Cortical Ablation Induces a Spreading Calcium-Dependent, Secondary Pathogenesis Which Can Be Reduced by Inhibiting Calpain

Many forms of acute brain injury are associated with a secondary, glutamate- and calcium-dependent cascade which greatly exacerbates the final damage. The calcium-dependent protease, calpain, has been implicated as an important variable in this pathogenic process. The present studies tested (i) if similar secondary degeneration occurs following surgical ablation of a discrete area within rat visual cortex, (ii) if activation of calpain contributes to the secondary degeneration by spreading into areas adjacent to the ablation, and (iii) if blocking calpain's proteolytic effects reduces the secondary degeneration attendant to the lesion. Antibodies selective for a protein fragment specifically generated by calpain were used to map areas in which the protease had been activated. Labeling was present 5 min after surgery in a narrow zone surrounding the ablated region. The volume of the immunopositive staining increased twofold within 24 h and fivefold by 48 h, at which time it was equivalent in size to the original lesion. This staining pattern significantly decreased in size at 5 days postsurgery. Application of calpain inhibitors to the ablation site immediately after surgery reduced the spread of calpain activation by approximately 80%. Following cortical ablation, the volume of the lateral geniculate nucleus ipsilateral to the cortical ablation shrank by 46 +/- 3% in control rats but only by 31 +/- 5% in animals given the calpain inhibitors. These results establish that (i) a secondary degenerative cascade is unleashed following discrete cortical surgery which expands into brain areas clearly outside the initial perturbation site, (ii) the gradual expansion of calpain activation contributes to the underlying secondary pathology, and (iii) blocking calpain activity substantially reduces atrophy in areas anatomically connected, but physically distal to the damaged zone. The possible utility of topical applications of calpain inhibitors, or analogously acting drugs, in minimizing the secondary effects of brain surgery is discussed.