Abstract

The recent biochemical demonstration of the association of the μ-opioid receptor (MOR) with Gα_s that increases after long-term morphine treatment (Mol Brain Res135:217–224, 2005) provides a new imperative for studying MOR-Gα_s interactions and the mechanisms that modulate it. A persisting challenge is to elucidate those neurochemical parameters modulated by long-term morphine treatment that facilitate MOR-Gα_s association. This study demonstrates that 1) Gα_s exists as a phosphoprotein, 2) the stoichiometry of Gα_s phosphorylation decreases after long-term morphine treatment, and 3) in vitro dephosphorylation of Gα_s increases its association with MOR. Furthermore, our data suggest that increased association of Gα_s with protein phosphatase 2A is functionally linked to the long-term morphine treatment-induced reduction in Gα_s phosphorylation. These findings are observed in MOR-Chinese hamster ovary and F11 cells as well as spinal cord, indicating that they are not idiosyncratic to the particular cell line used or a “culture” phenomenon and generalize to complex neural tissue. Taken together, these results indicate that the phosphorylation state of Gα_s is a critical determinant of its interaction with MOR. Long-term morphine treatment decreases Gα_s phosphorylation, which is a key mechanism underlying the previously demonstrated increased association of MOR and Gα_s in opioid tolerant tissue.