Structural Determinants for Activity of the Antidepressant Vortioxetine at Human and Rodent 5-HT ₃ receptors

Vortioxetine (VTX) is a recent antidepressant that targets a variety of serotonin receptors. We investigate the drug’s molecular mechanism of operation at serotonin 5-HT ₃ receptors (5-HT ₃ R), which features two mysterious properties: VTX acts differently on rodent and human 5-HT ₃ R; VTX appears to suppress any subsequent response to agonists. Using a combination of cryo-EM, electrophysiology, and molecular dynamics, we show that VTX stabilizes a resting inhibited state of the mouse 5-HT ₃ R and an agonist bound-like state of the human 5-HT ₃ R, in line with the functional profile of the drug. We report four human 5-HT ₃ R structures and show that the human receptor transmembrane domain is intrinsically fragile. We also explain the lack of recovery after VTX administration via a membrane partition mechanism.