Morphine tolerance attenuates the resolution of postoperative pain and enhances spinal microglial p38 and ERK phosphorylation

Persistent postoperative pain is a very common phenomenon which severely affects the lives of patients who develop it following common surgical procedures. Opioid analgesics are of limited efficacy in the treatment of persistent pain states because of side effects including antinociceptive tolerance. We have previously shown that surgical incision injury and morphine tolerance share similar mechanisms, including a central nervous system (CNS) role of spinal cord glia. We therefore hypothesized that prior chronic morphine exposure would inhibit the resolution of postoperative allodynia through increased glial ionized calcium-binding adaptor protein 1 (Iba1) and glial fibrillary acidic protein (GFAP) protein expression and mitogen activated protein kinase (MAPK) activation. To test this hypothesis, rats were implanted with subcutaneous osmotic minipumps on day zero, releasing saline or morphine for seven days preceding or seven days preceding and following paw incision surgery, which was completed on day seven. Thermal hyperalgesia and mechanical allodynia were assessed postoperatively every three days. Chronic morphine attenuated the resolution of postoperative thermal hyperalgesia and mechanical allodynia through day twenty. However, no changes in Iba1 or GFAP expression were observed in the spinal cord dorsal horn between groups. Assessment of MAPK protein phosphorylation revealed that chronic morphine administration enhanced both p38 and extracellular receptor kinase (pERK) phosphorylation compared to saline on day twenty. p-p38 and pERK immunofluorescence were only observed to colocalize with a marker of microglial cells and not with markers of astrocytes or neurons. Together, these data demonstrate that chronic morphine administration attenuates the resolution of postoperative allodynia in association with microglial p38 and ERK phosphorylation, independent of changes in Iba1 and GFAP expression.