Binding profile of the novel 5-HT1B/1D receptor antagonist, [H-3]GR 125,743, in guinea-pig brain: A comparison with [H-3]5-carboxamidotryptamine

Native brain 5-HT1B/1D receptors were studied using the novel antagonist, [H-3]GR 125,743 (N-[4-methoxy-3-(4-methylpiperazin-1-yl)phenyl]-3-methyl-4-(4-pyridyl)benzamide). In guinea-pig striatal membranes, [H-3]GR 125,743 displayed rapid association (t(1/2) = 4.5 min), high (90%) specific binding and high affinity (K-d = 0.29 nM), although B-max values (fmol/mg protein) varied according to brain region-striatum: 199; frontal cortex: 89; hippocampus: 79; cerebellum: 26. In frontal cortex, the B-max determined with [H-3]5-CT ([H-3]carboxamidotryptamine) was significantly higher (178; P < 0.05), suggesting that it also labels other binding sites. In striatal membranes, guanylylimidodiphosphate (GppNHp) inhibited [H-3]5-CT but not [H-3]GR 125,743 binding, suggesting that the latter has antagonist properties. Nevertheless, in competition binding experiments, the pK(i) values obtained with [H-3]GR 125,743 and [H-3]5-CT for 20 serotonergic ligands, including L 694,247 (2-[5-[3-(4-methylsulphonylamino)benzyl-1,2,4-oxadiazol-5-yl]-1H-indole-3-yl]ethylamine), GR 46,611 (3-[3-(2-dimethylamino-ethyl)-1H-indol-6-yl]-N-(4-methoxybenzyl)acrylamide), sumatriptan and alniditan, were highly correlated (r = 0.99). Ketanserin and ritanserin showed low affinity for [H-3]Gr 125,743 binding to guinea-pig striatal sites (K-i = 12600 and 369 nM), suggesting that 5-HT1D) receptors are predominantly labelled in this tissue. The present data indicate that [H-3]Gr 125, 743 is a useful tool for studying native 5-HT1B/1D receptors.