Characterization of sympathetic preganglionic neuron activity during fictive locomotion in the in vitro neonatal mouse spinal cord
Spinal cord injury (SCI) is a life-altering neurological condition, with lost movement and impaired body functions caused by loss of communication between the brain and spinal cord (SC). Electrical stimulation of the lumbar SC has emerged as a powerful intervention for people with SCI. Clinical trials have shown its ability to improve both movements and lost autonomic body functions (e.g., blood pressure regulation, sweating, whole body metabolism). Although promising, neural mechanisms mediating observed recoveries using this intervention remain unknown, thus limiting its use for the wider SCI population. We hypothesized that ascending neurons from the lumbar SC provide direct synaptic input onto thoracic sympathetic preganglionic neurons (SPN) that, in turn, give homeostatic support to locomotion and exercise. Our lab recently showed that lumbar locomotor-related neurons synapse on SPNs throughout the thoracic SC. Thus, this study aims to characterize activity patterns of SPNs before and during fictive locomotion in the in vitro neonatal mouse SC. We pre‐loaded fluorescent calcium dye into thoracic SPNs in the intermediolateral nucleus by applying reconstituted dye crystals to cut ends of thoracic ventral roots (VR). Using a high‐speed camera, we recorded fluorescent signals emitted from labeled SPNs at baseline (non‐locomotor) and during drug-induced fictive tonic and rhythmic VR activity. SPN calcium signals were then expressed as a change in fluorescence relative to baseline fluorescence intensity in labeled neurons. Our results indicate SPNs demonstrate rhythmic oscillations at baseline, which increase in amplitude during rhythmic VR activity. A higher proportion of SPNs in the T1-T7 segments show increased calcium fluorescence from baseline during tonic and rhythmic VR activity. In contrast, more SPNs in the T8-T13 segments demonstrate decreased fluorescence from their baseline. Fluorescence intensity changes <3% were considered insignificant. Finally, additional distinct SPNs become active with the appearance of tonic discharge or rhythmic locomotor activity. Together, these findings suggest there may be differences in SPN activity patterns at different rostrocaudal thoracic levels during VR locomotor activity. In the future, the results of this study may have practical implications for enhancing the efficacy of techniques like epidural stimulation to promote motor and specific autonomic recovery in people with SCI.
sympathetic preganglionic neurons, motor systems, calcium imaging, spinal cord injury, electrical stimulation