Effect of inflammation on recovery and pain after spinal cord injury

Texas A&M University, College Station TX

Investigators

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Abstract

? DESCRIPTION (provided by applicant): The tissue damage caused by a spinal cord injury (SCI) breaks down cellular barriers and alters the composition of the extracellular fluid, initiatig a chemical wave that impacts uninjured cells in the surrounding region, inducing a state that primes the cells to die through a form of pro-inflammatory programmed cell death (pyroptosis). In addition, damage to descending pathways disrupts the regulation of lower (caudal) spinal systems, allowing afferent signals to induce a state of neural over-excitation (central sensitization) that can foster chronic pain and promote cell death. Supporting this, research has shown that pain (nociceptive) input soon after SCI enhances tissue loss, undermines behavioral function, and promotes the development of chronic pain. It is hypothesized that nociceptive stimulation undermines recovery after SCI because it promotes the induction of pyroptosis and thereby enhances secondary tissue damage. Pyroptosis is mediated by a multi-protein complex called the inflammasome, which initiates the proteolytic cleavage of pro-caspase-1 to form caspase 1. Caspase 1 engages cellular signals that cause the release of pro- inflammatory cytokines (e.g., interleukin 1-ß [IL-1ß] and tumor necrosis factor [TNF]). Activation of the inflammasome is tied to a membrane channel (pannexin-1) and the adjoining P2X7 receptor (P2X7R). When engaged, the pannexin-1 channel allows IL-1ß and TNF to flow out of the cell. Calcium entry through the pannexin-1 channel initiates intracellular processes that lead to cell death through lysis. The current project will evaluate whether nociceptive input engages these processes and thereby leads to greater cell loss. A rodent (rat) model will be used. Subjects will receive a thoracic contusion injury. Prior work has shown that the application of a peripheral irritant (capsaicin) undermines behavioral recovery and enhances pain reactivity to tactile stimulation (allodynia). Aim 1 will establish the circumstances under which this occurs and assess tissue damage. It is hypothesized that spinal systems are especially vulnerable during the first few days after injury. Preliminary data indicate that nociceptive stimulation engages cellular signals indicative of pyroptosis (caspase 1, IL-1ß, IL-18). Aim 2 will examine how these processes unfold over time and the cell types affected. It is proposed that the spread of pyroptosis can be halted by a drug combination (probenecid plus Brilliant Blue-G [BBG]) that inhibits both the pannexin-1 channel and the P2X7R. Aim 3 will verify that this drug treatment attenuates the induction of pyroptosis in the acute stage. We also predict that it will reduce behavioral and cellular signs of central sensitization. After an effective regimen is derived, we will test whether it has a protective effect that blocks the adverse effect of nociceptive stimulation on long-term recovery. By discovering treatments that reduce cell loss after SCI, we hope to promote recovery. Further, because activation of the pannexin-1 channel can contribute to the development of central sensitization, interrupting this process should reduce chronic pain after injury.