Residual effects of prolonged cannabis administration on exploration and DRL performance in rats.

Chronic oral administration of cannabis extract to rats (daily delta 9-tetrahydrocannabinol dose 20 mg/kg) was examined for its residual effect on open field activity and DRL (differential reinforcement of low-rat responding) performance, following a 2-3 month drug-free period. Locomotor activity during the latter part of an open field test was markedly increased in rats previously treated for either 6 months or 3 months with the drug. The same treatments also produced a significant impairment on a DRL-20 task relative to control subjects' performance. These and other findings (impaired maze learning and facilitated two-way shuttle box avoidance) might mean that cannabis produces long-lasting hippocampal dysfunction in rats.