Tobacco smoking in humans and nicotine administration in animals decrease appetite and body weight. Many smokers report that they are reluctant to quit because they are afraid to gain weight, and subsets of smokers, including teenage girls in particular, report that they initiate smoking to control their weight. Nicotine, te primary psychoactive substance in tobacco, stimulates nicotinic acetylcholine receptors (nAChRs) on the pro- opiomelanocortin (POMC)-expressing neurons in the arcuate nucleus of the hypothalamus (ARC) that signal satiety, and this is required for the ability of nicotine to decrease food intake in mice. Our preliminary molecular genetic and behavioral studies have identified a specific cell type and circuit, that we can investigate further to evaluate the effect of nicotine and nicotine withdrawal on the changes in appetite that maintain smoking behavior and decrease motivation to quit in a large subset of individuals. The effects of nicotine on POMC neurons persist after chronic exposure, and the nAChRs involved are distinct from those that mediate the initial rewarding effects of the drug. Electrophysiological studies have shown that there are nAChRs on both POMC- neurons in the ARC, as well as neuropeptide Y (NPY)-expressing neurons that drive food intake; however, nicotinic currents in POMC neurons are larger than in NPY neurons. We have also found that nicotine administration modulates presynaptic GABA inputs to POMC neurons in ARC. Thus, we propose that differential activity and desensitization of nAChR subtypes on cell bodies and terminals in ARC may contribute to maintainenance of nicotine-dependent decreases in feeding behavior and that acetylcholine (ACh) signaling through these altered nAChR pathways could contribute to weight gain following withdrawal. Our hypothesis is that the effects of smoking on appetite are due, at least in part, to the ability of nicotine to activate, and differentially desensitize, distinct nAChR subtypes on POMC- and AGRP/NPY-expressing neurons and their presynaptic inputs, which could also lead to altered signaling of endogenous ACh at these receptors. Following chronic nicotine exposure, an imbalance of ACh signaling through nAChRs in the ARC could then lead to increases in food intake during the withdrawal period. In these studies we will: 1) use molecular biological and anatomical techniques to identify the nAChR subunits expressed on specific neuronal cell types in the ARC; 2) use electrophysiological and biochemical techniques to determine whether there are adaptations in nAChR signaling during nicotine administration and after withdrawal and use cell-type selective shRNA delivery to determine the contribution of specific nAChR subunits to neuronal currents and feeding behavior; 3) and use optogenetic and behavioral techniques to determine the effects of cholinergic signaling on food intake at baseline, during nicotine administration and after withdrawal.
Smoking decreases appetite and smokers often report that they smoke to control their weight. Understanding neurobiological mechanisms underlying effects of smoking and smoking cessation on appetite would facilitate development of novel treatments to help with smoking cessation. This study will identify critical molecular and circuit-level mechanisms contributing to nicotine-induced changes in food intake.
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