Differential Desensitization and Distribution of Nicotinic Acetylcholine Receptor Subtypes in Midbrain Dopamine Areas

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Abstract
Although many psychopharmacological factors contribute to nicotine addiction, midbrain dopaminergic systems have received much attention because of their roles in reinforcement and associative learning. It is generally thought that the mesocorticolimbic dopaminergic system is important for the acquisition of behaviors that are reinforced by the salient drives of the environment or by the inappropriate stimuli of addictive drugs. Nicotine, as obtained from tobacco, can activate nicotinic acetylcholine receptors (nAChRs) and excite midbrain neurons of the mesocorticolimbic system. Using midbrain slices from rats, wild-type mice, and genetically engineered mice, we have found differences in the nAChR currents from the ventral tegmental area (VTA) and the substantia nigra compacta (SNc). Nicotinic AChRs containing the α7 subunit (α7* nAChRs) have a low expression density. Electrophysiological analysis of nAChR currents, autoradiography of [125I]-α-bungarotoxin binding, and in situ hybridization revealed that α7* nAChRs are more highly expressed in the VTA than the SNc. In contrast, β2* nAChRs are move evenly distributed at a higher density in both the VTA and SNc. At the concentration of nicotine obtained by tobacco smokers, the slow components of current (mainly mediated by β2* nAChRs) become essentially desensitized. However, the minority α7* component of the current in the VTA/SNc is not significantly desensitized by nicotine in the range ≤100 nm. These results suggest that nicotine, as obtained from tobacco, can have multiple effects on the midbrain areas by differentially influencing dopamine neurons of the VTA and SNc and differentially desensitizing α7* and non-α7 nAChRs.