“Serotonin is a neurotransmitter that carries information across the synapse between one neuron and another. It is found in brain, gut, pineal gland, and blood vessels and causes blood vessels to narrow, reduces aggression, affects color pigmentation via melatonin, affects mood, emotion, sleep, appetite metabolite 5-HTP (excitotoxins also use 5-HTP receptors) and helps reduce pain.”
From the Third Edition of The Practicing Herbalist By Margi Flint