Cloning, expression, and pharmacological characterization of a novel humanhistamine receptor

Using a genomics-based reverse pharmacological approach for screening orpha n G-protein coupled receptors, we have identified and cloned a novel high-a ffinity histamine receptor. This receptor, termed AXOR35, is most closely r elated to the H3 histamine receptor, sharing 37% protein sequence identity. A multiple responsive element/cyclic AMP-responsive element-luciferase rep orter assay was used to identify histamine as a ligand for AXOR35. When tra nsfected into human embryonic kidney 293 cells, the AXOR35 receptor showed a strong, dose-dependent calcium mobilization response to histamine and H3 receptor agonists including imetit and immepip. Radioligand binding confirm ed that the AXOR35 receptor was a high-affinity histamine receptor. The pha rmacology of the AXOR35 receptor was found to closely resemble that of the H3 receptor; the major difference was that (R)-alpha -methylhistamine was a low potency agonist of the AXOR35 receptor. Thioperamide is an antagonist at AXOR 35. Expression of AXOR35 mRNA in human tissues is highest in periph eral blood mononuclear cells and in tissues likely to contain high concentr ations of blood cells, such as bone marrow and lung. In situ hybridization analysis of a wide survey of mouse tissues showed that mouse AXOR35 mRNA is selectively expressed in hippocampus. The identification and localization of this new histamine receptor will expand our understanding of the physiol ogical and pathological roles of histamine and may provide additional oppor tunities for pharmacological modification of these actions.