Search

In the CNS short episodes of acute hypoxia can result in a decrease in synaptic transmission which may be fully reversible upon re-oxygenation. Stabilization of hypoxia-inducible factor (HIF) by inhibition of prolyl hydroxylase domain (PHD) enzymes has been shown to regulate the cellular response to hypoxia and confer neuroprotection both in vivo and in vitro. Hypoxic preconditioning has become a novel therapeutic target to induce neuroprotection during hypoxic insults. However, there is little understanding of the effects of repeated hypoxic insults or pharmacological PHD inhibition on synaptic signaling. In this study we have assessed the effects of hypoxic exposure and PHD inhibition on synaptic transmission in the rat CA1 hippocampus. Field excitatory postsynaptic potentials (fEPSPs) were elicited by stimulation of the Schaffer collateral pathway. 30 min hypoxia (gas mixture 95% N 2 /5% CO 2 ) resulted in a significant and fully reversible decrease in fEPSP slope associated with decreases in partial pressures of tissue oxygen. 15-30 min of hypoxia was sufficient to induce stabilization of HIF in hippocampal slices. Exposure to a second hypoxic insult after 60 min resulted in a similar depression of fEPSP slope but with a significantly greater rate of recovery of the fEPSP. Prior single treatment of slices with the PHD inhibitor, dimethyloxalylglycine (DMOG) also resulted in a significantly greater rate of recovery of fEPSP post hypoxia. These results suggest that hypoxia and 'pseudohypoxia' preconditioning may improve the rate of recovery of hippocampal neurons to a subsequent acute hypoxia., (Copyright © 2018 Elsevier B.V. All rights reserved.)

The contributions of hypoxia and oxidative stress to the pathophysiology of acute ischemic stroke are well established and can lead to disruptions in synaptic signaling. Hypoxia and oxidative stress lead to the neurotoxic overproduction of reactive oxygen species (ROS) and the stabilization of hypoxia inducible factors (HIF). Compounds such as prolyl-4-hydroxylase domain enzyme inhibitors (PHDIs) have been shown to have a preconditioning and neuroprotective effect against ischemic insults such as hypoxia, anoxia, oxygen glucose deprivation (OGD) or H2O2. Therefore, this study explored the effects of two PHDIs, JNJ-42041935 (10 µM) and roxadustat (100 µM) on cell viability using organotypic hippocampal slice cultures. We also assessed the effects of these compounds on synaptic transmission during and post hypoxia, OGD and H2O2 application in isolated rat hippocampal slices using field recording electrophysiological techniques and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunit trafficking using immunohistochemistry. Our organotypic data demonstrated a protective role for both inhibitors, where slices had significantly less cell death post anoxia and OGD compared to controls. We also report a distinct modulatory role for both JNJ-42041935 and roxadustat on fEPSP slope post hypoxia and OGD but not H2O2. In addition, we report that application of roxadustat impaired long-term potentiation, but only when applied post-hypoxia. This inhibitory effect was not reversed with co-application of the cyclin-dependent kinase 5 (CDK-5) inhibitor, roscovitine (10 µM), suggesting a CDK-5 independent synaptic AMPAR trafficking mechanism. Both hypoxia and OGD induced a reduction in synaptic AMPA GluA2 subunits, the OGD effect being reversed by prior treatment with both JNJ-42041935 and roxadustat. These results suggest an important role for PHDs in synaptic signaling and plasticity during episodes of ischemic stress.