Acute and subacute effects of naturally occurring estrogens on luteinizing hormone secretion in the ovariectomized rat: Part 2

Abstract

Acute intravenous administration of the phytoestrogen genistein (G) blocks the gonadotropin-releasing hormone-(GnRH)-induced rise of luteinizing hormone (LH) in ovariectomized rats. The present experiments were performed to determine whether subacute administration of G or the mycoestrogens zearalenone and zearalenol would affect GnRH-induced or progesterone-induced LH secretion in ovariectomized rats. Charles River CD rats were ovariectomized and used 2 to 5 weeks later. Blood samples were obtained either via decapitation or via intraatrial cannulae three days after compounds were injected subcutaneously in sesame oil or corn oil vehicle. LH was measured by RIA. Pretreatment with estradiol benzoate suppressed LH levels at 1200 h, while G had no effect. Challenge with progesterone (8 mg/kg BW, sc) evoked LH release at 1600 h in rats pretreated with estradiol benzoate, but LH levels did not change in rats pretreated with G, zearalenone, or zearalenol. While GnRH-induced LH secretion was preserved in rats pretreated with estradiol, no LH response was detected in rats pretreated with the higher dose of G (8 mg/kg BW) or either dose of zearalenol (0.8 mg/kg BW or 8 mg/kg BW). We conclude that in the ovariectomized rat 1) subacute administration of G, zearalenone, or zearalenol do not inhibit tonic LH secretion, 2) G, zearalenone, and zearalenol do not provide 'estrogenic priming' for progesterone-induced LH secretion; however, 3) G and zearalenol do block GnRH-induced LH secretion. The seemingly selective neuroendocrine effects of these naturally-occurring dietary estrogens emphasize that actions of each putative estrogen must be characterized for each 'estrogenic' endpoint