Calcium mobilization elicited by two types of nicotinic acetylcholine receptors in mouse substantia nigra pars compacta

Nicotinic acetylcholine receptors (nAChRs) are expressed in the midbrain as cending dopaminergic system, a target of many addictive drugs. Here we asse ssed the intracellular Ca2+ level by imaging fura-2-loaded cells in substan tia nigra pars compacta in mouse brain slices, and we examined the influenc e on this level of prolonged exposures to nicotine using mice lacking the n AChR beta 2-subunit. In control cells, superfusion with nicotine (10-100 mu m) caused a long-lasting rise of intracellular Ca2+ level which depended o n extracellular Ca2+. This nicotinic response was almost completely absent in beta 2-/- mutant mice, leaving a small residual response to a high conce ntration (100 mu m) of nicotine which was inhibited by the alpha 7-subunit- selective antagonist, methyllycaconitine. Conversely, the alpha 7-subunit-s elective agonist choline (10 mm) caused a methyllycaconitine-sensitive incr ease in intracellular Ca2+ level both in wild-type and beta 2-/- mutant mic e. Nicotine-elicited Ca2+ mobilization was reduced by the Na+ channel block er tetrodotoxin (TTX) and by T-type Ca2+ channel blocking agents, whereas t he choline-elicited Ca2+ increase was insensitive to TTX. Neither nicotine nor choline produced Ca2+ increase following inhibition of the release of C a2+ from intracellular stores by dantrolene. These results demonstrate that in nigral dopaminergic neurons, nicotine can elicit Ca2+ mobilization via activation of two distinct nAChR subtypes: that of beta 2-subunit-containin g nAChR followed by activation of Na+ channel and T-type Ca2+ channels, and /or activation of alpha 7-subunit-containing nAChR. The Ca2+ influx due to nAChR activation is subsequently amplified by the recruitment of intracellu lar Ca2+ stores. This Ca2+ mobilization may possibly contribute to the long -term effects of nicotine on the dopaminergic system.