Characterization of the binding of the first selective radiolabelled histamine H3-receptor antagonist, [125I]-iodophenpropit, to rat brain.
1. The binding of the first selective radiolabelled histamine H3-receptor antagonist [125I]-iodophenpropit to rat cerebral cortex membranes was characterized. 2. [125I]-iodophenpropit, radiolabelled to a high specific activity of 1900 Ci mmol-1, saturably bound to a single class of sites with a KD of 0.57 +/- 0.16 nM (n = 4) and Bmax of 268 +/- 119 fmol mg-1 protein. 3. Specific binding at a concentration below 1 nM represented 50 to 60% of total binding. 4. Binding of [125I]-iodophenpropit to rat cerebral cortex membranes was readily displaced by histamine H3-agonists and antagonists. In contrast, the inhibitory potencies of selective histamine H1- and H2-receptor ligands were very low. 5. [125I]-iodophenpropit was biphasically displaced by the histamine H3-receptor antagonists, burimamide and dimaprit, which may indicate the existence of histamine H3-receptor subtypes. Other histamine H3-receptor antagonists showed a monophasic displacement. 6. Competition binding curves of H3-agonists were biphasic and showed a rightward shift upon the addition of the nonhydrolysable GTP analogue, guanosine 5'-o-(3-thio) triphosphate (GTP gamma S; 100 microM) which implicates the interaction of histamine H3-receptors with G-proteins. The affinities of the H3-receptor antagonists iodophenpropit, thioperamide and burimamide were not altered by GTP gamma S. 7. Histamine competition binding curves were shifted to the right by different nucleotides (100 microM) with a rank order of potency GTP gamma S > Gpp(NH)p, GTP. 8 In vitro autoradiographic studies revealed a heterogeneous distribution of [125I]-iodophenpropitbinding sites in rat brain, with highest densities observed in specific cerebral cortical areas and layers,the caudate-putamen complex, the olfactory tubercles, the hippocampal formation, the amygdala complex, the hypothalamic area and the mammillary bodies.9 It is concluded that the histamine H3-receptor antagonist, [125I]-iodophenpropit, meets the criteria fo ra suitable radioligand for histamine H3-receptor binding studies in rat brain.
Leiden/Amsterdam Center for Drug Research, Department of Pharmacochemistry, Faculty of Chemistry, Vrije Universiteit, The Netherlands.
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Jansen
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Wu
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Steinbusch
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Timmerman
D000818:Animals; D001345:Autoradiography; D001667:Binding, Competitive / Q000187:drug effects; D001921:Brain / Q000378:metabolism*; D002540:Cerebral Cortex / Q000187:drug effects / Q000378:metabolism; D006150:Guanine Nucleotides / Q000494:pharmacology; D017442:Histamine Agonists / Q000494:pharmacology; D006633:Histamine Antagonists / Q000493:pharmacokinetics*; D007093:Imidazoles / Q000493:pharmacokinetics*; D066298:In Vitro Techniques; D007457:Iodine Radioisotopes; D007550:Isothiuronium / Q000031:analogs & derivatives* / Q000493:pharmacokinetics; D008297:Male; D008566:Membranes / Q000187:drug effects / Q000378:metabolism; D009419:Nerve Tissue Proteins / Q000378:metabolism; D011869:Radioligand Assay; D051381:Rats; D017208:Rats, Wistar; D018100:Receptors, Histamine H3 / Q000378:metabolism
D006150:Guanine Nucleotides; D017442:Histamine Agonists; D006633:Histamine Antagonists; D007093:Imidazoles; D007457:Iodine Radioisotopes; D009419:Nerve Tissue Proteins; D018100:Receptors, Histamine H3; D007550:Isothiuronium; C077791:iodophenpropit