title: High-affinity agonist binding correlates with efficacy (intrinsic activity) at the human serotonin 5-HT2A and 5-HT2C 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-HT2A 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-HT2C (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-HT2C and 5-HT2A receptors, respectively. Antagonists labeled the high- and low-affinity states of each receptor with comparable affinities. Previously identified inverse agonists of the 5-HT2C 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-HT2A and 5-HT2C 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-HT2A or 5-HT2C 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.
authors.affiliation: CNS Diseases Research, DuPont Pharmaceuticals Research Laboratories, Wilmington, Delaware 19880, USA.
authors.forename: L W
authors.initials: LW
authors.lastname: Fitzgerald
authors.forename: D S
authors.initials: DS
authors.lastname: Conklin
authors.forename: C M
authors.initials: CM
authors.lastname: Krause
authors.forename: A P
authors.initials: AP
authors.lastname: Marshall
authors.forename: J P
authors.initials: JP
authors.lastname: Patterson
authors.forename: D P
authors.initials: DP
authors.lastname: Tran
authors.forename: G
authors.initials: G
authors.lastname: Iyer
authors.forename: W A
authors.initials: WA
authors.lastname: Kostich
authors.forename: B L
authors.initials: BL
authors.lastname: Largent
authors.forename: P R
authors.initials: PR
authors.lastname: Hartig
mesh_terms: D000662:Amphetamines / Q000378:metabolism; D001667:Binding, Competitive / Q000502:physiology; D002460:Cell Line; D004873:Ergolines / Q000378:metabolism; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000031:analogs & derivatives / Q000378:metabolism; D006801:Humans; D006868:Hydrolysis; D007536:Isomerism; D008024:Ligands; D008954:Models, Biological; D010716:Phosphatidylinositols / Q000378:metabolism; D011985:Receptors, Serotonin / Q000378:metabolism*; D011994:Recombinant Proteins; D012702:Serotonin Antagonists / Q000378:metabolism; D017366:Serotonin Receptor Agonists / Q000378:metabolism*; D013134:Spiperone / Q000031:analogs & derivatives / Q000378:metabolism
chemical_list: D000662:Amphetamines; D004873:Ergolines; D008024:Ligands; D010716:Phosphatidylinositols; D011985:Receptors, Serotonin; D011994:Recombinant Proteins; D012702:Serotonin Antagonists; D017366:Serotonin Receptor Agonists; C034882:guanosine 5'-O-(1-thiotriphosphate); D016244:Guanosine 5'-O-(3-Thiotriphosphate); D013134:Spiperone; C039167:3-N-methylspiperone; C015952:4-iodo-2,5-dimethoxyphenylisopropylamine; C031649:mesulergine