Medial deep dorsal glycinergic interneurons as a target for improving locomotion following spinal cord injury - ABSTRACT: Spinal cord injury (SCI) results in the devastating loss of motor, sensory, and autonomic function below the level of the lesion. Importantly, most SCIs occur in the cervical and thoracic levels, leaving the locomotor circuits in the lumbar segments, and sensory afferents below the injury, relatively intact. While these circuits may not be directly affected by the injury, the loss of descending control can lead to changes in excitability, afferent sprouting, and other compensatory changes. Regaining proper control of locomotor circuits after injury is crucial for restoration of efficient locomotion, yet many of the specific circuit rearrangements induced by SCI remain unknown. Inhibitory interneurons (INs) in the dorsal horn of the spinal cord finely tune incoming sensory afferent input, and are a possible target for modulating afferent transmission to locomotor circuits after injury. Specifically, INs regulating proprioceptive afferents are of great interest due to the importance of proprioception on locomotor recovery after SCI. Locomotor circuits are comprised of spinal INs that control the pattern and rhythm of locomotion, one population of which expresses the transcription factor Shox2. We have evidence that sensory afferent pathways to Shox2 INs are a point of plasticity after SCI and treatment with epidural stimulation. Specifically, the loss of the inhibitory sensory pathway to Shox2 INs after SCI suggests that inhibitory INs interposed between sensory afferents and Shox2 INs are a potential novel target for improving locomotion after SCI. Understanding the function of this population, and the mechanisms behind the plasticity of inhibitory sensory pathways to locomotor circuits may then allow for proper modulation after injury to restore proper control of locomotor circuits. The proposed experiments will test the overall hypothesis that medial laminae V/VI glycinergic INs are required for skilled locomotion, and an SCI-induced disruption of proprioceptive afferent input to them is responsible for the observed dysregulation of afferent pathways to Shox2 INs after SCI. Using a combination of whole-cell patch clamp, pharmacology, and histology we will identify known deep dorsal inhibitory populations that overlap with glycinergic INs interposed between sensory afferents and Shox2 INs. Then we will chemogenetically silence this population of glycinergic INs during skilled locomotor tasks. Together, these experiments will allow us to functionally identify medial lamina V/VI glycinergic INs. Next, we will use a combinatorial approach to investigate the mechanism underlying the loss of inhibitory sensory afferent pathways to Shox2 INs after SCI. Electrophysiology, neural tracing, and immunohistochemistry will be used to evaluate the proprioceptive afferent input to medial laminae V/VI glycinergic INs and, Shox2 INs after SCI. The findings from this proposal will provide insight into the role of a specific population of inhibitory INs, and the mechanistic underpinnings of plasticity within inhibitory sensory pathways to locomotor circuits. This knowledge will be vital to identify novel therapeutic targets, and guide strategies to restore locomotion after SCI.
