Histamine receptor

The histamine receptors are a class of G protein–coupled receptors which bind histamine as their primary endogenous ligand.

Histamine receptors are proteins that bind with histamine, a neurotransmitter involved in various physiological processes. There are four main types: H1, H2, H3, and H4. H1 receptors are linked to allergic responses, H2 to gastric acid regulation, H3 to neurotransmitter release modulation, and H4 to immune system function.

There are four known histamine receptors:
 * H1 receptor    H1 Receptors: These receptors are primarily located on smooth muscle cells, endothelial cells, and neurons. Activation of H1 receptors mediates various responses, including smooth muscle contraction (leading to bronchoconstriction, intestinal cramping), increased vascular permeability (resulting in edema), and stimulation of sensory nerve endings (causing itching and pain). H1 antagonists, commonly known as antihistamines, are used to alleviate symptoms of allergies and allergic reactions.
 * H2 receptor  H2 Receptors: Found mainly in the stomach lining (parietal cells), H2 receptors regulate gastric acid secretion by stimulating the production of hydrochloric acid. H2 antagonists (H2 blockers) are used to reduce stomach acid production and treat conditions like gastroesophageal reflux disease (GERD) and peptic ulcers.
 * H3 receptor   H3 Receptors: These receptors are predominantly located in the central nervous system (CNS), particularly in regions associated with neurotransmitter release and modulation. H3 receptors act as presynaptic autoreceptors and heteroreceptors, regulating the release of neurotransmitters such as dopamine, serotonin, norepinephrine, and acetylcholine. Modulation of H3 receptors is being explored as a potential target for various neurological and psychiatric disorders.
 * H4 receptor   H4 Receptors: Initially discovered on immune cells, particularly mast cells, eosinophils, and T cells, H4 receptors are involved in immune responses, including chemotaxis (cellular movement in response to chemical signals) and cytokine production. These receptors play a role in inflammation and allergic reactions. Research on H4 receptors is ongoing to better understand their involvement in immune-related disorders and to develop potential therapeutic interventions.

Comparison
There are several splice variants of H3 present in various species. Though all of the receptors are 7-transmembrane g protein coupled receptors, H1 and H2 are quite different from H3 and H4 in their activities. H1 causes an increase in PIP2 hydrolysis, H2 stimulates gastric acid secretion, and H3 mediates feedback inhibition of histamine.