Efficacy of selective 5‐HT6 receptor ligands determined by monitoring 5‐HT6 receptor‐mediated cAMP signaling pathways

Abstract

1 Two novel selective 5-HT6 receptor ligands E-6801 (6-chloro-N-(3-(2-(dimethylamino)ethyl)-1H-indol-5-yl)imidazo[2,1-b]thiazole-5-sulfonamide) and E-6837 (5-chloro-N-(3-(2-(dimethylamino)ethyl)-1H-indol-5-yl)naphthalene-2-sulfonamide) were investigated and compared to the putative 5-HT6 receptor antagonists SB-271046 (5-chloro-N-(4-methoxy-3-(piperazin-1-yl)phenyl)-3-methylbenzo [b] thiophene-2-sulfonamide) and Ro 04-06790 (N-(2,6-bis(methylamino)pyrimidin-4-yl)-4-aminobenzenesulfonamide) using a cAMP-mediated pathway. 2 Forskolin stimulation, to increase the magnitude of agonist cAMP responses, and site-directed mutagenesis of the 5-HT6 receptor, in order to yield constitutively active receptor, were applied. 3 5-HT (E max , % over basal: 200), E-6801 (120) and E-6837 (23) induced cAMP formation at the rat 5-HT6 receptor. In the copresence of forskolin, cAMP responses were more potent and enhanced to 294 (5-HT, % over forskolin), 250 (E-6801) and 207 (E-6837), respectively. 5-HT-mediated cAMP formation was dose-dependently blocked by SB-271046 (pA 2 : 8.7670.22) and Ro 04-6790 (pA 2 : 7.8970.10) and not affected by the copresence of forskolin. Both E-6801 and E-6837 yielded partial antagonism of the 5-HT response in the absence of forskolin, whereas antagonism was either completely absent (E-6801) or attenuated (E-6837) in the copresence of forskolin. Intrinsic activity of these 5-HT6 receptor ligands at a constitutively active human S267K 5-HT6 receptor in Cos-7 cells indicated similar efficacy (E max , % over basal) for 5-HT (97), E-6801 (91) and E-6837 (100), while Ro 04-6790 (-33) and SB-271046 (-39) were equi-efficacious inverse agonists. 4 The use of either forskolin or a constitutively active S267K 5-HT6 receptor enhances the resolution for monitoring the efficacy of 5-HT6 receptor ligands. E-6801 and E-6837 are potent partial agonists at the 5-HT6 receptor. Ro 04-6790 and SB-271046 appear to act as inverse agonists/antagonists.

E-6801, a 5-HT6 receptor agonist, improves recognition memory by combined modulation of cholinergic and glutamatergic neurotransmission in the rat

“…It is therefore an apparent paradox that E-6801, a full 5-HT 6 receptor agonist in in vitro studies (Holenz et al 2005;Romero et al 2006Romero et al , 2007, caused a dose-related restoration of natural forget- Fig. 5 The effects of E-6801 and donepezil on the discrimination ratio (calculated from novel object exploration/(novel+ familiar object exploration) in Fig.…”

“…More recently, several research groups have identified selective, high-affinity 5-HT 6 receptor full and partial agonists Glennon et al 2000;Holenz et al 2005;Mattsson et al 2005;Romero et al 2006Romero et al , 2007Schechter et al 2004Schechter et al , 2007. The few investigations carried out to date suggest that 5-HT 6 receptor agonists impair recognition memory in social recognition (Loiseau et al 2008) and short-and long-term memories in autoshaping tasks (Meneses et al 2008).…”

Involvement of an Altered 5-HT6 Receptor Function in Behavioral Symptoms of Alzheimer's Disease

“…Even though reductions in total levels of cAMP were found, probably representing neurodegeneration, it is notable that cAMP is generated by many different receptors, and the fraction generated by 5-HT 6 receptors is probably very small with respect to the total receptor stimulated levels of cAMP. Therefore, cAMP formation related to 5-HT 6 receptor activation was measured as the amount of cAMP formation after 5-HT 6 receptor activation by the selective agonist E-6801 [31]. It could be argued that the decreases in E-6801 induced cAMP formation found in AD may be due to the decreases in 5-HT 6 receptor density.…”

“…Patients were an autopsied subset of subjects included in a prospective study of behavioral changes in clinically diagnosed demented patients [15]. At entry to the study, assessment and diagnoses were made using CAMDEX [31], DMS-III-R criteria [1] and NINCDS-ADRA criteria [24]. Drug histories were recorded for all patients; 21 patients were taking tranquilizers (neuroleptics, sedative-hypnotics or antidepressants).…”

Neurotransmitters and Memory

“…In addition, 5-HT 6 receptor agonists might be useful for their promemory and/or antiamnesic effects (Woods et al, 2012) acting on the cholinergic and/or glutamatergic neurons or alternatively; 5-HT 6 receptor antagonists and agonists might operate via modulation of distinct intracellular signaling pathways (de Foubert et al, 2007;Liu and Robichaud, 2009;Romero et al, 2006;Woods et al, 2012). As already noted (see above), density and 5-HT 6 levels were significantly decreased in a cohort of AD patients (Marcos et al, 2008).…”