PHARMACOLOGICAL EVIDENCE FOR AN OMEGA-CONOTOXIN, DIHYDROPYRIDINE-INSENSITIVE NEURONAL CA2+ CHANNEL

Inactivation of N-type voltage-sensitive Ca2+ channels (VSCC) with omega-conotoxin (omega-CgTx) in tissue obtained from chicken brain produces a concentration dependent (0.01-0.1-mu-M) inhibition of K+-stimulated Ca2+ influx (DELTA-K+), the rise in [Ca2+]i and acetylcholine (ACh) release. In identical preparations from rat brain, Ca2+ influx and the rise in [Ca2+]i were only marginally affected by much higher (1-10-mu-M) concentrations of omega-CgTx. The release of ACh, however, was inhibited to the same degree with similar amounts of omega-CgTx as those used in chicken brain. An L-type VSCC inhibitor failed to affect any of these parameters alone, or to augment the effect of omega-CgTx. The results suggest that almost all the VSCC in chicken brain are of the N type and that these channels regulate neurotransmitter release. In rat brain, on the other hand, Ca2+ channels resistant to N- or L-type blockers account for almost 75% of the measurable Ca2+ influx and rise in [Ca2+]i. The conspicuous dissociation between the regulation of Ca2+ influx and ACh release demonstrated in rat brain by using omega-CgTx, suggest that neurotransmitter release is governed by only a small proportion of strategically located N-type, omega-CgTx sensitive, VSCC in the presynaptic terminal.