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Cetirizine crosses the blood–brain barrier only slightly, and for this reason, produces minimal sedation compared to many other antihistamines. [28] A positron emission tomography (PET) study found that brain occupancy of the H 1 receptor was 12.6% for 10 mg cetirizine, 25.2% for 20 mg cetirizine, and 67.6% for 30 mg hydroxyzine. [29]
The H 1 receptor is a histamine receptor belonging to the family of rhodopsin-like G-protein-coupled receptors. This receptor is activated by the biogenic amine histamine . It is expressed in smooth muscles , on vascular endothelial cells , in the heart, and in the central nervous system .
H1 receptors are linked to allergic responses, H2 to gastric acid regulation, H3 to neurotransmitter release modulation, and H4 to immune system function. There are four known histamine receptors: H 1 receptor H1 Receptors: These receptors are primarily located on smooth muscle cells , endothelial cells, and neurons.
H 1-antihistamines work by binding to histamine H 1 receptors in mast cells, smooth muscle, and endothelium in the body as well as in the tuberomammillary nucleus in the brain. Antihistamines that target the histamine H 1-receptor are used to treat allergic reactions in the nose (e.g., itching, runny nose, and
These agents also commonly have action at α-adrenergic receptors and/or 5-HT receptors. This lack of receptor selectivity is the basis of the poor tolerability profile of some of these agents, especially when compared with the second-generation H 1-antihistamines. Patient response and occurrence of adverse drug reactions vary greatly between ...
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Diphenhydramine is a potent antimuscarinic (a competitive antagonist of muscarinic acetylcholine receptors) and, as such, at high doses can cause anticholinergic syndrome. [69] The utility of diphenhydramine as an antiparkinson agent is the result of its blocking properties on the muscarinic acetylcholine receptors in the brain.
Levocetirizine is an antihistamine. It acts as an inverse agonist that decreases activity at histamine H1 receptors. This in turn prevents the release of other allergy chemicals and increases the blood supply to the area, providing relief from the typical symptoms of hay fever.