Histamine is a ubiquitous messenger molecule released from mast cells , enterochromaffin-like cells , and neurons. Its various actions are mediated by histamine receptors H1 , H2 , H3 and H4. The protein encoded by this gene is an integral membrane protein and belongs to the G protein-coupled receptor superfamily. It mediates the contraction of smooth muscles , the increase in capillary permeability due to contraction of terminal venules , the release of catecholamine from adrenal medulla , and neurotransmission in the central nervous system. It has been associated with multiple processes , including memory and learning , circadian rhythm , and thermoregulation. It is also known to contribute to the pathophysiology of allergic diseases such as atopic dermatitis , asthma , anaphylaxis and allergic rhinitis. Multiple alternatively spliced variants , encoding the same protein , have been identified. [provided by Ref
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