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Experimental Neurology (1)
Experimental neurology (1)
Journal of Neurotrauma (1)
Detloff, Megan Ryan (3)
Houlé, John D. (2)
Basso, D.Michele (1)
Fisher, Lesley C. (1)
Liu, Gang (1)
Longbrake, Erin E. (1)
McGaughy, Violetta (1)
Miller, Kassi N. (1)
Popovich, Phillip G. (1)
Santi, Lauren (1)
Wade, Rodel E. (1)
Year of Publication
Chronic At- and Below-Level Pain after Moderate Unilateral Cervical Spinal Cord Contusion in Rats
Wade, Rodel E.
Houlé, John D.
Journal of Neurotrauma
Chronic neuropathic pain is a significant consequence of spinal cord injury (SCI) that is associated with evoked pain, including allodynia and/or hyperalgesia. Allodynia is defined as a painful response to normally innocuous stimuli, and hyperalgesia occurs when there is an amplified pain response to normally noxious stimuli. We describe a model of a unilateral cervical level (C5) contusion injury where sensory recovery was assessed weekly for 6 weeks in 32 adult, female, Sprague-Dawley rats. Bilateral thermal hyperalgesia and tactile allodynia are detectable in the fore- and hindpaws as early as 7 days post-injury (dpi) and persist for at least 42 days. Paw withdrawal latency in response to a noxious thermal stimulus significantly intra-animal pre-operative values. Change in paw withdrawal latency plateaued at 21 dpi. Interestingly, bilateral forepaw allodynia develops in fewer than 40% of rats as measured by von Frey monofilament testing. Similar results occur in the hindpaws, where bilateral allodynia occurs in 46% of rats with SCI. The contralesional forepaw and both hindpaws of rats showed a slight increase in paw withdrawal threshold to tactile stimuli acutely after SCI, corresponding to ipsilesional forelimb motor deficits that resolve over time. That there is no difference among allodynic and non-allodynic groups in overall spared tissue or specifically of the dorsal column or ventrolateral white matter where ascending sensory tracts reside suggests that SCI-induced pain does not depend solely on the size or extent of the lesion, but that other mechanisms are in play. These observations provide a valid model system for future testing of therapeutic interventions to prevent the onset or to reduce the debilitating effects of chronic neuropathic pain after SCI.
central pain; mechanical allodynia; spinal cord injury; thermal hyperalgesia
Exercise modulates microRNAs that affect the PTEN/mTOR pathway in rats after spinal cord injury
Miller, Kassi N.
Houlé, John D.
We investigated microRNAs (miRs) associated with PTEN/mTOR signaling after spinal cord injury (SCI) and after hind limb exercise (Ex), a therapy implicated in promoting spinal cord plasticity. After spinalization, rats received cycling Ex 5 days/week. The expression of miRs, their target genes and downstream effectors were probed in spinal cord tissue at 10 and 31 days post injury. Ex elevated expression of miR21 and decreased expression of miR 199a-3p correlating with significant change in the expression of their respective target genes: PTEN mRNA decreased and mTOR mRNA increased. Western blotting confirmed comparable changes in protein levels. An increase in phosphorylated-S6 (a downstream effector of mTOR) within intermediate grey neurons in Ex rats was blocked by Rapamycin treatment. It thus appears possible that activity-dependent plasticity in the injured spinal cord is modulated in part through miRs that regulate PTEN and mTOR signaling and may indicate an increase in the regenerative potential of neurons affected by a SCI.
Remote activation of microglia and pro-inflammatory cytokines predict the onset and severity of below-level neuropathic pain after spinal cord injury in rats
Fisher, Lesley C.
Longbrake, Erin E.
Popovich, Phillip G.
Spinal cord injury (SCI) impairs sensory systems causing chronic allodynia. Mechanisms underlying neuropathic pain have been more extensively studied following peripheral nerve injury 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-α and IL-1β 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.
allodynia; p38; fractalkine; astrocytes; peripheral nerve injury
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