title: Increased tonic cannabinoid CB1R activity and brain region-specific desensitization of CB1R Gi/o signaling axis in mice with global genetic knockout of monoacylglycerol lipase.
abstract: In mammalian brain, monoacylglycerol lipase (MAGL) is the primary enzyme responsible for terminating signaling function of the endocannabinoid 2-arachidonoylglycerol (2-AG). Previous in vivo studies with mice indicate that both genetic and chronic pharmacological inactivation of MAGL result in 8-30-fold increase of 2-AG concentration in the brain, causing desensitization and downregulation of cannabinoid CB1 receptor (CB1R) activity, leading to functional and behavioral tolerance. However, direct evidence for reduced CB1R activity in the brain is lacking. In this study, we used functional autoradiography to assess basal and agonist-stimulated CB1R-dependent Gi/o protein activity in multiple brain regions of MAGL-KO mice in comparison to their wild-type (WT) littermates. In addition, the role of endogenous cannabinoids in basal CB1R signaling was assessed after comprehensive pharmacological blockade of 2-AG hydrolysis by determining the contents of endocannabinoids (eCBs) in WT and MAGL-KO brain tissues by LC/MS/MS technology. To show whether lack of MAGL cause compensatory alterations in the serine hydrolase activity, we compared serine hydrolase pattern of WT and MAGL-KO using activity-based protein profiling. Consistent with studies using chronic pharmacological MAGL inactivation in vivo, we observed a statistically significant decrease of CB1R-Gi/o signaling in most of the studied brain regions. In MAGL-KO brain sections, elevated 2-AG levels were mirrored to heightened basal CB1R-dependent Gi/o-activity, as well as, dampened agonist-evoked responses in several brain regions. The non-selective serine hydrolase inhibitor methylarachidonoylfluorophosphonate (MAFP) was able to significantly elevate 2-AG levels in brain sections of MAGL-KO mice, indicating that additional serine hydrolases possess 2-AG hydrolytic activity in MAGL-KO brain sections.
authors.affiliation: School of Medicine, Institute of Biomedicine/Physiology, University of Eastern Finland, P.O. Box 1627, 70211 Kuopio, Finland; School of Pharmacy, University of Eastern Finland, P.O. Box 1627, 70211 Kuopio, Finland. Electronic address: dina.navia-paldanius@uef.fi.
authors.forename: Dina
authors.initials: D
authors.lastname: Navia-Paldanius
authors.affiliation: School of Pharmacy, University of Eastern Finland, P.O. Box 1627, 70211 Kuopio, Finland. Electronic address: niina.aaltonen@uef.fi.
authors.forename: Niina
authors.initials: N
authors.lastname: Aaltonen
authors.affiliation: School of Pharmacy, University of Eastern Finland, P.O. Box 1627, 70211 Kuopio, Finland. Electronic address: marko.lehtonen@uef.fi.
authors.forename: Marko
authors.initials: M
authors.lastname: Lehtonen
authors.affiliation: School of Medicine, Institute of Biomedicine/Physiology, University of Eastern Finland, P.O. Box 1627, 70211 Kuopio, Finland. Electronic address: juha.savinainen@uef.fi.
authors.forename: Juha R
authors.initials: JR
authors.lastname: Savinainen
authors.affiliation: Institute of Molecular Biosciences, Karl Franzens Universität Graz, Heinrichstrasse 31/2, 8010 Graz, Austria. Electronic address: ulrike.taschler@uni-graz.at.
authors.forename: Ulrike
authors.initials: U
authors.lastname: Taschler
authors.affiliation: Institute of Molecular Biosciences, Karl Franzens Universität Graz, Heinrichstrasse 31/2, 8010 Graz, Austria. Electronic address: franz.radner@uni-graz.at.
authors.forename: Franz P W
authors.initials: FP
authors.lastname: Radner
authors.affiliation: Institute of Molecular Biosciences, Karl Franzens Universität Graz, Heinrichstrasse 31/2, 8010 Graz, Austria. Electronic address: robert.zimmermann@uni-graz.at.
authors.forename: Robert
authors.initials: R
authors.lastname: Zimmermann
authors.affiliation: School of Medicine, Institute of Biomedicine/Physiology, University of Eastern Finland, P.O. Box 1627, 70211 Kuopio, Finland. Electronic address: jarmo.laitinen@uef.fi.
authors.forename: Jarmo T
authors.initials: JT
authors.lastname: Laitinen
mesh_terms: D000818:Animals; D001921:Brain / Q000201:enzymology / Q000378:metabolism*; D051379:Mice; D018345:Mice, Knockout; D008994:Monoacylglycerol Lipases / Q000235:genetics*; D043884:Receptor, Cannabinoid, CB1 / Q000378:metabolism*; D015398:Signal Transduction
chemical_list: D043884:Receptor, Cannabinoid, CB1; D008994:Monoacylglycerol Lipases
keywords: 2-arachidonoylglycerol; Cannabinoid CB1 receptor; Desensitization; Endocannabinoid; Monoacylglycerol lipase-knockout; [(35)S]GTPγS autoradiography