Comparison of the pharmacological properties of human and rat histamine H 3-receptors

Abstract

Ligand pharmacology of histamine H 3-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 3-receptors (hH 3R and rH 3R, respectively), co-expressed with mammalian G proteins in Sf9 insect cell membranes. We compared a series of imidazole-containing H 3R ligands in radioligand binding and steady-state GTPase assays. H 3Rs similarly coupled to Gα i/o-proteins. Affinities and potencies of the agonists histamine, N α-methylhistamine and R-(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 rH 3R. The inverse agonists ciproxifan and thioperamide showed higher potency but lower efficacy at rH 3R. Clobenpropit was not species-selective. Strikingly, imoproxifan was almost full agonist at hH 3R, but an inverse agonist at rH 3R. Imoproxifan was docked into the binding pocket of inactive and active hH 3R- and rH 3R-models and molecular dynamic simulations were performed. Imoproxifan bound to hH 3R and rH 3R in E-configuration, which represents the trans-isomer of the oxime-moiety as determined in crystallization studies, and stabilized active hH 3R-, but inactive rH 3R-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 6.48 in TM VI. Collectively, the substantial species differences at H 3Rs are explained at a molecular level by the use of novel H 3R active-state models.