Degeneration of spared axons following partial white matter lesion: implications for optic nerve neuropathies

Neuroprotective therapy is a relatively new development in the approach to the treatment of acute and chronic brain damage. Though initially viewed in the framework of acute CNS injuries, the concept was recently extended to include chronic injuries, in which at any given time there are some neurons in an acute phase of degeneration coexisting with others that are healthy, marginally damaged, or dead. The healthy neurons and those that are only marginally damaged are the potential targets for neuroprotection. For the development of neuroprotective therapies, it is essential to employ an animal model in which the damage resulting from secondary degeneration can be quantitatively distinguished from primary degeneration. This is of particular relevance when the site of the damage is in the white matter (nerve fibers) rather than in the gray matter (cell bodies). In the present work we reexamine the concepts of secondary degeneration and neuroprotection in white matter lesions. Using a partial crush injury of the adult rat optic nerve as a model, we were able to assess both primary and secondary nerve damage. We show that neurons whose axons were not damaged or only marginally damaged after an acute insult will eventually degenerate as a consequence of their existence in the degenerative environment produced by the injury. This secondary degeneration does not occur in all of the neurons at once, but affects them in a stepwise fashion related to the severity of the damage inflicted. These findings, which may be applicable to the progression of acute or chronic neuropathy, imply that neuroprotective therapy may have a beneficial effect even if there is a time lag between injury and treatment.