Polarity-dependent adaptations of motoneuron electrophysiological properties after 5-wk transcutaneous spinal direct current stimulation in rats

Recently, it has been shown that spinal cord polarization considerably modulates motoneuron activity, with certain observed changes in motoneuron membrane and firing properties outlasting the duration of polarization. The purpose of this study was to determine whether repeated sessions of transcutaneous transspinal direct current stimulation (tsDCS) induce adaptive changes in motoneuron properties. In this study, adult male Wistar rats under isoflurane anesthesia were subjected to anodal (n = 6) or cathodal (n = 6) tsDCS (100 mu A, 15 min) 5 days per week for 5 wk. Sham control group rats (n = 6) served as a reference. Intracellular recordings from lumbar spinal motoneurons were performed under pentobarbital anesthesia 1 day after the final tsDCS session to analyze membrane and firing properties. Anodal polarization appeared to be effective in evoking significant adaptive changes toward the facilitation of motoneuron firing. When compared with the sham polarization group, these adaptations were expressed by the increased input resistance (P = 0.0077), decreased voltage threshold for spike generation (P = 0.0248) and doublet threshold (P = 0.0311), and increased maximum steady-state firing (SSF) frequency (P = 0.0073), SSF frequency range (P = 0.0075) and slope of the frequency-current relationship (P = 0.0111). However, the effects of cathodal polarization were modest and generally not significant in regard to the sham control. These novel findings support the existing knowledge on alterations in spinal neuronal network excitability in response to polarization and provide the direct evidence of adaptive neuroplasticity of spinal motoneurons in response to chronically applied tsDCS. NEW & NOTEWORTHY Transcutaneous spinal direct current stimulation applied systematically for 5 wk evoked polarity-dependent adaptations in the electrophysiological properties of rat spinal motoneurons. After anodal polarization sessions, motoneurons became more excitable and could evoke higher maximum discharge frequencies during repetitive firing than motoneurons in the sham polarization group. However, no significant adaptive changes of motoneuron properties were observed after repeated cathodal polarization in comparison with the sham control group.