Chronic Δ9-tetrahydrocannabinol treatment produces a time-dependent loss of cannabinoid receptors and cannabinoid receptor-activated G proteins in rat brain

Abstract

Chronic treatment of rats with Δ9-tetrahydrocannabinol (Δ9-THC) results in tolerance to its acute behavioral effects. In a previous study, 21-day Δ9-THC treatment in rats decreased cannabinoid activation of G proteins in brain, as measured by in vitro autoradiography of guanosine-5'-O- (3-[35S]thiotriphosphate) ([35S]GTPγS) binding. The present study investigated the time course of changes in cannabinoid-stimulated [35S]GTPγS binding and cannabinoid receptor binding in both brain sections and membranes, following daily Δ9-THC treatments for 3, 7, 14, and 21 days. Autoradiographic results showed time-dependent decreases in WIN 55212-2- stimulated [35S]GTPγS and [3H]WIN 55212-2 binding in cerebellum, hippocampus, caudate-putamen, and globus pallidus, with regional differences in the rate and magnitude of down-regulation and desensitization. Membrane binding assays in these regions showed qualitatively similar decreases in WIN 55212-2-stimulated [35S]GTPγS binding and cannabinoid receptor binding (using [3H]SR141716A), and demonstrated that decreases in ligand binding were due to decreases in maximal binding values, and not ligand affinities. These results demonstrated that chronic exposure to Δ9-THC produced time- dependent and region-specific downregulation and desensitization of brain cannabinoid receptors, which may represent underlying biochemical mechanisms of tolerance to cannabinoids.