title: Time-dependent effect of morphine and time-independent effect of MK-801, an NMDA antagonist, on the thermal hyperesthesia induced by unilateral constriction injury to the sciatic nerve in the rat.
abstract: BACKGROUND
  It is known that peripheral nerve injury induces time-dependent changes in dorsal horn function. The current study investigated the time dependency of the effects of intrathecal morphine and MK-801, an N-methyl-D-aspartate antagonist, on the thermal hyperesthesia evoked by unilateral constriction injury to the sciatic nerve in the rat.
  
  
  METHODS
  In rats with a unilateral constriction injury to the sciatic nerve, paw withdrawal latency against thermal stimulation for the injured paw was typically 3 s less than that for the uninjured paw during the first 5 weeks after the injury. Drugs were administered intrathecally 1 or 5 weeks after the nerve injury.
  
  
  RESULTS
  Intrathecal morphine increased the paw withdrawal latencies of both the injured paw and the uninjured paw in an equally dose dependent manner in the 1-week study. In the 5-week study, morphine increased the paw withdrawal latency of the uninjured paw in a dose-dependent manner, but not that of the injured paw. Intrathecal MK-801 increased the paw withdrawal latency of the injured paw to the level of the uninjured paw in a dose-dependent manner in both the 1- and 5-week studies.
  
  
  CONCLUSIONS
  These data indicate that (1) an N-methyl-D-aspartate receptor-mediated spinal facilitation may be the common mechanism maintaining the thermal hyperesthesia evoked by the constriction injury, and (2) the effects of intrathecal morphine on this thermal hyperesthesia are time-dependent.
authors.affiliation: Department of Anesthesiology, School of Medicine, Chiba University, Japan.
authors.forename: T
authors.initials: T
authors.lastname: Yamamoto
authors.forename: N
authors.initials: N
authors.lastname: Shimoyama
authors.forename: H
authors.initials: H
authors.lastname: Asano
authors.forename: T
authors.initials: T
authors.lastname: Mizuguchi
mesh_terms: D000818:Animals; D003251:Constriction, Pathologic; D016291:Dizocilpine Maleate / Q000008:administration & dosage / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D006358:Hot Temperature; D006941:Hyperesthesia / Q000209:etiology / Q000503:physiopathology*; D007278:Injections, Spinal; D008297:Male; D009020:Morphine / Q000008:administration & dosage / Q000494:pharmacology*; D009270:Naloxone / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D012018:Reflex / Q000187:drug effects*; D012584:Sciatic Nerve / Q000293:injuries*; D013997:Time Factors
chemical_list: D009270:Naloxone; D016291:Dizocilpine Maleate; D009020:Morphine