Effect of gonadectomy and gonadal steroid replacement on pituitary and plasma beta-endorphin levels in the rat

The effect of gonadectomy on β-endorphin-like immunoreactivity (βEP-LI) of the anterior (AP) and neurointermediate (NIL) lobes of the pituitary and plasma βEP concentrations has been evaluated in male and female rats 24 h, 2, 3, and 5 weeks post surgery, before and after institution of replacement therapy with gonadal steroids. Simultaneous determination of βEP-LI was also performed in the medial basal hypothalamus (MBH), median eminence (ME), and brain stem. In rats of both sexes, AP βEP-LI and plasma βEP concentrations were unaltered 24 h, 1, and 2 weeks after gonadectomy but were significantly reduced at 3 and 5 weeks. In the NIL, the reduction in βEP-LI occurred earlier, being already apparent 2 weeks after gonadectomy. In male rats, a 14-day replacement therapy with testosterone propionate (65 μg/rat sc, daily) instituted 3 weeks post surgery partially restored βEP titers in the AP and plasma but failed to do so in the NIL. In female rats, estradiol benzoate (10 μg/rat sc, daily) restored completely βEP titers in the AP but only partially in the NIL and plasma. In contrast to its effects on the pituitary and plasma, gonadectomy failed to alter βEP-LI in the MBH, ME, and brain stem and also ineffective was replacement therapy with testosterone propionate in male rats. Only treatment with estradiol benzoate increased βEP-LI in the MBH and ME of female rats. On the whole these data indicate that gonadal steroids play a major role on the secretion of pituitary but not central nervous system βEP stores, although their mechanism of action is presently unknown. © 1982 by The Endocrine Society.