Vol 5 n° 4 - Chronobiology and Mood Disorders
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he Earth’s rotation on its axis, the orbit of the Earth around the Sun, and the orbit of the Moon around the  Earth  induce  rhythmic  geophysical  phenomena. Organisms are thus affected by daily and seasonal vari- ations of many physical factors of their environment.The ability to adapt to this changing environment is an essen- tial survival mechanism. As a result, life has evolved to adapt to periodically changing environmental demands, and to anticipate them according to their predictability. Circadian and seasonal rhythms in biochemical, meta- bolic, endocrine, physiological, and behavioral processes are a fundamental feature of all living organisms, reflect- ing the need to ensure that biological functions occur at a given time of the day or year.The most obvious exam- ple is the fact that many animals are active only during the light period (diurnal species; human belong to this group) or in the dark period (nocturnal species), and are inactive during the other part of the day (sleep/wake cycle). Other rhythms, such as reproduction, diapause, hibernation, fur color changes, and migration, can also be given as examples. Precise timing is required at all levels from behavior to gene expression, and its dysregulation causes malfunction. Since the beginning of time, mankind has been fascinated by  the  sun  and  the  invariably  changing  seasons,  and ancient medical scripts include considerations on the vari- ation of disease through the seasons. Disorders of rhyth- micity are characteristic of—and may underlie—a variety B a s i c   r e s e a r c h 3 4 3 Melatonin in animal models Paul Pévet, PhD Keywords:  melatonin;  animal  model;  seasonal  function;  circadian  function; chronobiotic effect Author  affiliations:  Laboratoire  de  Neurobiologie  des  Rythmes,  UMR  7518 CNRS-Université Louis Pasteur, Strasbourg, France Address for correspondence:  Laboratoire de Neurobiologie des Rythmes, UMR  7518  CNRS-Université  Louis  Pasteur,  12  rue  de  l’Université,  67000 Strasbourg, France   (e-mail: pevet@neurochem.u-strasbg.fr) T Melatonin is a hormone synthesized and secreted during the night by the pineal gland. Its production is mainly dri- ven by the circadian clock, which, in mammals, is situated in the suprachiasmatic nucleus of the hypothalamus. The melatonin production and release displays characteristic daily (nocturnal) and seasonal patterns (changes in dura- tion proportional to the length of the night) of secretion. These rhythms in circulating melatonin are strong syn- chronizers for the expression of numerous physiological processes. In mammals, the role of melatonin in the con- trol of seasonality is well documented, and the sites and mechanisms of action involved are beginning to be iden- tified. The exact role of the hormone in the diurnal (cir- cadian)   timing   system   remains   to   be   determined. However, exogenous melatonin has been shown to affect the circadian clock. The molecular and cellular mecha- nisms involved in this well-characterized “chronobiotic” effect have also begun to be characterized. The circadian clock itself appears to be an important site for the entrain- ment effect of melatonin and the presence of melatonin receptors appears to be a prerequisite. A better under- standing of such “chronobiotic” effects of melatonin will allow clarification of the role of endogenous melatonin in circadian organization. © 2003, LLS SAS Dialogues Clin Neurosci. 2003;5:343-352. Copyright © 2003 LLS SAS.  All rights reserved www.dialogues-cns.org