Nerve root magnetic stimulation

Spinal cord injury (SCI) damages the neural pathways through the spinal canal, resulting in varying levels of motor and sensory dysfunction. However, previous research has found that there are still some intact nerve fibers that control limb movement after SCI, even in patients who have been clinically diagnosed with complete SCI (Weidner et al., 2001; Zijdewind and Thomas, 2003). Therefore, there is a potential to restore some degree of sensorimotor function through neuroplasticity under effective treatments. Existing conventional treatments, such as surgery, hyperbaric oxygen chamber treatment, neurotrophic drugs, and exercise training, can hardly achieve optimal efficacy and most of them have a limited time window of effectiveness. In recent years, transcranial magnetic stimulation (TMS) has been gradually proven to be an effective treatment for a variety of neuropsychiatric diseases, including SCI (Ganzer et al., 2018; Wagner et al., 2018), and the motor cortex has become a widely used stimulation target for the treatment of patients with movement disorders. However, to achieve the goal of a functional remodeling of locomotion, the involvement and restoration of superficial sensation, deep sensation, and proprioception are as important as limb movements, and both should be given great attention to reconstruct the sensorimotor neural circuit. Therefore, simply targeting the motor cortex is insufficient in the treatment of central nervous system disorders and there is an urgent need to innovate and reform the current magnetic stimulation pattern. In this context, we developed a dual-target protocol, including the motor cortex and the spinal nerve root of the targeted muscle, called neural circuit-magnetic stimulation (NC-MS). It was thought to fully mobilize the residual nerve fibers to target the sensorimotor neural circuit. Encouragingly, we have already validated the efficacy of promoting motor functional recovery in patients with chronic incomplete SCI who have experienced little progress in conventional rehabilitation (Mao et al., 2022). Moreover, to determine the target on the nerve root do promote functional recovery, we treated SCI rats with nerve root magnetic stimulation (NRMS) and demonstrated that NRMS can promote electrophysiological improvements in motor function, including increasing the neural activity in the sensory pathway (sensory evoked potential was recorded) and motor pathway (motor evoked potential was recorded), and enhancing the presynaptic inhibition effect on the spinal loop, and can facilitate synaptic reconstruction in sensorimotor cortex (Figure 1A) (Zheng et al., 2022). To further compare the efficacy of the novel stimulation on the nerve root versus NC-MS, we established SCI rat models treated with NRMS or NC-MS to explore their effects on motor function. Forty male Sprague-Dawley rats, aged 2–3 months, weighing 200–220 g, purchased from Shanghai Jiesijie Experimental Animal Farm (license No. SCXK [Hu] 2018–0004), were enrolled in this study. The study protocol was approved by the Animal Ethics Committee of Tongji Hospital Affiliated to Tongji University School of Medicine (approval No. 2019-DW-(036)). All animal experiments complied with the Animal Research: Reporting of In Vivo Experiments (ARRIVE) guidelines (Percie du Sert et al., 2020) and all procedures followed the National Research Council’s Guide for the Care and Use of Laboratory Animals (8th ed) (National Research Council, 2011). These rats were randomized into four groups (n = 10 per group): (1) SCI + sham stimulation group (SCI + SS), (2) SCI + 5 Hz NRMS group, (3) SCI + 10 Hz NRMS group, and (4) SCI + NC-MS group.Figure 1: Study protocol and results of NRMS/NC-MS treatment on SCI rat model.Note: (A) Nerve root magnetic stimulation (NRMS) perception. (B) Study protocol. (C) Changes in Basso-Beattie-Bresnahan (BBB) score pre- and post-SCI over the course of the treatment (NRMS/NC-MS). The mean ± standard deviation (SD) is shown for each group (n = 10 rats in each group). Statistical analyses were performed using two-way repeated measures analysis of variance followed by Tukey’s post hoc tests. *P < 0.05, **P < 0.01, vs. SCI + 5 Hz NRMS group; ## P < 0.01, ### P < 0.001, vs. SCI + 10 Hz NRMS group. MEP: motor evoked potential; NC-MS: neural circuit-magnetic stimulation; RMT: resting motor threshold; SCI: spinal cord injury; SEP: somatosensory evoked potential.These rats underwent 20-second clip compression on the spinal cord and nerve roots at the T10 vertebra using a 50 g aneurysm clip (Fine Science Tools, Heidelberg, Germany) as developed by Rivlin and Tator (Rivlin and Tator, 1978). The rats in the SCI + 5 Hz NRMS, SCI + 10 Hz NRMS, and SCI + NC-MS groups began to receive NRMS or NC-MS treatment from day 3 after the surgery using a figure-of-eight, custom-made coil (25 mm in diameter) for rodents linked with a MagPro R30 magnetic stimulator (MagVenture Co., Farum, Denmark). The stimulation sites were the nerve roots of the L5 lumbar segment (combined with the primary motor cortex for NC-MS treatment), targeting both the left and right gastrocnemius muscles on the paravertebral side. During stimulation, the rats were kept awake in the prone posture by a device made of plastic resin to avoid their escape. The resting motor threshold (RMT) was defined as the minimum intensity that elicited 5 motor evoked potentials > 0.05 mV in 10 consecutive, single TMS pulses at rest. Treatment was administered at 100% RMT in the SCI + 5 Hz NRMS group (500 pulses per day), 60% RMT in the SCI + 10 Hz NRMS group (2000 pulses per day), and 80% RMT in the SCI + NC-MS group (5000 and 1500 pulses per day respectively on the cortex and nerve root) 5 times weekly for 3 weeks. In the SCI + SS group, the coil was perpendicular to the spine followed by sham stimulation. Motor function was assessed by the Basso-Beattie-Bresnahan (BBB) scale (Basso et al., 1995, 1996), a classic assessment of locomotor function in rats, at various time points (before surgery and days 1, 3, 7, 14, and 21 following the procedure) (Figure 1B). Two to three rats were reared in a 2-m-diameter field and allowed to move freely for 5 minutes. Each hindlimb was evaluated on a scale of 0 to 21 based on the coordination, paw placement, joint movements, and toe clearance: 0 represented complete paralysis with no hindlimb movement, whereas 21 indicated unimpaired motor function as normal, uninjured rats under observation. The assessment was performed independently by two observers who were blind to the grouping of the animals. The average BBB score of both hindlimbs was calculated. To analyze the effect on the BBB score, four (group) by six (time) repeated measures analyses of variance were conducted, involving group [SCI + SS (= reference), SCI + 5 Hz NRMS, SCI + 10 Hz NRMS, and SCI + NC-MS], time (days 0, 1, 3, 7, 14, and 21), and group × time interaction effect on the BBB score [F(15,180) = 35.02, P < 0.000 1, η2 = 0.01934]. The results showed a significant effect of time, indicating a significant increase in the BBB score over the course of the treatment from day 0 to day 21 [F(2.191,78.89) = 5115, P < 0.0001, η2 = 0.9433; Figure 1C]. Moreover, there was also a substantial effect of groups [F(3,36) = 87.73, P < 0.0001, η2 = 0.02701] with significant differences at each time point following SCI between the two NRMS groups and SCI + SS group, as well as between the two NRMS groups and SCI + NC-MS group. However, the statistical variance between 5 Hz and 10 Hz NRMS groups over the course of the treatment was only observed on day 21 [t(18) = 2.788, P = 0.048, standard error (SE) = 0.6276], indicating that the improvement in BBB score in the NRMS groups was markedly different as the treatment period increased. Recently, a series of studies by our research group have preliminarily demonstrated the effect of NRMS, including its effect on functional remodeling in the recovery period of SCI and stroke (Gao et al., 2020; Zhao et al., 2020, 2023; Yang et al., 2021; Zheng et al., 2022), as well as on spasticity in the sequelae period. High-frequency NRMS may promote motor functional remodeling via three approaches, including increased ascending superficial sensory and proprioceptive input, modulation of spinal cord auto loop and activation of peripheral nerve and neuromuscular junction (Basso et al., 1996). For denervation, the high-frequency stimulation pattern plays an enhanced role in activating the nerve conduction pathways and transforming dysfunctional spinal circuits into functional states in the absence of brain input. In the spastic state, high-frequency stimulation plays an inhibitory role in the spinal cord anterior horn by the presynaptic inhibition of spinal loop so as to alleviate spasticity at least at the spinal cord level. In addition, NRMS can overcome the limitation of TMS that cannot be performed at the injury site of spinal cord where steel nails or steel plates are inserted. In short, NRMS is indeed groundbreaking and has a synergistic and safe rehabilitation modulation effect, especially in the early stages of the disease, but cannot replace TMS in terms of value. Since the parameters of nerve root stimulation in the NC-MS group differed from those in the two NRMS groups, these results do not confirm the hypothesis that the NC-MS protocol may have a synergistic and enhanced effect of NRMS and TMS, and further studies are underway. Nevertheless, the more optimal recovery of sensorimotor function by NRMS than natural recovery after SCI revealed that the cortical involvement of sensory signal input is highly needed in rehabilitation treatment. Therefore, NRMS provides a reliable strategy for modulating the sensorimotor neural circuit, thus providing a novel framework for future therapeutic approaches to central nervous system diseases.[15]