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

Authors


: Dr J.-P. Changeux, as above.
E-mail: changeux@pasteur.fr

Abstract

Nicotinic acetylcholine receptors (nAChRs) are expressed in the midbrain ascending dopaminergic system, a target of many addictive drugs. Here we assessed the intracellular Ca2+ level by imaging fura-2-loaded cells in substantia nigra pars compacta in mouse brain slices, and we examined the influence on this level of prolonged exposures to nicotine using mice lacking the nAChR β2-subunit. In control cells, superfusion with nicotine (10–100 μm) caused a long-lasting rise of intracellular Ca2+ level which depended on extracellular Ca2+. This nicotinic response was almost completely absent in β2–/– mutant mice, leaving a small residual response to a high concentration (100 μm) of nicotine which was inhibited by the α7-subunit-selective antagonist, methyllycaconitine. Conversely, the α7-subunit-selective agonist choline (10 mm) caused a methyllycaconitine-sensitive increase in intracellular Ca2+ level both in wild-type and β2–/– mutant mice. Nicotine-elicited Ca2+ mobilization was reduced by the Na+ channel blocker tetrodotoxin (TTX) and by T-type Ca2+ channel blocking agents, whereas the choline-elicited Ca2+ increase was insensitive to TTX. Neither nicotine nor choline produced Ca2+ increase following inhibition of the release of Ca2+ 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 β2-subunit-containing nAChR followed by activation of Na+ channel and T-type Ca2+ channels, and/or activation of α7-subunit-containing nAChR. The Ca2+ influx due to nAChR activation is subsequently amplified by the recruitment of intracellular Ca2+ stores. This Ca2+ mobilization may possibly contribute to the long-term effects of nicotine on the dopaminergic system.

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