Alterations of the crossed parabigeminotectal projection induced by neonatal eye removal in rats.

Projections of the parabigeminal nucleus to the contralateral superior colliculus and dorsal lateral geniculate nucleus were examined in normal adult pigmented rats and in adult rats from which one or both eyes had been removed at birth. In normal rats the crossed parabigeminotectal projection is restricted to the superficial layers in anterior and medial areas of colliculus, regions innervated also by the lower temporal portion of the ipsilateral retina. In unilaterally enucleated animals the crossed parabigeminotectal projection to the "denervated" colliculus is expanded, as is the retinal projection from the ipsilateral eye. In addition, there is a crossed parabigeminal projection to the "denervated" dorsal lateral geniculate nucleus in these rats. In bilaterally enucleated animals the parabigeminotectal projection is expanded, but not as greatly as in unilateral enucleation cases; there is a crossed parabigeminothalamic projection in these animals as well. The corresponding termination patterns of the contralateral parabigeminal nucleus and the ipsilateral retina in the normal superior colliculus may indicate a functional and/or developmental interdependence between the projections from these two regions. The existence of an expanded parabigeminotectal projection in bilaterally enucleated rats shows that a sustained ipsilateral retinotectal projection is not necessary for the establishment of a crossed parabigeminotectal projection, but points to the possibility that ipsilateral retinal input may constrain the parabigeminal projection to terminate within certain boundaries. The even greater expansion of the projection from the parabigeminal nucleus to the colliculus which receives an expanded projection from the ipsilateral retina of unilaterally enucleated rats suggests that the functional organization of the ipsilateral retinotectal projection may be capable of restricting the size of the terminal field of the crossed parabigeminotectal projection.