In the United States, tobacco use causes 400,000 deaths a year, making it the leading cause of premature death. Although multiple psychopharmacological effects contribute to tobacco abuse, nicotine is accepted as the primary component that reinforces the habit. Nicotine binds to nicotinic acetylcholine receptors (nAChRs). The rate of nicotine delivery and the time of exposure dictates how the populations of nAChRs distributes among functional states that are closed, ion conducting, desensitized, or long term inactivated. Smokers take in a small pulse of nicotine with each cigarette. These peaks of nicotine ride on a maintained low level of nicotine that increases with repeated cigarettes during the day. The research will test the following hypothesis: The pulse of nicotine could activate nAChRs that evoke dopamine release and produce """"""""rewarding"""""""" effects mediated mainly via the mesolimbic dopaminergic system. The sustained low levels of nicotine, however, could cause significant desensitization or longer-term inactivation of nAChRs that may underlie tolerance or aspects of withdrawal symptoms mediated by non-reward pathways. Patch-clamp electrophysiology, calcium measurements, and immunocytochemistry will be used to investigate the hypothesis. Microexplants of ventral tegmental area (VTA) neurons synapsing onto low density cultures of nucleus accumbens neurons will be examined to determine how nicotine evokes dopamine release. VTA brain slices and tissue cultures of VTA and habenula neurons will be used to understand the time scales and effects of nAChR desensitization and long-term inactivation as well as recovery from those states. Nicotinic AChR activation, leading to DA release by mesolimbic neurons, may provide the reinforcing reward that initiates tobacco abuse. A further drive to smoke may arise because smokers are medicating themselves with nicotine on longer time scales to control the degree to nAChR desensitization and inactivation that may underlie aspects of tolerance and withdrawal symptoms.
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