Suprachiasmatic nucleus: a central autonomic clock

Circadian rhythms are daily changes in behavior and physiology produced by the suprachiasmatic nucleus (SCN) even in the absence of external stimuli1, although photic input from the retina to the SCN entrains these changes to a 24-hour cycle. The SCN modulates autonomic and neuroendocrine function to prepare for diurnal or nocturnal changes in behavior, but its precise connections to the autonomic nervous system are unknown. We used viral transneuronal labeling2 to demonstrate extensive connections of the SCN with diverse types of sympathetic as well as parasympathetic motor systems. Double-virus transneuronal tracing showed connections of single SCN neurons to multiple autonomic systems. However, targets of SCN modulation seem limited to those that operate continuously under tonic, rather than phasic, control.