Synthesis and Biological Activity of Agonists for the Neuronal Tachykinin Receptor in Guinea Pig Ileum

The classification of tachykinin receptors into two subclasses, the SP-P and the SP-E receptors has been well established (1). Recently Laufer et. al. (2) characterized, in the guinea pig ileum, a third tachykinin receptor subclass, designated as SP-N receptor. The SP-N receptor is located on the enteric cholinergic neurons, and madiates the release of acetylcholine. We found that the SP analog (pGlu6,(N-Me)Phe8) SP_6-11 acts as a selective potent agonist for the SP-N receptor (EC₅₀=0.5 nM) while its potency for the SP-P receptor is much lower (EC₅₀=500 nM). Conceivably N-methylation of Phe8 in the hexapeptide sequence of substance P (SP), induces selectivity toward the SP-N receptor. Therefore we set to probe the constraints at the nitrogen of Phe7-Phe8 amide bond by changing the alkyl group at this site.