Involvement of glucocorticoid receptor in transcriptional response to morphine. Inhibition of morphine-induced transcription of selected genes by an antagonist of the glucocorticosteroid receptor, namely RU486. Five experimental groups were compared (see Materials and methods): vehicle and acute subcutaneous morphine 20 mg/kg (Veh/Mor); morphine preceded by injection of 20 mg/kg RU486 (RU20/Mor); morphine preceded by injection of 40 mg/kg RU486 (RU40/Mor); and control groups injected with saline and RU486 (RU40/Sal) or saline and vehicle (Veh/Sal). Gene expression in striatum of C57BL/6J mice was analyzed 4 hours after morphine treatment. The results of quantitative real-time reverse transcription polymerase chain reaction (qPCR) are presented as fold change of control group (Veh/Sal) with standard error. Differences between groups were analyzed by analysis of variance ANOVA following Bonferroni multiple comparison correction (n = 5 to 6; **P < 0.01, ***P < 0.001 versus control group; #P < 0.05, ##P < 0.01 versus morphine).