Sexual dimorhism, steroid-induced plasticity, and behavioral significance of the vasotocinergic innervation of the avian brain

This review focuses on the neuroanatomical distribution of vasotocin (the antidiuretic hormone) in the avian brain. This peptide is synthesized by a well described system of diencephalic magnocellular neurons, but, in addition, more recent studies demonstrated the presence of immunoreactive sexually dimorphic parvocellular groups or fibers in diencephalic and extradiencephalic regions of different species, including the Japanese quail. The main cluster has been detected in the male in a region considered as the avian homologue of the mammalian nucleus of the stria terminalis (nST). These cells are not visible in the female and this dimorphism has been confirmed also by in situ hybridization studies. Moreover, sexually dimorphic vasotocin-positive fibers are present in regions involved in the control of different aspects of reproduction, i.e. the nucleus preopticus medialis (copulatory behavior), the lateral septum (secretion of GnRH), and the nucleus intercollicularis (vocalization control). In the male the vasotocin-immunoreactivity in these regions is strictly testosterone-dependent: castration, or exposure to a short-day photoperiod decrease VT-immunoreactivity to female levels. Administration of estradiol-benzoate to embryos (a treatment that abolishes masculine sexual behavior) results in a dramatic decrease of the VT-immunoreactivity in all these brain regions of male quail. Behavioral experiments demonstrate that intracerebroventricular administration of vasotocin strongly inhibits male sexual behavior. These data suggest that this peptide in birds is not only the antidiuretic hormone, but also plays a central role in the control of diverse aspects of reproduction.