Single-molecule characterization of opioid receptor heterodimers reveals soluble µ-δ dimer blocker peptide alleviates morphine tolerance

Heterodimerization of opioid receptors (ORs), MOR, KOR, and DOR, is implied in their functional regulation and diversification, and thus its understanding is crucial for developing better analgesic treatments. However, our knowledge on OR heterodimerization/heterodimers remains limited. Here, using single-molecule imaging and functional analysis, we find that MOR, the main morphine receptor, repeatedly forms transient (≈250 ms) heterodimers with DOR every 1-10 seconds, but not with KOR, whereas DOR and KOR also form transient heterodimers. We obtain all the heterodimer-monomer equilibrium constants and rate constants with/without agonists. We identify the critical heterodimer binding sites in the extracellular domains, in addition to the less-specific transmembrane domains, and develop soluble peptide blockers for MOR-DOR and DOR-KOR heterodimerization, using amino-acid sequences mimicking the extracellular binding sites. With these peptide blockers, we dissect the monomer/dimer roles in OR internalization and signaling. The soluble MOR-DOR heterodimer blocker reduces the development of long-term morphine tolerance in mice.