The antiemetic effect of ondansetron in chemotherapy-induced vomiting is mediated by blocking:
- A Dopamine D2 receptors in the CTZ and stomach
- B 5-HT3 receptors in vagal afferents of the gut and the chemoreceptor trigger zone (area postrema) ✓
- C Histamine H1 receptors in the vestibular nuclei
- D Substance P/NK1 receptors in the nucleus tractus solitarius
Correct answer: B. 5-HT3 receptors in vagal afferents of the gut and the chemoreceptor trigger zone (area postrema)
Explanation
Cytotoxic chemotherapy triggers serotonin (5-HT) release from enterochromaffin cells in the gut mucosa; this activates 5-HT3 receptors on vagal afferent neurons (acute phase) and in the area postrema/CTZ (delayed phase). Ondansetron's highly selective 5-HT3 blockade at both peripheral and central sites dramatically reduces chemotherapy-induced nausea and vomiting. D2 antagonism is the mechanism of metoclopramide; NK1 blockade is the mechanism of aprepitant.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
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