Enhancing Motor Cortex Excitability in Mice Through Optogenetic Intermittent Theta Burst Stimulation

Abstract

The motor cortex (MCX) is integral to voluntary movement, sending signals from the brain to spinal motoneurons through the corticospinal tract. Enhancing MCX offers the potential for improving motor function in healthy individuals and aiding recovery post-stroke. Intermittent theta burst stimulation (iTBS) has shown promise in increasing MCX excitability in clinical settings, but its effects are often short-lived and lack spatiotemporal precision. Additionally, the use of invasive tools in animal models to study iTBS presents further challenges to its development. To overcome these limitations, this thesis will use a noninvasive approach that combines transcranial optogenetics with iTBS (opto-iTBS), offering high spatiotemporal precision and aiming to achieve more sustained enhancements in cortical excitability.

We hypothesized that opto-iTBS significantly increases cortical excitability and motor output in naïve mice. Specifically, we designed four noninvasive metrics, including tracking evoked forelimb movement, recording blood flow and epidural EEG signal, and using light-based motor mapping to track changes in the MCX pre- and post-opto-iTBS.

We established an iTBS protocol in mice using transcranial optogenetic stimulation. Using blue light stimulation in ketamine/xylazine anesthetized Thy1-ChR2 mice, we measured the time course of enhanced cortical excitability based on the amplitude of evoked forelimb movements and EEG recordings from the MCX. Stimulation was performed through a transcranial chronic window that provides optical access to the MCX in both hemispheres, thus allowing repeated assessments of iTBS effects. Laser Doppler imaging recorded blue light-evoked hemodynamic changes in the MCX during iTBS. Epidural EEG electrodes collected blue light evoked signals from around the stimulation region.

Our findings demonstrate that opto-iTBS significantly increases the optogenetically evoked contralateral forelimb movement amplitude for at least 85 minutes post-stimulation and expands the size and activation threshold of the motor map for a minimum of 10 minutes. Additionally, a significant increase in blood flow at the stimulation site in the MCX was observed during opto-iTBS induction.

Our noninvasive optogenetic stimulation approach enables longitudinal experiments that allow multi-target assessment, control of stimulation parameters, and within-subject measures of treatment effects. By elucidating optimal parameters and targets for iTBS, this research may identify therapeutic interventions to improve function following MCX stroke.