Antihistamines encyclopedia topics

Clinical: H₁- and H₂-receptor antagonists

H₁-receptor antagonists

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. Clinically, H₁ antagonists are used to treat allergic reactions.

Examples:

Diphenhydramine
Loratadine
Desloratadine
Meclizine
Quetiapine (this is a side-effect of a drug usually used as an antipsychotic)
Fexofenadine
Pheniramine
Cetirizine

H₂-receptor antagonists

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:

Experimental: H₃- and H₄-receptor antagonists

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.

H₃-receptor antagonists

Examples:

H₄-receptor antagonists

Examples:

Others

Inhibitors of histamine release

These agents (mast cell stabilizers) appear to stabilize the mast cells to prevent degranulation and mediator release.

Examples:

Cromoglicate (cromolyn)
Nedocromil

Other agents with antihistaminergic activity

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.