Role of the striatal cholinergic system in the pathophysiology of dystonia : a translational study – DysChol

Dystonia is defined as a syndrome of sustained muscular contractions leading to repetitive movements and abnormal postures. DYT1 dystonia is the most frequent form of primary dystonia, but the link between genomic mutation and expression of the phenotype remains largely unknown. The secondary forms of dystonia have highlighted the role of the basal ganglia, and particularly the putamen, in the pathophysiology of the disease. A line of evidence collected in genetic rodent models suggests that acetylcholine interneurones (Ach-I) of the putamen would play a critical role in the pathological plasticity that occurs at the glutamatergic cortico-striatal synapse but this data has not been demonstrated, so far, neither in humans nor in non-human primates. The aim of the present translational study is to demonstrate that dystonia is associated with an abnormal function of the ACh-I.
In a first study of functional imaging in humans, we will directly check this hypothesis in DYT1 dystonic patients using a marker of the vesicular transporter of acetylcholine, the [18F]-FEOBV, and TEP imaging. MRI images will also be acquired on a 3T GE apparatus allowing anatomical imaging and tensor diffusion techniques. Our primary outcome will be the quantification of marker fixation (binding potential i.e. BP) and its comparison between sex- and age-matched DYT1 dystonic patients (n = 20) and normal controls (n = 20). We postulate that the level of fixation will be increased by at least 40% in dystonic versus control subjects on the basis of preliminary data. Since a high phenotypic variability exists in DYT1 dystonia, we will then search for a correlation between the BP and the severity of dystonia, evaluated on the BFM rating scale. The tissular microstructure will also be studied with diffusion tensor imaging focusing on striatofugal fibers. These results will be correlated with the level of BP.
In a second experimental study, we will chronically infuse acetylcholine agonists in the sensorimotor putamen of monkeys to assess whether a chronic and reliable model of dystonia can be obtained by increasing ACh transmission in the putamen. Two internalized micro-pump systems will be connected to intracortical cannulas stereotaxically positioned over the two motor putamina. Acetylcholine agonists and sham (saline) will be continuously injected at a very low rate (1microl/h) for long periods of time (1 to 3 months) in each hemisphere. Previous data in rodents and the secretion-resorption cycle of cerebraospinal fluid in the monkey suggest that there will be any risk of intra-cerebral hypertension or local lesions but a careful monitoring of animal behavior will be performed. Animal behavior will be studied daily on a primate chair for clinical symptoms (blindly assessed videos) and on a primate behavioral platform for recording locomotor activity and specific behaviors. To study the effect of injections on neuronal activity, we will perform chronic single neuronal recordings in both the putamen and internal pallidum through microelectrodes glued to the injection cannulas. The electric signal will be recorded through a wireless multi-channel system in order to keep the monkey’s head free-moving. In addition, chronic electroencephalography will be performed through intra-skull electrodes and EMG recordings with intramuscular wire-electrodes. Finally, in order to study the plasticity of striatal neurones following sham or agonist injections, the binding of M1, M4, D1, D2 receptors will be investigated post-mortem and compared between the two hemispheres.
This innovative, clinically relevant, translational study, will potentially open up new therapeutic avenues by promoting the search for new drugs acting on the cholinergic system but devoted of the side-effects of existing drugs.