Ontogeny of vasotocinergic and mesotocinergic systems in the brain of the South African clawed frog Xenopus laevis

Abstract

For a better understanding of the development of neurotransmitter systems and of their putative functional significance during ontogenesis, the development of the vasotocin (AVT) and mesotocin (MST) systems in the brain of Xenopus laevis was studied by means of immunohistochemical techniques. Weakly immunoreactive fibers were already present at late embryonic stage 38 in the caudoventral part of the telencephalon and in the ventral part of the diencephalon. The earliest immunodetectable AVT and MST immunoreactive cell bodies were found in the developing preoptic area at late embryonic stage 43. At the end of the embryonic period (stage 45), AVT immunoreactive fibers have reached the future medial amygdala, the midbrain tegmentum, the median eminence and the neural lobe of the pituitary. When compared with AVT immunoreactive fibers, the development of MST fibers shows some temporal delay. During the premetamorphosis (stages 45-52), AVT immunoreactive cell bodies appear in the medial part of the suprachiasmatic nucleus, the dorsal infundibular region, and the midbrain tegmentum, whereas fibers can now be traced to the nucleus accumbens, the septum and the medial amygdala in the forebrain, to the midbrain tegmentum, the reticular formation, the raphe nuclei, and the solitary tract nucleus in the brainstem, and to the spinal cord. Further maturation of the AVT system during prometamorphosis (stages 53-58) includes the appearance of immunoreactive cell bodies in the lateral part of the suprachiasmatic nucleus, the ventral preoptic area, and the dorsal infundibular region. By the end of the metamorphosis (stage 65), the maturation of the AVT/MST systems reaches an almost adult-like pattern. It should be noted that in amphibians, in contrast to mammals, the early appearance of the AVT/MST systems, including their extensive extrahypothalamic component, suggests that the two neuropeptidergic systems may play a significant role during development.