histamine

n. (C₅H₉N₃) A chemical synthesized from the amino acid histidine and present in most body tissues which has four major receptor sites in the body. The first type of receptors (HI) occur in smooth muscles and the endothelium, where they cause vasodilation, smooth muscle contraction, swelling, rhinitis, pain, and itching. The second type of receptors (H2) are in the parietal lobe of the brain, where they are associated with the release of gastric acid. The third (H3) type of receptors are located throughout the brain and are associated with lowering the levels of acetylcholine, norepinephrine, and serotonin. The activity of the fourth (H4) sites, located in the thymus, small intestine, spleen, and colon, is not understood.