Concurrent variable-interval drug self-administration and the generalized matching law: a drug-class comparison.

It has previously been shown that self-administration of cocaine under concurrent variable-interval schedules is well described by the generalized matching law. That is, choice between two cocaine-maintained options was apportioned in accordance with relative frequency of reinforcement. However, the generality of this conclusion to drugs of other pharmacological classes has not been determined. In the present study, four male rhesus monkeys (Macaca mulatta) lever pressed under various pairs of concurrent variable-interval schedules with drug injection as the maintaining event. An opioid (alfentanil, 0.001 or 0.004 mg/kg/injection), a barbiturate (methohexital, 0.25 or 0.5 mg/kg/injection), and a psychomotor stimulant (cocaine, 0.05 mg/kg/injection) were selected as representatives of major classes of abused drugs and because of their relatively short duration of action. As has been found for cocaine, choice was well accounted for by the generalized matching law. There were no systematic differences in matching-law parameters across drugs and/or doses. As in earlier studies with drug and nondrug reinforcers, undermatching was a consistent finding. Therefore, the conclusion that relative reinforcement frequency is a crucial determinant of choice, as proposed by the generalized matching law, can be extended to behavior maintained by drugs from a variety of pharmacological classes.