Histamine is a ubiquitous messenger molecule released from mast cells, enterochromaffin-like cells, and neurons. Its various actions are mediated by histamine receptors Histamine H1 R, Histamine H2 R, Histamine H3 R and Histamine H4 R. HRH3 encodes one of the histamine receptors (H3) which belongs to the family 1 of G protein-coupled receptors. Histamine H3 R is an integral membrane protein and can regulate neurotransmitter release. It can also increase voltage-dependent calcium current in smooth muscles and innervates the blood vessels and the heart in cardiovascular system.