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

1 In smooth muscle contractility assays, many NK1 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. 2 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. 3 SP and septide gave sigmoid log concentration-response curves and were equipotent in inducing NK(1)r endocytosis. 4 The NK(1)r antagonists, CP-99994 (2S,3S)-3-(2-methoxybenzyl)amino-2-phenylpiperidine dihydrochloride and MEN-10581, cyclo(Leu psi/[CH2NH]Lys(benzyloxycarbonyl)-Gln-Trp-Phe-beta Ala) were both more potent in inhibiting endocytosis (50 x and 8 x greater respectively) against septide than against SP. 5 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.