5-Chloroindole; a useful pharmacological tool to discriminate low intrinsic activity partial agonists from antagonists of the 5-HT3 receptor

The human (h) 5-HT3A receptor subunit forms a functional homomeric receptor that is a useful model to study cys-cys loop ligand-gated ion channels, including the pharmacology of positive allosteric modulators. Various compounds have been identified as positive allosteric modulators of 5-HT3 receptors, including alcohols, volatile anaesthetics and indoles.
The present study used a FlexStation to monitor intracellular Ca2+ with fluorescent indicators in HEK293 cells stably transfected with the h5-HT3A subunit. Emax, pEC50 and Hill slope values in response to various 5-HT3 receptor agonists (mCPBG, 2-methyl-5-HT, DDP 733, quipazine, dopamine and the endogenous agonist, 5-HT) were consistent with the pharmacology of the human 5-HT3 receptor. In the presence of 5-chloroindole (cl-indole; 1-100 μM), 5-HT3 receptor agonist-induced responses were increased and prolonged (as apposed to the relatively rapid desensitization seen with agonist alone). Emax values determined for 5-HT increased to 117 ± 5% in the presence of Cl-indole (10 μM). In addition, the partial 5-HT3 receptor agonists, DDP 733, quipazine, 2-methyl-5-HT and mCPBG, appeared as full, or near full, agonists in the presence of 5-chloroindole (10 μM), with Emax value increases of 23 ± 5% to 71 ± 11%, 46 ± 9% to 115 ± 14%, 72 ± 8% to 121 ± 18%, and 89 ± 2% to 106 ± 10%, respectively. This potentiation was cl-indole concentration dependent. In contrast, the selective 5-HT3 receptor antagonists ramosetron, palonosetron, BRL 46470, ondansetron and alosetron, did not increase [Ca2+]i in the absence or presence of cl-indole.
Similar to previous studies, full agonists, like 5-HT, caused near complete desensitisation of the response; the response did not recover when cl-indole was added subsequently. However, after response to a partial agonist (DDP733), addition of cl-indole increased [Ca2+]i. These findings suggest that while the full desensitisation induced by full agonists is irreversible, the partial desensitisation induced by partial agonists can be reversed, at least partly, by cl-indole.
It is concluded that cl-indole is a useful tool to investigate the functional interaction of low intrinsic activity partial agonists with h5-HT3A receptors.