title: Activation of orexin 1 receptors in the periaqueductal gray of male rats leads to antinociception via retrograde endocannabinoid (2-arachidonoylglycerol)-induced disinhibition.
abstract: Orexin A and B are hypothalamic peptides known to modulate arousal, feeding, and reward via OX1 and OX2 receptors. Orexins are also antinociceptive in the brain, but their mechanism(s) of action remain unclear. Here, we investigated the antinociceptive mechanism of orexin A in the rat ventrolateral periaqueductal gray (vlPAG), a midbrain region crucial for initiating descending pain inhibition. In vlPAG slices, orexin A (30-300 nm) depressed GABAergic evoked IPSCs. This effect was blocked by an OX1 [1-(2-methylbenzoxazol-6-yl)-3-[1,5]naphthyridin-4-yl urea (SB 334867)], but not OX2 [N-acyl 6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline hydrochloride (compound 29)], antagonist. Orexin A increased the paired-pulse ratio of paired IPSCs and decreased the frequency, but not amplitude, of miniature IPSCs. Orexin A-induced IPSC depression was mimicked by (R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-napthalenylmethanone (WIN 55,212-2), a cannabinoid 1 (CB1) receptor agonist. 1-(2,4-Dichlorophenyl)-5-(4-iodophenyl)-4-methyl-N-(1-piperidyl)pyrazole-3-carboxamide (AM 251), a CB1 antagonist, reversed depressant effects by both agonists. Orexin A-induced IPSC depression was prevented by 1-[6-[[(17β)-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-1H-pyrrole-2,5-dione (U73122) and tetrahydrolipstatin, inhibitors of phospholipase C (PLC) and diacylglycerol lipase (DAGL), respectively, and enhanced by cyclohexyl[1,1'-biphenyl]-3-ylcarbamate (URB602), which inhibits enzymatic degradation of 2-arachidonoylglycerol (2-AG). Moderate DAGLα, but not DAGLβ, immunoreactivity was observed in the vlPAG. Orexin A produced an overall excitatory effect on evoked postsynaptic potentials and hence increased vlPAG neuronal activity. Intra-vlPAG microinjection of orexin A reduced hot-plate nociceptive responses in rats in a manner blocked by SB 334867 and AM 251. Therefore, orexin A may produce antinociception by activating postsynaptic OX1 receptors, stimulating synthesis of 2-AG, an endocannabinoid, through a Gq-protein-mediated PLC-DAGLα enzymatic cascade culminating in retrograde inhibition of GABA release (disinhibition) in the vlPAG.
authors: Graduate Institute and Department of Pharmacology, and Graduate Institute of Brain and Mind Sciences, College of Medicine, Graduate Institute of Zoology, Neurobiology and Cognitive Science Center, National Taiwan University, Taipei 100, Taiwan.
authors: Yu-Cheng
authors: YC
authors: Ho
authors: Hsin-Jung
authors: HJ
authors: Lee
authors: Li-Wei
authors: LW
authors: Tung
authors: Yan-Yu
authors: YY
authors: Liao
authors: Szu-Ying
authors: SY
authors: Fu
authors: Shu-Fang
authors: SF
authors: Teng
authors: Hsin-Tzu
authors: HT
authors: Liao
authors: Ken
authors: K
authors: Mackie
authors: Lih-Chu
authors: LC
authors: Chiou
mesh_terms: D000704:Analysis of Variance; D000818:Animals; D000831:Animals, Newborn; D001095:Arachidonic Acids / Q000494:pharmacology*; D048588:Benzoxazines / Q000494:pharmacology; D001583:Benzoxazoles / Q000494:pharmacology; D001713:Biphenyl Compounds / Q000494:pharmacology; D002121:Calcium Channel Blockers; D063385:Cannabinoid Receptor Modulators / Q000494:pharmacology*; D004195:Disease Models, Animal; D004558:Electric Stimulation; D063388:Endocannabinoids; D004791:Enzyme Inhibitors / Q000494:pharmacology; D004963:Estrenes / Q000494:pharmacology; D005989:Glycerides / Q000494:pharmacology*; D066298:In Vitro Techniques; D053444:Inhibitory Postsynaptic Potentials / Q000187:drug effects; D047908:Intracellular Signaling Peptides and Proteins / Q000378:metabolism; D007783:Lactones / Q000494:pharmacology; D008297:Male; D009025:Morpholines / Q000494:pharmacology; D009281:Naphthalenes / Q000494:pharmacology; D009287:Naphthyridines; D009433:Neural Inhibition / Q000187:drug effects / Q000502:physiology*; D009434:Neural Pathways / Q000187:drug effects / Q000502:physiology; D009479:Neuropeptides / Q000378:metabolism; D064446:Orexin Receptors; D000068797:Orexins; D000077403:Orlistat; D010146:Pain / Q000188:drug therapy* / Q000378:metabolism / Q000473:pathology; D010147:Pain Measurement / Q000187:drug effects; D018408:Patch-Clamp Techniques; D010487:Periaqueductal Gray / Q000187:drug effects / Q000502:physiology*; D010880:Piperidines / Q000494:pharmacology; D011720:Pyrazoles / Q000494:pharmacology; D011760:Pyrrolidinones / Q000494:pharmacology; D051381:Rats; D017208:Rats, Wistar; D043562:Receptors, G-Protein-Coupled / Q000037:antagonists & inhibitors / Q000378:metabolism*; D018013:Receptors, Neuropeptide / Q000037:antagonists & inhibitors / Q000378:metabolism*; D014508:Urea / Q000031:analogs & derivatives / Q000494:pharmacology; D005680:gamma-Aminobutyric Acid / Q000378:metabolism
chemical_list: C420062:1-(2-methylbenzoxazol-6-yl)-3-(1,5)naphthyridin-4-yl urea; D001095:Arachidonic Acids; D048588:Benzoxazines; D001583:Benzoxazoles; D001713:Biphenyl Compounds; D002121:Calcium Channel Blockers; D063385:Cannabinoid Receptor Modulators; D063388:Endocannabinoids; D004791:Enzyme Inhibitors; D004963:Estrenes; D005989:Glycerides; D047908:Intracellular Signaling Peptides and Proteins; D007783:Lactones; D009025:Morpholines; D009281:Naphthalenes; D009287:Naphthyridines; D009479:Neuropeptides; D064446:Orexin Receptors; D000068797:Orexins; D010880:Piperidines; D011720:Pyrazoles; D011760:Pyrrolidinones; D043562:Receptors, G-Protein-Coupled; D018013:Receptors, Neuropeptide; C501835:URB602; C060229:1-(6-((3-methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl)-1H-pyrrole-2,5-dione; C103505:AM 251; D005680:gamma-Aminobutyric Acid; C070417:(3R)-((2,3-dihydro-5-methyl-3-((4-morpholinyl)methyl)pyrrolo-(1,2,3-de)-1,4-benzoxazin-6-yl)(1-naphthalenyl))methanone; C094503:glyceryl 2-arachidonate; D014508:Urea; D000077403:Orlistat