Voltage-sensitive Ca2+ channels involved in nicotinic receptor-mediated [H-3]dopamine release from rat striatal synaptosomes

The potent nicotinic agonist anatoxin-a elicits mecamylamine-sensitive [H-3]dopamine release from striatal synaptosomes, and this action is both Na+ and Ca2+ dependent and is blocked by Cd2+. This suggests that stimulation of presynaptic nicotinic receptors results in Na+ influx and local depolarisation that activates voltage-sensitive Ca2+ channels, which in turn provide the Ca2+ for exocytosis. Here we have investigated the subtypes of Ca2+ channels implicated in this mechanism. [H-3]-Dopamine release evoked by anatoxin-a (1 mu M) was partially blocked by 20 mu M nifedipine, whereas KCl-evoked release was insensitive to the dihydropyridine. However, a Rb-86(+) efflux assay of nicotinic receptor function suggested that nifedipine has a direct effect on the receptor, discrediting the involvement of L-type channels. The N-type Ca2+ channel blocker omega-conotoxin GVIA (1 mu M) blocked anatoxin-a-evoked [H-3]dopamine release by 60% but had no significant effect on Rb-86(+) efflux; release evoked by both 15 and 25 mM KCl was inhibited by only 30%. The P-type channel blocker omega-agatoxin IVA (90 nM) also inhibited KCl-evoked release by similar to 30%, whereas anatoxin-a-evoked release was insensitive. The Q-type channel blocker omega-conotoxin MVIIC (1 mu M) had no effect on either stimulus. These results suggest that presynaptic nicotinic receptors on striatal nerve terminals promote [H-3]dopamine release by activation of N-type Ca2+ channels. In contrast, KCl-evoked [H-3]dopamine release appears to involve both N-type and P-type channels.