Abstract

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 nmand 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 andB _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 withK _d andB _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-HT₆receptor, which suggested the presence of 5-HT₆ receptors in porcine striatum.