Vol. 54, Issue 3, 577-583, September 1998

The 5-Hydroxytryptamine₆ Receptor-Selective radioligand [³H]Ro 63-0563 Labels 5-Hydroxytryptamine Receptor Binding Sites in Rat and Porcine Striatum

Frank G. Boess, Claus Riemer, Michael Bös, Jane Bentley, Anne Bourson, and Andrew J. Sleight

Pharma Division, Preclinical Research, F. Hoffmann-La Roche, Ltd., 4070 Basel, Switzerland

Ro 63-0563 [4-amino-N-(2,6 bis-methylamino-pyridin-4-yl)-benzene sulfonamide] is a high affinity 5-hydroxytryptamine₆ (HT₆) receptor antagonist with more than 100-fold selectivity for the 5-HT₆ receptor compared with 69 other receptors and binding sites. The present study describes the properties of [³H]Ro 63-0563, the first selective 5-HT₆ receptor radioligand. Specific binding of [³H]Ro 63-0563 (nonspecific binding defined in the presence of 10 µM methiothepin) to recombinant rat and human 5-HT₆ receptors was saturable, rapid, and reversible with equilibrium dissociation constants (K_d) of 6.8 nM and 4.96 nM, respectively. The pharmacological profile of the rat 5-HT₆ receptor labeled with [³H]Ro 63-0563 (methiothepin > D-lysergic acid diethylamide > clozapine ~ Ro 63-0563 > lisuride > ergotamine ~ Ro 04-6790 > 5-HT > amitriptyline ~ metergoline ~ mianserin ~ ritanserin > methysergide > mesulergine) was similar to that obtained by using either [³H]D-lysergic acide diethylamide or [³H]5-HT as radioligand. In equilibrium binding studies with rat striatal membranes, [³H]Ro 63-0563 labeled a single binding site with K_d and B_max values of 11.7 nM and 175 fmol/mg protein, respectively. In porcine striatal membranes, [³H]Ro 63-0563 also labeled a single binding site with K_d and B_max values of 8.0 nM and 130 fmol/mg protein, respectively. The affinities of 14 5-HT₆ receptor ligands at this binding site were similar to those found for the recombinant rat and human 5-HT₆ receptor, which suggested the presence of 5-HT₆ receptors in porcine striatum.