Long-term effects of retinal lesions on growth-associated protein 43 (GAP-43) expression in the visual system of adult cats

Time course of dorsolateral geniculate nucleus plasticity in adult monkeys with laser-induced retinal lesions

We studied changes in the expression of growth-associated protein 43 (GAP43), glial fibrillary acidic protein (GFAP), and calcium-binding proteins (calbindin [Cb] and parvalbumin [Pv]) in the dorsal lateral geniculate nucleus (dLGN) of four capuchin monkeys with laser-induced retinal lesions. The lesions were generated with the aid of a neodymium-YAG dual-frequency laser with shots of different intensity and at different survival time in each animal. The expression of these proteins in the layers of the dLGN was evaluated by performing histodensitometry of coronal sections throughout the nucleus. High-power laser shots administered at the border of the optic disc (OD)-injured fibers resulted in large scotomas. These lesions produced a devastating effect on fibers in this passage, resulting in large deafferentation of the dLGN. The time course of plasticity expressed in this nucleus varied with the degree of the retinal lesion. Topographically, corresponding portions of the dLGN were inferred by the extent of the ocular dominance column revealed by cytochrome oxidase histochemistry in flattened preparations of V1. In the region representing the retinal lesion, the expression of GFAP, GAP43, Pv, and Cb increased and decreased in the corresponding dLGN layers shortly after lesion induction and returned to their original values with different time courses. Synaptogenesis (indicated by GAP43 expression) appeared to be increased in all layers, while "cleansing" of the glial-damaged region (indicated by GFAP expression) was markedly greater in the parvocellular layers, followed by the magnocellular layers. Schematic drawings of optic discs laser lesions and of series of coronal sections of the dLGN, in three monkeys, depicting the areas of the nucleus deafferented by the lesions.

Retinal lesions induce layer-specific Fos expression changes in cat area 17

Quantitative analysis of the neuronal activity marker Fos revealed activity-dependent and lamina-specific changes in adult cat area 17, 14 days to 1 month after the induction of central retinal lesions. The supra- and infragranular layers were clearly differently engaged in the response to the visual deprivation, both inside and outside the lesion projection zone. The center of the LPZ exhibited an activity decrease in the extragranular layers, which was mainly reflected by an intracellular down-regulation of Fos rather than a decline in the number of Fos-immunoreactive nuclei. Interestingly, the infragranular layers displayed more Fos-immunoreactive neurons in experimental animals. This recruitment of an additional population of Fos expressing neurons in the subcortically projecting infragranular layers might have a protective function against neurodegeneration in the direct retinal target structures.

EXPERIENCE-DEPENDENT IMMEDIATE EARLY GENE EXPRESSION IN THE ADULT CENTRAL NERVOUS SYSTEM: EVIDENCE FROM ENRICHED-ENVIRONMENT STUDIES

Here I discuss evidence from our group's work that implicates the immediate early genes NGFI-A and arc as possible regulators of neuronal plasticity. The enriched environment (EE) paradigm has been demonstrated to induce neural plasticity in both developing and mature mammals. Others and we have recently demonstrated that adult rats placed within an enriched environment underwent central nervous system-wide increases in the expression levels for the IEGs NGFI-A and arc. The relationships between the altered expression profiles for both genes in response to an EE exposure, and their putative role in orchestrating network restructuring in response to enhanced environmental complexity are discussed.

Reconnecting neuronal networks in the auditory brainstem following unilateral deafening

When we disturbed the auditory input of the adult rat by cochleotomy or noise trauma on one side, several substantial anatomical, cellular, and molecular changes took place in the auditory brainstem. We found that: (1) cochleotomy or severe noise trauma both lead to a considerable increase of immunoreactivity of the growth-associated protein GAP-43 in the ventral cochlear nucleus (VCN) of the affected side; (2) the expression of GAP-43 in VCN is restricted to presynaptic endings and short fiber segments; (3) axon collaterals of the cholinergic medial olivocochlear (MOC) neurons are the path along which GAP-43 reaches VCN; (4) partial cochlear lesions induce the emergence of GAP-43 positive presynaptic endings only in regions tonotopically corresponding to the extent of the lesion; (5) judging from the presence of immature fibers and growth cones in VCN on the deafened side, at least part of the GAP-43 positive presynaptic endings appear to be newly formed neuronal contacts following axonal sprouting while others may be modified pre-existing contacts; and (6) GAP-43 positive synapses are formed only on specific postsynaptic profiles, i.e., glutamatergic, glycinergic and calretinin containing cell bodies, but not GABAergic cell bodies. We conclude that unilateral deafening, be it partial or total, induces complex patterns of reconnecting neurons in the adult auditory brainstem, and we evaluate the possibility that the deafness-induced chain of events is optimized to remedy the loss of a bilaterally balanced activity in the auditory brainstem.

Plasticity-driven gene expression in the rat retina

Animals exposed to an enriched environment display features of neural plasticity such as an increased brain volume, enhanced number of dendritic spines, as well as enlarged synapses. Here we report the first description of molecular plasticity in the mammalian retina, as revealed by gene expression. A marked upregulation of both NGFI-A and Arc, two candidate-plasticity genes, was observed in adult rats that had been exposed to an enriched environment for 3 weeks. This increase was paralleled by an increase in the expression of the late genes GAP-43 and Synapsin I, which also indicated changes in retinal connectivity. Our results suggest that both NGFI-A and Arc may regulate mechanisms of plasticity that had been invoked by heightened complexity of the visual environment.

Lesion-induced plasticity as a potential mechanism for recovery and rehabilitative training

Brain lesions not only cause a functional deficit in the lesion area, but also affect the structurally intact brain network connected to the lesion. In brain areas surrounding the lesion, as well as those remote from it, the structural and functional plasticity of the brain is increased because of an alteration of transmitter receptor expression and membrane properties of neurones. Within the penumbra of brain ischaemia, as well as after trauma, an additional perilesional dysfunctional zone is found that contributes to the neurological deficit. The lesion-induced plasticity can be used for adaptation, which also may restore function in the perilesional zone, if adequate rehabilitative training is performed.