Comparison of the pharmacological properties of human and rat histamine H3-receptors

Ligand pharmacology of histamine H3-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 H3-receptors (hH3R and rH3R, respectively), co-expressed with mammalian G proteins in Sf9 insect cell membranes. We compared a series of imidazole-containing H3R ligands in radioligand binding and steady-state GTPase assays. H3Rs similarly coupled to Gαi/o-proteins. Affinities and potencies of the agonists histamine, Nα-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 rH3R. The inverse agonists ciproxifan and thioperamide showed higher potency but lower efficacy at rH3R. Clobenpropit was not species-selective. Strikingly, imoproxifan was almost full agonist at hH3R, but an inverse agonist at rH3R. Imoproxifan was docked into the binding pocket of inactive and active hH3R- and rH3R-models and molecular dynamic simulations were performed. Imoproxifan bound to hH3R and rH3R in E-configuration, which represents the trans-isomer of the oxime-moiety as determined in crystallization studies, and stabilized active hH3R-, but inactive rH3R-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 Trp6.48 in TM VI. Collectively, the substantial species differences at H3Rs are explained at a molecular level by the use of novel H3R active-state models.

This paper documents substantial pharmacological differences between human and rat histamine H3-receptor. Most strikingly, in human H3-receptor, imoproxifan stabilizes an active conformation. In rat H3-receptor, imoproxifan stabilizes an inactive conformation.Figure optionsDownload as PowerPoint slide