title: Melatonin enhances antinociceptive effects of delta-, but not mu-opioid agonist in mice.
abstract: This present study examines the effect of melatonin on antinociceptive action induced by opioid agonists in mice using the tail-flick test. When injected either by intraperitoneal (i.p.) (1, 5, 25 mg/kg) or by intracerebroventricular (i.c.v.) (0.25, 0.5, 1 mg/kg) routes, melatonin significantly enhanced the delta-opioid agonist deltorphin I induced antinociception, but not mu-opioid agonist endomorphin-1. Further investigation showed that i.c.v. luzindole (0.5 mg/kg) (an antagonist of melatonin receptor) significantly antagonized the enhanced antinociceptive effect of i.c.v. melatonin. These results demonstrated that melatonin can specifically enhance the antinociception induced by specific opioid receptor agonist (i.e., delta opioid agonist) acting on melatonin receptor and that melatonin may have augmentation effect on analgesia with delta-, but not mu-opioid agonists in mice.
authors.affiliation: Department of Biochemistry and Molecular Biology, School of Life Science, Lanzhou University, 222 Tian Shui South Road, Lanzhou 730000, People's Republic of China.
authors.forename: Shi-rong
authors.initials: SR
authors.lastname: Li
authors.forename: Ting
authors.initials: T
authors.lastname: Wang
authors.forename: Rui
authors.initials: R
authors.lastname: Wang
authors.forename: Xu
authors.initials: X
authors.lastname: Dai
authors.forename: Qiang
authors.initials: Q
authors.lastname: Chen
authors.forename: Ren-de
authors.initials: RD
authors.lastname: Li
mesh_terms: D000701:Analgesics, Opioid / Q000494:pharmacology; D000818:Animals; D000927:Anticonvulsants / Q000494:pharmacology*; D004357:Drug Synergism; D007274:Injections, Intraperitoneal; D007276:Injections, Intraventricular; D008297:Male; D008550:Melatonin / Q000494:pharmacology*; D051379:Mice; D008815:Mice, Inbred Strains; D009619:Nociceptors / Q000187:drug effects* / Q000502:physiology; D009842:Oligopeptides / Q000494:pharmacology; D017288:Pain Threshold / Q000187:drug effects; D017465:Receptors, Opioid, delta / Q000819:agonists / Q000378:metabolism*; D017450:Receptors, Opioid, mu / Q000819:agonists / Q000378:metabolism*
chemical_list: D000701:Analgesics, Opioid; D000927:Anticonvulsants; D009842:Oligopeptides; D017465:Receptors, Opioid, delta; D017450:Receptors, Opioid, mu; C060208:deltorphin I, Ala(2)-; D008550:Melatonin