Abstract

Chronic μ-opioid agonist treatment leads to dependence with withdrawal on removal of agonist. At the cellular level withdrawal is accompanied by a supersensitization of adenylyl cyclase, an effect that requires inhibitory Gα proteins. Inhibitory Gα protein action is modulated by regulator of G protein signaling (RGS) proteins that act as GTPase activating proteins and reduce the lifetime of Gα-GTP. In this article, we use C6 glioma cells expressing the rat μ-opioid receptor (C6μ) to examine the hypothesis that Gαo alone can mediate μ-opioid agonist induced adenylyl cyclase supersensitivity and that endogenous RGS proteins serve to limit the extent of this supersensitization. C6μ cells were stably transfected with pertussis toxin (PTX)-insensitive Gαo that was either sensitive or insensitive to endogenous RGS proteins. Cells were treated with PTX to uncouple endogenous Gα proteins followed by exposure to the μ-opioid agonists [d-Ala²,N-Me-Phe⁴,Gly⁵-ol]-enkephalin or morphine. Supersensitization was observed in cells expressing wild-type Gα, but this was lost on PTX treatment. In cells expressing PTX-insensitive Gαo supersensitization was recovered, confirming that Gαo alone can support supersensitization. In cells expressing the RGS-insensitive mutant Gαo, there was a greater degree of supersensitization and the concentration of μ-agonist needed to achieve half-maximal supersensitization was reduced by 10-fold. The amount of supersensitization seen did not directly relate to the degree of acute inhibition of adenylyl cyclase. These results demonstrate a role for Gαo in adenylyl cyclase supersensitization after μ-agonist exposure and show that this action is modulated by endogenous RGS proteins.