Neuroscience edition:2005 location:Washington DC date:12-16 November 2005
For most neurons it is unclear how they sense anoxia and which molecular events trigger their initial response to O2 withdrawal. In the accompanying poster (Hepp, Gerich, Mueller) we show that inhibition of mitochondria does not necessarily hasten the onset of hypoxic spreading depression (HSD) and that it seems the mitochondrial depolarization or redox changes rather than accelerated ATP depletion that favor HSD onset. Since mitochondrial metabolism affects cytosolic redox status by generating reactive oxygen species (ROS) and oxidizing NADH, we asked, whether such redox changes modulate the susceptibility of hippocampal tissue slices to HSD. Application of H2O2 (1 mM) postponed HSD onset. Direct oxidation of sulfhydryl-groups by dithionitrobenzoic acid (2 mM) also caused a postponement of HSD, due to activation of BK channels. By contrast, inhibition of superoxide dismutase by diethyldithiocarbamate (1 mM) was without effect. Probing for the generation of ROS by using the redox-sensitive dyes dihydroethidium and dichlorofluorescein did not reveal a marked ROS generation during the short episodes of anoxia (3-5 min) which are sufficient to trigger HSD. Also, combined application of the antioxidants ascorbic adic (1 mM) and trolox (0.75 mM) did not induce any changes in HSD parameters. Reducing conditions, such as application of dithiothreitol (2 mM) which reduces sulfhydryl groups or increasing cellular NADH by inhibiting mitochondrial respiration hastened HSD. Also, inhibition of glutathione peroxidase by mercaptosuccinate (2 mM) hastened HSD onset. In conclusion, reducing conditions hasten HSD onset, while oxidizing conditions postponed it. Therefore, it might be a shift of cytosolic redox status to reducing conditions, caused by e.g. mitochondrial dysfunction and accumulation of NADH, possibly by involvement of other redox systems such as the glutathione system, which are involved in the modulation of the early anoxic response of CA1 neurons.