[125I]iodoproxyfan, a new antagonist to label and visualize cerebral histamine H3 receptors.

Iodoproxyfan, i.e., 3-(1H-imidazol-4-yl)propyl-(4-iodophenyl)-methyl ether, is a novel potent and selective histamine H3 receptor antagonist. [125I]Iodoproxyfan binding to membranes of the rat striatum was reversible and saturable. Specific binding defined with 1 microM (R)-alpha-methylhistamine corresponded to 65% of the total at 30 pM. Scatchard analysis indicated a Kd of 65 pM and maximal binding capacity of 78 fmol/mg of protein. The specificity of [125I]Iodoproxyfan binding to H3 receptors was demonstrated by its pharmacological profile. A series of H3 receptor agonists inhibited [125I]iodoproxyfan binding with a similar maximal effect and with the expected order of potency and stereoselectivity ratio. H3 receptor antagonists inhibited the specific binding with the expected Ki values. In the presence of guanylnucleotides, 40% of sites exhibited a approximately 40-fold lower affinity for histamine, indicating that the H3 receptor belongs to the superfamily of G protein-coupled receptors and revealing the existence of two populations of sites. Well contrasted autoradiographic pictures of total [125I]iodoproxyfan binding to sections of the rat brain were obtained in a short time and over a low nonspecific binding. The heterogenous distribution of H3 receptors with high labeling of anterior cerebral cortex, ventral striatum and other limbic areas was confirmed. In addition, a clearly distinguishable laminated pattern of labeling was evidenced in the cerebral cortex and hippocampal formation. Hence, this new probe should be useful for sensitive assay and localization of the H3 receptor.