Effect of repetitive magnetic stimulation on respiratory function after cervical spinal cord injury

Abstract

Introduction

Traumatic cervical spinal cord injuries (cSCI) damage respiratory pathways leading to life-threatening respiratory insufficiency. Inflammation is known to play a role in limiting neural regeneration after injury, thereby exacerbating respiratory dysfunction and refrain respiratory recovery. Repetitive Magnetic Stimulation (rMS) has emerged as a promising therapeutic intervention in SCI, demonstrating potential in reducing inflammation at the spinal level. In this project, we aim to explore the therapeutic potential of rMS in enhancing respiratory function by attenuating neuro-inflammation in a mouse cervical hemi-contusion (HC) model, which closely mimics the pathophysiology of human cSCI.

Methods

Eighteen adult Swiss male mice underwent hemi-contusion at the C3-C5 level (C3HC, n = 15) or laminectomy (n = 3) and received either high-frequency (10 Hz) rMS or sham treatment for 10 days (9 trains of 100 biphasic pulses, separated by 30 s intervals between trains delivered at 80% MO (percentage of maximum output of the stimulator), 900 stimulations per protocol). Animals that showed a reduction of more than 20% from baseline were divided into two groups: (1) C3HC + sham rMS (n = 5); (2) C3HC + rMS (n = 6). Respiratory function was assessed using whole-body plethysmography before (Day 0) and after C3HC (Day 7) and after intervention (Day 21) and provide amplitude (tidal volume, V_T) and respiratory frequency (f_R). Moreover, diaphragm activity was measured by in-situ electromyography (EMG). All spinal cord samples were collected for Immunofluorescence experiments.

Results

In the whole group of mice, SCI led to a mean decrease in V_T of 37% without differences between groups. Mice which received rMS, exhibited a recovery in V_T greater than spontaneous recovery as measured in the sham rMS group and there was a trend to significant difference between groups (rMS: D₂₁vs. D₇: 6.5 ± 1.1 μL/g vs. 4.1 ± 1.2 μL/g, sham rMS: D₂₁vs. D₇: 4.5 ± 0.5 μL/g vs. 3.3 ± 1.3 μL/g, 2 ways RM ANOVA, p = 0.05). There was no significant change observed for f_R. As a result, similar changes were observed for V_E than V_T after rMS or Sham but differences between groups were not significant. In addition, at D₂₁, the activity of the ipsilateral (injured side) hemidiaphragm tended to be lower than control values (Laminectomy) but only in the sham group, for both ventral and medial hemidiaphragms (Ventral: sham rMS vs. lami: 5.0 ± 1.4 vs. 7.3 ± 1.5 A. U., P = 0.09, Medial: sham rMS vs. lami: 4.7 ± 2.4 vs. 8.9 ± 3.3 A. U., P = 0.06). No differences were found with baseline in the rMS group, suggesting that hemidiaphragm EMG activity may have recovered faster in this group. No differences were found in this contralateral side.

Conclusion

Although preliminary, our results showed that rMS improved respiratory recovery after C3/4HC in mice and particularly tidal volume. The reduction in EMG activity that tended to be only present in the sham group would suggest that increased activation and/or excitability of phrenic nerve is one mechanism induced by rMS. However, this finding needs to be confirmed with a greater number of animals. Moreover, neuroinflammation at the spinal cord level (astrocyte, macrophage, microglia, and chondroitin sulfate proteoglycan) will be analyzed and correlations with changes in respiratory variables will be investigated in regard to treatment groups.