Actions of excitatory amino acid acid agonists and antagonists on the primary afferents of the vestibular system of the axolotl (Ambystoma mexicanum)

Pharmacological characterization and differential expression of NMDA receptor subunits in the chicken vestibular system during development

Previous studies demonstrated that in vitro preparations of the isolated vestibular system of diverse animal species still exhibit stable resting electrical activity and mechanically evoked synaptic transmission between hair cells and primary afferent endings. However, there are no reports related to their neurodevelopment. Therefore, this research aimed to examine whether NMDA receptors mediate these electrical signals in an isolated preparation of the chicken vestibular system at three developmental stages, E15, E18, and E21. We found that the spontaneous and mechanically evoked discharges from primary afferents of the posterior semicircular canal were modulated by agonists NMDA and glycine, but not by the agonist d-serine applied near the synapses. Moreover, the individually applied by bath perfusion of three NMDA receptor antagonists (MK-801, ifenprodil, and 2-naphthoic acid) or high Mg²⁺ decreased the resting discharge rate, the NMDA response, and the discharge rate of mechanically evoked activity from these primary afferents. Furthermore, we found that the vestibular ganglion shows a stage-dependent increase in the expression of NMDA receptor subunits GluN1, GluN2 (A-C), and GluN3 (A-B), being greater at E21, except for GluN2D, which was inversely related to the developmental stage. However, in the crista ampullaris, the expression pattern remained constant throughout development. This could suggest the possible existence of presynaptic NMDA receptors. Our results highlight that although the NMDA receptors are functionally active at the early embryonic stages of the vestibular system, NMDA and glycine reach their mature functionality to increase NMDA responses close to hatching (E21).

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.

Neuropharmacology of vestibular system disorders

This work reviews the neuropharmacology of the vestibular system, with an emphasis on the mechanism of action of drugs used in the treatment of vestibular disorders. Otolaryngologists are confronted with a rapidly changing field in which advances in the knowledge of ionic channel function and synaptic transmission mechanisms have led to the development of new scientific models for the understanding of vestibular dysfunction and its management. In particular, there have been recent advances in our knowledge of the fundamental mechanisms of vestibular system function and drug mechanisms of action. In this work, drugs acting on vestibular system have been grouped into two main categories according to their primary mechanisms of action: those with effects on neurotransmitters and neuromodulator receptors and those that act on voltage-gated ion channels. Particular attention is given in this review to drugs that may provide additional insight into the pathophysiology of vestibular diseases. A critical review of the pharmacology and highlights of the major advances are discussed in each case.

Acid-sensing ionic-channel functional expression in the vestibular endorgans

In the vestibular system, the electrical discharge of the afferent neurons has been found to be highly sensitive to external pH changes, and acid-sensing ionic-channels (ASIC) have been found to be functionally expressed in afferent neurons. No previous attempt to assay the ASIC function in vestibular afferent neurons has been done. In our work we studied the electrical discharge of the afferent neuron of the isolated inner ear of the axolotl (Ambystoma tigrinum) to determine the participation of proton-gated currents in the postransductional information processing in the vestibular system. Microperfusion of FMRF-amide significantly increased the resting activity of the afferent neurons of the semicircular canal indicating that ASIC currents are tonically active in the resting condition. The use of ASIC antagonists, amiloride and acetylsalicylic acid (ASA), significantly reduced the vestibular-nerve discharge, corroborating the idea that the afferent neurons of the vestibular system express ASICs that are sensitive to amiloride, ASA, and to FMRF-amide. The sensitivity of the vestibular afferent-resting discharge to the microperfusion of ASIC acting agents indicates the participation of these currents in the establishment of the afferent-resting discharge.

Acid-sensing ionic channels in the rat vestibular endorgans and ganglia

Acid-sensing ionic channels (ASICs) are members of the epithelial Na+ channel/degenerin (ENaC/DEG) superfamily. ASICs are widely distributed in the central and peripheral nervous system. They have been implicated in synaptic transmission, pain perception, and the mechanoreception in peripheral tissues. Our objective was to characterize proton-gated currents mediated by ASICs and to determine their immunolocation in the rat vestibular periphery. Voltage clamp of cultured afferent neurons from P7 to P10 rats showed a proton-gated current with rapid activation and complete desensitization, which was carried almost exclusively by sodium ions. The current response to protons (H+) has a pH0.5 of 6.2. This current was reversibly decreased by amiloride, gadolinium, lead, acetylsalicylic acid, and enhanced by FMRFamide and zinc, and negatively modulated by raising the extracellular calcium concentration. Functional expression of the current was correlated with smaller-capacitance neurons. Acidification of the extracellular pH generated action potentials in vestibular neurons, suggesting a functional role of ASICs in their excitability. Immunoreactivity to ASIC1a and ASIC2a subunits was found in small vestibular ganglion neurons and afferent fibers that run throughout the macula utricle and crista stroma. ASIC2b, ASIC3, and ASIC4 were expressed to a lesser degree in vestibular ganglion neurons. The ASIC1b subunit was not detected in the vestibular endorgans. No acid-pH-sensitive currents or ASIC immunoreactivity was found in hair cells. Our results indicate that proton-gated current is carried through ASICs and that ionic current activated by H+ contributes to shape the vestibular afferent neurons' response to its synaptic input.

Histamine (H3) receptors modulate the excitatory amino acid receptor response of the vestibular afferents

Although the effectiveness of histamine-related drugs in the treatment of peripheral and central vestibular disorders may be explained by their action on the vestibular nuclei, it has also been shown that antivertigo effects can take place at the peripheral level. In this work, we examined the actions of H3 histaminergic agonists and antagonists on the afferent neuron electrical discharge in the isolated inner ear of the axolotl. Our results indicate that H3 antagonists such as thioperamide, clobenpropit, and betahistine (BH) decreased the electrical discharge of afferent neurons by interfering with the postsynaptic response to excitatory amino acid agonists. These results lend further support to the idea that the antivertigo action of histamine-related drugs may be caused, at least in part, by a decrease in the sensory input from the vestibular endorgans. The present data show that the inhibitory action of the afferent neurons discharge previously described for BH is not restricted to this molecule but is also shared by other H3 antagonists.

Extracts of retina and brain that excite afferent fibers innervating hair cells contain a compound related to hydroxyphenylglycine-N-carbamoyl

To identify new neurotransmitter and modulator candidates that might be important in transmission from sensory hair cells to afferent nerves, we examined extracts of neural tissue for compounds that excite afferent fibers innervating hair cells. Here, we describe the extraction and purification from retina and brain of a potent, unstable, excitatory compound with pharmacological activity similar to glutamate on afferent fibers innervating hair cells. This compound, however, was clearly distinguished from glutamate, other common amino acids, and known endogenous glutamate-receptor agonists. After derivatization and analysis by gas chromatography-mass spectrometry, the major compound found in highly purified neuroactive chromatographic fractions had the same gas chromatographic elution time and mass spectrum as the compound formed by derivatization of L-p-hydroxyphenylglycine-N-carbamoyl. Hydroxyphenylglycine-N-carbamoyl, however, did not copurify with the neuroactive compound and was not neuroactive. We thus hypothesize that the detected compound was produced from a precursor, structurally related to L-p-hydroxyphenylglycine-N-carbamoyl, that was a major component of the neuroactive chromatographic fractions. Because several compounds related to hydroxyphenylglycine are known to act on glutamate receptors, such a compound is an interesting candidate to be an endogenous glutamate-receptor ligand in the mammalian nervous system.

Ca2+-activated K+-current density is correlated with soma size in rat vestibular-afferent neurons in culture

Vestibular-afferent neurons (VANs) transmit information about linear and angular accelerations during head movements from vestibular end organs to vestibular nuclei. In situ, these neurons show heterogeneous discharge patterns that may be produced by differences in their intrinsic properties. However, little is known about the ionic currents underlying their different firing patterns. Using the whole cell patch-clamp technique, we analyzed the expression of Ca(2+) and Ca(2+)-activated K(+) currents (I(KCa)) in primary cultured neurons isolated from young rats (p7-p10). We found two overlapping subpopulations of VANs classified according to low-threshold Ca(2+)-current [low-voltage-activated (LVA)] expression; LVA (-) neurons, formed by small cells, and LVA (+) neurons composed of medium to large cells. The I(KCa) in both cell-groups was carried through channels of high (BK), intermediate (IK), and low conductance (SK), besides a resistant channel to classical blockers (IR). BK was expressed preferentially in LVA (+) cells, whereas IR expression was preferentially in LVA (-) cells. No correlation between SK and IK expression with the soma size was found. Current-clamp experiments showed that BK participates in the adaptation of discharge and in the duration of the action potential, whereas SK and IK did not show a significant contribution to electrical discharge of cultured VANs. However, because of the low number of VANs in culture with repetitive firing it is difficult to interpret our results in terms of discharge patterns. Our results demonstrate that vestibular-afferent neurons possess different Ca(2+)-activated K(+) (K(Ca)) channels and that their expression, heterogeneous among the cells, would contribute to explain some of the differences in the electrical-firing properties of these neurons.

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.

Distribution of alpha-amino-3-hydroxy-5-methyl-4 isoazolepropionic acid and N-methyl-D-aspartate receptor subunits in the vestibular and spiral ganglia of the mouse during early development

We investigated the distribution of the glutamate receptor subunits, alpha-amino-3-hydroxy-5-methyl-4 isoazolepropionic acid (AMPA) GluR2 and GluR2/R3, and N-methyl-D-aspartate (NMDA) NR1, and the timing of their appearance during early development of the mouse vestibular and spiral ganglia. NMDA NR1 was the first to be expressed, in the statoacoustic ganglion neurons on E11. GluR2/R3 immunoreactivity was detected in these neurons on E12. This signal probably corresponded exclusively to GluR3, as no signal was obtained for GluR2 alone at this stage. The appearance of these proteins began much earlier than previously reported. GluR2 staining was observed later, on E14 in the vestibular neurons and on E17 in the spiral neurons. The sequence in which these three glutamate receptors appeared suggested possible differences in their roles in the establishment of neuronal circuitry in the inner ear sensory epithelia. The production of NR1 and GluR2/R3 began during the early period of neuron growth and fasciculation. GluR2 appeared later and its expression paralleled synaptogenesis in the vestibular sensory epithelia and in the organ of Corti.

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.

Betahistine produces post-synaptic inhibition of the excitability of the primary afferent neurons in the vestibular endorgans

Betahistine has been used to treat several vestibular disorders of both central and peripheral origin. The objective of this work was to study the action of betahistine in the vestibular endorgans. Experiments were done in wild larval axolotl (Ambystoma tigrinum). Multiunit extracellular recordings were obtained from the semicircular canal nerve using a suction electrode. Betahistine (10 microM to 10 mM; n = 32) inhibited the basal spike discharge of the vestibular afferent neurons with an IC50 of 600 microM. To define the site of action of betahistine, its interactions with the nitric oxide synthase inhibitor NG-nitro-L-arginine (3 microM) and with the cholinergic antagonists atropine (10 microM; n = 3) and d-tubocurarine (10 microM; n = 3) were studied. The action of betahistine when co-administered with these drugs was the same as that in control experiments, indicating that its effects did not include nitric oxide production or the activation of cholinergic receptors. In contrast, 0.01-1 mM betahistine reduced the excitatory action of kainic acid (10 microM; n = 6) and quiscualic acid (1 microM; n = 13). These results indicate that the action of betahistine on the spike discharge of afferent neurons seems to be due to a post-synaptic inhibitory action on the primary afferent neuron response to the hair cell neurotransmitter.

Nitric oxide in the afferent synaptic transmission of the axolotl vestibular system

This study was performed using intracellular and multiunit extracellular recording techniques in order to characterize the role of nitric oxide in the afferent synaptic transmission of the vestibular system of the axolotl (Ambystoma tigrinum). Bath application of nitric oxide synthase inhibitors N(G)-nitro-L-arginine (0.01microM to 10microM) and N-nitro-L-arginine methyl ester hydrochloride (0.1microM to 1000microM) elicited a dose-dependent decrease in the basal discharge of the semicircular canal afferent fibers. N(G)-Nitro-L-arginine also diminished the response to mechanical stimuli. Moreover, N(G)-nitro-L-arginine (1microM) produced a hyperpolarization associated with a decrease in the spike discharge and diminished the frequency of the excitatory postsynaptic potentials on afferent fibers recorded intracellularly. Nitric oxide donors were also tested: (i) S-nitroso-N-acetyl-DL-penicillamine (0.1microM to 100microM) increased the basal discharge and the response to mechanical stimuli. At the maximum effective concentration (100microM) this drug affected neither the amplitude nor the frequency of the excitatory postsynaptic potentials. However, it slightly depolarized the afferent neurons and decreased their input resistance. (ii) 3-Morpholino-sydnonimine hydrochloride did not significantly affect the basal discharge or the mechanically evoked peak response of afferent neurons at any of the concentrations used (1microM to 1000microM). However, after 10min of perfusion in the bath, 1microM and 10microM 3-morpholino-sydnonimine hydrochloride significantly modified the baseline of the mechanically evoked response, producing an increase in the mean spike discharge of the afferent fibers. These results indicate that nitric oxide may have a facilitatory role on the basal discharge and on the response to mechanical stimuli of the vestibular afferent fibers. Thus, nitric oxide probably participates in the sensory coding and adaptative changes of vestibular input in normal and pathological conditions.

Glycine induced calcium concentration changes in vestibular type I sensory cells

Glutamate is the neurotransmitter of the synapse between vestibular type I hair cells and the afferent nerve calyx. This calyx may also be involved in local feedback, which may modify sensory cell activity via N-methyl-D-aspartate (NMDA) receptors. Glycine is the co-agonist of glutamate in NMDA receptor activation. Both agents have been detected by immunocytochemistry in the nerve calyx. Glutamate and NMDA stimulations cause changes in the intracellular calcium concentration ([Ca(2+)](i)) of isolated type I sensory cells. We investigated the effect of glycine stimulation on [Ca(2+)](i) in guinea pig type I sensory cells by spectrofluorimetry with fura-2. Glycine application to isolated type I sensory cells induced a rapid and transient increase in [Ca(2+)](i). The fluorescence ratio increased by 55% above the resting level. The peak was reached in 9 s and the return to basal level took about 20 s. A specific antagonist of the glycine site on NMDA receptors, 7-chlorokynurenate (10 microM), decreased the calcium response to glycine by 60%. Glycine may activate NMDA receptors. Glycine may also activate the strychnine-sensitive glycine receptor-gated channel. Strychnine (50 microM) decreased the calcium response to glycine by 60%. Thus, glycine probably induces calcium concentration changes in type I vestibular sensory cells via NMDA receptors and/or glycine receptors.

Effects of dalargin on excitation induced by L-glutamate agonists in the frog vestibular organs

We have used an electrophysiological approach to investigate the action of a synthetic analog of leu-enkephalin dalargin (DAL) on chemically induced afferent activity in the frog vestibular organs. Administration of 5.0 microM kainic acid (KA), 5.0 microM (AMPA) and 50 microM NMDA produced an increase in the frequency of the resting discharge. Firing evoked by KA, AMPA or NMDA could be depressed by administration of 1 nM Dal by 55.5 +/- 9.9% (n = 10, p < 0.05), 64.5 +/- 11.2% (n = 13, p < 0.05) and 21.3 +/- 11.1% (n = 14, p = 0.051), respectively. Thus, the frequency decrease under NMDA was statistically non-significant. These results show that non-NMDA, but not NMDA subtypes of receptors are mostly involved in opioid action at the vestibular organs of the frog.

Opioid peptides as possible neuromodulators of the afferent synaptic transmission in the frog semicircular canal

Vestibular receptors of the frog, Rana temporaria, were examined for the effect of bath-applied opioid peptide leu-enkephalin, its synthetic analogue dalargin and the specific opiate antagonist naloxone. Multiunit afferent activity of the whole vestibular nerve was recorded in an in vitro preparation. Leu-enkephalin (0.005-100 nM) and dalargin (0.1-100 nM) depress the resting discharge frequency. Naloxone (10 nM-1 microM) antagonizes responses induced by leu-enkephalin and dalargin that suggests a specific action of opioid peptides. Leu-enkephalin and delargin inhibit the excitatory action of L-glutamate. The effects of opioid peptides on L-glutamate-induced responses are unaffected by Co2+ block of transmitter release from hair cells that could speak in favour of the postsynaptic nature of these responses. At the same time, the other possible site of action of opioid peptides, such as efferent system, can not be excluded. The results indicate that opiate receptors are present in hair cells and that the neurotransmitter L-glutamate is involved in opiate action at the peripheral vestibular system of the frog. We suggest that opioid peptides may act as a neuromodulator in this system.

The metabotropic glutamate receptors of the vestibular organs

This research sought to test the presence and function of metabotropic excitatory amino acid receptors (mGluR) in the frog semicircular canal (SCC). The mGluR agonist +/- 1-aminocyclopentane-trans-1,3-dicarboxylate (ACPD) produced an increase in afferent firing rates of the ampullar nerve of the intact posterior canal. This increase was not due to a stimulation of cholinergic efferent terminals or the acetylcholine (ACh) receptor, since atropine, in concentrations which blocked the response to exogenous acetylcholine, did not affect the response to ACPD. Likewise, ACPD effects were not due to stimulation of postsynaptic NMDA receptors, since the NMDA antagonist D(-)-2-amino-5-phosphonopentanoate (AP-5) did not affect the response to ACPD, reinforcing the reported selectivity of ACPD for mGluRs. When the SCC was superfused with artificial perilymph known to inhibit hair cell transmitter release (i.e. low Ca-high Mg), ACPD failed to increase afferent firing. This suggests that the receptor activated by ACPD is located on the hair cell. Pharmacological evidence suggested that the mGluRs involved in afferent facilitation belong to Group I (i.e. subtypes 1 and 5). In fact, the Group III agonist AP-4 had no effect, and the ACPD facilitatory effect was blocked by the Group I mGluR antagonists (S)-4-carboxyphenylglycine (CPG) and (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA). Additional pharmacological evidence supported the presence of Group I mGluRs. Interestingly, the mGluR antagonists, AIDA and 4CPG, by themselves did not affect the resting firing rates of ampullar afferents. This may suggest that the mGluRs are not involved in resting activity but perhaps only in evoked activity (as suggested in Guth et al. (1991) Hear. Res. 56, 69-78). In addition, the mRNA for the mGluR1 has been detected in hair cells of both SCC, utricle, and saccule. In summary, the evidence points to an mGluR localized to the hair cell (i.e. an autoreceptor) which may be activated to produce a positive feedback augmentation of evoked but not resting transmitter release and thus affect afferent activity.

The glutamate receptor subunit delta1 is highly expressed in hair cells of the auditory and vestibular systems

In the inner ear, fast excitatory synaptic transmission is mediated by ionotropic glutamate receptors, including AMPA, kainate, and NMDA receptors. The recently identified delta1 and delta2 glutamate receptors share low homology with the other three types, and no clear response or ligand binding has been obtained from cells transfected with delta alone or in combination with other ionotropic receptors. Studies of mice lacking expression of delta2 show that this subunit plays a crucial role in plasticity of cerebellar glutamatergic synapses. In addition, these mice show a deficit in vestibular compensation. These findings and the nature of glutamatergic synapses between vestibulocochlear hair cells and primary afferent dendrites suggest that delta receptors may be functionally important in the inner ear and prompted us to investigate the expression of delta receptors in the cochlea and peripheral vestibular system. Reverse transcription and DNA amplification by PCR combined with immunocytochemistry and in situ hybridization were used. Our results show that the expression of delta1 in the organ of Corti is intense and restricted to the inner hair cells, whereas delta1 is expressed in all spiral ganglion neurons as well as in their satellite glial cells. In the vestibular end organ, delta1 was highly expressed in both hair cell types and also was expressed in the vestibular ganglion neurons. The prominent expression of delta1 in inner hair cells and in type I and type II vestibular hair cells suggests a functional role in hair cell neurotransmission.

A microcomputer program for automated neuronal spike detection and analysis

A system for on-line spike detection and analysis based on an IBM PC/AT compatible computer, written in TURBO PASCAL 6.0 and using commercially available analog-to-digital hardware is described here. Spikes are detected by an adaptive threshold which varies as a function of signal mean and its variability. Since the threshold value is determined automatically by the signal-to-noise ratio analysis, the user is not actively involved in controlling its level. This program has been reliably used for the detection and analysis of the spike discharge of vestibular system afferent neurons. It generates the interval-joint distribution graph, the interval histogram, the autocorrelation function, the autocorrelation histogram, and phase-space graphs, thus, providing a complete set of graphical and statistical data for the characterization of the dynamics of neuronal spike activity. Data can be exported to other software such as Excel, Sigmaplot and MatLab, for example.

AMPA receptors in cultured vestibular ganglion neurons: detection and activation

The presence and the activity of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) glutamate receptors were investigated in mouse cultured vestibular ganglion neurons using immunocytochemistry and measurement of intracellular calcium concentration ([Ca2+]i) by spectrofluorimetry. Cultures of dissociated vestibular ganglia from 18 gestation day mouse embryos were grown in vitro for 3-4 days. Immunocytochemical labelling of AMPA receptor subunits GluR2/R3 and GluR4 was detected in neuron cell bodies and proximal neurites and more lightly in glial cells. There was no clear selective subcellular localization of the different subunits. For the GluR1 subunit a signal was observed only in some neurons and neurites and was weak. Vestibular ganglion neurons responded to fast application of 1 mM glutamate and 10 mM aspartate through unknown receptors by a transient increase in [Ca2+]i. The mean amplitude of this rapid increase was about nine times the resting level and recovery was complete within 30-45 s after the application. If separated by an interval of at least 10 min, consecutive applications produced similar calcium responses. AMPA (1 mM) application induced the same type of responses. Five minutes prior to the AMPA exposure, the application of a specific AMPA antagonist, 6,7-dinitroquinoxaline-2,3-dione (DNQX, 1.5 mM), in the external medium inhibited the response to AMPA. Chelation of external calcium by EGTA (1.5 mM) abolished the responses to drug applications, indicating that an influx of external calcium is involved in the [Ca2+]i increase. These observations suggest that heteromeric AMPA receptors are expressed in vestibular ganglion neurons in culture and play a functional role in their glutamate-induced depolarization. Experiments are in progress using specific AMPA and NMDA antagonists to characterize the participation of the two types of ionotropic glutamate receptors in the glutamate/aspartate-induced intracellular calcium response.

Antagonism of glutamate receptors by a chromatographic fraction from the exudate of the sea anemone Phyllactis flosculifera

In the search for new glutamate antagonists it seems promising to characterize the effects of venom from invertebrates that prey mainly on crustaceans. In this work, the exudate of the sea anemone Phyllactis flosculifera was used as a source of this type of compound. The action of chromatographic fraction D from P. flosculifera was tested upon microion-tophoretically evoked glutamate responses in intracellular recordings from central neurons of the land snail Zachrysia guanensis. Bath application of fraction D (2-8 mg/ml, n = 13) diminished both the excitatory and the inhibitory components of glutamate agonists in Z. guanensis neurons; this action was dose-dependent and partially reversible. Fraction D actions were also tested in the multiunit spontaneous and mechanically evoked responses of the glutamatergic junction between hair cells and afferent neurons of the axolotl Ambystoma tigrinum. Pressure ejection of fraction D in concentrations ranging from 0.5 to 2 mg/ml (n = 9) decreased the spontaneous and mechanically evoked activity of semicircular canal afferent neurons and the responses evoked by kainic acid and alpha-amino-3-hydroxy-5-methylisoxasole-4-propionic acid. This action was also dose-dependent and partially reversible. These results indicate that fraction D acts as a glutamate receptor antagonist in snail and amphibian neurons. Further studies are required to characterize the active compounds responsible for this action and its specificity upon the subtypes of glutamate receptors.

Cellular and subcellular localization of AMPA-selective glutamate receptors in the mammalian peripheral vestibular system

The cellular and subcellular distribution of AMPA-selective glutamate receptors in the mammalian peripheral vestibular system was examined using antibodies against peptides corresponding to the C-terminal portions of AMPA receptor subunits: GluR1, GluR2/R3 and GluR4. The light and electron microscopic immunocytochemical studies were carried out on Vibratome sections of rat and guinea pig vestibular sensory epithelial and ganglia. In the epithelium, GluR1 subunit immunoreactivity appeared as accumulations of patches outlining the baso-lateral periphery of the type I sensory cells. The GluR1-immunoreactive microareas were postsynaptically distributed on the membranes of calyceal afferent fibers. GluR2/R3 immunoreactivity was present in the sensory cells. GluR4 was not detected. In the vestibular ganglion, the neurons were densely stained with antibodies to GluR2/R3 and GluR4. The fibroblasts and the Schwann cells were also intensely stained with antibodies to GluR2/R3 and GluR4. In the sensory cells, the AMPA receptors, GluR2/R3, may function as (1) autoreceptors controlling afferent neurotransmitter release or (2) 'postsynaptic' receptors activated by the neurotransmitter release of the afferent calyx. The detection of GluR1 at postsynaptic sites in the afferent fibers provides anatomical evidence for the role of glutamate as a neurotransmitter of sensory cells. In the ganglion neurons, GluR2/R3 and GluR4 may represent reserve intracytoplasmic pools of receptor subunits in transit to the postsynaptic sites. In the Schwann cells, GluR2/R3 and GluR4 may be involved in neuronal-glial signalling at the nodes of Ranvier.

Functional support of glutamate as a vestibular hair cell transmitter in an amniote

Although hair cells in the cochlea and in the vestibular endorgans of anamniotes are thought to release glutamate or a similar compound as their transmitter, there is little evidence in amniotes (which, unlike anamniotes, possess both type I and II hair cells) as to the nature of the hair cell transmitters in the vestibular labyrinth. We have recorded extracellularly from single semicircular canal afferents in the turtle labyrinth maintained in vitro and have bath-applied a number of transmitter agonists and antagonists to relate the effects of these substances to the actions of the endogenous transmitter substances. Both glutamate and aspartate strongly excite the afferents while GABA and carbachol have negligible or weak effects. In contrast to its lack of effect on afferent activity in some anamniotes, N-methyl-D-aspartate (NMDA) was also found to excite these afferents. Kynurenic acid reversibly reduced the resting firing rates of the afferents and the increases in firing due to the application of glutamate and aspartate. These findings provide preliminary support for the hypothesis that glutamate (or a related compound) is also a vestibular hair cell transmitter in amniotes.

Influence of ATP and ATP agonists on the physiology of the isolated semicircular canal of the frog (Rana pipiens)

In the present study, the influence of extracellular ATP and ATP agonists in the physiology of the vestibular organs was examined, using the in vitro model of the isolated semicircular canal of the frog (Rana pipiens). The firing activity of the afferent nerve, the d.c. nerve potential and the transepithelial potential were measured in the absence and presence of mechanical stimulation of the sensory epithelium. Administration of ATP into the perilymphatic compartment, from 10(-12) to 10(-3) M, increased the firing rate of the afferent fibers recorded in the absence of mechanical stimulation. Recordings of the d.c. nerve potential indicated that the afferent fibers were hyperpolarized. The presence of the purine also modified the transepithelial potential. During mechanical stimulation of the sensory epithelium, both the evoked afferent firing and the evoked variation of the d.c. nerve potential were reduced in the presence of ATP. However, ATP did not effect the evoked modulation of the transepithelial potential, evoked by the mechanical stimulation. Administration of the P2x purinoceptor agonists, alpha, beta-methylene-ATP and beta, gamma-methylene-ATP, at concentrations between 10(-12) and 10(-3) M, did not significantly modify the different bioelectrical activities investigated. In contrast, 2-methylthio-ATP, a P2y purinoceptor agonist, more potent and efficacious than ATP in its effect on the spontaneous firing. Concurrently, no modification of the d.c. nerve potential, the transepithelial potential and their variation during mechanical stimulation was observed. In opposition to the ATP effect, the total amplitude of the evoked firing was increased in the presence of 2-methylthio-ATP. These data suggest that extracellular ATP, present in the perilymphatic compartment, may act as a neuromodulator in the vestibular physiology. The effects of the purine appear to be mediated by the activation of a P2y subtype of purinoceptor. The absence of an effect of ATP and 2-methylthio-ATP on the evoked variation of the transepithelial potential suggest that the purine did not affect the processes responsible for the generation of the receptor potential but more likely modified the mechanisms involved in the release of the neurotransmitter from the hair cells and/or acted on the afferent endings.

The effect of L-glutamate on the afferent resting activity in the cephalopod statocyst

The effects of bath application of L-glutamate and of excitatory amino acid agonists and antagonists on the resting activity of afferent crista fibers were studied in isolated preparations of the statocyst of the cuttlefish, Sepia officinalis. L-Glutamate (threshold 10(-5) M) and its agonists quisqualate and kainate (thresholds 10(-6) M) increased the resting activity in a dose-dependent manner. Glutamine (threshold 10(-5) M) was also excitatory, while D-glutamate had no effect. Also, no obvious excitatory effects were seen for NMDA and L-aspartate, nor was any antagonistic effect seen for the selective NMDA-receptor antagonist D-2-amino-5-phosphonovaleric acid (D-AP-5). The spider toxin Argiotoxin636 (threshold 10(-11) M), 2-amino-4-phosphonobutyric acid (AP-4), glutamic acid diethyl ester (GDEE), gamma-D-glutamylaminomethyl-sulfonic acid (GAMS), and kynurenic acid decreased the resting activity and effectively blocked or reversed the effect of L-glutamate and its non-NMDA agonists. Preliminary experiments with statocysts from the squid Sepioteuthis lessoniana and the octopod Octopus bimaculoides gave comparable results. All data show that in cephalopod statocysts L-glutamate, via non-NMDA receptors, has an excitatory effect on the activity of afferent fibers, an effect consistent with its possible function as a hair cell transmitter.

Evidence for NMDA receptor in the afferent synaptic transmission of the vestibular system

This study aimed to define the pharmacology and physiological role of the N-methyl-D-aspartate (NMDA) receptor in the synapse between the hair cells and primary afferent neurons in the vestibular system. The spontaneous and mechanically evoked spike discharges of vestibular nerve fibers were extracellularly recorded in isolated inner ear from the axolotl (Ambystoma tigrinum). Pressure ejection of NMDA (10(-6) to 10(-3) M) elicited a dose-dependent increase of the basal spike discharge from the vestibular nerve fibers. Extracellular magnesium antagonized the NMDA effect in a dose-dependent manner. D(-)-2-amino-5-phosphonovaleric acid (AP5, 10(-5) to 10(-3) M) and 7-chloro-kynurenic acid (7ClKyn, 10(-6) to 10(-3) M) inhibited the basal activity of the vestibular nerve fibers. 7ClKyn also diminished the responses elicited by the mechanical stimulation of the preparation. Glycine (10(-9) to 10(-6) M) applied by bath substitution enhanced the NMDA response, and the glycine agonist D-serine partially reversed the 7ClKyn inhibitory action. These results suggest that NMDA receptors participate in the generation of the basal spike discharge of vestibular system primary afferent neurons, but its activation is not critical for the response to brief mechanical stimuli.

Glutamate receptors on type I vestibular hair cells of guinea-pig

Afferent nerve calyces which surround type I vestibular hair cells (VHCI) have recently been shown to contain synaptic-like vesicles and to be immunoreactive to glutamate antibodies. In order to understand the physiological significance of these observations, the presence of glutamate receptors on type I vestibular sensory cells has been investigated. The effect of excitatory amino acids applied by iontophoresis was examined by spectrofluorimetry using fura-2 sensitive dye. Glutamate application caused a rapid and transient increase in intracellular calcium concentration ([Ca2+]i), in a dose-dependent manner. The ionotropic glutamate receptors agonists N-methyl-D-aspartic acid (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and quisqualic acid (QA) induced an increase of [Ca2+]i. The NMDA receptor antagonist 2-amino-5-phosphonovaleric acid and the AMPA receptor antagonist 6,7-dinitroquinoxaline-2,3-dione partially blocked the glutamate response, by 39 +/- 10 and 53 +/- 11% respectively. Metabotropic receptors were also revealed by the specific agonist trans-1-amino-cyclopentyl-1,3-dicarboxylate. The presence of different glutamate receptors on the VHCI membrane suggests two kinds of feedback. (i) At the base of the sensory cell, autoreceptors may locally control the synaptic transmission. (ii) At the apex, postsynaptic receptors may modulate sensory transduction from glutamate release at the upper part of the afferent nerve calyx. These feedbacks suggest presynaptic modulation of the vestibular hair cell response which could affect its sensitivity.

Glutamate, of the Endogenous Primary ?-Amino Acids, Is Specifically Released from Hair Cells by Elevated Extracellular Potassium

A hair cell (octavolateralis mechanoreceptor cell) sheet preparation from the trout saccular macula was superfused with bicarbonate-based physiological saline. Among the primary amine-containing compounds resolved by cation-exchange HPLC, glutamate alone was released in a statistically significant manner with elevation of extracellular [K+] from 3.5 to 14 mM in the presence of 1.8 mM calcium. Release of glutamate averaged 10.9 +/- 2.5 pmol (mean +/- SEM) over a 10-min period for a hair cell sheet preparation representing 20 micrograms of cell protein. No potassium-evoked release of glutamate was observed in 0 mM calcium/10 mM magnesium saline, suggesting calcium dependency. Because the sheet preparation, by the method of its isolation, contained only the hair cell as the intact cell type, release of glutamate, induced by relatively small increases in extracellular potassium, can be attributed directly to the receptor cell. The specific release of glutamate and its block by magnesium are consistent with the hypothesis that glutamate is one neurotransmitter/neuromodulator mediating receptoneural transmission in the octavolateralis periphery.

Streptomycin blocks the afferent synapse of the isolated semicircular canals of the frog

This study aimed to define the acute electrophysiological effects of the perilymphatic perfusion of streptomycin in the sensory apparatus of the semicircular canals of the frog. The ampullary DC potential, the vestibular nerve multiunit discharge, the nerve DC potential and the unitary EPSP activity were recorded in isolated semicircular canals of the frog (Rana esculenta L). The results demonstrated that perilymphatic microperfusion of streptomycin (0.1, 0.3, 1 and 3 mM) reduced both resting and mechanically evoked afferent discharge, while the response of the hair cells remains unchanged. Intracellular recordings from single afferent axons showed that the reduction of the afferent discharge was mainly due to a reduction of the amplitude, but not the frequency, of the EPSPs. These results indicate that streptomycin, when applied in the fluid bathing the synaptic pole of the sensory cells, can act as an antagonist of the vestibular afferent transmitter at the postsynaptic level.

Differential modulation of spontaneous and evoked neurotransmitter release from hair cells: some novel hypotheses

It has been generally accepted that even in the absence of mechanical stimulation of the transductional elements, a resting depolarizing current exists which is ultimately responsible for the spontaneous release of neurotransmitter. Movement of the transductional elements modulates this resting current and thereby the evoked release of neurotransmitter occurs. Recent data from our laboratory and others have led us to question whether the relationship between spontaneous and evoked neurotransmitter release is as simple as stated. Indeed, a variety of experimental manipulations appear to influence the two modes of release differently. Examination of our results and the results of others has led us to four hypotheses: 1. the two modes of neurotransmitter release are processed differently by the hair cells; 2. cyclic AMP is involved in spontaneous but not evoked neurotransmitter release; 3. there is a positive feedback step involving an excitatory amino acid and its receptor on the hair cell in evoked neurotransmitter release and; 4. different pools of calcium are involved according to the mode of release. Accordingly, there may be several biochemical steps between the transductional movement of the stereocilia at the apex of the hair cells and the ultimate release of the neurotransmitter at the base of these cells. Some of these biochemical steps are different depending on whether the mode of release is spontaneous or evoked. These biochemical steps may amplify or at least interact with the biophysical processes previously described in the hair cells.

Streptomycin blocks the postsynaptic effects of excitatory amino acids on the vestibular system primary afferents

It has been suggested that streptomycin might be an antagonist of the glutamate receptors, and that it selectively blocks quisqualic acid receptors. We studied whether streptomycin blocks the responses to excitatory amino acid agonists on the vestibular system primary afferents, and if it allows us to differentiate between kainate (KA) and quisqualate (QA) receptor mediated responses. The experiments were performed in the axolotl (Ambystoma tigrinum). Intra- and extracellular records of the electrical activity of semicircular canal afferent fibers were obtained. Drugs were applied by pressure ejection in volumes of 20 microliters in a 10 ml bath. Streptomycin (0.01-10 mM), induced a dose dependent reversible inhibition of the basal spike discharge of the afferent fibers. This coincided with a reduction in the amplitude of excitatory postsynaptic potentials (EPSP) recorded intracellularly in the afferent fibers. Streptomycin also blocked the excitatory action produced by KA and QA; increasing concentrations of streptomycin produced a rightward shift in the concentration-response curves for both KA and QA. This action persisted even in a high Mg2+ (10 mM), low Ca2+ (0.09 mM) Ringer solution, indicating its postsynaptic nature. These results show that streptomycin might be a non-selective excitatory amino acid (EAA) receptor antagonist.

Effects of excitatory amino acid antagonists on synaptic transmission in the ampullae of Lorenzini of the skate Raja clavata

1. The effects of excitatory amino acid antagonists on synaptic transmission in the ampullae of Lorenzini of the skate Raja clavata were studied. 2. At concentrations of 10(-3) to 10(-6) M. L-glutamic acid diethylester (GDEE) and L-glutamic acid dimethylester (GDME) decreased the resting afferent discharge frequency as well as the electrically evoked activities and depressed the responses to application of excitatory amino acids. 3. D-alpha-Aminoadipic acid (AA) and 2-amino-4 phosphonobutyric acid (APB) had practically no effect either on resting afferent discharge or on evoked afferent activity. 4. 2-Amino-5-phosphonovaleric acid (APV) reduced the resting afferent discharge and electrically evoked activity in the afferent fibres. APV blocked N-methyl-D-aspartate (NMDA) induced responses at a lower concentration than those induced by aspartate,. 5. Responses caused by NMDA were Mg 1(2+)-dependent; those to quisqualate (Q) application depended to a lesser degree on the Mg2+ concentration. 6. Cis-2,-3-piperidinedicarboxylic acid (PDA) blocked both the resting afferent discharge and the electrically evoked activity; it also reversibly blocked the postsynaptic, amino acid-induced responses. The L-glutamate (L-GLU) response was more resistant to the blockade than the L-aspartate (L-ASP) response. 7. Both kynurenic acid (KEN) and gamma-D-glutamylglycine (DGG) reduced the resting and evoked activities in the afferent fibres. L-ASP was more resistant to blockade by KEN than L-GLU. 8. These observations suggest a prominent physiological role of quisqualate-, kainate- and NMDA-preferring receptors in the ampullae of Lorenzini of Raja clavata.

Identification and localization of a kainate binding protein in the frog inner ear by electron microscopy immunocytochemistry

A kainate binding protein (KBP) was studied in Rana pipiens inner ear using monoclonal and polyclonal antibodies against affinity purified KBP from frog brain. The KBP identified and analyzed in inner ear tissue homogenates, with one- and two-dimensional immunoblots, was similar to the affinity purified KBP and to the antibody-identified frog brain KBP. As brain KBP, inner ear KBP had 5 main components in the molecular weight dimension, centered at Mr = 48,000; however, inner ear KBP had a greater abundance of the higher molecular weight components. Light and electron microscopy observations showed KBP immunostaining at two locations: (1) in the dendrites of the eight nerve afferent fibers contacting sensory hair cells, with the postsynaptic density being more intensely stained; and (2) on the cytoplasmic membrane of fibroblasts present in the inner ear connective tissue which displayed intense immunostaining. The presence of kainate (KA) binding sites in the inner ear was assessed using in vitro receptor autoradiography. [3H]KA binding sites were found in connective tissue areas confirming the immunocytochemistry results. The postsynaptic localization of the KBP in afferent endings, strongly supports it as being a component of the KA receptor complex. However, its presence on fibroblasts situated in the inner ear connective tissue makes its function hypothetical. The dual presence of the KBP on non-neuronal cells as well as at postsynaptic membrane sites suggests the existence of a family of proteins involved in KA binding and KA receptors with a complex organization.

Pre- and postsynaptic excitatory action of glutamate agonists on frog vestibular receptors

In order to investigate the localization and the type(s) of excitatory amino acid receptors in the frog vestibular system, the exogenous amino acid agonists Quisqualic acid, Kainic acid and N-methyl-D-aspartic acid were tested on the sensory organ of semicircular canals. Intracellular recordings of the resting discharge from single afferents showed that these agonists exerted a complex excitatory action consisting in a rapid and brief increase in frequency of both EPSPs and spikes, followed by a slower and longer lasting membrane depolarization. The progressive impairment of natural transmitter release achieved by adding Mg2+ or Co2+ in the bath caused a dose-dependent decrease of the agonist-induced afferent discharge, without substantially affecting axonal depolarization. These results suggest that the exogenous amino acid agonists act both pre- and postsynaptically on the vestibular organs. Quisqualic acid and kainic acid were much more potent than N-methyl-D-aspartic acid in inducing excitatory effects, suggesting that the amino acid receptors located on both hair cells and afferent endings are mainly of the non-NMDA type. The present findings, while not excluding that an excitatory amino acid may be the afferent transmitter, highlight its possible function as a presynaptic modulator of the afferent transmission in the frog vestibular system.

Argiotoxin-636 blocks effects of N-methyl-D-aspartate on lateral line of Xenopus laevis at concentrations which do not alter spontaneous or evoked neural activity

Activity of primary afferent neurons in acousticolateralis organs can be modulated by excitatory amino acids (EAA) and their antagonists, and EAA are among the better candidates for the transmitter(s) between hair cells and afferent neurons. Argiotoxin-636 (ATX) is a spider venom toxin that is a selective antagonist of the N-methyl-D-aspartate (NMDA) subtype of excitatory amino acid receptors in vertebrates. In the present study, the effects of NMDA alone and in combination with ATX on resting firing rate (spontaneous activity) and mechanically stimulated activity of lateral line afferent neurons of post-metamorphic Xenopus laevis were compared. Perfusion of NMDA (100 or 200 microM) typically produced a biphasic effect on spontaneous activity consisting of a transient increase followed by a decrease in firing rate. Mechanical stimulation applied after the initial response to NMDA produced an increase in firing activity that was not significantly different from the increase in firing activity evoked in the absence of NMDA. ATX at 1-2 microM did not alter either spontaneous or evoked activity, but blocked in a reversible manner both the excitatory and inhibitory actions of NMDA on spontaneous activity. These results suggest the presence of an NMDA receptor in the lateral line but do not support the hypothesis that the generation of action potentials by the afferent transmitter is solely dependent on activation of postsynaptic NMDA receptor-ion channels.

Voltage-gated and chemically gated ionic channels in the cultured cochlear ganglion neurone of the chick

1. Electrophysiological properties of ionic channels of isolated or cultured cochlear ganglion (CG) neurones from chick embryo were studied under voltage-clamp conditions using a patch electrode. 2. Tetrodotoxin-sensitive Na+ current was activated by a step depolarization more positive than -40 mV, and was inactivated rapidly. 3. Outward-going K+ current was activated by step depolarization to membrane potentials more positive than -62 mV. 4. Two types of Ca2+ currents were demonstrated, an inactivating and a non-inactivating type. The inactivating type was activated by step depolarizations more positive than -69 mV and was inactivated rapidly. The non-inactivating type was activated by step depolarizations more positive than -52 or -41 mV depending on the external divalent cation species. 5. The I-V relationship and the activation kinetics of the non-inactivating type Ca2+ channel was shifted in a positive direction along the voltage axis by 12 mV when extracellular 2.5 mM-Sr2+ or Ba2+ were replaced by Ca2+. This shift was not observed in the inactivating type Ca2+ channel. 6. The amplitude of peak current through the inactivating type Ca2+ channel was in the order of Ca2+ greater than Sr2+ greater than Ba2+. The order of relative permeability through the non-inactivating type estimated from the tail current amplitude was Ba2+ greater than Sr2+ greater than Ca2+. 7. After 5 days in culture, glutamate (30 microM), aspartate (100 microM), kainate (100 microM) and N-methyl-D-aspartic acid (NMDA; 100 microM) elicited ionic currents. The glutamate response was depressed by 1 mM-Mg2+ in a voltage-dependent manner at negative membrane potentials and was almost extinguished by amino-phosphonovalerate (APV) (0.1 mM). The major subtype of glutamate receptor could be of the NMDA type. 8. The permeability of the NMDA receptor channel to Na+ and Li+ was estimated from the reversal potential and was 1.0 and 0.7 compared with that of Cs+, respectively. 9. Divalent cations were more permeable than the monovalent cations through the NMDA receptor channel: PCa greater than or equal to PBa greater than PSr greater than PCs.