There are three main classes of neurokinin receptors: NK1R (the substance P preferring receptor), NK2R, and NK3R. These tachykinin receptors belong to the class I (rhodopsin-like) G-protein coupled receptor (GPCR) family. The various tachykinins have different binding affinities to the neurokinin receptors: NK1R, NK2R, and NK3R. These neurokinin receptors are in the superfamily of transmembrane G-protein coupled receptors (GPCR) and contain seven transmembrane loops. Neurokinin-1 receptor interacts with the Gαq-protein and induces activation of phospholipase C followed by production of inositol triphosphate (IP3) leading to elevation of intracellular calcium as a second messenger. Further, cyclic AMP (cAMP) is stimulated by NK1R coupled to the Gαs-protein. The neurokinin receptors are expressed on many cell types and tissues.