Locomotion

Locomotor testing was conducted as previously described [73]. Briefly, locomotor activity was performed in 40 × 40 × 35-cm chambers (Stoelting Co, Wood Dale, IL), and distance traveled was recorded using Anymaze video-tracking software. Under naïve conditions, mice were placed in the center of the chambers, and distance traveled was measured for 2 hours and analyzed in 10-minute bins. For pharmacological experiments, mice were handled and injected with vehicle for 3 days in order to minimize stress. For morphine experiments, mice were allowed to acclimatize to the chambers for 30 minutes before being administered morphine (s.c.) at a concentration of 1 mg/kg, 5 mg/kg, 10 mg/kg, or 20 mg/kg or saline and were recorded for 3 hours.