Abstract
G protein activation by different μ-selective opioid agonists was examined in rat thalamus, SK-N-SH cells, and μ-opioid receptor-transfected mMOR-CHO cells using agonist-stimulated guanosine-5′-O-(γ-thio)-triphosphate ([35S]GTPγS) binding to membranes in the presence of excess GDP. [d-Ala2,N-MePhe4,Gly5-ol]Enkephalin (DAMGO) was the most efficacious agonist in rat thalamus and SK-N-SH cells, followed by (in rank order) fentanyl = morphine ≫ buprenorphine. In mMOR-CHO cells expressing a high density of μ receptors, no differences were observed among DAMGO, morphine or fentanyl, but these agonists were more efficacious than buprenorphine, which was more efficacious than levallorphan. In all three systems, efficacy differences were magnified by increasing GDP concentrations, indicating that the activity state of G proteins can affect agonist efficacy. Scatchard analysis of net agonist-stimulated [35S]GTPγS binding revealed two major components responsible for agonist efficacy differences. First, differences in theK D values of agonist-stimulated [35S]GTPγS binding between high efficacy agonists (DAMGO, fentanyl, and morphine) and classic partial agonists (buprenorphine and levallorphan) were observed in all three systems. Second, differences in the B max value of agonist-stimulated [35S]GTPγS binding were observed between DAMGO and morphine or fentanyl in rat thalamus and SK-N-SH cells and between the high efficacy agonists and buprenorphine or levallorphan in all three systems. These results suggest that μ-opioid agonist efficacy is determined by the magnitude of the receptor-mediated affinity shift in the binding of GTP (or [35S]GTPγS) versus GDP to the G protein and by the number of G proteins activated per occupied receptor.
Footnotes
- Received May 28, 1996.
- Accepted September 27, 1996.
-
Send reprint requests to: Dr. Steven R. Childers, Department of Physiology and Pharmacology, Bowman Gray School of Medicine, Wake Forest University, Medical Center Boulevard, Winston-Salem, NC 27157. E-mail: childers{at}bgsm.edu
-
This work was partially supported by United States Public Health Service Grant DA02904 from the National Institute on Drug Abuse and a Young Investigator Award (D.E.S.) from the North Carolina Governor’s Institute on Alcohol and Substance Abuse.
- The American Society for Pharmacology and Experimental Therapeutics
MolPharm articles become freely available 12 months after publication, and remain freely available for 5 years.Non-open access articles that fall outside this five year window are available only to institutional subscribers and current ASPET members, or through the article purchase feature at the bottom of the page.
|