Ondansetron's anti-emetic mechanism involves blockade of 5-HT3 receptors located at which sites?
- A Chemoreceptor trigger zone (area postrema) and vagal afferents in the gut only
- B 5-HT3 receptors on vagal afferents in the gut, nucleus tractus solitarius, and chemoreceptor trigger zone ✓
- C Only the vomiting centre in the lateral reticular formation
- D D2 receptors in the CTZ plus 5-HT3 receptors only in the small intestine
Correct answer: B. 5-HT3 receptors on vagal afferents in the gut, nucleus tractus solitarius, and chemoreceptor trigger zone
Explanation
Ondansetron selectively antagonises 5-HT3 receptors at three key sites involved in the emetic reflex: peripheral vagal afferent endings in the GI mucosa (stimulated by enterochromaffin cell-released serotonin), afferent fibres projecting to the nucleus tractus solitarius (NTS), and the CTZ/area postrema in the brainstem. Blocking all three sites prevents the afferent arc of the emetic reflex. Ondansetron does not block D2 receptors — that is the mechanism of metoclopramide/domperidone.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.