Opioid peptide receptor studies. 16. Chronic morphine alters G-protein function in cells expressing the cloned mu opioid receptor.
Chronic morphine treatment results in functional uncoupling of the mu opioid receptor and its G protein in both cell culture and animal models. In the present study, Chinese hamster ovary (CHO) cells stably expressing the cloned human mu opioid receptor (hMOR-CHO cells) were incubated with 1 microM of morphine (or no drug) for 20 h. Subsequently, we assessed DAMGO- and morphine-stimulated [(35)S]-GTP-gamma-S binding and agonist-mediated inhibition of forskolin-stimulated cAMP accumulation. Using a single concentration of [(35)S]-GTP-gamma-S (0.05 nM), chronic morphine treatment did not significantly change basal [(35)S]-GTP-gamma-S binding, shifted the morphine EC(50) from 59 nM to 146 nM, and decreased the maximal stimulation (E(max)) from 201% to 177%. Similar results were observed with DAMGO. Binding surface analysis resolved two [(35)S]-GTP-gamma-S binding sites (high-affinity and low-affinity sites). In control cells, morphine stimulated [(35)S]-GTP-gamma-S binding by increasing the B(max) of the high-affinity site. In morphine-treated cells, morphine stimulated [(35)S]-GTP-gamma-S binding by decreasing the high-affinity K(d) without changing the B(max). Morphine treatment increased the EC(50) (5-11-fold) for agonist-mediated inhibition of forskolin-stimulated cAMP accumulation. These changes were not observed in cells expressing a mutant mu opioid receptor which does not develop morphine tolerance, suggesting that the changes in [(35)S]-GTP-gamma-S binding observed in hMOR-CHO cells result from the development of morphine tolerance.
Xu
Heng
H
Clinical Psychopharmacology Section, Intramural Research Program, National Institute on Drug Abuse, National Institutes of Health, Baltimore, Maryland 21224, USA.
Lu
Yi-Feng
YF
Rothman
Richard B
RB
D000262:Adenylyl Cyclases / Q000187:drug effects / Q000378:metabolism; D000701:Analgesics, Opioid / Q000494:pharmacology; D000818:Animals; D016466:CHO Cells; D018929:Cell Culture Techniques; D005576:Colforsin / Q000494:pharmacology; D006224:Cricetinae; D000242:Cyclic AMP / Q000378:metabolism; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)- / Q000494:pharmacology; D020536:Enzyme Activators / Q000494:pharmacology; D020962:Heterotrimeric GTP-Binding Proteins / Q000187:drug effects* / Q000378:metabolism*; D009020:Morphine / Q000494:pharmacology*; D009294:Narcotics / Q000494:pharmacology*; D017450:Receptors, Opioid, mu / Q000187:drug effects* / Q000378:metabolism*; D013462:Sulfur Radioisotopes
D000701:Analgesics, Opioid; D020536:Enzyme Activators; D009294:Narcotics; D017450:Receptors, Opioid, mu; D013462:Sulfur Radioisotopes; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; D005576:Colforsin; D009020:Morphine; D000242:Cyclic AMP; D020962:Heterotrimeric GTP-Binding Proteins; D000262:Adenylyl Cyclases