Histamine Neurotransmitter

Structure and properties

Histamine (C₅H₁₀N₃, M_r = 111.15 g/mol) is a biogenic amine (decarboxylated histidine). It is formed by L-histidine decarboxylase and plays a central role as a mediator in allergic reactions and in the immune system. It is found in mast cells, basophils, platelets and some neurons, where it is stored in vesicles and secreted from them. In drugs, it is present as hydrochloride or phosphate.

Receptors

H₁ to H₄

Effects

• Dilatation of the vessels
• Increased capillary permeability
• Smooth muscle contraction, e.g. bronchoconstriction, uterine contraction.
• Increase in mucus production
• Oedem
• Itching, pain
• Redness of the skin
• Increased gastric acid production in the vestibular cells
• Tachycardia, hypotension, cardiac arrhythmias.

Degradation of histamine

• Oxidative deamination by diaminooxidase (DAO, formerly histaminase), extracellularly.
• Ring methylation by histamine N-methyltransferase (HNMT), intracellular.

Indications

• As rubefaciens
• In homeopathy against allergy

Pathophysiology

• Allergic diseases: Hay fever, urticaria, et al.
• Pseudoallergy (food intolerance): histamine intolerance.
• Histamine intoxication when consuming fish that is spoiled (common in tuna, mackerel).

Antagonists:

• Antihistamines
• Mast cell stabilizers
• Capsaicin