Fig. 3.

Developmental decline of N- and R-type Ca²⁺ channels in the giant presynaptic terminal, the calyx of Held. Presynaptic Ca²⁺ currents (Ip_Ca) were evoked by a 10 msec depolarizing pulse from −80 mV holding potential to −10 mV under voltage clamp every 10 sec in the presence of TTX (0.1 μm) and TEA-Cl (10 mm). A, At P7, ω-CgTx (3 μm) reduced the amplitude ofIp_Ca by 23% (2), whereas ω-Aga-IVA (200 nm) by 66% (3). The fraction remaining after application of both toxins (11%) was abolished by Cd²⁺ (100 μm; 4). B, At P13, ω-CgTx had no effect on Ip_Ca, whereas ω-Aga-IVA almost completely abolishedIp_Ca (3) with no appreciable remaining Cd²⁺-sensitive component (4). C, The fraction ofIp_Ca blocked by ω-CgTx (N, ○), ω-Aga-IVA (P/Q, ■), and that insensitive to the toxins but blocked by Cd²⁺ (R, ▵) at three different postnatal ages. Symbols and error bars derived from five to eight cells at each age.