Axonal regeneration after crush injury of rat central nervous system fibres innervating peripheral nerve grafts.

Recent experimental studies in adult rodents indicate that neurons in many regions of the brain and spinal cord are capable of extensive axonal growth along peripheral nerve grafts inserted into the C.N.S. To explore further the capacity of damaged intrinsic C.N.S. neurons to initiate and sustain fibre growth we have studied the regenerative response of brain stem and spinal neurons to the crushing of their axons after such axons had already grown across peripheral nerve 'bridges' linking both these levels of the neuraxis. In adult rats, an autologous segment of sciatic nerve approximately 4 cm long was used to connect the medulla oblongata and the lower cervical spinal cord. After 6-42 weeks, when C.N.S. axons are known to have regenerated across these 'bridges', the nerve grafts were crushed near both their rostral and caudal insertions into the C.N.S. Axonal regeneration beyond the sites of injury was investigated 4-11 weeks after crush by retrogradely labelling C.N.S. neurons with horseradish peroxidase (HRP) applied 1 cm away from the injured site, along the assumed course of the C.N.S. fibres regrowing across the graft. The number and distribution of HRP-labelled neurons was found to be similar to that in rats with uncrushed grafts. To prove that such axonal regrowth from spinal and brain stem nerve cells did originate from injury of central nerve fibres innervating the graft and not by sprouting from undamaged C.N.S. neurons at both ends of the 'bridge', we first labelled with the fluorescent dye Fast Blue (FB) the cells whose axons were interrupted by the crush and, after two weeks, applied a second dye, Nuclear Yellow (NY) 1 cm beyond the site of injury. The presence of FB and NY double-labelled C.N.S. neurons in these animals, together with the results of the HRP-labelling experiments, suggest that central neurons whose axons innervate peripheral nerve grafts are capable of renewed growth after axonal injury. Under such experimental conditions these intrinsic C.N.S. neurons respond to axonal interruption in a manner that resembles the responses of cells that normally project along peripheral nerves. We believe this to be an additional indication of the powerful role in regeneration of interactions between neurons and the axonal environment.