The effects of kainic acid on the cochlear ganglion of the rat

Amitriptyline protects afferent synapses in the cochlea against excitotoxic trauma in vitro

Afferent synapses between inner hair cells (IHCs) and the type I spiral ganglion neurons (SGNs) in the cochlea provide over 95% of sensory signals for auditory perception in the brain. However, these afferent synapses are particularly vulnerable to damage, for example from excitotoxicity, and exposure to noise in the environment which often leads to noise-induced cochlear synaptopathy (NICS). In this study, we simulated excitotoxic trauma by incubating kainic acid, a non-desensitizing agonist for AMPA type glutamate receptors on cultured cochleae. The possible protective effects of amitriptyline against NICS were examined. We found that, in IHCs, amitriptyline reversed the decrease of Ca2+ current and exocytosis caused by excitotoxic trauma. In SGNs, amitriptyline promoted the recovery of neurite loss caused by excitotoxic trauma. Furthermore, we found that the protective effects of amitriptyline are likely mediated by suppressing apoptosis factors that were upregulated during excitotoxic trauma. In conclusion, our results suggest that amitriptyline could protect afferent synapses in the cochlea from NICS, making it a potential drug candidate for hearing protection.

Oxidative Stress Plays an Important Role in Glutamatergic Excitotoxicity-Induced Cochlear Synaptopathy: Implication for Therapeutic Molecules Screening

The disruption of the synaptic connection between the sensory inner hair cells (IHCs) and the auditory nerve fiber terminals of the type I spiral ganglion neurons (SGN) has been observed early in several auditory pathologies (e.g., noise-induced or ototoxic drug-induced or age-related hearing loss). It has been suggested that glutamate excitotoxicity may be an inciting element in the degenerative cascade observed in these pathological cochlear conditions. Moreover, oxidative damage induced by free hydroxyl radicals and nitric oxide may dramatically enhance cochlear damage induced by glutamate excitotoxicity. To investigate the underlying molecular mechanisms involved in cochlear excitotoxicity, we examined the molecular basis responsible for kainic acid (KA, a full agonist of AMPA/KA-preferring glutamate receptors)-induced IHC synapse loss and degeneration of the terminals of the type I spiral ganglion afferent neurons using a cochlear explant culture from P3 mouse pups. Our results demonstrated that disruption of the synaptic connection between IHCs and SGNs induced increased levels of oxidative stress, as well as altered both mitochondrial function and neurotrophin signaling pathways. Additionally, the application of exogenous antioxidants and neurotrophins (NT3, BDNF, and small molecule TrkB agonists) clearly increases synaptogenesis. These results suggest that understanding the molecular pathways involved in cochlear excitotoxicity is of crucial importance for the future clinical trials of drug interventions for auditory synaptopathies.

Normal behavioral discrimination of envelope statistics in budgerigars with kainate-induced cochlear synaptopathy

Cochlear synaptopathy is a common pathology in humans associated with aging and potentially sound overexposure. Synaptopathy is widely expected to cause "hidden hearing loss," including difficulty perceiving speech in noise, but support for this hypothesis is controversial. Here in budgerigars (Melopsittacus undulatus), we evaluated the impact of long-term cochlear synaptopathy on behavioral discrimination of Gaussian noise (GN) and low-noise noise (LNN) signals processed to have a flatter envelope. Stimuli had center frequencies of 1-3kHz, 100-Hz bandwidth, and were presented at sensation levels (SLs) from 10 to 30dB. We reasoned that narrowband, low-SL stimuli of this type should minimize spread of excitation across auditory-nerve fibers, and hence might reveal synaptopathy-related defects if they exist. Cochlear synaptopathy was induced without hair-cell injury using kainic acid (KA). Behavioral threshold tracking experiments characterized the minimum stimulus duration above which animals could reliably discriminate between LNN and GN. Budgerigar thresholds for LNN-GN discrimination ranged from 40 to 60ms at 30dB SL, were similar across frequencies, and increased for lower SLs. Notably, animals with long-term 39-77% estimated synaptopathy performed similarly to controls, requiring on average a ∼7.5% shorter stimulus duration (-0.7±1.0dB; mean difference ±SE) for LNN-GN discrimination. Decision-variable correlation analyses of detailed behavioral response patterns showed that individual animals relied on envelope cues to discriminate LNN and GN, with lesser roles of FM and energy cues; no difference was found between KA-exposed and control groups. These results suggest that long-term cochlear synaptopathy does not impair discrimination of low-level signals with different envelope statistics.

Histological Correlates of Auditory Nerve Injury from Kainic Acid in the Budgerigar (Melopsittacus undulatus)

PURPOSE

Loss of auditory nerve afferent synapses with cochlear hair cells, called cochlear synaptopathy, is a common pathology in humans caused by aging and noise overexposure. The perceptual consequences of synaptopathy in isolation from other cochlear pathologies are still unclear. Animal models provide an effective approach to resolve uncertainty regarding the physiological and perceptual consequences of auditory nerve loss, because neural lesions can be induced and readily quantified. The budgerigar, a parakeet species, has recently emerged as an animal model for synaptopathy studies based on its capacity for vocal learning and ability to behaviorally discriminate simple and complex sounds with acuity similar to humans. Kainic acid infusions in the budgerigar produce a profound reduction of compound auditory nerve responses, including wave I of the auditory brainstem response, without impacting physiological hair cell measures. These results suggest selective auditory nerve damage. However, histological correlates of neural injury from kainic acid are still lacking.

METHODS

We quantified the histological effects caused by intracochlear infusion of kainic acid (1 mM; 2.5 µL), and evaluated correlations between the histological and physiological assessments of auditory nerve status.

RESULTS

Kainic acid infusion in budgerigars produced pronounced loss of neural auditory nerve soma (60% on average) in the cochlear ganglion, and of peripheral axons, at time points 2 or more months following injury. The hair cell epithelium was unaffected by kainic acid. Neural loss was significantly correlated with reduction of compound auditory nerve responses and auditory brainstem response wave I.

CONCLUSION

Compound auditory nerve responses and wave I provide a useful index of cochlear synaptopathy in this animal model.

Animal models of hidden hearing loss: Does auditory-nerve-fiber loss cause real-world listening difficulties?

Afferent innervation of the cochlea by the auditory nerve declines during aging and potentially after sound overexposure, producing the common pathology known as cochlear synaptopathy. Auditory-nerve-fiber loss is difficult to detect with the clinical audiogram and has been proposed to cause 'hidden hearing loss' including impaired speech-in-noise perception. While evidence that auditory-nerve-fiber loss causes hidden hearing loss in humans is controversial, behavioral animal models hold promise to rigorously test this hypothesis because neural lesions can be induced and histologically validated. Here, we review recent animal behavioral studies on the impact of auditory-nerve-fiber loss on perception in a range of species. We first consider studies of tinnitus and hyperacusis inferred from acoustic startle reflexes, followed by a review of operant-conditioning studies of the audiogram, temporal integration for tones of varying duration, temporal resolution of gaps in noise, and tone-in-noise detection. Studies quantifying the audiogram show that tone-in-quiet sensitivity is unaffected by auditory-nerve-fiber loss unless neural lesions exceed 80%, at which point large deficits are possible. Changes in other aspects of perception, which were typically investigated for moderate-to-severe auditory-nerve-fiber loss of 50-70%, appear heterogeneous across studies and might be small compared to impairment caused by hair-cell pathologies. Future studies should pursue recent findings that behavioral sensitivity to brief tones and silent gaps in noise may be particularly vulnerable to auditory-nerve-fiber loss. Furthermore, aspects of auditory perception linked to central inhibition and fine neural response timing, such as modulation masking release and spatial hearing, may be productive directions for further animal behavioral research.

Experimental animal models of drug-induced sensorineural hearing loss: a narrative review

Objective

This narrative review describes experimental animal models of sensorineural hearing loss (SNHL) caused by ototoxic agents.

Background

SNHL primarily results from damage to the sensory organ within the inner ear or the vestibulocochlear nerve (cranial nerve VIII). The main etiology of SNHL includes genetic diseases, presbycusis, ototoxic agents, infection, and noise exposure. Animal models with functional and anatomic damage to the sensory organ within the inner ear or the vestibulocochlear nerve mimicking the damage seen in humans are employed to explore the mechanism and potential treatment of SNHL. These animal models of SNHL are commonly established using ototoxic agents.

Methods

A literature search of PubMed, Embase, and Web of Science was performed for research articles on hearing loss and ototoxic agents in animal models of hearing loss.

Conclusions

Common ototoxic medications such as aminoglycoside antibiotics (AABs) and platinum antitumor drugs are extensively used to induce SNHL in experimental animals. The effect of ototoxic agents in vivo is influenced by the chemical mechanisms of the ototoxic agents, the species of animal, routes of administration of the ototoxic agents, and the dosage of ototoxic agents. Animal models of drug-induced SNHL contribute to understanding the hearing mechanism and reveal the function of different parts of the auditory system in humans.

Excitotoxic damage to auditory nerve afferents and spiral ganglion neurons is correlated with developmental upregulation of AMPA and KA receptors

Excess release of glutamate at the inner hair cell-type I auditory nerve synapse results in excitotoxicity characterized by rapid swelling and disintegration of the afferent synapses, but in some cases, the damage expands to the spiral ganglion soma. Cochlear excitotoxic damage is largely mediated by α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPAR) and kainate receptor (KAR) and potentially N-methyl-D-aspartate receptors (NMDAR). Because these receptors are developmentally regulated, the pattern of excitotoxic damage could change during development. To test this hypothesis, we compared AMPAR, NMDAR and KAR immunolabeling and excitotoxic damage patterns in rat postnatal day 3 (P3) and adult cochlear cultures. At P3, AMPAR and KAR immunolabeling, but not NMDAR, was abundantly expressed on peripheral nerve terminals adjacent to IHCs. In contrast, AMPAR, KAR and NMDAR immunolabeling was minimal or undetectable on the SGN soma. In adult rats, however, AMPAR, KAR and NMDAR immunolabeling occurred on both peripheral nerve terminals near IHCs as well as the soma of SGNs. High doses of Glu and KA only damaged peripheral nerve terminals near IHCs, but not SGNs, at P3, consistent with selective expression of AMPAR and KAR expression on the terminals. However, in adults, Glu and KA damaged both peripheral nerve terminals near IHCs and SGNs both of which expressed AMPAR and KAR. These results indicate that cochlear excitotoxic damage is closely correlated with structures that express AMPAR and KAR.

Normal Tone-In-Noise Sensitivity in Trained Budgerigars despite Substantial Auditory-Nerve Injury: No Evidence of Hidden Hearing Loss

Loss of auditory-nerve (AN) afferent cochlear innervation is a prevalent human condition that does not affect audiometric thresholds and therefore remains largely undetectable with standard clinical tests. AN loss is widely expected to cause hearing difficulties in noise, known as "hidden hearing loss," but support for this hypothesis is controversial. Here, we used operant conditioning procedures to examine the perceptual impact of AN loss on behavioral tone-in-noise (TIN) sensitivity in the budgerigar (Melopsittacus undulatus; of either sex), an avian animal model with complex hearing abilities similar to humans. Bilateral kainic acid (KA) infusions depressed compound AN responses by 40-70% without impacting otoacoustic emissions or behavioral tone sensitivity in quiet. Surprisingly, animals with AN damage showed normal thresholds for tone detection in noise (0.1 ± 1.0 dB compared to control animals; mean difference ± SE), even under a challenging roving-level condition with random stimulus variation across trials. Furthermore, decision-variable correlations (DVCs) showed no difference for AN-damaged animals in their use of energy and envelope cues to perform the task. These results show that AN damage has less impact on TIN detection than generally expected, even under a difficult roving-level condition known to impact TIN detection in individuals with sensorineural hearing loss (SNHL). Perceptual deficits could emerge for different perceptual tasks or with greater AN loss but are potentially minor compared with those caused by SNHL.SIGNIFICANCE STATEMENT Loss of auditory-nerve (AN) cochlear innervation is a common problem in humans that does not affect audiometric thresholds on a clinical hearing test. AN loss is widely expected to cause hearing problems in noise, known as "hidden hearing loss," but existing studies are controversial. Here, using an avian animal model with complex hearing abilities similar to humans, we examined for the first time the impact of an experimentally induced AN lesion on behavioral tone sensitivity in noise. Surprisingly, AN-lesioned animals showed no difference in hearing performance in noise or detection strategy compared with controls. These results show that perceptual deficits from AN damage are smaller than generally expected, and potentially minor compared with those caused by sensorineural hearing loss (SNHL).

Effects of Kainic Acid-Induced Auditory Nerve Damage on Envelope-Following Responses in the Budgerigar (Melopsittacus undulatus)

Sensorineural hearing loss is a prevalent problem that adversely impacts quality of life by compromising interpersonal communication. While hair cell damage is readily detectable with the clinical audiogram, this traditional diagnostic tool appears inadequate to detect lost afferent connections between inner hair cells and auditory nerve (AN) fibers, known as cochlear synaptopathy. The envelope-following response (EFR) is a scalp-recorded response to amplitude modulation, a critical acoustic feature of speech. Because EFRs can have greater amplitude than wave I of the auditory brainstem response (ABR; i.e., the AN-generated component) in humans, the EFR may provide a more sensitive way to detect cochlear synaptopathy. We explored the effects of kainate- (kainic acid) induced excitotoxic AN injury on EFRs and ABRs in the budgerigar (Melopsittacus undulatus), a parakeet species used in studies of complex sound discrimination. Kainate reduced ABR wave I by 65-75 % across animals while leaving otoacoustic emissions unaffected or mildly enhanced, consistent with substantial and selective AN synaptic loss. Compared to wave I loss, EFRs showed similar or greater percent reduction following kainate for amplitude-modulation frequencies from 380 to 940 Hz and slightly less reduction from 80 to 120 Hz. In contrast, forebrain-generated middle latency responses showed no consistent change post-kainate, potentially due to elevated "central gain" in the time period following AN damage. EFR reduction in all modulation frequency ranges was highly correlated with wave I reduction, though within-animal effect sizes were greater for higher modulation frequencies. These results suggest that even low-frequency EFRs generated primarily by central auditory nuclei might provide a useful noninvasive tool for detecting synaptic injury clinically.

Expression and Localization of Kv1.1 and Kv3.1b Potassium Channels in the Cochlear Nucleus and Inferior Colliculus after Long-Term Auditory Deafferentation

Deafness affects the expression and distribution of voltage-dependent potassium channels (Kvs) of central auditory neurons in the short-term, i.e., hours to days, but the consequences in the expression of Kvs after long-term deafness remain unknown. We tested expression and distribution of Kv1.1 and Kv3.1b, key for auditory processing, in the rat cochlear nucleus (CN), and in the inferior colliculus (IC), at 1, 15 and 90 days after mechanical lesion of the cochlea, using a combination of qRT-PCR and Western blot in the whole CN, along with semi-quantitative immunocytochemistry in the AVCN, where the role of both Kvs in the control of excitability for accurate auditory timing signal processing is well established. Neither Kv1.1/Kv3.1b mRNA or protein expression changed significantly in the CN between 1 and 15 days after deafness. At 90 days post-lesion, however, mRNA and protein expression for both Kvs increased, suggesting that regulation of Kv1.1 and Kv3.1b expression is part of cellular mechanisms for long-term adaptation to auditory deprivation in the CN. Consistent with these findings, immunocytochemistry showed increased labeling intensity for both Kvs in the AVCN at day 90 after cochlear lesion. This increase argues that up-regulation of Kv1.1 and Kv3.1b in AVCN neurons may be required to adapt intrinsic excitability to altered input over the long term after auditory deprivation. Contrary to these findings in the CN, expression levels of Kv1.1 and Kv3.1b in the IC did not undergo major changes after cochlear lesion. In particular, there was no evidence of long-term up-regulation of either Kv1.1 or Kv3.1b, supporting that such post-lesion adaptive mechanism may not be needed in the IC. These results reveal that post-lesion adaptations do not necessarily involve stereotyped plastic mechanisms along the entire auditory pathway.

Effects of selective auditory-nerve damage on the behavioral audiogram and temporal integration in the budgerigar

Auditory-nerve fibers are lost steadily with age and as a possible consequence of noise-induced glutamate excitotoxicity. Auditory-nerve loss in the absence of other cochlear pathologies is thought to be undetectable with a pure-tone audiogram while degrading real-world speech perception (hidden hearing loss). Perceptual deficits remain unclear, however, due in part to the limited behavioral capacity of existing rodent models to discriminate complex sounds. The budgerigar is an avian vocal learner with human-like behavioral sensitivity to many simple and complex sounds and the capacity to mimic speech. Previous studies in this species show that intracochlear kainic-acid infusion reduces wave 1 of the auditory brainstem response by 40-70%, consistent with substantial excitotoxic auditory-nerve damage. The present study used operant-conditioning procedures in trained budgerigars to quantify kainic-acid effects on tone detection across frequency (0.25-8 kHz; the audiogram) and as a function of duration (20-160 ms; temporal integration). Tone thresholds in control animals were lowest from 1 to 4 kHz and decreased with increasing duration as in previous studies of the budgerigar. Behavioral results in kainic-acid-exposed animals were as sensitive as in controls, suggesting preservation of the audiogram and temporal integration despite auditory-nerve loss associated with up to 70% wave 1 reduction. Distortion-product otoacoustic emissions were also preserved in kainic-acid exposed animals, consistent with normal hair-cell function. These results highlight considerable perceptual resistance of tone-detection performance with selective auditory-nerve loss. Future behavioral studies in budgerigars with auditory-nerve damage can use complex speech-like stimuli to help clarify aspects of auditory perception impacted by this common cochlear pathology.

Persistent Auditory Nerve Damage Following Kainic Acid Excitotoxicity in the Budgerigar (Melopsittacus undulatus)

Permanent loss of auditory nerve (AN) fibers occurs with increasing age and sound overexposure, sometimes without hair cell damage or associated audiometric threshold elevation. Rodent studies suggest effects of AN damage on central processing and behavior, but these species have limited capacity to discriminate low-frequency speech-like sounds. Here, we introduce a new animal model of AN damage in an avian communication specialist, the budgerigar (Melopsittacus undulatus). The budgerigar is a vocal learner and speech mimic with sensitive low-frequency hearing and human-like behavioral sensitivity to many complex signals including speech components. Excitotoxic AN damage was induced through bilateral cochlear infusions of kainic acid (KA). Acute KA effects on cochlear function were assessed using AN compound action potentials (CAPs) and hair cell cochlear microphonics (CMs). Long-term KA effects were assessed using auditory brainstem response (ABR) measurements for up to 31 weeks post-KA exposure. KA infusion immediately abolished AN CAPs while having mild impact on the CM. ABR wave I, the far-field AN response, showed a pronounced 40-75 % amplitude reduction at moderate-to-high sound levels that persisted for the duration of the study. In contrast, wave I latency and the amplitude of wave V were nearly unaffected by KA, and waves II-IV were less reduced than wave I. ABR thresholds, calculated based on complete response waveforms, showed no impairment following KA. These results demonstrate that KA exposure in the budgerigar causes irreversible AN damage, most likely through excitotoxic injury to afferent fibers or synapses as in other species, while sparing ABR thresholds. Normal wave V amplitude, assumed to originate centrally, may persist through compensatory mechanisms that restore central response amplitude by downregulating inhibition. Future studies in this new animal model of AN damage can explore effects of this neural lesion, in isolation from hair cell trauma and threshold elevation, on central processing and perception of complex sounds.

c-Fos and Arc/Arg3.1 expression in auditory and visual cortices after hearing loss: Evidence of sensory crossmodal reorganization in adult rats

Cross-modal reorganization in the auditory and visual cortices has been reported after hearing and visual deficits mostly during the developmental period, possibly underlying sensory compensation mechanisms. However, there are very few data on the existence or nature and timeline of such reorganization events during sensory deficits in adulthood. In this study, we assessed long-term changes in activity-dependent immediate early genes c-Fos and Arc/Arg3.1 in auditory and neighboring visual cortical areas after bilateral deafness in young adult rats. Specifically, we analyzed qualitatively and quantitatively c-Fos and Arc/Arg3.1 immunoreactivity at 15 and 90 days after cochlea removal. We report extensive, global loss of c-Fos and Arc/Arg3.1 immunoreactive neurons in the auditory cortex 15 days after permanent auditory deprivation in adult rats, which is partly reversed 90 days after deafness. Simultaneously, the number and labeling intensity of c-Fos- and Arc/Arg3.1-immunoreactive neurons progressively increase in neighboring visual cortical areas from 2 weeks after deafness and these changes stabilize three months after inducing the cochlear lesion. These findings support plastic, compensatory, long-term changes in activity in the auditory and visual cortices after auditory deprivation in the adult rats. Further studies may clarify whether those changes result in perceptual potentiation of visual drives on auditory regions of the adult cortex.

Age-related cochlear synaptopathy: an early-onset contributor to auditory functional decline

Aging listeners experience greater difficulty understanding speech in adverse listening conditions and exhibit degraded temporal resolution, even when audiometric thresholds are normal. When threshold evidence for peripheral involvement is lacking, central and cognitive factors are often cited as underlying performance declines. However, previous work has uncovered widespread loss of cochlear afferent synapses and progressive cochlear nerve degeneration in noise-exposed ears with recovered thresholds and no hair cell loss (Kujawa and Liberman 2009). Here, we characterize age-related cochlear synaptic and neural degeneration in CBA/CaJ mice never exposed to high-level noise. Cochlear hair cell and neuronal function was assessed via distortion product otoacoustic emissions and auditory brainstem responses, respectively. Immunostained cochlear whole mounts and plastic-embedded sections were studied by confocal and conventional light microscopy to quantify hair cells, cochlear neurons, and synaptic structures, i.e., presynaptic ribbons and postsynaptic glutamate receptors. Cochlear synaptic loss progresses from youth (4 weeks) to old age (144 weeks) and is seen throughout the cochlea long before age-related changes in thresholds or hair cell counts. Cochlear nerve loss parallels the synaptic loss, after a delay of several months. Key functional clues to the synaptopathy are available in the neural response; these can be accessed noninvasively, enhancing the possibilities for translation to human clinical characterization.

Up-regulation of GAP-43 in the chinchilla ventral cochlear nucleus after carboplatin-induced hearing loss: correlations with inner hair cell loss and outer hair cell loss

Inner ear damage leads to nerve fiber growth and synaptogenesis in the ventral cochlear nucleus (VCN). In this study, we documented the relationship between hair cell loss patterns and synaptic plasticity in the chinchilla VCN using immunolabeling of the growth associated protein-43 (GAP-43), a protein associated with axon outgrowth and modification of presynaptic endings. Unilateral round window application of carboplatin caused hair cell degeneration in which inner hair cells (IHC) were more vulnerable than outer hair cells (OHC). One month after carboplatin treatment (0.5-5 mg/ml), we observed varying patterns of cochlear hair cell loss and GAP-43 expression in VCN. Both IHC loss and OHC loss were strongly correlated with increased GAP-43 immunolabeling in the ipsilateral VCN. We speculate that two factors might promote the expression of GAP-43 in the VCN; one is the loss of afferent input through IHC or the associated type I auditory nerve fibers. The other occurs when the medial olivocochlear efferent neurons lose their cochlear targets, the OHC, and may as compensation increase their synapse numbers in the VCN.

Ca2+ entry via AMPA-type glutamate receptors triggers Ca2+-induced Ca2+ release from ryanodine receptors in rat spiral ganglion neurons

Ryanodine receptor (RyR)-gated Ca2+ stores have recently been identified in cochlear spiral ganglion neurons (SGN) and likely contribute to Ca2+ signalling associated with auditory neurotransmission. Here, we identify an ionotropic glutamate receptor signal transduction pathway which invokes RyR-gated Ca2+ stores in SGN via Ca2+-induced Ca2+ release (CICR). Ca2+ levels were recorded in SGN in situ within rat cochlear slices (postnatal day 0-17) using the Ca2+ indicator fluo-4. RyR-gated Ca2+ stores were confirmed by caffeine-induced increases in intracellular Ca2+ which were blocked by ryanodine (100 microM) and were independent of external Ca2+. Glutamate evoked comparable increases in intracellular Ca2+, but required the presence of external Ca2+. Ca2+ influx via the glutamate receptor was found to elicit CICR via RyR-gated Ca2+ stores, as shown by the inhibition of the response by prior depletion of the Ca2+ stores with caffeine, the SERCA inhibitor thapsigargin, or ryanodine. The glutamate analogue AMPA (alpha-Amino-3-hydroxy-5-methylisoxazole-4-propionic acid) elicited Ca2+ responses that could be inhibited by caffeine. Glutamate- and AMPA-mediated Ca2+ responses were eliminated with the AMPA/Kainate receptor antagonist DNQX (6,7-dinitroquinoxaline-2,3-dione). These data demonstrate functional coupling between somatic AMPA-type glutamate receptors and intracellular Ca(2+) stores via RyR-dependent CICR in primary auditory neurons.

Trimetazidine modulates AMPA/kainate receptors in rat vestibular ganglion neurons

Trimetazidine (1[2,3,4-trimethoxy-benzyl] piperazine, 2 HCl) is an anti-ischemic agent frequently administered as a prophylactic treatment for episodes of angina pectoris and chorioretinal disturbances. It is also employed as a symptomatic treatment of vertigo but its mechanism of action is yet to be defined. Using Fura-2 fluorescence photometry and whole-cell patch-clamp recordings we investigated the effect of trimetazidine on the [Ca(2+)](i) and current responses induced by the application of non-N-methyl-D-aspartate (NMDA) receptor agonists on low density vestibular ganglion neuronal cultures explanted from 3 day s postnatal rats. Trimetazidine blocked the [Ca(2+)](i) and current responses induced by 100 microM applications of both kainate and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA). These responses were dependent on external Ca(2+) and were blocked by the voltage-dependent Ca(2+) channel blockers Ni(2+) and Cd(2+) . Trimetazidine only acts on the AMPA/kainate receptors and had no effect on K(+)-induced depolarizations. Dose-dependent curves were obtained for the inhibition by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and trimetazidine (IC(50) 7 microM and 0.7 microM) of kainate stimulations. After AMPA stimulation, dose-response inhibition curves showed an IC(50) of 3 microM for CNQX and 25 microM for trimetazidine. These results indicate that trimetazidine could be a potent antagonist of AMPA/kainate receptors in vestibular ganglion neurons. This may explain the protective role of trimetazidine in the inner ear suggesting an anti-excitotoxic activity.

Control of hair cell excitability by vestibular primary sensory neurons

In the rat utricle, synaptic contacts between hair cells and the nerve fibers arising from the vestibular primary neurons form during the first week after birth. During that period, the sodium-based excitability that characterizes neonate utricle sensory cells is switched off. To investigate whether the establishment of synaptic contacts was responsible for the modulation of the hair cell excitability, we used an organotypic culture of rat utricle in which the setting of synapses was prevented. Under this condition, the voltage-gated sodium current and the underlying action potentials persisted in a large proportion of nonafferented hair cells. We then studied whether impairment of nerve terminals in the utricle of adult rats may also affect hair cell excitability. We induced selective and transient damages of afferent terminals using glutamate excitotoxicity in vivo. The efficiency of the excitotoxic injury was attested by selective swellings of the terminals and underlying altered vestibular behavior. Under this condition, the sodium-based excitability transiently recovered in hair cells. These results indicate that the modulation of hair cell excitability depends on the state of the afferent terminals. In adult utricle hair cells, this property may be essential to set the conditions required for restoration of the sensory network after damage. This is achieved via re-expression of a biological process that occurs during synaptogenesis.

Physiology, pharmacology and plasticity at the inner hair cell synaptic complex

This report summarizes recent neuropharmacological data at the IHC afferent/efferent synaptic complex: the type of Glu receptors and transporter involved and the modulation of this fast synaptic transmission by the lateral efferents. Neuropharmacological data were obtained by coupling the recording of cochlear potentials and single unit of the auditory nerve with intra-cochlear applications of drugs (multi-barrel pipette). We also describe the IHC afferent/efferent functioning in pathological conditions. After acoustic trauma or ischemia, acute disruption of IHC-auditory dendrite synapses are seen. However, a re-growth of the nerve fibres and a re-afferentation of the IHC were completely done 5 days after injury. During this synaptic repair, multiple presynaptic bodies were commonly found, either linked to the membrane or "floating" in ectopic positions. In the meantime, the lateral efferents directly contact the IHCs. The demonstration that NMDA receptors blockade delayed the re-growth of neurites suggests a neurotrophic role of NMDA receptors in pathological conditions.

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.

Structure and innervation of the cochlea

The role of the cochlea is to transduce complex sound waves into electrical neural activity in the auditory nerve. Hair cells of the organ of Corti are the sensory cells of hearing. The inner hair cells perform the transduction and initiate the depolarization of the spiral ganglion neurons. The outer hair cells are accessory sensory cells that enhance the sensitivity and selectivity of the cochlea. Neural feedback loops that bring efferent signals to the outer hair cells assist in sharpening and amplifying the signals. The stria vascularis generates the endocochlear potential and maintains the ionic composition of the endolymph, the fluid in which the apical surface of the hair cells is bathed. The mechanical characteristics of the basilar membrane and its related structures further enhance the frequency selectivity of the auditory transduction mechanism. The tectorial membrane is an extracellular matrix, which provides mass loading on top of the organ of Corti, facilitating deflection of the stereocilia. This review deals with the structure of the normal mature mammalian cochlea and includes recent data on the molecular organization of the main cell types within the cochlea.

Riluzole rescues cochlear sensory cells from acoustic trauma in the guinea-pig

Acoustic trauma is the major cause of hearing loss in industrialised nations. We show in guinea-pigs that sound exposure (6 kHz, 120 dB sound pressure level for 30 min) leads to sensory cell death and subsequent permanent hearing loss. Ultrastructural analysis reveals that degeneration of the noise-damaged hair cells involved different mechanisms, including typical apoptosis, autolysis and, to a lesser extent, necrosis. Whatever the mechanisms, a common feature of noise damage to hair cells was mitochondrial alteration. Riluzole (2-amino-6-trifluoromethoxy benzothiazole) is a neuroprotective agent that prevents apoptosis- and necrosis-induced cell death. Perfusion of riluzole into the cochlea via an osmotic minipump prevents mitochondrial damage and subsequent translocation of cytochrome c, DNA fragmentation, and hair cell degeneration. This was confirmed by functional tests showing a clear dose-dependent reduction (ED(50)=16.8 microM) of permanent hearing loss and complete protection at 100 microM. Although less efficient than intracochlear perfusion, intraperitoneal injection of riluzole rescues the cochlea within a therapeutic window of 24 h after acoustic trauma.These results show that riluzole is able to prevent and rescue the cochlea from acoustic trauma. It may thus be an interesting molecule for the treatment of inner ear injuries.

Cochlear microphonic potentials: a new recording technique

A new instrumentation and a particular method for detecting and recording cochlear microphonic potentials (CMPs) are described here. The CMPs were recorded in rats by means of pure tones (4,000, 2,000, 1,000, 500, and 250 Hz) and intraepidermic electrodes; the electrocochleography technique was avoided. An experimental design that included the use of a glutamatergic agonist (kainic acid [KA]) and an aminoglycoside antibiotic (kanamycin [KANA]) was carried out to demonstrate the origin of the recorded potential. Morphological studies showed that KA selectively eliminated the afferent type I dendrites of the spiral ganglion, while the administration of KANA resulted in the absence of outer hair cells. When CMPs were recorded after KA administration, no alterations were detected. In contrast, KANA administration resulted in the absence of any selective electrophysiological activity corresponding to CMPs. All these results were compared with the recording of the compound action potential of the eighth nerve obtained by electrocochleography. These findings and the great specificity of the reproduction of the sound stimulus confirm that the CMPs can be recorded by the new equipment.

Functional recovery of hearing following ampa-induced reversible disruption of hair cell afferent synapses in the avian inner ear

Hair cells in the avian inner ear can regenerate after acoustic trauma or ototoxic insult, and significant functional recovery from hearing loss occurs. However, small residual deficits remain, possibly as a result of incomplete reestablishment of the hair cell neural synaptic contacts. The aim of the present study was to determine if intracochlear application of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA), an excitotoxic glutamate agonist, causes reversible disruption of hair cell neural contacts in the bird, and to what extent functional recovery occurs if synaptic contacts are reestablished. Compound action potential (CAP) responses to tone bursts were recorded to determine hearing thresholds during a recovery period of up to 4 months. Subsequently, the response properties of single auditory nerve fibers were analyzed in the same animals. Instillation of AMPA into the perilymph of the scala tympani led to immediate abolition of CAP thresholds. Partial recovery occurred over a period of 2-3 weeks, without further improvement of thresholds thereafter. High-frequency thresholds did not reach control values even after 3-4 months of recovery. Single-ganglion cell response properties, obtained 3-4 months after AMPA treatment, showed elevated thresholds at the fiber's characteristic frequency (CF) for units with CF above 0.3 kHz. Sharpness of tuning (Q(10 dB)) was reduced in units with CF above 0.4 kHz. The spontaneous firing rate was higher in units with CF above 0.18 kHz. The maximum sound-evoked discharge rate was also increased. Transmission electron micrographs of the basilar papilla showed that, following AMPA treatment, the nerve endings went through a sequence of swelling, degeneration and recovery over a period of 3-7 days. The process of neosynaptogenesis was completed 14 days after exposure. The present findings are strong evidence for a role of glutamate or a related excitatory amino acid as the afferent transmitter in the avian inner ear. In addition they show that functional recovery after disruption and regeneration of hair cell neural synapses, without apparent damage to the hair cells, is incomplete.

Reversible and irreversible damage to cochlear afferent neurons by kainic acid excitotoxicity

Kainic acid (KA) selectively damages afferent synapses that innervate, in chickens, mainly tall hair cells. To better understand the nature of KA-induced excitotoxic damage to the cochlear afferent neurons, KA, at two different concentrations (0.3 or 5 mM), was injected directly into the inner ear of adult chickens. Pathologic changes in the afferent nerve ending and cell body were evaluated with light and transmission electron microscopy at various time points after KA application. The compound action potential (CAP) and cochlear microphonic (CM) potential were recorded to monitor the physiologic status of the afferent neurons and hair cells, respectively. Hair cell morphology and function were essentially normal after KA treatment. However, afferent synapses beneath tall hair cells were swollen within 30 minutes after KA at both low (KA-L) and high (KA-H) doses. In the KA-L group, the swelling disappeared within 1 day and the morphology of the postsynaptic region returned to near normal condition. In the KA-H group, by contrast, the vacant region beneath tall hair cells remained evident even 20 weeks after KA. The number of cochlear ganglion neurons in the KA-H group decreased progressively from 1 to 8-20 weeks, whereas hair cells in the basilar papilla remained morphologically intact out to 20 weeks after KA. There was no significant change in neuron number in the KA-L group. Temporal changes in the CAP amplitude paralleled the anatomic changes, although the CAP only partially recovered. These results suggest that KA induces partially reversible damage to cochlear afferent neurons with low KA concentration; above this level, KA triggers irreversible, progressive neurodegeneration.

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

The role of AMPA receptors in cochlear synaptic transmission and excitotoxicity was investigated by comparing the actions of a selective AMPA antagonist GYKI 53784 (LY303070) with additional AMPA/kainate antagonists, GYKI 52466 and DNQX, and the NMDA antagonist, D-AP5, in several electrophysiological, neurotoxicological and histochemical tests. GYKI 53784 had the same potency as DNQX and was 10 times more potent than GYKI 52466 in reducing auditory nerve activity. The NMDA antagonist D-AP5 had no effect on auditory nerve activity. When single-fiber activity was blocked with GYKI 53784, the effects of AMPA or kainate were also antagonized. GYKI 53784 completely blocked excitotoxicity (i.e. destruction of the afferent nerve endings) induced by AMPA and kainate. The histochemical detection of Co(2+) uptake was used to study Ca(2+) influx within the primary auditory nerve cells. Application of AMPA induced no significant Co(2+) uptake into the cells, suggesting that these receptors normally have a very low permeability to Ca(2+). Application of kainate induced significant Co(2+) uptake that was blocked by the AMPA receptor antagonist GYKI 53784 suggesting that kainate stimulated Ca(2+) entry through AMPA receptor channels. Results suggest that AMPA-preferring receptors are functionally located at the sensory cell-afferent synapse whereas NMDA and kainate receptors are not.

Complementary roles of neurotrophin 3 and a N-methyl-D-aspartate antagonist in the protection of noise and aminoglycoside-induced ototoxicity

Recent progress has been made regarding the prevention of hearing loss. However, the complete protection of both hair cells and spiral ganglion neurons, with restored function, has not yet been achieved. It has been shown that spiral ganglion neuronal loss can be prevented by neurotrophin 3 (NT3) and hair cell damage by N-methyl-D-aspartate (NMDA) receptor antagonists. Here we demonstrate that the combined treatment with MK801, a NMDA antagonist, and NT3 protect both cochlear morphology and physiology from injury. Pretreatment with MK801 prevented hearing loss and the dendrites of the spiral ganglion neurons from swelling after noise-induced damage. The acute phase of insult with the aminoglycoside antibiotic amikacin resulted in swollen afferent dendrites beneath the inner hair cells. The chronic phase resulted in complete hair cell loss and near-complete loss of spiral ganglion neurons. This damage caused a near-complete loss of hearing sensitivity as displayed by elevated (>90-dB sound pressure levels) auditory brainstem response thresholds. The treatment of amikacin-exposed animals with MK801 gave only a partial protection of hearing. However, the combined treatment with NT3 and MK801 in the amikacin-comprised ear resulted in improved mean hearing within 20 dB of normal. Furthermore, hair cell loss was prevented in these animals and spiral ganglion neurons were completely protected. These results suggest that the NMDA antagonist MK801 protects against noise-induced excitotoxicity in the cochlea whereas the combined treatment of NT3 and MK801 has a potent effect on preserving both auditory physiology and morphology against aminoglycoside toxicity.

Excitotoxic effect of kainic acid on chicken otoacoustic emissions and cochlear potentials

Kainic acid (KA) is a potent glutamate analog that can temporarily or permanently damage glutamatergic neurons. The purpose of the present study was to determine the short- and long-term effects of KA on chicken otoacoustic emissions and cochlear potentials. A chronic electrode was used to record the compound action potential (CAP), cochlear microphonic (CM), and the slow, positive neural potential (SPNP), a predominantly dc response. The CM, CAP, SPNP, and distortion product otoacoustic emissions (DPOAEs) were recorded before and after infusing 10 microl of a low dose (KA-L, 0.3 mM) or high dose (KA-H, 5 mM) of KA into scala tympani. KA caused a rapid and large reduction in CAP and SPNP amplitude in both the KA-H and KA-L groups; however, the CM and DPOAEs were largely unchanged. The amplitude of the CAP and SPNP in the KA-L group began to recover around 1 week post-KA, but was approximately 50% below normal at 4 weeks post-KA. In contrast, the CAP and SPNP showed no signs of recovery in the KA-H group. The results suggest that KA has no effect on the CM and DPOAEs generated by the hair cells, but selectively damages the CAP generated by the cochlear ganglion neurons. The reduction in the avian SPNP suggests that the response originates in the cochlear afferent neurons, unlike the summating potential (SP) in mammals that is generated in hair cells.

A role of glutamate in drug-induced ototoxicity: in vivo microdialysis study combined with on-line enzyme fluorometric detection of glutamate in the guinea pig cochlea

The time course of the changes in perilymphatic glutamate was determined during the application of kanamycin and ethacrynic acid, which are known to damage the hair cells in the inner ear. For the continuous recording of glutamate, the microdialysis technique combined with an enzyme-linked fluorometric assay was used. In guinea pigs receiving a loading dose of 800 mg/kg of kanamycin subcutaneously followed 3 h later by an i.v. injection of 40 mg/kg of ethacrynic acid, a marked glutamate release was clearly found about 2 h after the injection of ethacrynic acid. Injection of kanamycin or ethacrynic acid alone did not produce any change in the perilymphatic glutamate. The morphological changes induced by the administration of both drugs indicated that the collapsing hair cells might release glutamate into the perilymphatic space. The present findings provide additional evidence that glutamate acts as an aggravating factor in aminoglycoside-induced ototoxicity.

Recovery of kainic acid excitotoxicity in chinchilla cochlea

The present study examines the recovery of the inner hair cell (IHC)/auditory nerve synapse following cochlear excitotoxicity induced by kainic acid (KA). Three hours after KA treatment, there was massive swelling of type I afferent endings under the IHCs. Five to ten days later, the pattern of IHC innervation appeared to be normal. Distortion-product otoacoustic emissions were normal during the whole experiment. The amplitude of the auditory nerve compound action potential (CAP) was significantly reduced immediately after KA treatment and then recovered over a 30-day period. However, it only took five days for the evoked response from the inferior colliculus (IC) to recover from a substantial depression. In contrast to amplitudes, thresholds for the CAP and IC recovered at the same rate and returned to normal within 5 days after KA. Single auditory nerve fibers were also assessed at various times after the KA treatment. Ten days after KA, these fibers had almost normal thresholds, tuning, spontaneous, and driven discharge rates. The results indicate that (1) excitotoxically damaged cochlear afferent neurons can rapidly regenerate and establish viable synapses with the IHCs, and (2) the central auditory system recovers more rapidly than the periphery.

Trimetazidine prevents cochlear lesions induced by intraperitoneal and perilymphatic administration of kainic acid

The protective activity of trimetazidine (TMZ) against cochlear neurotoxicity induced by intraperitoneal and intracochlear administration of kainic acid (KA) has been analyzed. The amplitude of the CAP N1 wave was significantly higher in KA rats pretreated with TMZ, independently of the administration route, than in those only treated with KA. However, CAP N1 amplitude of both TMZ pretreated and non-pretreated animals was always lesser than the N1 wave amplitude observed in the control group. The CAP N1 latency did not show any significant difference between KA and TMZ+KA groups except at high intensities of 8 and 12 kHz. As a complementary control, we have demonstrated that the intraperitoneal administration of TMZ (5 mg/kg) alone did not affect either the electrophysiological activity or the morphology of the auditory nerve. Morphological results fit well with electrophysiology. Some isolated swollen afferent fibers were observed in TMZ+KA cochleae, the swollen dendrites being sparser than in the KA only treated animals. In TMZ+KA animals, the cochlear apical coils were less affected than the basal coils. Our results are in agreement with recent clinical studies and suggest that TMZ could be an active drug on cochlear impairment linked to hypoxic-ischaemic syndromes.

Excitotoxic effect of kainic acid on chicken cochlear afferent neurons

The excitotoxic effects of kainic acid, a glutamate analog, on the auditory neurons in the chicken cochlea were assessed by light and transmission electron microscopy. Kainic acid was directly applied onto the round window of adult chickens and their cochleas were harvested 3 h after application. Transverse microscopic sections of the basilar papilla revealed swelling of afferent dendrites without any morphological changes in efferent endings. The regions of the basilar papilla damaged by kainic acid were localized in the apical 80% and primarily on the neural side where tall hair cells are located. The basal, abneural short hair cell region was devoid of damage. These results imply that glutamate is a primary neurotransmitter in chicken auditory afferent neurons that synapse on tall hair cells.

Role of nitric oxide in kainic acid-induced elevation of cochlear compound action potential thresholds

Nitric oxide (NO) has specifically been found to mediate the effects of excitatory amino acids in the central nervous system (CNS). Excitatory amino acids are the primary neurotransmitters at the cochlear hair cell afferent nerve synapse. Recent studies in our laboratory demonstrate that NO synthase is an active enzyme in the spiral ganglion cells of the cochlea. Given our current understanding of neurotransmission in the cochlea, it is reasonable to postulate that the actions of NO in cochlear neuronal tissue are similar to the actions of NO in the CNS, and that NO acts as a neurotransmitter/neuromodulator in the cochlea. In addition, NO is implicated as a mediator of excitotoxicity in the CNS and may therefore play a similar role in excitotoxicity in the cochlea. To further elucidate the role of NO in cochlear excitotoxicity, this study investigated the effects of 7-nitroindazole (7-NI), a competitive inhibitor of neuronal nitric oxide synthase, with regard to kainic acid (KA)-induced elevation of compound action potential (CAP) thresholds. KA is a conformationally restricted analog of glutamate with well-known excitotoxic effects on SGC's and previously described inhibitory actions on cochlear CAP thresholds. In anesthetized gerbils, CAP thresholds were recorded before and after cochlear perfusions with control solutions of artificial perilymph solution and test solutions of KA. 7-NI was administered i.p. prior to KA perfusion in an effort to block its depolarizing and toxic effects. Results showed that cochlear perfusion with KA caused significant elevation (p < 0.05) of the mean CAP threshold. This threshold shift was significantly reduced (p < 0.05) in animals pretreated with 7-NI. These results indicate that NO is involved in the toxic effects on CAP thresholds elicited by KA in the cochlea.

Faster recovery in central than in peripheral auditory system following a reversible cochlear deafferentation

Included among the exciting findings in auditory neuroscience are (i) central plasticity after peripheral injury and (ii) regeneration of auditory nerve fibres following excitotoxic damage. The present study extends our understanding of auditory system plasticity by examining changes in peripheral and central physiology as the cochlea recovers from temporary deafferentation due to excitotoxicity. Application of kainic acid (60 mM) to the round window membrane substantially depressed responses from both auditory nerve and brain stem (inferior colliculus), without affecting distortion-product otoacoustic emissions from the inner ear. The auditory nerve input/output functions recovered over a 30-day period whereas recovery of brainstem response amplitudes occurred within five days. In contrast to amplitudes, thresholds at both peripheral and central levels recovered simultaneously, within five days after kainic acid application. The results indicate that (i) cochlear afferent neurons can recover after excitotoxic damage; (ii) response threshold itself, either central or peripheral, is not sufficient to assess the integrity of the auditory periphery; (iii) the central auditory system can recover more rapidly than the periphery; and (iv) the system can maintain its function in the normal range as peripheral function continues to improve.

Ototoxicity of sodium nitroprusside

Nitric oxide (NO) not only has normal physiological roles like vasodilation and neurotransmission in the living organism, it could also have possible neurodestructive effects under certain pathological conditions. The present study aimed to determine whether direct exposure of guinea pig cochlea to a NO donor like sodium nitroprusside (SNP), or a nitric oxide synthase (NOS) inhibitor like N(G)-nitro-L-arginine methyl ester (L-NAME), would cause damage to the auditory hair cells. A piece of gelfoam was placed on the round window of the right ear of adult albino guinea pigs. It was then soaked with 0.1 ml of SNP (3.4 microM), 0.1 ml of L-NAME (9.3 microM or 18.5 microM) or 0.1 ml of injection water, the vehicle used to dissolve the above chemicals. Twelve animals receiving SNP were perfused 1 day, 2, 3 and 7 days later, with three animals being used for each survival period. Six animals receiving L-NAME were allowed to survive up to 7 days before perfusion. Eight animals receiving injection water or 0.45% saline were used as controls. With the scanning electron microscope, the inner and outer hair cells were counted over a 1 mm length of the basilar membrane in each turn of every cochlea. The results showed that, in animals treated with L-NAME at both concentrations stated, no significant loss of either inner or outer hair cells was noted in any part of the cochlea studied. However, as early as 1 day after SNP treatment, a striking loss of inner and outer hair cells was observed in the three lower turns of the cochlea. Damage to the outer hair cells was extended to the apical turn with increasing survival period, but no significant loss of inner hair cells was evident in the apical turn at any of the survival periods studied. To rule out the possibility that the effects were due to the presence of cyanide, a metabolite of SNP, hydroxycobalamin was introduced into the scala tympani of three animals through a cannula-osmotic pump device during SNP treatment. There was no significant difference in the results between the groups with and without hydroxycobalamin infusion 7 days after SNP treatment. The present study suggests that an excessive production of NO in the inner ear could lead to extensive loss of hair cells.

Immunocytochemical localization of a high-affinity glutamate-aspartate transporter, GLAST, in the rat and guinea-pig cochlea

Glutamate transporters play an important role in the reuptake of glutamate after its release from glutamatergic synapses. Four such transporters have so far been cloned from the rat brain. One, the glutamate-aspartate transporter GLAST, has been detected in the mammalian cochlea, in which the principal afferent synapse of the auditory nerve, between the inner hair cells and neurites of type I spiral ganglion neurons, has been suggested to be glutamatergic. The distribution of GLAST was therefore investigated to provide clues to the handling of glutamate in the cochlea. This was studied using light and electron microscopic immunocytochemistry in rats and guinea pigs with antibodies raised against synthetic peptides based on the sequence for GLAST. Significant immunoreactivity was found in the myelin sheath formed by satellite cells surrounding the type I spiral ganglion neurons, and along the plasma membranes of supporting cells around the inner hair cells; other cells in both locations were only weakly labelled, if at all. The absence of substantial numbers of synapses in the spiral ganglion suggests that GLAST is unlikely to be associated with the uptake of synaptic glutamate after release in this region. Immunoreactivity associated with the inner hair cells is consistent with the utilization of glutamate at the afferent synapse.

Efflux of glutamate into the perilymph of the cochlea following transient ischemia in the gerbil

Using a microdialysis technique followed by an enzyme cycling analysis, we measured changes in the glutamate levels in the perilymph of gerbil cochleae before, during and after transient ischemic insult. The basal glutamate level in perilymph was 0.35 +/- 0.22 pmol/microl. An almost immediate and continuous rise in the level of glutamate occurred after the ischemic insult, which advanced even further after recirculation; the average concentration was higher than 40 pmol/microl 55 min after recirculation. The compound action potentials (CAP) monitoring the auditory function totally disappeared after ischemic insult. However, CAP reappeared after recirculation; the threshold for acoustic stimulation was higher than that observed at the pre-ischemic state.

Recovery of structure and function of inner ear afferent synapses following kainic acid excitotoxicity

The present study was conducted to examine the re-establishment of IHC/VIII nerve synapses following kainic acid (KA) excitotoxicity and to discern if the re-organized afferent could render not only a normal auditory threshold but also a normal supra-threshold function. KA (60 mM) applied to the intact round window membrane in chinchilla destroyed postsynaptic endings of the auditory nerve, depressed the input-output (I/O) functions of auditory evoked potentials (EVP) and produced an average loss of sensitivity of over 80 dB at 4, 8, and 16 kHz, with less substantial losses (40-60 dB) at lower frequencies. However, there was no significant difference in 2f1-f2 distortion-product otoacoustic emissions (DPOAE) before and after the application of KA. The nerve endings went through a sequence of swelling, degeneration and recovery over a 3-5 day period at higher frequency. Auditory sensitivity and supra-threshold response returned accordingly. In contrast, complete recovery at lower frequencies (1 and 2 kHz) required more than 5 days. The results provide strong evidence that (1) excitotoxically damaged cochlear afferent neurons can recover and render both a normal EVP threshold and EVP I/O function and (2) afferent innervation to IHCs is not necessary for DPOAE generation.

Effects of kainic acid on the cochlear potentials and distortion product otoacoustic emissions in chinchilla

In absence of acoustic stimulation, the auditory nerve generates electrical noise with a spectral peak between 300 and 3000 Hz (Dolan et al., 1990). This electrical noise is eliminated when the dendrites of auditory nerve fibers are damaged by kainic acid (KA). We hypothesized that the KA-induced damage to the afferent dendrites might alter cochlear micromechanics or modify outer hair cell (OHC) electromotility. The KA-induced decrease in spontaneous electrical noise from the auditory nerve could conceivably reduce the spontaneous sounds recorded in the ear canal and the postulated change in cochlear micromechanics might alter distortion product otoacoustic emissions (DPOAE). To evaluate these hypotheses, we applied KA to the round window of the cochlea. KA reduced the spontaneous electrical noise recorded from the round window and significantly reduced the amplitude of the compound action potential (CAP) to tone bursts at 2, 4 and 8 kHz. KA caused only a slight reduction in the amplitude of the cochlear microphonic (CM) recorded from the round window: however, it had no effect on the spontaneous acoustic noise in the car canal or on 2 f1-f2 DPOAEs. These results suggest that the KA-induced reduction of electrical noise from the auditory nerve has no measurable effect on OHC electromotility as reflected in spontaneous otoacoustic emissions and that damage to the afferent dendrites has no effect on cochlear micromechanics as reflected in DPOAEs.

Chemical synaptic transmission in the cochlea

The last two decades have witnessed major progress in the understanding of cochlear mechanical functioning, and in the emergence of cochlear neurochemistry and neuropharmacology. Recent models describe active processes within the cochlea that amplify and sharpen the mechanical response to sound. Although it is widely accepted that outer hair cells (OHCs) contribute to these processes, the nature of the medial efferent influence on cochlear mechanics needs further clarification. Acetylcholine (ACh) is the major transmitter released onto OHCs during the stimulation of these efferents. The inhibitory influence of this system is mediated by post- and presynaptic nicontinic and muscarinic receptors and the role of other neuroactive substances [gamma-aminobutyric acid (GABA), calcitonin gene-related peptide (CGRP), adenosine 5'-triphosphate (ATP) or nitric oxide (NO)] remains to be determined. The inner hair cells (IHCs) that transduce the mechanical displacements into neural activity, release glutamate on receptor-activated channels of AMPA, kainate, and NMDA types. This synapse is in turn controlled and/or regulated by the lateral efferents containing a cocktail of neuroactive substances (ACh, GABA, dopamine, enkephalins, dynorphin, CGRP). This glutamatergic nature of the IHCs is responsible for the acute destruction of the nerve endings and subsequently for neuronal death, damage usually described in various cochlear diseases (noise-induced hearing losses, neural presbycusis and certain forms of sudden deafness or peripheral tinnitus). These pathologies also include a regrowth of new dendritic processes by surviving neurons up to IHCs. Understanding the subtle molecular mechanisms which underly the control of neuronal excitability, synaptic plasticity and neuronal death in cochlear function and disease is a very important issue for the development of future therapies.

Immunocytochemical localization of AMPA selective glutamate receptor subunits in the rat cochlea

The localization of subunits of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) selective glutamate receptor, termed Glutamate receptor (GluR) was examined in the rat cochlea using affinity purified polyclonal antibody to GluR subunits (GluR 1-4). GluR 2/3 and GluR 4 immunoreactive (IR) staining was observed in rat spiral ganglion cells, while GluR 1 IR was not. GluR 4 IR staining was also seen in puncta at inner and outer hair cell bases. These results suggest that GluR 2/3 and GluR 4 are components of excitatory amino acid synapses in the rat cochlea.

Identification of a glutamate/aspartate transporter in the rat cochlea

The neurotransmitter at the synapses between hair cells and spiral ganglion cells in the cochlea is probably L-glutamate or a similar excitatory amino acid. Glutamate uptake by nerve terminals and glial cells is an important component of neurotransmission at glutamatergic synapses of the central nervous system, for providing a reservoir of transmitter or transmitter precursors and the termination of the released glutamate. Hair cell synapses are not surrounded by glial cells, therefore, the uptake mechanism for glutamate in the cochlea may be unique. cDNA was synthesized from total RNA isolated separately from the rat organ of Corti, spiral ganglia, and lateral wall tissues. The expression of a glutamate/aspartate transporter (GLAST) was detected by DNA amplification with the polymerase chain reaction. The other two members of glutamate transporters in this family were not detected by this method. A partial cDNA encoding to GLAST was identified by sequence analysis in a rat cochlear cDNA library. Data concerning the expression and the molecular structure of the glutamate transporter GLAST in the cochlea may provide important information regarding the neurotransmission process at the hair cell-afferent synapses.

Excitatory amino acid antagonists protect cochlear auditory neurons from excitotoxicity

Since ischemic damage in the brain is linked to glutamate excitotoxicity, the effects of an acute exposure to glutamate, alpha-amino-3-hydroxy-5-methyl-4-isoxazole proprionic acid (AMPA) or N-methyl-D aspartate (NMDA) on the radial dendrites were compared with those occurring after a severe cochlear ischemia. Glutamate and AMPA, but not NMDA, produced a drastic swelling restricted to the radial dendrites below the inner hair cells (IHCs). At a concentration of 20 microM AMPA, a full electrophysiological recovery could be observed in some cochleas after washing the drug out. A prior perfusion of 6-7-dinitroquinoxaline-2,3-dione (DNQX, 50 microM) prevented the 25 microM AMPA-induced dendritic swelling. No protective effect of D-2-amino-5-phosphonopentanoate (D-AP5) could be observed. In the same way, ischemia (5-40 minutes) resulted in a clear swelling of the radial dendrites. While D-AP5 had no protective effects, 50 microM DNQX protected most of the radial dendrites from the ischemia-induced swelling, excepting those contacting the modiolar side of the IHCs. Finally, 50 microM DNQX + 50 microM D-AP5 resulted in a nearly complete protection of all the radial dendrites. Altogether, these results suggest that the acute swelling of radial dendrites primarily occurs via AMPA/kainate receptors. However, in radial dendrites contacting the inner hair cells on their modiolar side, NMDA receptors may be also involved.

Late developmental changes of the innervation densities of the myelinated fibres and the outer hair cell efferent fibres in the rat cochlea

The baso-apical distributions of the myelinated nerve fibres (representative for the inner hair cell afferent fibres) and the outer hair cell efferent fibres were studied during postnatal development of the rat cochlea. The myelinated fibres were counted in the primary osseos spiral lamina from semi-thin sections. The outer hair cell efferent fibres were counted in the tunnel of Corti by means of ultra-thin sections. The developmental changes of the myelinated fibres were investigated between 8 and 60 days after birth (DAB); those of the outer hair cell efferent fibres between 20 and 30 DAB. Between 12 DAB (onset of hearing) and 20 DAB the baso-apical distribution of the myelinated fibres does not change. Striking maturational changes occur later after the onset of hearing, between 20 and 30 DAB. The innervation density of the myelinated fibres increases in the lower middle region of the cochlea. In this region a maximum of innervation density appears. The efferent fibres to the outer hair cells show at 20 DAB a maximum of innervation density in the middle of the cochlea but between 20 and 30 DAB, the fibre density decreases in this region. During the same period the maximum of innervation density shifts towards the base. The change in the innervation densities of the myelinated fibres and the outer hair cell efferent fibres occurs late in development, after the onset of hearing, and after the organ of Corti shows an adult-like appearance.

Pathophysiology of the glutamatergic synapses in the cochlea

The synapses between the inner hair cells (IHCs) and the radial auditory dendrites are thought to be glutamatergic. Besides its fast excitatory properties, glutamate is known to be neurotoxic when released in excess or incompletely recycled. In the cochlea, this may occur in two pathological conditions: ischemia and noise trauma. We have further investigated the acute excitotoxicity (i.e. the swelling of type I afferent dendrites) by electron microscopy processing on guinea pig cochleas after an ischemic exposure lasting 5 to 40 min. The radial auditory dendrites reacted to ischemia in a time-dependent manner, with the swelling extending when the duration of ischemia increased. The type and the specificity of swelling were comparable to what acutely occurs after an exposure to glutamate analogs such as kainic acid or AMPA. A protection against this swelling was obtained by perfusing the cochlea with glutamate antagonists prior to ischemia. DNQX, an antagonist at AMPA/kainate receptors, had a powerful protective effect, and almost complete protection was obtained by perfusing both DNQX and D-AP5 (a NMDA antagonist). The latter results indicate that the two classes of glutamate receptors (AMPA/kainate and NMDA), both found to be electrophysiologically active at the IHC-auditory nerve synapse, are also involved in the excitotoxic processes. In addition, we also report data involving dopamine (its D2 agonist piribedil) a putative neurotransmitter at the lateral efferent synapses, in a postsynaptic protection of primary auditory neurons during transient ischemia. Altogether, these findings constitute a promising pharmacological approach of cochlear pathologies such as neural presbycusis.

Caroverine depresses the activity of cochlear glutamate receptors in guinea pigs: In vivo model for drug-induced neuroprotection?

With the aid of microiontophoretic techniques the action of caroverine, a quinoxaline-derivative, was tested on the receptor-linked depolarisation of the subsynaptic membrane of cochlear afferents. This membrane can be depolarised by the afferent transmitter agonist glutamate, mediated by NMDA and non-NMDA receptors and by acetylcholine, one of the different transmitter substances, released physiologically on axodendritic efferent synapses. Caroverine antagonized the membrane response to glutamate in an enduring but reversible manner. In contrast, the drug exhibited no effect on the depolarising action of acetylcholine. Therefore, the pharmacological profile of caroverine corresponded to the action of selective glutamate receptor antagonists. Since glutamate is likely to be the major mediator of neurotoxicity in the central nervous system, the selective glutamate-antagonism of caroverine is of particular interest, due to its putative neuroprotective competence. Caroverine is currently available clinically in some countries as a spasmolytic drug. Following these results it is proposed to test the drug for clinical efficacy in putatively glutamate-induced, excitotoxic disorders of the brain.

Intense sound increases the level of an unidentified amine found in perilymph

The hypothesis tested was that intense sound increases the levels of a substance such as glutamate, a putative neurotransmitter and neurotoxic substance, in the perilymph compartment of the cochlea. Artificial perilymph was perfused through the perilymphatic compartment of the guinea pig cochlea and the effluent collected during successive 10-min periods. The effects of perfusing an artificial perilymph containing normal levels of Na+ (NARP) were compared to the effects of perfusing an artificial perilymph containing very low concentrations of Na+ (VLNa). The effluent was collected during ambient noise and during increasing intensities of broad-band noise (10 min at 106, 112, 118 and 124 dB SPL). Levels of amines in the effluent were measured by HPLC utilizing precolumn o-phthalaldehyde (OPA) derivatization and fluorescence detection. VLNa increased the levels of glutamate and several other amines in effluent from the cochlea compared to levels obtained in NARP. Compared with its level during ambient room noise, the concentration of an unidentified amine labeled Unk 2.5 increased during intense noise (124 dB SPL). Intense noise induced no detectable changes in the concentrations of glutamate and fifteen other amines. The chemical identity and role of Unk 2.5 remain to be determined.

Bone repair in the otic capsule of the rabbit

The repair of 1 mm drill hole lesions in rabbit long bones and otic capsules was studied in undecalcified sections. Following surgery, bone formation was monitored during 12 weeks by intravital sequential time labeling with bone-seeking fluorochromes. In diaphyseal bone, initial repair by trabecular woven bone was accompanied by a transient remodeling response in primary repair bone and the surrounding compacta. In comparison, capsular defects showed a slightly deficient initial repair by woven bone which, once formed, persisted unchanged with no subsequent osteonal remodeling. In the surrounding capsule, the remodeling response was subtle and its duration much shorter. Moreover, the centrifugal distribution of bone remodeling units around inner ear spaces was preserved, suggesting that capsular repair is affected by the same morphogenetic field that controls perilabyrinthine growth, modeling and remodeling. The significance of these findings is briefly discussed in relation to otosclerosis and cochlear implant strategies.