Loss of cannabinoid-stimulated guanosine 5 '-O-(3-[S-35]thiotriphosphate) binding without receptor down-regulation in brain regions of anandamide-tolerant rats

The endogenous cannabinoid anandamide has been reported to produce well-def ined behavioral tolerance, but studies on the possible mechanisms underlyin g this process are few and often contradictory. The present study was desig ned to survey the cellular events involved in anandamide tolerance, in term s of the effects on receptor number, coupling with G proteins, and activati on of the cyclic AMP (cAMP) cascade. Chronic treatment of rats with anandam ide (20 mg/kg i.p. for 15 days) resulted in behavioral tolerance without an y change in cannabinoid receptor binding in the brain regions studied (stri atum, cortex, hippocampus, and cerebellum), suggesting that receptor down-r egulation was not involved in the development of anandamide behavioral tole rance. In contrast, prolonged exposure to anandamide significantly reduced agonist-stimulated guanosine 5'-O-(3-S-35]thiotriphosphate) binding in the same areas, with losses of >50%, suggesting that receptor desensitization m ay be part of the molecular mechanism underlying this tolerance. Finally, c oncerning the cAMP cascade-the most well-known intracellular signaling path ways activated by CB1 receptors-in the brain regions from rats tolerant to anandamide, we found no alteration in cAMP levels or in protein kinase A ac tivity. We propose that anandamide, unlike Delta (9)-tetrahydrocannabinol a nd other cannabinoids, does not alter the receptor system at multiple level s but that desensitization of the CB1 receptor might account for behavioral tolerance to the drug.