Stimulation in vivo of the secretion of prolactin and growth hormone by beta-endorphin.

Morphine sulfate (MS) and the opioid peptide beta-endorphin beta-LPH-(61-91) stimulate prolactin and growth hormone release in steroid-primed and non-treated male rats when injected intravenously or intracisternally. On a molar basis beta-endorphin is at least 20 times more potent than MS, whereas Met5-enkephalin (beta-LPH-(61-65)) and alpha-endorphin (beta-LPH-(61-76)) are devoid of activity at the dose injected (300 mug). The in vivo stimulatory effects of beta-endorphin on prolactin secretion are reversed by the opiate antagonist naloxone. The absence of in vitro effect of MS and beta-endorphin on prolactin and growth hormone secretion by cultured rat pituitary cells suggest that they have a central nervous system site of action.