Substance P (SP), Neurokinin A (NKA) and Neurokinin B (NKB) are all peptides belonging to the Tachykinin protein family. These three peptides which demonstrate a quite heterogeneity in their distribution exert their effect via three receptors: Neurokinin 1-3 receptors, members of the G-protein coupled receptor superfamily. However, Neurokinin 1 Receptor (NK₁) preferentially binds Substance P, Neurokinin 2 Receptor (NK₂) to NKA and Neurokinin 3 Receptor (NK₃) to NKB.