Abstract

Amphetamine, a catecholaminergic re-uptake-blocker, is able to improve neuroplastic mechanisms in humans. However, so far not much is known about the underlying physiological mechanisms. Here, we study the impact of amphetamine on NMDA receptor-dependent long-lasting excitability modifications in the human motor cortex elicited by weak transcranial direct current stimulation (tDCS). Amphetamine significantly enhanced and prolonged increases in anodal, tDCS-induced, long-lasting excitability. Under amphetamine premedication, anodal tDCS resulted in an enhancement of excitability which lasted until the morning after tDCS, compared to ∼1 h in the placebo condition. Prolongation of the excitability enhancement was most pronounced for long-term effects; the duration of short-term excitability enhancement was only slightly increased. Since the additional application of the NMDA receptor antagonist dextromethorphane blocked any enhancement of tDCS-driven excitability under amphetamine, we conclude that amphetamine consolidates the tDCS-induced neuroplastic effects, but does not initiate them. The fact that propanolol, a β-adrenergic antagonist, diminished the duration of the tDCS-generated after-effects suggests that adrenergic receptors play a certain role in the consolidation of NMDA receptor-dependent motor cortical excitability modifications in humans. This result may enable researchers to optimize neuroplastic processes in the human brain on the rational basis of purpose-designed pharmacological interventions.

Introduction

Learning depends critically on neuroplastic modifications in the efficacy of cortical neuronal processing. Long-lasting NMDA receptor-dependent cortical excitability and activity shifts, namely long-term potentiation (LTP), are involved in these processes (Rioult-Pedotti et al., 1998, 2000). In animals, dopaminergic and adrenergic mechanisms are important for consolidating and stabilizing NMDA receptor-dependent neuroplasticity (Ikegaya et al., 1997; Otani et al., 1998; Bailey et al., 2000) and learning (Stewart and Druhan, 1993).

Only indirect evidence for a similar process in the human brain is available so far. Amphetamine, which increases catecholaminergic transmitter availability, stabilizes use-dependent motor and improves sensory cortex plasticity (Bütefisch et al., 2002; Sawaki et al., 2002; Dinse et al., 2003), accelerates motor function recovery after stroke (Walker-Bateson et al., 1995), improves learning and consolidation of verbal material (Soetens et al., 1993, 1995) and can increase performance in subjects with low working memory capacity (Mattay et al., 2000). However, it has not yet been tested in humans whether this is due to an indirect stabilization of NMDA receptor-induced neuroplasticity through increased intracerebral availability of catecholamines.

Here we use the transcranial administration of weak direct currents (tDCS), which results in excitability changes of motor and visual cortices in the human. These changes evolve during tDCS but are stable for up to an h after stimulation termination, if the duration of stimulation is sufficiently long. Anodal tDCS increases, while cathodal tDCS decreases excitability. These excitability changes are of intracortical origin (Nitsche and Paulus, 2000, 2001; Antal et al., 2003; Nitsche et al., 2003a). It was shown directly in the animal and by pharmacological intervention in the human that the initial effect of DC stimulation is accomplished by a hyper- or depolarization of neuronal membranes (Purpura and McMurtry, 1965; Nitsche et al., 2003b), whereas the after-effects seem to be NMDA receptor-dependent (Liebetanz et al., 2002; Nitsche et al., 2003b). Moreover, tDCS is of functional relevance: in the motor cortex it has been shown to modulate use-dependent neuroplasticity (Rosenkranz et al., 2000) and to improve implicit motor learning (Nitsche et al., 2003c).

The aim of this study was to test if tDCS-generated, neuroplastic excitability shifts can be consolidated by an increase of intracerebral catecholamine availability induced by oral premedication with amphetamine.

Therefore, in separate experimental sessions, the effect of amphetamine on: (a) cortical excitability shifts during short-lasting tDCS, which does not induce any after-effects; (b) short-lasting, tDCS-induced after-effects; and (c) long-lasting, tDCS-induced after-effects was studied for cathodal and anodal stimulation. A direct influence of tDCS on β-adrenergic activity was tested by applying protocol (c) under propanolol (PROP), an unselective β-adrenoceptor-antagonist. By combined administration of amphetamine and dextromethorphane (DMO, a non-competitive NMDA receptor-antagonist) in the case of anodal tDCS-generated long-term effects, we tested the selective role of amphetamine in stabilizing, but not inducing, the respective excitability shifts.

Materials and Methods

Subjects

Five to twelve healthy subjects participated in each experiment (for details see Table 1). Six of the subjects participated in all experiments and thus received six AMP exposures. All gave written informed consent. The investigation was approved by the ethics committee of the University of Goettingen and conformed to the Declaration of Helsinki.

Current Stimulation of the Motor Cortex

Direct currents were transferred by a saline-soaked pair of surface sponge electrodes (35 cm2) and delivered by a specially developed, battery-driven, constant current stimulator (Schneider Electronic, Gleichen, Germany) with a maximum output of 2 mA. The motor-cortical electrode was fixed over the representational field of the right abductor digiti minimi muscle (ADM) as identified by transcranial magnetic stimulation (TMS) and the other electrode contralaterally above the right orbit. In the different experiments, the currents flowed continuously for 4 s (excitability shifts during tDCS), 7 min (short-lasting after-effects), or 9 (cathodal tDCS) and 13 (anodal tDCS) min (long-lasting after-effects) with an intensity of 1.0 mA (current density ∼0.03 mA/cm2). These stimulation durations have been demonstrated to elicit the intended durations in excitability shifts in former experiments (Nitsche and Paulus, 2000, 2001; Nitsche et al., 2003a). Nearly all subjects were able to feel the current flow as a slight itching sensation at both the anodal and cathodal electrodes.

Pharmacological Interventions

Two hours before the start of each experimental session, 20 mg amphetaminil (AMP), a precursor of amphetamine, which is completely metabolized into amphetamine (Honecker, 1975), 80 mg PROP, a combination of 20 mg AMP and 150 mg DMO or equivalent placebo (PLC) drugs were administered to the subjects orally (the different experiments are listed in Table 1). Two hours after oral intake, AMP, PROP and DMO were assumed to induce a stable plasma level (Honecker, 1975; Honecker et al., 1975; Silvasti et al., 1987; Karol et al., 2000) and to produce prominent effects in the central nervous system (Cruickshank and Neil-Dwyer, 1985; Bütefisch et al., 2002; Liebetanz et al., 2002; Nitsche et al., 2003b). To avoid cumulative drug effects, each experimental session was separated by at least 1 week. Subjects and the person conducting the experiment were blinded to the respective pharmacological condition. The subjects thus received identical numbers of pills in all experimental sessions, the number of pharmacologically active drugs being made up for with placebo pills where necessary. In order to keep the experimenter blinded with regard to the patient’s pharmacological condition, the drugs were administered by a different person.

Measurement of Motor-cortical Excitability

To detect current-driven changes of excitability, muscle-evoked potentials (MEPs) of the right ADM were recorded following stimulation of its motor-cortical representational field by single pulse TMS. These were induced using a Magstim 200 magnetic stimulator (Magstim Co., Whiteland, Dyfed, UK) and a figure-of-eight magnetic coil (diameter of one winding = 70 mm, peak magnetic field = 2.2 T). The coil was held tangentially to the skull, with the handle pointing backwards and laterally at 45° from midline. The optimal position was defined as the site where stimulation resulted consistently in the largest MEP. Surface EMG was recorded from the right ADM by use of Ag–AgCl electrodes in a belly-tendon montage. The signals were amplified and filtered with a time constant of 10 ms and a low-pass filter of 2.5 kHz. Signals were then digitized at an analogue-to-digital rate of 5 kHz and further relayed into a laboratory computer using the Neuroscan software collection (Neuroscan Inc., Herndon, VA) and conventional averaging software. The intensity of the stimulator output was adjusted for baseline recording so that the average stimulus led to an MEP of ∼1 mV, 5 min prior to DC stimulation.

Experimental Procedures

Each experiment was conducted in a repeated measurement design. The order of the conduction of the experiments was randomized between subjects. The subjects were seated in a reclining chair. First, the left motor-cortical representational field of the right ADM was identified by use of TMS (coil position which leads to the largest MEPs of ADM). One DC stimulation electrode, to which in the following the terms cathodal or anodal stimulation refer, was fixed at this position, and the other one was fixed on the forehead contralaterally, above the orbit.

In experiment 1 (intra-current excitability changes), a randomized series (0.1 Hz) of 15 TMS-evoked MEPs immediately before the end of a 4 s DC stimulation and another 15 MEPs without preceding DC stimulation were recorded. Anodal and cathodal DC stimulation were performed in one session in randomized order.

In experiment 2 (short-lasting after-effects), first, a baseline of TMS-evoked MEPs (20 stimuli) was recorded at 0.25 Hz. Afterwards anodal or cathodal tDCS was administered for seven min to elicit short-lasting after-effects. After termination of DC stimulation, 15 MEPs were recorded every fifth minute at 0.25 Hz up to 30 min after the end of tDCS.

Experiment 3 (long-lasting after-effects) differed from experiment 2 with regard to tDCS duration and time course covered by MEP measurements of cortical excitability after cessation of tDCS. Anodal tDCS was performed for 13 min and cathodal tDCS for 9 min; these are tDCS durations that modify cortical excitability for ∼1 h after the end of stimulation (Nitsche and Paulus, 2001; Nitsche et al., 2003a). After termination of tDCS, 15 MEPs were recorded at 0.25 Hz every fifth minute up to 30 min, then every 30th min up to 90 min (PRO, cathodal tDCS/AMP, anodal tDCS/AMP + DMO and PLC conditions) or 120 min (anodal tDCS/AMP). For the anodal condition (AMP only), tDCS was subsequently repeated after 240 min and the following day in the morning and in the afternoon. For the MEP measures on the second day, the recording electrodes were removed after the 240 min measurement, but their position was marked with a pen to guarantee consistent electrode positioning during the whole experiment.

To test if AMP alone changed cortical excitability, TMS stimulation intensity needed to elicit MEP amplitudes of ∼1 mV immediately before AMP intake was compared with that 2 h after AMP administration in 10 subjects.

Calculations and Statistics

MEP amplitude means were calculated in experiment 1 for the DC and non-current conditions (15 stimuli each). In experiments 2 and 3, MEP amplitude means were calculated for each time bin covering pre-tDCS baseline (20 stimuli) and post-tDCS time-points (15 stimuli). Post-stimulation MEP amplitude means were normalized to pre-current baselines in experiments 2 and 3.

For experiment 1, a repeated measures ANOVA was first calculated with the independent variables of drug condition, current flow and the dependent variable MEP amplitude. Then Student’s t-tests (paired samples, two-tailed, P < 0.05) were performed to test whether the values of the respective current and non-current conditions differed and if these differences depended on the drug conditions.

Regarding experiments 2 and 3, repeated measures ANOVAs (independent variables time course, current stimulation, drug condition, dependent variable MEP amplitude) were calculated, then Student’s t-tests (paired samples, two-tailed, level of significance P < 0.05) were performed to determine whether the MEP amplitudes before and after tDCS differed in each condition and if those differences were dependent on the drug conditions. For the AMP/PLC ANOVA of experiment 3, only MEPs up to 90 min after tDCS were included, since this time course covered AMP and PLC measures.

A separate repeated measures ANOVA was calculated for the AMP/DMO combination condition (comparison with AMP only).

For the 10 subjects who received MEP measures immediately before AMP intake in addition to the baseline measures immediately before tDCS, the respective TMS intensities were compared by Student’s t-tests (paired samples, two-tailed, level of significance P < 0.05).

Results

Effects of Amphetamine on tDCS-driven Excitability Shifts

With regard to cortical excitability changes during tDCS, the ANOVA revealed a significant main effect of tDCS (df = 3, F = 27.897, P < 0.001), but the main effect of the drug (df = 1, F = 0.695, P = 0.442) and interaction (df = 3, F = 1.507, P = 0.253) were not significant. As shown in Figure 1 and by the results of the t-tests, this is due to an excitability enhancement caused by anodal stimulation and an excitability reduction elicited by cathodal tDCS. These effects are identical in the AMP as well as in the PLC condition.

The ANOVA performed for the short-lasting after-effects revealed significant main effects of tDCS (df = 1, F = 111.665, P < 0.001) and time course (df = 7, F = 5.423, P < 0.001) and a significant interaction of current × time course (df = 7, F 17.412, P < 0.001). The main effect of drug condition as well as the remaining interactions were not significant (P > 0.25). Anodal tDCS increased cortical excitability in the PLC and AMP condition, however, the effect lasted for 15 min in the AMP condition, but only for 10 min under PLC. The difference between AMP and PLC was significant for 15 min. Also, the after-effects of cathodal tDCS tended to last longer under AMP as compared to PLC; however, here the difference between both conditions proved not to be significant (Fig. 2).

With regard to the long-lasting after-effects in the AMP/PLC condition, the ANOVA revealed significant main effects of tDCS (df = 1, F = 47.170, P = 0.002) and time course (df = 9, F = 2.717, P = 0.016) as well as a significant interaction of both variables (df = 9, F = 12.683, P < 0.001). The main effect of drug condition as well as the remaining interactions were not significant (P > 0.1). As shown by the post hoct-tests, anodal tDCS enhanced and cathodal tDCS diminished motor cortical excitability under AMP and PLC conditions. However, as compared to PLC, AMP prolonged the after-effects of anodal stimulation (30 min versus next morning) and decreased the duration and MEP amplitude shifts of the cathodal tDCS-induced excitability diminution. Hereby, the effects of AMP were much bigger and longer lasting with regard to the anodal tDCS condition. Whereas AMP prolonged the duration of the anodal stimulation-generated excitability enhancement until the morning after tDCS, the cathodal tDCS-induced excitability diminution was reduced significantly only at 20 min after stimulation by AMP (Fig. 3).

Effects of Amphetamine on tDCS-driven Excitability Changes under NMDA Receptor Blockade

Comparison of the ‘AMP-only’ and AMP/DMO conditions for the long-lasting after-effects of the anodal tDCS protocol showed significant main effects of drug condition (df = 1, F = 76.056, P = 0 0.001) and time course (df = 9, F = 3.328, P = 0.005) as well as a significant interaction of both variables (df = 9, F = 5.452, P < 0.001). This is due to a total loss of any excitability enhancement under AMP/DMO as compared to the AMP-only condition, as shown by the t-tests (Fig. 4).

AMP alone did not modulate the TMS-intensity needed for eliciting MEP amplitudes of 1 mV. Before and after AMP intake, 36 % of the maximum stimulator output intensity was needed (SEM ±4.528 before AMP, ±5.115 after AMP intake, t = 0.000).

Effects of Propanolol on tDCS-driven Excitability Modulations

For the long-lasting after-effects under PROP compared with PLC, the ANOVA revealed significant main effects due to the tDCS (df = 1, F = 39.876, P < 0.001) and time variables (df = 9, F = 2.352, P = 0.019) as well as a significant interaction of tDCS × time (df = 9, F = 21.760, P < 0.001). The main effect of drug condition and the remaining interactions were not significant (P > 0.089). According to the results of the post hoct-tests, anodal as well as cathodal after-effects were relatively shortened under PROP as compared to the PLC condition. Whereas in the latter condition, anodal tDCS elicited after-effects lasting for at least 30 min and cathodal tDCS resulted in excitability shifts lasting 60 min, under PROP, both were restricted to 20 min after tDCS (Fig. 5).

Discussion

The Amphetamine-induced Enhancements of Consolidation of tDCS-elicited Cortical Excitability Elevations are NMDA Receptor-dependent

In this study AMP selectively increased the duration of the anodal tDCS-induced motor cortical excitability enhancement as compared to PLC medication. This effect was relatively subtile for the short-lasting (7 min tDCS), but prominent for the long-lasting after-effects (9/13 min tDCS). It was restricted to the after-effects of anodal stimulation: tDCS eliciting no after-effects was not influenced by AMP. The necessary TMS output intensity to elicit MEP amplitudes of 1 mV magnitude was not modified by prior administration of AMP alone, thus a tDCS-independent effect of the drug on the amplitude of MEP can be ruled out. As suggested by a total loss of the after-effects from co-medication with DMO, a NMDA-antagonist, this effect is most likely caused by an NMDA receptor-dependent AMP-driven consolidation of the anodal tDCS-elicited cortical excitability enhancement and not due to a prominent direct effect of tDCS on catecholaminergic receptors.

Adrenaline may Contribute Relevantly to the Stabilization of Cortical Neuroplasticity

Since PROP, an unselective β-adrenergic antagonist, shortened the duration of the anodal tDCS after-effects, β-adrenergic receptor stimulation may be an important function of AMP for increasing consolidation of externally induced excitability enhancements. However, serotonergic and dopaminergic contributions were not tested in this study. Since it is known that dopamine via the D1 receptor facilitates NMDA-dependent excitability and facilitates NMDA-dependent LTP through cAMP-dependent mechanisms, similar to what has been found for the β-adrenergic receptor in hippocampus (Otmakhova and Liman, 1996, 1998; Bailey et al., 2000), a relevant additional contribution by dopamine to these effects seems plausible. Moreover, it was shown that even a single administration of AMP can induce prominent and long-term enhancements of cortical dopamine signaling (Vanderschuren et al., 1999). In this way, a prolonged dopaminergic activation could have stabilized the tDCS-induced, NMDA receptor-dependent, excitability enhancements. On the behavioral level, AMP exposure leads to sensitization, which is the progressive and enduring enhancement of the psychomotor and positive reinforcing effects of the drug caused by its application (Stewart and Badiani, 1993). Sensitization seems to depend on D1 receptor and — at least partly — on NMDA receptor activity (Hu et al., 2002; Pacchioni et al., 2002). It thus shares certain similarities with the phenomenon of LTP and long-term memory formation. Thus it seems possible that AMP is able to improve long-term memory formation via a mechanism similar to behavioral sensitization. Further studies have to be conducted to test this hypothesis more specifically.

A tendency for AMP to prolong the short-lasting after-effects of excitability-diminishing cathodal tDCS was also noticed. This could also be due to increased adrenergic activity, because antagonizing adrenergic receptors with PROP shortened the after-effects of cathodal tDCS. Alternatively, dopaminergic effects are possible candidates, since dopamine is known to consolidate long-lasting neuronal excitability reductions in animals (Otani et al., 1998). However, this AMP-effect was not significant compared to the PLC condition and could not be repeated for the AMP long-term excitability diminution induced by cathodal tDCS. It can be speculated that different catecholaminergic receptors — all indirectly activated by application of AMP — are functioning antagonistically to the consolidation of motor cortical excitability diminutions and thus the effect of adrenergic activation on the net long-lasting cathodal after-effects is reduced. Alternatively, the tendency to reverse the effect of AMP on short-term and long-term after-effects may be due to different contributions of different receptors, depending on the time course of excitability changes. Additional experiments are needed to clarify this issue.

However, since PROP diminished the duration of the tDCS-induced excitability elevations and diminutions similarly, a certain amount of adrenergic activity seems to be necessary for the consolidation of both neuroplastic mechanisms.

General Remarks

These results add important information to the understanding of the mechanisms of consolidation of tDCS-induced neuroplasticity in the human motor cortex: catecholaminergic transmitters and here especially adrenergic ones seem to contribute relevantly and the catecholaminergic effects are NMDA receptor-dependent. This is in line with knowledge gained from animal experimentation (Ikegaya et al., 1997). The contribution of the dopaminergic system to the consolidation process, which is probable due to animal experimentation, has to be explored in future studies.

This specific action of AMP could explain its positive effects on learning and neuroplasticity in humans (Soetens et al., 1993, 1995; Walker-Bateson et al., 1995; Bütefisch et al., 2002; Sawaki et al., 2002). So far, knowledge about the effects of amphetamine on human cortical function was mainly phenomenological. The results of this study offer a neurophysiological explanation of its mode of action. In this way, they establish a basis for further studies exploring the possibly beneficial effects of AMP on cortical functions in humans more systematically. Here, the combination of AMP with other therapeutical strategies which induce neuroplasticity could be promising to stabilize their effects. For example, AMP could increase the efficacy of motor and visual training strategies applied after stroke to restitute compromised functions. Moreover, the efficacy of externally neuroplasticity-evoking techniques, e.g. TMS-induced excitability enhancements of the dorsolateral prefrontal cortex, which have been shown to be effective in depression, could possibly be enhanced by by AMP. In this connection, it is important to note that AMP does not appear to have direct effects on cortical excitability as measured by TMS-evoked MEP, at least in the dosage applied here, but selectively consolidates them. This specific mode of action could be especially advantageous, since it may offer the possibility to selectively stabilize training- or stimulation-induced neuroplastic changes without inducing concurrent modifications, which could be maladaptive.

This study was supported in part by the German Ministry for Research and Education. We thank C. Crozier for improving the English of the manuscript.

Figure 1. AMP does not modulate motor cortical excitability shifts generated by short-lasting tDCS, which induces no after-effects. Shown are the mean MEP amplitudes ± standard error of mean (SEM) under AMP or PLC medication with/without tDCS for anodal and cathodal stimulation.

Figure 1. AMP does not modulate motor cortical excitability shifts generated by short-lasting tDCS, which induces no after-effects. Shown are the mean MEP amplitudes ± standard error of mean (SEM) under AMP or PLC medication with/without tDCS for anodal and cathodal stimulation.

Figure 2. AMP prolongs tDCS after-effects after 7 min DC stimulation, which causes short-lasting excitability enhancements. Shown are the mean ± SEM MEP amplitudes versus baseline across time following anodal or cathodal tDCS for AMP and placebo conditions. Fifteen minutes after anodal tDCS, MEP amplitudes are still enhanced under AMP, whereas under placebo medication they have reached baseline values. Asterisks indicate significant deviations of the post-tDCS MEP amplitudes from baseline values, crosses mark significant deviations of the respective AMP versus PLC conditions with regard to identical time points and tDCS polarities (Student’s t-test, two-tailed, paired samples, P < 0.05).

Figure 2. AMP prolongs tDCS after-effects after 7 min DC stimulation, which causes short-lasting excitability enhancements. Shown are the mean ± SEM MEP amplitudes versus baseline across time following anodal or cathodal tDCS for AMP and placebo conditions. Fifteen minutes after anodal tDCS, MEP amplitudes are still enhanced under AMP, whereas under placebo medication they have reached baseline values. Asterisks indicate significant deviations of the post-tDCS MEP amplitudes from baseline values, crosses mark significant deviations of the respective AMP versus PLC conditions with regard to identical time points and tDCS polarities (Student’s t-test, two-tailed, paired samples, P < 0.05).

Figure 3. AMP selectively prolongs the anodal tDCS after-effects of a 13 min stimulation period, which causes long-lasting excitability enhancements. The after-effects of cathodal tDCS are slightly reduced. Shown are the mean ± SEM MEP amplitudes versus baseline across time following anodal or cathodal tDCS for AMP and placebo conditions. On the morning following anodal tDCS, MEP amplitudes are still enhanced under AMP, whereas under placebo medication they have reached baseline values 60 min after tDCS. Asterisks indicate significant deviations of the post-tDCS MEP amplitudes from baseline values, crosses mark significant deviations of AMP versus PLC conditions with regard to identical time points and tDCS polarities (Student’s t-test, two-tailed, paired samples, P < 0.05). n.m., next morning; n.a., next afternoon; a, anodal tDCS; c, cathodal tDCS.

Figure 3. AMP selectively prolongs the anodal tDCS after-effects of a 13 min stimulation period, which causes long-lasting excitability enhancements. The after-effects of cathodal tDCS are slightly reduced. Shown are the mean ± SEM MEP amplitudes versus baseline across time following anodal or cathodal tDCS for AMP and placebo conditions. On the morning following anodal tDCS, MEP amplitudes are still enhanced under AMP, whereas under placebo medication they have reached baseline values 60 min after tDCS. Asterisks indicate significant deviations of the post-tDCS MEP amplitudes from baseline values, crosses mark significant deviations of AMP versus PLC conditions with regard to identical time points and tDCS polarities (Student’s t-test, two-tailed, paired samples, P < 0.05). n.m., next morning; n.a., next afternoon; a, anodal tDCS; c, cathodal tDCS.

Figure 4. Administration of the NMDA antagonist DMO eliminates any excitability enhancement following anodal tDCS under AMP. Shown are the mean ± SEM MEP amplitudes versus baseline across time following anodal tDCS for AMP and DMO versus only AMP after 13 min anodal tDCS. Ninety minutes after anodal tDCS, MEP amplitudes are still enhanced under AMP only, whereas under the combination of AMP and DMO, no excitability enhancement takes place. Asterisks indicate significant deviations of the post-tDCS MEP amplitudes from baseline values, crosses indicate significant deviation of AMP + DMO versus AMP-only with regard to identical time points (Student’s t-test, two-tailed, paired samples, P < 0.05).

Figure 4. Administration of the NMDA antagonist DMO eliminates any excitability enhancement following anodal tDCS under AMP. Shown are the mean ± SEM MEP amplitudes versus baseline across time following anodal tDCS for AMP and DMO versus only AMP after 13 min anodal tDCS. Ninety minutes after anodal tDCS, MEP amplitudes are still enhanced under AMP only, whereas under the combination of AMP and DMO, no excitability enhancement takes place. Asterisks indicate significant deviations of the post-tDCS MEP amplitudes from baseline values, crosses indicate significant deviation of AMP + DMO versus AMP-only with regard to identical time points (Student’s t-test, two-tailed, paired samples, P < 0.05).

Figure 5. The β-adrenergic antagonist PROP shortens the duration of the long-lasting after-effects following 13 min anodal and 9 min cathodal tDCS, but does not eliminate them. Shown are the mean ± SEM MEP amplitudes versus baseline across time following anodal or cathodal tDCS for PROP and placebo conditions. Thirty minutes after anodal tDCS MEP amplitudes are still enhanced under placebo medication, whereas under PROP they reach baseline values 25 min after stimulation. For the cathodal stimulation condition, MEP amplitudes are still diminished 60 min after tDCS, while they are at baseline level after 25 min under PROP. Asterisks indicate significant deviations of the post-tDCS MEP amplitudes from baseline values, crosses mark significant deviation of PROP versus PLC conditions with regard to identical time points and tDCS polarities (Student’s t-test, two-tailed, paired samples, P < 0.05). a, anodal tDCS; c, cathodal tDCS.

Figure 5. The β-adrenergic antagonist PROP shortens the duration of the long-lasting after-effects following 13 min anodal and 9 min cathodal tDCS, but does not eliminate them. Shown are the mean ± SEM MEP amplitudes versus baseline across time following anodal or cathodal tDCS for PROP and placebo conditions. Thirty minutes after anodal tDCS MEP amplitudes are still enhanced under placebo medication, whereas under PROP they reach baseline values 25 min after stimulation. For the cathodal stimulation condition, MEP amplitudes are still diminished 60 min after tDCS, while they are at baseline level after 25 min under PROP. Asterisks indicate significant deviations of the post-tDCS MEP amplitudes from baseline values, crosses mark significant deviation of PROP versus PLC conditions with regard to identical time points and tDCS polarities (Student’s t-test, two-tailed, paired samples, P < 0.05). a, anodal tDCS; c, cathodal tDCS.

Table 1


 Stimulation paradigms, drug dosages and subject characteristics of the experiments

tDCS-condition Drug Dosage (mg) TMS stimulation intensity (% of maximum stimulator output ± SD) MEP amplitude (µV) ± SD tDCS stimulation duration per cycle No. of subjects Age of subjects (years, mean ± SD) Gender of subjects (female/male) 
Intra-tDCS AMP 20 A: 53.333 ± 22.757 A:1008.930 ± 123.050  4s A/C  6 27.000 ± 4.336 3/3 
   C: 53.333 ± 22.757 C:1008.13 ± 73.155     
 PLC 40 A: 54.000 ± 17.844 A: 1033.840 ± 90.610  4s A/C  6 27.000 ± 4.336 3/3 
   C: 54.000 ± 18.254 C: 1077.700 ± 67.820     
Short-lasting after effects AMP 20 A: 43.833 ± 17.058 A: 979.386 ± 116.280  7 min. A/C  6 27.000 ± 4.336 3/3 
   C: 43.667 ± 16.825 C: 1031.779 ± 68.415     
 PLC 40 A: 43.667 ± 15.293 A: 1012.086 ± 64.613  7 min A/C  6 27.000 ± 4.336 3/3 
   C: 43.667 ± 14.720 C: 986.002 ± 98.049     
Long-lasting after-effects (a) AMP 20 A:41.889 ± 8.373 A: 974.367 ± 68.434 13 min A A:9 A:25.778 ± 1.986 A:6/3 
   C:38.000 ± 7.925 C: 1017.497 ± 59.308  9 min C C:6 C:27.286 ± 3.904 C:3/3 
 PRO 80 A: 42.750 ± 10.402 A: 995.423 ± 43.956 13 min A 12 26.167 ± 3.407 7/5 
   C: 43.000 ± 10.497 C: 1045.187 ± 53.240  9 min C    
 PLC/PRO 40 A: 44.833 ± 10.978 A: 974.322 ± 83.699 13 min A 12 26.167 ± 3.407 7/5 
   C: 45.417 ± 11.524 C: 1052.734 ± 38.199  9 min C    
 PLC/AMP 40 A:43.889 ± 6.030 A:1031.115 ± 123.609 13 min A A:9 A:25.778 ± 1.986 A:6/3 
   C:39.500 ± 4.680 C:1047.159 ± 55.963  9 min C C:6 C:27.286 ± 3.904 C:3/3 
Long-lasting after-effects (b) AMP 20 A:37.000 ± 4.528 A:972.950 ± 83.678 13 min A  5 27.500 ± 4.231 3/2 
 AMP/DMO 20/150 A:38.000 ± 7.649 A:1039.060 ± 61.833 13 min A  5 27.500 ± 4.231 3/2 
 PLC 40 A:41.600 ± 6.504 A:959.473 ± 13.044 13 min A  5 27.500 ± 4.231 3/2 
tDCS-condition Drug Dosage (mg) TMS stimulation intensity (% of maximum stimulator output ± SD) MEP amplitude (µV) ± SD tDCS stimulation duration per cycle No. of subjects Age of subjects (years, mean ± SD) Gender of subjects (female/male) 
Intra-tDCS AMP 20 A: 53.333 ± 22.757 A:1008.930 ± 123.050  4s A/C  6 27.000 ± 4.336 3/3 
   C: 53.333 ± 22.757 C:1008.13 ± 73.155     
 PLC 40 A: 54.000 ± 17.844 A: 1033.840 ± 90.610  4s A/C  6 27.000 ± 4.336 3/3 
   C: 54.000 ± 18.254 C: 1077.700 ± 67.820     
Short-lasting after effects AMP 20 A: 43.833 ± 17.058 A: 979.386 ± 116.280  7 min. A/C  6 27.000 ± 4.336 3/3 
   C: 43.667 ± 16.825 C: 1031.779 ± 68.415     
 PLC 40 A: 43.667 ± 15.293 A: 1012.086 ± 64.613  7 min A/C  6 27.000 ± 4.336 3/3 
   C: 43.667 ± 14.720 C: 986.002 ± 98.049     
Long-lasting after-effects (a) AMP 20 A:41.889 ± 8.373 A: 974.367 ± 68.434 13 min A A:9 A:25.778 ± 1.986 A:6/3 
   C:38.000 ± 7.925 C: 1017.497 ± 59.308  9 min C C:6 C:27.286 ± 3.904 C:3/3 
 PRO 80 A: 42.750 ± 10.402 A: 995.423 ± 43.956 13 min A 12 26.167 ± 3.407 7/5 
   C: 43.000 ± 10.497 C: 1045.187 ± 53.240  9 min C    
 PLC/PRO 40 A: 44.833 ± 10.978 A: 974.322 ± 83.699 13 min A 12 26.167 ± 3.407 7/5 
   C: 45.417 ± 11.524 C: 1052.734 ± 38.199  9 min C    
 PLC/AMP 40 A:43.889 ± 6.030 A:1031.115 ± 123.609 13 min A A:9 A:25.778 ± 1.986 A:6/3 
   C:39.500 ± 4.680 C:1047.159 ± 55.963  9 min C C:6 C:27.286 ± 3.904 C:3/3 
Long-lasting after-effects (b) AMP 20 A:37.000 ± 4.528 A:972.950 ± 83.678 13 min A  5 27.500 ± 4.231 3/2 
 AMP/DMO 20/150 A:38.000 ± 7.649 A:1039.060 ± 61.833 13 min A  5 27.500 ± 4.231 3/2 
 PLC 40 A:41.600 ± 6.504 A:959.473 ± 13.044 13 min A  5 27.500 ± 4.231 3/2 

‘tDCS condition’ refers to the different experiments: intra-tDCS MEP-measures (experiment 1), short-lasting (experiment 2) and long-lasting (experiment 3) after-effects. For each experiment, drugs and drug dosages are given. Mean TMS stimulation intensities to achieve non-tDCS (experiment 1) or pre-tDCS (experiments 2 and 3) MEP amplitudes of ∼1 mV were calculated for each experimental condition. They did not differ between the respective drug/PLC conditions (Student’s t-test, P > 0.05). Also non-current (experiment 1) and pre-tDCS (experiments 2 and 3) MEP amplitude means were identical for the drug/PLC conditions (Student’s t-test, P > 0.05). tDCS stimulation duration was 4 s in experiment 1. Here, stimulation was repeated 15 times for each tDCS condition, whereas in the remaining protocols only one DC stimulation per session was applied. As shown in the last rows, six to twelve subjects participated in each experiment; age and gender distribution were comparable between experiments. A, anodal tDCS; C, cathodal tDCS.

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