Introduction Transcranial direct current stimulation (tDCS) has been shown to modulate cortical excitability and to improve motor learning Nitsche and Paulus, 2000, Reis et al., 2009, Fritsch et al., 2010. To date there is no data available on the safety aspects of tDCS covering a broad range of stimulation intensities, thus limitations for dosages applied to humans are still based empirically on other types of electrical brain stimulation (Agnew and McCreery, 1987).

Objectives The aim of this study was to investigate lesion threshold, accompanying neurodegeneration and possible microstructural changes in glial cells in an animal model of tDCS.

Methods The tDCS electrode was placed in a screw tube surgically fixed on the skull above the left primary motor cortex of 47 Male Sprague Dawley rats. Anodal tDCS was applied in different intensities (254.8, 127.4, 63.7, 31.8, 15.9 and 8 A/sqm) or as sham stimulation for 20 min under slight isoflurane anesthesia. For each intensity animals were sacrificed either 24 h (acute effects) or 7 days (chronic effects) after intervention. Histological analyses of neurodegeneration (HE-staining, Fluoro-Jade C), astrocyte (anti-GFAP) and microglial (anti-CD11b [OX-42]) cell counts/density and/or morphological changes were evaluated.

Results Macroscopical lesions could be observed at tDCS intensity of 127 A/sqm and above whereas the lesion dose 50 (LD 50) for microscopical neurodegeneration was determined at 60 A/sqm. Threshold for damage in neuronal and glial cells and change in morphology (activation) was identical. These changes could be detected both in the acute and chronic group. Notably, signs of still ongoing neurodegeneration, though to a slighter extent, were seen 7 days after stimulation. Brain swelling, which was seen underneath the stimulation electrode 24 h following the highest intensity used, was not present one week after stimulation.