Circulating estrogens in a male songbird originate in the brain.

Gonadal steroids act on brain to regulate the development and expression of vertebrate reproductive behavior. In addition, steroid targets in brain are an integral part of the feedback regulation of gonadal steroidogenesis. The actions of androgens in brain are often mediated by enzymatic activation or inactivation of circulating hormone, including local conversion of androgen to estrogen. We now report that in zebra finches, the brain synthesizes large quantities of estrogen from androgen and releases this estrogen into blood. Since there appears to be no other significant source of estrogen synthesis in adult male zebra finches, it is likely that the brain directly controls plasma levels of this steroid by contributing most or all of the high levels of estrogen found in the circulation.