Redox modulation of NMDA receptor-mediated Ca2+ flux in mammalian central neurons

NMDA receptor-mediated Ca2+ flux was studied in cultured rat retinal ganglion cells and neocortical neurons. Intracellular free calcium (Ca2+]i was measured with fura-2 fluorescence imaging. Baseline Ca2+]i was 59 +/- 5 nM. In low Mg2+]o, 200 microM NMDA reversibly increased Ca2+]i to 421 +/- 70 nM. This rise in Ca2+]i was blocked by the NMDA antagonists APV (200 microM) or Mg2+]o (1 mM), but only slightly inhibited by the non-NMDA antagonist CNQX (10 microM). Chemical reduction with dithiothreitol (DTT) had no effect on resting Ca2+]i. However, DTT increased the NMDA-induced rise in Ca2+]i approximately 1.6-fold; the oxidizing agent dithiobisnitrobenzoic acid (DTNB) reversed this effect. In patch-clamp experiments, DTT increased NMDA-activated whole-cell conductance approximately 1.7-fold in low and high Ca2+]o. The Ca2+/Na+ permeability ratio of approximately 7 for NMDA channels remained unaltered by chemical reduction. Thus, redox modulation of the NMDA receptor/channel complex results in a dramatic alteration in current magnitude but no change in ionic permeabilities.