title: Cross-tolerance between mu opioid and alpha-2 adrenergic receptors, but not between mu and delta opioid receptors in the spinal cord of the rat.
abstract: Intrathecal administration of morphine, Tyr-D-Ser(otbu)-Gly-Phe-Leu-Thr or dexmedetomidine for 5 to 10 days rendered rats tolerant to the test drug as measured by both behavioral and electrophysiological tests. Tolerance to the alpha-2 adrenergic agonist dexmedetomidine required a longer induction time and was not as pronounced as the tolerance to the opioid agonists, probably because lower doses of dexmedetomidine relative to the ED50 dose were used to avoid sedation. In the behavioral studies we used the tail-flick test and in the electrophysiological studies recordings were made from dorsal horn nociceptive neurons under halothane anesthesia. After completion of the behavioral testing the same animals were then used in the electrophysiological study. Cross-tolerance developed clearly between the mu opioid agonist morphine and the alpha-2 adrenergic agonist dexmedetomidine, whereas no cross-tolerance was seen between the delta opioid agonist Tyr-D-Ser(otbu)-Gly-Phe-Leu-Thr and either morphine or dexmedetomidine. This is further evidence to support the assumption that in the dorsal horn the mu opioid and the alpha-2 adrenergic receptor are linked functionally, whereas the delta opioid receptor operates independently. These results have also important clinical implications indicating the potential of delta opioid agonists to restore analgesia in a morphine-tolerant patient.
authors.affiliation: Department of Pharmacology, University College London, United Kingdom.
authors.forename: E A
authors.initials: EA
authors.lastname: Kalso
authors.forename: A F
authors.initials: AF
authors.lastname: Sullivan
authors.forename: H J
authors.initials: HJ
authors.lastname: McQuay
authors.forename: A H
authors.initials: AH
authors.lastname: Dickenson
authors.forename: B P
authors.initials: BP
authors.lastname: Roques
mesh_terms: D000316:Adrenergic alpha-Agonists / Q000494:pharmacology; D000595:Amino Acid Sequence; D000818:Animals; D001522:Behavior, Animal / Q000187:drug effects; D004361:Drug Tolerance; D005071:Evoked Potentials; D007093:Imidazoles / Q000494:pharmacology; D008297:Male; D020926:Medetomidine; D008969:Molecular Sequence Data; D009020:Morphine / Q000494:pharmacology; D009474:Neurons / Q000187:drug effects / Q000502:physiology; D009842:Oligopeptides / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects*; D017465:Receptors, Opioid, delta / Q000187:drug effects*; D017450:Receptors, Opioid, mu / Q000187:drug effects*; D013116:Spinal Cord / Q000187:drug effects*
chemical_list: D000316:Adrenergic alpha-Agonists; D007093:Imidazoles; D009842:Oligopeptides; D011942:Receptors, Adrenergic, alpha; D017465:Receptors, Opioid, delta; D017450:Receptors, Opioid, mu; C054868:tyrosyl-seryl(O-tert-butyl)-glycyl-phenylalanyl-leucyl-threonine; D009020:Morphine; D020926:Medetomidine