ESTROGEN SYNTHESIS IN-VIVO IN THE ADULT ZEBRA FINCH - ADDITIONAL EVIDENCE THAT CIRCULATING ESTROGENS CAN ORIGINATE IN BRAIN

Aromatase activity is abundant in limbic and nonlimbic neural structur es in zebra finches, especially in the telencephalon near neural circu its that control singing behavior. Also, male songbirds can have high estrogen levels in blood. Because it is difficult to detect aromatase activity in other male tissues, we have postulated that the brain itse lf is the source of the estrogen present in the blood of males. Previo usly, we developed methods to measure estrogen synthesis in vivo by in jecting [H-3]androgen into the systemic circulation or directly into t issues and then determining the quantity of [H-3]estrogen entering or leaving the brain. Our results support our hypothesis that the brain i s the primary site of estrogen synthesis in males of this species. Her e, we confirm that [H-3]estrogen emanates from the brain after systemi c [H-3]androgen injection by showing that the presumptive estrogen in jugular plasma is significantly reduced by treatment with an aromatase inhibitor. In females, but not males, estrogens can be found in large amounts in carotid plasma after systemic androgen injection, presumab ly due to ovarian estrogen synthesis. However, carotid estrogen levels are variable in females, so it is difficult to determine if the femal e brain contributes estrogen to blood as is seen in males. The data pr esented here suggest that the adrenal of males is not an important sit e of aromatization, because [H-3]estrogens are undetected in the adren als of castrated or sham-castrated males after adrenal injections of [ H-3] androgen. Nevertheless, at least 2.8- to 17.3-fold more [H-3]estr ogen was present in the jugular than in the carotid of castrated males after systemic injection of [H-3]androgen. Because androgen levels we re in the physiological range, we conclude that the brain routinely en riches the estrogen content of blood in normal adult males of this spe cies.