INHIBITION OF THE ELECTROGENIC NA PUMP UNDERLIES DELAYED DEPOLARIZATION OF CORTICAL-NEURONS AFTER MECHANICAL INJURY OR GLUTAMATE

We previously characterized the electrophysiological response of corti cal neurons to a brief sublethal stretch-injury using an in vitro mode l of traumatic brain injury. This model revealed that cortical neurons undergo a stretch-induced delayed depolarization (SIDD) of their rest ing membrane potential (RMP) which is similar to 10 mV in magnitude. S IDD is dependent on N-methyl-D-aspartate (NIMDA) receptor activation, neuronal firing, and extracellular calcium for its induction but not i ts maintenance. SIDD was maximal 1 h after the insult and required inc ubation at 37 degrees C. The present study examined the mechanism medi ating SIDD and its relation to glutamate receptor activation. The Na p ump inhibitor ouabain was used to assess the contribution of the Na pu mp to tile RMP of control and stretched neurons using whole cell patch -clamp techniques. The nitric oxide(NO) synthase inhibitor N omega-nit ro-L-arginine and a polyethylene glycol conjugate of superoxide dismut ase were used to assess whether NO or superoxide anion, respectively, were involved in the induction of SIDD. Neurons were exposed to exogen ous glutamate in the absence of cell stretch to determine whether glut amate alone can mimic SIDD. We report that SIDD is mediated by Na pump inhibition and is likely to result from reduced energy levels since t he RMP of neurons dialyzed with a pipette solution containing 5 mM ATP were identical to controls. NO, but not superoxide anion, also may co ntribute to SIDD. A 3-min exposure to 10 mu M glutamate produced a SID D-like depolarization also associated with Na pump inhibition. The res ults suggest that Na pump inhibition secondary to alterations in cellu lar energetics underlies SIDD. Na pump inhibition due to glutamate exp osure may contribute to traumatic brain injury or neurodegenerative di seases linked to glutamate receptor activation.