Interaction of alpha-melanocyte-stimulating hormone with beta-endorphin to influence anterior pituitary hormone secretion in the female rat.

The interaction of alpha MSH and beta-endorphin on the secretion of PRL, GH, and LH was determined in the ovariectomized rat. The potent stimulatory effect on PRL release of injection of 20 ng (5.8 pmol) beta-endorphin into the third cerebral ventricle was completely blocked by 100 ng (60 pmol) alpha MSH. The same dose of alpha MSH partially blocked the effect of 150 ng (44 pmol) beta-endorphin on PRL secretion. Intraventricular injection of either 20 ng beta-endorphin or 100 ng alpha MSH had no effect by itself on plasma LH. However, coinjection of these doses of beta-endorphin and alpha MSH suppressed plasma LH levels significantly within 15 min. beta-Endorphin (150 ng) produced a significant suppression of plasma LH levels. Furthermore, coadministration of 100 ng alpha MSH with 150 ng beta-endorphin lowered plasma LH for a longer period of time than this dose of beta-endorphin alone. Low doses of beta-endorphin (20 ng) or alpha MSH (100 ng) alone or in combination did not alter plasma GH levels. A higher dose of 150 ng beta-endorphin produced a slight elevation of plasma GH. This effect was potentiated 5-fold when 150 ng beta-endorphin were injected in combination with 100 ng alpha MSH. These results indicate that alpha MSH acts as an antagonist to beta-endorphin in regard to the secretion of PRL, whereas it potentiates the effect of beta-endorphin in stimulation of GH and inhibition of LH secretion.