Failure of estradiol to ameliorate global ischemia-induced CA1 sector injury in middle-aged female gerbils

Global forebrain ischemia arising from brief occlusion of the carotid arteries in gerbils produces selective hippocampal CA1 neuronal loss. Pre-treatment with 17beta-estradiol ameliorates, in part, ischemia-induced damage in young animals. Because stroke and cardiac arrest are more likely to occur among elderly individuals, neuroprotective studies in older animals have compelling clinical relevance. We investigated whether estradiol would attenuate ischemia-induced hippocampal neuronal injury in middle-aged (12-14 months) male, intact female, ovariectomized (OVX) female and OVX females treated for 14 days with estradiol. Core temperature telemetry probes were also implanted at the time that estradiol was initiated. Ischemia was induced by bilateral occlusion of the common carotid arteries (5 min), during which time skull temperature was maintained under normothermic conditions. Estradiol blocked the modest spontaneous hyperthermia that normally follows ischemia. However, all four groups exhibited substantial neuronal cell loss in the CA1, assessed at 7 after ischemia. These findings indicate that estradiol pre-treatment under conditions that produce neuroprotection in young animals does not protect against ischemia-induced CA1 cell loss in middle-aged female gerbils.

Effects of estradiol on cognition and hippocampal pathology after lateral fluid percussion brain injury in female rats

Studies involving animal models of acute central nervous system (CNS) stroke and trauma strongly indicate that sex and/or hormonal status are important determinants of outcome after brain injury. The present study was undertaken to examine the ability of estradiol to protect hippocampal neurons from lateral fluid percussion brain injury. Sprague-Dawley female rats (211-285 g; n = 119) were ovariectomized, and a subset (n = 66) were implanted with 17beta-estradiol pellets to provide near physiological levels of estradiol. Animals were subjected to lateral fluid percussion brain injury or sham injury 1 week later. Activation of caspase-3 (n = 26) and TUNEL staining (n = 21) were assessed at 3 and 12 h after injury, respectively, in surviving control and estradiol-treated animals. Memory retention was examined using a Morris water maze test in a separate subset of animals (n = 43) at 8 days after injury. Activated caspase-3 and TUNEL staining were observed in the dentate hilus, granule cell layer, and CA3 regions in all injured rats, indicative of selective hippocampal cell apoptosis in the acute posttraumatic period. Estradiol did not significantly alter the number of hippocampal neurons exhibiting caspase-3 activity or TUNEL staining. Brain injury impaired cognitive ability, assessed at 1 week post-injury (p < 0.001). However, estradiol at physiological levels did not significantly alter injury-induced loss of memory. These data indicate that estradiol at physiological levels does not ameliorate trauma-induced hippocampal injury or cognitive deficits in ovariectomized female rats.

MAPK signaling is critical to estradiol protection of CA1 neurons in global ischemia

The importance of hormone therapy in affording protection against the sequelae of global ischemia in postmenopausal women remains controversial. Global ischemia arising during cardiac arrest or cardiac surgery causes highly selective, delayed death of hippocampal CA1 neurons. Exogenous estradiol ameliorates global ischemia-induced neuronal death and cognitive impairment in male and female rodents. However, the molecular mechanisms by which estrogens intervene in global ischemia-induced apoptotic cell death are unclear. Here we show that estradiol acts via the classical estrogen receptors, the IGF-I receptor, and the ERK/MAPK signaling cascade to protect CA1 neurons in ovariectomized female rats and gerbils. We demonstrate that global ischemia promotes early dephosphorylation and inactivation of ERK1 and the transcription factor cAMP-response element binding protein (CREB), subsequent down-regulation of the antiapoptotic protein Bcl-2, a known gene target of estradiol and CREB, and activation of caspase-3. Estradiol treatment increases basal phosphorylation of both ERK1 and ERK2 in hippocampal CA1 and prevents ischemia-induced dephosphorylation and inactivation of ERK1 and CREB, down-regulation of Bcl-2 and activation of the caspase death cascade. Whereas ERK/MAPK signaling is critical to CREB activation and neuronal survival, the impact of estradiol on Bcl-2 levels is ERK independent. These findings support a model whereby estradiol acts via the classical estrogen receptors and IGF-I receptors, which converge on activation of ERK/MAPK signaling and CREB to promote neuronal survival in the face of global ischemia.

Ischemic insults promote epigenetic reprogramming of mu opioid receptor expression in hippocampal neurons

Transient global ischemia is a neuronal insult that induces delayed, selective death of hippocampal CA1 pyramidal neurons. A mechanism underlying ischemia-induced cell death is activation of the gene silencing transcription factor REST (repressor element-1 silencing transcription factor)/NRSF (neuron-restrictive silencing factor) and REST-dependent suppression of the AMPA receptor subunit GluR2 in CA1 neurons destined to die. Here we show that REST regulates an additional gene target, OPRM1 (mu opioid receptor 1 or MOR-1). MORs are abundantly expressed by basket cells and other inhibitory interneurons of CA1. Global ischemia induces a marked decrease in MOR-1 mRNA and protein expression that is specific to the selectively vulnerable area CA1, as assessed by quantitative real-time RT-PCR, Western blotting, and ChIP. We further show that OPRM1 gene silencing is REST-dependent and occurs via epigenetic modifications. Ischemia promotes deacetylation of core histone proteins H3 and H4 and dimethylation of histone H3 at lysine-9 (H3-K9) over the MOR-1 promoter, an signature of epigenetic gene silencing. Acute knockdown of MOR-1 gene expression by administration of antisense oligodeoxynucleotides to hippocampal slices in vitro or injection of the MOR antagonist naloxone to rats in vivo affords protection against ischemia-induced death of CA1 pyramidal neurons. These findings implicate MORs in ischemia-induced death of CA1 pyramidal neurons and document epigenetic remodeling of expression of OPRM1 in CA1 inhibitory interneurons.

HIV-tat induces formation of an LRP-PSD-95- NMDAR-nNOS complex that promotes apoptosis in neurons and astrocytes

HIV infection of the central nervous system can result in neurologic dysfunction with devastating consequences in AIDS patients. NeuroAIDS is characterized by neuronal injury and loss, yet there is no evidence that HIV can infect neurons. Here we show that the HIV-encoded protein tat triggers formation of a macromolecular complex involving the low-density lipoprotein receptor-related protein (LRP), postsynaptic density protein-95 (PSD-95), N-methyl-d-aspartic acid (NMDA) receptors, and neuronal nitric oxide synthase (nNOS) at the neuronal plasma membrane, and that this complex leads to apoptosis in neurons negative as well as positive for NMDA receptors and also in astrocytes. Blockade of LRP-mediated tat uptake, NMDA receptor activation, or neuronal nitric oxide synthase significantly reduces ensuing neuronal apoptosis, suggesting that formation of this complex is an early step in tat toxicity. We also show that the inflammatory chemokine, CCL2, protects against tat toxicity and inhibits formation of the complex. These findings implicate the complex in HIV-induced neuronal apoptosis and suggest therapeutic targets for intervention in the pathogenesis of NeuroAIDS.

PSD-95 and PKC converge in regulating NMDA receptor trafficking and gating

Neuronal NMDA receptors (NMDARs) colocalize with postsynaptic density protein-95 (PSD-95), a putative NMDAR anchoring protein and core component of the PSD, at excitatory synapses. PKC activation and PSD-95 expression each enhance NMDAR channel opening rate and number of functional channels at the cell surface. Here we show in Xenopus oocytes that PSD-95 and PKC potentiate NMDA gating and trafficking in a nonadditive manner. PSD-95 and PKC each enhance NMDA channel activity, with no change in single-channel conductance, reversal potential or mean open time. PSD-95 and PKC each potentiate NMDA channel opening rate (k(beta)) and number of functional channels at the cell surface (N), as indicated by more rapid current decay and enhanced charge transfer in the presence of the open channel blocker MK-801. PSD-95 and PKC each increase NMDAR surface expression, as indicated by immunofluorescence. PKC potentiates NMDA channel function and NMDAR surface expression to the same final absolute values in the absence or presence of PSD-95. Thus, PSD-95 partially occludes PKC potentiation. We further show that Ser-1462, a putative phosphorylation target within the PDZ-binding motif of the NR2A subunit, is required for PSD-95-induced potentiation and partial occlusion of PKC potentiation. Coimmunoprecipitation experiments with cortical neurons in culture indicate that PKC activation promotes assembly of NR2 with NR1, and that the newly assembled NMDARs are not associated with PSD-95. These findings predict that synaptic scaffolding proteins and protein kinases convergently modulate NMDAR gating and trafficking at synaptic sites.

Activity bidirectionally regulates AMPA receptor mRNA abundance in dendrites of hippocampal neurons

Activity-dependent regulation of synaptic AMPA receptor (AMPAR) number is critical to NMDA receptor (NMDAR)-dependent synaptic plasticity. Using quantitative high-resolution in situ hybridization, we show that mRNAs encoding the AMPA-type glutamate receptor subunits (GluRs) 1 and 2 are localized to dendrites of hippocampal neurons and are regulated by paradigms that alter synaptic efficacy. A substantial fraction of synaptic sites contain AMPAR mRNA, consistent with strategic positioning and availability for "on-site" protein synthesis. NMDAR activation depletes dendritic levels of AMPAR mRNAs. The decrease in mRNA occurs via rise in intracellular Ca2+, activation of extracellular signal-regulated kinase/mitogen-activated protein kinase signaling, and transcriptional arrest at the level of the nucleus. The decrease in mRNA is accompanied by a long-lasting reduction in synaptic AMPAR number, consistent with reduced synaptic efficacy. In contrast, group I metabotropic GluR signaling promotes microtubule-based trafficking of existing AMPAR mRNAs from the soma to dendrites. Bidirectional regulation of dendritic mRNA abundance represents a potentially powerful means to effect long-lasting changes in synaptic strength.

RNA trafficking and local protein synthesis in dendrites: an overview

It is now widely accepted that mRNAs localize to dendrites and that translation of these mRNAs is regulated in response to neuronal activity. Recent studies have begun to reveal the underpinnings of these processes and to underscore the importance of local protein synthesis to synaptic remodeling and plasticity. When Steward and Levy (1982) first reported their observation of polyribosomes at the base of spines, the prevailing view was that all proteins were synthesized in the cell body and then transported to distal compartments of neurons. Steward and Levy's discovery, however, raised the intriguing possibility that mRNAs could be transported to synapses and locally translated in response to synaptic stimulation. This provided an elegant mechanism for spatially restricting gene expression within the neuron, such that individual synapses could independently regulate their morphology and efficacy, in a persistent, protein synthesis-dependent manner, in response to specific stimuli. It is now widely accepted that mRNAs do localize to dendrites and that translation of these mRNAs contributes to synaptic plasticity. As is evident from the collection of Mini-Reviews on dendritic protein synthesis in this issue of The Journal of Neuroscience, the field has evolved to focus on a series of key questions, including the following: (1) what mRNAs are present in dendrites? (2) How are these mRNAs transported from the nucleus into the dendrite? (3) How is translation of these mRNAs regulated by neuronal activity? and (4) What is the function of local translation of specific transcripts? In this brief introductory overview, we will consider each of these questions in turn.

Zinc-dependent multi-conductance channel activity in mitochondria isolated from ischemic brain

Transient global ischemia is a neuronal insult that induces delayed cell death. A hallmark event in the early post-ischemic period is enhanced permeability of mitochondrial membranes. The precise mechanisms by which mitochondrial function is disrupted are, as yet, unclear. Here we show that global ischemia promotes alterations in mitochondrial membrane contact points, a rise in intramitochondrial Zn2+, and activation of large, multi-conductance channels in mitochondrial outer membranes by 1 h after insult. Mitochondrial channel activity was associated with enhanced protease activity and proteolytic cleavage of BCL-xL to generate its pro-death counterpart, deltaN-BCL-xL. The findings implicate deltaN-BCL-xL in large, multi-conductance channel activity. Consistent with this, large channel activity was mimicked by introduction of recombinant deltaN-BCL-xL to control mitochondria and blocked by introduction of a functional BCL-xL antibody to post-ischemic mitochondria via the patch pipette. Channel activity was also inhibited by nicotinamide adenine dinucleotide, indicative of a role for the voltage-dependent anion channel (VDAC) of the outer mitochondrial membrane. In vivo administration of the membrane-impermeant Zn2+ chelator CaEDTA before ischemia or in vitro application of the membrane-permeant Zn2+ chelator tetrakis-(2-pyridylmethyl) ethylenediamine attenuated channel activity, suggesting a requirement for Zn2+. These findings reveal a novel mechanism by which ischemic insults disrupt the functional integrity of the outer mitochondrial membrane and implicate deltaN-BCL-xL and VDAC in the large, Zn2+-dependent mitochondrial channels observed in post-ischemic hippocampal mitochondria.

Ischemic insults direct glutamate receptor subunit 2-lacking AMPA receptors to synaptic sites

Regulated AMPA receptor (AMPAR) trafficking at excitatory synapses is a mechanism critical to activity-dependent alterations in synaptic efficacy. The role of regulated AMPAR trafficking in insult-induced synaptic remodeling and/or cell death is, however, as yet unclear. Here we show that brief oxygen-glucose deprivation (OGD), an in vitro model of brain ischemia, promotes redistribution of AMPARs at synapses of hippocampal neurons, leading to a switch in AMPAR subunit composition. Ischemic insults promote internalization of glutamate receptor subunit 2 (GluR2)-containing AMPARs from synaptic sites via clathrin-dependent endocytosis and facilitate delivery of GluR2-lacking AMPARs to synaptic sites via soluble N-ethylmaleimide-sensitive factor attachment protein receptor-dependent exocytosis, evident at early times after insult. The OGD-induced switch in receptor subunit composition requires PKC activation, dissociation of GluR2 from AMPA receptor-binding protein, and association with protein interacting with C kinase-1. We further show that AMPARs at synapses of insulted neurons exhibit functional properties of GluR2-lacking AMPARs. AMPAR-mediated miniature EPSCs exhibit increased amplitudes and enhanced sensitivity to subunit-specific blockers of GluR2-lacking AMPARs, evident at 24 h after ischemia. The OGD-induced alterations in synaptic AMPA currents require clathrin-mediated receptor endocytosis and PKC activation. Thus, ischemic insults promote targeting of GluR2-lacking AMPARs to synapses of hippocampal neurons, mechanisms that may be relevant to ischemia-induced synaptic remodeling and/or neuronal death.

Protein kinase A regulates calcium permeability of NMDA receptors

Calcium (Ca2+) influx through NMDA receptors (NMDARs) is essential for synaptogenesis, experience-dependent synaptic remodeling and plasticity. The NMDAR-mediated rise in postsynaptic Ca2+ activates a network of kinases and phosphatases that promote persistent changes in synaptic strength, such as long-term potentiation (LTP). Here we show that the Ca2+ permeability of neuronal NMDARs is under the control of the cyclic AMP-protein kinase A (cAMP-PKA) signaling cascade. PKA blockers reduced the relative fractional Ca2+ influx through NMDARs as determined by reversal potential shift analysis and by a combination of electrical recording and Ca2+ influx measurements in rat hippocampal neurons in culture and hippocampal slices from mice. In slices, PKA blockers markedly inhibited NMDAR-mediated Ca2+ rises in activated dendritic spines, with no significant effect on synaptic current. Consistent with this, PKA blockers depressed the early phase of NMDAR-dependent LTP at hippocampal Schaffer collateral-CA1 (Sch-CA1) synapses. Our data link PKA-dependent synaptic plasticity to Ca2+ signaling in spines and thus provide a new mechanism whereby PKA regulates the induction of LTP.

Induction of Dickkopf-1, a negative modulator of the Wnt pathway, is required for the development of ischemic neuronal death

Expression of Dickkopf-1 (Dkk-1), a secreted protein that negatively modulates the Wnt pathway, was induced in the hippocampus of gerbils and rats subjected to transient global cerebral ischemia as well as in cultured cortical neurons challenged with an excitotoxic pulse. In ischemic animals, the temporal and regional pattern of Dkk-1 expression correlated with the profile of neuronal death, as assessed by Nissl staining and Dkk-1 immunostaining in adjacent hippocampal sections. Treatment of ischemic animals with either Dkk-1 antisense oligonucleotides or lithium ions (which rescue the Wnt pathway acting downstream of the Dkk-1 blockade) protected vulnerable hippocampal neurons against ischemic damage. The same treatments protected cultured cortical neurons against NMDA toxicity. We conclude that induction of Dkk-1 with the ensuing inhibition of the canonical Wnt signaling pathway is required for the development of ischemic and excitotoxic neuronal death.

Acute and chronic estradiol treatments reduce memory deficits induced by transient global ischemia in female rats

Transient global ischemia induces selective, delayed neuronal death in the hippocampal CA1 and delayed cognitive deficits. Estrogen treatment ameliorates hippocampal injury associated with global ischemia. Although much is known about the impact of estrogen on neuronal survival, relatively little is known about its impact on functional outcome assessed behaviorally. We investigated whether long-term estradiol (21-day pellets implanted 14 days prior to ischemia) or acute estradiol (50 microg infused into the lateral ventricles immediately after ischemia) attenuates ischemia-induced cell loss and improves visual and spatial working memory in ovariectomized female rats. Global ischemia significantly impaired visual and spatial memory, assessed by object recognition and object placement tests at 6-9 days. Global ischemia did not affect locomotion, exploration, or anxiety-related behaviors, assessed by an open-field test at 6 days. Long-term estradiol prevented the ischemia-induced deficit in visual working memory, maintaining normal performance in tests with retention intervals of up to 1 h. Long-term estradiol also prevented ischemia-induced deficits in spatial memory tests with short (1 and 7 min), but not longer (15 min), retention intervals. Acute estradiol significantly improved visual memory assessed with short retention intervals, but did not prevent deficits in spatial memory. Acute estradiol significantly increased the number of surviving CA1 neurons, assessed either at 7 days after ischemia or after the completion of behavioral testing 9 days after ischemia. In contrast, chronic estradiol did not reduce CA1 cell death 9 days after ischemia. Thus, long-term estradiol at near physiological levels and acute estradiol administered after ischemic insult improve functional recovery after global ischemia. These findings have important implications for intervention in the neurological sequellae associated with global ischemia.

Blockade of calcium-permeable AMPA receptors protects hippocampal neurons against global ischemia-induced death

Transient global or forebrain ischemia induced experimentally in animals can cause selective, delayed neuronal death of hippocampal CA1 pyramidal neurons. A striking feature is a delayed rise in intracellular free Zn(2+) in CA1 neurons just before the onset of histologically detectable cell death. Here we show that alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptors (AMPARs) at Schaffer collateral to CA1 synapses in postischemic hippocampus exhibit properties of Ca(2+)/Zn(2+)-permeable, Glu receptor 2 (GluR2)-lacking AMPARs before the rise in Zn(2+) and cell death. At 42 h after ischemia, AMPA excitatory postsynaptic currents exhibited pronounced inward rectification and marked sensitivity to 1-naphthyl acetyl spermine (Naspm), a selective channel blocker of GluR2-lacking AMPARs. In control hippocampus, AMPA excitatory postsynaptic currents were electrically linear and relatively insensitive to Naspm. Naspm injected intrahippocampally at 9-40 h after insult greatly reduced the late rise in intracellular free Zn(2+) in postischemic CA1 neurons and afforded partial protection against ischemia-induced cell death. These results implicate GluR2-lacking AMPA receptors in the ischemia-induced rise in free Zn(2+) and death of CA1 neurons, although a direct action at the time of the rise in Zn(2+) is unproven. This receptor subtype appears to be an important therapeutic target for intervention in ischemia-induced neuronal death in humans.

Estrogen can act via estrogen receptor alpha and beta to protect hippocampal neurons against global ischemia-induced cell death

Estradiol at physiological concentrations intervenes in apoptotic death cascades and ameliorates neuronal death in experimental models of focal and global ischemia. The cellular targets that mediate estradiol protection of hippocampal neurons in global ischemia are, however, unclear. The present study examined the hypothesis that estradiol protects hippocampal neurons in ovariectomized rats via estrogen receptor (ER)alpha and/or beta. Estradiol (14 d pretreatment) afforded robust protection of CA1 neurons against global ischemia-induced death. The broad-spectrum ER antagonist ICI 182,780 (intracerebroventricularly, 0 and 12 h after ischemia) abolished estrogen protection, consistent with a role for ERs. To evaluate the potential roles of ERalpha vs. ERbeta in estrogen protection, we administered subtype-selective agonists for 14 d before and 7 d after ischemia. The ERalpha-selective agonist propyl pyrazole triol (PPT, 10 mg/kg) and ERbeta-selective agonist WAY 200070-3 (1 mg/kg) produced nearly complete protection of CA1 neurons in approximately 50% of the animals. PPT, but not WAY 200070-3, at doses used for protection, elicited lordosis, induced negative feedback inhibition of LH release, and reduced weight gain. These findings establish the efficacy of the PPT dose in neuroendocrine assays and specificity of WAY 200070-3 for ERbeta. We also examined the ability of estradiol and neuronal injury to regulate ERalpha and ERbeta expression. Both estradiol and global ischemia markedly increased ERalpha, but not ERbeta, protein in CA1. These data indicate that estradiol can act via ERalpha and ERbeta to protect CA1 neurons from global ischemia-induced death and that both estradiol and global ischemia modulate ERalpha expression in hippocampal CA1.

Postsynaptic density protein-95 regulates NMDA channel gating and surface expression

NMDA receptors (NMDARs) colocalize with postsynaptic density protein-95 (PSD-95), a multivalent synaptic scaffolding protein and core component of the postsynaptic density, at excitatory synapses. Although much is known about the identity and properties of scaffolding proteins, little is known about their actions on NMDAR function. Here we show that association of PSD-95 with NMDARs modulates channel gating and surface expression. PSD-95 increases the number of functional channels at the cell surface and channel opening rate of NMDARs, with little or no change in conductance, reversal potential, or mean open time. We show further that PSD-95 increases NMDAR surface expression by increasing the rate of channel insertion and decreasing the rate of channel internalization. The PDZ (PSD-95, discs large, zona occludens-1) binding motif at the distal end of the NR2 C-terminal tail is critical to the actions of PSD-95 on NMDAR function and surface expression. Given that activity bi-directionally modifies synaptic levels of PSD-95, our findings suggest a novel mechanism for activity-dependent regulation of NMDARs at central synapses.

Late calcium EDTA rescues hippocampal CA1 neurons from global ischemia-induced death

Transient global ischemia induces a delayed rise in intracellular Zn2+, which may be mediated via glutamate receptor 2 (GluR2)-lacking AMPA receptors (AMPARs), and selective, delayed death of hippocampal CA1 neurons. The molecular mechanisms underlying Zn2+ toxicity in vivo are not well delineated. Here we show the striking finding that intraventricular injection of the high-affinity Zn2+ chelator calcium EDTA (CaEDTA) at 30 min before ischemia (early CaEDTA) or at 48-60 hr (late CaEDTA), but not 3-6 hr, after ischemia, afforded robust protection of CA1 neurons in approximately 50% (late CaEDTA) to 75% (early CaEDTA) of animals. We also show that Zn2+ acts via temporally distinct mechanisms to promote neuronal death. Early CaEDTA attenuated ischemia-induced GluR2 mRNA and protein downregulation (and, by inference, formation of Zn2+-permeable AMPARs), the delayed rise in Zn2+, and neuronal death. These findings suggest that Zn2+ acts at step(s) upstream from GluR2 gene downregulation and implicate Zn2+ in transcriptional regulation and/or GluR2 mRNA stability. Early CaEDTA also blocked mitochondrial release of cytochrome c and Smac/DIABLO (second mitochondria-derived activator of caspases/direct inhibitor of apoptosis protein-binding protein with low pI), caspase-3 activity (but not procaspase-3 cleavage), p75NTR induction, and DNA fragmentation. These findings indicate that CaEDTA preserves the functional integrity of the mitochondrial outer membrane and arrests the caspase death cascade. Late injection of CaEDTA at a time when GluR2 is downregulated and caspase is activated inhibited the delayed rise in Zn2+, p75NTR induction, DNA fragmentation, and cell death. The finding of neuroprotection by late CaEDTA administration has striking implications for intervention in the delayed neuronal death associated with global ischemia.

Ischemic preconditioning: neuronal survival in the face of caspase-3 activation

Apoptosis is an evolutionarily conserved process critical to tissue development and tissue homeostasis in eukaryotic organisms and, when dysregulated, causes inappropriate cell death. Global ischemia is a neuronal insult that induces delayed cell death with many features of apoptosis. Ischemic preconditioning affords robust protection of CA1 neurons against a subsequent severe ischemic challenge. The molecular mechanisms underlying ischemic tolerance are unclear. Here we show that ischemia induces pronounced caspase-3 activity in naive neurons that die and in preconditioned neurons that survive. Preconditioning intervenes downstream of proteolytic processing and activation of caspase-3 (a protease implicated in the execution of apoptosis) and upstream of the caspase-3 target caspase-activated DNase (CAD, a deoxyribonuclease that catalyzes DNA fragmentation) to arrest neuronal death. We further show that global ischemia promotes expression of the pro-survival inhibitor-of-apoptosis (IAP) family member cIAP, but unleashes Smac/DIABLO (second mitochondria-derived activator of caspases/direct IAP-binding protein with low pI), a factor that neutralizes the protective actions of IAPs and promotes neuronal death. Preconditioning blocks the mitochondrial release of Smac/DIABLO, but not the ischemia-induced upregulation of IAPs. In the absence of Smac/DIABLO, cIAP halts the caspase death cascade and arrests neuronal death. These findings suggest that preconditioning preserves the integrity of the mitochondrial membrane, enabling neurons to survive in the face of caspase activation.

Characterization of two novel N-methyl-D-aspartate antagonists: EAA-090 (2-[8,9-dioxo-2,6-diazabicyclo [5.2.0]non-1(7)-en2-yl]ethylphosphonic acid) and EAB-318 (R-alpha-amino-5-chloro-1-(phosphonomethyl)-1H-benzimidazole-2-propanoic acid hydrochloride)

Two novel N-methyl-d-aspartate (NMDA) antagonists with unique chemical structures, EAA-090 (2-[8,9-dioxo-2, 6-diazabicyclo[5.2.0]non-1(7)-en2-yl]ethylphosphonic acid) and EAB-318 (R-alpha-amino-5-chloro-1-(phosphonomethyl)-1H-benzimidazole-2-propanoic acid hydrochloride), were compared with CGS-19755 (Selfotel) in ligand binding, electrophysiology, and neuroprotection assays. CGS-19755, EAA-090 and EAB-318 inhibited [(3)H]3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid binding to NMDA receptors with IC(50) values of 55, 28, and 7.9 nM, respectively. All three compounds decreased the duration of spontaneous synaptic currents and inhibited NMDA-activated currents in rat hippocampal neurons. IC(50) values for inhibition of current induced by 10 microM NMDA were 795, 477, and 69 nM for CGS-19755, EAA-090, and EAB-318, respectively. The NMDA antagonists protected chick embryo retina slices and cultured rat hippocampal and cortical neurons from glutamate- and NMDA-induced neurotoxicity. In experiments in which different NMDA receptor splice variants and subtypes were expressed in Xenopus oocytes, all three antagonists preferentially blocked NMDA-elicited currents mediated by N-methyl-d-aspartate receptor (NR)1 splice variants containing the N-terminal insertion. They also favored NR2A-versus NR2B- or NR2C-containing NMDA receptors, with EAA-090 showing the greatest selectivity. EAA-090 was 10 times more potent at blocking NR2A-versus NR2B- or NR2C-containing NMDA receptors. In addition to being the most potent NMDA antagonist, EAB-318 inhibited alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and kainate receptors. The combination of NMDA and AMPA/kainate block enabled EAB-318 to protect neurons against ischemia induced cell death.

Electrical coupling and neuronal synchronization in the Mammalian brain

Certain neurons in the mammalian brain have long been known to be joined by gap junctions, which are the most common type of electrical synapse. More recently, cloning of neuron-specific connexins, increased capability of visualizing cells within brain tissue, labeling of cell types by transgenic methods, and generation of connexin knockouts have spurred a rapid increase in our knowledge of the role of gap junctions in neural activity. This article reviews the many subtleties of transmission mediated by gap junctions and the mechanisms whereby these junctions contribute to synchronous firing.

Phosphorylation of the AMPA receptor GluR1 subunit is required for synaptic plasticity and retention of spatial memory

Plasticity of the nervous system is dependent on mechanisms that regulate the strength of synaptic transmission. Excitatory synapses in the brain undergo long-term potentiation (LTP) and long-term depression (LTD), cellular models of learning and memory. Protein phosphorylation is required for the induction of many forms of synaptic plasticity, including LTP and LTD. However, the critical kinase substrates that mediate plasticity have not been identified. We previously reported that phosphorylation of the GluR1 subunit of AMPA receptors, which mediate rapid excitatory transmission in the brain, is modulated during LTP and LTD. To test if GluR1 phosphorylation is necessary for plasticity and learning and memory, we generated mice with knockin mutations in the GluR1 phosphorylation sites. The phosphomutant mice show deficits in LTD and LTP and have memory defects in spatial learning tasks. These results demonstrate that phosphorylation of GluR1 is critical for LTD and LTP expression and the retention of memories.

Ischemic insults derepress the gene silencer REST in neurons destined to die

A subset of genes implicated in genetic and acquired neurological disorders encode proteins essential to neural patterning and neurogenesis. The gene silencing transcription factor neuronal repressor element-1 silencing transcription factor (REST)/neuron-restrictive silencer factor (NRSF) plays a critical role in elaboration of the neuronal phenotype. In neural progenitor and non-neural cells, REST acts by repression of a subset of neural genes important to synaptic plasticity and synaptic remodeling, including the AMPA receptor (AMPAR) subunit GluR2. Here we show that global ischemia triggers REST mRNA and protein expression. REST suppresses GluR2 promoter activity and gene expression in neurons destined to die. Because the GluR2 subunit governs AMPAR Ca2+ permeability, these changes are expected to have profound effects on neuronal survival. In keeping with this concept, acute knockdown of the REST gene by antisense administration prevents GluR2 suppression and rescues post-ischemic neurons from ischemia-induced cell death in an in vitro model. To our knowledge, our study represents the first example of ischemia-induced induction of a master transcriptional regulator gene and its protein expression critical to neural differentiation and patterning in adult neurons. Derepression of REST is likely to be an important mechanism of insult-induced neuronal death.

Hypothermia rescues hippocampal CA1 neurons and attenuates down-regulation of the AMPA receptor GluR2 subunit after forebrain ischemia

Brief forebrain ischemia in rodents induces selective and delayed neuronal death, particularly of hippocampal CA1 pyramidal neurons. Neuronal death is preceded by down-regulation specific to CA1 of GluR2, the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunit that limits Ca(2+) influx. This alteration is hypothesized to cause neurodegeneration by permitting a lethal influx of Ca(2+) and/or Zn(2+) through newly formed GluR2-lacking AMPA receptors. Two days of mild hypothermia induced 1 h after ischemia potently and lastingly protects against ischemic injury. We examined molecular mechanisms underlying hypothermia-induced neuroprotection. We report that hypothermia rescues most hippocampal CA1 neurons from ischemia-induced cell death and attenuates ischemia-induced down-regulation of mRNA encoding the AMPA receptor subunit GluR2. Ischemia induced a marked down-regulation of GluR2 mRNA and a small down-regulation of GluR1 mRNA in CA1 at 2 days, as assessed by quantitative in situ hybridization. The ischemia-induced changes in gene expression were cell-specific in that GluR2 was not significantly altered in CA3 or dentate gyrus. After ischemia treated by hypothermia GluR2 mRNA expression was modestly reduced at 2 days and exhibited complete recovery to control levels at 7 days. Hypothermia prevented ischemia induced changes in GluR1 mRNA expression. These findings suggest that intervention at the level of transcriptional regulation of the GluR2 gene may be a mechanism by which prolonged postischemic cooling rescues CA1 neurons otherwise "destined to die."