Nicotine consumption is regulated by a human polymorphism in dopamine neurons
Résumé
Smoking is the most important preventable cause of morbidity and mortality worldwide. Recent genome-wide association studies highlighted a human haplotype on chromosome 15 underlying the risk for tobacco dependence and lung cancer. Several polymorphisms in the CHRNA3-CHRNA5-CHRNB4 cluster coding for the nicotinic acetylcholine receptor nAChR) a3, a5 and beta 4 subunits were implicated. In mouse models, we define a key role in the control of sensitivity to nicotine for the a5 subunit in dopaminergic (DAergic) neurons of the ventral tegmental area (VTA). We first investigated the reinforcing effects of nicotine in drug-naive a5(-/-) mice using an acute intravenous nicotine self-administration task and ex vivo and in vivo electrophysiological recordings of nicotine-elicited DA cell activation. We designed lentiviral re-expression vectors to achieve targeted re-expression of wild-type or mutant a5 in the VTA, in general, or in DA neurons exclusively. Our results establish a crucial role for a5*-nAChRs in DAergic neurons. These receptors are key regulators that determine the minimum nicotine dose necessary for DA cell activation and thus nicotine reinforcement. Finally, we demonstrate that a single-nucleotide polymorphism, the non-synonymous a5 variant rs16969968, frequent in many human populations, exhibits a partial loss of function of the protein in vivo. This leads to increased nicotine consumption in the self-administration paradigm. We thus define a critical link between a human predisposition marker, its expression in DA neurons and nicotine intake.