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Circadian rhythms. The light-driven endogenous clock |
Cahiers Agricultures. Volume 5, Number 1, 9-21, Janvier-Février 1996, Synthèse
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 Résumé
 Summary
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Author(s) : Lise Martinet, Laboratoire de physiologie sensorielle, Inra, 78352 Jouy-en-Josas cedex, France.. |
Summary : The 24-hour and one-year rhythms observed in most biological, physiological and behavioural processes do not result from responses determined by variations in photoperiod, but from an internal system of temporal measurement: the circadian system. Part of an organism’s genetic make-up, this endogenous rhythmicity allows the temporal sequences of variations in the internal milieu to be organised for optimal adaptation to environmental changes. The system’s functioning is characterised by three stages: firstly, reception of the light signal\; secondly, on the CNS level, the cellular integration of the signal by a self-maintaining oscillator (circadian clock) of period close to 24 hours, the integration allowing the endogenous oscillation to be synchronised onto a 24-hour period\; and lastly, the effector Systems’ response to the clock’s signals modulating the amplitude of their activity on its own rhythm under conditions of temporal isolation (constant light or dark), or a 24-hour period under natural lighting conditions. The fundamental property of circadian rhythms is that a short stimulus of light can phase-shift them either forwards or backwards, depending on the time at which the stimulus is received during the 24-hour cycle. Interrupting the night by a pulse of light has allowed a phase response curve, of constant rate no matter what the species, to be plotted. In birds, photoreception occurs at the pineal gland and retina. The circadian clock is located in the pineal gland. In mammals, only the retina is photoreceptive, and the hypothalamic supra-chiasmatic nuclei (SCN) act as substrate to the clock. Circadian oscillation is a cell property since it is also observed in vitro in isolated chicken pinealocytes or isolated rat SCN neurones. Nevertheless, the molecular mechanism behind circadian oscillation remains obscure. On the other hand, in mammals at least, reception of the light signal by a specific population of retinal cells involves a chain reaction of events, certain steps of which are just beginning to be understood: release of a neurotransmitter, glutamate, by retinal projections at the SCN, induction of genes of early expression, and probably of late expression coding for neuropeptides. Amongst the various clock outputs, only that allowing the rhythmic synthesis of melatonin by the pineal gland has been properly elucidated. Released only at night, melatonin has feedback control over the clock via high-affinity receptors and controls seasonal functions by informing the organism of annual variations in day length. |
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