TOXIC NMDA-RECEPTOR ACTIVATION OCCURS DURING RECOVERY IN A TISSUE-CULTURE MODEL OF ISCHEMIA

In some animal models of reversible ischemia, there is a therapeutic w indow during early recovery when glutamate receptor antagonists can re scue neurons from injury. We have previously reported that organotypic cultures of the hippocampus can be protected by NMDA-receptor antagon ists during recovery from a brief period of simulated ischemia. To mod el ischemia, we have used potassium cyanide to inhibit oxidative metab olism and 2-deoxyglucose to inhibit glycolysis. To study the time cour se and mechanisms of delayed NMDA-receptor toxicity in more detail, we have extended these studies to dissociated cortical cultures, Injury was assessed by release of lactate dehydrogenase into the culture medi um, Metabolic inhibition for 15 min caused dose-dependent injury, Morp hologic signs of neuronal toxicity were delayed until the recovery per iod. MK-801 reduced injury significantly when present throughout the e xperiment, Surprisingly, MK-801 provided the same protection when admi nistration was delayed until after the end of the metabolic inhibition , blocking NMDA receptors only during recovery. To examine NMDA toxici ty during metabolic inhibition, the competitive NMDA-receptor antagoni st 3-(2-carboxypiperazin-4-yl) propyl-1-phosphonic acid was added duri ng exposure. The protective effect of NMDA-receptor blockade was compl etely lost if the antagonist was removed during 1 min of continuing se lective inhibition of oxidative metabolism. The toxic potency and effe ctiveness of glutamate were enhanced during metabolic inhibition, show ing that receptors were not inactivated by simulated ischemia, These r esults are consistent with the specific hypothesis that return of oxid ative metabolism triggers a critical period of toxic NMDA-receptor act ivation.