Elsevier

Neurobiology of Disease

Volume 62, February 2014, Pages 8-17
Neurobiology of Disease

Effects of prolyl-hydroxylase inhibition and chronic intermittent hypoxia on synaptic transmission and plasticity in the rat CA1 and dentate gyrus

https://doi.org/10.1016/j.nbd.2013.08.016Get rights and content

Highlights

  • We investigated the effects of intermittent hypoxia and PHD inhibition on synaptic plasticity in two hippocampal regions.

  • Chronic inhibition of PHDs for seven days impaired LTP in the CA1 but not dentate gyrus region of the hippocampus.

  • Chronic intermittent hyoxia also impaired LTP in the CA1 but not dentate gyrus region of the hippocampus.

  • Both PHD inhibition and chronic intermittent hyoxia improved the rate of neuronal recovery from acute hypoxic insult.

Abstract

Chronic 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 pharmacological 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; 50 mg/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 the dentate gyrus. These results suggest that LTP induction in the CA1 region is more sensitive to both CIH and DMOG treatments than the dentate gyrus.

Introduction

Chronic intermittent hypoxia (CIH) is a characteristic feature of sleep apnoea which can lead to significant memory deficits, as well as to cortical and hippocampal apoptosis (Gozal et al., 2001). It has been shown to cause neurocognitive deficits such as spatial learning impairments with increased cell death and structural changes to hippocampal and cortical regions (Cai et al., 2010, Gozal, 2013, Gozal et al., 2001, Klein et al., 2003, Nair et al., 2011, Row et al., 2002, Row et al., 2003). There is also evidence that these effects are correlated with impairments in synaptic plasticity, namely long-term potentiation (LTP) in the rodent hippocampus (Payne et al., 2004, Xie and Yung, 2012, Xie et al., 2010). Payne et al. (2004) showed that 3- and 7-day CIH treatments impaired population spike LTP (PS-LTP) in the CA1 region of rat hippocampal slices. Deficits in LTP due to CIH treatment were reversed with acute application of BDNF and 7-day in vivo treatment of BDNF in mice (Xie et al., 2010). Although these studies have shown CIH-induced impairments in synaptic plasticity in the hippocampal CA1 region no research has been carried out on the effects of CIH and other chronic–hypoxic treatments on synaptic transmission and plasticity in granule cells of the dentate gyrus. Previous work has demonstrated that the dentate gyrus is also susceptible to hypoxia but those certain blades of the dentate gyrus are more resistant to a decrease in oxygen availability than for example the CA1 region (Kreisman et al., 2000). Previous work has also shown differential susceptibility of the CA1 and CA3 regions of the hippocampus to intermittent hypoxia (Gozal et al., 2002).

CIH is a potent inducer of HIF-1α, a key regulator of the hypoxic response which promotes the transcription of numerous genes required for adaptation to decreased oxygen tension (Forsythe et al., 1996, Wang and Semenza, 1993). Under normoxic conditions, HIF-1α is hydroxylated on specific proline residues which targets HIF for proteosomal degradation (Jaakkola et al., 2001, Kaelin and Ratcliffe, 2008). This prolyl hydroxylation is mediated by three prolyl-4-hydroxylase domain proteins, PHD1, 2 and 3 during normoxia. During hypoxia, the loss of the co-factor, oxygen, inhibits PHD-mediated hydroxylation resulting in stabilization of HIF-1α. The discovery of PHDs as cellular regulators of the hypoxic response has led to a resurgence of hypoxic preconditioning as a therapeutic strategy (Siddiq et al., 2005). Pharmacological inhibition of PHDs prior to middle cerebral artery occlusion increases cerebral blood flow, delays neuronal injury and decreases infarct volume (Kunze et al., 2012, Nagel et al., 2011). Additionally, post-ischemic intervention with PHD inhibitors decreases neuronal damage and attenuates behavioural deficits associated with ischemia (Ogle et al., 2012). We have recently shown in isolated hippocampal slices that acute PHD inhibition using dimethyloxaloylglycine (DMOG) and other specific PHD inhibitors, can impair synaptic transmission and plasticity in the rat CA1 region (Batti et al., 2010, Corcoran et al., 2013). Furthermore these effects were shown to be mediated by the PHD2 isoform of the hydroxylase (Corcoran et al., 2013). It is therefore important to investigate the effects of chronic treatment with PHD inhibitors on synaptic plasticity.

In the present study we have compared the effects of 7-day CIH and DMOG treatment on synaptic transmission and plasticity in two regions of the rat hippocampus, namely by stimulation of the stratum radiatum of the CA1 region and stimulation of the medial perforant path of the dorsal dentate gyrus (suprapyramidal (upper) blade). We have investigated whether the effect of a hypoxic mimetic would have similar impairments on LTP as has previously been reported for CIH treated animals in the CA1 region (Payne et al., 2004, Xie et al., 2010). Since hypoxic preconditioning has been shown to alleviate neuronal damage and that especially associated with cerebral ischemia (see reviews by Dirnagl et al. (2009) and Eltzschig and Eckle (2011)), we also explored a putative preconditioning effect of CIH and PHD inhibition on the recovery of synaptic transmission in the dentate gyrus and CA1 regions following an acute hypoxic insult in isolated hippocampal slices.

Section snippets

Animals

Male Wistar rats (50–100 g) were used in these experiments. All experimental procedures were approved by the Animal Research Ethics Committee of the Biomedical Facility of University College Dublin, Ireland. Animals were grouped as either sham, CIH, DMOG (50 mg/kg; i.p.) or CIH + DMOG (50 mg/kg, i.p.) treated, with each group consisting of 5 animals. After 7 days of treatment animals were anesthetised using 5% isoflurane and decapitated by guillotine. Upon decapitation blood was taken and a

7-Day CIH and DMOG treatment increases haematocrit levels and alters the expression of HIF-1α, EPO and CREB

Haematocrit levels were significantly elevated in both CIH (black bar, 41.8 ± 1.9%, n = 5) and DMOG (hatched bar, 41.0 ± 0.7%, n = 5) treated groups when compared to shams (white bar, 29.8 ± 1.2%, n = 5; P < 0.01) indicating that hypoxia was induced in both treatment groups (Fig. 1B). Western blot analysis of total forebrain protein showed that HIF-1α, and EPO protein expression in both CIH (black bars) and DMOG (hatched bars, 50 mg/kg, i.p.) treated groups was significantly increased compared to control

Discussion

Our data provide evidence of a significant impairment of synaptic plasticity in the CA1 but not dentate gyrus region of the hippocampus in CIH and DMOG treated animals. This was associated with a decreased activation of CREB possibly due to reduced cyclic adenosine monophosphate (cAMP), availability. CIH and DMOG treated animals showed elevated haematocrit percentages indicated by increased red blood cell counts. This was associated with up-regulation of EPO expression in CIH and DMOG treated

Conclusion

In conclusion our study has shown that 7-day CIH impairs LTP in the CA1, but not the dentate gyrus, of rat hippocampal slices. Additionally, we provide novel evidence suggesting that chronic PHD inhibition (DMOG) impairs LTP in the CA1 region, but not the dentate gyrus, of the rat hippocampus. The impairments in LTP are not associated with any changes in synaptic excitability but rather with decreased CREB activity in the hippocampus. Despite CIH and DMOG treatment causing inhibitory effects on

Acknowledgments

The authors thank Ms. Fiona McDonald for the assistance with the generation of the CIH animal model. This work was supported by a grant from Science Foundation Ireland (SFI; 09/RFP/NES2450) to JOC. The Biospherix Oxycycler™ system was funded by the Health Research Board, Ireland (KDOH).

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