Dose-response effect of the MR antagonist spironolactone (a) and of the GR antagonist RU39305 (b) on the locomotor response to morphine (2 mg/kg). The i.c.v. administration of spironolactone (0, 100, 300, and 1000 ng/6 μl) had no effect on the locomotor response to morphine. In contrast, RU39305 (0, 50, 100, and 200 ng/6 μl) dose-dependently reduced the locomotor response to this drug. Each point represents the mean ± SEM of activity scores cumulated over 3 hr. Antagonist were administered 2 hr before the injection of morphine.