The suprachiasmatic nucleus controls the circadian rhythm of heart rate via the sympathetic nervous system.

The mammalian suprachiasmatic nucleus (SCN) controls a wide variety of circadian behavioral and physiological processes. The specific motor output pathway(s) by which these diverse processes are controlled are unknown. The only established motor output of this system is the regulation of pineal melatonin synthesis via the sympathetic nervous system. It is therefore possible that other peripheral circadian rhythms are regulated similarly. To address this issue, body temperature (BT), general activity (GA), wheel-running activity (WR), and heart rate (HR) were recorded in laboratory rats, and the effects of SCN lesion (SCNX) and of pharmacological sympathectomy with guanethidine (GUAN) on these multiple circadian rhythms were determined. The SCNX abolished circadian patterns in all motor outputs, whereas sham animals showed robust rhythms in all measures. In contrast, guanethidine, which depleted peripheral but not central catecholamine content, selectively reduced HR circadian rhythmicity. Other rhythms (BT, GA, and WR) were unaffected. These results suggest that the SCN influences some peripheral targets via circadian regulation of the sympathetic nervous system, and other circadian outputs are regulated via different, unknown pathways.