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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.

Assessments of somatosensory and motor cortical somatotopy in vivo can provide important information on sensorimotor physiology. Here, novel polyimide-based thin-film microelectrode arrays (72 contacts) implanted epidurally, were used for recording of somatosensory evoked potentials (SEPs) and somatosensory cortex somatotopic maps of the rat. The objective was to evaluate this method with respect to precision and reliability. SEPs and somatosensory maps were measured twice within one session and again after 8 days of rest. Additionally, motor cortex maps were acquired once to assess the spatial relationship between somatosensory and motor representations of fore- and hindlimb within one individual. Somatosensory maps were well reproduced within and between sessions. SEP amplitudes and latencies were highly reliable within one recording session (combined intraclass correlation 90.5%), but less so between sessions (21.0%). Somatosensory map geometry was stable within and between sessions. For the forelimb the somatosensory representation had a 30% overlap with the corresponding motor area. No significant overlap was found for the hindlimb. No evidence for cortical injury was found on histology (Nissl). Thin-film epidural electrode array technology enables a detailed assessment of sensorimotor cortex physiology in vivo and can be used in longitudinal designs enabling studies of learning and plasticity processes.

Stimulation mapping of motor cortex is an important tool for assessing motor cortex physiology. Existing techniques include intracortical microstimulation (ICMS) which has high spatial resolution but damages cortical integrity by needle penetrations, and transcranial stimulation which is non-invasive but lacks focality and spatial resolution. A minimally invasive epidural microstimulation (EMS) technique using chronically implanted polyimide-based thin-film microelectrode arrays (72 contacts) was tested in rat motor cortex and compared to ICMS within individual animals. Results demonstrate reliable mapping with high reproducibility and validity with respect to ICMS. No histological evidence of cortical damage and the absence of motor deficits as determined by performance of a motor skill reaching task, demonstrate the safety of the method. EMS is specifically suitable for experiments integrating electrophysiology with behavioral and molecular biology techniques.