Gilles de le Tourette Syndrome (GTS) is a hereditary, neuropsychiatric movement disorder with fundamental alterations in the functional dynamics of cortico-striatal circuitry¹. Motor and phonic tics are the hallmark features of GTS, with a majority of the patients presenting with associated conditions that include obsessive-compulsive and attention-deficit hyperactive behaviours. Pathophysiologically, alterations in (a) phasic dopamine transmission and (b) the density/binding potential of D2 receptors and dopamine transporters in striatal and cortical regions have suggested an abnormality in tonic and phasic dopamine release². Transient phasic release of dopamine depends on a glutamatergic excitatory drive via the activation of NMDA receptors³, and has been implicated with motor control, reward prediction errors and reinforcement seeking behaviour⁴. The dynamic regulation of dopaminergic firing is driven by (a) extra-synaptic dopaminergic concentrations, which regulate dopamine release by acting on D2 autoreceptors^5–7, and (b) GABAergic and glutamatergic input from cortical, striatal and mesencephalic regions⁸. Therefore, we hypothesized that glutamatergic signaling is related to pathophysiology of GTS, and aimed at investigating whether patients exhibit alterations in glutamate metabolism within regions implicated in GTS pathophysiology.3T MRS data were acquired from 37 GTS patients and 36 healthy controls on a Siemens MAGNETOM Verio using a 32-channel coil. Repeated acquisitions were obtained from 23 controls for test-retest reliability assessment and 15 patients following four weeks of pharmacotherapy with the D2 partial agonist aripiprazole. A landmark based GRE pre-scan (Auto-Align Head; AAH) was applied for automatic registration of all following protocols to the same geometry. T1-weighted images were acquired with MP2RAGE (TR=5s, TE=3.93ms, 1mm3 voxel dimensions). 1H-MRS spectra were obtained from 3 ROIs with PRESS (TE= 30ms, TR=3000ms, 80 or 128 averages). To minimize errors from bulk drift in the time lag between the anatomical and spectral acquisitions, single-shot ‘dummy’ spectra were localized on the MP2RAGE immediately after acquisition. The AAH sequence was applied again before each MRS acquisition to co-register the ‘dummy’ scan geometry to the newly defined space. Spectra were acquired from the anterior Mid-Cingulate Cortex (aMCC, 6.4 mL), the bi-lateral thalamus (7.2 mL) and the left striatum (3.4 mL) following FASTESTMAP shimming. To account for incoherent averaging due to head motion and temporal drifts in the B0 field, we implemented a non-linear least squares minimization operation to fit each signal average to a reference scan by adjusting the frequency and phase of the signal⁹. Absolute metabolite concentrations were calculated using the water signal as internal reference while considering compartmentation within the voxel¹⁰ (Fig. 1). Optimized masks including subcortical nuclei were generated from SP12 and FSL-FIRST. Relaxation effects of metabolites were ignored. Spectra were fit with LCModel¹¹ in a 0.3-3.67ppm range to avoid spurious residual signals above 3.7ppm. Inclusion criteria for good quality were: (a) correct voxel prescription; (b) SNR>10; (c) FWHM<11Hz; and (d) CRLB<50%12. Group differences were assessed using a repeated measure analysis of variance (2 times × 3 regions × 2 groups) followed by post-hoc independent- and paired-sample t-tests.We observed a significant time x group interaction (F = 22.39, p = 0.000127) for glutamate+glutamaine (Glx) concentrations. Post-hoc independent sample t-tests revealed significant reductions of Glx concentrations in the left striatum (t₆₁ =2.594, p = 0.0119) and the bilateral thalamus t₅₈ =2.189, p = 0.0325) of GTS patients in comparison to normal controls (Fig. 3). Following treatment with aripiprazole, patients exhibited significant increases in striatal Glx concentrations (t₁₀ =-3.241, p = 0.009) and a trend for increases in the thalamic voxel (t₁₁ =2.189, p = 0.072) when compared to baseline (Fig. 2). Multiple regression analysis revealed a significant negative correlation between left striatal Glx levels (r = -0.451, p = 0.011) and tic severity (Fig 4). Thalamic Glx levels were negatively correlated with the premonitory urges preceding tics (r = -0.434, p =0.023) and depression (r = -0.589, p = 0.002).Our results implicate glutamatergic metabolism in the pathophysiology of GTS and indicate a possibly dysfunctional astrocytic-neuronal coupling system, a notion that has been observed by genetic^13–15 and functional-based¹⁴ studies. Since the metabolic paths of glutamate, glutamine and GABA are intertwined, GTS patients may exhibit a perturbed balance between excitatory and inhibitory neurotransmission, which may ultimately affect the modulation of tonic and phasic dopamine release from the substania nigra pars compacta and the ventral tegmental area. This might have a profound effect on motor control, reward-prediction errors and goal-directed behaviour. Focal asymmetries in excitatory, inhibitory and modulatory neurotransmitter ratios in functionally distinct striatal regions may lead to the diverse symptomatology associated with GTS.