Multiple Tachykinin Binding Sites in Rat Brain and Peripheral Tissues

Based on the different rank order of potencies as well as the lack of complete cross-tachyphylaxis observed among different tachykinins and their analogues in several biological assays, we have previously proposed the existence of multiple subtypes of SP receptors in different tissues [1–3]. For the SP-P subtype, both physalaemin (PHY) and SP are active at nanomolar concentrations with eledoisin (E) and kassinin (KAS) more or less equipotent. On the other hand, for the SP-E subtype, KAS, E and neurokinin A (NKA) are active at nanomolar concentrations and they are several hundred times more active than PHY or SP. The distinction between SP-P and SP-E system was further delineated by the development of a SP-P selective agonist, SP-O-methyl ester (SPOMe), which is 100–1000 times less potent than substance P on SP-E systems [4].