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Copyright © The Physiological Society 2002 The mechanisms of interhemispheric inhibition in the human motor cortex Corresponding author R. Chen: Toronto Western Hospital, 5W-445, 399 Bathurst Street, Toronto, Ontario, Canada M5T 2S8. Email: robert.chen/at/uhn.on.ca Received January 25, 2002; Accepted May 23, 2002.  This article has been cited by other articles in PMC. | |||||||||||||
Abstract Transcranial magnetic stimulation can be used to non-invasively study inhibitory processes in the human motor cortex. Interhemispheric inhibition can be measured by applying a conditioning stimulus to the motor cortex resulting in inhibition of the contralateral motor cortex. Transcranial magnetic stimulation can also be used to demonstrate ipsilateral cortico-cortical inhibition in the motor cortex. At least two different ipsilateral cortico-cortical inhibitory processes have been identified: short interval intracortical inhibition and long interval intracortical inhibition. However, the relationship between interhemispheric inhibition and ipsilateral cortico-cortical inhibition remains unclear. This study examined the relationship between interhemispheric inhibition, short interval intracortical inhibition and long interval intracortical inhibition. First, the effect of test stimulus intensity on each inhibitory process was studied. Second, the effects of interhemispheric inhibition on short interval intracortical inhibition and long interval intracortical inhibition on interhemispheric inhibition were examined. Motor evoked potentials were recorded from the right first dorsal interosseous muscle in 11 right-handed healthy volunteers. For interhemispheric inhibition, conditioning stimuli were applied to the right motor cortex and test stimuli to the left motor cortex. For short interval intracortical inhibition and long interval intracortical inhibition, both conditioning stimuli and test stimuli were applied to the left motor cortex. With increasing test stimulus intensities, long interval intracortical inhibition and interhemispheric inhibition decreased, while short interval intracortical inhibition increased. Moreover, short interval intracortical inhibition was significantly reduced in the presence of interhemispheric inhibition. Interhemispheric inhibition was significantly reduced in the presence of long interval intracortical inhibition when matched for test motor evoked potential amplitude but the difference was not significant when matched for test pulse intensity. These findings suggest that both interhemispheric inhibition and long interval intracortical inhibition are predominately mediated by low threshold cortical neurons and may share common inhibitory mechanisms. In contrast, the mechanisms mediating short interval intracortical inhibition are probably different from those mediating long interval intracortical inhibition and interhemispheric inhibition although these systems appear to interact. | |||||||||||||
Transcranial magnetic stimulation (TMS) has been used to demonstrate at least three different cortico-cortical inhibitory processes: interhemispheric inhibition (IHI), short interval intracortical inhibition (SICI) and long interval intracortical inhibition (LICI). IHI can be demonstrated by applying a conditioning stimulus (CS) to the motor cortex, which inhibits the size of the motor evoked potential (MEP) produced by the test stimulus (TS) of the opposite motor cortex (Ferbert et al. 1992; Hanajima et al. 2001). This result is consistent with animal studies that show stimulation of the motor cortex inhibits the contralateral motor cortex several milliseconds later (Chang, 1953; Asanuma & Okuda, 1962; Matsunami & Hamada, 1984). IHI can be observed at interstimulus intervals (ISIs) between 6 and 50 ms (Ferbert et al. 1992; Gerloff et al. 1998). Conversely, SICI and LICI are cortico-cortical inhibitory processes observed within the ipsilateral motor cortex. In the SICI paradigm, pairing a subthreshold CS with a suprathreshold TS at short ISIs (1-5 ms) inhibits the MEP produced by the TS (Kujirai et al. 1993). LICI results in attenuation of the MEP when a suprathreshold CS is paired with a suprathreshold TS at long ISIs (50-200 ms) (Valls-Sole et al. 1992; Wassermann et al. 1996). Several lines of evidence suggest that these forms of cortico-cortical inhibition are mediated by cortical inhibitory neuronal mechanisms. For example, IHI is related to the activity of inhibitory interneurons and largely mediated by transcallosal pathways. This contention is supported by several findings. First, test responses evoked by small anodal electrical shock are not significantly inhibited by contralateral magnetic conditioning stimuli (Ferbert et al. 1992; Hanajima et al. 2001). Low intensity electrical stimuli excite descending pyramidal axons within the white matter that are not sensitive to changes in cortical excitability (Rothwell, 1997). Second, H-reflexes in the relaxed forearm flexor muscles are unaffected by conditioning stimuli to the ipsilateral hemisphere, suggesting that ipsilateral motor cortex stimulation does not change spinal excitability (Ferbert et al. 1992; Gerloff et al. 1998). Finally, reduced excitability of the contralateral motor cortex has been demonstrated directly by recordings of descending corticospinal volleys (Di Lazzaro et al. 1999). Similarly, evidence that SICI and LICI are mediated by cortical inhibitory interneurons include: absence of any change in spinal excitability (Fuhr et al. 1991); failure to suppress the response to double transcranial electrical stimulation (TES; Ferbert et al. 1992; Inghilleri et al. 1993; Kujirai et al. 1993), and marked reduction in the corticospinal waves evoked by TMS (Valls-Sole et al. 1992; Nakamura et al. 1997; Chen et al. 1999). Although these findings establish that IHI, SICI and LICI are all mediated by cortical inhibitory interneurons, other lines of evidence suggest that they are related to different subtypes of GABAergic receptors. For example, Sanger et al. (2001) found that SICI and LICI respond differentially to increasing TS intensities and that LICI inhibits SICI. Another important difference between SICI and LICI is that SICI is associated with a low intensity CS which produces shorter periods of cortical inhibition; whereas LICI is associated with a high intensity CS which produces longer periods of cortical inhibition. It is also known that GABAA receptor-mediated responses have lower activation thresholds and their inhibitory influence is brief (Davies et al. 1990; Sanger et al. 2001). Further, GABAB receptor-mediated responses have higher activation thresholds and their inhibitory influence is longer lasting (Deisz, 1999; Sanger et al. 2001). These findings have led researchers to suggest that SICI may be mediated by GABAA receptors while LICI may be mediated by GABAB receptors (Roick et al. 1993; Siebner et al. 1998; Werhahn et al. 1999). Neurons mediating IHI must arise from contralateral sites and travel to the opposite hemisphere to exert their inhibitory effects. Since inhibitory GABAergic neurons mainly serve local circuits (Somogyi et al. 1998), IHI is probably mediated through excitatory axons that cross the corpus callosum to act on local inhibitory neurons in the contralateral motor cortex (Berlucci, 1990). However, it is currently unknown whether IHI is related to SICI and LICI and therefore mediated by similar or different GABAergic mechanisms. One way to investigate whether experimental phenomena (i.e. SICI, LICI and IHI) share common mechanisms of action is to assess whether their profiles of response are similar or dissimilar under conditions of controlled perturbations. In these experiments, this is achieved in two ways; first, by a controlled manipulation of TS intensities on SICI, LICI and IHI and second by examining the impact of one inhibitory phenomenon on the other. This was accomplished by examining the interactions between SICI, LICI and IHI and intracortical facilitation (ICF) using a triple stimulation protocol. ICF was included because it may interact with these different inhibitory measures. Such methods have been used by Sanger et al. (2001) to examine the relationship between LICI and SICI. In the present experiments we use a similar approach to examine how IHI interacts with SICI and LICI. The findings will help us to understand how local inhibitory mechanisms are influenced by interhemispheric projections. | |||||||||||||
Methods Subjects Three experiments were conducted. In Experiments 1 and 3 we studied 11 healthy, right-handed volunteers (mean age = 36.4 years, s.d. = 9.9 years, range = 26–57 years; 8 male and 3 female). In Expt 2 we studied 10 healthy, right-handed volunteers (mean age = 36.2 years, s.d. = 11.8 years, range = 23–57 years; 9 males and 1 female), eight of whom also participated in Experiments 1 and 3. Handedness was confirmed using the Oldfield Handedness Inventory (Oldfield, 1971). Subjects were recruited through advertisements in the community and postings within the hospital. All subjects gave their written informed consent and the protocol was approved by the University Health Network Research Ethics Board in accordance with the declaration of Helsinki on the use of human subjects in experiments. Exclusion criteria included a self-reported comorbid medical illness or a history of drug or alcohol abuse.EMG recording Surface EMG was recorded from the right and left first dorsal interosseous (FDI) muscles with disposable disc electrodes placed in a tendon-belly arrangement over the bulk of the FDI muscle and the first metacarpo-phalangeal joint. The subject maintained relaxation throughout the experiment and the EMG was monitored on a computer screen and via speakers at high gain. The signal was amplified (Intronix Technologies Corporation Model 2024F, Bolton, Ontario, Canada), filtered (band-pass 2 Hz to 5 kHz), digitized at 5 kHz (Micro 1401, Cambridge Electronics Design, Cambridge, UK) and stored in a laboratory computer for offline analysis.TMS procedure This study involved three experiments. The first experiment examined the effects of test MEP size on SICI, LICI, ICF and IHI. The second experiment examined the effects of IHI on SICI and ICF. The third experiment examined the effects of LICI on IHI.TMS of the left motor cortex was performed with a 7 cm figure-of-eight coil and four Magstim 200 stimulators (The Magstim Company, Dyfed, UK) connected via three Bistim modules in a ‘pyramid’ setup. The output of each of the two pairs of Magstim 200 stimulators was connected to one Bistim module. The output from the two Bistim modules was directed to a third Bistim module that was connected to the TMS coil. This setup allowed us to deliver up to four pulses of different stimulus intensities through the same coil at very short interstimulus intervals. The power attenuation of the pyramid system is about 15 %, similar to a single Bistim system (personal communication, Dr R. Jalinous, Magstim Company). The coil was placed at the optimal position for eliciting motor-evoked potentials (MEPs) from the right FDI muscle. The optimal position was marked on the scalp with a felt pen to ensure identical placement of the coil throughout the experiment. The handle of the coil pointed backwards and was perpendicular to the presumed direction of the central sulcus, about 45 deg to the midsagittal line. The direction of the induced current was from posterior to anterior and was optimal to activate the motor cortex transsynaptically (Werhahn et al. 1994; Kaneko et al. 1996). TMS of the right motor cortex was performed with a 7 cm figure-of-eight coil and a Magstim Super Rapid stimulator. This stimulator produced bi-phasic current in the coil. The coil was placed at the optimal position for eliciting MEPs from the left FDI muscle. Stimulus intensity was set at 75 % of maximum stimulator output. The handle of the coil pointed forward and laterally about 45 deg to the midsagittal line. This orientation was chosen because in some subjects it was not possible to place both coils at the optimal positions with the handle pointed backwards and laterally due to the size of the coil. Previous studies in 11 normal subjects in our laboratory found no difference in the IHI between 75 % and 90 % of the stimulator output and between four coil orientations 90 deg apart (Yung & Chen, 2001). This section explains the various parameters used in the experiments. The MT is expressed as a percentage of maximum stimulator output and was defined as the lowest intensity that produced MEPs of > 50 μV in at least five out of ten trials with the muscles relaxed. SICI and ICF were tested using paired-TMS with a subthreshold CS preceding a suprathreshold TS. CS2 denotes a conditioning stimulus that occurred 2 ms prior to a TS and CS10 denotes a conditioning stimulus that occurred 10 ms prior to a TS. CS2 was chosen because it consistently leads to SICI (Kujirai et al. 1993; Chen et al. 1998) and largely avoids the phenomenon of I-wave facilitation (Ziemann et al. 1998; Chen & Garg, 2000), which may obscure SICI (Awiszus et al. 1999). CS10 was chosen because it consistently gives rise to ICF (Kujirai et al. 1993; Ridding et al. 1995). LICI was tested with the suprathreshold CS and TS (Valls-Sole et al. 1992). The CS precedes the TS by 100 ms and is termed CS100. CS100 was used because at this interval direct recording of corticospinal waves demonstrated reduced cortical excitability (Nakamura et al. 1997; Chen et al. 1999) without any change in spinal excitability (Fuhr et al. 1991). IHI was tested with a suprathreshold CS delivered to the left motor cortex followed by a suprathreshold TS delivered to the right motor cortex 10 ms later. This CS will be referred to as CCS10 (contralateral conditioning stimulus). CCS10 was chosen because it consistently leads to IHI (Ferbert et al. 1992). In all experiments the intensities of the TS were often adjusted to produce a target MEP size. An intensity of ‘TS 1 mV’ indicates a stimulator setting (determined to the nearest 1 % of the maximum stimulator output) that produces a peak-to-peak MEP amplitude of ≥ 1 mV in at least 5 out of 10 trials. Similarly, ‘TS 0.2 mV’ and ‘TS 4 mV’ indicate settings that produce peak-to-peak MEP amplitudes of ≥ 0.2 mV and ≥ 4 mV in at least 5 out of 10 trials, respectively. In Experiments 2 and 3, we compared the effects two inhibitory mechanisms together to that of one inhibitory mechanism alone. If we used the same test intensity throughout, the first inhibitory mechanism would decrease the test MEP amplitude upon which the second mechanism could operate. In order to match for test MEP amplitude, therefore, in some trials we increased the test stimulus intensity such that it would give a 1 mV test MEP in the presence of the first inhibitory mechanism. We then compared the effects of the second inhibitory mechanism on this 1 mV MEP to a 1 mV MEP that was elicited by a weaker single test pulse. Since both test MEP amplitude and test pulse intensity may be important in determining the degree of inhibition but it is not possible to match them at the same time, we designed our protocols to match for test MEP amplitude and test pulse intensity in different trials. Experiment 1: effects of test stimulus intensity on SICI, ICF, LICI and IHI In this experiment we examined the effects of different TS intensities on SICI, ICF, LICI and IHI. For SICI (CS2) and ICF (CS10), the intensity of the CS was set to 80 % of the MT (0.8 MT). For LICI the intensity of the suprathreshold CS100 was adjusted to produce a peak-to-peak MEP amplitude of about 1 mV and for IHI the CCS10 was set at 75 % of stimulator output. Each run consisted of 10 trials each of TS alone and four conditions with the conditioning stimulus preceding the test stimulus at different intervals (CS2, CS10, CS100, CCS10) delivered in random order. The time between trials was five seconds. Three TS intensities (TS 0.2 mV, TS 1 mV, and TS 4 mV) were studied in separate runs. Experiment 2: effects of IHI on SICI and ICF Here we investigated whether SICI and ICF are altered by IHI. Ten conditions were tested and are listed in Table 1 as 2A-2J. Each run consisted of 10 trials of each of the 10 conditions delivered in a random order (100 trials). Conditions 2A-2D were used to determine SICI, ICF and IHI for a 1 mV test MEP. Since IHI inhibits the test response, and SICI and ICF may be altered by an attenuated test MEP, for conditions 2E-2J the strength of the test stimulus was adjusted to produce 1 mV MEPs in the presence of an earlier CCS10 pulse. This test stimulus is referred to as ‘TS 1 mVCCS10'. This allowed us to match MEP amplitudes to produce a similar degree of corticospinal activation with and without preceding a CCS10. SICI and ICF in the presence of IHI were studied using three pulses in conditions 2I and 2J. We also measured SICI and ICF with the increased TS strength (TS 1 mVCCS10) in conditions 2F and 2G. Therefore, we designed this experiment to compare SICI and ICF in the presence of IHI (2I/2H and 2J/2H) to SICI and ICF in the absence of IHI matched for test MEP amplitude (i.e. TS 1 mV; 2B/2A and 2C/2A) and TS intensity (i.e. TS 1 mVCCS10; 2F/2E and 2G/2E).
Experiment 3: effects of LICI on IHI In this experiment we investigated the effects of LICI on IHI. Seven conditions were tested and are listed in Table 1 as 3A-3G. Each run consisted of 10 trials of each of the 7 conditions delivered in a random order (70 trials). LICI and IHI for a 1 mV test MEP were determined from conditions 3B and 3C. Since IHI may be affected by test MEP amplitude and CS100 inhibits the test MEP, the strength of the test stimuli was adjusted to produce 1 mV MEPs in the presence of the CS100 pulse in conditions 3D-3G. This test pulse is referred to as ‘TS 1 mVCS100'. The interaction between IHI and LICI were studied using three pulses in condition 3G. Therefore, we designed this experiment to compare IHI in the presence of LICI (3G/3F) to IHI in the absence of LICI matched for test MEP amplitude (i.e. TS 1 mV) (3B/3A) and test stimulus intensity (i.e. TS 1 mVCS100; 3E/3D). Data analysis The peak-to-peak MEP amplitude for each trial was measured offline. Inhibition or facilitation was expressed as a ratio of the conditioned to mean unconditioned MEP amplitude for each subject. Ratios less than one indicate inhibition, and ratios greater than one indicate facilitation. Values are expressed as mean ± standard deviation (s.d.).For Expt 1, the effects of TS intensity on SICI, LICI, ICF and IHI were evaluated by repeated-measures analysis of variance (ANOVA). If the effect of TS intensity was significant, Fisher's Protected Least Significant Difference (PLSD) post hoc test was used to detect differences among different TS intensities. Correlations between SICI and IHI were tested by Pearson product-moment correlation coefficients. In addition, it was found that the distribution for IHI values violated the assumptions of normality and homogeneity of variance and, therefore, was log transformed. For Expt 2, SICI and ICF alone at different test stimulus intensities (TS 1 mV and TS 1 mVCCS10) and in the presence of IHI were compared using repeated-measures ANOVA. For Expt 3, IHI alone at different test stimulus intensities (TS 1 mV and TS 1 mVCS100) and in the presence of LICI was compared using repeated-measures ANOVA. The threshold for significance was set at P < 0.05. | |||||||||||||
Results Experiment 1: effects of test stimulus intensity on SICI, ICF, LICI and IHI The MEP amplitude for TS alone was 0.28 ± 0.13 mV for TS 0.2 mV, 0.87 ± 0.33 mV for TS 1 mV and 3.85 ± 2.18 mV for TS 4 mV. The results are shown in Fig. 1. Separate within-group repeated measures ANOVA demonstrated that increasing the TS intensity, from 0.2 mV to 4 mV, resulted in a significant decrease in IHI (F = 18.91, d.f. = 2, 20, P < 0.001) and LICI (F = 8.90, d.f. = 2, 20, P = 0.002) and a significant increase in SICI (F = 4.65, d.f. = 2, 20, P = 0.02). Increasing the TS intensity resulted in a small reduction in ICF, although this was not statistically significant. Post hoc testing showed that both IHI and LICI were significantly greater at 0.2 mV and 1 mV TS intensity than at 4 mV TS intensity. In contrast, SICI demonstrated little inhibition at 0.2 mV and but marked inhibition at 1 and 4 mV. There was no significant correlation between these measures of inhibition (i.e. SICI, LICI and IHI) at each TS intensity.
Experiment 2: effects of IHI on SICI and ICF The MEP amplitude for TS 1 mV was 1.08 ± 0.49 mV (Table 1: condition 2A) and for TS 1 mVCCS10 was 2.42 ± 0.77 mV (2E). When a TS 1 mVCCS10 was preceded by CCS10 (2H), the test MEP amplitude was 1.05 ± 0.36 mV. Thus, the test MEP amplitudes for conditions 2A and 2H were matched. The IHI, SICI and ICF for a 1 mV test pulse was consistent with previous studies (Ferbert et al. 1992; Kujirai et al. 1993; Ziemann et al. 1996). Similar to the finding of Expt 1, IHI was higher with TS 1 mV (0.36 ± 0.05) compared to TS 1 mVCCS10 (0.49 ± 0.08, P = 0.04, paired t test). Figure 2 demonstrates the effects of combining CCS10 with CS2 in one representative subject and data for the entire sample are shown in Fig. 3. Compared to a TS alone (Fig. 2A), a preceding CS2 (Fig. 2B) or CCS10 (Fig. 2C) inhibited the test response. However, with CCS10 followed by CS2, there was little additional inhibition due to CS2 (Fig. 2D). The nature of the test MEP (TS 1 mV, TS 1 mVCCS10, CCS10-TS 1 mVCCS10; columns A, B and C in Fig. 3) had a significant effect on SICI (F = 9.57, d.f. = 2, 18, P = 0.01) (Fig. 3). Post hoc tests (PLSD) revealed a significant reduction in SICI in the presence of IHI compared to SICI alone when matched for either test MEP amplitude (i.e. TS 1 mV) (Fig. 3: A vs. C, P = 0.004) or test pulse intensity (i.e. TS 1 mVCCS10; Fig. 3: B vs. C, P = 0.0007) whereas SICI for the two TS intensities were not significantly different (Fig. 3: A vs. B). Moreover, the change in SICI in the presence of IHI (calculated as a ratio of SICI in the presence of IHI to SICI alone) was greater in subjects with a stronger IHI and the correlation was significant (r = 0.70, d.f. = 2, 20, P = 0.03; Fig. 4A). In contrast, the change in SICI in the presence of IHI (calculated as a ratio of SICI in the presence of IHI to SICI alone) was not related to the strength of SICI (r = 0.13, d.f. = 2, 20, P = 0.72; Fig. 4B). ICF was not significantly affected by IHI (Fig. 3).
For each subject we also examined whether the inhibitory CS (CCS10 or CS2) became facilitatory in the presence of each other. In three subjects the CS2 pulse caused facilitation of the test MEP in the presence of the CCS10 pulse. That is, the MEPs of condition 2J (CCS10-CS2-TS 1 mVCCS10) were larger than the MEPs of condition 2H (CCS10-TS 1 mVCCS10). In one of these three subjects this facilitation was statistically significant (t = 2.84, d.f. = 9, P = 0.02). Similarly, in three subjects the CCS10 pulse caused facilitation of the test MEP in the presence of the CS2 pulse with the MEPs of condition 2J (CCS10-CS2-TS 1 mVCCS10) larger than that of condition 2F (CS2-1 mVCCS10). In two of the three subjects this facilitation was statistically significant (t = 2.37, d.f. = 9, P = 0.04 and t = 2.26, d.f. = 9, P = 0.05). Experiment 3: effects of LICI on IHI The IHI and LICI for TS 1 mV were consistent with previous studies (Ferbert et al. 1992; Chen et al. 1997). The mean MEP amplitude for TS 1 mV alone was 1.56 ± 0.55 mV (condition 3A) and for the TS 1 mVCS100 test pulse was 3.01 ± 1.38 mV. With a TS 1 mVCS100 preceded by CS100 (3F), the mean MEP amplitude was 1.44 ± 0.43 mV, similar to TS 1 mV alone (3Aost-). Consistent with the results of Expt 1, LICI was greater with TS 1 mV (0.18 ± 0.16) than with TS 1 mVCS100 (0.53 ± 0.22; P = 0.002, paired t test). IHI was also greater with TS 1 mV (0.44 ± 0.23) than with TS 1 mVCS100 (0.63 ± 0.20; P = 0.014, paired t test). Figure 5 demonstrates the effects of combining a CS100 pulse with a CCS10 pulse in one representative subject. Compared to TS alone (Fig. 5A), a preceding CCS10 (Fig. 5B) inhibited the test response. In the presence of LICI, the CCS10 pulse no longer caused any inhibition (Fig. 5D compared to 5C). Data for the entire sample is shown in Fig. 6. The nature of the test MEP (TS 1 mV, TS 1 mVCS100, CS100-TS 1 mVCS100; columns A, B and C in Fig. 6) had a significant effect on IHI (F = 27.98, d.f. = 2, 18, P = 0.0001). Post-hoc tests (PLSD) revealed a significant reduction in IHI in the presence of LICI compared to IHI alone when matched for test MEP amplitude (i.e. TS 1 mV) (Fig. 6: A vs. C; P = 0.002) and a trend toward significance when matched for test pulse intensity (i.e. TS 1 mVCS100) (Fig. 6: B vs. C; P = 0.13).
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Discussion This study examined how IHI is related to SICI and LICI. In Expt 1, increasing TS intensities resulted in significantly less LICI and IHI but significantly greater SICI. Increasing TS intensity had no significant effect on ICF. In Expt 2 SICI was significantly reduced in the presence of IHI and this change in SICI was greater in subjects with stronger IHI. In Expt 3 IHI was significantly reduced in the presence of LICI when matched for test MEP amplitude but the difference was not significant when matched for test pulse intensity. A model that is consistent with our data is shown in Fig. 7.
Different neuronal populations mediate IHI and SICI Changing the TS intensity had opposite effects on SICI and IHI. Similar to a previous study (Sanger et al. 2001), we found that SICI increases with higher test stimulus intensity (i.e. 0.2 mV to 1 mV). In contrast, IHI decreases with increasing TS intensity, similar to the findings of Ferbert et al. (1992) in four subjects. These findings suggest that different neuronal circuits mediate IHI and SICI. The opposite effect of test MEP amplitude may be related to differences in activation thresholds or the location of cortical neurons mediating IHI and SICI. The cortical neurons mediating IHI may have lower activation thresholds than neurons mediating SICI. Alternatively, the neurons mediating IHI may be located at more superficial cortical layers than those mediating SICI. Another potential explanation to account for greater SICI with increasing TS intensities is related to the refractoriness of interneurons. The CS2 pulse may leave this inhibitory interneuron partially refractory to small test stimuli. Stronger test stimuli can overcome this refractoriness resulting in greater inhibition. Recent evidence suggests that there may be two phases of SICI with maximum inhibition at ISI of 1 ms and 2.5 ms (Fisher et al. 2002). Inhibition at ISI of 1 ms may be due to refractoriness whereas the inhibition as 2.5 ms is probably synaptic in origin. Since SICI at ISI of 2 ms is affected by GABAergic drugs (Ziemann et al. 1996) and voluntary activation (Ridding et al. 1995), it is more likely that this inhibition is predominately synaptic rather than due to neuronal refractoriness.IHI inhibits SICI In Expt 2 (Fig. 2 and Fig. 3), we found that SICI was significantly reduced in the presence of IHI. Moreover, the extent of this reduction correlated with IHI but not SICI (Fig. 4A and B), suggesting that the effect is probably due to IHI inhibiting SICI rather than SICI inhibiting IHI. A potential confounding factor is that the various test conditions may preferentially activate different populations of cortical or spinal neurons with different susceptibility to IHI and SICI. Since IHI preferentially inhibits low threshold neurons (Fig. 1), the test MEP produced by the CCS10-TS 1 mVCCS10 combination (condition 2H) may largely be mediated by high threshold cortical neurons. However, this cannot explain our results because neurons activated at higher intensities are inhibited by SICI to a similar degree as those activated at lower intensities (Fig. 1 and Fig. 3). The inhibition of SICI by IHI is similar to the inhibition of SICI by LICI demonstrated by Sanger et al. (2001). However, LICI almost completely abolished SICI (Sanger et al. 2001) while with IHI, SICI was reduced but still present (Fig. 3). The weaker inhibition of SICI by IHI compared to LICI may be related to the weaker MEP inhibition produced by IHI compared to LICI (Fig. 1).Another potential explanation to account for decreased SICI in the presence of IHI is an occlusion or saturation effect. This may occur if the same or overlapping populations of inhibitory interneurons mediate SICI and IHI. In the presence of IHI, fewer inhibitory interneurons would be available to be activated by the SICI leading to reduced SICI. This cannot be completely excluded but several observations argue against this explanation. First, the difference in response of SICI and IHI to higher TS intensities (Fig. 1) argues against the suggestion that the same or overlapping neuronal population mediates SICI and IHI. Second, in one subject the CS2 pulse in the presence of IHI and in two subjects the CCS10 pulse in the presence of SICI caused significant MEP facilitation. The occlusion model cannot explain this facilitation. Third, the occlusion model predicts that subjects with greater IHI and SICI will have larger reduction of SICI in the presence of IHI. Although the change in SICI in the presence of IHI correlated with IHI (Fig. 4A), there was no correlation with SICI (Fig. 4B). Similar neuronal populations may mediate IHI and LICI LICI and IHI may share common mechanisms for several reasons. The first relates to the effects of different test stimulus intensities on LICI and IHI (Expt 1, Fig. 1). Both LICI and IHI decrease with increasing test stimulus intensity, suggesting that the neuronal pathways that mediate both inhibitory measures predominately act on motor cortical neurons activated at low intensities. Second, subthreshold conditioning stimuli are required to activate SICI inhibitory pathways, whereas suprathreshold conditioning stimuli are required to activate LICI and IHI inhibitory pathways (Kujirai et al. 1993; Wassermann et al. 1996; Ziemann et al. 1996; Chen et al. 1998) suggesting that the activation thresholds for neurons mediating LICI and IHI are higher than the activation thresholds mediating SICI. A third line of evidence is the effect of voluntary contraction on these inhibitory paradigms. For both LICI (Valls-Sole et al. 1992; Wassermann et al. 1996) and IHI (Ferbert et al. 1992; Ridding et al. 2000) the extent of inhibition is similar at rest and during voluntary muscle contraction, whereas voluntary contraction markedly reduced SICI (Ridding et al. 1995). Fourth, both IHI and LICI inhibit SICI (Sanger et al. 2001). Differences also exist, however, between IHI and LICI. For example, the durations of IHI and LICI are different. LICI may last for 200 ms or longer depending on the conditioning stimulus intensity (Valls-Sole et al. 1992) while IHI last up to 50 ms (Gerloff et al. 1998; Yung & Chen, 2001). The longer duration of LICI compared to IHI may be related to stronger MEP inhibition elicited by LICI (Fig. 1). Nevertheless, these findings need to be confirmed in future studies.Site of ipsilateral inhibition Our results provide additional evidence that ipsilateral inhibition occurs at the cortical level (Ferbert et al. 1992; Di Lazzaro et al. 1999) in contrast to the findings of Gerloff et al. (1998) who suggested that ipsilateral inhibition may occur at subcortical sites and not necessarily though interhemispheric connections. Since both SICI (Kujirai et al. 1993; Nakamura et al. 1997; Di Lazzaro et al. 1998;) and LICI (Nakamura et al. 1997; Chen et al. 1999) are cortically mediated phenomena, our finding that contralateral motor cortex stimulation significantly influences SICI and LICI suggests that IHI occurs predominately at a cortical level.Possible role of GABAergic receptors Both physiological and pharmacological studies have suggested that LICI may be mediated by GABAB receptors whereas SICI probably involve GABAA receptors (Roick et al. 1993; Siebner et al. 1998; Werhahn et al. 1999). Our results suggest that similar populations of inhibitory neurons may mediate LICI and IHI. Therefore, IHI may be related to GABAB activity. This is consistent with the finding that lorazepam increased SICI but did not change IHI, suggesting that IHI is not related to GABAA activity (Ziemann et al. 1996).Interactions between LICI and IHI In Expt 3 we found that IHI was reduced in the presence of LICI when matched for TS 1 mV, but this difference was not significant when matched for TS 1 mVCS100. One explanation for this finding is that both IHI and LICI preferentially target low threshold cortical neurons. The MEPs produced by CS100-TS 1 mVCS100 (3F) pulse combination were probably mediated by higher threshold neurons than the MEPs produced by a test pulse alone (3A) even though they were matched for amplitude. Therefore, the reduced IHI in the presence of LICI (3G/3F) may be explained by these high threshold neurons being less sensitive to IHI. However, this is probably not the sole explanation. If LICI and IHI are mediated through an overlapping set of inhibitory neurons, the reduced inhibitory effects may be explained by a saturation effect. A third possibility is that the neurons mediating LICI and IHI inhibit one another. Single-cell recordings demonstrated a propensity for GABAB receptors to cause auto-inhibition (Rohrbacher et al. 1997). Therefore, cortical interneurons activated by LICI may cause auto-inhibition through GABAB presynaptic autoreceptors. Figure 7 depicts such as model. IHI may be mediated by contralateral excitatory inputs activating inhibitory interneurons that also mediate LICI. This hypothesis will have to be tested and refined in future studies.In conclusion, IHI is associated with changes in the inhibitory circuits in the contralateral motor cortex. Both IHI and LICI are predominately mediated by low threshold cortical neurons and IHI may suppress the contralateral motor cortex through mechanisms similar to LICI. The mechanisms mediating SICI may be different from those mediating LICI and IHI although the finding that IHI inhibits SICI suggests that these mechanisms interact. | |||||||||||||
Acknowledgments We thank Dr Peter Ashby for allowing us to use his equipment and for his comments on the manuscript as well as Dr Shitij Kapur for his comments on the manuscript. This work was supported by the Canadian Institutes of Health Research, the Canada Foundation for Innovation and the University Health Network Krembil Family Chair in Neurology. Dr Daskalakis was supported through a research training fellowship from the Ontario Mental Health Foundation and Canadian Psychiatric Research Foundation and Dr Chen is a Canadian Institutes of Health Research Scholar. | |||||||||||||
References
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