Is there hope that transpinal direct current stimulation corrects motoneuron excitability and provides neuroprotection in amyotrophic lateral sclerosis?

Abstract

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease of largely unknown pathophysiology, characterized by the progressive loss of motoneurons (MNs). We review data showing that in presymptomatic ALS mice, MNs display reduced intrinsic excitability and impaired level of excitatory inputs. The loss of repetitive firing specifically affects the large MNs innervating fast contracting muscle fibers, which are the most vulnerable MNs in ALS. Interventions that aimed at restoring either the intrinsic excitability or the synaptic excitation result in a decrease of disease markers in MNs and delayed neuromuscular junction denervation. We then focus on trans-spinal direct current stimulation (tsDCS), a noninvasive tool, since it modulates the activity of spinal neurons and networks. Effects of tsDCS depend on the polarity of applied current. Recent work shows that anodal tsDCS induces long-lasting enhancement of MN excitability and synaptic excitation of spinal MNs. Moreover, we show preliminary results indicating that anodal tsDCS enhances the excitatory synaptic inputs to MNs in ALS mice. In conclusion, we suggest that chronic application of anodal tsDCS might be useful as a complementary method in the management of ALS patients.

Keywords: excitability and excitation; intracellular recordings; neurodegenerative disease; spinal motoneurons.

FIGURE 1

Intrinsic excitability and synaptic excitation are depressed in the SOD1^G93A mice. (a) MN that fails to display a repetitive discharge in response to a slow triangular ramp of current (A₁) but that is still able to elicit a single spike in response to a short transient pulse (A₂). The blue trace is the injected current, the green trace is the intracellular recording, the gray trace is the EMG recorded in the triceps surae, the red trace is the force recorded at the tendon of the triceps surae. (b) MN displaying a repetitive firing in response to a slow triangular ramp (same arrangement as in (a). (c) Nonfiring MNs are found among the largest motor units (FF and FR with a twitch force larger than 1.3 mN). MNs indicated by an arrow correspond to the two examples in (a) and (b). Adapted from Martinez‐Silva et al. (2018). (d) Experimental arrangement for testing the size of the maximal Ia Excitatory Post‐Synaptic Potentials (EPSP). (e) Typical recordings in a wtSOD1 MN (E₁) and a SOD1^G93A MN (E₂). Lower traces are the intracellular recordings. Upper traces are the cord dorsum potentials showing the group I afferent volleys. (f) The maximal Ia EPSPs are significantly reduced in the MNs from SOD1^G93A mice (whereas resting potentials, input conductances and membrane time constants are unchanged). Adapted from Bączyk, Alami et al. (2020)

FIGURE 2

Excitatory synapses onto MNs are impaired in the SOD1^G93A mice and are restored through activation of the cAMP/PKA signaling pathway. (a) Drawing illustrating a normal excitatory synapse in a WT mouse. (b) In the presymptomatic SOD1^G93A mice (~ 50 days old), the GLUR subunits of the AMPA receptors and the scaffold proteins (Shank1, Homer) are less expressed in the postsynaptic side of excitatory synapses. At the same time, the presynaptic element does not seem affected. The postsynaptic disruption is responsible for a significant decrease of the EPSP amplitude (Bączyk, Alami et al., 2020). (c) Activation of the cAMP/PKA pathway, either through intracellular iontophoretic ejection of Sp‐AMP (a cAMP agonist) or through CNO‐activation of a DREADD G(s) specifically inserted in MN using an AAV9 vector, partially restores the synaptic impairment, entailing a firing increase and a burden decrease of disease markers such as misfSOD1, LC3A and p62 aggregates (Bączyk, Alami et al., 2020). The authors would like to thank Prof. Francesco Roselli who has drawn a preliminary draft of this figure and Dr. Marin Manuel who has prepared the final version

FIGURE 3

Schematic diagram of the experimental designs to investigate short‐term (a), persistent, long‐lasting (b) effects of acute tsDCS, and adaptive changes in response to chronic tsDCS application (c) in rat MNs

FIGURE 4

Summary of the changes in the frequency–current (f–I) relationship during rhythmic steady‐state firing (SSF) for MNs subjected to various polarization protocols. The linear relationship between the discharge frequency and injected current was assessed for each MN on the equation y = ax + b, where a determines the slope of the relationship in the primary range. Short‐term effects of anodal (a) and cathodal (b) polarization as in Figure 3a and Table 1 (columns 1 and 2) (Bączyk et al., 2019). (c) Long‐lasting effects of polarization as in Figure 3b and Table 1 (columns 3 and 4) (Bączyk et al., 2020a). (d) Chronic effects of polarization as in Figure 3c and Table 1 (column 5) (Bączyk et al., 2020b). Filled and open circles represent the average values for each group, while horizontal and vertical whiskers represent the SD values. “*” indicates significant effect of anodal polarization regarding the minimum and the maximum SSF current, the minimum and the maximum SSF frequency, and the f–I slope, at p < 0.05. “#” indicates significant effect of cathodal polarization for respective parameters, at p < 0.05. (a) and (b) RM ANOVA with a post hoc Tukey's test (for data with normal distribution and equal variance) or the Friedman tests with post hoc analysis of data with paired Kruskal–Wallis or Student's t‐test (for repeated nonparametric comparisons). (c) Two‐way ANOVA with a post hoc Tukey's test. (d) One‐way ANOVA with a post hoc Tukey's test

FIGURE 5

Short‐term and long‐lasting effects of polarization in SOD^G93A mice. (a) Monosynaptic EPSPs, evoked by stimulation of the triceps surae nerve, recorded from the same MN, before, and during anodal polarization. (b) as in (a), but records made in a different animal before and during cathodal polarization. (c) Examples of EPSPs recorded in the control (white), long‐lasting anodal (red), and long‐lasting cathodal (green) polarization groups. (d) Distribution of EPSP amplitudes within control, long‐lasting anodal, and long‐lasting cathodal polarization groups. Each data point represents a single MN, while box‐plots cover 25% of the upper and lower data range with horizontal lines showing the median. Notice a strong, 32% increase in EPSP amplitude following anodal polarization (without any change in input resistance). Difference in mean EPSPs amplitude is significant between control and long‐lasting anodal polarization groups (p < 0.01, Mann–Whitney test)