Two affinities for a single antagonist at the neuronal NK 1 tachykinin receptor: evidence from quantitation of receptor endocytosis

In smooth muscle contractility assays, many NK 1 receptor (NK 1 r) 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 NK 1 r. We used fluorescence immunohistochemistry and confocal microscopy to visualize agonist‐induced NK 1 r endocytosis and analyse agonist/antagonist interactions at native NK 1 r in neurons of the myenteric plexus of guinea‐pig ileum. SP and septide gave sigmoid log concentration‐response curves and were equipotent in inducing NK 1 r endocytosis. The NK 1 r antagonists, CP‐99994 (2S,3S)‐3‐(2‐methoxybenzyl)amino‐2‐phenylpiperidine dihydrochloride and MEN‐10581, cyclo(Leuψ[CH 2 NH]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 NK 1 r, and that a single antagonist can exhibit different affinities at a single NK 1 r 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