11.05.1 Synthesis, Receptors and Actions

5-HT is synthesised from the precursor tryptophan, an essential amino acid. Synthesised 5-HT is stored in granules which are released via exocytosis. Main storage sites include enterochromaffin cells (stomach and small intestine) and neurons both in the gastrointestinal tract (GIT) and central nervous system (CNS) in the raphe nucleus which is involved in the regulation of mood, appetite and sleep. 5-HT is actively transported into platelets (which do not synthesise 5-HT), then released upon platelet activation to induce vasoconstriction and assist haemostasis.

There are many subtypes of 5HT receptors, denoted 5-HT_1-7, all are GPCR except for 5-HT₃ which is a ligand-gated Na⁺-K⁺ channel. Many are sub-classified further. The effects of 5-HT include actions within the CNS leading to behavioural effects (sleep, anxiety, feeding) and excitation, constriction of cerebral blood vessels, contraction of GIT smooth muscle, gastric motility and peristalsis, centrally and peripherally-mediated emesis.