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 thecontrol of other important regulators by melatonin. Therefore, secondary signaling by factors influenced by melatoninhas to be taken into consideration, such as other humoral factors, components of circadian oscillators, sirtuins, andmicroRNAs. 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 recentdemonstration of melatonin formation in mitochondria of both plants and mammals as well as the presence of amammalian mitochondrial melatonin receptor open new aspects of melatonin’s intracellular actions.