Figure 1.

Exocytosis recorded by C_m measurements from single IHCs. (a) Simplified representation of an IHC, the measuring configuration, and an afferent IHC synapse (Inset) overlaid onto a Nomarski image of the explanted mouse organ of Corti (stereocilia of the row of IHCs are shown at the top of the image; cell bodies are covered by supporting cells). Previously suggested mechanisms of adaptation are indicated according to their localizations. Synaptic depression could be due to Ca²⁺ current inactivation, vesicle depletion, or postsynaptic glutamate receptor desensitization. (b) IHCs kept at 35°C in 2 mM [Ca²⁺]_e were stimulated by a 50-ms depolarizing sinusoidal voltage (1 kHz; 40 mV peak to peak added to a dc potential of −50 mV; Upper). Four C_m traces recorded from three cells in the whole-cell configuration (0.1 mM EGTA added to the pipette solution) were averaged and smoothed by box averaging (n = 3; Lower; horizontal lines represent C_m averages before and after stimulation). C_m estimation is not valid during the depolarization because of nonlinear conductance changes. (c) High time resolution plot of ΔC_m and membrane currents measured in response to depolarizations (to −15 mV) of different durations in the perforated-patch configuration. ΔC_m traces were smoothed by box averaging (n = 3), and current traces were smoothed by binomial smoothing (n = 5). Outward currents were (incompletely) inhibited by intracellular Cs⁺ and extracellular tetraethylammonium (35 mM). (Inset) Average of four ΔC_m traces in response to 4-ms depolarizations (to −15 mV). (d) Representative membrane current response to a 50-ms depolarization to −15 mV in the presence of intracellular (13 mM) and extracellular (35 mM) tetraethylammonium. Very little decline (inactivation) of Ca²⁺ current was observed under these conditions.