Effects of Chronic Treatment with Δ⁹-Tetrahydrocannabinol on Cannabinoid-Stimulated [³⁵S]GTPγS Autoradiography in Rat Brain

Abstract

Chronic Delta9-tetrahydrocannabinol (Delta9-THC) administration produces tolerance to cannabinoid effects, but alterations in signal transduction that mediate these changes are not yet known. The present study uses in vitro autoradiography of agonist-stimulated [35S]GTPgammaS binding to localize cannabinoid receptor-activated G-proteins after chronic Delta9-THC treatment. Cannabinoid (WIN 55212-2)-stimulated [35S]GTPgammaS binding was performed in brain sections from rats treated chronically with 10 mg/kg Delta9-THC for 21 d. Control animals received saline or an acute injection of Delta9-THC. Acute Delta9-THC treatment had no effect on basal or WIN 55212-2-stimulated [35S]GTPgammaS binding. After chronic Delta9-THC treatment, net WIN 55212-2-stimulated [35S]GTPgammaS binding was reduced significantly (up to 70%) in most brain regions, including the hippocampus, caudate-putamen, perirhinal and entorhinal cortex, globus pallidus, substantia nigra, and cerebellum. In contrast, chronic Delta9-THC treatment had no effect on GABAB-stimulated [35S]GTPgammaS binding. In membranes and brain sections, Delta9-THC was a partial agonist, stimulating [35S]GTPgammaS by only 20% of the level stimulated by WIN 55212-2 and inhibiting WIN 55212-2-stimulated [35S]GTPgammaS at high concentrations. Because the EC50 of WIN 55212-2-stimulated [35S]GTPgammaS binding and the KD of cannabinoid receptor binding were unchanged by chronic Delta9-THC treatment, the partial agonist actions of Delta9-THC did not produce the decrease in cannabinoid-stimulated [35S]GTPgammaS binding. These results suggest that profound desensitization of cannabinoid-activated signal transduction mechanisms occurs after chronic Delta9-THC treatment.