POSTMORTEM TRACING REVEALS THE ORGANIZATION OF HYPOTHALAMIC PROJECTIONS OF THE SUPRACHIASMATIC NUCLEUS IN THE HUMAN BRAIN

The suprachiasmatic nucleus (SCN) is a small structure considered to b e the site of the major circadian pacemaker of the mammalian brain. Di sturbances in human biological clock function may occur in several dis eases, such as Alzheimer's disease, sleep problems, and seasonal depre ssion. Since basic knowledge of the anatomical connections of the huma n SCN is limited due to the lack of suitable neuroanatomical tracing m ethods, the understanding of physiological mechanisms of human SCN fun ction has obviously been hampered. In the present study, the hypothala mic connections of the human SCN were revealed for the first time with a newly developed in vitro postmortem anterograde tracing method. The human SCN was found to be connected with nuclei in the hypothalamus t hat are involved in hormone secretion, cardiovascular regulation, and behavior activity These human SCN projections appear to follow the sam e general patterns as those in the rodent brain. This homology may ind icate an evolutionary conservation of the SCN projections from rodent to human. Through these connections, the human SCN may transmit its ci rcadian information to regulate hormone secretion, body temperature, a nd behavioral functions as it does in animal species. In addition, the postmortem tracing technique may be a valuable tool that will contrib ute to our understanding of anatomical connections in the human brain, and may have other applications in the research on the physiology and pathology of the human brain. (C) 1998 Wiley-Liss, Inc.