Estrogen augments hypothalamicβ-endorphin secretion and activates an inhibitoryβ-endorphin short-loop feedback system
The role of estrogen in the regulation of hypothalamicβ-endorphin hormone secretion is studied by determiningβ-endorphin concentration in pituitary portal plasma of ovariectomized rats in the presence or absence of this steroid and/or the opioid antagonist naloxone. Twenty-six hours following s.c. injection of 10 /µg estradiol benzoate (estrogen) or oil, rats anesthetized with Saffan (alphaxolone/alphadolone) underwent pituitary stalk exposure and hypophysectomy, after which pituitary portal blood was continuously collected and stored in 15 min aliquots from 1100–1400 h. At 1100 h, animals were given an initial bolus 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) or saline (0.8 ml/h) until 1400 h. Plasma samples were extracted and assayed by radioimmunoassay forβ-endorphin. Treatment with estrogen increased the meanβ-endorphin levels twofold as compared to oil-treated controls. Naloxone potentiated estrogen action ofβ-endorphin secretion, but did not affect basalβ-endorphin secretion. These results suggest that estrogen enhancedβ-endorphin secretion from the hypothalamus. Furthermore, the hypersecretion ofβ-endorphin induced by naloxone with, but not without, estrogen supports the existence of an estrogen-activated short-loop negative feedback mechanism regulatingβ-endorphin secretion.
KeywordsHypothalamicβ-endorphin secretion estrogen pituitary portal blood aphadolone/alphaxolone anesthesia ovariectomized rats short-loop feedback
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