Is there an optimal age for recovery from motor cortex lesions?
II. behavioural and anatomical consequences of unilateral motor cortex lesions
in perinatal, infant, and adult rats
Affiliations: Department of Psychology and Neuroscience, University
of Lethbridge, Lethbridge, AB, T1K 3M4 Canada | Okanagan University College, Kelowna, BC, V1V 1V7
Canada
Note: [] Corresponding author: Bryan Kolb, Dept. of Psychology and
Neuro-science, University of Lethbridge, Lethbridge, AB, Canada, T1K 3M4. Tel.:
+1 403 329 2405; Fax: +1 403 329 2555; E-mail: [email protected]
Abstract: Purpose: The purpose of this study was to compare the behavioural
and anatomical effects of unilateral motor cortex ablation in neonatal, infant,
and adult rats. Methods: Rats were given unilateral lesions of the motor cortex
on the day of birth (P1), at ten days of age (P10), or in adulthood. They were
trained on several motor tasks (skilled forelimb reaching, beam traversing,
tongue extension), general motor activity, and a test of spatial learning
(Morris water task). Results: Although all lesion groups were equally impaired
at skilled reaching with the forelimb contralateral to the lesion, rats with P1
lesions also were impaired at traversing a narrow beam and at learning the
Morris task. Gross anatomical analyses revealed that the P1 rats had smaller
brains than the other groups, a result that may account for the larger
behavioural deficits in the P1 group. Analysis of Golgi-Cox stained neurons
showed that relative to control groups, all lesion groups showed an increase in
dendritic length in the basilar dendrites of layer III pyramidal cells and,
paradoxically a decrease in length of the apical dendrites of the same cells.
Conclusions: The bilateral alterations in dendritic organization following the
motor cortex lesions suggest that there has been a bilateral reor-ganization of
intrinsic cortical connectivity following motor cortex lesions at any age.
These alterations in connectivity are likely not identical in the young and
adult animals, however, because relative to controls, both the young operated
groups, but not the adult group, showed a bilat-eral drop in spine density in
the basilar dendrites of layer V pyramidal cells. These findings are discussed
with respect to the idea that there may be critical ages in development in
which animals can use anatomical modifications to compensate for deficits
produced by cortical injury.
Keywords: recovery of function, corticospinal tract, motor cortex
