Localization of estrogen receptor alpha (ER alpha) in pyramidal neurons of the developing rat hippocampus

During development, estrogen has a variety of effects on morphological, biochemical and electrophysiological properties of hippocampal neurons. Correspondingly, estrogen receptor (ER) binding and mRNA increase transiently in the developing hippocampus. In this study, we used immunocytochemistry to determine the localization of the ER alpha subtype in the developing rat hippocampus. Nuclear staining was present in pyramidal cells and some interneurons of the CA1 and CA3 regions of the developing rat hippocampus. Little or no immunoreactivity was observed in postnatal day (P)0 animals (day of birth=P0), however, beginning on P4, ER alpha-immunoreactivity (ER alpha-ir) was visible and reached maximal levels by P10. These levels subsequently declined to low levels so that by P15, levels approximated those of adult females. Western blot analysis confirmed that this antibody recognized a 67 kDa protein, characteristic of the full-length ER alpha protein, in the hippocampus and pituitary. Furthermore, most of the ER alpha-immunopositive cells in the hippocampus were located in the pyramidal cell layer, and did not co-localize appreciably with gamma-aminobutyric acid (GABA) at any age examined. We conclude, based on the immunocytochemical localization of ER alpha, that the effects of estrogen on biochemistry and morphology of the developing hippocampus may be direct through the ER alpha subtype in hippocampal pyramidal cells.