The human cannabinoid CB(1) receptor stably expressed in Chinese hamster ovary cells provides a model system to predict the pharmacological effects of cannabinoids in man

Abstract

Since the cloning of the only known cannabinoid receptor in the human brain, CB₁, scientists have been trying to elucidate the mechanisms of action of this receptor. The possibility of yet unidentified cannabinoid receptors in human brain makes it important to be certain that any results obtained from studies of the CB₁ receptor are due strictly to this receptor. Therefore, the cloned human cannabinoid CB₁ receptor has been stably expressed in Chinese Hamster Ovary (CHO/CB₁) cells, which do not endogenously express and known cannabinoid receptors. Saturation binding studies using [³H]SR141716A and [³H]CP55,940 determined that this system expresses the human cannabinoid CB₁ receptor at approximately 1.8 pmoles of receptor/mg protein and that this expression system is able to differentially bind agonists (CP55,940) and antagonists (SR141716A). In the [³⁵S]GTPγS binding assay, CP55,940, WIN 55,212-2 and HU-210 are full agonists, methanandamide and 11-OH-Δ⁹-THC are partial agonists, Δ⁹-THC is a competitive antagonist and SR141716A and AM630 are inverse agonists. The results for the full and partial agonists are consistent with those reported for these: compounds in mammalian brain. The findings that Δ⁹-THC is a competitive antagonist and that SR141716A and AM630 are inverse agonists in the [³⁵S]GTPγS assay are novel. CP55,940 and WIN 55,212-2 were also shown to be full agonists by maximally inhibiting forskolin-stimulated cAMP formation in CHO/CB₁ cells. The finding that SR141716A is an inverse agonist was further supported by its ability to augment forskolin-stimulated cAMP formation. Interestingly, Δ⁹-THC was a partial agonist in this assay since it inhibited forskolin-stimulated cAMP formation, but to a lesser extent than either CP55,950, or WIN 55,212-2. This difference in the activity of Δ⁹-THC warrants further study. Pertussis toxin abolished the effects of cannabinoid ligands in these functional assays, confirming that the effects of these compounds are mediated through inhibitory Gᵢ/ₒ proteins. The data obtained from radioligand binding and functional assays with the human cannabinoid CB₁ receptor stably expressed in Chinese hamster ovary cells conclude that this is a model system to study the pharmacological effects of cannabinoids and to predict the actions of cannabinoids in man.