Evaluation of acute anodal direct current stimulation-induced effects on somatosensory-evoked responses in the rat

Transcranial direct current stimulation (tDCS) is a non-invasive tool used to treat brain disorders. The DC electric field is thought to modulate neuronal excitability and it has been reported to exert effects within the localized treatment area under the electrode, as well as in diffuse brain regions extending beyond the electrode. However, the manner in which tDCS influences neural transmission in the cortex and modulates neural activity in distant interconnected cortical regions remains unclear. Thus, the present study investigated the effects of anodal DCS (aDCS) on the forelimb-evoked sensory response that initially appears in the primary sensorimotor cortex (S1-M1) and then propagates to the secondary motor cortex (M2). When aDCS application was confined to the S1-M1 region, local field potential (LFP) recordings and voltage-sensitive dye (VSD) imaging revealed that the forelimb-evoked response in the S1-M1 was clearly enhanced. In contrast, the neural response in the M2 remained almost unchanged. On the other hand, aDCS application confined to the M2 increased the forelimb-evoked response in the M2 but not the S1-M1. Taken together, these results suggest that, when applied to the cortex, the aDCS may have intrinsic local effects, influencing afferent neural activity immediately underneath the stimulation site. Thus, the present results indicate that aDCS has less influence on neural activity in distant cortical areas interconnected to the stimulation site than at the stimulation site itself. Therefore, the findings do not support the idea of DCS remote activation via cortico-cortical connections, at least between the S1-M1 and M2 regions in rats.