Dexamethasone stimulation of running activity in the male rat

The effect of the synthetic glucocorticoid, dexamethasone-21-phosphate, added to the drinking water (0.1-20 μg/ml), on running wheel activity was studied in male rats. The rats were maintained in standard activity cages in a controlled environment with 12 hr light, 12 hr dark. After 2 weeks of equilibration in the activity cages, dexamethasone was started. One to three days later running activity increased. Peak activity (up to 10 times basal levels) was reached 5-7 days after the onset of dexamethasone. The increase in activity occurred entirely in the nocturnal phase. After stopping dexamethasone, activity continued at levels above the pre-treatment base line for 1-2 weeks if the dose was small (one μg/ml) and duration of dexamethasone administration was short. Selective depression of pituitary-adrenocortical function was produced in other rats by implantation of a cortisol acetate pellet in the median eminence of the hypothalamus. Activity was markedly reduced following implantation and was restored promptly by dexamethasone in the drinking water. It was concluded that dexamethasone markedly stimulates running activity in the rat. Since the increased activity occurs at night, it appears that glucocorticoids affect the magnitude of running activity but not its rhythmicity.