Phencyclidine in combination with pentobarbital: supra-additive effects on complex operant behavior in patas monkeys

In one component of a multiple schedule, patas monkeys acquired a different four-response chain each session by responding sequentially on three keys in the presence of four geometric forms (learning). In the other component, the four-response chain was the same each session (performance). The response chain in each component was maintained by food presentation under a fixed-ratio schedule. Errors produced a brief timeout but did not reset the chain. When administered alone, phencyclidine and pentobarbital, at the higher doses, generally decreased the overall response rate and increased the percent errors in both components. The performance component tended to be less sensitive than the learning component to the drug effects. When phencyclidine was administered in combination with pentobarbital, the phencyclidine dose-effect curves for both rate and accuracy generally shifted progressively to the left as the dose of pentobarbital was increased. In two of three monkeys, combinations of phencyclidine with a high dose of pentobarbital consistently produced greater rate-decreasing and error-increasing effects than expected from simple addition of the effects of each drug given alone. In other words, the phencyclidine-pentobarbital combinations produced supra-additive effects on responding under the multiple schedule.