Distribution and Biological Effects of Tritiated Estradiol Implanted in the Hypothalamo-Hypophysial Region of Female Rats

Abstract

Stereotaxic implants of approximately 1-2 [mu]g of H3-estradiol or estradiol-6,7-H3-acetate were placed unilaterally into the lateral portion of the median eminence (ME) or anterior pituitary (AP) in adult, normally cyclic Sprague-Dawley rats. The distribution of radioactivity within the brain and other tissues was determined by autoradiography and liquid scintillation counting. The radioactivity in the brain was still associated with estradiol as indicated by its movement in a thin-layer chromatographic system. Four to 5 days post implantation, measurable amounts of radioactivity could be counted as far as 2 mm from the implant site in the brain, and trace amounts of activity were detected in plasma and uterus. Significant amounts of radioactivity were counted in the anterior pituitary in rats with median eminence implants, with greater activity in the ipsilateral half of the gland. These animals also showed a marked elevation in plasma luteinizing hormone (LH) and a slight hypertrophy of the ipsilateral half of the adenohypophysis. Rats with pituitary implants, however, showed no elevation in plasma LH despite significant hypertrophy of the implanted lobe and the presence of radioactivity in the pituitary. By 18 days post implantation, significant unilateral hypertrophy of the adenohypophysis was found in both groups of animals, but no plasma LH was detectable at this time in either group. Implantation of blank tubing in the ME produced neither release of LH nor unilateral hypertrophy of the AP 5 days post implantation. The results indicate that estrogen exerts separate actions on the cells of the rat pituitary and median eminence. It stimulates the release of LH by a central action on the median eminence but induces pituitary hypertrophy by a direct action on the hypophysial cells to which it is carried by the hypothala-mohypophysial portal system.