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