Effects of prolyl-hydroxylase inhibition and chronic intermittent hypoxia on synaptic transmission and plasticity in the rat CA1 and 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).
References (49)
- et al.
Tumor necrosis factor-alpha impairs the recovery of synaptic transmission from hypoxia in rat hippocampal slices
J. Neuroimmunol.
(2010) - et al.
Hydroxylase inhibition reduces synaptic transmission and protects against a glutamate-induced ischemia in the CA1 region of the rat hippocampus
Neuroscience
(2010) - et al.
The regulation of AMP-activated protein kinase by H(2)O(2)
Biochem. Biophys. Res. Commun.
(2001) - et al.
Preconditioning and tolerance against cerebral ischaemia: from experimental strategies to clinical use
Lancet Neurol.
(2009) - et al.
Intermittent hypoxic exposure during light phase induces changes in cAMP response element binding protein activity in the rat CA1 hippocampal region: water maze performance correlates
Neuroscience
(2003) - et al.
Prolyl hydroxylase domain protein 2 regulates the intracellular cyclic AMP level in cardiomyocytes through its interaction with phosphodiesterase 4D
Biochem. Biophys. Res. Commun.
(2012) - et al.
Oxygen sensing by metazoans: the central role of the HIF hydroxylase
Mol. Cell
(2008) - et al.
Proteomic identification of a novel protein regulated in CA1 and CA3 hippocampal regions during intermittent hypoxia
Respir. Physiol. Neurobiol.
(2003) - et al.
Phospho-dependent functional modulation of GABA(B) receptors by the metabolic sensor AMP-dependent protein kinase
Neuron
(2007) - et al.
BDNF: a key regulator for protein synthesis-dependent LTP and long-term memory?
Neurobiol. Learn. Mem.
(2008)
Pharmacological inhibition of AMP-activated protein kinase provides neuroprotection in stroke
J. Biol. Chem.
Neurobiology of disease inhibition of prolyl hydroxylases by dimethyloxaloylglycine after stroke reduces ischemic brain injury and requires hypoxia inducible factor-1α
Neurobiol. Dis.
Group II and III metabotropic glutamate receptors modulate paired pulse depression in the rat dentate gyrus in vitro
Eur. J. Pharmacol.
Effect of intermittent hypoxia on long-term potentiation in rat hippocampal slices
Brain Res.
Hypoxia-inducible factor prolyl 4-hydroxylase inhibition. A target for neuroprotection in the central nervous system
J. Biol. Chem.
Characterization of hypoxia-inducible factor 1 and regulation of DNA binding activity by hypoxia
Biochemistry
Superoxide flashes in single mitochondria
Cell
Brain-derived neurotrophic factor rescues and prevents chronic intermittent hypoxia-induced impairment of hippocampal long-term synaptic plasticity
Neurobiol. Dis.
Chronic hypoxia leads to a glycolytic phenotype and suppressed HIF-2 signaling in PC12 cells
Biochim. Biophys. Acta
Three distinct mechanisms generate oxygen free radicals in neurons and contribute to cell death during anoxia and reoxygenation
J. Neurosci.
AMP-activated protein kinase: implications on ischemic diseases
BMB Rep.
Chronic intermittent hypoxia exposure induces memory impairment in growing rats
Acta Neurobiol. Exp.
Hypoxia inducible factor signalling mechanisms in the central nervous system
Acta Physiol.
A role for prolyl hydroxylase domain proteins in hippocampal synaptic plasticity
Hippocampus
Cited by (37)
Intermittent hypoxia increases ROS/HIF-1α ‘related oxidative stress and inflammation and worsens bleomycin-induced pulmonary fibrosis in adult male C57BL/6J mice
2021, International ImmunopharmacologyCitation Excerpt :Previous studies [28,31,32] revealed that repeated hypoxia-reoxygenation of IH can induce excess ROS, which plays an important role in up-regulating HIF-1α. For example, the levels of HIF-1α protein and ROS in the central nervous system were increased after mice exposed to IH, while antioxidants can inhibit this response [32]. Moreover, ROS was continuously activated and the level of HIF-1α still increased in IH states [33].
Intermittent Hypoxia causes targeted disruption to NMDA receptor dependent synaptic plasticity in area CA1 of the hippocampus
2021, Experimental NeurologyCitation Excerpt :Intermittent hypoxia (IH) is a hallmark of this condition that is capable of impairing spatial learning and memory (Arias-Cavieres et al., 2020; Goldbart et al., 2003; Gozal et al., 2001). Consistent with the correlation between deficits in cognitive performance and diminished hippocampal Long Term Potentiation (LTP) (Payne et al., 2004; Wall et al., 2014; Xie et al., 2010), IH impairs hippocampal LTP (Arias-Cavieres et al., 2020; Khuu et al., 2019; Wall et al., 2014; Xie et al., 2010). However, strengthening of synaptic transmission by LTP reflects only one aspect important for maintaining activity-dependent synaptic plasticity (Bliss and Collingridge, 1993; Citri and Malenka, 2008).
Huperzine A, reduces brain iron overload and alleviates cognitive deficit in mice exposed to chronic intermittent hypoxia
2020, Life SciencesCitation Excerpt :Moreover, we found that the decreased iron of HuA was owing to decline the TfR1 expression. During CIH exposing, hypoxia inducible factor 1 α (HIF-1α) could induce stable expression in hippocampus [41], and HIF-1α could promote iron mobilization and uptake, and inhibit FPN1 expression. The iron-uptake related proteins DMT1, TfR1 and iron storage protein Ferritin all contain hypoxia response element (HRE), and they are the target genes for HIF-1α [42].
- 1
These authors contributed equally.