Analysis of synaptic mechanisms in cochlear hair cells by measuring membrane capacitance

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We measured the membrane current possessing the properties of mechano-electric transducer current in isolated inner hair cells of guinea-pig cochlea. In free-standing hair bundle, depolarization to +80 mV evoked a stable outward current attributable to the opening of transducer channels and repolarization to -80 mV evoked a transient inward current indicating adaptation followed. Time constant of adaptation increased as the membrane potential depolarized. Dihydrostreptomycin diminished both the outward and inward currents.Cyclopiazonic acid (CPA) is a reticulum-like intracellular Ca^<2+> store depletory, which raises intracellular Ca^<2+> concentration. The effect of CPA on membrane currents in isolated inner hair cells (IHCs) from guinea-pig cochlea was investigated by the patch-clamp technique in the whole-cell configuration. Four out of eight IHCs showed an augmentation of the currents and the other four cells showed an inhibition of the currents by extracellular CPA application. The activation kinetics of outward currents were not changed by CPA. Three out of four IHCs obtained from the basal part of the cochlea demonstrated augmentation, whereas three out of four IHCs from the apical part demonstrated inhibition of the currents. This result suggests that Ca^<2+>-activated currents were dominant in the basal IHCs of the cochlea.