Clinically available NMDA receptor antagonists memantine and dextromethorphan reverse existing tolerance to the antinociceptive effects of morphine in mice

The tail-flick test was used to investigate the effects of chronic administ ration of the N-methyl-D-aspartate (NMDA) receptor antagonists, dextrometho rphan, 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 s hift of the cumulative dose-response curves. Co-administration of dextromet horphan, memantine or MRZ 2/579 between tests 1 and 2 dose-dependently (5-1 0 mg/kg) inhibited the development of morphine tolerance. In experiment 2, in which the effects on the reversal were investigated, morphine-tolerant m ice were treated b.i.d. for an additional 6 days (between tests 2 and 3) wi th vehicle+vehicle, NMDA receptor antagonist+vehicle, vehicle+morphine or N MDA receptor antagonist+morphine. Morphine-tolerant mice treated with vehic le+vehicle remained morphine tolerant, whereas this residual morphine toler ance was inhibited by administration of all three NMDA antagonists (each 10 mg/kg). Morphine-tolerant mice receiving vehicle+morphine injections demon strated an unchanged degree of antinociceptive tolerance. In these mice, th e co-administration of memantine and MRZ 2/579, but not dextromethorphan, r esulted in the reversal of morphine tolerance. In experiment 3, memantine a nd MRZ 2/579 (10 mg/kg) inhibited the acute antinociceptive effect of morph ine, but dextromethorphan did not. These data indicate that low-affinity, c linically available and/or therapeutically promising NMDA receptor antagoni sts may be used to inhibit ongoing morphine tolerance.