Subunit composition of functional nicotinic receptors in dopaminergic neurons investigated with knock-out mice.

Nicotinic acetylcholine receptors (nAChRs) expressed by dopaminergic (DA) neurons have long been considered as potential therapeutic targets for the treatment of several neuropsychiatric diseases, including nicotine and cocaine addiction or Parkinson's disease. However, DA neurons express mRNAs coding for most, if not all, neuronal nAChR subunits, and the subunit composition of functional nAChRs has been difficult to establish. Immunoprecipitation experiments performed on mouse striatal extracts allowed us to identify three main types of heteromeric nAChRs (alpha4beta2*, alpha6beta2*, and alpha4alpha6beta2*) in DA terminal fields. The functional relevance of these subtypes was then examined by studying nicotine-induced DA release in striatal synaptosomes and recording ACh-elicited currents in DA neurons fromalpha4, alpha6, alpha4alpha6, and beta2 knock-out mice. Our results establish that alpha6beta2* nAChRs are functional and sensitive to alpha-conotoxin MII inhibition. These receptors are mainly located on DA terminals and consistently do not contribute to DA release induced by systemic nicotine administration, as evidenced by in vivo microdialysis. In contrast, (nonalpha6)alpha4beta2* nAChRs represent the majority of functional heteromeric nAChRs on DA neuronal soma. Thus, whereas a combination of alpha6beta2* and alpha4beta2* nAChRs may mediate the endogenous cholinergic modulation of DA release at the terminal level, somato-dendritic (nonalpha6)alpha4beta2* nAChRs most likely contribute to nicotine reinforcement.