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Antihistamine: Difference between revisions
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[[File:Histamine.svg|200px|thumbnail|The chemical structure of histamine]] | [[File:Histamine.svg|200px|thumbnail|The chemical structure of histamine]] | ||
'''Antihistamines''' are a class of substances that inhibit the action of [[histamine]]. Antihistamines are commonly used to relieve allergies and to [[sedation|promote sleep]].<ref>Sicherer, | '''Antihistamines''' are a class of substances that inhibit the action of [[histamine]]. Antihistamines are commonly used to relieve allergies and to [[sedation|promote sleep]].<ref>{{cite book | vauthors=((Sicherer, S. H.)) | date= 2006 | title=Understanding and managing your child’s food allergies | publisher=Johns Hopkins University Press | series=A Johns Hopkins Press health book | isbn=9780801884917}} | ||
</ref> Recreationally, very high doses of most first-generation antihistamines can be used to induce [[delirium]] and achieve a [[hallucinogenic]] effect in which the user sees and hears fully-formed, extremely convincing hallucinations. However, this experience is typically considered highly unpleasant by most users. | |||
H1 antihistamines are classified as first- and second-generation compounds. First-generation compounds cross the [[blood–brain barrier]] (BBB) causing sedation and they commonly cause antimuscarinic anticholinergic effects such as [[delirium]], dry mouth and dysfunctional urine voiding. Second-generation compounds cross the BBB to a minimal degree and are less sedating and do not cause delirium.<ref name="H1-antihistamines">Histamine and H1-antihistamines: Celebrating a century of progress | https://www.sciencedirect.com/science/article/pii/S0091674911014084</ref> | H1 antihistamines are classified as first- and second-generation compounds. First-generation compounds cross the [[blood–brain barrier]] (BBB) causing sedation and they commonly cause antimuscarinic anticholinergic effects such as [[delirium]], dry mouth and dysfunctional urine voiding. Second-generation compounds cross the BBB to a minimal degree and are less sedating and do not cause delirium.<ref name="H1-antihistamines">{{cite journal | vauthors=((Simons, F. E. R.)), ((Simons, K. J.)) | journal=Journal of Allergy and Clinical Immunology | title=Histamine and H1-antihistamines: Celebrating a century of progress | volume=128 | issue=6 | pages=1139-1150.e4 | date=1 December 2011 | url=https://www.sciencedirect.com/science/article/pii/S0091674911014084 | issn=0091-6749 | doi=10.1016/j.jaci.2011.09.005}}</ref> | ||
The toxicity of recreational antihistamine use is poorly understood, although there is some evidence that abuse may cause cognitive deficits and other health issues.<ref>Gray, S. L., Anderson, M. L., Dublin, S., Hanlon, J. T., Hubbard, R., Walker, R., ... | The toxicity of recreational antihistamine use is poorly understood, although there is some evidence that abuse may cause cognitive deficits and other health issues.<ref>{{cite journal | vauthors=((Gray, S. L.)), ((Anderson, M. L.)), ((Dublin, S.)), ((Hanlon, J. T.)), ((Hubbard, R.)), ((Walker, R.)), ((Yu, O.)), ((Crane, P. K.)), ((Larson, E. B.)) | journal=JAMA Internal Medicine | title=Cumulative Use of Strong Anticholinergics and Incident Dementia: A Prospective Cohort Study | volume=175 | issue=3 | pages=401 | date=1 March 2015 | url=http://archinte.jamanetwork.com/article.aspx?doi=10.1001/jamainternmed.2014.7663 | issn=2168-6106 | doi=10.1001/jamainternmed.2014.7663}}</ref> | ||
==Pharmacology== | ==Pharmacology== | ||
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Most antihistamines act as [[Agonist#Agonists|inverse agonists]] on histamine [[receptors]], meaning they inhibit the action of histamine by preventing it from binding to them. They may also inhibit the enzymatic activity of histidine decarboxylase which catalyzes the transformation of histidine into [[histamine]].{{citation needed}} | Most antihistamines act as [[Agonist#Agonists|inverse agonists]] on histamine [[receptors]], meaning they inhibit the action of histamine by preventing it from binding to them. They may also inhibit the enzymatic activity of histidine decarboxylase which catalyzes the transformation of histidine into [[histamine]].{{citation needed}} | ||
First-generation antihistamines readily cross the [[blood–brain barrier]] (BBB) and occupy H1-receptors located on postsynaptic membranes of histaminergic neurons throughout the CNS. Most of these drugs have antimuscarinic anticholinergic effects, some have [[adrenaline|alpha-adrenergic]] blocking effects, and others can inhibit both histamine and [[5-HT]] activity. Second-generation H1-antihistamineshave significantly less affinity for muscarinic cholinergic and alpha-adrenergic receptors and cross the BBB to a minimal degree, penetrate poorly into the CNS, and typically occupy fewer than 20% of CNS H1-receptors.<ref | First-generation antihistamines readily cross the [[blood–brain barrier]] (BBB) and occupy H1-receptors located on postsynaptic membranes of histaminergic neurons throughout the central nervous system (CNS). Most of these drugs have antimuscarinic anticholinergic effects, some have [[adrenaline|alpha-adrenergic]] blocking effects, and others can inhibit both histamine and [[5-HT]] activity. Second-generation H1-antihistamineshave significantly less affinity for muscarinic cholinergic and alpha-adrenergic receptors and cross the BBB to a minimal degree, penetrate poorly into the CNS, and typically occupy fewer than 20% of CNS H1-receptors.{{citation needed}} | ||
The promotion of sleep by antihistamines with sedative properties may partially be due to the antagonism of [[histamine]] receptors in the ventrolateral preoptic area (VLPO) located in the hypothalamus. When the VLPO is stimulated, it increases the frequency of [[GABA|GABAergic]] activity within other wake-promoting sites of the brain as an inhibitory process to wakefulness. In contrast, the VLPO is inhibited by [[histamine|histaminergic]] activity, primarily from the tuberomammillary nucleus (TMN); deactivating the VLPO in order to promote wakefulness (primarily in the transition from sleep to wake).<ref>{{cite journal|last1=Yu Wei|first1=Liu|last2=Jing|first2=Li|last3=Jiang-Hong|first3=Ye|title=Histamine regulates activities of neurons in the ventrolateral preoptic nucleus|journal=The Journal of Physiology|volume=588|issue=21|year=2010|pages=4103-4116|issn=0022-3751|doi=10.1113/jphysiol.2010.193904}}</ref> | |||
==Examples== | ==Examples== | ||
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*[[Meclozine]] | *[[Meclozine]] | ||
*[[Promethazine]] | *[[Promethazine]] | ||
*[[Alimemazine]] | |||
===Second-generation antihistamines=== | ===Second-generation antihistamines=== | ||
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[[Category:Antihistamine| ]] | [[Category:Antihistamine| ]] | ||
[[Category:Pharmacology]] | [[Category:Pharmacology]] | ||
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