A novel agonist, phorbol 12-phenylacetate 13-acetate 20-homovanillate, abolishes positive cooperativity of binding by the vanilloid receptor

Abstract

Capsaicin binds to a specific recognition site, referred to as the vanilloid receptor, which it shares with the natural, ultrapotent agonist resiniferatoxin and with the competitive antagonist capsazepine. Upon binding to its receptor, capsaicin opens a cation channel leading to Ca 2+ influx. The binding of capsaicin or resiniferatoxin by the vanilloid receptor follows a sigmoidal saturation curve, indicative of positive cooperativity. The biological significance of this positive cooperative behaviour is unknown, as is the mechanism responsible for it. We have developed a novel ligand, phorbol 12-phenylacetate 13-acetate 20-homovanillate (PPAHV), which binds to cultured rat sensory neurons (with a K i of 3.1 ± 0.4 μM), and induces Ca 2+ uptake by them (with an ED 50 of 1.8 ± 0.3 μM) with similar affinities and in a non-cooperative manner (Hill coefficients are 0.99 and 1.06 for binding and Ca 2+ uptake, respectively). The behaviour of PPAHV thus contrasts with resiniferatoxin or capsaicin not only in the lack of cooperativity but also in the relative potencies for resiniferatoxin binding versus Ca 2+ uptake (resiniferatoxin is less potent and capsaicin is more potent for induction of Ca 2+ uptake than for binding). In further experiments in which the concentration of [ 3H]resiniferatoxin was varied, 1 μM PPAHV likewise reduced the cooperativity index that characterizes resiniferatoxin binding to rat spinal cord membranes from 2.3 ± 0.1 to 1.1 ± 0.2; in parallel experiments, neither capsaicin nor capsazepine (both at a concentration of 2 μM) affected binding cooperativity. Moreover, PPAHV (1 μM) turned the bi-phasic dissociation curve of resiniferatoxin into a monophasic curve, eliminating the second, slow-dissociation phase. The present results suggest that positive cooperativity is a ligand-induced feature rather than an inherent property of vanilloid receptors. A comparison of the spectrum of biological activity of ligands which bind to vanilloid receptors with different degrees of cooperativity may provide an approach to explore the functional significance of this binding behaviour.