Brain regional cannabinoid CB1 receptor signalling and alternative enzymatic pathways for 2-arachidonoylglycerol generation in brain sections of diacylglycerol lipase deficient mice

Endocannabinoids are the endogenous ligands of the G protein-coupled cannabinoid receptors. The principal brain endocannabinoid, 2-arachidonoylglycerol (2-AG), is enzymatically produced by postsynaptic neurons and then activates presynaptic CB1 receptors in a retrograde manner. The primary pathway for 2-AG generation is believed to be conversion from the diacylglycerols (DAGs) by two sn-1-specific lipases, DAGL alpha and DAGL beta. Previous studies with DAGL-deficient mice indicated that DAGL alpha is the major enzyme needed for retrograde synaptic 2-AG signalling. The current study investigated whether the CB1 receptor-mediated G(i/o) protein activity is altered in brain cryosections of DAGL-deficient mice when compared to wild-type mice and whether the sn-1-specific DAGLs are able to generate 2-AG in brain cryosections. Functional autoradiography indicated that brain regional CB1 receptor-G(i/o)-activity largely remained unaltered in DAGL alpha-knockout and DAGL beta-knockout mice when compared to wild-type littermates. Following comprehensive pharmacological blockade of 2-AG hydrolysis, brain sections generated sufficient amounts of 2-AG to activate CB1 receptors throughout the regions endowed with these receptors. As demonstrated by LC/MS/MS, this pool of 2-AG was generated via tetrahydrolipstatin-sensitive enzymatic pathways distinct from DAGL alpha or DAGL beta. We conclude that in addition to the sn-1-specific DAGLs, additional 2-AG generating enzymatic pathways are active in brain sections. (C) 2013 Elsevier B.V. All rights reserved.