Interaction of central and peripheral clocks in physiological regulation

In mammals, circadian rhythms of physiology and behavior are regulated by a complex network of cellular molecular oscillators distributed throughout the brain and peripheral tissues. A master clock in the hypothalamic suprachiasmatic nuclei (SCN) synchronizes internal time with the external light-dark cycle, thus entraining the overall rhythmicity of the organism. Recent findings have challenged the dominant role of the SCN in physiological regulation and it becomes increasingly evident that close interaction between different central and peripheral clocks is necessary to maintain robust circadian rhythms of physiology and metabolism. In this review, we summarize recent findings regarding circadian organization in the SCN and in other central and peripheral tissues. We outline the communication pathways between different tissue clocks and, exemplified by the regulation of glucocorticoid release from the adrenal gland and glucose homeostasis in the blood, characterize the interaction between different clocks in the regulation of physiological processes.