8. Gronseth GS, Barohn RJ. Practice parameter: thymectomy for autoimmune myasthenia gravis (an evidence-based review): report of the Quality Standards Subcommittee of the American Academy of Neurology. Neurology 2000;55:7–15. 9. Berrih S, Morel E, Gaud C, et al. Anti-AChR antibodies, thymic histology, and T cell subsets in myasthenia gravis. Neurology 1984;34:66 –71. 10. Schluep M, Willcox N, Ritter MA, et al. Myasthenia gravis thymus: clinical, histological and culture correlations. J Autoimmunity 1988;1:445– 467. 11. Mu¨ller-Hermelink HK, Marx A. Pathological aspects of malignant and benign thymic disorders. Ann Med 1999;31:5–14. 12. Roxanis I, Micklem K, McConville J, et al. Thymic myoid cells and germinal center formation in myasthenia gravis; possible roles in pathogenesis. J Neuroimmunol 2002:125:185–197. 13. Wekerle H, Ketelsen UP. Intrathymic pathogenesis and dual genetic control of myasthenia gravis. Lancet 1977;1:678 – 680. 14. Willcox N, Schluep M, Ritter MA, Newsom-Davis J. The thymus in seronegative myasthenia gravis patients. J Neurol 1991; 238:256 –261. 15. Verma PK, Oger JJ. Seronegative generalized myasthenia gravis: low frequency of thymic pathology. Neurology 1992;42: 586 –589. 16. Flores KG, Li J, Sempowski GD, et al. Analysis of the thymic perivascular space during aging. J Clin Invest 1999;104: 1031–1039. 17. Mesnard-Rouiller L, Bismuth J, Wakkach A, et al. Thymic myoid cells express high levels of muscle genes. J Neuroimmunol 2004;148:97–105. 18. Plested CP, Tang T, Spreadbury I, et al. AChR phosphorylation and indirect inhibition of AChR function in seronegative MG. Neurology 2002;59:1682–1688. 19. Flores KG, Li J, Hale LP. B cells in epithelial and perivascular compartments of human adult thymus. Hum Pathol 2001;32: 926 –934.
Subthalamic Stimulation Activates Internal Pallidus: Evidence from cGMP Microdialysis in PD Patients Alessandro Stefani, MD,1,2 Ernesto Fedele, PhD,3,4 Salvatore Galati, MD,2 Olimpia Pepicelli, PhD,3 Stefania Frasca, MD,2 Mariangela Pierantozzi, MD, PhD,1,2 Antonella Peppe, MD, PhD,1 Livia Brusa, MD, PhD,2 Antonio Orlacchio, MD, PhD,1,2 Atticus H. Hainsworth, PhD,5 Giuseppe Gattoni, BSc,2 Paolo Stanzione, MD,1,2 Giorgio Bernardi, MD,1,2 Maurizio Raiteri, MD, PhD,3,4 and Paolo Mazzone, MD6
Parkinson’s disease patients benefit from deep brain stimulation (DBS) in subthalamic nucleus (STN), but the basis for this effect is still disputed. In this intraoperative microdialysis study, we found elevated cGMP extracellular concentrations in the internal segment of the globus pallidus, despite negligible changes in glutamate levels, during a clinically effective STN-DBS. This supports the view that a clinically beneficial effect of STN-DBS is paralleled by an augmentation (and not an inactivation) of the STN output onto the GPi. Ann Neurol 2005;57:448 – 452
Although subthalamic nucleus (STN) deep brain stimulation (DBS) has been accepted as a powerful clinical option in advanced Parkinson’s disease (PD) patients, we lack a precise understanding of the mechanisms explaining DBS efficacy. Initially, it was inferred that DBS acts through the inactivation of STN cells due to high-frequency stimulation (HFS). This view was supported by animal1 and human studies.2 Several findings, however, have challenged this view. First, the ability of human STN cells to fire at a peculiarly high
From 1Instituto di Ricovero e Cura a Carrattere Scientificio (IRCCS) Fondazione S. Lucia; 2Clinica Neurologica, Universita` Tor Vergata; 3Dipartmento Medicina Sperimentale, Sezione di Farmacologia e Tossicologia, Universita` degli Studi di Genova; 4Centro di Eccellenza per la Ricerca Biomedica, Universita` degli Studi di Genova, Genova, Italy; 5Pharmacology Research Group, the Leicester School of Pharmacy De Montfort University, Leicester, United Kingdom; and 6Divisione di Neurochirurgia, Ospedale CTO, Rome, Italy. Received Aug 12, 2004, and in revised form Dec 17. Accepted for publication Dec 20, 2004. Published online Feb 24, 2005, in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/ana.20402 Address correspondence to Prof. Stanzione, I.R.C.C.S. Fondazione S. Lucia, Via Ardeatina 306. 00179, Rome, Italy. E-mail:
[email protected]
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frequency rate, either spontaneously or during passive movement3,4 rendered the inactivation hypothesis unlikely. Second, both functional magnetic resonance imaging and positron emission tomography studies inferred that STN stimulation promotes an increase in blood oxygenation level dependent signal5 and blood flow6 in ipsilateral basal ganglia stations. In addition, previous neurochemical data in rodents showed an increase of glutamate release (and not a decrease) on target stations during the delivery of HFS in STN.7 Finally, STN-DBS promoted an increase of internal globus pallidus (GPi) firing activity in a primate model, suggesting that STN-DBS may reinforce and synchronize, and not suppress, the excitatory drive from STN.8 Similarly, HFS changed the spontaneous irregular firing into a high-frequency bursting activity in rat STN slices,9 a result at variance with the decrease reported in humans.2 Here, we took advantage of our experience in intraoperative microdialysis10 to analyze whether STN-DBS might induce biochemical changes in GPi according to an increased or to a decreased glutamate release from the STN fibers. Given possible limitations in determining extracellular glutamate by microdialysis,10 we measured both glutamate and its second messenger cGMP, which is not influenced by Krebs’s cycle activity, in different surgical sessions. Indeed, it has been extensively demonstrated that extracellular cGMP, monitored by intracerebral microdialysis, exactly reflects its intracellular counterpart and represents a close marker of the activity of the glutamate receptor/nitric oxide synthase/soluble guanylyl cyclase pathway in different brain regions of rodents (for reviews, see Fedele and Raiteri11 and Pepicelli and colleagues12). Fourteen advanced PD patients were included in this study according to selection criteria previously reported.4,10,13,14 In each patient, stimulating permanent electrodes were implanted both in the STN and in the GPi.10,13,14 Patient clinical characteristics are reported in Table 1. Written, informed consent was obtained from each patient who participated in the study. The local ethic committee approved the protocol and consent form describing the risks and potential benefits of the study. In brief, STN and GPi target areas were identified preoperatively by means of ventriculography and intraoperatively by means of single-unit recordings performed on two different trajectories, each performed with a multielectrode holder.13 After target identification, one of the recording electrodes in the GPi trajectory was replaced by a microdialysis probe and infusion (flow rate 5l/min) for stabilization (90 minutes) was begun.10 During stabilization, the permanent stimulating electrode (Medtronic mod 3389) was implanted in ipsilateral STN. After STN electrode insertion and stabilization, we started the collection of 10-minute fraction samples of 50l each. The first five fractions were
Fig. (top panel) Time course of (triangles) glutamate (n ⫽ 4 patients) and (circles) cGMP (n ⫽ 5 patients) extracellular concentrations in the GPi during 1 hour of clinically efficacious STN-DBS (top bar). (Dotted line represents the average Unified Parkinson’s Disease Rating Scale clinical score of rigidity and akinesia of the cotralateral arm: 0 ⫽ normal; 12 ⫽ maximum score). Note the clear increase in cGMP extracellular concentrations while glutamate did not change. (bottom panel) Time course of (triangles) glutamate (n ⫽ 4 patients) and (circles) cGMP (n ⫽ 5 patients) extracellular concentrations after insertion of the DBS electrode in the STN (arrow: electrode insertion). Note the absence of relevant changes of both glutamate and cGMP extracellular concentrations in the GPi.
utilized for basal evaluation either for glutamate (five patients) or for cGMP (nine different patients). All procedures were without additional risks for the patient, because guide tubes for microrecordings were already placed. STN-DBS then was switched on at a rate of 130Hz, pulse width of 110s, and voltage was progressively increased by steps of 0.1V up to the appearance of side effects (paresthaesia or dystonia or myoclonus). Then, voltage stimulation was decreased by 10% or more to obtain disappearance of clinical side effects. Clinical changes, contralateral upper limb rigidity, and akinesia were continuously assessed by an expert neurologist using selected items of Unified Parkinson’s Disease Rating Scale15 (rigidity 0 – 4, finger tapping 0 – 4, hand movement 0 – 4, total 0 –12), while remaining blind to the stimulus intensity between 0 and 3V. In all patients, an improvement of the sum of
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Table 1. Demographic and Clinical Data of the Patients Age (yr)
Disease Duration (yr)
LD Therapy (yr)
LTTS Duration (yr)
LD Therapy before DBS (mg)
68 60 52 49 59 57.6 ⫾ 3.23
10 23 10 15 12 14.0 ⫾ 2.42
10 23 7 13 12 13.0 ⫾ 2.70
5 19 6 8 2 8.0 ⫾ 2.91
750 800 600 900 1,000 810.0 ⫾ 67.82
56 63 58 54 61 58.4 ⫾ 1.63
13 10 8 9 11 10.2 ⫾ 0.86
12 10 7 7 9 9.0 ⫾ 0.94
2 5 3 2 4 3.2 ⫾ 0.58
800 1,000 750 800 1,000 870.0 ⫾ 53.85
65 63 55 64 61.7 ⫾ 2028 59.1 ⫾ 1.44
10 15 8 11 11.0 ⫾ 1.47 11.7 ⫾ 1.04
10 12 8 11 10.2 ⫾ 0.85 10.7 ⫾ 4.06
5 9 4 6 6.0 ⫾ 1.08 5.7 ⫾ 4.37
900 850 1,000 1,300 1012.5 ⫾ 100.77 889.2 ⫾ 44.83
Patient No. Glutamate 1 2 3 4 5 Mean ⫾ SEM High cGMP 6 7 8 9 10 Mean ⫾ SEM Low cGMP 11 12 13 14 Mean ⫾ SEM Total Mean ⫾ SEM
Data are divided according to the Grouping Reported Patients in which glutamate was assessed (Glut); patients in which cGMP was assessed and found at reliable levels (high cGMP); patients in which cGMP was assessed but found at not reliable levels (low cGMP). No significant differences were found among the three groups.
rigidity and akinesia of the upper contralateral arm of more than 30% was achieved without side effects within the first 10 minutes of stimulation at an intensity ranging between 2 and 3V, while the first dialysis fraction during stimulation was collected. STN-DBS intensity was kept constant thereafter and 10-minute fractions were collected during 1 hour of stimulation, while clinical efficacy was continuously monitored to ensure persistence of the clinical effects. Then, STNDBS was discontinued and 10-minute fractions were collected for 1 additional hour. In each patient, microdialysis, coupled to STN-DBS, was performed only in one hemisphere. In the second hemisphere, microdialysis was performed as reported above, including insertion of the STN electrode, but with no voltage stimulus to assess possible changes of glutamate and cGMP due to the surgical procedure. Glutamate and cGMP concentrations were determined by high-performance liquid chromatography and by a commercially available radioimmunoassay, respectively.10,12 The detection limit for cGMP radioimmunoassay is 1fmol/100L and the antiserum cross-reactivity for cAMP and other common nucleotides is less than 0.001, using the acetylation protocol. In each group of patients, significance of glutamate and cGMP changes were assessed by performing in each subject the mean of single determinations obtained under basal conditions, during STNDBS (or after electrode insertion) and during recovery conditions. Then, means were compared by Friedman analysis of variance followed by Wilcoxon test.
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No significant changes in extracellular glutamate concentrations were observed during or after STNDBS or because of insertion of the stimulating electrode (Table 2). cGMP extracellular concentrations were examined in nine subjects. Four of them exhibited cGMP basal values (in the first five fractions) below the minimal detectable concentration and were, therefore, eliminated (data not shown). In the other five patients, cGMP basal concentrations were clearly detectable (⬎1fmol/ fraction) and were relatively similar among patients (see Table 2). STN-DBS produced a significant increase in cGMP extracellular concentrations in the GPi (mean, ⫹646%). In each patient, the increase in cGMP was significant from the first or the second fraction after DBS-ON (see Table 2). Noticeably, these changes paralleled the amelioration of clinical performance (Fig.). After cessation of DBS, extracellular cGMP concentrations remained elevated for 10 to 20 minutes and returned to prestimulation values within 20 to 30 minutes in all the five subjects. The insertion of the stimulating electrode “per se” did not produce any change in the contralateral hemisphere. Our results show that a functional activation of the STN-GPi pathway occurs when STN-DBS is clinically efficacious. The clear increase in cGMP extracellular levels, detected by the GPi probe, likely results from an increase of glutamate release from STN fibers terminating in the GPi. In agreement with our findings, a shift to a bursting activity in the STN, induced by intranu-
Table 2. Biochemical Changes of Glutamate and cGMP Extracellular Concentrations before, during, and after STN-DBS Basal Fractions Glutamate Patient No. 1 2 3 4 5 Mean SD SEM cGMP
STN-DBS
Recovery
1
2
3
4
5
Mean
1
2
3
4
5
6
25.00 75.00 25.00 43.75 50.00 43.75 20.73 10.36
50.00 25.00 50.00 37.50 25.00 37.50 12.50 6.25
25.00 50.00 25.00 43.75 75.00 43.75 20.73 10.36
25.00 25.00 50.00 37.50 50.00 37.50 12.50 6.25
50.00 50.00 25.00 43.75 50.00 43.75 10.83 5.41
35.00 45.00 35.00 41.25 50.00 41.25 6.50 3.25
50.00 75.00 50.00 50.00 25.00 50.00 17.68 8.84
25.00 50.00 75.00 43.75 25.00 43.75 20.73 10.36
25.00 25.00 50.00 43.75 75.00 43.75 20.73 10.36
50.00 75.00 25.00 43.75 25.00 43.75 20.73 10.36
25.00 50.00 25.00 37.50 50.00 37.50 12.50 6.25
25.00 50.00 25.00 37.50 50.00 37.50 12.50 6.25
6 1.50 1.30 7 1.20 1.50 8 1.40 1.30 9 1.70 1.50 10 2.50 1.30 Mean 1.66 1.38 SD 0.50 0.11 SEM 0.22 0.05 cGMP under the minimal 11 0.01 0.10 12 0.30 0.20 13 0.10 0.20 14 0.20 0.30 Mean 0.15 0.20 SD 0.13 0.08 SEM 0.06 0.04
1.20 1.20 1.50 1.34 1.50 1.30 1.20 1.34 1.20 1.20 1.40 1.30 1.60 1.70 1.50 1.60 1.50 1.50 1.20 1.60 1.40 1.38 1.36 1.44 0.19 0.22 0.15 0.14 0.08 0.10 0.07 0.06 detectable concentration 0.02 0.20 0.10 0.09 0.30 0.20 0.10 0.22 0.10 0.30 0.10 0.16 0.20 0.30 0.10 0.22 0.16 0.25 0.10 0.17 0.12 0.06 0.00 0.06 0.06 0.03 0.00 0.03
1
2
3
4
5
6
25.00 25.00 50.00 31.25 25.00 31.25 10.83 5.41
25.00 25.00 50.00 43.75 75.00 43.75 20.73 10.36
50.00 75.00 25.00 50.00 50.00 50.00 17.68 8.84
50.00 25.00 50.00 50.00 75.00 50.00 17.68 8.84
25.00 25.00 25.00 31.25 50.00 31.25 10.83 5.41
25.00 50.00 25.00 37.50 50.00 37.50 12.50 6.25
Mean 6.50 21.00 20.20 20.10 18.90 20.60 17.88 23.30 1.40 1.80 1.70 1.80 1.60 1.60 1.65 1.90 2.00 2.50 3.00 3.50 4.00 6.00 3.50 7.00 2.00 3.40 3.20 3.40 3.20 2.90 3.02 3.50 2.50 3.00 3.50 3.00 3.50 4.00 3.25 2.50 2.88 6.34 6.32 6.36 6.24 7.02 5.86a 7.64 2.06 8.22 7.79 7.71 7.13 7.76 6.76 8.97 0.92 3.67 3.48 3.45 3.19 3.47 3.02 4.01
1.40 1.50 1.50 3.50 4.50 2.48 1.43 0.64
1.30 1.40 1.40 1.60 1.70 1.48 0.16 0.07
1.50 1.50 1.20 1.50 1.30 1.40 0.14 0.06
1.30 1.30 1.40 1.40 1.20 1.32 0.08 0.04
1.40 1.40 1.50 1.20 1.50 1.40 0.12 0.05
33.33 54.17 41.67 42.71 41.67 42.71 7.44 3.72
Friedman ANOVA
33.3 37.5 37.5 40.6 54.2 40.6 8.0 4.0
N
Mean 5.0 1.5 2.3 2.1 2.1 2.6 p ⬍ 0.001 1.4 0.6
Expunged Expunged Expunged Expunged
Only the increase of cGMP showed significance ( p ⬍ 0.001). a
p ⬍ 0.05 post hoc Wilcoxon.
STN-DBS ⫽ subthalamic nucleus–deep brain stimulation; ANOVA ⫽ analysis of variance; SD ⫽ standard deviation; SEM ⫽ standard error of the mean; NS ⫽ not significant.
clear HFS, has been shown in vitro9 as well as a STNDBS–induced augmentation of GPi discharge8 in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine–treated monkeys. Our findings may represent a biochemical counterpart of these electrophysiological data. Both the increase of firing rate and the shift to a bursting activity can promote a larger glutamate release that, in turn, leads to increased cGMP extracellular concentrations in GPi. Microdialysis, however, does not discriminate between a net increase of STN firing activity or a change of the STN firing pattern, as a possible explanation for the observed biochemical changes. Other research groups have recently reported a large decrease in spontaneous STN firing activity during STN-DBS in humans (associated with a “burst-like” firing pattern).2 Clearly, STN-DBS may have differential effects on STN cell bodies and STN fibers projecting to the GPi,9,16 finally inducing an increase in firing rate and cGMP extracellular concentrations in GPi, whereas STN cell activity is decreased. From the data presented here, it seems unlikely that STN-DBS clinical efficacy reflects a DBS-mediated inhibition of STN cells, unless this is somehow coupled with axonal stimulation.17,18 Previous studies have emphasized the possibility that pharmacological and electrical manipulations of basal ganglia circuitry may affect the same target area (e.g.
thalamus) by different mechanisms 13. The partially additive effects of DBS and dopaminergic therapy in implanted patients suggest that part of the mechanisms activated by these two procedures are overlapping, that is, possibly on the final target within thalamus, independently of the mechanisms activated within the basal ganglia. Dopaminergic therapy is reported to decrease the GPi inhibitory output to the thalamus,19,20 whereas STN-DBS may modulate the GPi–thalamic pathway by directly interfering with its fibers passing above the STN.21 Another aspect of our work is the striking insensitivity of GPi glutamate concentrations to STN-DBS, compared with the changes in cGMP. However, it has to be borne in mind that microdialysis can detect neurochemical changes occurring at the synaptic levels only if they are sufficiently reflected into the extracellular compartment. Thus, small, though physiologically relevant, glutamate variations in the synaptic cleft might not be picked up by this technique, taking also into account the very efficient reuptake of this neurotransmitter into nerve terminals and, especially, into glial cells. Alternatively, the high basal levels of metabolic-derived glutamate might obscure changes in the amino acid levels of neuronal origin. On the contrary, cGMP has not been reported to originate from metabolic reactions, but, as said above, it is a well-
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known amplification product of the nitric oxide synthase/soluble guanylyl cyclase pathway that, in the central nervous system, is particularly linked to glutamatergic receptors. These findings indicate that basal glutamate concentration is not a suitable parameter for studying glutamatergic transmission in human beings,10 and cGMP appears a more suitable tool. Extracellular cGMP, in vivo, is mainly controlled by N-methyl-D-aspartate, ␣-amino-3-hydroxy-5-methyl-4isoxazole propionic acid, and metabotropic glutamate receptors but also influenced by other transmitters (GABA, acetylcholine, and neuropeptides).11,12 Hence, simultaneous DBS-induced changes in multiple transmitters cannot be excluded currently, although the increased GPi firing during STN-DBS8 suggests that the reported cGMP augmentation is linked to excitatory transmitter release. Alternatively, the STN-DBS–mediated increase in cGMP might be caused by a decreased inhibitory endogenous tone onto nitric oxide synthase– containing neurons, as found in the hippocampus of rodents.11 Finally, note that (1) not all the studied PD patients exhibited cGMP extracellular concentrations detectable by radioimmunoassay methods, and (2) one patient, on the contrary (see Table 2) showed strikingly high DBSdependent cGMP response. These findings could be related to the different clinical conditions (ie, disease duration, previous therapy, endogenous nitric oxide/ phosphodiesterases) of the studied patients or, alternatively, to peculiar subregional difference inside each patient’s GPi. In this respect, larger series of different cohort of patients are currently under evaluation. In conclusion, clinically effective STN-DBS produced a marked (⬎6-fold) increase in cGMP extracellular concentrations within the GPi, indicative of increased glutamatergic transmission or an increase of glutamate effectiveness on pallidal neurons. This work has been supported by a grant from Ministero della Sanita` (RF 02-220, P.S., A.S.).
References 1. Benazzouz A, Piallat B, Pollak P, Benabid AL. Responses of substantia nigra pars reticulata and globus pallidus complex to high frequency stimulation of the subthalamic nucleus in rats: electrophysiological data. Neurosci Lett 1995;189:77– 80. 2. Welter ML, Houeto JL, Bonnet AM, et al. Effects of highfrequency stimulation on subthalamic neuronal activity in parkinsonian patients. Arch Neurol 2004;61:89 –96. 3. Levy R, Dostrovsky JA, Lang AE, et al. Effects of apomorphine on subthalamic nucleus and globus pallidus internus neurons in patients with Parkinson’s disease. J Neurophysiol 2001;86: 249 –260. 4. Stefani A, Bassi A, Mazzone P, et al. Subdyskinetic apomorphine responses in globus pallidus and subthalamus of parkinsonian patients: lack of clear evidence for the “indirect pathway.” Clin Neurophysiol 2002;113:91–100.
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5. Jech R, Urgosik D, Tintera J, et al. Functional magnetic resonance imaging during deep brain stimulation: a pilot study in four patients with Parkinson’s disease. Mov Disord 2001;16: 1126 –1132. 6. Hershey T, Revilla FJ, Wernle AR, et al. Cortical and subcortical blood flow effects of subthalamic nucleus stimulation in PD. Neurology 2003;23:816 – 821. 7. Windels F, Bruet N, Poupard A, et al. Effects of high frequency stimulation of subthalamic nucleus on extracellular glutamate and GABA in substantia nigra and globus pallidus in the normal rat. Eur J Neurosci 2000;12:4141– 4146. 8. Hashimoto T, Elder CM, Okun MS, et al. Stimulation of the subthalamic nucleus changes the firing pattern of pallidal neurons. J Neurosci 2003;23:1916 –1923. 9. Garcia L, Audin J, D’Alessandro G, et al. Dual effect of highfrequency stimulation on subthalamic neuron activity. J Neurosci 2003;23:8743– 8751. 10. Fedele E, Mazzone P, Stefani A, et al. Microdialysis in parkinsonian patient basal ganglia: acute apomorphine-induced clinical and electrophysiological effects not paralleled by changes in the release of neuroactive amino acids. Exp Neurol 2001;167: 356 –365. 11. Fedele E, Raiteri M. In vivo studies of the cerebral glutamate receptor/NO/cGMP pathway. Prog Neurobiol 1999;58: 89 –120. 12. Pepicelli O, Raiteri M, Fedele E. The NOS/sGC pathway in the rat central nervous system: a microdialysis overview. Neurochem Int 2004;45:787–797. 13. Peppe A, Pierantozzi M, Bassi A, et al. Stimulation of subthalamic nucleus compared with the globus pallidus internus in patients with Parkinson’s disease. J Neurosurg 2004;101: 195–200. 14. Mazzone P, Brown P, DiLazzaro V, et al. Bilateral implantation in globus pallidus internus and in subthalamic nucleus in Parkinson’s disease. Neuromodulation 2005;8:1– 6. 15. Fahn S, Elton RL, and Members of the UPDRS development committee. The Unified Parkinson’s disease rating scale. In: Fahn S, Mardsen CD, Goldstein M, Calne DB, editors. Recent developments in Parkinson’s disease. Florham Park, NJ: MacMillan Healthcare Information, 1987:153–163. 16. Nowak LG, Bullier J. Axons, but not cell bodies are activated by electrical stimulation in cortical grey matter. Exp Brain Res 1998;118:477– 488. 17. McIntyre CC, Savasta M, Kerkerian-Le Goff L, Vitek JL. Uncovering the mechanism(s) of action of deep brain stimulation: activation, inhibition, or both. Clin Neurophysiol 2004;115: 1239 –1248. 18. Filali M, Hutchison WD, Palter VN, et al. Stimulationinduced inhibition of neuronal firing in human subthalamic nucleus. Exp Brain Res 2004;156:274 –281. 19. Stefani A, Stanzione P, Bassi A, et al. Effects of increasing doses of apomorphine during stereotaxic neurosurgery in Parkinson’s disease: clinical score and internal globus pallidus activity. J. Neural Transm. 1997;104:895–904. 20. Hutchinson WD, Levy R, Dostrovsky JO, et al. Effects of apomorphine on globus pallidus neurons in parkinsonian patients. Ann Neurol 1997;42:767–775.
