Résumés
Résumé
Les rythmes biologiques font partie des propriétés fondamentales du vivant. Ils peuvent être ajustés à 24 heures par divers facteurs, dont l’alternance jour/nuit. Toutefois, en l’absence de repère temporel, les rythmes perdurent avec une très grande précision, indiquant que l’organisme possède un système endogène de mesure du temps. Chez les mammifères, l’horloge centrale est située dans les noyaux suprachiasmatiques (NSC) de l’hypothalamus. Nous nous proposons ici de faire le point sur les mécanismes moléculaires qui sous-tendent la rythmicité et la remise à l’heure de cette horloge, en mettant l’accent sur son organisation cellulaire et fonctionnelle. Effectivement, des données récentes amènent à repenser l’horloge non pas comme une entité uniforme, mais comme un ensemble cohérent d’unités fonctionnelles distinctes et interconnectées. Le rôle des NSC comme générateur fondamental de la rythmicité sera également discuté à la lumière de résultats récents.
Summary
Biological rhythms represent a fundamental property of various living organisms. In particular, circadian rhythms, i.e. rhythms with a period close to 24 hours, help organisms to adapt to environmental daily rhythms. Although various factors can entrain or reset rhythms, they persist even in the absence of external timing cue, showing that their generation is endogenous. Indeed, the suprachiasmatic nucleus (SCN) of the hypothalamus is considered to be the main circadian clock in mammals. Isolated SCN neurons have been shown to display circadian rhythms, and in each cell, a set of genes, called « clock genes », are devoted to the generation and regulation of rhythms. Recently, it has become obvious that the clock located in the SCN is not homogenous, but is rather composed of multiple functional components somewhat reminiscent of its neurochemical organization. The significance and implications of these findings are still poorly understood but pave the way for future exciting studies. Here, current knowledge concerning these distinct neuronal populations and the ways through which synchronization could be achieved, as well as the potential role of neuropeptides in both photic and non-photic resetting of the clock, are summarized. Finally, we discuss the role of the SCN within the circadian system, which also includes oscillators located in various tissues and cell types.
Parties annexes
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