title: Nerve injury induces a tonic bilateral mu-opioid receptor-mediated inhibitory effect on mechanical allodynia in mice.
abstract: BACKGROUND
  Mice lacking the mu-opioid receptor gene have been used to characterize the role of mu-opioid receptors in nociception and the analgesic actions of opioid agonists. In this study, the authors determined the role of mu-opioid receptors in neuropathic pain behaviors and the effectiveness of mu- and kappa-opioid receptor agonists on this behavior in mice.
  
  
  METHODS
  The authors studied the behavioral responses of mu-opioid receptor knockout and wild-type mice to thermal and mechanical stimuli before and after neuropathic pain induced by unilateral ligation and section of the L5 spinal nerve. Response to mechanical stimuli was evaluated by determining the frequency of hind paw withdrawal to repetitive stimulation using a series of von Frey monofilaments. Thermal hyperalgesia was assessed by determining the paw withdrawal latencies to radiant heat and frequency of hind paw withdrawal to cooling stimuli. The effects of systemic morphine, the kappa-opioid agonist U50488H, and naloxone on responses to mechanical and thermal stimuli were also studied in spinal nerve-injured mice.
  
  
  RESULTS
  After spinal nerve injury, wild-type mice developed increased responsiveness to mechanical, heat, and cooling stimuli ipsilateral to nerve injury. mu-Opioid receptor knockout mice not only had more prominent mechanical allodynia in the nerve-injured paw, but also expressed contralateral allodynia to mechanical stimuli. Hyperalgesia to thermal stimuli was similar between mu-opioid knockout and wild-type animals. Morphine decreased mechanical allodynia dose dependently (3-30 mg/kg subcutaneous) in wild-type mice--an effect that was attenuated in the heterozygous mice and absent in the homozygous mu-opioid knockout mice. The kappa-opioid agonist U50488H (3-10 mg/kg subcutaneous) attenuated mechanical allodynia in wild-type, heterozygous, and homozygous mu-opioid mice. Naloxone in wild-type mice resulted in enhanced ipsilateral and contralateral allodynia to mechanical stimuli that resembled the pain behavior observed in mu-opioid receptor knockout mice.
  
  
  CONCLUSIONS
  The authors' observations indicate that (1) unilateral nerve injury induces a bilateral tonic activation of endogenous mu-opioid receptor-mediated inhibition that attenuates mechanical allodynia but not thermal hyperalgesia, (2) both mu- and kappa-opioid agonists attenuate neuropathic pain in mice, and (3) the antihyperalgesic actions of morphine are mediated primarily via mu-opioid receptors.
authors.affiliation: Department of Anesthesiology and Critical Care Medicine, Department of Neurosurgery, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21287, USA.
authors.forename: Heikki
authors.initials: H
authors.lastname: Mansikka
authors.forename: Chengshui
authors.initials: C
authors.lastname: Zhao
authors.forename: Rishi N
authors.initials: RN
authors.lastname: Sheth
authors.forename: Ichiro
authors.initials: I
authors.lastname: Sora
authors.forename: George
authors.initials: G
authors.lastname: Uhl
authors.forename: Srinivasa N
authors.initials: SN
authors.lastname: Raja
mesh_terms: D019900:3,4-Dichloro-N-methyl-N-(2-(1-pyrrolidinyl)-cyclohexyl)-benzeneacetamide, (trans)-Isomer / Q000494:pharmacology; D000818:Animals; D004195:Disease Models, Animal; D051379:Mice; D009020:Morphine / Q000494:pharmacology; D009270:Naloxone / Q000494:pharmacology; D010146:Pain / Q000188:drug therapy / Q000523:psychology*; D011930:Reaction Time; D017450:Receptors, Opioid, mu / Q000502:physiology*; D013127:Spinal Nerves / Q000293:injuries*
chemical_list: D017450:Receptors, Opioid, mu; D009270:Naloxone; D019900:3,4-Dichloro-N-methyl-N-(2-(1-pyrrolidinyl)-cyclohexyl)-benzeneacetamide, (trans)-Isomer; D009020:Morphine