Vagus Nerve Stimulation Paired With Rehabilitative Training Enhances Motor Recovery After Bilateral Spinal Cord Injury to Cervical Forelimb Motor Pools

Abstract

Closed-loop vagus nerve stimulation (VNS) paired with rehabilitative training has emerged as a strategy to enhance recovery after neurological injury. Previous studies demonstrate that brief bursts of closed-loop VNS paired with rehabilitative training substantially improve recovery of forelimb motor function in models of unilateral and bilateral contusive spinal cord injury (SCI) at spinal level C5/6. While these findings provide initial evidence of the utility of VNS for SCI, the injury model used in these studies spares the majority of alpha motor neurons originating in C7-T1 that innervate distal forelimb muscles. Because the clinical manifestation of SCI in many patients involves damage at these levels, it is important to define whether damage to the distal forelimb motor neuron pools limits VNS-dependent recovery. In this study, we assessed recovery of forelimb function in rats that received a bilateral incomplete contusive SCI at C7/8 and underwent extensive rehabilitative training with or without paired VNS. The study design, including planned sample size, assessments, and statistical comparisons, was preregistered prior to beginning data collection ( https://osf.io/ysvgf/ ). VNS paired with rehabilitative training significantly improved recovery of volitional forelimb strength compared to equivalent rehabilitative training without VNS. Additionally, VNS-dependent enhancement of recovery generalized to 2 similar, but untrained, forelimb tasks. These findings indicate that damage to alpha motor neurons does not prevent VNS-dependent enhancement of recovery and provides additional evidence to support the evaluation of closed-loop VNS paired with rehabilitation in patients with incomplete cervical SCI.

Keywords: cervical; plasticity; rehabilitation; spinal cord injury; vagal nerve stimulation; vagus nerve stimulation.

Conflict of interest statement

Competing Financial Interests: MPK has a financial interest in MicroTransponder, Inc., which is developing VNS for stroke. RLR is a co-owner of Vulintus, Inc., which makes rodent behavioral testing systems, and owner of Teliatry, which is developing a novel VNS device. All other authors declare no conflicts of interest.

Figures

Figure 1.. Timeline and Experimental Design.

(A) Illustration of experimental timeline for each subject in the study. (B) Example of forelimb pull forces recorded during rehabilitative training. Five trials are shown. Animals in the VNS+Rehab group received a short 0.5 s burst of VNS (red blocks) paired with trials in which pull force exceeded the stimulation threshold. Animals in the Rehab group performed equivalent rehabilitative training without VNS.

Figure 2.. SCI causes bilateral damage between spinal levels C7 and C8.

(A) Damage to gray matter of the spinal cord is centered between C7 and C8, and extends rostrally and caudally. No difference in lesion extent were observed between VNS+Rehab or Rehab groups. Additional lesion quantification can be found in Fig. S1. (B,C) Representative examples from a range of levels illustrating the extent of damage through the cord. Substantial damage was observed near the lesion epicenter, with minimal injury to levels above and below. Scale bars indicate 3 mm in panel B and and 500 μm in panel C.

Figure 3.. VNS paired with rehabilitative training enhances recovery after C7/8 SCI.

(A) VNS paired with rehabilitative training (VNS+Rehab) significantly improves recovery of volitional forelimb strength compared to equivalent rehabilitative training without VNS (Rehab) after bilateral SCI at C7/8. VNS-dependent benefits remain on week 14 after the cessation of stimulation, indicating lasting recovery. (B) Similarly, VNS+Rehab significantly increases hit rate on the isometric pull task compared to Rehab after SCI, measured as the percentage of trials on which peak forelimb pull force exceeds 120g. * p

Figure 4.. VNS-dependent recovery generalizes to similar, but untrained, forelimb tasks.

(A) SCI reduces the total number of forepaw contacts on the cylinder task. VNS paired with rehabilitative training on the isometric pull task significantly increases spontaneous forelimb use during exploration compared to rehabilitative training without VNS, indicative of a generalization of forelimb recovery to a similar task. (B) Pairing VNS with training on the isometric pull task significantly increases grip strength compared to rehabilitative training without VNS. Unpaired t-tests across groups at each time point; ** denotes p