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and -ß and progesterone receptor immunoreactivity after ovariectomy and exposure to oestradiol benzoate, tamoxifen or raloxifene in the rat: correlation with LH secretion
Departments of Cellular Biology, Physiology and Immunology, University of Córdoba, Córdoba, Spain
1 Department of Comparative Pathology, University of Córdoba, Córdoba, Spain
(Requests for offprints should be addressed to J E Sánchez-Criado; Email: fi1sacrj{at}uco.es)
* (J E Sánchez-Criado and J Martín de las Mulas contributed equally to this work)
The selective oestrogen receptor modulator (SERM) tamoxifen (TX) has agonist/antagonist actions on LH secretion in the rat. Whereas in the absence of oestrogens TX elicits progesterone receptor (PR)-dependent GnRH self-priming, it antagonizes oestrogen-stimulatory action on LH secretion. The aim of these experiments was to explore whether TX treatment-induced differential expression of oestrogen receptor (ER)
and ERß in the gonadotrope may determine its agonist effect on LH secretion. In the first experiment, basal LH secretion, GnRH-stimulated LH secretion and PR-dependent GnRH self-priming were determined in incubated pituitaries from ovariectomized (OVX) rats treated with oestradiol benzoate (EB), TX or raloxifene (RX). Cycling rats in metoestrus or pro-oestrus were used as basic controls. As in pro-oestrus, pituitaries from OVX rats treated with EB exhibited GnRH-stimulated LH secretion, immunohistochemical PR expression and GnRH self-priming. While RX had no effect on these parameters, TX induced PR expression and GnRH self-priming. GnRH self-priming was absent in pituitaries incubated with the antiprogestin ZK299. In the second experiment, we evaluated the immunohistochemical expression of ER and ERß in gonadotropes of cycling rats and OVX rats treated with EB, TX or RX. We found that while ER expression was similar in all six groups, ER expression was oestrous cycle dependent. Moreover, ER expression in gonadotropes of TX-treated rats was as high as that found in pro-oestrus, while ER expression in the gonadotropes of RX-treated rats was lower than in metoestrous or pro-oestrous pituitaries. These results suggest that, in the absence of the cognate ligand, TX, unlike RX, may regulate LH secretion through the ER subtype in gonadotropes.
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