Mechanism of Action
Histamine exerts its actions by combining with specific cellular histamine receptors. The four histamine receptors that have been discovered in humans and animals are designated H1 through H4, and are all G protein-coupled receptors (GPCR). Histamine biology is a series of weak interactions. In all of the known physiological reactions, the histamine backbone is unchanged.
In the H2 receptor mechanism, histamine is protonated at the end-chain amine group. This amine group interacts with aspartic acid in the transmembrane domains of cells. The other nitrogens in the molecule interact with threonine and aspartic acid in different transmembrane domains. This is a three pronged interaction. It brings the transmembrane domains close to each other, causing a signal transduction cascade.
Histamine receptors in insects, like Drosophila melanogaster, are histamine-gated chloride channels that function in inhibition of neurons. Histamine-gated chloride channels are implicated in neurotransmission of peripheral sensory information in insects, especially in photoreception/vision. Two receptors subtypes have been identified in Drosophila, HClA and HClB. There are no known GPCRs for histamine in insects.
| Type | Location | Function |
| H1 histamine receptor | Found on smooth muscle, endothelium, and central nervous system tissue | Causes, bronchoconstriction, bronchial smooth muscle contraction, vasodilation, separation of endothelial cells (responsible for hives), and pain and itching due to insect stings; the primary receptors involved in allergic rhinitis symptoms and motion sickness; sleep and appetite suppression. |
| H2 histamine receptor | Located on parietal cells and vascular smooth muscle cells | Primarily involved in vasodilation. Also stimulate gastric acid secretion |
| H3 histamine receptor | Found on central nervous system and to a lesser extent peripheral nervous system tissue | Decreased neurotransmitter release: histamine, acetylcholine, norepinephrine, serotonin |
| H4 histamine receptor | Found primarily in the basophils and in the bone marrow. It is also found on thymus, small intestine, spleen, and colon. | Plays a role in chemotaxis. |
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