<prompt> Which of the following selectively acts on 5HT3 receptors?OndansetronBupropionSumatriptanRenzapride  <response>: Ans. a Ondansetron Ref. KDT 6th163, 167.Ondansetron is a selective 5-HT3 antagonist which inhibits vomiting by blocking these receptors in brainstem as well as in gut wall. 2-Methyl 5-HT is a selective 5-HT3 agonist..5HT3 blockers Include:- Ondansetron, granisetron, dolasetron, palonosetron, and ramosetron. Palonosetron is the most potent 5HT3 blocker. Palonosetron has longest 12 life whereas Ondansetron has shortest 12 life. Dolasetron may prolong QT interval. Uses:- They are DOC for chemotherapy and radiotherapy induced vomiting.- Also effective in treating hyperemesis of pregnancy and posoperative nausea. Vomiting due to cisplatin most emetogenic anticancer drug can occur in 24 hrs or delayed by 2 days. DOC for former presentation is 5HT3 blocker whereas for the later DOC is aprepitant substance P antagonist. 