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

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