Locomotor activity was measured with an image analysis system (Ethovision 3.1, Noldus Information Technology BV, Netherlands) as previously described11. Briefly, each animal was placed into a square open field box (40 cm × 40 cm × 45 cm) and monitored with an overhead video camera and video tracking software. After recording baseline activity for 30 min, the animal was given an intraperitoneal (i.p.) injection of cocaine (15 mg/kg) and monitored for up to 60 min after injection. The distance travelled during each 10-min period was analyzed. The data are expressed as a percentage of baseline activity.