Rebalancing Descending Motor Circuit Development after Perinatal Brain Injury
DOI: Pending DOI
Synopsis
Cerebral palsy results from perinatal brain injury, disrupting the development of corticospinal motor pathways. Neonatal hypoxic-ischemic (H-I) injury may increase brainstem-driven motor activity, contributing to spasticity and impaired spinal inhibition. We tested whether temporal interference stimulation (TIS) could restore corticospinal–brainstem balance in neonatal mice. H-I injury was induced at P9, followed by 7 days of targeted TIS beginning at P11. At P21, H-reflex testing showed reduced supraspinal inhibition and increased spinal excitability after H-I injury, while TIS improved rate-dependent depression. Motor evoked potential latency was prolonged after H-I injury but normalized with TIS. These findings suggest early noninvasive neuromodulation may improve motor circuit development after perinatal brain injury.
Background
Cerebral palsy (CP), the most common cause of lifelong motor disability in children, arises from perinatal brain injury that disrupts the normal developmental trajectory of descending motor circuits. During early postnatal life, motor control normally transitions from predominantly brainstem-driven activity to refined, lateralized corticospinal modulation through activity-dependent competition, pruning, and myelination. After central motor injury, contributions from brainstem pathways-particularly reticulospinal and related medial descending projections - appear to increase relative to corticospinal influence, leading to exaggerated reflex gains, tonic activation, and co-contraction typical of spasticity [1]. In contrast, corticospinal signals become relatively weak or poorly coordinated, reducing phasic inhibitory control over spinal circuits. This supraspinal imbalance is associated with exaggerated stretch reflexes and hyperexcitability of spinal networks after damage to inhibitory supraspinal control [2, 3]. We propose that perinatal hypoxic-ischemic injury distorts this developmental transition, leading to persistent dominance of tonic brainstem drive, impaired corticospinal refinement, and maladaptive stabilization of spinal circuits. To restore the balance between corticospinal and brainstem motor control after perinatal brain injury, we attempted to differentially modulate these structures using temporal interference stimulation (TIS) by varying the spatial focus and temporal organization of the stimulation waveform.
Methods
At P9, brain injury was induced in neonatal C57BL6 mice of both sexes using the modified hypoxia-ischemia Vannucci model, consisting of permanent left carotid artery occlusion followed by 10% O2 hypoxia for 1 hour. Sham control mice underwent the same surgical manipulations but without ligation and hypoxia. Moderate cases were selected in which a unilateral brain lesion measured 15-30% and sparing of the motor cortex was present, as determined by T2-weighted MRI.
At P11, mice were randomly assigned to groups receiving TIS twice daily for 15 min over 7 days. TIS was delivered noninvasively using a 2-source surface-electrode montage optimized to steer subthreshold electric fields with spatial focus on 1) subcortical white matter/internal capsule, 2) rubro-/reticulospinal projections in deep subcortical motor pathways, or 3) sham stimulation with patterned, pulse-modulated waveforms (2.00 and 2.01 kHz; Δf = 10 Hz). Finite‑element electric field modeling in Sim4Life was used to optimize and validate electrode configurations that produce maximal envelope fields in the targeted white matter, based on an MRI-derived P12 neonatal mouse head model. For internal capsule stimulation, high-contrast envelope bursts were used to enhance corticospinal timing and transmission. For the reticulospinal tract target, envelope contrast and duty cycle were reduced, and temporal jitter was introduced to generate diffuse, low-salience modulation that avoids reinforcing tonic brainstem drive.
At P21, supraspinal presynaptic inhibition was assessed using the H-reflex Hmax/Mmax ratio and rate-dependent depression. Brainstem contribution was examined using a startle-modulated H-reflex paradigm, with loud acoustic startle stimuli delivered before H-reflex elicitation. Functional effects of early-life TIS on corticospinal tract organization were assessed by motor evoked potential (MEP) latency in response to a single pulse delivered by the animal TMS device and a dedicated, actively cooled 25 mm coil (Soterix Medical Inc.) in lightly anesthetized P21 mice.
Results
The methodology and measurements of the Hoffman reflex in the hind limbs of P21 mice after neonatal H-I are shown in Figure 1. EMG responses to n. Ischiadicus stimulation were recorded with needle electrodes in the m. triceps surae in lightly anesthetized mice. A. Representative response curves. M-wave amplitude increased, and H-wave amplitude decreased with increasing stimulation intensity (blue - 0.7 mA, red - 1.3 mA). B. With increased stimulation rate, rate-dependent depression of the H-wave was close to 0% in controls (green) but reduced to 35% in H-I pups with brain injury (blue), indicating decreased supraspinal inhibitory control. RDD increased after 5 days of swimming with TIS (orange). C-D H-reflex pairs before (red curve) and after 100 dB 1 kHz tone startle (blue curve) in P21 controls (C) and H-I (D). The ratio of H-wave after startle to H-wave before increased in H-I pups with brain injury, Dunnett’s post hoc p=0.038, ANOVA F(2,12)=3.56, p=0.061.
Conduction time evoked by a suprathreshold TMS pulse and recorded on EMG from the m. gastrocnemius was 5.38±0.24 ms in sham controls, 5.56±0.8 ms in H-I, and 5.12±0.27 ms in the H-I + TIS group. MEP latency was longer than in controls ( post hoc Dunnett’s p=0.021). The MEP latency in the H-I + TIS group did not differ from that of the sham control group.
Discussion
Overall, the study demonstrates that neonatal hypoxic-ischemic injury leads to persistent enhancement of brainstem-mediated motor regulation and impaired corticospinal refinement during postnatal development. Early application of spatially focused temporal interference stimulation appears capable of modulating these maladaptive developmental processes, partially restoring inhibitory control of spinal circuits and normalizing corticospinal conduction. These findings support the potential of noninvasive neuromodulation during early developmental windows as a therapeutic strategy to prevent or reduce motor dysfunction after perinatal brain injury. Further studies are needed to determine the long-term functional effects, underlying mechanisms, and safety of deep-brain temporal interference stimulation, particularly when targeting brainstem structures in the developing nervous system.
Disclosures
Nothing to disclose by any authors.
References
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