An imbalance between cells’ intrinsic excitability and synaptic excitation levels underlies the spinal motoneuron (MN) pathophysiology in Amyotrophic Lateral Sclerosis. Recently, a transient restoration of the deficient Ia synaptic excitation of spinal MNs in the presymptomatic SOD1 G93A mice was achieved by applying a single trans-spinal direct current stimulation (tsDCS) session. Here we investigate whether two-week repeated tsDCS applied to presymptomatic SOD1 G93A animals can provoke neuroplasticity, i.e. permanently alter spinal MN synaptic excitation levels and in this way affect intracellular metabolic pathways and disease progression. Anodal, cathodal, or sham polarisation of 100 µA was applied to P30-P35 SOD1 G93A male mice, and passive membrane properties and Ia excitatory post-synaptic potential (EPSP) characteristics were investigated by intracellular recordings of spinal MNs in vivo. A second cohort of animals was used to test the impact of our intervention on Ia synapse morphology, intracellular metabolic pathways activity, and disease markers. Anodal tsDCS evoked a strong increase in maximal Ia EPSPs amplitudes, coupled with a significant upregulation of GlurR4 subunits of AMPA receptors at the Ia synapse. The cathodal polarisation failed to induce any significant alteration to Ia synapse morphology, but increased the input resistance of MNs. However, changes in MN electrophysiological profile and Ia synapse morphology did not translate into alterations of intracellular molecular pathways activity and did not decrease the cellular burden of the disease. Our results indicate a strong polarity-dependent plasticity of spinal MNs in SOD1 G93A mice in response to tsDCS, which however does not alter disease dynamics.

In Amyotrophic Lateral Sclerosis (ALS), alterations of spinal motoneurons’ excitability form the hallmark of their degeneration. Trans spinal direct current stimulation (tsDCS) is based on the delivery of low-intensity DCS to the spinal column in order to alter spinal circuit excitability. Recently, this technique was applied to the management of ALS in the SOD1 G93A mice and resulted in a reduction of disease biomarkers and extended mouse survival. While indirect evidence suggests that these effects can be linked to a decrease in MNs excitability following tsDCS, this has never been directly confirmed. Therefore, in this study, we have utilized in vivo sharp intracellular recordings of spinal MN to directly investigate the impact of DCS on MN intrinsic excitability in SOD1 G93A mice. Electrophysiological properties of MNs recorded before DCS were compared to the properties of MNs recorded one hour after DCS application using linear mixed-effect models. We have found that direct DCS application significantly increases MN peak and plateau input resistance (by 31 and 35% respectively); however, this was not linked to any significant change to MN threshold and firing properties. Both computational modelling and in vivo recordings of the EF field indicate that our results may be explained by the low density of the DC field at the MN recording site. While our results indicate that invasive DCS is not efficient in modifying MN excitability, it may be effective in altering the excitability of afferent fibres traversing the dorsal column close to the DCS electrode.