Fig. 2.

Calcium-sensitive currents in calyx terminals. A) Peak outward currents measured during depolarizing voltage steps to +20 mV are plotted versus time for a P15 calyx exposed to three short applications of low Ca²⁺ solution (indicated by bars). Outward currents showed a partial recovery after each application. The gradual decline in outward current is likely due to rundown (see Methods). B) Currents in response to a voltage step to +20 mV are shown for control and low Ca²⁺ condition in a P15 calyx. The membrane potential was held at −80 mV and stepped to −130 mV for 40 ms before stepping from −90 mV to +20 mV in 10 mV increments (only part of protocol is shown, for complete voltage protocol refer to Fig. 1A). The same voltage protocol was used in figures 3–5. The Ca²⁺-sensitive component, obtained by subtracting the current in low Ca²⁺ from control is also shown (dashed line). Zero-current level is indicated by the dotted line. C) Current-voltage (I-V) plots for steady-state currents at a series of potentials for control (filled circles) and low Ca²⁺ (open squares) for the cell shown in B. The subtracted currents (triangles) represent the current sensitive to Ca²⁺ removal and activate above ~ −40 mV