1. Biological amine
  1. Binds to, and activates 5-HT receptors.
  2. Tissue and organ effects as follows:
    • CVS
      • Potent vasoconstrictor, except in the heart and skeletal muscles, where it causes vasodilation.
      • Small positive inotropic and chronotropic action.
      • May elicit the chemoreceptor reflex/Bezold Jarish reflex, where serotonin activates 5HT3 receptors on the heart, leading to profound hypotension and bradycardia.
    • Immunological function
      • Found in platelets – assists with platelet aggregation
    • Nervous system
      • CNS: functions as a neurotransmitter
      • 5HT3 receptors participate in the vomiting reflex.
      • Implicated in depression/migraine. Also implicated in mood/pain perception/sleep
    • Respiratory
      • Causes bronchoconstriction and hyperventilation
    • GIT
      • Prokinetic and secretory effect, resulting in diarrhea.
  1. Formed from tryptophan
    • After formation, it is rapidly stored or broken down by MAO to 5HIAA.
  2. Serotonin can be found in
    • Enterochromaffin cells in the GIT (90%)
    • Platelets in blood stream
    • Serotonergic neurons in the CNS
  3. The amount of 5-HIAA may be used to detect carcinoid syndrome.
  1. Excessive serotonin leads to a potentially fatal syndrome called serotonin syndrome. It is a predictable (rather than idiosyncratic) reaction. It is similar in presentation to MH and NMS, but the pathophysiology and treatment are different.
Synd. Precipitant Presentation Therapy
SS SSRIs, MAOIs, triptans, ondansetron, St. John’s Wort, morphine, pethidine, linezolid, tramadol. HTN, hyper-reflexia, tremor, hyperthermia, diarrhea, mydriasis, agitation and coma (onset occurs within hours).Also clonus, increased bowel sounds. Sedation (BZDs), paralysis, intubation and sedation. Possibly cyproheptadine or chlorpromazine (5HT blockade)
NMS Anti-psychotic drugs that block D2. Acute, severe parkinsonism. Hypertension, hyperthermia, normal/reduced bowel sounds. Onset 1-3 days. Diphenhydramine, sedation and cooling.
MH Volatile anaesthetics, suxemethonium Hyperthermia, muscle rigidity, HTN, tachycardia. Onset within minutes. Dantrolene