title: Analgesic effects of Tyr-W-MIF-1: a mixed mu2-opioid receptor agonist/mu1-opioid receptor antagonist.
abstract: Tyr-W-MIF-1 (Tyr-Pro-Trp-Gly-NH2) is a naturally occurring neuropeptide that displays high selectivity for mu-opioid receptors. Recently, intrathecal (i.t.) Tyr-W-MIF-1 was shown to induce potent analgesia mediated through spinal mu2-opioid receptors in mice. In the current study, we investigated the supraspinal analgesic effects of Tyr-W-MIF-1 using intracerebroventricular (i.c.v.) administration in mice. I.c.v. Tyr-W-MIF-1 induced a dose-dependent analgesic response with an ED50 of 31.4 micrograms that was antagonized by i.c.v. naloxone (ED50 = 4.46 nmol) and the mu-opioid receptor antagonist beta-funaltrexamine but not by the mu1-opioid receptor-selective antagonist naloxonazine. I.t. naloxone (ED50 = 0.12 nmol), however, was nearly 40-fold more potent than i.c.v. naloxone at antagonizing i.c.v. Tyr-W-MIF-1-induced analgesia. Tyr-W-MIF-1 also possesses antagonist activity at mu1-opioid receptors in brain. Coadministration of i.c.v. Tyr-W-MIF-1 with i.c.v. morphine or i.c.v. [D-Ala2, MePhe4, Gly(ol)5]enkephalin (DAMGO) significantly decreased the analgesic response to either drug administered alone. Thus, Tyr-W-MIF-1 functions as a mixed mu2-opioid receptor agonist/mu1-opioid receptor antagonist after i.c.v. administration in mice.
authors.affiliation: Tulane University School of Medicine, New Orleans, LA 70146, USA.
authors.forename: K A
authors.initials: KA
authors.lastname: Gergen
authors.forename: J E
authors.initials: JE
authors.lastname: Zadina
authors.forename: D
authors.initials: D
authors.lastname: Paul
mesh_terms: D000700:Analgesics / Q000494:pharmacology*; D000818:Animals; D004305:Dose-Response Relationship, Drug; D007276:Injections, Intraventricular; D009075:MSH Release-Inhibiting Hormone / Q000031:analogs & derivatives* / Q000037:antagonists & inhibitors / Q000494:pharmacology; D008297:Male; D051379:Mice; D008815:Mice, Inbred Strains; D009020:Morphine / Q000037:antagonists & inhibitors / Q000494:pharmacology; D009270:Naloxone / Q000494:pharmacology; D009292:Narcotic Antagonists / Q000494:pharmacology*; D009294:Narcotics / Q000494:pharmacology; D010147:Pain Measurement / Q000187:drug effects; D017450:Receptors, Opioid, mu / Q000819:agonists* / Q000037:antagonists & inhibitors*
chemical_list: D000700:Analgesics; D009292:Narcotic Antagonists; D009294:Narcotics; D017450:Receptors, Opioid, mu; C077532:tyrosyl-prolyl-tryptophyl-glycinamide; D009270:Naloxone; D009020:Morphine; D009075:MSH Release-Inhibiting Hormone