Evidence that the cannabinoid CB1 receptor is a 2-arachidonoylglycerol receptor - Structure-activity relationship of 2-arachidonoylglycerol ether-linked analogues, and related compounds

An endogenous cannabimimetic molecule, 2-arachidonoylglycerol, induces a ra pid, transient increase in intracellular free Ca2+ concentrations in NG108- 15 cells through a cannabinoid CB1 receptor-dependent mechanism. We examine d the activities of 24 relevant compounds (2-arachidonoylglycerol, its stru ctural analogues, and several synthetic cannabinoids). We found that 2-arac hidonoylglycerol is the most potent compound examined so far: its activity was detectable from as low as 0.3 nM, and the maximal response induced by 2 -arachidonoylglycerol exceeded the responses by others. Activities of HU-21 0 and CP55940, potent cannabinoid receptor agonists, were also detectable f rom as low as 0.3 nM, whereas the maximal responses induced by these compou nds were low compared with 2-arachidonoylglycerol, Anandamide was also foun d to act as a partial agonist in this assay system. We confirmed that free arachidonic acid failed to elicit a response. Furthermore, we found that a metabolically stable ether-linked analogue of 2-arachidonoylglycerol posses ses appreciable agonistic activity, although its activity was apparently lo wer than that of 2-arachidonoylglycerol. We also confirmed that pretreating cells with various cannabinoid receptor agonists nullified the response in duced by 2-arachidonoylglycerol, whereas pretreating cells with other neuro transmitters or neuromodulators did not affect the response. These results strongly suggested that the cannabinoid CB1 receptor is originally a 2-arac hidonoylglycerol receptor, and 2-arachidonoylglycerol is the intrinsic phys iological ligand for the cannabinoid CB1 receptor.