Ondansetron mechanisms/effects

The vomiting centre and the nucleus of the tractus solitarius contain many muscarinic cholinergic and histamine receptors, and the CTZ is rich in dopamine receptors drugs that block these receptors are effective antiemetics. The precise role and location of S-HTj receptors (see ondansetron, below) in relation to emesis remains to be defined but both central and peripheral mechanisms may be involved. 

Phenothiazines such as prochlorperazine, thiethylperazine, and chlorpromazine see Chapter 18) are among the most commonly used antinauseants and antiemetics. Their principal mechanism of action is dopamine receptor antagonism at the CTZ. Compared to metoclopramide or ondansetron see above), these drugs do not appear to be as uniformly effective in cancer chemotherapy-induced emesis. On the other hand, they also possess antihistaminic and anticholinergic activities, which are of value in other forms of nausea, such as motion sickness. 

A placebo-controlled, crossover study in 26 healthy subjects found that both intravenous granisetron 3 mg and tropisetron 5 mg blocked the analgesic effect of a single 1 oral dose of paracetamol given 90 minutes later. The pharmacokinetics of paracetamol were unaffected by the two drugs. The interaction was thought to involve the serotonergic system, see Mechanism, in Opioids + Antiemetics Ondansetron , p.l61. 

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