High-affinity agonist binding correlates with efficacy (intrinsic activity) at the human serotonin 5-HT2A and 5-HT2C receptors: Evidence favoring theternary complex and two-state models of agonist action

Many modern models of receptor-G protein function assume that there is a di rect relationship between high-affinity agonist binding and efficacy. The v alidity of this assumption has been recently questioned for the serotonin 5 -HT2A receptor. We examined the intrinsic activities of various ligands in activating phosphoinositide hydrolysis and measured their respective bindin g affinities to the high- and low-affinity states of the 5-HT2C (VNV isofor m) and 5-HT2A receptors. Ligand binding affinities for the high-affinity st ate of the receptors were determined using 1-(4-[I-125]iodo-2,5-dimethoxyph enyl)-2-aminopropane, whereas [H-3]mesulergine and N-[H-3]methylspiperone w ere used, in the presence of excess guanine nucleotide [guanosine 5'-O-(3-t hiotriphosphate)], to define binding to the low-affinity state of the 5-HT2 C and 5-HT2A receptors, respectively. Antagonists labeled the high- and low -affinity states of each receptor with comparable affinities. Previously id entified inverse agonists of the 5-HT2C receptor behaved as silent antagoni sts 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-HT2A and 5-HT2C receptors was highly correlated (r(2) = 0.86 and 0.96, respectively) with their intrinsi c activities. These data suggest that high-affinity agonist states can acco unt for agonist efficacy at human 5-HT2A or 5-HT2C receptors without the ne ed for considering additional transition or active states of the receptor-l igand complex. The procedure described herein may expedite drug discovery e fforts by predicting intrinsic activities of ligands solely from ligand bin ding assays.