Effects of neonatal enucleation on the organization of callosal linkages in striate cortex of the rat.

Lewis and Olavarria ([1995] J. Comp. Neurol. 361:119-137) showed that the mediolateral organization of callosal linkages differs markedly between medial and lateral regions of striate cortex in the rat. Thus, callosal fibers originating from medial regions of striate cortex interconnect loci that are mirror-symmetric with respect to the midsagittal plane. In contrast, fibers from lateral regions of striate cortex show a reversed pattern of connections: tracer injections into the 17/18a border produce retrograde cell labeling in regions medial to the contralateral 17/18a border, whereas injections placed somewhat medial to the 17/18a border label cells located at the contralateral 17/18a border. Based on the interpretation that callosal fibers from lateral striate cortex connect retinotopically corresponding loci (Lewis and Olavarria [1995] J. Comp. Neurol. 361:119-137) we propose here that the development of the reversed pattern of connections in lateral portions of striate cortex is guided by activity-dependent cues originating from spontaneously active ganglion cells in temporal retina. In the present study we have attempted to falsify this hypothesis by investigating the effects of neonatal bilateral enucleation on the organization of callosal linkages in striate cortex of the rat. Once enucleated rats reached adulthood, we studied the mediolateral organization of callosal connections by placing small injections of different fluorescent tracers into different loci within medial and lateral striate cortex. The analysis of the distribution of retrogradely labeled callosal cells indicated that connections from lateral portions of striate cortex were no longer organized in a reversed fashion, rather, they resembled the mirror image pattern normally found in the medial callosal region, i.e., injections at the 17/18a border produced labeled cells at the opposite 17/18a border, whereas injections into slightly more medial regions produced labeled cells in the opposite, mirror-symmetric location. In addition, we found that enucleation does not alter the organization of callosal linkages in medial portions of striate cortex. Thus, by showing that enucleation significantly changes the pattern of connections from lateral portions of striate cortex, the present study does not falsify, but rather strengthens the hypothesis that interhemispheric correlated activity driven from the temporal retinal crescent guides the normal development of reversed callosal linkages in lateral portions of rat striate cortex. Furthermore, the present study shows that, in the absence of the eyes, the pattern of callosal linkages in lateral portions of striate cortex resembles the mirror image pattern normally found only in medial striate cortex.