Membrane-initiated actions of estradiol (E2) in the regulation of LH secretion in ovariectomized (OVX) ewes

Abstract

BackgroundWe demonstrated that E2 conjugated to BSA (E2BSA) induces a rapid membrane-initiated inhibition of LH secretion followed hours later by a slight increase in LH secretion. Whether these actions of E2BSA are restricted to the pituitary gland and whether the membrane-initiated pathway of E2BSA contributes to the up-regulation of the number of GnRH receptors during the positive feedback effect of E2 were evaluated here. We have shown that the suppression of LH secretion induced by E2 and E2BSA is the result of a decreased responsiveness of the pituitary gland to GnRH. In this study we further tested the ability of E2BSA to decrease the responsiveness of the pituitary gland to GnRH under the paradigm of the preovulatory surge of LH induced by E2.MethodsFor the first experiment GnRH and LH secretions were determined in samples of pituitary portal and jugular blood, respectively, in ewes treated with 12 mg E2BSA. In the second experiment, the number of GnRH receptors was quantified in ewes 12 h after administration of 25 micrograms E2 (the expected time for the increase in the number of GnRH receptors and the positive feedback effect of E2 in LH secretion) or 12 mg E2BSA. In the third experiment, the preovulatory-like surge of LH was characterized in ewes injected with 25 micrograms E2 alone or followed 8 h later (before the beginning of the LH surge) with 60 mg E2BSA.Resultsa) the decrease in LH secretion induced by E2BSA was not accompanied by changes in the pulsatile pattern of GnRH, b) E2BSA increased the number of GnRH receptors, and c) the presence of E2BSA in E2-treated ewes delayed the onset, reduced the length, and decreased the amount of LH released during the preovulatory surge of LH.Conclusionsa) the rapid suppression of LH secretion induced by E2BSA is mediated only via a direct action on the pituitary gland, b) E2 acting via a membrane-initiated pathway contributes to increase the number of GnRH receptors and, c) administration of E2BSA near the beginning of the pre-ovulatory surge of LH delays and reduces the magnitude of the surge.

Testosterone rapidly increases ejaculate volume and sperm density in competitively breeding goldfish through an estrogenic membrane receptor mechanism

“…We placed each male into a separate 5 gallon holding tank and measured milt volume again at 1 hr post-injection. E2:BSA, which cannot pass through the cell membrane, has been used in previous studies to assess the rapid effects of estradiol on cell surface receptors in preparations of fish testes (Loomis and Thomas, 2000) and neural tissue in several mammalian species (e.g., Abe and Terasawa, 2005;Arreguin-Arevalo et al, 2010;Temple et al, 2004). We conducted this experiment in June, when fish were in breeding condition.…”

Does a Nonclassical Signaling Mechanism Underlie an Increase of Estradiol-Mediated Gonadotropin-Releasing Hormone Receptor Binding in Ovine Pituitary Cells?1

“…Following the transient suppression of LH secretion, a preovulatory-like surge of LH was evident at approximately 12 h post-E2 infusion. In contrast, the E2-PEP and E2-BSA conjugates did not induce a preovulatory-like surge of LH; however, the mean LH measured during the surge window was significantly greater than that measured during the preinfusion period, suggesting that sensitivity of the pituitary gland may have increased at that time even through there was no increase in GnRH secretion [46].…”

“…The linkage of the large BSA molecule to the C-6 position of E2 has been shown to have differential effects in cells relative to free E2 [76,77]; however, in the present study, it was shown that E2-BSA increased GnRHR numbers in a manner similar to free E2. Other concerns have been raised that the bulkiness of BSA could alter the ability of binding of E2-BSA to the ER and alter the biological outcome [46,76]. That a greater concentration of E2-BSA was required to increase the numbers of GnRHR is not surprising because binding of E2-BSA to the soluble E2 receptor is only about 7% that of E2 (Arreguin-Arevalo and Nett, unpublished results).…”

“…Our laboratory has conducted studies to refute these claims. Administration of E2-BSA to ewes causes a rapid nongenomic effect as observed by a decrease in secretion of LH; however, the preovulatory surge release of LH that is considered to be a genomic effect is not observed [16,46]. The results from these studies provide evidence that the diethyl ether extraction method removes free E2 from the conjugated form and that E2-BSA is stable in vivo for a number of hours and does not appear to be degraded by proteolysis because genomic effects were not observed.…”

“…The absence of a full LH surge in animals treated with the BSAconjugated E2 [16] likely reflects the loss of the second component as a result of either an inability of the E2 conjugates to cross the blood-brain barrier or the lack of E2 membrane signaling to GnRH neurons [46]. In contrast, if E2 activation of membrane-signaling events are involved in the synthesis of GnRHR, it would explain the discrete increase in LH secretion observed during the expected preovulatory-like surge of LH in sheep treated with membrane-impermeable E2-BSA [16].…”