Unilateral ablation of telencephalon induces appearance of contralateral cortical and subcortical projections to thalamic nuclei.

This study sought to investigate the afferent connections of the thalamus in the rat following massive telencephalic lesions. After unilateral removal of the telencephalon, horseradish peroxidase (HRP) was injected into the thalamic nuclei ipsilateral or, for control purposes, contralateral to the lesioned side. Injection of HRP into the ventral or posterior thalamus ipsilateral to the lesioned telencephalon led to retrograde transport and HRP-labeling of cells in unilateral projection areas in the cortex, diencephalon and mesencephalon and, in addition, unexpectedly, in their mirror-image sites in the hemisphere contralateral to the injected thalamus. HRP-positive cells in the contralateral hemisphere were found whenever HRP was injected 7 days postlesion, when the animals had ceased to exhibit spontaneous turning behavior, but not when it was injected immediately after the lesion. We favor the hypothesis that the development of contralateral afferents to the thalamic nuclei represents a morphological substrate of the behavioral reorganization necessitated by the asymmetrical removal of the telencephalon.