CALPAIN ACTIVATION CONTRIBUTES TO DENDRITIC REMODELING AFTER BRIEF EXCITOTOXIC INJURY IN-VITRO

The calcium-dependent protease calpain may contribute to neuronal deat h in acute neurological insults and may be activated very early in the neuronal injury cascade. We assessed the role of calpain in a model o f rapid, reversible dendritic injury in murine cortical cultures. Brie f sublethal NMDA exposure (10-30 mu M for 10 min) resulted in focal sw ellings, or varicosities, along the length of neuronal dendrites as vi sualized with the lipophilic membrane tracer Dil or with immunostainin g using antibodies to the somatodendritic protein MAP2. These varicosi ties appeared within minutes of NMDA exposure and recovered spontaneou sly within 2 hr after NMDA removal. Addition of the calpain inhibitors MDL28,170, calpain inhibitors I and II, and leupeptin (all 1-100 mu M ) had little effect on the development of NMDA-induced dendrite injury . However, the resolution of varicosities was substantially delayed by addition of calpain inhibitors after sublethal excitotoxic exposure. Using Western blots and immunocytochemistry, we observed reactivity fo r a calpain-specific spectrin proteolytic fragment during the period o f recovery from dendritic swelling, but not during its formation. Spec trin breakdown product immunoreactivity could be blocked by the calpai n inhibitor MDL28,170 and appeared in neuronal cell bodies and neurite s in a time course that paralleled dendritic recovery. These observati ons suggest that calcium-dependent proteolysis contributes to recovery of dendritic structure after NMDA exposure. Calpain activation is not necessarily detrimental and may play a role in dendritic remodeling a fter neuronal injury.