Mechanism of action of a 5-HT3 antagonist
Binding of an agonist at the 5-HT binding site causes a conformational change and activation of the 5-HT3 receptor. As a ligand gated ion channel this permits the movement of positively charged ions from the synaptic cleft into the cytoplasm. Binding of an antagonist at the 5-HT binding site prevents this activation and cell depolarisation is inhibited.
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References
Histamine, serotonin and the ergot alkaloids. In: Basic and clinical pharmacology, 8th edition. Katzung BG. USA: The McGraw Hill Companies, Inc, 2001:265–291.