Pharmacological differences between human and guinea pig histamine H ₁ receptors: Asn⁸⁴ (2.61) as key residue within an additional binding pocket in the H₁ receptor

We tested several histamine H₁ receptor (H₁R) agonists and antagonists for their differences in binding affinities between human and guinea pig H₁Rs transiently expressed in African green monkey kidney (COS-7) cells. Especially, the bivalent agonist histaprodifen-histamine dimer (HP-HA) shows a higher affinity for guinea pig than for human H₁Rs. Based on the structure of HP-HA, we have further identified VUF 4669 [7-(3-(4-(hydroxydiphenylmethyl)piperidin-1-yl)propoxy)-4- oxochroman-2-carboxylic acid] as a guinea pig-preferring H₁R antagonist, demonstrating that the concept of species selectivity is not limited to agonists. To delineate the molecular mechanisms behind the observed species selectivity, we have created mutant human H₁Rs in which amino acids were individually replaced by their guinea pig H₁R counterparts. Residue Asn⁸⁴ (2.61) in transmembrane domain (TM) 2 seemed to act as a selectivity switch in the H₁R. Molecular modeling and site-directed mutagenesis studies suggest that Asn⁸⁴ interacts with the conserved Tyr⁴⁵⁸ (7.43) in TM7. Our data provide the first evidence that for some H₁R ligands, the binding pocket is not only limited to TMs 3, 4, 5, and 6 but also comprises an additional pocket formed by TMs 2 and 7.