Safety limits of cathodal transcranial direct current stimulation in rats

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Abstract

Objective

The aim of this rat study was to investigate the safety limits of extended transcranial direct current stimulation (tDCS). tDCS may be of therapeutic value in several neuro-psychiatric disorders. For its clinical applicability, however, more stable effects are required, which may be induced by intensified stimulations.

Methods

Fifty-eight rats received single cathodal stimulations at 1–1000 μA for up to 270 min through an epicranial electrode (3.5 mm2). Histological evaluation (H&E) was performed 48 h later. A threshold estimate was calculated from volumes of DC-induced lesions.

Results

Brain lesions occurred at a current density of 142.9 A/m2 for durations greater than 10 min. For current densities between 142.9 and 285.7 A/m2, lesion size increased linearly with charge density; with a calculated zero lesion size intercept of 52400 C/m2. Brains stimulated below either this current density or charge density threshold, including stimulations over 5 consecutive days, were morphologically intact.

Conclusion

The experimentally determined threshold estimate is two orders of magnitude higher than the charge density currently applied in humans (171–480 C/m2). In relation to transcranial DC stimulation in humans the rat epicranial electrode montage may provide for an additional safety margin.

Significance

Although these results cannot be directly transferred to humans, they encourage the development intensified tDCS protocols. Further animal studies are required, before such protocols can be applied in humans.

Introduction

Weak direct current stimulation is capable of inducing lasting alterations of cortical excitability. The direction of these excitability shifts is determined by current polarity, while their stability is controlled by stimulation duration as well as by current intensity (Bindman et al., 1962, Creutzfeldt et al., 1962, Landau et al., 1963, Purpura and McMurtry, 1965). During recent years, the technique of transcranial direct current stimulation (tDCS) has been successfully transferred to the human cortex (Nitsche and Paulus, 2000, Nitsche and Paulus, 2001, Nitsche et al., 2003a) and has increasingly gained interest among clinical neuroscientists.

Clinical and functional studies propose beneficial effects of tDCS in several neurological and psychiatric disorders including patients with chronic stroke (Hummel et al., 2005, Hummel et al., 2006, Fregni et al., 2005), epilepsy (Fregni et al., 2006a, Liebetanz et al., 2006a), chronic pain (Fregni et al., 2006b), Parkinson’s disease (Fregni et al., 2006c) and major depression (Fregni et al., 2006d, Boggio et al., 2007). Other studies aimed at investigating pharmacological interactions or functional aspects of the induced after effect excitability changes (Liebetanz et al., 2002, Liebetanz et al., 2006b, Antal et al., 2004, Antal et al., 2006, Nitsche et al., 2004a, Nitsche et al., 2004b, Nitsche et al., 2004c, Nitsche et al., 2005, Nitsche et al., 2006, Kincses et al., 2004, Accornero et al., 2007). Although functional and preliminary clinical effects may indicate its therapeutic potential, the after effects of tDCS so far have been limited in their duration to a few hours.

Clinicians would potentially aim for excitability changes lasting up to several weeks, or at least several days. As early as the 1960s Lynn Bindman found that after effects can be enhanced by using repeated DC stimulations (Bindman et al., 1964). Although the latter approach has hitherto not been approved in humans, two clinical studies applied tDCS in a repetitive design, assuming that the induced effects would become more prominent and stable (Fregni et al., 2006a, Fregni et al., 2006d). Finally, in addition to physical parameters, tDCS after effects have been stabilized for at least 24 h when combined with pharmacological interventions (Nitsche et al., 2004b, Nitsche et al., 2004c, Nitsche et al., 2005, Nitsche et al., 2006).

However, a major concern for treatment with tDCS is related to its safeness, particularly when paradigms with increased intensities or prolonged stimulation durations are applied. While current safety considerations for the application of tDCS in humans are based on measurements of neuron-specific enolase, MRI and EEG data from human tDCS studies (Nitsche et al., 2003a, Nitsche et al., 2004d, Iyer et al., 2005) as well as on the adoption of safety limits stated by Agnew and McCreery (1987) for pulsed electrical stimulation, no studies are available which explored the safety limits of tDCS systematically. Moreover, most animal studies investigated only behavioural and neurophysiologic effects but not specifically the safety aspects of DC stimulation. Therefore, only little is known about potential harmful effects of continuous weak DC stimulation. At present, our incomplete knowledge about potentially deleterious effects limits the development of intensified tDCS regimes, which could prove to be therapeutically relevant in neurological and psychiatric disorders which would benefit from long-lasting changes of cortical excitability. To partly close this gap, we present here a first experimental approach to the potentially harmful effects and safety limits of cathodal tDCS in a recently introduced tDCS rat model (Liebetanz et al., 2006a, Liebetanz et al., 2006b).

Section snippets

Material and methods

All experiments were conducted in accordance with the “Guide for the Care and Use of Laboratory Animals of the NIH” and were ethically approved by the Government of Lower Saxony. The experiments were performed on 62 Wistar rats of both sexes (mean body weight 310 ± 24 g). Animals were kept in single cages under standard laboratory conditions, with food and water ad libitum.

Threshold for brain lesion of cathodal tDCS

Histological analysis of the escalating stimulation regime revealed no signs of any current-induced neurotrauma from single tDCS when applying current strength intensities from 1 to 100 μA at stimulation durations from 10 to 270 min (see Table 1). However, when applied above a certain threshold, tDCS resulted in a focal brain lesion directly beneath the epicranial cathode (Fig. 2). The earliest visible signs of such lesions were detected in brains which received cathodal tDCS for 10 min at a

Discussion

The results of this systematic animal safety study demonstrate that cathodal tDCS is able to cause severe neuronal damage when it is applied above a certain charge density. For cathodal current densities between 142.9 and 287 A/m2, no pathological brain lesions were observed below a charge density threshold of 52400 C/m2. This threshold is at least 2 orders of magnitude higher than those charge densities currently being applied in clinical studies (171–480 C/m2). Moreover, for cathodal current

Acknowledgements

We thank C. Bunker for revising the English and A. Dettmar for technical assistance. This project was supported by the Deutsche Forschungsgemeinschaft (DL) (Grant No. Li/1016/3-1).

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