1. Activation of orexin 1 receptors in the periaqueductal gray of male rats leads to antinociception via retrograde endocannabinoid (2-arachidonoylglycerol)-induced disinhibition.
- Author
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Ho YC, Lee HJ, Tung LW, Liao YY, Fu SY, Teng SF, Liao HT, Mackie K, and Chiou LC
- Subjects
- Analysis of Variance, Animals, Animals, Newborn, Benzoxazines pharmacology, Benzoxazoles pharmacology, Biphenyl Compounds pharmacology, Calcium Channel Blockers, Disease Models, Animal, Electric Stimulation, Enzyme Inhibitors pharmacology, Estrenes pharmacology, In Vitro Techniques, Inhibitory Postsynaptic Potentials drug effects, Intracellular Signaling Peptides and Proteins metabolism, Lactones pharmacology, Male, Morpholines pharmacology, Naphthalenes pharmacology, Naphthyridines, Neural Inhibition drug effects, Neural Pathways drug effects, Neural Pathways physiology, Neuropeptides metabolism, Orexin Receptors, Orexins, Orlistat, Pain metabolism, Pain pathology, Pain Measurement drug effects, Patch-Clamp Techniques, Periaqueductal Gray drug effects, Piperidines pharmacology, Pyrazoles pharmacology, Pyrrolidinones pharmacology, Rats, Rats, Wistar, Receptors, G-Protein-Coupled antagonists & inhibitors, Receptors, Neuropeptide antagonists & inhibitors, Urea analogs & derivatives, Urea pharmacology, gamma-Aminobutyric Acid metabolism, Arachidonic Acids pharmacology, Cannabinoid Receptor Modulators pharmacology, Endocannabinoids, Glycerides pharmacology, Neural Inhibition physiology, Pain drug therapy, Periaqueductal Gray physiology, Receptors, G-Protein-Coupled metabolism, Receptors, Neuropeptide metabolism
- 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.
- Published
- 2011
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