title: Role of morphine maintenance dose in the development of tolerance and its attenuation by an NMDA receptor antagonist.
abstract: BACKGROUND
  Current research shows that N-methyl-d-aspartate (NMDA) receptor antagonists attenuate the development of morphine tolerance in rodent antinociceptive assays.
  
  
  OBJECTIVE
  The purpose of this study was to determine the role of morphine maintenance dose in the attenuation of morphine tolerance by the competitive NMDA receptor antagonist, LY235959.
  
  
  METHODS
  A rat warm-water tail-withdrawal procedure was used to measure the antinociceptive effects of morphine and LY235959. In this procedure, the distal 8 cm of each rat's tail is immersed in 40 degrees (non-noxious) and 55 degrees C (noxious) water, and the latency to remove the tail is recorded.
  
  
  RESULTS
  Morphine (0.3-10 mg/kg, SC) produced dose-dependent increases in tail-withdrawal latencies from the 55 degrees C water. Following determination of the morphine dose-effect curves, rats were administered chronically one of three doses of morphine (10, 20, or 40 mg/kg) either alone or in combination with LY235959 (1.0, 3.0, or 5.6 mg/kg, SC) twice daily for 7 days. Chronic administration of 10, 20, and 40 mg/kg morphine produced rightward shifts in the morphine dose-effect curves of approximately 3-, 6-, and 12-fold, respectively. When LY235959 (1.0-5.6 mg/kg) was co-administered with 10 mg/kg morphine, the development of morphine tolerance was attenuated in a dose-dependent manner, with complete prevention observed following 3.0 mg/kg LY235959. LY235959 (1.0, 3.0 mg/kg) also attenuated the development of tolerance to 20 and 40 mg/kg morphine; however, tolerance was not completely prevented. Administering 3.0 mg/kg LY235959 along with 20 and 40 mg/kg morphine was functionally equivalent to treating rats with half the amount of morphine.
  
  
  CONCLUSIONS
  These data suggest that the maintenance dose of morphine, and thus the magnitude of tolerance, can determine the effectiveness of an NMDA receptor antagonist to attenuate morphine tolerance.
authors.affiliation: Department of Psychology, CB# 3270, Davie Hall, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-3270, USA. rmallen@md.unc.edu
authors.forename: R M
authors.initials: RM
authors.lastname: Allen
authors.forename: L A
authors.initials: LA
authors.lastname: Dykstra
mesh_terms: D000818:Animals; D004305:Dose-Response Relationship, Drug; D004361:Drug Tolerance; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology*; D007546:Isoquinolines / Q000494:pharmacology; D008297:Male; D009020:Morphine / Q000008:administration & dosage / Q000494:pharmacology*; D009294:Narcotics / Q000008:administration & dosage / Q000494:pharmacology*; D009619:Nociceptors / Q000187:drug effects; D010146:Pain / Q000517:prevention & control; D010147:Pain Measurement; D051381:Rats; D017207:Rats, Sprague-Dawley; D011930:Reaction Time; D016194:Receptors, N-Methyl-D-Aspartate / Q000037:antagonists & inhibitors*
chemical_list: D018691:Excitatory Amino Acid Antagonists; D007546:Isoquinolines; D009294:Narcotics; D016194:Receptors, N-Methyl-D-Aspartate; D009020:Morphine; C088643:LY 235959