ABSTRACT: 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 ?5, ?3, and ?4 nicotinic acetylcholine receptor (nAChR) subunits, to nicotine addiction. ?4*nAChRs have been implicated in nicotine withdrawal, aversion, and reinforcement. Here we show that ?4*nAChRs also are involved in non-nicotine-mediated responses that may predispose to addiction-related behaviors. ?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 (VTA) in mice. We found that, at low nicotine doses, ?4KO self-administer less than wild-type (WT) mice. Conversely, at high nicotine doses, this was reversed and ?4KO self-administered more than WT mice, whereas ?4-overexpressing mice avoided nicotine injections. Viral expression of ?4 subunits in medial habenula (MHb), interpeduncular nucleus (IPN), and VTA of ?4KO mice revealed dose- and region-dependent differences: ?4*nAChRs in the VTA potentiated nicotine-mediated rewarding effects at all doses, whereas ?4*nAChRs in the MHb-IPN pathway, limited VTA-ICSA at high nicotine doses. Together, our findings indicate that the lack of functional ?4*nAChRs result in deficits in reward sensitivity including increased ICSA at high doses of nicotine that is restored by re-expression of ?4*nAChRs in the MHb-IPN. These data indicate that ?4 is a critical modulator of reward-related behaviors.SIGNIFICANCE STATEMENT Human genetic studies have provided strong evidence for a relationship between variants in the CHRNA5-CHRNA3-CHRNB4 gene cluster and nicotine addiction. Yet, little is known about the role of ?4 nicotinic acetylcholine receptor (nAChR) subunit encoded by this cluster. We investigated the implication of ?4*nAChRs in anxiety-, food reward- and nicotine reward-related behaviors. Deletion of the ?4 subunit gene resulted in an addiction-related phenotype characterized by low anxiety, high novelty-induced response, lack of sensitivity to palatable food rewards and increased intracranial nicotine self-administration at high doses. Lentiviral vector-induced re-expression of the ?4 subunit into either the MHb or IPN restored a "stop" signal on nicotine self-administration. These results suggest that ?4*nAChRs provide a promising novel drug target for smoking cessation.
