Phosphorylation of Gαs Influences Its Association with the μ-Opioid Receptor and Is Modulated by Long-Term Morphine Exposure

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Abstract
The recent biochemical demonstration of the association of the μ-opioid receptor (MOR) with Gαs that increases after long-term morphine treatment (Mol Brain Res135:217–224, 2005) provides a new imperative for studying MOR-Gαs 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-Gαs association. This study demonstrates that 1) Gαs exists as a phosphoprotein, 2) the stoichiometry of Gαs phosphorylation decreases after long-term morphine treatment, and 3) in vitro dephosphorylation of Gαs increases its association with MOR. Furthermore, our data suggest that increased association of Gαs with protein phosphatase 2A is functionally linked to the long-term morphine treatment-induced reduction in Gαs 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 Gαs is a critical determinant of its interaction with MOR. Long-term morphine treatment decreases Gαs phosphorylation, which is a key mechanism underlying the previously demonstrated increased association of MOR and Gαs in opioid tolerant tissue.

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