A dose–response study of estradiol's effects on the developing zebra finch song system

To gauge the sensitivity of the female zebra finch song system to estradiol (E2), we used subcutaneous implants to administer various doses of E2 to hatchling female zebra finches. Four different doses of E2 were administered: 50, 15, 5 and 0-μg via subcutaneous silicon “ropes” at hatching, and the brains were examined in adulthood. Further, we examined whether masculinization was all-or-none once a threshold was reached or if the morphology of the song system would show a graded response to the various doses of E2. Finally, we asked if the various dependent measures – volume of song nuclei, neuron size, and neuron number – would show differential sensitivity to E2. Fifteen micrograms was sufficient to masculinize many aspects of the song system and was often as effective as 50-μg, causing a dramatic difference relative to the 0-μg group. Different aspects of the song system seemed differentially sensitive to the effects of E2: volumes of song control nuclei, the size of RA neurons, and the number of HVC neurons were significantly masculinized by 15-μg E2, but the number of RA neurons and HVC and lMAN soma sizes required 50-μg. The results suggest that several developmental processes are influenced by E2, possibly because of multiple sites of action or multiple processes that respond to E2.