How do 5-HT3 receptor antagonists work as antiemetics and a typical side effect?

• A. Block dopamine receptors in the CNS; sedation as side effect
• B. Block serotonin receptors in the chemoreceptor trigger zone and gut; headache or constipation is common
• C. Inhibit acetylcholine at muscarinic receptors; dry mouth as common
• D. Activate GABA receptors; dizziness as common

Answer: (B)

The main concept is that 5-HT3 receptor antagonists block serotonin signaling involved in triggering vomiting. Serotonin is released in the gut in response to stimuli like chemotherapy, and it activates 5-HT3 receptors on vagal afferents to send emetic signals to the brain. These drugs also block 5-HT3 receptors in the brain’s chemoreceptor trigger zone, where emetic pathways can be activated. By blocking these receptors both peripherally in the gut and centrally in the CTZ, they prevent the cascade that leads to nausea and vomiting, making them effective antiemetics, especially for chemotherapy-induced nausea.

A typical side effect you’ll often see with these agents is headache, along with constipation. Headache is a common CNS-related reaction, while blocking 5-HT3 signaling in the gut reduces motility, which can lead to constipation. This profile contrasts with choices that describe blocking dopamine receptors (which tends to cause sedation) or muscarinic acetylcholine receptors (which can cause dry mouth), or activating GABA receptors (which can cause dizziness).