Characterization of a 5-HT3–ELIC Chimera Revealing the Sites of Action of Modulators

Cys-loop receptors are major sites of action for many important therapeutically active compounds, but the sites of action of those that do not act at the orthosteric binding site or at the pore are mostly poorly understood. To help understand these, we here describe a chimeric receptor consisting of the extracellular domain of the 5-HT3A receptor and the transmembrane domain of a prokaryotic homologue, ELIC. Alterations of some residues at the coupling interface are required for function, but the resulting receptor expresses well and responds to 5-HT with a lower EC50 (0.34 μM) than that of the 5-HT3A receptor. Partial agonists and competitive antagonists of the 5-HT3A receptor activate and inhibit the chimera as expected. Examination of a range of receptor modulators, including ethanol, thymol, 5-hydroxyindole, and 5-chloroindole, which can affect the 5-HT3A receptor and ELIC, suggest that these compounds act via the transmembrane domain, except for 5-hydroxyindole, which can compete with 5-HT at the ort...