AXONAL INJURY CAUSED BY FOCAL CEREBRAL-ISCHEMIA IN THE RAT

The susceptibility of axons to blunt head injury is well established. However, axonal injury following cerebral ischemia has attracted less attention than damage in gray matter. We have employed immunocytochemi cal methods to assess the vulnerability of axons to cerebral ischemia in vivo. Immunocytochemistry was performed using antibodies to a synap tosomal-associated protein of 25 kDa (SNAP25), which is transported by fast anterograde transport; the 68-kDa neurofilament subunit (NF68kD) ; and microtubule-associated protein 5 (MAPS) on sections from rats su bjected to 30 min and 1, 2, and 4 h of ischemia induced by permanent m iddle cerebral artery (MCA) occlusion. After 4 h of occlusion, there w as increased SNAP25 immunoreactivity, which was bulbous in appearance, reminiscent of the axonal swellings that occur following blunt head i njury. Increased SNAP25 immunoreactivity was present in circumscribed zones in the subcortical white matter and in the axonal tracts at the border of infarction, a pattern similar to that previously described f or amyloid precursor protein. Although less marked, similar changes in immunoreactivity in axons were evident following 2 h of ischemia. MAP S and NF68kD had striking changes in immunoreactivity in axonal tracts permeating the caudate nucleus within the MCA territory at 4 h. The a ppearance was roughened and disorganized compared with the smooth regu lar staining in axons within the nonischemic areas. Profiles reminisce nt of axonal bulbs were evident in MAPS-stained sections. The changes seen with NF68kD and MAP5 were also evident at 2 h but were more subtl e at 1 h. There were no changes in axonal immunoreactivity with SNAP25 or NF68kD at 30 min after MCA occlusion. Altered immunoreactivity fol lowing ischemia using SNAP25, MAPS, and NF68kD provides further eviden ce for the progressive breakdown of the axonal cytoskeleton following an ischemic insult. NF68kD and MAP5 appear to be sensitive markers of the structural disruption of the cytoskeleton, which precedes the subs equent accumulation of SNAP25 within the damaged axons. Axonal cytoske letal breakdown and disruption of fast axonal transport, which are wel l-recognized features of traumatic brain injury, are also sequalae of an ischemic insult.