Distribution of vasopressin, oxytocin and vasoactive intestinal polypeptide in the hypothalamus and extrahypothalamic regions of tree shrews.

Vasopressin (VP), oxytocin (OXT) and vasoactive intestinal polypeptide (VIP) in the brain modulate physiological and behavioral processes in many vertebrates. Day-active tree shrews, the closest relatives of primates, live singly or in pairs in territories that they defend vigorously against intruding conspecifics. However, anatomy concerning peptidergic neuron distribution in the tree shrew brain is less clear. Here, we examined the distribution of VP, OXT and VIP immunoreactivity in the hypothalamus and extrahypothalamic regions of tree shrews (Tupaia belangeri chinensis) using the immunohistochemical techniques. Most of VP and OXT immunoreactive (-ir) neurons were found in the paraventricular nucleus (PVN) and supraoptic nucleus (SON) of the hypothalamus. In addition, VP-ir or OXT-ir neurons were scattered in the preoptic area, anterior hypothalamic areas, dorsomedial hypothalamic nucleus, stria terminalis, bed nucleus of the stria terminalis and medial amygdala. Interestingly, a high density of VP-ir fibers within the ventral lateral septum was observed in males but not in females. Both VP-ir and VIP-ir neurons were found in different subdivisions of the suprachiasmatic nucleus (SCN) with partial overlap. VIP-ir cells and fibers were also scattered in the cerebral cortex, anterior olfactory nucleus, amygdala and dentate gyrus of the hippocampus. These findings provide a comprehensive description of VIP and a detailed mapping of VP and OXT in the hypothalamus and extrahypothalamic regions of tree shrews, which is an anatomical basis for the participation of these neuropeptides in the regulation of circadian behavior and social behavior.