Mechanism of action on ondensetron -

Ans. is 'd' i.e., 5HT3 antagonist Ondansetron* It is the prototype of a distinct class of antiemetic drugs developed to control cancer chemotherapy/radiotherapy induced vomiting, and later found to be highly effective in PONV and disease/drug 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 drugs/radiation 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.