Co-expression of mu and delta opioid receptors as receptor-G protein fusions enhances both mu and delta signalling via distinct mechanisms

J Neurochem. 2008 May;105(3):865-73. doi: 10.1111/j.1471-4159.2008.05215.x. Epub 2008 Jan 7.


Mu and delta opioid receptors (MORs and DORs) were co-expressed as fusion proteins between a receptor and a pertussis insensitive mutant Galpha(i/o) protein in human embryonic kidney 293 cells. Signalling efficiency was then monitored following inactivation of endogenous Galpha(i/o) proteins by pertussis toxin. Co-expression resulted in increased delta opioid signalling which was insensitive to the mu specific antagonist d-Phe-Cys-Tyr-d-Trp-Arg-Thr-Pen-Thr-NH2. Under these conditions, mu opioid signalling was also increased and insensitive to the delta specific antagonist Tic-deltorphin. In this latter case, however, no G protein activation was observed in the presence of the delta specific inverse agonist N,N(CH3)2-Dmt-Tic-NH2. When a MOR fused to a non-functional Galpha subunit was co-expressed with the DOR-Galpha protein fusion, delta opioid signalling was not affected whereas mu opioid signalling was restored. Altogether our results suggest that increased delta opioid signalling is due to enhanced DOR coupling to its tethered Galpha subunit. On the other hand, our data indicate that increased mu opioid signalling requires an active conformation of the DOR and also results in activation of the Galpha subunit fused the DOR.

MeSH terms

  • Analgesics, Opioid / pharmacology
  • Cell Line
  • Central Nervous System / metabolism
  • GTP-Binding Protein alpha Subunits, Gi-Go / genetics
  • GTP-Binding Protein alpha Subunits, Gi-Go / metabolism*
  • Humans
  • Narcotic Antagonists / pharmacology
  • Neurons / metabolism
  • Nociceptors / metabolism
  • Opioid Peptides / metabolism
  • Pain / genetics
  • Pain / metabolism
  • Pain / physiopathology
  • Protein Binding / physiology
  • Protein Conformation
  • Receptors, G-Protein-Coupled / genetics
  • Receptors, G-Protein-Coupled / metabolism*
  • Receptors, Opioid, delta / drug effects
  • Receptors, Opioid, delta / genetics
  • Receptors, Opioid, delta / metabolism*
  • Receptors, Opioid, mu / drug effects
  • Receptors, Opioid, mu / genetics
  • Receptors, Opioid, mu / metabolism*
  • Recombinant Fusion Proteins / genetics
  • Recombinant Fusion Proteins / metabolism*
  • Signal Transduction / genetics*


  • Analgesics, Opioid
  • Narcotic Antagonists
  • Opioid Peptides
  • Receptors, G-Protein-Coupled
  • Receptors, Opioid, delta
  • Receptors, Opioid, mu
  • Recombinant Fusion Proteins
  • GTP-Binding Protein alpha Subunits, Gi-Go