Neurotrophin-Mediated Potentiation of Neuronal InjuryM. MARGARITA BEHRENS,* UTA STRASSER, DOUG LOBNER, AND LAURA L. DUGANCenter for the Study of the Nervous System Injury and Department of Neurology, WashingtonUniversity School of Medicine, St. Louis, Missouri

KEY WORDS neurotrophins; BDNF; NGF; ischemia; excitotoxicity; apoptosis; free radicals

ABSTRACT The neurotrophins are a diverse family of peptides which activate specific tyrosinekinase-linked receptors. Over the past five decades, since the pioneering work of Levi-Montalciniand colleagues, the critical role that neurotrophins play in shaping the developing nervous systemhas become increasingly established. These molecules, which include the nerve growth factor(NGF)-related peptides, NGF, brain-derived neurotrophic factor (BDNF), NT-4/5 and NT-3, promotedifferentiation and survival in the developing nervous system, and to a lesser extent in the adultnervous system. As survival-promoting molecules, neurotrophins have been studied as potentialneuroprotective agents, and have shown beneficial effects in many model systems. However, asurprising ‘‘dark side’’ to neurotrophin behavior has emerged from some of these studies implyingthat, under certain pathological conditions, neurotrophins may exacerbate, rather than alleviate,injury. How neurotrophins cause these deleterious consequences is a question which is onlybeginning to be answered, but initial work supports altered free radical handling or modification ofglutamate receptor expression as possible mechanisms underlying these effects. This review willfocus on evidence suggesting that neurotrophins may enhance injury under certain circumstancesand on the mechanisms behind these injury-promoting aspects. Microsc. Res. Tech. 45:276–284,1999. r 1999 Wiley-Liss, Inc.

INTRODUCTIONThe neurotrophins are a family of growth factors that

promote survival and differentiation of the nervoussystem. Among them, the members of the family ofnerve growth factor (NGF) including NGF, brain-derived neurotrophic factor (BDNF), and the neuro-trophins 4/5 (NT-4/5) and 3 (NT-3) act via specifictyrosine kinase receptors Trk A, Trk B, and Trk C, and acommon low affinity receptor, p75NTR (reviewed byBarbacid, 1994). During development of the peripheralnervous system, the principal mechanism underlyingneuronal survival is the competition for limitingamounts of neurotrophins (Oppenheim et al., 1991).Synthesized by target tissues, the neurotrophins pro-vide the survival support of sensory and sympatheticneurons (reviewed by Levi-Montalcini, 1987; Snider,1994; Ip and Yancopoulos, 1996; Lewin and Bardee,1996), where development of each specific subpopula-tion of neurons appears to undergo sequential depen-dence on distinct neurotrophins (reviewed by Davies,1997; Conover and Yancopoulos, 1997). This trophicsupport plays a critical role in the determination ofneuronal survival by preventing programmed cell death,or apoptosis (Wyllie et al., 1980), a prominent feature ofthe developing nervous system. Paradoxically, althoughhigh affinity sites for NGF require expression of bothTrk A and p75NTR (Barker and Shooter, 1994; Verdi etal., 1994; Lee et al., 1994), activation of p75NTR in theabsence of Trk A has been shown to promote apoptosis,probably through increased intracellular ceramide lev-els (reviewed by Bredesen and Rabizadeh, 1997; Chaoet al., 1998).

The role of neurotrophins in the survival of develop-ing central neurons is less clear. Recent studies in mice

carrying targeted mutations in the genes coding foreither the neurotrophins or its receptors revealed nogross abnormalities in the different populations ofcentral neurons (Crowley et al., 1994; Ernfors et al.,1994a,b; Klein et al., 1994), although effects in pheno-type regulation were demonstrated (Jones et al., 1994).Furthermore, neurotrophins were shown to regulatethe type and number of afferent synapses by promotingthe survival of discrete neuronal subpopulations (Hy-man et al., 1991; Arenas and Persson, 1994), alter thesynaptic connectivity of the developing cortex (Castrenet al., 1992; Maffei et al., 1992; Cabelli et al., 1995), andalso to modify synaptic strength in adult animals(reviewed by Thoenen, 1995). This supports the emerg-ing concept that central neuronal survival may involveinput from multiple overlapping neurotrophin systems.

NEURONAL INJURY: NECROSISAND APOPTOSIS

Apoptosis, characterized morphologically by cellshrinkage, chromatin condensation, and internucleo-somal DNA fragmentation, follows a temporally definedpattern culminating in the activation of a family ofcysteine proteases called caspases (reviewed by Thorn-berry and Lazebnik, 1998). Cell surface presentation ofintracellular molecules (Fadok et al., 1992) triggers therapid sequestration of these dying cells by phagocytesor other neighboring cells, thus preventing cell lysis-induced inflammatory reactions. This process was re-

*Correspondence to: M. Margarita Behrens, Center for the Study of theNervous System Injury and Department of Neurology, Box 8111, WashingtonUniversity School of Medicine, 660 S. Euclid Ave., St. Louis, MO 63110.E-mail, behrensm@neuro.wustl.edu

Received 12 November 1998; accepted in revised form 11 December 1998

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r 1999 WILEY-LISS, INC.

cently proposed to be mediated by DOCK180, themammalian homolog of C. elegans CED-5 (Wu andHorvitz, 1998). Prevention of cell lysis in the nervoussystem is of particular importance, since the release ofexcitotoxic concentrations of glutamate from dying cellsresults in the overstimulation of ionotropic glutamatereceptors leading to rapid cell body and dendriticswelling (Rothman and Olney, 1987; Choi, 1988, 1992),characteristics of a necrotic death.

The notion of excitotoxic-mediated neuronal necrosishas been strengthened by cumulative evidence fromexperimental animal models indicating that excitotoxicglutamate release into the extracellular space is a keyevent leading to necrosis during postischemic andtraumatic injuries (Faden, 1993; Choi and Rothman,1990). Furthermore, in cultured neurons even mildexcitotoxic insults tend to trigger necrosis, accompa-nied by prominent acute cell swelling, increase inintracellular calcium concentration, and insensitivityto protein synthesis inhibitors (Gwag et al., 1997). Thisrapidly triggered, glutamate receptor-mediated toxicitywas not prevented by antiapoptotic interventions suchas neurotrophin treatments (Koh et al., 1995) or inhibi-tors of caspases (Gottron et al., 1997a). In addition,neurons from bax-gene knockout mice were not pro-tected against excitotoxic treatments, either in vitro orin vivo (Choi, 1996; Gottron et al., 1997b; but see Xianget al., 1998), suggesting that apoptosis and necrosishave different execution pathways.

Although apoptosis and necrosis are commonly re-garded as conceptually distinct modes of cell death, ithas been proposed that they may represent the twoextreme ends of a death continuum (Choi, 1996; Leistand Nicotera, 1998). Growing evidence suggests thatthe definition of whether the death is apoptotic ornecrotic will be defined by the energy conditions of thecell (reviewed by Green and Reed, 1998; Leist andNicotera, 1998). Apoptosis-inducing conditions trig-gered necrosis when intracellular ATP concentrationswere reduced below 50%, whereas restoration of ATPlevels through glycolysis in the presence of mitochon-drial inhibitors was sufficient to restore the apoptosisprogram (Eguchi et al., 1997; Leist et al., 1997; Lieber-thal et al., 1998). Thus, in neurons expressing highlevels of glutamate receptor function, excitotoxin-mediated calcium overload will induce depolarization,glutamate release, and overactivation of the excitotoxicmechanisms, which will in turn induce a further compro-mise of energy levels, collapse of mitochondria, andfinally necrosis (Novelli et al., 1988; Isaev et al., 1996;White and Reynolds, 1996). In the absence of thishighly energy-consuming excitotoxic injury, intracellu-lar energy levels may be maintained and indolent levelsof excitotoxicity may lead to apoptosis (Choi, 1996).Indeed, glutamate exposure can induce apoptosis inyoung neurons (Kure et al., 1991), or both apoptosis andnecrosis in older neurons (Ankarcrona et al., 1995;Bonfoco et al., 1995; Tenneti et al., 1998). Althoughthese effects could be attributed to glutathione deple-tion and oxidative stress in young neurons (Murphy etal., 1989, 1990; Ratan et al., 1994), or a low level ofglutamate receptor function in culture, they supportthe hypothesis that apoptosis and necrosis may beconcurrently triggered under mild excitotoxic conditions.

Recent evidence shows that different kinds of celldeath may indeed coexist in several diseases andinjuries of the nervous system (reviewed by Johnson etal., 1995; Choi, 1996). For example, in the core ofischemic regions necrotic cell death is prevalent,whereas toward the border regions, where energy deple-tion and excitotoxic stimulation is less severe, apoptoticneuronal death has been shown to occur (Li et al.,1995). Furthermore, exposure of cultured cortical neu-rons to oxygen-glucose deprivation conditions in thepresence of glutamate receptor antagonists inducedapoptosis instead of necrosis, which was prevented bycaspase inhibitors (Gwag et al., 1995a; Gottron et al.,1997a).

A more direct role for apoptosis in ischemic braininjury was recently suggested by results showing thatexpression of dominant negative mutations of the inter-leukin-converting enzyme (ICE) in transgenic mice(Friedlander et al., 1997; Hara et al., 1997a), as well asinhibition of the ICE family of proteases with z-VAD.FMK (Hara et al., 1997b) attenuated ischemic braindamage. Activation of caspase-3, considered to be theprotease playing the major role in neuronal apoptosis(reviewed by Pettmann and Henderson, 1998), wasrecently shown to occur during apoptosis induced byexperimental cerebral ischemia (Namura et al., 1998).

NEUROTROPHINS IN NEURONAL DEATH:PREVENTION OF APOPTOSIS

There is now extensive literature demonstrating theability of neurotrophins to attenuate neuronal apopto-sis (reviewed by Lewin and Barde, 1996; Dragunow etal., 1997b), and given this clear evidence for theirantiapoptotic effects they could be considered the mol-ecules of choice in the treatment of neuronal death aftertraumatic injury or stroke. Indeed, neurotrophins andother growth factors have been shown to reduce theneuronal damage induced by exposure to excitotoxins,glucose deprivation, or ischemia (Shigeno et al., 1991;Davies and Beardsall, 1992; Frim et al., 1993; Chengand Mattson, 1994; Cheng et al., 1994, 1997; Holtzmanet al., 1996). Furthermore, the survival-promoting ef-fect of neurotrophins is well documented for manydifferent models of central or peripheral nervous sys-tem injury. For example, both NGF and NT-3 preventedexperimental cisplatin sensory neuropathy (Apfel et al.,1992; Gao et al., 1995), BDNF and NGF prevented thedegeneration of basal forebrain cholinergic neuronsinduced by fimbria-fornix lesions (Hefti, 1986; Widmeret al., 1993), and attenuated neuronal injury in aneonatal model of hypoxic-ischemic brain injury (Holtz-man et al., 1996; Cheng et al., 1997). In addition,axotomy-induced motoneuron death (Sendtner et al.,1992; Yan et al., 1992) and motoneuron degeneration inwobbler mice (Mitsumoto et al., 1994; Ikeda et al., 1995)were prevented by BDNF administration. The extent ofthe apoptotic component in these types of injuriesprobably reflects the effectiveness of neurotrophin treat-ments.

NEUROTROPHINS IN NEURONALEXCITATION: POTENTIATION

OF EXCITOTOXICITYDuring the past 10 years, increasing evidence has

accumulated showing that synaptic activity and stimu-

277NEUROTROPHIN-MEDIATED NEURONAL INJURY

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lation of glutamate receptors results in an increasedexpression of BDNF and its receptor Trk B (Zafra et al.,1990; Thoenen et al., 1991; Lindefors et al., 1992;Bessho et al., 1993; Favaron et al., 1993; Gwag andSpringer, 1993; Ghosh et al., 1994; Wetmore et al.,1994; Falkenberg et al., 1996; Dragunow et al., 1997a;Hughes et al., 1998; Pellegri et al., 1998). Also, inpathological conditions such as cerebral ischemia andkindling, BDNF mRNA was increased by glutamate-mediated mechanisms (Lindvall et al., 1992; Ernfors etal., 1991). On the other hand, prolonged treatment withneurotrophins were shown to induce NMDA-receptorsubunit expression in neurons (Muzet and Dupont,1996; Small et al., 1998; Ying et al., 1995) and NGFtreatment induced NMDAR1 mRNAexpression in PC12cells (Schubert et al., 1992), perhaps in part by increas-ing promoter activity (Bai and Kusiak, 1997), suggest-ing that this mechanism might be a general effect ofneurotrophins.

More recently, a short-term effect of neurotrophinswas postulated. Original reports suggesting a role forBDNF in long-term potentiation (LTP) (Dragunow etal., 1993) were later confirmed by the impairment inLTP induction in BNDF knockout mice (Korte et al.,1995; Patterson et al., 1996). BDNF and NT-4 wereshown to increase the amplitude of postsynaptic cur-rents in hippocampus (Levine et al., 1995); both BDNFand NGF enhanced the excitatory postsynaptic cur-rents in rat visual cortex (Carmignoto et al., 1997;Akaneya et al., 1997); and BDNF was shown to enhanceNMDA-mediated synaptic transmission in culturedhippocampal neurons (Song et al., 1998; Levine et al.,1998; Jarvis et al., 1997). These effects also extend to invivo situations, where it was recently shown that acuteintrahippocampal infusion of BDNF induced a lastingpotentiation of synaptic transmission in adult rats(Messaoudi et al., 1998). Possible explanations for thisenhancement of synaptic transmission by BDNF are 1)protein synthesis induction (Kang and Schuman, 1996)and increased expression of NMDA receptor subunits(Muzet and Dupont, 1996; Small et al., 1998; Ying et al.,1995), or 2) presynaptic mechanisms (Li et al., 1998;Gottschalk et al., 1998) involving inhibition of GABAer-

gic transmission (Kim et al., 1994) or increased trans-mitter release (Kang and Schuman, 1995; Sakai et al.,1997; Wang and Poo, 1997), and/or 3) postsynapticmechanisms involving the enhancement of NMDA-receptor activity by phosphorylation of receptor sub-units, as described for hippocampal cultures (Suen etal., 1997; Lin et al., 1998). This phosphorylation isprobably mediated by activation of the Src-kinase fam-ily member Fyn, which was shown to bind to Trk B(Iwasaki et al., 1998) and to phosphorylate and activatethe NMDA receptor (Suzuki and Okumura-Naji, 1995;Grant, 1996; Kohr and Seeburg, 1996).

Given the enhancement of synaptic glutamate-mediated transmission by BDNF, it is not surprisingthat, besides its protective effects against the apoptoticcomponent of neuronal injury, BDNF might actuallyenhance excitotoxic neuronal damage. In addition tothe above arguments, this hypothesis is also supportedby the findings that acute treatment with BDNF in-duces an increase in intracellular free calcium(Berninger et al., 1993), as well as NMDA-mediatedrises in intracellular free calcium (Jarvis et al., 1997),and increases in IP3 and cAMP production (Knipper etal., 1993), all situations that may increase vulnerabilityto an excitotoxic insult by activation of glutamatereceptors.

Direct evidence that prolonged treatment with neuro-trophins renders neurons more sensitive to excitotoxicinsults has also been presented: BDNF enhanced gluta-mate-receptor mediated cell death in cerebellar granu-lar cell cultures (Fernandez-Sanchez and Novelli, 1993),as well as in cultures of rat hippocampal neurons(Prehn, 1996), and pretreatment with neurotrophinspotentiated the vulnerability of cortical neurons toexcitotoxic neuronal necrosis (Fig. 1) (Koh et al., 1995;Ying et al., 1995; Lobner et al., 1995; Samdami et al.,1997). For each of these cell types, however, oppositefindings were also reported: in hippocampal cultures, itwas reported that BDNF protected neurons againstglutamate-induced excitotoxicity (Mattson et al., 1995),and in cultures from cortical neurons, BDNF reducedneuronal injury induced by glutamate (Kume et al.,1997). In cerebellar granule neurons, glutamate is ableto induce neuronal apoptosis as well as necrosis (Ankarc-rona et al., 1995) and in these cells exposure to BDNFprevents glutamate-mediated apoptosis, probablythrough downregulation of NMDA receptor expression(Brandoli et al., 1998). An earlier report (Fernandez-Sanchez and Novelli, 1993) could not find a protectiveeffect of BDNF against excitotoxicity in this system,suggesting that the protective effect might be depen-dent on the severity of the insult, determining the fateof the neuron as necrotic or apoptotic. One possibleexplanation for these conflicting results could be thelevels of glutamate receptor expression in differentculture systems.

NEUROTROPHINS AND MITOCHONDRIALFREE RADICALS: POTENTIATION

OF OXIDATIVE DAMAGEOxidative stress refers to the pathological outcome of

an imbalance in the production of free radicals and theability of cells to defend against them. Mitochondrialoxidative metabolism, nitric oxide, phospholipid metabo-lism, and proteolytic pathways are all potential sources

Fig. 1. Chronic treatment of cortical neurons with BDNF enhancesinjury produced by oxygen-glucose deprivation. One day after applica-tion of BDNF (100 ng/ml), cortical cultures were exposed to 40 minutesof oxygen-glucose deprivation (OGD), and evaluated by differentialinterference contrast microscopy on a Noran confocal microscope usinga 60X water-immersion Nikon objective (1.2 NA). Control cultures(CTRL) were maintained in oxygenated, glucose-rich medium for 40minutes and showed smooth neuronal membranes, no cell swelling,and nuclei that are not well-delineated from the cytoplasm. Culturesexposed to 40 minutes OGD in the absence of BDNF pretreatment(center) demonstrated occasional neurons with irregular plasma mem-branes, cellular swelling, and distinctly delimited nuclei. In culturestreated with BDNF, swelling and disintegration of the plasma mem-branes was evident in the majority of neurons and, in addition, manyneurons demonstrated nuclei that were condensed and pyknotic. (Kohet al., 1995).

Fig. 2. Expression of BDNF and Trk B proteins in cortical neurons.Near-pure cortical neuronal cultures were grown as described (Koh etal., 1995). After 8 days, in vitro cultures were fixed for immunocyto-chemistry. BDNF and Trk B expression was detected by confocalmicroscopy using anti-BDNF or anti-Trk B specific antibodies (SantaCruz Biotechnology, Santa Cruz, CA), and Cy3-conjugated secondaryantibodies (Jackson ImmunoResearch Laboratories, West Grove, PA).Bar 5 20 µm.

279NEUROTROPHIN-MEDIATED NEURONAL INJURY

of intracellular free radicals. A net increase in reactiveoxygen species can produce damage to lipids, proteins,and DNA and induce necrosis or apoptosis (reviewed byRichter et al., 1995; Jacobson, 1996). The susceptibilityof the central nervous system to free radical-mediatedinjury is related to its high energy requirements,reliance on aerobic metabolism, high concentration ofiron, and high levels of polyunsaturated fatty acids(Cao et al., 1988; Hall and Braughler, 1989; Floyd,1990; Ikeda and Long, 1990). Under normal conditions,mitochondria produce small amounts of free radicals,which increase under several injury conditions such asexcitotoxicity (Dugan et al., 1995; Murakami et al.,1998) and in neurodegenerative diseases such as Alzhei-mer’s, Parkinson’s, and amyotrophic lateral sclerosis(reviewed by Gorman et al., 1996).

Neurotrophin treatments in vitro and in vivo havebeen shown to change the expression of certain antioxi-dant enzymes. For example, NGF increases g-glutamyl-cysteine synthetase, glutathione peroxidase, and cata-lase activity in PC12 cells (Jackson et al., 1990; Pan andPerez-Polo, 1993), while BDNF increases glutathionereductase levels in mesencephalic (Spina et al., 1992)and hippocampal neurons (Mattson et al., 1995), sug-gesting an injury-preventing role of neurotrophins infree radical-mediated injury. Furthermore, short expo-sure to neurotrophins has been shown to reduce mito-chondrial free radical production through a mitogen-activated protein kinase-dependent mechanism (Duganet al., 1997).

Embryonic cortical cultures express Trk B and BDNFproteins (Fig. 2) and BDNF has been shown to berequired in the activity-dependent survival of theseneurons (Ghosh et al., 1994; Koh et al., 1995). Activa-tion of voltage-sensitive calcium channels in thesecultures promotes neuronal survival probably throughan activity-dependent release of endogenous BDNF(Ghosh et al., 1994). Unexpectedly, in this system aswell as in striatal cultures, long-term exposure toneurotrophins has been shown to potentiate direct freeradical-mediated neuronal death (Gwag et al., 1995b;Park et al., 1998). This potentiation required proteinsynthesis and did not involve glutamate receptors.When mitochondrial free radical production was ana-lyzed, BDNF pretreatment of cortical neurons showedan enhancement of free radical production under basalconditions (Lobner D, Dugan LL, Behrens MM, andChoi DW, unpublished). The mechanism of this BDNF-mediated increase in mitochondrial free radical produc-tion is not certain, but could be related to alteredmitochondrial function. Enhanced free radical genera-tion induced by neurotrophins, however, may haveimplications for modifying intracellular signaling viaredox mechanisms, as recently suggested (Abe et al.,1998; Finkel, 1998). In addition, a role for neuronalnitric oxide synthase (NOS) was suggested to mediatethe injury-potentiating effects of neurotrophins. It wasshown that prolonged treatment with BDNF inducedthe expression of nNOS in cortical neuronal cultures,and that inhibitors of NOS such as NG-Nitro-L-arginineprevented BDNF-mediated potentiation of excitotoxic-ity (Lobner et al., 1995; Samdani et al., 1997). Undersuch conditions, normal levels of nitric oxide productionstimulated by increased intracellular calcium causedby oxygen-glucose deprivation, combined with in-

creased mitochondrial free radical production, leads tothe production of the highly toxic peroxinitrite andenhanced cell death (Darley-Usmar et al., 1995).

The ability of the free radical scavenger, trolox, toattenuate BDNF potentiation of cell death induced byoxidative damage (Gwag et al., 1995b; Park et al., 1998)or oxygen-glucose deprivation (Lobner et al., 1995)could suggest a more broad effect of neurotrophins inthe handling or production of free radicals. Thesefindings are consistent with those showing that anotherneurotrophin, NGF, potentiated glutamate-mediatedcell death in PC12 cells, through inhibition of cystineuptake leading to glutathione depletion and free radical-mediated death (Pan and Perez-Polo, 1993). Alterationsin free radical handling appear to play a key role inneurotrophin actions, and differences in culture condi-tions might lead to variations in the cells capacity tohandle free radical production (Samdani et al. 1997),which might explain some of the conflicting results.

An event now recognized as important to apoptosis inmany systems is the mitochondrial permeability transi-tion (MPT). First characterized in isolated mitochon-dria (Hunter et al., 1976), the MPT represents anabrupt increase of permeability of the mitochondrialinner membrane to solutes of low molecular mass(reviewed by Bernardi, 1996). The rapid change ofpermeability associated with MPT causes membranedepolarization, uncoupling of oxidative phosphoryla-tion, release of intra-mitochondrial ions and metabolicintermediates, and mitochondrial swelling. MPT iscaused by the opening of a megapore in the innermitochondrial membrane that allows entry of bothanionic and cationic solutes into the matrix (Szabo andZoratti, 1991). Exposure to oxidative injury may ini-tiate opening of the megapore, and inhibition of freeradical formation can block MPT (Kowaltowski et al.,1996; reviewed by Lemasters et al., 1998).

Oxidative processes occurring at the mitochondriamay be important steps on the way to both apoptoticand excitotoxic cell death, where mitochondrial func-tion and the energy state of the cell appear to be critical(reviewed by Green and Reed, 1998). Mitochondrialcalcium uptake, free radical production, and feed-forward loss of mitochondrial membrane potential oc-cur rapidly during N-methyl-D-aspartate (NMDA) re-ceptor-mediated injury (Dugan et al., 1995; Reynoldsand Hastings, 1995; Budd and Nicholls, 1996; Schinderet al., 1996), and acute treatment with mitochondrialelectron transport inhibitors (Budd and Nicholls, 1996;Stout et al., 1998) or anti-oxidants (Monyer et al., 1990;Lafon-Cazal et al., 1993; Dugan et al., 1996) decreasedNMDA-induced neuronal death. In neurons showingenhanced mitochondrial free radical production, as inBDNF-treated cortical neurons, overstimulation of glu-tamate receptors may lead to further mitochondrialdysfunction, thus enhancing neuronal vulnerability toan excitotoxic insult.

Recent findings support the hypothesis of a dualaction of neurotrophins in neuronal survival and death.BDNF, although a survival factor for Purkinje cells,induced the death of these cells when co-cultured withgranule cells (Morrison and Mason, 1998). Further-more, a similar dual effect has been postulated for themodel of retinal ganglion cell death following transec-tion of the rat optic nerve, where the protective effects

280 M. MARGARITA BEHRENS ET AL.

of BDNF were enhanced by combination treatmentswith free radical scavengers or NOS inhibitors (Klockeret al., 1998; Isenmann et al., 1998).

In conclusion, while it is clear that neurotrophins arecapable of attenuating neuronal death, the potentialharmful effects should not be underestimated. If thelatter could be identified and eliminated, the beneficialproperties of neurotrophins could be enhanced and,thus, become a powerful therapeutic strategy in thetreatment of the nervous system injuries and disease.

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