The Effects of Nerve Injury and Synaptic Preservation on Motoneuron Activity An

Peripheral nerve injury currently has a poor prognosis that often results in motor deficits such as discoordination and co-contraction of muscle antagonists (Brushart, 2011; Horstman et 2019). Because of this, studying the effects of peripheral nerve injury on the morphology and functional connectivity of the central nervous system (CNS) is of utmost importance. Our study centers around the anatomical changes that occur after peripheral nerve injury in the rat spinal cord, namely the degradation of Ia input defined by the expression of the vesicular glutamate transporter 1 (VGLUT1). This study is based on the proposed microglia-dependent mechanism of permanent synaptic loss from the Rotterman et al. (2019) paper. We ask if 1) we can suppress microglia accumulation with minocycline, a tetracycline antibiotic, and 2) if using minocycline can help preserve the Ia sensory afferent synapses after nerve injury. We first retrogradely labeled the medial gastrocnemius motor pool in 15 adult Wistar rats. One week later, we transected the medial gastrocnemius nerve in the left hindlimb. Rats were either treated with vehicle or minocycline for 14 days following injury. Control animals were also produced. At 14 days post injury, animals were perfused, spinal cords were collected and subsequently sectioned. Using immunohistochemistry (IHC), we labeled VGLUT1 synapses on these injured motor neurons and imaged them with confocal microscopy for subsequent reconstruction and analysis. We found four main results: 1) treatment with minocycline for 14 days after nerve injury does not seem to prevent microglia proliferation, 2) the chromalytic reaction (somatic expansion) that commonly occurs after axotomy did not seem to occur in the minocycline-treated animals, 3) there was partial preservation of somatic VGLUT1 synapses in the minocycline-treatment animals, and 4) there was complete dendritic VGLUT1 synapse preservation in the minocycline-treated animals. Although there were limitations to the study with regards to the methods of counting microglia, the study produced robust conclusions that will aid in the development of further research. Future studies should be conducted on the efficacy of minocycline preserving the synapses, the molecular mechanisms underlying minocycline’s effects, and the potential recovery of nerve function. The conclusions from this study will to nerve injury literature, and therefore help us develop better medical treatments for peripheral nerve injury while furthering our knowledge of the central nervous in = 16, 25%ile = 32, 75%ile = 49.5, and 95%ile = 70 cells. The one-way ANOVA of the three conditions resulted in a statistically significant p-value of < 0.0000001. Through the post-hoc (Tukey) analyses, the control group had significantly less microglia when compared to the cut + vehicle and cut + minocycline treatment groups (both p-values < 0.0000001). When comparing the vehicle and minocycline nerve cut groups, there is no statistical significance between the microglia accumulation around the injury motor neurons (p-value = 0.0929). the results indicate a significant loss of VGLUT1 boutons in the minocycline-treated rats as compared to the controls (p-value = 0.00393), however, the preservation of synapses in the minocycline-treated rats was significantly different than the vehicle treated rats (p-value = 0.00106). This result, in addition to the preserved somatic surface area, indicates the potential effects of minocycline on the inflammation response, even if microglia proliferation was not reduced in these animals.