<prompt> Mechanism of action on ondensetron -RANK ligand inhibitorMMDA antagonistNK 1 receptor antagonist5 HT3 antagonist  <response>: Ans. is d i.e., 5HT3 antagonist Ondansetron It is the prototype of a distinct class of antiemetic drugs developed to control cancer chemotherapyradiotherapy induced vomiting, and later found to be highly effective in PONV and diseasedrug associated vomiting as well. It blocks the depolarizing action of 5-HT exerted through 5-HT3 receptors on vagal afferents in the g.i.t. as well as in NTS and CTZ. Cytotoxic drugsradiation produce nausea and vomiting by causing cellular damage release of mediators including 5-HT from intestinal mucosa activation of vagal afferents in the gut emetogenic impulses to the NTS and CTZ. Ondansetron blocks emetogenic impulses both at their peripheral origin and their central relay. It does not block dopamine receptors. Apomorphine or motion sickness induced vomiting is not suppressed. A weak gastrokinetic action due to 5-HT3 blockade has been detected, but this is clinically insignificant. A minor 5-HT4 antagonistic action has also been shown, but seems to have no clinical relevance. 