Clock-Talk: Interactions between central and peripheral circadian oscillators in mammals

In mammals, including humans, nearly all physiological processes are subject to daily oscillations that are governed by acircadian timing system with a complex hierarchical structure. The central pacemaker, residing in the suprachiasmatic nucleus(SCN) of the ventral hypothalamus, is synchronized daily by photic cues transmitted from the retina to SCN neurons via theretinohypothalamic tract. In turn, the SCN must establish phase coherence between self-sustained and cell-autonomousoscillators present in most peripheral cell types. The synchronization signals (Zeitgebers) can be controlled more or lessdirectly by the SCN. In mice and rats, feeding-fasting rhythms, which are driven by the SCN through rest-activity cycles, are the most potent Zeitgebers for the circadian oscillators of peripheral organs. Signaling through the glucocorticoid receptorand the serum response factor also participate in the phase entrainment of peripheral clocks, and these two pathwaysare controlled by the SCN independently of feeding-fasting rhythms. Body temperature rhythms, governed by the SCNdirectly and indirectly through rest-activity cycles, are perhaps the most surprising cues for peripheral oscillators. Althoughthe molecular makeup of circadian oscillators is nearly identical in all cells, these oscillators are used for different purposes inthe SCN and in peripheral organs.