Recovery from early cortical lesions in rats: 6. Cortical noradrenaline, cortical thickness, and development of spatial learning after frontal lesions or hemidecortications

Abstract

Spatial learning, cortical thickness, cortical noradrenaline, and striatal dopamine were assessed in rats with bilateral or unilateral frontal cortex lesions or hemidecortications at 4 days of age. Training in a spatial learning task began at 25 days of age and was repeated at 60 and 150 days of age. The results showed that, relative to littermate controls, rats with bilateral frontal lesions were severely impaired at spatial learning, even when tested as young juveniles (around 25 days). The frontal lesions produced a 50% decrease in noradrenaline in the remaining cortex and a thinning of the remaining cortex. However, although the cortical thickness and noradrenaline levels were correlated with one another, neither was correlated with behavior. Hemidecortication also produced a deficit in spatial learning in young and older animais alike, but, in contrast to the frontal lesions, hemidecortication led to an increase in noradrenaline in the remaining hemisphere and a small increase in cortical thickness. Neither of the latter measures was correlated with behavior. The results suggest that the neocortex is necessary for spatial navigation, even in young animais with an immature cortex. Furthermore, it appears that although neonatal lesions alter noradrenaline in the cortex, this noradrenergic alteration probably does not account for the severe behavioral effects of early lesions in rats.