Background: \- Central nicotinic acetylcholine receptors (nAChRs) are primary targets for the action of nicotine. In addition to being involved in tobacco dependence, they are also involved in a variety of brain disorders, including Alzheimer's and Parkinson's diseases. Researchers are interested in developing better ways to study the action of nAChRs to improve treatments for smoking cessation and other problems affected by these receptors. These new study methods may involve different approaches to positron emission tomography (PET) scanning, which can show brain activity related to nAChRs. Objectives: \- To evaluate appropriate and useful doses of radiotracers used in PET scanning of nAChRs in the brains of nonsmokers/former smokers, light smokers, and heavy smokers. Eligibility: \- Individuals between 18 and 50 years of age who fall into one of the following groups: (1) nonsmokers or former smokers who have not smoked for the past 2 years, (2) light/situational smokers, or (3) heavy smokers (at least 15 cigarettes/day for the past 2 years). Design: * Each participant will undergo up to three PET studies, given approximately1 month apart. Each study will take approximately 8 hours to complete. * Participants will provide urine and breath samples before the study and at the start of the study, which will be tested for chemicals that may interfere with the study. * Depending on the study, some of the smoking participants may receive a nicotine patch to wear during the PET scan. * On the day of the study, participants will receive a dose of a radiotracer (a drug used in PET scanning) given either as a single injection or as an injection followed by a continuous infusion, and will have a series of PET scans over the next 7 hours and provide blood samples during that time. * Participants will return for a follow-up visit 1 month after the end of the study.
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