Stretch-induced injury of cortical neurons in culture

The cellular mechanisms underlying traumatic brain injury (TBI) are not well understood. To gain insight to the processes which may contribute to the pathophysiology of TBI a novel cellular model of injury has been adopted. This model, developed by Dr. Earl F. Ellis (Dept. of Pharmacology & Toxicology, MCV), involves culturing cortical neurons on a stretchable silastic membrane. A 50 ms pressure pulse is applied to the cultured cells which causes a rapid and reversible deformation of the silastic membrane. This deformation stretches adherent neurons and astrocytes mimicking some of the forces encountered in vivo. After mechanical pertubation, current-clamp recordings were made from neurons in the whole-cell patch clamp configuration. Recordings of membrane potential revealed that following sublethal deformation, neuronal resting membrane potential (RMP) is depolarized by approximately 10 mV without a loss of input resistance. This depolarization is not evident immediately following stretch but appears to be maximal by 1 hour after the insult. The RMP of neurons recovers to control levels by 24 hours. This stretch-induced delayed depolarization (or SIDD) is dependent on the amount of cell stretch. The induction but not the maintenance of SIDD requires neuronal firing, the presence of extracellular calcium, and NMDA receptor activation. SIDD occurs without the clear involvement of any second messenger system or the production of reactive oxygen species. Reduction of electrogenic Na{dollar}sp+{dollar}, K{dollar}sp+{dollar}-ATPase (Na pump) activity mediates SIDD. A reduction of intracellular ATP levels underlies the loss of Na pump function. Inhibition of the Na pump appears to be mediated via loss of ATP at a modulatory site that does not require ATP hydrolysis. Mitochondrial dysfunction is proposed to mediate the reduction in cellular ATP levels. SIDD represents the first report of a delayed alteration in neuronal membrane properties in response to trauma. The pharmacology for the induction of SIDD shows similarities with that seen for glutamate-induced delayed neuronal death. Sublethal glutamate exposure produces a long-lasting depolarization that appears to be associated with a loss of the Na pump suggesting that these phenomena have common underlying mechanisms. These results suggest mitochondrial dysfunction produced via excessive NMDA receptor activation contributes to SIDD and is likely to contribute to the pathophysiology of TBI and other related pathologies.