Thermal (54°C hot plate) antinociceptive response s in WT and βarr2-KO mice in response to chronic infusion over a 5 day period with (A) morphine (48 mg/kg/day, s.c.; two-way ANOVA with Bonferroni post-hoc analysis for genotype effect compared at at each day: P<0.0001, n=9 WT, 8 KO); (B) methadone (96 mg/kg/day, s.c; n=7 WT, 6 KO); (C) fentanyl (3.2 mg/kg/day, s.c., n=5 WT, 5 KO); or (D) oxycodone (25 mg/kg/day, s.c., n=6 WT, 6 KO). Responses were measured on day 1, 3, and 5 following pump implantation. Data are presented as the mean ± S.E.M. For B-D, two-way ANOVA revealed no differences for genotype effect (P>0.05) and for A-D, one-way ANOVA for day effect reveals differences for WT: *P<0.05, **P<0.01, ***P<0.001; for KO: #P<0.05, ##p<0.01, ###P<0.001, Bonferroni post-hoc analysis.