Direct current stimulation-induced synaptic plasticity in the sensorimotor cortex: structure follows function

BACKGROUND Non-invasive direct current stimulation (DCS) of the brain induces functional plasticity in vitro and facilitates motor learning across species. The effect of DCS on structural synaptic plasticity is currently unknown. OBJECTIVE This study addresses the effects and the underlying mechanisms of anodal DCS on structural plasticity and morphology of dendritic spines in the sensorimotor cortex (M1/S1). METHODS A DCS electrode setup was combined with a chronic cranial window over M1/S1 in transgenic Thy1-GFP mice, to allow for in vivo 2-photon microscopy and simultaneous DCS. Contralateral electrical forepaw stimulation (eFS) was used to mimic the second synapse specific input, a previously shown requirement to induce functional plasticity by DCS. Changes in spine density and spine morphology were compared between DCS/eFS and sham, as well as two control conditions (sham-DCS/eFS, DCS/sham-eFS). Furthermore, the role of BDNF for stimulation-induced changes in spine density was assessed in heterozygous Thy1-GFP x BDNF+/- mice. RESULTS Combined DCS/eFS rapidly increased spine density during stimulation and changes outlasted the intervention for 24 h. This effect was due to increased survival of original spines and a preferential formation of new spines after intervention. The latter were morphologically characterized by larger head sizes. The DCS-induced spine density increase was absent in mice with reduced BDNF expression. CONCLUSION Previous findings of DCS-induced functional synaptic plasticity can be extended to structural plasticity in M1/S1 that similarly depends on a second synaptic input (eFS) and requires physiological BDNF expression. These findings show considerable parallels to motor learning-induced M1 spine dynamics.

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