In the CNS, neurones are highly sensitive to the availability of oxygen. In conditions where oxygen availability is decreased, neuronal function can be altered, leading to injury and cell death. Hypoxia has been implicated in a number of central nervous system pathologies including stroke, head trauma and neurodegenerative diseases. Cellular responses to oxygen deprivation are complex and result in activation of short‐ and long‐term mechanisms to conserve energy and protect cells. Failure of synaptic transmission can be observed within minutes following this hypoxia. The acute effects of hypoxia on synaptic transmission are primarily mediated by altering ion fluxes across membranes, pre‐synaptic effects of adenosine and other actions at glutamatergic receptors. A more long‐term feature of the response of neurones to hypoxia is the activation of transcription factors such as hypoxia‐inducible factor. The activation of hypoxia‐inducible factor is governed by a family of dioxygenases called hypoxia‐inducible factor prolyl 4 hydroxylases (PHDs). Under hypoxic conditions, PHD activity is inhibited, thereby allowing hypoxia‐inducible factor to accumulate and translocate to the nucleus, where it binds to the hypoxia‐responsive element sequences of target gene promoters. Inhibition of PHD activity stabilizes hypoxia‐inducible factor and other proteins thus acting as a neuroprotective agent. This review will focus on the response of neuronal cells to hypoxia‐inducible factor and its targets, including the prolyl hydroxylases. We also present evidence for acute effects of PHD inhibition on synaptic transmission and plasticity in the hippocampus.

Publication informationNeurobiology of Disease, 62 (February 2014): 8-17 Publisher Elsevier Item record/more information http://hdl.handle.net/10197/4791 Publisher's statementThis is the author's version of a work that was accepted for publication in Neurobiology of Disease. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Neurobiology of Disease (In press (2013) AbstractChronic intermittent hypoxia (CIH) is an underlying component of obstructive sleep apnoea and has been shown to have deleterious and damaging effects on central neurons and to impair synaptic plasticity in the CA1 region of the rat hippocampus. CIH has previously been shown to impair synaptic plasticity and working memory. CIH is a potent inducer of hypoxia inducible factor (HIF), a key regulator in a cell's adaptation to hypoxia that plays an important role in the fate of neurons during ischemia. Levels of HIF-1α are regulated by the activity of a group of enzymes called HIF-prolyl 4-hydroxylases (PHDs) and these have become potential pharmalogical targets for preconditioning against ischemia. However little is known about the effects of prolyl hydroxylase inhibition and CIH on synaptic transmission and plasticity in sub-regions of the hippocampus. Male Wistar rats were treated for 7-days with either saline, CIH or PHD inhibition (dimethyloxaloylglycine, DMOG; 50mg/kg, i.p.). At the end of treatment all three groups showed no change in synaptic excitability using paired pulse paradigms. However long-term potentiation (LTP) was impaired in the CA1 region of the hippocampus in both CIH and DMOG treated animals. LTP induced in the dentate gyrus was not significantly affected by either CIH or DMOG treatment. We also investigated the effect of 7-day CIH and DMOG treatment on the recovery of synaptic transmission following an acute 30 min hypoxic insult. CIH treated animals showed an improved rate of recovery of synaptic transmission following re-oxygenation in both the CA1 and dentate gyrus. These results suggest that LTP induction in the CA1 region is more sensitive to both CIH and DMOG treatment than the dentate gyrus.

Hypoxia-inducible factors (HIFs) are key transcriptional regulators that play a major role in oxygen homeostasis. HIF activity is tightly regulated by oxygen-dependent hydroxylases, which additionally require iron and 2-oxoglutarate as cofactors. Inhibition of these enzymes has become a novel target to modulate the hypoxic response for therapeutic benefit. Inhibition of prolyl-4-hydroxylase domains (PHDs) have been shown to delay neuronal cell death and protect against ischemic injury in the hippocampus. In this study we have examined the effects of prolyl hydroxylase inhibition on synaptic transmission and plasticity in the hippocampus. Field excitatory postsynaptic potentials (fEPSPs) and excitatory postsynaptic currents (EPSCs) were elicited by stimulation of the Schaffer collateral pathway in the CA1 region of the hippocampus. Treatment of rat hippocampal slices with low concentrations (10 µM) of the iron chelator deferosoxamine (DFO) or the 2-oxoglutarate analogue dimethyloxalyl glycine (DMOG) had no effect on fEPSP. In contrast, application of 1 mM DMOG resulted in a significant decrease in fEPSP slope. Antagonism of the NMDA receptor attenuated the effects of DMOG on baseline synaptic signalling. In rat hippocampal slices pretreated with DMOG and DFO the induction of longterm potentiation (LTP) by tetanic stimulation was strongly impaired. Similarly, neuronal knockout of the single PHD family member PHD2 prevented murine hippocampal LTP. Pre-conditioning of PHD2 deficient hippocampi with either DMOG, DFO or the PHD specific inhibitor JNJ-42041935, did not further decrease LTP suggesting that DMOG and DFO influences synaptic plasticity primarily by inhibiting PHDs rather than unspecific effects. These findings provide striking evidence for a modulatory role of PHD proteins on synaptic plasticity in the hippocampus.

Leptin, a peptide hormone released by adipose tissue, acts on the hypothalamus to control cravings and appetite. Leptin also acts to decrease taste responses to sweet substances, though there is little detailed information regarding where leptin acts in the taste transduction cascade. The present study examined the effects of leptin on sweet-evoked responses and neuro transmitter release from isolated taste buds. Our results indicate that leptin moderately decreased sweet-evoked calcium mobilization in isolated mouse taste buds. We also employed Chinese hamster ovary biosensor cells to examine taste transmitter release from isolated taste buds. Leptin reduced ATP and increased serotonin release in response to sweet stimulation. However, leptin has no effect on bitter-evoked transmitter release, further showing that the action of leptin is sweet specific. Our results support those of previous studies, which state that leptin acts on taste tissue via the leptin receptor, most likely on Type II (Receptor) cells, but also possibly on Type III (Presynaptic) cells.

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 hypoxiainducible 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 signalling. 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 collatoral pathway. 30 min hypoxia (gas mixture 95% N2/5% CO2) 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.