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

We tested several histamine H(1) receptor (H(1)R) and antagonists for their differences in agonists binding affinities between human and guinea pig H(1)Rs 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 H(1)Rs. 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 H(1)R 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 H(1)Rs in which amino acids were individually replaced by their guinea pig H(1)R counterparts. Residue Asn(84) (2.61) in transmembrane domain (TM) 2 seemed to act as a selectivity switch in the H(1)R. Molecular modeling and site-directed mutagenesis studies suggest that Asn(84) interacts with the conserved Tyr(458) (7.43) in TM7. Our data provide the first evidence that for some H(1)R 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.