Histamine and antihistamines

Histamine is one of the most extensively studied biological amines in medicine. It stimulates smooth muscle contraction and gastric acid secretion, increases vascular permeability, functions as a neurotransmitter, and plays various roles in immunomodulation, allergy, inflammation, haematopoiesis and cell proliferation. Histamine exerts its effects through four receptors, designated H1-H4. H1 and H2 receptors are widely distributed, H3 receptors are mainly presynaptic, and H4 receptors are mainly haematopoietic. H1 antihistamines are classified as first- and second-generation compounds. First-generation compounds lack specificity and cross the blood-brain barrier causing sedation. Second-generation compounds are less sedating and highly specific. H1 antihistamines have well-documented anti-allergic and anti-inflammatory effects and are well established in the treatment of a variety of allergic disorders. First-generation antihistamines are also used in the treatment of vestibular disorders and can be used as sedatives, sleeping aids and anti-emetics. H2 antihistamines are widely used in the treatment of gastric acid-related disorders; however, proton pump inhibitors are becoming the drugs of first choice in some of these disorders. H3 antihistamines are expected to be of potential value in the treatment of some cognitive disorder. H4 antihistamines could be of potential therapeutic benefit in the management of various immune and inflammatory disorders.