Despite substantial degradation, 2‐arachidonoylglycerol is a potent full efficacy agonist mediating CB 1 receptor‐dependent G‐protein activation in rat cerebellar membranes

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. 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 CB 1 receptors using agonist‐stimulated [ 35 S]‐GTPγS binding to cerebellar membranes and whole brain sections. Degradation of endocannabinoids under experimental conditions was monitored by HPLC. 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 ∼2.5 fold over basal. The superior efficacy of 2‐AG over all other compounds became evident by increasing GDP (10 −5 and 10 −4   M ). 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. 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. These data establish that even under conditions of substantial degradation, 2‐AG is a full efficacy agonist, clearly more potent than AEA, in mediating CB 1 receptor‐dependent G‐protein activity in native membranes.British Journal of Pharmacology (2001) 134 , 664–672; doi: 10.1038/sj.bjp.0704297