The mammalian circadian system is comprised of a central clock in the suprachiasmatic nucleus (SCN) and a network of peripheral oscillators located in all of the major organ systems. The SCN is traditionally thought to be positioned at the top of the hierarchy, with SCN lesions resulting in an arrhythmic organism. However, recent work has demonstrated that the SCN and peripheral tissues generate independent circadian oscillations in Per1 clock gene expression in vitro. In the present study, we sought to clarify the role of the SCN in the intact system by recording rhythms in clock gene expression in vivo. A practical imaging protocol was developed that enables us to measure circadian rhythms easily, noninvasively, and longitudinally in individual mice. Circadian oscillations were detected in the kidney, liver, and submandibular gland studied in about half of the SCN-lesioned, behaviorally arrhythmic mice. However, their amplitude was decreased in these organs. Free-running periods of peripheral clocks were identical to those of activity rhythms recorded before the SCN lesion. Thus, we can report for the first time that many of the fundamental properties of circadian oscillations in peripheral clocks in vivo are maintained in the absence of SCN control.
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