[Identification and characterization of histamine H4 receptor]

Recently, we and other groups have identified cDNA encoding the novel histamine H4 receptor. All of the groups have initially found a clue for the H4 receptor-nucleotides sequence in the human draft genomic DNA database. The primary structure of H4 receptor reveals the highest homology with H3 receptor among known G-protein coupled receptors (37.4%). H4 receptor binds to histamine with high affinity, which results in the down-regulation of intracellular cAMP level. H4 receptor is activated not only by histamine, but also R-(alpha)-methylhistamine (H3 receptor agonist), clobenpropit (H3 receptor antagonist), clozapine (neuroleptic) and other histaminergic compounds, while it is antagonized by thioperamide (H3 receptor antagonist). The H4 receptor is localized in the peripheral blood leukocytes, spleen, thymus, small intestine, colon, bone marrow and so on. The tissue distribution of the H4 receptor and known physiological function of histamine tempts us to speculate about its function as an immune modulator. Although there needs much additional work on characterization of the H4 receptor, the discovery of this receptor subtype will unveil a new phase for determining the physiological role of histamine.