After alcohol-seeking behavior has been established, the brain undergoes certain adaptive changes to continue functioning despite the presence of alcohol. As a consequence of this adaptation, however, certain abnormalities occur in the brain when alcohol is removed. Thus, periods of abstinence are marked by feelings of discomfort and craving, motivating continued alcohol consumption. This kind of motivation--based not on reward but on avoidance of painful stimuli--is called negative reinforcement. Both positive and negative reinforcement are involved in the maintenance of alcoholism.
Physical dependence in alcoholism is the need for continued alcohol consumption to avoid a withdrawal syndrome that generally occurs from 6 to 48 hours after the last drink. Withdrawal symptoms include anxiety, agitation, tremor, elevated blood pressure, and, in severe cases, seizures. The withdrawal syndrome is distinct from the ongoing process of negative reinforcement described above, although both phenomena result from adaptation of the nervous system.
Source: The National Institute on Alcohol Abuse and Alcoholism (http://www.niaaa.nih.gov)
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