Novel Allosteric Modulation of the Human 5-HT₃A Receptor by the Clinically Used Drugs Piperazine and Hydroxyzine

The 5-HT₃ receptor is a serotonin-gated cation channel of the pentameric ligand-gated ion channel family and plays a key role in fast synaptic transmission within the central, peripheral, and enteric nervous systems. Dysregulation of this receptor has been linked to neurological, psychiatric, and gastrointestinal disorders. While competitive 5-HT₃ antagonists, primarily setrons, are clinically used to alleviate gastrointestinal symptoms, there is a growing interest in developing new negative allosteric modulators (NAMs) for this receptor. Here, we applied a drug-repositioning strategy targeting the human 5-HT₃A receptor. Using macroscopic current recordings, we identified a previously unrecognized NAM activity of two clinically used drugs, piperazine and its derivative hydroxyzine, on the 5-HT₃A receptor. Computational analyses suggested two distinct binding sites at subunit interfaces: one in the lower extracellular domain and another in the upper transmembrane domain. Functional characterization of receptors carrying mutations at key residues within these sites (E75, E208, S248, D293) showed that some mutations abolished drug-induced inhibition, whereas others enhanced the inhibitory effect of hydroxyzine. Altogether, these results validate the proposed allosteric sites and demonstrate that both contribute to the actions of these compounds. This work uncovers a new functional connection between 5-HT₃A receptors and piperazine-derived drugs, opening avenues for therapeutic repurposing.