Estrogen synthesis and secretion in the brown-headed cowbird (Molothrus ater)

Estrogens exert profound effects on vertebrate physiology and behavior. In most vertebrates, including birds, estrogens derived from ovarian tissue circulate at high levels during discrete periods of reproductive activity, and estrogen levels in males are low. In some songbirds (Passeriformes) plasma estrogens are high in both males and females. In the zebra finch, aromatase (estrogen-synthetase) is expressed abundantly in several regions of the male and female telencephalon and contributes to peripheral estrogen titers. To determine if this pattern of neural aromatase and estrogen synthesis is found in other songbirds, we have examined the patterns of estrogen synthesis in various tissues of another songbird, the common North American cowbird (Molothrus ater). Radiolabeled aromatizable androgenic substrate was injected in vivo or provided in vitro to telencephalic and gonadal tissue from adult male and female cowbirds. Estrogenic products were assayed in blood from the carotid artery and jugular vein, and in the telencephalon, ovary, and testes. Additionally, the presence of aromatase mRNA was studied in the brain using in situ hybridization. Radiolabeled androgenic substrate, injected in vivo, was readily converted to estrogens with higher amounts in the jugular compared to carotid blood, suggesting that the brain contains relatively high levels of aromatase. Further, radiolabeled androgens, provided in vitro to telencephalic, ovarian, and testicular tissue, resulted in the formation of radiolabeled estrogens. Aromatase mRNA is distributed widely in several areas of the cowbird telencephalon including the hippocampus, caudomedial neostriatum (including Field L), and nucleus taeniae. This pattern of neural aromatase expression resembles what we found previously in the zebra finch. Telencephalic aromatase may be characteristic of passerine songbirds and may function to provide local estrogenic cues to estrogen-sensitive neural loci, and/or contribute to peripheral estrogen titers in male and female songbirds.