Estrogen lowers immunoreactive beta-endorphin in the neurointermediate lobe of the rat pituitary

The effects of ovariectomy and sex steroids on tissue levels of immunoreactive beta-endorphin (ir-beta EP) were examined. Adult female Sprague-Dawley rats were sham operated, ovariectomized (OVX), adrenalectomized (ADRX), or ADRX-OVX. Animals were given estradiol (E2) or vehicle (oil) by six daily im injections, and tissue levels of immunoreactive beta-endorphin (ir-beta EP) determined. In OVX animals, neurointermediate lobe (NIL) content of ir-beta EP was double that in controls; plasma ir-beta EP was modestly but significantly raised, but levels in the anterior pituitary (AP) remained unaltered. The effects of ovariectomy on NIL and plasma ir-beta EP were reversed by E2 in a dose-related manner. ADRX-OVX animals showed elevations of AP and NIL ir-beta EP to levels double those in SHAM, and plasma levels 5 times higher. E2 administration to ADRX-OVX rats normalized ir-beta EP in NIL, but not that in AP nor plasma. These findings suggest that the ovary may exert a specific tonic influence on NIL ir-beta EP in the rat, and that this inhibition appears to be mediated, at least in part, through ovarian estrogen.