical and laboratory animalstudies is that morphine loses its efficacy in neuropathic painpatients and rats. That is, a state akin to tolerance prevails,despite the lack of prior opioid exposure, in other words “naiveopioid tolerance”. Importantly, spinal inhibition of proinflam-matory cytokines abolishes morphine resistance in neuropathicanimals. That is, in the presence of proinflammatory cytokineinhibition, the analgesic efficacy of even acute morphine wasrestored in rats with neuropathic pain (Raghavendra et al., 2002).This link has been strengthened by studies of “anti-analgesia”.Specifically, it has been demonstrated that prior sub-analgesicdoses of morphine reduce subsequent analgesia produced by ananalgesic dose of morphine. These effects are mediated by glialactivation since propentofylline reinstates normal analgesia(Wu et al., 2005). Moreover, p38MAPK activation is also involved,again strengthening the ties to neuropathic pain. What remainsunclear in this case is what glial products mediate these anti-analgesic responses.

In addition, while speculative, these data suggest a potentialsolution to what has come to be called “paradoxical” opioid-induced pain enhancement (King et al., 2005; Mao, 2002). Theobservation in humans as well as laboratory animals is thatrepetitive opioid exposure leads to the development of abnormalsensitivity to pain. These effects were observed even whenopioid exposure was kept constant over time in order to avoid“mini-withdrawals” between subsequent opioid doses (Mao,2002; Vanderah et al., 2000). This “paradoxical” opioid-inducedpain enhancement was interpreted to mean that “prolongedopioid treatment not only results in a loss of opioid antinoci-ceptive efficacy but also leads to activation of a pronociceptivesystemmanifested as reduction of nociceptive thresholds” (Mao,2002). To date, evidence points to the involvement of NO,dynorphin, and glutamatergic NMDA receptors (Mao, 2002; Kinget al., 2005). Given (a) the progressive activation of glia withrepeated opioid exposure and (b) the evidence that dynorphin,


NMDA, and NO all activate glia and induce the release ofproinflammatory cytokines (Holguin et al., 2004; Laughlin et al.,2000; Liu et al., 2006), exploringwhether glial activationwill solvethe “paradox” of opioid-induced abnormal pain sensitivitywould seemwarranted for both theoretical and practical clinicalreasons.

3.2. Opioid-induced glial activation contributes to opioidreward, dependence and withdrawal

In addition to morphine tolerance, evidence has now accruedthat glia may be importantly involved in morphine dependence/withdrawal as well. In dependent populations, continuedexposure to opioids is required to avoid a withdrawal syndromethat would otherwise occur when opioid dosing ceases. The firstindications that glia contribute to the development of morphinedependence came from the work of Raghavendra et al. (2002,2004a) and Johnston et al. (2004). The endpoint measured inthese studies was the pain enhancement that naturally occursupon cessation of chronically administered opioids; that is,opioid withdrawal induced pain facilitation. These studiesreported that withdrawal-induced pain enhancement is blockedby: (a) drugs or IL-10 (an anti-inflammatory cytokine) that blockglial proinflammatory cytokine production, or (b) IL-1 receptorantagonist (Johnston et al., 2004; Raghavendra et al., 2002, 2004a).

These data raised the question of whether glia may beinvolved in morphine phenomena mediated by the brain, inaddition to spinal cord. To test for brain involvement independence/withdrawal, each dose of morphine in a multi-dayregimen was co-administered with AV411 (ibudilast), a blood–brain barrier permeable glial activation inhibitor. After the lastdrug dose, withdrawal was precipitated by administering anopioid antagonist. Rats that were not given the glial inhibitorexhibited robust brain mediated withdrawal signs over time,whereas rats whose glia were inhibited during morphineexposure did not (Ledeboer et al., 2007; Lewis et al., 2006). Also,while this systemic morphine regimen upregulated astrocyticand microglial activation markers throughout the brain andspinal cord in morphine treated rats in the absence of AV411,AV411 maintained glial activation at near basal levels inmorphine-treated rats (Ledeboer et al., 2007; Lewis et al., 2006).In addition, glia may contribute to morphine reward. Microin-jection of media from activated astrocyte cultures into thenucleus accumbens or cingulate cortex increased morphineconditioned place preference, an experimental measure of drugreward. In addition, in vivo administration of the glial modula-tory drug propentofylline reduced morphine conditioned placepreference (Narita et al., 2006). Taken together, these data clearlysuggest that glia, in addition to regulating pathological pain,opioid analgesia, and opioid tolerance, now should be consid-ered as contributing to the phenomena of morphine reward andmorphine dependence/withdrawal as well.

3.3. Opioid-induced glial activation opposes acute opioidanalgesia: involvement of non-classical opioid receptors

It is also clear that, while the initial investigations reviewedabove focused on the glial activating effects of chronicmorphine, glia modulate the acute effects of opioid analgesiaas well. Indeed, they do so in a surprising way.

Opioids (met- and leu-enkephalin) stimulate the release ofIL-1 frommicroglial cultures (Kowalski et al., 2002) and blockingspinal proinflammatory cytokine increases the magnitude andduration of acute analgesia to morphine (Johnston et al., 2004;Shavit et al., 2005) andmethadone (Hutchinson et al., in press-b).Similarly, administration of a neutral dose of IL-1 (i.e., a lowdoseexerting no detectable effects on pain responsivity on its own)abolished, whereas knocking out IL-1 signaling potentiated andprolonged, morphine analgesia (Shavit et al., 2005). Intriguingly,administration of antagonists against TNF, IL-1, or IL-6 imme-diately upon apparent cessation of morphine analgesia rapidly“reinstates” analgesia suggesting that morphine-induced glialrelease of proinflammatory cytokines “masks” ongoing mor-phine analgesia by exerting an opposing effect (Coats et al., inpress; Hutchinson et al., 2006; Shavit et al., 2005). Thus IL-1 andother proinflammatory cytokines oppose the ability of acutemorphine to suppress pain.

A point worth emphasizing here is that it has been assumedthatmorphine activates glia via classical opioid receptors. Basedon the recent work of Hutchinson et al. which documented thatopioids could alter peripheral immune function via non-classicalreceptors (Hutchinson and Somogyi, 2005, 2004a,b; Wu et al.,2006a,b,c), we have begun exploring the parallel issue forglia. Unlike neuronal opioid receptors, which are stereoselec-tive, glial opioid receptors are not. Indeed, neuronally inactive[+]-methadone upregulates mRNA for IL-1, TNF, and IL-6 to atleast as great a degree as does [-]-morphine and [-]-methadoneupon their intrathecal administration (Coats et al., in press).This set of findings suggests that different receptorsmustmediatethe (classical) pain suppressive effects of morphine than its(non-classical) pain enhancing effects. The clinical implicationsof this difference are enormous as it predicts that it should bepossible to separate the neuronally mediated pain suppressiveeffects of opioids from their glial activating, pain enhancingeffects by either (a) structurally modifying opioids to preventtheir binding to the (as yet unidentified) glial non-stereoselec-tive opioid receptor or (b) co-administering an [+]-opioid antag-onist which would block glial activation while allowing opioidactions on neurons to remain unaltered. Indeed, [+]-naloxone(which is inactive at neuronal opioid receptors) potentiatesmorphine analgesia, delays the development of morphinetolerance, decreases [-]-naloxone-precipitated withdrawal beha-viors (Coats et al., in press), and blocks morphine-induced anti-analgesia (Wu et al., 2006a,b, 2004a, 2005). Intriguingly, there isadditional evidence of non-classical opioid activity of degradedendogenous opioids (Vanderah et al., 2000) suggesting that non-classical opioid receptors possibly mediate counter regulatorysystems of pain control. Thus early indications are that pre-venting opioid activation of gliamay well be a clinically relevantstrategy for increasing opioid analgesia while decreasing thenegative consequences of repeated opioids.

4. Evidence for immune/glial regulation ofpain from studies of human chronic pain patients

As the balance of the expression of proinflammatory cytokinesrelative to anti-inflammatory cytokines is of major importancein defining their impact, it is of interest to explorewhether eitherelevated proinflammatory cytokines or suppressed anti-


inflammatory cytokines may influence human pain or respon-sivity to opioids. To our knowledge, there have been virtually nopublished reports exploring whether cytokines regulate opioidanalgesia, tolerance, or dependence in people. All that is knownis that (a) IL-1ra polymorphisms that can alter the balance of IL-1and IL-1ra may influence postoperative morphine consumption(Bessler et al., 2006), and (b) the anti-inflammatory cytokine IL-4induces mu- and delta-opioid receptor transcription via IL-4responses elements in these opioid gene promoters (Borneret al., 2004; Kraus et al., 2001). In humans, a single nucleotidepolymorphism within the IL-4 response element reduces itstranscriptional activating potential by 50%, suggesting that apolymorphism in this region could affect human opioid receptorexpression (Kraus et al., 2001).

On the other hand, a few human studies have recentlyappeared which suggest that higher proinflammatory cytokinesand/or lower anti-inflammatory cytokines may indeed beassociated with enhanced pain states. Backonja et al. (2006)reported lower anti-inflammatory cytokine expression andelevated pro-inflammatory cytokine expression in plasma and/or lumbosacral CSF of chronic pain patients with eitherneuropathic pain or fibromyalgia. Elevated glial activationmarkers, elevated proinflammatory cytokine and chemokinelevels, and suppressed levels of anti-inflammatory cytokinelevels in lumbosacral CSF have also been observed in complexregional pain syndrome patients, relative to controls (Alexanderet al., 2005, 2007). Similarly, chronic widespread pain (includingfibromyalgia) is associated with significantly lower gene expres-sion and lower serum protein concentrations for both anti-inflammatory cytokines studied (IL-10 and IL-4) (Uceyler et al.,2006). Elevated serum proinflammatory cytokines in serum and/or painful skin of fibromyalgia patients (reviewed inUceyler et al.,2006) and complex regional pain syndrome patients have beenreported as well (reviewed in Manning, in press).

Another avenue for exploring the potential influence of pro-or anti-inflammatory cytokines on human chronic pain comesfrom the study of gene polymorphisms. There are obviousconstraints on interpretation of such studies, as they do notimply whether the altered cytokine profiles impact the under-lying pathogenesis vs. resultant pain modulation. However,these studies may prove informative. Gene polymorphismsexpected to elevate proinflammatory cytokine production havebeen suggested to be risk factors for lowback pain (Karppinen, inpress; Solovieva et al., 2004), arthritis pain (Oenet al., 2005; Zhanget al., 2002b), discogenic pain (Noponen-Hietala et al., 2005),postoperative pain (Bessler et al., 2006), pain of burning mouthsyndrome (Guimaraes et al., 2006), and pain of inflammatoryboweldisease (Yucesoyetal., 2003).While little information isyetavailable regarding the importance of anti-inflammatory cyto-kine polymorphisms in human pain, a low IL-10 producinggenotype has been linked to chronic pelvic pain (Shoskes et al.,2002) as well as rheumatoid arthritis and painful juvenileidiopathic arthritis (Fife et al., 2006).

Complementary to the data reviewed above are scatteredclinical reports of the efficacy of cytokinemodulating compoundson pain. For example, thalidomide and its successors (i.e.,lenalidomide) can inhibit the production of proinflammatorycytokines and elevate anti-inflammatory cytokines (Manning, inpress). As such, thalidomide (which readily crosses the blood–brain barrier, allowing it to potentially influence glial function)

and lenalidomide (whose actions are restricted to the periphery)may be intriguing compounds for clinical pain control. Clinically,there is precedence for examining TNF inhibitors as infliximab (aTNF inhibitor) reduced both proinflammatory cytokine levels andpain in a study of 2 complex regional pain syndrome (CRPS)patients (Huygen et al., 2004). Several small open label studieshave supported the conclusion that thalidomide may be moder-ately to dramatically effective in chronic pain states, such asCRPS (Ching et al., 2003; Manning, in press). The only study oflenalidomide inCRPSpatients to date also reported amoderate tomarked reduction in pain (Manning, in press).

In addition to CRPS, infliximab has been reported to beefficacious in the treatment of herniated disc-induced sciatica(reviewed in Karppinen, in press). The pain-relieving effectoccurred within 3 h and lasted throughout the 3-month and1-year follow-up. Similarly, infusion of the TNF inhibitorentanercept likewise had marked beneficial effects on lumbarradicular pain (reviewed in Karppinen, in press). However,such positive results are not universally found (reviewed inKarppinen, in press).

Thus exploration of cytokine regulation of human chronicpain is in its infancy, and exploration of cytokine regulation ofopioid efficacy in humans is as yet virtually non-existent.However, what little data exist to date are supportive of theconclusions drawn from animal studies reviewed above.

Given the above, however, it may off-hand appear quitepuzzling that chronic pain is not simply, easily controlled bycommon anti-inflammatories, such as inhibitors of prostaglan-dins or administration of steroids (Hempenstall et al., 2005;Kingery, 1997). The resolution of this seeming paradoxmay lie inthe fact that what is classically thought of as inflammatory anddamaging in the periphery (such as PGE2) andwhat is classicallythought of as anti-inflammatory in the periphery (such asadrenal steroids), may not be in the CNS. For example, in theCNS, PGE2 can exert anti-inflammatory and neuroprotectiveeffects including suppression of proinflammatory cytokines andenhancement of the production of anti-inflammatory cytokines(Aloisi et al., 1999; Liu et al., 2005). In parallel, there is a growingliterature supporting that steroidal “anti-inflammatories” (de-fined by their peripheral actions) may well not be anti-inflammatory in the CNS. For example, intrathecal steroidsdecrease the expression of glial glutamate transporters andincrease spinal neuronal expression of NMDA receptors, effectspredicted to increase the excitability of neurons (Lim et al., 2005).Furthermore, adrenal steroids increase microglial activationincluding the induction of proinflammatory cytokines (Franket al., in press) and exert multiple proinflammatory effects inCNS (Sorrells and Sapolsky, 2007).

5. Conclusions

This review has approached immune/glial regulation of painand opioid actions at multiple levels. Regarding pain amplifica-tion, this review has described how immune and/or glial cellsare a natural and inextricable part of (a) peripheral nerves,wherevarious types of immune cells are in intimate contact with nervefibers and can alter axonal anatomy and function; (b) dorsal rootganglia, where the function of sensory neuronal cell bodies are


modulated by ensheathing satellite glial cells and immune cells;and (c) spinal cord, where glial cells formdynamic networks thatmaintain and spread excitation. This review has also attemptedto explore, using proinflammatory cytokines as a primeexample, the multitude of ways that products released bythese non-neuronal cells can enhance neuronal excitability.Finally, it extended the issue of enhanced neuronal excitabilityto an exploration of how activated glia dysregulate the actions ofopioids. Here again, the focus was on proinflammatory cyto-kines, which suppress the ability of opioids to control pain andcontribute to opioid tolerance and dependence/withdrawal.

There are several major points worth re-emphasizing. Firstand foremost is the importance of immunology as regardsneuropathic pain. Classical immune cells (macrophages, Tlymphocytes, mast cells, dendritic cells), immunocompetentcells that can release substances classically thought of asimmune-cell products (endothelial cells, fibroblasts, keratino-cytes), and immune-like glial cells (Schwann cells of peripheralnerve, satellite glial cells of DRG, microglia, and astrocytes of theCNS)must all be considered as contributors to neuropathic pain.The importance of immune and glial activation to neuropathicpain has potentially profound implications both for the under-standing of how such pain states occur as well as for thedevelopment of novel therapies to treat such pain. Therecognition of immune and glial involvement in such patholog-ical pain offers hope for novel approaches to pain control.

Indeed, new treatments for neuropathic pain that targetthese non-neuronal cells (Ledeboer et al., 2007, in press;Manning, in press) have recently entered clinical trialssponsored by Celgene and Avigen. Celgene is testing lenali-domide, a compound that, at most, minimally crosses theblood–brain barrier (so likely effective for peripheral immunecells but not CNS glia) and has greater potency thanthalidomide in its ability to suppress proinflammatory cyto-kine production and increase IL-10 (Manning, in press). It iscurrently being tested in 2 large multicenter, randomized,placebo-controlled studies for neuropathic pain arising fromcomplex regional pain syndrome type I and for chronic painfulradiculopathy (Manning, in press). In contrast to the Celgenecompound, Avigen's AV411 (ibudilast) does cross the blood–brain barrier with excellent partitioning to CNS sites. This is acompound that has been used clinically for many years inAsia for allergy and post-stroke dizziness (Ledeboer et al.,2007). In animal studies, AV411 suppresses proinflammatorycytokines and increases IL-10 in both peripheral immune cellsand CNS glia (Ledeboer et al., 2007). The Avigen safety,tolerability and, pharmacokinetic study enrolled 18 healthyadult volunteers (10male, 8 female) in Adelaide, Australia. Thedouble blind, placebo-controlled study, with single dose and2-week repeat dose phases, was designed to evaluate thesafety, tolerability, and pharmacokinetics of AV411. In this 20-day study, volunteers were randomized to receive either60 mg/day of AV411 or placebo in a 3:1 ratio. The standardhuman dose for ibudilast is 10 mg given three times per day.There were no serious adverse events in the study and mostcommon events involved headache and nausea. A small phaseIIa study in Australia involving the treatment of diabeticneuropathic pain patients, also at dose levels higher-than-approved in Asia, is currently enrolling (Avigen press release,January, 2007).

The second major point worth re-emphasizing is theimportance of immunology as regards the efficacy of opioids.Although only recognized within the past ∼5 years, the data arestartling in their consistency across laboratories and paradigms.As such, they warrant attention. It is already clear that gliaregulate morphine analgesia, tolerance, and dependence/with-drawal (Hutchinson et al., in press-a). While it is also clear thatsuch effects are not limited to morphine, it is as yet unknownhow pervasive this glial regulation of pharmaceuticalsmay turnout to be. Certainly the fact that glia are also implicated in effectsof drugs such asmethamphetamine (Narita et al., 2006) suggeststhat glia may ultimately be found to exert pervasive effects onphenomena now thought of as purely neuronal.

The third major point is the pervasiveness of proinflamma-tory cytokines to neuropathic pain and dysregulation of opioidactions. In terms of neuropathic pain, TNF, IL-1, and IL-6 arewell-established contributors to pain enhancement at the site ofperipheral nerve injury and in DRG, as well as in spinal cord.While beyond the scope of this review, it may well be that theseproinflammatory cytokines are important to pain amplificationat supraspinal sites, as well (Oka et al., 1994; Raghavendra et al.,2004b). It certainly is the case that glial activation at both spinaland supraspinal sites regulates the actions of opioids. While theinvolvement of proinflammatory cytokines in the regulation ofopioids is now clearly established in spinal cord, their potentialrole in brain remains to be explored.

The lastmajor point is that investigation of immune and glialregulation of neuronal function is still in its infancy. Whileremarkable progress has beenmade over the past decade, muchmore work needs to be done. This is, in part, because it has onlyrecently been recognized that glia residing in various sites in theCNS are not all the same. Rather, their receptor expression andfunction reflect their microenvironment and, as such, must beunderstood in this context. Much remains to be learned aboutthe dynamics of immune and glial regulation of neuronalfunctions, not only as regards to pain and opioids, but also farmore broadly as well.

R E F E R E N C E S

Abbadie, C., Lindia, J.A., Cumiskey, A.M., Peterson, L.B., Mudgett,J.S., Bayne, E.K., DeMartino, J.A., MacIntyre, D.E., Forrest, M.J.,2003. Impaired neuropathic pain responses in mice lacking thechemokine receptor CCR2. Proc. Natl. Acad. Sci. U. S. A. 100,7947–7952.

Ahn, S.H., Cho, Y.W., Ahn, M.W., Jang, S.H., Sohn, Y.K., Kim, H.S.,2002. mRNA expression of cytokines and chemokines inherniated lumbar intervertebral discs. Spine 27, 911–917.

Aicher, S.A., Sharma, S., Cheng, P.Y., Pickel, V.M., 1997. TheN-methyl-D-aspartate (NMDA) receptor is postsynaptic tosubstance P-containing axon terminals in the rat superficialdorsal horn. Brain Res. 772, 71–81.

Alexander, G.M., van Rijn, M.A., van Hilten, J.J., Perreault, M.J.,Schwartzman, R.J., 2005. Changes in cerebrospinal fluid levelsof pro-inflammatory cytokines in CRPS. Pain 116, 213–219.

Alexander, G.M., Perreault, M.J., Reichenberger, E., Schwartzman,R.J., 2007. Changes in immune and glial markers in the CSF ofpatients with complex regional pain syndrome. Brain Behav.Immun. 21, 668–676.

Aloisi, F., De Simone, R., Columba-Cabezas, S., Levi, G., 1999.Opposite effects of interferon-gamma and prostaglandin E2 ontumor necrosis factor and interleukin-10 production in


microglia: a regulatory loop controlling microglia pro- andanti-inflammatory activities. J. Neurosci. Res. 56, 571–580.

Backonja, M., Muller, D., Coe, C., 2006. Neuropathic pain and levelsof cytokines and IL-10 in the blood and cerebrospinal fluid.Proc. of the 11th World Congress on Pain, p. 1647 (P1150).

Baldwin, R.L., Stolowitz, M.L., Hood, L., Wisnieski, B.J., 1996.Structural changes of tumor necrosis factor alpha associatedwith membrane insertion and channel formation. Proc. Natl.Acad. Sci. U. S. A. 93, 1021–1026.

Beattie, E.C., Stellwagen, D., in press. AMPAR synaptic and surfacelocalization is altered by glial-derived TNFalpha: relevance tohyperalgesia and central sensitization. In: J.A. DeLeo, L.S.Sorkin, L.R. Watkins (Eds.), Immune and Glial Regulation ofPain, IASP Press, Seattle, p.

Beggs, S., Trang, T., Salter, M.W., in press. Role of ATP andmicroglia in enhanced pain states. In: J.A. Deleo, L.S. Sorkin,L.R. Watkins (Eds.), Glial and Immune Regulation of Pain, IASPPress, Seattle, p.

Berger, U.V., Hediger, M.A., 2000. Distribution of the glutamatetransporters GLAST and GLT-1 in rat circumventricular organs,meninges, and dorsal root ganglia. J. Comp. Neurol. 421, 385–399.

Besong, G., Battaglia, G., D'Onofrio, M., Di Marco, R., Ngomba, R.T.,Storto, M., Castiglione, M., Mangano, K., Busceti, C.L., Nicoletti,F.R., Bacon, K., Tusche, M., Valenti, O., Conn, P.J., Bruno, V.,Nicoletti, F., 2002. Activation of group III metabotropicglutamate receptors inhibits the production of RANTES inglial cell cultures. J. Neurosci. 22, 5403–5411.

Bessler, H., Shavit, Y., Mayburd, E., Smirnov, G., Beilin, B., 2006.Postoperative pain, morphine consumption, and geneticpolymorphism of IL-1beta and IL-1 receptor antagonist.Neurosci. Lett. 404, 154–158.

Beuche, W., Friede, R.L., 1984. The role of non-resident cells inWallerian degeneration. J. Neurocytol. 13, 767–796.

Biber, K., Dijkstra, I., Trebst, C., De Groot, C.J., Ransohoff, R.M.,Boddeke, H.W., 2002. Functional expression of CXCR3 incultured mouse and human astrocytes and microglia.Neuroscience 112, 487–497.

Binns, B.C., Huang, Y., Goettl, V.M., Hackshaw, K.V., Stephens Jr.,R.L., 2005. Glutamate uptake is attenuated in spinal deep dorsaland ventral horn in the rat spinal nerve ligation model.Brain Res. 1041, 38–47.

Bolay, H., Moskowitz, M.A., 2002. Mechanisms of pain modulationin chronic syndromes. Neurology 59, S2–S7.

Borner, C., Woltje, M., Hollt, V., Kraus, J., 2004. STAT6 transcriptionfactor binding sites with mismatches within the canonical5′-TTC…GAA-3′ motif involved in regulation of delta- andmu-opioid receptors. J. Neurochem. 91, 1493–1500.

Burke, J.G., Watson, R.W., McCormack, D., Dowling, F.E., Walsh,M.G., Fitzpatrick, J.M., 2002. Intervertebral discs whichcause low back pain secrete high levels of proinflammatorymediators. J. Bone Jt. Surg., Br. 84, 196–201.

Campbell, J.N., 2001. Nerve lesions and the generation of pain.Muscle Nerve 24, 1261–1273.

Campbell, J.N., Meyer, R.A., 2006. Mechanisms of neuropathic pain.Neuron 52, 77–92.

Chacur, M., Milligan, E.D., Gazda, L.S., Armstrong, C., Wang, H.,Tracey, K.J., Maier, S.F., Watkins, L.R., 2001. A new model ofsciatic inflammatory neuritis (SIN): induction of unilateral andbilateral mechanical allodynia following acute unilateralperi-sciatic immune activation in rats. Pain 94, 231–244.

Chacur, M., Milligan, E.D., Sloan, E.M., Wieseler-Frank, J.,Barrientos, R.M., Martin, D., Poole, S., Lomonte, B., Gutierrez,J.M., Maier, S.F., Cury, Y., Watkins, L.R., 2004. Snake venomphospholipase A2s (Asp49 and Lys49) induce mechanicalallodynia upon peri-sciatic administration: involvement ofspinal cord glia, proinflammatory cytokines andnitric oxide. Pain 108, 180–191.

Chapman, G., Moores, K., Harrison, D., Campbell, C.A., Stewart,B.R., Strijbos, P.J.L.M., 2000. Fractalkine cleavage from neuronal

membranes represents an acute event in the inflammatoryresponse to excitotoxic brain damage. J. Neurosci.20 (RC87), 1–5.

Chattopadhyay, S., Myers, R.R., Janes, J., Shubayev, V., 2007.Cytokine regulation of MMP-9 in peripheral glia: implicationsfor pathological processes and pain in injured nerve. BrainBehav. Immun. 21, 561–568.

Cherkas, P.S., Huang, T.Y., Pannicke, T., Tal, M., Reichenbach, A.,Hanani, M., 2004. The effects of axotomy on neurons andsatellite glial cells in mouse trigeminal ganglion. Pain 110,290–298.

Chessell, I.P., Hatcher, J.P., Bountra, C., Michel, A.D., Hughes, J.P.,Green, P., Egerton, J., Murfin, M., Richardson, J., Peck, W.L.,Grahames, C.B., Casula, M.A., Yiangou, Y., Birch, R., Anand, P.,Buell, G.N., 2005. Disruption of the P2X7 purinoceptor geneabolishes chronic inflammatory and neuropathic pain. Pain114, 386–396.

Ching, D.W., McClintock, A., Beswick, F., 2003. Successfultreatment with low-dose thalidomide in a patient with bothBehcet's disease and complex regional pain syndrome type I:case report. J. Clin. Rheumatol. 9, 96–98.

Clark, A.K., D'Aquisto, F., Gentry, C., Marchand, F., McMahon, S.B.,Malcangio, M., 2006. Rapid co-release of interleukin 1beta andcaspase 1 in spinal cord inflammation. J. Neurochem. 99,868–890.

Clatworthy, A.L., Castro, G.A., Budelmann, B.U., Walters, E.T., 1994.Induction of a cellular defense reaction is accompanied by anincrease in sensory neuron excitability in Aplysia. J. Neurosci.14, 3263–3270.

Coats, B.D., Hutchinson, M.R., Watkins, L.R., in press. Opioidinactive (+)-naloxone potentiates morphine analgesia: furtherevidence that opioid analgesia is opposed by opioid inducedglial activation. Proc. Soc. Neurosci.

Copray, J.C., Mantingh, I., Brouwer, N., Biber, K., Kust, B.M., Liem,R.S., Huitinga, I., Tilders, F.J., Van Dam, A.M., Boddeke, H.W.,2001. Expression of interleukin-1 beta in rat dorsal root ganglia.J. Neuroimmunol. 118, 203–211.

Costigan, M., Mannion, R.J., Kendall, G., Lewis, S.E., Campagna, J.A.,Coggeshall, R.E., Meridith-Middleton, J., Tate, S., Woolf, C.J.,1998. Heat shock protein 27: developmental regulation andexpression after peripheral nerve injury. J. Neurosci. 18,5891–5900.

Cuellar, J.M., Montesano, P.X., Carstens, E., 2004. Role of TNF-alphain sensitization of nociceptive dorsal horn neurons induced byapplication of nucleus pulposus to L5 dorsal root ganglion inrats. Pain 110, 578–587.

Cui, J.G., Holmin, S., Mathiesen, T., Meyerson, B.A., Linderoth, B.,2000. Possible role of inflammatory mediators in tactilehypersensitivity in rat models of mononeuropathy. Pain88, 239–248.

Cui, Y., Chen, Y., Zhi, J.L., Guo, R.X., Feng, J.Q., Chen, P.X., 2006.Activation of p38 mitogen-activated protein kinase in spinalmicroglia mediates morphine antinociceptive tolerance.Brain Res. 1069, 235–243.

Dai, Y., Moriyama, T., Higashi, T., Togashi, K., Kobayashi, K.,Yamanaka, H., Tominaga, M., Noguchi, K., 2004.Proteinase-activated receptor 2-mediated potentiation oftransient receptor potential vanilloid subfamily 1 activityreveals a mechanism for proteinase-induced inflammatorypain. J. Neurosci. 24, 4293–4299.

Dame, J.B., Juul, S.E., 2000. The distribution of receptors for thepro-inflammatory cytokines interleukin (IL)-6 and IL-8 in thedeveloping human fetus. Early Hum. Dev. 58, 25–39.

Davalos, D., Grutzendler, J., Yang, G., Kim, J.V., Zuo, Y., Jung, S.,Littman, D.R., Dustin, M.L., Gan,W.B., 2005. ATPmediates rapidmicroglial response to local brain injury in vivo. Nat. Neurosci.8, 752–758.

De, A., Krueger, J.M., Simasko, S.M., 2003. Tumor necrosis factoralpha increases cytosolic calcium responses to AMPA


and KCl in primary cultures of rat hippocampal neurons.Brain Res. 981, 133–142.

Devor, M., 2006. Sodium channels andmechanisms of neuropathicpain. J. Pain 7, S3–S12.

Di Virgilio, F., 2006. Purinergic signalling between axons andmicroglia. Novartis Found. Symp. 276, 253–258 discussion259–262, 275–281.

Doita, M., Kanatani, T., Harada, T., Mizuno, K., 1996.Immunohistologic study of the ruptured intervertebral disc ofthe lumbar spine. Spine 21, 235–241.

Dublin, P., Hanani, M., 2007. Satellite glial cells in sensoryganglia: their possible contribution to inflammatory pain.Brain Behav. Immun. 21, 592–598.

Durrenberger, P.F., Facer, P., Casula, M.A., Yiangou, Y., Gray, R.A.,Chessell, I.P., Day, N.C., Collins, S.D., Bingham, S., Wilson, A.W.,Elliot, D., Birch, R., Anand, P., 2006. Prostanoidreceptor EP1 and Cox-2 in injured human nerves and a ratmodel of nerve injury: a time-course study. BMC Neurol. 6, 1.

Eliav, E., Herzberg, U., Ruda, M.A., Bennett, G.J., 1999. Neuropathicpain from an experimental neuritis of the rat sciatic nerve. Pain83, 169–182.

Fife, M.S., Gutierrez, A., Ogilvie, E.M., Stock, C.J., Samuel, J.M.,Thomson, W., Mack, L.F., Lewis, C.M., Woo, P., 2006. Novel IL10gene family associations with systemic juvenile idiopathicarthritis. Arthritis Res. Ther. 8, R148.

Finnerup, N.B., Otto, M., McQuay, H.J., Jensen, T.S., Sindrup, S.H.,2005. Algorithm for neuropathic pain treatment: an evidencebased proposal. Pain 118, 289–305.

Frank, M.G., Baratta, M.V., Watkins, L.R., Maier, S.F., in press.Glucocorticoids modulate CD200: a neural inhibitor ofmicroglial activation. Proc. Soc. Neurosci.

Frisen, J., Risling, M., Fried, K., 1993. Distribution and axonalrelations of macrophages in a neuroma. Neuroscience55, 1003–1013.

Garrison, C.J., Dougherty, P.M., Kajander, K.C., Carlton, S.M., 1991.Staining of glial fibrillary acidic protein (GFAP) in lumbar spinalcord increases following a sciatic nerve constriction injury.Brain Res. 565, 1–7.

Garrison, C.J., Dougherty, P.M., Carlton, S.M., 1994. GFAPexpression in lumbar spinal cord of naive and neuropathic ratstreated with MK-801. Exp. Neurol. 129, 237–243.

George, A., Marziniak, M., Schafers, M., Toyka, K.V., Sommer, C.,2000. Thalidomide treatment in chronic constrictiveneuropathy decreases endoneurial tumor necrosisfactor-alpha, increases interleukin-10 and has long-termeffects on spinal cord dorsal horn met-enkephalin. Pain 88,267–275.

George, A., Kleinschnitz, C., Zelenka, M., Brinkhoff, J., Stoll, G.,Sommer, C., 2004. Wallerian degeneration after crush orchronic constriction injury of rodent sciatic nerve is associatedwith a depletion of endoneurial interleukin-10 protein. Exp.Neurol. 188, 187–191.

Gold, M.S., 2000. Spinal nerve ligation: what to blame for the painand why. Pain 84, 117–120.

Gordh, T., Chu, H., Sharma, H.S., 2006. Spinal nerve lesion altersblood-spinal cord barrier function and activates astrocytes inthe rat. Pain 124, 211–221.

Grassi, F., Mileo, A.M., Monaco, L., Punturieri, A., Santoni, A.,Eusebi, F., 1994. TNF-alpha increases the frequency ofspontaneous miniature synaptic currents in cultured rathippocampal neurons. Brain Res. 659, 226–230.

Guimaraes, A.L., de Sa, A.R., Victoria, J.M., de Fatima Correia-Silva,J., Gomez, M.V., Gomez, R.S., 2006. Interleukin-1beta andserotonin transporter gene polymorphisms in burning mouthsyndrome patients. J. Pain 7, 654–658.

Guo, J.D., Wang, H., Zhang, Y.Q., Zhao, Z.Q., 2006. Distinct effects ofD-serine on spinal nociceptive responses in normal andcarrageenan-injected rats. Biochem. Biophys. Res. Commun.343, 401–406.

Guo, W., Wang, H., Watanabe, M., Shimizu, K., Zou, S., LaGraize,S.C., Wei, F., Dubner, R., Ren, K., 2007. Glial-cytokine-neuronalinteractions underlying the mechanisms of persistent pain.J. Neurosci. 27, 6006–6018.

Hains, B.C., Waxman, S.G., 2006. Activated microglia contribute tothe maintenance of chronic pain after spinal cord injury.J. Neurosci. 26, 4308–4317.

Hanani, M., 2005. Satellite glial cells in sensory ganglia: from formto function. Brain Res. Brain Res. Rev. 48, 457–476.

Hashizume, H., Rutkowski, M.D., Weinstein, J.N., DeLeo, J.A., 2000.Central administration of methotrexate reduces mechanicalallodynia in an animal model of radiculopathy/sciatica. Pain87, 159–169.

Haydon, P.G., Carmignoto, G., 2006. Astrocyte control of synaptictransmission and neurovascular coupling. Physiol. Rev. 86,1009–1031.

Hayes, A.G., Hawcock, A.B., Hill, R.G., 1984. The depolarising actionof capsaicin on rat isolated sciatic nerve. Life Sci. 35, 1561–1568.

Haynes, S.E., Hollopeter, G., Yang, G., Kurpius, D., Dailey, M.E., Gan,W.B., Julius, D., 2006. The P2Y12 receptor regulates microglialactivation by extracellular nucleotides. Nat. Neurosci. 9,1512–1519.

Heblich, F., England, S., Docherty, R.J., 2001. Indirect actions ofbradykinin on neonatal rat dorsal root ganglion neurones: a rolefor non-neuronal cells as nociceptors. J. Physiol. 536, 111–121.

Hempenstall, K., Nurmikko, T.J., Johnson, R.W., A'Hern, R.P., Rice,A.S., 2005. Analgesic therapy in postherpetic neuralgia: aquantitative systematic review. PLoS Med. 2, e164.

Herzberg, U., Sagen, J., 2001. Peripheral nerve exposure to HIV viralenvelope protein gp120 induces neuropathic pain and spinalgliosis. J. Neuroimmunol. 116, 29–39.

Ho, T.W., McKhann, G.M., Griffin, J.W., 1998. Human autoimmuneneuropathies. Annu. Rev. Neurosci. 21, 187–226.

Holguin, A., Biedenkapp, J., Campisi, J., Wieseler-Frank, J.,O'Connor, K.A., Milligan, E.D., Maksimova, E., Bravmann, C.,Hansen, M.K., Martin, D., Fleshner, M., Maier, S.F.,Watkins, L.R.,2004. HIV-1 gp120 stimulates proinflammatorycytokine-mediated pathological pain via activation of nitricoxide synthase-I (nNOS). Pain 110, 517–530.

Holguin, A., Frank, M.G., Biedenkapp, J.C., Nelson, K., Lippert, D.,Watkins, L.R., Rudy, J.W., Maier, S.F., 2007. Characterization ofthe temporo-spatial effects of chronic bilateralintrahippocampal cannulae on interleukin-1beta.J. Neurosci. Methods 161, 265–272.

Holmes, G.M., Hebert, S.L., Rogers, R.C., Hermann, G.E., 2004.Immunocytochemical localization of TNF type 1 and type 2receptors in the rat spinal cord. Brain Res. 1025, 210–219.

Homma, Y., Brull, S.J., Zhang, J.M., 2002. A comparison of chronicpain behavior following local application of tumor necrosisfactor alpha to the normal and mechanically compressedlumbar ganglia in the rat. Pain 95, 239–246.

Honore, P., Wade, C.L., Zhong, C., Harris, R.R., Wu, C., Ghayur, T.,Iwakura, Y., Decker, M.W., Faltynek, C., Sullivan, J., Jarvis, M.F.,2006a. Interleukin-1alphabeta gene-deficient mice showreduced nociceptive sensitivity in models of inflammatory andneuropathic pain but not post-operative pain. Behav. Brain Res.167, 355–364.

Honore, P.M., Donnelly-Roberts, D., Namovic, M., Hsieh, G., Zhu, C.,Mikusa, J., Hernandez, G., Zhong, C., Gauvin, D., Chandran, P.,Harris, R., Perez-Medrano, A., Carroll, W., Marsh, K., Sullivan, J.,Faltynek, C., Jarvis, M.F., 2006b. A-740003 (N-(1-{[(cyanoimino)(5-quinolinylamino) methyl]amino}-2,2-dimethylpropyl)-2-(3,4-dimethoxyphenyl)acetamide, a noveland selective P2X7 receptor antagonist dose-dependentlyreduces neuropathic pain in the rat. J. Pharmacol. Exp. Ther.319, 1376–1385.

Hu, P., McLachlan, E.M., 2002. Macrophage and lymphocyteinvasion of dorsal root ganglia after peripheral nerve lesions inthe rat. Neuroscience 112, 23–38.


Hu, P., Bembrick, A.L., Keay, K.A., McLachlan, E.M., 2007. Immunecell involvement in dorsal root ganglia and spinal cord afterchronic constriction or transection of the rat sciatic nerve.Brain Behav. Immun. 21, 559–616.

Hughes, R.A., Allen, D., Makowska, A., Gregson, N.A., 2006.Pathogenesis of chronic inflammatory demyelinatingpolyradiculoneuropathy. J. Peripher. Nerv. Syst. 11, 30–46.

Hutchinson, M.R., Somogyi, A.A., 2004a. Relationship between4,5-epoxymorphinan structure and in vitro modulation of cellproliferation. Eur. J. Pharmacol. 494, 251–262.

Hutchinson, M.R., Somogyi, A.A., 2004b. (S)-(+)-methadone is moreimmunosuppressive than the potent analgesic(R)-(-)-methadone. Int. Immunopharmacol. 4, 1525–1530.

Hutchinson, M.R., Somogyi, A.A., 2005. Characterisation of the invitro modulation of splenocyte proliferation bynon-4,5-epoxymorphinan opioids. Int. Immunopharmacol.5, 1713–1722.

Hutchinson, M.R., Milligan, E.D., Jekich, B.M., Coats, B.D.,Lewis, S., Maier, S.F., Watkins, L.R., 2006. Elevations ofspinal proinflammatory cytokines are mediated vianon-classical opioid mechanisms. Proc. Soc. NeuroImmunePharmacol. (poster WP-13, presented at Santa Fe, NW 2006).

Hutchinson, M.R., Bland, S.T., Johnson, K.W., Rice, K.C., Maier, S.F.,Watkins, L.R., in press-a. Opioid-induced glial activation:mechanisms of activation and implications for opioid analgesia,dependence, and reward. The Scientific World Journal.

Hutchinson, M.R., Coats, B.D., Lewis, S.S., Maier, S.F., Watkins, L.R.,in press-b. Spinal proinflammatory cytokines oppose accrueopioid analgesia following intrathecal and subcutaneousopioid administration. Proc. Soc. Neurosci.

Huygen, F.J., Niehof, S., Zijlstra, F.J., van Hagen, P.M., van Daele,P.L., 2004. Successful treatment of CRPS 1 with anti-TNF. J. PainSymptom Manage. 27, 101–103.

Inoue, K., 2006. The function of microglia through purinergicreceptors: neuropathic pain and cytokine release. Pharmacol.Ther. 109, 210–226.

Inoue, K., Koizumi, S., Nakajima, K., Hamanoue, M., Kohsaka, S.,1994. Modulatory effect of plasminogen on NMDA-inducedincrease in intracellular free calcium concentration in ratcultured hippocampal neurons. Neurosci. Lett. 179, 87–90.

Inoue, M., Rashid, M.H., Fujita, R., Contos, J.J., Chun, J., Ueda, H.,2004. Initiation of neuropathic pain requires lysophosphatidicacid receptor signaling. Nat. Med. 10, 712–718.

Irnich, D., Tracey, D.J., Polten, J., Burgstahler, R., Grafe, P., 2002. ATPstimulates peripheral axons in human, rat andmouse-differential involvement of A(2B) adenosine and P2Xpurinergic receptors. Neuroscience 110, 123–129.

Jimenez-Andrade, J.M., Peters, C.M., Mejia, N.A., Ghilardi, J.R.,Kuskowski, M.A., Mantyh, P.W., 2006. Sensory neurons andtheir supporting cells located in the trigeminal, thoracic andlumbar ganglia differentially express markers of injuryfollowing intravenous administration of paclitaxel in the rat.Neurosci. Lett. 405, 62–67.

Jin, X., Gereau 4th, R.W., 2006. Acute p38-mediated modulation oftetrodotoxin-resistant sodium channels in mouse sensoryneurons by tumor necrosis factor-alpha. J. Neurosci. 26,246–255.

John, G.R., Lee, S.C., Song, X., Rivieccio, M., Brosnan, C.F., 2005.IL-1-regulated responses in astrocytes: relevance to injury andrecovery. Glia 49, 161–176.

Johnston, I.N., Milligan, E.D., Wieseler-Frank, J., Frank, M.G.,Zapata, V., Campisi, J., Langer, S., Martin, D., Green, P., Fleshner,M., Leinwand, L., Maier, S.F., Watkins, L.R., 2004. A role forproinflammatory cytokines and fractalkine in analgesia,tolerance, and subsequent pain facilitation induced by chronicintrathecal morphine. J. Neurosci. 24, 7353–7365.

Jou, I., Lee, J.H., Park, S.Y., Yoon, H.J., Joe, E.H., Park, E.J., 2006.Gangliosides trigger inflammatory responses via TLR4 in brainglia. Am. J. Pathol. 168, 1619–1630.

Kagan, B.L., Baldwin, R.L., Munoz, D., Wisnieski, B.J., 1992.Formation of ion-permeable channels by tumor necrosisfactor-alpha. Science 255, 1427–1430.

Karppinen, J., in press. New perspectives into sciatica. In: J.A.DeLeo, L.S. Sorkin, L.R. Watkins (Eds.), Immune and GlialRegulation of Pain, IASP Press, Seattle, p.

Kawakami, M., Tamaki, T., Hashizume, H., Weinstein, J.N., Meller,S.T., 1997. The role of phospholipase A2 and nitric oxide inpain-related behavior produced by an allograft of intervertebraldisc material to the sciatic nerve of the rat. Spine 22, 1074–1079.

Kawakami, M., Matsumoto, T., Kuribayashi, K., Tamaki, T., 1999.mRNA expression of interleukins, phospholipase A2, and nitricoxide synthase in the nerve root and dorsal root ganglioninduced by autologous nucleus pulposus in the rat. J. Orthop.Res. 17, 941–946.

Kawakami, M., Tamaki, T., Hayashi, N., Hashizume, H.,Matsumoto, T., Minamide, A., Kihira, T., 2000a. Mechanicalcompression of the lumbar nerve root alters pain-relatedbehaviors induced by the nucleus pulposus in the rat. J. Orthop.Res. 18, 257–264.

Kawakami, M., Tamaki, T., Matsumoto, T., Kuribayashi, K.,Takenaka, T., Shinozaki, M., 2000b. Role of leukocytes inradicular pain secondary to herniated nucleus pulposus. Clin.Orthop. Relat. Res. 268–277.

Kawakami, M., Matsumoto, T., Tamaki, T., 2001. Roles ofthromboxane A2 and leukotriene B4 in radicular pain inducedby herniated nucleus pulposus. J. Orthop. Res. 19, 472–477.

King, T., Ossipov, M.H., Vanderah, T.W., Porreca, F., Lai, J., 2005. Isparadoxical pain induced by sustained opioid exposure anunderlying mechanism of opioid antinociceptive tolerance?Neurosignals 14, 194–205.

Kingery, W.S., 1997. A critical review of controlled clinical trials forperipheral neuropathic pain and complex regional painsyndromes. Pain 73, 123–139.

Kleibeuker, W., Ledeboer, A., Eijkelkamp, N., Watkins, L.R., Maier,S.F., Zijlstra, J., Heijnen, C.J., Kavelaars, A., 2007. A role for Gprotein-coupled receptor kinase 2 in mechanical allodynia.Eur. J. Neurosci. 25, 1696–1704.

Kowalski, J., Gabryel, B., Labuzek, K., Herman, Z.S., 2002.Methionine-enkephalin and leucine-enkephalin increaseinterleukin-1 beta release in mixed glia cultures.Neuropeptides 36, 401–406.

Kraus, J., Borner, C., Giannini, E., Hickfang, K., Braun, H., Mayer, P.,Hoehe, M.R., Ambrosch, A., Konig, W., Hollt, V., 2001.Regulation of mu-opioid receptor gene transcription byinterleukin-4 and influence of an allelic variation within aSTAT6 transcription factor binding site. J. Biol. Chem.276, 43901–43908.

Kreutzberg, G.W., 1996. Microglia: a sensor for pathological eventsin the CNS. Trends Neurosci. 19, 312–318.

Kreutzberg, G.W., Barron, K.D., 1978. 5′-Nucleotidase of microglialcells in the facial nucleus during axonal reaction. J. Neurocytol.7, 601–610.

Laughlin, T.M., Bethea, J.R., Yezierski, R.P., Wilcox, G.L., 2000.Cytokine involvement in dynorphin-induced allodynia. Pain84, 159–167.

Lavand'homme, P.M., Eisenach, J.C., 2003. Perioperativeadministration of the alpha2-adrenoceptor agonist clonidineat the site of nerve injury reduces the development ofmechanical hypersensitivity and modulates local cytokineexpression. Pain 105, 247–254.

Ledeboer, A., Sloane, E.M., Milligan, E.D., Frank, M.G., Mahony, J.H.,Maier, S.F., Watkins, L.R., 2005. Minocycline attenuatesmechanical allodynia and proinflammatory cytokineexpression in rat models of pain facilitation. Pain 115, 71–83.

Ledeboer, A., Mahoney, J.H., Milligan, E.D., Martin, D., Maier, S.F.,Watkins, L.R., 2006. Spinal cord glia and interleukin-1 do notappear to mediate persistent allodynia induced byintramuscular acidic saline in rats. J. Pain 7, 757–767.


Ledeboer, A., Hutchinson, M.R., Watkins, L.R., Johnson, K.W., 2007.AV411 (ibudilast): a new class therapeutic candidate forneuropathic pain and opioid withdrawal syndromes. ExpertOpin. Investig. Drugs 16, 935–950.

Ledeboer, A., Liu, T., Shumilla, J.A., Mahoney, J.H., Vargas, J.A.,Sultzbaugh, L., Sanftner, L.M., Watkins, L.R., Johnson, K.W.,in press. The glial modulatory drug AV411 attenuatesmechanical allodynia in rat models of neuropathic pain.Neuron Glia Biol.

Lewis, S.S., Hutchinson, M.R., Coats, B.D., Brzeski, A.L., Maier, S.F.,Watkins, L.R., Johnson, K.W., 2006. AV411, a blood brain barrierpermeable glial activation inhibitor, reduces morphinewithdrawal behaviors in rats. Proc. Soc. Neurosci. (Poster No.765.2).

Li, L., Xian, C.J., Zhong, J.H., Zhou, X.F., 2002. Effect of lumbar 5ventral root transection on pain behaviors: a novel rat modelfor neuropathic pain without axotomy of primary sensoryneurons. Exp. Neurol. 175, 23–34.

Lim, G., Wang, S., Zeng, Q., Sung, B., Yang, L., Mao, J., 2005.Expression of spinal NMDA receptor and PKCgamma afterchronic morphine is regulated by spinal glucocorticoidreceptor. J. Neurosci. 25, 11145–11154.

Lindenlaub, T., Sommer, C., 2003. Cytokines in sural nerve biopsiesfrom inflammatory and non-inflammatory neuropathies. ActaNeuropathol. 105, 593–602.

Lindia, J.A., McGowan, E., Jochnowitz, N., Abbadie, C., 2005. Inductionof CX3CL1 expression in astrocytes andCX3CR1 inmicroglia in thespinal cord of a rat model of neuropathic pain. J. Pain 6, 434–438.

Liu, T., Knight, K.R., Tracey, D.J., 2000a. Hyperalgesia due to nerveinjury-role of peroxynitrite. Neuroscience 97, 125–131.

Liu, T., van Rooijen, N., Tracey, D.J., 2000b. Depletion ofmacrophages reduces axonal degeneration and hyperalgesiafollowing nerve injury. Pain 86, 25–32.

Liu, B., Li, H., Brull, S.J., Zhang, J.M., 2002. Increased sensitivity ofsensory neurons to tumor necrosis factor alpha in rats withchronic compression of the lumbar ganglia. J. Neurophysiol. 88,1393–1399.

Liu, D., Wu, L., Breyer, R., Mattson, M.P., Andreasson, K., 2005.Neuroprotection by the PGE2 EP2 receptor in permanent focalcerebral ischemia. Ann Neurol. 57, 758–761.

Liu, W., Wang, C.H., Cui, Y., Mo, L.Q., Zhi, J.L., Sun, S.N., Wang, Y.L.,Yu, H.M., Zhao, C.M., Feng, J.Q., Chen, P.X., 2006. Inhibition ofneuronal nitric oxide synthase antagonizes morphineantinociceptive tolerance by decreasing activation of p38MAPK in the spinal microglia. Neurosci. Lett. 410, 174–177.

Liu, Y.L., Zhou, L.J., Hu, N.W., Xu, J.T., Wu, C.Y., Zhang, T., Li, Y.Y.,Liu, X.G., 2007. Tumor necrosis factor-alpha induces long-termpotentiation of C-fiber evoked field potentials in spinal dorsalhorn in rats with nerve injury: the role of NF-kappa B, JNK andp38 MAPK. Neuropharmacology 52, 708–715.

Ma, W., Eisenach, J.C., 2003. Cyclooxygenase 2 in infiltratinginflammatory cells in injured nerve is universally up-regulatedfollowing various types of peripheral nerve injury.Neuroscience 121, 691–704.

Ma, W., Quirion, R., 2006a. Increased calcitonin gene-relatedpeptide in neuroma and invading macrophages is involved inthe up-regulation of interleukin-6 and thermal hyperalgesiain a rat model of mononeuropathy. J. Neurochem. 98,180–192.

Ma, W., Quirion, R., 2006b. Targeting invadingmacrophage-derived PGE2, IL-6 and calcitonin gene-relatedpeptide in injured nerve to treat neuropathic pain. Expert Opin.Ther. Targets 10, 533–546.

Ma, C., Greenquist, K.W., Lamotte, R.H., 2006. Inflammatorymediators enhance the excitability of chronicallycompressed dorsal root ganglion neurons. J. Neurophysiol. 95,2098–2107.

Maier, S.F., Watkins, L.R., 1998. Cytokines for psychologists:implications of bidirectional immune-to-brain communication

for understanding behavior,mood, and cognition. Psychol. Rev.105, 83–107.

Manning, D.C., in press. Immunomodulatory therapies forcomplex regional pain syndrome: thalidomide and beyond. In:J.A. DeLeo, L.S. Sorkin, L.R. Watkins (Eds.), Immune and GlialRegualtion of Pain, IASP Press, Seattle, p.

Mao, J., 2002. Opioid-induced abnormal pain sensitivity:implications in clinical opioid therapy. Pain 100, 213–217.

Mao, J., Sung, B., Ji, R.R., Lim, G., 2002. Chronic morphine inducesdownregulation of spinal glutamate transporters: implicationsin morphine tolerance and abnormal pain sensitivity.J. Neurosci. 22, 8312–8323.

Martineau, M., Baux, G., Mothet, J.P., 2006. D-serine signalling inthe brain: friend and foe. Trends Neurosci. 29, 481–491.

Maves, T.J., Pechman, P.S., Gebhart, G.F., Meller, S.T., 1993. Possiblechemical contribution from chromic gut sutures producesdisorders of pain sensation like those seen in man. Pain54, 57–69.

Mayer, D.J., Mao, J., Holt, J., Price, D.D., 1999. Cellular mechanismsof neuropathic pain, morphine tolerance, and theirinteractions. Proc. Natl. Acad. Sci. U. S. A. 96, 7731–7736.

McKay, S.M., McLachlan, E.M., 2004. Inflammation of rat dorsalroot ganglia below a mid-thoracic spinal transection.NeuroReport 15, 1783–1786.

McLachlan, E.M., Hu, P., in press. Inflammation of dorsal rootganglia: satellite cell activation and immune cell recruitmentafter nerve injury. In: J.A. DeLeo, L.S. Sorkin, L.R. Watkins(Eds.), Immune and Glial Regulation of Pain, IASP Press,Seattle, p.

McMahon, S.B., Cafferty, W.B., Marchand, F., 2005. Immune andglial cell factors as pain mediators and modulators. Exp.Neurol. 192, 444–462.

McQuay, H., Carroll, D., Jadad, A.R., Wiffen, P., Moore, A., 1995.Anticonvulsant drugs for management of pain: a systematicreview. BMJ 311, 1047–1052.

McQuay, H.J., Tramer, M., Nye, B.A., Carroll, D., Wiffen, P.J., Moore,R.A., 1996. A systematic review of antidepressants inneuropathic pain. Pain 68, 217–227.

Meller, S.T., Dyskstra, C., Grzybycki, D., Murphy, S., Gebhart, G.F.,1994. The possible role of glia in nociceptive processing andhyperalgesia in the spinal cord of the rat. Neuropharmacology33, 1471–1478.

Milligan, E.D., Zapata, V., Chacur, M., Schoeniger, D., Biedenkapp,J., O'Connor, K.A., Verge, G.M., Chapman, G., Green, P., Foster,A.C., Naeve, G.S., Maier, S.F., Watkins, L.R., 2004. Evidence thatexogenous and endogenous fractalkine can induce spinalnociceptive facilitation in rats. Eur. J. Neurosci.20, 2294–2302.

Milligan, E., Zapata, V., Schoeniger, D., Chacur, M., Green, P., Poole,S., Martin, D., Maier, S.F., Watkins, L.R., 2005. An initialinvestigation of spinal mechanisms underlying painenhancement induced by fractalkine, a neuronally releasedchemokine. Eur. J. Neurosci. 22, 2775–2782.

Moalem, G., Tracey, D.J., 2006. Immune and inflammatorymechanisms in neuropathic pain. Brain Res. Brain Res. Rev. 51,240–264.

Moalem, G., Xu, K., Yu, L., 2004. T lymphocytes play a role inneuropathic pain following peripheral nerve injury in rats.Neuroscience 129, 767–777.

Moalem, G., Grafe, P., Tracey, D.J., 2005. Chemical mediatorsenhance the excitability of unmyelinated sensory axons innormal and injured peripheral nerve of the rat. Neuroscience134, 1399–1411.

Morioka, N., Inoue, A., Hanada, T., Kumagai, K., Takeda, K., Ikoma,K., Hide, I., Tamura, Y., Shiomi, H., Dohi, T., Nakata, Y., 2002.Nitric oxide synergistically potentiates interleukin-1beta-induced increase of cyclooxygenase-2 mRNA levels,resulting in the facilitation of substance P releasefrom primary afferent neurons: involvement of


cGMP-independent mechanisms. Neuropharmacology43, 868–876.

Moumdjian, R.A., Antel, J.P., Yong, V.W., 1991. Origin ofcontralateral reactive gliosis in surgically injured rat cerebralcortex. Brain Res. 547, 223–228.

Murata, Y., Rydevik, B., Takahashi, K., Takahashi, I., Olmarker, K.,2004. Macrophage appearance in the epineurium andendoneurium of dorsal root ganglion exposed to nucleuspulposus. J. Peripher. Nerv. Syst. 9, 158–164.

Murata, Y., Onda, A., Rydevik, B., Takahashi, I., Takahashi, K.,Olmarker, K., 2006. Changes in pain behavior andhistologic changes caused by application of tumor necrosisfactor-alpha to the dorsal root ganglion in rats. Spine 31,530–535.

Murphy, P.G., Borthwick, L.S., Johnston, R.S., Kuchel, G.,Richardson, P.M., 1999. Nature of the retrograde signal frominjured nerves that induces interleukin-6 mRNA in neurons.J. Neurosci. 19, 3791–3800.

Myers, R.R., Heckman, H.M., Rodriguez, M., 1996. Reducedhyperalgesia in nerve-injured WLD mice: relationship to nervefiber phagocytosis, axonal degeneration, and regeneration innormal mice. Exp. Neurol. 141, 94–101.

Myers, R.R., Campana, W.M., Shubayev, V.I., 2006. The role ofneuroinflammation in neuropathic pain: mechanisms andtherapeutic targets. Drug Discov. Today 11, 8–20.

Narita, M., Miyatake, M., Shibasaki, M., Shindo, K., Nakamura, A.,Kuzumaki, N., Nagumo, Y., Suzuki, T., 2006. Direct evidence ofastrocytic modulation in the development of rewarding effectsinduced by drugs of abuse. Neuropsychopharmacology 31,2476–2488.

Nicol, G.D., Lopshire, J.C., Pafford, C.M., 1997. Tumor necrosisfactor enhances the capsaicin sensitivity of rat sensoryneurons. J. Neurosci. 17, 975–982.

Noponen-Hietala, N., Virtanen, I., Karttunen, R., Schwenke, S.,Jakkula, E., Li, H., Merikivi, R., Barral, S., Ott, J., Karppinen, J.,Ala-Kokko, L., 2005. Genetic variations in IL6 associate withintervertebral disc disease characterized by sciatica. Pain 114,186–194.

O'Connor, K.A., Hansen, M.K., Rachal Pugh, C., Deak, M.M.,Biedenkapp, J.C., Milligan, E.D., Johnson, J.D., Wang, H.,Maier, S.F., Tracey, K.J., Watkins, L.R., 2003. Furthercharacterization of high mobility group box 1 (HMGB1) as aproinflammatory cytokine: central nervous system effects.Cytokine 24, 254–265.

Obata, K., Tsujino, H., Yamanaka, H., Yi, D., Fukuoka, T.,Hashimoto, N., Yonenobu, K., Yoshikawa, H., Noguchi, K., 2002.Expression of neurotrophic factors in the dorsal rootganglion in a rat model of lumbar disc herniation. Pain 99,121–132.

Obata, H., Eisenach, J.C., Hussain, H., Bynum, T., Vincler, M., 2006.Spinal glial activation contributes to post-operativemechanical hypersensitivity in the rat. J. Pain 7, 816–822.

Obreja, O., Rathee, P.K., Lips, K.S., Distler, C., Kress, M., 2002. IL-1beta potentiates heat-activated currents in rat sensoryneurons: involvement of IL-1RI, tyrosine kinase, and proteinkinase C. FASEB J. 16, 1497–1503.

Obreja, O., Biasio, W., Andratsch, M., Lips, K.S., Rathee, P.K.,Ludwig, A., Rose-John, S., Kress, M., 2005. Fast modulation ofheat-activated ionic current by proinflammatory interleukin 6in rat sensory neurons. Brain 128, 1634–1641.

Oen, K., Malleson, P.N., Cabral, D.A., Rosenberg, A.M., Petty, R.E.,Nickerson, P., Reed, M., 2005. Cytokine genotypes correlatewith pain and radiologically defined joint damage in patientswith juvenile rheumatoid arthritis. Rheumatology (Oxford)44, 1115–1121.

Ohtori, S., Takahashi, K., Moriya, H., Myers, R.R., 2004. TNF-alphaand TNF-alpha receptor type 1 upregulation in glia andneurons after peripheral nerve injury: studies in murine DRGand spinal cord. Spine 29, 1082–1088.

Oka, T., Aou, S., Hori, T., 1994. Intracerebroventricular injection ofinterleukin-1b enhances nociceptive neuronal responses of thetrigeminal nucleus caudalis in rats. Brain Res. 656, 236–244.

Okamoto, K., Martin, D.P., Schmelzer, J.D., Mitsui, Y., Low, P.A.,2001. Pro- and anti-inflammatory cytokine gene expression inrat sciatic nerve chronic constriction injury model ofneuropathic pain. Exp. Neurol. 169, 386–391.

Okuda, T., Ishida, O., Fujimoto, Y., Tanaka, N., Inoue, A., Nakata, Y.,Ochi, M., 2006. The autotomy relief effect of a silicone tubecovering the proximal nerve stump. J. Orthop. Res. 24, 1427–1437.

Olsson, Y., 1990. Microenvironment of the peripheral nervoussystem under normal and pathological conditions. Crit. Rev.Neurobiol. 5, 265–311.

Opree, A., Kress, M., 2000. Involvement of the proinflammatorycytokines tumor necrosis factor-alpha, IL-1 beta, and IL-6 butnot IL-8 in the development of heat hyperalgesia: effects onheat-evoked calcitonin gene-related peptide release from ratskin. J. Neurosci. 20, 6289–6293.

Owens, T., Babcock, A.A., Millward, J.M., Toft-Hansen, H., 2005.Cytokine and chemokine inter-regulation in the inflamed orinjured CNS. Brain Res. Brain Res. Rev. 48, 178–184.

Owolabi, S.A., Saab, C.Y., 2006. Fractalkine and minocycline alterneuronal acivity in the spinal cord dorsal horn. FEBS Lett. 580,4306–4310.

Ozaktay, A.C., Kallakuri, S., Takebayashi, T., Cavanaugh, J.M., Asik,I., Deleo, J.A., Weinstein, J.N., 2006. Effects of interleukin-1 beta,interleukin-6, and tumor necrosis factor on sensitivity of dorsalroot ganglion and peripheral receptive fields in rats. Eur. SpineJ. 1–9.

Park, J.S., Gamboni-Robertson, F., He, Q., Svetkauskaite, D.,Kim, J.Y., Strassheim, D., Sohn, J.W., Yamada, S., Maruyama, I.,Banerjee, A., Ishizaka, A., Abraham, E., 2006. High mobilitygroup box 1 protein interacts with multiple Toll-like receptors.Am. J. Physiol., Cell Physiol. 290, C917–C924.

Peters, C.M., Ghilardi, J.R., Keyser, C.P., Kubota, K., Lindsay, T.H.,Luger, N.M., Mach, D.B., Schwei, M.J., Sevcik, M.A., Mantyh,P.W., 2005. Tumor-induced injury of primary afferentsensory nerve fibers in bone cancer pain. Exp. Neurol. 193,85–100.

Peters, C.M., Jimenez-Andrade, J.M., Jonas, B.M., Sevcik, M.A.,Koewler, N.J., Ghilardi, J.R., Wong, G.Y., Mantyh, P.W., 2006.Intravenous paclitaxel administration in the rat induces aperipheral sensory neuropathy characterized by macrophageinfiltration and injury to sensory neurons and theirsupporting cells. Exp. Neurol. 203, 42–54.

Piao, Z.G., Cho, I.H., Park, C.K., Hong, J.P., Choi, S.Y., Lee, S.J., Lee, S.,Park, K., Kim, J.S., Oh, S.B., 2006. Activation of glia andmicroglial p38 MAPK in medullary dorsal horn contributes totactile hypersensitivity following trigeminal sensory nerveinjury. Pain 121, 219–231.

Qiu, Z., Sweeney, D.D., Netzeband, J.G., Gruol, D.L., 1998. Chronicinterleukin-6 alters NMDA receptor-mediated membraneresponses and enhances neurotoxicity in developing CNSneurons. J. Neurosci. 18, 10445–10456.

Raghavendra, V., Rutkowski, M.D., DeLeo, J.A., 2002. The role ofspinal neuroimmune activation in morphinetolerance/hyperalgesia in neuropathic and sham-operatedrats. J. Neurosci. 22, 9980–9989.

Raghavendra, V., Tanga, F., DeLeo, J.A., 2003. Inhibition ofmicroglial activation attenuates the development but notexisting hypersensitivity in a rat model of neuropathy.J. Pharmacol. Exp. Ther. 306, 624–630.

Raghavendra, V., Tanga, F.Y., DeLeo, J.A., 2004a. Attenuation ofmorphine tolerance, withdrawal-induced hyperalgesia,and associated spinal inflammatory immune responses bypropentofylline in rats. Neuropsychopharmacology 29,327–334.

Raghavendra, V., Tanga, F.Y., DeLeo, J.A., 2004b. Complete Freundsadjuvant-induced peripheral inflammation evokes glial


activation and proinflammatory cytokine expression in theCNS. Eur. J. Neurosci. 20, 467–473.

Raivich, G., 2005. Like cops on the beat: the active role of restingmicroglia. Trends Neurosci. 28, 571–573.

Ramer, M.S., Murphy, P.G., Richardson, P.M., Bisby, M.A., 1998.Spinal nerve lesion-inducedmechanoallodynia and adrenergicsprouting in sensory ganglia are attenuated ininterleukin-6 knockout mice. Pain 78, 115–121.

Rappert, A., Bechmann, I., Pivneva, T., Mahlo, J., Biber, K., Nolte, C.,Kovac, A.D., Gerard, C., Boddeke, H.W., Nitsch, R., Kettenmann,H., 2004. CXCR3-dependent microglial recruitment is essentialfor dendrite loss after brain lesion. J. Neurosci. 24, 8500–8509.

Reeve, A.J., Patel, S., Fox, A., Walker, K., Urban, L., 2000.Intrathecally administered endotoxin or cytokines produceallodynia, hyperalgesia and changes in spinal cord neuronalresponses to nociceptive stimuli in the rat. Eur. J. Pain4, 247–257.

Ren, W.H., Guo, J.D., Cao, H., Wang, H., Wang, P.F., Sha, H., Ji, R.R.,Zhao, Z.Q., Zhang, Y.Q., 2006. Is endogenous D-serine in therostral anterior cingulate cortex necessary for pain-relatednegative affect? J. Neurochem. 96, 1636–1647.

Romero-Sandoval, A., Eisenach, J.C., 2006. Perineural clonidinereduces mechanical hypersensitivity and cytokine productionin established nerve injury. Anesthesiology 104, 351–355.

Said, G., Hontebeyrie-Joskowicz, M., 1992. Nerve lesions inducedby macrophage activation. Res. Immunol. 143, 589–599.

Samad, T.A., Wang, H., Broom, D.C., Woolf, C.J., 2004. Centralneuroimmune interactions after peripheral inflammation:interleukin-1b potentiates synaptic transmission in the spinalcord. Proc. Soc. Neurosci. 34, 511–517.

Schafers, M., Geis, C., Svensson, C.I., Luo, Z.D., Sommer, C., 2003a.Selective increase of tumour necrosis factor-alpha in injured andspared myelinated primary afferents after chronic constrictiveinjury of rat sciatic nerve. Eur. J. Neurosci. 17, 791–804.

Schafers, M., Lee, D.H., Brors, D., Yaksh, T.L., Sorkin, L.S., 2003b.Increased sensitivity of injured and adjacent uninjured ratprimary sensory neurons to exogenous tumor necrosisfactor-alpha after spinal nerve ligation. J. Neurosci.23, 3028–3038.

Scholz, J., Broom, D.C., Youn, D.H., Mills, C.D., Kohno, T., Suter,M.R., Moore, K.A., Decosterd, I., Coggeshall, R.E., Woolf, C.J.,2005. Blocking caspase activity prevents transsynapticneuronal apoptosis and the loss of inhibition in lamina II of thedorsal horn after peripheral nerve injury. J. Neurosci. 25,7317–7323.

Shamash, S., Reichert, F., Rotshenker, S., 2002. The cytokinenetwork of Wallerian degeneration: tumor necrosisfactor-alpha, interleukin-1alpha, and interleukin-1beta.J. Neurosci. 22, 3052–3060.

Shan, S., Hong, C., Mei, H., Ting-Ting, L., Hai-Li, P., Yu-Qiu, Z., 2006.New Evidence for the Involvement of Spinal FractalkineReceptor in Pain Facilitation and Spinal Glia.

Shavit, Y., Wolf, G., Goshen, I., Livshits, D., Yirmiya, R., 2005.Interleukin-1 antagonizes morphine analgesia and underliesmorphine tolerance. Pain 115, 50–59.

Shieh, C.C., Jarvis, M.F., Lee, C.-H., Perner, R.J., 2006. P2X receptorligands and pain. Expert Opin. Ther. Pat. 16, 1113–1127.

Shoskes, D.A., Albakri, Q., Thomas, K., Cook, D., 2002. Cytokinepolymorphisms in men with chronic prostatitis/chronic pelvicpain syndrome: association with diagnosis and treatmentresponse. J. Urol. 168, 331–335.

Shubayev, V.I., Myers, R.R., 2001. Axonal transport of TNF-alpha inpainful neuropathy: distribution of ligand tracer and TNFreceptors. J. Neuroimmunol. 114, 48–56.

Sindrup, S.H., Jensen, T.S., 1999. Efficacy of pharmacologicaltreatments of neuropathic pain: an update and effect related tomechanism of drug action. Pain 83, 389–400.

Sindrup, S.H., Jensen, T.S., 2000. Pharmacologic treatment of painin polyneuropathy. Neurology 55, 915–920.

Solovieva, S., Leino-Arjas, P., Saarela, J., Luoma, K., Raininko, R.,Riihimaki, H., 2004. Possible association of interleukin 1 genelocus polymorphisms with low back pain. Pain 109, 8–19.

Sommer, C., Schafers, M., 1998. Painful mononeuropathy inC57BL/Wld mice with delayed wallerian degeneration:differential effects of cytokine production and nerveregeneration on thermal and mechanical hypersensitivity.Brain Res. 784, 154–162.

Sommer, C., Galbraith, J.A., Heckman, H.M., Myers, R.R., 1993.Pathology of experimental compression neuropathy producinghyperesthesia. J. Neuropathol. Exp. Neurol. 52, 223–233.

Sommer, C., Marziniak, M., Myers, R.R., 1998. The effect ofthalidomide treatment on vascular pathology andhyperalgesia caused by chronic constriction injury of rat nerve.Pain 74, 83–91.

Sommer, C., Petrausch, S., Lindenlaub, T., Toyka, K.V., 1999.Neutralizing antibodies to interleukin 1-receptor reduce painassociated behavior in mice with experimental neuropathy.Neurosci. Lett. 270, 25–28.

Song, P., Zhao, Z.Q., 2001. The involvement of glial cells in thedevelopment of morphine tolerance. Neurosci. Res. 39,281–286.

Sorkin, L.S., Doom, C.M., 2000. Epineurial application of TNF elicitsan acute mechanical hyperalgesia in the awake rat. J. Peripher.Nerv. Syst. 5, 96–100.

Sorkin, L.S., Xiao, W.H., Wagner, R., Myers, R.R., 1997. Tumournecrosis factor-alpha induces ectopic activity in nociceptiveprimary afferent fibres. Neuroscience 81, 255–262.

Sorkin, L.S., Hammaker, D., Boyle, D., Vail, E., Svensson, C.I.,Zatooni, M., Wei, H., Firestein, G.S., 2006. Loss of MKK3affects measures of chronic pain in mice. Proc. Soc.Neurosci. 2006.

Sorkin, L.S., Firestein, G.S., Schafers, M., in press. Immune cells inperipheral nerve. In: J.A. DeLeo, L.S. Sorkin, L.R. Watkins (Eds.),Immune and Glial Regulation of Pain, IASP Press, Seattle, p.

Sorrells, S.F., Sapolsky, R.M., 2007. An inflammatory review ofglucocorticoid actions in the CNS. Brain Behav. Immun.21, 259–272.

Sperlagh, B., Baranyi, M., Hasko, G., Vizi, E.S., 2004. Potent effect ofinterleukin-1 beta to evoke ATP and adenosine release from rathippocampal slices. J. Neuroimmunol. 15, 33–39.

Stellwagen, D., Beattie, E.C., Seo, J.Y., Malenka, R.C., 2005.Differential regulation of AMPA receptor and GABA receptortrafficking by tumor necrosis factor-alpha. J. Neurosci. 25,3219–3228.

Stievano, L., Piovan, E., Amadori, A., 2004. C and CX3C chemokines:cell sources and physiopathological implications. Crit. Rev.Immunol. 24, 205–228.

Svensson, C.I., Marsala, M., Westerlund, A., Calcutt, N.A.,Campana,W.M., Freshwater, J.D., Catalano, R., Feng, Y., Protter,A.A., Scott, B., Yaksh, T.L., 2003. Activation of p38mitogen-activated protein kinase in spinal microglia is acritical link in inflammation-induced spinal pain processing.J. Neurochem. 86, 1534–1544.

Svensson, C.I., Hua, X.Y., Powell, H.C., Lai, J., Porreca, F., Yaksh,T.L., 2005. Prostaglandin E2 release evoked by intrathecaldynorphin is dependent on spinal p38 mitogen activatedprotein kinase. Neuropeptides 39, 485–494.

Sweitzer, S.M., Hickey, W.F., Rutkowski, M.D., Pahl, J.L., DeLeo, J.A.,2002. Focal peripheral nerve injury induces leukocytetrafficking into the central nervous system: potentialrelationship to neuropathic pain. Pain 100, 163–170.

Tai, Y.H., Wang, Y.H., Wang, J.J., Tao, P.L., Tung, C.S., Wong, C.S.,2006. Amitriptyline suppresses neuroinflammation andup-regulates glutamate transporters in morphine-tolerantrats. Pain 124, 77–86.

Takahashi, M., Kawaguchi, M., Shimada, K., Konishi, N., Furuya, H.,Nakashima, T., 2004. Peri-sciatic administration ofindomethacin early after nerve injury can attenuate the


development of tactile allodynia in a rat model of L5 singlespinal nerve injury. Neurosci. Lett. 356, 37–40.

Takeda, K., Sawamura, S., Tamai, H., Sekiyama, H., Hanaoka, K.,2005. Role for cyclooxygenase 2 in the development andmaintenance of neuropathic pain and spinal glial activation.Anesthesiology 103, 837–844.

Takeda, M., Tanimoto, T., Kadoi, J., Nasu, M., Takahashi, M.,Kitagawa, J., Matsumoto, S., 2007. Enhanced excitability ofnociceptive trigeminal ganglion neurons by satellite glialcytokine following peripheral inflammation. Pain 129, 155–166.

Takeuchi, H., Jin, S., Wang, J., Zhang, G., Kawanokuchi, J., Kuno, R.,Sonobe, Y., Mizuno, T., Suzumura, A., 2006. Tumor necrosisfactor-alpha induces neurotoxicity via glutamate release fromhemichannels of activated microglia in an autocrine manner.J. Biol. Chem. 281, 21362–21368.

Tanaka, T., Minami, M., Nakagawa, T., Satoh, M., 2004. Enhancedproduction of monocyte chemoattractant protein-1 in thedorsal root ganglia in a rat model of neuropathic pain: possibleinvolvement in the development of neuropathic pain.Neurosci. Res. 48, 463–469.

Tanga, F.Y., Raghavendra, V., DeLeo, J.A., 2004. Quantitativereal-time RT-PCR assessment of spinal microglial andastrocytic activation markers in a rat model of neuropathicpain. Neurochem. Int. 45, 397–407.

Tanga, F.Y., Nutile-McMenemy, N., DeLeo, J.A., 2005. The CNS roleof Toll-like receptor 4 in innate neuroimmunity and painfulneuropathy. Proc. Natl. Acad. Sci. U. S. A. 102, 5856–5861.

Tawfik, V.L., DeLeo, J.A., in press. Modulating glial activation inopioid tolerance and neuropathic pain: a role for glutamatetransporters. In: J.A. DeLeo, L.S. Sorkin, L.R. Watkins (Eds.),Immune and Glial Regulation of Pain, IASP Press, Seattle, p.

Tawfik, V.L., Lacroix-Fralish, M.L., Bercury, K.K.,Nutile-McMenemy, N., Harris, B.T., Deleo, J.A., 2006. Inductionof astrocyte differentiation by propentofylline increasesglutamate transporter expression in vitro: heterogeneity of thequiescent phenotype. Glia 54, 193–203.

Tawfik, V.L., Nutile-McMenemy, N., LaCroix-Fralish, M.L., DeLeo,J.A., 2007. Efficacy of propentofylline, a glial modulating agent,on existing mechanical allodynia following peripheral nerveinjury. Brain Behav. Immun. 21, 677–685.

Theodosiou, M., Rush, R.A., Zhou, X.F., Hu, D., Walker, J.S., Tracey,D.J., 1999. Hyperalgesia due to nerve damage: role of nervegrowth factor. Pain 81, 245–255.

Tikka, T.M., Koistinaho, J.E., 2001. Minocycline providesneuroprotection against N-methyl-D-aspartate neurotoxicityby inhibiting microglia. J. Immunol. 166, 7527–7533.

Trang, T., Beggs, S., Salter, M.W., 2006. Purinoceptors in microgliaand neuropathic pain. Pflugers Arch. 452, 645–652.

Tsuda, M., Shigemoto-Mogami, Y., Koizumi, S., Mizokoshi, A.,Kohsaka, S., Salter, M.W., Inoue, K., 2003. P2X4 receptorsinduced in spinal microglia gate tactile allodynia after nerveinjury. Nature 424, 778–783.

Twining, C.M., Sloane, E.M., Milligan, E.D., Chacur, M., Martin, D.,Poole, S., Marsh, H., Maier, S.F., Watkins, L.R., 2004. Peri-sciaticproinflammatory cytokines, reactive oxygen species, andcomplement induce mirror-image neuropathic pain in rats.Pain 110, 299–309.

Uceyler, N., Valenza, R., Stock, M., Schedel, R., Sprotte, G., Sommer,C., 2006. Reduced levels of antiinflammatory cytokines inpatients with chronic widespread pain. Arthritis Rheum. 54,2656–2664.

Uceyler, N., Tscharke, A., Sommer, C., 2007. Early cytokineexpression in mouse sciatic nerve after chronic constrictionnerve injury depends on calpain. Brain Behav. Immun. 21,553–560.

van der Goot, F.G., Pugin, J., Hribar, M., Fransen, L., Dunant, Y., DeBaetselier, P., Bloc, A., Lucas, R., 1999. Membrane interaction ofTNF is not sufficient to trigger increase in membraneconductance in mammalian cells. FEBS Lett. 460, 107–111.

van der Laan, L., Kapitein, P., Verhofstad, A., Hendriks, T., Goris,R.J., 1998. Clinical signs and symptoms of acute reflexsympathetic dystrophy in one hindlimb of the rat, induced byinfusion of a free-radical donor. Acta Orthop. Belg. 64, 210–217.

Vanderah, T.W., Gardell, L.R., Burgess, S.E., Ibrahim, M., Dogrul, A.,Zhong, C.M., Zhang, E.T., Malan Jr., T.P., Ossipov, M.H., Lai, J.,Porreca, F., 2000. Dynorphin promotes abnormal pain andspinal opioid antinociceptive tolerance. J. Neurosci. 20, 7074–7079.

Vargas, M.E., Singh, S.J., Barres, B.A., 2006. Why is Walleriandegeneration so slow in the CNS? In: Barres, B.A., Raff, M. (Eds.),Glia in Health and Disease, Cold Spring Harbor Laboratories.Cold Spring Harbor Laboratories, p. 130.

Verge, G.M., Milligan, E.D., Maier, S.F., Watkins, L.R., Naeve, G.S.,Foster, A.C., 2004. Fractalkine (CX3CL1) and fractalkine receptor(CX3CR1) distribution in spinal cord and dorsal root gangliaunder basal and neuropathic pain conditions. Eur. J. Neurosci.20, 1150–1160.

Viviani, B., Bartesaghi, S., Gardoni, F., Vezzani, A., Behrens, M.M.,Bartfai, T., Binaglia, M., Corsini, E., Di Luca, M., Galli, C.L.,Marinovich, M., 2003. Interleukin-1beta enhances NMDAreceptor-mediated intracellular calcium increase throughactivation of the Src family of kinases. J. Neurosci. 23,8692–8700.

Viviani, B., Gardoni, F., Bartesaghi, S., Corsini, E., Facchi, A., Galli,C.L., Di Luca, M., Marinovich, M., 2006. Interleukin-1betareleased by gp120 drives neural death through tyrosinephosphorylation and trafficking of NMDA receptors. J. Biol.Chem. 281, 30212–30222.

von Banchet, G.S., Kiehl, M., Schaible, H.G., 2005. Acute andlong-term effects of IL-6 on cultured dorsal root ganglionneurones from adult rat. J. Neurochem. 94, 238–248.

Watkins, L.R., Maier, S.F., 2000. The pain of being sick: implicationsof immune-to-brain communication for understanding pain.Annu. Rev. Psychol. 51, 29–57.

Watkins, L.R., Maier, S.F., 2002. Beyond neurons: evidence thatimmune and glial cells contribute to pathological pain states.Physiol. Rev. 82, 981–1011.

Watkins, L.R., Maier, S.F., 2003. Glia: a novel drug discovery targetfor clinical pain. Nat. Rev. Drug Discov. 2, 973–985.

Watkins, L.R., Wiertelak, E.P., Goehler, L.E., Smith, K.P., Martin, D.,Maier, S.F., 1994. Characterization of cytokine-inducedhyperalgesia. Brain Res. 654, 15–26.

Watkins, L.R., Hansen, M.K., Nguyen, K.T., Lee, J.E., Maier, S.F.,1999. Dynamic regulation of the proinflammatory cytokine,interleukin-1 beta: molecular biology for non-molecularbiologists. Life Sci. 65, 449–481.

Watkins, L.R., Hutchinson, M.R., Johnston, I.N., Maier, S.F., 2005.Glia: novel counter-regulators of opioid analgesia. TrendsNeurosci. 28, 661–669.

Werry, E.L., Liu, G.J., Bennett, M.R., 2006. Glutamate-stimulatedATP release from spinal cord astrocytes is potentiated bysubstance P. J. Neurochem. 99, 924–936.

White, F.A., Bhangoo, S.K., Miller, R.J., 2005. Chemokines: integratorsof pain and inflammation. Nat. Rev. Drug Discov. 4, 834–844.

Wieseler, J., Jekich, B.M., Mahoney, J.H., Lewis, S., Coats, B.D.,Maier, S.F., Watkins, L.R., 2006. Spinal cord meninges: a playerin exaggerated pain states? Proc. Soc. Neurosci. 2006.

Wieseler, J., Jekich, B.M., Bland, S., Maier, S.F.,Watkins, L.R., 2007. Anovel immune-to CNS communication pathway: cells of themeninges surrounding the spinal cord CSF space produceproinflammatory cytokines in response to an inflammatorystimulus. Brain Behav. Immun. 21, 711–719.

Wilkinson, M.F., Earle, M.L., Triggle, C.R., Barnes, S., 1996.Interleukin-1beta, tumor necrosis factor-alpha, and LPSenhance calcium channel current in isolated vascular smoothmuscle cells of rat tail artery. FASEB J. 10, 785–791.

Woolf, C.J., 2004. Dissecting out mechanisms responsible forperipheral neuropathic pain: implications for diagnosis andtherapy. Life Sci. 74, 2605–2610.


Wu, H.E., Thompson, J., Sun, H.S., Leitermann, R.J., Fujimoto, J.M.,Tseng, L.F., 2004a. Nonopioidergic mechanism mediatingmorphine-induced antianalgesia in the mouse spinal cord.J. Pharmacol. Exp. Ther. 310, 240–246.

Wu, Y., Willcockson, H.H., Maixner, W., Light, A.R., 2004b. Suramininhibits spinal cord microglia activation and long-termhyperalgesia induced by formalin injection. J. Pain 5, 48–55.

Wu, H.E., Thompson, J., Sun, H.S., Terashvili, M., Tseng, L.F., 2005.Antianalgesia: stereoselective action of dextro-morphine overlevo-morphine on glia in the mouse spinal cord. J. Pharmacol.Exp. Ther. 314, 1101–1108.

Wu, H.E., Sun, H.S., Cheng, C.W., Terashvili, M., Tseng, L.F., 2006a.Dextro-naloxone or levo-naloxone reverses the attenuation ofmorphine antinociception induced by lipopolysaccharide inthe mouse spinal cord via a non-opioid mechanism. Eur. J.Neurosci. 24, 2575–2580.

Wu, H.E., Sun, H.S., Cheng, C.W., Tseng, L.F., 2006b. p38Mitogen-activated protein kinase inhibitor SB203580 reversesthe antianalgesia induced by dextro-morphine or morphinein the mouse spinal cord. Eur. J. Pharmacol. 550, 91–94.

Wu, H.E., Sun, H.S., Terashivili, M., Schwasinger, E., Sora, I., Hall,F.S., Uhl, G.R., Tseng, L.F., 2006c. Dextro- and levo-morphineattenuate opioid delta and kappa receptor agonist producedanalgesia in mu-opioid receptor knockout mice. Eur. J.Pharmacol. 531, 103–107.

Xie, W.R., Deng, H., Li, H., Bowen, T.L., Strong, J.A., Zhang, J.M.,2006. Robust increase of cutaneous sensitivity, cytokineproduction and sympathetic sprouting in rats with localizedinflammatory irritation of the spinal ganglia. Neuroscience142, 809–822.

Xie, Y.F., Zhang, S., Chiang, C.Y., Hu, J.W., Dostrovsky, J.O., Sessle,B.J., 2007a. Involvement of glia in central sensitization intrigeminal subnucleus caudalis (medullary dorsal horn).Brain Behav. Immun. 21, 634–641.

Xie, Y.F., Zhang, S., Chiang, C.Y., Hu, J.W., Dostrovsky, J.O., Sessle,B.J., 2007b. Involvement of glia in central sensitization intrigeminal subnucleus caudalis (medullary dorsal horn). BrainBehav. Immun. 21, 634–641.

Yucesoy, B., Kashon, M.L., Luster, M.I., 2003. Cytokinepolymorphisms in chronic inflammatory diseases withreference to occupational diseases. Curr. Mol. Med. 3, 39–48.

Zelenka, M., Schafers, M., Sommer, C., 2005. Intraneural injectionof interleukin-1beta and tumor necrosis factor-alpha into ratsciatic nerve at physiological doses induces signs ofneuropathic pain. Pain 116, 257–263.

Zhang, J., De Koninck, Y., 2006. Spatial and temporal relationshipbetween monocyte chemoattractant protein-1 expression andspinal glial activation following peripheral nerve injury.J. Neurochem. 97, 772–783.

Zhang, J.M., Li, H., Liu, B., Brull, S.J., 2002a. Acute topicalapplication of tumor necrosis factor alpha evokes proteinkinase A-dependent responses in rat sensory neurons.J. Neurophysiol. 88, 1387–1392.

Zhang, X., Llamado, L., Pillay, I., Price, P., Will, R., 2002b.Interleukin-1 gene polymorphism disease activity and bonemineral metabolism in rheumatoid arthritis. Chin. Med. J.(Engl.) 115, 46–49.

Zhang, X.F., Han, P., Faltynek, C.R., Jarvis, M.F., Shieh, C.C.,2005. Functional expression of P2X7 receptors innon-neuronal cells of rat dorsal root ganglia. Brain Res.1052, 63–70.

Zhuang, Z.-Y., Kawasaki, Y., Wen, Y.-R., Huang, J., Ji, R.-R., inpress. Role of the CX3CR1/p38 MAPK pathway in spinalmicroglia for the development of neuropathic pain followingnerveinjury-induced cleavage of fractalkine, Brain Behav. Immun.

Zimmermann, M., 2001. Pathobiology of neuropathic pain. Eur. J.Pharmacol. 429, 23–37.

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