Functional Nicotinic Acetylcholine Receptors Containing α6 Subunits Are on GABAergic Neuronal Boutons Adherent to Ventral Tegmental Area Dopamine Neurons

Diverse nicotinic acetylcholine receptor (nAChR) subtypes containing different subunit combinations can be placed on nerve terminals or soma/dendrites in the ventral tegmental area (VTA). nAChR alpha 6 subunit message is abundant in the VTA, but alpha 6(star)-nAChR cellular localization, function, pharmacology, and roles in cholinergic modulation of dopaminergic (DA) neurons within the VTA are not well understood. Here, we report evidence for alpha 6 beta 2(star)-nAChR expression on GABA neuronal boutons terminating on VTA DA neurons. alpha-Conotoxin (alpha-Ctx) MII labeling coupled with immunocytochemical staining localizes putative alpha 6(star)-nAChRs to presynaptic GABAergic boutons on acutely dissociated, rat VTA DA neurons. Functionally, acetylcholine (ACh) induces increases in the frequency of bicuculline-, picrotoxin-, and 4-aminopyridine-sensitive miniature IPSCs (mIPSCs) mediated by GABA(A) receptors. These increases are abolished by alpha 6(star)-nAChR-selective alpha-Ctx MII or alpha-Ctx PIA (1 nM) but not by alpha 7 (10 nM methyllycaconitine) or alpha 4(star) (1 mu M dihydro-beta-erythroidine)-nAChR-selective antagonists. ACh also fails to increase mIPSC frequency in VTA DA neurons prepared from nAChR beta 2 knock-out mice. Moreover, ACh induces an alpha-Ctx PIA-sensitive elevation in intraterminal Ca2+ in synaptosomes prepared from the rat VTA. Subchronic exposure to 500 nM nicotine reduces ACh-induced GABA release onto the VTA DA neurons, as does 10 d of systemic nicotine exposure. Collectively, these results indicate that alpha 6 beta 2(star)-nAChRs are located on presynaptic GABAergic boutons within the VTA and modulate GABA release onto DA neurons. These presynaptic alpha 6 beta 2(star)-nAChRs likely play important roles in nicotinic modulation of DA neuronal activity.