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

Background and Purpose The 5‐HT3 receptor is a prototypical member of the Cys‐loop ligand‐gated ion channel (LGIC) superfamily and an established therapeutic target. In addition to activation via the orthosteric site, receptor function can be modulated by allosteric ligands. We have investigated the pharmacological action of Cl‐indole upon the 5‐HT3A receptor and identified that this positive allosteric modulator possesses a novel mechanism of action for LGICs. Experimental Approach The impact of Cl‐indole upon the 5‐HT3 receptor was assessed using single cell electrophysiological recordings and [3H]‐granisetron binding in HEK293 cells stably expressing the 5‐HT3 receptor. Key Results Cl‐indole failed to evoke 5‐HT3A receptor‐mediated responses (up to 30 μM) or display affinity for the [3H]‐granisetron binding site. However, in the presence of Cl‐indole, termination of 5‐HT application revealed tail currents mediated via the 5‐HT3A receptor that were independent of the preceding 5‐HT concentration but were antagonized by the 5‐HT3 receptor antagonist, ondansetron. These tail currents were absent in the 5‐HT3AB receptor. Furthermore, the presence of 5‐HT revealed a concentration‐dependent increase in the affinity of Cl‐indole for the orthosteric binding site of the human 5‐HT3A receptor. Conclusions and Implications Cl‐indole acts as both an orthosteric agonist and an allosteric modulator, but the presence of an orthosteric agonist (e.g. 5‐HT) is a prerequisite to reveal both actions. Precedent for ago‐allosteric action is available, yet the essential additional presence of an orthosteric agonist is now reported for the first time. This widening of the pharmacological mechanisms to modulate LGICs may offer further therapeutic opportunities.