Reparative mechanisms in the cerebellar cortex.

In the adult brain, different neuronal populations display different degrees of plasticity. Here, we describe the highly different plastic properties of inferior olivary neurones and Purkinje cells. Olivary neurones show a basal expression of growth-associated proteins, such as GAP-43 and Krox24/EGR-1, and remarkable remodelling capabilities of their terminal arbour. They also regenerate their transected neurites into growth-permissive territories and may reinnervate the lost target. Sprouting and regrowing olivary axons are able to follow specific positional information cues to establish new connections according to the original projection map. In addition, they set a strong cell body reaction to injury, which in specific olivary subsets is regulated by inhibitory target-derived cues. In contrast, Purkinje cells do not have a constitutive level of growth-associated genes, and show little cell body reaction, no axonal regeneration after axotomy, and weak sprouting capabilities. Block of myelin-derived signals allows terminal arbour remodelling, but not regeneration, while selective over-expression of GAP-43 induces axonal sprouting along the axonal surface and at the level of the lesion. We suggest that the high constitutive intrinsic plasticity of the inferior olive neurones allows their terminal arbour to sustain the activity-dependent ongoing competition with the parallel fibres in order to maintain the post-synaptic territory, and possibly underlies mechanisms of learning and memory. Such a plasticity is used also as a reparative mechanism following axotomy. In contrast, in Purkinje cells, poor intrinsic regenerative capabilities and myelin-derived signals stabilise the mature connectivity and prevent axonal regeneration after lesion.