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. 2015 Jan 28;15:12. doi: 10.1186/1471-2253-15-12

Figure 2.

Figure 2

Effect of melatonin on the morphine tolerance. The development of the tolerance to morphine’s analgesic effect was assessed by the hindpaw thermal withdrawal latency at 60 min after co-administration (10 mg/kg melatonin and/or 10 mg/kg morphine) from day 1 to 14 (A), as well as MPAE% of morphine at 60 min after administration of morphine (20 mg/kg) alone on day 15 (B). A: Injection of morphine (10 mg/kg) significantly increased the thermal withdrawal latency in rats receiving administration of morphine on day 1. However, such an analgesic effect gradually decreased and then disappeared (tolerance) from day 3 to 14 after repeated treatment of morphine. Co-administration of 10 mg/kg melatonin reversed the analgesic effect of morphine. B: The MPAE% of morphine (20 mg/kg) significantly decreased in rats receiving repeated administration of morphine on day 15. However, the decreased MPAE% owing to morphine tolerance was reversed by consecutive 14 days co-administration of morphine with melatonin. MPAE% = [(test latency - basal latency)/(20–basal latency) × 100% (20 s as the cut-off time). *P < 0.05 and **P < 0.01, as compared with the SAL-SAL group at the same time point; # P <0.05 and ## P < 0.01, as compared with the MOR-SAL group at the same time point. Data are presented as mean ± SD for 6 rats per group.