Central histamine function
Histamine is synthesised in brain from histidine by the enzyme histidine decarboxylase (HDC).
Histamine exerts its actions by binding one of the histamine receptors. The histamine receptors that have been discovered are all G protein-coupled receptors.
In the CNS, all histamine receptors H1, H2 and H3 occur. H1 is involved in regulation of vigilance, attention, sleep, feeding, and motion
Outside the brain the different histamine receptors fulfil other functions. H1 is important in inflammatory reactions (vasodilation, bronchoconstriction, pain, itching). The H2 receptor is localised in parietal cells in the stomach and plays a role in gastric acid release.