Phylogeny of tachykinin receptor localization in the vertebrate central nervous system: apparent absence of neurokinin-2 and neurokinin-3 binding sites in the human brain.

Binding of [125I]Bolton-Hunter labeled tachykinins substance P (BHSP), neurokinin A (BHNKA) and eledoisin (BHELE) to brain sections from several vertebrates was investigated by receptor autoradiography. Densities of BHSP binding sites were low in fish brain, increased in lower vertebrates, were high in birds and rodents, and relatively constant in cat, monkey and human. In contrast, BHELE binding site densities were moderate in fish brain and high in frog, snake, chick, pigeon, mouse and rat brain. Low and very low densities were localized in guinea pig and cat, while no significant BHELE specific binding was found in monkey and human brain. BHSP and BHELE binding sites were distinctly distributed in the vertebrate brains analyzed. Each ligand showed a characteristic regional distribution which was similar from species to species. The affinity profiles of tachykinins for BHSP and BHELE binding sites as analyzed on frog, chick and rat brain sections, corresponded to the NK1 and NK3 receptor types, respectively. No BHNKA binding sites could be detected in any vertebrate brain investigated. In conclusion, marked species variations exist in the density and distribution of tachykinin receptor types in the vertebrate brain. Thus, neurokinin A receptors (NK2 type) seem to be absent in the vertebrate central nervous system and, while substance P receptors (NK1 type) appear to be preserved and increase in density during evolution, the contrary seems to happen for the eledoisin receptors (NK3 type) which are more abundant in lower vertebrates and apparently absent in primate, particularly human brain.