INTRODUCTION: Trans-spinal direct current stimulation (tsDCS) is a non-pharmacological neuromodulatory technique evoking functional changes in spinal circuit activity. However, the effects of tsDCS are highly variable between interventions, and the application parameters responsible for its effectiveness remain largely unknown. It was recently suggested that the tsDCS electrode configuration, rather than current dose, plays a crucial role in producing the neuromodulatory effects.

AIM(S): This study aims to establish the impact of electrode arrangement on tsDCS effects on passive membrane, threshold, and firing properties of spinal motoneurons (MNs).

METHOD(S): Due to the invasive nature of the investigations, all experiments were performed on B6SJL WT mice under general anesthesia. 15 minutes of anodal or cathodal tsDCS of 60 uA was applied to the mice in rostro-caudal (RC - two rectangular electrodes placed on the animal’s back) or dorso-ventral (DV - rectangular electrode on the back, and crocodile clip attached diagonally to the abdominal skin) arrangement. In vivo intracellular recordings of spinal MNs were performed before and up to 1h after tsDCS application.

RESULTS: In both setups, cathodal tsDCS was the most prominent in affecting MNs' electrophysiological properties. In RC, it led to an increase in MNs’ peak and plateau input resistance, subthreshold ion channel activity, and firing frequency gain. In the DV arrangement, it caused more modest changes, increasing MNs' peak and plateau RIN, and membrane time constant. On the other hand, anodal tsDCS tended to increase MNs’ input resistance only in the RC setup.

CONCLUSIONS: These results indicate that different electrode configurations during tsDSC application lead to diversified alterations in MNs' intrinsic excitability and firing properties. Cathodal tsDCS in the RC configuration evokes the strongest alterations in MNs' passive membrane and firing properties.

FINANCIAL SUPPORT: NCN 2022/04/Y/NZ4/00117.