Modulation of behavioral response to morphine by RU486 pretreatment. (a) Effects of acute morphine administration (20 mg/kg, subcutaneously) on locomotor activity were analyzed in glucocorticoid antagonist-treated C57BL/6J mice. Mice were injected with 20 mg/kg RU486 (RU20). After 30 min, the mice were injected with morphine (Mor) or saline (Sal). Column bars correspond to the mean number of photocell counts per group with standard error (n = 6 to 8) collected from 0 to 180 min after administration of morphine. (b) Influence of glucocorticoid receptor antagonist pretreatment on the development of opioid physical dependence was analyzed in C57BL/6J mice. RU486 (20 mg/kg) was administered 30 min before acute injection of morphine (100 mg/kg, subcutaneously). The next day animals were injected with an additional dose of morphine (20 mg/kg, subcutaneously) following by the injection of naloxone (15 mg/kg, subcutaneously). The level of morphine dependence is presented as mean naloxone-precipitated withdrawal jumping response (number of jumps per 30 min after naloxone injection) with standard error (n = 5 to 6). Control animals were treated with vehicle (Veh) and saline (Sal). The data presented in panels a and b were analyzed using analysis of variance following Tukey's post hoc test (*P < 0.05; ns, not significant).