THE 5-HYDROXYTRYPTAMINE, RECEPTOR-SELECTIVE RADIOLIGAND [H-3]RO-63-0563 LABELS 5-HYDROXYTRYPTAMINE RECEPTOR-BINDING SITES IN RAT AND PORCINE STRIATUM

Ro 63-0563 [4-amino-N-(2,6 bis-methylamino-pyridin-4-yl)benzene sulfon amide] is a high affinity 5-hydroxytryptamine(6) (HT6) receptor antago nist with more than 100-fold selectivity for the 5-HT6 receptor compar ed with 69 other receptors and binding sites. The present study descri bes the properties of [H-3]Ro 63-0563, the first selective 5-HT6 recep tor radioligand. Specific binding of [H-3]Ro 63-0563 (nonspecific bind ing defined in the presence of 10 mu M methiothepin) to recombinant ra t and human 5-HT6 receptors was saturable, rapid, and reversible with equilibrium dissociation constants (K-d) of 6.8 nM and 4.96 nM, respec tively. The pharmacological profile of the rat 5-HT6 receptor labeled with [H-3]Ro 63-0563 (methiothepin > D-lysergic acid diethylamide > cl ozapine similar to Ro 63-0563 > lisuride > ergotamine similar to Ro 04 -6790 > 5-HT > amitriptytine similar to metergoline similar to mianser in similar to ritanserin > methysergide > mesulergine) was similar to that obtained by using either [H-3]D-lysergic acide diethylamide or [H -3]5-HT as radioligand. In equilibrium binding studies with rat striat al membranes, [H-3]Ro 63-0563 labeled a single binding site with K-d a nd B-max values of 11.7 nM and 175 fmol/mg protein, respectively. In p orcine striatal membranes, [H-3]Ro 63-0563 also labeled a single bindi ng site with K-d and B-max values of 8.0 nM and 130 fmol/mg protein, r espectively. The affinities of 14 5-HT6 receptor ligands at this bindi ng site were similar to those found for the recombinant rat and human 5-HT6 receptor, which suggested the presence of 5-HT6 receptors in por cine striatum.