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Chondroitin sulfate proteoglycans are major constituents of the extracellular matrix and form perineuronal nets. Information regarding the growth-inhibitory activity of these molecules after injury is rapidly expanding. However, less is known about their physiological role in the adult undamaged CNS. Here, we investigated the function of chondroitin sulfate proteoglycans in maintaining the proper structure of Purkinje axons in the cerebellum of adult rats. To this end, we examined the morphology and distribution of intracortical Purkinje neurites after intraparenchymal injection of chondroitinase ABC. Staining with the lectin Wisteria floribunda agglutinin or 2B6 antibodies showed that this treatment efficiently removed chondroitin sulfate proteoglycans from wide areas of the cerebellar cortex. In the same sites, there was a profuse outgrowth of terminal branches from the Purkinje infraganglionic plexus, which invaded the deeper regions of the granular layer. In contrast, myelinated axon segments were not affected and maintained their normal relationship with oligodendroglial sheaths. Purkinje axon sprouting was first evident at 4 d and increased further at 7 d after enzyme application. Within 42 d, the expression pattern of chondroitin sulfate proteoglycans gradually recovered, whereas axonal modifications progressively regressed. Our results show that, in the absence of injury or novel external stimuli, degradation of chondroitin sulfate proteoglycans is sufficient to induce Purkinje axon sprouting but not the formation of long-lasting synaptic contacts. Together with other growth-inhibitory molecules, such as myelin-associated proteins, chondroitin sulfate proteoglycans restrict structural plasticity of intact Purkinje axons to maintain normal wiring patterns in the adult cerebellar cortex.

In the last few years Purkinje cells have become a most interesting model to investigate cellular/molecular mechanisms of axon regeneration and plasticity. Adult Purkinje cells are most peculiar for their weak cell body response to axotomy, which is accompanied by a strong resistance to injury and a virtually absolute inability to regenerate severed neurites, even in the presence of favourable environmental conditions. The same neurons show a vigorous intrinsic inclination toward axonal sprouting and structural plasticity, which can be elicited by removing extrinsic growth-inhibitory cues. These features gradually develop during early postnatal life, but the underlying mechanisms and biological significance remain unclear. This article reviews recent studies aimed at addressing these questions with respect to the general issue of brain repair. Indeed, understanding the reasons for the extremely poor regenerative capacity of Purkinje cells will be most important to elucidate basic biological mechanisms of axon regeneration and plasticity, and to promote circuit rewiring in the adult CNS.

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Mammalian visual cortex is immature at birth and develops gradually during defined postnatal temporal windows. In the present work, we studied the maturation of astrocytes in developing mouse visual cortex (VC). The cellular distribution and the level of glial fibrillary acidic protein (GFAP) were analyzed by immunohistochemistry and Western blotting. Experiments were performed at different postnatal ages: postnatal day 12 (P12), before eye opening; P24, corresponding roughly to the peak of the critical period for monocular deprivation, and P60, after the end of the critical period. At P12, GFAP immunoreactivity (IR) was distributed throughout all cortical layers. At P24, there was a prominent localization of GFAP IR in layers I, II, and VI, while cortical layers III, IV, and V contained no longer GFAP IR cells. No differences were found in GFAP IR between P24 and P60. Western blot analysis revealed a reduction of GFAP expression in the VC at P24 with respect to P12 and no significant difference between P60 and P24. These results show that GFAP expression is modulated during early postnatal development. To know whether visual experience influences the maturation pattern of GFAP expression, mice were dark-reared from P12 to P24. Dark rearing did not change the distribution and the expression of GFAP. Our results indicate that maturation of GFAP expression occurs early in postnatal development in mouse VC. In addition, we showed that GFAP development is not affected by visual deprivation.

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