The antinociceptive triterpene β-amyrin inhibits 2-arachidonoylglycerol (2-AG) hydrolysis without directly targeting CB receptors.

Abstract

Background and purpose:  Pharmacological activation of cannabinoid CB(1) and CB(2) receptors is a therapeutic strategy to treat chronic and inflammatory pain. It was recently reported that the natural product mixture α/β-amyrin selectively binds to the cannabinoid CB(1) receptor with a subnanomolar K(i) value (133 pM). Orally administered α/β-amyrin inhibited inflammatory and persistent neuropathic pain in mice both CB(1) and CB(2) receptor-dependently. Here we investigated effects of amyrins on the major targets of the endocannabinoid system. Experimental approach:  We measured CB receptor binding interactions of α- and β-amyrin in validated binding assays using hCB(1) and hCB(2) transfected CHO-K1 cells and further explored potential effects on endocannabinoid transport in U937 cells and breakdown in BV2 cell and pig brain homogenates, as well as purified enzymes. Key results:  We were unable to detect significant binding of neither α- nor β-amyrin to hCB receptors in our assays (K(i ) >10 µM). Intriguingly, the triterpene β-amyrin potently inhibited 2-arachidonoyl glycerol (2-AG) hydrolysis in pig brain homogenates, but had no effect on anandamide degradation. Although β-amyrin only weakly inhibited purified human MAGL it also inhibited α,β-hydrolases (ABHDs) and more potently inhibited 2-AG breakdown than α-amyrin and the MAGL inhibitor pristimerin in BV2 cell and pig brain homogenates. Conclusions and implications:  We propose that β-amyrin exerts its analgesic and anti-inflammatory pharmacological effects via indirect cannabimimetic mechanisms involving the inhibition of degradation of the endocannabinoid 2-AG without interacting directly with CB receptors. Triterpenoids appear to offer a very broad and largely unexplored scaffold for inhibitors of 2-AG enzymatic degradation. © 2012 The Authors. British Journal of Pharmacology © 2012 The British Pharmacological Society.