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.

The mammalian circadian clock is known to be entrained by both a daily light-dark cycle and daily feeding cycle. However, the mechanisms of feeding-induced entrainment are not as fully understood as those of light entrainment. To elucidate the first step of entrainment of the liver clock, we identified the circadian clock gene(s) that show both phase advance and acute change of gene expression during the early term of the daytime refeeding schedule in mice. The expressions of liver Per2 and Rev-erbα genes were phase-advanced within 1 day of refeeding. Additionally, the upregulation of Per2 mRNA and down-regulation of Rev-erbα mRNA were induced within 2 hours, not only by food intake but also by insulin injection in intact mice. These expression changes by food intake were not revealed in streptozotocin-treated insulin-deficient mice, but insulin injection was able to recover the impairment of Per2 and Rev-erbα gene expression. Furthermore, we demonstrated using an ex vivo luciferase monitoring system that insulin injection during the daytime causes a phase advance of liver Per2 expression rhythm in Per2::luciferase knock-in mice. In embryonic fibroblasts from Per2::luciferase knock-in mice, insulin infusion caused an acute increase of Per2 gene expression and a similar phase advance of Per2 expression rhythm. Our results indicate that an acute change of Per2 and Rev-erbα gene expression mediated by refeeding-induced insulin secretion is a critical step mediating the early phase of feeding-induced entrainment of the liver clock.

Peripheral circadian clocks in mammals are strongly entrained by light-dark and eating cycles. Their physiological functions are maintained by the synchronization of the phase of organs via clock gene expression patterns. However, little is known about the adaptation of peripheral clocks to the timing of multiple daily meals. Here, we investigated the effect of irregular eating patterns, in terms of timing and volume, on their peripheral clocks in vivo. We found that the phase of the peripheral clocks was altered by the amount of food and the interval between feeding time points but was unaffected by the frequency of feeding, as long as the interval remained fixed. Moreover, our results suggest that a late dinner should be separated into 2 half-dinners in order to alleviate the effect of irregular phases of peripheral clocks.

BackgroundRecent studies on humans and rodents have suggested that the timing of food intake plays an important role in circadian regulation and metabolic health. Consumption of high-fat foods during the inactive period or at the end of the awake period results in weight gain and metabolic syndrome in rodents. However, the distinct effects of breakfast size and the breakfast/dinner size ratio on metabolic health have not yet been fully examined in mice.MethodsWe examined whether the parameters of metabolic syndrome were differentially affected in mice that consumed a large meal at the beginning of the awake period (breakfast; one meal group) and a relatively smaller meal at end of the awake period (dinner; two meals group). The mice of each group were provided equal food volume per day.ResultsMice on one meal exhibited an increase in body weight gain, hyperinsulinemia, hyperleptinemia, and a decrease of gene expression associated with β-oxidation in adipose tissue and liver compared with those on two meals. The circadian expression pattern of the Clock gene in mice on one meal was disturbed compared with those on two meals.ConclusionsIn conclusion, a bigger breakfast with a smaller dinner (two meals per day) but not breakfast only (one meal per day) helps control body weight and fat accumulation in mice on a high-fat meals schedule. The findings of this study suggest that dietary recommendations for weight reduction and/or maintenance should include information on the timing and quantity of dietary intake.

No abstract