Effects of low and high doses of morphine on thermal pain withdrawal latencies in differently aged WT and D3KO. Displayed data represent changes in withdrawal latency compared to respective control vehicle injections, with dashed lines representing relative vehicle control levels. A. Effects of low morphine (2 mg/kg) in WT. Treatment with low morphine significantly increased thermal withdrawal latencies at all ages, and there was no significant difference between age groups. B. Effects of low morphine (2 mg/kg) in D3KO. Treatment with low morphine had no significant effect at all age, and there was no significant difference between the groups. C. Effects of high morphine (5 mg/kg) in WT. Treatment with high morphine significantly increased thermal withdrawal latencies at all ages, and this effect was significantly increased at 1 year of age over the effects at 2 months and 2 years of age. D. Effects of high morphine (5 mg/kg) in D3KO. Similar to WT, treatment with high morphine significantly increased thermal withdrawal latencies at all ages, but this effect was significantly increased at 2 months of age over the effects at 1 year and 2 years of age. Abbreviations and colors as in Figure 1. *: denotes significant difference between groups; #: denotes significant difference from vehicle control (dashed line).