and improved mood. It has also been shown to act as a positive reinforcer of smoking; for example, people smoke only tobacco that contains nicotine, and regular smokers modify their smoking behavior to maintain a particular concentration of nicotine in the body (Heishman et al., 1997). Nicotine dependence has also been defined as meeting three of the seven criteria for dependence in the Diagnostic and Statistical Manual of Mental Disorders–IV during the preceding year (American Psychiatric Association, 2000). The 2001–2002 National Epidemiologic Survey on Alcohol and Related Conditions (NESARC) found that 24.9% of the US adult population currently smoked cigarettes and 12.8% of adults were nicotine-dependent; the latter group consumed 57.5% of all cigarettes smoked (Grant et al., 2004).

Biology of Nicotine Reinforcement

The biology of nicotine addiction is reviewed in detail elsewhere (Benowitz, 2009). A few key aspects of the biology are mentioned here. Nicotine acts on the brain by binding to nicotinic cholinergic receptors that are normally activated by endogenously released acetylcholine. Brain-imaging studies demonstrate that nicotine acutely increases activity in the prefrontal cortex, thalamus, and visual system (Brody, 2006). It results in the release of a variety of neurotransmitters of which the most important is dopamine, which appears to be critical in drug-induced reward (Dani and De Biasi, 2001; Nestler, 2005) and signaling of a pleasurable experience—this is necessary for the reinforcing effects of nicotine and other drugs of abuse (Nestler, 2005). The decrease in brain-reward function experienced during nicotine withdrawal is an essential component of nicotine addiction and a key barrier to abstinence.

Psychoactive Effects of Nicotine and Nicotine Withdrawal

The nicotine in tobacco induces stimulation and pleasure while reducing stress and anxiety. Smokers come to use nicotine to modulate their levels of arousal and for mood control in daily life. Smoking may also improve concentration, reaction time, and the performance of some tasks. When one stops smoking, the following nicotine-withdrawal symptoms may emerge: irritability, depressed mood, restlessness, anxiety, problems in getting along with friends and family, difficulty in concentrating, increased hunger and eating, insomnia, and craving for tobacco (Hughes and Hatsukami, 1986). Most smokers experience withdrawal symptoms when they are unable to smoke. Withdrawal in untreated smokers produces mood disturbances comparable in intensity with those seen in psychiatric outpatients (Hughes, 2006). One withdrawal symptom seen in connection with nicotine and other drugs of



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