Two affinities for a single antagonist at the neuronal NK₁ tachykinin receptor: evidence from quantitation of receptor endocytosis

In smooth muscle contractility assays, many NK1 receptor (NK1r) antagonists inhibit responses to the neurotransmitter, substance P (SP), and its analogue, septide, with markedly different potency, leading to the proposal that there is a septide‐preferring receptor related to the NK1r. We used fluorescence immunohistochemistry and confocal microscopy to visualize agonist‐induced NK1r endocytosis and analyse agonist/antagonist interactions at native NK1r in neurons of the myenteric plexus of guinea‐pig ileum. SP and septide gave sigmoid log concentration‐response curves and were equipotent in inducing NK1r endocytosis. The NK1r antagonists, CP‐99994 (2S,3S)‐3‐(2‐methoxybenzyl)amino‐2‐phenylpiperidine dihydrochloride and MEN‐10581, cyclo(Leuψ[CH2NH]Lys(benzyloxycarbonyl)‐Gln‐Trp‐Phe‐βAla) were both more potent in inhibiting endocytosis (50× and 8× greater respectively) against septide than against SP. The results suggest that SP and septide interact differently with the NK1r, and that a single antagonist can exhibit different affinities at a single NK1r population, depending on the agonist with which it competes. Thus it may not be necessary to posit a separate septide‐preferring tachykinin receptor. British Journal of Pharmacology (1999) 126, 131–136; doi:10.1038/sj.bjp.0702285