NEUROPATHIC PAIN AFTER SPINAL CORD INJURY and ITS RELATIONSHIP TO MICROGLIA and ATP RECEPTOR UPREGULATION.

Abstract

Purpose/Hypothesis: Spinal cord injury (SCI) often causes neuropathic pain (chronic allodynia and hyperalgesia) but the underlying neu-roanatomical mechanisms are unknown. In peripheral nerve injury models of pain, allodynia developed after upregulation of ATP receptors, specifically P2X4, on dorsal horn microglia. Since astrocyte networks communicate through ATP receptors, altered microglial and astrocytic responses may underlie neuropathic pain after SCI. Therefore, we investigated if allodynia and hyperalgesia after SCI is associated with greater microglial and astrocytic response and the upregulation of P2X4 receptors in the dorsal horn. Number of Subjects: Thirty-seven adult, female, Sprague-Dawley rats were used. Materials/Methods: Rats were randomly assigned to: naive, spinal nerve ligation (SNL), laminectomy, mild (0.5 mm displacement) or moderate (1.1 mm displacement) SCI groups. To rule out cellular responses due to surgery, we included laminectomy and mild SCI groups because they do not historically develop pain. We assessed allodynia and hyperalgesia with von Frey hair and plantar heat tests for 5 weeks after surgery. SCI severity was classified by the amount of white matter spared at the lesion epicenter (WMS) and was analyzed via myelin staining. L5 cross-sections were stained for microglia (OX42), astrocytes (GFAP) and P2X4 ATP receptors, and positively labeled tissue was quantified. To determine whether intracellular signals which cause more activated microglia, astrocytes or ATP receptors occurred in rats with neuropathic pain, mRNA was quantified in a subset of L5 spinal cords (n=2) from naive, SNL, and moderate SCI groups. Results: Lesion severity was within expected limits with greater WMS in the mild (26.995.43%) vs. moderate SCI (2.840.475%) (p < .01). Significant allodynia and hyperalgesia developed in the L5 der-matome for moderate SCI and SNL groups (p < .01). These groups had more activated microglia (p < .01) but not astrocytes (p>.01) in the L5 dorsal horn. Allodynia was positively correlated with microglial activation (p < .05). Interestingly, there was no significant upregulation of P2X4 ATP receptors in the dorsal horn of rats exhibiting neuropathic pain. Conclusions: Robust microglial activation occurs only in rats which develop neuropathic pain, indicating that microglia but not astrocytes or P2X4 ATP receptors are key mediators in the development and maintenance of neuropathic pain. Clinical Relevance: Modulating the microglial response after SCI may prove to be effective at limiting or preventing neuropathic pain after SCI in humans.