The tachykinin receptor 1 (TACR1) also known as neurokinin 1 receptor (NK1R) or substance P receptor (SPR) is a G protein coupled receptor found in the central nervous system and peripheral nervous system. The endogenous ligand for this receptor is Substance P, although it has some affinity for other tachykinins. The protein is the product of the TACR1 gene. Properties Tachykinins are a family of neuropeptides that share the same hydrophobic C-terminal region with the amino acid sequence Phe-X-Gly-Leu-Met-NH2, where X represents a hydrophobic residue that is either an aromatic or a beta-branched aliphatic. The N-terminal region varies between different tachykinins. The term tachykinin originates in the rapid onset of action caused by the peptides in smooth muscles. Substance P (SP) is the most researched and potent member of the tachykinin family. It is an undecapeptide with the amino acid sequence Arg-Pro-Lys-Pro-Gln-Gln-Phe-Phe-Gly-Leu-Met-NH2. SP binds to all three of the tachykinin receptors, but it binds most strongly to the NK1 receptor. Tachykinin NK1 receptor consists of 407 amino acid residues, and it has a molecular weight of 58,000. NK1 receptor, as well as the other tachykinin receptors, is made of seven hydrophobic transmembrane (TM) domains with three extracellular and three intracellular loops, an amino-terminus and a cytoplasmic carboxy-terminus. The loops have functional sites, including two cysteines amino acids for a disulfide bridge, Asp-Arg-Tyr, which is responsible for association with arrestin and, Lys/Arg-Lys/Arg-X-X-Lys/Arg, which interacts with G-proteins. The binding site for substance P and other agonists and antagonists is found between the second and third transmembrane domains. The NK-1 receptor is found on human chromosome 2 and is located on the cell's surface as a cytoplasmic receptor. Distribution and function The NK1 receptor can be found in both the central and peripheral nervous system. It is present in neurons, brainstem, vascular endothelial cells, muscle, gastrointestinal tracts, genitourinary tract, pulmonary tissue, thyroid gland and different types of immune cells. The binding of SP to the NK1 receptor has been associated with the transmission of stress signals and pain, the contraction of smooth muscles and inflammation. NK1 receptor antagonists have also been studied in migraine, emesis and psychiatric disorders. In fact, aprepitant has been proved effective in a number of pathophysiological models of anxiety and depression. Other diseases in which the NK1 receptor system is involved include asthma, rheumatoid arthritis and gastrointestinal disorders. Mechanism SP is synthesized by neurons and transported to synaptic vesicles;