β2 nAChR activation on VTA DA neurons is sufficient for nicotine reinforcement in rats.

Mesolimbic nicotinic acetylcholine receptor (nAChRs) activation is necessary for nicotine reinforcement behavior, but it is unknown whether selective activation of nAChRs in the dopamine (DA) reward pathway is sufficient to support nicotine reinforcement. In this study, we tested the hypothesis that activation of β2-containing (β2*) nAChRs on VTA neurons is sufficient for intravenous nicotine self-administration (SA). We expressed β2 nAChR subunits with enhanced sensitivity to nicotine (referred to as β2Leu9'Ser) in the VTA of male Sprague Dawley rats, enabling very low concentrations of nicotine to selectively activate β2* nAChRs on transduced neurons. Rats expressing β2Leu9'Ser subunits acquired nicotine SA at 1.5 μg/kg/infusion, a dose too low to support acquisition in control rats. Saline substitution extinguished responding for 1.5 μg/kg/inf, verifying that this dose was reinforcing. β2Leu9'Ser nAChRs also supported acquisition at the typical training dose in rats (30 μg/kg/inf) and reducing the dose to 1.5 μg/kg/inf caused a significant increase in the rate of nicotine SA. Viral expression of β2Leu9'Ser subunits only in VTA DA neurons (via TH-Cre rats) also enabled acquisition of nicotine SA at 1.5 μg/kg/inf, and saline substitution significantly attenuated responding. Next, we examined electrically-evoked DA release in slices from β2Leu9'Ser rats with a history of nicotine SA. Single-pulse evoked DA release and DA uptake rate were reduced in β2Leu9'Ser NAc slices, but relative increases in DA following a train of stimuli were preserved. These results are the first to report that β2* nAChR activation on VTA neurons is sufficient for nicotine reinforcement in rats. Significance Statement Nicotinic acetylcholine receptor (nAChR) pharmacology and neurobiology in the dopamine reward pathway is complex and it has been a challenge to identify the minimum receptor/circuit combination(s) giving rise to nicotine dependence. This study reveals that activation of β2-containing nAChRs in ventral tegmental area dopamine neurons is sufficient to support acquisition and maintenance of nicotine self-administration in rats. This work, which employs a gain-of-function approach, complements and extends prior loss-of-function experiments demonstrating the importance of these receptors in several nicotine-related behaviors. This study 1) affirms the importance of β2 nAChRs in nicotine reinforcement, and 2) provides a useful in vivo approach for developing nicotine dependence therapeutics with either nicotinic or non-nicotinic mechanisms of action.