[Melatonin: what for?].

Service de Biochimie Médicale et Biologie Moléculaire, Faculté de Médecine Pitié-Salpêtrière, 91 boulevard de l'Hôpital, 75013 Paris.
Bulletin de l'Académie nationale de médecine (Impact Factor: 0.22). 06/2005; 189(5):879-89; discussion 889-91.
Source: PubMed


Melatonin (N-acetyl-5 methoxy-tryptamine) is a hormone secreted mainly by the pineal gland or epiphysis; it is also produced, but in much smaller quantities, by the retina. The key enzyme in melatonin synthesis is N-acetyl-transferase (NAT) whose activity increases during darkness. This explains why melatonin is mainly secreted at night. The circadian rhythm of melatonin synthesis is controlled by the suprachiasmatic nuclei--the "biologic clock". This clock itself functions rhythmically, being entrained by environmental synchronizers. According to the time of exposure, light has two effects on melatonin secretion, either suppressing or shifting its secretion (phase-response curve). Exposure to light in the morning advances the phase of the rhythm, while exposure in the evening delays it. Melatonin works in the opposite way, acting as a light signal transducer, and relaying information on the duration of day and night. Exogenous melatonin can synchronize some circadian rhythms, such as body core temperature. Current research is focusing on the properties of melatonin (and of light) as a resynchronizing agent in situations such as aging, blindness, shift work, night work, phase-advanced or phase-delayed sleep syndrome, and jet lag. Melatonin also has strong antioxidant properties (stronger than those of vitamin E), and an oncostatic action. Interestingly, three recent epidemiological studies show that women who work exclusively at night for long periods have a significantly elevated relative risk of breast cancer (RR 1.1-1.6). Other studies are required to confirm these results.

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    ABSTRACT: In the present study we tried to document alterations of some circadian rhythms in intensive care unit patients. The genes involved in the generation and regulations of the physiological circadian rhythms have been now identified in several species, including human beings. Patients in intensive care units often develop symptoms of circadian rhythm disorders. The effect of coma or surgery on the biological clock remains unknown.The present study determines the 24 hour expression of clock genes: Per(s), Clock, Bmal1 and CRY1, in peripheral blood leucocytes. Plasma melatonin concentration was also measured in five comatose and ten non comatose patients. Cosinor and student's t-test were used as statistical tools to test inter-group differences in the obtained time series.The Per(s) gene mRNA exhibits a circadian expression in comatose patients while the rhythmicity of some studied genes disappeared in non comatose patients. The 24h profiles of melatonin in both groups were greatly disturbed. These results suggest that surgical stress and intensive care environment affect the peripheral clock as well as the endogenous hormones in human.
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