Nicotine-evoked transmitter release from synaptosomes: functional association of specific presynaptic acetylcholine receptors and voltage-gated calcium channels

It has previously been shown that nicotine-evoked dopamine release from rat striatal synaptosomes and nicotine-evoked norepinephrine release from hippocampal synaptosomes are mediated by distinct nicotinic acetylcholine receptor (nAChR) subtypes. In the present study, the functional association of these nicotinic receptors with specific subtypes of voltage-gated calcium channels was examined. Cd2+ (200 muM), as well as omega -conotoxin MVIIC (5 muM), blocks similar to 85% of nicotine-evoked dopamine release from striatal synaptosomes, indicating a major involvement of calcium channels. Furthermore, the toxin-susceptibility suggests that these calcium channels contain alpha (1A) and/or alpha (1B) subunits. Inhibition of nicotine-evoked dopamine release by conotoxins a-MII and omega -GVIA is additive and indicates that presynaptic alpha3 beta2 nAChRs are functionally coupled to alpha (1A), but not alpha (1B), calcium channel subtypes. Conversely, insensitivity to alpha -AuIB and sensitivity to omega -MVIIC indicate that non-alpha3 beta2/alpha3 beta4-containing nAChRs are functionally coupled to alpha (1 beta)-containing calcium channels. In contrast, Cd2+ blocks only 65% of nicotine-evoked norepinephrine release from hippocampal synaptosomes, indicating that a substantial fraction of this release occurs through mechanisms not involving calcium channels. This Cd2+-insensitive component of release is blocked by alpha -AuIB and therefore appears to be triggered by Ca2+ flowing directly through the channels of presynaptic alpha3 beta4 nAChRs. Thus, these data indicate that different presynaptic termini can have distinctive functional associations of specific nAChRs and voltage-gated calcium channels.