title: Orexins/hypocretins: pain regulation and cellular actions.
abstract: Orexin A and B (also named hypocretin 1 and 2) are 33 and 28 amino acid-containing neuropeptides, respectively, derived from prepro-orexin (prepro-hypocretin) which is localized in the the lateral and perifonical areas of the hypothalamus. Two G-protein coupled receptor subtypes, OX1 and OX2, were identified. Orexin-containing fibers and OX receptors are widely distributed in the central nervous system. Orexins have been implicated in the arousal, rewarding, energy homeostasis, autonomic central control and antinociceptive systems. Subtype-selective peptide agonists and antagonists and non-peptide antagonists, but not non-peptide agonists, have been developed. This review summarizes the studies investigating the antinociceptive effects of orexins in various animal models of pain, including trigeminovascular pain, and their cellular mechanisms. Orexins are antinociceptive at both spinal and supraspinal levels. The antinociceptive effect of orexin A is comparable to opioids but orexin B is less or not effective. This effect is opioid-independent and mainly mediated through OX1 receptors. Some animal studies suggest that endogenous orexins may be released during postoperative and inflammatory, but not acute, pain states, or during some stress conditions, which may contribute to stress-induced analgesia. Purinergic P(2X) and glycine receptors are proposed to be involved in orexin-induced spinal antinociception. The supraspinal sites of actions might involve the posterior hypothalamus, which contributes to the trigeminovascular nociception, and the ventrolateral periaqueductal gray, which mediates descending pain inhibition. Endocannobinoids and nociceptin/orphanin FQ were found to interplay with orexins in nocicpetive processing. Further studies are required to elucidate the receptor subtype-specific mechanism(s) and clinical implications of orexin-induced antinociception.
authors.affiliation: Department of Pharmacology, College of Medicine, National Taiwan University. No. 1, Jen-Ai Rd., Section 1, Taipei 100, Taiwan. lcchiou@ntu.edu.tw
authors.forename: Lih-Chu
authors.initials: LC
authors.lastname: Chiou
authors.forename: Hsin-Jung
authors.initials: HJ
authors.lastname: Lee
authors.forename: Yu-Cheng
authors.initials: YC
authors.lastname: Ho
authors.forename: Shih-Pin
authors.initials: SP
authors.lastname: Chen
authors.forename: Yan-Yu
authors.initials: YY
authors.lastname: Liao
authors.forename: Chia-Hau
authors.initials: CH
authors.lastname: Ma
authors.forename: Pi-Chuan
authors.initials: PC
authors.lastname: Fan
authors.forename: Jong-Ling
authors.initials: JL
authors.lastname: Fuh
authors.forename: Shuu-Jiun
authors.initials: SJ
authors.lastname: Wang
mesh_terms: D000700:Analgesics / Q000494:pharmacology*; D000818:Animals; D002490:Central Nervous System / Q000187:drug effects* / Q000378:metabolism / Q000502:physiology; D004195:Disease Models, Animal; D006801:Humans; D047908:Intracellular Signaling Peptides and Proteins / Q000494:pharmacology* / Q000502:physiology; D009479:Neuropeptides / Q000494:pharmacology* / Q000502:physiology; D064446:Orexin Receptors; D000068797:Orexins; D010146:Pain / Q000188:drug therapy* / Q000503:physiopathology; D043562:Receptors, G-Protein-Coupled / Q000819:agonists* / Q000037:antagonists & inhibitors* / Q000378:metabolism; D018013:Receptors, Neuropeptide / Q000819:agonists* / Q000037:antagonists & inhibitors* / Q000378:metabolism
chemical_list: D000700:Analgesics; C000596229:HCRT protein, human; D047908:Intracellular Signaling Peptides and Proteins; D009479:Neuropeptides; D064446:Orexin Receptors; D000068797:Orexins; D043562:Receptors, G-Protein-Coupled; D018013:Receptors, Neuropeptide