Which of the following selectively acts on 5HT3 receptors?

Ans. a (Ondansetron ) (Ref. KDT 6th/163, 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 1/2 life whereas Ondansetron has shortest 1/2 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).