Many animals use the variation in duration of melatonin production each day as a seasonal clock.[20] In animals including humans[21] the profile of
melatonin synthesis and secretion is affected by the variable duration of night in summer as compared to winter. The change in duration of secretion
thus serves as a biological signal for the organization of daylength-dependent (photoperiodic) seasonal functions such as reproduction, behavior, coat
growth and camouflage coloring in seasonal animals.[21] In seasonal breeders that do not have long gestation periods and that mate during longer
daylight hours, the melatonin signal controls the seasonal variation in their sexual physiology, and similar physiological effects can be induced by
exogenous melatonin in animals including mynah birds[22] and hamsters.[23]
In mammals, melatonin is biosynthesized in four enzymatic steps from the essential dietary amino acid tryptophan, with serotonin produced at the third
step. Melatonin is secreted into the blood by the pineal gland in the brain. Known as the "hormone of darkness," it is secreted in darkness in both
day-active (diurnal) and night-active (nocturnal) animals.[24] It may also be produced by a variety of peripheral cells such as bone marrow
cells,[25][26] lymphocytes, and epithelial cells. Usually, the melatonin concentration in these cells is much higher than that found in the blood, but
it does not seem to be regulated by the photoperiod.
|
