Cytosolic free calcium and NAD/NADH redox state in the cat cortex during in vivo activation of NMDA receptors.

Activation of the N-methyl-D-aspartate (NMDA) receptors and the concomitant Ca2+ entry have been implicated in neuronal injury in a variety of pathological states. The effects of extracellular Mg2+ concentrations and D,L-2-amino-5-phosphonovaleric acid (APV), a competitive NMDA receptor antagonist on the NMDA-induced responses were investigated in vivo. In vivo fluorometric measurements were made of changes in cytosolic free Ca2+ ([Ca2+]i) and NADH fluorescence directly from the cat cortex using indo-1, a fluorescent Ca2+ indicator. Changes in [Ca2+]i were assessed utilizing the ratio of indo-1 emission at two wavelengths (400 and 506 nm) during excitation with ultraviolet light (340 nm). Application of 100 microM NMDA to the cortex produced a significant increase in the [Ca2+]i signal ratio at physiological concentrations of Mg2+ (1.2 mM). This increase was enhanced in the absence of Mg2+ and was completely blocked either at 5 mM Mg2+ or in the presence of 50 microM APV. The NAD/NADH redox state was initially oxidized, which was also blocked by either high Mg2+ or APV. The application of NMDA elicited characteristic electroencephalogram (EEG) changes consisting of a marked reduction in amplitude and regular spikes (17-20 Hz). These EEG changes did not appear in the presence of APV. In addition to NMDA receptor antagonists, the level of extracellular Mg2+ is a potent physiological modulator of the NMDA response.