Tissue injury leads rapidly to an exaggerated response to noxious stimuli applied to the injury site. A primary mechanism underlying this behaviorally defined hyperalgesia is nitiation of facilitated processing in the spinal dorsal horn. Previous work on this grant showed that one component of this facilitation arises from lipid mediator release by a constitutively active P38MAPK-PLA2-COX2 cascade. Of importance, we showed that i) the acute facilitation was mediated sy P38MAPK&, but not P38MAPKa, ii) P38MAPKB is present in microglia, not neurons, iii) peripheral njury activates (phosphorylates) P38MAPK in microglia and iv) down regulation of P38MAPKR., but not P38MAPKaby use of intrathecal (IT) antisense (AS) prevents hyperalgesia. These findings ndicate, unexpectedly, an immediate role for this constitutively expressed MAPK isozyme and spinal microglia in short latencied facilitated states initiated by acute afferent input. The results suggest hypotheses focusing on the role of P38MAPK and non-neuronal cells that acutely regulate spinal excitability in pain states arising from peripheral injury. 1) Acute activation of spinal P38 MAPK by tissue injury results from release of afferent transmitters (glutamate, SP, VIP), ATP and chemokines released from neurons and non-neurons, and circulating products released by inflammation (IPS). Accordingly, activation should be initiated by IT delivery of the respective agonists and the activation by peripheral injury will be blocked by spinal injection of respective antagonists. 2) Activation of spinal P38MAPKB will acutely activate the COX-2 cascade leading to spinal prostaglandins (PG)formation. This release will be examined in vivo using spinal dialysis after down regulation of P38MAPKa or P38MAPKB, with the respective IT AS. 3) Given the role of microglia, spinal blockade of activation using agents such as minocycline or pentoxifylline will block: P38 activation, evoked release of spinal PG, COX2 upregulation and hyperalgesia. These studies thus pursue the theme initiated by studies considering the constitutive spinal COX-PG cascade. Further in contrast to the thinking that microglial contributions occur only in the face of major changes in trophic activity this work points to the contribution of spinal microglia in the ongoing regulation of spinal activity.
The association of components of THE P38/PLA2/COX cascade with dorsal horn microglia provides: i) insights into distinct mechanisms of chronic pain processing and ii) provides functional correlates for the development of novel targets for persistent pain states.
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