Asymmetrical effects of cortical island lesions in the rat.

Baseline spontaneous activity of male Sprague-Dawley rats was established in home running wheel cages 3 weeks prior to receiving cortical circumscription lesions. Circumscriptions 2 mm in diameter and 2 mm deep were produced by means of a rotating microknife and were centered approximately 9.0 mm anterior to ear bar zero. Circular knife cuts of the right, but not the homologous left cortex produced long-lasting depletions of ipsilateral and contralateral fronto- and posterio-cortical NE. Right hemisphere lesions produced a 35% increase in activity while left hemispheric lesions uniformly decreased spontaneous home cage running wheel activity by 35%. These results are consistent with our hypothesis that injury to transcortical pathways (perhaps noradrenergic fibers) are sufficient to produce lateralized spontaneous hyperactivity. The possible existence of competing excitatory and inhibitory mechanisms in the left frontal cerebral cortex is discussed.