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

Abstract

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 µm), as well as ω‐conotoxin MVIIC (5 µm), blocks ∼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 α_1A and/or α_1B subunits. Inhibition of nicotine‐evoked dopamine release by conotoxins α‐MII and ω‐GVIA is additive and indicates that presynaptic α3β2 nAChRs are functionally coupled to α_1A, but not α_1B, calcium channel subtypes. Conversely, insensitivity to α‐AuIB and sensitivity to ω‐MVIIC indicate that non‐α3β2/α3β4‐containing nAChRs are functionally coupled to α_1B‐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 α‐AuIB and therefore appears to be triggered by Ca²⁺ flowing directly through the channels of presynaptic α3β4 nAChRs. Thus, these data indicate that different presynaptic termini can have distinctive functional associations of specific nAChRs and voltage‐gated calcium channels.