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 be tween medial and lateral regions of striate cortex in the rat. Thus, c allosal fibers originating from medial regions of striate cortex inter connect loci that are mirror-symmetric with respect to the midsagittal plane. In contrast, fibers from lateral regions of striate cortex sho w a reversed pattern of connections: tracer injections into the 17/18a border produce retrograde cell labeling in regions medial to the cont ralateral 17/18a border, whereas injections placed somewhat medial to the 17/18a border label cells located at the contralateral 17/18a bord er. Based on the interpretation that callosal fibers from lateral stri ate cortex connect retinotopically corresponding loci (Lewis and Olava rria [1995] J. Comp. Neurol. 361:119-137) we propose here that the dev elopment 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 t he effects of neonatal bilateral enucleation on the organization of ca llosal linkages in striate cortex of the rat. Once enucleated rats rea ched adulthood, we studied the mediolateral organization of callosal c onnections by placing small injections of different fluorescent tracer s into different loci within medial and lateral striate cortex. The an alysis of the distribution of retrogradely labeled callosal cells indi cated that connections from lateral portions of striate cortex were no longer organized in a reversed fashion, rather, they resembled the mi rror image pattern normally found in the medial callosal region, i.e., injections at the 17/18a border produced labeled cells at the opposit e 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 show ing that enucleation significantly changes the pattern of connections from lateral portions of striate cortex, the present study does not fa lsify, but rather strengthens the hypothesis that interhemispheric cor related activity driven from the temporal retinal crescent guides the normal development of reversed callosal linkages in lateral portions o f rat striate cortex. Furthermore, the present study shows that, in th e absence of the eyes, the pattern of callosal linkages in lateral por tions of striate cortex resembles the mirror image pattern normally fo und only in medial striate cortex. (C) 1995 Wiley-Liss, Inc.