Chapter

Circadian clocks and their molecular organization

Saurabh Sahar and Paolo Sassone-Corsi

in Seasonal Affective Disorder

Second edition

Published on behalf of Oxford University Press

Published in print October 2009 | ISBN: 9780199544288
Published online February 2013 | e-ISBN: 9780191754593 | DOI: http://dx.doi.org/10.1093/med/9780199544288.003.0001
Circadian clocks and their molecular organization

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The earth's rotation around its axis leads to day–night cycles, which affect the physiology of all living organisms. Circadian (from the Latin words circa dies meaning “about a day”) clocks are intrinsic, time-tracking systems that enable organisms to anticipate environmental changes (e.g., food availability and predator pressure for animals), thereby adapting their behavior and physiology to the appropriate time of day (Schibler and Sassone-Corsi 2002). Feeding behavior, sleep–wake cycles, hormonal levels, and body temperature are just a few examples of physiological variations dependent on the time of day. The three integral parts of circadian clocks are: 1) an input pathway that includes detectors to receive environmental cues (or Zeitgebers) and transmit them to the central oscillator, 2) a central oscillator that keeps circadian time and generates rhythm, and 3) output pathways through which the rhythms are manifested via control of various metabolic, physiological, and behavioral processes. Distinguishing characteristics of circadian clocks include that they are entrainable, self-sustained, and temperature compensated (Merrow et al. 2005).

Circadian clocks are present in almost all the tissues in mammals. The master or “central” clock is located in the hypothalamic suprachiasmatic nucleus (SCN) containing 10–15,000 neurons. Clocks outside the SCN are referred to as “peripheral clocks”. The master clock synchronizes these peripheral clocks to ensure temporally coordinated physiology (Cermakian and Sassone-Corsi 2000). The SCN clock can function autonomously, without any external input, but it can be reset by environmental cues such as light. The molecular machinery that regulates these circadian rhythms comprises a set of genes, known as “clock” genes, the products of which interact to generate and maintain the rhythms.

Chapter.  3661 words.  Illustrated.

Subjects: Psychiatry

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