β4-nicotinic receptors are critically involved in reward-related behaviors and self-regulation of nicotine reinforcement.

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 alpha 5, alpha 3, and beta 4 nicotinic acetylcholine receptor (nAChR) subunits, to nicotine addiction. beta 4*nAChRs have been implicated in nicotine withdrawal, aversion, and reinforcement. Here we show that beta 4*nAChRs also are involved in non-nicotine-mediated responses that may predispose to addiction-related behaviors. beta 4 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 directly injecting nicotine into the ventral tegmental area (VIA) in mice. We found that, at low nicotine doses, beta 4KO self-administer less than wild-type (WT) mice. Conversely, at high nicotine doses, this was reversed and beta 4KO self-administered more than WT mice, whereas beta 4-overexpressing mice avoided nicotine injections. Viral expression of beta 4 subunits in medial habenula (MHb), interpeduncular nucleus (IPN), and VTA of beta 4KO mice revealed dose- and region-dependent differences: beta 4*nAChRs in the VTA potentiated nicotine-mediated rewarding effects at all doses, whereas beta 4*nAChRs in the MHb-IPN pathway, limited VTA-ICSA at high nicotine doses. Together, our findings indicate that the lack of functional PP nAChRs result in deficits in reward sensitivity including increased ICSA at high doses of nicotine that is restored by re-expression of beta 4*nAChRs in the MHb-IPN. These data indicate that beta 4 is a critical modulator of reward-related behaviors.