Adrenal influence on pituitary secretion of thyrotropin and prolactin in rats

Basel plasma levels and pituitary content of TSH and PRL were measured in young male rats after bilateral adrenalectomy with and without corticosterone replacement. Adrenal ablation for a period of 4 days resulted in the arrest of body weight gain, a remarkable hemoconcentration, the slight and insignificant increase in plasma TSH and PRL, but no significant changes in the pituitary content of TSH and PRL and the hypothalamic content of catecholamines, as compared with the control animals. Adequate glucocorticoid replacement in adrenalectomized rats, as determined by the plasma level of corticosterone, did not result in a comparable weight gain, as in the control group, but did reduce plasma protein to almost normal levels. The plasma levels of TSH and PRL in the adrenalectomized animals were significantly suppressed, although the pituitary contents of TSH and PRL were not different from the control values. Four days after adrenalectomy, pituitary stimulation tests with TRH, chlorpromazine, and metoclopramide produced much higher levels of plasma TSH and PRL in the adrenalectomized rats than in the control group. Replacement of corticosterone in the adrenalectomized rats diminished the excess of TSH and PRL responsiveness. These results suggest that adrenal glands may not be directly involved in the hypothalamic control of the pituitary content of TSH and PRL, as they are in the case of ACTH. However, physiological levels of glucocorticoids clearly exert an inhibitory effect on TSH and PRL release by the pituitary gland in response to provocative stimulation.