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OHC piezoelectricity can overcome membrane capacitance damping to improve OHC high frequency responses. The OHC piezoelectric response showed a high-pass property and was increased as the stimulus frequency was increased. The cut-frequency was 70-90 kHz, mainly limited by the recording system. Simultaneous administrations of electronic and mechanical (piezoelectric) stimulation to the OHC, which mimics the OHC suffered electronic (receptor current through transduction channels) and mechanical (the vibration of the basilar membrane) stimulations in vivo, generated the flat response up to 80 kHz. Abolishment of piezoelectricity eliminated this high frequency enhancement. Like a regular cell, the sole electronic frequency response of the OHC was low-pass; the cut-frequency was ~1 kHz. Finally, as computer modeling expected, the resonant peaks were also visible in the responses to electronic-me- chanical stimulation. Our results indicate that OHC electromotility can perform at high frequency effec- tively to contribute active cochlear mechanics in whole mammalian auditory frequency range (Supported by NIH DC05989 and the Research Foundation of American Tinnitus Association).

Pre- and Postsynaptic Changes Underlying the Maturation of Inner Hair Cell Ribbon Syn- apses Do Not Depend on the Onset of Hearing. (Abstract of ARO Meeting Denver, Colorado) Michel Eybalin1, Nicole Renard1, Annelies Schrott-Fischer2, Jean-Luc Puel1 1INSERM U583, Montpellier, France, 2ENT Department, Innsbruck, Austria

Inner hair cell (IHC) synapses play a key role in the auditory physiology as they ensure transmission of sound stimuli to first auditory neurons. Glutamate is the neurotransmitter responsible for this fast synap- tic transmission which essentially involves AMPA receptors. The glutamate release is dependent on L- type Ca2+ channels with Cav1.3 subunit and occurs at synapses equipped with a dense ribbon thought to mediate the continuous and rapid recruitment of its attached vesicles to the release sites. Despite the importance of the IHC synapse, the cellular and molecular machineries underlying its function are still largely unknown despite their elucidation is of prime importance to gain insight into the occurrence of tinnitus and most forms of deafness. Using immunocytochemistry, we have studied the expression of a selected set of presynaptic proteins (SNAP25, cysteinestring protein, Rab3 and synaptogyrin) during the postnatal maturation of the rodent cochlea and found that, with the exception of Rab3, they were only detected starting postnatal days 10 and 12, when the first, immature, cochlear potentials can be recorded. During the same postnatal period, we also found that the composition and pharmacological properties of the postsynaptic AMPA receptors changed. GluR2 replaced GluR1 at postnatal day 10, switching the potential composition of AMPA recep- tors from GluR1/3/4 to GluR2/3/4 and their pharmacology to calcium impermeability. Finally, we have checked the expression of GluR2 and the 4 presynaptic proteins in the cochlea of the deaf Cav1.3 knock out mice and found that they were all expressed at adult IHC synapses suggesting that their expression was not dependent of the first sound stimuli transduced by IHCs.

Quinine Induced Tinnitus-like Behavior Using a Startled Reflex Paradigm. (Abstract of ARO Meeting Denver, Colorado) Edward Lobarinas, Wei Sun, Lei Wei, Richard Salvi University at Buffalo

The anti-malarial drug, quinine, has been reported to induce tinnitus when administered at high doses and has been used to investigate the neural and biochemical mechanisms underlying tinnitus. Previous- ly, schedule induced polydipsia avoidance conditioning (SIP-AC) was used to evaluate the presence of tinnitus in rats treated with high doses of both salicylate and quinine. Recently, the effects of quinine on tinnitus-like behavior were evaluated using a high-throughput behavioral assay, gap pre-pulse inhibition of acoustic startle (GPIAS), which can be used to estimate tinnitus pitch. GPIAS was used to measure

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