The quinoxalinediones DNOX, CNOX and two related congeners suppress hair cell-to-auditory nerve transmission

Vestibular Testing-New Physiological Results for the Optimization of Clinical VEMP Stimuli

Both auditory and vestibular primary afferent neurons can be activated by sound and vibration. This review relates the differences between them to the different receptor/synaptic mechanisms of the two systems, as shown by indicators of peripheral function-cochlear and vestibular compound action potentials (cCAPs and vCAPs)-to click stimulation as recorded in animal studies. Sound- and vibration-sensitive type 1 receptors at the striola of the utricular macula are enveloped by the unique calyx afferent ending, which has three modes of synaptic transmission. Glutamate is the transmitter for both cochlear and vestibular primary afferents; however, blocking glutamate transmission has very little effect on vCAPs but greatly reduces cCAPs. We suggest that the ultrafast non-quantal synaptic mechanism called resistive coupling is the cause of the short latency vestibular afferent responses and related results-failure of transmitter blockade, masking, and temporal precision. This "ultrafast" non-quantal transmission is effectively electrical coupling that is dependent on the membrane potentials of the calyx and the type 1 receptor. The major clinical implication is that decreasing stimulus rise time increases vCAP response, corresponding to the increased VEMP response in human subjects. Short rise times are optimal in human clinical VEMP testing, whereas long rise times are mandatory for audiometric threshold testing.

Use of the guinea pig in studies on the development and prevention of acquired sensorineural hearing loss, with an emphasis on noise

Guinea pigs have been used in diverse studies to better understand acquired hearing loss induced by noise and ototoxic drugs. The guinea pig has its best hearing at slightly higher frequencies relative to humans, but its hearing is more similar to humans than the rat or mouse. Like other rodents, it is more vulnerable to noise injury than the human or nonhuman primate models. There is a wealth of information on auditory function and vulnerability of the inner ear to diverse insults in the guinea pig. With respect to the assessment of potential otoprotective agents, guinea pigs are also docile animals that are relatively easy to dose via systemic injections or gavage. Of interest, the cochlea and the round window are easily accessible, notably for direct cochlear therapy, as in the chinchilla, making the guinea pig a most relevant and suitable model for hearing. This article reviews the use of the guinea pig in basic auditory research, provides detailed discussion of its use in studies on noise injury and other injuries leading to acquired sensorineural hearing loss, and lists some therapeutics assessed in these laboratory animal models to prevent acquired sensorineural hearing loss.

Intracochlear drug delivery: Fluorescent tracer evaluation for quantification of distribution in the cochlear partition

Measurement of drug distribution in the inner ear has important roles in the design of local delivery methods, such as direct, intracochlear delivery, and in assessment of emerging drug candidates in preclinical animal models. Sampling methods have been used in the past to measure drug concentrations in the cochlear fluids, but these methods provide no direct information about drug distribution in the cochlear tissues. In this work, we evaluated four fluorescent markers that simulate drug distribution in the organ of Corti after intracochlear delivery to the cochlea's scala tympani compartment. Our hypothesis is that ultimately, a cocktail comprising several fluorescent drug surrogates or fluorescently-tagged drugs, each with differing distribution, spreading, and clearance behavior, can be used to evaluate both transient and cumulative drug distributions associated with different delivery techniques. In this study, FITC-dextran, Qtracker™ 655, gentamicin Texas-Red, and FM 1-43 FX were each evaluated as candidate markers by direct intracochlear infusion into guinea-pig cochleae. Distribution of the markers was measured using fluorescence confocal microscopy imaging of cochlear whole mount dissections from animals sacrificed 3 h after the tracer-infusion. For all four tracers, strong fluorescence was observed in the tissue sections near the base, but only Qtracker™-655, gentamicin Texas-Red (GTTR) and FM 1-43 FX exhibited any specificity in labelling of the sensory hair cells. Therefore, these substances represent leading candidates for the quantification drug distribution achieved by different delivery approaches to the scala tympani.

A fluorescence-based imaging approach to pharmacokinetic analysis of intracochlear drug delivery

Advances in microelectromechanical systems (MEMS) technologies are enhancing the development of intracochlear delivery devices for the treatment of hearing loss with emerging pharmacological therapies. Direct intracochlear delivery addresses the limitations of systemic and intratympanic delivery. However, optimization of delivery parameters for these devices requires pharmacokinetic assessment of the spatiotemporal drug distribution inside the cochlea. Robust methods of measuring drug concentration in the perilymph have been developed, but lack spatial resolution along the tonotopic axis or require complex physiological measurements. Here we describe an approach for quantifying distribution of fluorescent drug-surrogate probe along the cochlea's sensory epithelium with high spatial resolution enabled by confocal fluorescence imaging. Fluorescence from FM 1-43 FX, a fixable endocytosis marker, was quantified using confocal fluorescence imaging of whole mount sections of the organ of Corti from cochleae resected and fixed at several time points after intracochlear delivery. Intracochlear delivery of FM 1-43 FX near the base of the cochlea produces a base-apex gradient of fluorescence in the row of inner hair cells after 1 h post-delivery that is consistent with diffusion-limited transport along the scala tympani. By 3 h post-delivery there is approximately an order of magnitude decrease in peak average fluorescence intensity, suggesting FM 1-43 FX clearance from both the perilymph and inner hair cells. The increase in fluorescence intensity at 72 h post-delivery compared to 3 h post-delivery may implicate a potential radial transport pathway into the scala media.

Microfabricated reciprocating micropump for intracochlear drug delivery with integrated drug/fluid storage and electronically controlled dosing

The anatomical and pharmacological inaccessibility of the inner ear is a major challenge in drug-based treatment of auditory disorders. This also makes pharmacokinetic characterization of new drugs with systemic delivery challenging, because efficacy is coupled with how efficiently a drug can reach its target. Direct delivery of drugs to cochlear fluids bypasses pharmacokinetic barriers and helps to minimize systemic toxicity, but anatomical barriers make administration of multiple doses difficult without an automated delivery system. Such a system may be required for hair-cell regeneration treatments, which will likely require timed delivery of several drugs. To address these challenges, we have developed a micropump for controlled, automated inner-ear drug delivery with the ultimate goal of producing a long-term implantable/wearable delivery system. The current pump is designed to be used with a head mount for guinea pigs in preclinical drug characterization experiments. In this system, we have addressed several microfluidic challenges, including maintaining controlled delivery at safe, low flow rates and delivering drug without increasing the volume of fluid in the cochlea. By integrating a drug reservoir and all fluidic components into the microfluidic structure of the pump, we have made the drug delivery system robust compared to previous systems that utilized separate, tubing-connected components. In this study, we characterized the pump's unique infuse-withdraw and on-demand dosing capabilities on the bench and in guinea pig animal models. For the animal experiments, we used DNQX, a glutamate receptor antagonist, as a physiological indicator of drug delivery. DNQX suppresses compound action potentials (CAPs), so we were able to infer the distribution and spreading of the DNQX over time by measuring the changes in CAPs in response to stimuli at several characteristic frequencies.

Microfabricated infuse-withdraw micropump component for an integrated inner-ear drug-delivery platform

One of the major challenges in treatment of auditory disorders is that many therapeutic compounds are toxic when delivered systemically. Local intracochlear delivery methods are becoming critical in emerging treatments and in drug discovery. Direct infusion via cochleostomy, in particular, is attractive from a pharmacokinetics standpoint, as there is potential for the kinetics of delivery to be well-controlled. Direct infusion is compatible with a large number of drug types, including large, complex molecules such as proteins and unstable molecules such as siRNA. In addition, hair-cell regeneration therapy will likely require long-term delivery of a timed series of agents. This presents unknown risks associated with increasing the volume of fluid within the cochlea and mechanical damage caused during delivery. There are three key requirements for an intracochlear drug delivery system: (1) a high degree of miniaturization (2) a method for pumping precise and small volumes of fluid into the cochlea in a highly controlled manner, and (3) a method for removing excess fluid from the limited cochlear fluid space. To that end, our group is developing a head-mounted microfluidics-based system for long-term intracochlear drug delivery. We utilize guinea pig animal models for development and demonstration of the device. Central to the system is an infuse-withdraw micropump component that, unlike previous micropump-based systems, has fully integrated drug and fluid storage compartments. Here we characterize the infuse-withdraw capabilities of our micropump, and show experimental results that demonstrate direct drug infusion via cochleostomy in animal models. We utilized DNQX, a glutamate receptor antagonist that suppresses CAPs, as a test drug. We monitored the frequency-dependent changes in auditory nerve CAPs during drug infusion, and observed CAP suppression consistent with the expected drug transport path based on the geometry and tonotopic organization of the cochlea.

Intracochlear drug delivery systems

INTRODUCTION

Advances in molecular biology and in the basic understanding of the mechanisms associated with sensorineural hearing loss and other diseases of the inner ear are paving the way towards new approaches for treatments for millions of patients. However, the cochlea is a particularly challenging target for drug therapy, and new technologies will be required to provide safe and efficacious delivery of these compounds. Emerging delivery systems based on microfluidic technologies are showing promise as a means for direct intracochlear delivery. Ultimately, these systems may serve as a means for extended delivery of regenerative compounds to restore hearing in patients suffering from a host of auditory diseases.

AREAS COVERED

Recent progress in the development of drug delivery systems capable of direct intracochlear delivery is reviewed, including passive systems such as osmotic pumps, active microfluidic devices and systems combined with currently available devices such as cochlear implants. The aim of this article is to provide a concise review of intracochlear drug delivery systems currently under development and ultimately capable of being combined with emerging therapeutic compounds for the treatment of inner ear diseases.

EXPERT OPINION

Safe and efficacious treatment of auditory diseases will require the development of microscale delivery devices, capable of extended operation and direct application to the inner ear. These advances will require miniaturization and integration of multiple functions, including drug storage, delivery, power management and sensing, ultimately enabling closed-loop control and timed-sequence delivery devices for treatment of these diseases.

Kinetics of reciprocating drug delivery to the inner ear

Reciprocating drug delivery is a means of delivering soluble drugs directly to closed fluid spaces in the body via a single cannula without an accompanying fluid volume change. It is ideally suited for drug delivery into small, sensitive and unique fluid spaces such as the cochlea. We characterized the pharmacokinetics of reciprocating drug delivery to the scala tympani within the cochlea by measuring the effects of changes in flow parameters on the distribution of drug throughout the length of the cochlea. Distribution was assessed by monitoring the effects of DNQX, a reversible glutamate receptor blocker, delivered directly to the inner ear of guinea pigs using reciprocating flow profiles. We then modeled the effects of those parameters on distribution using both an iterative curve-fitting approach and a computational fluid dynamic model. Our findings are consistent with the hypothesis that reciprocating delivery distributes the drug into a volume in the base of the cochlea, and suggest that the primary determinant of distribution throughout more distal regions of the cochlea is diffusion. Increases in flow rate distributed the drug into a larger volume that extended more apically. Over short time courses (less than 2h), the apical extension, though small, significantly enhanced apically directed delivery of drug. Over longer time courses (>5h) or greater distances (>3mm), maintenance of drug concentration in the basal scala tympani may prove more advantageous for extending apical delivery than increases in flow rate. These observations demonstrate that this reciprocating technology is capable of providing controlled delivery kinetics to the closed fluid space in the cochlea, and may be suitable for other applications such as localized brain and retinal delivery.

Development of a microfluidics-based intracochlear drug delivery device

BACKGROUND

Direct delivery of drugs and other agents into the inner ear will be important for many emerging therapies, including the treatment of degenerative disorders and guiding regeneration.

METHODS

We have taken a microfluidics/MEMS (MicroElectroMechanical Systems) technology approach to develop a fully implantable reciprocating inner-ear drug-delivery system capable of timed and sequenced delivery of agents directly into perilymph of the cochlea. Iterations of the device were tested in guinea pigs to determine the flow characteristics required for safe and effective delivery. For these tests, we used the glutamate receptor blocker DNQX, which alters auditory nerve responses but not cochlear distortion product otoacoustic emissions.

RESULTS

We have demonstrated safe and effective delivery of agents into the scala tympani. Equilibration of the drug in the basal turn occurs rapidly (within tens of minutes) and is dependent on reciprocating flow parameters.

CONCLUSION

We have described a prototype system for the direct delivery of drugs to the inner ear that has the potential to be a fully implantable means for safe and effective treatment of hearing loss and other diseases.

Local drug delivery with a self-contained, programmable, microfluidic system

The development and optimization of many new drug therapies requires long-term local delivery with controlled, but variable dosage. Current methods for chronic drug delivery have limited utility because they either cannot deliver drugs locally to a specific organ or tissue, do not permit changes in delivery rate in situ, or cannot be used in clinical trials in an untethered, wearable configuration. Here, we describe a small, self-contained system for liquid-phase drug delivery. This system enables studies lasting several months and infusion rates can be programmed and modified remotely. A commercial miniature pump is integrated with microfabricated components to generate ultralow flow rates and stroke volumes. Solutions are delivered in pulses as small as 370 nL, with pulses delivered at any interval of 1 min or longer. A unique feature of the system is the ability to infuse and immediately withdraw liquid, resulting in zero net volume transfer while compounds are exchanged by mixing and diffusion with endogenous fluid. We present in vitro results demonstrating repeatability of the delivered pulse volume for nearly 3 months. Furthermore, we present in vivo results in an otology application, infusing into the cochlea of a guinea pig a glutamate receptor antagonist, which causes localized and reversible changes in auditory sensitivity.

Dopamine transporter is essential for the maintenance of spontaneous activity of auditory nerve neurones and their responsiveness to sound stimulation

Dopamine, a neurotransmitter released by the lateral olivocochlear efferents, has been shown tonically to inhibit the spontaneous and sound-evoked activity of auditory nerve fibres. This permanent inhibition probably requires the presence of an efficient transporter to remove dopamine from the synaptic cleft. Here, we report that the dopamine transporter is located in the lateral efferent fibres both below the inner hair cells and in the inner spiral bundle. Perilymphatic perfusion of the dopamine transporter inhibitors nomifensine and N-[1-(2-benzo[b]thiophenyl)cyclohexyl]piperidine into the cochlea reduced the spontaneous neural noise and the sound-evoked compound action potential of the auditory nerve in a dose-dependent manner, leading to both neural responses being completely abolished. We observed no significant change in cochlear responses generated by sensory hair cells (cochlear microphonic, summating potential, distortion products otoacoustic emissions) or in the endocochlear potential reflecting the functional state of the stria vascularis. This is consistent with a selective action of dopamine transporter inhibitors on auditory nerve activity. Capillary electrophoresis with laser-induced fluorescence (EC-LIF) measurements showed that nomifensine-induced inhibition of auditory nerve responses was due to increased extracellular dopamine levels in the cochlea. Altogether, these results show that the dopamine transporter is essential for maintaining the spontaneous activity of auditory nerve neurones and their responsiveness to sound stimulation.

Inner ear drug delivery via a reciprocating perfusion system in the guinea pig

Rapid progress in understanding the molecular mechanisms associated with cochlear and auditory nerve degenerative processes offers hope for the development of gene-transfer and molecular approaches to treat these diseases in patients. For therapies based on these discoveries to become clinically useful, it will be necessary to develop safe and reliable mechanisms for the delivery of drugs into the inner ear, bypassing the blood-labyrinthine barrier. Toward the goal of developing an inner ear perfusion device for human use, a reciprocating microfluidic system that allows perfusion of drugs into the cochlear perilymph through a single inlet hole in scala tympani of the basal turn was developed. The performance of a prototype, extracorporeal reciprocating perfusion system in guinea pigs is described. Analysis of the cochlear distribution of compounds after perfusion took advantage of the place-dependent generation of responses to tones along the length of the cochlea. Perfusion with a control artificial perilymph solution had no effect. Two drugs with well-characterized effects on cochlear physiology, salicylate (5 mM) and DNQX (6,7-Dinitroquinoxaline-2,3-dione; 100 and 300 microM), reversibly altered responses. The magnitude of drug effect decreased with distance from the perfusion pipette for up to 10 mm, and increased with dose and length of application.

A method for intracochlear drug delivery in the mouse

The confluence of two rapidly emerging research arenas - development of mouse models of human deafness and inner ear drug therapy for treatment and prevention of hearing loss - provides an opportunity for unprecedented approaches to study and treat deafness. Toward such goals, we have developed a method for intracochlear drug delivery in the mouse. The bulla was exposed using a ventral approach and the stapedial artery cauterized. An opening made into the inferior-medial aspect of the bulla, where the basal cochlear wall fuses with tympanic bulla, provided direct access to the scala tympani without separately opening the bulla or elevating auditory response thresholds. Cochlear responses, assayed by frequency-specific effects on ABRs and DPOAEs, were stable with infusion (1 microl/h) of an artificial perilymph solution (80 min). The glutamate receptor antagonist, CNQX (100 microM; 175 min), reduced ABR responses without affecting DPOAEs. Salicylate (5mM; 165 min) altered both. Both drugs had greatest effects at high frequencies, but distributed throughout the cochlea and were reversible. The safe delivery of drugs into the cochlea by this approach has immediate application in the study and treatment of various forms of human hearing loss that can be modeled in the mouse.

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.

The simultaneous in vivo perilymphatic perfusion of avian auditory and vestibular end organs

Perilymphatic perfusion is a method that allows the control of fluid parameters throughout the perilymphatic space of the inner ear. We have evaluated a new method for continuous perilymphatic perfusion of the auditory and vestibular end organs with artificial perilymph (APL) in chickens. Perfusate temperature (39.0 degrees C), pH (7.4), osmolarity (328 +/- 2 mosm), and flow rate (2 microl/min) were carefully controlled. Independent functional tests of vestibular and auditory sensory systems were made throughout perfusion periods by recording peripheral compound action potentials (CAPs). The recordings provided a means of monitoring the status of hair cell transduction, synaptic transmission and collective primary afferent activation in response to auditory or vestibular gravity receptor stimuli. Auditory and vestibular responses were stable during perfusion. No significant changes occurred in vestibular or auditory CAP amplitudes during long-term perfusion (50-80 min, n=7) and responses remained stable in one animal perfused for over 3 h. To our knowledge, there have been no reports evaluating vestibular function under these conditions. This technique enables us to systematically study receptor pharmacology in the peripheral vestibular and auditory systems virtually simultaneously in vivo. The model is well suited for use in the study of the pharmacology and toxicology of inner ear sensory systems.

NMDA receptor blockage protects against permanent noise-induced hearing loss but not its potentiation by carbon monoxide

While a clear role has been proposed for glutamate as a putative neurotransmitter at the inner hair cell type I spiral ganglion cell synapse, the possible role of excessive glutamate release in cochlear impairment and of NMDA receptors in such a process is uncertain. The present study compares the protective effects of (+)-MK-801, an NMDA receptor antagonist, and the relatively inactive isomer (-)-MK-801 against permanent noise-induced hearing loss (NIHL). The study also asks whether (+)-MK-801 can protect against the NIHL potentiation by carbon monoxide (CO). Rats (n = 6) were exposed to 100-dB, 13.6-kHz octave-band noise for 2 h after receiving injection of (+)-MK-801 hydrogen maleate (1 mg/kg), (-)-MK-801 hydrogen maleate (1 mg/kg), or saline. Other groups of animals were exposed to the combination of noise and CO (1200 ppm) after receiving (+)-MK-801 or saline. Additional subjects received (+)-MK-801, saline or CO exposure alone. Compound action potential (CAP) threshold sensitivities were compared 4 weeks after the exposures. The results show significant protection by (+)-MK-801 against the permanent CAP threshold elevation induced by noise alone, but no protective effect of (-)-MK-801. (+)-MK-801 produced limited protection against threshold shifts induced by the combination of noise and CO. Outer hair cell (OHC) loss was not protected by (+)-MK-801 administration. The data suggest that NMDA receptor stimulation may play a role in NIHL resulting from fairly mild noise exposure. The data do not support a role for NMDA receptor stimulation in the potentiation of NIHL that results from simultaneous exposure to CO and noise.

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.

Contribution of glutamate receptors to spontaneous and stimulus-evoked discharge in afferent fibers innervating hair cells of the Xenopus lateral line organ

The relative contributions of NMDA (N-methyl-D-aspartate) and non-NMDA glutamate receptors to spontaneous and stimulus-evoked transmission at the hair cell/afferent fiber synapse were determined in the Xenopus laevis lateral line organ. The non-NMDA receptor antagonist, CNQX (6-cyano-7-nitroquinoxaline-2,3-dione), reversibly reduced both spontaneous and stimulus-evoked discharge rate with an EC(50) of 0.5 microM. NMDA receptor antagonism with the combination of chlorokynurenic acid (100 microM) and elevated magnesium (1.1 mM), or elevated magnesium alone, blocked responses to NMDA without significantly altering spontaneous or stimulus-evoked discharge rate or the responses to kainate. All non-NMDA receptor agonists tested increased discharge rate at low concentrations and, at higher concentrations, increased, then suppressed discharge rate. The EC(50)s were: domoic acid (2.4 mcM)<quisqualic acid (6 mcM)<kainic acid (18 mcM)<AMPA (82 mcM)<<glutamate (1150 mcM). NMDA and ibotenic acid also produced an increase in discharge followed by a suppression, but the suppressive phase of the response predominated and maximum increases in discharge rates were low compared to effects of the non-NMDA agonists. The EC(50)s were: NMDA (148 mcM)<ibotenic acid (463 mcM). The EC(50) for the suppression of afferent discharge that followed the initial excitatory effect was similar to the EC(50) for excitation. Perfusion with active concentrations of kainate, AMPA, or NMDA did not alter the threshold for electrical stimulation of these nerve fibers. We conclude that most of the postsynaptic signal normally seen in afferent fibers is mediated by non-NMDA receptors.

Hearing loss and glutamate efflux in the perilymph following transient hindbrain ischemia in gerbils

The mechanism underlying ischemia-induced hearing loss was studied in gerbils with transient hindbrain ischemia. Occlusion of the vertebral arteries caused an increase in the concentration of glutamate in the perilymph and elevated the compound action potential (CAP) threshold to 24.6 dB at 5 minutes. the CAP threshold subsequently recovered on reperfusion, gradually reaching 8.3 dB 120 minutes after reperfusion. Under electron microscopy, afferent dendrites of the cochlear nerve in contact with inner hair cells exhibited abnormal swelling 5 minutes after ischemia/reperfusion. These morphological changes were not observed in cochleas treated with an alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)/kainate-type glutamate receptor antagonist, 6-7-dinitroquinoxaline-2,3-dione (DNQX), before hindbrain ischemia; an N-methyl-D-aspartate (NMDA)-type receptor antagonist, D-2-amino-5-phosphonopentanoate (D-AP5), was ineffective. Moreover, the histopathological alterations noted 5 minutes after reperfusion were spontaneously ameliorated 120 minutes after ischemia/reperfusion. These findings suggest that the ischemia-induced increase in extracellular glutamate concentration with subsequent activation of AMPA/kainate receptors is responsible for neurite degeneration and hearing loss in the early stages following transient hindbrain ischemia.

AMPA-preferring glutamate receptors in cochlear physiology of adult guinea-pig

1. The present study was designed to determine which glutamate (Glu) receptors are involved in excitatory neurotransmission at the first auditory synapse between the inner hair cells and the spiral ganglion neurons. 2. The Glu receptors present at the membrane level were investigated on isolated spiral ganglion neuron somata from guinea-pigs by whole-cell voltage-clamp measurements. Glu and AMPA induced a fast onset inward current that was rapidly desensitized, while kainate induced only a non-desensitizing, steady-state current. NMDA induced no detectable current. 3. To further discriminate between the AMPA and kainate receptors present, we used the receptor-specific desensitization blockers, cyclothiazide and concanavalin A. While no effect was observed with concanavalin A, cyclothiazide greatly enhanced the Glu-, AMPA- and kainate-induced steady-state currents and potentiated Glu-induced membrane depolarization. 4. To extrapolate the results obtained from the somata to the events occurring in situ at the dendrites, the effects of these drugs were evaluated in vivo. Cyclothiazide reversibly increased spontaneous activity of single auditory nerve fibres, while concanavalin A had no effect, suggesting that the functional Glu receptors on the somata may be the same as those at the dendrites. 5. The combination of a moderate-level sound together with cyclothiazide increased and subsequently abolished the spontaneous and the sound-evoked activity of the auditory nerve fibres. Histological examination revealed destruction of the dendrites, suggesting that cyclothiazide potentiates sound-induced Glu excitotoxicity via AMPA receptors. 6. Our results reveal that fast synaptic transmission in the cochlea is mainly mediated by desensitizing AMPA receptors.

Implication of NMDA type glutamate receptors in neural regeneration and neoformation of synapses after excitotoxic injury in the guinea pig cochlea

In the adult mammalian cochlea, the ability of nerve fibres to regenerate has been observed following disruption of the organ of Corti by various means, or transsection of the cochlear nerve in the internal auditory meatus. Based upon the implication of glutamate as a neurotransmitter at synapses between sensory hair cells and terminal dendrites of the auditory nerve in the mammalian cochlea, we have developed, in a previous study, an in vivo model of neural regeneration and formation of synapses after the destruction of the afferent nerve endings by local application of the glutamate agonist alpha-amino-3-hydroxy-5-methyl-isoxazol-propionic acid (AMPA). In situ hybridization experiments performed during the re-innervation process revealed an overexpression of mRNA coding for NR1 subunit of N-methyl-D-aspartate (NMDA) receptors in the spiral ganglion neurons, suggesting that these receptors are implicated in neural regenerative processes. The present study has been designed to study the functional implication of NMDA receptors in the regrowth and synaptic repair of auditory dendrites in the guinea pig cochlea, by blocking the NMDA receptors during the period of normal functional recovery. In a first set of experiments, we recorded compound action potential after acute perilymphatic perfusion of cumulative doses (0.03-10mM) of DL 2-amino-5-phosphonovalerate (D-AP5), a NMDA antagonist, to determine the efficiency of the drug. In a second set of experiments, the auditory dendrites were destroyed by local application of the glutamate agonist AMPA. The blockage of NMDA by the antagonist D-AP5 applied with an osmotic micropump delayed the functional recovery and the regrowth of auditory dendrites. The findings of our study support the hypothesis that, in addition to acting as a fast transmitter, glutamate has a neurotrophic role via the activation of NMDA receptors.

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.

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.

Glutamate neurotoxicity in the developing rat cochlea is antagonized by kynurenic acid and MK-801

Glutamate (Glu) is neurotoxic in the neonatal rat cochlea, producing hearing impairment which is largely due to the death of spiral ganglion cells, whereas the receptor hair cells are spared. Dendritic processes of the spiral ganglion are postsynaptic to the primary afferent synapse of the auditory system. The experiments reported here were designed to test whether this apparent excitotoxicity can be blocked by Glu antagonists. The broad-spectrum antagonist kynurenic acid (KYNA) was coadministered with Glu initially to determine whether the high-frequency hearing deficit caused by Glu may be mediated by excitatory amino acid receptors. Subsequently, the N-methyl-D-aspartate (NMDA)-specific receptor blocker MK-801 was used to test whether NMDA receptors may be involved in the effect. Both antagonists partially blocked the high-frequency hearing impairment caused by Glu. The blocker-alone control groups exhibited mid-frequency effects of unknown origin. The significant antagonism of Glu-induced impairment is consistent with the hypothesis that Glu or a similar excitatory amino acid is an important afferent transmitter in the cochlea.

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

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

Magnitude of the negative summating potential varies with perilymph calcium levels

Previous results demonstrated that nimodipine, an L-type of Ca2+ channel antagonist, abolished the negative summating potential (SP) recorded from anesthetized guinea pigs (Bobbin et al., 1990), suggesting that Ca2+ is involved in generation of the negative SP. Therefore we examined the effect of changing concentrations of perilymph Ca2+ on this cochlear potential. Perilymph spaces of guinea pig cochleae were perfused with artificial perilymph solutions containing zero mM Ca2+, zero mM Ca2+ with 2 mM EGTA, 30 mM Mg2+ and increasing levels of Ca2+ (2, 4, 8, 16 mM) at a rate of 2.5 microliters/min for 10 min. Immediately after each period of perfusion the compound action potential of the auditory nerve (CAP), cochlear microphonics (CM) and the negative SP evoked by 10 kHz tone bursts of varying intensities were recorded from a wire inserted in the basal turn scala vestibuli. Decreasing the level of Ca2+ decreased the magnitude of the negative SP, whereas increasing the level of Ca2+ progressively increased the magnitude of the negative SP. Mg2+ (30 mM) suppressed the CAP to the same extent as zero mM Ca2+ with 2 mM EGTA, but only slightly increased the magnitude of the negative SP. These results support the hypothesis that Ca2+ and L-type Ca2+ channels are involved in the function of the hair cells and the generation of the negative SP. Mg2+ appears to be a selective antagonist of the Ca2+ channel involved in transmitter release.

Changing cation levels (Mg2+, Ca2+, Na+) alters the release of glutamate, GABA and other substances from the guinea pig cochlea

We examined the effects of changes in cation levels (increased Mg2+ concentration combined with low Ca2+ concentration, and two low concentrations of Na+) on the perilymph levels of gamma-aminobutyric acid (GABA), glutamate (Glu), aspartate (Asp) and other substances. Artificial perilymph solutions containing normal (5 mM) and high (50 mM) levels of K+ were perfused through the perilymphatic compartment of the guinea pig cochlea to examine basal release (5 mM K+) and depolarization-induced release (50 mM K+). Each of the two K+ concentrations were contained in four different solutions: [I] normal artificial perilymph (NARP; NaCl, 137 mM; CaCl2, 2 mM; MgCl2, 1 mM;); [II] high Mg2+ (20 mM)/low Ca2+ (0.1 mM) (HMgLCa); [III] low Na+ (117 mM; LNa), and [IV] very low Na+ (NaCl, 0 mM; VLNa). The effluent was collected and assayed for eighteen primary amines by HPLC. Compared with NARP, the HMgLCa group had an increase in the high K(+)-induced release of Asp and Glu with no change in GABA. VLNa increased the normal K+ levels of Asp, Glu and GABA up to those observed with high K+ in NARP. VLNa increased the high K+ levels of Asp and Glu over fivefold compared with the high K+ levels in NARP, but decreased GABA. We ascribe the results to an interference with either a Na(+)-dependent uptake processes or a Na+/Ca2+ exchange carrier.

Glutamate neurotoxicity in the developing rat cochlea: physiological and morphological approaches

The neurotoxic effects of exogenous glutamate were studied in the rat cochlea. Glutamate-treated rats (4 g/kg/day i.p., postnatal days 2-9) exhibited electrophysiologically-measured elevations in high frequency thresholds usually associated with hair cell loss in the basal region of the cochlea. While surface preparations of the organ of Corti revealed no loss of hair cells, there was a dramatic and selective reduction of neurons in the basal, high frequency-related portion of the spiral ganglion. This sensitivity of developing spiral ganglion cells to the neurotoxicity of glutamate is consistent with the hypothesis that glutamate or a structurally related substance is a neurotransmitter at afferent synapses of cochlear hair cells.

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

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

Glutamate receptors in afferent cochlear neurotransmission in guinea pigs

With the aid of microinotophoretic techniques we tested the action of the transmitter candidate glutamate (Glu) at the afferent synapses of inner hair cells (IHC) in guinea pigs. In order to determine the various types of glutamate receptors, further agonistic excitatory amino acids (EAA) as well as competitive EAA-antagonists were used. Applied perisynaptically, Glu, aspartate, N-methyl-D-aspartate (NMDA), quisqualate (Q) and kainate (K) activate the subsynaptic, phasic firing activity of the afferent dendrites. The NMDA-induced activation is augmented by simultaneous application of glycine. The firing rate induced by Glu and NMDA is blocked by the specific NMDA-antagonist D-2-amino-7-phosphonoheptanoate (AP-7). Furthermore, activity induced by Glu and Q decreases under the influence of the selective Q-antagonist glutamic acid diethylester (GDEE). These results are consistent with the hypothesis that Glu acts as a possible afferent neurotransmitter of the IHC. This neurotransmission is mediated by postsynaptic EAA-receptor subpopulations which are sensitive to NMDA, Q and K. The activity of the NMDA-receptors depends, however, on the amount of glycine available. Our data suggest that the afferent synapses of the IHC possess functional properties which are equivalent to the properties of glutamatergic NMDA-sensitive and NMDA-non-sensitive synapses in the central nervous system.

Electrophysiological evidence for the presence of NMDA receptors in the guinea pig cochlea

An excitatory amino acid, possibly L-glutamate, which probably acts as a neurotransmitter at the inner hair cell-afferent fiber synapses in the cochlea. In the present study, we have used an electrophysiological approach to investigate at this level the presence of a major type of excitatory amino acid receptor, namely the glutamatergic receptor for which N-methyl-D-aspartate is a selective agonist. Our results show that, when N-methyl-D-aspartate and the antagonist 2-amino-5-phosphonovalerate are perfused through the perilymphatic scalae, they induced, by different mechanisms, a significant reduction of the amplitude of the compound action potential and an increase of the N1 latency, both predominant at high intensity tone burst stimulations. No significant difference was found in the presence or absence of Mg2+ in the artificial perilymph used as a vehicle. A further slight N-methyl-D-aspartate-induced decrease of the amplitude of the compound action potential, although non significant, was observed when the Mg2(+)-free perilymph contained 100 or 1000 microM glycine. In all the experimental conditions, no effect was observed on the cochlear microphonic potential. This observation is consistent with an action of N-methyl-D-aspartate and 2-amino-5-phosphonovalerate at receptors located on the auditory nerve dendrites contacting the inner hair cells. In conclusion, our results suggest the presence of N-methyl-D-aspartate receptors in the cochlea.

N-methyl-D-aspartate-induced oscillations in excitatory afferent neurotransmission in the guinea pig cochlea

With the aid of microiontophoretic techniques we tested the action of N-methyl-D-aspartate (NMDA) and an NMDA antagonist in the dendritic region of inner hair cell afferent fibers. In the majority of units tested NMDA enhanced the spontaneous firing rate. Furthermore, the activation of NMDA receptors triggered unusual depolarization patterns: (1) slow-frequency oscillation with a periodicity of about 3/min; (2) a fast oscillatory pattern of burst firing with a rhythmic interburst frequency of about 2/s. These findings provide evidence for NMDA receptor properties in the cochlea similar to those of analog channels in the central nervous system.

Alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid electrophysiological and neurotoxic effects in the guinea-pig cochlea

We have recorded cochlear potentials after perilymphatic perfusion of cumulative doses of the excitatory amino acid alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) which selectively recognizes the non-N-methyl-D-aspartate ionotropic receptor formerly known as the quisqualate receptor. Our results show that AMPA (1-80 microM) caused a significant suppression of the amplitude of the compound action potential evoked by acoustic stimulation. A total elimination of this potential at the 100 microM concentration was observed in all animals. In no case was the cochlear microphonic potential, a hair cell receptor potential, affected by AMPA. Histological examinations were performed either at the end of the physiological studies or on cochleas perfused for 10 min with a single dose of AMPA (50 or 100 microM). In both experimental conditions, a selective dendritic swelling or radial afferent nerve endings under the sensory inner hair cells was observed. No damage was found in both types of hair cells supporting cells, lateral and medial efferent fibers and spiral afferent nerve ending on the outer hair cells. The occurrence of the radial dendrite swelling was prevented when 6,7-dinitroquinoxaline-2,3-dione (500 microM) was perfused in the cochlea 10 min prior, then concomitantly with AMPA. The present study strongly suggests that non-N-methyl-D-aspartate receptors, possibly of the AMPA subtype, are involved in the synaptic transmission between the inner hair cells and the primary auditory neurons. They provide further support for the hypothesis that L-glutamate, or another excitatory amino acid, acts as an inner hair cell neurotransmitter.

Autoradiographic studies of selective amino acid uptake by neural and nonneural elements in the gerbil cochlea

The cochlea is well suited for studies of the uptake properties of auditory neurons and nonneuronal supporting cells. Probe concentrations of radioisotopically labeled amino acids, including putative neurotransmitters and their precursors, breakdown products, and blockers, can be introduced via the natural, fluid-filled channels of the inner ear. Uptake patterns can be mapped at cellular and intracellular levels using light and electron microscopic autoradiographic methods. The procedures for introduction of label, fixation, plastic embedment, and light and electron microscopic autoradiography are described with special reference to the cochlea. Labeling patterns observed with over 20 amino acids are summarized for hair cells, spiral ganglion neurons, efferents, and nonneural elements of the stria vascularis, limbus, and modiolus. Limitations on the interpretation of results and their implications for the general usefulness of the methods are discussed.

Nimodipine, an L-channel Ca2+ antagonist, reverses the negative summating potential recorded from the guinea pig cochlea

Nimodipine, an L-type Ca2+ channel antagonist, was tested using sound-evoked cochlear potentials in guinea pigs to investigate whether these channels are involved in cochlear function. Perilymph spaces of guinea pig cochleae were perfused with artificial perilymph solutions containing 0.1-10 microM nimodipine at a rate of 2.5 microliters/min for 10 min. The cochlear potentials evoked by 10 kHz tone bursts of varying intensities were recorded from the basal turn of the scala vestibuli. Cochlear perfusion of nimodipine resulted in reversible, dose-related suppression of the compound action potential of the auditory nerve (CAP; N1-P1), a prolongation of N1 latency at suprathreshold levels, an elevated CAP threshold, a decrease in N1 latency at a constant amplitude measured at CAP threshold, a reduction in cochlear microphonics (CM), and a reduction of the negative summating potential (SP) to a point where it became positive (i.e., a reversal of SP). The endocochlear potential (EP) was not affected. These results support the hypothesis that L-type Ca2+ channels are directly involved in the operation of the organ of Corti. We speculate that L-type Ca2+ channels are integrally involved in generation of a negative summating potential and the dc motion of the cochlear partition described by others.

A M3 muscarinic receptor coupled to inositol phosphate formation in the rat cochlea?

Various neuroactive substances, including excitatory and inhibitory amino acids, biogenic amines and neuropeptides, were tested for their ability to stimulate the inositol phosphate (IPs) cascade in the presence of lithium in the rat cochlea. Among them, only the muscarinic agonists (carbachol and oxotremorine M) were able to stimulate the IPs formation in 12-day-old rat cochleas. The carbachol-elicited IPs formation was inhibited by muscarinic antagonists with the following relative order of potency: atropine greater than 4-DAMP much greater than pirenzepine greater than methoctramine = AF-DX 116. This pharmacological profile suggests that the activation of the M3 muscarinic receptor subtype is responsible for the increase in IPs synthesis in the rat cochlea. However, an interaction with a m5 receptor subtype could not be completely excluded. The unusual link of only one receptor subtype with the phosphoinositide breakdown in the cochlea, as opposed to the usual existence of several receptors coupled to this transduction system in other organs such as the brain, suggest a unique role for muscarinic agonists in the cochlea.

Potassium induced release of GABA and other substances from the guinea pig cochlea

Gamma-aminobutyric acid (GABA) has been proposed as a neurotransmitter of a subset of efferent nerve fibers in the mammalian cochlea. We tested this hypothesis by examining if GABA was released by high concentrations of K+ from the guinea pig cochlea. Artificial perilymph solutions containing either normal K+ (5 mM) or high K+ (50 mM) were perfused through the perilymphatic compartment of the guinea pig cochlea while collecting the effluent. Nineteen primary amines including GABA were quantified in the effluent by HPLC. This was carried out in normal animals and in animals pretreated with ethacrynic acid and kanamycin to destroy the organ of Corti. Significantly greater levels of GABA, taurine, glutamate, aspartate, glycine and three unidentified substances appeared in effluent collected during exposure of the cochlea to solutions containing higher K+ than normal K+. Compared to normal animals, destruction of the organ of Corti significantly decreased the K(+)-induced release of GABA, taurine, glutamate, aspartate, glycine and one of the unidentified substances; although significant release of glutamate and taurine still occurred in the destroyed ears. The release of GABA is consistent with it being a neurotransmitter in the cochlea. In addition the results: confirm the release of glutamate and taurine from the organ of Corti; suggest that additional substances may be released; and demonstrate the release of glutamate and taurine from tissue other than the organ of Corti.

Blockade of synaptic transmission from hair cells to auditory afferent nerves by 6-cyano-2,3-dihydroxy-7-nitroquinoxaline, a selective non-NMDA receptor antagonist

6-Cyano-2,3-dihydroxy-7-nitroquinoxaline (CNQX) is a new, potent and selective competitive antagonist for the non-N-methyl-D-aspartate (non-NMDA) type of excitatory amino acids receptors. We studied the effect of CNQX on the compound auditory nerve action potential (CAP), cochlear microphonics (CM), summating potential (SP) and endocochlear potential (EP). CNQX in doses of 10-20 microM reduced the CAP magnitude and increased the N1 latency without affecting the CM, SP, or EP. Parallel shifts of CAP amplitude- and latency-intensity functions were observed. The CAP suppressed by 10 microM CNQX was completely reversed by a 10-min washout with artificial perilymph. As 10 microM and 20 microM CNQX seem to exert a selective antagonism for non-NMDA receptors, results indicate that non-NMDA receptors play a major role in synaptic transmission from hair cells to auditory afferent nerves.

Immunocytochemical localization of glutamate immunoreactivity in the guinea pig cochlea

The localization of glutamate immunoreactivity was examined in the guinea pig cochlea using affinity purified polyclonal antibodies to glutamate and immunoperoxidase post-embedding staining techniques on one micron plastic sections. Glutamate immunoreactive staining was seen in both inner and outer hair cells and in spiral ganglion cells and auditory nerve fibers. These results support the hypothesis that glutamate may function as the hair cell transmitter or as a precursor to the transmitter and add further support for an excitatory amino acid as the transmitter of the auditory nerve.