Ondansetron acts by:: (A) Action on CTZ, (B) 5-HT3 antagonism, (C) D1 and D2 receptor antagonism, (D) Increasing GIT motility
Answer is A. Ondansetron blocks the depolarizing action of serotonin through 5-HT3 receptors on vagal afferents in the gut as well as in NTS and CTZ. It donot block dopamine receptors or ACH receptors.( ref KDT 6/e p646)