Evidence for an important role of protein phosphatases in the mechanism of morphine tolerance

Acute morphine antinociception has been shown to be blocked by very low picogram doses of okadaic acid indicating that inhibition of protein phosphatase PP2A allows for increases in phosphorylation to inhibit antinociception. Comparative studies in morphine tolerant animals have not been reported. In the present study, we showed a significant increase in the total phosphatase activity in the periaqueductal gray matter (PAG) from morphine-pelleted versus placebo-pelleted mice, 72-h after pellet implantation. This supports our hypothesis that phosphatase activity is increased in tolerance as a compensatory mechanism for the increase in kinase activity during the development of tolerance. We also demonstrated that i.c.v. administration of the phosphatase inhibitor okadaic acid (3 pmol/mouse; a dose tested to be inert in placebo-pelleted mice) enhanced the level of morphine antinociceptive tolerance assessed by the tail immersion test, 72-h following pellet implantation. This was supported by the fact that the same treatment with okadaic acid blocked the increase in phosphatase activity in PAG of morphine tolerant mice indicating that selective inhibition of PP2A contributes to enhanced levels of morphine tolerance. We have previously reported that PKC or PKA inhibitors reversed morphine antinociceptive tolerance in mice. The current study shows that i.c.v. administration of the PKC inhibitors bisindolylmaleimide I or Go6976 reversed the enhanced level of morphine tolerance induced by okadaic acid treatment to the same level of tolerance observed in non-okadaic acid-treated tolerant mice. However, the PKA inhibitor PKI-(14-22)-amide only partially reversed the enhancement of morphine tolerance induced by okadaic acid. Our data suggest an important role for the balance between kinases and phosphatases in modulating tolerance levels. Further studies will be directed towards a better understanding of the role of different phosphatase isoforms in morphine tolerance.