Influence of intravenous self-administered psychomotor stimulants on performance of rhesus monkeys in a multiple schedule paradigm.

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Rhesus monkeys were trained to complete three multiple schedules. The schedules consisted of three components: a fixed interval (component 1), a variable interval (component 2), and a fixed ratio (component 3). During components 1 and 2, pressing lever 1 was always reinforced by food delivery. During component 3, pressing lever 2 resulted in either food delivery or intravenous infusions of saline solution, solutions of cocaine, of d-amphetamine, of phenmetrazine, or fenetylline. In schedule I, animals were presented with all three components independent of key-pressing behavior during components 1 and 2. In schedule II the availability of component 2 was dependent on completion of component 1. Component 3 was made available only on completion of component 2. Noncompletion of components 1 or 2 resulted in time-out of 15 and 10 min, respectively. Schedule III was identical with schedule II, except that in schedule III the completion of components was indicated only by a change in the lever lights. The influence of self-administered drugs on behavior in all three components was evaluated. Self-administration of psychomotor stimulants impaired the performance of animals and delayed completion of components 1 and 2 of schedules I, II, and III. The effects on behavior were similar with low drug intake in schedule III, moderate intake in schedule II, and high drug intake in schedule I. These effects were strong with self-administration of phenmetrazine, moderate with self-administration of cocaine and d-amphetamine, and weak with self-administration of fenetylline.