Laboratory of Neurobiology of Rhythms, Centre National de la Recherche Scientifique (UMR7518), University Louis Pasteur, Neuroscience Federation (IFR37), 12 rue de l'université, F-67000 Strasbourg, France.
Published: May 2003
AI Article Synopsis
Article Abstract
The molecular clockwork in mammals involves various clock genes with specific temporal expression patterns. Synchronization of the master circadian clock located in the suprachiasmatic nucleus (SCN) is accomplished mainly via daily resetting of the phase of the clock by light stimuli. Phase shifting responses to light are correlated with induction of Per1, Per2 and Dec1 expression and a possible reduction of Cry2 expression within SCN cells. The timing of peripheral oscillators is controlled by the SCN when food is available ad libitum. Time of feeding, as modulated by temporal restricted feeding, is a potent 'Zeitgeber' (synchronizer) for peripheral oscillators with only weak synchronizing influence on the SCN clockwork. When restricted feeding is coupled with caloric restriction, however, timing of clock gene expression is altered within the SCN, indicating that the SCN function is sensitive to metabolic cues. The components of the circadian timing system can be differentially synchronized according to distinct, sometimes conflicting, temporal (time of light exposure and feeding) and homeostatic (metabolic) cues.
Circadian rhythms have evolved to synchronize gene expression, physiology, and behavior with time-of-day changes in the external environment. In every mammalian cell exists a core clock mechanism that consists of a transcriptional-translational feedback loop that drives rhythmic gene expression. Circadian disruption, as observed in shift workers and genetic mouse models, contributes to the onset and progression of cardiometabolic disorders.
: Chronic liver diseases such as hepatic tumors can affect the brain through the liver-brain axis, leading to neurotransmitter dysregulation and behavioral changes. Cancer patients suffer from fatigue, which can be associated with sleep disturbances. Sleep is regulated via two interlocked mechanisms: homeostatic regulation and the circadian system.
Arctic Seasonal Timekeeping Initiative, Arctic Chronobiology and Physiology Research Group, Department of Arctic and Marine Biology, UiT-The Arctic University of Norway, Tromsø, Norway.
Circadian rhythms synchronize the internal physiology of animals allowing them to anticipate daily changes in their environment. Arctic habitats may diminish the selective advantages of circadian rhythmicity by relaxing daily rhythmic environmental constraints, presenting a valuable opportunity to study the evolution of circadian rhythms. In reindeer, circadian control of locomotor activity and melatonin release is weak or absent, and the molecular clockwork is reportedly non-functional.
This contribution highlights the scientific development of two intertwined disciplines, photoneuroendocrinology and circadian biology. Photoneuroendocrinology has focused on nonvisual photoreceptors that translate light stimuli into neuroendocrine signals and serve rhythm entrainment. Nonvisual photoreceptors first described in the pineal complex and brain of nonmammalian species are luminance detectors.
Exercise physiology and circadian biology are distinct and long-standing fields. Recently they have seen increased integration, largely due to the discovery of the molecular components of the circadian clock and recognition of human exercise performance differences over time-of-day. Circadian clocks, ubiquitous in cells, regulate a daily tissue specific program of gene expression that contribute to temporal patterns of physiological functions over a 24-h cycle.
