Global cerebral ischemia: synaptic and cognitive dysfunction

Cardiopulmonary arrest is one of the leading causes of death and disability, primarily occurring in the aged population. Numerous global cerebral ischemia animal models induce neuronal damage similar to cardiac arrest. These global cerebral ischemia models range from vessel occlusion to total cessation of cardiac function, both of which have allowed for the investigation of this multifaceted disease and detection of numerous agents that are neuroprotective. Synapses endure a variety of alterations after global cerebral ischemia from the resulting excitotoxicity and have been a major target for neuroprotection; however, neuroprotective agents have proven unsuccessful in clinical trials, as neurological outcomes have not displayed significant improvements in patients. A majority of these neuroprotective agents have specific neuronal targets, where the success of future neuroprotective agents may depend on non-specific targets and numerous cognitive improvements. This review focuses on the different models of global cerebral ischemia, neuronal synaptic alterations, synaptic neuroprotection and behavioral tests that can be used to determine deficits in cognitive function after global cerebral ischemia.

Neuronal SIRT1 (Silent Information Regulator 2 Homologue 1) Regulates Glycolysis and Mediates Resveratrol-Induced Ischemic Tolerance

BACKGROUND AND PURPOSE

Resveratrol, at least in part via SIRT1 (silent information regulator 2 homologue 1) activation, protects against cerebral ischemia when administered 2 days before injury. However, it remains unclear if SIRT1 activation must occur, and in which brain cell types, for the induction of neuroprotection. We hypothesized that neuronal SIRT1 is essential for resveratrol-induced ischemic tolerance and sought to characterize the metabolic pathways regulated by neuronal Sirt1 at the cellular level in the brain.

METHODS

We assessed infarct size and functional outcome after transient 60 minute middle cerebral artery occlusion in control and inducible, neuronal-specific SIRT1 knockout mice. Nontargeted primary metabolomics analysis identified putative SIRT1-regulated pathways in brain. Glycolytic function was evaluated in acute brain slices from adult mice and primary neuronal-enriched cultures under ischemic penumbra-like conditions.

RESULTS

Resveratrol-induced neuroprotection from stroke was lost in neuronal Sirt1 knockout mice. Metabolomics analysis revealed alterations in glucose metabolism on deletion of neuronal Sirt1, accompanied by transcriptional changes in glucose metabolism machinery. Furthermore, glycolytic ATP production was impaired in acute brain slices from neuronal Sirt1 knockout mice. Conversely, resveratrol increased glycolytic rate in a SIRT1-dependent manner and under ischemic penumbra-like conditions in vitro.

CONCLUSIONS

Our data demonstrate that resveratrol requires neuronal SIRT1 to elicit ischemic tolerance and identify a novel role for SIRT1 in the regulation of glycolytic function in brain. Identification of robust neuroprotective mechanisms that underlie ischemia tolerance and the metabolic adaptations mediated by SIRT1 in brain are crucial for the translation of therapies in cerebral ischemia and other neurological disorders.

Protein kinase C epsilon regulates mitochondrial pools of Nampt and NAD following resveratrol and ischemic preconditioning in the rat cortex

Preserving mitochondrial pools of nicotinamide adenine dinucleotide (NAD) or nicotinamide phosphoribosyltransferase (Nampt), an enzyme involved in NAD production, maintains mitochondrial function and confers neuroprotection after ischemic stress. However, the mechanisms involved in regulating mitochondrial-localized Nampt or NAD have not been defined. In this study, we investigated the roles of protein kinase C epsilon (PKCɛ) and AMP-activated protein kinase (AMPK) in regulating mitochondrial pools of Nampt and NAD after resveratrol or ischemic preconditioning (IPC) in the cortex and in primary neuronal-glial cortical cultures. Using the specific PKCɛ agonist ψɛRACK, we found that PKCɛ induced robust activation of AMPK in vitro and in vivo and that AMPK was required for PKCɛ-mediated ischemic neuroprotection. In purified mitochondrial fractions, PKCɛ enhanced Nampt levels in an AMPK-dependent manner and was required for increased mitochondrial Nampt after IPC or resveratrol treatment. Analysis of intrinsic NAD autofluorescence using two-photon microscopy revealed that PKCɛ modulated NAD in the mitochondrial fraction. Further assessments of mitochondrial NAD concentrations showed that PKCɛ has a key role in regulating the mitochondrial NAD(+)/nicotinamide adenine dinucleotide reduced (NADH) ratio after IPC and resveratrol treatment in an AMPK- and Nampt-dependent manner. These findings indicate that PKCɛ is critical to increase or maintain mitochondrial Nampt and NAD after pathways of ischemic neuroprotection in the brain.

Effect of cardiac arrest on cognitive impairment and hippocampal plasticity in middle-aged rats

Cardiopulmonary arrest is a leading cause of death and disability in the United States that usually occurs in the aged population. Cardiac arrest (CA) induces global ischemia, disrupting global cerebral circulation that results in ischemic brain injury and leads to cognitive impairments in survivors. Ischemia-induced neuronal damage in the hippocampus following CA can result in the impairment of cognitive function including spatial memory. In the present study, we used a model of asphyxial CA (ACA) in nine month old male Fischer 344 rats to investigate cognitive and synaptic deficits following mild global cerebral ischemia. These experiments were performed with the goals of 1) establishing a model of CA in nine month old middle-aged rats; and 2) to test the hypothesis that learning and memory deficits develop following mild global cerebral ischemia in middle-aged rats. To test this hypothesis, spatial memory assays (Barnes circular platform maze and contextual fear conditioning) and field recordings (long-term potentiation and paired-pulse facilitation) were performed. We show that following ACA in nine month old middle-aged rats, there is significant impairment in spatial memory formation, paired-pulse facilitation n dysfunction, and a reduction in the number of non-compromised hippocampal Cornu Ammonis 1 and subiculum neurons. In conclusion, nine month old animals undergoing cardiac arrest have impaired survival, deficits in spatial memory formation, and synaptic dysfunction.

Increased BDNF protein expression after ischemic or PKC epsilon preconditioning promotes electrophysiologic changes that lead to neuroprotection

Ischemic preconditioning (IPC) via protein kinase C epsilon (PKCɛ) activation induces neuroprotection against lethal ischemia. Brain-derived neurotrophic factor (BDNF) is a pro-survival signaling molecule that modulates synaptic plasticity and neurogenesis. Interestingly, BDNF mRNA expression increases after IPC. In this study, we investigated whether IPC or pharmacological preconditioning (PKCɛ activation) promoted BDNF-induced neuroprotection, if neuroprotection by IPC or PKCɛ activation altered neuronal excitability, and whether these changes were BDNF-mediated. We used both in vitro (hippocampal organotypic cultures and cortical neuronal-glial cocultures) and in vivo (acute hippocampal slices 48 hours after preconditioning) models of IPC or PKCɛ activation. BDNF protein expression increased 24 to 48 hours after preconditioning, where inhibition of the BDNF Trk receptors abolished neuroprotection against oxygen and glucose deprivation (OGD) in vitro. In addition, there was a significant decrease in neuronal firing frequency and increase in threshold potential 48 hours after preconditioning in vivo, where this threshold modulation was dependent on BDNF activation of Trk receptors in excitatory cortical neurons. In addition, 48 hours after PKCɛ activation in vivo, the onset of anoxic depolarization during OGD was significantly delayed in hippocampal slices. Overall, these results suggest that after IPC or PKCɛ activation, there are BDNF-dependent electrophysiologic modifications that lead to neuroprotection.

Cognitive Deficits after Cerebral Ischemia and Underlying Dysfunctional Plasticity: Potential Targets for Recovery of Cognition

Cerebral ischemia affects millions of people worldwide and survivors suffer from long-term functional and cognitive deficits. While stroke and cardiac arrest are typically considered when discussing ischemic brain injuries, there is much evidence that smaller ischemic insults underlie neurodegenerative diseases, including Alzheimer's disease. The "regenerative" capacity of the brain relies on several aspects of plasticity that are crucial for normal functioning; less affected brain areas may take over function previously performed by irreversibly damaged tissue. To harness the endogenous plasticity mechanisms of the brain to provide recovery of cognitive function, we must first understand how these mechanisms are altered after damage, such as cerebral ischemia. In this review, we discuss the long-term cognitive changes that result after cerebral ischemia and how ischemia alters several plasticity processes. We conclude with a discussion of how current and prospective therapies may restore brain plasticity and allow for recovery of cognitive function, which may be applicable to several disorders that have a disruption of cognitive processing, including traumatic brain injury and Alzheimer's disease.

Physical Exercise Improves Cognitive Outcomes in 2 Models of Transient Cerebral Ischemia

BACKGROUND AND PURPOSE

Preclinical studies suggest that exercise can enhance cognition after cerebral ischemia but often use long training regiments and test cognition during or acutely after training. The cognitive changes may result from enhanced physical fitness and may only provide acute benefit. We sought to determine whether a short period of exercise after cerebral ischemia could improve cognitive outcomes when measured days after completion of exercise training in 2 cerebral ischemia models.

METHODS

Focal or global cerebral ischemia was induced in Sprague-Dawley rats. Rats recovered (3-4 days) then were subject to no exercise (0 m/min), mild (6 m/min), moderate (10 m/min), or heavy (15-18 m/min) treadmill exercise (5-6 days). Cognition was tested 8 to 10 days after the last exercise session with hippocampal-dependent contextual fear conditioning.

RESULTS

A short training period of moderate exercise enhanced cognitive function for a week after exercise completion in both models of cerebral ischemia.

CONCLUSIONS

Utilization of this exercise paradigm can further the elucidation of exercise-mediated factors involved in cognitive recovery independent of changes in physical fitness.

Recurrent Hypoglycemia Exacerbates Cerebral Ischemic Damage in Diabetic Rats via Enhanced Post-Ischemic Mitochondrial Dysfunction

Diabetes significantly increases the risk of stroke and post-stroke mortality. Recurrent hypoglycemia (RH) is common among diabetes patients owing to glucose-lowering therapies. Earlier, we showed that RH in a rat model of insulin-dependent diabetes exacerbates cerebral ischemic damage. Impaired mitochondrial function has been implicated as a central player in the development of cerebral ischemic damage. Hypoglycemia is also known to affect mitochondrial functioning. The present study tested the hypothesis that prior exposure of insulin-treated diabetic (ITD) rats to RH exacerbates brain damage via enhanced post-ischemic mitochondrial dysfunction. In a rat model of streptozotocin-induced diabetes, we evaluated post-ischemic mitochondrial function in RH-exposed ITD rats. Rats were exposed to five episodes of moderate hypoglycemia prior to the induction of cerebral ischemia. We also evaluated the impact of RH, both alone and in combination with cerebral ischemia, on cognitive function using the Barnes circular platform maze test. We observed that RH exposure to ITD rats leads to increased cerebral ischemic damage and decreased mitochondrial complex I activity. Exposure of ITD rats to RH impaired spatial learning and memory. Our results demonstrate that RH exposure to ITD rats potentially increases post-ischemic damage via enhanced post-ischemic mitochondrial dysfunction.

Protein kinase C epsilon delays latency until anoxic depolarization through arc expression and GluR2 internalization

Global cerebral ischemia is a debilitating injury that damages the CA1 region of the hippocampus, an area important for learning and memory. Protein kinase C epsilon (PKCɛ) activation is a critical component of many neuroprotective treatments. The ability of PKCɛ activation to regulate AMPA receptors (AMPARs) remains unexplored despite the role of AMPARs in excitotoxicity after brain ischemia. We determined that PKCɛ activation increased expression of a protein linked to learning and memory, activity-regulated cytoskeleton-associated protein (arc). Also, arc is necessary for neuroprotection and confers protection through decreasing AMPAR currents via GluR2 internalization. In vivo, activation of PKCɛ increased arc expression through a BDNF/TrkB pathway, and decreased GluR2 mRNA levels. In hippocampal cultured slices, PKCɛ activation decreased AMPAR current amplitudes in an arc- and GluR2-dependent manner. Additionally, PKCɛ activation triggered an arc- and GluR2 internalization-dependent delay in latency until anoxic depolarization. Inhibiting arc also blocked PKCɛ-mediated neuroprotection against lethal oxygen and glucose deprivation. These data characterize a novel PKCɛ-dependent mechanism that for the first time defines a role for arc and AMPAR internalization in conferring neuroprotection.

Sex-Dependent Differences in Physical Exercise-Mediated Cognitive Recovery Following Middle Cerebral Artery Occlusion in Aged Rats

Stroke remains a leading cause of death and disability in the United States. No current treatments exist to promote cognitive recovery in survivors of stroke. A previous study from our laboratory determined that an acute bout of forced treadmill exercise was able to promote cognitive recovery in 3 month old male rats after middle cerebral artery occlusion (MCAo). In this study, we tested the hypothesis that 6 days of intense acute bout of forced treadmill exercise (physical exercise – PE) promotes cognitive recovery in 11–14 month old male rats. We determined that PE was able to ameliorate cognitive deficits as determined by contextual fear conditioning. Additionally, we also tested the hypothesis that PE promotes cognitive recovery in 11–13 month old reproductive senescent female rats. In contrast to males, the same intensity of exercise that decrease cognitive deficits in males was not able to promote cognitive recovery in female rats. Additionally, we determined that exercise did not lessen infarct volume in both male and female rats. There are many factors that contribute to higher stroke mortality and morbidities in women and thus, future studies will investigate the effects of PE in aged female rats to identify sex differences.

Abstract TP267: Protein Kinase C Epsilon-activation Mediates Ischemic Neuroprotection by Activating an Activity-regulated Cytoskeleton-associated Protein-dependent Mechanism

Introduction: Cerebral ischemia can trigger cell death in the CA1 region of the hippocampus, an area important for memory. Protein kinase C epsilon (PKCε) activation prior to ischemia protects the CA1 from injury by modulating neurotransmission. The underlying mechanism needs further study.

Hypothesis: PKCε-induced neuroprotection increases latency until anoxic depolarization (AD) through an activity-regulated cytoskeleton-associated protein (arc)-dependent mechanism of regulating AMPAR currents.

Results: In vivo activation of PKCε by the PKCε-activator ψε-Receptor of Activated C Kinase (ψεRACK) increased BDNF 5.99 +/- 0.11 fold and TrkB phosphorylation levels 2.94 +/- 0.32 fold (enhancers of arc protein levels) (n = 4, p < 0.005, t-test). Arc mRNA and protein was increased 143.97 +/-7.68 % and 1.91 +/- 0.22 fold (n = 9, p < 0.005, t-test). Inhibition of arc using antisense oligodeoxynucleotides (arc AS ODNs) in cultured slices blocked PKCε-mediated neuroprotection against lethal oxygen and glucose deprivation (OGD) from 35.91 +/- 5.97 to 74.93 +/- 4.24 % cell death (n = 6, p < 0.005, ANOVA, Bonferroni). ΨεRACK decreased AMPAR-mediated mEPSC amplitude to 12.75 +/- 0.35 pA from 14.80 +/- 0.39 pA (n = 20, p < 0.01, ANOVA, Bonferroni). This effect was arc-dependent. Additionally, ψεRACK treatment increased latency until AD from 29.27 +/- 3.6 min 50.77 +/- 5.08 min (n = 13, p < 0.01, ANOVA, Bonferroni). This increase was arc-dependent, and required AMPAR internalization. Inhibiting internalization reduced AD latency from 54.5 +/- 8.40 min to 22.3 + 5.17 min (n = 6, p <0.005, t-test).

Conclusion: Arc expression is necessary for neuroprotection afforded by PKCε-activation, modulates excitatory synaptic strength, and increases latency until AD.

Methods: Western blot: Proteins (40 μg) were separated on a 12% SDS-PAGE gel. Immunofluorescence : 30 μm sections were incubated with 1:500 NeuN and 1:50 arc in PBS with 0.8% triton. Cultured slices preparation : sections from P9-11 rats were plated on inserts and cultured for 14 days. PI measurements of cell death : Slices were incubated in medium with 2 μg/mL PI. mEPSC measurements : Whole-cell voltage clamp was performed. AD: OGD, perfusate was switched to glucose free media, and bubbled with a 95% N 2 and 5% CO 2 gas.

Abstract TP98: Moderate Physical Exercise Promotes Cognitive Recovery Following Focal Cerebral Ischemia in Rats

The majority of stroke patients suffer long-term disabilities impacting both motor and cognitive function. Physical exercise has been shown to improve functional recovery following focal cerebral ischemia by facilitating brain plasticity. However, the exercise intensity that promotes optimal cognitive recovery needs closer examination. This study seeks to analyze the effects of forced treadmill exercise at various speeds on spatial memory tests after focal ischemia in rats. We hypothesized that, a moderate intensity treadmill regiment will better ameliorate cognitive deficits and improve functional recovery after focal ischemia compared to the high or mild intensity groups. Three-month-old male Sprague Dawley rats were acclimated to the investigator and treadmill for three days. On day four, rats underwent focal cerebral ischemia using the middle cerebral artery occlusion model for 90 minutes or sham surgery and were randomly assigned to an exercise intensity group. Rats had three days of recovery post-surgery followed by forced treadmill exercise for six days. The rats were exposed to a two-minute warm-up period at 5 m/min and then exercised for a period of 30 minutes at 6 m/min for mild, 10 m/min for moderate, and 18 m/min for heavy. Finally, the rats were subjected to contextual fear conditioning where rats were placed in a fear conditioning chamber for 8 minutes with a 2 second 1.5 mA shock delivered at minute 7.5 then returned to chamber 24 hours later. Fear conditioning data showed that, following a stroke rats froze 16.48 +/- 2.75%, compared to the moderate intensity group where rats froze 63.57 +/- 10.72 % (S.E.M, n = 10, p < 0.01, one-way ANOVA, Bonferroni post-hoc). Mild and heavy exercise groups froze 43.3 +/- 10.75% and 38.26 +/- 10.53% respectively. In conclusion, focal cerebral ischemia impairs cognitive function, however moderate intensity physical exercise has the most beneficial impact on cognitive recovery compared to other exercise intensities.