The 5-Hydroxytryptamine₆Receptor-Selective radioligand [³H]Ro 63–0563 Labels 5-Hydroxytryptamine Receptor Binding Sites in Rat and Porcine Striatum

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 microM methiothepin) to recombinant rat and human 5-HT6 receptors was saturable, rapid, and reversible with equilibrium dissociation constants (Kd) 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 approximately Ro 63-0563 > lisuride > ergotamine approximately Ro 04-6790 > 5-HT > amitriptyline approximately metergoline approximately mianserin approximately 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 Kd and Bmax 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 Kd and Bmax 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.

5‐HT6 antagonists as potential treatment for cognitive dysfunction

“…The distribution of receptor protein in the rat brain as determined by immunocytochemistry Gerard et al, 1996] is in good agreement with the mRNA results. The distribution pattern has been further confirmed by autoradiographic studies using selective ligands [Boess et al, 1998;Hirst et al, 2003;Roberts et al, 2002]. 5-HT 6 receptor distribution in human brain parallels that found in rats Hirst et al, 2003;Kohen et al, 2001].…”

“…1) including 5-HT 1, 5-methoxytryptamine 2, LSD 3, methysergide 4, metergoline 5, and methiothepin 8, bind to the 5-HT 6 with high affinity (Fig. 1) as do a number of typical and atypical antipsychotic agents including clozapine 6, loxapine 7, and ritanserin 11 and tricyclic antidepressants such as mianserin 9 and amitryptyline 10 [ Boess et al, 1998;Monsma et al, 1993;Roth et al, 1994]. The binding affinity of these compounds is very similar among different species including human, rat, and mouse ( Fig.…”

Pharmacologic mechanisms of serotonergic regulation of dopamine neurotransmission

“…Activation of the 5-HT6 receptor in artificial expression systems (Ruat et al, 1993) and striatal tissues (Boess et al, 1998) enhances adenylate cyclase activity. In situ hybridization (Gerard et al, 1996) and immunohistochemical studies (Gerard et al, 1997) demonstrate that 5-HT6 receptors are abundant in DA terminal areas including the striatum, NA, hippocampus, and frontal cortex.…”

Radiosynthesis and Characterization of 11C-GSK215083 as a PET Radioligand for the 5-HT6 Receptor

“…Tissue-section receptor autoradiography studies have characterized the binding and distribution of the radioligands 125 I-SB-258585 and 3 H-Ro-63-0563 in various species (9,10). These studies have consistently revealed heterogeneous binding throughout the brain, with the highest levels found in the striatum, moderate levels in the cerebral cortex, and the lowest levels in the cerebellum.…”