Long‐lasting modifications of motoneuron firing properties by trans‐spinal direct current stimulation in rats

Abstract

Trans-spinal direct current stimulation (tsDCS) is a novel neuromodulatory technique that has been used during neurological rehabilitation and sports to modulate muscle activation. However, the physiological mechanisms that underly the longlasting functional effects of polarization are not yet fully understood, nor are their relationships with specific neuronal populations. The acute facilitatory and depressive effects of anodal and cathodal polarization on motoneurons have been recently demonstrated, and the aim of this study was to determine whether tsDCS-evoked modulations of motoneuron properties are able to persist over several hours. Intracellular recordings from multiple antidromically identified rat motoneurons were performed both before and after the application of tsDCS (0.1 mA for 15 min), at various time points up to 180 min after the offset of anodal or cathodal tsDCS. The examined effects of anodal polarization included decreased rheobase, voltage threshold, the minimum and maximum currents necessary for rhythmic firing, increased rhythmic firing frequencies and the slope of the f-I relationship. The majority of these facilitatory changes to threshold and firing properties were sustained for 30-60 min after polarization. In contrast, the significant effects of cathodal polarization were absent, except the short-lasting decreased ability for motoneurons to induce rhythmic activity. This study provides direct evidence that a single polarization session can alter the electrophysiological properties of motoneurons for at least one hour and provides a basis for the further use of tsDCS techniques under conditions where the sustained modification of motoneuron firing is desired.

The plasticity of nerve fibers: the prolonged effects of polarization of afferent fibers

“…The lack of effects on the stimulusresponse curve of the M-wave (143) would argue against effects secondary to changes in the excitability of motoneurons. Nevertheless, as tsDCS was shown to decrease the threshold for activation of rodent motoneurons to intracellular current pulses (148,149), the contribution of such changes cannot be ruled out.…”

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

“…Columns 1 and 2 indicate short-term effects (see Figure 3a), based on data recorded from single MNs during, and 15 min after tsDCS (0.1 mA) application, compared to control recordings before the onset of polarization (averaged across MNs in anodal [N = 10] or cathodal [N = 10] polarization groups, Bączyk et al, 2019); columns 3 and 4 present long-lasting effects (see Figure 3b), based on data averaged across separate groups of neurons, recorded during the first 30 min (N = 22 for anodal tsDCS, N = 21, for cathodal tsDCS), and between 30 and 60 min (N = 21 for anodal tsDCS, N = 22, for cathodal tsDCS) after the offset of tsDCS (0.1 mA), respectively, compared to the prepolarization group (N = 36) from which records were made prior to the onset of tsDCS (Bączyk et al, 2020a); column 5 shows chronic effects (see Figure 3c), based on data averaged for MNs recorded after repeated transcutaneous application of anodal (N = 39) or cathodal (N = 43) tsDCS (0.5 mA, 15 min daily, for 5 weeks), compared to the sham control group (N = 41; Bączyk et al, 2020b F I G U R E 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).…”

“…Significant long‐lasting effects of anodal tsDCS causing potentiation of firing of MNs were shown to persist in MNs up to 60 min after the offset of polarization (Bączyk et al, 2020a; Table 1, Figure 4c). The effects of cathodal polarization were less prominent and shorter‐lasting, and were not observed 30 min after the offset of tsDCS.…”

“…(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)…”