Ontogenesis of the retinohypothalamic tract, vasoactive intestinal polypeptide- and peptide histidine isoleucine-containing neurons and melatonin binding in the hypothalamus of the mink.

The development of the retinohypothalamic tract (RHT) labeled with cholera toxin and of the vasoactive intestinal polypeptide (VIP) and peptide histidine isoleucine (PHI) systems was studied in the hypothalamus of neonatal mink by using immunohistochemistry. Retinal fibers were observed in the suprachiasmatic nucleus (SCN) from birth and were adult-like by day 14. VIP and PHI immunoreactivity was also present from birth. Melatonin binding was studied by autoradiography using [125I]melatonin as a ligand. A specific binding was detected in near-term fetal and neonate brains in the olfactory epithelium, various thalamic nuclei, the pineal gland, and the pars tuberalis of the pituitary gland, but never in the SCN. These results are discussed in the context of the potential role of daylight cycles and/or melatonin in entraining circadian rhythms in neonate mink.