Striatal dopamine depletion induces forelimb motor impairments and disrupts forelimb movement representations within the motor cortex

While limb motor deficits of Parkinson's disease are well characterized, the effects of striatal dopamine depletion on the motor cortex is poorly understood. We therefore aimed to 1) examine the effects of striatal dopamine depletion on forelimb function and cortical motor map topography and 2) explore potential relationships between forelimb function and cortical movement representations in an animal model of PD. Twenty-four male Long Evans rats were randomized to control or 6-hydroxydopamine (6-OHDA) groups. Animals in the 6-OHDA group underwent four unilateral 6-OHDA infusions into the striatum to induce striatal dopamine depletion. Four weeks later, animals were tested on a comprehensive battery of behavioral limb motor tasks followed by intracortical microstimulation to derive high-resolution topographic maps of forelimb movement representations. Standard tyrosine hydroxylase (TH) immunohistochemistry was performed and near infrared densitometry analysis utilized to assess TH depletion. Unilateral striatal dopamine depletion induced significant reductions in limb motor function that were reflected neurophysiologically as a reduction in cortical forelimb movement representations. Voluntary forelimb use, pasta handling, sunflower seed manipulation, and forelimb motor maps were all significantly impaired in 6-OHDA animals. A positive correlation was observed between forelimb function and motor map size, as well as two negative correlations between TH depletion with 1) motor map size and 2) forelimb function. The results clearly show how dysfunction within the basal ganglia thalamocortical loop resulting from nigrostriatal dopamine depletion disrupts corticospinal function.