Unlike the absence of β-arrestin2, which enhances reinforcement by morphine, dasatinib had no effect. (A) Morphine (3 and 10 mg/kg subcutaneously) caused a dose-dependent preference of wild-type (WT) mice (n = 8) for the paired environment (*P < 0.01, one-way ANOVA, post hoc Dunnett test). Morphine preference was lacking in µ^–/– mice (n = 7). (B) After conditioning, the duration of occupancy of the morphine-paired environment increased significantly compared with the saline-paired environment in WT mice at all three 5-min intervals (*P < 0.0001, two-way ANOVA, post hoc Bonferroni test; n = 8). (C) µ^–/– mice (n = 7) exhibited no morphine conditioned place preference at any stage during the 15-min test period. (D) µ^+/– mice (n = 8) also exhibited no morphine (10 mg/kg) conditioned place preference. (E) By contrast, µ^+/– mice (n = 8) exhibited conditioned place preference to a higher dose of morphine (30 mg/kg; *P < 0.05 for 300 to 600 s, P < 0.001 for 600 to 900 s, two-way ANOVA, post hoc Bonferroni test). (F) Morphine (10 and 30 mg/kg) caused a dose-dependent preference of µ^+/– mice (n = 8) for the paired environment (*P < 0.05, one-way ANOVA, post hoc Dunnett test). (G) Morphine preference occurred at a lower dose in β-arr2^–/– mice (*P < 0.01, one-way ANOVA, post hoc Dunnett test; n = 8). (H) By contrast, dasatinib had no effect on morphine preference (*P < 0.01, one-way ANOVA, post hoc Dunnett test; n = 8).