Opioid agonist and antagonist treatment differentially regulates immunoreactive mu-opioid receptors and dynamin-2 in vivo

Eur J Pharmacol. 2004 Sep 13;498(1-3):87-96. doi: 10.1016/j.ejphar.2004.07.052.


Opioid agonists and antagonists can regulate the density of mu-opioid receptors in whole animal and in cell culture. High intrinsic efficacy agonists (e.g., etorphine), but not lower intrinsic efficacy agonists (e.g., morphine), produce mu-opioid receptor down-regulation and can alter the abundance of mu-opioid receptor mRNA. Conversely, opioid antagonists substantially increase the density of mu-opioid receptors without changing its mRNA. Mu-opioid receptor up-regulation has been associated with decreases in the trafficking protein dynamin-2, whereas mu-opioid receptor down-regulation produces an increase in dynamin-2 abundance. To probe the differences between opioid agonist and antagonist-induced mu-opioid receptor regulation, the current study determined changes in mu-opioid receptor density using a combined radioligand binding ([3H] DAMGO) and quantitative Western blotting approach in mouse spinal cord. Furthermore, the differences between intermittent and continuous dosing protocols were evaluated. Continuous (7-8 days) s.c. infusions of naloxone (5 mg/kg/day) or naltrexone (15 mg s.c. implant pellet) increased mu-opioid receptor density in radioligand binding assays (approximately +80%) in mouse spinal cord and down-regulated dynamin-2 abundance (approximately -30%), but had no effect on the abundance of immunoreactive mu-opioid receptor. Continuous (7 days) s.c. infusion of etorphine (200 microg/kg/day) decreased immunoreactive mu-opioid receptor (approximately -35%) and [3H] DAMGO binding (approximately -30%), and concurrently increased dynamin-2 abundance (approximately +40%). Continuous (7 days) morphine infusion (40 mg/kg/day plus 25 mg s.c. implant pellet) had no effect on any outcome measure. Delivery of the same daily dose of etorphine or naloxone using intermittent (every 24 h for 7 days) s.c. administration had no effect on immunoreactive mu-opioid receptor, [3H] DAMGO binding or dynamin-2 abundance. These data indicate that mu-opioid receptor density, determined in radioligand binding assays, and immunoreactive dynamin-2 abundance are regulated by continuous, but not intermittent, opioid ligand treatment. Furthermore, the differential regulation of mu-opioid receptor abundance by agonists and antagonists in immunoblotting assays contrasts with changes in [3H] DAMGO binding. Taken together, these results suggest that etorphine-induced down-regulation may depend upon mu-opioid receptor degradation and changes in dynamin-2-mediated receptor trafficking. Conversely, antagonist-induced up-regulation does not require an increase in mu-opioid receptor synthesis and may entail conversion of receptors to an appropriate conformation to bind ligand, as well as changes in receptor trafficking.

Publication types

  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Analgesics, Opioid / pharmacology
  • Animals
  • Binding, Competitive / drug effects
  • Blotting, Western
  • Dynamin II / metabolism*
  • Enkephalin, Ala(2)-MePhe(4)-Gly(5)- / metabolism
  • Etorphine / pharmacology
  • Male
  • Mice
  • Morphine / pharmacology
  • Naloxone / pharmacology
  • Narcotic Antagonists / pharmacology
  • Radioligand Assay
  • Receptors, Opioid, mu / agonists
  • Receptors, Opioid, mu / antagonists & inhibitors
  • Receptors, Opioid, mu / metabolism*
  • Spinal Cord / drug effects
  • Spinal Cord / metabolism


  • Analgesics, Opioid
  • Narcotic Antagonists
  • Receptors, Opioid, mu
  • Enkephalin, Ala(2)-MePhe(4)-Gly(5)-
  • Naloxone
  • Etorphine
  • Morphine
  • Dynamin II