CHARACTERIZATION OF THE BINDING OF THE FIRST SELECTIVE RADIOLABELED HISTAMINE H-3 RECEPTOR ANTAGONIST, [I-125] IODOPHENPROPIT, TO RAT-BRAIN

1 The binding of the first selective radiolabelled histamine H-3-receptor antagonist [I-125]-iodophenpropit to rat cerebral cortex membranes was characterized. 2 [I-125]-iodophenpropit, radiolabelled to a high specific activity of 1900 Ci mmol(-1), saturably bound to a single class of sites with a K-D of 0.57 +/- 0.16 nM (n = 4) and B-max 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 [I-125]-iodophenpropit to rat cerebral cortex membranes was readily displaced by histamine H-3-agonists and antagonists. In contrast, the inhibitory potencies of selective histamine H-1- and H-2-receptor ligands were very low. 5 [(125)]-iodophenpropit was biphasically displaced by the histamine H-3-receptor antagonists, burimamide and dimaprit, which may indicate the existence of histamine H-3-receptor subtypes. Other histamine H-3-receptor antagonists showed a monophasic displacement. 6 Competition binding curves of H-3-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 mu M) which implicates the interaction of histamine H-3-receptors with G-proteins, The affinities of the H-3-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 mu M) with a rank order of potency GTP gamma S>Gpp(NH)p, GTP. 8 In vitro autoradiographic studies revealed a heterogeneous distribution of [I-125]-iodophenpropit binding 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 H-3-receptor antagonist, [I-125]-iodophenpropit, meets the criteria for a suitable radioligand for histamine H-3-receptor binding studies in rat brain.