ESTROGEN AUGMENTS HYPOTHALAMIC BETA-ENDORPHIN SECRETION AND ACTIVATESAN INHIBITORY BETA-ENDORPHIN SHORT-LOOP FEEDBACK-SYSTEM

The role of estrogen in the regulation of hypothalamic beta-endorphin hormone secretion is studied by determining beta-endorphin concentrati on in pituitary portal plasma of ovariectomized rats in the presence o r absence of this steroid and/or the opioid antagonist naloxone. Twent y-six hours following s.c. injection of 10 mu g estradiol benzoate (es trogen) or oil, rats anesthetized with Saffan (alphaxolone/alphadolone ) underwent pituitary stalk exposure and hypophysectomy, after which p ituitary portal blood was continuously collected and stored in 15 min aliquots from 1100-1400 h. At 1100 h, animals were given an initial bo lus iv injection of naloxone or saline (naloxone, 2 mg/kg, or saline, 0.1 ml) and then infused (iv) continuously with naloxone (2 mg/kg/h) o r saline (0.8 ml/h) until 1400 h. Plasma samples were extracted and as sayed by radioimmunoassay for beta-endorphin. Treatment with estrogen increased the mean beta-endorphin levels twofold as compared to oil-tr eated controls. Naloxone potentiated estrogen action of beta-endorphin secretion, but did not affect basal beta-endorphin secretion. These r esults suggest that estrogen enhanced beta-endorphin secretion from th e hypothalamus. Furthermore, the hypersecretion of beta-endorphin indu ced by naloxone with, but not without, estrogen supports the existence of an estrogen-activated short-loop negative feedback mechanism regul ating beta-endorphin secretion.