Serotonin is a biologically important signalling molecule best known as a neurotransmitter in the central nervous system and a signalling amine in peripheral tissues. Its chemical name is 5‑hydroxytryptamine (5‑HT). Serotonin is synthesised from the essential amino acid tryptophan and participates in many physiological processes. In technical contexts it is described as a neurotransmitter, but that single label understates the variety of roles 5‑HT performs throughout the body.

Distribution and basic properties

Serotonin is present across vertebrate species and in many non‑vertebrate organisms. In humans the largest pool is found in the gastrointestinal tract, where enterochromaffin cells produce serotonin that influences gut motility and secretion. Another important peripheral reservoir is in blood platelets: platelets take up serotonin and release it during clotting, affecting vascular tone and haemostasis. A separate pool exists in the central nervous system, where serotonergic neurons modulate mood, sleep, appetite and cognition. Because serotonin does not readily cross the blood–brain barrier, central and peripheral pools are largely independent.

Synthesis, receptors and mechanisms

Biochemically, serotonin is produced by two enzymatic steps from tryptophan: first hydroxylation to 5‑hydroxytryptophan by tryptophan hydroxylase, then decarboxylation to 5‑HT. Serotonin signals through a diverse family of receptors (conventionally labelled 5‑HT1 through 5‑HT7), which include ion channels and G‑protein coupled receptors. This receptor diversity allows 5‑HT to produce different effects depending on cell type and location.

Physiological and clinical significance

Serotonin influences multiple organ systems. In the brain it is associated with mood regulation and is a therapeutic target in mood and anxiety disorders; many antidepressant drugs act by increasing synaptic serotonin levels (for example, selective serotonin reuptake inhibitors). Serotonin also affects sleep, appetite, memory and pain perception. Peripherally, it modulates gut motility, vascular tone and platelet function. Drugs that target specific serotonin receptors are used in clinical practice—for instance, triptans act on 5‑HT1 receptors to treat migraine, and 5‑HT3 antagonists are used to control chemotherapy‑related nausea.

History, ecology and notable facts

The substance now called serotonin was identified and characterised during the mid‑20th century; earlier work had noted a blood‑derived factor that constricted blood vessels and a peptide‑like compound in gut tissue, later recognised as the same molecule. Serotonin or serotonin‑like compounds are found in fungi, plants and invertebrates and can play ecological roles: in some seeds they may affect passage through animal guts, and in certain venoms serotonin contributes to pain and local tissue effects. Studies in a range of animals have also linked serotonin signalling to social behaviours and dominance hierarchies, illustrating the molecule's influence beyond simple mood modulation.

Safety, disturbances and distinctions

Because serotonin affects many systems, both deficiency and excess can cause problems. Low serotonergic function has been associated with mood disorders, though human behaviour is multi‑determined and causation is complex. Excessive serotonergic activity can produce serotonin syndrome, a potentially serious condition marked by autonomic instability and neuromuscular symptoms. Serotonin is often compared with other neurotransmitters such as dopamine: a useful distinction is that serotonin is broadly implicated in mood regulation, inhibition and internal state, while dopamine is central to reward, motivation and motor control—though the systems interact closely.

Further reading and resources