Effects of the putative dopamine D-3 receptor agonist 7-OH-DPAT in rhesus monkeys trained to discriminate cocaine from saline

These studies were designed to evaluate the effects of the putative dopamine D-3 receptor agonist 7-hydroxy-N,N-di-n-propyl-2-aminotetraline (7-OH-DPAT), alone and in combination with cocaine, in four rhesus monkeys trained to discriminate cocaine (0.4 mg/kg, IM) from saline under a fixed-ratio 30 schedule of food presentation. Under these conditions, cumulative doses of cocaine (0.013-1.3 mg/kg) produced a dose-dependent and complete generalization to the training dose of cocaine in all monkeys, while producing only minimal effects on response rates. The discriminative stimulus effects of cocaine were antagonized by the non-selective dopamine receptor antagonist flupenthixol (0.018 mg/kg, IM) in all four monkeys. The effects of 7-OH-DPAT (0.01-1.8 mg/kg) were inconsistent across monkeys. In two of the four monkeys (monkeys L990 and L958), 7-OH-DPAT consistently and completely generalized to cocaine and decreased response rates in a dose-dependent manner. Both the cocaine-like discriminative stimulus effects and rate-decreasing effects of 7-OH-DPAT were antagonized by flupenthixol in these two monkeys. Pretreatment with low doses of 7-OH-DPAT (0.01-0.032 mg/kg) had no effect on the cocaine dose-effect curve in monkeys L990 and L958; however, higher doses of 7-OH-DPAT (0.032-0.32 mg/kg) shifted the cocaine dose-effect curve to the left. In the other two monkeys (monkeys 150F and 89B036), 7-OH-DPAT produced a dose-dependent decrease in response rates but did not consistently generalize to cocaine. Flupenthixol did not antagonize the rate-decreasing effects of 7-OH-DPAT in these two monkeys, and pretreatment with 7-OH-DPAT (0.1-0.32 mg/kg) produced a decrease in response rates but had no effect on the cocaine dose-effect curve. Time-course experiments revealed that 7-OH-DPAT (0.32 mg/kg) displayed a slower onset and a longer duration of effect than the training dose of cocaine. Finally, the D-3/D-2 dopamine agonist quinpirole completely generalized to cocaine in three monkeys, and partially in the fourth monkey. Quinpirole showed the highest potency in those monkeys in which 7-OH-DPAT consistently generalized to cocaine. The results of the present study suggest that, in rhesus monkeys, 7-OH-DPAT produces cocaine-like effects and may modulate the discriminative stimulus effects of cocaine in some monkeys.