Superoxide Production Increases in Nucleus Tractus Solitarius (NTS) Neurons in Rat Brain Slices during Acute Normobaric Hyperoxia and Hypoxia

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Conference Proceeding

Publication Date

11-17-2008

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Abstract

We previously reported that hyperbaric hyperoxia stimulates firing rate of putative CO2-chemoreceptors in the solitary complex of the dorsocaudal medulla oblongata in rat brain slices (JAP 95: 910-921, 2003). We next reported that the typical control level of 95%O2 is a greater source of redox stress than ≤ 40%O2 leading to increased cell death in brain slices (J. Neurophysiol. 98:1030-1041, 2007). In the present study we used 20-40%O2 as the control to test the hypothesis that normobaric hyperoxia and hypoxia increase the rate of superoxide production (·O2-) in NTS neurons. Brain slices (400μm, 36-37oC) were maintained using 1- or 2-sided superfusion. Brainstem neurons maintained in 20-40%O2 (5%CO2, balance N2) exhibited i) whole-cell/intracellular activity for many hours, ii) CO2 chemosensitivity (10-15%CO2) and iii) were stimulated by hyperoxia (60-95%O2). ·O2- production was measured (3 min intervals) using the fluorogenic probe, dihydroethidium (2.5μM), continuously loaded via the superfusate. The rate of ·O2- production (slope of fluorescence intensity units/min, FIU/min) increased during acute hyperoxia (20 to 95%O2, 15-20min). Likewise, FIU/min increased during hypoxia (40/20% to 0%O2, 10-20min). ·O2- production during hypoxia was dependent on a lower threshold tissue pO2 that is estimated to be well below 20 Torr based on measurements of tissue slice pO2. ·O2- production during hypoxia was repeatedly induced using 95%N2-5%CO2 during either 1) 1-sided slice superfusion or 2) in combination with an O2-scavenger (1mM Na2SO3) during 2-sided slice superfusion. Myxothiazol (10μM; an inhibitor of Complex III) decreased ·O2- production during hypoxia but had little effect during hyperoxia. This suggests that mitochondrial Complex III is the primary source of ·O2- during hypoxia but not hyperoxia in NTS neurons. Preliminary experiments in CA1 hippocampus and Inferior olive indicate that these neurons do not increase their rate of ·O2- production during hypoxia/Na2SO3. We posit that the similar pattern of ·O2- production in NTS neurons activated by hypoxia and hyperoxia renders these cardio-respiratory neurons vulnerable to redox stimulation and/or stress during sleep disordered breathing (episodic hypoxia, reoxygenation and rebound hyperoxia) and during exposure to normobaric and hyperbaric hyperoxia

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Presented at the Society for Neuroscience Annual Meeting, Washington, DC, November 15-19, 2008.

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