Locomotion in response to an injection of morphine. For the test of locomotion in response to an injection of morphine, the methods of Kest et al. (2002a,b); and Schulteis et al. (1997) were used. Mice received a single injection (i.p.) of morphine sulfate (50 mg/kg in isotonic saline at a volume of 10 ml/kg) and were immediately placed into the activity chambers. Horizontal distance traveled and rearing were recorded in 15 min intervals throughout the 3 h test and also expressed as totals over the 3 h.
