Motor Representations in the Intact Hemisphere of the Rat Are Reduced After Repetitive Training of the Impaired Forelimb

Abstract

Background During recovery from a unilateral cortical stroke, spared cortical motor areas in the contralateral (intact) cerebral cortex are recruited. Pre-clinical studies have demonstrated that compensation with the less-impaired limb may have a detrimental inhibitory effect on the intact cortical hemisphere and could impede recovery of the more-impaired limb. However, evidence from detailed neurophysiological mapping studies in animal models is lacking. Objectives The present study examines neurophysiological changes in the intact hemisphere of the rat following a unilateral ischemic infarct to cortical forelimb motor areas. Methods Eight rats were trained for two weeks on a reach and retrieval task prior to an ischemic infarct induced by the vasoconstrictor, endothelin-1 injected into the cortical grey matter encompassing the two forelimb motor representations, the caudal forelimb area (CFA) and the rostral forelimb area (RFA). Animals were randomly assigned to an Infarct/Training group (n=4) or an Infarct/No Training group (i.e., spontaneous recovery, n=4). After a five-week post-infarct period, motor areas of the intact hemisphere (CFA and RFA) were characterized using intracortical microstimulation techniques. The resulting maps of evoked movements were compared to maps derived from CFA and RFA in normal rats (Normal, n=5; Normal/Training, n=4). Results Compared with the Normal/No Training group, CFA representations were significantly smaller in the Infarct/Training group but not in the Infarct/No Training group. No significant differences were found in RFA. Conclusions Repetitive training of the more-impaired forelimb during the post-infarct recovery period reduces the size of motor representations in the intact hemisphere.

The Effect of Lesion Size on the Organization of the Ipsilesional and Contralesional Motor Cortex

“…To date, the few studies that looked at the organization of cortical motor maps in the contralesional hemisphere have found no effect of lesion. 19 , 34 , 35 The failure to identify changes in the cRFA in these studies may be explained by the restricted range of lesion sizes or by lesion location.…”

“…Controls did not undergo the familiarization period as this was shown to have no effect on motor maps. 19 …”

Improved methods for chronic light-based motor mapping in mice: automated movement tracking with accelerometers, and chronic EEG recording in a bilateral thin-skull preparation

“…During light-based motor mapping (LBMM), the cortical region representing forelimb muscles can be identified by sequentially stimulating different cortical points and recording the evoked electromyographic (EMG) activity from the contralateral forelimb (Ayling et al, 2009; Hira et al, 2009). This method complements previously available tools, such as intracortical microstimulation (ICMS) where an electrode is lowered into deep cortical layers and limb or body movements are evoked through current injection (Asanuma and sakata, 1967; Li and Waters, 1991; Kleim et al, 1998; Monfils et al, 2005; Tennant et al, 2011; Young et al, 2011; Barbay et al, 2012). LBMM offers two distinct advantages over traditional ICMS: the effect of light-based stimulation is more specific as it is restricted to cells expressing the light-sensitive opsin, and light-based stimulation may be performed repeatedly, without the need for a craniotomy.…”

Displacement of Sensory Maps and Disorganization of Motor Cortex After Targeted Stroke in Mice

“…In the future, bilateral studies of reorganization could take advantage of the spared hemisphere to expand the mapped area. 33 Performing motor mapping in the hours or days after stroke may also reveal greater reductions in motor output. 34 We have demonstrated the feasibility of longitudinal sensorimotor mapping and characterized the spontaneous cortical reorganization that occurs in the absence of any intervention.…”