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 reproducti ve 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 abundan tly in several regions of the male and female telencephalon and contri butes to peripheral estrogen titers. To determine if this pattern of n eural aromatase and estrogen synthesis is found in other songbirds, we have examined the patterns of estrogen synthesis in various tissues o f another songbird, the common North American cowbird (Molothrus aces) . Radiolabeled aromatizable androgenic substrate was injected in vivo or provided in vitro to telencephalic and gonadal tissue from adult ma le 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 subs trate, injected in vivo, was readily converted to estrogens with highe r amounts in the jugular compared to carotid blood, suggesting that th e brain contains relatively high levels of aromatase. Further, radiola beled androgens, provided in vitro to telencephalic, ovarian, and test icular tissue, resulted in the formation of radiolabeled estrogens. Ar omatase mRNA is distributed widely in several areas of the cowbird tel encephalon including the hippocampus, caudomedial neostriatum (includi ng Field L), and nucleus taeniae. This pattern of neural aromatase exp ression resembles what we have found previously in the zebra finch. Te lencephalic aromatase may be characteristic of passerine songbirds and may function to provide local estrogenic cues to estrogen-sensitive n eural loci, and/or contribute to peripheral estrogen titers in male an d female songbirds. (C) 1997 Academic Press.