Circadian Clocks, Food Intake, and Metabolism - Progress in Molecular Biology and Translational Science - page 114

2.4. Peripheral organs and most brain regions: Clocks
entrainable by mealtime
Time when food is eaten has potent phase-resetting effects on the clockwork
of all peripheral tissues studied so far, including liver, white adipose tissue,
gastrointestinal tract, heart, lung, and kidne y 70-72 ( Fig. 5.2 ). The character-
istics of the feeding-fasting cycles are critical because both food volume and
interval of food deprivation matter to reset the liver oscillations. 73 However,
the nature of the feeding-associated signals capable of resetting peripheral
oscillators is not yet fully identified. Hormones, such as glucocorticoids
and metabolites, like glucose, as well as nutrient sensors, like AMPK, are
good candidates. 57,74,75 In the liver, transient upregulation of Per2 and
Dec1 transcription is observed in the first hour after feeding. 76 Moreover,
refeeding-induced insulin secretion leads not only to an upregulation of
Per2 expression but also to downregulated Rev - erva mRNA hepatic levels. 77
Timing of circadian oscillations is markedly modified by restricted
feeding in many, but not all, cerebral regions out of the SCN. For example,
daytime feeding modifies

the phase of molecular oscillations in the

Figure 5.2 The suprachiasmatic nuclei (SCN) contain the master clock that controls
sleep - wake cycle and hormonal rhythms. The SCN are the conductor of the many sec-
ondary clocks/oscillators in the brain and peripheral organs, in part via temporal mes-
sages transmitted by nervous pathways (dotted arrows). Light and feeding time act as
synchronizers (filled arrows) at different levels of the multi-oscillatory circadian network.