Electrophysiology-based analysis of human histamine H4 receptor pharmacology using GIRK channel coupling in Xenopus oocytes

The recently cloned histamine H4 receptor is expressed predominantly in haematopoietic cells and has been found to modulate the function of mast cells, eosinophils, dendritic cells and T lymphocytes. It represents an attractive target for pharmacological interventions against a number of inflammatory and autoimmune disorders. In the present work we used two-electrode voltage-clamp to demonstrate histamine H4 receptor modulation of G protein-coupled inward rectifier potassium (GIRK) channels heterologously expressed in Xenopus oocytes. In accordance with earlier findings in other effector systems, full agonism by histamine and (R)-α-methylhistamine, partial agonism by clobenpropit and inverse agonism by thioperamide were observed. Furthermore, in oocytes injected with low amounts of receptor cRNA, clobenpropit apparently acted as a neutral antagonist. We also used the high temporal resolution afforded by this system to study the differential time courses of response deactivation upon ligand washout for clobenpropit and (R)-α-methylhistamine. GIRK channels represent a novel effector system for histamine H4 receptor modulation, which may be of physiological relevance and prove useful in the development of compounds targeting this receptor.