The modulatory action of estradiol (E₂) on the GnRH network can be exerted indirectly on presynaptic neurons or directly on estrogen receptors (ERs) located within GnRH hypothalamic neurons. Using the GnRH-producing GT1-7 cell line, we have investigated whether E₂ is able to modify the response of these cells to norepinephrine (NE) stimulation. A 48-h exposure of GT1-7 cells to 10 nM E₂ reduced NE-induced cAMP accumulation. However, 15-min exposure was enough to induce this inhibitory action, provided that a hormone-free period of 48 h after steroid treatment was allowed. Furthermore, this effect was mimicked by E₂ coupled to (E-BSA), indicating that it may be exerted through a membrane-mediated mechanism. In addition, competition experiments using E-BSA coupled to fluorescein isothiocyanate (FITC) revealed the presence of cell membrane-binding sites for E₂. Binding of E-BSA coupled to FITC was blocked by preincubation of cells with either E₂, antiestrogen ICI 182 780, or tamoxifen. Moreover, fluorescence staining of non-permeabilized cells with antibodies against receptors α and β confirmed the presence of both receptor subtypes at the cell membrane. To determine the nature of the ER involved in this response, specific agonists for ERα 4,4′,4′′-(4-propyl-[¹H]pyrazole-1,3,5-triyl)tris-phenol (PPT) and ERβ 2,3-bis(4-hydroxyphenyl)-propionitrile (DPN) were used. Since PPT, but not DPN, reproduced the effect of E₂, it is suggested that estrogen-induced modulatory action on NE responsiveness was mediated by the ERα isoform. Taken together, these results indicate that E₂ modulates the adrenergic sensitivity of GT1-7 cells by a mechanism compatible with the activation of membrane-associated ERs.

In the rat, administration of tamoxifen (TX) in the absence of oestrogen (E) induces LHRH self-priming, the progesterone receptor (PR)-dependent property of LHRH that increases gonadotrope responsiveness to itself. The oestrogen-dependent PR can be phosphorylated/activated by progesterone (P₄) and, in the absence of the cognate ligand, by intracellular LHRH signals, particularly cAMP/protein kinase A. We have recently found that oestradiol-17β (E₂), acting on a putative membrane estrogen receptor-α in the gonadotrope, inhibits this agonist action of TX. This study investigated the mechanism by which E₂ inhibits TX-elicited LHRH self-priming using both incubated pituitaries from TX-treated ovariectomized (OVX) rats and anterior pituitary cells from OVX rats cultured with TX. It was found that (1) in addition to the inhibitory effect on TX-elicited LHRH self-priming, E₂ blocked P₄ and adenylyl cyclase activator forskolin augmentation of LHRH-stimulated LH secretion, and (2) E₂ did not affect the increasing action of TX on gonadotrope PR expression or pituitary cAMP content. Furthermore, inhibition of protein phosphatases with okadaic acid suppressed E₂ inhibition of TX-elicited LHRH-induced LH secretion, while stimulation of protein phosphatases with ceramide blocked TX-induced LHRH self-priming. Together, these results indicated that membrane ER-mediated E₂ inhibition of the TX-stimulated LHRH self-priming pathway involves a blockade of gonadotrope PR phosphorylation/activation, but not a deficient response of PR to phosphorylases. Results also suggested that the inhibitory effect of E₂on TX-induced LHRH self-priming is exerted through modulation of cellular protein phosphatase activity in the gonadotrope.

This study aimed to investigate whether the administration of recombinant porcine somatotropin (rpST) late in gestation is associated with increased rates of obstetric and neonatal complications in primiparous sows. From days 80 to 114 of gestational age, 20 primiparous sows were randomly assigned to receive an intramuscular injection of either saline or 6 mg rpST/day. Throughout pregnancy, sows were fed 2.5 to 3 kg/day of a corn-soybean diet (14 MJ ME/kg). Of 111 piglets born to control sows and 109 piglets born to treated sows, 8.1% and 17.4% piglets respectively died intrapartum (P=0.04). Glucose blood levels in sows and live-born piglets in the rpST-treated group were significantly higher than in their corresponding controls. Birth weight of live-born piglets in the treated group was 1.4 ± 0.1 kg versus 1.3 ± 0.1 kg in the control group (P<0.0001). Birth weight of piglets born dead was also higher in the former than in the latter group (P<0.0001). No evidence of teratogenicity was observed in either of the groups. In conclusion, rpST administration in late pregnancy to primparous sows increased the rate of neonatal deaths and was associated with higher blood glucose levels in both sows and piglets.