Disruption of corticospinal plasticity by 6-hydroxydopamine as measured by intracortical microstimulation in rats

Recovery of function is associated with increased spine density in cortical pyramidal cells after frontal lesions and/or noradrenaline depletion in neonatal rats

Rats were given medial frontal lesions at 7 days of age and were tested as adults on tests of forelimb use, forelimb tactile sensitivity, tongue use, hindleg use, and in a spatial navigation task. The brains were processed with a modified Golgi-Cox procedure and dendritic arborization and spine density was measured. The animals showed recovery only on the spatial task and this was associated with an increase in the number of spines per unit length of dendrite. We also reanalyzed Golgi-Cox stained material from an experiment in which animals were depleted of cortical noradrenaline (NA) in infancy and then given frontal lesions on day 7. The NA depletion blocked the recovery from frontal lesions. Analysis of dendritic morphology showed that in otherwise intact rats, NA depletion decreased dendritic arbor but increased spine density to the level of frontal operates. Depleted frontal-operates showed no additional increase in spine density and also showed a decrease in dendritic arborization. These results suggest that recovery from neonatal cortical injury and from neonatal noradrenaline depletion may be supported by changes in both the dendritic arborization and the spine density in the remaining cortex.

Functional recovery of forelimb response capacity after forelimb primary motor cortex damage in the rat is due to the reorganization of adjacent areas of cortex

Functional recovery after brain damage has been described frequently and different mechanisms have been proposed to account for the observed recovery. One possible mechanism involves the capacity of one part of the brain to take over the function of another. A possible area for this to take place is in the cerebral cortex, where a variety of reorganizational processes have been described after different manipulations. We show in the present study that the forelimb force and response capacity of the rat, which becomes highly impaired after the bilateral ablation of the forelimb primary motor cortex, is recovered when the animals receive an electrical stimulation in the ventral tegmental nucleus contingent to each forelimb response in the task. Microstimulation mapping of the cortical areas adjacent to the forelimb primary motor cortex revealed the appearance of an area located caudolaterally to the forelimb primary motor cortex, where forelimb movements could be evoked in recovered animals but to a lesser extent in non-recovered animals. A positive and significant correlation was observed between the size of the reorganized forelimb area and the behavioral performance of the animals. Ablation of the forelimb reorganized area in recovered animals reinstated the forelimb behavioral impairment, while the same lesion in normal animals had no effect on the behavioral performance. The results indicate that recovery after bilateral forelimb primary motor cortex ablation may be due to the organization of specific adjacent areas in the cortex.

Effects of neonatal hemispherectomy on location and number of corticospinal neurons in the rat

Corticospinal neurons in the rat normally project to the contralateral spinal cord only. After neonatal hemispherectomy, the remaining single hemisphere forms both ipsilateral and contralateral projections. The distribution and number of corticospinal neurons projecting to the contralateral and to the ipsilateral spinal cord were studied in the hemispherectomized rat. The method used was that of retrograde labelling of corticospinal neurons by unilateral spinal cord injection of horseradish peroxidase conjugated to wheat germ agglutinin (WGA-HRP). The establishment of both crossed and uncrossed corticospinal projections by one cerebral hemisphere is accomplished in two ways. (1) Some neurons located within the confines of normal motor cortex send axons to ipsilateral spinal cord, where normally only contralateral projections occur. (2) There is expansion of the areas of cerebral cortex that contain corticospinal neurons. This expansion is most pronounced near the frontal pole, but it occurs at all borders of motor cortex except at its posterior limit, where the border with visual cortex persists unchanged. Only corticospinal neurons forming uncrossed projections occur in aberrant sites, while both cells with crossed and those with uncrossed spinal axons are intermingled in cortex that normally contains corticospinal neurons. The findings suggest that plasticity of the corticospinal motor system in the rat occurs to some extent at the expense of impaired localization of functions in cerebral cortex, and that following focal brain injury there may be an increase in areas that subserve multiple functions.