Molecular mechanisms of inverse agonism via κ-opioid receptor–G protein complexes

Opioid receptors, a subfamily of G protein-coupled receptors (GPCRs), are key therapeutic targets. In the canonical GPCR activation model, agonist binding is required for receptor–G protein complex formation, while antagonists prevent G protein coupling. However, many GPCRs exhibit basal activity, allowing G protein association without an agonist. The pharmacological impact of agonist-free receptor–G protein complexes is poorly understood. Here we present biochemical evidence that certain κ-opioid receptor (KOR) inverse agonists can act via KOR–Gi protein complexes. To investigate this phenomenon, we determined cryo-EM structures of KOR–Gi protein complexes with three inverse agonists: JDTic, norBNI and GB18, corresponding to structures of inverse agonist-bound GPCR–G protein complexes. Remarkably, the orthosteric binding pocket resembles the G protein-free 'inactive' receptor conformation, while the receptor remains coupled to the G protein. In summary, our work challenges the canonical model of receptor antagonism and offers crucial insights into GPCR pharmacology. This study uncovers that certain κ-opioid receptor inverse agonists form receptor–G protein complexes, even in inactive states, challenging the classic GPCR activation model.