Effects of anodal tDCS on lumbar propriospinal system in healthy subjects

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Abstract

Objective

It has recently been shown that transcranial direct current stimulation (tDCS) (1) can modify lumbar spinal network excitability and (2) decreases cervical propriospinal system excitability. Thus the purpose of this series of experiments was to determine if anodal tDCS applied over the leg motor cortex area induces changes in lumbar propriospinal system excitability. To that end, the effects of anodal tDCS and sham tDCS on group I and group II propriospinal facilitation of quadriceps motoneurones were studied in healthy subjects.

Methods

Common peroneal nerve group I and group II quadriceps H-reflex facilitation was assessed in 15 healthy subjects in two randomised conditions: anodal tDCS condition and sham tDCS condition. Recordings were performed before, during and after the end of the cortical stimulation.

Results

Compared to sham, anodal tDCS decreases significantly CPN-induced group I and II quadriceps H-reflex facilitation during and also after the end of the cortical stimulation.

Conclusions

Anodal tDCS induces (1) modulation of lumbar propriospinal system excitability (2) post-effects on spinal network.

Significance

These results open a new vista to regulate propriospinal lumbar system excitability in patients and suggest that anodal tDCS would be interesting for neuro-rehabilitation of patients with central nervous system lesions.

Highlights

► Anodal tDCS decreases lumbar propriospinal system excitability in healthy subjects. ► For the first time, the results of this series of experiments show that anodal tDCS induces post effects on spinal networks. ► These results suggest that anodal tDCS could be of interest in neuro-rehabilitation in order to improve locomotion in patients with central nervous system lesions.

Introduction

Propriospinal lumbar system seems to play a key role in lower limb motor function enabling integration of the descending command en route to the motoneurones with the peripheral afferent feedback from the active limb. It has already been shown that the excitability of this system can be modulated by cortical stimulation (Iglesias et al., 2008). Indeed Iglesias et al. showed in healthy subjects that a single pulse of TMS decreases propriospinal facilitation of quadriceps motoneurones mediated by group I and group II afferents. If this finding is interesting from physiological point of view, this tool can however not be so easily used in clinical routine to modulate propriospinal lumbar system of patients (Priori et al., 2009). Another cortical stimulation method, more easier to use in clinical routine, is now available: transcranial direct current stimulation (tDCS). This tool allows to modulate brain excitability in humans (Priori et al., 1998, Priori, 2003, Nitsche and Paulus, 2000). This modification of brain excitability depends on current polarity, and anodal tDCS, when applied over motor cortex, increases motor cortex excitability (Nitsche and Paulus, 2000, Lang et al., 2005, Nitsche et al., 2005, Furubayashi et al., 2008). The effects of tDCS should however not be considered only in regards to cortical circuits but in regards to spinal circuits. Indeed, it has been demonstrated recently that anodal tDCS modifies spinal network excitability in healthy subjects (Roche et al., 2009, Roche et al., 2011).

At cervical spinal level, disynaptic inhibition directed from Extensor Carpi Radialis (ECR) to Flexor Carpi Radialis (FCR) is increased by anodal tDCS but not modified by cathodal or sham tDCS. It has also been demonstrated that cathodal tDCS decreases ipsilateral cervical propriospinal system excitability (Bradnam et al., 2011).

At lumbar spinal level, reciprocal Ia inhibition directed from Tibialis Anterior (TA) to soleus (SOL) is decreased by anodal tDCS but not by cathodal tDCS, and anodal tDCS increases homonymous recurrent inhibition to SOL α motoneurones (Roche et al., 2011).

Taken together, these results suggest that tDCS (particularly in anodal polarity) may also have an impact on propriospinal lumbar system.

The behaviour of the propriospinal lumbar system can be assessed by studying the facilitation induced in quadriceps motoneurones by TA group I and group II afferent stimulation (Marque et al., 2001a, Marque et al., 2001b). Therefore, the main purpose of this series of experiments was to determine if anodal tDCS applied over the leg motor cortex area induces changes in lumbar propriospinal system excitability. To that end, the effects of anodal tDCS and sham tDCS on group I and group II propriospinal facilitation of quadriceps motoneurones were studied in healthy subjects.

Section snippets

General experimental setup

Fifteen healthy subjects (aged from 24 to 59 years; 8 women, 7 men) were included in the study. Subjects consuming central nervous system-acting medication or recreational drugs were excluded. All subjects gave their written consent before participation. This study was performed in accordance with the ethical codes of the World Medical Association (Declaration of Helsinki) and was approved by the local ethic committee (CPP Ile de France VI-Pitié-Salpêtrière). During all the experiments, the

Anodal tDCS

Fig. 2 illustrates the time course of CPN-induced facilitation of the quadriceps H-reflexes in a representative subject (▴) before tDCS. The early facilitation (group I) began at the conditioning-test interval of 8 ms, reached a peak at 11 ms and ended at 13 ms, whereas the late facilitation (group II) reached a peak at 16 ms and ended at 18 ms. For all subjects, the two ISIs corresponding to the early peak of facilitation mediated by group I afferents and the late peak of facilitation mediated by

Discussion

The main finding of this series of experiments is that anodal tDCS decreases CPN-induced group I and II quadriceps H-reflex facilitation. Moreover, the results of this series of experiment indicate for the first time that anodal tDCS can also induce post-effects on lumbar spinal network excitability. Indeed, up to now, post effects after anodal tDCS when applied over motor cortex have only been described at cortical level (Nitsche and Paulus, 2000, Lang et al., 2005, Nitsche et al., 2005, Power

Clinical implications

Marque et al., 2001a, Marque et al., 2001b showed that lumbar propriospinal system excitability is enhanced in the affected side of hemiplegic patients compared to the unaffected side and healthy subjects (Marque et al., 2001b). Indeed these authors showed that this increased facilitation involves the early peak (mediated by group I afferents) and the late peak (mediated by group II afferents) and suggested that this increased excitability could play a role in spasticity and be involved in some

Acknowledgements

The authors wish to express their gratitude to Max Westby for reading and commenting upon the manuscript and to Geneviève Bard for her technical support. This work was supported by grants from INSERM and MESR (Er 6 UPMC), APHP, CICIT Garches, IRME, FRM and Foundation Motrice.

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