title: Time-dependent melatonin analgesia in mice: inhibition by opiate or benzodiazepine antagonism.
abstract: The aim of this study was to determine whether melatonin-induced analgesia in mice exhibits the time dependency known to occur for several other effects of the hormone, and to analyze to what extent the activity of melatonin can be inhibited by the opiate antagonist naloxone or the central-type benzodiazepine (BZP) antagonist Ro 15-1788. Analgesia was assessed with the hot plate procedure. There was a significant diurnal variation in the pain threshold, with an increase in latency during the dark phase of the daily photo period. Melatonin (20-40 mg/kg i.p.) exhibited maximal analgesic effects at late evening (20:00 h). The administration of naloxone or Ro 15-1788 at 20:00 h, although unable by themselves to modify pain threshold, blunted the analgesic response to melatonin. Significant increases in the latency of the hot plate response were found after diazepam injection, an effect blocked by Ro 15-1788 or naloxone. These results indicate that time-dependent melatonin analgesia is sensitive to opioid or central-type BZP antagonism.
authors.affiliation: Departamento de Fisiologia, Facultad de Medicina, Universidad de Buenos Aires, Argentina.
authors.forename: D A
authors.initials: DA
authors.lastname: Golombek
authors.forename: E
authors.initials: E
authors.lastname: Escolar
authors.forename: L J
authors.initials: LJ
authors.lastname: Burin
authors.forename: M G
authors.initials: MG
authors.lastname: De Brito Sánchez
authors.forename: D P
authors.initials: DP
authors.lastname: Cardinali
mesh_terms: D000700:Analgesics; D000818:Animals; D001569:Benzodiazepines / Q000037:antagonists & inhibitors*; D002940:Circadian Rhythm / Q000502:physiology; D005442:Flumazenil / Q000494:pharmacology; D007700:Kinetics; D008027:Light; D008297:Male; D008550:Melatonin / Q000037:antagonists & inhibitors / Q000494:pharmacology*; D051379:Mice; D009270:Naloxone / Q000494:pharmacology; D009292:Narcotic Antagonists / Q000494:pharmacology*; D010147:Pain Measurement; D011930:Reaction Time / Q000187:drug effects; D013997:Time Factors
chemical_list: D000700:Analgesics; D009292:Narcotic Antagonists; D001569:Benzodiazepines; D009270:Naloxone; D005442:Flumazenil; D008550:Melatonin