Vincent David et al in Journal of Neuroscience

Nicotine addiction, through smoking, is the principal cause of preventable mortality worldwide. Human genome-wide association studies have linked polymorphisms in the CHRNA5-CHRNA3-CHRNB4 gene cluster, coding for the a5, a3, and b4 nicotinic acetylcholine receptor (nAChR) subunits, to nicotine addiction. b4*nAChRs have been implicated in nicotine withdrawal, aversion, and reinforcement. Here we show that b4*nAChRs also are involved in non-nicotine mediated responses that may predispose to addiction-related behaviors. b4 knock-out (KO) male mice show increased novelty-induced locomotor activity, lower baseline anxiety, and motivational deficits in operant conditioning for palatable food rewards and in reward-based Go/No-go tasks. To further explore reward deficits we used intracranial self-administration (ICSA) by injecting nicotine directly into the ventral tegmental area (VTA) in mice. We found that, at low nicotine doses, b4KO self-administer less than wild-type (WT) mice. Conversely, at high nicotine doses, this was reversed and b4KO self-administered more than WT mice, whereas b4-overexpressing mice avoided nicotine injections. Viral expression of b4 subunits in medial habenula (MHb), interpeduncular nucleus (IPN), and VTA of b4KO mice revealed dose- and region- dependent differences: b4*nAChRs in the VTA potentiated nicotine-mediated rewarding effects at all doses, whereas b4*nAChRs in the MHb-IPN pathway limited VTA-ICSA at high nicotine doses. Together, our findings indicate that the lack of functional b4*nAChRs result in deficits in reward sensitivity including increased ICSA at high doses of nicotine. Normal, WT levels of nicotine ICSA are restored by re-expression of b4*nAChRs in the MHb-IPN. These data indicate that the b4 subunit is a critical modulator of reward-related behaviors, and that β4*nAChRs may provide a promising novel drug target for smoking cessation.