Immediate and after effects of transcranial direct-current stimulation in the mouse primary somatosensory cortex (original) (raw)

Transcranial direct-current stimulation (tDCS) is a non-invasive brain stimulation technique consisting in the application of weak electric currents on the scalp. Although previous studies have demonstrated the clinical value of tDCS for modulating sensory, motor, and cognitive functions, there are still huge gaps in the knowledge of the underlying physiological mechanisms. To define the immediate impact as well as the after effects of tDCS on sensory processing, we first performed electrophysiological recordings in primary somatosensory cortex (S1) of alert mice during and after administration of S1-tDCS, and followed up with immunohistochemical analysis of the stimulated brain regions. During the application of cathodal and anodal transcranial currents we observed polarity-specific bidirectional changes in the N1 component of the sensory-evoked potentials (SEPs) and associated gamma oscillations. On the other hand, 20 min of cathodal stimulation produced significant aftereffects including a decreased SEP amplitude for up to 30 min, a power reduction in the 20-80 Hz range and a decrease in gamma event related synchronization (ERS). In contrast, no significant changes in SEP amplitude or power analysis were observed after anodal stimulation except for a significant increase in gamma ERS after tDCS cessation. The polarity-specific differences of these after effects were corroborated by immunohistochemical analysis, which revealed an unbalance of GAD 65-67 immunoreactivity between the stimulated versus non-stimulated S1 region only after cathodal tDCS. These results highlight the differences between immediate and after effects of tDCS, as well as the asymmetric after effects induced by anodal and cathodal stimulation. Transcranial direct-current stimulation (tDCS) is a safe and well tolerated neuromodulatory technique 1-4 that relies on the application of constant weak electrical currents on the scalp during several minutes through strategically positioned electrodes 5,6. Most studies using tDCS deliver a low-current intensity (from conventional 1-2 mA up to currents of 4 mA) between two rubber electrodes (25-35 cm 2) placed on the scalp for 10-20 min 1,3,7. Given its ability to modulate neuronal excitability, tDCS has attracted the attention of basic and clinical neuroscientists that have investigated its potential to modulate brain function 8 and treat a variety of neurological conditions such as epilepsy 9 , attention deficit hyperactivity disorder (ADHD) 10 or ataxia 11 among others (for a review see 12-14). From a mechanistic point of view, the effects of tDCS on cortical excitability can be separated into immediate and after effects. Immediate effects, appearing at the very moment of electric field application, are related to changes in membrane polarization caused by redistribution of charges in the cells in presence of the externally applied electric field 15,16. On the other hand, after effects observed following current cessation require several minutes of stimulation to develop and involve plasticity mechanisms 17. Recently, in vitro models have been successfully used to show that different neuronal features such as the orientation of the somatodendritic axis with