Serotonin (5-hydroxytryptamine (5-HT)) is a monoamine neurotransmitter that controls various behavioral processes, including circadian rhythms, sleep, mating behavior, learning, and aggression through its binding to the G-coupled serotonin receptors. It is primarily synthesized in the cytosol of serotonergic neurons from L-tryptophan and then loaded into synaptic vesicles. The first step is catalyzed by Trhn in neuronal tissues and by Hn in non-neuronal tissues. Serotonin is also the precursor of another monoamine neurotransmitter, melatonin, which regulates the photoperiod synchronization of physiological events by binding to melatonin receptors and may also act as a free radical scavenger and antioxidant. (Adapted from FBrf0255924.)