Tachykinin receptors exert their effects through the binding of guanine nucleotide-binding regulator proteins (G-proteins). There are three tachykinin receptors; NK1, NK2, and NK3, and each has a preferred ligand; substance P (SP), neurokinin A (NKA), and neurokinin B (NKB), respectively. Tachykinins are widely distributed in the central nervous system, however, their expression patterns are distinct. NK1 and NK3 are widely distributed in the central nervous system, while NK2 is localized in the smooth muscle of the gastrointestinal, respiratory, and urinary tracts and some discrete regions of the CNS such as the prefrontal cortex and the hippocampus.
