Despite substantial degradation, 2-arachidonoylglycerol is a potent full efficacy agonist mediating CB1 receptor-dependent G-protein activation in rat cerebellar membranes

1 Two endocannabinoids, arachidonoyl ethanolamide (AEA) and 2-arachidonoylglycerol (2-AG) bind and activate G-protein-coupled cannabinoid receptors, but limited data exist on their relative ability to activate G-proteins. 2 Here we assess agonist potency and efficacy of various cannabinoids, including 2-AG, HU-310 (2-arachidonoyl glyceryl ether, a third putative endocannabinoid), HU-313 (another ether analogue of 2-AG), AEA, R-methanandamide (an enzymatically stable analogue of AEA), and CP-55,940 at rat brain CB1 receptors using agonist-stimulated [S-35]-GTP gammaS binding to cerebellar membranes and whole brain sections. Degradation of endocannabinoids under experimental conditions was monitored by HPLC. 3 To enhance efficacy differences, agonist dose-response curves were generated using increasing GDP concentrations. At 10(-6) M GDP, all compounds, except HU-313, produced full agonists responses similar to2.5 fold over basal. The superior efficacy of 2-AG over all other compounds became evident by increasing GDP (10(-5) and 10(-4) M). 4 In membrane incubations, 2-AG was degraded by 85% whereas AEA and HU-310 were stable. Pretreatment of membranes with phenylmethylsulphonyl fluoride inhibited 2-AG degradation, resulting in 2 fold increase in agonist potency. Such pretreatment had no effect on AEA potency. 5 Responses in brain sections were otherwise consistent with membrane binding data, but 2-AG evoked only a weak signal in brain sections, apparently due to more extensive degradation. 6 These data establish that even under conditions of substantial degradation, 2-AG is a full efficacy agonist, clearly more potent than AEA, in mediating CB1 receptor-dependent G-protein activity in native membranes.