Endocannabinoid 2-AG metabolism in rat cerebral cortex during physiological aging

Abstract

The ‘‘cannabinoid system’’ is a cell communication mechanism which involves the interaction of endogenous ligands, membrane receptors, and signal inactivation processes. 2-arachidonoilglycerol (2-AG), which is synthesized and released in response either to an increase in intracellular calcium or to the action of metabotropic agonists, is one of the endogenous ligands of cannabinoid receptors CB1 and CB2 that mediate its signalling coupled to G proteins. The enzymes responsible for its synthesis are diacylglycerol lipase (DAGL) and lysophosphatidate phosphohydrolase (LPAase). Its hydrolysis is carried out principally by the enzyme monoacylglycerol lipase (MAGL), although other enzymes may be involved in its breakdown such as fatty acid amide hydrolase (FAAH) and serine hydrolase ABHD. Although it is well known that endocannabionoids play a role as neuroprotectors in pathological senescent processes, their role in physiological senescent processes has not been fully elucidated to date. We thus suggest that 2-AG synthesis and hydrolysis enzymes both of which control its level, could be regulated in physiological aging. To approach this hypothesis we firstly characterized the enzymatic activities involved in 2-AG synthesis and hydrolysis in membrane, soluble and synaptosomal fractions from adult (3 months) and aged (28 months) rat cerebral cortex (CC). CC fractions were isolated by differential centrifugation and synaptosomes were purified in ficoll gradients. DAGL, MAGL and LPAase activities were assayed using tritium radiolabeled substrates, and their products monoacyl[3 H]glycerol y [3 H]glycerol were quantified by liquid scintillation from organic or aqueous phase, respectively. Our observations showed that: (i) LPAasa activity is the most active pathway for 2-AG synthesis; (ii) there is a decrease in LPAase activity and a redistribution of DAGL activity from the soluble to the membrane fraction as a result of aging; (iii) 2-AG hydrolysis in adult membrane is carried out by ABHD and by MAGL while ABHD is the only enzyme responsible for cannabinoid hydrolysis in aged membrane; (iv) DAGL activity is low while LPAase activity is high in aged synaptosomes; (v) MAGL, FAAH and ABHD are responsible for 2-AG hydrolysis in adult synaptosomes; (vi) MAGL is responsible, almost exclusively, for 2-AG hydrolysis in aged synaptosomes. Results from the present study reveal a precise regulation of 2-AG metabolism, which is, in turn, modified in physiological aging