The specific estrogen receptor antagonist ICI 182,780 masculinizes development of the zebra finch syrinx

In zebra finches, the vocal organ (syrinx) is larger in males compared to females. The exact mechanism responsible for this sex difference is not known, but it may be related to steroid hormones. Previous studies have demonstrated that treatment with estradiol feminizes the mass as well as fiber size of the two largest syrinx muscles (ventralis and dorsalis) in males. Treating females with the aromatase inhibitor fadrozole, however, does not induce masculinization. As an alternative approach to further clarify this paradoxical effect of estrogens on syrinx development, we administered the specific estrogen receptor antagonist ICI 182,780 during the first 25 days post-hatching. Daily injections of this drug significantly increased ventralis and dorsalis muscle fiber size in both sexes. Data also demonstrate that in males, the ventralis muscle makes an earlier contribution to the sex difference in syrinx mass by becoming dimorphic prior to dorsalis. Taken together, these data suggest that estrogens can influence development of the syrinx by feminizing morphology of this tissue. However, the lack of reported sex differences during development in steroid receptors, plasma steroid levels, and aromatase enzyme, indicate that hormones are not solely responsible for sex differences in this organ. Thus, similar to the neural forebrain regions that control song, complete sexual differentiation of the zebra finch syrinx likely involves additional factors.