The synergistic effect of combining physical activity and anodal tDCS boosts motor cortical interhemispheric plasticity in mice

In recent years, transcranial direct current stimulation (tDCS), one of the most encouraging noninvasive neuromodulatory techniques, has been combined with other types of interventions, such as physical or cognitive training. The idea is that the coupling of tDCS and an endogenous brain activation (e.g. motor activity) may have a synergistic addictive effect with respect to single interventions. Nevertheless, the mechanisms responsible for this synergistic effect are not clear and animal studies on this topic are still missing. In both humans and rodents, anodal tDCS (A-tDCS) increases neural activation and plasticity of the primary motor cortex (M1). However, the effects of combining M1 A-tDCS and physical activity have not been studied yet. Moreover, the motor cortices are highly interconnected, and this thesis is based on the hypothesis that tDCS might affect motor cortex functional connectivity when coupled with its physiological activation. For these reasons, we evaluated the effects of the combination between monolateral A-tDCS and moderate physical activity in the stimulated and non-stimulated motor cortices, in C57/Bl6 young mice. To this aim we measured cell activation by the immediate early gene (IEG) c-Fos expression, functional connectivity by local field potential (LFP) M1-M1 coherence analysis and structural plasticity by the mean spine density with Golgi-Cox technique. We found that, when A-tDCS is applied in combination with walking, mice displayed higher cell activation and mean spine density in layer II/III, in the directly stimulated hemisphere but also in the contralateral M1. Functional connectivity is also affected by the coupling, with an enhanced M1-M1 synchronization in the theta rhythm, which is associated to locomotion. This inter-hemispheric boosting effect does not occur when mice receive the stimulation alone, supporting the idea that tDCS effects strongly depend on the ongoing network activity. Thus, motor activity physiologically activates the motor network which, in turn, determine the tDCS-induced plasticity effects. All this data sustains the idea that coupling tDCS and endogenous activation leads to the synergistic boosting effects of the coupling, as observed in human studies. Of note, physiological aging has been associated to a reduction of M1 plasticity and changes in tDCSinduced plasticity effects in humans. Hence, the second aim of this investigation was to verify the eventual synergistic effects on cell activation and structural plasticity in elderly mice. Our findings reveal that both the activation and the mean spine density in layer II/III resemble the ones in young adult mice, indicating that the synergistic effects of the combo are still efficacious, even if the motor network plasticity is physiologically reduced.