Oestrogen receptor alpha and beta in female rat pituitary cells: an immunochemical study

Estradiol is a critical factor in the anterior pituitary secretory activity of mammalian females. Previous reports have demonstrated the presence of oestrogen receptor alpha (ERalpha) and beta (ERbeta) in specific anterior pituitary cells from ovariectomized rats, as well as in the whole anterior pituitary at particular stages of the rat oestrous cycle. However, the ERalpha and ERbeta distribution patterns in specific hormone producing cells of the anterior pituitary during the oestrous cycle remain to be clarified. The purpose of this study was to determine the cellular and subcellular distribution of both ER-subtypes during the rat oestrous cycle, using immunochemistry at light- and electron-microscope levels. ERalpha-immunoreactive (ir) cells mainly corresponded to PRL-ir cells and, to a lesser extent, to TSH-, FSH- and GH-ir cells. ERbeta-ir cells corresponded to a few GH-, PRL- and FSH-ir cells, whichever the phase of the cycle. ERalpha-ir was found either in the cytoplasm and/or the nucleus, depending on the phase of the oestrous cycle, while ERbeta-ir was always detected in the cytoplasm. Both ER-subtypes were immunoreactives inside the rough endoplasmic reticulum (RER), secretory vesicles (SV) and free in the cytosol. The highest number of ERalpha-ir cells was consistently found at pro-oestrus midday and the lowest at metaoestrous, while the number of ERbeta-ir cells was low in all stages of the cycle. These results indicate that the genomic actions of oestrogen in the anterior pituitary cells during the oestrous cycle are mediated by ERalpha. However, the localization of ERalpha and ERbeta in the RER and SV suggest a different translational and/or post-translational pathway, which could be involved in non-genomic mechanisms.