Figure 1

Effect of morphine pretreatment on the thermal pain thresholds in rats. Noxious radiant heat was used as painful stimulation, which was randomly applied to the plantar surface of the rats' hindpaws. Baseline paw withdrawal latency was measured. Morphine (5 mg/kg) or equivalent volume of NS (n = 8) was injected intraperitoneally and pain-related activities were recorded 10 min later. For NAL + MOR treatment, rats were pre-treated with 4 mg/kg naloxone 10 min prior to administration of morphine (n = 4). The ANOVA revealed that pre-treatment with morphine produced significantly longer withdrawal latency than that of any other session (P < 0.001). The effect of morphine was antagonized by naloxone pretreatment prior to morphine administration. NAL: naloxone; MOR: morphine; NS: normal saline.