The influence of short-interval intracortical facilitation when assessing developmental changes in short-interval intracortical inhibition

Neuroscience 312 (2016) 19–25

THE INFLUENCE OF SHORT-INTERVAL INTRACORTICAL FACILITATION WHEN ASSESSING DEVELOPMENTAL CHANGES IN SHORT-INTERVAL INTRACORTICAL INHIBITIONI L. A. SCHNEIDER, M. R. GOLDSWORTHY, J. P. COLE, M. C. RIDDING AND J. B. PITCHER * Robinson Research Institute, Paediatrics & Reproductive Health, School of Medicine, University of Adelaide, Adelaide 5005, Australia

INTRODUCTION In humans, motor cortical output is the result of complex interactions between inhibitory and excitatory processes (Chen, 2004). The balance of these processes plays an important role in the preparation and execution of motor tasks (Ridding et al., 1995; Sinclair and Hammond, 2008) and in the modulation of activity-dependent neuroplasticity in the human primary motor cortex (M1) (Sanes and Donoghue, 2000). Transcranial magnetic stimulation (TMS) is a neurophysiological technique that can be employed to study cortical circuitry in the intact human brain. Paired-pulse TMS, in particular, can be applied according to well-established parameters to test inhibitory and excitatory neuronal circuits within M1 (Rothwell, 1997). However, the majority of studies investigating these circuits in humans have included only adults, and as a consequence less is known about the balance of intracortical excitation and inhibition in children and adolescents. Short-interval intracortical inhibition (SICI) is an established paired-pulse TMS measure of intracortical inhibition (Kujirai et al., 1993). The amplitude of the motor-evoked potential (MEP) elicited by a supratheshold TMS pulse (S2) is suppressed if preceded by 1–6 ms by a subthreshold conditioning stimulus (S1). This inhibition is thought to be mediated by the gamma-aminobutyric acid (GABA) A receptor (Ziemann et al., 1996; Di Lazzaro et al., 2006; Florian et al., 2008), with pharmacological studies demonstrating that SICI is increased by drugs that augment the transmission of GABA (Ziemann et al., 1996, 1996), but unaffected by drugs that block voltage-gated sodium channels (Ziemann et al., 1996; Chen et al., 1997). Studies of the maturation of the GABAergic system have shown that during fetal neurodevelopment, GABA initially behaves as an excitatory neurotransmitter, but switches to become inhibitory during the early postnatal period due to a complex series of events, including the lowering of intracellular chloride concentrations (Jensen et al., 2000; Ganguly et al., 2001; Ben-Ari, 2002; Swanwick et al., 2006; Klueva et al., 2008; Ben-Ari et al., 2012). Little is known about the signaling that instigates this change or the time course of this postnatal transformation in humans. There is, however, some evidence that GABAergic inhibitory circuits undergo maturation during childhood, and some authors have

Abstract—Objective: Measures of short-interval intracortical inhibition (SICI) can be contaminated by excitatory influences of short-interval intracortical facilitation (SICF), unless examined at individually-optimized interstimulus intervals (ISIs). We hypothesized that age-related differences in SICF would explain previously reported reduced SICI in children and adolescents compared with adults. Methods: Fifty-one participants, aged 8–29 years, underwent transcranial magnetic stimulation. SICF curves were constructed to determine the ISI at which SICF was minimal (i.e. the first trough). SICI curves were constructed at this individually-determined ISI with conditioning stimulus (S1) intensities of 60–110% of active motor threshold. Results: There was no effect of age on the ISI corresponding with the SICF trough. However, there was a main effect of age on the amplitude of the conditioned motor-evoked potential at the different ISIs, such that children aged 8–12 years demonstrated greater SICF than those aged 16–18 and 19–21 years. There was no effect of age on SICI, and no interaction between age group and S1 intensity. Conclusions: Compared with that in older adolescents and young adults, SICF is enhanced in children aged 8–12 years. Surprisingly, this enhanced SICF does not appear to reduce the degree of SICI that can be evoked at the first trough in this age group. Significance: This is the first report of enhanced SICF in young children. It remains possible that enhanced SICF may have confounded earlier reports of reduced SICI in children less than 8 years. Ó 2015 IBRO. Published by Elsevier Ltd. All rights reserved.

Key words: children, transcranial magnetic stimulation, motor-evoked potential, paired-pulse TMS, gammaaminobutyric acid. I

Work completed at the Robinson Research Institute, Paediatrics & Reproductive Health, School of Medicine, University of Adelaide. *Corresponding author. Tel: +61-08-83131301; fax: +61-0883131355. E-mail address: [email protected] (J. B. Pitcher). Abbreviations: aMT, active motor threshold; ANCOVA, analysis of covariance; EMG, electromyogram; FDI, first dorsal interosseous; GABA, gamma-aminobutyric acid; ISI, interstimulus interval; M1, primary motor cortex; MEP, motor-evoked potential; rMT, resting motor threshold; SI, stimulation intensity; SICF, short-interval intracortical facilitation; SICI, short-interval intracortical inhibition; TMS, transcranial magnetic stimulation. http://dx.doi.org/10.1016/j.neuroscience.2015.10.057 0306-4522/Ó 2015 IBRO. Published by Elsevier Ltd. All rights reserved. 19

20

L. A. Schneider et al. / Neuroscience 312 (2016) 19–25

hypothesized that reduced inhibition in early life enhances neuroplasticity and thereby facilitates motor learning (Mall et al., 2004; Walther et al., 2009). Two previous studies have both reported reduced SICI in children and adolescents when compared to adults (Mall et al., 2004; Walther et al., 2009). Other studies have compared SICI in healthy children and children with attention-deficit-hyperactivity disorder and/or tic disorder, and while these studies did not compare SICI in children and adults, they provide evidence of intracortical inhibitory processes in 8–17-year-old children and adolescents (Moll et al., 1999, 2000, 2001). However, measures of SICI can be contaminated by facilitatory influences unless tested with stimulation parameters individually optimized to minimize this (Peurala et al., 2008). The strength of intracortical facilitatory circuits can be tested using a paired-pulse TMS paradigm, where S1 is suprathreshold and S2 is sub- or suprathreshold (Ziemann et al., 1998). Strong facilitatory interaction between the paired stimuli is seen when the interstimulus interval (ISI) is
1.5,
2.5, and
4.5 ms, and this has been termed short-interval intracortical facilitation (SICF) (Tokimura et al., 1996; Ziemann et al., 1998; Di Lazzaro et al., 1999; Hanajima et al., 2002; Ilic et al., 2002). In between these discrete SICF peaks are troughs, where little or no facilitation occurs. Under certain conditions, measurement of SICI can be contaminated by SICF and the consequence is a net response, reflecting the summation of SICI and SICF (Peurala et al., 2008). Consequently, these authors suggested that the contamination of SICI by SICF should be carefully avoided by assessing SICI at an individually-determined ISI that corresponds with a SICF trough. SICF is believed to be the product of S2 directly exciting the axon initial segments of excitatory intracortical interneurons previously depolarized, and made hyperexcitable, by S1 (Hanajima et al., 2002; Ilic et al., 2002). While the reduced SICI in children and adolescents compared with adults has been attributed to protracted maturation of GABAergic inhibition (Mall et al., 2004; Walther et al., 2009), the developmental trajectory of SICF circuitry has not been studied. Due to the overlap between SICI and SICF measures, it is possible that protracted SICF maturation could, at least in part, contribute to the previously reported age-related differences in SICI. We hypothesized that the reduced SICI previously reported in children and adolescents is confounded by the influence of concomitant SICF. Therefore, we aimed to examine the influence of age on the ISI at which the first SICF trough was evident in participants aged 8–29 years. Thereafter, we constructed each individual’s SICI curve at their individually-determined ISI, using S1 intensities ranging from 60% to 110% of active motor threshold (aMT), and assessed the degree of SICI induced.

EXPERIMENTAL PROCEDURES

have previously shown persistent reductions in corticomotor excitability in children and adolescents born prior to 37 completed weeks of gestation (Pitcher et al., 2012). Only right-handed participants were included in the study [handedness was confirmed with the Edinburgh Handedness Inventory Oldfield, 1971]. Prior to their inclusion in the study, participants were screened for contraindications to TMS (Rossi et al., 2009), and provided written informed consent. Parents/caregivers provided consent for participants <18 years of age, and accompanied them to the experimental session. Exclusion criteria also included any history of perinatal brain injury or neuropathy. All procedures were approved by the Women’s and Children’s Health Network and University of Adelaide Human Research Ethics Committees, and all procedures were conducted in accordance with the Declaration of Helsinki (2008 revision). Recording procedures Participants were seated in an armchair with their hands and forearms supported. Adhesive Ag/AgCl electrodes were applied to the skin overlying the right first dorsal interosseous (FDI) hand muscle, using a belly-tendon montage. Electromyogram (EMG) signals were amplified (1000), bandpass filtered (20–1000 Hz) (D360; Digitimer, Welwyn Garden City, UK) and then digitized at 5.1 kHz (CED 1401; Cambridge Electronic Design, Cambridge, UK), before being stored on a computer for offline analysis. TMS Motor cortical excitability was assessed with single- and paired-pulse TMS, applied to the left hemisphere through a figure-of-eight coil (90 mm external wing diameter) connected to two monophasic Magstim 2002 magnetic stimulators coupled using a Magstim Bistim module (Magstim Co, Whitland, UK). The coil was oriented with the handle pointing posterolaterally at a 45° angle to the sagittal plane (i.e. posterior-anterior current flow across M1). The optimal scalp site for consistently evoking MEPs in the FDI was determined and marked with a water-soluble pen. The resting motor threshold (rMT) was determined as the lowest TMS intensity required to evoke MEPs of at least 50 lV peak-to-peak amplitude in the resting FDI, in at least five of ten consecutive trials. The aMT was assessed while the subject maintained a voluntary contraction of approximately 10% of their maximal voluntary contraction for FDI. The aMT was determined as the lowest TMS intensity required to evoke MEPs of at least 200 lV peak-to-peak amplitude in the active FDI, in at least five of ten consecutive trials. The TMS intensity that evoked MEPs of
1 mV peak-to-peak amplitude (SI1 mV) was also determined.

Subjects

SICF

A total of 51 neurologically healthy subjects (mean age ± SD, 17.7 ± 4.4 years, range: 8–29 years, 20 males) participated in this study. All were born at term, as we

SICF was assessed utilizing a standard paired-pulse TMS paradigm (Ziemann et al., 1998; Peurala et al., 2008). S1 was set at SI1 mV, and was applied prior to S2, which was

21

L. A. Schneider et al. / Neuroscience 312 (2016) 19–25

applied at an intensity of 90% rMT. The ISI between S1 and S2 ranged from 1.5 to 2.3 ms (increments of 0.2 ms c.f. (Peurala et al., 2008). MEPs were recorded at five different ISIs (paired-pulse stimuli) and S1 alone (single pulse). Single and paired-pulse stimuli were delivered pseudo-randomly in a single block comprising 72 trials (i.e. 12 per condition) separated by an inter-trial interval of 6 s (±10%). The average peak-to-peak MEP amplitude for each condition was calculated immediately after the block so that the SICF trough could be identified. SICI SICI was measured utilizing standard paired-pulse TMS techniques (Kujirai et al., 1993). S1 intensities ranged from 60% to 110% aMT (increments of 10% aMT). S2 was set at SI1 mV. The ISI was optimized for each participant based on the trough of their SICF curve to enable examination of inhibitory effects with minimal influence of facilitatory influences (Peurala et al., 2008). Six blocks of stimuli were administered (one S1 intensity per block) that each comprised 24 trials; 12 S2 alone and 12 paired-pulse stimuli. The inter-trial interval was 6 s (±10%). Blocks, and stimuli within each block, were presented in a pseudo-randomized order. Data analysis Trials were excluded from the analysis if EMG activity exceeded 5 lV in the 100 ms prior to S1 or during the interval between S1 and S2. Peak-to-peak MEP amplitude was obtained from the 40 ms of EMG activity that commenced 15 ms after S2 was applied. For all analyses of SICF and SICI, the mean conditioned MEP amplitude was quantified as a ratio of the mean testalone MEP amplitude. Participant age was treated as a categorical variable based on five groupings: 8–12 years (M = 10.5 ± 1.4 years, N = 8), 13–15 years (M = 14.0 ± 0.63 years, N = 6), 16–18 years (M = 17.0 ± 0.94 years, N = 10), 19–21 years (M = 19.4 ± 0.51 years, N = 14), 22–29 years (M = 23.0 ± 2.1 years, N = 12). These groupings were based upon previous reports regarding maturation of rMT (Eyre et al., 2001; Pitcher et al., 2012) and puberty/developmental considerations – i.e. 8–12 years corresponds with primary school age and borderline motor threshold maturity and includes early puberty, 13–15 years corresponds with lower secondary school age and puberty, 16–18 years corresponds with upper secondary school age and late teens, 19– 21 years corresponds with undergraduate University age/ young adults, and 22–29 years corresponds with young adulthood. All statistical analyses were performed with IBM SPSS Statistics 21 (IBM SPSS, Armonk, NY, USA). The Shapiro–Wilk test of normality revealed that SICF and SICI data were not normally distributed, and therefore they were log-transformed. Furthermore, one subject from the 16–18-year-old group was excluded because their SICF data were extreme outlying values (>3 SD of the mean). The group sizes and mean ages presented above reflect the sample with this outlier removed (i.e. the original N = 51). Motor thresholds and SI1 mV were compared across age groupings utilizing a one-way analysis of

variance (ANOVA) with polynomial contrasts and Bonferroni correction for multiple comparisons. Likewise, SI1 mV and the ISI corresponding with the SICF trough were compared across age groupings utilizing a one-way ANOVA. SICF data were analyzed with repeated-measures ANOVA (ANOVARM) with ISI (five levels) as the withinsubjects factor and AGE GROUP (five levels) as the between-subjects factor. Similarly, SICI data were analyzed with ANOVARM with S1 INTENSITY (six levels) as the within-subjects factor and AGE GROUP as the between-subjects factor. In both SICF and SICI analyses, SEX (two levels) was included as a between-subjects factor, with rMT (for analysis of SICF) and aMT (for analysis of SICF and SICI) included as covariates. Subject to statistically significant main effects or interactions, post hoc paired t tests were performed with Bonferroni correction for multiple comparisons. Greenhouse–Geisser correction was utilized for those analyses in which the assumption of circularity was violated (Mauchly’s test of Sphericity). Additional repeated-measures analysis of covariance (ANCOVARM) were performed on both the SICF and SICI data with ISI (five levels) (for SICF analysis) and S1 intensity (six levels) (for SICI analysis) as the within-subjects factor, and AGE entered as a continuous covariate. Data are presented as mean ± standard deviation, except where indicated otherwise, and statistical significance was accepted at P 6 0.05.

RESULTS Single-pulse TMS measures Table 1 shows the group means for single-pulse TMS measures. In 8–12-year olds, rMT was higher compared with all other age groups, with the exception of the 13– 15-year-old group. aMT did not differ between groups, nor did test-alone MEP amplitudes for both SICI and SICF recordings. SICF Fig. 1 shows the SICF curve for each age group. Neither rMT (F(1, 43) = 2.77, P = 0.10) nor aMT (F(1, 43) = 0.40, P = 0.53) influenced SICF at any ISI. There was a main effect of ISI (F(2.17, 97.55) = 29.52, P < 0.001), such that the pattern of facilitation was U-shaped. Post hoc analyses revealed that the mean amplitude of conditioned MEPs evoked using a 1.5-ms ISI (ISI1.5) was larger than the mean amplitudes evoked using ISIs of 1.7 ms (ISI1.7), 1.9 ms (ISI1.9), 2.1 ms (ISI2.1), and 2.3 ms (ISI2.3) (all P < 0.05 after Bonferroni correction). Conditioned MEP amplitudes were larger for ISI1.7 than ISI1.9 and ISI2.1, and smaller for ISI1.9 than those evoked using ISI2.3 (P < 0.05 after Bonferroni correction). Finally, the amplitude of conditioned MEPs at ISI2.1 was smaller than that evoked at ISI2.3 (P < 0.05 after Bonferroni correction). There was a main effect of AGE GROUP (F(4, 45) = 3.44, P = 0.02), but no AGE GROUP*ISI interaction (F(8.67, 97.55) = 0.83, P = 0.59). Post hoc analyses revealed that the mean amplitude of the conditioned MEPs across the different ISIs was greater for children aged 8–12 years, when compared

22

L. A. Schneider et al. / Neuroscience 312 (2016) 19–25

Table 1. Motor thresholds, stimulus intensities and test MEP characteristics by age group Age group

* y

8–12 years N=8

13–15 years N=6

16–18 years N = 10

19–20 years N = 14

22–29 years N = 12

TMS measure (M ± SD) rMT (% maximal stimulator output) aMT (% maximal stimulator output) SI1 mV (% maximal stimulator output)

61.9 ± 7.3 40.8 ± 5.8 72.3 ± 9.1

49.2 ± 6.3 35.7 ± 10.3 61.7 ± 11.9

45.4 ± 10.3y 35.1 ± 8.1 56.3 ± 14.7*

45.4 ± 5.6y 33.5 ± 5.1 55.9 ± 10.9*

48.3 ± 9.9y 34.2 ± 5.4 58.3 ± 10.9

Test-alone MEP amplitude (mV) SICF SICI

0.71 ± 0.54 0.76 ± 0.52

1.23 ± 0.74 1.30 ± 0.67

1.27 ± 0.67 0.91 ± 0.27

1.16 ± 0.61 1.07 ± 0.31

1.12 ± 0.74 1.31 ± 0.63

Denotes statistically significant difference when compared to 8–12-year-old group (P 6 .05). Denotes statistically significant difference when compared to 8–12-year-old group (P 6 .01).

Fig. 1. Short-interval intracortical facilitation (SICF) curve for each age group showing facilitation of the conditioned motor-evoked potential (MEP) as a function of the interstimulus interval (ISI; ms). Facilitation was expressed as a ratio of the conditioned MEP: testalone MEP and then log-transformed. A value of 0 (dashed line) indicates no difference between the mean peak-to-peak MEP amplitude evoked by the test-alone and paired-pulse trials. A value >0 indicates facilitation of the conditioned MEP. The 8–12-year-old group exhibited significantly greater facilitation, when compared to the 13–15, 16–18, and 19–21-year-old groups. Error bars represent standard error of the mean.

with all other age groups, except 22–29-year olds (P < 0.05 after Bonferroni correction). There was no main effect of SEX (F(1, 40) = 0.22, P = 0.64), and no interactions of SEX with ISI (F(2.14, 85.69) = 0.46, P = 0.65) or AGE GROUP (F(4, 40) = 0.78, P = 0.55). The mean ISI corresponding with the SICF trough was 1.98 ± 0.16 ms. Across all participants, the SICF trough was most frequently observed at 2.1 ms (
47% of participants), followed by 1.9 ms (
35% of participants). One-way ANOVA indicated no difference in the ISI corresponding with the first SICF trough when the age groups were compared (F(4, 49) = 0.64, P = 0.64) (Fig. 2). ANCOVARM indicated a significant effect of age on the mean peak-to-peak amplitude of the conditioned MEP at the different ISIs (F(1, 48) = 5.0, P = 0.03). SICI There was a main effect of S1 intensity on the mean amplitude of conditioned MEPs (F(3.95, 177.64) = 25.39, P < 0.001) whereby increasing S1 intensity was associated with increased inhibition of the conditioned

Fig. 2. The mean interstimulus interval (ISI; ms) corresponding with the short-interval intracortical facilitation (SICF) trough for each age group. There were no age-related differences in the ISI at which the SICF trough occurred. Error bars represent standard error of the mean.

MEP. There was no main effect of AGE GROUP (F(4, 45) = 0.69, P = 0.60) and no AGE GROUP*S1 INTENSITY interaction (F(15.79, 177.64) = 1.05, P = 0.41) (Fig. 3). There was also no main effect of SEX (F(1, 40) = 0.11, P = 0.75), and no interaction of SEX with S1 INTENSITY (F(3.90, 155.85) = 0.16, P = 0.96) or AGE GROUP (F(4, 40) = 0.83, P = 0.51). However, subjects with the highest aMTs showed the greatest SICI (F(1,39) = 16.48, P < 0.001), specifically at S1 intensities of 70% (t = 4.71, P 6 0.001), 80% (t = 3.36, P = 0.002), 90% (t = 2.93, P = 0.006), and 110% (t = 2.66, P = 0.01) aMT. This was not influenced by SEX (F(3.78, 147.34) = 0.11, P = 0.98) or P = 0.51). AGE GROUP (F(15.11, 147.34) = 0.95, ANCOVARM indicated no significant effect of age on conditioned MEP amplitudes at the different S1 intensities (F(1, 44) = 0.03, P = 0.86).

DISCUSSION The aim of this study was to determine if age-related differences in the ISI corresponding with the first SICF trough could explain the previously reported reduced SICI in children and adolescents when compared to

L. A. Schneider et al. / Neuroscience 312 (2016) 19–25

Fig. 3. Short-interval intracortical inhibition (SICI) as a function of conditioning stimulus intensity for each of the age groups. Inhibition was expressed as a ratio of the conditioned motor-evoked potential (MEP): test-alone MEP and then log-transformed. A value of 0 (dashed line) indicates no difference between the mean peak-to-peak MEP amplitude evoked by the test-alone and paired-pulse trials. A value <0 indicates inhibition of the conditioned MEP. There was no effect of age on SICI. Error bars represent standard error of the mean.

adults. Compared with adolescents and young adults, SICF was greater in children aged 8–12 years. Conversely, the ISI at which SICF was smallest, and the degree of SICI evoked at that ISI, was comparable across all age groups, regardless of S1 stimulus intensities. This unexpected finding suggests that, despite increased SICF, the degree of SICI that can be evoked in 8–12-year-old children is not different to adolescents and young adults. It is not clear why SICI was not reduced against a background influence of enhanced SICF in 8–12-year olds. Even though SICI was assessed at the individually determined ISI at which SICF had the least influence, SICF was still significantly greater at this ISI in the young children compared with their older counterparts. Our findings suggest that the GABAergic mechanisms responsible for SICI are able to overcome the opposing influence of glutamatergic SICF, without a noticeable decrement in inhibition. There may be a threshold of excess SICF above which SICI is impaired; measuring SICI at a several ISIs where the degree of SICF is greater than the first trough may more fully elucidate this. Alternatively, this SICF/SICI ‘‘mismatch” may be greater in children younger than 8 years, and we failed to capture it with our sample. Two previous studies have reported reduced SICI in children aged 6–10 years compared with young adults (Mall et al., 2004; Walther et al., 2009). Walther et al. (2009) assessed SICI in FDI at ISIs of 1, 3 and 5 ms and showed that, in adolescents aged 11–17 years, SICI was reduced at 5 ms compared with adults. Prior to this, they reported reduced SICI at 2 ms ISI in children aged 6–10 years (Mall et al., 2004). Given that our data do not necessarily support age-related differences in SICF as the mechanism underlying the reduced SICI reported previously in children, it is possible that methodological factors, at least in part, explain the discordance between our SICI findings and those of previous studies. Most

23

importantly, these previous studies were performed in younger children than our cohort. Determining the earliest age at which SICI reaches adult levels was not our aim. But despite these previous reports also finding reduced SICI in 11–17-year-old adolescents, it may be that agerelated differences in SICI are only consistently evident in younger children than we assessed. We chose to record a SICI intensity curve at a single ISI, rather than testing a range of ISIs at a set intensity, as with previous studies. SICI at 3 ms is likely contaminated with SICF (Peurala et al., 2008). However, SICI at 2 ms ISI is unlikely to be significantly contaminated with SICF, and in fact the mean ISI coinciding with the SICF trough in our participants was 1.99 ms. Peurala and colleagues (Peurala et al., 2008) only recorded SICF out to 4.5 ms ISI, so the amount of SICF likely at 5 ms (i.e. where reductions in SICI have previously been reported in adolescents) is unknown. We also used a figure-of-eight coil rather than a circular coil (Mall et al., 2004). Viewed from above, a clockwise current in a round coil positioned over the vertex induces an anterior-to-posterior current flow in the left hemisphere but, also, a posteriorto-anterior current flow in the right hemisphere. This may induce different effects on the conditioned MEP, partly because of differences in D- and I-wave activation, and also because of likely interhemispheric effects (Rosler et al., 1989; Di Lazzaro et al., 2002). When compared with when using a figure-of-eight coil, Di Lazzaro et al. (2002) have shown differences in the composition of the descending volley when a round coil is used for TMS, including a larger descending volley for a given stimulus intensity, a greater likelihood of evoking a D-wave (thought to be due to activation closer to the neuronal cell body due to its latency difference), and eliciting smaller I1 but larger I3 waves. It is these I3 waves that appear most sensitive to the SICI protocols (Di Lazzaro et al., 1999). So it is possible that a number of methodological differences contributed to the different findings. Walther et al. (2009) used one conditioning stimulus intensity and set it relative to rMT. Orth et al. (2003) showed that the threshold for induction of SICI is more closely correlated with aMT than with rMT and that the degree of SICI correlates with S1 intensity when expressed relative to aMT (Orth et al., 2003). In our study, rMT was higher in children under 14 years compared with all other ages, but there were no between-group differences in aMT, and the range of S1 intensities used to assess SICI were all set relative to each individual’s aMT. It is possible that Walther et al. (2009) findings are a result of higher rMTs in their young children, which were as young as 6 years of age, whereas our youngest children were 8 years old. Alternatively, SICI circuits may reach maturity between 6 and 8 years of age, but we were unable to assess this possibility in our sample. It has previously been shown, in adults, that neuroplasticity and motor learning are enhanced in the presence of reduced intracortical inhibition (Ziemann et al., 2001) and consequently, it has been proposed that reduced SICI in children might enhance their motor learning capacity (Mall et al., 2004; Walther et al., 2009). We

24

L. A. Schneider et al. / Neuroscience 312 (2016) 19–25

have previously shown that long-term depression-like neuroplasticity, evoked with continuous theta burst stimulation (Huang et al., 2005, 2006), is enhanced in termborn adolescents compared with young adults (Pitcher et al., 2012). To our knowledge, there is no evidence that enhanced neuroplasticity in children and adolescents is either due to, or enhanced by, reduced SICI. Our results suggest that any enhancement of neuroplasticity in children is likely limited to early childhood and unlikely to be associated with a developmental trajectory whereby SICI remains reduced into adolescence. While the results of this study do not support the protracted development of SICI processes, there is evidence in the cognitive neurodevelopmental literature that inhibitory processes in other cortical regions, particularly those involved in behavioral control, have a more protracted maturation (Paus et al., 1999). In this domain, inhibition is defined as an executive function involving the ability to withhold an impulsive response or cancel a planned action (Vara et al., 2014). This inhibition is often assessed using go/no go paradigms and brain imaging, where the subject makes a decision to respond or not depending upon the cue presented. These studies implicate the dorsolateral prefrontal cortex as not only a key player in behavioral inhibition, but to also have a protracted maturational trajectory well into late adolescence (Paus et al., 1999; Vidal et al., 2012). Our finding supports the earlier maturation of M1 intracortical inhibitory circuitry, which fits with the known posterior-to-anterior developmental trajectory previously described for the cerebral cortex (Tzarouchi et al., 2009), and the earlier maturation of lower order areas such as M1, compared with the delayed maturation of higher order areas such as prefrontal and frontal cortex with greater integrative functions (Gogtay et al., 2004; Tzarouchi et al., 2009). Unlike Peurala et al. (2008) who assessed SICF at 16 ISIs between 1.5 and 4.5 ms, we only assessed SICF at 5 ISIs between 1.5 and 2.3 ms. The main reason for this was one of practicality, to minimize the experimental time for the children. Peurala et al. demonstrated two troughs; one at 1.9–2.1 ms and a second at 3.3–3.9 ms. We chose to determine the first trough only, because these authors also showed that SICI is greatest when assessed at the ISI for the first trough than at the ISI for the second trough (Peurala et al., 2008). However, we cannot rule out that there may be age-related differences in the ISIs at which the second trough appears, and age-related differences in the SICI evoked at these ISIs. There are limitations associated with this study that warrant discussion. The sample size is relatively small, and we were only able to assess a small number of younger children. Notwithstanding this limitation, the youngest participants (8 years) in the study demonstrated SICI, which indicates that inhibitory circuitry is mature, at least in some children, by this age. It is possible that SICI matures between 6 and 8 years of age, which would explain the difference between our findings and those of previous studies (Mall et al., 2004; Walther et al., 2009). While future studies should concentrate on this younger age group, there are a number of challenges. First, rMT is higher in young

children and can remain so until late adolescence (Garvey et al., 2003; Pitcher et al., 2015) and therefore, many may not have a sufficiently low rMT to allow SICF or SICI assessment. Second, long duration TMS studies in young children, whom often have a limited attention span, can be difficult to achieve.

CONCLUSIONS When stimulating at an ISI at which SICF is minimized, the degree of SICI that can be evoked in children aged 8–12 years is similar to that evoked in young adults. This is despite their having greater SICF. So, contrary to our original hypothesis, increased SICF in young children does not appear to reduce the degree of SICI that can be evoked. These findings imply that children’s superior motor neuroplasticity is unlikely to be related to any delayed maturation of primary motor intracortical inhibitory circuits persisting beyond early childhood, although this remains to be confirmed experimentally.

CONFLICT OF INTEREST None of the authors have potential conflicts of interest to be disclosed. Acknowledgments—These studies were funded by grants from the Australian National Health and Medical Research Council (ID: 565344) and the Women’s and Children’s Hospital Research Foundation (to JBP).

REFERENCES Ben-Ari Y (2002) Excitatory actions of GABA during development: the nature of the nurture. Nat Rev Neurosci 3:728–739. Ben-Ari Y, Khalilov I, Kahle KT, Cherubini E (2012) The GABA excitatory/inhibitory shift in brain maturation and neurological disorders. Neuroscientist 18:467–486. Chen R (2004) Interactions between inhibitory and excitatory circuits in the human motor cortex. Exp Brain Res 154:1–10. Chen R, Samii A, Canos M, Wassermann EM, Hallett M (1997) Effects of phenytoin on cortical excitability in humans. Neurology 49:881–883. Di Lazzaro V, Rothwell JC, Oliviero A, Profice P, Insola A, Mazzone P, Tonali P (1999) Intracortical origin of the short latency facilitation produced by pairs of threshold magnetic stimuli applied to human motor cortex. Exp Brain Res 129:494–499. Di Lazzaro V, Oliviero A, Meglio M, Cioni B, Tamburrini G, Tonali P, Rothwell JC (2000) Direct demonstration of the effect of lorazepam on the excitability of the human motor cortex. Clin Neurophysiol 111:794–799. Di Lazzaro V, Oliviero A, Pilato F, Saturno E, Insola A, Mazzone P, Tonali PA, Rothwell JC (2002) Descending volleys evoked by transcranial magnetic stimulation of the brain in conscious humans: effects of coil shape. Clin Neurophysiol 113:114–119. Di Lazzaro V, Pilato F, Dileone M, Ranieri F, Ricci V, Profice P, Bria P, Tonali PA, Ziemann U (2006) GABA(A) receptor subtype specific enhancement of inhibition in human motor cortex. J Physiol 575:721–726. Eyre J, Taylor J, Villagra F, Smith M, Miller S (2001) Evidence of activity-dependent withdrawal of corticospinal projections during human development. Neurology 57:1543–1554. Florian J, Muller-Dahlhaus M, Liu Y, Ziemann U (2008) Inhibitory circuits and the nature of their interactions in the human motor cortex a pharmacological TMS study. J Physiol 586:495–514.

L. A. Schneider et al. / Neuroscience 312 (2016) 19–25 Ganguly K, Schinder AF, Wong ST, Poo M (2001) GABA itself promotes the developmental switch of neuronal GABAergic responses from excitation to inhibition. Cell 105:521–532. Garvey MA, Ziemann U, Bartko JJ, Denckla MB, Barker CA, Wassermann EM (2003) Cortical correlates of neuromotor development in healthy children. Clin Neurophysiol 114:1662–1670. Gogtay N, Giedd JN, Lusk L, Hayashi KM, Greenstein D, Vaituzis AC, Nugent TF, Herman DH, Clasen LS, Toga AW, Rapoport JL, Thompson PM (2004) Dynamic mapping of human cortical development during childhood through early adulthood. Proc Natl Acad Sci USA 101:8174–8179. Hanajima R, Ugawa Y, Terao Y, Enomoto H, Shiio Y, Mochizuki H, Furubayashi T, Uesugi H, Iwata NK, Kanazawa I (2002) Mechanisms of intracortical I-wave facilitation elicited with paired-pulse magnetic stimulation in humans. J Physiol 538:253–261. Huang Y-Z, Edwards MJ, Rounis E, Bhatia KP, Rothwell JC (2005) Theta burst stimulation of the human motor cortex. Neuron 45:201–206. Huang Y-Z, Chen R-S, Rothwell JC, Wen H-Y (2007) The after-effect of human theta burst stimulation is NMDA receptor dependent. Clin Neurophysiol 118:1028–1032. Ilic TV, Meintzschel F, Cleff U, Ruge D, Kessler KR, Ziemann U (2002) Short-interval paired-pulse inhibition and facilitation of human motor cortex: the dimension of stimulus intensity. J Physiol 545:153–167. Jensen K, Jensen MS, Bonefeld BE, Lambert JDC (2000) Developmental increase in asynchronous GABA release in cultured hippocampal neurons. Neuroscience 101:581–588. Klueva J, Meis S, de Lima AD, Voigt T, Munsch T (2008) Developmental downregulation of GABAergic drive parallels formation of functional synapses in cultured mouse neocortical networks. Dev Neurobiol 68:934–949. Kujirai T, Caramia MD, Rothwell JC, Day BL, Thompson PD, Ferbert A, Wroe S, Asselman P, Marsden CD (1993) Corticocortical inhibition in human motor cortex. J Physiol 471:501–519. Mall V, Berweck S, Fietzek UM, Glocker FX, Oberhuber U, Walther M, Schessl J, Schulte-Monting J, Korinthenberg R, Heinen F (2004) Low level of intracortical inhibition in children shown by transcranial magnetic stimulation. Neuropediatrics 35:120–125. Moll GH, Wischer S, Heinrich H, Tergau F, Paulus W, Rothenberger A (1999) Deficient motor control in children with tic disorder: evidence from transcranial magnetic stimulation. Neurosci Lett 272:37–40. Moll GH, Heinrich H, Trott GE, Wirth S, Rothenberger A (2000) Deficient intracortical inhibition in drug-naive children with attention-deficit hyperactivity disorder is enhanced by methylphenidate. Neurosci Lett 284:121–125. Moll GH, Heinrich H, Trott GE, Wirth S, Bock N, Rothenberger A (2001) Children with comorbid attention-deficit-hyperactivity disorder and tic disorder: evidence for additive inhibitory deficits within the motor system. Ann Neurol 49:393–396. Oldfield RC (1971) The assessment and analysis of handedness: the Edinburgh inventory. Neuropsychologia 9:97–113. Orth M, Snijders AH, Rothwell JC (2003) The variability of intracortical inhibition and facilitation. Clin Neurophysiol 114:2362–2369. Paus T, Zijdenbos A, Worsley K, Collins DL, Blumenthal J, Giedd JN, Rapoport JL, Evans AC (1999) Structural maturation of neural pathways in children and adolescents: in vivo study. Science 283:1908–1911. Peurala SH, Mueller-Dahlhaus JFM, Arai N, Ziemann U (2008) Interference of short-interval intracortical inhibition (SICI) and short-interval intracortical facilitation (SICF). Clin Neurophysiol 119:2291–2297.

25

Pitcher JB, Schneider LA, Burns NR, Drysdale JL, Higgins RD, Ridding MC, Nettelbeck TJ, Haslam RR, Robinson JS (2012) Reduced corticomotor excitability and motor skills development in children born preterm. J Physiol 590:5827–5844. Pitcher JB, Riley AM, Doeltgen SH, Kurylowicz L, Rothwell JC, McAllister SM, Smith AE, Clow A, Kennaway DJ, Ridding MC (2012) Physiological evidence consistent with reduced neuroplasticity in human adolescents born preterm. J Neurosci 32:16410–16416. Pitcher JB, Doeltgen SH, Goldsworthy MR, Schneider LA, Vallence A-M, Smith AE, Semmler JG, McDonnell MN, Ridding MC (2015) A comparison of two methods for estimating 50% of the maximal motor evoked potential. Clin Neurophysiol. http://dx.doi.org/ 10.1016/j.clinph.2015.02.011. Ridding MC, Taylor JL, Rothwell JC (1995) The effect of voluntary contraction on corticocortical inhibition in human motor cortex. J Physiol 487:541–548. Rosler KM, Hess CW, Heckmann R, Ludin HP (1989) Significance of shape and size of the stimulating coil in magnetic stimulation of the human motor cortex. Neurosci Lett 100:347–352. Rossi S, Hallett M, Rossini PM, Pascual-Leone A, T.M.S.C.G. Safety (2009) Safety, ethical considerations, and application guidelines for the use of transcranial magnetic stimulation in clinical practice and research. Clin Neurophysiol 120:2008–2039. Rothwell JC (1997) Techniques and mechanisms of action of transcranial stimulation of the human motor cortex. J Neurosci Methods 74:113–122. Sanes JN, Donoghue JP (2000) Plasticity and primary motor cortex. Annu Rev Neurosci 23:393–415. Sinclair C, Hammond GR (2008) Reduced intracortical inhibition during the foreperiod of a warned reaction time task. Exp Brain Res 186:385–392. Swanwick CC, Murthy NR, Mtchedlishvili Z, Sieghart W, Kapur J (2006) Development of gamma-aminobutyric acidergic synapses in cultured hippocampal neurons. J Comp Neurol 495:497–510. Tokimura H, Ridding MC, Tokimura Y, Amassian VE, Rothwell JC (1996) Short latency facilitation between pairs of threshold magnetic stimuli applied to human motor cortex. Electroencephalogr Clin Neurophysiol: Electromyogr Motor C 101:263–272. Tzarouchi LC, Astrakas LG, Xydis V, Zikou A, Kosta P, Drougia A, Andronikou S, Argyropoulou MI (2009) Age-related grey matter changes in preterm infants: an MRI study. NeuroImage 47:1148–1153. Vara AS, Pang EW, Vidal J, Anagnostou E, Taylor MJ (2014) Neural mechanisms of inhibitory control continue to mature in adolescence. Dev Cogn Neurosci 10:129–139. Vidal J, Mills T, Pang EW, Taylor MJ (2012) Response inhibition in adults and teenagers: spatiotemporal differences in the prefrontal cortex. Brain Cogn 79:49–59. Walther M, Berweck S, Schessl J, Linder-Lucht M, Fietzek UM, Glocker FX, Heinen F, Mall V (2009) Maturation of inhibitory and excitatory motor cortex pathways in children. Brain Dev Jpn 31:562–567. Ziemann U, Lonnecker S, Steinhoff BJ, Paulus W (1996) The effect of lorazepam on the motor cortical excitability in man. Exp Brain Res 109:127–135. Ziemann U, Lonnecker S, Steinhoff BJ, Paulus W (1996) Effects of antiepileptic drugs on motor cortex excitability in humans: a transcranial magnetic stimulation study. Ann Neurol 40:367–378. Ziemann U, Tergau F, Wassermann EM, Wischer S, Hildebrandt J, Paulus W (1998) Demonstration of facilitatory I wave interaction in the human motor cortex by paired transcranial magnetic stimulation. J Physiol 511:181–190. Ziemann U, Muellbacher W, Hallett M, Cohen LG (2001) Modulation of practice-dependent plasticity in human motor cortex. Brain 124:1171–1181.

(Accepted 28 October 2015) (Available online 3 November 2015)