Nicotinic acetylcholine receptor function in the mesolimbic dopamine system

Tobacco addiction is a serious threat to public health in the United States and abroad, and development of new therapeutic approaches is a major priority. Nicotine, the primary psychoactive compound in cigarette smoke, activates and/or desensitizes nicotinic acetylcholine receptors (nAChRs) throughout the brain. nAChRs in the mesolimbic dopamine (DA) pathway are crucial for the rewarding and reinforcing properties of nicotine in rodent models, suggesting that they may be key mediators of nicotine’s action in humans. Although it is known that α4 and α6 nAChR subunits are important components of mesolimbic DA pathway nAChRs, the precise composition and/or stoichiometry of nAChRs in this pathway that are sufficient and/or necessary for nicotine reward remains unknown. Furthermore, the changes in the brain following nicotine exposure that give rise to reward behavior are poorly understood. This proposal will test the hypothesis that activation of α4α6β2* (*denotes that other subunits may be present in the nAChR pentamer) nAChRs in the mesolimbic DA pathway is sufficient for nicotine’s rewarding effects. Confirming or refuting this hypothesis will result in further identification of the relevant nAChR subtypes in the mesolimbic DA pathway that mediate nicotine’s biophysical and/or behavioral effects. Using a combination of behavioral, electrophysiological, and biochemical techniques, we will address our hypothesis with three specific aims. In Specific Aim 1, we will test the idea that activation of α4α6β2* nAChRs is sufficient for nicotine reward-like behavior. Using mice which express α6* nAChRs that are hypersensitive to nicotine, in parallel with a compound with modest functional selectivity for α6β2* nAChRs, we will determine whether selective activation of these receptors is sufficient for nicotine CPP. In Specific Aim 2, we will use patch clamp electrophysiology to test the hypothesis that stimulation of α4α6β2* nAChRs is sufficient to activate ventral tegmental area (VTA) DA neurons and strengthen glutamatergic input to these cells. In Specific Aim 3, we directly measure changes in nAChR and glutamatergic receptor function in DA neurons from animals exposed to nicotine. The proposed studies will enhance our understanding of the key nAChRs in the mesolimbic DA pathway that are involved in nicotine addiction. Identifying the nAChRs that are sufficient for nicotine reward should lead to a more rational and focused effort to develop new smoking cessation therapeutics.