Abstract
Within- and between-subjects experimental designs were used to assess the effects of morphine in mice. In the within-subjects design, mice were tested for activity and responsivity to pain before and after an injection of morphine (15 mg/kg), and the effect of morphine was calculated as a difference between pre- and post-injection tests. In the between-subjects design, mice were tested for activity and responsivity to pain after an injection of either morphine or saline and the effect of morphine was calculated as a difference between morphine- and saline-injected animals. The between-subjects design was more sensitive than the within-subjects design in detecting morphine-induced locomotor and analgesic effects across four repeated injections of morphine.