Agonist-induced μ opioid receptor phosphorylation and functional desensitization in rat thalamus

By metabolically labeling tissue slices from striatum and thalamus with [ 32P]orthophosphoric acid and immunoprecipitating the receptor with μ receptor-specific antiserum, we found that the endogenous μ receptor in the brain tissue did undergo phosphorylation. The phosphorylation occurred at basal level (no drug treatment) and was enhanced with DAMGO-treatment. The enhancement of the phosphorylation was blocked by naloxone. Morphine stimulation also increased the phosphorylation, but the amount of enhancement was less than that caused by DAMGO-treatment. μ receptor phosphorylation in the thalamus was much greater than the striatum, while no phosphorylation of the μ receptor in the cerebellum was detected, even with DAMGO treatment. The extent of μ receptor phosphorylation identified in the thalamus, striatum and cerebellum is consistent with the previous studies of μ receptor distribution. The time course and dose–response studies demonstrated that μ receptor phosphorylation was a rapid event, exhibited a positive dose-dependent response, and was similar to that observed in the cloned μ receptor in CHO cells. Furthermore, we correlated the change of μ receptor phosphorylation with the desensitization of the μ receptor function, specifically, inhibition of adenylyl cyclase activity in the thalamus of morphine-tolerant rats. We found that in the thalamus of rats chronically treated with morphine, the enhancement of μ receptor phosphorylation in basal and DAMGO-treated samples paralleled the desensitization of DAMGO-mediated inhibition of adenylyl cyclase. Our results suggest that μ receptor phosphorylation in vivo may play an important role in the modulation of mu receptor function following both acute exposure to morphine and during the development of morphine tolerance.