High-affinity agonist binding correlates with efficacy (intrinsic activity) at the human serotonin 5-HT(2A) and 5-HT(2C) receptors: Evidence favoring the ternary complex and two-state models of agonist action

Abstract

Many modern models of receptor-G protein function assume that there is a direct relationship between high-affinity agonist binding and efficacy. The validity of this assumption has been recently questioned for the serotonin 5- HT(2A) receptor. We examined the intrinsic activities of various ligands in activating phosphoinositide hydrolysis and measured their respective binding affinities to the high- and low-affinity states of the 5-HT(2C) (VNV isoform) and 5-HT(2A) receptors. Ligand binding affinities for the high-affinity state of the receptors were determined using 1-(4-[125I]iodo-2,5- dimethoxyphenyl)2-aminopropane, whereas [3H]mesulergine and N- [3H]methylspiperone were used, in the presence of excess guanine nucleotide [guanosine 5'-O-(3-thiotriphosphate)], to define binding to the low-affinity state of the 5-HT(2C) and 5-HT(2A) receptors, respectively. Antagonists labeled the high- and low-affinity states of each receptor with comparable affinities. Previously identified inverse agonists of the 5-HT(2C) receptor behaved as silent antagonists in our systems even when the receptor was overexpressed at a relatively high density. In contrast, the ability of agonists to bind differentially to the high- and low-affinity states of the 5-HT(2A) and 5-HT(2C) receptors was highly correlated (r2 = 0.86 and 0.96, respectively) with their intrinsic activities. These data suggest that high- affinity agonist states can account for agonist efficacy at human 5-HT(2A) or 5-HT(2C) receptors without the need for considering additional transition or active states of the receptor-ligand complex. The procedure described herein may expedite drug discovery efforts by predicting intrinsic activities of ligands solely from ligand binding assays.