Abstract

For several GPCRs, discrimination between agonism and antagonism is possible on the basis of thermodynamics parameters, such as binding enthalpy and entropy. In this study, we analyze whether agonists and antagonists can also be discriminated thermodynamically at the histamine H₁ receptor (H₁R). Because previous studies revealed species differences in pharmacology between human H₁R (hH₁R) and guinea pig H₁R (gpH₁R), we analyzed a broad spectrum of H₁R antagonists and agonists at hH₁R and gpH₁R. [³H]Mepyramine competition binding assay were performed at five different temperatures in a range from 283.15 to 303.15 K. In addition, we performed a temperature-dependent three-dimensional quantitative structure activity relationship study to predict binding enthalpy and entropy for histaprodifen derivatives, which can bind to H₁R in two different orientations. Our studies revealed significant species differences in binding enthalpy and entropy between hH₁R and gpH₁R for some antagonists and agonists. Furthermore, in some cases, we found changes in heat capacity of the binding process that were different from zero. Differences in flexibility of the ligands may be responsible for this observation. For most ligands, the binding process to hH₁R and gpH₁R is clearly entropy-driven. In contrast, for the endogenous ligand histamine, the binding process is significantly enthalpy-driven at both species isoforms. Thus, a definite discrimination between antagonism and agonism based on thermodynamic parameters is possible for neither hH₁R nor gpH₁R, but thermodynamic analysis of ligand-binding may be a novel approach to dissect agonist- and antagonist-specific receptor conformations.