Circadian rhythms.

The neurobiological substratum of circadian rhythmicity encompasses three levels of integration: firstly, generation of time signals by circadian pacemakers; secondly, entrainment of pacemakers by environmental influences; thirdly, coupling of circadian pacemakers among themselves and with target systems responsible for the expression of overt rhythms. From recent contributions, the notion that circadian organization results from the interaction of independent oscillators and pathways has been strengthened. In addition, recent evidence supports the existence of circadian rhythmicity in single isolated neurons. New information was produced on the gene control of circadian rhythm generation in Drosophila, as well as interesting advances in the understanding of neuronal mechanisms involved in the generation, entrainment and coupling of circadian rhythms in various species.