Exploration of Differences in Metabolite Concentrations by 7Teslas NMR Spectroscopy in Striatum and Subthalamic Nuclei in de Novo Parkinsonian Patients and Control Subjects

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Initially, the exploration of brain metabolism by Nuclear Magnetic Resonance Spectroscopy (MRS) of the high magnetic field proton (1H) (11.7T) applied to acute and chronic animal models of Parkinson's disease (PD) showed glutamatergic hyperactivity within the striatum, one of the components of the basal ganglia. Interestingly, acute administration of L-dopa and acute, subchronic and chronic deep brain stimulation of the subthalamic nucleus (STN) normalizes these neurochemical profiles. Investigators also show an increase in glutamate levels in the STN ipsilateral to the substantia nigra pars compacta (SNpc) damaged by the neurotoxin, expected phenomenon, but also and surprisingly in the STN controlateral to the lesion. A degeneration of dopaminergic neurons is also observed in the controlateral SNpc at the lesion suggesting that the hyperglutamatergy of the controlateral STN to the lesion could promote neuronal death in the SNpc and thus participate in the progression and lateralization of the PD. Using 3T MRS in PD patients, as in other studies in humans, investigators do not see changes in glutamate and glutamine levels in the putamen of Parkinsonian patients. This difference between animal and human studies can be explained: 1. by the different rate of progression between PD in humans and animal models with plasticity phenomena limiting glutamatergic hyperactivity, 2. by the effect of treatment in PD masking changes in glutamate metabolism, 3. by limiting sensitivity in the detection of metabolites (Glutamate, glutamine, GABA) at 3T. The 7T 1H MRS improves the dispersion of chemical shifts of the metabolites studied, increases the sensitivity of the measurement, makes it possible to select regions of interest of smaller volumes (1 cm3) and thus limits the magnetic susceptibility effects that degrade the quality of the measured signal. This makes it possible to reliably separate glutamate and glutamine peaks. In this context, investigators propose to study the metabolic changes in a homogeneous group of de novo Parkinsonian patients, naive to any treatment intended to replace the missing dopamine. The gain in spatial resolution, contrast and signal will allow better characterization of localized anomalies in small-volume structures such as basal ganglia, putamen and STN.

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