Histamine

Histamine is formed from the amino acid histidine and is stored in high concentrations in vesicles in mast cells, enterochromaffin cells in the gut, some neurons, and a few other cell types.
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Histamine H1 receptor—This Gq-coupled receptor is important in smooth muscle effects, especially those caused by IgE-mediated responses. Inositol trisphosphate (IP3) and diacylglycerol (DAG)are the second messengers.
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H2 receptor—This Gs-coupled receptor mediates gastric acid secretion by parietal cells in the stomach. It also has a cardiac stimulant effect. A third action is to reduce histamine release from mast cells—a negative feedback effect.
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H3 receptor—This Gi-coupled receptor appears to be involved mainly in presynaptic modulation of histaminergic neurotransmission in the central nervous system (CNS).