role of gene regulation during vestibular compensation : an integrative approach

52

Role of Gene Regulation duringVestibular Compensation

An Integrative Approach

CAREY D. BALABAN

Departments of Otolaryngology and Neurobiology, University of Pittsburgh,Eye & Ear Institute, Pittsburgh, Pennsylvania 15213, USA

ABSTRACT: Identification of the role of gene regulation in vestibular compensa-tion is one aspect of a larger issue: the identification of molecular bases forplasticity in multiple vestibulo-ocular, vestibulo-spinal, vestibulo-collic, andvestibulo-autonomic responses. To achieve this goal, it is incumbent on investi-gators to examine molecular events within the contexts of the single neuron, thelocation of the neuron in pathways, and the timing of the molecular eventsrelative to behavioral compensation. Hence, the goal of identifying molecularbases for a particular compensatory response (e.g., the disappearance of spon-taneous nystagmus in the light or the disappearance of static head tilt) requirescareful attention to the time course of physiologic compensation and the loca-tion of the effects within central pathways that have the potential to affect theresponses. The effects of impeding these site-specific and time-specific changescan then be tested to determine their role in the compensatory process. A con-sideration of the recent literature on molecular events related to the resolutionof spontaneous nystagmus in the light indicates that a meaningful approach tothese issues requires a broadening of our conceptual approach. Specifically,one must consider the roles of transcriptional, translational, and post-translational events on the turnover of critical signaling substrates for vestibu-lar compensation.

KEYWORDS: Gene regulation; Vestibular compensation; Plasticity; Neuron;Spontaneous nystagmus

More than a century ago, Bechterew1 presented experimental evidence for a centralprocess that produced behavioral compensation for unilateral, serial bilateral, andsimultaneous bilateral damage to the vestibular periphery. Subsequent studies havedocumented and catalogued numerous electrophysiological and neurochemical cor-relates of behavioral compensation. Despite their critical role in recovery from uni-lateral injury and, more generally, the maintenance of balance function throughoutthe life of an organism, we must admit that our knowledge of molecular bases forcompensatory processes is rudimentary. This seemingly harsh critique is true for all

Address for correspondence: Carey D. Balaban, Ph.D., Departments of Otolaryngology andNeurobiology, University of Pittsburgh, Eye & Ear Institute, 203 Lothrop Street, Pittsburgh, PA15213. Voice: (412) 647-2298; fax: (412) 647-0108.

[email protected]

53BALABAN: ROLE OF GENE REGULATION

of us (including the author) because our empirical approach has focused narrowly onidentifying isolated changes in a particular substrate in a particular site or cell pop-ulation. As a result, we are biased to interpret any difference in mRNA, protein, orsubstrate levels in compensating animals as prima facie evidence of its directinvolvement in compensatory processes. It is undeniable that this approach providesinsight into aspects of the compensatory process. However, one is reminded of thedictum that a narrow empirical approach “bears many flowers, but no fruit”. In lightof the profound advances in knowledge of the mammalian genome, it is imperativeto consider conceptual problems that need to be addressed in order to prospectivelyapproach the question of how interactive, regulated changes in gene expressionmaintain vestibular function and restore function after unilateral injury.

The obstacles to a more complete understanding of the molecular bases for ves-tibular compensation have been both technical and conceptual. Vestibular compen-sation is a dynamic phenomenon that is produced by multiple processes. Theseprocesses occur on different timescales, involve different regions of the brain, andutilize different biochemical substrates in each region. Hence, a systematic approachto identifying bases for vestibular compensation requires attention to (1) temporalgradients in compensation of different behavioral signs and symptoms of peripheralvestibular injury, (2) involvement of different brain pathways in different compo-nents of behavioral compensation, and (3) spatiotemporal gradients of biologicalresponses within these brain circuits. A consideration of these issues provides anappropriate context for interpreting results of recent studies of molecular bases forvestibular compensation.

TEMPORAL GRADIENTS IN COMPENSATION

Vestibular compensation after unilateral labyrinthectomy has been describedtypically as consisting of an acute (or critical) phase, a partially compensated (ordynamic recovery) phase, and a compensated phase.1–5 During the acute phase,humans or animals show spontaneous nystagmus in the light, postural symptoms(e.g., falling and neck torsion), and sensations of vertigo. During the second phase,symptomatic resolution occurs and the achievement of maximum recovery marks thebeginning of the compensated phase. A key property of the dynamic recovery pro-cess, though, is that different vestibular responses compensate at different rates andreach different endpoints of recovery.6,7 For example, Llinas and Walton4 reportedthat an initial, marked improvement in tonic head deviation occurs within 1 h inalbino rats, but that spontaneous nystagmus resolves over a longer time course. Inhumans, spontaneous nystagmus in the light resolves within days, but nystagmus canbe unmasked when visual fixation is abolished by Frenzel lenses or in darkness.Compensation of the dynamic performance of the horizontal vestibulo-ocular reflex,on the other hand, proceeds more slowly and is incomplete; the response remainsasymmetric to brief, rapid head accelerations. Ocular tilt (eye torsion) only compen-sates partially and the axis of eye rotation remains misaligned with respect to the axisof head rotation. By contrast, vertigo resolves completely.

The effects of visual deprivation on compensation of vestibulo-ocular reflexesprovide a rationale for the assertion that each hallmark of vestibular compensation

54 ANNALS NEW YORK ACADEMY OF SCIENCES

involves multiple compensatory processes. Although visual deprivation does notaffect the loss of spontaneous nystagmus in darkness, it impairs the recovery ofdynamic performance of the VOR.8,9 It is unclear whether vision plays a critical rolein the rapid loss of spontaneous nystagmus in the light. However, Fetter et al.9

reported that a prior occipital lobectomy attenuated the resolution of spontaneousnystagmus in the light.

The time course and completeness of each aspect of compensation are importantconsiderations for detecting and interpreting molecular bases for vestibular compen-sation. During the period of active compensation, transient molecular changes areexpected to contribute to dynamic processes that lead to behavioral endpoints ofcompensation. Therefore, paradigms to detect the molecular events must bedesigned on the basis of the time course of each compensatory process.

MODULAR PATHWAYS AND COMPENSATION

Recent basic anatomical and physiological studies of central vestibular pathwaysare consistent with the hypothesis6 that these temporal gradients and differences infunctional recovery are a product of partially independent neural substrates for dif-ferent vestibular responses. These studies indicate that vestibular responses are con-trolled by parallel pathways that include afferents from the vestibular periphery,regions within the vestibular nuclei, and specific zonal projections from cerebellarcortex. Horizontal canal–ocular reflexes appear to be mediated by specific groups ofcells in the rostral medial vestibular nucleus and ventral lateral vestibular nucleusthat receive inhibitory inputs from specific zones in the flocculo-nodular lobe. Ver-tical canal–ocular reflexes appear to be mediated by neurons in the superior vestib-ular nucleus and group y, which receive inhibitory input from other flocculo-nodularlobe zones. Lateral vestibulo-spinal tract neurons in the lateral vestibular nucleus, onthe other hand, are inhibited by a specific zone of Purkinje cells in the cerebellar an-terior lobe vermis. Hence, modifications of specific vestibular nuclear and flocculo-nodular lobe pathways are likely to be engaged in suppression of spontaneousnystagmus in the light, suppression of nystagmus in the dark, and dynamic vestibulo-ocular reflex gain compensation. In contrast, compensatory changes in gait and pos-ture are likely to involve other vestibular nuclear pathways and the cerebellar vermis.Visual fixation changes may additionally involve the caudal fastigial nucleus andposterior vermis (lobules VI–VII).

The effects of preexisting cerebellar lesions on vestibular compensation suggestthat mechanisms for vestibulo-ocular and vestibulo-spinal (postural) recovery arepathway-specific. Igarashi and Ishikawa10 reported that monkeys with a lesion of theposterior vermis showed impaired recovery from gait deviations after labyrinthecto-my, but a normal (control) time course for the reduction of nystagmus slow-phaseeye velocity in darkness. The converse was true for monkeys with nodulus and uvulaablation: the reduction of slow-phase eye velocity was impaired, but locomotorrecovery was identical to the control group. Furman et al.11 reported similar findingsfor a patient with a preexisting cerebellar infarction that included the uvula andpyramis.


SPATIAL AND TEMPORAL GRADIENTS OF GENE AND PROTEINEXPRESSION DURING VESTIBULAR COMPENSATION:

UNDERSTANDING FUNCTIONAL CONTEXTS

The problem of identifying molecular bases for vestibular compensation (andother forms of neuronal plasticity) is manifested operationally as a goal of isolatinga cascade of obligatory, “machine-like” events that lead from maladaptive to adap-tive performance of a neuron or connected group of neurons. The primary contextfor understanding the role of regulated gene expression in vestibular compensationis the response of individual neurons, glial cells, endothelial cells, or macrophagesto changes in extrinsic signals. An initial level of inquiry is to identify important oressential genes and proteins for compensation, their basal levels of expression, andtheir responses during the compensation process. It is also important to realize thatcellular context for compensation will vary with the neuron. Immediately after uni-lateral surgical labyrinthectomy, a cell in Scarpa’s ganglion may respond initially torepair direct injury to its primary dendrite. A vestibular nucleus neuron can be influ-enced potentially by factors such as changes in synaptic transmission from damagedprimary afferents, biochemical responses of afferents to injury (e.g., neurotrophinrelease), and activity related to movement from other sensory modalities. A Purkinjecell in the flocculo-nodular lobe, on the other hand, may respond to parallel fiberactivity reflecting altered primary and secondary vestibular information, activity re-flecting the generation of spontaneous nystagmus, and retinal slip signals generatedby the spontaneous nystagmus. However, even within a functional circuit, each celltype may show different patterns of responses at any given time because they (1) mayexpress different combinations of intracellular and intercellular signaling proteinsand (2) are responding to different combinations of chemical signal. From theperspective of the cell, then, one is faced with both (1) the challenge of identifyingthe critical substrates in each cell population and (2) the challenge of discriminatingcritical responses that directly mediate compensation from unrelated and secondaryresponses.

CONSIDERATIONS FOR EXPERIMENTAL DESIGN

The practical design of experimental investigations of vestibular compensationmust be sensitive to the time course and central loci of changes in gene and proteinregulation that produce different compensatory effects. These issues determine thesensitivity of the study for detecting the key features of biological substrates for ves-tibular compensation:

(1) Methods for inducing vestibular injury and species selection for the exper-imental model.

(2) Timing of biochemical changes in relation to behavioral compensation.(3) Location of changes in pathways involved in components of vestibular

compensation.(4) Identification of changes in expression of mRNAs, translation products,

posttranslationally modified products, and other substrates.(5) Identification of the polarity of changes (up- or downregulation).


Technical Considerations: Labyrinthectomy Methods and Species Selection

It seems clear from our brief exploration of compensatory processes that strate-gies to detect transient neurochemical changes during vestibular compensation re-quire careful consideration of a variety of experimental design issues. Four mainissues need to be addressed regarding the labyrinthectomy per se. First, because thecompensatory mechanisms are engaged by injury, damage should be complete,instantaneous, and synchronous in all animals. Surgical labyrinthectomy meets thiscriterion because the primary damage is restricted both temporally and spatially.Second, anesthetic agents during labyrinthectomy should be rapid-acting, rapidlymetabolized. Third, anesthetic agents should be selected for minimal inference withneurochemical substrates of interest. Fourth, sham-operated controls must be em-ployed to control for confounded effects of anesthesia and surgical trauma at shortsurvival times. For example, there is evidence from studies in vitro that halothanemay have direct effects on activation or translocation of some protein kinase C(PKC) isoforms.12,13 Both sham-operated animals and vehicle controls (for drugtreatments) are essential to control for anesthetic and surgical effects in studies ofthe role of PKC in vestibular compensation.14 It is obvious that similar concernsarise in studies of other biochemical substrates.

The selection of species and strains of animals is another important consider-ation. Timescales and relative capacities for compensation may also vary across spe-cies. These variations may extend to even the most basic of compensatory responses.For example, rats show an extremely consistent time course for the disappearance ofspontaneous nystagmus in the light.14–16 However, Faulstich et al.17 reported thatthis early form of compensation showed profound individual variations in wild-typeC57/b16 mice, which precluded its use as a dependent variable. Albinism in somestrains of laboratory animals is another issue to be considered: for example, albinorabbits show inverted optokinetic nystagmus in the anterior visual field18 and differ-ences in retinopretectal projections from pigmented animals.19 Finally, patency ofthe cochlear aqueduct in many mammalian species is a contraindication to the use ofchemical toxins in the inner ear. It is well known that perilymph is confluent withcerebrospinal fluid in the subarachoid space.20,21 It has been reported recently thatgentamicin affects the brain stem near the opening of the cochlear aqueduct22 andthat adenoviral injections into one cochlea result in transfection of cells in the brainstem and the contralateral cochlea.23 These findings show that there is a potentialperilymph–cerebrospinal fluid route for diffusion of drugs that enter the perilymphthrough the round window, such as arsanilic acid,24 aminoglycosides, or 100%ethanol.16,25 The problem should be particularly acute for the flocculus, which liesextremely close to the site of diffusion. It is obvious that central effects of such com-pounds would be confounded variables in studies of early components of neuralcompensation.

Timing and Location of Biochemical Responseswith Regard to Behavioral Compensation

Our discussion to this point leads to the premise that identification of functionalbiochemical bases for vestibular compensation requires an understanding of changesin regulated expression of cell constituents within the context of the location of the


neurons in vestibular pathways. The behavioral and neurobiological data indicatethat alterations in gene and protein expression will likely vary in both location andtiming as a function of the involvement of different groups of neurons in compensa-tory processes. For example, one would expect to find the earliest, transient changesin rats in pathways mediating the initial resolution of tonic head position, which iscompleted largely within the first 30 min after surgical labyrithectomy.4 During theinitial 24 h after labyrinthectomy, one would expect to find transient changes in geneand protein expression in regions of the flocculo-nodular lobe and vestibular nucleithat produce visual suppression of spontaneous nystagmus. Changes that produceslower features of compensation (e.g., suppression of spontaneous nystagmus in thedark and partial recovery of dynamic vestibulo-ocular control, postural control, andlocomotion) would be expected to occur along more prolonged timescales. The ini-tial changes are also expected to be highly consistent between animals at short timeintervals after damage to the vestibular periphery and then to become more variablewith recovery time. Hence, one expects to see a dynamic sequence of neurochemicalchanges during compensation. This concept is supported by the observation thatthere are different patterns of regional CNQX and MK-801 binding among ratvestibular nuclear regions during the first month after labyrinthectomy.26

A simple temporal association between a biochemical effect and vestibular com-pensation is only the first condition for identifying a potential contribution to thecompensatory process. The basic challenge is to identify critical biochemical eventsthat (1) are triggered by a specific “upstream” mechanism or signal that producesbehavioral compensation and (2) are essential for “downstream” compensatoryevents. As a result, experiments using either specific pharmacological challenges(e.g., antisense mRNA, enzyme inhibitors, or receptor antagonists) or geneticallyaltered animals are needed to establish that particular biochemical events are essen-tial for appearance of compensation within the appropriate time frame. A potentialconfounding effect is a functional redundancy: if multiple substrates (e.g., multipleisoforms of an enzyme) are interchangeable for a particular function, the critical roleof each substrate may be difficult to isolate experimentally.

Potential Biochemical Mechanisms and Substrates for Vestibular Compensation

The initial problem in identifying biochemical substrates for vestibular compen-sation is identifying (1) substrates that change dynamically during the compensatoryprocess and (2) whether the changes are at the level of transcription, translation, orposttranslational modification of proteins. The approach to identifying regulatedchanges in gene and protein expression is thus posited on the recognition that allneurochemical constituents [mRNAs, proteins (including cytoskeletal elements,receptors, enzymes, and glycoproteins), lipids, and transmitters] are regulated sub-strates, not phenotypic markers. The desire to categorize and classify cells biases in-vestigators to view many patterns of mRNA or protein expression as immutablephenotypic features (i.e., “genetic markers”) of particular cell populations. This biasis unfortunate because it ignores the more important issue of understanding factorsthat regulate the selective expression of that moiety by particular populations ofneurons. For example, because calcium/calmodulin-dependent cyclic nucleotidephosphodiesterase (CAM-PDE) is expressed by all Purkinje cells in normal ani-mals,27 one may suspect that it is a phenotypic marker for that cell type. Such a con-


clusion is erroneous, though, because Purkinje cell CAM-PDE expression isdownregulated selectively by climbing fiber ablation.28 It is important to rememberthat functional plasticity of neuronal circuits likely reflects molecular plasticity ofpopulations of neurons within those circuits. Hence, mechanisms for regulatingneuronal molecular plasticity constitute the signaling substrates for vestibularcompensation.

Analysis of the regulation of molecular constituents of neurons after labyrinthec-tomy requires consideration of the contributions of transcription, translation, andposttranslational modifications (including degradation) of protein moieties, as wellas effects of those protein moieties on substrates within the cell. Implicit in this state-ment is the recognition that changes in mRNA expression are not an obligatoryinitial component of a functional biochemical response during vestibular compensa-tion. It is not generally appreciated that changes in mRNA expression are not anecessary condition for changes in protein expression. For example, previous studiesof regulation of PKC expression in cell lines indicated that prolonged PKC activa-tion leads to a downregulation by proteolysis, without affecting PKC mRNA expres-sion.29 Even changes in expression of immediate early response genes (e.g.,members of the fos and jun families) are regulated by multiple intracellular signalingpathways, including the PKC and mitogen-activated protein kinase (MEK) path-ways.30,31 Hence, the issue of understanding biochemical bases for vestibular com-pensation requires a focus on the more global issue of the role of regulation ofturnover kinetics of intracellular mRNA, protein expression, and other signalingsubstrates (e.g., enzyme products) in the compensatory process. This question is tan-tamount to investigation of the effects of the regulation of turnover kinetics of eachrelevant transcribed mRNA, the translation product(s), and downstream events oncell physiology.

Polarity of Biochemical Changes

It is important to remember that responses of cells to biological challenges mayinvolve both upregulatory and downregulatory changes in different biochemical con-stituents. For example, modification of a specific class of serotonin receptors canmodify expression of different combinations of GABA receptor subunits in differentbrain regions;32 both up- and downregulation were observed. Our challenge is toidentify spatially and temporally distinct patterns of up- and downregulatorychanges that produce vestibular compensation.

POTENTIAL CHANGES IN REGULATED EXPRESSION DURINGEARLY VESTIBULAR COMPENSATION: RESOLUTION OF

SPONTANEOUS NYSTAGMUS IN THE LIGHT

One example is provided by events that are associated with resolution of sponta-neous nystagmus in the light. In Long-Evans rats, the frequency of spontaneous fastphases declines after surgical labyrinthectomy with an exponential time constant ofapproximately 12 h.14,15 One expects that indications of the underlying biochemicalmechanisms will be most detectable during the time window when the greatest de-gree of compensation occurs (i.e., the first 12 h after labyrinthectomy). Biochemical


responses that are related to the compensation process are expected to be transient;some responses that maintain compensated function may be permanent.

Within 3 h after labyrinthectomy in rats, a number of specific, transient changesin mRNA or protein expression have been noted at sites in the inferior olive, medialvestibular nucleus, nucleus prepositus hypoglossi, and the flocculo-nodular lobe thatcontribute to vestibulo-ocular reflex control. Within the contralateral inferior olive,increases were seen in c-fos mRNA,33–35 Fos protein,33,36 and BDNF mRNA,35

particularly in the dorsal cap, ventrolateral outgrowth, and beta nucleus. Parallelchanges in expression of the same mRNAs and Fos were also observed in the ipsi-lateral medial vestibular nucleus and contralateral nucleus prepositus hypo-glossi.33,35,36 These changes appear to be preceded by a bilateral increase inexpression of phosphorylated cAMP/calcium response element binding protein(pCREB) in the vestibular nuclei, which peaked within 1 h and resolved more slowlyon the ipsilateral side.37 These vestibular nucleus effects are correlated temporallywith the development of a decreased efficacy of postsynaptic responses of medialvestibular nucleus neurons to GABA agonists within 4 h after labyrinthectomy.38

During the same time frame, a bilateral decrease is seen in flocculus glutamatereceptor δ-2 mRNA.39 In addition, transient, lateralized zonal changes in PKC α, δ,and γ expression have been observed in flocculo-nodular lobe Purkinje cells at 3 and6 h after unilateral labyrinthectomy.14,15 Since these temporally restricted biochem-ical responses are located in pathways that contribute to vestibulo-ocular reflex con-trol, they are all candidates for molecular substrates that facilitate the resolution ofspontaneous nystagmus in the light (FIG. 1).

The location of these early molecular responses in vestibular pathways involvingthe flocculo-nodular lobe is consistent with data suggesting that the vestibulo-cerebellum contributes to the rapid disappearance of spontaneous nystagmus in thelight after labyrinthectomy. The disappearance of spontaneous nystagmus in thelight is retarded in cats40 and rats41 in animals with preexisting flocculus lesions.Inspection of the data presented by Kitahara et al.41 indicates that the time constantfor the resolution of nystagmus increased from approximately 12 h to approximately30 h in rats with a flocculus ablation on the same side as the later labyrinthectomy.Further, Johnston et al.42 reported recently that an intact flocculus is necessary forthe emergence of changes in rat ipsilateral medial vestibular nucleus excitabilitywithin 4 h after labyrinthectomy.38 These findings raise two distinct, but notmutually exclusive, hypotheses regarding flocculo-nodular lobe contributions torapid compensatory changes after unilateral labyrinthectomy. First, the findings areconsistent with a static, tonic, or “trophic” contribution of flocculo-nodular lobePurkinje cell activity to neurochemical plasticity in VN. Second, the data are consis-tent with the hypothesis that regionally specific, transient biochemical responses ofthe inferior olive and flocculo-nodular lobe are components of essential processesfor rapid loss of spontaneous nystagmus in the light.

The early changes in the distribution of PKC-positive flocculo-nodular lobePurkinje cells14,15 and the distribution of increased BDNF mRNA expression in theinferior olive35 are consistent with responses to retinal slip activity that is secondaryto spontaneous nystagmus (FIG. 2). In the dorsal cap and ventrolateral outgrowth,BDNF mRNAs are upregulated contralaterally and downregulated ipsilaterally inthe acute postlabyrinthectomy period.35 The increases in BDNF mRNA expressionoccur in sites that are expected to be excited by directionally selective retinal slip


signals from the contralateral eye43 during slow phases of spontaneous nystagmus.By contrast, the ipsilateral decreases in expression are localized in sites that areexpected to show decreased neuronal discharges. From this perspective, the contem-poraneous zonal changes in flocculo-nodular lobe Purkinje cell PKCα, PKCγ, andPKCδ expression15,44 appear to represent an ipsilateral downregulation due toclimbing fiber activation and a contralateral upregulation due to inhibition of climb-ing fiber activity. Consistent with this view, climbing fiber ablation produces an up-regulation of the number of PKCδ-immunopositive Purkinje cells in the flocculo-nodular lobe.44 However, the data also suggest that additional signals regulatePurkinje cell PKC expression during this acute recovery period.

Current data raise the hypothesis that baseline levels of climbing fiber activitymay coregulate Purkinje cell intracellular signal transduction via mechanismsrequiring coordinated contributions of both PKC and PKG. Specifically, climbingfiber ablation results in an increase of Purkinje cell expression of PKC44 and a con-comitant decrease of expression of CAM-PDE.28 Results of our studies suggest thatreduced inferior olivary activity increases Purkinje cell PKC expression in flocculo-

FIGURE 1. Schematic diagram of neurochemical changes that accompany the earlycompensation for spontaneous nystagmus in the light, which occurs largely within the first24 h after labyrinthectomy. The chart is based upon studies of compensation for unilaterallabyrinthectomy in rats and the results were observed within 6 h of the lesion. Several tem-porally and regionally specific responses are shown on a diagram of vestibular nuclear andcerebellar pathways related to vestibulo-ocular reflexes.


nodular lobe zones.15,44 Decreased Purkinje cell CAM-PDE expression would beexpected to reduce calcium/calmodulin-dependent hydrolysis of cGMP (activatedby increased intracellular calcium levels) and, hence, to produce a more robust orprolonged activation of PKG by accumulated intracellular cGMP. Thus, a prolongeddecrease in climbing fiber activity may increase the contributions of both PKC andPKG to protein phosphorylation, while a prolonged increase in climbing fiber activ-ity may effectively decrease PKC- and PKG-mediated phosphorylation. However,pharmacologic evidence suggests that PKC and PKG (or PKA) may affect vestibularcompensation on different timescales. We14 reported that decreases in spontaneousnystagmus in the light were attenuated markedly for at least 8 h by a single intra-cerebroventricular injection of a selective PKC inhibitor (bisindolylmaleimide I)immediately prior to unilateral labyrinthectomy. By contrast, we reported that anequipotent dose of a selective PKG and PKA inhibitor [N-(2-aminoethyl)-5-chloro-naphthalene-1-sulfonamide HCl] produced only a delayed and modest attenuation of

FIGURE 2. Hypothetical mechanisms for changes in zonal expression of PKC iso-forms by flocculo-nodular lobe Purkinje cells during early compensation for spontaneousnystagmus in the light. This diagram shows only responses in the contralateral dorsal capand ventrolateral outgrowth (DC-VLO) of the inferior olive and the ipsilateral flocculo-nodular lobe. Responses of opposite direction are expected in the ipsilateral DC-VLO andcontralateral flocculus due to suppression of baseline activation-dependent proteolysis ofPKC that results from depression (or suppression) of baseline climbing fiber activity. Seetext for further details.


compensation for spontaneous nystagmus. This finding is consistent with findingsfrom other examples of neuronal plasticity45 that indicate that different intracellularsignaling pathways may mediate early and later adaptive processes.

The findings to date are consistent with the hypothesis that cerebellar long-termdepression (LTD) may be engaged during the early compensation for spontaneousnystagmus in the light in rats. Climbing fiber activation, calcium fluxes, and activa-tion of both PKC and PKG are essential components of cerebellar LTD.46–49 Theabsence of both cerebellar LTD and rapid vestibulo-ocular reflex adaptation in trans-genic mice that express the pseudosubstrate PKC inhibitor PKC[19–31]49 is consis-tent with an important functional role of Purkinje cell PKC expression in the earlieststages of vestibular compensation. A simple hypothesis, then, is that availability ofPKC within the flocculo-nodular lobe Purkinje cells may be one rate-limiting stepfor early compensation for spontaneous nystagmus in the light. However, one mustremain fully cognizant of the fact that this is only one component of multiplemechanisms that are termed collectively vestibular compensation.

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role of gene regulation during vestibular compensation : an integrative approach

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