Figure 1. Presynaptic Ca²⁺ current recorded from the calyx of Held presynaptic terminal in wild-type and knockout mice.

A and B, pharmacological dissection of presynaptic Ca²⁺ current (I_pCa). A, ω-conotoxin GVIA (ω-CgTx; 2 μm), ω-agatoxin IVA (ω-AgaTx; 200 nm) and Cd²⁺ (100 μm) were applied in this sequence to block I_pCa (evoked by a 10 ms depolarizing pulse) in wild-type (WT) and knockout (KO) mice. Sample records are I_pCa (average of 6 records) before (a) and after (b) application of ω-CgTx, after ω-AgaTx application (c), and after Cd²⁺ application (d) (superimposed). B, percentage of P/Q-type (filled bar) and N-type (open bar) current components in WT and KO mice. Data are derived from 8 calyces each. The N-type current in KO mice was 443 ± 52 pA, whereas that in WT was 46 ± 21 pA. C, current–voltage relationships of I_pCa in WT (•, n = 9) and KO (○, n = 11) mice. Sample records show I_pCa evoked by depolarizing steps to −40, −20, 0 and +20 mV (superimposed). D, activation and inactivation curves. Activation curves (circles) were obtained from tail currents evoked by a 10 ms depolarizing pulse stepping to various membrane potentials. Data obtained from WT (•, n = 9) and KO (○, n = 11) mice were normalized to the maximal value, plotted against voltage, and fitted by the Boltzmann function I= 1/(I_max+ exp(V_½−V)/k), where V_½ and k represent the half-activation voltage and slope factor, respectively. Five data points between −5 and +15 mV from WT mice were excluded from the activation curve fit because inactivation, proceeding during 10 ms pulse, attenuated the tail current amplitude. Resting inactivation of I_pCa was assessed using a protocol (right bottom) comprising conditioning prepulses of various amplitudes (500 ms in duration) followed by a test pulse (10 ms, to −10 mV). To formulate inactivation curves, amplitudes of I_pCa evoked by the test pulse were normalized to that evoked after a −130 mV conditioning pulse, and plotted against the conditioning voltage, both in WT (▪, n = 6) and KO (□, n = 8) mice. Sample records show I_pCa evoked by the test pulse after conditioning prepulses (10 mV incremental steps from −130 to −20 mV, 12 records superimposed for both WT and KO mice). Lines in A and C were eye-fitted. Error bars indicate s.e.m.