Structural plasticity in the motor cortex: Differential impacts of learning and Li-iTBS on dendritic spine dynamics.

Low-intensity intermittent theta burst stimulation (Li-iTBS) is a non-invasive brain stimulation technique that uses magnetic pulses to modulate neuronal activity. When applied immediately before a learning event (priming), Li-iTBS has been shown to enhance performance in skilled motor tasks. While its behavioural effects can be long-lasting, the underlying mechanisms remain unclear. We investigated how Li-iTBS priming influences structural synaptic plasticity in the mouse motor cortex. Using longitudinal in vivo two-photon microscopy, we tracked dendritic spine changes in pyramidal neurons of Thy1-GFP-M mice. Mice received 10 days of daily Li-iTBS or sham stimulation immediately before training on a skilled reaching task. Daily imaging captured changes in spine stability, gains, and losses. All mice improved in task performance over the 10 days. In the learning-only group, synaptic spine gains decreased during the initial three days of training, with no changes in losses, stable spines, survival, or density. In contrast, mice that received Li-iTBS before training showed a modest but significant increase in spine gains from day four onward. Moreover, newly formed spines in the early learning phase exhibited reduced 48-hour survival in the learning-only group compared to baseline. This decrease was mitigated by Li-iTBS. These findings suggest that Li-iTBS modulates learning-related synaptic plasticity when used as a priming stimulus, enhancing spine formation and stabilisation. This supports the potential of Li-iTBS as a tool for influencing experience-dependent plasticity and may inform future clinical and preclinical neuromodulation strategies.