Marijuana effects on sensitivity to reinforcement in humans

Under controlled laboratory conditions, eight adult subjects smoked placebo and three different potencies of marijuana cigarettes ranging in Delta(9) THC content. Immediately following, smoking, subjects were exposed to a laboratory task that provided concurrently available response options. One option systematically decreased in reinforcement frequency throughout the session, and thus required a reallocation of behavior to the non-decreasing option to maximize monetary earnings. After smoking the two highest doses (1.77% and 3.58% Delta(9) THC) subjects earned fewer reinforcers and allocated a higher proportion of responding to the decreasing option, compared with placebo and the lowest dose. The difference in reinforcers earned could not be accounted for by a change in response rates. Quantitative and graphical analyses revealed that the higher doses produced considerable periods of time spent on the decreasing option despite earning few, reinforcers. The data are discussed with regard to marijuana effects on dopamine/cannabinoid systems and adaptive behavior change. (C) 2002 American College of Neuropsychopharmacology. Published by Elsevier Science Inc.