title: Endogenous regulator of g protein signaling proteins reduce {mu}-opioid receptor desensitization and down-regulation and adenylyl cyclase tolerance in C6 cells.
abstract: Chronic exposure of cells to mu-opioid agonists leads to tolerance which can be measured by a reduced ability to activate signaling pathways in the cell. Cell signaling through inhibitory G proteins is negatively regulated by RGS (regulator of G protein signaling) proteins. Here we examine the hypothesis that the GTPase accelerating activity of RGS proteins, by altering the lifetime of Galpha and Gbetagamma, plays a role in the development of cellular tolerance to mu-opioids. C6 glioma cells were stably transfected with mu-opioid receptor and pertussis toxin (PTX)-insensitive Galpha(o) that was either sensitive or insensitive to endogenous RGS proteins. Cells were treated with PTX to uncouple endogenous Galpha proteins followed by exposure to the mu-opioid agonists [d-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin (DAMGO) or morphine. Receptor desensitization as measured by agonist-stimulated [(35)S]GTPgammaS binding and receptor down-regulation as measured by [(3)H]diprenorphine binding were increased in cells expressing RGS-insensitive Galpha(o). Exposure to high concentrations of morphine or the peptidic mu-opioid agonist DAMGO led to a tolerance to inhibit adenylyl cyclase activity in both cell types with a rapid (30 min) and a slower component. Using a submaximal concentration of DAMGO to induce a reduced level of tolerance, a shift in the concentration-effect curve for DAMGO to inhibit adenylyl cyclase activity was seen in the cells expressing RGS-insensitive Galpha(o), but not in the cells expressing RGS-sensitive Galpha(o), which can be partly explained by an increased supersensitization of the adenylyl cyclase response. The results show that RGS proteins endogenously expressed in C6 cells reduce agonist-induced mu-opioid receptor desensitization, down-regulation, and sensitivity to tolerance to inhibit adenylyl cyclase activity.
authors.affiliation: Department of Pharmacology, University of Michigan Medical School, 1301 Medical Science Research Building III, 1150 West Medical Center Drive, Ann Arbor, MI 48109-0632, USA.
authors.forename: Mary J
authors.initials: MJ
authors.lastname: Clark
authors.forename: John R
authors.initials: JR
authors.lastname: Traynor
mesh_terms: D000262:Adenylyl Cyclases / Q000502:physiology*; D000701:Analgesics, Opioid / Q000494:pharmacology; D045744:Cell Line, Tumor; D002478:Cells, Cultured; D000242:Cyclic AMP / Q000378:metabolism; D015536:Down-Regulation / Q000187:drug effects*; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)- / Q000494:pharmacology; D020558:GTP Phosphohydrolases / Q000378:metabolism; D005910:Glioma / Q000378:metabolism; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000494:pharmacology; D006801:Humans; D037342:Pertussis Toxin / Q000494:pharmacology; D020710:RGS Proteins / Q000235:genetics*; D017450:Receptors, Opioid, mu / Q000187:drug effects*
chemical_list: D000701:Analgesics, Opioid; D020710:RGS Proteins; D017450:Receptors, Opioid, mu; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D000242:Cyclic AMP; D037342:Pertussis Toxin; D020558:GTP Phosphohydrolases; D000262:Adenylyl Cyclases