Abstract
Circadian rhythms are essential for the survival of all organisms, enabling them to predict daily changes in the environment and time their behaviour appropriately. The molecular basis of such rhythms is the circadian clock, a self-sustaining molecular oscillator comprising a transcriptional–translational feedback loop. This must be continually readjusted to remain in alignment with the external world through a process termed entrainment, in which the phase of the master circadian clock in the suprachiasmatic nuclei (SCN) is adjusted in response to external time cues. In mammals, the primary time cue, or “zeitgeber”, is light, which inputs directly to the SCN where it is integrated with additional non-photic zeitgebers. The molecular mechanisms underlying photic entrainment are complex, comprising a number of regulatory factors. This review will outline the photoreception pathways mediating photic entrainment, and our current understanding of the molecular pathways that drive it in the SCN.
Highlights
The molecular light responses of the suprachiasmatic nuclei (SCN) are regulated throughout the day/night cycle such that the SCN responds to resetting signals in a time-dependent manner
Lack of CK1ε leads to faster re-entrainment following both phase advances and delays of the light/dark cycle [88]. This suggests that CK1ε limits the light-induced accumulation of PER in the SCN, which in turn limits the size of behavioural phase shifts
It is clear that the molecular basis of photic entrainment in the SCN is complex
Summary
The clock must be continually readjusted to remain in alignment with the external world. This is achieved through a process termed entrainment using external time cues called “zeitgebers”. Within the SCN, light information is integrated with signals from a range of other non-photic zeitgebers including food, temperature and sleep, to align the biological and environmental day [5]. Such an alignment allows organisms to deliver the correct materials, in the correct concentration to the correct organ systems at the optimal time of day.
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