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.

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.