Circadian clocks are intrinsic time-tracking systems that endow organisms with a survival advantage. The core of the circadian clock mechanism is a cell-autonomous and self-sustained oscillator called a cellular clock, which operates via a transcription-/translation-based negative feedback loop. Under natural conditions, circadian clocks are entrained to a 24-hour day by environmental time cues, most commonly light. In mammals, circadian clocks are regulated by cellular clocks located in the central nervous system, such as the suprachiasmatic nucleus (SCN), and in other peripheral tissues. Importantly, mammals have no photoreceptors in the peripheral tissues; therefore the effect of light on peripheral clocks is indirect. By striking contrast, zebrafish peripheral cellular clocks are directly light responsive. This characteristic of the zebrafish cellular clock has contributed to the identification of molecules and signaling pathways that are involved in the light-dependent regulation of the cellular clock. Here, selected light-dependent regulatory mechanisms of circadian clocks in mammals and zebrafish are described.
Part of the book: Chronobiology