Histamine Receptor Antagonists

This is the parent class for drugs said to block a histamine receptor of any subtype. Histamine acts through distinct G protein-coupled receptors with different clinical meaning: through H1 it contracts smooth muscle, increases capillary permeability and mediates central neurotransmission regulating circadian rhythms, emotional and locomotor activities and cognitive functions; through H2, predominantly in the gastric mucosa, it couples to Gs, raises cyclic AMP and drives parietal-cell hydrochloric acid secretion. The subtype blocked decides whether a drug is used as an antihistamine or as a gastric acid suppressant, so prefer the specific H1 or H2 class wherever the subtype is known. Note that the cited parent class is broader than its own name: among its 157 members are benztropine, hyoscyamine, scopolamine and methantheline, whose primary action is antimuscarinic, and pitolisant, an H3 inverse agonist rather than a plain antagonist.