Springer Online Journal Archives 1860-2000
Abstract The effects on schedule-controlled operant behavior of the D2 receptor agonist, quinpirole, and the D1 agonist, SKF 38393, were assessed alone and in combination with selective dopamine-receptor antagonists. Squirrel monkeys (Saimiri sciureus) were trained to press a response key under fixed-interval and fixed-ratio schedules of food reinforcement. The fixed-interval schedule maintained relatively low rates of responding that increased up to food presentation. The fixed-ratio schedule maintained relatively constant high rates of responding. Quinpirole increased rates and disrupted the temporal pattern of responding under the fixed-interval schedule at doses (0.1–1.0 mg/kg) that decreased rates of responding under the fixed-ratio schedule. Under the fixed-interval schedule, the D2 antagonists, spiperone (0.003–0.006 mg/kg) and haloperidol (0.003–0.01 mg/kg), and the D1 antagonist, SCH 23390 (0.03 mg/kg), shifted the quinpirole dose-effect curve to the right. The maximal effects of quinpirole were decreased at the highest doses of the antagonists. However, only spiperone antagonized effects of quinpirole on the rates of responding under the fixed-ratio schedule. The D1 agonist, SKF 38393, dose-dependently (1.0–10.0 mg/kg) decreased rates of responding under both schedules. Those effects were not antagonized by any doses studied of either spiperone (0.003 mg/kg) or SCH 23390 (0.003–0.3 mg/kg). Rather, both antagonists enhanced the effects of SKF 38393. The present study suggests significant differences between the effects of D1 and D2 agonists on schedule-controlled behavior, and differences in the antagonist actions of the D2 antagonists haloperidol and spiperone. Further, the “selective” dopamine D1 agonist, SKF 38393, has behavioral effects that cannot be antagonized by either a D1 or D2 antagonist, suggesting that some other mechanism has a significant role in mediating its behavioral effects.
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