Nausea is a complex process that is intended to protect the body by preventing potentially toxic substances from being vomited and entering the body. In addition, vomiting can occur during cancer treatment and chemotherapy (cytostatic-induced vomiting), after operations (post-operative), during pregnancy and kinetosis (motion sickness) and can be treated with antiemetics. Several regions of the body are involved in the nausea.
One of them is the area postrema. This is a brain region in the brain stem which, together with other nuclei, form the nausea centre. It has the chemo-receptive trigger zone (CTZ).
This is a group of neurons that has receptors located in front of the blood–brain barrier. Normally, the brain is surrounded by a layer of very dense cell assemblies (blood-brain barrier), which prevents toxins from entering the brain. To ensure that the brain is nevertheless informed about toxic substances that have entered the bloodstream, there are the circumventricular organs (a group of specialized brain areas that have contact with the bloodstream, i.e. not through the blood-brain barrier).
These include the area postrema as part of the vomiting centre. The gastrointestinal tract has afferent nerves that run to the brain and activate the vomiting centre via the nucleus tractus solitarii (NTS, a brain region that is closely involved in the choking and vomiting reflex) in the presence of toxins. The organ of equilibrium and the inner ear activate the vomiting centre in the presence of kinetoses (motion sickness, motion sickness).
Active ingredients and mode of action
So-called D2-receptor antagonists act centrally (in the brain) by inhibiting the dopamine receptors of the area postrema and peripherally by activating the upper digestive tract. They can be divided into antiemetics that cross the blood-brain barrier (perphenazine, alizapride, droperidol) and those that act mainly peripherally, outside the central nervous system (metoclopramide and domperidone). Metoclopramide (MCP) also acts as a 5-HT3 receptor antagonist and increases stomach and small intestine motility (increased number of muscle movements).
They can be used for kinetosis, post-operative vomiting and cytostatic-induced vomiting (vomiting caused by chemotherapy). H1 receptor antagonists block type 1 histamine receptors and are also used in the treatment of allergies and as a hypnotic (drug that promotes sleep and calming). Antiemetics used include promethazine and diphenhydramine, which are among the first generation histamine antagonists (these reach the vomiting centre via the blood-brain barrier).
Ondansetron, Granisetron, Tropisetron are used for post-operative vomiting and cytostatic-induced vomiting. NK1 receptor antagonists inhibit the neurokinin receptor 1 in the vomiting centre. The only active substance currently used is aprepitant.
It is usually used in combination with 5-HT3 antagonists and dexamethasone. Apreptinate must not be used during pregnancy and lactation. The main effect of glucocorticoids (e.g. dexamethasone) is not the antiemetic effect, although they can be used for this purpose. The antiemetic effect of glucocorticoids has not been conclusively clarified.