The molecular mechanisms of the circadian clock

All living organisms have an endogenous clock which is essential to a circa dian regulation of various physiological processes. A dysfunction of this c lock can lead to a variety of diseases or syndromes including sleep disorde rs. In mammals, the central pacemaker is located in the suprachiasmatic nuc leus of the anterior hypothalamus. Our understanding of the molecular mecha nisms underlying the circadian clock have greatly increased over the past f ew years. In all the organisms which have been studied; the establishment a nd the perpetuation of the rhythms rely on negative feedback loops, in whic h clock factors accumulate to a certain level, then repress the transcripti on of their own gene, and of other genes. Recent studies are focusing (1) o n the identification of new clock genes and on defining their respective ro les; (2) on peripheral clocks which appear to be intrinsic to many tissues or cultured cells and on the search for the photoreceptors involved in rece iving signals from the environment.