Figure 3.

Locomotor sensitization to morphine. Locomotor activity was initially assessed in WT and βarr2-KO mice during a 30 min habituation period and for 90 min after an acute dose of morphine (10 mg/kg, s.c.). After this observation, mice received one daily dose of morphine (10 mg/kg, s.c.) for 6 additional days in their home cage. After 1 d of rest, the mice were then challenged again with the same dose of morphine (10 mg/kg, s.c.), and locomotor activity was assessed (WT-S, βarr2-KO-S). Locomotor activity was increased significantly after this regimen in both WT and βarr2-KO mice, indicating that sensitization to morphine developed in both genotypes (for sensitization, F_(1,36) = 5.68, p < 0.05; for genotype, F_(1,36) = 3.78, p = 0.06; n = 10 of each genotype.)