title: Clinically available NMDA receptor antagonists memantine and dextromethorphan reverse existing tolerance to the antinociceptive effects of morphine in mice.
abstract: The tail-flick test was used to investigate the effects of chronic administration of the N-methyl-D-aspartate (NMDA) receptor antagonists, dextromethorphan, memantine and MRZ 2/579, on the development and reversal of morphine tolerance in mice in three separate experiments. Experiment 1 investigated the effects of NMDA receptor antagonists on the development of tolerance. Morphine (10 mg/kg for 6 days, twice daily) produced a 5.9-fold rightward shift of the cumulative dose-response curves. Co-administration of dextromethorphan, memantine or MRZ 2/579 between tests 1 and 2 dose-dependently (5-10 mg/kg) inhibited the development of morphine tolerance. In experiment 2, in which the effects on the reversal were investigated, morphine-tolerant mice were treated b.i.d. for an additional 6 days (between tests 2 and 3) with vehicle+vehicle, NMDA receptor antagonist+vehicle, vehicle+morphine or NMDA receptor antagonist+morphine. Morphine-tolerant mice treated with vehicle+vehicle remained morphine tolerant, whereas this residual morphine tolerance was inhibited by administration of all three NMDA antagonists (each 10 mg/kg). Morphine-tolerant mice receiving vehicle+morphine injections demonstrated an unchanged degree of antinociceptive tolerance. In these mice, the co-administration of memantine and MRZ 2/579, but not dextromethorphan, resulted in the reversal of morphine tolerance. In experiment 3, memantine and MRZ 2/579 (10 mg/kg) inhibited the acute antinociceptive effect of morphine, but dextromethorphan did not. These data indicate that low-affinity, clinically available and/or therapeutically promising NMDA receptor antagonists may be used to inhibit ongoing morphine tolerance.
authors.affiliation: Institute of Pharmacology, Polish Academy of Sciences, Kraków. nfpopik@cyf-kr.edu.pl
authors.forename: P
authors.initials: P
authors.lastname: Popik
authors.forename: E
authors.initials: E
authors.lastname: Kozela
authors.forename: W
authors.initials: W
authors.lastname: Danysz
mesh_terms: D000701:Analgesics, Opioid / Q000494:pharmacology*; D000818:Animals; D003517:Cyclopentanes / Q000494:pharmacology; D003915:Dextromethorphan / Q000494:pharmacology*; D004361:Drug Tolerance; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology*; D008297:Male; D008559:Memantine / Q000494:pharmacology*; D051379:Mice; D009020:Morphine / Q000494:pharmacology*; D016194:Receptors, N-Methyl-D-Aspartate / Q000037:antagonists & inhibitors*; D013623:Tail
chemical_list: D000701:Analgesics, Opioid; D003517:Cyclopentanes; D018691:Excitatory Amino Acid Antagonists; D016194:Receptors, N-Methyl-D-Aspartate; D003915:Dextromethorphan; D009020:Morphine; C114144:neramexane; D008559:Memantine