Motor cortex neuromodulation activates molecular signaling necessary for corticospinal axon growth and muscle response plasticity in intact rats and in rats with bilateral cervical spinal cord injury

STUDY PURPOSE: The purpose of this study was to determine the effects of short-term and long-term repetitive multi-pulse stimulation (rMPS) and intermittent theta burst stimulation (iTBS) neuromodulation applied to motor cortex, on corticospinal tract (CST) axon structural remodeling, physiological connectivity (motor evoked potential, MEP) and activation of axon growth-promoting molecular signaling (mTOR, PTEN and Jak/Stat) in rats without injury and after moderate, bilateral cervical spinal cord contusion injury. We used adult, female Sprague Dawley rats between two and three months of age. We initially conducted neuromodulation experiments in uninjured animals to understand the effects of neuromodulation alone, without any interference from an injury-dependent response. Next, we applied neuromodulation to spinal cord injured rats to determine the effects on the damaged corticospinal system. Our overall goal was to identify molecular predictors of neuromodulation-dependent CST axon growth and physiological plasticity in intact animals and those with SCI. DATA COLLECTED: Data are presented in relation to figures 2, 4, 5, 6, and 8 in the article in press (Zareen et al. Molecular signaling predicts corticospinal axon growth state and muscle response plasticity induced by neuromodulation; PNAS; 2024). Motor cortex neuromodulation was delivered for 1-day (short-term) and 10 consecutive days (long-term). Assessments were made at different days post-neuromodulation: immediately after neuromodulation, 10-days later and 30-days, to assay for persistence.
Fig_2A and Fig_2B present quantified CST axon length in C7 spinal cord segment after normalizing to the number of biotinylated dextran-amine (BDA)-labeled (anterograde axon tracer) cST axons in the dorsal column. This was done in uninjured rats, in confirmation of our previous publication in spinal cord injured rats.
Fig 4A, 4B, 4C and 4D present electrophysiological data as MEPs recorded from the extensor carpi radialis (ECR) muscle of uninjured rats in response to an electrical stimulus to the forelimb motor cortex. MEP was analyzed as the area under the electromyographic response curve (AUC), after no stimulation, short- and long-term rMPS or iTBS.
Fig_5 presents activation of mTOR and Jak/Stat signaling and deactivation of the mTOR antagonist, PTEN, after short-term rMPS and iTBS in uninjured rats. mTOR pathway activation results in the phosphorylated S6 (pS6). Phosphorylation of PTEN (pPTEN), leads to PTEN deactivation. Jak/Stat pathway activation results in phosphorylation of Stat3 (pStat3). Activation is determined by examining the ratio of the phospho-protein to the total protein. The levels of total S6, total PTEN and total Stat3 were also examined and presented as ratios of S6 to GAPDH, PTEN to GAPDH and Stat3 to GAPDH.
Fig_6 experimental data are presented in the same format as in Fig_5, but after long-term rMPS and iTBS.
Fig_8 experimental data are presented in the same format as in Figures 5 and 6, but after spinal cord injury and long-term rMPS and iTBS. DATA USAGE NOTES: