Kyle M. Baumbauer, Ph.D. - US grants

? DESCRIPTION (provided by applicant): Spinal cord injury (SCI) can result in profound loss of function and constitutes a significant financial burden for patients. In addition, the majority of patients develop chronic pain that is difficult to manage and are forced to develop coping strategies for dealing with the pain. The literature examining pain following SCI has been built around what happens at or near the site of injury - within the spinal cord. However, there is evidence that the cells within the spinal cord are not the only cells in the nociceptive system impacted by SCI. In fact, sensory neurons involved in the transmission of pain signals to the central nervous system, so-called nociceptors, play an integral role in the ensuing pain. There are few physiological/mechanistic studies examining the impact of SCI on sensory neuron function, but what has been published has shown that sensory neurons exhibit spontaneous activity following SCI. Spontaneous activity is characterized by neuronal firing independent of peripheral stimulation and has been shown to contribute to behavioral hypersensitivity to both mechanical and thermal stimulation. Interestingly, topical application of substances like capsaicin or lidocaine to the skin helps to alleviate pain in both humans and rodents by targeting the afferents within the skin. In general, we know that the majority of sensory neurons impacted by SCI are peptidergic, expressing peptides such as CGRP and substance P. A minority of the neurons are nonpeptidergic and do not express CGRP or substance P. We also know that the neurons that exhibit spontaneous activity are sensitive to capsaicin and possess the TRPV1 receptor, and that spontaneous activity can be reduced by silencing the nociceptor specific sodium channel, Nav1.8. These discoveries are instrumental for our understanding of how SCI impacts nociceptor function, but what is lacking is a way to selectively target specific nociceptors affected by SCI. Here, we propose a way of targeting specific sensory neurons for both scientific study and for the eventual development of therapeutic interventions. We will utilize an ex vivo skin/nerve/DRG/spinal cord preparation that leaves the entire peripheral sensory circuit intact. We will electrophysiologically characterize neurons from SCI, sham, and naïve mice to determine how cells respond independent of stimulation or in response to mechanical, heat, and cold stimulation. Following recordings cells will be injected with a fluorescent dye, collected, and individual cells will be subjected to real-time RT-PCR to measure level of gene expression. Gene expression profiles will be created for all of the afferents examined, and analysis will be performed to capture any injury-induced alterations in gene expression and neuronal function. With this knowledge we can then identify targets responsible for hypersensitivity, and guide development of novel therapeutic interventions for the treatment of SCI-induced pain.

ABSTRACT Spinal cord injury (SCI) can result in profound loss of function the majority of patients experience persistent pain that is difficult to manage. Most of what is known about the mechanisms underlying pain following SCI has focused on changes occurring within the spinal cord. However, recent work has shown that primary afferents also contribute to the initiation and maintenance of SCI-induced chronic pain. Of particular significance is the finding that afferents exhibit spontaneous activity (SA) following SCI, and fire independent of peripheral stimulation. SA has also been shown to contribute to behavioral indices of SCI-induced chronic pain. The mechanisms underlying the emergence of SA in sensory neurons have not been clearly elucidated, but a population of afferents that express the sodium channel, Nav1.8, and the TRPV1 receptor appear to be vulnerable to the effects of SCI. These afferents are thought to be peptidergic, suggesting that this population of nociceptors is primarily responsible for the emergence of pain following SCI. However, recent work from our laboratory has shown that the SCI impacts both peptidergic and nonpeptidergic afferents. Using single cell qPCR, we have shown that the expression of a number of pain-relevant genes is increased in both populations of afferents within 24 hr of SCI, and that increased gene expression corresponds to the onset of SA. Of these targets, Acid Sensing Ion Channel Subunit 3 (ASIC3) exhibits the largest increase in expression following SCI. Given that ASIC3 shows a rapid increase in expression following SCI in both populations of afferents, we hypothesize that ASIC3 functions to relay the onset of SCI and initiates afferent sensitization and subsequent behavioral sensitivity. To examine the role of ASIC3 in afferent sensitization and behavioral sensitivity following injury, SCI, sham, and naïve mice will receive injections of targeting or nontargeting antisense oligonucleotides (ASOs) at the time of SCI (Aim 1) or 6 days following injury (Aim 2). We will then assess behavioral sensitivity to mechanical stimulation of the hindpaw, followed by ex vivo characterization of neuronal responses to mechanical, heat, and cold stimulation 1, 7, 14, and 28 days following SCI. Recorded cells will then be collected and levels of ASIC3 mRNA expression will be evaluated using single cell qPCR. We predict that administration of ASIC3 targeting siRNA will attenuate the development of SCI-induced mechanical sensitivity and afferent sensitization. We further propose that given the nature of SCI and the potential for early intervention, targeting ASIC3 at the time of injury may be effective at disrupting the processes underlying the development of persistent pain following SCI.