The selective AMPA receptor antagonist GYKI 53784 blocks action potential generation and excitotoxicity in the guinea pig cochlea

Ryanodine receptors and BK channels act as a presynaptic depressor of neurotransmission in cochlear inner hair cells

Ryanodine receptors (RyRs) are known to contribute to the regulation of free cytosolic calcium concentration. This family of intracellular calcium channels plays a significant role in calcium-induced-calcium-release (CICR), and have been implicated in calcium-dependent processes requiring exquisite spatio-temporal regulation. In order to characterize the importance of these intracellular calcium channels in cochlear physiology, we perfused the guinea pig cochlea with antagonistic concentrations of ryanodine. The distortion products of the cochlear microphonic and the compound action potential of the auditory nerve were reversibly inhibited by ryanodine (IC(50)=27.3 microm, Hill coefficient=1.9), indicating an action at the cochlear amplifier. Single auditory nerve fibre recordings showed that ryanodine slightly increased spontaneous firing rates by 22%, suggesting an excitatory effect of ryanodine. This paradoxical effect could be explained by an inhibitory action of ryanodine on presynaptic BK channels of inner hair cells (IHC). Indeed, perfusing iberiotoxin also increased the spontaneous firing activity of the auditory nerve fibres. Furthermore, whole-cell patch-clamp recordings demonstrated that ryanodine inhibits BK currents at the IHC level. Conversely, immunohistochemistry demonstrated a strong expression of RyR in IHCs and, more particularly, below the cuticular plate where membranous BK channels are highly expressed. Overall, the study demonstrated a key role for RyR and CICR in signal transduction at the IHCs. We therefore propose that coupled RyR--BK channels act to suppress the fast neurotransmission in IHCs.

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

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

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.

Transient Ca2+-permeable AMPA receptors in postnatal rat primary auditory neurons

Fast excitatory transmission in the nervous system is mostly mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors whose subunit composition governs physiological characteristics such as ligand affinity and ion conductance properties. Here, we report that AMPA receptors at inner hair cell (IHC) synapses lack the GluR2 subunit and are transiently Ca2+-permeable before hearing onset as evidenced using agonist-induced Co2+ accumulation, Western blots and GluR2 confocal microscopy in the rat cochlea. AMPA (100 microM) induced Co2+ accumulation in primary auditory neurons until postnatal day (PND) 10. This accumulation was concentration-dependent, strengthened by cyclothiazide (50 microM) and blocked by GYKI 52466 (80 microM) and Joro spider toxin (1 microM). It was unaffected by D-AP5 (50 microM), and it could not be elicited by 56 mM K+ or 1 mM NMDA + 10 microM glycine. Western blots showed that GluR1 immunoreactivity, present in homogenates of immature cochleas, had disappeared by PND12. GluR2 immunoreactivity was not detected until PND10 and GluR3 and GluR4 immunoreactivities were detected at all the ages examined. Confocal microscopy confirmed that the GluR2 immunofluorescence was not located postsynaptically to IHCs before PND10. In conclusion, AMPA receptors on maturing primary auditory neurons differ from those on adult neurons. They are probably composed of GluR1, GluR3 and GluR4 subunits and have a high Ca2+ permeability. The postsynaptic expression of GluR2 subunits may be continuously regulated by the presynaptic activity allowing for variations in the Ca2+ permeability and physiological properties of the receptor.

Chronic excitotoxicity in the guinea pig cochlea induces temporary functional deficits without disrupting otoacoustic emissions

Brief cochlear excitotoxicity produces temporary neural swelling and transient deficits in auditory sensitivity; however, the consequences of long-lasting excitotoxic insult have not been tested. Chronic intra-cochlear infusion of the glutamate agonist AMPA (a-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) resulted in functional deficits in the sound-evoked auditory brainstem response, as well as in behavioral measures of hearing. The electrophysiological deficits were similar to those observed following acute infusion of AMPA into the cochlea; however, the concentration-response curve was significantly shifted as a consequence of the slower infusion rate used with chronic cochlear administration. As observed following acute excitotoxic insult, complete functional recovery was evident within 7 days of discontinuing the AMPA infusion. Distortion product otoacoustic emissions were not affected by chronic AMPA infusion, suggesting that trauma to outer hair cells did not contribute to AMPA-induced deficits in acoustic sensitivity. Results from the current experiment address the permanence of deficits induced by chronic (14 day) excitotoxic insult as well as deficits in psychophysical detection of longer duration acoustic signals.

In vitro reconstitution of signal transmission from a hair cell to the growth cone of a chick vestibular ganglion cell

Signal transmission from a chick hair cell to the growth cone of a vestibular ganglion cell was examined by placing an acutely dissociated hair cell on the growth cone of a cultured vestibular ganglion cell. Electrical stimuli were applied to the hair cell while monitoring the intracellular Ca(2+) concentration ([Ca(2+)](i)) at the growth cone or recording whole-cell currents from the vestibular ganglion cell. Electrical stimulation of the hair cell induced [Ca(2+)](i) increases at the growth cone and inward currents in the vestibular ganglion cell. The [Ca(2+)](i) increase was blocked by 6-cyano-7-nitroquinoxaline (CNQX) (10 microM) but not by 2-amino-5-phosphonovaleric acid (APV; 50 microM). Glutamate (100 nM-300 microM) applied to the vestibular ganglion cell by the Y-tube method induced inward currents which were also antagonized by CNQX, but not by APV. These results indicate that the electrical stimulation of a hair cell induced glutamate or glutamate like agent release from the hair cell, which activated non-N-methyl-D-aspartate receptors at the growth cone of the vestibular ganglion cell, followed by action potentials and [Ca(2+)](i) elevation in the vestibular ganglion cell. This is the first demonstration of in vitro reconstitution of functional signal transmission from a hair cell to a vestibular ganglion cell.

The afferent synapse of cochlear hair cells

Mechanosensory hair cells of the cochlea must serve as both transducers and presynaptic terminals, precisely releasing neurotransmitter to encode acoustic signals for the postsynaptic afferent neuron. Remarkably, each inner hair cell serves as the sole input for 10-30 individual afferent neurons, which requires extraordinary precision and reliability from the synaptic ribbons that marshal vesicular release onto each afferent. Recent studies of hair cell membrane capacitance and postsynaptic currents suggest that the synaptic ribbon may operate by simultaneous multi-vesicular release. This mechanism could serve to ensure the accurate timing of transmission, and further challenges our understanding of this synaptic nano-machine.

Salicylate induces tinnitus through activation of cochlear NMDA receptors

Salicylate, the active component of aspirin, is known to induce tinnitus. However, the site and the mechanism of generation of tinnitus induced by salicylate remains unclear. Here, we developed a behavioral procedure to measure tinnitus in rats. The behavioral model was based on an active avoidance paradigm in which rats had to display a motor task (i.e., to jump on a climbing pole when hearing a sound). Giving salicylate led to a decrease in the percentage of correct responses (score) and a drastic increase in the number of false positive responses (i.e., animals execute the motor task during a silent period). Presentation of the sound at a constant perceptive level prevents decrease of the score, leading to the proposal that score is related to hearing performance. In contrast, the increase of false positive responses remained unchanged. In fact, animals behaved as if they hear a sound, indicating that they are experiencing tinnitus. Mefenamate in place of salicylate also increased the number of false positive responses, suggesting that salicylate-induced tinnitus is related to an inhibition of cyclooxygenase. One physiological basis of salicylate ototoxicity is likely to originate from altered arachidonic acid metabolism. Because arachidonic acid potentiates NMDA receptor currents, we tested the involvement of cochlear NMDA receptors in the occurrence of tinnitus. Application of NMDA antagonists into the perilymphatic fluids of the cochlea blocked the increase in pole-jumping behavior induced by salicylate, suggesting that salicylate induces tinnitus through activation of cochlear NMDA receptors.

Glutamate transporters in the guinea-pig cochlea: partial mRNA sequences, cellular expression and functional implications

In the cochlea, glutamate plays a major role in synaptic transmission between the inner hair cell and the primary auditory neurons. Extracellular glutamate concentration must be regulated to prevent excitotoxicity. This regulation is mediated by excitatory amino acid transporters, membrane proteins that remove glutamate from the synaptic cleft. In this study, we investigated the distribution and activity of three excitatory amino acid transporters subtypes in the guinea-pig cochlea: glutamate aspartate transporter, glutamate transporter and excitatory amino acid carrier. A partial messenger ribonucleic acid sequence was determined for each of these transporters, by polymerase chain reaction with degenerate primers, using guinea-pig brain complementary deoxyribonucleic acid as the template. Primers specific for each transporter were then designed and used to screen a dissected organ of Corti complementary deoxyribonucleic acid library. The cellular distribution of each transporter was examined by immunocytochemistry. We investigated the functional consequences of inhibiting glutamate uptake by recording cochlear potentials during intracochlear perfusion with either l-trans-pyrrolidine-2,4-dicarboxylic acid or dihydrokainate. At the end of the electrophysiological session, cochleas were processed for electron microscopy. Only the glutamate aspartate transporter messenger ribonucleic acid was detected in the organ of Corti. Consistently, glutamate aspartate transporter protein was detected in the inner hair cell-supporting cells and in the ganglion of Corti satellite cells. Glutamate transporter and excitatory amino acid carrier were found in the afferent auditory neurons. Only intracochlear perfusions with l-trans-pyrrolidine-2,4-dicarboxylic acid resulted in a dose-dependent decrease in the amplitude of the cochlear compound action potential, leaving cochlear microphonic potential unaffected. After l-trans-pyrrolidine-2,4-dicarboxylic acid perfusion, cochleas displayed a swelling of the afferent endings typical of excitotoxicity. [(-)1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-4,5-dihydro-3-methylcarbamyl-2,3-benzodiazepine], a selective alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor antagonist protects the cochlea against l-trans-pyrrolidine-2,4-dicarboxylic acid effect.

Negative allosteric modulation of AMPA-preferring receptors by the selective isomer GYKI 53784 (LY303070), a specific non-competitive AMPA antagonist

GYKI 53784 or LY303070 [(-)1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-4,5-dihydro-3-methylcarbamoyl-2,3-benzodiazepine] belongs to a new family of 2,3-benzodiazepine compounds (also called homophtalazines) selective and non-competitive antagonists at alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) receptors. These compounds include the original GYKI-52466, its more potent derivative GYKI 53655 and the active isomer of the latter, GYKI 53784. This review summarizes current knowledge of this novel AMPA antagonist: GYKI 53784. GYKI 53784 is the most potent of the compounds in the 2,3-benzodiazepine class, blocking AMPA receptor-mediated responses. In contrast to the compounds of the quinoxalinedione family, that block AMPA as well as kainate receptors, GYKI 53784 does not block the activation of kainate receptors. Furthermore, GYKI 53784 does not act at the same receptor site as positive AMPA modulators (i.e., cyclothiazide, BDP-12, 1-BCP or aniracetam). GYKI 53784 is a powerful neuroprotective agent in both in vitro and in vivo models of AMPA receptor-mediated excitotoxicity. In contrast to NMDA receptor antagonists, whose favorable clinical actions are compromised by important side effects such as the impairment of memory functions, the selective AMPA antagonist, GYKI 53784, may be of potential clinical value, both in acute (stroke and trauma) and chronic (Alzheimer's disease, epilepsy) neurological disorders.

Caffeine and ryanodine demonstrate a role for the ryanodine receptor in the organ of Corti

The hypothesis that the release of Ca(2+) from ryanodine receptor activated Ca(2+) stores in vivo can affect the function of the cochlea was tested by examining the effects of caffeine (1-10 mM) and ryanodine (1-333 microM), two drugs that release Ca(2+) from these intracellular stores. The drugs were infused into the perilymph compartment of the guinea pig cochlea while sound (10 kHz) evoked cochlear potentials and distortion product otoacoustic emissions (DPOAEs; 2f(1)-f(2)=8 kHz, f(2)=12 kHz) were monitored. Caffeine significantly suppressed the compound action potential of the auditory nerve (CAP) at low intensity (56 dB SPL; 3.3 and 10 mM) and high intensity (92 dB SPL; 10 mM), increased N1 latency at high and low intensity (3 and 10 mM) and suppressed low intensity summating potential (SP; 10 mM) without an effect on high intensity SP. Ryanodine significantly suppressed the CAP at low intensity (100 and 333 microM) and at high intensity (333 microM), increased N1 latency at low intensity (33, 100 and 333 microM) and at high intensity (333 microM) and suppressed low intensity SP (100 and 333 microM) and increased high intensity SP (333 microM). The cochlear microphonic (CM) evoked by 10 kHz tone bursts was not affected by caffeine at high or low intensity, and ryanodine had no effect on it at low intensity but decreased it at high intensity (10, 33, 100 and 333 microM). In contrast, caffeine (10 mM) and ryanodine (33 and 100 microM) significantly increased CM evoked by l kHz tone bursts and recorded from the round window. Caffeine (10 mM) and ryanodine (100 microM) reversibly suppressed the cubic DPOAEs evoked by low intensity primaries. Overall, low intensity evoked responses were more sensitive and were suppressed to a greater extent by both drugs. This is consistent with the hypothesis that release of Ca(2+) from ryanodine receptor Ca(2+) stores, possibly in outer hair cells and supporting cells, affects the function of the cochlear amplifier.

Role of L-type Ca2+ channels in transmitter release from mammalian inner hair cells. II. Single-neuron activity

Previously reported changes in the gross sound-evoked cochlear potentials after intracochlear perfusion of nimodipine suggest that dihydropyridine-sensitive Ca2+ channels (L-type) control the sound-evoked release of transmitter from the inner hair cells of the mammalian cochlea. In the present study, we combined recording of the action potentials of single primary auditory afferent neurons with intracochlear perfusion to further investigate the role of voltage-gated Ca2+ channels at this synapse. Spontaneous action potential firing rates were depressed by the L-type channel blocker nimodipine, but were elevated by S(-) BAY K8644, an L-type channel agonist. Sound-evoked responses of single primary afferents were depressed by nimodipine in a manner that was consistent with a block at the inner hair cell-afferent dendrite synapse. Perfusions with solutions containing the N-type channel blocker conotoxin GVIA did not differ in their effects from control artificial perilymph perfusions. The results extend the conclusions of the earlier study by showing that L-type Ca2+ channels are primarily responsible for controlling both spontaneous and sound-evoked transmitter release from inner hair cells. In addition it was found that afferent neurons with widely different spontaneous firing rates were all sensitive to nimodipine and to BAY K8644, suggesting that the multiple synaptic outputs of each inner hair cell are under the control of only one major type of Ca2+ channel.

The inner hair cell synaptic complex: physiology, pharmacology and new therapeutic strategies

Within the cochlea, the sensory inner hair cells (IHCs), which transduce mechanical displacement of the basilar membrane into neural activity, release glutamate to act on postsynaptic receptor channels located on dendrites of primary auditory neurons. In turn the activity of the postsynaptic auditory dendrites is modulated by a variety of lateral efferent neurotransmitters. This presentation reviews the most recent findings obtained at the IHC synaptic complex with an original technique, namely coupling auditory nerve single unit recordings with multibarrel intracochlear perfusions. Two types of results are emphasized: (1) in physiological conditions, the activity of auditory nerve fibers involves AMPA, but not kainate or NMDA receptors, and (2) this activity is tonically modulated by dopamine, one of the lateral efferent neurotransmitters. With the increasing knowledge of molecular mechanisms involved at the first synaptic complex in the cochlea, it is now possible to envisage local treatments for spiral ganglion neurons. These treatments, available experimentally, may be used in the near future: either to protect spiral ganglion neurons against excitotoxic injury (traumatic and/or ischemic sudden deafness), or to prevent excitotoxic-induced hyperexcitability (probably the starting point of most posttraumatic tinnitus), or to delay neuronal death (neural presbycusis).

Dopamine inhibition of auditory nerve activity in the adult mammalian cochlea

Efferent feedback systems provide a means for modulating the input to the central nervous system. The lateral olivocochlear efferents modulate auditory nerve activity via synapses with afferent dendrites below sensory inner hair cells. We examined the effects of dopamine, one of the lateral olivocochlear neurotransmitters, by recording compound and single unit activity from the auditory nerve in adult guinea pigs. Intracochlear application of dopamine reduced the compound action potential (CAP) of the auditory nerve, increased the thresholds and decreased the spontaneous and driven discharge rates of the single unit fibres without changing their frequency-tuning properties. Surprisingly, dopamine antagonists SCH-23390 and eticlopride decreased CAP amplitude as did dopamine. In some units, both SCH-23390 and eticlopride increased the basal activity of auditory nerve fibres leading to an improvement of threshold sensitivity and a decrease of the maximum driven discharge rates to sound. In other units, the increase in firing rate was immediately followed by a marked reduction to values below predrug rates. Because CAP reflects the summed activity of auditory nerve fibres discharging in synchrony, both the decrease in sound-driven discharge rate and the postexcitatory reduction account for the reduction in CAP. Ultrastructural examination of the cochleas perfused with eticlopride showed that some of the afferent dendrites were swollen, suggesting that the marked reduction in firing rate may reflect early signs of excitotoxicity. Results suggest that dopamine may exert a tonic inhibition of the auditory nerve activity. Removal of this tonic inhibition results in the development of early signs of excitotoxicity.

PPADS, an ATP antagonist, attenuates the effects of a moderately intense sound on cochlear mechanics

Increasing attention is being given to the role of neurotransmitters and other signaling substances in the damage induced by intense sound to the cochlea. Adenosine triphosphate (ATP) is one example of a putative neurotransmitter that may alter cochlear mechanics during sound exposure. The purpose of the present study was to test the hypothesis that endogenous extracellular ATP has a role in the generation of the changes in cochlear mechanics induced by moderate intense sound exposure. Guinea pigs were exposed to either: (1) a perilymphatic administration of pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 1 mM), an ATP antagonist; (2) a moderately intense sound (6 kHz tone, 95 dB SPL, 15 min); or (3) a combination of the PPADS and the sound. The effects on the cubic distortion product otoacoustic emissions (DPOAEs; 2f1-f2) were monitored using three sets of equal level primaries (f1=9.25 kHz, f2=10.8 kHz, 2f1-f2=7.7 kHz; f1=7.2 kHz, f2=8.4 kHz, 2f1-f2=6 kHz; f1=5.55 kHz, f2=6.5 kHz, 2f1-f2=4.6 kHz). PPADS alone had no effect on the cubic DPOAEs monitored. The intense sound alone suppressed all three cubic DPOAEs. The combination of PPADS with the intense sound induced a suppression of the cubic DPOAEs that was equal to or greater than induced by the intense sound alone at f2=10.8 kHz but was equal to or less than induced by the intense sound at f2=8.4 and 6.5 kHz. After washing the PPADS out of the cochlea with artificial perilymph, all three cubic DPOAEs were suppressed less in the PPADS with intense sound treatment group than in the intense sound alone group. The PPADS appeared to provide protection from the intense sound. Results are consistent with the hypothesis that extracellular ATP is involved in the changes in cochlear mechanics induced by moderately intense sound exposure.