Structures rich in 5-HT are:

• Gastrointestinal tract (chromaffin cells and enteric neurons)
• Platelets
• Central nervous system

The metabolism of 5-HT closely parallels that of noradrenaline. Serotonin is formed from dietary tryptophan, which is converted to 5-hydroxytryptophan by tryptophan hydroxylase, then to 5-HT by a non-specific decarboxylase.

5-HT is transported into 5-HT containing cells by a specific transport system.
Degradation occurs mainly by monoamine oxidase (MAO), forming 5-HIAA, which is excreted in urine.

• increased gastrointestinal motility
• direct excitation of smooth muscle and indirect action via enteric neurons
• contraction of other smooth muscle (e.g. bronchi, uterus)
• vascular effects
  • constriction (direct and via sympathetic innervation)
  • dilatation (endothelium dependent)
• platelet aggregation
• stimulation of peripheral nociceptive nerve endings
• excitation/inhibition of CNS neurons
• increased microvascular permeability

• peristalsis
• vomiting
• platelet aggregation and haemostasis
• inflammatory mediator
• sensitisation of nociceptors
• microvascular control

• control of appetite
• sleep
• mood
• hallucinations
• stereotyped behaviour
• pain perception
• vomiting