The behavioral effects of nicotine have been defined as both stimulant and depressant, effects that are influ-enced by the present mental status and expectations of the smoker. Smokers may feel alert and relaxed. Nicotine produces myriad effects on the central nervous system (CNS), almost all of which appear to be medi-ated through nicotinic receptors. Additionally, nicotine influences multiple neuronal systems. One of its most prominent effects is stimulated release of dopamine, particularly in the nucleus accumbens, which is a major component of the reward system. Nicotine also stimu-lates the release of endogenous opioids and glucocorti-coids.
Tolerance to nicotine’s effects develops rapidly and most likely involves multiple processes, although the pattern and extent of tolerance development is not identical for all of nicotine’s effects. It has been pro-posed that rapid tolerance or desensitization occurs to the behavioral or reinforcing effects of nicotine. These effects are of such a short duration that a smoker con-tinually cycles between a sensitized and desensitized state. This notion is consistent with the fact that drugs with high abuse liability have a rapid onset and short duration of action.
Regardless of the mechanism of tolerance, nicotine is a highly addicting drug. Even though most individuals are unaware of nicotine’s reinforcing properties when smoking, many individuals feel intense, long-lasting craving when attempting to stop. Although most smok-ers wish to quit, only about one-third attempt to do so each year.
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