Figure 4.

The depth of the mitochondrial NADH dip depends on the magnitude of the excitation-induced energetic load. (A) Cells were stimulated with varying activation strengths over a constant stimulation period of 10 seconds. (B) Cytosolic NADH transients. (C) Mitochondrial NADH transients. (D) Relative changes in mitochondrial (solid line) and cytosolic (dashed line) NADH concentrations. (E) Cytosolic NAD/NADH ratios. (F) Mitochondrial NAD/NADH ratios. (G) FAD transients (dotted line, dashed line, solid line, and dash-dotted line) are almost identical in shape but differ in magnitude depending on the midpoint potential of the binding enzyme moiety. (H) Mitochondrial NADH transient (solid black line) and average NADH transients (gray dashed line) obtained by weighing the transients shown in B and D by the relative volumes of mitochondria and cytosol and the fluorescence efficiency of NADH in these compartments. The inlet in A shows experimentally determined dip sizes:¹³ (1) control, (2) inhibition of postsynaptic currents and postsynaptic action potentials by the drugs 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo[f]quinoxaline-2,3-dione and D-(-)-2-Amino-5-phosphonopentanoic acid (NBQX+AP5), (3) inhibition of presynaptic transmitter release and downstream postsynaptic events by Cd²⁺, and (4) complete blocking of excitability by tetrodotoxin (TTX).