Drug Self-Administration by Laboratory Animals: Control by Schedules of Reinforcement

A common factor underlying all concepts of drug dependence is the persistent maintenance of behavior that leads to drug self-administration (I, 2). Early studies of drug self-administration by laboratory animals involved morphine and employed subjects that were made physiologically (physically) dependent by repeated injec­ tions prior to the initiation of self-administration behavior (3-9). This emphasis on physiological dependence strengthened the belief that drugs were self-administered by laboratory animals chiefly because their administration terminated or postponed the characteristic withdrawal syndrome induced by drug abstinence. While physio­ logical dependence may profoundly influence drug self-administration behavior, it is not necessary for the maintenance of that behavior. With psychomotor stimulant drugs, for example, no clear signs of withdrawal occur after chronic administration is terminated, yet these drugs are persistently self-administered by laboratory ani­ mals (10-16). Even with drugs such as narcotic analgesics, barbiturates, and ethanol that can induce physiological dependence, the initiation of drug self-administration by drug-naive animals must result from other factors (17). Moreover, many experi­ ments have shown that self-administration can be maintained by these drugs in the absence of any demonstrable physiological dependence when access to drug is limited to short periods of time each day and when small doses of drug are used (18-28). Considerations such as these have prompted the investigation of behavioral factors underlying drug dependence.