1. Following opioid-induced antinociception in mice, hyperalgesic responses may be observed. The present study was designed to evaluate the effect of different N-methyl-d-aspartate (NMDA) receptor modulators (magnesium, dextromethorphan, d-serine) on the development of morphine-induced hyperalgesia in mice. The tail-flick test was used to assess the effects of morphine alone and in combination with the NMDA receptor modulators. 2. Administration of a single low dose (2 mg/kg) of morphine to mice produced antinociception that was followed by hyperalgesia. 3. Administration of magnesium sulphate (5 mg/kg) and d-serine (10 mg/kg) alone produced a transient antinociceptive response, whereas dextromethorphan (10 mg/kg) alone produced a prolonged antinociceptive response that had a relatively delayed onset after 4 h. 4. When coadministered with morphine, the NMDA receptor blockers magnesium (2 mg/kg) and dextromethorphan (2 and 5 mg/kg) and the NMDA receptor agonist d-serine (2, 5 and 10 mg/kg), maintained the duration of the antinociceptive response to morphine and inhibited the development of the hyperalgesic response. Coadministration of dextromethorphan (10 mg/kg) with morphine produced antinociception at 30-120 min and at 4-24 h. 5. The results of the present study suggest that coadministration of low-dose NMDA receptor antagonists, as well as the NMDA receptor agonist d-serine, with morphine can inhibit morphine-induced hyperalgesia.

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