Parkinson’s disease (PD) has been believed to result from the dopamine deficiency in the substantial nigra pars compacta (SNc) that leads to functional changes in the basal ganglia-thalamocortical circuit.1,2 According to the model, the cardinal symptoms of bradykineisa and akinesia is proposed to result from overactivity of the basal ganglia output nucleus, globus pallidus internal (GPi), leading to depression of thalamic neurons and motor cortical areas.1,2 Supportive data come from microelectrode recording studies that an increased neuronal firing in GPi not only observed in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated monkeys,3-5 but also observed in patients with PD.6,7 Lesions or stimulation in GPi of PD patients result in improvement of parkinsonian symptoms.8 Moreover, imaging study found an increase in regional cerebral blood flow (rCBF) in premotor cortical areas during GPi stimulation, which improved rigidity and bradykinesia.9 In accordance with the model, the neuronal firing in the motor thalamus, particularly in the pallidal receiving areas, the ventral oral posterior (Vop), are possibly decreased in PD.
Essential tremor (ET) is a movement disorder characterized by monosymptomatic action or postural tremor with a frequency of 4-12 Hz.10 Studies on the animal model of ET and patients with ET11-13 suggest that the inferior olive nucleus-cerebellum-thalamus-cortex pathway is likely to involve in pathophysiology of ET. According to the hypothesis, the ET is supposed to associate with the overactivity of the cerebellar. Thus, the ventral intermediate (Vim), the cerebellar receiving area of thalamus, should show an increased neuronal activity.14
In addition, parkinsonian tremor and ET can be effectively treated by thalamotomy or thalamic deep brain stimulation (DBS)15,16 suggesting that the thalamus is involved in pathophysiology of PD and ET. Although several studies report the neurophysiological findings in the thalamus of PD and ET, the role of thalamus in pathophysiology of PD and ET remains unclear.
The present study takes advantage of microelectrode-guided stereotactic surgery for PD and ET symptoms to further explore the neuronal activity of Vop/Vim in a relative large patient group of PD and ET.
This study was carried out by retrospective analysis of data gathered during the physiological localization for a thalamic lesion or stimulation to relieve parkinsonian tremor (n=20) and ET (n=16) from the period of 2005 to 2007.
Patients with PD (12 males and 8 females, average age: 56.5 years) had disease duration of (5.8±2.1) years and Hoehn and Yahr score was 2-4 at operation.17 The diagnosis of PD was based on medical history, physical and neurological examinations, response to levodopa or dopaminergic drugs, laboratory tests, and MRI scans to exclude other diseases. All patients had disabling tremor and their tremor scores of Unified Parkinson’s Disease Rating Scale (UPDRS) III was ≥8 at medication “off” state (the total tremor scores were 28).18
Patients with ET (10 males and 6 females, average age: 51.2 years) had disease duration of (18.4±6.2) years and their action tremor score of Fahn-Tolosa-Marin Tremor Rating Scale (FAHN) were ≥10 (the total scores of tremor were 48).19 Eleven patients had family history, 7 patients respond to alcohol, and 9 patients took beta-adrenergic receptor antagonist.
The study was approved by the Ethics Committee of Xuanwu Hospital, Capital Medical University, China according to the Declaration of Helsinki. Written informed consents were obtained from all patients.
Surgical procedure and electrophysiology
Prior surgery procedure, patient had been off all medications for at least 12 hours. The thalamic exploration was performed as a standard stereotactic surgery procedure using the Cosman-Roberts-Wells (CRW) frame (Radionics, Burlington, MA, USA). The frame coordinates of anterior (AC) and posterior (PC) of commissures were measured by using sagittal magnetic resonance imaging (Siemens 1.5 Tesla, Sonata, Germany). In the present study, the coordinates were 0 to 2 mm superior to the AC-PC line, 4 to 8 mm anterior to the PC, and 12 to 15 mm lateral to the midline. These coordinates were then used to reconstruct the location of nucleus based on the stereotactic atlas of Schaltenbrand & Wahren.20
Physiological confirmation of the target location was then performed under local anesthesia by microelectrode recording. The microelectrode was advanced through a burr hole 2.5 cm away from the midline anterior to the coronal suture. Tip size of tungsten microelectrode ranged from 10 to 20 μm and resistances from 0.1 to 0.5 MΩ at 1000 Hz. The initial trajectories were always focused on the ventral causal (Vc) thalamic nuclei because the response of neurons in the region to somatosensory stimulation provide the most reliable landmark to guide the procedure.
Extracellular action potential signals were amplified (×20 000-50 000), filtered (at bandpass of 200 Hz-10 kHz), and fed to an audio monitor with an AC Amplifier (FHC, Inc., Bowdoinham, ME, USA). The signal was sampled at 7.5 kHz, displayed on a computer screen and an oscilloscope (HITACHI, V-1560, Japan). The electromyogram (EMG) was recorded simultaneously using surface electrodes from the extensor carpi radialis (ECR) and flexor carpi radialis (FCR) muscles and the tibialis anterior (TA) muscles on the controlateral side. The signals were amplified (×5000-10 000) and filtered (at bandpass of 100 Hz-1 kHz) (FHC, Inc.). A four-channel recording was done using PolyView program (Astro-Med, Inc., RI, USA) and the data were stored in a computer (Pentium III) for off-line analysis.
Throughout the surgical procedure, all patients were required to be awake and conscious to cooperate with the neurosurgeon.
For neuronal data analysis, only spikes (negative upward) having a signal-to-noise ratio greater than 2:1 were used. Action potentials were confirmed to arise from a single neuron by amplitude and waveform criteria. The confirmation method included examining whether the waveform of the action potential was constant as verified by displaying the waveform on the screen at a time window for at least 15-20 seconds. The interspike intervals (ISIs) were measured and ISI histograms were used to estimate the pattern of neuronal discharges. The mean firing rate and standard deviation (SD) were calculated. The degree of regularity of neuronal discharge was determined by calculating coefficient of variation (CV) of ISIs (CV = SD of ISIs/mean ISIs).
The Spike 2 (Cambridge Electronic Design, UK), PolyView Program (Astro-Med. Inc., USA), and Origin 7.0 (OriginLab Corporation, USA) were used for the data analysis.
In the present study, the location of Vim was defined as a region just anterior to the border of Vc (<3 mm). The location of Vop was defined as a region localized at anterior to the border of Vc (>3 mm), according to the standard human anatomy atlas of Schaltenbrand and Wharen.20
SPSS 12.0 (SPSS Inc., USA) and Origin 7.0 programs were used for statistical analysis. The data were expressed as mean ± standard deviation (SD) or median (minimum-maximum). The relationship between tremor-related neuronal activity and limb tremor was examined by correlation test.
Mann-Whitney test or Kruskal-Wallis (K-W) test was used to compare the neurons with tonic firing and irregular discharge of ET and PD patients. The chi-square test was used for different neuronal firing patterns. For all tests, significance was set at P <0.05.
A total of 510 thalamic neurons were analyzed in the 36 patients. Of these neurons, 38.6% (n=197) neurons show that the rhythmic bursting activity correlated with the limb tremor, defined as tremor related neuronal activity; 29.6% neurons (n=151) show regular tonic firing and 31.8% neurons (n=162) show irregular discharge.
Neuronal activity in the thalamus of PD
Three hundred and twenty-three neurons were obtained from PD patients (n=20), of which, 46.7% (n=151) were tremor related neuronal activity with bursting activity that correlated with limbs tremor (4-6 Hz) (R: 0.72, P <0.01); 22.9% (n=74) were tonic firing and 30.4% (n=98) were irregular discharge.
ISI analysis showed that the mean spontaneous firing rate (MSFR) of neurons with tonic firing (n=74) was 26.7 (3.4-68.3) Hz and mean ISI were 24 (1-668) ms. The MSFR of neurons with irregular firing (n=98) were 13.9 (3.0-58.1) Hz and mean ISI were 36 (2-1465) ms.
Figure 1 shows the representative neuronal activity of tremor related neuronal activity, neurons with tonic firing and irregular discharge in PD. The ISI histograms show that the MSFR of these three types of neuron were 22.3 Hz, 38.4 Hz, and 16.7 Hz, respectively.
Further analysis found that the MSFR of neurons (n=62) in Vop was 11.7 Hz lower than the MSFR of similar neurons (n=32) in Vim (16.3 (3.0-65.1) Hz vs. 28.0 (6.5-68.3) Hz) (Mann-Whitney test 566, P <0.05; Table 1). The data suggest that neuronal firing rate in Vop was reduced in PD as predicted by the model of pathophysiology of PD.1,2
Neuronal activity in the thalamus of ET
One hundred and eighty-seven neurons were obtained from ET patients (n=16), of which, 24.6% (n=46) were tremor related neuronal activity (R: 0.67, P <0.01); 41.2% (n=77) were tonic firing, and 34.2% (n=64) were irregular discharge.
The analysis of ISI found that the MSFR of neurons with tonic firing (n=77) were 48.8 (19.0-135.5) Hz and mean ISI were 17 (2-239) ms. The MSFR of neurons with irregular discharge (n=64) were 26.3 (8.7-84.7) Hz and mean ISI were 22 (2-728) ms.
Figure 2 shows the representative neuronal activity of tremor related neuronal activity, tonic firing, and irregular discharge in the thalamus in ET patients. The ISI histograms show that the MSFR of the three neuronal types were 34.8 Hz, 75.6 Hz, and 22.1 Hz, respectively.
Further analysis found that the MSFR of neurons (n=75) in Vop was 15.1 Hz lower than the MSFR of similar neurons (n=24) in Vim (34.8 (8.7-135.5) Hz vs. 49.9 (18.3-94.2) Hz) (Mann-Whitney test 533, P <0.05; Table 1). The data suggest that neuronal firing in Vim was increased in ET as proposed by the model of pathophysiology of ET14.
Different neuronal firing rates between PD and ET
Further analysis found that the MSFR of neurons with tonic firing (n=74) were 26.7 (3.4-68.3) Hz and mean ISI were 24 (1-668) ms in PD whereas the MSFR of neurons with tonic firing (n=77) were 48.8 (19.0-135.5) Hz and mean ISI were 17 (2-239) ms in ET. Similarly, the MSFR of irregular neurons (n=98) were 13.9 (3.0-58.1) Hz and mean ISI were 36 (2-1465) ms in PD, and the MSFR of irregular neurons (n=64) were 26.3 (8.7-84.7) Hz and mean ISI were 22 (2-728) ms in ET. K-W test showed that there was a significance reached among the MSFR of tonic firing and irregular discharge in the thalamus of PD and ET (80.838, P <0.05). Further comparison showed that MSFR of the tonic firing and irregular discharge in PD was lower than those similar neuronal firing in ET (both P<0.01).
Moreover, it was found that the MSFR of neurons of 16.3 (3.0-65.1) Hz (n=62) in Vop of PD were lower than those neurons of 34.8 (8.7-135.5) Hz (n=75) in Vop of ET (Figure 3). Similarly, the MSFR of neurons of 28.0 (6.5-68.3) Hz (n=32) in Vim of PD was lower than those neurons of 49.9 (18.3-94.2) Hz (n=24) in Vim of ET (Figure 3). K-W test demonstrated that there was a significance reached among the MSFR of the Vop and Vim neurons in PD and ET patients (65.373, P <0.05). Further comparison showed that MSFR of the Vop and Vim neurons in PD was lower than those similar neurons in ET (both P<0.01).
The results suggest that a decrease in an overall activity of the thalamic neurons is revealed in PD whereas an increase in an overall activity of the thalamic neurons might be involved in ET.
Additionally, no significant differences were found when comparing the CV of ISI of neurons with tonic firing and irregular discharge reached between in PD and in ET.
Neuronal firing patterns and distributions in the thalamus of PD and ET
Auto-correlation analysis of tremor related neuronal activity found that the oscillatory frequency at ranged of 4-6 Hz was for PD (n=98) whereas the oscillatory frequency at ranged at 4-8 Hz was for ET (n=43) demonstrates in Figure 4. The results confirmed the characteristics of parkinsonian rest tremor and action or posture tremor of ET often seen in the clinic.
In the meanwhile, the distribution of tremor related neuronal activity, tonic firing and irregular discharge in the Vop/Vim were further analyzed. The percentage of three neuronal types was significantly different between ET and PD (χ2 value: 28.952, P <0.05; Table 2)
In consistent with previous studies,21-24 this study demonstrates that a large number of tremor related neuronal activities exhibit in the Vop/Vim indicating that the incidence of bursting activity increases in both of PD and ET. In particular, the auto-correlograms confirm the clinical futures of PD and ET that the oscillatory frequency at 4-6 Hz is for parkinsonian tremor whereas the oscillatory frequency at 4-8 Hz is for ET. Lesions or stimulation in the region where tremor related neuronal activities localized abolish tremor immediately, supporting the view that the thalamus is involved in the pathophysiology of parkinsonian tremor and ET.25
The significant finding of the present study is that the MSFR of pooled neurons of tonic firing and irregular discharge in Vop/Vim in patients with PD is significantly lower than those of similar neurons in patients with ET (26.7 Hz vs. 48.8 Hz; 13.9 Hz vs. 26.3 Hz). In addition, the significant differences also found that the MSFR of neurons of tonic firing and irregular discharge in Vop as well as in Vim in PD was lower than the MSFR of the similar neurons in Vop and Vim in ET, respectively (16.3 Hz vs. 34.8 Hz, 28.0 Hz vs. 49.9 Hz) (Figure 3). In this study, the decreased neuronal firing rate observed in Vop/Vim, particularly in Vop in PD patients is consistent with the popular model of dysfunction of basal ganglia.1,2 The model has been proposed that the hyperactivity of GPi should lead to the decreased firing rates in the Vop in the parkinsonian state. The prediction has been supported by studies of either MPTP treated-monkeys or PD patients that has been shown to result in an increase in GPi firing rate in the MPTP treated-parkinsonian monkeys3-5 and GPi firing rate in PD patients are similar to those in MPTP-treated monkeys,6,7 suggesting that they are elevated compared with normal primate. Therefore, it is reasonable to presume that the firing rates of the neurons in GPi of the PD patients in this study are increased in comparison to the patient with ET. 23 Furthermore, the present results are also consistent with recent findings that a small decrease of neuronal activity has been observed in Vop in PD;26,27 a decrease in an overall activity of the thalamic neurons was revealed in PD.26,28
In addition, according to the prediction of basal ganglia model, the reduced firing rate in Vop observed in PD in this study would be in relation to suppressed activity of premotor cortical areas receiving input from Vop. The present results, thus, also consistent with the recent functional imaging studies that the activity of supplementary motor area (SMA) of the cortex, a major projection target of the neurons in Vop, decreased in PD patients.14,29
In contrast, this study found that the MSFR of neurons in Vop/Vim, in particular, in Vim, the cerebellar receiving area of thalamus in ET patients was significantly greater than those in PD patients and normal primates.27 The results were similar to recent studies of ET patients that neuronal firing rate was increased in Vim in ET as compared with PD patients and pain patients,23 supporting the view that the cerebellar-thalamo-cortical pathway may be a possible cause of tremor in ET.11-13 Animal model of ET has been proposed that the olivocerebellar circuit may be involved in tremor generation of ET.30 The evidence obtained from recent human studies also suggests that the cerebellothalamocortical pathway is involved in the pathophysiology of ET. Functional imaging studies from ET patients during tremor and at rest without tremor have shown increased bilateral cerebellar activities, deep cerebellar nuclei activation, 16 and increased blood flow in the contralateral thalamus during tremor.16 Anatomically, cerebellar input to the human motor thalamus is mainly to Vim and is excitatory in nature, then overactivity from the cerebellar should increase the activity in Vim,31 similar to the findings in the present study and two other studies.22,23
With respect to the result of significantly different distributions of neuronal firing pattern in the Vop/Vim in PD and ET, one explanation is that pathophysiological changes in PD and ET likely result in either changes in neuronal firing rate or changes in neuronal firing patterns, or both. The different proportions of three neuronal firing patterns might also be explained to relate to different motor symptoms involved in PD and ET. Additionally, this study also found a decrease in an overall activity of the thalamic neurons in PD and an increase in an overall activity of the thalamic neurons in ET. The phenomena might be explained by anatomical studies that pallidal afferents to the motor thalamus innervate not only mostly the Vop, but also the regions of the Vim;32 whereas cerebellar afferents to the motor thalamus innervate primarily Vim, possibly a part of Vop.33 Thus, it is not surprise that an overall decreased thalamic neuronal activity would be obtained from PD and an overall increased thalamic neuronal activity would be obtained from ET. Clearly, further investigations should be carried out to explain these issues.
We would like to thank Drs. HU Yong-sheng, ZHU Hong-wei, TAO Wei, YU Tao, MA Kai, YAN Xiao-ming, ZHANG Xiao-hua and CAI Li-xin who provide assistance to the study.
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