In vivo and in vitro localization of brain-derived neurotrophic factor, fibroblast growth factor-2 and their receptors in the bullfrog vestibular end organs

The inner ear sensory epithelia of vertebrates are composed mainly of supporting cells and hair cells (HCs). Brain-derived neurotrophic factor (BDNF) and fibroblast growth factor-2 (FGF-2) are trophins that are believed to play an essential role in the development and innervation of inner ear epithelia. Both trophins also may play a crucial role in the maintenance and regeneration of hair cells in the adult vertebrate ear. In the bullfrog vestibular system, hair cells are produced throughout life, and the epithelia regenerates following ototoxicity. The expression of BDNF and FGF-2 in the vestibular organs of the adult bullfrog was investigated at a cellular level both in histological sections and in vitro in dissociated cell cultures. In histological sections of the crista ampullaris, in situ hybridization and immunocytochemical techniques demonstrated that HCs express both BDNF and its receptor trkB, while the supporting cells express the receptor trkB alone. Following dissociation and in vitro cell culture no changes in the pattern of BDNF and trkB receptor were observed. Immunocytochemical studies demonstrated that in vivo hair cells express FGF-2 and the receptors FGFR-1 and FGFR-2 while supporting cells do not express either molecule. Following dissociation, HCs continue to express FGF-2 and its two receptors, while supporting cells upregulate the expression of FGF-2 and its receptor FGFR-2. These data confirm the potential role of BDNF and FGF-2 trophic regulation of the sensory epithelia of the adult inner ear. The findings suggest that BDNF has a role in the maintenance of the vestibular epithelia while FGF-2 may regulate the proliferation of supporting cells.

PTEN is not altered in sporadic vestibular schwannomas

AIMS

To investigate the role of the tumour suppressor gene PTEN in the tumorigenesis and growth of sporadic vestibular schwannomas, and to characterize the cellular distribution of the PTEN protein in relation to the MIB-1 proliferation index in these tumour.

METHODS AND RESULTS

Immunoexpression of the PTEN protein was observed within the neoplastic Schwann cells in 21 out of 30 sporadic schwannomas examined (70%). PTEN expression was consistently stronger in Antoni A areas than in Antoni B areas. High levels of PTEN immmunoexpression in schwannomas were associated with an increased MIB-1 labelling index. Occasionally, vascular endothelial cells also showed PTEN immunoreaction. By polymerase chain reaction-single strand conformation polymorphism screening, no mutations were found in the complete protein coding region of the PTEN gene.

CONCLUSIONS

The PTEN tumour suppressor gene is expressed in the majority of sporadic schwannomas. The maintained expression of the PTEN protein, together with the lack of detectable mutations in this gene, suggests that the function of the PTEN tumour suppressor gene is not altered in sporadic vestibular schwannomas.

Subcellular immunolocalization of NMDA receptor subunit NR1, 2A, 2B in the rat vestibular periphery

The immunohistochemical localization of the NMDA glutamate receptor subunits NR1, NR2A, and NR2B was investigated in the rat vestibular periphery at the light and electron microscopy level using specific antipeptide antibodies. The afferent calyceal terminals and nerve fibers innervating type I vestibular hair cells were strongly NR1, NR2A, and NR2B immunoreactive. Under electron microscopy, the basolateral type I hair cell membrane was NR1 immunoreactive. The type II hair cell and its afferent boutons were NR1, NR2A, and NR2B non-immunoreactive. Nearly all of Scarpa's ganglion neurons were NR1 immunoreactive, but there was a subset of NR2A non-immunoreactive neurons. Additionally, the larger sized Scarpa's ganglia neurons were NR2B immunoreactive, while the smaller neurons were non-immunoreactive. These findings are strong evidence for functional NMDA receptor mediation or modulation of afferent excitatory neurotransmission from type I but not type II vestibular hair cells to the primary afferent nerve. The receptor subtype(s) may be a combination of NR1/NR2A, NR1/NR2B, and/or NR1/NR2A/NR2B.

Localization of efferent neurotransmitters in the inner ear of the homozygous Bronx waltzer mutant mouse

Naturally occurring mutant mice provide an excellent model for the study of genetic malformations of the inner ear. Mice homozygous for the Bronx waltzer (bv/bv) mutation are severely hearing impaired or deaf and exhibit a 'waltzing' gait. Functional aspects of cochlear and vestibular efferents in the bv/bv mutant mouse are not well known. The present study was designed to evaluate several candidates of efferent neurotransmitters or neuromodulators including choline acetyltransferase (ChAT), gamma-aminobutyric acid (GABA), and calcitonin gene-related peptide (CGRP) in the inner ear of the bv/bv mutant mouse. Ultrastructural investigations at both light and electron microscopic level were performed. Ultrastructural morphologic evaluations of the cochlea and the vestibular end-organs were also undertaken. It is demonstrated that ChAT, GABA and CGRP immunoreactivities are present in the cochlea and in vestibular end-organs of bv/bv mutant mice. In the organ of Corti, immunoreactivity of ChAT, GABA and CGRP is confined to the inner spiral fibers, tunnel-crossing fibers, and the vesiculated nerve endings synapsing with outer hair cells. Interestingly, immunoreactivity was detectable even where inner hair cells appeared missing. Results also revealed malformations of the outer hair cells with synaptic contacts to efferent nerve endings consistently intact. In the neurosensory epithelia of the vestibular end-organs, the presence of ChAT, GABA, and CGRP immunoreactivity was localized at the vestibular efferents, with the exception of the macula of saccule. In one 8-month-old macula of utricle where the depletion of hair cells appeared highest, ChAT immunostaining was still discernible. Ultrastructural investigation demonstrated that vesiculated efferent nerve endings make synaptic contact with the outer hair cells in the organ of Corti and with type II hair cells in the vestibular end-organs. The present study provides further support that the efferent system in the bv/bv mutant inner ear is morphologically as well as functionally mature. These findings also demonstrate that if and when the onset of efferent degeneration in the bv/bv mutant inner ear occurs, it transpires subsequent to pathological conditions in the hair cells. The present findings give further indication that the efferent systems of the bv/bv mutant inner ear are independent of the afferent systems in many aspects including development, maturation as well as degeneration.

Connexins and gap junctions in the inner ear

Mutations in the genes for three different isotypes of the gap junction channel protein connexin are associated with deafness. This indicates an important role for gap junctions in auditory function and provides an opportunity to explore structure-function relationships in the connexin molecule. We have been examining the distribution of gap junctions and the pattern of connexin expression in the mature inner ear and during development, and the effect of specific mutations on the processing and functionality of the expressed connexin proteins in an in vitro system.

Expression patterns of aquaporins in the inner ear: evidence for concerted actions of multiple types of aquaporins to facilitate water transport in the cochlea

Water transport between the perilymph and endolymph is important in regulations of volume and osmotic pressure of the inner ear labyrinth. It is now known that expression of water channels (aquaporins or AQPs) in the cell membrane dramatically increases the ability of water to cross epithelial cells. The aims of the current study were to investigate the cellular localization of AQPs by immunolabeling, and to study the developmental expression and relative abundance of various subtypes of AQPs. We report here that AQP3, AQP7 and AQP9 were expressed in the inner ear. Specific subtypes of AQPs were found in discrete regions expressed by both epithelial cells and fibrocytes in cochlear and vestibular organs. Semi-quantitative measurements showed that AQP4 and AQP1 were the two most abundantly expressed AQP subtypes in the inner ear, and their expressions were dramatically upregulated during development. These data showed a highly localized and largely non-overlapping distribution pattern for different subtypes of AQPs in the inner ear, suggesting the existence of regional subtype-specific water transport pathways, and global regulation of water transport in the inner ear may require concerted actions of multiple types of AQPs.

Role of cholinergic mossy fibers in vestibular nuclei in the development of vestibular compensation

Several lines of evidence have suggested that acetylcholine is a possible neurotransmitter/neuromodulator involved in vestibular compensation. However, details of cholinergic effective sites during vestibular compensation remain unclear. In this study, we selectively damaged the rat vestibulo-floccular cholinergic mossy fibers using ethylcholine mustard aziridinium ion. In these animals, unilateral labyrinthectomy caused more severe vestibulo-ocular deficits, especially in the initial stage. These findings suggest that the vestibulo-floccular cholinergic mossy fibers serve to restore the balance between intervestibular nuclear activities in order to induce vestibular compensation in the initial stage.

The contribution of nitric oxide to vestibular compensation: are there species differences?

Nitric oxide (NO) has been implicated in the processes by which animals recover from peripheral vestibular damage ("vestibular compensation"). However, there is little systematic data available on the effects of NO inhibition on the vestibular compensation process. In the present study we administered the nitric oxide synthase (NOS) inhibitor NG-nitro-L-arginine methyl ester (L-NAME) using a subcutaneous osmotic minipump and examined its effects on the compensation of spontaneous nystagmus (SN), yaw head tilt (YHT) and roll head tilt (RHT) in guinea pigs. Following unilateral labyrinthectomy (UL), treatment with 5, 10, 50 or 100 mM L-NAME had no effect on the expression of any of these symptoms or their rate of compensation. By contrast, pre-UL treatment with 100 mM L-NAME resulted in a decrease in SN frequency at 10 h post-UL and an increase in its rate of compensation. Lower concentrations had no effect on SN. Pre-UL treatment with L-NAME had no significant effect on YHT or RHT at any particular time point. Analysis of NOS activity demonstrated that the highest concentration of L-NAME inhibited NOS activity in the contralateral vestibular nucleus complex, bilateral cerebellum and bilateral cortices. These results suggest that L-NAME may have different effects on vestibular compensation in guinea pigs compared to other species, such as the rat and frog.

Ultrastructural evaluation of calcitonin gene-related peptide immunoreactivity in the human cochlea and vestibular endorgans

Calcitonin gene-related peptide (CGRP) is a neuropeptide widely distributed in the peripheral and central nervous system. Demonstrated in the efferent systems of the mammalian cochlea and vestibule, immunoreactive patterns of CGRP may vary by species. There is, however, no information in the literature investigating CGRP localization in the human cochlea. In the present study, the ultrastructural localization of CGRP immunoreactivity was evaluated in the human inner ear with immunoelectron microscopy. It was found that, in human cochlea, CGRP immunoreactivity was located in unmyelinated nerve fibres of the spiral lamina, inner spiral fibres beneath inner hair cells, tunnel spiral fibres, tunnel crossing fibres and outer radial fibres. In endorgans of human vestibule, CGRP immunoreactivity was located in vesiculated nerve fibres and bouton-type nerve terminals which were seen to contact afferent nerve chalices surrounding type I sensory cells and afferent nerve fibres, or to form an en passant contact with afferent dendrites. CGRP immunoreactivity appeared to be confined to efferent systems in all cases. This study presents evidence that CGRP could serve a role in neurotransmission or neuroregulation in both cochlear and vestibular efferent systems of human.

Calpain immunoreactivity and morphological damage in chinchilla inner ears after carboplatin

Carboplatin produces an unusual pattern of damage in the chinchilla inner ear, characterized by early destruction of type I afferent fibers and preferential loss of type I hair cells in the vestibular end organs and inner hair cells (IHCs) in the cochlea. In the present study, we investigated a potential role of calpains, a family of calcium-activated proteases, in carboplatin ototoxicity. Chinchillas received carboplatin (100 mg/kg IP) and were sacrificed 12, 24, 48, or 72 h later for morphological evaluation or immunocytochemistry. Nerve fibers and myelin were the initial sites of increased calpain immunoreactivity (IR) and morphological damage. At 12 h, granular immunoreactive puncta were present within nerve fibers and their myelin sheaths in the spiral ganglion. In the habenula perforata, dense reaction product was present in large vacuoles in the myelin surrounding the nerve fibers. At 24 h, nerve fibers and myelin were destroyed in the habenula, and those in the spiral ganglion showed increased calpain IR and morphological damage. At 72 h, nerve fibers and myelin were completely destroyed. Calpain IR was not a prominent feature of IHCs, type I vestibular hair cells, or ganglion cells at any time after carboplatin. The results show a correlation between calpain IR and carboplatin-induced axon and myelin degeneration. We propose that calpain-induced axonopathy and myelinopathy are primary features of carboplatin ototoxicity in chinchilla.

Leupeptin protects cochlear and vestibular hair cells from gentamicin ototoxicity

Calpains, a family of calcium-activated proteases that breakdown proteins, kinases, phosphatases and transcription factors, can promote cell death. Since leupeptin, a calpain inhibitor, protected against hair cell loss from acoustic overstimulation, we hypothesized that it might protect cochlear and vestibular hair cells against gentamicin (GM) ototoxicity. To test this hypothesis, mouse organotypic cultures from the cochlea, maculae of the utricle and the crista of the semicircular canal (P1-P3) were treated with different doses of GM (0.1-3 mM) alone or in the presence of leupeptin (0.1-3 mM). The percentage of outer hair cells (OHCs) and inner hair cells (IHCs) decreased with increasing doses of GM between 0.1 and 3 mM. The addition of 1 mM of leupeptin significantly reduced GM-induced damage to IHCs and OHCs; this protective effect was dose-dependent. GM also significantly reduced hair cell density in the crista and utricle in a dose-dependent manner between 0.1 and 3 mM. The addition of 1 mM of leupeptin significantly reduced hair cell loss in the crista and utricle for GM concentrations between 0.1 and 3 mM. These results suggest that one of the early steps in GM ototoxicity may involve calcium-activated proteases that lead to the demise of cochlear and vestibular hair cells.

Glial fibrillary acidic protein expression and promoter activity in the inner ear of developing and adult mice

The intermediate filament glial fibrillary acidic protein (GFAP) is a classic marker for several types of glial cells, including astrocytes and nonmyelinating Schwann cells. The pattern of expression of GFAP in the postnatal murine inner ear, from postnatal day 3 (P3) to P38, was studied by anti-GFAP immunostaining in wild-type mice as well as in two lines of transgenic mice expressing either beta-galactosidase (LacZ) or green fluorescent protein (GFP) under the control of the GFAP promoter. Analysis of protein and promoter activity shows that several classes of supporting cells in the sensory epithelia, as well as Schwann cells and satellite cells express GFAP. Early after birth, all cochlear supporting cells express GFAP, in a gradient decreasing in intensity from base to apex. After P15, GFAP expression in the organ of Corti is mostly restricted to supporting cells of the inner hair cell area (i.e., inner border and inner phalangeal cells) and outer hair cell area (i.e., Deiters' cells). A small population of limbic cells also showed expression in a base-to-apex gradient. In the vestibular organs, high expression was detected in supporting cells in extrastriolar regions of the utricular macula and in the canal ampullae, with weaker staining in the saccular macula. These results suggest that supporting cells of the inner ear have important similarities to glial cells and may play roles similar to those of astrocytes or Schwann cells in supporting the normal development and maintenance of neurons and sensory cells of the inner ear.

Role of substance P in the peripheral vestibular and auditory system

The central role of substance P (SP) has attracted growing interest in the past two decades. One of the important physiological functions of SP and other tachykinins is that of a neurotransmitter in primary afferent neurons. Recent immunocytochemical, biochemical and electrophysiological investigations on various neurotransmitters support the hypothesis that SP has a similar function in the vestibular and auditory systems of all mammals including humans. The purpose of this review is to give an overview of the distribution and concomitant physiological functions of this peptide in these sensory systems.

Neuronal feedback between brain and inner ear for growth of otoliths in fish

Previous investigations revealed that fish inner ear otolith growth (concerning otolith size and calcium-incorporation) depends on the amplitude and the direction of gravity, suggesting the existence of a (negative) feedback mechanism. In search for the regulating unit, the vestibular nerve was unilaterally transected in neonate swordtail fish (Xiphophorus helleri) which were subsequently incubated in the calcium-tracer alizarin-complexone. Calcium incorporation ceased on the transected head sides, indicating that calcium uptake is neurally regulated.

P2X2 receptor mediates stimulation of parasensory cation absorption by cochlear outer sulcus cells and vestibular transitional cells

Cochlear outer sulcus cells (OSC) and vestibular transitional cells (VTC) are part of the parasensory epithelium in the inner ear and are located in homologous positions between the sensory hair cells and the cation secretory epithelial cells in the cochlea and the vestibular labyrinth. OSC are known to sustain a reabsorptive transepithelial current and to contain an immunoreactivity for P2X(2) purinergic receptors. This study addresses whether OSC and VTC share functional similarities and extends this hypothesis to the question of whether both cell types contain functional P2X(2) receptors. The current density (I(sc)) was recorded with the vibrating probe technique and was found to be similar in VTC and OSC. Both gadolinium and flufenamic acid reduced I(sc) in VTC, as reported previously for OSC. I(sc) was stimulated by extracellular ATP but not by selective agonists of P2Y receptors. Purinergic receptor agonists increased I(sc) with a potency order of ATP > 2'- and 3'-O-(4-benzoyl-benzoyl)adenosine 5'-triphosphate alpha,beta-methyleneadenosine 5'-triphosphate in both OSC and VTC. In the presence of suramin (100 micrometer) or gadolinium (100 micrometer), the responses of ATP were inhibited significantly in both OSC and VTC. This pharmacological profile is consistent with that of the P2X(2) receptor. These results demonstrate that VTC participate in vestibular parasensory cation absorption and that both OSC and VTC regulate their parasensory cation flux via P2X(2) receptors, which would regulate the endolymphatic concentration of the current-carrying ion species in auditory and vestibular transduction.

Induction of apoptotic pathway in the vestibule of cisplatin (CDDP)-treated guinea pigs

BACKGROUND

During the process of apoptosis, double-stranded DNA is broken into single-stranded DNA by the action of caspases and caspase-activated deoxyribonuclease. We immunohistochemically examined the apoptotic changes induced by cisplatin in the vestibule of guinea pigs.

MATERIALS AND METHODS

Cisplatin (10 mg/kg b.w.) was intraperitoneally injected into guinea pigs and, 3 days after the injection, the animals were sacrificed by intracardiac perfusion of fixative. The temporal bones were then removed and immunohistochemically stained for caspase-activated deoxyribonuclease or caspase 3.

RESULTS

Both caspase-activated deoxyribonuclease and caspase 3 were observed in the dark cell area, transitional area and the sensory epithelium.

CONCLUSION

These findings suggest that apoptosis is involved in the vestibular dysfunction of the CDDP-treated patients.

Vestibular histofluorescence could be due to accumulation of both the antibiotic and its derivative, streptidine, after acute streptomycin treatment in the guinea pig

Acute treatment with 300 mg/kg of pigmented guinea pigs with streptomycin sulfate induces an elevation of endogenous fluorescence in vestibular ampullary cristae. Fluorescence accumulates in all compartments of the epithelium, i.e., vestibular sensory and supporting cells and nerve fibers of the stroma and it was very intense 1 and 12 hours after its administration. Fluorescence decreased to control levels 24 hours following streptomycin injection. Fluorescence levels were very low either in untreated animals or in animals injected with saline physiological solution. To investigate whether this fluorescence was an intrinsic property of the antibiotic or whether it was due to a derivative of it, or both, an in vitro fluorescence spectrum was performed with 100 microM solutions of streptomycin or streptidine, or both, dissolved in various buffer solutions at 488 nm of excitation. A discrete level of fluorescence was observed in the spectrum regardless of media when separate solutions of both streptomycin or streptidine were studied. Fluorescence notably increased at 522-532 nm when the solutions contained both streptomycin and streptidine together. These results suggest that streptidine putatively derived from streptomycin may contribute to the observed fluorescence accumulation in vestibular preparations after acute treatment. Thus, these metabolic properties of the inner ear which transform streptomycin into streptidine, something never considered earlier, could be claimed as partially responsible for converting a therapeutic agent into a compound which could be as harmful as STP to the inner ear.

Role of gene regulation during vestibular compensation: an integrative approach

Identification of the role of gene regulation in vestibular compensation is one aspect of a larger issue: the identification of molecular bases for plasticity in multiple vestibulo-ocular, vestibulo-spinal, vestibulo-collic, and vestibulo-autonomic responses. To achieve this goal, it is incumbent on investigators to examine molecular events within the contexts of the single neuron, the location of the neuron in pathways, and the timing of the molecular events relative to behavioral compensation. Hence, the goal of identifying molecular bases for a particular compensatory response (e.g., the disappearance of spontaneous nystagmus in the light or the disappearance of static head tilt) requires careful attention to the time course of physiologic compensation and the location of the effects within central pathways that have the potential to affect the responses. The effects of impeding these site-specific and time-specific changes can then be tested to determine their role in the compensatory process. A consideration of the recent literature on molecular events related to the resolution of spontaneous nystagmus in the light indicates that a meaningful approach to these issues requires a broadening of our conceptual approach. Specifically, one must consider the roles of transcriptional, translational, and posttranslational events on the turnover of critical signaling substrates for vestibular compensation.

Immunolocalization of ClC-K chloride channel in strial marginal cells and vestibular dark cells

Secretion of K(+) into endolymph depends on a particular constellation of ion transport proteins in the apical and basolateral membranes of strial marginal cells and vestibular dark cells. One fundamental component is the large chloride conductance of the basolateral membrane, which recycles chloride taken up by the Na(+)-K(+)-Cl(-) cotransporter in the same membrane. Evidence has been reported recently that supports ClC-K, a channel subunit previously thought to be specific to the kidney, as being the molecular entity underlying this conductance. We have isolated protein from the gerbil kidney, stria vascularis and vestibular labyrinth and found by Western blot analysis a 60 kDa band, a 48 kDa band and 54 and 70 kDa bands, respectively, specifically labeled by ClC-K antibody. Subsequent immunohistochemical observations of the inner ear tissues with a confocal microscope on fluorescently labeled tissue sections showed the staining to be restricted to the basolateral region of strial marginal cells and vestibular dark cells. The cochlear staining was distinct from the distribution of the Kir4.1 (KCNJ10) K(+) channel, known to be present only in strial intermediate cells. These findings support the contention that ClC-K is an important component of the basolateral Cl(-) conductance that participates in K(+) secretion by these epithelia.

Monoclonal L-citrulline immunostaining reveals nitric oxide-producing vestibular neurons

Nitric oxide is an unstable free radical that serves as a novel messenger molecule in the central nervous system (CNS). In order to understand the interplay between classic and novel chemical communication systems in vestibular pathways, the staining obtained using a monoclonal antibody directed against L-citrulline was compared with the labeling observed using more traditional markers for the presence of nitric oxide. Brainstem tissue from adult rats was processed for immunocytochemistry employing a monoclonal antibody directed against L-citrulline, a polyclonal antiserum against neuronal nitric oxide synthase, and/or NADPH-diaphorase histochemistry. Our findings demonstrate that L-citrulline can be fixed in situ by vascular perfusion, and can be visualized in fixed CNS tissue sections by immunocytochemistry. Further, the same vestibular regions and cell types are labeled by NADPH-diaphorase histochemistry, by the neuronal nitric oxide synthase antiserum, and by our anti-L-citrulline antibody. Clusters of L-citrulline-immunoreactive neurons are present in subregions of the vestibular nuclei, including the caudal portion of the inferior vestibular nucleus, the magnocellular portion of the medial vestibular nucleus, and the large cells in the ventral tier of the lateral vestibular nucleus. NADPH-diaphorase histochemical staining of these neurons clearly demonstrated their multipolar, fusiform and globular somata and long varicose dendritic processes. These results provide support for the suggestion that nitric oxide serves key roles in both vestibulo-autonomic and vestibulo-spinal pathways.

Effects of vestibular nerve transection on the calcium incorporation of fish otoliths

Previous investigations revealed that the growth of fish inner ear otoliths (otolith size and calcium-incorporation) depends on the amplitude and the direction of gravity, suggesting the existence of a (negative) feedback mechanism. In search for the regulating unit, the vestibular nerve was transacted unilaterally in neonate swordtail fish (Xiphophorus helleri) which were subsequently incubated in the calcium-tracer alizarin-complexone. Calcium incorporation ceased on the transacted head sides, indicating that calcium uptake is neurally regulated. Grant numbers: 50 WB 9533, 50 WB 9997.

Spatiotemporal expression of otogelin in the developing and adult mouse inner ear

Using a PCR-based subtractive method on cDNA from 2-day-old mouse cochlea, we identified a gene encoding otogelin, Otog, an inner ear specific glycoprotein expressed in all acellular structures. Here, we provide evidence that otogelin is detected as early as embryonic day 10 in the otic vesicle. At this stage, otogelin is detected in the epithelial cells which do not overlap with the myosin VIIA-expressing cells, namely the precursors of the hair cells, thus arguing for an early commitment of the two cell populations. Analysis of otogelin spatiotemporal cell distribution allows a molecular tracing for the contribution of the cochlear and vestibular inner ear supporting cells to the formation of the acellular structures. Throughout embryonic and adult life, the expression of the otogelin gene as monitored by LacZ inserted into Otog, and the abundance of the protein are greater in the vestibule than in the cochlea. In adult, otogelin is still produced by the vestibular supporting cells, which argues for a continuous process of otogelin renewal in the otoconial membranes and cupulae. In contrast, in the tectorial membrane, otogelin should be a long-lasting protein since both the otogelin gene and protein were almost undetectable in adult cochlear cells. The data are consistent with the requirement for otogelin in the attachment of the otoconial membranes and cupulae to their corresponding sensory epithelia as revealed in Otog -/- mice.

Glycerol affects vestibular evoked myogenic potentials in Meniere's disease

OBJECTIVES

to show that abnormal vestibular evoked myogenic potentials on the sternocleidomastoid muscle (SCM) in patients with unilateral Meniere's disease are caused by endolymphatic hydrops.

SUBJECTS

six normal volunteers and 17 patients with unilateral Meniere's disease were examined.

METHODS

click-evoked myogenic potentials were recorded with surface electrodes over each SCM. Responses evoked by clicks recorded after oral administration of glycerol (1.3 g/kg body weight) were compared with those recorded before administration.

RESULTS

the change rate of the p13-n23 amplitude was calculated. The mean+standard deviation (S.D.) of the change rate was 3.52+14.6% in normal subjects. On the unaffected side of patients the change rates were within the normal range (within the mean+/-2S.D.) in 13 patients, and three ears showed significant decrease. Only one ear showed significant increase. On the affected side, five ears showed significant increase of the amplitude while two ears showed significant decrease after oral administration of glycerol. Effects on evoked myogenic potentials were independent of those on pure tone hearing.

CONCLUSION

vestibular evoked myogenic potentials in some patients with unilateral Meniere's disease were improved by oral administration of glycerol. This result suggests that abnormal vestibular evoked myogenic potentials in patients with unilateral Meniere's disease could result from endolymphatic hydrops.

Effects of betahistine and of its metabolites on vestibular sensory organs

Betahistine is widely used in the treatment of peripheral and central vestibular disorders. Till now the anti-vertigo effect of the drug was though to be mainly due to an action of betahistine on inner ear or cerebral microcirculation or on some structures of the CNS, chiefly the vestibular nuclei. Vertigo, however is, in most cases, of peripheral origin but it remains unknown whether betahistine, or some of its metabolities, may directly affect the vestibular system at peripheral level. Pharmacokinetic studies have in fact demonstrated that betahistine is transformed, mainly at the hepatic level, in aminoethylpyridine (M1), hydroxyethylpyridine (M2) and, finally, in pyridylacetic acid (M3) which is excreted with the urine. All these substances are therefore present in the body fluids of subjects treated with betahistine, and thus might have pharmacological effects. The goal of the present study was to investigate whether betahistine or some of its metabolites could exert any effect on vestibular receptors. To this end, the effects of the drugs (10(-7)-10(-2) M) have been examined on frog semicircular canals, an animal model well suited for this purpose. The effects of betahistine and of its metabolites have been evaluated by recording ampullar receptor activity both at rest and during mechanical stimulation of the sensory organ. The results demonstrated that both betahistine and one of its metabolites, the aminoethylpyridine (M1), exert effects quite similar on ampullar receptors; both these substances in fact could reduce greatly ampullar receptor resting discharge but had scanty effects on mechanically-evoked responses. This observation might justify betahistine and possibly M1 anti-vertigo effects. In fact vertigo is normally due to uncontrolled changes in vestibular receptor resting discharge. It is therefore probable that any factor able to reduce vestibular receptor resting firing rate and, in consequence, its variations, may have, as final effect, an anti-vertigo action. The observation that betahistine and M1 have similar effects might be of some clinical interest. In fact, on the basis of our data, the hypothesis may be put forward that the anti-vertigo action of betahistine is at first achieved by betahistine itself and then sustained and prolonged in time by M1.

Adult rat otic placode-derived neurons and sensory epithelium express all four erbB receptors: a role in regulating vestibular ganglion neuron viability

The erbB receptor family consists of erbB1/epidermal growth factor receptor, erbB2/neu, erbB3, and erbB4, all of which have been implicated in cell proliferation, differentiation, and survival in several tissues. In the nervous system, these family members can function in a trophic capacity for certain subpopulations of neurons and some types of non-neuronal cells. Vestibular sensory epithelial cells and vestibular ganglion neurons are derived from ectodermal otic placode and are essential components of the peripheral vestibular system, the sensory system for balance. Recent studies in mammals suggest that certain ligands of the epidermal growth factor receptor can induce proliferation of vestibular sensory epithelial cells. We now show that vestibular ganglion neurons and vestibular sensory epithelial cells express all four erbB receptors in adult rats. Cultured vestibular ganglion neurons also expressed all four erbB family members and were therefore used to analyze the effects of modulating erbB signaling on differentiated vestibular ganglion neurons. Transforming growth factor-alpha (a ligand for epidermal growth factor receptor) and sensory and motor neuron-derived factor (a ligand for erbB3 and erbB4) promoted vestibular ganglion neuron viability, whereas epidermal growth factor (another ligand for epidermal growth factor receptor) did not. Glial growth factor 2 (another ligand for erbB3 and erbB4) and an antibody that blocks erbB2/neu-mediated signaling inhibited vestibular ganglion neuron viability. Collectively, these observations indicate that erbB signaling regulates the viability of differentiated otic placode-derived cells in mammals and suggest that exogenous modulation of erbB signaling in peripheral vestibular tissues may prove therapeutically useful in peripheral vestibular disorders.