Biphasic effects of δ9-tetrahydrocannabinol on variable interval schedule performance in rats

Four rats were trained to barpress for water reinforcement under a variable interval 60 sec schedule. Nine acute administrations of (–)⁹-trans-tetrahydrocannabinol, in amounts ranging from 0.25 to 16.0 mg/kg, produced dose-related effects on responding; overall response rate increased at lower doses, while higher doses produced ataxia and a complete suppression of responding. Increased response rates reflected changes both in response spacing and in the lengths of post-reinforcement pauses. It was concluded that marihuana has a biphasic effect on variable interval water-reinforced behavior in rats.The authors thank Reid Vandell for his assistance in collecting the data. Research supported by National Institute of Mental Health Grant MH20363-01 to D. P. Ferraro. Synthetic ⁹-THC obtained by approval of the FDA-NIMH Psychotomimetic Agents Advisory Committee. The animals involved in this study were maintained in accordance with Guide for Laboratory Animal Facilities and Care as published by the National Academy of Sciences-National Research Council.