Abstract

Many of the pharmacological effects of Δ⁹-tetrahydrocannabinol are mediated through CB₁ and CB₂ cannabinoid receptors. However, with the discovery of endogenous cannabinoids, some discrepancies have arisen. Furthermore, unlike the CB₁ receptor, the sequences of the mouse and human CB₂ receptor are divergent, raising the possibility of species specificity. The gene for the rat CB₂ receptor was cloned, expressed, and its properties compared with those of mouse and human CB₂ receptors. Sequence analysis of the coding region of the rat CB₂genomic clone indicates 90% nucleic acid identity (93% amino acid identity) between rat and mouse and 81% nucleic acid identity (81% amino acid identity) between rat and human. The rat CB₂receptor was stably expressed in human embryonic kidney-293 cells to examine its pharmacology. The rat CB₂ showed low affinity for anandamide, an endogenous ligand shown to act at the CB₁ receptor. In contrast, high-affinity binding for SR144528 (CB₂-selective antagonist) as well as several cannabinoid receptor agonists was observed. Coupling to adenylate cyclase was observed. Aspects of the pharmacology of palmitoylethanolamide were also examined. It bound to CB₁and CB₂ receptors with low affinity and stimulated GTPγS binding in the cerebellum and CB₂-expressing cell lines with low potency. The data in this study suggest that the discrepancies in affinities between rat and human may represent species differences. The rat CB₂ receptor genomic clone will be a useful tool for studying the function and regulation of CB₂ in rats.