Neurobiology of circadian rhythms

Adaptation in the temporal environment is key to survival. This is achieved by the manifestation of periodicity in occurrence of vital behavioural and physiological processes at regular intervals--the biological rhythms. Biological rhythms (= biological clocks) are ubiquitous, can be demonstrated persisting at any level of organization in the living world, and are generated and controlled by some central pacemaker(s), mostly located in the brain. In mammals, the suprachiasmatic nucleus (SCN) of the anterior hypothalamus is the principal site of the endogenous circadian pacemaker, regulating many daily physiological and behavioural functions, although other neural structures could also be contributing to the circadian timekeeping system. In other vertebrates, the neural site(s) of the circadian pacemaker is(are) still unclear. An organism without brain can have the biological clock, as well, for fully functional 24-hour temporal organization has been identified in several invertebrates, including unicellular Paramecium and Gonyaulax as well as filamentous fungus, Neurospora. This article attempts to provide an update of the informations which have accumulated over the past decade about understanding of the neurophysiological and molecular bases of circadian rhythms in animals.