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International audience; 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.

International audience; Characterization of synaptic transmission between the inner ear sensory cells and primary neuron dendrites has been hampered by the limited access to the postsynaptic terminals. Because direct physiological recording of postsynaptic currents are difficult to achieve, no information regarding the synaptic and dendritic events are available. This is due to the small size of the postsynaptic afferent nerve endings that do not allow a clear identification, and thus compromise direct electrophysiological recordings of the buttons. To study the physiology of afferent nerve endings, we have developed a two-photon imaging technique in cochlear and vestibular slice preparations from neonatal rats and turtles. This technique is based on a retrograde labeling of afferent nerve endings with high-affinity calcium-sensitive dyes. Dye filling was achieved by 6 h application of the dextran-amine conjugate of calcium green-1. Calcium changes were measured in afferent nerve endings in line scan and time lap mode. To address recording in a near-physiological situation, iontophoretic application of K+ was performed in the area of the stereocilia whereas glutamate was applied at the basal pole of sensory hair cells. Both types of application cause a reversible and sustained increase of Ca2+ in the button of afferent nerve fibers. Typical recordings are presented and potential interests for pharmacological studies of inner ear sensory cell synapses are discussed.

International audience; 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.