Among tobacco's 3500 different constituents, nicotine has been considered the most likely responsible for tobacco's addictive effects (7). When smoked, tobacco releases
From: Molecular Biology of Drug Addiction Edited by: R. Maldonado © Humana Press Inc., Totowa, NJ
nicotine, which readily enters the blood circulatory system and rapidly accumulates in the brain, exerting its neuroactive action. To date, the neurobiology of nicotine dependence is only partially understood; the knowledge about the underlying molecular mechanisms so far accumulated is based mostly on findings obtained in animal models of nicotine dependence and, to a lesser extent, on observations in humans. According to a well-accepted psychopharmacological model (8), substance use or drug-seeking behavior is controlled by four main processes: the positive reinforcing effects, the aversive effects, the discriminative effects, and the stimulus-conditioned effects of the drug. In more explicit terms, an individual consumes nicotine because it:
1. Produces pleasure (positive emotions)
2. Reduces the aversion due to abstinence (negative emotions)
3. Signals discrimination about its presence (assuming motivational value)
4. Promotes, as a conditioned stimulus, the unconditioned learned component of the drug-taking response
In the real world all these processes are interacting in determining the behavior of individuals smoking tobacco, and have been modeled according to the different weight given to certain processed (9,10). The common factor to all these models is the presence of compulsive drug-taking.
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