Steroid receptors in the adult zebra finch syrinx: a sex difference in androgen receptor mRNA, minimal expression of estrogen receptor alpha and aromatase.

The zebra finch syrinx (sound production organ) is a sexually dimorphic component of the song system. Only male zebra finches sing, and in parallel, the overall mass and size of fibers in the two largest syrinx muscles are greater in males than females. Despite these obvious sexual dimorphisms, little is known about the role of steroid hormones in the maintenance of the structure and/or function of the syrinx. In this report, we used in situ hybridization to assess the expression of androgen receptor (AR), estrogen receptor alpha (ERalpha), and aromatase (AROM) mRNAs in the syrinx of adult male and female zebra finches. Increased AR mRNA expression was noted in males compared to females in two regions, over the ventralis muscle and in a band of connective tissue neighboring cartilage (perichondria). In contrast, we did not detect specific ERalpha or AROM mRNA expression within the syrinx. However, substantial ERalpha mRNA was present in oviduct, and aromatase mRNA was expressed at high levels in ovary. In parallel, an assay for AROM detected activity in ovary, but not in syrinx tissue from males or females. Taken together, these data suggest that the adult syrinx is sensitive to androgens; that sex differences in function and morphology of the syrinx may in part be due to increased expression of AR in males compared to females. In contrast, estrogen receptor alpha and AROM appear to have limited roles.