Developmental changes in the distribution of oestrogen receptor mRNA expressing cells in the forebrain of female, male and masculinized female zebra finches.

In female zebra finches, oestrogens induce a male-like development of forebrain song nuclei such as the hyperstriatalis ventrale, pars caudal (HVC) and robustus archistriatalis (RA) that are normally highly sexually dimorphic. The distribution of oestrogen receptor (ER) mRNA in the vocal control system of male, female and oestrogen-treated female zebra finches was assessed between the day of hatching and adulthood using in situ hybridizations with a zebra finch-specific ER probe. ER mRNA was first expressed in the caudal forebrain at posthatching (P) day P5 to P7. At this age, ER mRNA-expressing cells lined the lateral ventricle. This band of labelling extended into the medial portion of HVC from P15 onwards. ER mRNA was found in the shell of the RA from P20 onwards. ER were not expressed in other vocal control areas at any age. Thus oestrogens seem to induce ER-mediated differentiation of the vocal control system via action in the medial HVC, in the area along the lateral ventricle and in the shell of the RA. This differentiation does not involve oestrogen-induced upregulation of ER in vocal control areas. Since the distribution of ER mRNA in the caudal forebrain was similar in juvenile males and females, ER pattern does not explain the sex-specific differentiation of the song system.