Interaction of brain cannabinoid receptors with guanine nucleotide binding protein - A radioligand binding study

The binding of a classical cannabinoid agonist, [H-3]R (+)-(2,3-dihydro-5-methyl-3-[(4-morpholinyl)methyl]pyrol[1,2,3- de]-1,4-benzoxazin-6-yl)(1-napthalenyl)methanone monomethanesulfonate ([H-3] WIN55212-2), and a selective cannabinoid receptor (CB1) antagonist, N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4- methyl-1H-pyrazole-3-carboxamide hydrochloride ([H-3]SR141716A), to rat cannabinoid receptors was evaluated using rat cerebellar membranes. Guanine nucleotides inhibited [3H]WIN55212-2 binding by approximately 50% at 10 mu M and enhanced [H-3]SR141716A binding very slightly. In the same tissue, the binding of guanosine 5'-O-[gamma-[S-35]thio]triphosphate ([S-35]GTP-gamma-S) was characterized and the influence of cannabinomimetics evaluated on this binding. Cannabinoid receptor agonists enhanced [S-35]GTP-gamma-S binding, whereas SR141716A was devoid of action by itself but antagonized the action of cannabinoid receptor agonists. The good correlation obtained between the half maximum efficient concentration (EC50) values in [S-35]GTP-gamma-S binding and the IC50 values [H-3]WIN55212-2 binding shows that [S-35]GTP-gamma-S binding could be a good functional assay for brain cannabinoid receptors. (C) 1997 Elsevier Science Inc.