We assessed the effects of anodal/cathodal direct current stimulation (DCS) applied epidurally over the cerebellum. We studied the excitability of both the motor cortex and the anterior horn of the spinal cord in adult rats under continuous anesthesia. We also investigated the effects on the spatial representation of a couple of agonist/antagonist muscles on primary motor cortex. Moreover, we evaluated the effects on the afferent inhibition in a paradigm of conditioned corticomotor responses. Anodal DCS of the cerebellum (1) decreased the excitability of the motor cortex, (2) reduced the excitability of waves, as shown by the decrease of both mean /mean ratios and persistence of waves, (3) exerted a “smoothing effect” on corticomotor maps, reshaping the representation of muscles on the motor cortex, and (4) enhanced the afferent inhibition of conditioned motor evoked responses. Cathodal DCS of the cerebellum exerted partially reverse effects. DCS of the cerebellum modulates the excitability of both motor cortex and spinal cord at the level of the anterior horn. This is the first demonstration that cerebellar DCS tunes the shape of corticomotor maps. Our findings provide a novel mechanism by which DCS of the cerebellum exerts a remote neuromodulatory effect upon motor cortex. 1. Introduction The dynamic modulation of the excitability of the motor cortex is critical for motor control. It depends on several elemental parameters: the excitability of single cells, the synaptic strength, and the balance between excitatory cells and inhibitory cells [1, 2]. The cerebellum is one of the subcortical structures modulating the excitability of the motor cortex and the spinal cord [3]. It is presumed that defects in the tuning of the excitability of the corticomotor responses contribute to the sensorimotor learning deficits in cerebellar patients [4]. Since we currently lack efficient therapies in numerous forms of cerebellar disorders encountered during daily practice, there is a need to identify novel strategies that might be used to antagonize cerebellar motor deficits. The mechanisms of these deficits depend on the type of cerebellar damage. Indeed, cerebellar cortex inhibits strongly cerebellar nuclei, which themselves stimulate contralateral motor cortex and ipsilateral anterior horn of the spinal cord. Therefore, cerebellar cortical lesions induce a disinhibition of cerebellar nuclei, which results in an excitatory overdrive along the dentatothalamocortical pathway [5]. When the lesion is extensive and includes cerebellar nuclei, such as in hemicerebellar
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