Sciatic Nerve Electrical Stimulation Enhances Locomotor Recovery in Rats Following Spinal Cord Contusion

Introduction

Spinal cord injury (SCI) is a devastating condition that results in severe disabilities, affecting over 300 000 individuals in the United States with approximately 18 000 new injuries occurring annually. ¹ While significant progress has been made in medical treatment for the consequences of SCI and surgical treatments for associated spinal instability, ² there are limited treatment options for significant locomotor recovery. The impact of SCI on quality of life and the associated socioeconomic burden necessitates the development of effective treatment strategies to promote functional recovery.

Neuromuscular electrical stimulation (NMES) has emerged as a promising approach in post-SCI rehabilitation.^3,4 NMES aims to facilitate or replace volitional movement, thereby enhancing neural motor control and reducing disability. So far, NMES in SCI rehabilitation has primarily been used as a supplementary tool in combination with other therapeutic strategies, ⁴ and the efficacy of NMES as a standalone treatment to improve locomotor function after SCI remains a subject of debate. ⁵

Our previous research demonstrated improved functional outcome following repetitive daily NMES in a stroke model. ⁶ Building on these findings, we investigated whether peripheral nerve electrical stimulation applied to the sciatic nerves could affect functional recovery in a rat model of SCI. We aimed to study the effects of bilateral stimulation of the sciatic nerves on hindlimb locomotor function after SCI, and how well long-term stimulation would be tolerated. The sciatic nerve was chosen because it controls crucial movements of the hindlimbs and limb motion could be easily observed during electrical stimulation. This study was undertaken to contribute to the growing body of knowledge on neuromodulation after SCI, and potentially inform the development of novel therapeutic approaches for patients with spinal cord injuries.

Materials and Methods

Experimental Design

The experimental timeline is illustrated in Figure 1.

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Figure 1.

Experimental timeline.

Surgical Procedures

Spinal Cord Contusion Injury

The spinal cord injury procedure was adapted from Krishna et al. ⁷ Rats were anesthetized with ketamine/xylazine (75/5 mg/ml, i.p). A laminectomy at T9-T11 was performed to expose the underlying spinal cord. A contusion injury was induced at the T10 level vertebra using the Precision Systems and Instrumentation (PSI) Infinite Horizon (IH) Impactor IH-0400, controlled by IH computer software (PSI, Lexington, KY). The impactor tip (3.0 mm diameter) was applied at a speed of 4 cm/s with a dwell time of 0.3 seconds and a force of 150 kDynes. Post-SCI care is detailed in the Supplemental (Supplemental Materials and Methods).

Electrode Placement

Immediately following the SCI procedure, and under the same anesthesia, all rats received bilateral implantation of an in-house designed stimulator electrode cuff on the sciatic nerves (Figure 2A). Details for the electrode cuff design are described in the Supplemental (Supplemental Materials and Methods, Figure 1A). The sciatic nerves were carefully isolated from surrounding tissues and nerve cuffs (Figure 2B1, Supplemental Materials and Methods, Figure 1A) were placed around each sciatic nerve and secured with a 6.0 silk suture (Figure 2B1, 2B2). The wire leads were passed subcutaneously to the skull, where a connector was attached using a screw and dental cement.

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Figure 2.

Sciatic nerve electrode cuff placement. (A) Schematic of surgical procedures for nerve cuff implantation and stimulation. (B1) An intraoperative photograph showing the cuff enclosing the sciatic nerve. (B2) An image of the custom-made electrode cuff used in this report. (C) Schematic diagram of electrical stimulation circuit setup and (D) Schematic demonstration of stimulation parameters.

Electrical Stimulation Protocol

The electrical stimulation of bilateral sciatic nerves was adapted from our previous report. ⁶ A Grass SD9 stimulator, isolated from the ground, was used to deliver the electrical pulses.

The stimulating voltage applied to the nerve was measured with an oscilloscope recording across the stimulating electrode. The stimulating current was measured by the voltage drop across a 1000 Ω resistor in line with the stimulating electrodes and applying ohms law to determine the current (Figure 2C).

The stimulation voltage was gradually increased until muscle contraction was observed (threshold, average 1.2 ± 0.2 mA). The threshold currents were measured, and 1.5 times the threshold current was applied for stimulation.

Threshold currents were recorded for possible nerve damage assessment. It was first recorded after completion of the implantation, and it was conducted daily during the treatment sessions until 6 weeks post-SCI. Stimulating parameters included pulses that were 1.0 msec in duration, and that were delivered every 2 seconds in 1-second trains of 5 Hz (Figure 2D). Daily 30-minute stimulation sessions began 3 days after SCI surgery and continued (Monday – Friday) for a total of 6 weeks. Rats in the SCI/sham group had the exact same placement of the electrodes post-SCI, were placed in the stimulation boxes for the same time period, but did not receive any electrical stimulation. Each stimulation session occurred at least 2 hours after the behavioral tests, and rats were placed in the stimulation box in the same order at the same time daily.

Behavioral Assessment

Rats were video-recorded and assessed weekly using the Basso, Beattie, and Bresnahan (BBB) locomotor rating scale. ⁸ The BBB scale is a 21-point ordinal scale that evaluates hindlimb motor function, ranging from complete paralysis (score 0) to normal locomotion (score 21). Rats were video-recorded in the same order at the same time daily for future analysis of the BBB scores. Rats scoring 8 or higher on the BBB scale at 3 days post-SCI (prior to the start of treatment) were removed from the study.

Results

Electrical Stimulation Improved Functional Outcome Post-SCI

Pre-SCI baseline BBB analysis showed no significant difference between groups with the score of 21 (Figure 3). Three days post-SCI (prior to treatment), both SCI/Sham (1.7 ± 0.9) and SCI/EStim (2.6 ± 1.1) rats exhibited marked deficits showing minimal hindlimb movement, with no significant difference between groups (P > .05). Repeated-measures 2-way ANOVA revealed a significant effect of time (F(7,105) = 136.49, P < .0001), treatment (F(1,15) = 7.2, P < .05) and a treatment by time interaction (F(7,105) = 5.17, P < .0001) (Figure 3). The overall recovery analysis revealed that rats receiving daily bilateral electrical stimulation performed significantly better than those receiving no stimulation. At the end of behavioral testing, SCI/Sham rats showed intervals of uncoordinated stepping (average BBB scores = 10.6 ± 1.5) while rats receiving daily electrical stimulation showed coordinated stepping (average BBB scores = 15.4 ± 2.2). Significantly better BBB score outcomes by SCI/EStim rats were observed first at 7 days post-SCI and persisted until the end of the study (P < .05).

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Figure 3.

BBB locomotor scale analysis showed no difference between groups at baseline. There were marked deficits 3 days post-SCI before treatment started without group differences. However, the SCI/EStim group exhibited significant improvement greater than the SCI/Sham group (P < .001, two-way repeated measure ANOVA). Post hoc Bonferroni analysis showed that rats receiving electrical stimulation exhibited early and significant improvement starting 7 days post-SCI that persisted until the end of the study (*P < .05, **P < .01 error bars indicate ± SEM).

Discussion

Our results demonstrate that daily bilateral direct sciatic nerve stimulation following thoracic spinal cord contusion in adult rats leads to significantly earlier and overall improved hindlimb locomotor function. These findings suggest that peripheral nerve stimulation may be a promising standalone therapeutic approach for enhancing motor function after SCI.

The observed improvements in BBB scores as early as 7 days post-SCI in the stimulation group suggest that early intervention with electrical stimulation may be crucial for maximizing functional recovery. This rapid improvement could be attributed to several factors, including the preservation of neuromuscular junctions which may prevent muscle atrophy, ⁹ reduction in neuroinflammation, ¹⁰ modulation of spinal cord excitability ⁵ and induction of trophic factor release, ³ all known to occur as a result of electrical nerve stimulation. Additionally, electrical stimulation may promote the reorganization of neural circuits both at the spinal and supraspinal levels. ¹¹ This reorganization could involve the strengthening of existing connections, the formation of new synapses, or the recruitment of alternative pathways to compensate for the damaged spinal cord tissue.^12,13 In this regard, our earlier study of electrical stimulation in the upper extremity following stroke correlated functional recovery with cortical efferent plasticity, indicating the establishment of new neural connections as the underlying basis for better outcome post-stroke. ⁶ In addition, a recent study reported off-site stimulation of sciatic nerves restored diaphragm function in a cervical SCI model. ¹⁴ Further research is necessary to elucidate the precise mechanisms underlying the observed functional improvements in this study.

Our findings have several important clinical implications. Our results indicate that peripheral nerve stimulation alone may be effective in promoting functional recovery after SCI. This could simplify rehabilitation protocols and reduce the need for complex, multimodal interventions. The observed early improvements in BBB scores highlight the importance of initiating neuromodulation therapies as soon as possible after SCI. This may inform clinical decision-making regarding the timing of interventions in SCI patients. While this study used implanted electrodes, the principles demonstrated here could be applied to develop less invasive stimulation protocols, such as transcutaneous electrical nerve stimulation, or even electro-stimulation generated by an ultrasound-activated implant.^10,15 Lastly, the positive effects of peripheral nerve stimulation suggest that it could be a valuable component of combinatorial therapies for SCI, potentially enhancing the efficacy of other interventions such as cell transplantation, pharmacological treatments ² or biologics ¹⁶ that have shown promise in current clinical trials.

Conclusion

This study demonstrates that sciatic nerve electrical stimulation significantly enhances functional recovery in a rat model of thoracic spinal cord contusion suggesting it may be a promising standalone therapeutic approach. These findings provide a foundation for further research into the mechanisms of action and potential clinical applications of peripheral nerve stimulation in SCI rehabilitation.

Supplemental Material

Supplemental Material

Supplemental Material

Supplemental Material

Supplemental Material