Anomalous uncrossed retinal projections fail to activate superior colliculus neurons in rabbits unilaterally enucleated by fetal surgery.

Previous studies have shown that unilateral enucleation of rabbit pups produces an aberrant uncrossed retinotectal projection. These fibers failed to drive collicular units when stimulated with either light or electric shock. The present study attempts to assess the possibility that enucleation at earlier stages of development would lead to a greater degree of morphological and physiological reorganization in the uncrossed retinotectal projection. Rabbit fetuses were unilaterally enucleated at day 20 or 25 of gestation. Birth is at day 31. After 3 months, the degree of reorganization of the uncrossed retinotectal projection was assessed using the following techniques: (1) autoradiographic demonstration of the projection from the remaining eye, (2) electrophysiological recording of collicular unit activity, and (3) a combination of these methods. Autoradiographic data indicated a much greater expansion of the anomalous uncrossed projection in fetally enucleated animals than in those enucleated at birth. Labelled terminals were found to occupy more than the anterior third of the ipsilateral colliculus and were distributed to the entire depth of the stratum griseum superficiale and stratum opticum. Electrode penetrations within the boundaries of the expanded projection failed to locate collicular units which could be driven by either light stimulation of the eye or electric shock of the optic nerve. Only a few cells encountered in the lateral border area, receiving the normal uncrossed retinal projection, could be driven by light stimulation. These negative findings are in contrast to the data reported for the rat and hamster where anomalous retinal projections are capable of forming functional connections. Further comparative studies of reorganization are needed.