The effects of a D1 and a D2 dopamine antagonist on behavior maintained by cocaine or food.

The purpose of the present experiment was to determine whether a D1 or a D2 dopamine antagonist could alter responding maintained by cocaine at doses that did not affect responding maintained by food. Rhesus monkeys were trained to press a lever in daily experimental sessions under a 3 component multiple schedule of reinforcement. In the first and third components, food was available under a fixed-ratio 30/time-out 2 min (FR30/TO 2) schedule. In the second component, cocaine was available under identical schedule conditions. Each component lasted 15 minutes and there was a 15-minute TO between components. When behavior was stable, rates of responding for injections of saline or several doses of cocaine were determined by making each of these solutions available in the second component for at least 4 sessions. After dose-response determinations for cocaine had been determined, a dose of cocaine that maintained maximal rates of responding was available in daily sessions. When behavior was again stable in all 3 components, monkeys were injected daily before the session with each of several doses of the D1 antagonist SCH 23390 or the D2 antagonist pimozide for the same number of sessions that had been required for responding to decline to low levels when the monkeys were allowed to self-administer saline. Both antagonists caused a dose-related decrease in responding for both cocaine and food. Each antagonist decreased responding for food at the same doses that decreased responding for cocaine. Thus both a D1 and a D2 dopamine antagonist decreased behavior maintained by cocaine but only at doses that also decreased behavior maintained by another reinforcer, food.