Physiological and anatomic evidence for regulation of the heart by suprachiasmatic nucleus in rats

The suprachiasmatic nucleus (SCN) is the mammalian biological clock that ge nerates the daily rhythms in physiology and behavior. Light can phase shift the rhythm of the SCN but can also acutely affect SCN activity and output, e.g., output to the pineal. Recently, multisynaptic SCN connections to oth er organs were also demonstrated. Moreover, they were shown to affect those organs functionally. The aim of the present study was to investigate the r ole of the SCN in the regulation of the heart. First, we demonstrated that heart rate (HR) in SCN-intact, but not SCN-lesioned (SCNx), male Wistar rat s had a clear circadian rhythm, which was not caused by locomotor activity. Second, we demonstrated that light at night reduces HR in intact but not i n SCNx rats. Finally, we demonstrated the presence of a multisynaptic auton omic connection from SCN neurons to the heart with the retrograde pseudorab ies virus tracing technique. Together, these results demonstrate that the S CN affects the heart in rats and suggest that this is mediated by a neurona l mechanism.