Recent Advances in Animal Models of Drug Addiction

Drug addiction is defined as a compulsion to take a drug with loss of control in limiting intake (33). It is considered a chronic disorder because the risk of relapse remains high even after completion of treatment and prolonged abstinence. In 1968, the term drug dependence replaced that of addiction in the nomenclature of theWorld Health Organization and the American Psychiatric Association. Defined as a cluster of cognitive, behavioral, and physiologic symptoms indicative of an individual continuing substance use despite significant substance related problems, this term has become the accepted diagnostic term for compulsive use of a psychoactive substance. When defined as described, it is analogous to the term addiction. However, this term should not be confused with physical or psychic dependence, conditions in which the cessation or reduction of drug usage results in a withdrawal syndrome.Withdrawal and tolerance often are associated with compulsive drug use; however, they are not required for drug addiction. Although individuals suffering from chronic pain may develop tolerance to the analgesic effects of an opiate and experience withdrawal symptoms, they do not exhibit signs of compulsive drugseeking behavior. The concept of reinforcement has provided the cornerstone for current theories and animal models of drug addiction. A reinforcer is defined operationally as ‘‘any event that increases the probability of a response’’ and often is used interchangeably with ‘‘reward.’’ In general, drugs function as positive or conditioned reinforcers by virtue of their rewarding effects, and reward often connotes additional attri-