Serotonin (5-hydroxytryptamine, 5-HT)

Function

Serotonin is a signaling hormone and neurotransmitter involved in mood regulation, gastrointestinal function, vascular signaling, platelet activity, appetite control, sleep-related physiology, and nervous system communication. Although serotonin is widely recognized for its central nervous system activity, most serotonin in the body is produced within the gastrointestinal tract where it regulates intestinal motility and digestive signaling.

Serotonin contributes to emotional processing, behavioral adaptation, satiety signaling, circadian coordination, and autonomic nervous system regulation. In peripheral tissues, serotonin participates in vascular tone control, platelet aggregation, smooth muscle contraction, and communication between enteric neurons and gastrointestinal tissues.

Production

Serotonin is synthesized from the amino acid tryptophan through conversion into 5-hydroxytryptophan by tryptophan hydroxylase followed by decarboxylation through aromatic amino acid decarboxylase. Enterochromaffin cells of the gastrointestinal tract produce the majority of systemic serotonin, while neurons within the raphe nuclei produce serotonin used for central nervous system signaling.

Platelets do not synthesize serotonin directly but absorb circulating serotonin and store it within granules for release during vascular injury and coagulation signaling. Serotonin production therefore reflects integration between gastrointestinal physiology, nutrient availability, and neural signaling systems.

Regulation

Serotonin synthesis is regulated by tryptophan availability, neuronal activity, circadian rhythm, gut microbiota interactions, inflammatory signaling, stress pathways, and enzymatic activity of tryptophan hydroxylase. Light exposure, sleep-wake timing, hormonal signaling, and autonomic nervous system activity also influence serotonergic pathways.

Serotonin acts through multiple receptor families including G-protein-coupled receptors and ligand-gated ion channels distributed throughout nervous tissue, gastrointestinal tract, vascular structures, and immune cells. Intracellular signaling pathways involve cyclic AMP systems, calcium signaling, and ion-channel modulation. Reuptake transporters and monoamine oxidase enzymes regulate serotonin clearance and signaling duration. Through these integrated endocrine and neural systems, serotonin coordinates mood-related signaling, gastrointestinal communication, vascular responsiveness, and circadian physiological adaptation.