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

Abstract

Selective radioligands for histamine H 3 receptors have been used to characterize H 3 receptor pharmacology by radioligand binding assays and to determine H 3 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 H 3 receptor. The binding of [ 3H]A-317920 to rat cortical and cloned H 3 receptors revealed fast on- and slower off-rate kinetics with calculated K d 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 H 3 receptors with low binding to non-H 3 sites, including cytochrome P450. These findings show that [ 3H]A-317920 is a potent rat H 3 receptor antagonist radioligand and has utility for studying H 3 receptor pharmacology.