RTP4 expression is regulated by chronic morphine administration, and this affects abundance of MOPr and MOPr-DOPr heteromers. (A and B) Male C57BL/6 mice (5–6/group) were administered saline or morphine (10 mg/kg, s.c.) once a day for 10 days. On the 11th day, brain regions were collected and RT-qPCR was performed. Data were normalized with GAPDH, an internal control. Data are the mean ± S.D. n = 5–6. **P < 0.01 vs. control group (saline), two-way ANOVA and Bonferroni test (A) or two-way multivariate ANOVA (B). (C and D) Male C57BL/6 mice (5–6/group) were administered saline or morphine (10 mg/kg, s.c.) once a day for 10 days. On the 11th day, the hypothalamus, cortex, NAcc, and VTA were collected. Membrane protein (10 µg) was subjected to ELISA using receptor-specific antibodies. Data are the mean ± S.D. n = 3–6. *P < 0.05; **P < 0.01 vs. control group (saline), unpaired t test with Welch’s correction (C) or two-way multivariate ANOVA (D). PFC = prefrontal cortex.