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The Chemoreceptor trigger zone (CTZ) is an area of the brain which receives inputs from blood-borne drugs or hormones, and communicates with the Vomiting center, to initiate vomiting.

The CTZ is close to the area postrema on the floor of the fourth ventricle and is (necessarily) outside of the blood-brain barrier.

The neurotransmitters implicated in the control of nausea and vomiting include acetylcholine, dopamine, Histamine (H-1 receptor), substance P (NK-1 receptor), and 5-HT (5-HT3 receptor). There are also opioid receptors present which may be the mechanism by which opiates cause nausea and vomiting.

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