The effects of neonatal or adult enucleation on the final adult pattern of the rat visual corticocollicular (C-Co) connection were studied using the anterograde tracer biotinylated dextranamine 10,000 (BDA) iontophoretically injected in the primary visual cortex. In control animals, column-shaped terminal fields limited to a small portion of the collicular surface were observed. Synaptic boutons were present in all superficial strata of the superior colliculus (SC), with the highest density in the ventral part of the stratum griseum superficiale (SGS). Neonatal enucleation caused a considerable expansion of the contralateral visual C-Co terminal fields, which occupied almost the entire collicular surface, suggesting that axonal sprouting had occurred. In addition, terminal boutons tended to localize more dorsally in these cases compared with controls. Following enucleation in adult animals, no changes were observed with respect to the extension of the terminal fields, although a plastic reaction leading to an increase in the bouton density in the stratum zonale (SZ) and upper SGS was found, reflecting a process of reactive synaptogenesis at these levels. These results show that both neonatal and adult visual C-Co fibers react in response to retinal ablation, although this reaction shows distinct characteristics. Molecular factors, such as growth-associated cytoskeletal proteins operating in the cortical origin, and extracellular matrix components and myelin-associated axonal growth inhibitors acting on the collicular target very likely account for these differences.
Copyright 2002 Wiley-Liss, Inc.