Figure 2.

Torus type II hair cells respond to ACh. (A,B) Family of current responses in a type II hair cell near the torus before (A) and 10 s after the application of 100 μM ACh (B). The cell was first held at −67 mV for 56 ms, then stepped to a voltage from −137 to 43 mV in 10 mV intervals for 250 ms, and then back to −67 mV for 100 ms (protocol at top). Steady state current amplitudes were measured at t = 300 ms (triangle). Red traces indicate currents elicited by a voltage step to −17 mV. Dashed line in (B) is zero-current level. (C) Current recorded from the same cell during a ramp protocol, where the cell was first held at −67 mV, stepped to −130 mV for 100 ms, and then ramped to 40 mV over 400 ms, under control condition (circles), application of 100 μM ACh (squares), or after wash (triangles). (D) Current–voltage (I–V) relationship of ACh-sensitive current during voltage step protocol (triangles) was compared to ACh-sensitive currents generated during the ramp protocol (solid line). When voltage steps were used, the ACh-sensitive current was generated by subtracting control current (A, lower panel) from the ACh current (B) at steady-state. When ramp protocol was used, ACh-sensitive current was generated by subtracting the control current from ACh current. In order to plot the I–V relationship, time points were linearly converted to voltage by multiplying by the ramp speed. (E) Gap-free current clamp recording under zero-current conditions were acquired from a torus type II hair cell during the superfusion of external solution and 100 μM ACh. Segments of application indicated in boxes along the time axis. Arrowhead indicates a post-hyperpolarization depolarization. Dashed line demarcates the mean pre-ACh zero-current membrane potential. Inset: Several hyperpolarizing fluctuations observed during post-ACh washout are shown on an expanded time scale (scale bars: 500 ms, 5 mV).