Plastic reaction of the rat visual corticocollicular connection after contralateral retinal deafferentiation at the neonatal or adult stage: Axonal growth versus reactive synaptogenesis

Abstract

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.

Retinal Input Instructs Alignment of Visual Topographic Maps

“…Previous studies have shown that removal of retinal input during development or adulthood results in increased corticocollicular plasticity (García del Caño et al, 2002), and that anopthalmic mice have broader corticocollicular TZs compared to WT controls (Khachab and Bruce, 1999). However, neither of these studies performed a detailed analysis of the topographic organization of V1 projections.…”

Impact of peripheral hearing loss on top-down auditory processing

“…Adult enucleated rats showed no terminal field expansion, indicating that the plastic mechanisms giving rise to axonal sprouting/deactivated pruning are restricted to a developmental window (Figure 5C). Interestingly, the adult enucleated animals also showed increased synaptic terminal density in the stratum zonale, suggesting that the plastic processes involved in reactive synaptogenesis are maintained into adulthood (García Del Caño et al, 2002). …”

“…B) Expanded terminal field of rat enucleated in early postnatal life. C) Terminal field of rat enucleated in adulthood, demonstrating a lack of expansion (obtained with permission from (García Del Caño et al, 2002)). SG = stratum griseum superficiale, SO = stratum opticum, SZ = stratum zonale.…”

Differential regulation of the neuronal isoform of nitric oxide synthase in the superior colliculus and dorsal lateral geniculate nucleus of the adult rat brain following eye enucleation

“…It is tempting to speculate that nNOS regulation in the current model may depend on glutamate receptor expression. There are several reports on the organization of the glutamatergic pathways in the visual system, leading to the conclusion that different visual areas express different pools of glutamate receptor subtypes (Cirone and Salt, 2000), that glutamate subunits seem to be differentially regulated after unilateral enucleation and other experimental procedures (Chalmers and McCulloch, 1991; Pires et al, 2000), and that there is an increase of glutamatergic boutons in the SC, originating from the visual cortex, after retinal removal in the adult rat (Garcia del Cano et al, 2002). Therefore, as activation of glutamate receptors are undoubtedly related to the regulation of activity and expression of nNOS (reviewed by Esplugues, 2002; Kew and Kemp, 2005), it is possible that deafferentation of visual areas may cause upregulation of glutamate receptors and consequently of nNOS.…”