We examined the effect of p38 mitogen-activated protein kinase (MAPK) inhibitors in models of nociception and correlated this effect with localization and expression levels of p38 MAPK in spinal cord. There was a rapid increase in phosphorylated p38 MAPK in spinal cord following intrathecal administration of substance P or intradermal injection of formalin. Immuncytochemisty revealed that phosphorylated p38 MAPK-immunoreactive cells were predominantly present in laminae I-IV of the dorsal horn. Double-staining with markers for neurons, microglia, astrocytes and oligodendrocytes unexpectedly revealed co-localization with microglia but not with neurons or other glia. Pretreatment with p38 MAPK inhibitors (SB20358 or SD-282) had no effect on acute thermal thresholds. However, they attenuated hyperalgesia in several nociceptive models associated with spinal sensitization including direct spinal activation (intrathecal substance P) and peripheral tissue inflammation (intraplantar formalin or carrageenan). Spinal sensitization, manifested by enhanced expression of cyclo-oxygenase-2 and inflammation-induced appearance of Fos-positive neurons, was blocked by pretreatment, but not post-treatment, with p38 MAPK inhibitors. Taken together, these results indicate that spinal p38 MAPK is involved in inflammation-induced pain and that activated spinal microglia play a direct role in spinal nociceptive processing.

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Long-lasting pain may partly be a consequence of ongoing neuroinflammation, in which astrocytes play a significant role. Following noxious stimuli, increased inflammatory receptor activity, influences in Na(+)/K(+)-ATPase activity and actin filament organization occur within the central nervous system. In astrocytes, the Ca(2+) signaling system, Na(+) transporters, cytoskeleton, and release of pro-inflammatory cytokines change during inflammation. The aim of this study was to restore these cell parameters in inflammation-reactive astrocytes. We found that the combination of (1) endomorphin-1, an opioid agonist that stimulates the Gi/o protein of the μ-opioid receptor; (2) naloxone, an opioid antagonist that inhibits the Gs protein of the μ-opioid receptor at ultralow concentrations; and (3) levetiracetam, an anti-epileptic agent that counteracts the release of IL-1β, managed to activate the Gi/o protein and Na(+)/K(+)-ATPase activity, inhibit the Gs protein, and decrease the release of IL-1β. The cell functions of astrocytes in an inflammatory state were virtually restored to their normal non-inflammatory state and it could be of clinical significance and may be useful for the treatment of long-term pain.

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