A comparison of noninternalizing (herkinorin) and internalizing (DAMGO) mu-opioid agonists on cellular markers related to opioid tolerance and dependence

Synapse. 2007 Mar;61(3):166-75. doi: 10.1002/syn.20356.


Previous studies established that Tyr-D-Ala-Gly-N-Me-Phe-Gly-ol (DAMGO) and (2S,4aR,6aR,7R,9S,10aS,10bR)-9-(Benzoyloxy)-2-(3-furanyl)dodecahydro-6a,10b-dimethyl-4,10-dioxo-2H-naphtho-[2,1-c]pyran-7-carboxylic acid methyl ester (herkinorin) are fully efficacious mu-agonists. Herkinorin (HERK), unlike DAMGO, does not recruit beta-arrestin and promote mu-receptor internalization, even in cells that over express beta-arrestin. We hypothesized that chronic HERK and DAMGO treatment will differentially affect cellular markers of tolerance and dependence. CHO cells expressing the cloned human mu-receptor were treated for 20 h with 10 microM DAMGO, HERK, morphine, or medium. Both DAMGO and HERK acted as full agonists in the [(35)S]GTP-gamma-S binding assay with E(MAX) values of 230% and EC(50) values of 12.8 and 92.5 nM, respectively. In the cAMP assay, DAMGO and HERK had similar E(MAX) values of approximately 80% and EC(50) values of 3.23 and 48.7 nM, respectively. Chronic exposure to both drugs produced moderate tolerance to both drugs ( approximately 2 to 5 fold) in the [(35)S]GTP-gamma-S binding assay. In the cAMP assay, chronic DAMGO produced tolerance to both drugs ( approximately 3 to 4 fold). Chronic HERK eliminated the ability of either drug to inhibit forskolin-stimulated cAMP accumulation. Chronic DAMGO increased, and chronic HERK decreased, forskolin-stimulated cAMP accumulation. Naloxone, after chronic HERK (but not DAMGO) induced a large increase in forskolin-stimulated cAMP accumulation. Viewed collectively with published data, the current data indicate that both internalizing and noninternalizing mu-agonists produce cellular signs of tolerance and dependence.

Publication types

  • Comparative Study
  • Research Support, N.I.H., Extramural
  • Research Support, N.I.H., Intramural

MeSH terms

  • Analgesics, Opioid / pharmacology
  • Animals
  • Binding, Competitive / drug effects
  • Binding, Competitive / physiology
  • CHO Cells
  • Cell Membrane / drug effects*
  • Cell Membrane / metabolism
  • Colforsin / pharmacology
  • Cricetinae
  • Cricetulus
  • Cyclic AMP / analysis
  • Cyclic AMP / metabolism
  • Drug Tolerance / physiology*
  • Endocytosis / drug effects
  • Endocytosis / physiology
  • Enkephalin, Ala(2)-MePhe(4)-Gly(5)- / pharmacology*
  • Furans / pharmacology*
  • Guanosine 5'-O-(3-Thiotriphosphate) / metabolism
  • Humans
  • Narcotic Antagonists / pharmacology
  • Opioid-Related Disorders / metabolism*
  • Opioid-Related Disorders / physiopathology
  • Pyrones / pharmacology*
  • Radioligand Assay
  • Receptors, Opioid, mu / agonists*
  • Receptors, Opioid, mu / genetics
  • Receptors, Opioid, mu / metabolism


  • 9-(benzoyloxy)-2-(3-furanyl)dodecahydro-6a,10b-dimethyl-4,10-dioxo-2H-naphtho(2,1-c)pyran-7-carboxylic acid methyl ester
  • Analgesics, Opioid
  • Furans
  • Narcotic Antagonists
  • Pyrones
  • Receptors, Opioid, mu
  • Enkephalin, Ala(2)-MePhe(4)-Gly(5)-
  • Colforsin
  • Guanosine 5'-O-(3-Thiotriphosphate)
  • Cyclic AMP