Vasotocin immunoreactivity in goldfish brains: characterizing primitive circuits associated with social regulation

The neuroanatomical characteristics of vasotocin (VT) and vasopressin (VP) systems have been described in numerous species from diverse vertebrate groups, including teleost fish. However, there are very few teleost species in which VT's effects on social behavior have been established and the neural circuits associated with that regulation fully described. We have previously shown that VT inhibits social approach behaviors in goldfish via actions in the hindbrain. Here we further describe that primitive VT circuit, as well as others throughout the goldfish brain that might contribute to social regulation in this species. In particular, we highlight forebrain projections to the dorsal telencephalon that have not been described previously in any teleost, projections to limbic forebrain areas, most notably the medial division of the dorsal telencephalon and the ventral telencephalon, and midbrain projections to the optic tectum and torus semicircularis that have rarely been described. However, the most dense VT projection in goldfish is to the hindbrain, particularly to motor divisions of the vagal complex and to area postrema, which we argue might influence social approach behaviors through autonomic regulatory mechanisms. Because hindbrain VT projections are some of the most primitive in vertebrates, we suggest they might represent an ancestral mechanism through which VT influenced social behavior.