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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
Author(s) -
Fitzgerald Lawrence W.,
Conklin Deborah S.,
Krause Carol M.,
Marshall Anne P.,
Patterson John P.,
Tran David P.,
Iyer Geeta,
Kostich Walter A.,
Largent Brian L.,
Hartig Paul R.
Publication year - 1999
Publication title -
journal of neurochemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.75
H-Index - 229
eISSN - 1471-4159
pISSN - 0022-3042
DOI - 10.1046/j.1471-4159.1999.0722127.x
Subject(s) - receptor , agonist , inverse agonist , intrinsic activity , 5 ht receptor , endogenous agonist , chemistry , ligand (biochemistry) , serotonin , g protein coupled receptor , biochemistry , dopamine receptor d1
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‐[ 125 I]iodo‐2,5‐dimethoxyphenyl)‐2‐aminopropane, whereas [ 3 H]mesulergine and N ‐[ 3 H]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 over‐expressed 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 ( r 2 = 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.