Anti-Analgesic Effect of the Mu/Delta Opioid Receptor Heteromer Revealed by Ligand-Biased Antagonism

Abstract

Delta (DOR) and mu opioid receptors (MOR) can complex as heteromers, conferring functional properties in agonist binding, signaling and trafficking that can differ markedly from their homomeric counterparts. Because of these differences, DOR/MOR heteromers may be a novel therapeutic target in the treatment of pain. However, there are currently no ligands selective for DOR/MOR heteromers, and, consequently, their role in nociception remains unknown. In this study, we used a pharmacological opioid cocktail that selectively activates and stabilizes the DOR/MOR heteromer at the cell surface by blocking its endocytosis to assess its role in antinociception. We found that mice treated chronically with this drug cocktail showed a significant right shift in the ED50 for opioid-mediated analgesia, while mice treated with a drug that promotes degradation of the heteromer did not. Furthermore, promoting degradation of the DOR/MOR heteromer after the right shift in the ED50 had occurred, or blocking signal transduction from the stabilized DOR/MOR heteromer, shifted the ED50 for analgesia back to the left. Taken together, these data suggest an anti-analgesic role for the DOR/MOR heteromer in pain. In conclusion, antagonists selective for DOR/MOR heteromer could provide an avenue for alleviating reduced analgesic response during chronic pain treatment.

Critical Role for Gi/o-Protein Activity in the Dorsal Striatum in the Reduction of Voluntary Alcohol Intake in C57Bl/6 Mice

“…Here, our findings indicate that activation of dorsal striatal G i/o -coupled receptors, either via endogenous DORs or by virally expressed DREADDs, is sufficient to decrease voluntary alcohol intake in C57Bl/6 male mice. As β-arrestin-2 recruitment is associated with rapid internalization of DORs in vitro and in vivo [where DORs are degraded upon internalization ( 36 , 37 , 64 )], we hypothesize that β-arrestin-2 recruitment to DORs by SNC80 can lead to rapid desensitization of endogenously expressed DORs, resulting in increased alcohol similar to that observed in DOR KO mice ( 18 ). In addition, SNC80-induced β-arrestin-2 recruitment may lead to β-arrestin-dependent signaling events ( 78 ), such as increased phosphorylation of ERK ( 79 , 80 ).…”

“…In the second infusion test week, TAN-67 was infused, followed by SNC80 infusion in the third infusion test week. This specific order of drug infusion was determined based upon the in vitro β-arrestin-2 recruitment profiles of TAN-67 and SNC80 (Figure 3 ) and previously published work on SNC80’s ability to cause rapid DOR internalization [and potential degradation ( 64 )] in vitro and in vivo (in the striatum) ( 20 , 37 ). Thus, we infused TAN-67 first to prevent potential SNC80-induced desensitization of the DOR system and we did not counterbalance our drug infusions, thus limiting our conclusions on how observed SNC80 responses may be confounded by potential inflammation upon repeated drug infusion into this brain region.…”

In vivo opioid receptor heteromerization: where do we stand?

“…Treatment with a low dose of naltriben pharmacologically stabilized MOP/DOP receptors at the cell membrane in transfected HEK 293 cells by blocking methadone‐promoted MOP/DOP receptor internalization without affecting MOP/DOP signalling (Milan‐Lobo and Whistler, *). In vivo , chronic administration of the MOP receptor selective agonist methadone with a low dose of the DOP receptor selective antagonist naltriben resulted in reduced thermal analgesia that could be restored by promoting internalization with methadone or by blocking signalling with the antagonist naltrindole (Milan‐Lobo et al ., *).…”

Challenges for opioid receptor nomenclature: IUPHAR Review 9

“…Towards this end, reagents that allow detection and evaluation of the endogenous OP heterodimers are being generated and these have begun to show promising results; δ−μ heterodimer‐selective antibodies have been useful in revealing morphine‐induced up‐regulation of this heterodimer in the brain and in demonstrating heterodimer‐directed signal trafficking (Rozenfeld and Devi, ; Gupta et al ., ). Ligands selectively targeting the heterodimer have helped demonstrate allosteric modulation of ligand binding and signalling by heterodimerization (Gomes et al ., 2011; 2013) as well as the exploration of the pharmacological properties of heterodimers in vivo (Daniels et al ., ; Milan‐Lobo et al ., ). Finally, cell‐permeable peptides that selectively disrupt the δ−μ heterodimer have helped address the contribution of this heterodimer to opioid pharmacology (He et al ., ).…”