The role of defensins in the excitability of the peripheral vestibular system in the frog: Evidence for the presence of communication between the immune and nervous systems

Defensins are one of the major groups of endogenous peptides that are considered to be important antibiotic-like effectors of host innate and adaptive antimicrobial immunity. The current study investigated the electrophysiological effects of externally applied human and rabbit defensins (HNP-1 and RNP-1, correspondingly) on afferent neurotransmission in the frog semicircular canals (SCC). Application of HNP-1 and RNP-1 induces a concentration-dependent decrease in resting activity. Threshold concentrations for both substances were of the order of 0.0001 nM. The firing evoked by L-glutamate (L-Glu) and its agonists alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA), kainate and N-methyl-D-aspartate (NMDA) and (1S, 3R)-1-aminocyclopentane-trans-1,3-dicarboxilic acid (ACPD) could be inhibited by HNP-1, suggesting that defensins exert inhibitory control over both ionotropic and metabotropic glutamate receptors. HNP-1 considerably inhibited the L-glutamate/high Mg2+ -induced increase in frequency, thus, demonstrating its postsynaptic site of action. Acetylcholine (ACh) responses under HNP-1 did not differ from the frequency increase induced by ACh alone, and the ACh antagonist atropine left the response to HNP-1 intact. The specific opioid receptor antagonist naloxone (Nal) antagonized the inhibitory response evoked by HNP-1. The results obtained support the evidence for the recruitment of defensins in communication between the immune and nervous systems, and on the potential of sensory receptors to participate in the inflammatory response.

An in vitro model system to study gene therapy in the human inner ear

The confined fluid-filled labyrinth of the human inner ear presents an opportunity for introduction of gene therapy reagents designed to treat hearing and balance dysfunction. Here we present a novel model system derived from the sensory epithelia of human vestibular organs and show that the tissue can survive up to 5 days in vitro. We generated organotypic cultures from 26 human sensory epithelia excised at the time of labyrinthectomy for intractable Meniere's disease or vestibular schwannoma. We applied multiply deleted adenoviral vectors at titers between 10(5) and 10(8) viral particles/ml directly to the cultures for 4-24 h and examined the tissue 12-96 h post-transfection. We noted robust expression of the exogenous transgene, green fluorescent protein (GFP), in hair cells and supporting cells suggesting both were targets of adenoviral transfection. We also transfected cultures with a vector that carried the genes for GFP and KCNQ4, a potassium channel subunit that causes dominant-progressive hearing loss when mutated. We noted a positive correlation between GFP fluorescence and KCNQ4 immunolocalization. We conclude that our in vitro model system presents a novel and effective experimental paradigm for evaluation of gene therapy reagents designed to restore cellular function in patients who suffer from inner ear disorders.

Immunocytochemical localization of ubiquitin A-52 protein in the mouse inner ear

The ubiquitin A-52 residue ribosomal protein fusion product 1 (UbA52) is a gene highly expressed specifically in the inner ear. Through cellular localization we immunocytochemically investigated its function in the inner ear. In the adult mouse, UbA52 protein was distributed in the strial marginal cells and vestibular dark cells, which regulate the endolymphatic ion homeostasis. In the developing mouse cochlea, no significant staining was observed from birth to postnatal day 3, whereas after postnatal day 6, strong UbA52-immunoreactivities were observed in strial marginal cells. Endolymphatic K concentration is elevated between postnatal days 3-8: therefore, our results indicate that UbA52 may have a functional role in regulation of ion secretion in the inner ear.

Thyroid hormone receptor alpha1 is a critical regulator for the expression of ion channels during final differentiation of outer hair cells

Cochlear outer hair cells (OHCs) terminally differentiate prior to the onset of hearing. During this time period, thyroid hormone (TH) dramatically influences inner ear development. It has been shown recently that TH enhances the expression of the motor protein prestin via liganded TH receptor beta (TRbeta) while in contrast the expression of the potassium channel KCNQ4 is repressed by unliganded TRalpha1. These different mechanisms of TH regulation by TRalpha1 or TRbeta prompted us to analyse other ion channels that are required for the final differentiation of OHCs. We analysed the onset of expression of the Ca(2+) channel Ca(V)1.3, and the K(+) channels SK2 and BK and correlated the results with the regulation via TRalpha1 or TRbeta. The data support the hypothesis that proteins expressed in rodents prior to or briefly after birth like Ca(V)1.3 and prestin are either independent of TH (e.g. Ca(V)1.3) or enhanced through TRbeta (e.g. prestin). In contrast, proteins expressed in rodents later than P6 like KCNQ4 ( approximately P6), SK2 ( approximately P9) and BK ( approximately P11) are repressed through TRalpha1. We hypothesise that the precise regulation of expression of the latter genes requires a critical local TH level to overcome the TRalpha1 repression.

Differential expression of otoferlin in brain, vestibular system, immature and mature cochlea of the rat

Mutations of the human otoferlin gene lead to an autosomal recessive nonsyndromic form of prelingual, sensorineural deafness (deafness autosomal recessive 9, DFNB9). Several studies have demonstrated expression of otoferlin in the inner ear and brain, and suggested a role of otoferlin in Ca(2+)-triggered exocytosis. So far, otoferlin expression profiles were solely based on the detection of mRNA. Here, we analysed the expression of otoferlin protein and mRNA using immunohistochemistry, in situ hybridization and RT-PCR in neonatal and mature Wistar rat tissue. In agreement with previous studies, otoferlin expression was found in the brain and in inner and vestibular hair cells. Otoferlin mRNA and protein was, however, also detected in mature outer hair cells of low-frequency processing cochlear turns and in auditory nerve fibres. In outer, inner and vestibular hair cells, otoferlin was subcellularly localized at a considerable distance from the presumed active release sites. Double-staining with the synaptic ribbon marker, C-terminal binding protein 2 (CtBP2), or the presynaptic Ca(2+)-channel, Ca(v)1.3, both assumed to mark the sites of vesicle fusion and transmitter release, did not colocalize with otoferlin expression and thus do not necessarily support a selected role of otoferlin in Ca(2+)-triggered exocytosis. The widespread distribution of otoferlin in neurons, nerve fibres and hair cells, and its subcellular distribution extending beyond the regions of synaptic vesicle fusion, i.e. coenrichment with the cytosolic Golgi matrix protein 130 (GM130) in inner hair cells or the early endosomal autoantigen 1 (EEA1) in outer hair cells support instead the idea of a more ubiquitous role of otoferlin in early/recycling endosome trans-Golgi network dynamics.

Sphingosine 1-phosphate (S1P) signaling is required for maintenance of hair cells mainly via activation of S1P2

Hearing requires the transduction of vibrational forces by specialized epithelial cells in the cochlea known as hair cells. The human ear contains a finite number of terminally differentiated hair cells that, once lost by noise-induced damage or toxic insult, can never be regenerated. We report here that sphingosine 1-phosphate (S1P) signaling, mainly via activation of its cognate receptor S1P2, is required for the maintenance of vestibular and cochlear hair cells in vivo. Two S1P receptors, S1P2 and S1P3, were found to be expressed in the cochlea by reverse transcription-PCR and in situ hybridization. Mice that are null for both these receptors uniformly display progressive cochlear and vestibular defects with hair cell loss, resulting in complete deafness by 4 weeks of age and, with complete penetrance, balance defects of increasing severity. This study reveals the previously unknown role of S1P signaling in the maintenance of cochlear and vestibular integrity and suggests a means for therapeutic intervention in degenerative hearing loss.

Developmental expression of Kcnq4 in vestibular neurons and neurosensory epithelia

Sensory signal transduction of the inner ear afferent neurons and hair cells (HCs) requires numerous ionic conductances. The KCNQ4 voltage-gated M-type potassium channel is thought to set the resting membrane potential in cochlear HCs. Here we describe the spatiotemporal expression patterns of Kcnq4 and the associated alternative splice forms in the HCs of vestibular labyrinth. Whole mount immunodetection, qualitative and quantitative RT-PCR were performed to characterize the expression patterns of Kcnq4 transcripts and proteins. A topographical expression and upregulation of Kcnq4 during development was observed and indicated that Kcnq4 is not restricted to either a specific vestibular structure or cell type, but is present in afferent calyxes, vestibular ganglion neurons, and both type I and type II HCs. Of the four alternative splice variants, Kcnq4_v1 transcripts were the predominant form in the HCs, while Kcnq4_v3 was the major variant in the vestibular neurons. Differential quantitative expression of Kcnq4_v1 and Kcnq4_v3 were respectively detected in the striolar and extra-striolar regions of the utricle and saccule. Analysis of gerbils and rats yielded results similar to those obtained in mice, suggesting that the spatiotemporal expression pattern of Kcnq4 in the vestibular system is conserved among rodents. Analyses of vestibular HCs of Bdnf conditional mutant mice, which are devoid of any innervation, demonstrate that regulation of Kcnq4 expression in vestibular HCs is independent of innervation.

WDR1 expression in the normal and noise-damaged chick vestibule

Unlike mammals, avian cochlear hair cells can regenerate after acoustic overstimulation. The WDR1 gene is one of the genes suspected to play an important role in this difference. In an earlier study, we found that the WDR1 gene is over-expressed in the chick cochlea after acoustic overstimulation. The aim of this study was to compare the expression of WDR1 before and after acoustic overstimulation in the chick vestibule. Seven-day-old chicks were divided into three groups: normal group, damage group, and regeneration group. The damage and regeneration group was exposed to 120 dB SPL white noise for 5-6 hours. The damage group was euthanized shortly after the impulse, but the regeneration group was allowed to recover for 2 days. The utricle, saccule, and the three ampullae of each semicircular canal were dissected and immunohistochemically stained with anti-WD40 repeat protein 1 antibody. For quantitative analysis, immunoreactive densities were measured and quantitative real-time RT PCR was performed. WD40 repeat protein 1 expression was elevated in all the semicircular canals and utricle, two days after an acoustic overstimulation (P=0.001). WDR1 mRNA expression was 1.34 times higher in the regeneration group compared to the normal group, but it was not statistically significant. Exceptionally, WD40 repeat protein 1 expression did not increase in the saccule of the regeneration group. Elevated WDR1 expression in the avian vestibule may have a role in the hair cell regenerating ability as in the avian cochlea. A similar mechanism of hair cell regeneration may exist in the avian cochlea and vestibule.

Efferent neurotransmitters in the human cochlea and vestibule

CONCLUSION

Current neurotransmission models based on animal studies on the mammalian inner ear not always reflect the situation in human. Rodents and primates show significant differences in characteristics of efferent innervation as well as the distribution of neuroactive substances.

OBJECTIVE

Immunohistochemistry demonstrates the mammalian efferent system as neurochemically complex and diverse: several neuroactive substances may co-exist within the same efferent terminal. Using light and electron microscopic immunohistochemistry, this study presents a comparative overview of the distribution patterns of choline acetyltransferase (ChAT), the acetylcholine synthesizing enzyme, GABA, CGRP, and enkephalins within the peripheral nerve fiber systems of the human inner ear.

MATERIALS AND METHODS

Human temporal bones were obtained post mortem and prepared according to a pre-embedding immunohistochemical technique to detect immunoreactivities to ChAT, GABA, CGRP, leu- and met-enkephalins at the electron microscopic level.

RESULTS

Immunoreactivities of all the antigens were present within both the lateral and medial efferent systems of the cochlea, whereas only ChAT, GABA, and CGRP were detected in efferent pathways of the vestibular end organs.

Edaravone protects the vestibular periphery from free radical-induced toxicity in response to perilymphatic application of (±)-α-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid

Intracochlear infusion of (+/-)-alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA) was performed with a syringe pump in guinea pigs, and peripheral vestibular dysfunction was induced. Animals were administered edaravone systemically or topically. In the systemic application group, animals were administered edaravone once a day for 7 days after AMPA infusion. In the topical application group, edaravone-soaked gelfoam was placed on the round window membrane just after, 12 h after or 24 h after AMPA infusion. Spontaneous nystagmus was observed after AMPA infusion. Immunohistochemistry for 4-hydroxy-2-nonenal (4-HNE), a marker of free radical-induced lipid peroxidation, was performed 24 h after AMPA infusion. In addition, caloric tests were performed to evaluate vestibular function 1 week after AMPA infusion. Animals in both groups showed decreased spontaneous nystagmus, but results were not significant. Animals treated topically with edaravone within 12 h of AMPA infusion showed normal morphology of the ampullar sensory epithelia of the lateral semicircular canals and showed a good response to the caloric tests. 4-HNE immunoreactivity in the sensory epithelia was very low in these animals. In contrast, untreated animals and animals treated with edaravone systemically or topically 24 h after AMPA infusion showed morphologic hair cell damage, reduced caloric response and remarkable 4-HNE immunoreactivity in the sensory epithelia. These results indicate that topical application of edaravone within 12 h after damage protects the vestibular periphery from free radical-induced toxicity in response to intracochlear infusion of AMPA.

Expression of nicotinic acetylcholine receptor subunit alpha9 in type II vestibular hair cells of rats

AIM

To explore the cell specific existence of alpha 9 AChR in the vestibular type II hair cells (VHC II) of rats.

METHODS

To detect the expression of alpha 9 AChR messenger RNA (mRNA) in the vestibular endorgans and single VHC II of rats by using the reverse transcription polymerase chain reaction (RT-PCR) technique and the single cell RT-PCR technique, respectively.

RESULTS

It was shown that alpha 9 AChR mRNA was detected in the vestibular endorgans. By using single-cell RT-PCR, mRNA encoding alpha 9 AChR was also detected in the VHC II of the rats. Sequence analysis of the PCR products confirmed identity to corresponding cDNA sequence in the predicted region.

CONCLUSION

We established a method which could effectively detect the cell specific expression of mRNA in an individual VHC. Present data confirm that alpha 9 AChR mRNA is expressed in the VHC II of rats and indicates that alpha 9 AChR may function as a mediator of efferent cholinergic signaling in mammalian VHC.

[Expression of mACh receptor ml-m5 subunits in the vestibular endorgans of rat]

OBJECTIVE

To investigate the expression of mACh receptor ml - m5 subunits in the vestibular endorgans of rat.

METHOD

Gene specific primers used were based upon the rat mACh receptor ml-m5 subunits sequence. Reverse transcription polymerase chain reaction (RT-PCR) was carried out on the vestibular endorgans. The PCR products were directly sequenced.

RESULT

It was shown that mACh receptor ml - m5 subunits mRNA was detected and sequenced from vestibular endorgans by RT-PCR technique. Sequence analysis of the PCR products confirmed identity to rat mACh receptor ml--m5 subunits cDNA sequence in the predicted region.

CONCLUSION

Present data confirmed the expression of ACh receptor ml-m5 subunits in the vestibular endorgans of rat, giving the direct evidence to date that ACh is one of the important neurotransmitters in the peripheral vestibular system of rat.

Computational model of vectorial potassium transport by cochlear marginal cells and vestibular dark cells

Cochlear marginal cells and vestibular dark cells transport potassium into the inner ear endolymph, a potassium-rich fluid, the homeostasis of which is essential for hearing and balance. We have formulated an integrated mathematical model of ion transport across these epithelia that incorporates the biophysical properties of the major ion transporters and channels located in the apical and basolateral membranes of the constituent cells. The model is constructed for both open- and short-circuit situations to test the extremes of functional capacity of the epithelium and predicts the steady-state voltages, ion concentrations, and transepithelial currents as a function of various transporter and channel densities. We validate the model by establishing that the cells are capable of vectorial ion transport consistent with several experimental measurements. The model indicates that cochlear marginal cells do not make a significant direct contribution to the endocochlear potential and illustrates how changes to the activity of specific transport proteins lead to reduced K(+) flux across the marginal and dark cell layers. In particular, we investigate the mechanisms of loop diuretic ototoxicity and diseases with hearing loss in which K(+) and Cl(-) transport are compromised, such as Jervell and Lange-Nielsen syndrome and Bartter syndrome, type IV, respectively. Such simulations demonstrate the utility of compartmental modeling in investigating the role of ion homeostasis in inner ear physiology and pathology.

Inner ear therapy for neural preservation

A gradual loss of auditory neurons often occurs following sensorineural hearing loss. Since the cochlear implant must stimulate the remaining auditory neuron population, it would be beneficial to preserve as many auditory neurons as possible. Neurotrophic factors protect auditory neurons from degradation after sensorineural hearing loss in experimental animals, but have not yet been translated into the clinical setting. Current experimental and clinical techniques for drug delivery to the inner ear are examined in this review, covering the routes for drug delivery to the cochlea and the delivery systems used to introduce them. Duration of treatment, drug diffusion, effectiveness and safety are discussed with references to how they may be translated to the implementation of neurotrophic factor treatment for neural preservation.

MicroRNA gene expression in the mouse inner ear

MicroRNAs (miRNAs) are small non-coding RNAs that function through the RNA interference (RNAi) pathway and post-transcriptionally regulate gene expression in eukaryotic organisms. While miRNAs are known to affect cellular proliferation, differentiation, and morphological development, neither their expression nor roles in mammalian inner ear development have been characterized. We have investigated the extent of miRNA expression at various time points throughout maturation of the postnatal mouse inner ear by microarray analysis. Approximately one third of known miRNAs are detected in the inner ear, and their expression persists to adulthood. Expression of such miRNAs is validated by quantitative PCR and northern blot analysis. Further analysis by in situ hybridization demonstrates that certain miRNAs exhibit cell-specific expression patterns in the mouse inner ear. Notably, we demonstrate that miRNAs previously associated with mechanosensory cells in zebrafish are also expressed in hair cells of the auditory and vestibular endorgans. Our results demonstrate that miRNA expression is abundant in the mammalian inner ear and that certain miRNAs are evolutionarily associated with mechanosensory cell development and/or function. The data suggest that miRNAs contribute substantially to genetic programs intrinsic to development and function of the mammalian inner ear and that specific miRNAs might influence formation of sensory epithelia from the primitive otic neuroepithelium.

Survey of inward ionic currents acquired by the cochleovestibular ganglion of the early-aged embryonic chick

The acquisition of ion channels is critical to the formation of neuronal pathways in the peripheral and central nervous systems. This study describes the different types of inward currents (Ii) recorded from the soma of isolated cochleovestibular ganglion (CVG) cells of the embryonic chicken, Gallus gallus. Cells were isolated for whole-cell tight-seal recording from embryonic day (ED) 3, an age when the CVG is a cell cluster, to ED 9, an age when the cochlear and vestibular ganglia (CG, VG) are distinct structures. Results show Na+ and Ca2+ currents (INa and ICa) are acquired by ED 3, although INa dominates with greater density levels that peak by ED 6-7 in VG neurons. In the CG, INa acquisition is slower, reaching peak values by ED 8-9. Isolation of ICa, using Ba2+ as the charge carrier, showed both transient (IBaT)- and sustained (IBaL)-type currents on ED 3. Unlike INa, IBa density varied with age and ganglion. Total IBa increased steadily, showing a decline only in CG cells on ED 8-9 as a result of a decrease in IBaT. IBaL density increased over time, reaching a maximum on ED 6-7 in VG cells, followed by a decline on ED 8-9. In comparison, IBaL in CG neurons, did not increase significantly beyond mean values measured on ED 5. The early onset of these currents and the variations in Ca2+ channel expression between the ganglia suggests that intracellular signals relevant to phenotypic differentiation begin within these early time frames.

Disruption and restoration of cell-cell junctions in mouse vestibular epithelia following aminoglycoside treatment

The intracellular junction complexes, which consist of tight junctions (TJ), adherens junctions (AJ), and desmosomes, mediate cell-cell adhesion in epithelial cells. E-cadherin, which is a major component of AJ, plays a role not only in the maintenance of cell-cell junctions, but also in repressing cell proliferation. In this study, we examined changes of E-cadherin expression in mouse vestibular epithelia following local application of neomycin using immunohistochemistry and western blotting, and morphology of cell-cell junctions by transmission electron microscopy (TEM). Immunohistochemistry and western blotting revealed down-expression of E-cadherin and its consecutive recovery. TEM demonstrated temporal disruption of cell-cell junctions. Morphology of cell-cell junctions was more rapidly restored than recovery of E-cadherin expression. Transient disruption of cell-cell junctions and down-expression of E-cadherin is a rational response for the deletion of dying hair cells, and may be associated with a limited capacity for cell proliferations in mammalian vestibular epithelia following their rapid restoration.

Balanced levels of Espin are critical for stereociliary growth and length maintenance

Hearing and balance depend on microvilli-like actin-based projections of sensory hair cells called stereocilia. Their sensitivity to mechanical displacements on the nanometer scale requires a highly organized hair bundle in which the physical dimension of each stereocilium is tightly controlled. The length and diameter of each stereocilium are established during hair bundle maturation and maintained by life-long continuing dynamic regulation. Here, we studied the role of the actin-bundling protein Espin in stereociliary growth by examining the hair cell stereocilia of Espin-deficient jerker mice (Espn(je)), and the effects of transiently overexpressing Espin in the neuroepithelial cells of the organ of Corti cultures. Using fluorescence scanning confocal and electron microscopy, we found that a lack of Espin results in inhibition of stereociliary growth followed by progressive degeneration of the hair bundle. In contrast, overexpression of Espin induced lengthening of stereocilia and microvilli that mirrored the elongation of the actin filament bundle at their core. Interestingly, Espin deficiency also appeared to influence the localization of Myosin XVa, an unconventional myosin that is normally present at the stereocilia tip at levels proportional to stereocilia length. These results indicate that Espin is important for the growth and maintenance of the actin-based protrusions of inner ear neuroepithelial cells.

Differential expression of espin isoforms during epithelial morphogenesis, stereociliogenesis and postnatal maturation in the developing inner ear

The espins are a family of multifunctional actin cytoskeletal proteins. They are present in hair cell stereocilia and are the target of mutations that cause deafness and vestibular dysfunction. Here, we demonstrate that the different espin isoforms are expressed in complex spatiotemporal patterns during inner ear development. Espin 3 isoforms were prevalent in the epithelium of the otic pit, otocyst and membranous labyrinth as they underwent morphogenesis. This espin was down-regulated ahead of hair cell differentiation and during neuroblast delamination. Espin also accumulated in the epithelium of branchial clefts and pharyngeal pouches and during branching morphogenesis in other embryonic epithelial tissues, suggesting general roles for espins in epithelial morphogenesis. Espin reappeared later in inner ear development in differentiating hair cells. Its levels and compartmentalization to stereocilia increased during the formation and maturation of stereociliary bundles. Late in embryonic development, espin was also present in a tail-like process that emanated from the hair cell base. Increases in the levels of espin 1 and espin 4 isoforms correlated with stereocilium elongation and maturation in the vestibular system and cochlea, respectively. Our results suggest that the different espin isoforms play specific roles in actin cytoskeletal regulation during epithelial morphogenesis and hair cell differentiation.

Asymmetric gene expression in the brain during acute compensation to unilateral vestibular labyrinthectomy in the Mongolian gerbil

Commercial microarrays were used to identify transcriptome expression within vestibular related brain regions (vestibular brainstem and cerebellum, and caudotemporal cortical regions) during the acute period of recovery following unilateral surgical vestibular labyrinth ablation in the gerbil. As a representative model of vestibular compensation, vestibular lesions in the gerbil produced activation in a common set of genes related to vestibular compensation. The total RNA was prepared and amplified using Affymetrix Gene Chip probes from the Rat U34 Neurobiology and R230, and Mouse M430 gene sets, resulting in GCRMA summarized data from S+AA software. Matched rat and mouse genes from gerbil hybridization produced good interspecies synteny. Multiple gene target trends supported global increases in neuron excitability throughout the vestibular brainstem and cerebellum. We focused further on gene expression with anatomically asymmetric activation relative to the lesion, indicative of involvement in rebalancing central vestibular tone during the vestibular compensation process. Cluster analysis revealed distinct spatial (regional and ipsi-contra) and temporal patterns. The asymmetric genes were part of well-defined neuron-related networks and included multiple members of the glutamate and GABA neurotransmitter systems. Transcripts for D3 dopamine, glycine, and some GABA receptor signals increased quickly in the ipsilesional vestibular complex and then increased gradually in the contralateral region, restoring the expression symmetry. Alternatively, the NMDA binding subunit decreased gradually over the acute compensation period in the contralateral vestibular complex. There was evidence for numerous associations between signaling systems with PKC as one possible mediator between early changes in GABA and progressive changes in NMDA signaling. These data begin to define the compensatory response at the level of molecular cascades.

Comparative analysis ofGata3 andGata2 expression during chicken inner ear development

The inner ear is a complex sensory organ with hearing and balance functions. Gata3 and Gata2 are expressed in the inner ear, and to gain more insight into their roles in otic development, we made a detailed expression analysis in chicken embryos. At early stages, their expression was highly overlapping. At later stages, Gata2 expression became prominent in vestibular and cochlear nonsensory epithelia. In contrast to Gata2, Gata3 was mainly expressed in the developing sensory epithelia, reflecting the importance of this factor in the sensory-neural development of the inner ear. While the later expression patterns of both Gata3 and Gata2 were highly conserved between chicken and mouse, important differences were observed especially with Gata3 during early otic development, providing indications of divergent molecular control during placode invagination in mice and chickens. We also found indications that the regulatory hierarchy observed in mouse, where Gata3 is upstream of Gata2 and Fgf10, could be conserved in chicken.

Cellular distribution of d-serine, serine racemase and d-amino acid oxidase in the rat vestibular sensory epithelia

Glutamate is the main neurotransmitter at the synapses between sensory cells and primary afferents in the peripheral vestibular system. Evidence has recently been obtained demonstrating that the atypical amino acid D-serine is the main endogenous co-agonist of the N-methyl-D-aspartate receptors in the CNS. We studied the distribution of D-serine and its synthesizing and degrading enzymes, serine racemase and d-amino acid oxidase in the rat vestibular sensory epithelium using immunocytochemistry. D-serine, serine racemase and D-amino acid oxidase were localized in the transitional cells, which are parasensory cells located between the sensory epithelium and the dark cells. The dark cells expressed only serine racemase. D-Serine was also detected in the supporting cells of the sensory epithelium. These cells, which are in close contact with glutamatergic synapses, express GLAST, a glial specific transporter for glutamate. They may have similar functions to glial cells in the CNS and thus expression of D-serine suggests a neuromodulator role for D-serine at the glutamatergic synapses in the peripheral vestibular system. Our data also indicate that the metabolism of D-serine is not restricted to glial cells suggesting that the amino acid may play an additional role in the peripheral nervous system.

Expression of Cochlin in the Vestibular Organ of Rats

The COCH gene mutated in autosomal dominant sensorineural deafness (DFNA9) encodes cochlin, a major constituent of the inner ear extracellular matrix. Cochlin constitutes 70% of the inner ear protein and cochlin isoforms can be classified into three subgroups, p63s, p44s and p40s. Symptoms of some DFNA9 patients are consistent with those of Ménière's disease. Here, we report the expression of cochlin in the vestibular organ of rats using isoform-specific antibodies that recognize all three isoforms. Cochlin is highly expressed in the stromata of the maculae of otolithic organs and cristae of semicircular canals, and in the channels in the bony labyrinth that transmit the dendritic innervation to the cristae and maculae. Cochlin cannot be detected in the sensory cells, dark cells, nor in the acellular structures, otolithic membrane or in the cupula. These findings support the theory that deposition of acidophilic substance in the inner ear caused by mutation of cochlin can induce a secondary retrograde dendritic degeneration of the vestibular nerves.

CGRP Expression in the Vestibular Periphery after Transient Blockage of Bilateral Vestibular Input

This study aimed to establish an animal model of reversible bilateral vestibular disorders that is suitable for examining the mechanisms of vestibular plasticity, and to observe the changes in the plasticity of vestibular efferent systems. Tetrodotoxin (TTX) was infused continuously for 7 days into the bilateral perilymph of guinea pig cochlea. We assessed the vestibulo-ocular reflex (VOR) for evaluating the vestibular function. We also investigated the changes in calcitonin gene-related peptide (CGRP) immunoreactivity in vestibular end organs to observe the changes in the plasticity of vestibular systems. The VOR was completely eliminated by TTX administration and returned to the preoperative levels within 120 h after TTX discontinuation. An obvious increase in the number of CGRP-immunoreactive fibers was observed within the neurosensory epithelia of the maculae and cristae. An animal model of reversible bilateral vestibular disorders was established and used for investigating the plasticity of the vestibular nervous system.