Distribution of GABA-like immunoreactivity in guinea pig vestibular cristae ampullaris

Activation of GABAB receptors results in excitatory modulation of calyx terminals in rat semicircular canal cristae

Previous studies have found GABA in vestibular end organs. However, existence of GABA receptors or possible GABAergic effects on vestibular nerve afferents has not been investigated. The current study was conducted to determine whether activation of GABAB receptors affects calyx afferent terminals in the central region of the cristae of semicircular canals. We used patch-clamp recording in postnatal day 13-18 (P13-P18) Sprague-Dawley rats of either sex. Application of GABAB receptor agonist baclofen inhibited voltage-sensitive potassium currents. This effect was blocked by selective GABAB receptor antagonist CGP 35348. Application of antagonists of small (SK)- and large-conductance potassium (BK) channels almost completely blocked the effects of baclofen. The remaining baclofen effect was blocked by cadmium chloride, suggesting that it could be due to inhibition of voltage-gated calcium channels. Furthermore, baclofen had no effect in the absence of calcium in the extracellular fluid. Inhibition of potassium currents by GABAB activation resulted in an excitatory effect on calyx terminal action potential firing. While in the control condition calyces could only fire a single action potential during step depolarizations, in the presence of baclofen they fired continuously during steps and a few even showed repetitive discharge. We also found a decrease in threshold for action potential generation and a decrease in first-spike latency during step depolarization. These results provide the first evidence for the presence of GABAB receptors on calyx terminals, showing that their activation results in an excitatory effect and that GABA inputs could be used to modulate calyx response properties.NEW & NOTEWORTHY Using in vitro whole cell patch-clamp recordings from calyx terminals in the vestibular end organs, we show that activation of GABAB receptors result in an excitatory effect, with decreased spike-frequency adaptation and shortened first-spike latencies. Our results suggest that these effects are mediated through inhibition of calcium-sensitive potassium channels.

Excitatory actions of GABA in developing chick vestibular afferents: effects on resting electrical activity

The aim of this study was to characterize the effect of γ-aminobutyric acid (GABA) in the resting multiunit activity of the vestibular afferents during development using the isolated inner ear of embryonic and postnatal chickens (E15-E21 and P5). GABA (10(-3) to 10(-5) M; n = 133) and muscimol (10(-3) M) elicited an increase in the frequency of the basal discharge of the vestibular afferents. We found that GABA action was dose-dependent and inversely related to animal age. Thus, the largest effect was observed in embryonic ages such as E15 and E17 and decreases in E21 and P5. The GABAA receptor antagonists, bicuculline (10(-5) M; n = 10) and picrotoxin (10(-4) M; n = 10), significantly decreased the excitatory action of GABA and muscimol (10(-3) M). Additionally, CNQX 10(-6) M, MCPG 10(-5) M and 7ClKyn 10(-5) M (n = 5) were co-applied by bath substitution (n = 5). Both the basal discharge and the GABA action significantly decreased in these experimental conditions. The chloride channel blocker 9-AC 0.5 mM produced an important reduction in the effect of GABA 10(-3) (n = 5) and 10(-4) M (n = 5). Thus, our results suggest an excitatory role of GABA in the resting activity of the vestibular afferents that can be explained by changes in the gradient of concentration of Cl(-) during development. We show for the first time that the magnitude of this GABA effect decreases at later stages of embryonic and early postnatal development. Taking into account the results with glutamatergic antagonists, we conclude that GABA has a presynaptic action but is not the neurotransmitter in the vestibular afferent synapses, although it could act as a facilitator of the spontaneous activity and may regulate glutamate release.

Modalities of GABA and glutamate neurotransmission in the vertebrate inner ear vestibule

GABA and glutamate have been postulated as afferent neurotransmitters at the sensory periphery inner ear vestibule in vertebrates. GABA has fulfilled the main criteria to act as afferent neurotransmitter but may also be a putative efferent neurotransmitter, mainly due to cellular localization of its synthesizing enzyme glutamate decarboxylase derived from biochemical, immunocytochemical, in situ hybridization and molecular biological techniques, whereas glutamate afferent neurotransmission role is supported mainly by pharmacological evidences. GABA and Glu could also act as afferent co-neurotransmitters based upon immunocytochemical techniques. This multiplicity was not considered earlier and postulates a peripheral modulation of afferent information being sent to higher vestibular centers. In order to make a definitive cellular assignation to these putative neurotransmitters it is necessary to have evidences derived from immunocytochemical and pharmacological experiments in which both substances are tested simultaneously.

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.

Effects of selected pharmacological agents on avian auditory and vestibular compound action potentials

Glutamate is currently the consensus candidate for the hair cell transmitter in the inner ear of vertebrates. However, other candidate transmitter systems have been proposed and there may be differences in this regard for auditory and vestibular neuroepithelia. In the present study, perilymphatic perfusion was used to deliver prescribed concentrations of ten drugs to the interstitial fluids of the inner ear of hatchling chickens (n = 124). Dose-response curves were obtained for four of these pharmacological agents. The work was carried out in part to distinguish further the neuroepithelial chemical receptors mediating auditory and vestibular compound action potentials (CAPs). Kainic acid (KA) eliminated both auditory and vestibular responses. D-alpha-Aminoadipic acid (DAA) and dizocilpine maleate (MK-801), both NMDA-specific antagonists, failed to alter vestibular CAPs at any concentration. MK-801 significantly and selectively reduced auditory CAPs at concentrations equal to or greater than 1 mM. Similarly, kynurenic acid (4-hydroxyquinoline-2-carboxylic acid, 1 mM), a glutamate antagonist, significantly reduced auditory but not vestibular CAPs. A non-NMDA glutamate receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), reduced vestibular CAPs significantly but only at the highest concentration tested (1 mM). In contrast, CNQX reduced auditory responses at concentration as low as 1 microM. The CNQX concentration effective in reducing auditory CAPs by 50% (EC(50)) was approximately 20 microM. Glutamate (1 mM) as well as alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA), a glutamate agonist, significantly reduced auditory CAPs (AMPA EC(50)=100 microM). Bicuculline, a GABA(A) receptor antagonist, and L-NAME, a nitric oxide synthase inhibitor, failed to alter responses from either modality. These findings support the hypothesis that glutamate receptors mediate auditory CAPs in birds. However, the results underscore a remarkable difference in sensitivity of the vestibular neuroepithelium (here gravity receptors) to non-NMDA receptor antagonists. The basis of the vestibular insensitivity to glutamate blockers is unknown but it may reflect differences in receptors themselves, differences in the transmission modes available to vestibular synapses or differences in the access of compounds to vestibular neuroepithelial receptors from the interstitial-perilymphatic fluid spaces.

Gamma-aminobutyric acid is present in a spatially discrete subpopulation of hair cells in the crista ampullaris of the toadfish Opsanus tau

Although gamma-aminobutyric acid (GABA) and glutamate are known to be present in the vestibular sensory epithelia of a variety of species, the functional relationship between these two transmitters is not clear. The present study addresses the three-dimensional spatial distribution of GABA and glutamate immunoreactivity in the vestibular labyrinth of the oyster toadfish by using whole end organs labeled by immunofluorescence with monoclonal anti-GABA and/or antiglutamate antibodies and visualized as whole mounts by multiphoton confocal microscopy. We find glutamate-immunoreactive hair cells present throughout the sensory epithelium. In contrast, prominent GABA immunoreactivity is restricted to a small population of hair cells located in the central region of the crista. Double immunofluorescence reveals two distinct staining patterns in GABA-labeled hair cells. Most ( approximately 80%) GABA-labeled cells show trace levels of glutamate, appropriate for the metabolic/synthetic role of cytoplasmic glutamate. The remainder of the GABA-stained cells contain substantial levels of both GABA and glutamate, suggesting transmitter colocalization. In the toadfish utricle, glutamatergic hair cells are present throughout the macula. GABA-immunoreactive hair cells follow the arc of the striola, and most GABA-labeled receptor cells coexpress glutamate. The localization of GABA was explored in other species as well. In the pigeon, GABAergic hair cells are present throughout the crista ampullaris. Our findings demonstrate that multiple, neurochemically distinct types of hair cells are present in vestibular sensory epithelia. These observations, together with the excitatory activity generally associated with 8th nerve afferent fibers, strongly suggest that GABA serves an important, specific, and complex role in determining primary afferent response dynamics.

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.

Glutamate-like immunoreactivity during hair cell recovery after gentamicin exposure in the chinchilla vestibular sensory periphery

OBJECTIVE

Determine the expression of glutamate by immunohistochemistry in normal and recovering vestibular hair cells in the chinchilla crista ampullaris after gentamicin ototoxicity.

STUDY DESIGN

In five groups of three animals each, ototoxicity was produced by placing gentamicin (50 microg)-impregnated Gelfoam pellets within the perilymphatic space of the superior semicircular canal. Animals were sacrificed at 1, 2, 4, 8, and 16 weeks after treatment. A group of normal (n=3) animals was also processed.

METHODS

For the detection of glutamate the inner ears of these animals were dissected, and the horizontal cristae ampullaris embedded in plastic. Two-micron-thick tissue sections were obtained and incubated with monoclonal antibodies against glutamate. The immunoreaction was detected using the avidinbiotinylated-complex technique and diaminobenzidine was the chromogen.

RESULTS

Normal sensory epithelia demonstrated type I and type II hair cells with moderate glutamate-like immunoreactivity. Supporting cells demonstrated no glutamate-like immunoreactivity. Afferent nerve fibers and calyxes surrounding type I hair cells demonstrated strong glutamate-like immunoreactivity. At 1 and 2 weeks after treatment the few type II hair cells surviving ototoxic treatment (15%-18%) contained moderate glutamate-like immunoreactivity, supporting cells showed no immunoreactivity, and nerve terminals and fibers displayed strong immunoreactivity. At 4 and 8 weeks after treatment, recovered hair cells (80%) had greater glutamate-like immunoreactivity when compared with normal hair cells, supporting cells displayed no glutamate-like immunoreactivity, and afferent fibers contained strong glutamate-like immunoreactivity. At 16 weeks, glutamate-like immunoreactivity in hair cells returned to normal level.

CONCLUSION

Glutamate may be used as an indicator of hair cell differentiation and as an index of the molecular recovery of hair cells after ototoxicity.

Expression of gamma-aminobutyric acid(A) receptor subunits in the vestibular system

OBJECTIVES

Profile the expression of genes encoding GABAA receptor subunits in the vestibular end organs of a rat.

MATERIALS AND METHODS

Using a combination of reverse transcription followed by polymerase chain reaction (PCR) with gene-specific primers, expression of mRNAs encoding 13 individual GABA(A) receptor subunits was examined.

RESULTS

PCR amplification products representing subunit gene expression for alpha1-6, beta1-3, and gamma1-3, but not for delta, subunits were amplified, suggesting multiple molecular levels of regulation of vestibular GABA(A) receptor expression. Nucleotide sequencing confirmed the identity of rat vestibular end-organs subunit cDNAs.

CONCLUSION

These results give the most direct evidence to date that GABAA receptors composed of the detected subunits are expressed in the mammalian vestibular system, giving new support to previous investigations implicating GABA as a vestibular neurotransmitter.

Ultrastructural localization of GABA-like immunoreactivity in the human utricular macula

In the vertebrate vestibular periphery, gamma-aminobutyric acid (GABA) has long been presumed to be a neurotransmitter candidate. However, experimental reports about the localization and function of GABA in the vestibular systems of vertebrates are contradictory. In addition, there is no information in the literature concerning the localization of GABA in the human vestibular periphery. The present study investigates the ultrastructural localization of GABA-like immunoreactivity in the human utricular macula. A modified pre-embedding immunostaining electron microscopy technique was applied using two different commercially available polyclonal antibodies to GABA. GABA-like immunoreactivity is confined to the vesiculated nerve fibers and terminals of the human vestibular neurosensory epithelia. The GABA-containing nerve terminals make asymmetrical axo-dendritic synapses with the afferent chalices surrounding the type I sensory hair cells. Type I and type II hair cells as well as afferent chalices are devoid of GABA-like immunoreactive staining. The present study demonstrates that GABA exists in the human vestibular periphery, and that GABA is a neurotransmitter candidate of the human efferent vestibular system.

Ultrastructural localization of GABA-like immunoreactivity in the vestibular periphery of the rat

Gamma-aminobutyric acid (GABA) is presumed to be a neurotransmitter candidate in the vestibular periphery of mammals. However, experimental reports about the localization of GABA in afferents or efferents of the vestibular systems are contradictory. It is an open question whether there are species differences in the amammalian vestibular system. The present study was designed to investigate the ultrastructural localization of GABA-like immunoreactivity in the vestibular periphery of the rat. A modified preembedding immunoelectron microscopy technique was applied using a polyclonal antibody to GABA as a marker. GABA-like immunoreactivity was revealed in the vestibular periphery of the rat, confined to the vesiculated nerve fibers and terminals of the rat vestibular neurosensory epithelia. Type I hair cells and type II hair cells as well as efferent chalices are devoid of GABA-like immunoreactive staining. These findings indicate that GABA is a neurotransmitter candidate of the efferent vestibular system of the rat.

Histological evidence for hair cell regeneration after ototoxic cell destruction with local application of gentamicin in the chinchilla crista ampullaris

Two experiments were conducted to study the ototoxic effects of local gentamicin (GM) administration and the subsequent hair cell (HC) regeneration process in the chinchilla cristae ampullares (CA). In the first experiment, 3 different doses of GM (0.1, 0.2 and 1.2 mg) were administered by surgical implantation of GM-soaked Gelfoam pledgets in the perilymphatic space in the otic capsule of the left superior semicircular canal. The CA was histologically processed for light-microscopic examination. In the second experiment, 6 groups of 2 chinchillas each were treated with 0.1 mg of GM. To document cell proliferation and HC regeneration, Alzet micro-osmotic pumps were implanted in each chinchilla to deliver bromodeoxyuridine (BrdU) at 125 micrograms/h for 1 week. Chinchillas were subsequently killed at 1 and 4 days and 1, 2, 4 and 8 weeks post-treatment (PT). The CA was processed for light microscopy and BrdU immunocytochemistry. In the first experiment the smallest dose produced damage restricted to HCs alone, while the medium and large doses produced severe damage in the sensory epithelium, including supporting cells and HCs. Results in the second experiment demonstrated that at 1 and 4 days PT the HCs showed extensive damage, including clumping of nuclear material. By 4 days PT the supporting cell nuclei lost their monolayer configuration. Calyceal terminals appeared empty, and vacuolized remnants of nerve calyces were evident in the basal portion. At 1 week PT complete disappearance of HCs from the sensory epithelium was evident, and there was cytoplasmic extrusion into the endolymphatic space. At 2 weeks PT there was complete HC loss, the supporting cell nuclei were scattered randomly in the crista, and the nerve fibers were retracted from the sensory epithelium. At 4 weeks PT there was evidence of sensory epithelium repair and HC regeneration. Short cells resembling type-II HCs were evident in the surface of the sensory epithelium. At 8 weeks PT the number of HCs increased in a uniform fashion on the surface of the sensory epithelium, and the supporting cell nuclei were realigned on the basal membrane. Nerve fibers with growth cones penetrated the basal membrane. Supporting cell proliferation was evident by the presence of mitotic figures and BrdU immunoreactivity in the chromatin material of dividing cells at 2 weeks PT. The labeling was more evident in newly formed cells at 4 and 8 weeks PT. These results demonstrate that in chinchillas the vestibular organs have the capacity of self-repair and the process includes HC regeneration after local administration of GM. The overall process involves changes in different cells in the sensory epithelium and neural elements, all of which show modifications with an orderly pattern.

Immunocytochemical localization of aspartate and glutamate in the peripheral vestibular system

Controversy exists concerning the identity of the neurotransmitter in the mammalian peripheral vestibular system. Several candidates have been proposed, including the excitatory amino acids glutamate and aspartate and the inhibitory amino acid gamma-aminobutyric acid (GABA). Previous studies have demonstrated vestibuloneural electrophysiological activity associated with glutamate and aspartate. Paraffin sections of rat vestibular ganglia and end-organs were examined for the presence of glutamate-like and aspartate-like immunoreactivity. Our results demonstrate the presence of both aspartate-like and glutamate-like immunoreactivity in vestibular hair cells, peripheral vestibular nerve fibers, and vestibular ganglion cells. Minimal immunoreactivity was noted in the tissues surrounding these cells. These data add support to the hypothesis that the excitatory amino acids glutamate and aspartate are involved in vestibular neurotransmission.

Immunohistochemical localization of GABAA receptors in the mammalian crista ampullaris

The distribution of GABAA receptor-like immunoreactivity in the hamster, rat, and mouse crista ampullaris was determined by use of a monoclonal antibody to the beta 2 and beta 3 subunits of the GABAA receptor. In the crista ampullaris, punctate staining was seen associated with the calyces surrounding vestibular type I hair cells. Afferent nerve fibers approaching the hair cell layer were often observed to be immunoreactive. Hair cells, supporting cells, and cells in the transitional and dark cell regions were not immunoreactive. The distribution of staining of calyces appeared to be relatively uniform in all regions (crest and slope) of the crista. In addition, cell bodies located in the vestibular ganglion were immunoreactive. The association of GABAA receptor-like immunoreactivity with the afferent nerve calyx and cell body of the vestibular ganglion cells suggests that GABA may act to modify afferent nerve transmission at the calyceal afferent nerve ending.

Expression of substance P, CGRP, and GABA in the vestibular periphery, with special reference to species differences

The present study was conducted to elucidate species differences in the distribution of neuroactive substances, including substance P (SP), calcitonin gene-related peptide (CGRP) and gamma-aminobutyric acid (GABA), in the vestibular periphery of various animals (chicken, pigeon, rat, guinea pig and squirrel monkey). SP-like immunoreactivity was found in a subset of the primary vestibular afferents with no marked species differences in the staining pattern. In contrast, the localization of CGRP- and GABA-like immunoreactivities in the efferent nerve fibers varied according to species. This difference in the distribution pattern of neuroactive substances, found in the efferent system, may indicate that each species has a chemically (and probably functionally) distinct efferent system which is related to its specific environment and/or evolution.

Nitric oxide in the rat vestibular system

Nitric oxide is known to function as a neurotransmitter in the central nervous system. It is also known to be involved in the central nervous system excitatory amino acid neurotransmission cascade. Activation of excitatory amino acid receptors causes an influx of calcium, which activates nitric oxide synthase. The resulting increase in intracellular nitric oxide activates soluble guanylate cyclase, leading to a rise in cyclic guanosine monophosphate. The excitatory amino acids glutamate and aspartate are found in the vestibular system and have been postulated to function as vestibular system neurotransmitters. Although nitric oxide has been investigated as a neurotransmitter in other tissues, no published studies have examined the role of nitric oxide in the vestibular system. Neuronal NADPH-diaphorase has been characterized as a nitric oxide synthase. This enzyme catalyzes the conversion of L-arginine to L-citrulline, producing nitric oxide during the reaction. We used a histochemical stain characterized by Hope et al. (Proc Natl Acad Sci 1991;88:2811) as specific for neuronal nitric oxide synthase to localize the enzyme in the rat vestibular system. An immunocytochemical stain was used to examine rat inner ear tissue for the presence of the enzyme's end product, L-citrulline, thereby demonstrating nitric oxide synthase activity. Staining of vestibular ganglion sections showed nitric oxide synthase presence and activity in ganglion cells and nerve fibers. These results indicate the presence of active nitric oxide synthase in these tissues and suggest modulation of vestibular neurotransmission by nitric oxide.

Neurofilament proteins form an annular superstructure in guinea-pig type I vestibular hair cells

Neurofilaments, the neuron-specific intermediate filaments, are composed of three immunochemically distinct subunits: NF-L, NF-M and NF-H that can be either phosphorylated or unphosphorylated. In mammals, the distribution of these subunits has been described in vestibular ganglion neurons, but there are no reports on the presence of neurofilaments in vestibular hair cells. We investigated, by immunocytochemistry, neurofilaments in vestibular hair cells from rat and guinea-pig using antibodies against the three subunits and to dephosphorylated NF-H (clone SMI 32, recognizes also NF-M on immunoblots), on Vibratome sections of the vestibular end-organs and on isolated hair cells. Various immunostaining protocols were used, as appropriate for the method of observation: laser scanning confocal microscopy (immunofluorescence) and transmission electron microscopy (immunoperoxidase, pre-embedding technique). In rat and guinea-pig cristae and utricles, neurofilament immunoreactivity was observed in axons inside and below the sensory epithelia. In guinea-pig, in addition to this staining, intensely immunoreactive annular structures were found in the basal regions of hair cells. These rings were detected with anti-NF-L, -NF-M and -dephosphorylated NF-H/M antibodies, but not with anti-phosphorylation-independent NF-H. Ring-containing hair cells were present in all regions of the sensory epithelia but were more abundant in the peripheral areas. All levels of observation (Vibratome and thin sections, and isolated hair cells) showed that only the guinea-pig type I hair cells contained a neurofilament ring. High-resolution observations showed that the ring was located below the nucleus, often close to smooth endoplasmic reticulum and the cell membrane.

Expression of GABAA receptor gamma 1 and gamma 2 subunits in the peripheral vestibular system of the rat

The distribution of GABAA receptor gamma 1 and gamma 2 subunits in the rat vestibular ganglion cells and end-organs was examined by using immunohistochemical techniques. Both gamma 1 and gamma 2 subunit-like immunoreactivities were observed in most vestibular ganglion cell bodies and peripheral terminal endings in the vestibular sensory epithelia. These results indicate that GABAA receptors are expressed in the vestibular afferent terminal endings and therefore suggest that GABA in addition to glutamate is a neurotransmitter which effects on vestibular afferents.

Localization of chat-like immunoreactivity in the vestibular endorgans of the rat

In vertebrates acetylcholine (ACh) has been generally considered as a neurotransmitter of the vestibular efferent system. The precise localization and innervation of the cholinergic nerve endings in the vestibular sensory periphery is still unknown. We examined choline acetyltransferase (ChAT)-like immunoreactivity in all five endorgans of the rat vestibule with light and electron microscopy using a modified pre-embedding immunostaining technique. The results were: (1) ChAT-like immunoreactivity was widespread in all five endorgans of the vestibule and confined to the vesiculated efferent nerve endings. (2) Two types of ChAT-like immunostained nerve endings can be identified according to their size and innervation pattern: a large nerve ending and a small--middle size one. (3) Vestibular endorgans differ in their ChAT-like immunoreactivity: staining is dense in the macula of the utricule and the three ampullary cristae, but less so in the macula of the saccule. (4) We found also a regional difference of the ChAT-like immunostaining in ampullary crista. ChAT-like immunostained nerve endings were predominant in the periphery close to the semilunar plane, and less in density in the central area. These findings demonstrate that ACh is a major neurotransmitter in the vestibular efferent system.

Cellular distribution of parvalbumin immunoreactivity in the peripheral vestibular system of three rodents

The cellular distribution of parvalbumin immunoreactivity in the vestibular peripheral system of mouse, rat, and guinea pig was investigated by light and electron microscopy. Parvalbumin was found in all neurons of the vestibular ganglia of these species but in the sensory epithelia immunoreactivity was restricted to type I hair cells localized exclusively in the central areas. The very intense staining pattern was similar in the cristae ampullares and utricles of all three species but a faint immunoreaction was also detectable in sensory cells of peripheral areas of rat cristae. The parvalbumin-immunoreactive type I sensory cells are connected by nerve fibres of the calyx unit type which are known selectively to contain calretinin.

GABAB-mediated modulation of ionic conductances in type I hair cells isolated from guinea-pig semicircular canals

Mammalian vestibular type I hair cells (VIHCs) are innervated by an afferent synaptic calyx which contains vesicles and is immunoreactive for GABA. We describe here the effects of GABA on electrophysiological properties and on cytosolic free-calcium levels ([Ca2+]i) of VIHCs isolated from guinea-pig ampullae. Whole-cell tight-seal macroscopic currents recorded from VIHCs showed that 100 microM GABA induced a decrease in the outward currents elicited by depolarizing membrane potentials. These are known to comprise potassium calcium-dependent currents. This effect was mimicked by baclofen, a GABAB agonist, and was not affected by picrotoxin, a GABAA antagonist. GABA also induced an increase in the inward current elicited at hyperpolarized membrane potentials in 50% of the tested cells. Single channel recording in cell-attached patches revealed that externally applied GABA produced a decrease and an increase in the open probability of 170 pS and 45 pS and of 15 pS channels, respectively. In imaging experiments using the dye Fura-2 to measure [Ca2+]i, the only reversible modulation of [Ca2+]i observed in response to GABA application was a decrease. These results demonstrate a modulation of calcium and potassium conductances by GABA, via GABAB receptors, in guinea-pig VIHCs.

Immunocytochemical evidence for an afferent GABAergic neurotransmission in the guinea pig vestibular system

To implicate gamma-aminobutyric acid (GABA) as an afferent neurotransmitter (AN), the localization of GABA synthesizing and degradation enzymes; L-glutamate decarboxylase (GAD) and GABA transaminase (GABA-T) was investigated by light and electron microscopy immunocytochemistry in guinea pig vestibular cristae and ganglion cells (GC). GAD-like immunoreactivity was exclusively confined to the sensory hair cell (HC) cytoplasm, suggesting that GAD synthesizes GABA in the HC. GABA-T like immunoreactivity was found within HC, nerve calyces, nerve fibers, and GC, suggesting its participation in terminating transmitter action. These results demonstrate the existence of a GABAergic system in the guinea pig vestibule and strongly support GABA as a vestibular AN.