CHARACTERIZATION OF NICOTINE RECEPTORS: ROLE IN TOLERANCE AND DEPENDENCE

Virginia Commonwealth University, Richmond VA

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Abstract

Nicotine's high addiction liability is well recognized, as is the fact that smoking cessation strategies, including behavioral modification and pharmacotherapies, have low success rates. Understanding the biological basis of nicotine dependence is essential if we are to develop new treatment strategies. Tremendous progress has been made in the past decade regarding the actions of nicotine at ion-gated cholinergic receptors (nA ChR). Of the large number of possible nAChR subtypes, o402 and <x7 receptors have received the most attention and have been implicated in a wide range of nicotine's effects, including reward, cognition and pain. Importantly, nicotine addiction has been modeled in mice, in conditioned place preference (CPP) for rewarding effects and chronic infusion of nicotine via minipumps to induce somatic and affective signs of physical dependence upon treatment end. Evidence is emerging that reward and physical dependence are mediated through different nicotinic receptor subtypes and may involve receptors made of aj5 and ct[unreadable] subunits. Nicotine can also manipulate levels of endogenous cannabinoids and THC can blunt some nicotine effects. It is our premise that activation of selective nicotinic receptors activates the endocannabinoid system that in turn contributes to nicotine dependence. Our first goal is to determine what role a4p2 and ct7 receptors, and receptors that contain either a5 or a(J subunits, play in development and expression of rewarding and withdrawal signs associated with nicotine dependence. We will examine nicotine CPP in mice lacking or overexpressing a402 and a? receptors as well as mice with an a5 deletion. Complementary experiments will be conducted in wild-type mice by establishing CPP with selective agonists for these receptor subtypes or using selective antagonists (in particular a6 antagonists) to block nicotine's effects. Genetically modified mice will be chronically infused with nicotine to assess somatic and affective signs upon drug withdrawal. Additionally, during withdrawal, we will challenge wild-type animals with receptor selective agonists and antagonists to assess their effects on somatic and affective signs. We will also assess the effects of varenicline, a partial agonist for a4p2 receptors now in clinical trials for smoking cessation, in the CPP and chronic infusion paradigms. Secondly, we will pursue our premise that the CB1 cannabinoid receptor and endocannabinoids contribute to nicotine rewarding effects and possibly to affective withdrawal signs. We will study nicotine CPP and nicotine withdrawal in mice lacking the CB1 receptor and in those lacking fatty acid amidohydrolase, the enzyme necessary for inactivation of the endocannabinoid anandamide. We will complement these studies by conducting the same experiments in wild type mice challenged with CB1 receptor antagonist or inhibitors of the fatty acid amidohydrolase. Levels of anandamide and 2- arachidonoylglycerol in brain areas of reward during times of nicotine CPP and withdrawal will be studied.