The Mu and Delta Opioid Receptors: Mixed Efficacy Ligands and Receptor Trafficking.

Abstract

Opioids are widely used in the treatment of pain; however, the development of tolerance and dependence limit clinical use. The co-administration of a µ opioid receptor (MOR) agonist with a δ opioid receptor (DOR) agonist or antagonist produces analgesia with reduced tolerance and dependence. Elucidating how MOR and DOR interact has the potential to change opioid analgesic treatment. This work has two branches: firstly, cyclic MOR agonist/DOR antagonist peptides were synthesized. Models of MOR and DOR indicate that the active state conformation of both receptors is narrower than the inactive conformations. DOR has bulkier residues occluding its binding pocket; the active and inactive state conformations of DOR are narrower than their MOR counterparts. Increased steric bulk prevents binding to the active state conformation of DOR and is tolerated in the more open MOR active and DOR inactive conformations, producing MOR agonist/DOR antagonist ligands. Analogues were synthesized with bulkier residues and both the size and stereochemistry of the cycle were varied to explore the active and inactive state MOR and DOR conformations. The structure activity relationships found in the cyclic ligands were then translated to linear peptides, as they are easier to synthesize and have higher yields. However, the flexibility of the linear peptides allowed for more compact binding poses, resulting in MOR/DOR agonist ligands. Promising ligands from both series were glycosylated for improved bioavailability. Secondly, selective fluorescent MOR agonist and DOR antagonist ligands were synthesized and used as probes to explore MOR trafficking in response to DOR and DOR antagonists. The presence of both receptors in the same cell did not alter the trafficking of agonist-bound MOR or antagonist-bound DOR when only one ligand was present. The presence of both ligands in cells that expressed both receptors increased the internalization of agonist-bound MOR, without changing the trafficking of antagonist-bound DOR. Internalization of MOR is involved in receptor recycling and can lead to an increase in surface expressed receptor. By increasing surface expression of MOR, antagonist-bound DOR may reduce the development of tolerance to MOR agonists.