Fig. 4.

Morphine (MRP)-induced locomotor activity in wild-type (WT) and dopamine D₃ receptor knock-out (D₃KO) mice.A, Time course changes in the MRP-induced locomotor-enhancing effect in WT and D₃KO mice. Eachpoint represents the mean activity counts for 10 min with SEM of 7–8 mice. B, Total locomotor activity of the MRP-induced locomotor-enhancing effect in WT and D₃KO mice. Each column represents the mean activity for 180 min with SEM of 7–8 mice. **p < 0.01, ***p < 0.001 versus each saline group.^### p < 0.001 versus MRP-treated WT group.