In ovo exposure to ethynylestradiol demasculinizes the vasotocin system in the medial preoptic nucleus in male Japanese quail.

Ethynylestradiol (EE2) is an estrogen that is used predominantly in oral contraceptives and contaminates the environment. In ovo exposure to EE2 impairs copulatory behavior in male Japanese quail, although the mechanisms responsible remain undetermined. The medial preoptic nucleus (POM) plays a pivotal role in male copulatory behaviors and is sexually dimorphic, with a higher number of vasotocin (VT) neuronal fibers in males. In this study, the effects of in ovo EE2 exposure on the VT neuronal fibers of the POM were examined to test the hypothesis that impaired copulatory behaviors caused by EE2 are due to demasculinization of the POM in male birds. Japanese quail embryos were treated with vehicle or EE2 by injecting it into the egg yolk. After the male embryos were hatched and sexually matured, the POM in VT-immunostained and Nissl-stained brain sections, as well as the performance of male sexual behaviors, were analyzed to evaluate the effects of EE2. The analysis of the POM revealed that EE2 treatment significantly decreased the number of VT-immunopositive fibers in the POM of adult male quail, but it did not significantly change the volume of the POM. EE2 dramatically reduced the number and incidence of copulatory behaviors, including neck grabs, mount attempts, mounts, and cloacal contact movements, but it did not affect courtship behaviors, including strutting and crowing. Our results suggest that in ovo EE2 exposure induces demasculinization of the VT system in the POM. The effect of EE2 may be responsible for reducing copulatory behaviors in male Japanese quail. SIGNIFICANT STATEMENT: Embryonic exposure to ethynylestradiol demasculinizes the vasotocin system in the preoptic area, resulting in reduced copulatory behavior in adult male Japanese quail.