Female-biased sex difference in vasotocin-immunoreactive neural structures in the developing quail brain

The bed nucleus of the stria terminalis pars medialis (BSTM), medial preoptic nucleus (POM), and lateral septal region (LS) exhibit more vasotocin-immunoreactive (VT-ir) neural structures in male than in female adult quail. VT-ir cells and fibers in these regions are sensitive to gonadal steroids only in males. The insensitivity of adult female VT-ir neural structures to sex steroids is attributed to estradiol exposure during a critical period in embryonic life. Although the VT-ir system has been intensively examined in adult quail, information is limited in embryos and juveniles. Therefore, we herein investigated the development of VT-immunoreactive neural structures from embryonic day (E) 9 to adulthood with a particular focus on the BSTM, POM and LS of both sexes. VT-ir neural structures were more evident in female than in male embryos from E9 (BSTM and POM) and E11 (LS). This sex difference disappeared between E15 and post-hatch day 1 in the BSTM and POM, and during the first week of life in the LS. Male-biased sex differences in VT-ir structures appeared at puberty. Female-biased sexual dimorphism in the density of the VT-ir structures of BSTM was reflected by the stronger expression of VT mRNA in females than in males. However, the density of VT mRNA somata was comparable in the two sexes. The exposure of male embryos to estradiol resulted in the feminization of VT-ir neural structures in the BSTM, but not in the POM or LS at E11. Collectively, these results suggest that sex differences in VT-ir neural structures changes drastically throughout quail life. In embryos, endogenous estradiol may stimulate the expression of VT in females, resulting in a robust sex difference in VT-ir cells and fibers in favor of this sex.