Plastic changes in the vibrissa motor cortex in adult rats after output suppression in the homotopic cortex

After motor cortex damage, the unaffected homotopic cortex shows changes in motor output. The present experiments were designed to clarify the nature of these interhemispheric effects. We investigate the vibrissa motor cortex (VMC) output after activity suppression of the homotopic area in adult rats. Comparison was made of VMC output after lidocaine inactivation (L-group) or quinolinic acid lesion (Q-group) of the homotopic cortex. In the Q-group, VMC mapping was performed 3 days (Q3Ds group), 2 weeks (Q2Ws group) and 4 weeks (Q4Ws group) after cortical lesion. In each animal, VMC output was assessed by mapping movements induced by intracortical microstimulation (ICMS) in both hemispheres (hemisphere ipsilateral and contralateral to injections). Findings demonstrated that, in the L-group, the size of vibrissal representation was 39.5% smaller and thresholds required to evoke vibrissa movement were 46.3% higher than those in the Control group. There was an increase in the percentage of ineffective sites within the medial part of the VMC and an increase in the percentage of forelimb sites within the lateral part. Both the Q3Ds group and the L-group led to a similar VMC reorganization (Q3Ds vs. L-group, P > 0.05). In the Q2Ws group the VMC representation showed improvement in size (83.4% recovery compared with controls). The VMC showed recovery to normal output at 4 weeks after lesion (Control vs. Q4Ws group, P > 0.05). These results suggest that the VMC of the two hemispheres continuously interact through excitatory influences, preserving the normal output and inhibitory influences defining the border with the forelimb representation.