Evidence that the Loss of the Voltage-Dependent Mg²⁺ Block at the N-Methyl-D-Aspartate Receptor Underlies Receptor Activation During Inhibition of Neuronal Metabolism

Abstract: In this study, the importance of the Mg²⁺ blockade of the N-methyl-D-aspartate (NMDA) receptor during metabolic stress was examined in embryonic day 13 chick retina. Retina exposed to mild conditions of metabolic stress (i.e., blockade of glycolysis with 1 mM iodoacetate for 30 min) underwent acute histological somal and neuritic swelling and an increase in γ-aminobutyric acid (GABA) release into the medium. These acute signs of metabolic stress were eliminated by NMDA antagonists present during pharmacological blockade of glycolysis, occurred in the absence of a net increase in extracellular glutamate or aspartate, and were not affected by the presence or absence of Ca²⁺ in the incubation medium. One possible explanation for the activation of NMDA receptors in the absence of an increase in extracellular ligand is that NMDA sensitivity during metabolic stress may be governed at the receptor level. Depolarization of membrane potential during metabolic stress may result in the loss of the Mg²⁺ blockade from the NMDA receptor channel, resulting in an increased potency for glutamate. To test this, the dose-response characteristics for NMDA, glutamate, and kainate in the presence or absence of extracellular Mg²⁺ and the effects of Mg²⁺ on metabolic inhibition were examined. The potency for NMDA- or glutamate-mediated acute toxicity was enhanced two- to fivefold in the absence of Mg²⁺. Omission of Mg²⁺ greatly decreased the minimal concentration of agonist needed to produce acute excitotoxicity; 25 versus 5 μM for NMDA and 300 versus 10 μM for glutamate in 1.2 or zero Mg²⁺, respectively. Elevating external Mg²⁺ to 20 mM completely protected against NMDA-mediated acute toxic effects. In contrast, varying external Mg²⁺ had no effect on kainate-induced toxicity. Acute toxicity caused by inhibition of metabolism was not potentiated in the absence of Mg²⁺ but was attenuated by elevating extracellular Mg²⁺. The protective effect of Mg²⁺ during metabolic inhibition was not additive with NMDA antagonists, suggesting that the action of Mg²⁺ was at the level of the NMDA receptor. These findings are consistent with the hypothesis that the Mg²⁺ block is lifted during metabolic inhibition and may be the primary event resulting in NMDA receptor activation.