Evidence that some long‐lasting effects of direct current in the rat spinal cord are activity‐independent

Abstract

The effects of trans-spinal direct current (DC) stimulation (tsDCS) on specific neuronal populations are difficult to elucidate, as it affects a variety of neuronal networks. However, facilitatory and depressive effects on neurons processing information from the skin and from muscles can be evaluated separately when weak (0.2-0.3 μA) DC is applied within restricted areas of the rat spinal cord. The effects of such local DC application were recently demonstrated to persist for at least 1 h, and to include changes in the excitability of afferent fibres and their synaptic actions. However, whether these effects require activation of afferent fibres in spinal neuronal pathways during DC application, i.e. whether they are activity-dependent or activity-independent, remained an open question. The aim of the present study was to address this question by analysing the effects of local DC application on monosynaptic actions of muscle and skin afferents (extracellular field potentials) and afferent fibre excitability. The results revealed that long-lasting post-polarization changes evoked without concomitant activation of afferent fibres replicate changes evoked by stimuli applied during, before and after polarization. The study leads to the conclusion that the reported effects are activity-independent. As this conclusion applies to the local effects of DC application in at least two spinal pathways and to the effects of both cathodal and anodal polarization, it indicates that some of the more widespread effects of trans-spinal and trans-cranial stimulation (both tsDCS and transcranial DC stimulation) may be activity-independent. The results may therefore contribute to the design of more specific DC applications in clinical practice.

Direct current stimulation boosts synaptic gain and cooperativity in vitro

“…Thus, a targeted brain region may be influenced by both direct modulation of local neuronal processes and by polarizing afferent brain regions (Bolzoni & Jankowska, ; Jankowska et al . ). The diversity of experimental findings is well quantified by a model of synaptic dynamics (Figs and E ) – though we cannot exclude contributions from additional factors such as changes in polarization sensitivity by active neurons (Antal et al .…”

“…; Bolzoni & Jankowska, ; Jankowska et al . ). tDCS is typically applied adjunct to a task (corresponding to sustained neuronal activity) in clinical and performance enhancement applications.…”

“…Low-intensity direct current stimulation (DCS) produces a small change in membrane potential (<1 mV; Radman et al 2009), which is not sufficient to induce spiking activity in resting cortical pyramidal cells. DCS, while weak, modulates synaptic efficacy at rest as measured by single evoked responses in humans (Nitsche & Paulus, 2000;Priori, 2003) as well as in animals (Bikson et al 2004;Kabakov et al 2012;Marquez-Ruiz et al 2012;Rahman et al 2013;Bolzoni & Jankowska, 2015;Jankowska et al 2016). tDCS is typically applied adjunct to a task (corresponding to sustained neuronal activity) in clinical and performance enhancement applications.…”

Direct current stimulation modulates the excitability of the sensory and motor fibres in the human posterior tibial nerve, with a long‐lasting effect on the H‐reflex

“…These experiments followed the main procedures described in Jankowska et al . (). All experiments were approved by the Regional Ethics Committee for Animal Research (Göteborgs Djurförsöksetiska Nämnd) and followed NIH and EU guidelines for animal care.…”

“…Depending on the experimental conditions, both anodal and cathodal stimulations were found to induce a block of conduction (Bhadra & Kilgore, ). However, when local cathodal polarization was applied in the CNS, cathodal block was only found at excessive current intensities, while at low intensities cathodal DC consistently increased the excitability of all tested fibres (Baczyk & Jankowska, ; Bolzoni & Jankowska, ; Jankowska et al ., , ). The effects of cathodal DC on human peripheral fibres analysed by Burke et al .…”