Dopaminergic modulation of motor maps in rat motor cortex: an in vivo study.

While the primary motor cortex (M1) is know to receive dopaminergic projections, the functional role of these projections is poorly characterized. Here, it is hypothesized that dopaminergic signals modulate M1 excitability and somatotopy, two features of the M1 network relevant for movement execution and learning. To test this hypothesis, movement responses evoked by electrical stimulation using an electrode grid implanted epidurally over the caudal motor cortex (M1) were assessed before and after an intracortical injection of D1- (R-(+),8-chloro,7-hydroxy,2,3,4,5,-tetra-hydro,3-methyl,5-phenyl,1-H,3-benzazepine maleate, SCH 23390) or D2-receptor (raclopride) antagonists into the M1 forelimb area of rats. Stimulation mapping of M1 was repeated after 24 h. D2-inhibition reduced the size of the forelimb representation by 68.5% (P<0.001). Movements thresholds, i.e., minimal currents required to induce movement responses increased by 37.5% (P<0.001), and latencies increased by 35.9% (P<0.01). Twenty-4 h after the injections these effects were reversed. No changes were observed with D1-antagonist or vehicle. By enhancing intracortical excitability and signal transduction, D2-mediated dopaminergic signaling may affect movement execution, e.g. by enabling task-related muscle activation synergies, and learning.