Structure of the δ-opioid receptor bound to naltrindole

The X-ray crystal structure of the mouse δ-opioid receptor in complex with the subtype-selective antagonist naltrindole is reported. Four papers in this issue of Nature present the long-awaited high-resolution crystal structures of the four known opioid receptors in ligand-bound conformations. These G-protein-coupled receptors are the targets of a broad range of drugs, including painkillers, antidepressants, anti-anxiety agents and anti-addiction medications. Brian Kobilka’s group reports the crystal structure of the µ-opioid receptor bound to a morphinan antagonist and the δ-opioid receptor bound to naltrindole. Raymond Stevens’ group reports on the κ-opioid receptor bound to the selective antagonist JDTic, and the nociceptin/orphanin FQ receptor bound to a peptide mimetic. In an associated News and Views, Marta Filizola and Lakshmi Devi discuss the implications of these landmark papers for research on the mechanisms underlying receptor function and drug development. The opioid receptor family comprises three members, the µ-, δ- and κ-opioid receptors, which respond to classical opioid alkaloids such as morphine and heroin as well as to endogenous peptide ligands like endorphins. They belong to the G-protein-coupled receptor (GPCR) superfamily, and are excellent therapeutic targets for pain control. The δ-opioid receptor (δ-OR) has a role in analgesia, as well as in other neurological functions that remain poorly understood1. The structures of the µ-OR and κ-OR have recently been solved2,3. Here we report the crystal structure of the mouse δ-OR, bound to the subtype-selective antagonist naltrindole. Together with the structures of the µ-OR and κ-OR, the δ-OR structure provides insights into conserved elements of opioid ligand recognition while also revealing structural features associated with ligand-subtype selectivity. The binding pocket of opioid receptors can be divided into two distinct regions. Whereas the lower part of this pocket is highly conserved among opioid receptors, the upper part contains divergent residues that confer subtype selectivity. This provides a structural explanation and validation for the ‘message–address’ model of opioid receptor pharmacology4,5, in which distinct ‘message’ (efficacy) and ‘address’ (selectivity) determinants are contained within a single ligand. Comparison of the address region of the δ-OR with other GPCRs reveals that this structural organization may be a more general phenomenon, extending to other GPCR families as well.