Figure 3. Properties of voltage-gated Ca²⁺ currents in endbulb terminals.

A, P/Q-type VGCC account for ∼86% of I_Ca(V) in endbulb terminals. I_Ca(V) was elicited every 3.9 s by depolarizing voltage steps (10 ms, from V_h = –80 to 0 mV) while ω-AgaTX (200 nm) was applied to the external solution as indicated by the bar. Time course of block (Aa) and individual sweeps (Ab) representing the amplitude values at arrowheads. B and C, current–voltage relationship of I_Ca(V) in endbulb and calyx terminals. B, I_Ca(V) was elicited in an endbulb terminal by step depolarizations (10 ms duration) from V_h = –80 mV to the potentials indicated next to each trace. C, I–V curves obtained from peak amplitudes of I_Ca(V) from 13 endbulb (black) and 14 calyx (grey) terminals. The smooth red curves represent fits to the data points using a modified form of the constant-field equation (see Methods eqn (2)). D, normalized tail current integrals of 10 ms step depolarizations plotted as a function of V_m to estimate the steady-state activation parameter . The smooth red curves represents fits to the data points using a squared Boltzmann function with half-activation voltages V_0.5 = −24.4 mV (endbulb) and –17.4 mV (calyx) and steepness factors κ = 9.63 mV (endbulb) and 8.23 mV (calyx).