title: The synergistic antinociceptive interactions of endomorphin-1 with dexmedetomidine and/or S(+)-ketamine in rats.
abstract: Spinal administration of the endogenous mu-opioid agonist peptide, endomorphin-1, results in antinociception in rodents, but there are few data about its interaction with other antinociceptive drugs. We investigated the antinociceptive interactions at the spinal level of endomorphin-1 with the N-methyl-D-aspartate antagonist S(+)-ketamine, the alpha2-adrenoceptor agonist dexmedetomidine, or both in awake rats. Nociception was assessed by the tail-flick test. Dose-response curves were determined for endomorphin-1 (0.6-50 microg), for dexmedetomidine (0.1-10 microg), for mixtures of S(+)-ketamine (30 or 100 microg) with endomorphin-1 (2-18 microg) or of endomorphin-1 with dexmedetomidine in a fixed ratio (4:1), and for the triple combination of the three drugs after intrathecal administration. Endomorphin-1 and dexmedetomidine both produced dose-dependent antinociception. The coadministration of 100 microg S(+)-ketamine significantly enhanced the antinociceptive effect of 6 microg endomorphin-1. Isobolographic analysis of the combinations of endomorphin-1 and dexmedetomidine revealed a synergistic interaction between these drugs. The 80% effective dose for the triple combination was significantly less than that for either binary combination. These data indicate that S(+)-ketamine and dexmedetomidine, acting via different receptors, produce synergistic antinociceptive interaction with endomorphin-1 at the spinal level. Furthermore, the triple combination of an opioid agonist, an alpha2-adrenoceptor agonist, and an N-methyl-D-aspartate receptor antagonist shows potent antinociceptive activity.
  
  
  CONCLUSIONS
  The coadministration of the N-methyl-D-aspartate antagonist receptor antagonist, S(+)-ketamine, or the specific alpha2-adrenoceptor agonist, dexmedetomidine, significantly enhances the antinociceptive effect of the endogenous mu-opioid agonist, endomorphin-1, at the spinal level. The triple combination of the three drugs causes a further improved antinociception.
authors.affiliation: Department of Physical Therapy, Faculty of Health Science, University of Szeged, Szeged, Hungary. horvath@phys.szote.u-szeged.hu
authors.forename: G
authors.initials: G
authors.lastname: Horvath
authors.forename: G
authors.initials: G
authors.lastname: Joo
authors.forename: I
authors.initials: I
authors.lastname: Dobos
authors.forename: W
authors.initials: W
authors.lastname: Klimscha
authors.forename: G
authors.initials: G
authors.lastname: Toth
authors.forename: G
authors.initials: G
authors.lastname: Benedek
mesh_terms: D000316:Adrenergic alpha-Agonists / Q000494:pharmacology*; D000465:Algorithms; D000701:Analgesics, Opioid / Q000494:pharmacology*; D000778:Anesthetics, Dissociative / Q000494:pharmacology*; D000818:Animals; D020927:Dexmedetomidine / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D004357:Drug Synergism; D007278:Injections, Spinal; D007649:Ketamine / Q000494:pharmacology*; D008297:Male; D009842:Oligopeptides / Q000494:pharmacology*; D017288:Pain Threshold / Q000187:drug effects; D051381:Rats; D017208:Rats, Wistar; D018341:Receptors, Adrenergic, alpha-2 / Q000187:drug effects
chemical_list: D000316:Adrenergic alpha-Agonists; D000701:Analgesics, Opioid; D000778:Anesthetics, Dissociative; D009842:Oligopeptides; D018341:Receptors, Adrenergic, alpha-2; C105293:endomorphin 1; D020927:Dexmedetomidine; D007649:Ketamine