High-frequency repetitive transcranial magnetic stimulation enhances layer II/III morphological dendritic plasticity in mouse primary motor cortex.

High-frequency repeated transcranial magnetic stimulation (HF-rTMS) is a safe non-invasive neuromodulatory technique and there is a body of evidence shows that it can modulate plasticity in different brain areas. One of the most interesting application of HF-rTMS is the modulation of plasticity in primary motor cortex (M1) to promote recovery after brain injuries. However, the underlying mechanism by which HF-rTMS modulates motor cortex plasticity remain to be investigated. In this study, we investigated the effects of HF-rTMS treatment on morphological plasticity of pyramidal neurons in layer II/III (L2/3) of the primary motor cortex in mice. Our results show that the treatment did not increase anxiety in mice in the open field test and the elevated plus-maze test. Treated mice displayed increased total spine density in apical and basal dendrites, with a predominance of thin spines. The treatment also increased dendritic complexity, as assessed by Sholl analysis at both apical and basal dendrites. Collectively, the results show that HF-rTMS induced remarkable changes in dendritic complexity in primary motor cortex L2/3 connections which may strengthen corticocortical connections increasing integration of information across cortical areas. The data support the use of HF-rTMS as a circuit-targeting neuromodulation strategy.