The high co-morbidity between alcohol (ethanol) and nicotine abuse suggests that nicotinic acetylcholine receptors (nAChRs), thought to underlie nicotine dependence, may also be involved in alcohol dependence. The β2* nAChR subtype serves as a potential interface for these interactions since they are the principle mediators of nicotine dependence and have recently been shown to modul6ate some acute responses to ethanol. Therefore, the aim of this study was to more fully characterize the role of β2* nAChRs in ethanol-responsive behaviors in mice after acute exposure to the drug. We conducted a battery of tests in mice lacking the β2 coding gene (Chrnb2) or pretreated with a selective β2* nAChR antagonist for a range of ethanol-induced behaviors including locomotor depression, hypothermia, hypnosis, and anxiolysis. We also tested the effect of deletion on voluntary escalated ethanol consumption in an intermittent access two-bottle choice paradigm to determine the extent of these effects on drinking behavior. Our results showed that antagonism of β2* nAChRs modulated some acute behaviors, namely by reducing recovery time from hypnosis and enhancing the anxiolytic-like response produced by acute ethanol in mice. Chrnb2 deletion had no effect on ethanol drinking behavior, however. We provide further evidence that β2* nAChRs have a measurable role in mediating specific behavioral effects induced by acute ethanol exposure without affecting drinking behavior directly. We conclude that these receptors, along with being key components in nicotine dependence, may also present viable candidates in the discovery of the molecular underpinnings of alcohol dependence.

Background Ethanol and nicotine abuse are two leading causes of preventable mortality in the world, but little is known about the pharmacological mechanisms mediating co-abuse. Few studies have examined the interaction of the acute effects of ethanol and nicotine. Here, we examine the effects of nicotine administration on the duration of ethanol-induced loss of righting reflex (LORR) and characterize the nature of their pharmacological interactions in C57BL/6J mice. Methods We assessed the effects of ethanol and nicotine and the nature of their interaction in the LORR test using isobolographic analysis after acute injection in C57BL/6J male mice. Next, we examined the importance of receptor efficacy using nicotinic partial agonists varenicline and sazetidine. We evaluated the involvement of major nAChR subtypes using nicotinic antagonist mecamylamine and nicotinic α4 and α7 knockout mice. The selectivity of nicotine’s actions on ethanol-induced LORR was examined by testing nicotine’s effects on the hypnotic properties of ketamine and pentobarbital. We also assessed the development of tolerance after repeated nicotine exposure. Lastly, we assessed if the effects of nicotine on ethanol-induced LORR extends to hypothermia and ethanol intake in the Drinking in the Dark (DID) paradigm. Results We found that acute nicotine injection enhances ethanol’s hypnotic effects in a synergistic manner and that receptor efficacy plays an important role in this interaction. Furthermore, tolerance developed to the enhancement of ethanol’s hypnotic effects by nicotine after repeated exposure of the drug. α4* and α7 nAChRs seem to play an important role in nicotine-ethanol interaction in the LORR test. In addition, the magnitude of ethanol-induced LORR enhancement by nicotine was more pronounced in C57BL/6J than DBA/2J mice. Furthermore, acute nicotine enhanced ketamine and pentobarbital hypnotic effects in the mouse. Finally, nicotine enhanced ethanol-induced hypothermia but decreased ethanol intake in the DID test. Conclusion Our results demonstrate that nicotine synergistically enhances ethanol-induced LORR in the mouse.

Evidence suggests that there is an association between polymorphisms in the α5 nicotinic acetylcholine receptor (nAChR) subunit and risk of developing alcohol dependence in humans. The α5 nAChR subunit has also recently been shown to modulate some of the acute response to ethanol in mice. The aim of the current study was to further characterize the role of α5-containing (α5*) nAChRs in acute ethanol responsive behaviors, ethanol consumption and ethanol preference in mice. We conducted a battery of tests in male α5 knockout (KO) mice for a range of ethanol-induced behaviors including hypothermia, hypnosis, and anxiolysis. We also investigated the effects of α5* nAChR on ethanol reward using the Conditioned Place Preference (CPP) assay. Further, we tested the effects of gene deletion on drinking behaviors using the voluntary ethanol consumption in a two-bottle choice assay and Drinking in the Dark (DID, with or without stress) paradigm. We found that deletion of the α5 nAChR subunit enhanced ethanol-induced hypothermia, hypnosis, and an anxiolytic-like response in comparison to wild-type controls. The α5 KO mice showed reduced CPP for ethanol, suggesting that the rewarding properties of ethanol are decreased in mutant mice. Interestingly, Chrna5 gene deletion had no effect on basal ethanol drinking behavior, or ethanol metabolism, but did decrease ethanol intake in the DID paradigm following restraint stress. Taken together, we provide new evidence that α5 nAChRs are involved in some but not all of the behavioral effects of ethanol. Our results highlight the importance of nAChRs as a possible target for the treatment of alcohol dependence.

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