ß4-Nicotinic Receptors Are Critically Involved in Reward-Related Behaviors and Self-Regulation of Nicotine Reinforcement.

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 α5, α3, and ß4 nicotinic acetylcholine receptor (nAChR) subunits, to nicotine addiction. ß4*nAChRs have been implicated in nicotine withdrawal, aversion, and reinforcement. Here we show that ß4*nAChRs also are involved in non-nicotine-mediated responses that may predispose to addiction-related behaviors. ß4 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, ß4KO self-administer less than wild-type (WT) mice. Conversely, at high nicotine doses, this was reversed and ß4KO self-administered more than WT mice, whereas ß4-overexpressing mice avoided nicotine injections. Viral expression of ß4 subunits in medial habenula (MHb), interpeduncular nucleus (IPN), and VTA of ß4KO mice revealed dose- and region-dependent differences ß4*nAChRs in the VTA potentiated nicotine-mediated rewarding effects at all doses, whereas ß4*nAChRs in the MHb-IPN pathway, limited VTA-ICSA at high nicotine doses. Together, our findings indicate that the lack of functional ß4*nAChRs result in deficits in reward sensitivity including increased ICSA at high doses of nicotine that is restored by re-expression of ß4*nAChRs in the MHb-IPN. These data indicate that ß4 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 ß4 nicotinic acetylcholine receptor (nAChR) subunit encoded by this cluster. We investigated the implication of ß4*nAChRs in anxiety-, food reward- and nicotine reward-related behaviors. Deletion of the ß4 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 ß4 subunit into either the MHb or IPN restored a "stop" signal on nicotine self-administration. These results suggest that ß4*nAChRs provide a promising novel drug target for smoking cessation.