Primate models of dystonia.

Several models of dystonia have emerged from clinical studies providing a comprehensive explanation for the pathophysiology of this movement disorder. However, several points remain unclear notably concerning the specific role of brainstem, basal ganglia nuclei and premotor cortex. We review data collected in sub-human primate to see whether they might provide new insights into the pathophysiology of dystonia. As in human patients, lesions of the putamen induce dystonia, as well as pharmacological manipulations of the dopaminergic system. In addition, primate studies revealed that lesions in brain stem areas involved in the control of muscular tone and GABAergic manipulations in various basal ganglia nuclei or thalamus also lead to dystonia. Moreover, there is a dramatic disruption in the processing of proprioceptive information with abnormal large receptive fields in the basal ganglia, thalamus, primary somesthetic cortex and premotor cortex of dystonic monkeys. These data highlight the idea that dystonia is associated with aberrant sensory representations interfering with motor control. Considering that the supplementary motor area (SMAp) is the target of basal ganglia projections within the motor loop, we propose a model of dystonia in which abnormal excitability, associated with alteration in sensory receptive fields within the SMAp, leads to an abnormal synchronization between primary motor cortex columns. Such a phenomenon might account for the co-contractions of antagonist muscles favored by action and the abnormal postures observed in dystonia.