Tachykinins and tachykinin receptors: structure and activity relationships

In addition to the classical neurotransmitters, acetylcholine and noradrenaline, a wide number of peptides with neurotransmitter activity have been identified in the past few years. Among them, the tachykinins substance P (SP), neurokinin A (NKA) and neurokinin B (NKB) appear to act as mediators of nonadrenergic, noncholinergic (NANC) excitatory neurotransmission. Tachykinins interact with specific membrane proteins, belonging to the family of G protein-coupling cell membrane receptors. Until now, three tachykinin receptors termed NK1 (NK1R), NK2 (NK2R) and NK3 (NK3R) have been cloned in different species. A large amount of reports suggests that these peptides are involved in nociception and neuroimmunomodulation, and in the development of different diseases such as bronchial asthma, inflammatory bowel syndrome and psychiatric disorders. Tachykinin receptor antagonists are therefore promising, therapeutically relevant agents. However, and in spite of extensive research, the obtention of selective antagonists of tachykinin receptors have revealed very difficult. An understanding of how ligands interact with their receptors is essential to permit a rational design of compounds acting selectively at the tachykinin receptor level. The major aim of the present article is to review the structure-activity data that exist for tachykinins and their receptors, with the purpose of getting insight into basic structural requirements that determine ligand/receptor interaction.