Alterations in neurofilaments associated with reactive brain changes and axonal sprouting following acute physical injury to the rat neocortex

In order to study the changes in axons related to acute localized physical trauma, a 25 gauge needle was inserted into the somatosensory cortex of ana esthetized adult rats. Animals were examined over 11 time points. From 30 m in to 14 days postinjury. Initially. the central needle tract was surrounde d by 'reactive' abnormal axons characterized by their bulb- or ring-like im munoreactivity for neurofila ments. Quantification demonstrated that these structures reached a peak density at 24 h postinjury, followed by a gradual decrease over 2 weeks. By 5 days postinjury, long axons showing high level s of neurofilament labelling were localized to the lesion area, either alig ned parallel to the tract edges or extending into the bridge of tissue form ing between the tract edges. Double-labelling demonstrated a close associat ion between sprouting axons and ferritin-labelled microglia. Immunolabellin g for GAP43 also demonstrated the presence of sprouting axons within this t issue bridge. Ultrastuctural examination showed that sprouting axons contai ned a high density of neurofilaments, with a leading edge lacking these fil aments. Injury to the adult neocortex is associated with reactive and sprou ting changes within axons, coordinated with the proliferation of microglia and wound healing. These data also support a role for neurofilaments in axo nal sprouting following brain injury.