Use of an inverse agonist radioligand [3H]A-317920 reveals distinct pharmacological profiles of the rat histamine H3 receptor

Selective radioligands for histamine H3 receptors have been used to characterize H3 receptor pharmacology by radioligand binding assays and to determine H3 receptor distribution by tissue autoradiography. Here we report the synthesis and receptor binding characterization of [3H]A-317920 (furan-2-carboxylic acid(2-{4-[3-([3,5-3H]4-cyclopropanecarbonyl-phenoxy)-propyl]-piperazin-1-yl}-1-methyl-2-oxo-ethyl)-amide), a high affinity inverse agonist radioligand for the rat H3 receptor. The binding of [3H]A-317920 to rat cortical and cloned H3 receptors revealed fast on- and slower off-rate kinetics with calculated Kd values in agreement with those determined in saturation binding assays (0.2 nM for both receptors). Further, we compared [3H]A-317920 with the agonist [3H](N)-α-methylhistamine ([3H]NαMH) as radioligand tools to study receptor pharmacology. Agonists and antagonists displaced [3H]NαMH with one-site binding characteristics and Hill slopes approached unity. In contrast, although antagonists exhibited one-site binding, [3H]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 [3H]NαMH binding. Unlike [125I]iodoproxyfan, [3H]A-317920 exhibits potent and selective binding to rat H3 receptors with low binding to non-H3 sites, including cytochrome P450. These findings show that [3H]A-317920 is a potent rat H3 receptor antagonist radioligand and has utility for studying H3 receptor pharmacology.