Distribution of vasotocin- and mesotocin-like immunoreactivities in the brain of the South African clawed frog Xenopus-laevis.

In order to obtain more insight into primitive and derived conditions of neuropeptidergic systems in vertebrates, in particular amphibians, we have studied immunohistochemically the distribution of vasotocin (AVT) and mesotocin (MST) neuronal elements in the brain of the South African clawed frog Xenopus laevis. Apart from a well-developed hypothalamohypophysial system, the antibodies revealed the existence of extrahypothalamic AVT- and MST-immunoreactive cell groups as well as extensive extrahypothalamic networks of immunoreactive fibres, thus confirming the phylogenetic constancy of this condition in vertebrates. The wide distribution of AVT- and MST-immunoreactive fibres throughout the brains of amphibians suggests that the two neuropeptidergic systems are involved not only in hypothalamohypophysial interactions, but also, as in mammals, in a variety of other brain functions. In particular, the relationship of AVT- and MST-immunoreactive fibres with catecholaminergic cell bodies was noted. The present study has underscored once again that considerable differences in relative densities of AVT- and MST-immunoreactive fibres occur between species, even within a single order of vertebrates.