Rewiring of spinal respiratory neural network via cervical glutamatergic interneurons preserves respiratory function in progressive cervical spinal cord compression injury (CSM)

Abstract

Cervical spinal cord injury (SCI) disrupts the spinal respiratory neural circuitry leading to the loss of diaphragmatic and intercostal muscle function. While acute traumatic SCI leads to significant ventilatory impairment, CSM, in which the cervical spinal cord is progressively compressed over time, results in milder respiratory dysfunction despite the significant disruption of the cervical neural network. The chronic and progressive nature of this disease modifies the respiratory neural network to limit respiratory insufficiency.Using a unique mouse animal model of CSM, we found a significant loss of PMNs (retrograde labeled with CTb) that innervate the main inspiratory muscle, yet these animals do not exhibit severe respiratory insufficiency similar to what is observed in human CSM patients. Progressively increased Vglut2 positive boutons on the preserved PMNs indicates that despite the significant loss in the number of PMNs, respiratory motor output is maintained via compensatory and progressive increases in glutamatergic input onto preserved PMNs. While the PMNs were decreased following CSM there was an increase in the number of prephrenic cervical interneurons labeled with pseudorabies virus‐152 (PRV‐152). We set out to specifically delineate the extent of glutamatergic presynaptic inputs directly arising from rVRG neurons and those that are relayed via prephrenic cervical interneurons in CSM. To assess direct monosynaptic excitatory glutamatergic projections arising directly from rVRG and cervical spinal cord onto PMNs, a modified rabies virus (Rb) missing the glycoprotein but carrying a GFP protein (Rb‐ΔG‐GFP) with adeno associated virus expressing the Rabies‐Glycoprotein (RG, essential for subsequent trans‐synaptic transfection), was injected into the diaphragm of CSM and sham Vglut2::cre;tdtomato mice. We observe significantly altered input from glutamatergic monosynaptic inputs from rVRG neurons and those from prephrenic cervical interneurons onto PMNs during CSM. In conclusion, this study provides novel insights into the alterations in spinal respiratory networks that occur in the setting of CSM and a greater understanding of the neural control of breathing and the compensatory changes that occur in neural circuits with chronic, graduated compression.Support or Funding InformationPVA