Comparison of the pharmacological properties of human and rat histamine H3-receptors
Résumé
Ligand pharmacology of histamine H-receptors is species-dependent. In previous studies, two amino acids in transmembrane domain 3 (TM III) were shown to play a significant role. In this study, we characterized human and rat histamine H-receptors (hHR and rHR, respectively), co-expressed with mammalian G proteins in Sf9 insect cell membranes. We compared a series of imidazole-containing HR ligands in radioligand binding and steady-state GTPase assays. HRs similarly coupled to Gα-proteins. Affinities and potencies of the agonists histamine, -methylhistamine and -(α)-methylhistamine were in the same range. Imetit was only a partial agonist. The pharmacology of imetit and proxifan was similar at both species. However, impentamine was more potent and efficacious at rHR. The inverse agonists ciproxifan and thioperamide showed higher potency but lower efficacy at rHR. Clobenpropit was not species-selective. Strikingly, imoproxifan was almost full agonist at hHR, but an inverse agonist at rHR. Imoproxifan was docked into the binding pocket of inactive and active hHR- and rHR-models and molecular dynamic simulations were performed. Imoproxifan bound to hHR and rHR in -configuration, which represents the -isomer of the oxime-moiety as determined in crystallization studies, and stabilized active hHR-, but inactive rHR-conformations. Large differences in electrostatic surfaces between TM III and TM V cause differential orientation of the oxime-moiety of imoproxifan, which then differently interacts with the rotamer toggle switch Trp in TM VI. Collectively, the substantial species differences at HRs are explained at a molecular level by the use of novel HR active-state models.
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