An endocannabinoid mechanism for stress-induced analgesia

Stress activates the neural systems that inhibit the sensation of pain, inducing natural analgesia. Some of these pathways involve natural opiates but now an endocannabinoid-based equivalent has been found. The enzymes that break up these natural marijuana-like compounds may be important therapeutic targets for stress-related disorders. Acute stress suppresses pain by activating brain pathways that engage opioid or non-opioid mechanisms. Here we show that an opioid-independent form of this phenomenon, termed stress-induced analgesia1, is mediated by the release of endogenous marijuana-like (cannabinoid) compounds in the brain. Blockade of cannabinoid CB1 receptors in the periaqueductal grey matter of the midbrain prevents non-opioid stress-induced analgesia. In this region, stress elicits the rapid formation of two endogenous cannabinoids, the lipids 2-arachidonoylglycerol2 (2-AG) and anandamide3. A newly developed inhibitor of the 2-AG-deactivating enzyme, monoacylglycerol lipase4,5, selectively increases 2-AG concentrations and, when injected into the periaqueductal grey matter, enhances stress-induced analgesia in a CB1-dependent manner. Inhibitors of the anandamide-deactivating enzyme fatty-acid amide hydrolase6, which selectively elevate anandamide concentrations, exert similar effects. Our results indicate that the coordinated release of 2-AG and anandamide in the periaqueductal grey matter might mediate opioid-independent stress-induced analgesia. These studies also identify monoacylglycerol lipase as a previously unrecognized therapeutic target.