Effects of Estrogen and Ectopic Pituitary Transplants on the Responsiveness of Pituitary Prolactin Release to Luteinizing Hormone-Releasing Hormone and Dopamine

The responsiveness of prolactin release to regulatory inputs depends on the functional state of the lactotrophs. In the present study, we have examined the effects of luteinizing hormone-releasing hormone (LHRH; 10^–7 or 10^–6 M) on the release of prolactin in vitro from hyperplastic pituitaries of estrogen-treated male Fischer rats, ectopic pituitary transplants and in situ pituitaries of grafted and control rats. The effects of dopamine (10^–8 or 10^–7 M) in this system were also examined. The extent of inhibition of prolactin release by dopamine was not related to the amounts of prolactin secreted under basal conditions or to plasma prolactin levels. LHRH significantly suppressed prolactin secretion in all groups but its effect was most pronounced in the ectopic pituitary transplants and in the hyperplastic pituitaries of animals after chronic exposure to estrogen followed by a period of recovery. Thus, the effects of LHRH on prolactin release appear to be related to the secretory activity and/or to the absolute or relative number of the lactotrophs.