Figure 1.

Stimulus-evoked field potentials and O₂ and NADH changes. A, Diagram of the electrode positions and area imaged. B, Inset: Specimen field potentials evoked at 0.33 Hz, in the absence and presence of TTX. Main panel: Trace from inset without TTX minus the trace in TTX, annotated to show how field potential amplitudes were measured. C, Decrease of extracellular [O₂] evoked by 10 s of 20 Hz stimulation. D, Biphasic NADH response (change in fluorescence/fluorescence at the start of stimulation, ΔF/F0) evoked by 10 s of 20 Hz stimulation in the stratum radiatum (s.r.), stratum pyramidale (s.p.), and stratum oriens (s.o.). E–G, Resting NADH fluorescence (E), decrease of NADH fluorescence evoked by 20 Hz stimulation (F), and subsequent increase of NADH fluorescence above resting level (G) in the s.r., s.p., and s.o. All p values are from paired t tests. H, How the rates of glycolysis, the TCA cycle, and oxidative phosphorylation determine whether the NADH level falls or rises. An increase in the rate of oxidative phosphorylation will lower [NADH]. An increase in the rate of glycolysis (in astrocytes or in neurons) will increase [NADH], if not balanced by NADH consumption in the conversion of pyruvate to lactate, as will an increase in the rate of the tricarboxylic acid cycle. Lactate dehydrogenase is proposed to convert pyruvate to lactate in astrocytes, and lactate to pyruvate in neurons.