Chronic ▵9‐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 ▵⁹-tetrahydrocannabinol (▵⁹-THC) results in tolerance to its acute behavioral effects. In a previous study, 21-day ▵⁹-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 ▵⁹-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 ▵⁹-THC produced time-dependent and region-specific down-regulation and desensitization of brain cannabinoid receptors, which may represent underlying biochemical mechanisms of tolerance to cannabinoids.