Histamine and antihistamines

Histamine is one of the most important and extensively studied biological molecules in the human body. It plays a constitutive role within almost every bodily system, but most notably within the stomach, where it regulates acid secretion, the central nervous system, where it acts as a neurotransmitter, the cardiovascular system, where it affects cardiac output and vascular permeability, and it has a well-established role in allergy and anaphylaxis. Histamine exerts its effects through four distinct receptor subtypes: H1, H2, H3 and H4. Predominantly, though not exclusively, these take the form of G-protein-coupled receptors. Clinically used antihistamines demonstrate inverse agonism to the histamine receptor and drugs are available with activity at H1, H2 and H3 receptors. H1 antihistamines are used in the treatment of allergy, and are classified as either first or second generation. First-generation antihistamines have significant sedative side effects. H2 antihistamines are predominantly used for the treatment of gastrooesophageal reflux and peptic ulcer disease; however, the most widely used of these, ranitidine, has been withdrawn from use due to (impurity related) safety concerns. H3 antihistamines have been explored for the treatment of neurological disease and to date the only licensed H3 antihistamine is used for the treatment of narcolepsy. Multiple uses have been suggested for H4 antihistamines, including immunomodulation, the treatment of asthma and even as an analgesic. However, no (commercially available) drug exists as of yet.