Regenerative events in the olivocerebellar pathway.

Mature neurons display a wide range of regenerative capabilities. As a general rule, peripheral neurons have the highest regenerative abilities both in the form of terminal sprouting and of axonal elongation following axotomy, regardless of the distance of the lesion from the cell body. In contrast, in central neurons reactive sprouting has been demonstrated in a limited number of neuronal populations and this type of growth may be dependent on the constitutive presence of specific growth-associated proteins. Central axon elongation is critically dependent on the presence of suitable environment and on the intrinsic capabilities of each neuronal population. These capabilities are controlled at least in part by repressive signals that are mainly located along the axons. They are more easily disclosed when a short axon stump is left after axotomy. The adult olivary neurons offer a unique model in the central nervous system for their remarkable plastic properties: i) they undergo extensive remodeling of their terminal arborizations following target manipulations or under the influence of electrical activity; ii) they are capable of axonal regeneration in a suitable environment; iii) their response to injury does not depend on the distance of the axotomy from the cell body. In this respect they are similar to peripheral neurons and likely their target cells are the main source of the repressive signals control-ling growth genes. The demonstration that this pathway is also able to find the proper target cells provides a striking example of how the mature brain may be repaired through appropriate manipulations.