Nicotine is the pharmacologic agent that acts on the central nervous system (CNS). Its actions are seen in the brain where it operates on cholin-ergic receptors. The cigarette is a very fast and effective delivery system and effects occur rapidly after a single inhalation of tobacco smoke. Nicotine quickly crosses the blood-brain barrier and, once in the brain, interacts with brain receptors. Nicotine alters moods and acts on pleasure-seeking receptors in the brain, including dopamine and serotonin. The nicotine alkaloid affects numerous body systems: It raises blood pressure and the heart rate. It also affects the peripheral nervous system (PNS) and both stimulant and depressive effects are observed in cardiovascular, endocrine, gastrointestinal, and skeletal systems.
Initial exposure to nicotine is not a pleasant experience, often causing sickness, intoxication, and disruptions in physiologic functioning. After a period of daily smoking (assumed to be at least a few weeks), the body adapts to nicotine and the unpleasant effects are less pronounced. Tolerance develops and physical dependence occurs. Smokers are free to self-administer the dose of nicotine they desire, and tolerance increases so that the amount of nicotine used per day continues to increase. The level of dependence is strongly related to the dose of nicotine.
As a smoker becomes physically dependent on, that is, addicted to, smoking, the smoker feels normal, comfortable, and effective when taking nicotine, and dysphoric, uncomfortable, and ineffective when deprived of nicotine. The process of dependence development weakens the ability of the person to achieve and sustain even short-term abstinence. Thus, in the nicotine-dependent person, "normal" function depends on nicotine, and the removal of nicotine results in impairment.
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