ABSTRACT
Tachykinins (TKs) are a family of peptides that share the common C-terminal sequence Phe-Xaa-Gly-Leu-Met-NH2 where Xaa is an aromatic or a branched aliphatic amino acid. The three TK receptors have been isolated and cloned from various species. They all belong to the superfamily of rhodopsin-like G-protein-coupled receptors with seven hydrophobic transmembrane-spanning segments, which form the ligand recognition site; phosphoinositide generation is the main second-messenger system linked to their activation. The common C-terminal sequence of TK is essential for their interaction with neurokinin-1 (NK-1), NK-2, and NK-3 receptors, while some effects of substance P, not shared by other TKs, are mediated by its N-terminal region. A few studies have described the possible existence of differences between central and peripheral NK-1 receptors in both rat and guinea pig species. The pharmacological heterogeneity of the NK-2 receptors may be entirely species-related: however, examples of intraspecies heterogeneity have been described as well, suggesting the existence of true receptor subtypes.
