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The influence of lorazepam on somatosensory-evoked fast frequency (600 Hz) activity in MEG
Brain Research 874 (2000) 10–14 www.elsevier.com / locate / bres
Research report
The influence of lorazepam on somatosensory-evoked fast frequency (600 Hz) activity in MEG a, a a a a Jens Haueisen *, Thomas Heuer , Hannes Nowak , Joachim Liepert , Cornelius Weiller , b c Yoshio Okada , Gabriel Curio a
¨ Jena, Philosophenweg 3, 07740 Jena, Germany Department of Neurology, Biomagnetisches Zentrum, Friedrich-Schiller-Universitat b Departments of Neurology and Physiology, University of New Mexico School of Medicine, Albuquerque, NM, USA c ¨ , Berlin, Germany Neurophysics Group, Department of Neurology, Klinikum Benjamin Franklin, Freie Universitat Accepted 23 May 2000
Abstract The generators of high frequency bursts (600-Hz activity) detected at the parietal scalp over the primary somatosensory cortex after electrical stimulation of peripheral nerves are not yet known. We investigated the influence of benzodiazepine on the somatosensoryevoked 600-Hz activity by means of neuromagnetic measurements and source analysis. After oral administration of lorazepam, the latency of the 600-Hz burst activity was increased; specifically later peaks were delayed more than earlier peaks. In contrast, the latency of the concurrent primary cortical low frequency response (N20m) was not significantly changed. The source strengths of both N20m and 600-Hz bursts were significantly increased. Our results provide evidence for two components of the 600-Hz activity with a different generator structure. 2000 Elsevier Science B.V. All rights reserved. Theme: Sensory systems Topic: Somatosensory cortex and thalamocortical relationships Keywords: Magnetoencephalography (MEG); Primary somatosensory cortex; Median nerve; Oscillatory burst; 600 Hz wavelets; Lorazepam
1. Introduction Electrical stimulation of peripheral nerves evokes high frequency activity, within the time interval of the first cortical response after stimulation [4,5]. The dominant frequency of this activity is around 600 Hz, thus it is called 600 Hz activity [6]. Applying digital filtering one can reliably extract the 600-Hz activity from the underlying slower response which contains mainly frequencies up to about 300 Hz [7]. The generator of the 600-Hz activity is not yet known. Thalamo-cortical fibers [7,15], cortical interneurons [10], and also heterogeneous generator structures [5] have been proposed. It was found that the 600-Hz activity is attenuated or totally disappears in sleep [8,10,20]. Studies applying different stimulation frequencies [15] and double stimuli with varying interstimulus intervals [9] found a differential effect on the latency of *Corresponding author. Tel.: 149-3641-935-338; fax: 149-3641-935355. E-mail address: [email protected] (J. Haueisen).
the 600-Hz burst activity: the latency of later peaks changed more than the latency of earlier peaks. Benzodiazepines are known to influence both latency and amplitude of evoked potentials by enhancing GABAergic transmission [1,2,19], and early cortical responses are less influenced than later components. However, the influence of benzodiazepine on the 600-Hz activity is not yet known. Therefore, a comparison of the benzodiazepine influence on N20m and 600 Hz activity might provide new insights into the generation of the 600-Hz activity, especially since inhibitory cortical interneurons have been proposed as generators [10].
2. Methods Neuromagnetic fields were recorded over the somatosensory cortex with a 31-channel biomagnetometer (Philips, Hamburg, Germany) contralateral to electrical median nerve stimulation (according to the IFCN recom-
0006-8993 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0006-8993( 00 )02534-8
J. Haueisen et al. / Brain Research 874 (2000) 10 – 14
mendations [17], stimulation rate: 4 Hz, stimulation strength: motor plus sensor threshold) in two groups of healthy right-handed male volunteers (group A: benzodiazepine group, five subjects, age 32.263.9 years, oral administration of lorazepam; group B: control group, five subjects, age 29.664.4 years). All subjects had no history of neurological disorders. No subject was taking any medication. The duration of one measurement was 25 min (6000 epochs with 250-ms interstimulus interval). The stimulated arm was well covered to prevent cooling throughout the recording session. A total of 6000 epochs were offline averaged and Wiener-filtered using a Daubechies 12-wavelet filter in order to optimize signal-tonoise ratio. Each subject in group A received 3 mg lorazepam (three ¨ tablets Wyeth Tavor 1.0, Wyeth Pharma GmbH, Munster, Germany). We obtained one measurement before drug administration (three subjects on the same day prior to drug intake, two subjects on a separate day). Five min after drug intake the first measurement was started. At the very end of this measurement or after this measurement all group A subjects reported the first drug effects. A second measurement was started 55 min after drug intake (afterward referred to as 1 h after intake). At that time, all group A subjects reported a clear effect of the drug. All subjects except one were measured a third time starting 2 h after drug intake. All group A subjects stayed 4–5 h after drug intake and reported that after 3–4 h the influence of the drug started diminishing. Two out of the five subjects reported antirecall effects [14]. The measurements in group B (control group) were performed in similar manner as in group A but without administration of lorazepam. For each subject four measurements were performed (on average three measurements on one day and the fourth on a separate day). Since we analyzed only the earliest components of the somatosensory-evoked field which are considered to be exogenous, we did not use placebos for the control group. The 600-Hz activity is attenuated or totally disappears in sleep [8,10,20]. Therefore, the subjects were kept awake and attentive to the stimuli as much as possible. All subjects were ask to count random breaks in the delivery of the stimuli (nine to 12 breaks, 1 s duration). Additionally, all subjects were instructed to lift the left (unstimulated) arm as soon as they felt a change in the stimulus. Moreover, the examiner asked the subject five or six times during the measurement whether the delivery of the stimuli had changed. The subjects were instructed to lift the left arm as a response if something had changed. A one-compartment boundary element method (BEM) model (linear potential approximation: 7 mm triangle side length) [11] was constructed from a T1-weighted MRI data set of the head (256 slices with 1 mm thickness) for each subject. Neuromagnetic data were sampled with 5 kHz (1500 Hz antialiasing low pass filter), baseline corrected, and digital-
11
ly filtered (N20m: 20–300 Hz, 600-Hz bursts: 450–750 Hz). A singular value decomposition (SVD) over the time interval of the N20m was used to extract the strongest of the orthogonal signal components, which was then used in all dipole localizations. Dipole localizations were performed at the peak of the N20m and the largest peak in the 600-Hz bursts employing the BEM model. All source localizations were carried out with the help of the software CURRY (NeuroScan, Sterling, VA, USA). For the evaluation of the changes caused by lorazepam, we calculated the latency and dipole strength differences between single measurements. In group A, we calculated the differences between the measurements 5 min, 1 and 2 h after lorazepam administration and the measurement prior to administration. In group B, we subtracted latencies and dipole strengths of each measurement from every other measurement (four measurements yield six subtraction results) and computed the average (S.D.) of all subtraction results. All time instants for latency and dipole estimations were determined in the square root of mean global field power (MGFP) trace of the 31 channels. MGFP was ]]]2 defined by œo 31 i 51 m i for each time point, where m i is the magnetic field value in channel i. Latency and dipole analysis was restricted to the first part of the 600-Hz burst activity (p1 in Fig. 1), since the second part (p2) was not detectable in all subjects. Latency and amplitude differences were tested with a two-tailed t-test.
3. Results Our main finding was a systematic delay in the latency of the 600-Hz burst peaks. Fig. 1 illustrates this finding for one subject, and Table 1 provides the mean latency changes for group A (benzodiazepine) and group B (control). Two h after lorazepam administration the latency shift was approximately 0.4 ms for the third and fourth peak of the 600-Hz activity (dotted line in Fig. 1; third row in Table 1). The latency shift of 0.4 ms corresponded to almost one half of the wave length of the entire oscillation. Furthermore, lorazepam caused a significant latency shift for the third and fourth 600 Hz peak almost 1 h after administration. The first and second 600 Hz peak showed a smaller (but significant) latency shift for group A than the third and fourth peak 2 h after administration. The mean N20m latency was shifted by 0.1 ms (S.D. 0.12) 2 h after benzodiazepine intake (Table 1), which was not significant. The MGFP traces were used for the latency estimations. Additionally, we checked the MEG channel with the strongest signal and found the same latency shift. Confirming the results of Curio et al. [6] and Hashimoto et al. [10], we found a close co-localization of single dipoles for the two frequency bands investigated. The difference between N20m and 600 Hz peak dipole locali-
J. Haueisen et al. / Brain Research 874 (2000) 10 – 14
12
Fig. 1. MGFP in fT (31 channels) of unfiltered data (upper four traces) and 450–750 Hz bandpass filtered data (lower four traces) prior to and after benzodiazepine intake (subject 2). The measurement prior to benzodiazepine intake was obtained on a separate day, thus amplitudes cannot be compared to the other three traces (see text). The first and second part of the 600-Hz activity are indicated by p1 and p2. Please note the different scaling for the upper and lower four traces.
zation was for group A: 20.7 (3.9), 3.3 (2.5), 21.2 (3.9) and for group B: 21.0 (2.5), 1.8 (3.9), 21.1 (4.3) (values x, y, z, mean (S.D.) in mm, x: medial to lateral, y: posterior to anterior, z: inferior to superior). Due to the signal-tonoise ratio the dipole localizations were more stable (lower S.D. across measurements) for N20m than for the 600-Hz activity. Thus, the relatively large S.D. values in the
differences above were caused by the lower signal-to-noise ratio of the 600-Hz activity. The projection of the source localization results onto the individual MRI showed the expected location in or close to Brodmann area 3b (maximum distance from area 3b was below 7 mm for the N20m). Furthermore, lorazepam had no significant influence on the dipole localization.
Table 1 Mean (S.D.) of latency differences in ms for N20m and 600 Hz peaks Group
Measurements subtracted
N20m
600 Hz burst peaks 1
2
A (lorazepam)
5 min2prior 1 h2prior 2 h2prior
0.00 0.00 0.10
(0) (0) (0.12)
0.00 0.10 0.23*
(0) (0.1) (0.13)
B (controls)
Measurements 1–4 a
0.02
(0.02)
0.00
(0)
a
Number of subjects 3
20.02 0.12 0.32* 0.00
4
(0.04) (0.11) (0.15)
0.04 0.24** 0.43**
(0.09) (0.09) (0.13)
(0)
0.01
(0.05)
Measurements 1–2, 1–3, 1–4, 2–3, 2–4, and 3–4 averaged per subject, then averaged over all subjects. * P,0.05 (group A vs. group B). ** P,0.005 (group A vs. group B).
0.00 0.20** 0.37** 20.02
(0) (0) (0.05)
5 5 4
(0.08)
5
J. Haueisen et al. / Brain Research 874 (2000) 10 – 14
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Table 2 Mean (S.D.) of dipole strength differences in nAm for dipoles at peak N20m and largest 600 Hz peak Group
Measurements subtracted
N20m
600 Hz
Number of subjects
A (lorazepam)
5 min2prior 1 h2prior 2 h2prior
20.49 (1.73) 2.97 (2.25) 2.13** (2.28)
20.05 (0.13) 20.18 (0.33) 0.31* (0.34)
5 5 4
B (controls)
Measurements 1–4 a
20.78 (92.65)
0.13 (0.24)
5
a
Measurements 1–2, 1–3, 1–4, 2–3, 2–4, and 3–4 averaged per subject, then averaged over all subjects. * P,0.05 (paired t-test of pooled data of 1 and 2 h against prior). ** P,0.005 (paired t-test of pooled data of 1 and 2 h against prior).
The source strengths of both N20m and 600 Hz burst and correspondingly the amplitudes of the magnetic fields were increased after intake of benzodiazepine (Table 2). This increase was significant if the data of the measurements 1 and 2 h after administration were pooled together and tested against the measurement prior to administration using a paired t-test (Table 2). Neither for the N20m nor for the 600-Hz peak it reached significance when tested against the control group. For this analysis, we considered only the first part of the 600-Hz burst activity which was evident in every subject tested. In two subjects of group A, we found a second part of the 600-Hz burst activity (p2 in Fig. 1). This part diminished after benzodiazepine intake in both subjects. For those five subjects in both groups where we found both parts of the 600-Hz activity, p1 was approximately in the time interval of the ascending slope of the N20m, while p2 was in the time interval of the descending slope of the N20m.
4. Discussion Benzodiazepines act on GABA receptors types by facilitating the interaction of GABA with the receptor. Of the large number of GABAA receptor subtypes, the most common throughout the brain is the a 1 b 2 g2 (BZ 1 ) receptor. It is suggested that benzodiazepine-induced sedation results from the widespread decrease in neuronal activity mediated primarily through this receptor [16]. There is also evidence that feedback or recurrent inhibition in the motor cortex is mediated by GABAA [3]. The enhanced GABAergic transmission can increase both the tonic level of inhibition and the phasic level of inhibition in all relay nuclei including the thalamus, as well as in the cortex. A higher tonic level of GABA activity should increase the threshold for firing. This should lead to a general increase in latency of the 600-Hz activity, as found in this study. Furthermore, both the source strength of the 600-Hz burst and the N20m was increased under lorazepam. This concordant change of N20 and 600 Hz burst is in contrast to reported alterations in sleep with an increased N20m but a decreased burst [10] and points to the possibility that the high frequency burst might reflect
the upstroke of fast EPSPs in cortical neurons generated by a barrage of synchronous action potentials carried in thalamo-cortical afferents, as discussed in Ref. [5]. Benzodiazepines could also increase the effect of GABA released after phasic excitatory inputs to the GABA neurons due to the stimulus. This should increase the amount of inhibition that starts about 5–10 ms after the input to GABA neurons. This could explain the different behaviour of the first (p1) and second part (p2) of the 600-Hz activity. The first part should be less affected by benzodiazepine than the second part as it seems to be the case in this study (see Fig. 1). A functional dissociation between a first and a second burst part was described also in a recent study using stimulus rate variations between 0.5 and 25 Hz [13]. Emori et al. [9] found a systematic latency shift of the 600-Hz activity when applying double stimuli with varying interstimulus intervals. In their study, the latency of the earlier peaks of the 600-Hz burst activity were less shifted than the latency of later peaks. We found a similar effect for the latencies of the 600-Hz peaks caused by lorazepam. The underlying mechanism for the differential latency delays could be the same in both studies if the 600-Hz oscillations are caused by a network loop of neurons (and perhaps interneurons). In such a loop, small initial delays would add up and later oscillation peaks would be influenced by the delay of all previous oscillations. The network loop could consist of neurons in the thalamus if the thalamo-cortical projection neurons are responsible for the generation of the 600-Hz activity [7,15]. Alternatively, the loop could be entirely cortical if the 600-Hz are, for example, generated by modulations of the backpropagating action potentials in the pyramidal neurons [18]. There are some technical limitations in this study. The sampling frequency of our measurements was 5 kHz, i.e. a time resolution of 0.2 ms. Although the latency shift observed was highly significant, it was close to the time resolution limit. Subsequent studies should therefore use a higher sampling rate. The increase in source strength reached a significant level only if the data of the measurements 1 and 2 h after administration were pooled together and tested against the measurement prior to administration. The increase did not reach a significant level when tested against the control
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J. Haueisen et al. / Brain Research 874 (2000) 10 – 14
group. This could be at least partially explained by the fact that dipole strength estimations usually exhibit a relatively large inter-subject variability. Possible reasons for this variability are, for example, shortcomings in the head modeling due to the unknown in vivo resistivity values [12]. Thus, although significant the resulting increase in source strength needs to be interpreted with caution. We used five subjects in both groups, and we have observed the second part (p2) only in two subjects in group A and three subjects in group B. Thus, we restricted dipole and latency analysis to the first part (p1) of the 600-Hz activity. We observed that the second part of the 600-Hz activity with a latency between 20 and 25 ms always diminished after benzodiazepine intake. However, in some previous measurements we have seen that this second part of the 600-Hz activity could probably be influenced by other factors, such as vigilance or attention to the stimulus. In conclusion, our results provide evidence for a heterogeneous generator structure of the 600-Hz activity which is sensitive to manipulations of GABAergic mechanisms.
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
References [1] D. Bansi, M. Krug, J. Schmidt, Der Einfluß von Psychopharmaka ¨ auf die durch Zahnpulpareizung ausgeloste kortikale Potentiale und ¨ langanhaltende posttetanische Erregbarkeitszustande, Acta Biol. Med. Ger. 35 (1976) 613–625. [2] R. Berchou, S. Chayasirisobhon, V. Green, K. Mason, The pharmacodynamic properties of lorazepam and methylphenidate drugs on event-related potentials and power spectral analysis in normal subjects, Clin. Electroencephalogr. 17 (1986) 176–180. [3] W. Chen, J.-J. Zhang, G.-Y. Hu, C.-P. Wu, GABA receptor-mediated feedback inhibition in pyramidal neurons of cat motor cortex, Neurosci. Lett. 198 (1995) 123–126. [4] R.Q. Cracco, J.B. Cracco, Somatosensory evoked potential in man: far-field potentials, Electroencephalogr. Clin. Neurophysiol. 41 (1976) 460–466. ¨ [5] G. Curio, B.-M. Mackert, K. Abraham-Fuchs, W. Harer, Highfrequency activity (600 Hz) evoked in the human primary somatosensory cortex: a survey of electric and magnetic recordings, in: C. Pantev (Ed.), Oscillatory Event-Related Brain Dynamics, Plenum Press, New York, 1994, pp. 205–218. [6] G. Curio, B.-M. Mackert, M. Burghoff, R. Koetitz, K. Abraham¨ Fuchs, W. Harer, Localization of evoked neuromagnetic 600 Hz
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Research report
The influence of lorazepam on somatosensory-evoked fast frequency (600 Hz) activity in MEG a, a a a a Jens Haueisen *, Thomas Heuer , Hannes Nowak , Joachim Liepert , Cornelius Weiller , b c Yoshio Okada , Gabriel Curio a
¨ Jena, Philosophenweg 3, 07740 Jena, Germany Department of Neurology, Biomagnetisches Zentrum, Friedrich-Schiller-Universitat b Departments of Neurology and Physiology, University of New Mexico School of Medicine, Albuquerque, NM, USA c ¨ , Berlin, Germany Neurophysics Group, Department of Neurology, Klinikum Benjamin Franklin, Freie Universitat Accepted 23 May 2000
Abstract The generators of high frequency bursts (600-Hz activity) detected at the parietal scalp over the primary somatosensory cortex after electrical stimulation of peripheral nerves are not yet known. We investigated the influence of benzodiazepine on the somatosensoryevoked 600-Hz activity by means of neuromagnetic measurements and source analysis. After oral administration of lorazepam, the latency of the 600-Hz burst activity was increased; specifically later peaks were delayed more than earlier peaks. In contrast, the latency of the concurrent primary cortical low frequency response (N20m) was not significantly changed. The source strengths of both N20m and 600-Hz bursts were significantly increased. Our results provide evidence for two components of the 600-Hz activity with a different generator structure. 2000 Elsevier Science B.V. All rights reserved. Theme: Sensory systems Topic: Somatosensory cortex and thalamocortical relationships Keywords: Magnetoencephalography (MEG); Primary somatosensory cortex; Median nerve; Oscillatory burst; 600 Hz wavelets; Lorazepam
1. Introduction Electrical stimulation of peripheral nerves evokes high frequency activity, within the time interval of the first cortical response after stimulation [4,5]. The dominant frequency of this activity is around 600 Hz, thus it is called 600 Hz activity [6]. Applying digital filtering one can reliably extract the 600-Hz activity from the underlying slower response which contains mainly frequencies up to about 300 Hz [7]. The generator of the 600-Hz activity is not yet known. Thalamo-cortical fibers [7,15], cortical interneurons [10], and also heterogeneous generator structures [5] have been proposed. It was found that the 600-Hz activity is attenuated or totally disappears in sleep [8,10,20]. Studies applying different stimulation frequencies [15] and double stimuli with varying interstimulus intervals [9] found a differential effect on the latency of *Corresponding author. Tel.: 149-3641-935-338; fax: 149-3641-935355. E-mail address: [email protected] (J. Haueisen).
the 600-Hz burst activity: the latency of later peaks changed more than the latency of earlier peaks. Benzodiazepines are known to influence both latency and amplitude of evoked potentials by enhancing GABAergic transmission [1,2,19], and early cortical responses are less influenced than later components. However, the influence of benzodiazepine on the 600-Hz activity is not yet known. Therefore, a comparison of the benzodiazepine influence on N20m and 600 Hz activity might provide new insights into the generation of the 600-Hz activity, especially since inhibitory cortical interneurons have been proposed as generators [10].
2. Methods Neuromagnetic fields were recorded over the somatosensory cortex with a 31-channel biomagnetometer (Philips, Hamburg, Germany) contralateral to electrical median nerve stimulation (according to the IFCN recom-
0006-8993 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0006-8993( 00 )02534-8
J. Haueisen et al. / Brain Research 874 (2000) 10 – 14
mendations [17], stimulation rate: 4 Hz, stimulation strength: motor plus sensor threshold) in two groups of healthy right-handed male volunteers (group A: benzodiazepine group, five subjects, age 32.263.9 years, oral administration of lorazepam; group B: control group, five subjects, age 29.664.4 years). All subjects had no history of neurological disorders. No subject was taking any medication. The duration of one measurement was 25 min (6000 epochs with 250-ms interstimulus interval). The stimulated arm was well covered to prevent cooling throughout the recording session. A total of 6000 epochs were offline averaged and Wiener-filtered using a Daubechies 12-wavelet filter in order to optimize signal-tonoise ratio. Each subject in group A received 3 mg lorazepam (three ¨ tablets Wyeth Tavor 1.0, Wyeth Pharma GmbH, Munster, Germany). We obtained one measurement before drug administration (three subjects on the same day prior to drug intake, two subjects on a separate day). Five min after drug intake the first measurement was started. At the very end of this measurement or after this measurement all group A subjects reported the first drug effects. A second measurement was started 55 min after drug intake (afterward referred to as 1 h after intake). At that time, all group A subjects reported a clear effect of the drug. All subjects except one were measured a third time starting 2 h after drug intake. All group A subjects stayed 4–5 h after drug intake and reported that after 3–4 h the influence of the drug started diminishing. Two out of the five subjects reported antirecall effects [14]. The measurements in group B (control group) were performed in similar manner as in group A but without administration of lorazepam. For each subject four measurements were performed (on average three measurements on one day and the fourth on a separate day). Since we analyzed only the earliest components of the somatosensory-evoked field which are considered to be exogenous, we did not use placebos for the control group. The 600-Hz activity is attenuated or totally disappears in sleep [8,10,20]. Therefore, the subjects were kept awake and attentive to the stimuli as much as possible. All subjects were ask to count random breaks in the delivery of the stimuli (nine to 12 breaks, 1 s duration). Additionally, all subjects were instructed to lift the left (unstimulated) arm as soon as they felt a change in the stimulus. Moreover, the examiner asked the subject five or six times during the measurement whether the delivery of the stimuli had changed. The subjects were instructed to lift the left arm as a response if something had changed. A one-compartment boundary element method (BEM) model (linear potential approximation: 7 mm triangle side length) [11] was constructed from a T1-weighted MRI data set of the head (256 slices with 1 mm thickness) for each subject. Neuromagnetic data were sampled with 5 kHz (1500 Hz antialiasing low pass filter), baseline corrected, and digital-
11
ly filtered (N20m: 20–300 Hz, 600-Hz bursts: 450–750 Hz). A singular value decomposition (SVD) over the time interval of the N20m was used to extract the strongest of the orthogonal signal components, which was then used in all dipole localizations. Dipole localizations were performed at the peak of the N20m and the largest peak in the 600-Hz bursts employing the BEM model. All source localizations were carried out with the help of the software CURRY (NeuroScan, Sterling, VA, USA). For the evaluation of the changes caused by lorazepam, we calculated the latency and dipole strength differences between single measurements. In group A, we calculated the differences between the measurements 5 min, 1 and 2 h after lorazepam administration and the measurement prior to administration. In group B, we subtracted latencies and dipole strengths of each measurement from every other measurement (four measurements yield six subtraction results) and computed the average (S.D.) of all subtraction results. All time instants for latency and dipole estimations were determined in the square root of mean global field power (MGFP) trace of the 31 channels. MGFP was ]]]2 defined by œo 31 i 51 m i for each time point, where m i is the magnetic field value in channel i. Latency and dipole analysis was restricted to the first part of the 600-Hz burst activity (p1 in Fig. 1), since the second part (p2) was not detectable in all subjects. Latency and amplitude differences were tested with a two-tailed t-test.
3. Results Our main finding was a systematic delay in the latency of the 600-Hz burst peaks. Fig. 1 illustrates this finding for one subject, and Table 1 provides the mean latency changes for group A (benzodiazepine) and group B (control). Two h after lorazepam administration the latency shift was approximately 0.4 ms for the third and fourth peak of the 600-Hz activity (dotted line in Fig. 1; third row in Table 1). The latency shift of 0.4 ms corresponded to almost one half of the wave length of the entire oscillation. Furthermore, lorazepam caused a significant latency shift for the third and fourth 600 Hz peak almost 1 h after administration. The first and second 600 Hz peak showed a smaller (but significant) latency shift for group A than the third and fourth peak 2 h after administration. The mean N20m latency was shifted by 0.1 ms (S.D. 0.12) 2 h after benzodiazepine intake (Table 1), which was not significant. The MGFP traces were used for the latency estimations. Additionally, we checked the MEG channel with the strongest signal and found the same latency shift. Confirming the results of Curio et al. [6] and Hashimoto et al. [10], we found a close co-localization of single dipoles for the two frequency bands investigated. The difference between N20m and 600 Hz peak dipole locali-
J. Haueisen et al. / Brain Research 874 (2000) 10 – 14
12
Fig. 1. MGFP in fT (31 channels) of unfiltered data (upper four traces) and 450–750 Hz bandpass filtered data (lower four traces) prior to and after benzodiazepine intake (subject 2). The measurement prior to benzodiazepine intake was obtained on a separate day, thus amplitudes cannot be compared to the other three traces (see text). The first and second part of the 600-Hz activity are indicated by p1 and p2. Please note the different scaling for the upper and lower four traces.
zation was for group A: 20.7 (3.9), 3.3 (2.5), 21.2 (3.9) and for group B: 21.0 (2.5), 1.8 (3.9), 21.1 (4.3) (values x, y, z, mean (S.D.) in mm, x: medial to lateral, y: posterior to anterior, z: inferior to superior). Due to the signal-tonoise ratio the dipole localizations were more stable (lower S.D. across measurements) for N20m than for the 600-Hz activity. Thus, the relatively large S.D. values in the
differences above were caused by the lower signal-to-noise ratio of the 600-Hz activity. The projection of the source localization results onto the individual MRI showed the expected location in or close to Brodmann area 3b (maximum distance from area 3b was below 7 mm for the N20m). Furthermore, lorazepam had no significant influence on the dipole localization.
Table 1 Mean (S.D.) of latency differences in ms for N20m and 600 Hz peaks Group
Measurements subtracted
N20m
600 Hz burst peaks 1
2
A (lorazepam)
5 min2prior 1 h2prior 2 h2prior
0.00 0.00 0.10
(0) (0) (0.12)
0.00 0.10 0.23*
(0) (0.1) (0.13)
B (controls)
Measurements 1–4 a
0.02
(0.02)
0.00
(0)
a
Number of subjects 3
20.02 0.12 0.32* 0.00
4
(0.04) (0.11) (0.15)
0.04 0.24** 0.43**
(0.09) (0.09) (0.13)
(0)
0.01
(0.05)
Measurements 1–2, 1–3, 1–4, 2–3, 2–4, and 3–4 averaged per subject, then averaged over all subjects. * P,0.05 (group A vs. group B). ** P,0.005 (group A vs. group B).
0.00 0.20** 0.37** 20.02
(0) (0) (0.05)
5 5 4
(0.08)
5
J. Haueisen et al. / Brain Research 874 (2000) 10 – 14
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Table 2 Mean (S.D.) of dipole strength differences in nAm for dipoles at peak N20m and largest 600 Hz peak Group
Measurements subtracted
N20m
600 Hz
Number of subjects
A (lorazepam)
5 min2prior 1 h2prior 2 h2prior
20.49 (1.73) 2.97 (2.25) 2.13** (2.28)
20.05 (0.13) 20.18 (0.33) 0.31* (0.34)
5 5 4
B (controls)
Measurements 1–4 a
20.78 (92.65)
0.13 (0.24)
5
a
Measurements 1–2, 1–3, 1–4, 2–3, 2–4, and 3–4 averaged per subject, then averaged over all subjects. * P,0.05 (paired t-test of pooled data of 1 and 2 h against prior). ** P,0.005 (paired t-test of pooled data of 1 and 2 h against prior).
The source strengths of both N20m and 600 Hz burst and correspondingly the amplitudes of the magnetic fields were increased after intake of benzodiazepine (Table 2). This increase was significant if the data of the measurements 1 and 2 h after administration were pooled together and tested against the measurement prior to administration using a paired t-test (Table 2). Neither for the N20m nor for the 600-Hz peak it reached significance when tested against the control group. For this analysis, we considered only the first part of the 600-Hz burst activity which was evident in every subject tested. In two subjects of group A, we found a second part of the 600-Hz burst activity (p2 in Fig. 1). This part diminished after benzodiazepine intake in both subjects. For those five subjects in both groups where we found both parts of the 600-Hz activity, p1 was approximately in the time interval of the ascending slope of the N20m, while p2 was in the time interval of the descending slope of the N20m.
4. Discussion Benzodiazepines act on GABA receptors types by facilitating the interaction of GABA with the receptor. Of the large number of GABAA receptor subtypes, the most common throughout the brain is the a 1 b 2 g2 (BZ 1 ) receptor. It is suggested that benzodiazepine-induced sedation results from the widespread decrease in neuronal activity mediated primarily through this receptor [16]. There is also evidence that feedback or recurrent inhibition in the motor cortex is mediated by GABAA [3]. The enhanced GABAergic transmission can increase both the tonic level of inhibition and the phasic level of inhibition in all relay nuclei including the thalamus, as well as in the cortex. A higher tonic level of GABA activity should increase the threshold for firing. This should lead to a general increase in latency of the 600-Hz activity, as found in this study. Furthermore, both the source strength of the 600-Hz burst and the N20m was increased under lorazepam. This concordant change of N20 and 600 Hz burst is in contrast to reported alterations in sleep with an increased N20m but a decreased burst [10] and points to the possibility that the high frequency burst might reflect
the upstroke of fast EPSPs in cortical neurons generated by a barrage of synchronous action potentials carried in thalamo-cortical afferents, as discussed in Ref. [5]. Benzodiazepines could also increase the effect of GABA released after phasic excitatory inputs to the GABA neurons due to the stimulus. This should increase the amount of inhibition that starts about 5–10 ms after the input to GABA neurons. This could explain the different behaviour of the first (p1) and second part (p2) of the 600-Hz activity. The first part should be less affected by benzodiazepine than the second part as it seems to be the case in this study (see Fig. 1). A functional dissociation between a first and a second burst part was described also in a recent study using stimulus rate variations between 0.5 and 25 Hz [13]. Emori et al. [9] found a systematic latency shift of the 600-Hz activity when applying double stimuli with varying interstimulus intervals. In their study, the latency of the earlier peaks of the 600-Hz burst activity were less shifted than the latency of later peaks. We found a similar effect for the latencies of the 600-Hz peaks caused by lorazepam. The underlying mechanism for the differential latency delays could be the same in both studies if the 600-Hz oscillations are caused by a network loop of neurons (and perhaps interneurons). In such a loop, small initial delays would add up and later oscillation peaks would be influenced by the delay of all previous oscillations. The network loop could consist of neurons in the thalamus if the thalamo-cortical projection neurons are responsible for the generation of the 600-Hz activity [7,15]. Alternatively, the loop could be entirely cortical if the 600-Hz are, for example, generated by modulations of the backpropagating action potentials in the pyramidal neurons [18]. There are some technical limitations in this study. The sampling frequency of our measurements was 5 kHz, i.e. a time resolution of 0.2 ms. Although the latency shift observed was highly significant, it was close to the time resolution limit. Subsequent studies should therefore use a higher sampling rate. The increase in source strength reached a significant level only if the data of the measurements 1 and 2 h after administration were pooled together and tested against the measurement prior to administration. The increase did not reach a significant level when tested against the control
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group. This could be at least partially explained by the fact that dipole strength estimations usually exhibit a relatively large inter-subject variability. Possible reasons for this variability are, for example, shortcomings in the head modeling due to the unknown in vivo resistivity values [12]. Thus, although significant the resulting increase in source strength needs to be interpreted with caution. We used five subjects in both groups, and we have observed the second part (p2) only in two subjects in group A and three subjects in group B. Thus, we restricted dipole and latency analysis to the first part (p1) of the 600-Hz activity. We observed that the second part of the 600-Hz activity with a latency between 20 and 25 ms always diminished after benzodiazepine intake. However, in some previous measurements we have seen that this second part of the 600-Hz activity could probably be influenced by other factors, such as vigilance or attention to the stimulus. In conclusion, our results provide evidence for a heterogeneous generator structure of the 600-Hz activity which is sensitive to manipulations of GABAergic mechanisms.
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