Effects of light cues on re-entrainment of the food-dominated peripheral clocks in mammals.

Although the light/dark (LD) cycle is a weak Zeitgeber in peripheral clocks compared with food stimuli, whether the effect of light cues on the re-entrainment of peripheral clocks can be masked by that of the dominating food cue remains unknown. In the present study, the individual reversal of LD cycle for 7 days could not obviously affect circadian patterns of core clock genes (Bmal1, Cry1, Per1, and Dec1) in the liver and heart of restricted-fed rats. In contrast, reversing the feeding schedule together with the LD cycle markedly enhanced the re-entrainment of peripheral clocks compared with reversal of the feeding regimen alone. In addition, LD reversal alone for 7 days contrarily regulated the expression levels of Cry1, Per1, and Dec1 in the liver and heart. Moreover, daytime restricted feeding not only induced different phase shift rates of the four examined clock genes but also led to reversed phase shift direction in their resetting processes in these two peripheral clocks. Furthermore, the resetting sequences of these genes were also disparate between these two peripheral clocks. These observations suggest that the mechanisms underlying the liver and heart clocks are distinct, which may distinguish them from each other in the SCN-synchronized peripheral system.