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Anti-Emetics Mechanism of Action

Anti-emetics: Mechanism of action Migraine, chemotherapy, post-operative, gastroenteritis nausea Chemotherapy, post-operative, gastroenteritis nausea Vestibular nausea Chemotherapy nausea D2 receptor antagonists (e.g. metoclopramide) Migraineurs are hypersensitive to dopamine D2 receptor antagonists bind to D2 (dopamine) receptors in brain ↓ Binding of endogenous dopamine to dopamine receptors Emetogenic stimuli (e.g. chemotherapy) in blood and cerebrospinal fluid Chemoreceptor trigger zone in area postrema of 4th ventricle that lies outside blood-brain barrier Emetogenic stimuli in the bloodstream stimulate dopamine receptors in area postrema D2 receptor antagonists bind to D2 (dopamine) receptors in chemoreceptor trigger zone ↓ Binding of dopamine in area postrema 5HT-3 receptor antagonists (e.g. ondansetron) Emetogenic stimuli (e.g. chemotherapy) in blood and cerebrospinal fluid Area postrema has no blood- brain barrier Emetogenic stimuli in the bloodstream stimulate serotonin receptors in area postrema 5HT-3 receptor antagonists bind to 5HT-3 (serotonin) receptors in chemoreceptor trigger zone in area postrema of 4th ventricle ↓ Binding of serotonin in area postrema Emetogenic stimuli (e.g. toxins, chemotherapy) and pathologies in gastrointestinal (GI) tract ↑ Local release of serotonin from enterochromaffin cells of GI tract 5HT-3 (serotonin) receptor antagonists bind to 5HT-3 receptors on vagal afferent (sensory) fibres in GI tract ↓ Binding of endogenous serotonin to vagal afferent fibres ↓ Innervation of medulla via vagal afferent fibres H1 receptor antagonists (e.g. dimenhydrinate) Brain receives conflicting information about body’s movement from visual, vestibular, proprioceptive senses Motion sickness (nausea triggered by slow lateral or vertical movement) ↑ Activation of histaminergic neuronal system in hypothalamus Antihistamine H1 receptor antagonists bind to H1 (histamine) receptors in chemoreceptors trigger zone in area postrema ↓ Binding of endogenous histamine in area postrema Muscarinic antagonists (e.g. scopolamine) Motion sickness activates vestibular afferent fibres Vestibular afferents release acetylcholine Muscarinic receptor antagonists bind to muscarinic receptors in posterior cerebellum ↓ Binding of endogenous acetylcholine to posterior cerebellum ↓ Diffusion of acetylcholine into 4th ventricle ↓ Binding of acetylcholine to area postrema Cannabinoids* (e.g. synthetic tetrahydrocannabinol) Emetogenic stimuli (e.g. chemotherapy) in blood and CSF Area postrema has no blood- brain barrier Emetogenic stimuli stimulate serotonin receptors in area postrema Cannabinoids bind to 5HT-3 (serotonin) receptors in area postrema ↓ Binding of dopamine to area postrema *Still under research ↓ Stimulation of vomiting center in medulla by emetogenic stimuli Coordination of respiratory, gastrointestinal, abdominal muscle contraction ↓ Vomiting Authors: Catherine Jung Reviewers: Claire Song Shahab Marzoughi Sylvain Coderre* * MD at time of publication Legend: Pathophysiology Mechanism Sign/Symptom/Lab Finding Complications Published Aug 25, 2024 on www.thecalgaryguide.com