CIRCADIAN TIMING BY ENDOGENOUS OSCILLATORS IN THE NERVOUS SYSTEM: TOWARD CELLULAR MECHANISMS

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Abstract
The basic properties of circadian rhythms such as oscillator type, entrainment to daily light-dark (LD) cycles, temperature compensation of the period length and free-running periodicity are remarkably similar in eucaryotic organisms. All circadian oscillators may be based on identical principles found at the cellular level. Animals have a multioscillator organization, with brain centers and related structures serving as sources of rhythmicity and coordination. Oscillators and driven activities are coupled by secretion or by direct neuronal connection. Oscillators of the multioscillatory systems also are coupled. The cellular requirements for the circadian oscillator appear to be as generally uniform among various organisms as the basic properties. Ions and ion transport are important in the timing mechanism, as is protein synthesis on the eucaryotic ribosome. Although no concise model of the circadian oscillator encompassing protein synthesis, ions, and membranes was offered, progress in analysis of the mechanisms was made by genetic selection, screening of biochemical mutants, organ and tissue culturing, biochemical isolation of components and chemical-pulse probing of the cellular oscillator.