Interaction between tissue oxygen tension and NADH imaging during synaptic stimulation and hypoxia in rat hippocampal slices

Oxygen and NADH are essential components in the production of ATP in the CNS. This study examined the dynamic interaction between tissue oxygen tension (pO2) and NADH imaging changes within hippocampal tissue slices, during metabolic stresses including hypoxia and synaptic activation. The initiation of abrupt hypoxia (from 95% O2 to 95% N2) caused a rapid decrease in pO2, onset of hypoxic spreading depression (hsd; at 6.7±1.3 mm Hg; n=15), and a monophasic increase in NADH. Provided that reoxygenation was prompt, synaptic responses, pO2 and NADH levels returned to baseline following hsd. Longer hypoxia caused irreversible neuronal dysfunction, an increase in pO2 beyond baseline (due to decreased tissue demand), and hyperoxidation of NADH (10±2% decrease below baseline; n=7). Synaptic activation in ambient 95% O2 caused a decrease or 'initial dip' in pO2 and a biphasic NADH response (oxidation followed by reduction). The oxidizing phase of the NADH response was mitochondrial as it was synchronous with the 'initial' dip in pO2. Following slow graded reductions in ambient oxygen levels to 8%, four of seven slices developed hsd following synaptic stimulation. The hypoxic threshold for graded oxygen reductions occurred at 7.9±5.8 mm Hg O2 (n=7). Our hypoxic threshold range (6.7-7.9 mm Hg O2 from abrupt and graded oxygen reduction, respectively) correlates well with reported in vivo values of <12 mm Hg O2. The major findings of this study include: 1) determination of the critical physiological threshold of pO2 (based upon hsd), which is a marker of imminent neuronal death if oxygen is not rapidly restored; 2) NADH hyperoxidation and an increase in pO2 beyond baseline levels following longer periods of hypoxia; and 3) the occurrence of a pO2 'dip' during synaptic stimulation, which correlates with the early oxidizing phase of the biphasic NADH response.