5‐Chloroindole: a potent allosteric modulator of the 5‐HT₃ receptor

Abstract

BACKGROUND AND PURPOSEThe 5-HT3 receptor is a ligand-gated ion channel that is modulated allosterically by various compounds including colchicine, alcohols and volatile anaesthetics. However the positive allosteric modulators (PAMs) identified to date have low affinity, which hinders investigation because of non-selective effects at pharmacologically active concentrations. The present study identifies 5-chloroindole (Cl-indole) as a potent PAM of the 5-HT3 receptor. EXPERIMENTAL APPROACH5-HT3 receptor function was assessed by the increase in intracellular calcium and single-cell electrophysiological recordings in HEK293 cells stably expressing the h5-HT3A receptor and also the mouse native 5-HT3 receptor that increases neuronal contraction of bladder smooth muscle. KEY RESULTSCl-indole (1-100 μM) potentiated agonist (5-HT) and particularly partial agonist [(S)-zacopride, DDP733, RR210, quipazine, dopamine, 2-methyl-5-HT, SR57227A, meta chlorophenyl biguanide] induced h5-HT3A receptor-mediated responses. This effect of Cl-indole was also apparent at the mouse native 5-HT3 receptor. Radioligand-binding studies identified that Cl-indole induced a small (∼twofold) increase in the apparent affinity of 5-HT for the h5-HT3A receptor, whereas there was no effect upon the affinity of the antagonist, tropisetron. Cl-indole was able to reactivate desensitized 5-HT3 receptors. In contrast to its effect on the 5-HT3 receptor, Cl-indole did not alter human nicotinic α7 receptor responses. CONCLUSIONS AND IMPLICATIONSThe present study identifies Cl-indole as a relatively potent and selective PAM of the 5-HT3 receptor; such compounds will aid investigation of the molecular basis for allosteric modulation of the 5-HT3 receptor and may assist the discovery of novel therapeutic drugs targeting this receptor. LINKED ARTICLESRecent reviews on allosteric modulation can be found at:

Novel mechanism of modulation at a ligand‐gated ion channel; action of 5‐Cl‐indole at the 5‐HT3A receptor

“…We previously demonstrated that when 5‐HT 3 A receptors were completely desensitized by prolonged application of 5‐HT, subsequent application of Cl‐indole generated an inward current indicative of Cl‐indole being able to reactivate desensitized receptors (Newman et al , ) (see also Figure ). We therefore tested whether Cl‐indole could modulate receptor resensitization using a dual pulse protocol; a single prolonged application of 5‐HT (10 μM, 20 s) induced receptor desensitization (Figure A).…”

“…As well as orthosteric agonism, allosteric modulation of the 5‐HT 3 receptor has been reported by compounds as diverse as alcohols (Downie et al , ; Bentley and Barnes, ; Stevens et al , ), cannabinoids (Barann et al , ), indole derivatives (Parker et al , ; Van Hooft et al , ; Newman et al , ), terpenes (Lansdell et al , ; Ziemba et al , ), benzamides (Jørgensen et al , ) and the class of novel negative allosteric modulators typified by PU02 (Trattnig et al , ). A number of these modulators appear to have their effect via a site (or sites) in the transmembrane domain [e.g.…”

“…We have recently described a selective allosteric modulator of the 5‐HT 3 receptor, 5‐chloro‐indole (Cl‐indole; (Newman et al , ). Our study suggested that Cl‐indole slows the decay of 5‐HT 3 A receptor currents and that it can reactivate desensitized 5‐HT 3 A receptors in the continued presence of 5‐HT (Newman et al , ). In the light of the mixed modes of action described for indole derivatives, we undertook a study to further explore the mode of action of Cl‐indole on the modulation of the 5‐HT 3 receptor.…”

Activation of Human 5-Hydroxytryptamine Type 3 Receptors via an Allosteric Transmembrane Site

“…For example, positive allosteric modulation of 5-HT 3 Rs has been observed with alcohols (Machu and Harris, 1994), volatile anesthetics (Machu and Harris, 1994), colchicine (de OliveiraPierce et al, 2009), and 5-chloroindole (Newman et al, 2013). In addition, menthol (a monoterpene) has been shown to act as an allosteric inhibitor of human 5-HT 3 Rs (Ashoor et al, 2013).…”

“…Examples of such compounds include verapamil (Hargreaves et al, 1996), cannabinoids (Barann et al, 2002), cannabidiol (Yang et al, 2010), hydrocortisone (Corradi et al, 2011), menthol (Ashoor et al, 2013), and PU02 (6-[(1-naphthylmethyl)thio]-9H-purine) (Trattnig et al, 2012). A variety of positive allosteric modulators of 5-HT 3 Rs have also been identified, including alcohols (Zhou and Lovinger, 1996), 5-hydroxyindole (van Hooft et al, 1997) colchicine (de Oliveira-Pierce et al, 2009), and 5-chloroindole (Newman et al, 2013).…”