Effects of N-Methyl-d-Aspartate on [Ca2+]i and the Energy State in the Brain by 19F- and 31P-Nuclear Magnetic Resonance Spectroscopy

The effects of N-methyl-D-aspartate (NMDA) on the free intracellular Ca2+ concentration ( Ca2+]i) and the energy state in superfused cerebral cortical slices have been studied using 19F- and 31P-nuclear magnetic resonance spectroscopy. Ca2+]i was measured using the calcium indicator 1,2-bis(2-amino-5-fluorophenoxy)ethane-N,N,N',N'-tetraacetic acid (5FBAPTA). NMDA (10 microM) in the absence of extracellular Mg2+ caused the expected rise in Ca2+]i but produced an impairment of the energy state: the phosphocreatine (PCr) content was decreased by 42%, and the Pi/PCr ratio was increased by 55%. There was no detectable change in ATP or free intracellular Mg2+ concentration. Increasing the NMDA concentration in the superfusing medium to 100 or 400 microM caused no further increase in Ca2+]i or further decrease in PCr content, but the Pi/PCr ratio continued to rise. The impairment of the energy state preceded the effect on Ca2+]i, and these changes were irreversible on return to control conditions. Repeating the experiments in the presence of 1.2 mM extracellular Mg2+ resulted in similar changes in the energy state, with no change in Ca2+]i. The possibilities that the effects were due to membrane depolarisation or to the presence of 5FBAPTA within the tissues were eliminated. The results suggest that low concentrations (10 microM) of NMDA produce an impaired energy state independent of the presence of extracellular Mg2+ and that the decreased energy state is not due to the changes in Ca2+]i, which are seen only in the absence of extracellular Mg2+.