Inhibition of voltage-dependent calcium channels by omega-conotoxin MVIIC (omega-CTx-MVIIC) was studied in various types of rat neurons. When studied with 5 mM Ba2+ as charge carrier, omega-CTx-MVIIC block of N-type calcium channels in sympathetic neurons was potent, with half-block at 18 nM. Block of N-type channels had a rapid onset (tau approximately 1 sec at 1 microM omega-CTx-MVIIC) and quick reversibility (tau approximately 30 sec). The rate of block was proportional to toxin concentration, consistent with 1:1 binding of toxin to channels, with a rate constant (k on) of approximately 1 X 10(6) M-1. sec-1. Both potency and rate of block were reduced dramatically with increasing concentrations of extracellular Ba2+ omega-CTx-MVIIC also blocked P-type calcium channels in cerebellar Purkinje neurons, but both development and reversal of block were far slower than for N-type channels. The rate of block was proportional to toxin concentration, with k on -1.5 x 10(3) M-1. sec-1 at 5 mM Ba2+. From this value and an unblocking time constant of approximately 200 min, a dissociation constant of approximately 50 nM was estimated. Thus, block of P-type channels is potent but very slow. In hippocampal CA3 pyramidal neurons, omega-CTx-MVIIC blocked approximately 50% of the high-threshold calcium channel current; one component (approximately 20%) was blocked with the rapid kinetics expected for N-type channels, whereas the other component was blocked slowly. The component blocked slowly was reduced but not eliminated by preexposure to 200 nM or 1 microM omega-Aga-IVA.

The effects of the spider toxin w-Aga-WA were studied on Ca channel currents in rat central neurons. In hippocampal CA1 pyramidal neurons, o-Aga-IIIA blocked-70% of the high-threshold Ca currents and had no effect on low-threshold T-type current. Occlusion experiments with blockers of L-, N-, and P-type Ca currents showed that w-Aga-IIIA abolished dihydropyridine-sensitive L-type current and blocked a substantial fraction of the w-conotoxin (CgTX)-sensitive N-type and w-Aga-IVA-sensitive P-type Ca currents. The highthreshold current remaining with saturating concentrations of nimodipine; CgTX, and w-Aga-IVA was also partially blocked by w-Aga-MA in a variety of central neurons. Block of P-type current by o-Aga-IIIA was investigated in more detail in cerebellar Purkinje neurons. Block was potent (&-0.5 nM), but incomplete and voltage dependent. Tail current activation curves showed that channel gating is shifted in the depolarizing direction by-7 mV. The instantaneous current-voltage curve for P-type current was also altered; the toxin reduced Ba-carried inward currents by-40% and had little effect on Cs-carried outward currents. The partial, voltage-dependent reduction of P-type Ca current can be accounted for by a combination of toxin effects on channel permeation and gating. [Key words: Ca channels, CA 7 pyramidal neurons, Purkinje neurons, ion channel block, spider toxin, w-Aga-I/IA] Neurons possess multiple types of Ca channels (