Expression and functional properties of canine, rat, and murine histamine H4 receptors in Sf9 insect cells

The histamine H-4 receptor (H4R) is expressed on cells of the immune system including eosinophils, dendritic cells, and T cells and plays an important role in the pathogenesis of bronchial asthma, atopic dermatitis, and pruritus. Analysis of the H4R in these diseases depends on the use of animal models. However, there are substantial pharmacological differences between various H4R species orthologs. The purpose of this study was to analyze the pharmacological properties of canine, rat, and murine H4R in comparison to human H4R expressed in Sf9 insect cells. Only hH(4)R and cH(4)R exhibited a sufficiently high [H-3] histamine affinity for radioligand binding studies. Generally, cH(4)R exhibited lower ligand-affinities than hH(4)R. Similarly, in high-affinity GTPase studies, ligands were more potent at hH(4)R than at other H4R species orthologs. Unlike the other H4R species orthologs, hH(4)R exhibited high agonist-independent (constitutive) activity. Most strikingly, the prototypical H4R antagonist (1-[(5-chloro-1H-indol-2-yl) carbonyl]-4-methylpiperazine) (JNJ7777120) exhibited partial agonistic activity at cH(4)R, rH(4)R, and mH(4)R, whereas at hH(4)R, JNJ7777120 was a partial inverse agonist. H4R agonists from the class of N-G-acylated imidazolylpropylguanidines and cyanoguanidines exhibited substantial differences in terms of affinity, potency, and efficacy among H4R species orthologs, too. The species-dependent pharmacological profiles are not due to the highly variable amino acid sequence position 341. Finally, H4R species orthologs differ from each other in terms of regulation by NaCl. Collectively, there are profound pharmacological differences between H4R species orthologs. Most importantly, caution must be exerted when interpreting pharmacological effects of "the prototypical H4R antagonist" JNJ7777120 as H4R antagonism.