The role of pineal gland in circadian rhythms regulation

In the presented article we try to find a synthesis of the current knowledge on circadian rhythms. We pick up the most prominent oscillations in human physiology and review the current knowledge of their regulation. Circadian oscillations in the parameters of internal environment are driven by the pineal gland. A biochemical pathway in the pineal transforms tryptophan through serotonin to the final product--the indolamine melatonin. Its plasma level is high at night and low during the day. Melatonin, easily penetrating through biological barriers, thus carries phase of day information to all peripheral tissues. Light exposure of retina alters (via neural pathways connecting retina to pineal gland) the amount of serotonin metabolized to melatonin. This physiological mechanism of adjustment of the endogenous clock to the solar day can be altered in some circumstances, leading to pathologic symptoms. The best known diseases caused by breakdown in circadian regulation are seasonal affective disorders and jet-lag syndrome. (Fig. 2, Ref. 71.)