Serotonin (5-hydroxytryptamine; 5-HT) produces its effects through a variety of membrane-bound receptors both in the central and peripheral nervous system (CNS/PNS) as well as in several non-neuronal tissues (e.g., gastrointestinal tract, cardiovascular system, and blood).
The serotoninergic system is one of the oldest neurotransmitter/hormone systems in evolution; this may explain why 5-HT interacts with such a diversity of receptors of both the 7-transmembrane receptors and the ligand-gated ion channels superfamilies, similarly to acetylcholine, GABA, and glutamate, but with a larger number and diversity of receptor subtypes. 5-HT was discovered in the gut in the 1930s and called enteramine, and then rediscovered in the 1940s in the blood and called serotonin, based on its vasoconstrictor features.