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

Abstract

Native brain 5-HT 1B/1D receptors were studied using the novel antagonist, [ 3H]GR 125,743 ( N-[4-methoxy-3-(4-methylpiperazin-1-yl)phenyl]-3-methyl-4-(4-pyridyl)benzamide). In guinea-pig striatal membranes, [ 3H]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 [ 3H]5-CT ([ 3H]carboxamidotryptamine) was significantly higher (178; P<0.05), suggesting that it also labels other binding sites. In striatal membranes, guanylylimidodiphosphate (GppNHp) inhibited [ 3H]5-CT but not [ 3H]GR 125,743 binding, suggesting that the latter has antagonist properties. Nevertheless, in competition binding experiments, the p K i values obtained with [ 3H]GR 125,743 and [ 3H]5-CT for 20 serotonergic ligands, including L 694,247 (2-[5-[3-(4-methylsulphonylamino)benzyl-1,2,4-oxadiazol-5-yl]-1 H-indole-3-yl]ethylamine), GR 46,611 (3-[3-(2-dimethylamino-ethyl)-1 H-indol-6-yl]- N-(4-methoxybenzyl)acrylamide), sumatriptan and alniditan, were highly correlated ( r=0.99). Ketanserin and ritanserin showed low affinity for [ 3H]GR 125,743 binding to guinea-pig striatal sites ( K i=12600 and 369 nM), suggesting that 5-HT 1B (rather than 5-HT 1D) receptors are predominantly labelled in this tissue. The present data indicate that [ 3H]GR 125,743 is a useful tool for studying native 5-HT 1B/1D receptors.