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 nave 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.
The majority of patients with spinal cord injury develop medically resistant to treatment and is often rated as severe. To date, there is a lack of effectiv treatments, in part because the mechanisms underlying SCI- induced pain are not well understood. The current application shifts the focus away from the central nervous system to identify the unique patterns of gene expression in sensory neurons impacted to identify novel targets for therapeutic intervention.
