The neuroprotective effects of gamma-vinyl GABA in transient global ischemia: a morphological study with early and delayed evaluations

An Overview of Stroke: Mechanism, In vivo Experimental Models Thereof, and Neuroprotective Agents

BACKGROUND

Stroke is one of the causes of death and disability globally. Brain attack is because of the acute presentation of stroke, which highlights the requirement for decisive action to treat it.

OBJECTIVE

The mechanism and in-vivo experimental models of stroke with various neuroprotective agents are highlighted in this review.

METHOD

The damaging mechanisms may proceed by rapid, nonspecific cell lysis (necrosis) or by the active form of cell death (apoptosis or necroptosis), depending upon the duration and severity and of the ischemic insult.

RESULTS

Identification of injury mediators and pathways in a variety of experimental animal models of global cerebral ischemia has directed to explore the target-specific cytoprotective strategies, which are critical to clinical brain injury outcomes.

CONCLUSION

The injury mechanism, available encouraging medicaments thereof, and outcomes of natural and modern medicines for ischemia have been summarized. In spite of available therapeutic agents (thrombolytics, calcium channel blockers, NMDA receptor antagonists and antioxidants), there is a need for an ideal drug for strokes.

[Long-term effect of oligodendrocyte precursor cell transplantation on a rat model of white matter injury in the preterm infant]

OBJECTIVE

To investigate the long-term effect of oligodendrocyte precursor cell (OPC) transplantation on a rat model of white matter injury (WMI) in the preterm infant.

METHODS

A total of 80 Sprague-Dawley rats aged 3 days were randomly divided into sham-operation group, model control group, 5-day ventricular/white matter transplantation group, 9-day ventricular/white matter transplantation group, 14-day ventricular/white matter transplantation group (n=10 each). All groups except the sham-operation group were treated with right common carotid artery ligation and hypoxia for 80 minutes to establish a rat model of WMI in the preterm infant. OPCs were prepared from the human fetal brain tissue (10-12 gestational weeks). At 5, 9, and 14 days after modeling, 3×10⁵ OPCs were injected into the right lateral ventricle or white matter in each transplantation group, and myelin sheath and neurological function were evaluated under an electron microscope at ages of 60 and 90 days.

RESULTS

Electron microscopy showed that at an age of 60 days, each transplantation group had a slight improvement in myelin sheath injury compared with the model control group; at an age of 90 days, each transplantation group had significantly thickened myelin sheath and reduced structural damage compared with the model control group, and the 14-day transplantation groups had the most significant changes. There were no significant differences in the degree of myelin sheath injury between the ventricular and white matter transplantation groups at different time points. At an age of 60 or 90 days, the transplantation groups had a significantly higher modified neurological severity score (mNSS) than the sham-operation group and a significantly lower mNSS than the model control group (P<0.05).

CONCLUSIONS

OPC transplantation may have a long-term effect in the treatment of WMI in the preterm infant, and delayed transplantation may enhance its therapeutic effect.

Hypoxia and GABA shunt activation in the pathogenesis of Alzheimer's disease

We have previously observed that the conversion of mild cognitive impairment to definitive Alzheimer's disease (AD) is associated with a significant increase in the serum level of 2,4-dihydroxybutyrate (2,4-DHBA). The metabolic generation of 2,4-DHBA is linked to the activation of the γ-aminobutyric acid (GABA) shunt, an alternative energy production pathway activated during cellular stress, when the function of Krebs cycle is compromised. The GABA shunt can be triggered by local hypoperfusion and subsequent hypoxia in AD brains caused by cerebral amyloid angiopathy. Succinic semialdehyde dehydrogenase (SSADH) is a key enzyme in the GABA shunt, converting succinic semialdehyde (SSA) into succinate, a Krebs cycle intermediate. A deficiency of SSADH activity stimulates the conversion of SSA into γ-hydroxybutyrate (GHB), an alternative route from the GABA shunt. GHB can exert not only acute neuroprotective activities but unfortunately also chronic detrimental effects which may lead to cognitive impairment. Subsequently, GHB can be metabolized to 2,4-DHBA and secreted from the brain. Thus, the activation of the GABA shunt and the generation of GHB and 2,4-DHBA can have an important role in the early phase of AD pathogenesis.

Anesthetics, cerebral protection and preconditioning

BACKGROUND AND OBJECTIVES

Several studies demonstrate that cerebral preconditioning is a protective mechanism against a stressful situation. Preconditioning determinants are described, as well as the neuroprotection provided by anesthetic and non-anesthetics agents.

CONTENT

Review based on the main articles addressing the pathophysiology of ischemia-reperfusion and neuronal injury and pharmacological and non-pharmacological factors (inflammation, glycemia, and temperature) related to the change in response to ischemia-reperfusion, in addition to neuroprotection induced by anesthetic use.

CONCLUSIONS

The brain has the ability to protect itself against ischemia when stimulated. The elucidation of this mechanism enables the application of preconditioning inducing substances (some anesthetics), other drugs, and non-pharmacological measures, such as hypothermia, aimed at inducing tolerance to ischemic lesions.

Anesthetics, cerebral protection and preconditioning

BACKGROUND AND OBJECTIVES

Several studies demonstrate that cerebral preconditioning is a protective mechanism against a stressful situation. Preconditioning determinants are described, as well as the neuroprotection provided by anesthetic and non-anesthetics agents.

CONTENT

Review based on the main articles addressing the pathophysiology of ischemia-reperfusion and neuronal injury and pharmacological and non-pharmacological factors (inflammation, glycemia, and temperature) related to the change in response to ischemia-reperfusion, in addition to neuroprotection induced by anesthetic use.

CONCLUSIONS

The brain has the ability to protect itself against ischemia when stimulated. The elucidation of this mechanism enables the application of preconditioning inducing substances (some anesthetics), other drugs, and non-pharmacological measures, such as hypothermia, aimed at inducing tolerance to ischemic lesions.

Gastrodia elata Blume water extracts improve insulin resistance by decreasing body fat in diet-induced obese rats: vanillin and 4-hydroxybenzaldehyde are the bioactive candidates

BACKGROUND

Insulin resistance is a common symptom of metabolic diseases such as obesity, type 2 diabetes and hyperlipidemia.

AIM OF THE STUDY

We investigated whether Gastrodia elata Blume water extract(GEB), containing phenolic compounds, had a beneficial action on insulin resistance in male Sprague-Dawley rats fed a high fat diet(HFD) and determined how this effect was produced. In addition, the bioactive candidates involved were identified.

METHODS

Rats fed HFD were daily administered with 0.3 g GEB(GEB-L), 1 g GEB(GEB-H), or 1 g cellulose(control) per kg body weight for 8 weeks, while rats in the fourth group were fed a low fat diet(LFD). In vitro study, 4 major components of GEB were tested for their impact on fat accumulation.

RESULTS

Rats in the control group exhibited a higher weight gain of epididymal and retroperitoneal fat pads than those fed LFD, while GEB prevented such an increment in a dose-dependent manner. GEB-H significantly decreased energy intake partly through potentiating STAT3 phosphorylation and attenuating AMPK phosphorylation in the hypothalamus. GEB-H also increased energy expenditure with the increase in fat oxidation. GEB-H increased whole body glucose disposal rates and decreased hepatic glucose output compared to the control. Among the major components of GEB, 4-hydroxybenzaldehyde and vanillin decreased triglyceride accumulation by modulating the expression of genes involved in fat metabolism in 3T3-L1 adipocytes. They increased insulin-stimulated glucose uptake to reduce insulin resistance.

CONCLUSIONS

GEB-H, mainly as a result of the action of 4-hydroxybenzaldehyde and vanillin, reduces insulin resistance by decreasing fat accumulation in adipocytes by activating fat oxidation and potentiating leptin signaling in diet-induced obese rats.

Rodent neonatal bilateral carotid artery occlusion with hypoxia mimics human hypoxic-ischemic injury

We report a new clinically relevant model of neonatal hypoxic-ischemic injury in a 10-day-old rat pup. Bilateral carotid artery occlusion and 8% hypoxia (1 to 15 mins, BCAO-H) was induced with varying degrees of injury (mild, moderate, severe), which was quantified using magnetic resonance imaging including diffusion-weighted and T2-weighted imaging at 24 h and 21/28 days. We developed a magnetic resonance imaging-based rat pup severity score and compared 3D ischemic injury volumes/rat pup severity score with histology and behavioral testing. At 24 h, hypoxic-ischemic injury was observed in 17/27 animals; long-term survival was 81%. Magnetic resonance imaging lesion volumes did not correlate with hypoxia duration but correlated with rat pup severity score, which was used to classify animals into mild (n=21), moderate (n=6), and severe (n=10) groups with average brain lesion volumes of 0.9%, 33.2%, and 56.3%, respectively. Histology confirmed lesion location and histologic scoring correlated with the rat pup severity score. We also found excellent correlation between injury severity and multiple behavioral tasks. Bilateral carotid artery occlusion and hypoxia in the P10 rat pup is an excellent model of neonatal hypoxic-ischemic injury because it induces diffuse global injury similar to the term infant. This model can produce graded injury severity, similar to that seen in human neonates, but manipulation with hypoxia duration is unpredictable.

Vanillin, 4-hydroxybenzyl aldehyde and 4-hydroxybenzyl alcohol prevent hippocampal CA1 cell death following global ischemia

Mongolian gerbils subjected to transient global ischemia exhibit neuroprotection against ischemic neuronal cell death in the hippocampal CA1 region when treated with vanillin, 4-hydroxybenzyl aldehyde (4-HBAL) and 4-hydroxybenzyl alcohol (4-HBA), which are active components of Gastrodia elata Blume. Pre- and post-insult vanillin, 4-HBAL and 4-HBA treated-animals showed a significant increase in neuronal survival (66.32%, 43.21% and 64.58%, respectively) compared to vehicle-treated animals. Animals exhibited a gender difference in this neuroprotective effect. To study the neuroprotective mechanism of 4-HBA, we investigated N-methyl-d-aspartate (NMDA) receptor 1 (NR1), 8-hydroxy-2'-deoxyguanosine (8-OHdG), and gamma-aminobutyric acid transaminase (GABA-T) immunoreactivity at various times after ischemic insults. Treatment with 4-HBA did not affect NR1 expression levels, down-regulated 8-OHdG immunoreactivity, and increased GABA-T expression levels after global ischemia, suggesting that 4-HBA inhibited NR1 stimulation. Moreover, GABA-T was rapidly increased in the early stage after ischemia, which might enhance the survival of cells by supplying energy to the CA1 region. These results suggest that 4-HBA inhibits oxidative stress and excitotoxicity for at least 12 h and suppresses neuronal death in CA1 region. Diethyl ether fractions of GE scavenged hydroxyl radical (OH.) and showed antioxidant activity on lipid peroxidation. Vanillin and 4-HBA treatment blocked oxidative damage in PC12 cells. The neuroprotective effect has therapeutic significance and these compounds need to be evaluated for potential use in protecting against neuronal cell damage during stroke.

The co-treatments of vigabatrin and P2X receptor antagonists protect ischemic neuronal cell death in the gerbil hippocampus

During transient global ischemia, the excessive accumulation of intracellular Ca2+ induced by several episodes triggers delayed neuronal death within the vulnerable CA1 region of the hippocampus after ischemia-reperfusion insults. Although P2X receptors provide an additional source of Ca2+ entry, little data are available that these receptors could modulate the performance of the ischemic neuronal death. Therefore, we investigated the roles of the P2X receptor in the ischemic neuronal damage associated with various sequelae of transient ischemia, and the effects of their antagonist on the ischemic insults. As the results, ischemic insults increased P2X receptor expression in the gerbil hippocampus. Neither vigabatrin (VGB) nor P2X receptor antagonists (suramin, pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid) protected against the delayed neuronal death in the CA1 region of the hippocampus after ischemia. However, the co-treatments of VGB and P2X receptor antagonists effectively prevent ischemia-induced neurodegeneration. Therefore, these findings suggest that blockade of the P2X receptor accompanied by activation of GABAergic inhibition may play an important role in the neuroprotection against ischemic insults.

Correlations between neuronal loss, decrease of memory, and decrease expression of brain-derived neurotrophic factor in the gerbil hippocampus during normal aging

It is known that the hippocampus has vital functions in learning and memory, behavioral regulation, and activity-dependent synaptic plasticity, and that the hippocampus contains high levels of brain-derived neurotrophic factor (BDNF). In the present study, we followed age-dependent changes of BDNF immunoreactivity and protein level in the gerbil hippocampus to identify the correlation between BDNF and aging. BDNF immunoreactivity and its protein level significantly increased at postnatal month (PM) 12 in the hippocampus and thereafter reduced. At PM 24, BDNF immunoreactivity in the hippocampal CA1 region and dentate gyrus was similar to that in the PM 1 group, whereas BDNF immunoreactivity in the CA2/3 region at PM 24 was higher than that at PM 1. In the PM 24 group, an age-related neuronal loss and the decrease of reference and working memory were observed. In conclusion, our results suggest that observed reduction in BDNF and reference memory may be associated with age-dependent neuronal loss in the hippocampal CA1 region.

GABA-mediated effects of some taurine derivatives injected i.c.v. on rabbit rectal temperature and gross motor behavior

Some synthetic taurine analogues, namely ethanolamine-O-sulphate (EOS), N,N-dimethyltaurine (DMT), N,N,N-trimethyltaurine (TMT) and 2-aminoethylphosphonic acid (AEP) were shown to interact with rabbit brain GABA(A)- or GABA(B)-receptors, while (+/-)piperidine-3-sulfonic acid (PSA) inhibited the activity of rabbit brain 4-aminobutyrate transaminase. This suggests that they behave like direct/indirect GABA agonists or GABA antagonists and affect thermoregulation and gross motor behaviour (GMB) which are under GABA control. In the present study micromole (1.2-48) amounts of these compounds were i.c.v. injected in conscious, restrained rabbits while monitoring rectal temperature (RT), ear skin temperature (EST) and GMB. AEP, EOS, DMT and TMT induced a dose-related hyperthermia, ear vasoconstriction and excitation of GMB, while PSA induced a dose-related hypothermia, ear vasodilation and inhibition of GMB. EOS antagonized in a dose-related fashion hypothermia induced by 60 nmol THIP, a GABA(A) agonist, while AEP, DMT and TMT counteracted that induced by 8 nmol R(-)Baclofen, a GABA(B) agonist. In conclusion, EOS and AEP, DMT, TMT seem to act as GABA(A) and GABA(B) antagonists, respectively, while PSA behaves like an indirect GABA agonist, all affecting the central mechanisms which drive rabbit thermoregulation.

Mechanisms of Brain Injury after Global Cerebral Ischemia

Cerebral ischemia results in a rapid depletion of energy stores that triggers a complex cascade of cellular events such as cellular depolarization and Ca2+ influx, resulting in excitotoxic cell death. The critical determinant of severity of brain injury is the duration and severity of the ischemic insult and early restoration of CBF. Induced therapeutic hypothermia following CA is the only strategy that has demonstrated improvement in outcomes in prospective, randomized clinical trials. Although pharmacologic neuro-protection has been disappointing thus far in a variety of experimental animal models, further research efforts are directed at using some agents that demonstrate marginal or moderate efficacy in combination with hypothermia. Although the signal transduction pathways and intracellular molecular events during cerebral ischemia and reperfusion are complex, potential therapeutic neuroprotective strategies hold promise for the future.

A systematic review of currently available pharmacological neuroprotective agents as a sole intervention before anticipated or induced cardiac arrest

We conducted a Medline search for controlled studies evaluating currently available drugs for pharmacological neuroprotection. They had to be administered prior to transient global cerebral ischaemia without further non-pharmacological measures. We deliberately excluded focal ischaemia since its pathophysiology is substantially different from global ischaemia. A total of 45 articles conducted exclusively in laboratory animals met these criteria. The following classes of agents were evaluated: anaesthetics, GABAergic drugs, calcium-antagonists, anticonvulsives, sodium-channel blockers, potassium-channel activators, NMDA-receptor antagonists, hormones, vasodilators, dopamine- and alpha2-agonists, magnesium, xanthine oxidase- and cyclooxygenase inhibitors, a nootropic, a protease inhibitor, and immunosuppressants. Some of them were applied chronically and others administered via clinically impracticable routes. The available literature favours isoflurane, phenytoin, lamotrigine, magnesium, and potentially, nimodipine, and flunarizine. If factors like costs, toxicity, side effects, route and mode of application are considered, isoflurane and MgSO4 that have also been safely applied to patients with compromised left ventricular pump function are advantageous but their true role in human neuroprotection remains unclear.

Protective effects of valproic acid against hypoxic-ischemic brain injury in neonatal rats

Although controversial, protective and therapeutic effects of valproic acid in various types of cellular injury suggest a potential role for this agent in hypoxic-ischemic brain injury. We therefore investigated the effects of valproic acid in an experimental model of neonatal hypoxic-ischemic brain injury. To examine the effect of valproic acid in this condition, hypoxic-ischemic brain injury was induced in 7-day-old rat pups by ligation of the right common carotid and then the pups were exposed to 1 hour of hypoxia in 8% oxygen. Low (200 mg/kg/day) and high (400 mg/kg/day) doses of valproic acid were administered in a 5-day regimen. Neuropathologic evaluation was performed using the hippocampus, cerebral cortex, and basal ganglia in the coronal plane. The 5-day regimen of valproic acid administration resulted in some protective and therapeutic effects on the brain damage and neuronal apoptosis in both hemispheres in a dose-dependent manner. Administration of valproic acid also decreased the percentage of apoptotic neurons in the contralateral hemisphere (P < .05). These results suggest that valproic acid can have therapeutic and protective effects in hypoxic-ischemic brain injury.

Differential neuroprotective effects for three GABA-potentiating compounds in a model of hypoxia–ischemia

Clomethiazole (CMZ) is a GABA(A)-potentiating compound; however, it is unclear whether this mode of action is responsible for its neuroprotective effects in animal models of ischemia. This study compared the neuroprotective efficacies of muscimol and midazolam, two potent GABA(A)-potentiating compounds, to that of CMZ in a model of hypoxia-ischemia (H-I). To establish a neuroprotective profile for CMZ, CMZ (60, 95, or 125 mg kg-1, i.p.) was administered to post-natal day 25 male rats at numerous post-hypoxic time points and the rats were sacrificed 1 or 4 weeks later. Varying degrees of histological protection were evident when CMZ was administered 1, 2, or 3 h post-hypoxia with the 125 mg kg-1 dose producing complete histological protection if administered 3 h post-hypoxia. To determine whether midazolam or muscimol could match the protection provided by CMZ administered 3 h post-hypoxia, H-I rats received varying doses of these compounds 3 h post-hypoxia and were sacrificed 1 week later. Under identical conditions, no dose of muscimol or midazolam provided equivalent neuroprotection to that provided by CMZ. In fact, muscimol showed no neuroprotective ability whatsoever. Thus, CMZ, administered as late as 3 h post-hypoxia, was able to completely prevent H-I-induced cell death while a full dose range of other GABA-potentiating agents did not. Such direct comparison of these compounds in this model suggests the mechanism underlying the protective effects of CMZ may not rely solely on GABA(A)-potentiating properties. Elucidation of a novel mechanism of action for CMZ may expose new therapeutic targets in stroke treatment.

Expression and changes of galanin in neurons and microglia in the hippocampus after transient forebrain ischemia in gerbils

In the present study, we investigated chronological changes of galanin (GAL), well known as the potassium channel opener, immunoreactivity and GAL protein level in the hippocampus of the gerbil at the various times after 5 min transient forebrain ischemia. In the sham-operated group, weak GAL immunoreactivity was found in non-pyramidal cells. At 12 h after ischemia-reperfusion, the number of GAL-immunoreactive neurons and GAL immunoreactivity were significantly increased in the hippocampus compared to 3 h after ischemic insult, especially in the hippocampal CA1 region. Thereafter the number of GAL-immunoreactive neurons and GAL immunoreactivity decrease time-dependently in the hippocampus. Four days after transient ischemia, GAL immunoreactivity was low as compared with the sham-operated group. At this time point after ischemic insult, GAL immunoreactivity was shown in microglia in the CA1 region because delayed neuronal death happened in the CA1 pyramidal cells. The result of Western blot showed the pattern of GAL expression similar to that of immunohistochemical data. These results suggest that the early increase of GAL in the CA1 pyramidal cells may be associated with the reduction of the excitotoxic damage, that long-lasting enhanced expression of endogenous GAL at 12 h-2 days after ischemia may be associated with efflux of potassium ion into the extracellular space, and that GAL expression in microglia 4 days after ischemia may be associated with reduction of ischemic damage.

Ischemia-Related Changes in Galanin Expression in the Dentate Hilar Region after Transient Forebrain Ischemia in Gerbils

Although galanin (GAL) protects hippocampal neurons from ischemic damage, no study has examined ischemia-related changes in endogenous GAL in the hippocampal dentate gyrus. We investigated the chronological changes of GAL, well-known as the potassium channel opener, expression in the dentate gyrus at various times after 5 min of transient forebrain ischemia in gerbils. A few GAL-immunoreactive (IR) neurons were found in the polymorphic layer of the sham-operated group. Three hours after ischemia-reperfusion, the pattern of GAL immunoreactivity was similar to that of the sham-operated group and the number of GAL-IR neurons and immunoreactivity were highest 12 h after ischemic insult. At this time, GAL-IR neurons in the polymorphic layer showed strong GAL immunoreactivity. Thereafter, GAL-IR neurons and immunoreactivity significantly decreased in the dentate hilar region. Four days after ischemic insult, GAL-IR neurons were not detectable. In addition, the results of a Western blot study showed a pattern of GAL expression similar to the immunohistochemical changes. GAL protein content also was highest 12 h after ischemia. In conclusion, the increased expression of endogenous GAL in the dentate gyrus after ischemia is related to response to the ischemic damage.

Isch�mie und Hypoxie

In hypoxic or ischemic states, the receptors of the ganglion cells are overstimulated by release of neurotransmitters. Glutamate and GABA (gamma-aminobutyric acid) are the decisive neurotransmitters in the retina. It is presumed that the extent of cell death depends on the degree of depolarization, which in turn is determined by the amount of excitatory (glutamate) or inhibitory (GABA) receptors of the corresponding ganglion cell. The assumption is that the receptor profile of the individual ganglion cells determines the sensitivity of these cells to hypoxia or ischemia, i.e., the time up to cell death, and thus represents the underlying cause of the different rates of cell death in primary chronic open-angle glaucoma. Research on this receptor profile could be of pivotal importance for the approach to neuroprotective treatment of primary chronic open-angle glaucoma.

Age-related changes of γ-aminobutyric acid transaminase immunoreactivity in the hippocampus and dentate gyrus of the Mongolian gerbil

We investigated the age-related changes of gamma-aminobutyric acid transaminase (GABA-T, a GABA degradation enzyme) in the hippocampus and dentate gyrus of the gerbil at postnatal month 1 (PM 1), PM 3, PM 6, PM 12, and PM 24. Age-related changes of GABA-T immunoreactivity were distinct in the hippocampal CA1 region and in the dentate gyrus. GABA-T immunoreactivity was weak at PM 1, but at PM 3, it had increased significantly, and then increased further. Between PM 6 and PM 12, strong GABA-T immunoreactivity was found in nonpyramidal cells (GABAergic) in the stratum pyramidale of the CA1 region, and at PM 6, strong GABA-T immunoreactivity was found in neurons of the dentate gyrus subgranular zone. At PM 24, CA1 pyramidal cells showed strong GABA-T immunoreactivity. Western blot analysis showed a pattern of GABA-T expression similar to that shown by immunohistochemistry at various ages. In conclusion, our results suggest that the age-related changes of GABA-T provide important information about the aged brain with GABA dysfunction.

Antiepileptic drugs in neuroprotection

Antiepileptic drugs (AEDs) are designed to prevent and suppress seizure activity. Their effects on calcium influx and molecular cascades contributing to necrotic and apoptotic neuronal death, however, suggests that they have functions other than just suppression of excitability. The neuroprotective effects of 20 AEDs currently in use or being investigated in Phase II - III clinical trials for treatment of epilepsy are reviewed. Data analyses is complicated by several factors. Firstly, the available data on the neuroprotective effects of different AEDs varies largely. Secondly, most of the evidence demonstrating neuroprotective effects comes from stroke models and it is uncertain whether these data can be extrapolated to other conditions, such as status epilepticus (SE) or traumatic brain injury. Thirdly, data obtained in adult animals cannot be extrapolated to young animals without caution. For example, AEDs protecting adult brain from stroke or SE-induced injury can cause apoptosis in immature brain. Finally, data comparison is complicated by the variability in study designs and methodologies between studies. With these caveats in mind, an analysis of the available data suggests that AEDs with different mechanisms of action can have mild-to-moderate neuroprotective effects. It is difficult, however, to associate the neuroprotective effects with a favourable functional outcome. For example, it is difficult to conclude that administration of AEDs during the latency phase would have an effect on the molecular cascades underlying epileptogenesis. The few favourable data demonstrating a decrease in the incidence of epilepsy after SE are probably related to the administration of AEDs during SE, which resulted in modification/alleviation of the insult itself and consequently, reduced its epileptogenecity. These experimental data, however, are clinically important because they show that early intervention of SE has an effect on long-term functional outcome. These observations emphasise the need to use additional outcome measures, such as markers of normal development or cognitive performance, when the benefits of neuroprotection achieved by the use of neuroprotective AEDs are assessed.

Coactivation of GABA A and GABA B Receptor Results in Neuroprotection During In Vitro Ischemia

Background and Purpose— The possible neuroprotective effect of endogenous γ-aminobutyric acid (GABA) on the irreversible electrophysiological changes induced by in vitro ischemia on striatal neurons was investigated. In particular, the aim of the study was the characterization of the neuroprotective action of 2 antiepileptic drugs increasing GABAergic transmission such as tiagabine, a GABA transporter inhibitor, and vigabatrin, an irreversible inhibitor of GABA transaminase.

Methods— Extracellular field potential recordings were obtained from rat corticostriatal slice preparations. In vitro ischemia was delivered by switching to an artificial cerebrospinal fluid solution in which glucose was omitted and oxygen was replaced with N 2 .

Results— An irreversible loss of the field potentials recorded from striatal neurons was observed after 10 minutes of ischemia in control solution. Conversely, tiagabine and vigabatrin partially prevented the ischemia-induced field potential loss. Surprisingly, both GABA A and GABA B receptor antagonists blocked these effects. Accordingly, neuroprotection could be obtained only when GABA A and GABA B receptor agonists were coapplied, but not when a single agonist was given in isolation.

Conclusions— Antiepileptic drugs targeting GABAergic transmission can exert neuroprotective effects against ischemia by increasing endogenous GABA levels and via the activation of both GABA A and GABA B receptors.

Antiepileptic drugs as a possible neuroprotective strategy in brain ischemia

Several new antiepileptic drugs (AEDs) have been introduced for clinical use recently. These new AEDs, as did the classic AEDs, target multiple cellular sites both pre- and postsynaptically. The major common goal of the pharmacological treatment using AEDs is to counteract abnormal brain excitability by either decreasing excitatory transmission or enhancing neuronal inhibition. Interestingly, an excessive release of excitatory amino acids and a reduced neuronal inhibition also occur in brain ischemia. Thus, recently, the use of AEDs as a possible neuroprotective strategy in brain ischemia is receiving increasing attention, and many AEDs have been tested in animal models of stroke, providing encouraging results. Experimental studies utilizing global or focal ischemia in rodents have provided insights into the possible neuroprotective action of the various AEDs. However, the implication of these studies in the treatment of acute stroke in humans is not always direct. In fact, various clinical studies with drugs targeting the same voltage- and ligand-gated channels modulated by most of the AEDs failed to show neuroprotection. The differential mechanisms that underlie the development of focal ischemic injury in experimental animal models versus human stroke require further investigation to open a new therapeutic perspective for neuroprotection that might be applicable in the future.

Spatial and temporal alterations in the GABA shunt in the gerbil hippocampus following transient ischemia

In the present study, we have identified the alteration in the expressions of GABA shunt-associated enzymes and the GABA transporter in order to determine the relationship between the neuronal damage and GABA metabolism following ischemia. At 30 min post-ischemia, the immunoreactivities of the glutamic acid decarboxylase (GAD) isoforms were markedly elevated in the CA1 region, as compared with the sham operated group. At 3-12 h post-ischemia, their immunoreactivities recovered at the sham level. These patterns were similarly observed up to 12 h following ischemia insult. However, the intensity of GAD67 was markedly increased at 24 h post-ischemic insult. The temporal changes in GABA transporter 1 (GAT-1) expressions were similar to that of GAD67, but not GAD65, expression, at least prior to 12 h after ischemic insults. GAT-1 immunoreactivity was significantly elevated in the CA1 region posterior to 12 h post-ischemia. Both succinic semialdehyde dehydrogenase (SSADH) and succinic semialdehyde reductase (SSAR) immunoreactivities were not altered in GABAergic neurons following ischemia. In contrast, in pyramidal cells, both SSADH and SSAR immunoreactivities showed chronological alterations in the CA1 region. Thus, our findings suggest that the differential alterations of GABA metabolism may be one of the important factors in neuronal damages induced by ischemia.

Tolerance against ischemic neuronal injury can be induced by volatile anesthetics and is inducible NO synthase dependent

BACKGROUND AND PURPOSE

We tested whether volatile anesthetics induce neuroprotection that is maintained for a prolonged time.

METHODS

Rats were pretreated for 3 hours with 1 minimal anesthetic concentration of isoflurane or halothane in normal air (anesthetic preconditioning [AP]). The animals were subjected to permanent middle cerebral artery occlusion (MCAO) at 0, 12, 24, or 48 hours after AP. Halothane-pretreated animals were subjected to MCAO 24 hours after AP. Histological evaluation of infarct volumes was performed 4 days after MCAO. Cerebral glucose utilization was measured 24 hours after AP with isoflurane. Primary cortical neuronal cultures were exposed to 1.4% isoflurane for 3 hours. Oxygen-glucose deprivation (OGD) was performed 24 hours after AP. Injury was assessed 24 hours later by measuring the release of lactate dehydrogenase into the medium 24 hours after OGD.

RESULTS

Isoflurane anesthesia at 0, 12, and 24 hours before MCAO or halothane anesthesia 24 hours before MCAO significantly reduced infarct volumes (125+/-42 mm3, P=0.024; 118+/-51 mm3, P=0.008; 120+/-49 mm3, P=0.009; and 121+/-48 mm3, P=0.018, respectively) compared with control volumes (180+/-51 mm3). Three hours of isoflurane anesthesia in rats did not have any effect on local or mean cerebral glucose utilization measured 24 hours later. Western blot analysis from cortical extracts of AP-treated animals revealed an increase of the inducible NO synthase (iNOS) protein beginning 6 hours after AP. The iNOS inhibitor aminoguanidine (200 mg/kg IP) eliminated the infarct-sparing effect of AP. In cultured cortical neurons, isoflurane exposure 24 hours before OGD decreased the OGD-induced release of lactate dehydrogenase by 49% (P=0.002).

CONCLUSIONS

Pretreatment with volatile anesthetics induces prolonged neuroprotection in vitro and in vivo, a process in which iNOS seems to be critically involved.

Efficacy of current antiepileptics to prevent neurodegeneration in epilepsy models

Results of experiments performed in animal epilepsy models and human epilepsy during the past decade indicate that the epileptic brain is not a stable neuronal network, but undergoes modifications caused by the underlying etiology and/or recurrent seizures. In many forms of epilepsy, such as temporal lobe epilepsy, the underlying etiologic factor triggers a cascade of events (epileptogenesis) leading to spontaneous seizures and cognitive decline. In some patients, the condition progresses, due in part to recurrent seizures. The current treatment of epilepsy focuses exclusively on preventing or suppressing seizures, which are symptoms of the underlying disease. Now, however, we are beginning to understand the underlying neurobiology of the epileptic process, as well as factors that might predict the risk of progression in individual patients. Thus, there are new opportunities to develop neuroprotective and antiepileptogenic treatments for patients who, if untreated, would develop drug-refractory epilepsy associated with cognitive decline. These treatments might improve the long-term outcome and quality-of-life of patients with epilepsy. Here we review the available data regarding the neuroprotective effects of antiepileptic drugs (AEDs) at different phases of the epileptic process. Analysis of published data suggests that initial-insult modification and prevention of the progression of seizure-induced damage are candidate indications for treatment with AEDs. An understanding of the molecular mechanisms underlying the progression of epileptic process will eventually show what role AEDs have in the neuroprotective and antiepileptogenic treatment regimen.

Delayed postischemic hypothermia improves long-term behavioral outcome after cerebral hypoxia-ischemia in neonatal rats

Hypothermia may be an ideal neuroprotective intervention in hypoxic-ischemic encephalopathy after perinatal asphyxia. The present study describes the long-term effects of prolonged resuscitative whole-body hypothermia initiated 2 h after hypoxic-ischemic injury on brain morphology and neuropsychological behavior in 7-d-old rats. After right common carotid artery ligation and exposure to hypoxia of 8% O(2) for 105 min, 10 animals were kept normothermic at 37 degrees C and 10 animals were cooled to 30 degrees C rectal temperature for 26 h, starting 2 h after the hypoxic-ischemic insult. All hypoxic-ischemic animals were gavage fed to guarantee long-term survival. Neuroprotection was evaluated by magnetic resonance imaging and behavioral testing. Hypothermia significantly reduced the final size of cerebral infarction by 23% at 6 wk after the insult. The most extended tissue rescue was found in the hippocampus (21%, p = 0.031), followed by the striatum (13%, p = 0.143) and the cortex (11%, p = 0.160). Cooling salvaged spatial memory deficits verified at 5 wk of recovery with Morris Water Maze test; whereas circling abnormalities after apomorphine injection and sensory motor dysfunctions on rotating treadmill improved, yet did not reach statistical significance. When compared with controls, hypoxic-ischemic animals performed worse in all behavioral tests. Hypothermia did not influence functional outcome in controls. Significant correlations between behavioral performance and corresponding regional brain volumes were found. We conclude that 26 h of mild to moderate resuscitative hypothermia leads not only to brain tissue rescue, but most important to long-lasting behavioral improvement throughout brain maturation despite severity of injury and delayed onset of cooling.

Propofol versus isoflurane anesthesia under hypothermic conditions: effects on intracranial pressure and local cerebral blood flow after diffuse traumatic brain injury in the rat

BACKGROUND

The aim of this study was to compare the cerebral protective effects of two known protective anesthetics, isoflurane and propofol, when these were used in combination with moderate hypothermia (33-34 degrees C) after diffuse traumatic brain injury (TBI) in the rat. We assessed cerebral protection by measuring local cerebral blood flow (LCBF), mean arterial blood pressure (MABP), cerebral perfusion pressure (CPP) and intracranial pressure (ICP).

METHODS

Sixteen female Wistar rats weighing 275 to 350 g were anesthetized and subjected to an accelerated-impact weight-drop model of diffuse TBI. Hypothermia (33-34 degrees C) was induced 45 minutes after TBI (baseline), and was maintained for 180 minutes. The isoflurane group (n = 8) received 70% N(2)O in O(2), and isoflurane at 0.9 +/- 0.04%. The propofol group (n = 8) received 70% N(2)O in O(2) and a propofol infusion (12 mg/kg/hr). LCBF was measured by laser Doppler flowmeter. MABP, ICP, and brain and rectal temperatures were measured every 15 minutes from baseline through 180 minutes. Blood gas and hematocrit testing was also done at baseline and every 60 minutes thereafter to assess the animals' physiological state.

RESULTS

In the isoflurane group, MABP and CPP decreased significantly from baseline to 180 minutes (p < 0.05 and p < 0.01, respectively), and MABP was significantly lower than the pressure in the propofol group from 45 minutes through 180 minutes (p < 0.05, p < 0.01). ICP and LCBF remained unchanged in this group. In the propofol group, from baseline to 180 minutes, CPP increased to maximum 120 +/- 8 mmHg at 75 minutes from 98 +/- 5 mmHg (p < 0.05) and ICP fell from 18 +/- 2 mmHg to 7 +/- 1 mmHg (p < 0.01); and the latter was significantly lower than ICP in the isoflurane group (p < 0.05, p < 0.01, p < 0.001). LCBF in this group was significantly higher than LCBF in the isoflurane group in the last 30 minutes of the experiment (p < 0.05). The propofol group showed no change in MABP over the course of the experiment.

CONCLUSION

In the clinical setting, propofol anesthesia may be better for use in combination with hypothermia in cases of traumatic brain injury, as it reduces ICP and increases CPP under these conditions.

Chronic elevation of brain GABA levels beginning two days after status epilepticus does not prevent epileptogenesis in rats

Vigabatrin (VGB) treatment is neuroprotective in various models of status epilepticus (SE) and delays the development of kindling via mechanisms that are assumed to relate to the elevation of GABA levels in the brain. Here, we tested the hypothesis that a chronic elevation of brain GABA levels obtained by VGB treatment prevents the development of spontaneous seizures (i.e. epilepsy) following SE in rats. Self-sustained SE (SSSE) was induced by stimulating the lateral nucleus of the amygdala. Two days later, chronic VGB (75 mg/kg/day) or saline treatment was started via subcutaneous osmotic minipumps. The development of spontaneous seizures was monitored once a week (24 h at a time) using video-EEG recording. Rats were perfused for histology either at the end of the 10-week drug treatment, or later at the end of an 8-week drug-free follow-up period. Before perfusion for histology, spatial learning and memory perform was tested in the Morris water-maze. Spontaneous seizures were observed in 55% (6/11) of the saline-treated and 73% (8/11) of the VGB-treated rats during the 10-week treatment period. Seizure frequency, severity, and duration were similar in VGB-treated rats and controls during and after the drug-treatment period. VGB treatment did not decrease neuronal damage in various temporal lobe regions or mossy fiber sprouting. VGB treatment also did not attenuate spatial learning or memory impairments. These findings indicate that the augmentation of GABAergic neurotransmission by VGB does not prevent the development of epilepsy when treatment is started 2 days after SE.

Elevation of the gamma-aminobutyric acid transaminase expression in the gerbil CA1 area after ischemia-reperfusion damage

gamma-Aminobutyric acid-transaminase (GABA-T) plays an important role in the metabolism of GABA, particularly in the neurons or glial cells. The present study was undertaken to determine the alteration of GABA-T expression in the gerbil hippocampus after ischemia-reperfusion. In the sham, GABA-T(+) neurons were scattered in the hippocampus proper and dentate gyrus. The intensity of the GABA-T immunoreactivity had nearly disappeared in the interneurons at 12 h after ischemia. In contrast, 24 h post-ischemia the dramatic augmentation of GABA-T immunoreactivity in the pyramidal cells was observed in the CA1 area but not in the CA2 or CA3 areas. Forty-eight hours after ischemia-reperfusion, its immunoreactivity was preserved in the CA1 neurons. These results suggest that the over-expression of GABA-T in the CA1 area may be related to delayed neuronal death after ischemia-reperfusion insult.

GABA potentiation: a logical pharmacological approach for the treatment of acute ischaemic stroke

It has been shown that enhancing the function of the major inhibitory neurotransmitter GABA decreases glutamatergic activity in the brain. Since increased glutamatergic activity is the major primary event that results in cell death following an acute hypoxic-ischaemic stroke, GABAmimetic drugs might therefore be expected to be neuroprotective. This review examines the evidence that GABAergic function is acutely depressed following an ischaemic insult, and also reviews the data that suggest that increasing cerebral GABA concentration has a neuroprotective effect, as does the administration of some (but not all) GABAmimetic agents. The GABA uptake inhibitor CI-966, the GABA(A) agonist muscimol and the GABA(A)mimetic clomethiazole have all been shown to be neuroprotective in animal models of stroke when given after the ischaemic insult. In contrast, benzodiazepines and particularly barbiturates, although potent GABA(A) potentiators, have shown little promise as neuroprotectants. The diversity of GABA(A) receptor subtypes and the in vivo efficacy of certain GABA(A) receptor ligands in animal models of stroke suggests that GABAmimetic drugs are an undervalued approach to stroke therapy.

GABA(A)-mediated toxicity of hippocampal neurons in vitro

In the present study, we examined whether the elevation of GABA by gamma-vinyl-GABA protects cultured rat fetal hippocampal neurons against toxicity induced by a 20-min incubation with 100 microM L-glutamate. Neither a 24-h pretreatment nor posttreatment with gamma-vinyl-GABA (100 microM) had any neuroprotective effects, as determined by counting microtubule-associated protein-2 positive cells and lactate dehydrogenase assay 24 h after the glutamate treatment. Unexpectedly, gamma-vinyl-GABA alone induced a 20% loss of microtubule-associated protein-2-positive cells in a culture that was grown in medium containing 25 mM KCl. The toxic effect of gamma-vinyl-GABA was mimicked by a 24-h treatment with GABA (100 microM) and the GABA(A) receptor agonist, muscimol (10 microM), but not the GABA(B) receptor agonist, baclofen (10 microM). The GABA(A) receptor antagonist, bicuculline (10 microM), protected against gamma-vinyl-GABA and GABA-evoked toxicity. Neither gamma-vinyl-GABA nor GABA was toxic in culture medium containing 15 mM KCl. These data indicate that, under depolarizing conditions, an increased GABA level is toxic for a subpopulation of developing hippocampal neurons in vitro. The effect is GABA(A) receptor-mediated. These data provide a new view for understanding neurodegenerative processes, and raise a question of the safety of therapies aimed at increasing GABA concentration following brain insults, especially in immature brains.

Glial glutamate transporter GLT-1 down-regulation precedes delayed neuronal death in gerbil hippocampus following transient global cerebral ischemia

Glial (GLT-1 and GLAST) and neuronal (EAAC1) high-affinity transporters mediate the sodium dependent glutamate reuptake in mammalian brain. Their dysfunction leads to neuronal damage by allowing glutamate to remain in the synaptic cleft for a longer duration. The purpose of the present study is to understand their contribution to the ischemic delayed neuronal death seen in gerbil hippocampus following transient global cerebral ischemia. The protein levels of these three transporters were studied by immunoblotting as a function of reperfusion time (6 h to 7 days) following a 10 min occlusion of bilateral common carotid arteries in gerbils. In the vulnerable hippocampus, there was a significant decrease in the protein levels of GLT-1 (by 36-46%, P < 0.05; between 1 and 3 days of reperfusion) and EAAC1 (by 42-68%, P < 0.05; between 1 and 7 days of reperfusion). Histopathological evaluation showed no neuronal loss up to 2 days of reperfusion but an extensive neuronal loss (by approximately 84%, P < 0.01) at 7 days of reperfusion in the hippocampal CA1 region. The time frame of GLT-1 dysfunction (1-3 days of reperfusion) precedes the initiation of delayed neuronal death (2-3 days of reperfusion). This suggests GLT-1 dysfunction as a contributing factor for the hippocampal neuronal death following transient global cerebral ischemia. Furthermore, decreased EAAC1 levels may contribute to GABAergic dysfunction and excitatory/inhibitory imbalance following transient global ischemia.

Global ischemia and behavioural deficits

Global cerebral ischemia in rodents is an established model in experimental research on cerebral ischemia which is characterized morphologically by a selective neuronal damage in the hippocampus, striatum and cortex. Using this model many studies have been performed to examine the pathophysiology of ischemic neuronal damage. Based upon these results it has been analysed whether substances which interact with the pathophysiological processes reduce the ischemic neuronal damage. Besides the morphological changes global ischemia leads to functional changes which can be assessed by behavioural studies. The Morris water maze examines the animals' abilities to learn, remember and go to a place in space only defined by its position relative to distal extramaze cues. In this test ischemic animals display a deficit in spatial learning as revealed by an increase in latency and in swim distance in the escape trials and a deficit in spatial memory as shown by reduced quadrant time and crossings over the former platform position during the probe trial. In several studies it could be demonstrated that neuroprotective strategies which reduce ischemic neuronal damage also attenuate or even completely prevent the ischemia-induced behavioural deficits in the water maze. Transplantation of fetal tissue which can also be used to achieve morphological recovery following global ischemia results in an amelioration of the ischemia-induced deficit. Thus, the water maze can clearly show that transplanted tissue can be functionally relevant. Data from the water maze seem to be a valuable completion to morphology which is especially important with respect to the relevance of experimental studies for clinical trials.

Sensorimotor function and neuropathology five to six weeks after hypoxia-ischemia in seven-day-old rats

Various therapeutic interventions after hypoxia-ischemia (HI) have been shown to reduce brain injury in the short-term perspective, but it remains uncertain whether such findings are accompanied by long-term functional and structural improvements. HI was induced in 7-d-old rats as follows. The left carotid artery was ligated, and the rat was exposed to 100 min of hypoxia (7.70% oxygen in nitrogen). At postnatal d 42 the rats were assessed using four sensorimotor tests. The results were correlated with the extent of brain damage expressed as volume of deficit of the left hemisphere as percent of the right hemisphere. In the grip-traction test, the time to falling was 2.2 times shorter in the HI animals compared with controls (p < 0.01). Asymmetries of limb-placing and foot-faults (p < 0.001) were detected in HI animals, and the motor function was abnormal in the postural reflex test (p < 0.001). We found a moderate correspondence between functional and neuropathologic outcome (r = 0.842, p < 0.001). A set of four easily performed sensorimotor tests is presented for the long-term evaluation of neurologic function in the 7-d-old rat model of HI.

Low-dose vigabatrin (gamma-vinyl GABA)-induced damage in the immature rat brain

The antiepileptic drug, vigabatrin, inhibits GABA transaminase, thus elevating GABA levels in the brain. In adult animal experiments, high-dose (200 mg/kg/day) chronic vigabatrin administration is associated with potentially reversible myelin vacuolation, a phenomenon not documented in humans. We hypothesized that vigabatrin might adversely affect myelination in the developing brain. Rats were given vigabatrin in doses comparable to those used clinically (15-50 mg/kg/day), from age 12 to 16 days. The rats were killed at age 19-20 days. We observed decreased myelin staining in the external capsule, axonal degeneration in white matter, evidence of glial cell death in the white matter, and reactive astrogliosis in the frontal cortex. We did not detect myelin vacuolation. These findings indicate that vigabatrin can have adverse and potentially irreversible effects on the developing rat brain. The mechanism of damage could be direct toxicity of vigabatrin or an indirect effect mediated through elevated GABA levels. Vigabatrin has been recommended as a treatment for some forms of childhood epilepsy; therefore, further studies are needed to assess the risks in children.