MEDIZINISCHE HOCHSCHULE HANNOVER
Klinik für Psychiatrie, Sozialpsychiatrie und Psychotherapie
Elemental and Neurochemical Based Analysis of the Pathophysiological
Mechanisms of Gilles de la Tourette syndrome
INAUGURALDISSERTATION
zur Erlangung des Grades einer Doktorin oder eines Doktors der Naturwissenschaften Doctor rerum naturalium -
Dr. rer. nat.
vorgelegt von
Ahmad Seif Kanaan B.Sc., M.Sc
geb. am 17/05/1986, Amman, Jordanien
Hannover, Deutschland (2017)
Promotionsordnung der Medizinischen Hochschule Hannover
für die Erlangung des Grades einer Doktorin/eines Doktors der Naturwissenschaften (Doctor rerum naturalium) Letzte Änderung in der 523. Sitzung Senatssitzung vom 15.07.2015
Wissenschaftliche Betreuung: Prof. Dr. med. Kirsten Müller-Vahl
Wissenschaftliche Zweitbetreuung: Prof. Dr. rer. nat. Claudia Grothe
1. Erst-Gutachterin/Gutachter: Prof. Dr. med. Kirsten Müller-Vahl
2. Gutachterliche Stellungnahme durch: Prof. Dr. rer. nat. Claudia Grothe
3. Gutachterin/Gutachter: Prof Dr. Phil. Florian Beißner
Tag der mündlichen Prüfung: 12.07.2017
Promotionsordnung der Medizinischen Hochschule Hannover
für die Erlangung des Grades einer Doktorin/eines Doktors der Naturwissenschaften (Doctor rerum naturalium) Letzte Änderung in der 523. Sitzung Senatssitzung vom 15.07.2015
Acknowledgements
I warmly thank that patients and their families for their selfless contribution of time and effort to further the understanding of their affliction — a disease that makes them lead difficult, stigmatized and action packed lives. I particularly admired the determination of many of the patients who had to travel vast distances across Germany to undergo multiple investigations.
My most enduring acknowledgment goes to my supervisors Kirsten Müller-Vahl and Harald E. Möller who were the true pillars of this work. I sincerely thank them both for their excellent mentorship and guidance, and for giving me the opportunity to tackle absorbing ideas while traveling between Leipzig and Hannover.
My deep gratitude also goes to Alfred Anwander for teaching me how to think like an image scientist and for always keeping the door open for discussion. I sincerely thank Sarah Gerasch for her significant contribution in recruiting the patients and acquiring the clinical data. I am also grateful to Daniel Margulies for giving me the opportunity to discover image analysis in his laboratory in the initial stages of my PhD.
My deep admiration and gratitude go to Isabel Garcia-Garcia who provided ample ideas and reviewed multiple drafts of initial works. Thank you for crossing my path, for teaching me how to conduct statistical analyses with a smile and for being there when it counted.
For their tremendous support in image and clinical data acquisition, I sincerely thank Saskia Czerwonatis, Claudia Pelke, Leonie Lampe, Tomas Goucha, Sieglinde Remane, Nicole Pampus, Christiane, Driedger-Garbe and Cornelia Gerbothe. I especially thank Andre Pampel and Torsten Schlumm for their support in developing the imaging se- quences.
To Jamie Near, Berkin Bilgic and Andreas Schäfer, thank you for your openness to collaborate and for providing technical support in finding solutions to challenging imaging problems.
I would also like to thank my colleagues in the NMR group: Ricardo Metere, Tobias Lenich, Miguel Martinez Maestro, Henrick Marschner, Maria Guidi, Jakob Georgi and all other members for sharing this experience with me.
To the TS-EUROTRAIN community: Nacho Gonzalez, Francesca Rizzo, Ester Nespoli, Muhammad Sulaman Nawaz, Sam Padmanabhuni, John Alexander, Nuno Nogueira, Natalie Forde, Luca Pagliaroli, Sarah Fan, Joanna Widomska, Rayan Houssari, Peristera Paschou, Andrea Ludolph, Danielle Cathe, Pieter Hoekstra, Zeynep Tümer, Csaba Barta, Jeffrey Glennon, Bastian Hengerer, Hrienn Steffanson, Jeremiah Scharf, I thank you all for making this journey so cheerful and all the more worthwhile.
I also warmly thank Professor Mary Robertson for our illuminating discussions and for inspiring me to keep going forward. To Jim Leckman, I thank you for appreciating my work and bestowing me with the honor of the 2016 Professor Mary Robertson Award.
To my family Seif, Shireen, Mira, Farah and Hussam, I deeply thank you for your love, understanding and everlasting encouragement.
Abstract
Elemental and neurochemical based analysis of the pathophysiological mechanisms of Gilles de le Tourette syndrome.
Ahmad Seif Kanaan
Gilles de la Tourette syndrome is a developmental neuropsychiatric movement disorder characterized by the presence of tics and associated comorbid conditions. As current treatment strategies are often unsatisfactory and associated with significant adverse ef- fects, there is an urgent need in further elucidating the nature of GTS pathophysiology to accelerate the drug discovery and development process. Neurochemically, previous work has indicated that the clinical manifestations of GTS are primarily driven by putative ab- normalities in dopamine and γ-Aminobutyric-acid (GABA). Given the spatio-temporal and metabolic interdependence exhibit by the neurotransmitter glutamate with dopamine and GABA, respectively, we hypothesized the glutamatergic signalling is related to the pathophysiology of GTS. On a finer scale, considering the critical role exhibited by the el- ement iron in varied biochemical processes sustaining typical neurochemical synthesis and trafficking throughout the lifespan, we additionally postulated that GTS patients exhibit abnormalities in iron metabolism. Utilizing a multi-parametric, quantitative Magnetic Resonance Imaging approach in vivo, we investigated the role of glutamate and iron using 1H-Magnetic Resonance Spectroscopy and Quantitative Susceptibility Mapping, respectively, for the first time. To achieve these aims, two methodological investigations were initially conducted to obtain quantitative neurochemical and magnetic susceptibil- ity measurements of sufficient precision to identify rather subtle changes. Imaging, spec- troscopic and clinical data were acquired from a relatively large and well-characterized sample of adult patients with GTS and age/gender matched healthy controls. To in- terrogate the influence of treatment on neurochemical and clinical characteristics of the study sample, we employed a longitudinal study design in which the patients were in- vited to undergo treatment with the commonly used antipsychotic aripiprazole. At the neurochemical level, we report significant reductions in the concentrations of spec- troscopic glutamatergic signalling markers in the striatum and the thalamus in GTS.
These reductions correlated with tic severity and were normalized with aripiprazole treat- ment. At the elemental level, we report significant reductions in subcortical magnetic susceptibility which is regarded as surrogate index for iron content. Reductions were specific to subcortical nuclei key in coordinating mechanisms of motor and non-motor habit formation, and were mirrored by decreases in serum ferritin levels. Importantly, significant associations were observed between striatal susceptibility and glutamatergic neurotransmission as indexed by the glutamine:glutamate ratio. Clinically, treatment with aripiprazole led to significant reductions of tic severity in the patient sample, and additionally led to an approximate 50% reduction in OCD diagnosis. Our results indi- cate that patients with GTS exhibit an abnormality in the flux of metabolites in the GABA-glutamate-glutamine cycle, thus implying perturbations in astrocytic-neuronal coupling systems that maintain the subtle balance between excitatory and inhibitory neurotransmission within subcortical nuclei. These abnormalities may be driven or fur- ther compounded by the observed abnormalities in iron metabolism. Chronic pertur- bations in the subcortical GABA-glutamate-glutamine cycle flux could lead to spatially focalized alterations in excitatory, inhibitory and modulatory subcortical neurochemical ratios that would have a profound influence on the neuroplastic mechanisms involved
ii
in reinforcement learning and habit formation systems, which are governed by striatal neurons that code the serial order of syntactic natural behaviour. This work sheds a new light on the neurobiological basis of GTS and provides novel clues that may prove critical in the future development of functionally selective pharmacological modulators that target multiple neurochemical systems.
Zusammenfassung
Elemental and neurochemical based analysis of the pathophysiological mechanisms of Gilles de le Tourette syndrome.
Ahmad Seif Kanaan
Das Gilles de la Tourette-Syndrom (GTS) ist eine neuropsychiatrische Entwicklungsstör- ung, die durch das Bestehen von Tics und psychiatrischen Komorbiditäten gekennze- ichnet ist. Die heute zur Verfügung stehenden Behandlungsmöglichkeiten sind häufig unzureichend wirksam und führen oft zu klinisch relevanten Nebenwirkungen. Die Er- forschung der Ursachen des GTS könnte daher auch die Entwicklung neuer Behand- lungsmöglichkeiten mit sich bringen. Die Mehrzahl der vorliegenden Studienergebnisse weist auf eine dem GTS zugrunde liegende Störung im dopaminergen und GABAer- gen Neurotransmitter-System hin. Wegen der engen Wechselwirkungen zwischen dem dopaminergen und GABAergen System einerseits und dem glutamatergen System an- dererseits, haben wir die Vermutung aufgestellt, dass möglicherweise auch das gluta- materge System in die Pathophysiologie des GTS involviert sein könnte. Vor dem Hin- tergrund, dass das Spurenelement Eisen an zahlreichen biologischen Prozessen beteiligt ist inklusive der Synthese und Funktion zahlreicher Neurotransmitter, haben wir darüber hinaus die Hypothese aufgestellt, dass bei Patienten mit GTS eine Störung im Eisen- stoffwechsel vorliegen könnte. Mit Hilfe des Einsatzes multiparametrischer quantitativer Magnet-Resonanz-Tomographie-Techniken haben wir erstmals regionale Konzentratio- nen von Glutamat (mittels 1H-Kernspinresonanzspektroskopie, MRS) und Eisen (mit- tels quantitativer Suszeptibilitätskartierung, QSM) bei Patienten mit GTS gemessen. Im Rahmen unserer Studie wurden bei einer großen Gruppe erwachsener Patienten mit GTS sowie einer alters- und geschlechts-gematchten Gruppe gesunder Kontrollpersonen um- fangreiche klinische und bildgebende Daten erhoben. Um zusätzlich den Einfluss einer medikamentösen Behandlung sowohl auf neurochemische als auch auf klinische Parameter untersuchen zu können, haben wir eine Langzeitstudie durchgeführt und eine Subgruppe der Patienten vor und nach einer Behandlung mit dem etablierten Antipsychotikum Arip- iprazol untersucht. Die wesentlichen Ergebnisse diese Studie sind: (a)bei Patienten mit GTS findet sich im Vergleich zu gesunden Kontrollen eine mittels MRS nachgewiesene sig- nifikante Reduktion glutamaterger Marker im Striatum und Thalamus. Diese korreliert mit der Schwere der Tics und normalisiert sich durch eine Behandlung mit Aripipra- zol;(b) mittels QSM ist eine signifikante Verminderung der subkortikalen Suszeptibilität nachweisbar, welche als Index für den Eisengehalt gewertet wird. Interessanterweise fan- den sich diese Verminderungen besonders in jenen Hirnarealen, die als Schlüsselgebiete für die motorische Koordination und Bildung bewegungsunabhängiger Gewohnheiten angesehen werden. Parallel konnte im Serum eine Verminderung der Ferritin-Spiegel nachgewiesen werden; (c) schließlich konnten wir einen Zusammenhang zwischen der im Striatum gemessenen Suszeptibilität und der glutamatergen Neurotransmission fest- stellen; (d) die Behandlung mit Aripiprazol führte zu einer signifikanten Verminderung der Tics und Zwänge. Unsere Ergebnisse belegen, dass bei Patienten mit GTS Verän- derungen im Zusammenspiel der Transmitter GABA, Glutamin und Glutamat bestehen.
Dies wiederum deutet auf eine Störung im Zusammenspiel zwischen Astrozyten und Neu- ronen hin, welches für die feine Balance im Zusammenwirken zwischen inhibitorischen und exzitatorischen Neurotransmittern in subkortikalen Kerngebieten entscheidend ist.
Es kann spekuliert werden, dass diese Veränderungen auf eine Störung im Eisenstoffwech- sel in der frühen Hirnentwicklung zurückzuführen sind. Die gefundenen Veränderungen
iv
sowohl im glutamatergen Transmitter-System als auch im Eisenstoffwechsel erlauben neue Einblicke in die Pathogenese des GTS und eröffnen möglicherweise zukünftig neue Behandlungsmöglichkeiten dieser komplexen Erkrankung.
Declaration of Authorship
I, Ahmad Seif Kanaan, declare that this thesis titled: "Elemental and neurochemical based analysis of the pathophysiological mechanisms of Gilles de la Tourette syndrome"
and the work presented in it, unless otherwise stated, are my own. I confirm that:
This work was done wholly or mainly while in candidature for a research degree at this University.
Where any part of this thesis has previously been submitted for a degree or any other qualification at this University or any other institution, this has been clearly stated.
Where I have consulted the published work of others, this is always clearly at- tributed.
Where I have quoted from the work of others, the source is always given. With the exception of such quotations, this thesis is entirely my own work.
I have acknowledged all main sources of help.
Where the thesis is based on work done by myself jointly with others, I have made clear exactly what was done by others and what I have contributed myself.
Signed: Ahmad Seif Kanaan
Date: 06.06.2017
vi
Contents
Acknowledgements i
Abstract ii
Zusammenfassung iv
Declaration of Authorship vi
List of Figures xi
List of Tables xiii
Abbreviations xiv
Symbols xvii
I INTRODUCTION 2
1 Overview 3
2 Gilles de la Tourette Syndrome 6
2.1 A brief history . . . 6
2.2 Clinical Phenomenology . . . 9
2.3 Etiological Basis . . . 11
2.4 Pathophysiology: functional anatomy . . . 12
2.4.1 Structural neuroimaging studies. . . 12
2.4.2 Functional neuroimaging studies . . . 12
2.4.3 Key points gleaned from neuroimaging studies. . . 13
2.5 Pathophysiology: neurochemical aspects . . . 15
2.5.1 Mico-Circuitry Of the Basal Ganglia . . . 15
2.5.2 Dopamine . . . 17
2.5.3 GABA . . . 20
2.5.4 Glutamate. . . 21
2.5.5 Other neurotranmistters . . . 22
2.6 Pathophysiology: elemental aspects . . . 23
2.7 Treatment . . . 24
3 Objectives 26 3.1 Primary objectives . . . 26
3.2 Study Design . . . 27
3.3 Data acquisition . . . 28
3.4 A note on treatment with aripiprazole . . . 29
3.5 Ethics . . . 30
vii
Table of Contents viii
II METHODS 31
4 Magnetic Resonance Imaging and Spectroscopy 32
4.1 Nuclear Magnetic Resonance Imaging. . . 32
4.1.1 Principles of Nuclear Magnetic Resonance Imaging . . . 32
4.1.2 NMR properties of biological tissue environments . . . 34
4.1.3 NMR signal localization . . . 34
4.1.4 Pulse sequence design . . . 36
4.1.5 Image reconstruction . . . 36
4.1.6 Encoding physio-chemical properties in images . . . 38
4.2 Magnetic Resonance Spectroscopy . . . 40
4.2.1 Principles of in-vivo NMR Spectroscopy . . . 40
4.2.2 Acquisition of1H-MRS spectra . . . 41
4.2.3 Time domain signal processing . . . 43
4.2.4 Spectral quantitation . . . 45
4.2.5 1H-MRS metabolites of the human brain . . . 48
4.3 Quantitative Susceptibility Mapping . . . 50
4.3.1 Magnetic Susceptibility . . . 50
4.3.2 Phase Imaging . . . 51
4.3.3 Susceptibility Weighted Imaging . . . 52
4.3.4 Quantitative Susceptibility Mapping . . . 53
4.3.5 Sources of magnetic susceptibility contrast . . . 56
5 Clinical Assessment 58 5.1 Tics, premonitory urges and quality of life . . . 58
5.2 Obsessive Compulsive Disorder . . . 59
5.3 Attention defecit-hyperactivity disorder . . . 59
5.4 Depression . . . 59
5.5 Anxiety . . . 59
5.6 GTS subgroup classification . . . 60
III Methodological Investigations 61 6 Absolute metabolite quantitation 62 6.1 Introduction . . . 62
6.2 Methods . . . 63
6.3 Results. . . 64
6.4 Discussion . . . 65
6.5 Conclusions . . . 67
7 Coil Combination 68 7.1 Introduction . . . 68
7.2 Methods . . . 69
7.3 Results. . . 70
7.4 Discussion . . . 71
7.5 Conclusions . . . 73
Table of Contents ix
IV Pathophysiological Investigations 74 8 Neurochemical Investigation of pathophysiology 75
8.1 Abstract . . . 75
8.2 Introduction . . . 76
8.3 Materials and Methods . . . 79
8.3.1 Population Sampling . . . 79
8.3.2 Magnetic Resonance Data Acquisition . . . 81
8.3.3 Absolute Metabolite Quantitation . . . 82
8.3.4 Statistical Analysis . . . 85
8.4 Results. . . 86
8.4.1 Demographic and clinical characteristics . . . 86
8.4.2 Test-Retest Reliability . . . 86
8.4.3 Degree of tic-urges and tic-suppression during MR data acquisition 88 8.4.4 Group Differences in Metabolite Concentrations. . . 89
8.4.5 Correlation of Metabolite Concentrations with Clinical Variables . 96 8.4.6 Comparison of Glutamate/Glutamine separation at 3T and 7T . . 97
8.4.7 Influence of head displacement on spectral measures . . . 99
8.4.7.1 Intra-group differences in head motion . . . 99
8.4.7.2 Inter-group differences in head motion . . . 100
8.4.7.3 Influence of head motion on voxel compartmentation and absolute metabolite quantitation . . . 103
8.5 Discussion . . . 105
8.5.1 Altered Glutamate-Glutamine Cycling in GTS . . . 105
8.5.2 The Role of Functionally Selective Modulators in the Adaptive Stabilization of Neurotransmitter Systems in GTS . . . 108
8.5.3 The Influence of Subcortical GABA-Glu-Gln Cycling Abnormali- ties on Dopaminergic Signalling and the Phenomenology of Tics . . 110
8.5.4 Methodological Limitations and Future Directions . . . 112
8.6 Conclusions . . . 113
9 Elemental investigation of pathophysiology 114 9.1 Abstract . . . 114
9.2 Introduction . . . 115
9.3 Materials and Methods . . . 118
9.3.1 Population Sampling . . . 118
9.3.2 Measurement of serum Ferritin . . . 118
9.3.3 Magnetic Resonance Imaging and Spectroscopy . . . 119
9.3.4 Quantitative Susceptibility Mapping . . . 119
9.3.5 Masking of Subcortical Matter Nuclei . . . 120
9.3.6 Quality Control . . . 121
9.3.7 Statistical Analysis . . . 122
9.4 Results. . . 124
9.4.1 Group differences in surrogate measures of iron . . . 124
9.4.2 Magnetic susceptibility correlations with ferritin, Gln:Gln and clin- ical measures . . . 127
9.5 Discussion . . . 130
9.5.1 Disturbed iron homeostasis in GTS . . . 130
9.5.2 Disruptions in iron homeostasis influence mechanisms of subcorti- cal neurochemical signaling . . . 131
Table of Contents x
9.5.3 Limitations and future directions . . . 133
10 Investigation of the influence of aripiprazole on clinical status 134 10.1 Abstract . . . 134
10.2 Introduction . . . 135
10.3 Methods . . . 137
10.3.1 Population Sampling . . . 137
10.3.2 Clinical Assessment . . . 137
10.3.3 Statistical Analysis . . . 137
10.4 Results. . . 138
10.4.1 Patient characteristics at baseline . . . 138
10.4.2 Patient characteristics during treatment with aripiprazole . . . 140
10.4.2.1 Tics and premonitory urges . . . 140
10.4.2.2 Psychiatric comorbidities and quality of life . . . 140
10.4.3 Comparison of clinical characteristics of patients electing for- and against-treatment. . . 142
10.4.4 Adverse Effects and continuation of treatment . . . 143
10.4.5 Serum levels of aripiprazole . . . 143
10.5 Discussion . . . 143
10.5.1 Efficacy of aripiprazole on tics and premonitory urges . . . 144
10.5.2 GTS subgroup classification . . . 145
10.5.3 Efficacy of aripiprazole on associated comorbid conditions . . . 145
10.5.4 Influence of aripiprazole on quality of life . . . 146
10.5.5 Adverse effects of aripiprazole . . . 147
10.5.6 Decision factors for treatment with aripiprazole . . . 147
10.5.7 Characteristics of the Sample and serum levels of aripiprazole . . 148
10.5.8 Limitations . . . 148
10.6 Conclusions . . . 149
V CONCLUSIONS 150 11 Key findings and significance 151 11.1 Pathological glutamatergic neurotransmission in GTS. . . 152
11.2 Subcortical iron reductions associated with glutamatergic neurotransmis- sion in GTS . . . 153
11.3 Aripiprazole improves associated comorbid conditions in addition to tics in GTS . . . 154
11.4 Significance . . . 156
VI Bibliography 157
VII APPENDIX 184
A Related publications 185
B Contribution of authours 188
C Research compliance certificates 190
D MR Imaging sequence parameters 197
E Author portfolio 213
List of Figures
2.1 Une leçon clinique á la Salpêtriére. . . 7
2.2 Georges Gilles de la Tourette . . . 8
2.3 Clinical manifestations of GTS . . . 10
2.4 Major cortico-striato-thalamo-cortical circuits in the human brain . . . 17
2.5 Selective dysfunction of basal ganglia subterritories . . . 18
2.6 Interactions between the major neurochemical systems in the cortico- striato-thalamo-cortical pathway. . . 19
3.1 Illustration of the longitudinal study design of the project. . . 28
4.1 Basic Physics of the NMR signal . . . 33
4.2 Selective excitation of an image slice by applying a shaped RF pulse and field gradient at the same time. . . 35
4.3 Conventional MRI pulse sequence diagrams . . . 37
4.4 MR image reconstruction from k-space . . . 37
4.5 The relationship between TR/TE and the encoding of physio-chemical tissue properties as image contrasts. . . 39
4.6 Chemical shift properties of N-Acetylaspartate . . . 41
4.7 The effect of echo time on metabolite detectability . . . 43
4.8 1H-MRS time domain signal processing. . . 44
4.9 Referencing methods for absolute metabolite quantitation . . . 46
4.10 The linear combination basis spectrum fitting model . . . 47
4.11 GABA, Glutamate, Glutamine cycling . . . 49
4.12 Magnetic Susceptibility of diamagnetic and paramagnetic material . . . . 50
4.13 Tissue phase imaging . . . 52
4.14 Susceptibility-Weighted Imaging . . . 53
4.15 Quantitative Susceptibility Mapping Image processing . . . 55
4.16 Correspondence between Perls’ stain and QSM in revealing iron deposition in the deep grey matter nuclei . . . 57
6.1 Test-retest reliability of commonly used tissue segmentation methods . . . 66
6.2 Test-retest reliability of 1H-MRS absolute metabolite quantitation . . . . 67
7.1 Tissue phase estimation using adaptive coil combination and ESPIRiT- SVD from multichannel data. . . 71
7.2 QSM differences following multi-channel coil combination with the adap- tive and ESPIRiT-SVD algorithms . . . 72
8.1 Voxel localization and spectral data pre-processing . . . 83
8.2 Spatial overlap of test-retest voxel localization . . . 88
8.3 Spectral localization, fitting and statistical analysis . . . 90
xi
List of Figures xii
8.4 Representative cingular 1H-MRS spectra of the frequency and phase-drift
corrected data. . . 93
8.5 Representative thalamic1H-MRS spectra of the frequency and phase-drift corrected data. . . 94
8.6 Representative striatal 1H-MRS spectra of the frequency and phase-drift corrected data. . . 95
8.7 Correlation between absolute metabolite concentrations and clinical mea- sures . . . 96
8.8 1H spectra achieved at 3T and 7T to inspect Glutamate/Glutamine sep- aration. . . 98
8.9 Consistency of head motion across scanning sessions . . . 100
8.10 The effect of subject movement on within voxel tissue content and metabo- lite concentration. . . 104
8.11 LCModel individual metabolite fitting . . . 106
8.12 Astrocytic-neuronal coupling and the homeostasis of glutamatergic and GABAergic neurotransmission . . . 109
8.13 Local circuit model of subcortical connectivity . . . 111
9.1 Processing and analysis framework utilized to obtain high-quality quanti- tative susceptibility maps and 1H-MR spectra. . . 120
9.2 QQ plots of the multivariate outlier detection technique implemented via squared Mahalanobis distance. . . 123
9.3 QSM Data quality. . . 125
9.4 Nucleus segmentation quality and group comparison statistics.. . . 126
9.5 Ferritin group differences and correlations with susceptibility. . . 128
9.6 Correlational analysis between susceptibility and Gln:Glu . . . 129
10.1 Distribution of YGTSS-TTS scores at baseline and followup in the differ- ent subgroups . . . 140
10.2 Psychiatric comorbidity subclassification at baseline and following treat- ment with aripiprazole . . . 141
10.3 Prevalence of comorbidities in patients that elected for- and against-treatment with aripiprazole. . . 142
10.4 Adverse effects reported by the patients following the administration of aripiprazole. . . 143
10.5 Prevalence of comorbidities in patients that elected for- and against-treatment with aripiprazole. . . 144
List of Tables
6.1 Test-retest tissue fraction estimates of SPM, FSL and Freesurfer (FSU) . . 65 7.1 Statistical comparison between magnetic susceptibility values achieved
with the adaptive and ESPIRiT-SVD coil combination methods . . . 71 8.1 Demographic and clinical characteristics of the 1H-HMRS study sample
included in the final analysis. . . 80 8.2 Water T1 and T2 relaxation times and relative water content (α) in GM,
WM and CSF . . . 85 8.3 Test-Retest Reliability of absolute metabolite quantitation . . . 87 8.4 Control vs. GTS group comparison of absolute metabolite concentrations 91 8.5 GTS Off- and On-treatment group comparison of absolute metabolite con-
centrations . . . 92 8.6 Influence of frequency/phase drift correction on spectral measures. . . 102 8.7 Comparison of frequency/phase corrected spectral data between high and
low motion control groups . . . 102 9.1 Statistical comparisons of magnitude image data quality metrics. . . 122 9.2 Demographic and clinical characteristics of the QSM study sample in-
cluded in the final analysis . . . 124 9.3 Statistical comparison of magnetic susceptibility within general regions of
interest . . . 127 9.4 Statistical comparison of magnetic susceptibility within distinct subcorti-
cal nuclei . . . 127 10.1 Subgroup classification of the whole study sample based on comorbidities 138 10.2 Clinical characteristics of the whole study sample at baseline and following
treatment with aripirazole . . . 139
xiii
Abbreviations
3D three-dimensional
AAH auto-align head
ADHD attention deficit hyperactivity disorder aMCC anterior mid-cingulate cortex
ANTS advanced normalization tools
BAI beck anxiety inventory
BDI-II beck depression inventory II CAARS conners’ adult ADHD rating scales
COV coefficient of variation
Cho choline compounds
Cre (phospoho)creatine
CRLB cramér-rao lower bound
CSF cerebro- spinal fluid
CSTC cortico-striato-thalamo-cortical D1 dopamine type-1 like receptor D2 dopamine type-2 like receptor
DOF degrees of freedom
DOPA dihydroxyphenylalanine
EFC entropy focus criterion
EPI echo planar imaging
ESPIRiT eigenvalue approach to autocalibrating parallel MRI FASTESTMAP fast, noniterative shimming of spatially localized signals
FD framewise displacement
FID free induction decay
FLASH fast low-angle shot
xiv
Abbreviations xv
FWHM full width at half maximum
GABA γ-aminobutyric acid
Gln glutamine
Glu glutamate
Glx glutamate plus glutamine
GM grey matter
GPe globus pallidus external segment GPi globus pallidus internal segment GTS gilles de la tourette syndrome
GRE gradient echo
MADRS montgomery-äsberg depression rating scale m-Ins myo-inositol
MP2RAGE magnetization-prepared 2 rapid gradient echo
MR magnetic resonance
MRS magnetic resonance spectroscopy
MSN medium-sized spiny neuron
NAA n-acetylaspartate
NMDA n-methyl-D-aspartate
NMR nuclear magnetic resonance OCB obsessive- compulsive behavior OCD obsessive-compulsive disorder
OCI-R obsessive-compulsive inventory, revised PET positron emission tomography
PRESS point-resolved spectroscopy PUTS premonitory urge for tics scale RCT randomized controlled trial
ROI region of interest
rs-fMRI resting-state functional magnetic resonance imaging RVTRS rush video-based tic rating scale
SD standard deviation
SHARP sophisticated harmonic artifact reduction on phase data
SMA supplementary motor area
SNc substantia nigra pars compacta
Abbreviations xvi
SNr substantia nigra pars reticulata
SNR signal-to-noise ratio
STEAM stimulated echo acquisition mode SVD singular value decomposition SVS single voxel spectroscopy TCA tricarboxylic acid cycle TKD thresholded k-space division
WM white matter
WURS-k wender Utah rating scale
Y-BOCS yale-brown obsessive compulsive scale YGTSS yale global tic severity scale
YGTSS-GS ygtss global score YGTSS-TTS ygtss total tic score
QI1 quality index 1
QOL quality of life scale
QSM quantiative susceptibility mapping
Symbols
B0 static magnetic field B1 applied magnetic field cm metabolite concentration
c0w water concentration in bulk water
dx,dy,dz translational displacement from the center along the x-, y-, andz-axis fε relative volume fraction of tissue type εwithin a voxel
Im metabolite signal intensity Iw water signal intensity
Nm number of protons in the molecule contributing to the metabolite signal P error probability
Rε relaxation factor of tissue type ε r correlation coefficient
S signal
Sref reference signal
T1 longitudinal relaxation time T2 transverse relaxation time Te echo time
Tr repetition time
α,β,γ rotation angles about the x-,y-, andz-axis aε relative water content in tissue type ε
ξ scaling factor accounting for partial voluming
ϕ phase
ω larmour frequency χ magnetic susceptibility
xvii
Symbols 1
Part I
INTRODUCTION
2
Chapter 1
Overview
Gilles de la Tourette syndrome (GTS) is a multifaceted and anomalous disorder that wavers along the fine margin between neurology and psychiatry with an enshrouding set of symptoms. The pathophysiology of GTS remains enigmatic, as tics — the hallmark feature of GTS — are usually joined by related symptoms such as echo- and copro- phenomena, as well as comorbid conditions that include attention deficit/hyperactivity disorder (ADHD), obsessive-compulsive disorder (OCD) and depression. Although a range of pharmacological, behavioral and surgical therapeutic approaches are currently being used to manage the symptomatology of GTS, there is currently no cure for the manifesting motor and non-motor features. This is largely due to the lack of a compre- hensive pathophysiological model of GTS, which may be attributable to the presence of a large number of inconsistent findings and the dearth of studies utilizing multi-parametric approaches.
Considering the complex phenomenology of GTS, genetic analyses have not revealed a precise abnormality exhibiting Mendelian inheritance, but have rather pointed to a more-complex polygenic pattern of inheritance that leads to variabilities in a number of systems. Observations from (a) pharmacological investigations, (b) histopathological analyses of post mortem specimens, (c) neurophysiological investigations, (d) surgical and pharmacological animal model studies, and (e) several morphometric, functional and nuclear imaging studies have provided support for the pathological involvement of the basal ganglia and cortico-striatal-thalamo-cortical (CSTC) pathways.
At the neurochemical level, abnormalities in dopaminergic neurotransmission are widely considered as a primary abnormality in GTS. However, in view of the strict spatio- temporal synergy exhibited between excitatory, inhibitory and modulatory neurotrans- mitter systems that drive typical motor and non-motor behaviour, several groups have posited that other neurochemical systems may exhibit perturbations as well. Along this
3
Overview 4
line of reasoning, more recent studies have implicated other neurotransmitter systems that most prominently include the γ-Aminobutyric acid (GABA) system. Although GABA exhibits both spatio-temporal as well close metabolic links to glutamate, inves- tigations on the role of the glutamatergic system in GTS have not yet been undertaken in vivo.
At the elemental level, one unifying feature exhibited by the dopaminergic, GABAer- gic and glutamatergic neurotransmitter systems is that the enzymes involved in their metabolism and the production of their receptors/transporters require iron for typical function. Along this line, preliminary work investigating serum ferritin concentrations have indicated that iron may be related to GTS pathophysiology. Nevertheless, in vivo investigations of the role of iron in GTS pathophysiology are currently lacking.
With recent advances in quantitative Magnetic Resonance Imaging (MRI) techniques, a unique perspective that allows the in vivo measurement of biochemical and physio- chemical tissue properties has been afforded, thus paving the road for undertaking more incisive investigations of disease specific changes at multiple-scales. Proton Magnetic Resonance Spectroscopy (1H-MRS) and Quantitative Susceptibility Mapping (QSM) are two such approaches that allow the measurement of steady-state metabolite quantities and the intrinsic physical property of magnetism in matter. Using 1H-MRS, for exam- ple, several neurochemicals could be quantified, including surrogate measures related to glutamatergic signalling. On the other hand, QSM is a novel contrast mechanism that provides surrogate quantitative measures of specific biomarkers such as iron.
Consequently,the overarching goal of this thesis is to investigate GTS pathophys- iology using multi-parametric, quantitative MRI approaches at multiple-scales, in order to offer novel perspectives of pathophysiology that could potentially pave the road for the design of new therapeutic approaches. In this regard, the secondary aim of this work was to investigate the influence of an established pharmacological agent (aripirazole) on neurochemistry and clinical status in GTS.
This thesis is composed of 5 separate Parts that are organized into 11 Chapters as follows:
• Part I: INTRODUCTION: The state of the art of the clinical phenomenology and pathophysiology of GTS are first introduced (Chapter 2) in order to qualify the hypotheses and the objectives of this work (Chapter 3).
• Part II: METHODS: A succinct overview of the utilized imaging and clinical methods is presented in Chapter 4 andChapter 5, respectively.
Overview 5
• Part III: METHODOLOGICAL INVESTIGATIONS: Two methodological investigations were initially conducted to improve the accuracy of the performed magnetic resonance measurements. These included:
– Chapter 6: Investigation of the test-retest reliability of 1H-MRS absolute metabolite quantitation with partial volume correction. The results of this investigation led to improvements in the accuracy of absolute metabolite quantiation and served as a benchmark for the neurochemical investigation of pathophysiology presented in Chapter 8.
– Chapter 7: Investigation of the influence of different coil-combination al- gorithms on QSM. This work led to conclusion that vendor provided phase data should not be used for QSM reconstruction since they contain an arti- fact that may bias group comparisons. Therefore, this work led to significant enhancements that permitted the elemental investigation of pathophysiology presented in Chapter 9.
• Part IV: PATHOPHYSIOLOGICAL INVESTIGATIONS: The main ob- jectives of this work are presented as three separate investigations of GTS patho- physiology and are organized as follows:
– Chapter 8: Investigation of the role of the glutamatergic system in the pathophysiology of GTS using single voxel 1H-MRS.
– Chapter 9: Investigation of the role of iron in the pathophysiology of GTS using QSM and serum ferritin analysis as surrogate measures of iron.
– Chapter 10: Investigation of the influence of treatment with aripiprazole on the motor and non-motor features manifested in GTS.
• Part V: KEY FINDINGS AND SIGNIFICANCE: A summary of all the key findings and a discussion of their significance is presented in the concluding chapter (Chapter 11).
Chapter 2
Gilles de la Tourette Syndrome
This chapter provides an overview of Gilles de la Tourette syndrome pathophysiology with a specific focus on neurochemical aspects.
2.1 A brief history
The 19th century was a remarkable period in the history of neuroscience that saw the characterization of novel disorders of the brain and the clear documentation of previously recognized ones. Much of this is owed to the work of Jean-Martin Charcot (1825–
1893) who spent 33 years studying the nervous system and teaching students at the Pitié-Salpêtriére Hospital in Paris. His reputation attracted many bright students who, themselves, later become pioneers in various fields. This list contains names that include Sigmund Freud, Joseph Babinski, Pierre Janet and George Gilles de la Tourette, among others (Figure 2.1).
Under his mentorship, Gilles de la Tourette (1857–1904) began studying various neu- rological disorders before focusing on "obscure" movement disorders. At the age of 28, Gilles de la Tourette published a landmark article [1] about a bizarre condition that exhibited stereotyped movements, phonic symptoms, premonitory sensations, echo- and copro-phenomena, which he referred to as that "maladie de tics" (Figure 2.2). Owing to Tourette’s pioneering work in which he documented this condition as a distinct neuro- logical disorder, Charcot bestowed the eponym "Gilles de la Tourette syndrome" (GTS) in his honor. Interestingly, there is some discourse in the current academic arena of who should be the true bearer of the eponym. Twelve years before his 1885 publication, descriptions of symptoms similar to that of Tourette’s were outlined in a monograph by Armand Trousseau (1801–1867) [2]. The true nature of events and reason why Charcot
6
Gilles de la Tourette Syndrome 8
Figure 2.2: Georges Gilles de la Tourette. Georges Albert Édouard Brutus Gilles de la Tourette (30 October 1857 – 26 May 1904) was a French physician and the namesake of Gilles de la Tourette syndrome. He began his medical studies at Poitiers in 1873 before relocating to Paris where he became a student of the influential neurologist Jean-Martin Charcot, the director of the Salpêtriére Hospital. Tourette studied and lectured in psychotherapy, hysteria and hypnosis, before describing the symptoms of what he called the "maladie des tics" in his work "Étude sur une affection nerveuse caractérisé e par de l’incoordination motrice accompagnée d’ècholalie et de coprolalie"
(https://en.wikipedia.org/wiki/Georges_Gilles_de_la_Tourette).
instrumental in promoting information and increasing the public discourse about GTS in the following decades.
Nevertheless, funding the study of GTS remained scarce, as it was understood to be a rare disorder with very low prevalence rates. Largely owing to the significant public response towards several published articles highlighting GTS, the 1980s saw a marked increase in research funding and the inclusion of the disorder in the third edition of the Diagnostic and Statistical Manual of Mental Disorders. Notwithstanding the large number of pub- lished works investigating GTS over the past three decades, current funding opporunities and scientific output are still overshadowed by other disorders such as Parkinson’s disease and Autism. In the year 2000, a pan European Society for the study of GTS (ESSTS) was established by Marie Robertson and Anne Korsgaard with the aims of increasing collaborative research and public awareness across Europe. The establishment of ESSTS was a significant step forward towards securing new funding opportunities and establish- ing new European guidelines for the(a)clinical characterization [5],(b)pharmacological
Gilles de la Tourette Syndrome 9
treatment [6], (c) behavioral treatment [7], and surgical treatment [8] of patients with GTS and other tic disorders. As a result of the endeavors of many scientists and physi- cians working within such societies, we now have a much better understanding of the pathophysiology of GTS and have much better ways of treating it. Current knowledge of clinical, etiological, pathological and interventional aspects of GTS are summarized in the sections below.
2.2 Clinical Phenomenology
In popular culture, Gilles de la Tourette syndrome (GTS) is frequently regarded as a dis- order in which individuals utter obscene, socially inappropriate or derogatory statements without any volitional control. This is partly true as only a minority of patients suffering from GTS exhibit this involuntary swearing condition known as coprolalia. In essence, GTS is a multifaceted disorder that exhibits both motor and non-motor clinical features.
Specifically, GTS is a childhood-onset, neuropsychiatric movement disorder with rela- tively high heritability and prevalence rates [9] estimated at about 1% of the general population, thus rendering it as one of the most common movement disorders [10]. Tics are the cardinal features of GTS and are defined as rapid, non-rhythmic, stereotyped in- voluntary movements or utterances that are misplaced in context and time [9,11]. Tics usually follow a waxing and waning course of severity and usually improve or go into complete remission in adulthood [12,13]. Most tics are preceded by unpleasant sensory sensations (premonitory urges) which are relieved by executing the tic [14]. Psychiatric comorbidity in GTS is very common with lifetime prevalence rates of any psychiatric co- morbidity estimated at 86% [15,16]. These comorbid conditions most commonly include attention deficit hyperactivity disorder (ADHD) (≈50%), obsessive compulsive disorder (OCD) (20-60%), depression and anxiety, leading many to suggest an overlap in their pathophysiological mechanisms [17,18] (Figure2.3).
In the clinical setting, GTS is diagnosed if an individual exhibits: (a)multiple motor tics and one or more vocal tics; (b) a waxing and waning course of tic intensity, frequency and severity; (c)the persistence of tics for at least one year following first onset; (d) tic onset before the age of 18; and (e) the presence of tics that are not caused by another medical condition or substance (DSM-5, 307.23, ICD-10 F95.2) [19]. The typical age of onset of tics is between 6 to 8 years of age, where peak severity is usually reached between ages 10-14. While the majority of patients exhibit significant improvements over the course of their adolescent period, a small number of patients do not improve and in most of these cases, the patients take on a negative course of illness in which comorbid psychiatric conditions surface. In addition to the common conditions associated with
Gilles de la Tourette Syndrome 11
GTS (ADHD, OCD, depression, anxiety), other conditions that could surface include self-injurious behavior, impulsivity, rage attacks, sleep problems and learning disorders [18]. Such associated conditions often impair a patients’ qualify of life and are usually the reason why patients seek treatment.
Different therapeutic methods are currently being used to manage the symptomatol- ogy of GTS. These include: (a) behavioral therapy, (b) pharmacological treatment, (c) deep brain stimulation, and (d) alternative medicine [20]. Nevertheless, current treat- ment strategies are strictly palliative and are often ineffective and unsatisfactory. Conse- quently, a better understanding of the pathophysiological mechanisms of GTS could pave a new road for designing better treatments. Currently, there is no generally accepted etiological or pathophysiological model of GTS. In general, current data suggests that GTS has a complex genetic background where it is likely caused by genetic susceptibility factors that interact with the environment to confer the total risk of acquiring a complex phenotype [21].
2.3 Etiological Basis
The etiology of GTS is still not well characterized though much research is currently being undertaken in this area. Research into the genetic basis of GTS has been marked by difficulties in the replication of original findings, largely due to the low power of individual studies. Nevertheless, the heredity of GTS is undisputed in current literature [22, 23].
Akin to other complex psychiatric disorders exhibiting a diverse symptomatology, current data indicates that GTS is not caused by single gene effects that exhibit classic Mendelian patterns of inheritance, but exhibits a complex etiological basis likely caused by the complex interaction of multiple genetic variants with environmental factors. The complex interplay of multiple genetic variants is believed to be a primary contributor to the diverse phenotypes exhibited in GTS [22,23], though an increasing body of evidence has provided support for a role of environmental factors in the onset and natural course of the disease (e.g. smoking during pregnancy, delivery complications etc.) [24,25]. Nevertheless, there remains much debate on the role of environmental influences in GTS. For example, there has been much discourse and speculation on the role of post-streptococcal infection in tic onset [24,26,27].
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2.4 Pathophysiology: functional anatomy
On a systems level, major strides have been achieved in the field over the past two decades, though a generally accepted pathophysiological model of GTS remains elusive.
Although evidence from structural and functional Magnetic Resonance Imaging (MRI) studies is varied, these studies have repeatedly highlighted abnormalities within the lim- bic, associative and motor cortico-striatal-thalamo-cortical (CSTC) networks [28]. A summary of structural and functional neuroimaging studies is highlighted in the follow- ing subsections.
2.4.1 Structural neuroimaging studies
Despite the abundance of structural MRI studies, there are many inconsistencies in the literature. The diversity of results can be explained by variabilities in sample size, patient characteristics (e.g. age, presence of comorbidities, neuroleptic use), imaging methods (e.g. region-of-interest based vs. whole brain), and statistical analysis methods [29,30].
Nonetheless, there are some consistencies in structural investigations of GTS patients and these include: (a) a reduction in the volume of the caudate in both children and adults [31–34], and (b) cortical thinning in the sensorimotor, prefrontal and cingulate cortices [32, 35–37]. It is of note to mention that the specificity of cortical thinning seems to be dependent on the phenotype. For example, Worbe et al. [37] showed that patients with simple tics (cortical thinning in primary sensory and motor cortices) exhibit a different structural profile relative to patients with complex tics (cortical thinning in premotor, prefrontal and associative areas). Overall, structural MRI indicate that GTS is associated with dysfunction in associative, limbic and motor CTSC regions [28,29,38].
2.4.2 Functional neuroimaging studies
Evidence from task based functional MRI (fMRI) studies on the involvement of specific regions in the pathophysiology of GTS is relatively scarce and ambiguous. Task fMRI studies can be classified into four groups; investigations of motor behaviour, tic gener- ation, tic suppression and executive function. Groups investigating mechanisms of tic generation and control in GTS have compared (a) simple vs. complex tics [39]; (b) tic imitation vs. real tics [40]; (c) periods of tic suppression vs. periods of free ’ticking’
[41]; (d) pre-monitory urge periods and tic generation periods [42]; (e) suppression of eye blink in healthy controls vs. GTS patients [43]. Although these studies show that an extensive network of premotor, primary motor and sensorimotor networks are involved
Gilles de la Tourette Syndrome 13
in tic generation, a critical distillation of the studies only ties the Supplementary Mo- tor Area (SMA) to tic generation [29]. A few other groups have aimed at investigating motor performance in GTS patients. These studies are limited and conflicting, as some groups showed over activations in the SMA and the sensorimotor cortex during a finger tapping task [6,44], while others show no group differences for the same task [45]. There is much debate on whether executive function is impaired in GTS. For example, (a) some behavioral studies have demonstrated impairments in several domains of executive function (response inhibition, selective attention, cognitive flexibility) [46–48];(b)others have argued that such impairments are driven by comorbid conditions [49]; (c) while a third group of studies presented evidence of enhanced performance by GTS patients on cognitive tasks [50,51].
Moreover, studies that implemented fMRI paradigms designed to investigate response inhibition in GTS patients also exhibited inconsistencies. First, using a Stroop task to investigate the neural correlates of response inhibition, Marsh et al. [52] demonstrated that tic severity and the persistence of GTS symptoms may be due to disrupted matura- tion of fronto-striatal circuits involved in self-regulatory control, since prefrontal activity correlated with tic severity and there was no age related change in frontostriatal cir- cuitry as exhibited by the control group. Second, also using a Stroop task in addition to a go/no-go task, Debes et al. [45] demonstrated that there were no differences between healthy controls and patients, however, activity in the posterior cingulate and superior temporal gyrus correlated with OCD symptoms. This suggests that response inhibition symptoms may be due to comorbidity as argued by Ozonoff [49] and [53]. Third, similar to Debes [45] using a go/no-go task, Hershey et al. [54] also failed to find any group difference between controls and patients. Fourth, using the Simon task, Raz et.al [55]
showed that increased activation in frontostriatal circuitry was associated with increased task related frontostriatal activity associated with better task performance. However, in contrast to the results of Marsh [52], Raz et al. [55] showed that frontostriatal activity correlates with age and tic severity. In conclusion, fMRI studies on response inhibition are highly inconsistent and further investigation of the neural correlates of GTS patients is warranted.
2.4.3 Key points gleaned from neuroimaging studies
Despite the substantial potential benefit afforded by structural and functional neuroimag- ing methods to further our understanding of the pathophysiological mechanisms under- lying the GTS spectrum, a series of recently published papers have indicated that the quality of neuroimaging data is key in performing unbiased group comparisons [56–64].
One crucial factor that must be considered for the faithful evaluation of brain imaging
Gilles de la Tourette Syndrome 14
data is related to motion artifacts that ensue as a result of displacements in head position within the head-coil during MRI data acquisition. As patients with GTS are essentially characterized by movement, the disruption of the acquired MRI signal leading to mo- tion contaminated data is inevitable. The presence of motion artifacts in MRI data has been shown to: (a) lead to spurious correlations in estimates of functional connectivity [60, 65, 66] and (b) influence cortical thickness and grey-matter-volume morphometric estimates [56, 67]. While the inconsistencies in the published works investigating GTS pathophysiology may have been driven by variabilities in: (a) sample sizes, which range between N=10–60; (b) the clinical characteristics of the study samples; and (c) the status of psychoactive drug use, the inclusion of low-quality data may have also led to significant biases in the observed results.
Notwithstanding the aforementioned limitations which may have led to the inconsistent observations between different studies, these studies were crucial in implicating specific brain regions that exhibit altered structural or functional characteristics in GTS. Key points gleaned from structural and functional MRI studies investigating GTS pathophys- iology are summarized below:
• Both children and adult patients with GTS exhibit reductions in the volume of the caudate, which correlates negatively with tic-severity.
• Patients with simple tics exhibit cortical thinning in primary sensory and motor cortices, while patients with complex tics exhibit cortical thinning in premotor, prefrontal and associative areas.
• Results from fMRI studies of motor and cognitive control are generally inconsistent and may be biased by motion artifacts, small sample size and variabilities in the clinical characteristics of the study samples.
• Premonitory urges are associated with abnormalities in the supplementary motor area.
Gilles de la Tourette Syndrome 15
2.5 Pathophysiology: neurochemical aspects
A large body of imaging, spectroscopic and post-mortem studies investigating neuro- chemical aspects of GTS pathophysiology have been published to date. Given that ini- tial success of the dopaminergic antagonist haloperidol in reducing tic symptoms, these studies have mainly focused on the dopaminergic system. More recent works, however, have highlighted a role for other neurochemical systems in GTS. In the following sec- tion, an overview of neurochemical interactions in the basal ganglia in relation to GTS pathophysiology is presented.
2.5.1 Mico-Circuitry Of the Basal Ganglia
The basal ganglia are group of subcortical nuclei that act as a cohesive functional unit allowing the tight regulation of motor, cognitive and limbic functions. The main com- ponents of the basal ganglia are the Striatum (caudate and putamen), the Substantia Nigra (pars compacta — SNr and pars reticulata — SNr), the Subthalamic Nucleus (STN) and the Globus pallidus interna (GPi) and externa (GPe) (Figure2.4) [68]. With the exception of the excitatory glutamatergic (—) STN projections, all intrinsic and output projections of the basal ganglia areγ-Aminobutyric acid (GABA)ergic (—) and inhibitory. The striatum and the STN are the main input structures and they receive glutamatergic excitatory signals from the cerebral cortex, the brainstem, and the limbic system. On the other hand, the GPi/SNr complex is the main output system project- ing GABAergic inhibitory neurons onto the thalamic sub-nuclei which feed back to the cortex. The GPe and the SNc hold intrinsic functions and provide the striatum with important modulatory signals [69].
In current anatomical models of internal basal ganglia circuitry, two antagonistic striato- thalamic pathways exist. A direct and an indirect pathway are organized in a way that allows for the selection or inhibition of competing actions [70]. In this model, the striatum is linked to the GPi/SNr output complex via a monosynaptic direct pathway emanating from set of distinct GABAergic medium-sized spiny neurons (MSNs) within the striatum. In the direct pathway, excitatory cortical input to the striatum results in the release of inhibitory signals from the GPi/SNr to the thalamus. This releases the ’breaks’ off the thalamus allowing it to emit its excitatory signals that facilitate the execution of movement. The direct pathway can be summarized as follows:
Cortex—Striatum —SNr/GPi···· Thalamus —Cortex.
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The indirect pathway on the other hand, is polysynaptic where a different population of striatal MSNs delivers input to the GPi/SNr output complex via the GPe and its intercalated STN. In this case, cortical excitability of the striatum leads to a decrease in the tonic inhibition of the STN by the GPe. As such, the STN is free to release its glutamatergic excitatory signals to the output nuclei furthering the inhibition of the thalamus and leading to a net hypokinetic state. The indirect pathway can be summarized as follows:
Cortex — Striatum—GPe···· STN—SNr/GPi—Thalamus···· Cortex.
Overall, the indirect pathway leads to the inhibition of movement while the direct path- way leads to the facilitation of movement.
Macroscopically, early models of basal ganglia physiology have posited a "funneling and selection" function of its nuclei [71]. In the late 1980s, however, this model has been replaced by a segregated circuit model in which the basal ganglia are seen as components of five segregated and parallel re-entrant loops, over which information sent from specific cortical areas is processed topographically and is integrated within its internal circuitry [69,72]. These circuits fall into three major domains (motor, limbic and associative) and include the sensorimotor, oculomotor, dorsolateral prefrontal, lateral orbitofrontal, and anterior cingulate limbic loops [69,70,72] (Figure 2.4).
Disequilibrium in the natural mechanics of the inbound CSTC loops is a major factor in the emergence of both motor and non-motor features exhibited by various neuropsychi- atric and movement disorders [74,75]. Along this line, previous work has indicated that spatially focalized alterations in neurochemical ratios play a major role in the emergence of diverse motor, limbic and associative features (Figure2.5). Keeping in mind that GTS is not only centered on tics, where the majority of the patients exhibit a varied motor and behavioral symptomatology, early hypotheses have postulated that the expression of tics and its accompanying behavioral features is a result of atypical neurotransmis- sion within specific sub-territories of the basal ganglia, which consequentially lead to the aberrant integrative interplay of CSTC circuitry [76–78]. As the basal ganglia provide a mechanism for the selection of an action from a competing response [72,79], fundamen- tal alterations in the functional dynamics of CSTC circuitry could cause selection errors that are believed to lead to the expression of motor and non-motor features as exhibited in GTS [75,78].
Gilles de la Tourette Syndrome 18
Figure 2.5: Selective dysfunction of basal ganglia subterritories. Utilizing local injections of the GABAergic antagonist bicuculline, a causal link between the neurochemical activity of specific functional subterritories of the basal ganglia and clin- ical manifestations observed in movement and behavioral disorders were demonstrated [75,80,81]. The authors showed that the emergence of symptoms associated with GTS (e.g. tics, stereotypy, attentional deficits) is dependent on the spatial location of the
neurochemical alteration. Figure retrieved from [75].
Several lines of evidence have suggested a strong role for dopamine in GTS pathophysiol- ogy [31,89]. Dopaminergic dysfunction in GTS is supported by(a)initial clinical obser- vations of improvements in tic-like symptoms following the administration of dopamin- ergic antagonists (pimozide, fluphenazine, haloperidol, risperidone, aripiprazole), syn- thesis blockers (α-methyl-para-tyrosine) or monoamine depletion drugs (tetrabenazine);
and the exacerbation of symptoms following the administration of dopaminergic stimu- lants (L-DOPA, central stimulants) [20,90–94]; (b) results from varied nuclear imaging studies that show alterations in dopamine transporter and receptor function in stri- atal and extra-striatal regions [95–110]; (c) increased dopamine concentrations in cere- brospinal fluid [89, 111] and (d) altered dopamine levels as revealed by post-mortem studies [100, 110, 112]. Given this data, multiple dopaminergic hypotheses have thus been posited on the role of dopamine in GTS pathophysiology. These hypotheses include:
(a) dopaminergic hyper-innervation within the striatum; (b) a presynaptic abnormality in aromatic L-amino acid dihydroxyphenylalanine (DOPA) decarboxylase, which is in- volved in the catalysis of L-DOPA to dopamine; (c) super-sensitivity of postsynaptic
Gilles de la Tourette Syndrome 20
2.5.3 GABA
GABA is the primary inhibitory neurotransmitter in the human brain. There are two main types of GABAergic neurons that include (a) projections neurons and (b) in- terneurons [122]. Although the majority of GABAergic striatal neurons are of the spiny projection type (i.e. MSNs), striatal GABAergic interneurons produce a strong inhibitory influence over MSNs, regulating their output to the GPi and the GPe in the direct and indirect pathways, respectively [123].
Considering that GTS is in essence a disorder exhibiting pathological mechanisms of inhibitory control, various authors have posited that the primary inhibitory neurotrans- mitter GABA may have an important role in GTS pathophysiology [31]. For example, the burst-like disinhibition of thalamo-cortical projections which would ultimately facil- itate hyperkinetic behaviour (ie. tics), could be a result of alterations in GABAergic striatal projection neurons (ie. MSNs) or striatal as well cortical interneurons. Given that pallidal GABAergic neurons exhibit a potent influence over dopaminergic neurons of the substantia nigra, abnormalities in GABAergic pallidal projection could also have an important role in GTS pathophysiology.
Although preliminarypost mortem work investigating GABA levels in various brain re- gions failed to find any significant differences between patients and controls [124], more recent work has demonstrated that GABA does exhibit abnormalities in GTS. Utilizing unbiased immunocytochemical techniques, Kalanithiet al. [125] demonstrated profound alterations in the density and distribution of a specific type of GABAergic interneurons.
Specifically, higher densities of parvalbumin-positive GABAergic interneurons were found in the GPi, whereas lower densities were observed in the GPe, caudate and putamen.
The authors were able to replicate and extend their work in another study, in which they showed that different subtypes of interneurons also exhibit reductions [126]. In another study, Lerneret al. [127] utilized Positron Emission Tomography with injection of the ra- dioligand [11C]flumazenil to visualize the distribution ofGABAAreceptors. The authors demonstrated that patients with GTS exhibit widespread abnormalities in the GABAer- gic system, as they observed(a)reductions in the binding capacity ofGABAAreceptors in the ventral striatum, thalamus, amygdala and right insula, and (b) reductions in the substantia nigra, periaqueductal gray, posterior cingulate cortex and cerebellum. More recent work utilizing Magnetic Resonance Spectroscopy expanded on these findings by demonstrating GABAergic reductions in(a)striatal, cingulate and sensorimotor regions in pediatric sample [128,129], and(b) elevations in the supplementary motor area in an adult sample [130]. In general, this work provides strong support for an abnormality in GABAergic neurotransmission in GTS.
Gilles de la Tourette Syndrome 21
2.5.4 Glutamate
Given its wide distribution as the brain’s primary excitatory neurotransmitter and its essential role in the normal mechanics of CSTC circuitry, a pathophysiological role for glutamatergic neurotransmission in GTS has recently been postulated [131]. A role for glutamate in GTS pathophysiology can be indirectly drawn from the fact that (a) glu- tamate and GABA exhibit close metabolic links, in which the penta-carbon skeleton of glutamate acts a precursor for the synthesis of GABA and(b) glutamate and dopamine exhibit extensive interactions at different levels of cortico-striato-thalamo-cortical cir- cuitry [131] (Figure2.6). With respect to the interactions of glutamate and dopamine, it has been suggested that the co-transmission of both neurotransmitters is possible in cen- tral monoaminergic neurons such as in the ventral tegmental area (VTA) [132]. Second, mesocortical dopaminergic inputs from the VTA are able to directly and indirectly alter pyramidal neuron excitability in prefrontal cortical regions [133]. Third, descending cor- tical glutamatergic afferents, modulate dopaminergic VTA and SNc neurons which feed back to the striatal matrisomes and striosomes, respectively [134, 135]. Fourth, corti- cal glutamatergic afferents and dopaminergic SNc projections converge towards striatal MSNs, where dopamine is able to modulate glutamatergic neurotransmission depending on the type of receptor it targets.
On the other hand, direct evidence of glutamatergic abnormalities in GTS is drawn from pathophysiological and genomic studies. Pathophysiologically, early postmortem studies demonstrated a reduction in glutamate levels in the GPi, GPe, and SNr [124]. This evidence correlates with volumetric MRI analysis that indicated a reduction in the size of the left GP [136], although a direct link was never made. Moreover, two genetic studies have also highlighted a role for glutamate in GTS. One large multigenerational family genome scan identified a role for 5p13 - an area that overlaps with glial glutamate transporter gene1 [137]. Another genome scan using sibling pairs identified a missense variant in E219D, a highly conserved residue that results in increased glutamate uptake [138,139]. Additionally, inference can also be drawn from studies that investigated the role of glutamate in OCD since there is a high degree of pathophysiological overlap. Other genetic studies also implicated SLC1A1 gene (glutamate transporter) and GRIN2B gene (glutamate related gene expressing a subunit of the NMDA glutamate receptor) in OCD [140–142], which exhibits a phenomenological overlap with GTS pathophysiology.
In a Magnetic Resonance Spectroscopy study conducted on an adolescent patient sample, DeVitoet al. [143] did not find any differences in the concentration of glutamate in vari- ous brain regions. However, the authors demonstrated reduced levels of N-acetylaspartate and choline in the left putamen, and reduced creatine levels in the putamen bilaterally.
Nevertheless, this study had several limitations as (a) 50% of their patient group was
Gilles de la Tourette Syndrome 22
sedated using chloral hydrate and this was not accounted for; (b) the impact of comor- bidities and medication was not included in the analysis; (c) only a male population was investigated. In conclusion, the authors suggested that further studies are needed to replicate their results and to investigate the impact of comorbidity and the effect of psychotropic medication on metabolite levels.
In conclusion, though insufficient, some published work points to a role for glutamate in GTS. It is not known whether GTS patients exhibit a hyper- or a hypo-glutamatergic state. For example, tics can be induced either by (a) excess dopaminergic stimulation of the striatum or by (b)excess cortical glutamatergic input onto the striatum. In both of these scenarios, hyperkinetic effects could be induced [131]. Interrogating the role of glutamate in GTS will possibly have direct implications on novel interventional strategies [131].
2.5.5 Other neurotranmistters
Considering that typical motor and non-motor behaviour is driven by strict spatio- temporal interactions between various neurochemical systems, it is possible that neu- rotransmitter systems other than the primary excitatory, inhibitory and modulatory systems may be involved. Though preliminary, some work has indicated links between the cholinergic, serotonergic and histaminergic neurotransmitter systems and GTS. The strongest possible link is related to the cholinergic system. Initial immunocytochemical work indicated that patients with GTS exhibit alterations in the density and distribution of cholinergic interneurons [126], which may have drastic downstream consequences on the GABAergic projection and interneuron populations that they influence. This notion was demonstrated by the targeted ablation of striatal cholinergic neurons in a rat model, in which tic-like stereotypies were induced following acute stress of d-amphetamine chal- lenges [144].
Studies investigating the role of serotonin in GTS have revealed: (a)reductions of sero- tonin metabolite levels in the cerebrospinal fluid [111, 145, 146] and the basal ganglia [124] in some but not all patients;(b) normal metabolite levels in the cortex [112];(c) a decrease in postmortem density of 5HT-1A receptor levels in frontal and occipital areas [110]; (d) a reduction in the binding capacity of 5HT transporters in association with OCD [147]; (iv) a negative correlation between vocal tics and 5HT transporter binding potential in the midbrain and thalamic areas [148]. Other nuclear imaging studies in- vestigating serotonin receptor binding exhibited variables results, where some showed no change [108] to increases in multiple regions [149]. In summary, these findings indicate
