Chronic Morphine Augments Gβγ/GsαStimulation of Adenylyl Cyclase: Relevance to Opioid Tolerance
- 1Department of Biochemistry, State University of New York, Health Science Center at Brooklyn, Brooklyn, New York 11203 (S.C., M.R., A.R.G.), and 2Department of Pharmacological and Physiological Sciences, University of Chicago, Chicago, Illinois 60637 (S.-Z.Y., W.-J.T.)
Abstract
In the current study, we investigated the neurochemical basis for the previously reported predominance of stimulatory μ-opioid signaling in guinea pig longitudinal muscle/myenteric plexus (LMMP) preparations after chronic in vivo morphine exposure. As expected, recombinant Gsα (rGsα) dose-dependently stimulated adenylyl cyclase (AC) activity in LMMP membranes obtained from opioid naive as well as tolerant LMMP tissue. However, the magnitude of the increase was significantly greater in the latter than in the former. The Gβγ blocking peptide QEHA (50 μm) essentially abolished stimulation by rGsα in LMMP membranes obtained from both opioid naive and tolerant animals. Interestingly, after partial blockade by lower QEHA concentrations, the incremental AC stimulation by rGsα in tolerant LMMP membranes was no longer observed, indicating augmented Gβγ stimulatory responsiveness. Concomitant changes in the content of AC isoform protein are consistent with these biochemical observations. After chronic systemic morphine, AC protein is augmented significantly (56%). This increment is most likely to be composed of AC isoforms that are stimulated by Gβγ. This is the first demonstration in a complex mammalian tissue that persistent activation of opioid receptors results in augmented Gβγ/Gsα AC stimulatory interactiveness. The relevance of such changes to the manifestation of opioid tolerance is discussed.
Footnotes
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Send reprint requests to: Dr. Alan Gintzler, Box 8, Department of Biochemistry, SUNY HSCB, 450 Clarkson Avenue, Brooklyn, NY 11203.
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↵1 Current affiliation: United States Department of Agriculture, ARS, NPA, Roman L. Hruska U.S. Meat Animal Research Center, Clay Center, Nebraska 68933.
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This work was supported in part by Aaron Diamond Foundation Grant HRI 817–533 (M.R., A.R.G.) and National Institutes of Health Grant GM53459 (W.J.T). M.R. was an Aaron Diamond Foundation Postdoctoral Fellow. S.C., M.R., and S.-Z.Y contributed equally to this article.
- Abbreviations:
- AC
- adenylyl cyclase
- LMMP
- longitudinal muscle/myenteric plexus
- EGTA
- ethylene glycol bis(β-aminoethyl ether)-N,N,N′,N′-tetraacetic acid
- HEPES
- 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid
- SDS
- sodium dodecyl sulfate
- TCA
- trichloroacetic acid
- GTPγS
- guanosine-5′-O-(3-thio)triphosphate
- DTT
- dithiothreitol
- BSA
- bovine serum albumin
- SSC
- standard saline citrate
- TES
- N-Tris[hydroxymethyl] methyl-2-aminoethane sulfonic acid
- rGsα
- recombinant Gsα
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- Received April 17, 1998.
- Accepted July 13, 1998.
- The American Society for Pharmacology and Experimental Therapeutics
