Brain regional cannabinoid CB(1) receptor signalling and alternative enzymatic pathways for 2-arachidonoylglycerol generation in brain sections of diacylglycerol lipase deficient mice

Abstract

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, DAGLα and DAGLβ. Previous studies with DAGL-deficient mice indicated that DAGLα is the major enzyme needed for retrograde synaptic 2-AG signalling. The current study investigated whether the CB1 receptor-mediated Gi/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-Gi/o-activity largely remained unaltered in DAGLα-knockout and DAGLβ-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 DAGLα or DAGLβ. We conclude that in addition to the sn-1-specific DAGLs, additional 2-AG generating enzymatic pathways are active in brain sections.

Keywords: 2-AG; 2-Arachidonoylglycerol; 2-arachidonoylglycerol; AEA; BSA; Cannabinoid CB(1) receptor; DAGL; Diacylglycerol lipase-knockout; FAAH; KO; LC/MS/MS; MAFP; MAGL; Methylarachidonoylfluorophosphonate; N-arachidonoylethanolamide/anandamide; PLA; PLC; THL; Tetrahydrolipstatin; WT; [(35)S]GTPγS autoradiography; bovine serum albumin; diacylglycerol lipase; eCB; endocannabinoid; fatty acid amide hydrolase; knockout; liquid chromatography/tandem mass spectrometry; methylarachidonoylfluorophosphonate; monoacylglycerol lipase; phospholipase A; phospholipase C; tetrahydrolipstatin; wild-type.