title: Molecular architecture of endocannabinoid signaling at nociceptive synapses mediating analgesia.
abstract: Cannabinoid administration suppresses pain by acting at spinal, supraspinal and peripheral levels. Intrinsic analgesic pathways also exploit endocannabinoids; however, the underlying neurobiological substrates of endocannabinoid-mediated analgesia have remained largely unknown. Compelling evidence shows that, upon exposure to a painful environmental stressor, an endocannabinoid molecule called 2-arachidonoylglycerol (2-AG) is mobilized in the lumbar spinal cord in temporal correlation with stress-induced antinociception. We therefore characterized the precise molecular architecture of 2-AG signaling and its involvement in nociception in the rodent spinal cord. Nonradioactive in situ hybridization revealed that dorsal horn neurons widely expressed the mRNA of diacylglycerol lipase-alpha (DGL-alpha), the synthesizing enzyme of 2-AG. Peroxidase-based immunocytochemistry demonstrated high levels of DGL-alpha protein and CB(1) cannabinoid receptor, a receptor for 2-AG, in the superficial dorsal horn, at the first site of modulation of the ascending pain pathway. High-resolution electron microscopy uncovered postsynaptic localization of DGL-alpha at nociceptive synapses formed by primary afferents, and revealed presynaptic positioning of CB(1) on excitatory axon terminals. Furthermore, DGL-alpha in postsynaptic elements receiving nociceptive input was colocalized with metabotropic glutamate receptor 5 (mGluR(5)), whose activation induces 2-AG biosynthesis. Finally, intrathecal activation of mGluR(5) at the lumbar level evoked endocannabinoid-mediated stress-induced analgesia through the DGL-2-AG-CB(1) pathway. Taken together, these findings suggest a key role for 2-AG-mediated retrograde suppression of nociceptive transmission at the spinal level. The striking positioning of the molecular players of 2-AG synthesis and action at nociceptive excitatory synapses suggests that pharmacological manipulation of spinal 2-AG levels may be an efficacious way to regulate pain sensation.
authors.affiliation: Institute of Experimental Medicine, Hungarian Academy of Sciences, Szigony u. 43., H-1083 Budapest, Hungary.
authors.forename: Rita
authors.initials: R
authors.lastname: Nyilas
authors.forename: Laura C
authors.initials: LC
authors.lastname: Gregg
authors.forename: Ken
authors.initials: K
authors.lastname: Mackie
authors.forename: Masahiko
authors.initials: M
authors.lastname: Watanabe
authors.forename: Andreas
authors.initials: A
authors.lastname: Zimmer
authors.forename: Andrea G
authors.initials: AG
authors.lastname: Hohmann
authors.forename: István
authors.initials: I
authors.lastname: Katona
mesh_terms: D000698:Analgesia; D000818:Animals; D001095:Arachidonic Acids / Q000378:metabolism*; D063385:Cannabinoid Receptor Modulators / Q000378:metabolism*; D063388:Endocannabinoids; D005989:Glycerides / Q000378:metabolism*; D007091:Image Processing, Computer-Assisted; D007150:Immunohistochemistry; D017403:In Situ Hybridization; D008071:Lipoprotein Lipase / Q000378:metabolism; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D046529:Microscopy, Electron, Transmission; D009619:Nociceptors / Q000378:metabolism / Q000648:ultrastructure; D010146:Pain / Q000378:metabolism*; D017729:Presynaptic Terminals / Q000378:metabolism / Q000648:ultrastructure; D051381:Rats; D017207:Rats, Sprague-Dawley; D017208:Rats, Wistar; D043884:Receptor, Cannabinoid, CB1 / Q000235:genetics / Q000378:metabolism; D064529:Receptor, Metabotropic Glutamate 5; D018094:Receptors, Metabotropic Glutamate / Q000378:metabolism / Q000648:ultrastructure; D015398:Signal Transduction / Q000502:physiology*; D013116:Spinal Cord / Q000378:metabolism; D013569:Synapses / Q000378:metabolism* / Q000648:ultrastructure
chemical_list: D001095:Arachidonic Acids; D063385:Cannabinoid Receptor Modulators; D063388:Endocannabinoids; D005989:Glycerides; D043884:Receptor, Cannabinoid, CB1; D064529:Receptor, Metabotropic Glutamate 5; D018094:Receptors, Metabotropic Glutamate; C094503:glyceryl 2-arachidonate; D008071:Lipoprotein Lipase