Voltage-Sensitive Ca2+ Channels Involved in Nicotinic Receptor-Mediated [3H]Dopamine Release from Rat Striatal Synaptosomes

Abstract: The potent nicotinic agonist anatoxin-a elicits mecamylamine-sensitive ³H]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. ³H]Dopamine release evoked by anatoxin-a (1 µM) was partially blocked by 20 µM nifedipine, whereas KCl-evoked release was insensitive to the dihydropyridine. However, a ⁸⁶Rb⁺ 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 ω-conotoxin GVIA (1 µM) blocked anatoxin-a-evoked ³H]dopamine release by 60% but had no significant effect on ⁸⁶Rb⁺ efflux; release evoked by both 15 and 25 mM KCl was inhibited by only 30%. The P-type channel blocker ω-agatoxin IVA (90 nM) also inhibited KCl-evoked release by ～30%, whereas anatoxin-a-evoked release was insensitive. The Q-type channel blocker ω-conotoxin MVIIC (1 µM) had no effect on either stimulus. These results suggest that presynaptic nicotinic receptors on striatal nerve terminals promote ³H]dopamine release by activation of N-type Ca²⁺ channels. In contrast, KCl-evoked ³H]dopamine release appears to involve both N-type and P-type channels.