Delta-9-tetrahydrocannabinol potentiates the disruptive effects of phencyclidine on repeated acquisition in monkeys.

Patas monkeys acquired a different four-response chain each session by responding sequentially on three keys or levers in the presence of four discriminative stimuli (geometric forms or numerals). The response chain was maintained by food presentation under a fixed-ratio schedule. Errors produced a brief timeout but did not reset the chain. Each day there were four 15-min sessions, with a 10-min intersession interval. Cumulative dose-effect curves for phencyclidine were obtained by giving an IM injection before each of the four sessions; successive injections increased the cumulative dose by 1/4 log-unit steps. When phencyclidine was administered alone, overall response rate decreased and percent errors increased with increasing doses. When delta-9-tetrahydrocannabinol (THC) was administered PO before the first session at a dose that was ineffective when given alone, the phencyclidine dose-effect curves for both rate and accuracy tended to shift to the left. After pretreatment with a higher dose of THC, which decreased rate in one of three subjects without affecting accuracy when given alone, the rate-decreasing and error-increasing effects of phencyclidine were generally even more pronounced. The results indicate that THC potentiates the disruptive effects of phencyclidine on complex operant behavior in monkeys.