Distribution analysis of the two chicken estrogen receptor-alpha isoforms and their transcripts in the hypothalamus and anterior pituitary gland

Estrogen plays a key role in the control of reproductive behavior and in th e regulation of the neuroendocrine system. To elucidate the mechanisms by w hich it controls these functions it is important to understand how estrogen ic effects are mediated. We have investigated the distribution of the two i soforms of the chicken estrogen receptor alpha (cER-alpha) protein; the pre viously characterized cER-alpha 66 and a new N-terminal truncated isoform, cER-alpha 61. Immunolocalization demonstrated the presence of cER-alpha 66 protein in hypothalamic areas, principally the nucleus septalis lateralis, bed nucleus striae terminalis medialis, nucleus preopticus medialis, and nu cleus infundibuli hypothalami, and in the anterior pituitary gland. When th e distribution of ER-alpha immunoreactive cells was compared using the anti bodies H 222 (directed against the hormone-binding domain) and ER 221 (dire cted against the 21-amino acid N-terminus), no apparent differences could b e detected. Because this immunocytochemical approach was not able to distin guish whether full-length cER-alpha 66 is the only isoform observed in the ER-positive regions or whether both cER-alpha receptor isoforms are present , SI nuclease assays were performed to compare the relative abundance in th ese regions of the two distinct classes of cER-alpha mRNA variants (A1-D an d A2), which encode the cER-alpha 66 and cER-alpha 61 protein isoforms, res pectively. in cockerels and hens, both variants of cER-alpha mRNA are expre ssed in the anterior pituitary gland and basal hypothalamus with a dominanc e of the mRNA that encodes cER-alpha 66, whereas the mRNA that encodes cER- alpha 61 was not detectable in the anterior hypothalamus. Therefore, becaus e both receptor isoforms differ in their ability to modulate estrogen targe t gene expression in a promoter and cell type-specific manner, these differ ences may mediate the pleiotropic actions of estrogen in reproductive behav ior and neuroendocrine functions.