Effects of pentobarbital and d-amphetamine on oral phencyclidine self-administration in rhesus monkeys.

Three rhesus monkeys self-administered phencyclidine (0.25 mg/ml) during daily 3-hr sessions. Water was also available under a concurrent fixed-ratio (FR) 16 schedule. In Experiment 1, saline or three doses of pentobarbital (2.5, 5 or 10 mg/kg) were injected 10 min before phencyclidine (and water) self-administration sessions. The 2.5 mg/kg pentobarbital dose increased phencyclidine-maintained responding, the 5 mg/kg dose produced mixed effects among the three monkeys, and the 10 mg/kg dose consistently decreased phencyclidine-maintained responding. Subsequently, a saccharin solution (0.03% wt/vol) replaced phencyclidine, and the pentobarbital pretreatment procedure was repeated. Pentobarbital produced dose-related decreases in saccharin-maintained responding. In Experiment 2, saline or three doses of d-amphetamine (0.05, 0.1 or 0.2 mg/kg) were injected 10 min before the phencyclidine self-administration sessions. The 0.05 mg/kg dose produced increases in phencyclidine-maintained responding, while the two higher doses produced dose dependent decreases in responding. When a saccharin solution (0.03%, wt/vol) replaced phencyclidine during the daily sessions, d-amphetamine produced only dose-related decreases in saccharin-maintained responding. These results indicate that pentobarbital and d-amphetamine have a biphasic effect on phencyclidine-maintained behavior; low doses increased responding and high doses decreased responding.