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

Chronic Δ 9 -tetrahydrocannabinol (Δ 9 -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 [ 35 S]GTPγS binding to localize cannabinoid receptor-activated G-proteins after chronic Δ 9 -THC treatment. Cannabinoid (WIN 55212-2)-stimulated [ 35 S]GTPγS binding was performed in brain sections from rats treated chronically with 10 mg/kg Δ 9 -THC for 21 d. Control animals received saline or an acute injection of Δ 9 -THC. Acute Δ 9 -THC treatment had no effect on basal or WIN 55212-2-stimulated [ 35 S]GTPγS binding. After chronic Δ 9 -THC treatment, net WIN 55212-2-stimulated [ 35 S]GTPγS 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 Δ 9 -THC treatment had no effect on GABA B -stimulated [ 35 S]GTPγS binding. In membranes and brain sections, Δ 9 -THC was a partial agonist, stimulating [ 35 S]GTPγS by only 20% of the level stimulated by WIN 55212-2 and inhibiting WIN 55212-2-stimulated [ 35 S]GTPγS at high concentrations. Because the EC 50 of WIN 55212-2-stimulated [ 35 S]GTPγS binding and the K D of cannabinoid receptor binding were unchanged by chronic Δ 9 -THC treatment, the partial agonist actions of Δ 9 -THC did not produce the decrease in cannabinoid-stimulated [ 35 S]GTPγS binding. These results suggest that profound desensitization of cannabinoid-activated signal transduction mechanisms occurs after chronic Δ 9 -THC treatment.