Human brain contains vasopressin and vasoactive intestinal polypeptide neuronal subpopulations in the suprachiasmatic region.

The suprachiasmatic nuclei (SCN) and retinohypothalamic tract ( RHT ) in the anterior hypothalamus have been postulated to play an important role in the timing of daily biological rhythms in mammals. Although physiological studies have described circadian rhythms in man, the presence of an RHT or SCN has not been conclusively demonstrated in the human brain. Immunocytochemical identification of distinct ventral vasoactive intestinal polypeptide (VIP) containing and dorsal vasopressin containing neuronal subpopulations in the human suprachiasmatic region provides correlative evidence of neuronal clusters which are homologous to discrete cell groups in the SCN of other mammalian species. Manipulation of the circadian system has been used to treat some affective illnesses and other physiological timing disorders. Characterization of the neural substrates underlying human circadian rhythms could be useful in the development of future treatment modalities and is essential for understanding normal human circadian organization.