Antiandrogen blocks estrogen-induced masculinization of the song system in female zebra finches

Song behavior and the neural song system that serves it are sexually dimorphic in zebra finches. In this species, males sing and females normally do not. The sex differences in the song system include sex differences in the proportion of neurons that express androgen receptors, which is higher in specific brain regions of males. Estradiol (E-2) administered in early development profoundly masculinizes the song system of females, including the proportion of neurons expressing androgen receptors. We examined whether or not the expression of these androgen receptors was causally related to the E-2-induced masculinization of this system by coadministering Flutamide, which blocks androgen action at the receptor, along with E-2 at hatching. E-2 alone had its usual masculinizing effect on the female song system, measured in adulthood: increasing the size of song nuclei, the size of neurons in HVC, RA, and IMAN, and the number of neurons in HVC. E-2's masculinizing action, however, was significantly diminished on all measures by coadministering Flutamide. Indeed, females receiving both E-2 and Flutamide were never significantly more masculine than controls on any measure. Flutamide alone had no effect. Our results strongly suggest that the activation of androgen receptors is necessary for the E-2-induced masculinization of the song system in females. (C) 2002 Wiley Periodicals, Inc.