The influence of endogenous estrogen on transcranial direct current stimulation: A preliminary study

Transcranial direct current stimulation (tDCS) is a non-invasive neuromodulatory technique. Responses to tDCS differ substantially between individuals. Sex hormones that modulate cortical excitability, such as estrogen, may contribute to this inter-individual variability. The influence of estrogen on tDCS after-effects has not yet been researched. This study aimed to investigate whether endogenous estrogen levels influence cortical response to tDCS. Data from 15 male and 14 female healthy adults were analyzed. Males completed one experimental session. Females completed two, one during the early follicular phase of the menstrual cycle when estrogen was low, one during the mid-luteal phase when estrogen was high. Each session comprised 15-min of anodal tDCS delivered to the left dorsolateral prefrontal cortex (DLPFC). Response to stimulation was assessed using electroencephalography with DLPFC transcranial magnetic stimulation (TMS) administered before, immediately after, and 20-min after tDCS. Changes in amplitudes of N120 and P200 components of TMS-evoked potentials over time were compared between males, women with low estrogen and women with high estrogen. Blood assays verified estrogen levels. Women with high estrogen demonstrated a significant increase in P200 amplitude at both time points and change over time was greater for the high estrogen group compared with males. No significant differences were observed between males and women with low estrogen, or between women with low and high estrogen. These preliminary results indicate that, greater neuroplastic response to DLPFC tDCS is seen in highest compared with lowest estrogen states, suggesting that endogenous estrogen levels contribute to inter-individual variability of tDCS outcomes.