Anatoxin-a-evoked [ 3H]dopamine release from rat striatal synaptosomes

Abstract

Presynaptic nicotinic acetylcholine receptors on striatal nerve terminals modulate the release of dopamine. Using rat striatal synaptosomes loaded with [ 3H]dopamine, we have characterized the action of the selective nicotinic agonist, (±)anatoxin-a, with respect to [ 3H]dopamine release, in order to explore the mechanisms coupling nicotinic receptor activation to exocytosis. Anatoxin-a evoked [ 3H]dopamine release in a concentration-dependent and mecamylamine-sensitive manner, EC 50 = 0.11 μM. The maximum [ 3H]dopamine release elicited by anatoxin-a was only 20% of the maximum elicited by KC1 depolarization; there was no additivity between anatoxin-a and sub-maximal concentrations of KC1. Both agents stimulated Ca 2+-dependent release that was equally sensitive to inhibition by 200 μM Cd 2+. This result suggests that anatoxin-a-stimulated exocytosis is mediated by Ca 2+ influx via voltage-sensitive Ca 2+ channels, with little contribution from Ca 2+ entering directly through the nicotinic receptor channel. This view is supported by the abolition of anatoxin-a-evoked [ 3H]dopamine release in Na +-depleted medium. A partial (40%) inhibition by tetrodotoxin was observed. These data suggest that activation of presynaptic nicotinic acetylcholine receptors by anatoxin-a results in an influx of Na +, producing sufficient local depolarization to open voltage-sensitive Ca 2+ and Na + channels. The latter may then amplify the response, activating further Ca 2+ channels. The particular voltage-sensitive Ca 2+ channels involved remain to be determined.