Antihistamines are drugs which treat allergic rhinitis and other allergies. However, this is not always possible as some substances, such as pollen, are carried in the air, thus making allergic reactions caused by them generally unavoidable. The two largest classes of antihistamines are H 1 -antihistamines and H 2 -antihistamines. Antihistamines that target the histamine H 1 -receptor are used to treat allergic reactions in the nose e.
They are sometimes also used to treat motion sickness or vertigo caused by problems with the inner ear. Antihistamines that target the histamine H 2 -receptor are used to treat gastric acid conditions e. H 1 -antihistamines work by binding to histamine H 1 receptors in mast cellssmooth allergy med antihistamine antiand endothelium in the body as anti cholesterol medications as in the tuberomammillary nucleus in the brain; H 2 -antihistamines bind to histamine H 2 receptors in the upper gastrointestinal tractprimarily in the stomach.
Histamine receptors exhibit constitutive activityso antihistamines can function as either a neutral receptor antagonist or an inverse agonist at histamine receptors. Histamine produces increased vascular permeability, causing fluid to escape from capillaries into tissueswhich leads to the classic symptoms of an allergic reaction ó a runny nose and watery eyes. Histamine also promotes angiogenesis.
Antihistamines suppress the histamine-induced wheal response swelling and flare response vasodilation by blocking the binding of histamine to its receptors or reducing histamine receptor activity on nervesvascular smooth muscleglandular cells, endotheliumand mast cells. Itchingsneezingand inflammatory responses are suppressed by antihistamines that act on H1-receptors. H 1 -antihistamines refer to compounds that inhibit the activity of the H 1 receptor.
The vast majority of marketed H 1 -antihistamines are receptor antagonists and allergy med antihistamine anti a minority of marketed compounds are inverse allergy med antihistamine anti at the receptor.
Sedation is a common side effect of H 1 -antihistamines that readily cross the bloodóbrain barrier ; some of these drugs, such as diphenhydramine and doxylamineare therefore used to treat insomnia. A combination of these effects, and in some cases metabolic ones as well, lead to most first-generation antihistamines having analgesic-sparing potentiating effects on opioid analgesics and to some extent with non-opioid ones as well.
The most commonly used for the purpose include hydroxyzinepromethazine enzyme induction especially helps with codeine and similar prodrug opioidsphenyltoloxamineorphenadrineand tripelennamine ; some may also have intrinsic analgesic properties of their own, orphenadrine being an example. Second-generation antihistamines cross the bloodóbrain barrier to a much lower degree than the first-generation antihistamines.
Their main benefit is they primarily affect peripheral histamine receptors and therefore are less sedating. However, high doses can still induce drowsiness through acting on the central nervous system. Some second-generation antihistamines, allergy med antihistamine anti, notably cetirizinecan interact with CNS psychoactive drugs such as bupropion and benzodiazepines.
Examples of H 1 antagonists include:. The H 1 receptor inverse agonists include: H 2 -antihistamines, like H 1 -antihistamines, occur as inverse agonists and neutral antagonists.
They act on H 2 histamine receptors found mainly in the parietal cells of the gastric mucosa, which are part of the endogenous signaling pathway for gastric acid secretion, allergy med antihistamine anti.
Normally, histamine acts on H 2 to stimulate acid secretion; drugs that inhibit H 2 signaling thus reduce the secretion of gastric acid. H 2 -antihistamines are among first-line therapy to treat gastrointestinal conditions including peptic ulcers and gastroesophageal reflux disease. Some formulations are available over the counter. Most side effects are due to cross-reactivity with unintended receptors. Cimetidine, for example, is notorious for antagonizing androgenic testosterone and DHT receptors at high doses.
These are experimental agents and do not yet have a defined clinical use, allergy med antihistamine anti, although a number of drugs are currently [ when? H 3 -antihistamines have a stimulant and nootropic effect, whereas H 4 -antihistamines appear to have an immunomodulatory role. An H 3 -antihistamine is a classification of drugs used to inhibit the action of histamine at the H 3 receptor.
H 3 receptors are primarily found in the allergy med antihistamine anti and are inhibitory autoreceptors located on histaminergic nerve terminals, which modulate the release of histamine. Histamine release in the allergy med antihistamine anti triggers secondary release of excitatory neurotransmitters such as glutamate and acetylcholine via stimulation of H 1 receptors in the cerebral cortex.
Consequently, unlike the H 1 -antihistamines which are sedating, H 3 -antihistamines have stimulant and cognition-modulating effects. Inhibit the action of histidine decarboxylase:. Mast cell stabilizers are drugs which prevent mast cell degranulation. Currently most people who use an antihistamine to treat allergies use a second generation drug. The first generation of antihistamine drugs became available in the s. Antihistamines can vary greatly in cost, allergy med antihistamine anti.
The United States government removed two second generation antihistamines, terfenadine and astemizolefrom the market based on evidence that they could cause heart problems. Not much published research exists which compares the efficacy and safety of the various antihistamines available. Most studies of antihistamines reported on people who are younger, so the effects on people over age 65 are not as well understood. From Wikipedia, the free encyclopedia.
Antihistamine Drug class Histamine structure. World Allergy Organ J. The H1-receptor is a transmembrane protein belonging to the G-protein coupled receptor family, allergy med antihistamine anti. Signal transduction from the extracellular to the intracellular environment occurs as the GCPR becomes activated after binding of a specific ligand or agonist. Importantly, because antihistamines can theoretically behave as inverse agonists or neutral antagonists, they are more properly described as H1-antihistamines rather than H1-receptor antagonists.
Clinical and Experimental Allergy. Retrieved 8 October Int J Exp Pathol. The Journal of Allergy and Clinical Immunology. J Eur Acad Dermatol Venereol. Retrieved 28 January The Journal of Organic Chemistry. Neurophysiological characterization and broad preclinical efficacy in cognition and schizophrenia of a potent and selective histamine H3 receptor antagonist". The Journal of Pharmacology and Experimental Therapeutics. Allergy and asthma allergy med antihistamine anti. Clinical pharmacology in drug development.
Histamine receptor agonist Histamine receptor antagonist H 1 H allergy med antihistamine anti H 3. Opioid modulator Opioid receptor agonist Opioid receptor antagonist Enkephalinase inhibitor. Cofactor see Enzyme cofactors Precursor see Amino acids. Retrieved from " https: Use dmy dates from October All articles with vague or ambiguous time Vague or ambiguous time from June Views Read Edit View history.