Chapter 13 - Impact of chronic transcranial random noise stimulation (tRNS) on GABAergic and glutamatergic activity markers in the prefrontal cortex of juvenile mice
Introduction
Transcranial random noise stimulation (tRNS) delivers a painless, weak current at random, constantly changing frequencies usually between 101 and 640 Hz. Previous research has highlighted the benefit of random noise and in particular it has shown that tRNS can alter cortical activity during (Snowball et al., 2013) and after stimulation (Terney et al., 2008), with some behavioral and neural effects lasting up to several months (Brevet-Aeby et al., 2019; Cappelletti et al., 2013; Frank et al., 2018; Herpich et al., 2019; Pasqualotto, 2016; Snowball et al., 2013).
Compared to more familiar methods, such as transcranial magnetic stimulation, transcranial direct current stimulation (tDCS) or transcranial alternating current stimulation (tACS), tRNS is considered the most comfortable intervention technique for participants, which is a key advantage for use with cognitive training and effective blinding (i.e., whether the participant receives sham or active tRNS). For example, the 50% perception threshold for tDCS was set at 0.4 mA, and at 1.2 mA in the case of tRNS (Ambrus et al., 2010). In addition, tRNS exhibited long-lasting effects in several studies (Brevet-Aeby et al., 2019; Cappelletti et al., 2013; Frank et al., 2018; Herpich et al., 2019; Pasqualotto, 2016; Snowball et al., 2013). tRNS is also polarity-independent, with both electrodes (Terney et al., 2008), or at least one (Snowball et al., 2013) inducing excitatory effects (when the current is set to 1 mA). tRNS is less sensitive to cortical folding than other neurostimulation methods (Terney et al., 2008), reducing the impact of anatomical variations between participants.
One of the suggested mechanisms in tRNS is stochastic resonance (Fertonani and Miniussi, 2017; Harty and Cohen Kadosh, 2019; Terney et al., 2008; van der Groen and Wenderoth, 2016). According to this framework in non-ideal and non-linear systems, such as the brain, noise can be beneficial. This fact has been shown in a number of research fields, including perception, ecology, and engineering (McDonnell and Abbott, 2009; McDonnell and Ward, 2011). In all of these cases, when noise is applied to a subthreshold signal/input it will improve performance/output. Critically, the noise needs to be at a specific level to yield optimal gain as the addition of too much noise can be non-beneficial. The main assumption is that tRNS induces noise in the neural system and as a consequence will improve the signal-to-noise ratio at the neuronal level and the sensitivity of the neurons (Fertonani et al., 2011). This idea is based on the most prevalent hypothesis of noise-induced improvement in multiple disciplines (McDonnell and Abbott, 2009). In the present experiment, the prediction is that random noise-based neurostimulation should influence the brain excitability non-linearly. That is, the changes in the tRNS parameters should follow an inverted U-function in that the output metric is very small for high and no noise (sham stimulation), while low noise level provides more optimal output (McDonnell and Abbott, 2009).
It is surprising that despite the promising results of tRNS in human-based research, in some cases even more than more popular methods such as tDCS and tACS (Berger et al., 2019; Brem et al., 2018; Fertonani et al., 2011; Simonsmeier et al., 2018) its mechanisms are relatively unclear, and are scarce compared to other methods such as tDCS and tACS. In addition, its safety based on animal-based research is lacking. This is a point of potential concern as tRNS is now being used in the case of neurodevelopmental disorders (Berger et al., 2019; Looi et al., 2017). Therefore, the motivation in this study was to mirror a promising protocol of tRNS that was used during a cognitive training in atypical developing children, in order to examine its effects at vGLUT1 and GAD 65–67 that are involved in neuroplasticity and its safety in terms of pathological changes induced by tRNS.
To do so we used the protocol published by Looi et al. (2017), who administrated chronic tRNS in the shape of 9 tRNS sessions during 5 weeks to children with mathematical learning difficulties. Juvenile mice were randomly distributed in three different groups: (1) “tRNS 1 ×” group: receiving tRNS at the same density current as in Looi et al., (2017) (0.3 A/m2, 20 min, n = 5), (2) “tRNS 100 ×” group: receiving tRNS at two order of magnitude higher than those commonly used in human tRNS experiments (30.0 A/m2, 20 min, n = 6), and (3) “sham” group: receiving the same tRNS than tRNS 1 × group except for time duration that was 15 s instead of 20 min (n = 5).
Section snippets
Animals
Experiment was carried out on 6 weeks old males C57 mice (University of Seville, Spain) weighing 28–35 g. Before and after surgery, the animals were kept in the same room but placed in independent cages. The animals were maintained on a 12-h light/12-h dark cycle with continuously controlled humidity (55 ± 5%) and temperature (21 ± 1 °C). All experimental procedures were carried out in accordance with European Union guidelines (2010/63/CE) and following Spanish regulations (RD 53/2013) for the use of
Results
To explore for potential changes in the excitation/inhibition balance in PFC after exposition to tRNS, young mice (6 weeks old) were prepared for chronic stimulation in alert head restrained condition (Fig. 1A). The three first days after surgery recovery were used to habituate the mice to the treadmill and head-fixed condition. During tRNS sessions, the animals were placed over a treadmill and the head fixed to the recording table by means of the implanted head-holding system. The plastic tube
Discussion
The present histological results in young mice suggest that tRNS applied at low-density currents is capable of increasing excitability by decreasing GABA levels in a focalized way. These results could be of crucial importance for human tRNS studies suggesting that a decrease in GABA levels could be mediating the behavioral enhancement observed in previous studies (Terney et al., 2008; Fertonani et al., 2011; Cappelletti et al., 2013; Snowball et al., 2013; Pasqualotto, 2016; van der Groen and
Funding
This work was supported by grants from the Spanish MINECO-FEDER (BFU2014-53820-P and BFU2017-89615-P) to J.M-R. C.A.S-L was in receipt of an FPU grant from the Spanish Government (FPU13/04858).
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2022, Neuroscience and Biobehavioral ReviewsCitation Excerpt :tDCS research suggests that LTP-like mechanisms occur after more than 3 min of stimulation (Chaieb et al., 2015; Terney et al., 2008). tRNS-induced plasticity is also GABAa receptor sensitive (Chaieb et al., 2015), and a recent study has suggested that GABA levels can be reduced after prolonged stimulation with tRNS in juvenile mice (Sánchez-León et al., 2021). It is unknown if similar effects occur in the human brain, but this could plausibly be tested by measuring GABA levels following tRNS with magnetic resonance spectroscopy (MRS) (Puts and Edden, 2012).