Extended Signaling by Melatonin

The whole spectrum of melatonin's actions cannot be directly deduced from its primary signaling via its membranebound G protein-coupled receptors. This is not so much a matter of possibly existing other binding sites, but rather of the control of other important regulators by melatonin. Therefore, secondary signaling by factors influenced by melatonin has to be taken into consideration, such as other humoral factors, components of circadian oscillators, sirtuins, and microRNAs. Transmission of microRNAs by exosomes contributes to the actions of melatonin. In plants, melatoninbinding proteins are being investigated, but additionally interactions with phytohormones are likely. The recent demonstration of melatonin formation in mitochondria of both plants and mammals as well as the presence of a mammalian mitochondrial melatonin receptor open new aspects of melatonin's intracellular actions.