Androgen receptor, estrogen receptor alpha, and estrogen receptor beta show distinct patterns of expression in forebrain song control nuclei of European starlings

In songbirds, singing behavior is controlled by a discrete network of inter connected brain nuclei known collectively as the song control system. Both the development of this system and the expression of singing behavior in ad ulthood are strongly influenced by sex steroid hormones. Although both andr ogenic and estrogenic steroids have effects, androgen receptors (AR) are mo re abundantly and widely expressed in song nuclei than are estrogen recepto rs (ER alpha) The recent cloning of a second form of the estrogen receptor in mammals, ER beta, raises the possibility that a second receptor subtype is present in songbirds and that estrogenic effects in the song system may be mediated via ER beta. We therefore cloned the ER beta complementary DNA (cDNA) from a European starling preoptic area-hypothalamic cDNA library and used in situ hybridization histochemistry to examine its expression in for ebrain song nuclei, relative to the expression of AR and ER alpha messenger RNA(mRNA), in the adjacent brain sections. The starling ER beta cDNA has a n open reading frame of 1662-bp, predicted to encode a protein of 554 amino acids. This protein shares greater than 70% sequence identity with ER beta in other species. We report that starling ER beta is expressed in a variet y of tissues, including brain, pituitary, skeletal muscle, liver, adrenal, kidney, intestine, and ovary. Similar to reports in other songbird species, we detected AR mRNA-containing cells in several song control nuclei, inclu ding the high vocal center (HVc), the medial and lateral portions of the ma gnocellular nucleus of the anterior neostriatum, and the robust nucleus of the archistriatum. We detected ER alpha expression in the medial portion of HVc (also called paraHVc) and along the medial border of the caudal neostr iatum. ER beta was not expressed in HVc, in the medial and lateral portions of the magnocellular nucleus of the anterior neostriatum, in the robust nu cleus of the archistriatum, or in area X. In contrast, ER beta mRNA-contain ing cells were detected in the caudomedial neostriatum and medial preoptic area in a pattern reminiscent of P450 aromatase expression in the same brai n regions in other songbirds. These data suggest that estrogenic effects on the song system are not mediated via ER beta-producing cells within song n uclei. Nonetheless, the overlapping expression of ER beta- and aromatase-pr oducing cells in the caudomedial neostriatum suggests that locally synthesi zed estrogens may act via ER beta, in addition to ER alpha, to mediate seas onal or developmental effects on nearby song nuclei (e.g. HVc).