Decreased Glutamate Receptor 2 Expression and Enhanced Epileptogenesis in Immature Rat Hippocampus after Perinatal Hypoxia-Induced Seizures

Abstract

Hypoxic encephalopathy is the most common cause of neonatal seizures and can lead to chronic epilepsy. In rats at postnatal days 10-12 (P10-12), global hypoxia induces spontaneous seizures and chronically decreases seizure threshold, thus mimicking clinical aspects of neonatal hypoxia. We have shown previously that the acute and chronic epileptogenic effects of hypoxia are age-dependent and require AMPA receptor activation. In this study, we aimed to determine whether hypoxia-induced seizures and epileptogenesis are associated with maturational and seizure-induced changes in AMPA receptor composition and function. Northern and Western blots indicated that glutamate receptor 2 (GluR2) mRNA and protein expression were significantly lower in neocortex and hippocampus at P10-12 compared with adult. After hypoxia-induced seizures at P10, GluR2 mRNA was significantly decreased within 48 hr, and GluR2 protein was significantly decreased within 96 hr. AMPA-induced Co(2+) uptake by neurons in hippocampal slices indicated higher expression of Ca(2+)-permeable AMPA receptors in immature pyramidal neurons compared with adult. In slices obtained 96 hr after hypoxia-induced seizures, AMPA-induced Co(2+) uptake was significantly increased compared with age-matched controls, and field recordings revealed increased tetanus-induced afterdischarges that could be kindled in the absence of NMDA receptor activation. In situ end labeling showed no acute or delayed cell death after hypoxia-induced seizures. Our results indicate that susceptibility to hypoxia-induced seizures occurs during a developmental stage in which the expression of Ca(2+)-permeable AMPA receptors is relatively high. Furthermore, perinatal hypoxia-induced seizures induce increased expression of Ca(2+)-permeable AMPA receptors and an increased capacity for AMPA receptor-mediated epileptogenesis without inducing cell death.

Mechanisms of seizure-induced ‘transcriptional channelopathy’ of hyperpolarization-activated cyclic nucleotide gated (HCN) channels

“…In essence, functional Ca 2+ -permeable AMPA receptors and activation of CaM Kinase II were required for many hours following the seizures, and blocking them during the seizures did not interfere with the ability of seizures to regulate HCN1 expression. This sequence of events, coupled by the findings of the current work and evidence from other groups (Friedman et al, 1994;Sanchez et al, 2001), suggests the following scenario:…”

“…This raised the possibility that CaM Kinase II was activated through Ca 2+ -entry via newly formed channels, and specifically through Ca 2+ -permeable AMPA receptors. Ca 2+ -permeable AMPA receptors are known to be formed newly after seizures, as shown in several models (e.g., Friedman et al, 1994;Sanchez et al, 2001). This occurs by seizure-induced reduction of GluR2 mRNA and protein levels, leading to formation of GluR2-lacking AMPA channels that are permeable to Ca 2+ .…”

“…(a) First, seizure-like events lead to reduced expression of the GluR2 subunit of AMPA receptors (Friedman et al, 1994;Sanchez et al, 2001;Richichi et al, 2005). This reduction in mRNA and protein levels leads to preferential formation of GluR2-lacking, Ca 2+ -permeable AMPA receptors, a process that takes several hours.…”

Epigenetic Mechanisms in Stroke and Epilepsy

“…A follow-up study by Dingledine and colleagues using a related animal model of status epilepticus, showed that seizures promote deacetylation of core histone protein H4 (a mark of gene repression) at the RE1 site of the gria2 promoter (gene encoding the AMPAR subunit GluA2), while promoting an increase in acetylation of H4 (a mark of open chromatin and active gene transcription) at the promoter of brain-derived neurotrophic factor BDNF (Huang et al, 2002;Tsankova et al, 2004). Although GluA2 expression was decreased, leading to an increase in GluA2-lacking, Ca 2 + -permeable AMPARs at CA3 synapses and neuronal death in CA3 Sanchez et al, 2001;Huang et al, 2002), BDNF expression was increased (Kokaia et al, 1995;Binder et al, 1999;Huang et al, 2002;Tsankova et al, 2004). Alterations in expression of these proteins contribute to the pathophysiology of recurrent seizures.…”

GluR2 knockdown reveals a dissociation between [Ca2+]i surge and neurotoxicity

“…In keeping with this, the downregulation of the GluR2 subunit was recently not correlated with neurotoxicity in the vulnerable CA1 region of hippocampus of immature rats following hypoxia-induced seizures (Sanchez et al, 2001). In addition, microinfusing GluR2(B)-AS-ODNs directly into the hippocampus of adult rats is insufficient to induce neuronal cell death unless the animals succumb to seizures also suggesting that other factors are responsible for cell death (Friedman and Velísková, 1998;Friedman et al, submitted).…”