Two-week ovariectomized (OVX) rats were injected over three days with 25 μg oestradiol benzoate (EB), 3 mg tamoxifen (TX) and 0.2 ml oil and their pituitaries were harvested for incubation experiments. Pituitaries from EB-and TX-treated OVX rats exhibited GnRH self-priming when incubated with their corresponding ligand. However, incubation of pituitaries with different ligands yielded divergent results: when pituitaries from EB-treated rats were incubated with 10⁻⁷ M TX they displayed GnRH self-priming, whereas incubation of pituitaries from TX-treated rats with 10⁻⁸ M oestradiol-17β (E₂) blocked GnRH self-priming. Further studies to analyse the latter finding revealed that: (a) E₂ inhibited TX-induced GnRH self-priming in a dose-dependent manner while 10⁻⁸ M oestradiol-17α did not; (b) co-incubation of E₂ with the pure anti-oestrogen ICI 182,780, but not with the selective oestrogen receptor modulator TX, reversed the E₂ inhibitory effect; (c) the oestrogen receptor (ER)-α selective agonist propylpyrazole triol, but not the ERβ selective agonist diarylpropionitrile, mimicked the inhibitory effect of E₂; (d) the analogue membrane-impermeable conjugated E₂-BSA also inhibited TX-induced GnRH self-priming; and (e) a 15-min exposure of the pituitaries to E₂ was sufficient to inhibit the GnRH self-priming elicited by TX. Although other explanations may exist, altogether these results suggested that E₂, via an ER different from classical ER, inhibits the GnRH self-priming elicited by TX.

The specific role of each oestrogen receptor (ER) isoform (α and β ) and site (nucleus and plasma membrane) in LH release was determined in ovariectomized (OVX) rats injected over 6 days (days 15–20 after OVX) with a saturating dose (3 mg/day) of tamoxifen (TX), a selective ER modulator with nuclear ERα agonist actions in the absence of oestrogen. This pharmacological effect of TX was demonstrated by the fact that it was blocked by the selective ERα antagonist methyl-piperidinopyrazole. Over the past 3 days of the 6-day TX treatment, rats received either 25 μg/day oestradiol benzoate (EB), 1.5 mg/day selective ERα agonist propylpyrazole triol (PPT) and the selective ERβ agonist diarylpropionitrile (DPN), or a single 3 mg injection of the antiprogestin onapristone (ZK299) administered on day 20. Blood samples were taken to determine basal and progesterone receptor (PR)-dependent LH-releasing hormone (LHRH)-stimulated LH secretion and to evaluate LHRH self-priming, the property of LHRH that increases gonadotrope responsiveness to itself. Blood LH concentration was determined by RIA and gonadotrope PR expression by immunohistochemistry. Results showed that i) EB and DPN potentiated the negative feedback of TX on basal LH release; ii) DPN reduced TX-induced PR expression; iii) EB and PPT blocked TX-elicited LHRH self-priming and iv) ZK299 reduced LHRH-stimulated LH secretion and blocked LHRH self-priming. These observations suggest that oestrogen action on LH secretion in the rat is exerted at the classic ERα pool and that this action might be modulated by both ERβ and membrane ERα through their effects on PR expression and action respectively.

Hyperstimulation of ovarian function with human FSH (hFSH) attenuates the preovulatory surge of LH. These experiments aimed at investigating the mechanism of ovarian-mediated FSH suppression of the progesterone (P₄) receptor (PR)-dependent LH surge in the rat. Four-day cycling rats were injected with hFSH, oestradiol benzoate (EB) or vehicle during the dioestrous phase. On pro-oestrus, their pituitaries were studied for PR mRNA and protein expression. Additionally, pro-oestrous pituitaries were incubated in the presence of oestradiol-17β (E₂), and primed with P₄ and LH-releasing hormone (LHRH), with or without the antiprogestin RU486. After 1 h of incubation, pituitaries were either challenged or not challenged with LHRH. Measured basal and LHRH-stimulated LH secretions and LHRH self-priming were compared with those exhibited by incubated pituitaries on day 4 from ovariectomized (OVX) rats in metoestrus (day 2) injected with hFSH and/or EB on days 2 and 3. The results showed that: i) hFSH lowered the spontaneous LH surge without affecting basal LH and E₂ levels, gonadotroph PR-A/PR-B mRNA ratio or immunohistochemical protein expression; ii) incubated pro-oestrous pituitaries from hFSH-treated rats did not respond to P₄ or LHRH, and lacked E₂-augmenting and LHRH self-priming effects and iii) OVX reversed the inhibitory effects of hFSH on LH secretion. It is concluded that under the influence of hFSH, the ovaries produce a non-steroidal factor which suppresses all PR-dependent events of the LH surge elicited by E₂. The action of such a factor seemed to be due to a blockade of gonadotroph PR action rather than to an inhibition of PR expression.

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.