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Antihistamines |
A histamine antagonist is an agent which serves to inhibit the release or action of histamine. Antihistamine can be used to describe any histamine antagonist, but it is usually reserved for the classical antihistamines that act upon the H1 histamine receptor.
Antihistamines are used as treatment for allergies. Allergies are caused by an excessive response of the body to allergens, materials that are sometimes omnipresent, such as the pollen released by grasses and trees. An allergic reaction indicates an excessive release, by the body, of histamines.
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In common use, the term antihistamine refers only to H1 antagonists, also known as H1 antihistamines. It has been discovered that these H1-antihistamines are actually inverse agonists at the histamine H1-receptor, rather than antagonists per se.1 Clinically, H1 antagonists are used to treat allergic reactions.
Examples:
H2 antagonists, like H1 antagonists, are also inverse agonists and not true antagonists. H2 histamine receptors are found principally in the parietal cells of the gastric mucosa. H2 antagonists are used to reduce the secretion of gastric acid, treating gastrointestinal conditions including peptic ulcers and gastroesophageal reflux disease.
Examples:
These are experimental agents and do not yet have a defined clinical use, although a number of drugs are currently in human trials. H3-antagonists have a stimulant and nootropic effect, and are being investigated for the treatment of conditions such as ADHD, Alzheimer's Disease and schizophrenia, while H4-antagonists appear to have an immunomodulatory role and are being investigated as antiinflammatory and analgesic drugs.
Examples:
Examples:
These agents (mast cell stabilizers) appear to stabilize the mast cells to prevent degranulation and mediator release.
Examples:
Many drugs, used for other indications, possess unwanted antihistaminergic activity. These include tricyclic antidepressants and antipsychotics. One drug initially developed as an antipsychotic, but now commonly used for its antihistaminic effect, is promethazine.
Large doses of vitamin C are known to alleviate shock by inhibiting deaminizing proteins which release histamine. 2
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