CHARACTERIZATION OF A PROLONGED REGENERATIVE ATTEMPT BY DIFFUSELY INJURED AXONS FOLLOWING TRAUMATIC BRAIN INJURY IN ADULT CAT - A LIGHT ANDELECTRON-MICROSCOPIC IMMUNOCYTOCHEMICAL STUDY

Traumatic brain injury in animals and humans is well known to cause ax onal damage diffusely scattered throughout the brain without evidence of other brain parenchymal change. This observation has prompted some to posit that such damaged axons are well positioned to mount a regene rative attempt. The present study uses an immunocytochemical marker sp ecific for regenerating neurites to explore this issue. Further, in an attempt to expedite and enhance any potential regenerative effort, th is study evaluates the efficacy of intrathecally applied nerve growth factor. Three sets of experiments were performed in adult cats. One gr oup of animals was subjected to moderate fluid percussion brain injury and followed for 7 or 14 days post injury, with the continuous intrav entricular infusion of nerve growth factor delivered by implanted osmo tic pumps. These animals were compared to a second group of time-match ed, sham-operated animals receiving artificial cerebrospinal fluid inf usion. To assess axonal damage immunohistochemical staining for the lo w molecular weight neurofilament subunit (NF-L) was carried out using an NR4 monoclonal antibody. To localize axons exhibiting a regenerativ e response immunohistochemical staining for the growth associated prot ein GAP43 was employed. In sham controls, at the light microscopic lev el NF-L-immunoreactive axonal swellings were numerous at 7 days, but b y 14 days post injury their frequency declined markedly. In contrast, GAP43-immunoreactive, disconnected reactive axonal swellings were rare ly observed at 7 days but were numerous at 14 days. Ultrastructural an alysis at 14 days post injury of carefully matched sections revealed r eactive axons demonstrating sprouting consistent with a regenerative e ffort. Analysis of tissue from animals of 14 days of survival indicate d that supplementation with nerve growth factor did not appear to enha nce the capacity of damaged brain axons to mount a regenerative attemp t. Rather, it appears that regenerative efforts seen reflect a spontan eous response. A third group of adult cats, subjected to the same inju ry but not subjected to osmotic pump implantation was allowed to survi ve for 22-28 days. Animals in this group also demonstrated GAP43 immun oreactivity in reactive axonal swellings in the brain stem. This study demonstrates that diffusely injured axons can mount a sustained regen erative attempt that is associated with a reorganization of their cyto skeleton and accompanied by an up-regulation of growth-associated prot eins.