Trigger zone

The area postrema is a circumventricular brain region positioned on the dorsal surface of the medulla on the floor of the fourth ventricle. The blood-brain barrier and the cerebrospinal fluid-brain barrier are absent in this region and consequently many substances that do not pass across capillaries in other regions of the brain can do so in the area postrema. The chemoreceptor trigger zone (CTZ), located in the lateral area postrema is sensitive to blood-borne emetogens. Nerves from the CTZ connect with the vomiting centre. 

ExCDDIs certainly improve the efficacy and duration of action of levodopa so that it can be given in a smaller dose (e.g. 25%) and generally in a 4 1 ratio, levodopa ExCDDI. As might be expected, some DA side-effects such as dyskinesia and psychoses are worse, but hypotension is less (no peripheral effects of DA) and vomiting is actually much reduced or abolished. This is because the chemoreceptor trigger zone of the vomiting centre while in the brain is on the blood side of the blood-brain barrier and will not be stimulated since no DA is formed peripherally (Fig. 15.5). That an... 

With most DA agonists there are the other expected signs of increased DA activity such as hallucinations, psychosis and hypotension which can be worse than with levodopa. Fortunately vomiting can be countered by giving the DA antagonist domperidone. This does not cross the blood-brain barrier and so counteracts only the peripheral (chemoreceptor trigger zone) effect of the DA agonist (Fig. 15.5). 

Opiates activate the chemoreceptor trigger zone in the medulla (by disinhibition) to cause nausea and vomiting, and cough suppression also occurs because of the inhibitory effects of opiates on the brainstem nuclei in the cough reflex pathway. Dextromethorphan is the non-opiate isomer of the opiate levorphanol and is an effective cough suppressant. 

Chemoreceptor trigger zone Located in the area postrema of the fourth ventricle of the brain, it is exposed to cerebrospinal fluid and blood and is easily stimulated by circulating toxins to induce nausea and vomiting. 

The answer is d. (Hardman, p 414.) Antipsychotic agents, particularly prochlorperazine, are also useful as anti emetic agents, thought to be due to dopamine blockade at the stomach and at the chemoreceptor trigger zone of the medulla. 

Stimulation of chemoreceptor trigger zone Nausea, vomiting... 

Motilium contains domperidone, v/hich is a dopamine antagonist and acts on the chemoreceptor trigger zone. It is ineffective in motion sickness. Stugeron contains cinnarizine Avomine and Phenergan contain promethazine and Kwells contains hyoscine hydrobromide. Cinnarizine and promethazine are antihistamines, which are indicated in motion sickness and hyoscine hydrobromide is an antimuscarinic agent that is also used in motion sickness. 

Metoclopramide is ineffective in motion sickness, as it acts selectively on the chemoreceptor trigger zone. Metoclopramide is effective in treating vomiting associated with gastroduodenal, biliary and hepatic disease, and postoperative vomiting. 

Metoclopramide is a dopamine receptor antagonist, which acts selectively on the chemoreceptor trigger zone. 

Q53 Domperidone may be used to prevent motion sickness. Domperidone is a dopamine antagonist that acts at the chemoreceptor trigger zone. 

Domperidone is a dopamine antagonist that acts on the chemoreceptor trigger zone. It can therefore be used as an anti-emetic in nausea and vomiting, for example, to counteract side-effects of cytotoxic therapy and to treat nausea associated with dopaminergic drugs used in Parkinson s disease. Unlike hyoscine butlybromide and antihistamines, domperidone is ineffective in motion sickness. 

Pharmacokinetics Mechanism is obscure, but may be mediated through the chemoreceptor trigger zone direct impulses to vomiting center are not inhibited. 

Metoclopramide (Region, Clopro, Octamide) [Antiemetic/ Dopamine Antagonist] Uses Tx NA Action T UGI motility, blocks dopamine in chemoreceptor trigger zone Dose Adults. 10 mg IV, 10-20 mg IM Peds. 1-2 mg/kg IV/IM Caution [B, -] Drugs w/ extrapyramidal ADRs Contra ... 

Uses NA Action -I- Medullary chemorec tor trigger zone Dose Adults. 

Antipsychotic agents are also used for a diversity of other indications like hiccups, Tourette s syndrome, aggressive behavior in children and the elderly and alcohol withdrawal syndrome. Some of them are also used in anesthesia as they can potentiate the sedative, analgesics or anesthetic effects of other agents. Antipsychotics which are mainly active by blocking dopamine activity have also an effect on chemoreceptor trigger zone and may therefore be used as anti-emetics. 

The vomiting centre in the hypothalamus receives impulses from the chemo-effector trigger zone (CETZ) and from cortical centres such as emotional, visual and olfactory areas, and from peripheral sources, including the inner ear and gastrointestinal tract. While mainly muscarinic mechanisms operate at the vomiting centre, dopaminergic mechanisms predominate at the CETZ. The CETZ can be stimulated by a multitude of xenobiotics, including medicines. 

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