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Superoxide (·O₂^–) Production in CA1 Neurons of Rat Hippocampal Slices Exposed to Graded Levels of Oxygen

¹Department of Molecular Pharmacology and Physiology, College of Medicine, University of South Florida, Tampa, Florida; and ²Department of Neuroscience, Cell Biology and Physiology, College of Medicine, Wright State University, Dayton, Ohio

Submitted 21 September 2006; accepted in final form 31 May 2007

Neuronal signaling, plasticity, and pathologies in CA1 hippocampal neurons are all intimately related to the redox environment and, thus tissue oxygenation. This study tests the hypothesis that hyperoxic superfusate (95% O₂) causes a time-dependent increase in superoxide anion (·O₂^–) production in CA1 neurons in slices, which will decrease as oxygen concentration is decreased. Hippocampal slices (400 µm) from weaned rats were incubated with the fluorescent probe dihydroethidium (DHE), which detects intracellular ·O₂^– production. Slices were loaded for 30 min using 10 µM DHE and maintained using one-sided superfusion or continuously loaded using 2.5 µM DHE and maintained using two-sided superfusion (36°C). Continuous loading of DHE and two-sided superfusion gave the highest temporal resolution measurements of ·O₂^– production, which was estimated by the increase in fluorescence intensity units (FIUs) per minute (FIU/min ± SE) over 4 h. Superoxide production (2.5 µM DHE, 2-sided superfusion) was greatest in 95% O₂ (6.6 ± 0.4 FIU/min) and decreased significantly during co-exposure with antioxidants (100 µM melatonin, 25 µM MnTMPyP) and lower levels of O₂ (60, 40, and 20% O₂ at 5.3 ± 0.3, 3.3 ± 0.1, and 1.6 ± 0.2 FIU/min, respectively). CA1 cell death after 4 h (ethidium homodimer-1 staining) was greatest in 95% O₂ and lowest in 40 and 20% O₂. CA1 neurons generated evoked action potentials in 20% O₂ for >4 h, indicating viability at lower levels of oxygenation. We conclude that ·O₂^– production and cell death in CA1 neurons increases in response to increasing oxygen concentration product (= PO₂ x time). Additionally, lower levels of oxygen (20–40%) and antioxidants should be considered to minimize superoxide-induced oxidative stress in brain slices.