Immunoreactivity to vasoactive intestinal polypeptide (VIP) in the hypothalamus of the domestic fowl, Gallus domesticus

The distribution of VIP-immunoreactive neurons and fibers was detected in the hypothalamus of the chick by immunohistochemistry and light microscopy. A large amount of VIP cellular bodies was localized in the anterior and medial area of the hypothalamus with the highest density of cells in supraoptic, magnocellular preoptic, suprachiasmatic and paraventricular nuclei. Only few VIP-immunoreactive neurons were observed in the caudal section of infundibuli nucleus. A considerable concentration of VIP-positive fibers was also detected in the external layer of the anterior and posterior median eminence. Their presence might have origin both from the neurons of the infundibuli nucleus and from the cells of the paraventricular nucleus. Few VIP-immunoreactive fibers were revealed in the organum vasculosum of the lamina terminalis. These results tend to suggest that VIP may play more than one role in the hypothalamic regions, particularly in the preoptico-hypothalamic area. The presence of this peptide in the median eminence supports even more the hypothesis that it may be released into the portal circulation and transported to the pars distalis of the pituitary gland.