Opioid infidelity: novel opioid peptides with dual high affinity for delta- and mu-receptors

Deltorphins represent the paragon of delta-opioid-receptor ligands of natural origin, since they exceed the affinity and selectivity of the endogenous enkephalins by orders of magnitude. A series of opioid peptides have been developed in which the change in a single amino acid causes an extraordinary increase in mu-receptor binding while maintaining high affinity for the delta-receptor. The peptides appear to have a similar extended conformation in solution with a type-I beta-turn in the N-terminus region, suggesting that tertiary architecture plays a pivotal role in enabling the peptide to bind indiscriminately to mu- and delta-receptors. These dual-affinity peptide ligands can serve to mask delta- and mu-receptors while mapping kappa-receptors in the nervous system, to provide an understanding of the differences and similarities in the structure of the binding domains of delta- and mu-receptors, and might lead to a comprehensive new regime for the clinical management of acute and chronic pain.