Glucocorticoid suppression of pituitary prolactin release in the nonhuman primate.

The administration of 8 mg dexamethasone daily for 8 days significantly lowered basal and TRH-stimulated PRL levels in ovariectomized rhesus monkeys. A single acute injection of dexamethasone attenuated the total amount of PRL released after TRH stimulation, although basal and peak PRL levels were not significantly different from those found in controls. In an attempt to determine the site of action of glucocorticoids on PRL secretion, baboon pituitaries were incubated with cortisol. Cortisol at a concentration of 3 x 10(-5) M significantly inhibited PRL secretion in vitro, while a concentration of 3 x 10(-4) M was needed to block TRH-stimulated prolactin release. In conclusion, glucocorticoids can suppress pituitary PRL secretion by a direct action on the pituitary gland, although a hypothalamic site of action may also be involved.