Synchronization of the mammalian central and peripheral circadian clocks

The circadian clock is an endogenous, self-sustained biological rhythm. Different organisms possess different period of the circadian clock, ranging from 20 to 28 h. Synchronizations of the circadian clock contribute to the organisms’ fitness. This synchrony refers not only to the synchronization between the body clock and the environment, but also to that between the central and peripheral clocks. The synchronization is a process that the organism’s circadian clock is reset by the outside environment. Being synchronized often makes the organism oscillate in a period of 24 h, that of the earth’s rotation. More importantly, being synchronized changes the phase of the organism’s clock, for example activity onset, to a stable phase relationship to the environmental time cues, like the sunrise. Pittendrigh had raised the phase response curve (PRC) model to explain how light synchronizes the circadian clock by phase shifting the clock. His PRC model successfully predicts entrainment on the basis of period changes. Light pulse is considered as the most effective zeitgeber that can synchronize an organism’s circadian clock. Phase response curves show distinct similarities from diverse organisms, including both diurnal and nocturnal animals. Better understanding on these synchronizations are built on recent advances in the following aspects, the molecular feedback loops, the novel photoreceptor melanopsin and its expressing cells, and the hierarchical structures constituting the mammalian clock system. This review summarizes recent progresses on topics of the synchronizations of both the body clock and the environment, and the central and peripheral clocks. It discusses how light and food cues mediate the synchronization of the clock system, followed with introducing main human disorders in which desynchronized body clocks are involved. Light pulse transmits the environmental time cues to the suprachiasmatic nucleus (SCN) through the retino-hypothalamic tract (RHT). SCN is located in the hypothalamus and serves as the central oscillator in mammals. Food is another important zeitgeber that synchronizes the circadian clock. Feeding behavior passes the timing information to the central oscillator, SCN, by taking advantages of the circulating hormones and metabolites. After receiving information from either light or food, SCN integrates the information and passes them along to peripheral clocks that locate in different organs, finally reaching to a coherent circadian system for the entire multicellular organism. Therefore, this review offers basic knowledge to further researches on the clock synchronization, and expects people understand that keeping our body clocks in synchrony is important to our health.