Use of an inverse agonist radioligand [ super(3)H]A-317920 reveals distinct pharmacological profiles of the rat histamine H sub(3) receptor

Selective radioligands for histamine H sub(3) receptors have been used to characterize H sub(3) receptor pharmacology by radioligand binding assays and to determine H sub(3) receptor distribution by tissue autoradiography. Here we report the synthesis and receptor binding characterization of [ super(3)H]A-317920 (furan-2-carboxylic acid(2-{4-[3-([3,5- super(3)H]4-cyclopropanecarbonyl-phenoxy)-propyl]- piperazin-1-yl}-1-methyl-2-oxo-ethyl)-amide), a high affinity inverse agonist radioligand for the rat H sub(3) receptor. The binding of [ super(3)H]A-317920 to rat cortical and cloned H sub(3) receptors revealed fast on- and slower off-rate kinetics with calculated K sub(d) values in agreement with those determined in saturation binding assays (0.2 nM for both receptors). Further, we compared [ super(3)H]A-317920 with the agonist [ super(3)H](N)- alpha -methylhistamine ([ super(3)H]N alpha MH) as radioligand tools to study receptor pharmacology. Agonists and antagonists displaced [ super(3)H]N alpha MH with one-site binding characteristics and Hill slopes approached unity. In contrast, although antagonists exhibited one-site binding, [ super(3)H]A-317920 displacement by agonists was best fit by two- site binding models, and the potencies of the high affinity, GDP-sensitive sites correlated with the potencies defined in [ super(3)H]N alpha MH binding. Unlike [ super(125)I]iodoproxyfan, [ super(3)H]A-317920 exhibits potent and selective binding to rat H sub(3) receptors with low binding to non-H sub(3) sites, including cytochrome P450. These findings show that [ super(3)H]A-317920 is a potent rat H sub(3) receptor antagonist radioligand and has utility for studying H sub(3) receptor pharmacology.