Hormonal Rhythms as Internal Time-Givers (pp. 29-31)
Authors: Etienne Challet
Abstract: Main biological functions are time-controlled by a circadian system synchronized by environmental factors, especially ambient light. The master clock localized in the suprachiasmatic nucleus of the hypothalamus controls the phase of secondary clocks in the brain and in peripheral organs, such as liver, heart and adrenal glands. Timed meal can induce phase-shifts of most of the secondary clocks, but not the master clock, which is mainly reset by light (Figure 1).
Hormonal rhythms are tightly regulated by the circadian system, including temporal cues coming from the master clock and from local clocks in the endocrine glands. Glucocorticoid secretion from the adrenal glands is opposite in phase between diurnal and nocturnal animals, with daily peak at dawn and dusk, respectively . Melatonin, sometimes called the hormone of darkness, is always secreted from the pineal gland at night in both diurnal and nocturnal animals . Adipocyte-derived leptin is rhythmically released in plasma with low and high nocturnal levels in diurnal and nocturnal rodents, respectively .
This view point aims to emphasize that, besides being clock-controlled, most hormonal rhythms can exert feedback action on the circadian clocks, and to some extent modulate the phase and/or the amplitude of their oscillations.
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