Remote activation of microglia and pro-inflammatory cytokines predict the onset and severity of below-level neuropathic pain after spinal cord injury in rats

Exp Neurol. 2008 Aug;212(2):337-47. doi: 10.1016/j.expneurol.2008.04.009. Epub 2008 Apr 20.

Abstract

Spinal cord injury (SCI) impairs sensory systems causing chronic allodynia. Mechanisms underlying neuropathic pain have been more extensively studied following peripheral nerve injury (PNI) than after central trauma. Microglial activation, pro-inflammatory cytokine production and activation of p38 MAP kinase pathways may induce at-level allodynia following PNI. We investigated whether midthoracic SCI elicits similar behavioral and cellular responses below the level of injury (lumbar spinal cord; L5). Importantly, we show that anatomical connections between L5 and supraspinal centers remain intact after moderate SCI allowing direct comparison to a well-established model of peripheral nerve injury. We found that SCI elicits below-level allodynia of similar magnitude to at-level pain caused by a peripheral nerve injury. Moreover, the presence of robust microglial activation in L5 cord predicted allodynia in 86% of rats. Also increased phosphorylation of p38 MAP kinase occurred in the L5 dorsal horn of allodynic rats. For below-level allodynia after SCI, TNF-alpha and IL-1beta increased in the L5 dorsal horn by 7 dpo and returned to baseline by 35 dpo. Interestingly, IL-6 remains at normal levels early after SCI and increases at chronic time points. Increased levels of pro-inflammatory cytokines also occurred in the thalamus after SCI-induced allodynia. These data suggest that remote microglial activation is pivotal in the development and maintenance of below-level allodynia after SCI. Fractalkine, a known activator of microglia, and astrocytes were not primary modulators of below-level pain. Although the mechanisms of remote microglial activation are unknown, this response may be a viable target for limiting or preventing neuropathic pain after SCI in humans.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Analysis of Variance
  • Animals
  • Behavior, Animal
  • Cytokines / genetics
  • Cytokines / metabolism*
  • Disease Models, Animal
  • Female
  • Functional Laterality
  • Hyperalgesia / etiology
  • Microglia / physiology*
  • Mitogen-Activated Protein Kinase Kinases / genetics
  • Mitogen-Activated Protein Kinase Kinases / metabolism
  • Nerve Tissue Proteins / genetics
  • Nerve Tissue Proteins / metabolism
  • Neuralgia* / etiology
  • Neuralgia* / metabolism
  • Neuralgia* / pathology
  • Rats
  • Rats, Sprague-Dawley
  • Severity of Illness Index
  • Spinal Cord Injuries / complications*
  • Stilbamidines
  • Time Factors

Substances

  • 2-hydroxy-4,4'-diamidinostilbene, methanesulfonate salt
  • Cytokines
  • Nerve Tissue Proteins
  • Stilbamidines
  • Mitogen-Activated Protein Kinase Kinases