title: Memantine and dizocilpine interactions with antinociceptive or discriminative stimulus effects of morphine in rats after acute or chronic treatment with morphine.
abstract: BACKGROUND
  Memantine is a N-methyl-D-aspartic acid receptor (NMDAR) channel blocker that binds to dizocilpine sites and appears well tolerated during chronic use. Published studies suggest NMDAR antagonists prevent development of tolerance to effects of morphine by blocking NMDAR hyperactivation.
  
  
  OBJECTIVE
  We sought to compare effects of memantine to those of the more frequently studied dizocilpine and to evaluate memantine as a potential adjunct to modify tolerance to mu-opioid receptor agonists.
  
  
  METHODS
  Sprague-Dawley rats were trained to discriminate morphine (3.2 mg/kg) and saline under fixed ratio 15 schedules of food delivery. Potency and maximal stimulus or rate-altering effects of cumulative doses of morphine were examined 30 min after pretreatment with dizocilpine (0.032-0.1 mg/kg) or memantine (5-10 mg/kg) and after chronic treatment with combinations of dizocilpine or memantine and morphine, 10 mg/kg twice daily, for 6 to 14 days. Effects of dizocilpine or memantine on morphine antinociception were examined in a 55 °C water tail-withdrawal assay with drug treatments parallel to those in discrimination studies.
  
  
  RESULTS
  Acutely, memantine attenuated while dizocilpine potentiated the stimulus and antinociceptive effects of morphine. Neither chronic dizocilpine nor memantine blocked tolerance to the stimulus effects of morphine. In contrast, combined treatment with dizocilpine (0.1 mg/kg) blocked tolerance to antinociceptive effects of lower (0.1~3.2 mg/kg) but not higher doses of morphine, whereas memantine did not block tolerance.
  
  
  CONCLUSIONS
  Memantine and dizocilpine interacted differently with morphine, possibly due to different NMDAR binding profiles. The lack of memantine-induced changes in morphine tolerance suggests that memantine may not be a useful adjunct in chronic pain management.
authors.affiliation: Department of Pharmacology and Neuroscience, Texas Tech University Health Sciences Center, Lubbock, TX 79430, USA.
authors.forename: Yukun
authors.initials: Y
authors.lastname: Chen
authors.forename: Marianne
authors.initials: M
authors.lastname: Evola
authors.forename: Alice M
authors.initials: AM
authors.lastname: Young
mesh_terms: D000701:Analgesics, Opioid / Q000008:administration & dosage / Q000494:pharmacology; D000818:Animals; D004193:Discrimination Learning / Q000187:drug effects; D004195:Disease Models, Animal; D016291:Dizocilpine Maleate / Q000008:administration & dosage / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D004334:Drug Administration Schedule; D004347:Drug Interactions; D004361:Drug Tolerance; D018691:Excitatory Amino Acid Antagonists / Q000008:administration & dosage / Q000494:pharmacology*; D008297:Male; D008559:Memantine / Q000008:administration & dosage / Q000494:pharmacology*; D009020:Morphine / Q000008:administration & dosage / Q000494:pharmacology*; D010146:Pain / Q000188:drug therapy; D051381:Rats; D017207:Rats, Sprague-Dawley; D016194:Receptors, N-Methyl-D-Aspartate / Q000037:antagonists & inhibitors; D017450:Receptors, Opioid, mu / Q000819:agonists; D012055:Reinforcement Schedule
chemical_list: D000701:Analgesics, Opioid; D018691:Excitatory Amino Acid Antagonists; D016194:Receptors, N-Methyl-D-Aspartate; D017450:Receptors, Opioid, mu; D016291:Dizocilpine Maleate; D009020:Morphine; D008559:Memantine