In songbirds, singing behavior is controlled by a discrete network of interconnected brain nuclei known collectively as the song control system. Both the development of this system and the expression of singing behavior in adulthood are strongly influenced by sex steroid hormones. Although both androgenic and estrogenic steroids have effects, androgen receptors (AR) are more abundantly and widely expressed in song nuclei than are estrogen receptors (ERα). The recent cloning of a second form of the estrogen receptor in mammals, ERβ, raises the possibility that a second receptor subtype is present in songbirds and that estrogenic effects in the song system may be mediated via ERβ. We therefore cloned the ERβ complementary DNA (cDNA) from a European starling preoptic area-hypothalamic cDNA library and used in situ hybridization histochemistry to examine its expression in forebrain song nuclei, relative to the expression of AR and ERα messenger RNA (mRNA), in the adjacent brain sections. The starling ERβ cDNA has an open reading frame of 1662-bp, predicted to encode a protein of 554 amino acids. This protein shares greater than 70% sequence identity with ERβ in other species. We report that starling ERβ is expressed in a variety 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, including the high vocal center (HVc), the medial and lateral portions of the magnocellular nucleus of the anterior neostriatum, and the robust nucleus of the archistriatum. We detected ERα expression in the medial portion of HVc (also called paraHVc) and along the medial border of the caudal neostriatum. ERβ was not expressed in HVc, in the medial and lateral portions of the magnocellular nucleus of the anterior neostriatum, in the robust nucleus of the archistriatum, or in area X. In contrast, ERβ mRNA-containing cells were detected in the caudomedial neostriatum and medial preoptic area in a pattern reminiscent of P450 aromatase expression in the same brain regions in other songbirds. These data suggest that estrogenic effects on the song system are not mediated via ERβ-producing cells within song nuclei. Nonetheless, the overlapping expression of ERβ- and aromatase-producing cells in the caudomedial neostriatum suggests that locally synthesized estrogens may act via ERβ, in addition to ERα, to mediate seasonal or developmental effects on nearby song nuclei.
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