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A Review of Endogenous and Exogenous Melatonin
Melatonin was first isolated from the bovine pineal gland in 1958.1 In humans, it is the main hormone synthesized and secreted by the pineal gland. It is produced from a pathway that includes both tryptophan and serotonin. Melatonin displays high lipid and water solubility, which allows it to diffuse easily through most cell membranes, including the blood-brain barrier. Its half-life is about 30 minutes, and it is cleared mostly through the liver and subsequently excreted in the urine as urinary 6-sulfatoxymelatonin.
In humans and most diurnal mammals, melatonin is secreted at night with a robust circadian rhythm and maximum plasma levels that occur around 3 to 4 am. The daily rise of melatonin secretion correlates with a subsequent increase in sleep propensity about 2 hours before the person's regular bedtime. The time before this secretion is the least likely for sleep to occur, and when it starts, the propensity for sleep increases greatly as the "sleep gate" opens. The rhythmic release of melatonin is regulated by the central circadian rhythm generator- the suprachiasmatic nucleus (SCN) of the anterior hypothalamus.
Most of the chronobiotic and hypnotic effects of melatonin are mediated through 2 receptors: MT1 and MT2. Both subtypes have high density in the SCN, but they are also spread throughout other sites in the brain and other organs, which indicates that melatonin likely affects other biological systems. Given this distribution, it is not surprising that melatonin appears to have a number of effects on human biology that have not been fully elucidated, including regulating the sleep-wake cycle and acting as a neurogenic/neuroprotective agent.
It appears that the function of melatonin is to mediate dark signals and provide night information, a "hormone of darkness," rather than be the hormone of sleep. It has also been thought to be an "endogenous synchronizer" that stabilizes and reinforces various circadian rhythms in the body.2 Although direct hypnotic effects have been seen, melatonin's effect on sleep appears more involved in the circadian rhythm of sleep-wake regulation. The phase shifting effects of melatonin appear to be due to the MT2 receptor, while the MT1 receptor is more related to sleep onset.
Melatonin and the circadian rhythm of the sleep-wake cycle
The daily...