Melatonin is a hormone involved in the synchronization of circadian rhythms in vertebrates. Melatonin synthesis is inhibited by light, for that reason the maximal level of the content of this hormone is found at night. When melatonin circadian rhythm is synchronized by photoperiod, its information about external environment is transmitted to circadian oscillators that contain the elements to detect the melatonin as melatonin membrane receptors. This signal could be a conservative pathway. We hypothesized the possibility that, in invertebrates, melatonin interacts with membrane receptors to send a signal of darkness to the circadian system.
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