Investigations of the Pathophysiology of Gilles de la Tourette Syndrome. Part 2: 7T MRI

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Gilles de la Tourette Syndrome (GTS) is characterized by the presence of motor and vocal tics, which have been defined as rapid, habitual, burst-like movements or utterances that typically mimic fragments of normal behavior. Patients often report unpleasant premonitory urge sensations preceding tics that are relieved by their execution. Although the therapeutic spectrum for GTS has recently been expanding, current treatment strategies are often unsatisfactory, thus provoking the need for further elucidation of the underlying pathophysiology.

In current models of GTS pathophysiology, symptoms are thought to arise as a result of the inappropriate activation of specific clusters of striatal neurons, which lead to a burst-like disinhibition of thalamocortical output. The bulk of current literature suggests a dysregulated dopaminergic system. This is supported by clinical evidence of improvements in tics following the administration of dopamine antagonists, synthesis blockers or depletion drugs, and the exacerbation of symptoms following the administration of dopaminergic stimulants. Dopamine drives movement by activating a direct, net excitatory basal ganglia pathway involving the dopamine receptor D1 or an indirect, net inhibitory basal ganglia pathway involving the dopamine receptor D2. Currently, the vast majority of the antipsychotics used for the treatment of tics in GTS aim at the D2 receptor, with aripiprazole, risperidone and pimozide being selective D2 receptor antagonists and haloperidol being mainly a D2 receptor antagonist. However, recent randomized controlled trials further indicate promising results for the selective dopamine receptor D1 antagonist ecopipam.

Methodologically varied work has revealed that patients with GTS exhibit alterations in (i) D2 receptor density or binding, (ii) Dopamine Active Transporter (DAT) density/binding, and (iii) phasic dopamine transmission in striatal and cortical regions. A very small number of post-mortem examinations further suggest potential abnormalities in D1 (and D2 and DAT) receptor densities in cortical regions. While this would be in line with the therapeutic efficiency of selective D1 receptor antagonists, thorough experimental verification is missing. In particular, D1 receptors in GTS patients have not yet been investigated in vivo, suggesting a need for additional research.

Both postsynaptic and presynaptic mechanisms have been postulated to offer explanations of the above observations:

• Supersensitive postsynaptic dopamine receptors were proposed, in particular to explain findings of reduced levels of homovanillic Acid (HVA) in cerebrospinal fluid (CSF) in GTS despite the premise of a hyperdopaminergic system. The validity of this view has been questioned as HVA levels may be confounded by medication, and previous positron emission tomography (PET) studies on dopamine receptors (although likely involved in the neurobiology of GTS) have produced inconsistent results.
• Dopamine hyperinnervation, that is, an overabundance of striatal dopamine terminals was suggested to reflect observations of generally increased binding to the DAT and to the vesicular monoamine transporter type 2 (VMAT2).
• Tonic-phasic dysfunction assumes reduced tonic dopamine levels as well as a hyperresponsive (spike-dependent) phasic dopaminergic system. A low tonic dopamine tone could be caused by an overactive DAT preventing efficient spillage to the extrasynaptic space and/or altered presynaptic dopamine D2 autoreceptor binding.

Apart from such considerations specific to dopamine one can postulate that if a dopaminergic abnormality were present, other neurotransmitter systems would exhibit perturbations as well. In particular, this is suggested by (i) the close synergy exhibited between excitatory, inhibitory and modulatory neurotransmitter systems within the striatum and throughout the brain; and (ii) the interdependent metabolic relationship exhibited between glutamate (Glu), and γ-aminobutyric acid (GABA) via the non-neuroactive metabolic intermediate glutamine (Gln). Moreover, an irregular afferent modulation of dopaminergic nuclei would have profound effects on tonic/phasic dopaminergic release in the striatum and the control of subsequent thalamocortical output. Consistently, separate groups have demonstrated that adult patients with GTS exhibit alterations within the GABAergic system in cortical regions using in vivo proton (1H) magnetic resonance spectroscopy (MRS) and subcortical regions using PET. Employing 1H MRS, the investigators recently found reductions in striatal concentrations of Gln and the sum of Glu plus Gln (Glx) in GTS patients as well as negative correlations between striatal Gln and actual tic severity and between thalamic Glu and premonitory urges. While these findings do not rule out alternative mechanisms, they lend support to the hypothesis of an alteration in the dynamics of the tonic/phasic dopaminergic signaling because chronic perturbations in the subcortical GABA-Glu-Gln cycle flux could lead to spatially focalized alterations in excitatory and inhibitory neurotransmitter ratios.

Another aspect of dopamine neurobiology that has recently gained interest in the context of neuroimaging is the relation to brain iron. Besides supporting myelination and cellular respiration, brain iron is crucial for the synthesis of neurotransmitters, in particular dopamine. It is stored primarily as ferritin and co-localizes with dopamine vesicles having the greatest concentration in the dopamine-rich basal ganglia and midbrain. As the major brain iron compounds have (super)paramagnetic properties, they can be detected via susceptibility-sensitive magnetic resonance (MR) techniques, such as quantitative susceptibility mapping (QSM) or measurements of the effective or the reversible transverse relaxation rates, R2* or R2', respectively. Recent multimodal imaging targeted at (normal) developmental changes of the striatal dopamine system demonstrated that R2'-based estimates of tissue iron content were associated with carbon-11 [11C]dihydrotetrabenazine PET of presynaptic vesicular dopamine. This suggests that susceptibility-sensitive MR imaging (MRI), which does not require an intravenously applied radiotracer, might serve as a proxy for obtaining information on dopamine that could substitute measurements of HVA in CSF without sharing the same confounds. More recently, the investigators already obtained preliminary indications of disturbed iron homeostasis in GTS patients as evidenced by reduced serum ferritin and magnetic susceptibility in the striatum and further subcortical structures.

Objectives and Hypotheses

In continuation of previous investigations, the investigators plan to perform examinations with MRI and MRS in GTS patients in comparison to age- and sex-matched healthy controls within Part 2 of this combined study. This includes (indirect) information on the interplay of different neurotransmitter systems (Glu, and GABA), as well as on the role of brain iron in GTS.

In particular, the investigators plan to use (i) iron-sensitive MRI techniques, such as QSM and R2* mapping; and (ii) 1H MRS with standard single-voxel techniques and spectral-editing methods, for obtaining neurochemical profiles and quantitative information on Glu, Gln, and GABA in the striatum and cortical areas.

Neuropsychological tests:

At the time of the MR and/or PET exams, an established, comprehensive test battery will be performed with all patients for a detailed clinical assessment, including the severity of tics or the presence of comorbidities. These tests may be performed online as a video conference questionnaire and include:

• DSM-IV-symptom list for attention deficit hyperactivity disorder (ADHD), rage attack questionnaire (RAQ), Pittsburgh sleep quality index (PSQI);
• Clinical ratings : Yale global tic severity scale (YGTSS-R), Yale-Brown obsessive-compulsive scale (Y-BOCS), clinical global impression scale (CGI).
• Self-assessment questionnaires: adult tic questionnaire (ATQ), Beck depression inventory (BDI), Beck anxiety inventory (BAI), Conners' adult ADHA rating scales (CAARS), autism spectrum quotient (AQ); premotory urge for tics scale (PUTS), GTS quality of life scale (GTS-QOL).

MR exams within Part 2 (7T MR):

MR scanning at 7 Tesla will be performed to exploit the increased sensitivity available at the higher magnetic field. In particular, this will improve the sensitivity of susceptibility-sensitive MR techniques, which permits to achieve sub-millimeter spatial resolutions for better segmentation of small subcortical structures, such as the subtantia nigra, subthalamic nucleus or red nucleus. Similarly, improved sensitivity and spectral resolution at 7 T will expand the range of accessible metabolites, which allows reliable separation of Glu and Gln as well as improved detectability of GABA. The 7T sub-study comprises the following acquisitions:

• Scout acquisition for automated alignment of imaging or spectroscopy volumes ("auto align").
• Structural MR scan ("MP2RAGE") for image registration, tissue segmentation, morphometry of cortical and subcortical structures, and for measuring the longitudinal relaxation time T1 to obtain information on myelin and brain iron content.
• Susceptibility-sensitive acquisition ("multi-echo FLASH") for measurements of the magnetic susceptibility (QSM) and R2* to obtain information on brain iron and myelin. Note that the MP2RAGE and multi-echo FLASH scans may be combined into a single, simultaneous acquisition of all parameters (susceptibility, T1 and R2*) depending on the results of initial tests in healthy volunteers.
• Single-voxel proton MRS of the basal ganglia and anterior cingulate cortex with both direct and edited acquisitions to obtain concentration estimates of local Glu, Gln, and GABA.

10ml of venous blood will be collected from all subjects (GTS patients and healthy controls) at the time of the exam for subsequent measurement of blood ferritin levels. These levels will be compared with the results from susceptibility-related measures of brain iron.

GTS Group:

Inclusion criteria

• GTS according to DSM-IV-TR criteria
• mild or moderate tics
• drug-free for a minimum of 4 weeks prior to the exam

Exclusion criteria

• severe tics of the head and/or face
• psychiatric medication within 4 weeks prior to the exam
• consumption of alcohol during 24 hours prior to the exam
• consumption of cannabis during 24 hours prior to the exam
• pregnancy
• general contra-indications for MRI exams

Control Group:

Inclusion criteria

• no known neurological or psychiatric disease

Exclusion criteria

• psychiatric medication within 4 weeks prior to the exam
• consumption of alcohol during 24 hours prior to the exam
• consumption of cannabis during 24 hours prior to the exam
• pregnancy
• general contra-indications for MRI exams