Fig. 2.

Effect of the N-methyl-d-aspartate receptor antagonist, MK-801, and the melanocortin-1 receptor antagonist, MSG606, on morphine hyperalgesia in male and female mice. Naltrexone-pelleted B6 and CD-1 mice of both sexes were assayed for nociception on the tail-withdrawal test on Day 0 and began receiving continuous morphine infusion at cumulative daily doses of 40.0 mg/kg. Withdrawal latencies were again obtained on infusion Day 5 and, consistent with the earlier data, confirmed that the mice were hyperalgesic. Mice were then immediately injected with MK-801 (A), MSG606 (B, C), or their corresponding vehicle and retested for the next 60 min. Symbols are mean ± SEM withdrawal latencies obtained immediately before (time 0) and after injection using 8–10 mice in every group. *Indicates significant increase in latency relative to own baseline (time 0) value (Bonferroni corrected P < 0.05).