Daily oscillations in liver function: diurnal vs circadian rhythmicity

The rodent suprachiasmatic nucleus (SCN), a site in the brain that contains a light-entrained biological (circadian) clock, has been thought of as the master oscillator, regulating processes as diverse as cell division, reproductive cycles, sleep, and feeding. However, a second circadian system exists that can be entrained by meal feeding and has an influence over metabolism and behavior. Recent advances in the molecular genetics of circadian clocks are revealing clock characteristics such as rhythmic clock gene expression in a variety of non-neural tissues such as liver. Although little is known regarding the function of these clock genes in the liver, there is a large literature that addresses the capabilities of this organ to keep time. This time-keeping capability may be an adaptive function allowing for the prediction of mealtime and therefore improved digestion and energy usage. Consequently, an understanding of these rhythms is of great importance. This review summarizes the results of studies on diurnal and circadian rhythmicity in the rodent liver. We hope to lend support to the hypothesis that there are functionally important circadian clocks outside of the brain that are not light- or SCN-dependent. Rather, these clocks are largely responsive to stimuli involved in nutrient intake. The interaction between these two systems may be very important for the ability of organisms to synchronize their internal physiology.