Schedule-induced self-injection of drugs

Studies of acquisition and maintenance patterns of drug intake, including dependence, necessitate voluntary intake of drugs. Voluntary intake in animal studies is difficult to achieve because of aversive taste factors associated with most drugs, and involuntary or forced choice methods obscure the behavior which is the object of study. The schedule-induced polydipsia paradigm has been used to induce oral ingestion of large volumes of alcohol, barbiturate and other drug solutions. We have developed a method of schedule-induced self-injection which allows the study of acquisition and maintenance of drug intake behavior with changing environments free from the interference of taste factors or imbalances due to excessive water intake. In this paper we review our findings on the acquisition and maintenance patterns of amphetamine, methadone, heroin, alcohol, nicotine, cocaine, delta 9-THC and haloperidol. For all drugs except amphetamine, the combination of schedule and nutritional deprivation leads to the highest rates of drug intake as compared to controls. The schedule does not appear to be a potent factor at 90% and free feeding weight, and drug intake is the result of interaction of environmental factors and pharmacological properties of the drugs, rather than the effects of drug or environmental factors separately. The maintenance patterns of nicotine, cocaine, heroin and alcohol are also discussed and the advantages of schedule-induced self-injection over schedule-induced polydipsia methods are presented.