Plastic response of the retrospleniocollicular connection after removal ofretinal inputs in neonatal rats - An anterograde tracing study

The effects of neonatal enucleation on the final adult pattern of retrosple niocollicular connection in the rat was studied using the anterograde trace r biotin-dextranamine 10,000 (BDA) iontophoretically injected in different anteroposterior locations of the retrosplenial cortex. Retrosplenial affere nts are normally distributed in all collicular layers beneath the stratum g riseum superficiale (SGS) throughout almost the entire rostrocaudal and lat eromedial collicular axes. Neonatal enucleation caused an invasion of lower SGS by abundant retrosplenial afferents, whose distribution remained unalt ered in intermediate and deep collicular layers. Axons entering the deaffer ented SGS showed variable morphologies and arborization patterns. Some of t hem ran lateromedially close to the SGS-stratum opticum (-SO) limit, giving rise to many collaterals which invaded the lower part of the SGS; whereas others formed narrow terminal arbors, mostly branching in the SO. In the in termediate layers, synaptic profiles were mainly found close to the borders of acetylcholinesterase (AChE) patches in both control and enucleated anim als, indicating that neonatal enucleation does not alter the final pattern of retrospleniocollicular afferents to these collicular regions. The result s presented here demonstrate that neonatal enucleation leads to the develop ment of an aberrant projection from the retrosplenial cortex to the deaffer ented superficial layers of the superior colliculus. These results provide new information regarding the reorganization of connections subsequent to n eonatal enucleation and suggest that, in enucleated animals, nonvisual mult isensorial information could be relayed to central circuits which in intact animals belong to the visual system.