Mechanism(s) by which adrenalectomy and corticosterone influence prolactin release in the rat.

Abstract

The possible direct effect of corticosterone on release of pituitary prolactin was examined in a system using incubation for 8 h. Corticosterone at either 0·1 or 1 μg/ml medium had no significant effect on in-vitro prolactin release but 10 or 100 μg/ml medium produced a significant inhibition of release of prolactin. Release of LH, FSH and thyroid-stimulating hormone were not altered by 0·1, 1 or 10 μg corticosterone/ml, indicating that its action at the concentration of 10 μg/ml was specific on release of prolactin. Corticosterone injected at doses of 1 or 5 mg/kg into hypophysectomized rats with two pituitary grafts underneath the kidney capsule produced a significant fall in serum levels of prolactin when compared with control hypophysectomized rats with two pituitary grafts. Examination with the electron microscope showed that about one third of the lactotrophes from adrenalectomized rats after corticosterone injection exhibited patterns which suggested a decrease in protein synthesis when compared with lactotrophes from adrenalectomized rats given only the vehicle injection. These observations indicated that inhibition of release of prolactin by corticosterone could be exerted directly on the pituitary gland, and that the rise of serum levels of prolactin after adrenalectomy might have been due to the removal of direct inhibition by corticosterone. Male rats were adrenalectomized and 2–3 weeks later, concentrations of dopamine and noradrenaline in the medial basal hypothalamus were measured and found not to be different from values in intact rats. Dopamine metabolism also was not altered in the median eminence. The dopaminergic agonist, l-DOPA, inhibited, and the antagonists, pimozide and haloperidol, stimulated release of prolactin in both adrenalectomized and intact rats. Serotonin (5-HT) metabolism in the medial basal hypothalamus and anterior hypothalamus of adrenalectomized rats was not significantly different from values in intact rats, but a higher concentration of 5-HT was observed in the medial basal hypothalamus of adrenalectomized rats when compared with the values in intact rats. A serotonergic agonist, fluoxetine, and an antagonist, cyproheptadine, had no apparent effect on release of prolactin in intact rats, but fluoxetine produced a significant rise, and cyproheptadine, a significant lowering of serum levels of prolactin in adrenalectomized rats. These results suggest that 5-HT, but not dopamine, may be involved in the rise of prolactin after adrenalectomy.