Abstract

Histamine, endogenously formed by decarboxylation of histidine, is stored preformed and released during allergic, reactions. Histamine is also responsible for gastric acid secretion, functions as a neurotransmitter in the brain, and may have a physiological role in the cardiovascular system. The actions of histamine are mediated through H₁, and H₃ receptors. The H₁ receptor mediates most of histamine's effects in allergic diseases and H₁ receptor antagonists are used in the treatment of such diseases as allergic rhinitis and urticaria. Classical H₁ antagonists, primarily all substituted ethylamines, cause sedation, whereas second-generation H₁ antagonists do not. Both types of H₁ antagonists are described. The most important effect mediated through H₂ receptors is gastric acid secretion. H₂ agonists and antagonists are described. The antagonists are available for the treatment of peptic ulcer disease and esophageal reflux. The H₃ receptor is present in the brain and in peripheral tissues modulating the release of a variety of neurotransmitters. H₃ agonists and antagonists are described. No clear therapeutic indications have been reported for H₃ ligands.