Protective effect of synaptic inhibition during cerebral ischemia in rats and rabbits

The Activation of GABAAR Alleviated Cerebral Ischemic Injury via the Suppression of Oxidative Stress, Autophagy, and Apoptosis Pathways

Ischemic stroke is a devastating disease leading to neurologic impairment. Compounding the issue is the very limited array of available interventions. The activation of a γ-aminobutyric acid (GABA) type A receptor (GABAAR) has been reported to produce neuroprotective properties during cerebral ischemia, but its mechanism of action is not yet fully understood. Here, in a rat model of photochemically induced cerebral ischemia, we found that muscimol, a GABAAR agonist, modulated GABAergic signaling, ameliorated anxiety-like behaviors, and attenuated neuronal damage in rats suffering cerebral ischemia. Moreover, GABAAR activation improved brain antioxidant levels, reducing the accumulation of oxidative products, which was closely associated with the NO/NOS pathway. Notably, the inhibition of autophagy markedly relieved the neuronal insult caused by cerebral ischemia. We further established an oxygen-glucose deprivation (OGD)-induced PC12 cell injury model. Both in vivo and in vitro experiments demonstrated that GABAAR activation obviously suppressed autophagy by regulating the AMPK-mTOR pathway. Additionally, GABAAR activation inhibited apoptosis through inhibiting the Bax/Bcl-2 pathway. These data suggest that GABAAR activation exerts neuroprotective effects during cerebral ischemia through improving oxidative stress and inhibiting autophagy and apoptosis. Our findings indicate that GABAAR serves as a target for treating cerebral ischemia and highlight the GABAAR-mediated autophagy signaling pathway.

A retrospect and outlook on the neuroprotective effects of anesthetics in the era of endovascular therapy

Studies on the neuroprotective effects of anesthetics were carried out more than half a century ago. Subsequently, many cell and animal experiments attempted to verify the findings. However, in clinical trials, the neuroprotective effects of anesthetics were not observed. These contradictory results suggest a mismatch between basic research and clinical trials. The Stroke Therapy Academic Industry Roundtable X (STAIR) proposed that the emergence of endovascular thrombectomy (EVT) would provide a proper platform to verify the neuroprotective effects of anesthetics because the haemodynamics of patients undergoing EVT is very close to the ischaemia–reperfusion model in basic research. With the widespread use of EVT, it is necessary for us to re-examine the neuroprotective effects of anesthetics to guide the use of anesthetics during EVT because the choice of anesthesia is still based on team experience without definite guidelines. In this paper, we describe the research status of anesthesia in EVT and summarize the neuroprotective mechanisms of some anesthetics. Then, we focus on the contradictory results between clinical trials and basic research and discuss the causes. Finally, we provide an outlook on the neuroprotective effects of anesthetics in the era of endovascular therapy.

Symmetric and Asymmetric Synapses Driving Neurodegenerative Disorders

In 1959, E. G. Gray described two different types of synapses in the brain for the first time: symmetric and asymmetric. Later on, symmetric synapses were associated with inhibitory terminals, and asymmetric synapses to excitatory signaling. The balance between these two systems is critical to maintain a correct brain function. Likewise, the modulation of both types of synapses is also important to maintain a healthy equilibrium. Cerebral circuitry responds differently depending on the type of damage and the timeline of the injury. For example, promoting symmetric signaling following ischemic damage is beneficial only during the acute phase; afterwards, it further increases the initial damage. Synapses can be also altered by players not directly related to them; the chronic and long-term neurodegeneration mediated by tau proteins primarily targets asymmetric synapses by decreasing neuronal plasticity and functionality. Dopamine represents the main modulating system within the central nervous system. Indeed, the death of midbrain dopaminergic neurons impairs locomotion, underlying the devastating Parkinson’s disease. Herein, we will review studies on symmetric and asymmetric synapses plasticity after three different stressors: symmetric signaling under acute damage—ischemic stroke; asymmetric signaling under chronic and long-term neurodegeneration—Alzheimer’s disease; symmetric and asymmetric synapses without modulation—Parkinson’s disease.

Cerebroprotection for Acute Ischemic Stroke: Looking Ahead

We search for ischemic stroke treatment knowing we have failed-intensely and often-to translate mechanistic knowledge into treatments that alleviate our patients' functional impairments. Lessons can be derived from our shared failures that may point to new directions and new strategies. First, the principle criticisms of both preclinical and clinical assessments are summarized. Next, previous efforts to develop single-mechanism treatments are reviewed. Finally, new definitions, novel approaches, and different directions are presented. In previous development efforts, the basic science and preclinical assessment of candidate treatments often lacked rigor and sufficiency; the clinical trials may have lacked power, rigor, or rectitude; or most likely both preclinical and clinical investigations were flawed. Single-target agents directed against specific molecular mechanisms proved unsuccessful. The term neuroprotection should be replaced as it has become ambiguous: protection of the entire neurovascular unit may be called cerebral cytoprotection or cerebroprotection. Success in developing cerebroprotection-either as an adjunct to recanalization or as stand-alone treatment-will require new definitions that recognize the importance of differential vulnerability in the neurovascular unit. Recent focus on pleiotropic multi-target agents that act via multiple mechanisms of action to interrupt ischemia at multiple steps may be more fruitful. Examples of pleiotropic treatments include therapeutic hypothermia and 3K3A-APC (activated protein C). Alternatively, the single-target drug NA-1 triggers multiple downstream signaling events. Renewed commitment to scientific rigor is essential, and funding agencies and journals may enforce quality principles of rigor in preclinical science. Appropriate animal models should be selected that are suited to the purpose of the investigation. Before clinical trials, preclinical assessment could include subjects that are aged, of both sexes, and harbor comorbid conditions such as diabetes or hypertension. With these new definitions, novel approaches, and renewed attention to rigor, the prospect for successful cerebroprotective therapy should improve.

Restoration of early deficiency of axonal guidance signaling by guanxinning injection as a novel therapeutic option for acute ischemic stroke

Despite of its high morbidity and mortality, there is still a lack of effective treatment for ischemic stroke in part due to our incomplete understanding of molecular mechanisms of its pathogenesis. In this study, we demonstrate that SHH-PTCH1-GLI1-mediated axonal guidance signaling and its related neurogenesis, a central pathway for neuronal development, also plays a critical role in early stage of an acute stroke model. Specifically, in vivo, we evaluated the effect of GXNI on ischemic stroke mice via using the middle cerebral artery embolization model, and found that GXNI significantly alleviated cerebral ischemic reperfusion (I/R) injury by reducing the volume of cerebral infarction, neurological deficit score and cerebral edema, reversing the BBB permeability and histopathological changes. A combined approach of RNA-seq and network pharmacology analysis was used to reveal the underlying mechanisms of GXNI followed by RT-PCR, immunohistochemistry and western blotting validation. It was pointed out that axon guidance signaling pathway played the most prominent role in GXNI action with Shh, Ptch1, and Gli1 genes as the critical contributors in brain protection. In addition, GXNI markedly prevented primary cortical neuron cells from oxygen-glucose deprivation/reoxygenation damage in vitro, and promoted axon growth and synaptogenesis of damaged neurons, which further confirmed the results of in vivo experiments. Moreover, due to the inhibition of the SHH-PTCH1-GLI1 signaling pathway by cyclopropylamine, the effect of GXNI was significantly weakened. Hence, our study provides a novel option for the clinical treatment of acute ischemic stroke by GXNI via SHH-PTCH1-GLI1-mediated axonal guidance signaling, a neuronal development pathway previously considered for after-stroke recovery.

Mechanisms of action of acute and subacute sphenopalatine ganglion stimulation for ischemic stroke

BACKGROUND

Sphenopalatine ganglion stimulation (SPG-Stim) for ischemic stroke, starting 8-24 h after onset and continuing through five days in a pooled analysis of two recent, randomized, sham-controlled trials, improved outcome of acute ischemic stroke patients with confirmed cortical involvement. As a neuromodulatory therapy, SPG-Stim differs substantially from existing pharmacologic (lytic and antiplatelets) and device (endovascular thrombectomy) acute ischemic stroke treatments.

AIM

Focused review of SPG anatomy, physiology, and neurovascular and neurobiologic mechanisms of action mediating benefit of SPG-Stim in acute ischemic stroke.

SUMMARY OF REVIEW

Located posterior to the maxillary sinus, the SPG is the main source of parasympathetic innervation to the anterior circulation. Preclinical and human studies delineate four distinct mechanisms of action by which the SPG-Stim may confer benefit in acute ischemic stroke: (1) collateral vasodilation and enhanced cerebral blood flow, mediated by release of neurotransmitters with vasodilatory effects, nitric oxide, and acetylcholine, (2) stimulation frequency- and intensity-dependent stabilization of the blood-brain barrier, reducing edema (3) direct acute neuroprotection from activation of the central cholinergic system with resulting anti-inflammatory, anti-apoptotic, and anti-excitatory effects; and (4) neuroplasticity enhancement from enhanced central cholinergic and adrenergic neuromodulation of cortical networks and nitrous oxide release stimulating neurogenesis.

CONCLUSION

The benefit of SPG-Stim in acute ischemic stroke is likely conferred not only by potent collateral augmentation, but also blood-barrier stabilization, direct neuroprotection, and neuroplasticity enhancement. Further studies clarifying the relative contribution of these mechanisms and the stimulation protocols that maximize each may help optimize SPG-Stim as a therapy for acute ischemic stroke.

Increased GABAergic inhibitory function against ischemic long-term potentiation in the CA1 region of the hippocampus

Potentiation of N-methyl-D-aspartate receptor (NMDAR)-mediated excitatory synaptic plasticity around 1 h after brief exposure to anoxia/aglycemia is called ischemic long-term potentiation (iLTP), which is considered a pathological form of synaptic response during the early phase of ischemic stroke. It is known that GABAergic inhibitory transmission is also an important molecular process involved in synaptic plasticity and learning memory. However, whether GABAergic transmission is involved in iLTP and early-phase plasticity in ischemic stroke remains unknown. In this study, iLTP was found to be induced in the hippocampal Schaffer-collateral pathway by exposure to oxygen glucose deprivation (OGD). Western blot analysis was conducted to analyze excitatory synaptic receptors and inhibitory synaptic receptors following OGD. The β3 subunit of the GABA_A receptor (GABA_AR) was markedly reduced, whereas the GluN2B subunit of the NMDAR was increased in the hippocampal area in the OGD group. Using extracellular recording, we demonstrated that application of GABA_AR agonist midazolam could abolish the hippocampal iLTP. Moreover, midazolam had no significant effect on the increase in NMDAR subunit GluN2B, but ameliorated the reduction in the β3 subunit of GABA_AR after OGD. In summary, our results indicated that hippocampal GABA_AR reduction promoted synaptic potentiation after OGD. Activation of GABAergic inhibitory transmission function could inhibit iLTP; thus, modulation of GABAergic function is a protective treatment method in the acute phase of synaptic plasticity in ischemic stroke.

Review : Neuroprotection in Brain Ischemia: An Update (Part II

Ischemic brain injury produced by stroke or cardiac arrest is a major cause of human neurological disability. Steady advances in the neurosciences have elucidated the pathophysiological mechanisms of brain ischemia and have suggested many therapeutic approaches to achieve neuroprotection of the acutely ischemic brain that are directed at specific injury mechanisms. In the second portion of this two-part review, the following potential therapeutic approaches to acute ischemic injury are considered: 1) modulation of nonglutamatergic neurotransmission, including monoaminergic systems (dopamine, norepinephrine, serotonin), γ-aminobutyric acid, and adenosine; 2) mild-to-moderate therapeutic hypothermia; 3) calcium channel antagonism; 4) an tagonism of oxygen free radicals; 5) modulation of the nitric oxide system; 6) antagonism of cytoskeletal proteolysis; 7) growth factor administration; 8) therapy directed at cellular mediators of injury; and 9) the rationale for combination pharmacotherapy. The Neuroscientist 1:164-175, 1995

Enhanced phasic GABA inhibition during the repair phase of stroke: a novel therapeutic target

Ischaemic stroke is the leading cause of severe long-term disability yet lacks drug therapies that promote the repair phase of recovery. This repair phase of stroke occurs days to months after stroke onset and involves brain remapping and plasticity within the peri-infarct zone. Elucidating mechanisms that promote this plasticity is critical for the development of new therapeutics with a broad treatment window. Inhibiting tonic (extrasynaptic) GABA signalling during the repair phase was reported to enhance functional recovery in mice suggesting that GABA plays an important function in modulating brain repair. While tonic GABA appears to suppress brain repair after stroke, less is known about the role of phasic (synaptic) GABA during the repair phase. We observed an increase in postsynaptic phasic GABA signalling in mice within the peri-infarct cortex specific to layer 5; we found increased numbers of α1 receptor subunit-containing GABAergic synapses detected using array tomography, and an associated increased efficacy of spontaneous and miniature inhibitory postsynaptic currents in pyramidal neurons. Furthermore, we demonstrate that enhancing phasic GABA signalling using zolpidem, a Food and Drug Administration (FDA)-approved GABA-positive allosteric modulator, during the repair phase improved behavioural recovery. These data identify potentiation of phasic GABA signalling as a novel therapeutic strategy, indicate zolpidem’s potential to improve recovery, and underscore the necessity to distinguish the role of tonic and phasic GABA signalling in stroke recovery.

Combination of mild hypothermia with neuroprotectants has greater neuroprotective effects during oxygen-glucose deprivation and reoxygenation-mediated neuronal injury

Co-treatment of neuroprotective reagents may improve the therapeutic efficacy of hypothermia in protecting neurons during ischemic stroke. This study aimed to find promising drugs that enhance the neuroprotective effect of mild hypothermia (MH). 26 candidate drugs were selected based on different targets. Primary cultured cortical neurons were exposed to oxygen-glucose deprivation and reoxygenation (OGD/R) to induce neuronal damage, followed by either single treatment (a drug or MH) or a combination of a drug and MH. Results showed that, compared with single treatment, combination of MH with brain derived neurotrophic factor, glibenclamide, dizocilpine, human urinary kallidinogenase or neuroglobin displayed higher proportion of neuronal cell viability. The latter three drugs also caused less apoptosis rate in combined treatment. Furthermore, co-treatment of those three drugs and MH decreased the level of reactive oxygen species (ROS) and intracellular calcium accumulation, as well as stabilized mitochondrial membrane potential (MMP), indicating the combined neuroprotective effects are probably via inhibiting mitochondrial apoptosis pathway. Taken together, the study suggests that combined treatment with hypothermia and certain neuroprotective reagents provide a better protection against OGD/R-induced neuronal injury.

Role of inhibition in chronic cerebral hypoperfusion

Chronic reductions in cerebral blood flow (CBF) of between 25 and 50% maintained for 26 weeks impair neuronal function, through a mechanism which is not known, but which is now explored. Increased GABAergic synaptic inhibition may play a role, as inhibitory interneurons are known to be relatively resistant to acute ischaemic insults. The phenomenon of tetanus-induced longterm potentiation (LTP) was previously found to be impaired in this setting, and was thus examined in the in vitro rat hippocampus in the presence of bicuculline, a specific GABA(A) antagonist, to evaluate the role of inhibition in the impairment of LTP in chronic cerebral hypoperfusion (CCH). Nine Sprague-Dawley rats aged 8-10 weeks had arteriovenous fistulae (AVF) surgically constructed to reduce CBF to between 25 and 50%. Ten animals were used as age-matched controls. After a further 26 weeks, 400 mum hippocampal slices were prepared. Tetanic stimulation was used in order to attempt to induce LTP. In vitro extracellular field potentials from control and AVF slices with 5 x 10(-)6 M bicuculline exposure and subsequent tetanic stimulation were compared. There was no statistical difference between the responses of the two groups in either scenario (P > 0.05), although LTP was in general more difficult to induce (only occurring in 60% of control animals). Possible causes of this are discussed. It is concluded that increased GABAergic synaptic inhibition does not play a role in impairment of neuronal function seen after 26 weeks of non-infarctional CCH.

Vagus nerve stimulation reduces infarct size in rat focal cerebral ischemia: an unlikely role for cerebral blood flow

We sought to investigate the effect of cervical vagus nerve stimulation (VNS) on cerebral blood flow (CBF), infarct volume, and clinical outcome in a model of middle cerebral artery occlusion in rats. Electrical stimulation of the right and left vagus nerves was initiated 30min after the induction of the right-sided ischemia and lasted for 1h. Infarct size measurement revealed that the volume of ischemic damage was 41-45% smaller in animals receiving stimulation as compared with control animals. Both the right and left VNS caused subtle reduction in CBF during each 30-s stimulation period that quickly returned back to the baseline level at the end of each stimulation cycle. There was no significant effect of VNS on CBF during the entire 1-h stimulation period. The effect of VNS on tissue outcome was associated with better neurological outcome at both 1- and 3-day time points after the induction of ischemia. These findings suggest that VNS-induced protection against acute ischemic brain injury is not primarily mediated by changes in CBF, stimulation of both the right and left nerve have comparable effects, and VNS is effective after ipsilateral and contralateral focal ischemia.

Ischemic insult to cerebellar Purkinje cells causes diminished GABAA receptor function and allopregnanolone neuroprotection is associated with GABAA receptor stabilization

Cerebellar Purkinje cells (PC) are particularly vulnerable to ischemic injury and excitotoxicity, although the molecular basis of this sensitivity remains unclear. We tested the hypothesis that ischemia causes rapid down-regulation of GABA(A) receptors in cerebellar PC, thereby increasing susceptibility to excitotoxicity. Oxygen-glucose deprivation (OGD) caused a decline in functional GABA(A) receptors, within the first hour of re-oxygenation. Decreased amplitude of miniature inhibitory post-synaptic potentials confirmed that OGD caused a significant decrease in functional synaptic GABA(A) receptors and quantitative Western blot analysis demonstrated the loss of GABA(A) receptor current was associated with a decline in total receptor protein. Interestingly, the potent neuroprotectant allopregnanolone (ALLO) prevented the decline in GABA(A) receptor current and protein. Consistent with our in vitro data, global ischemia in mice caused a significant decline in total cerebellar GABA(A) receptor protein and PC specific immunoreactivity. Moreover, ALLO provided strong protection of PC and prevented ischemia-induced decline in GABA(A) receptor protein. Our findings indicate that ischemia causes a rapid and sustained loss of GABA(A) receptors in PC, whereas ALLO prevents the decline in GABA(A) receptors and protects against ischemia-induced damage. Thus, interventions which prevent ischemia-induced decline in GABA(A) receptors may represent a novel neuroprotective strategy.

Neuroprotection of GluR5-containing kainate receptor activation against ischemic brain injury through decreasing tyrosine phosphorylation of N-methyl-D-aspartate receptors mediated by Src kinase

Previous studies indicate that cerebral ischemia breaks the dynamic balance between excitatory and inhibitory inputs. The neural excitotoxicity induced by ionotropic glutamate receptors gain the upper hand during ischemia-reperfusion. In this paper, we investigate whether GluR5 (glutamate receptor 5)-containing kainate receptor activation could lead to a neuroprotective effect against ischemic brain injury and the related mechanism. The results showed that (RS)-2-amino-3-(3-hydroxy-5-tert-butylisoxazol-4-yl) propanoic acid (ATPA), a selective GluR5 agonist, could suppress Src tyrosine phosphorylation and interactions among N-methyl-D-aspartate (NMDA) receptor subunit 2A (NR2A), postsynaptic density protein 95 (PSD-95), and Src and then decrease NMDA receptor activation through attenuating tyrosine phosphorylation of NR2A and NR2B. More importantly, ATPA had a neuroprotective effect against ischemia-reperfusion-induced neuronal cell death in vivo. However, four separate drugs were found to abolish the effects of ATPA. These were selective GluR5 antagonist NS3763; GluR5 antisense oligodeoxynucleotides; CdCl(2), a broad spectrum blocker of voltage-gated calcium channels; and bicuculline, an antagonist of gamma-aminobutyric acid A (GABA(A)) receptor. GABA(A) receptor agonist muscimol could attenuate Src activation and interactions among NR2A, PSD-95 and Src, resulting the suppression of NMDA receptor tyrosine phosphorylation. Moreover, patch clamp recording proved that the activated GABA(A) receptor could inhibit NMDA receptor-mediated whole-cell currents. Taken together, the results suggest that during ischemia-reperfusion, activated GluR5 may facilitate Ca(2+)-dependent GABA release from interneurons. The released GABA can activate postsynaptic GABA(A) receptors, which then attenuates NMDA receptor tyrosine phosphorylation through inhibiting Src activation and disassembling the signaling module NR2A-PSD-95-Src. The final result of this process is that the pyramidal neurons are rescued from hyperexcitability.

Theanine prevents memory impairment induced by repeated cerebral ischemia in rats

The present study investigated the neuroprotective effect of gamma-glutamylethylamide (theanine), a component Japanese green tea (Camellia sinensis), on memory impairment induced by twice-repeated cerebral ischemia in rats. Theanine was injected i.p. immediately after the first occlusion. Theanine (0.3 and 1 mg/kg) significantly prevented the impairment of spatial memory in rats subjected to repeated cerebral ischemia, 7 days after the second reperfusion. Moreover, theanine (1 mg/kg) significantly inhibited the decrease in the number of surviving cells in the hippocampal CA1 field in the same rats. These results suggest that theanine prevents memory impairment induced by repeated cerebral ischemia, in part by protecting against neuronal cell death, and that it might be useful for preventing cerebrovascular disease.

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.

Failure of ischemic neuroprotection by potentiators of gamma-aminobutyric acid

INTRODUCTION

Potentiators of inhibitory neurotransmission may provide a neuroprotective effect on cerebral tissue exposed to ischemia, without inducing toxic side effects. Topiramate and vigabatrin enhance the action of gamma-aminobutyric acid (GABA), and each has side effect profiles known to be well tolerated through their clinical use as anticonvulsant medications. We assessed the potential benefit through GABA activation by these drugs on infarct size and functional recovery following focal cerebral ischemia in mice.

METHODS

Silicon-coated suture was advanced through the internal carotid artery of 89 halothane-anesthetized mice to temporarily occlude the right middle cerebral artery for either 45 minutes (topiramate), or 120 minutes (vigabatrin). Animals were treated either at the time of reperfusion with topiramate (100 mg/kg, 40 mg/kg, or saline control), or two hours before arterial occlusion with vigabatrin, (1000 mg/kg, 500 mg/kg, or saline control). Neurological outcome was measured 24 hours after ischemia using a 28-point functional examination score. Infarct volume was estimated by summing area maps of stained slices of infarcted hemispheres.

RESULTS

Functional examination scores at 24 hours were similar between the high dose topiramate group, the low dose topiramate group, and the control group. Similarly, no differences were noted between examination scores of high dose vigabatrin, low dose vigabatrin, and control. Consistent sized right hemisphere infarcts were noted within each group on histological examination. Mean infarct volumes did not differ between groups treated with high dose topiramate, low dose topiramate, or control. Infarct volumes of animals treated with saline control were slightly larger than that of high dose vigabatrin and low dose vigabatrin groups, but the difference did not reach significance.

CONCLUSION

Treatment with these two potentiators of GABA did not result in significant differences in outcome following focal cerebral ischemia, by either functional or histological measures. These results do not support a substantial neuroprotective role of GABA following ischemia in this mouse suture model.

Preconditioned resistance to oxygen-glucose deprivation-induced cortical neuronal death: alterations in vesicular GABA and glutamate release

Central neurons exposed to several types of sublethal stress, including ischemia, acquire resistance to injury induced by subsequent ischemic insults, a phenomenon called ischemic preconditioning. We modeled this phenomenon in vitro, utilizing exposure to 45 mM KCl to reduce the vulnerability of cultured murine cortical neurons to subsequent oxygen-glucose deprivation. Twenty-four hours after preconditioning, cultures exhibited enhanced depolarization-induced, tetanus toxin-sensitive GABA release and a modest decrease in glutamate release. Total cellular GABA levels were unaltered. Inhibition of GABA degradation with the GABA transaminase inhibitor (+/-)-gamma-vinyl GABA, or addition of low levels of GABA, muscimol, or chlormethiazole to the bathing medium, mimicked the neuroprotective effect of preconditioning against oxygen-glucose deprivation-induced death. However, neuronal death was enhanced by higher levels of these manipulations, as well as by prior selective destruction of GABAergic neurons by kainate. Finally, selective blockade of GABA(A) receptors during oxygen-glucose deprivation or removal of GABAergic neurons eliminated the neuroprotective effects of prior preconditioning. Taken together, these data predict that presynaptic alterations, specifically enhanced GABA release together with reduced glutamate release, may be important mediators of ischemic preconditioning, but suggest caution in regard to interventions aimed at increasing GABA(A) receptor activation.

Amphetamines and related drugs in motor recovery after stroke

Studies in laboratory animals indicate that the rate and extent of functional recovery after focal brain injury can be modulated by drugs affecting specific central neurotransmitters. Preliminary clinical studies suggest that similar drug effects may occur in humans recovering from stroke. Combined with principles derived from the laboratory, these clinical studies provide important insights to guide the rational design of trials aimed at determining the clinical use of this approach to improving poststroke recovery.

gamma-Aminobutyric acid(A) neurotransmission and cerebral ischemia

In this review, we present evidence for the role of gamma-aminobutyric acid (GABA) neurotransmission in cerebral ischemia-induced neuronal death. While glutamate neurotransmission has received widespread attention in this area of study, relatively few investigators have focused on the ischemia-induced alterations in inhibitory neurotransmission. We present a review of the effects of cerebral ischemia on pre and postsynaptic targets within the GABAergic synapse. Both in vitro and in vivo models of ischemia have been used to measure changes in GABA synthesis, release, reuptake, GABA(A) receptor expression and activity. Cellular events generated by ischemia that have been shown to alter GABA neurotransmission include changes in the Cl(-) gradient, reduction in ATP, increase in intracellular Ca(2+), generation of reactive oxygen species, and accumulation of arachidonic acid and eicosanoids. Neuroprotective strategies to increase GABA neurotransmission target both sides of the synapse as well, by preventing GABA reuptake and metabolism and increasing GABA(A) receptor activity with agonists and allosteric modulators. Some of these strategies are quite efficacious in animal models of cerebral ischemia, with sedation as the only unwanted side-effect. Based on promising animal data, clinical trials with GABAergic drugs are in progress for specific types of stroke. This review attempts to provide an understanding of the mechanisms by which GABA neurotransmission is sensitive to cerebral ischemia. Furthermore, we discuss how dysfunction of GABA neurotransmission may contribute to neuronal death and how neuronal death can be prevented by GABAergic drugs.

Dose-dependent neuroprotection with tiagabine in a focal cerebral ischemia model in rat

Enhancement of the inhibitory activity of GABA may protect ischemic neurons. We evaluated the neuroprotective effect of tiagabine, a novel GABA agonist, in reversible focal cerebral ischemia rats subjected to 2h middle cerebral artery (MCA) occlusion. Tiagabine was given at 10, 20 and 40 mg/kg, i.p., 2 h after the onset of reperfusion. We found that post-ischemia treatment with tiagabine improved neurobehavioral outcome and reduced brain infarction volume in a dose-dependent manner. The data suggest that post-ischemic administration of tiagabine is neuroprotective in the focal cerebral ischemia model.

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.

Synergistic combinatorial stroke therapy: A quantal bioassay of a GABA agonist and a glutamate antagonist

We sought to prolong the window for stroke treatment using synergistic combinatorial therapy. We used the intraluminal filament occlusion model in rats to cause focal cerebral ischemia and a quantal bioassay to measure efficacy. The GABA agonist muscimol and the glutamate antagonist MK-801 were used alone and in combination at various times after ischemia onset. At progressively longer treatment delay intervals (30, 60, 75, 120, 240, and 360 min), higher doses of the single drugs were required to achieve neuroprotection. In contrast, the combination 1.0 mg/kg muscimol plus 0.5 mg/kg MK-801 was effective at all delay intervals studied except the longest (P < 0.05 at each time). After 240 min from ischemia onset, the combination was more effective than either single agent (P < 0.05 for each drug dose), suggesting synergism. The neuroprotective effect could not be demonstrated using morphometry. The treatment effects were probably not due to hypothermia because brain temperatures recorded in awake, unregulated subjects remained normo- or slightly hyperthermic following all treatments. Awake subjects kept on a heating pad exhibited mild brain hyperthermia. The combination caused a drop and MK-801 caused a significant increase in mean arterial blood pressure (main effects F(5,172) = 29, P < 0.0001). The combination of a GABA agonist and glutamate antagonist appears to possess synergistic neuroprotective effects when treatment is delayed up to 240 min following the onset of cerebral ischemia. Temperature regulation causes hyperthermia in awake subjects. The quantal bioassay is one method suitable for studies of synergistic stroke therapy.

Diazepam-induced neuroprotection: dissociating the effects of hypothermia following global ischemia

Global cerebral ischemia produces hippocampal CA1 neuronal loss which in turn leads to deficits in memory related tasks. Previous studies have shown that the benzodiazepine diazepam is effective at attenuating this cell death and the related behavioural impairments. However these studies have been confounded by diazepam-induced hypothermia. In this study we sought to determine the neuroprotective efficacy of diazepam in the absence of hypothermia. Diazepam (10 mg/kg) was administered to two groups of gerbils at 30 and 90 min following a 5-min ischemic insult. In one group the brain temperature was monitored for 24 h post-ischemically but not regulated. In the second group, post-ischemic brain temperature was maintained at 36.5 degrees C to counteract the hypothermia produced by diazepam. Both behaviour (open field performance) and CA1 cell counts from these groups were compared to those from sham/normal, no drug ischemic and vehicle ischemic groups at 10 days survival. In animals treated with diazepam without temperature regulation, there was significant histological and behavioural protection at 10 days compared to untreated ischemic animals. Preventing hypothermia in diazepam-treated animals resulted in a decrease in the number of cells surviving (from 41.2 to 31.6% of sham) and abolished behavioural protection. Diazepam appears to have limited ability to attenuate neuronal loss and its neuroprotective efficacy is augmented by the concurrent hypothermic actions of the drug itself.

Attenuated neurotransmitter release and spreading depression-like depolarizations after focal ischemia in mutant mice with disrupted type I nitric oxide synthase gene

Nitric oxide (NO) plays a complex role in the pathophysiology of cerebral ischemia. In this study, mutant mice with disrupted type I (neuronal) NO synthase (nNOS) were compared with wild-type littermates after permanent focal ischemia. Cerebral blood flow in the central and peripheral zones of the ischemic distribution were measured with laser doppler flowmetry. Simultaneously, microdialysis electrodes were used to measure extracellular amino acid concentrations and DC potential in these same locations. Blood flow was reduced to <25 and 60% of baseline levels in the central and peripheral zones, respectively; there were no differences in nNOS mutants versus wild-type mice. Within the central ischemic zone, DC potentials rapidly shifted to -20 mV in all mice. In the ischemic periphery, spreading depression (SD)-like waves of depolarization were observed. SD-like events were significantly fewer in the nNOS mutant mice. Concurrent with these hemodynamic and electrophysiological perturbations, extracellular elevations in amino acids occurred after ischemia. There were no detectable differences between wild-type and mutant mice in the ischemic periphery. However, in the central zone of ischemia, elevations in glutamate and GABA were significantly lower in the nNOS mutants. Twenty-four hour infarct volumes in the nNOS mutant mice were significantly smaller than in their wild-type littermates. Overall, the number of SD-like depolarizations and the integrated efflux of glutamate were significantly correlated with infarct size. These results suggest that NO derived from the nNOS isoform contributes to tissue damage after focal ischemia by amplifying excitotoxic amino acid release in the core and deleterious waves of SD-like depolarizations in the periphery.

Induction of neurosteroid synthesis by NMDA receptors in isolated rat retina: a potential early event in excitotoxicity

Here we investigated the possible regulation of neurosteroidogenesis by N-methyl-D-aspartic acid (NMDA) receptor activation and addressed the hypothesis that neurosteroid synthesis may be involved in acute excitotoxicity. In the isolated retina, exposure to NMDA modified pregnenolone and pregnenolone sulphate formation. This effect was dose and time dependent, the synthesis being increased by relatively moderate NMDA doses (1-100 microM) within 30 min exposure and reduced to its control value by 60 min or by raising drug concentrations. NMDA-stimulated neurosteroid synthesis was blocked by (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclo-hepten-5,10-imine hydrogen maleate (MK-801) and 3(2-carboxypiperazine-4-yl)propyl-1-phosphonic acid (CPP), depended on extracellular calcium and reproduced by glutamate. Lactate dehydrogenase (LDH) release and morphological analysis revealed that retinal cell viability was not significantly affected after 30 min exposure to 50 microM NMDA, but severe cell damage occurred by 60 min. When the GABAA (gamma-aminobutyric acid) receptor agonist muscimol (1-1000 microM), known to activate retinal neurosteroidogenesis, was added together with NMDA, no additional increase in neurosteroid synthesis was observed, and NMDA-induced LDH release remained unchanged. However, exposure to a high concentration of muscimol alone (500 microM) provoked a similar degree of toxicity to NMDA. By contrast, bicuculline abolished the increase in neurosteroidogenesis and LDH release. Similarly, pretreatment with R (+)-p-aminoglutethimide (AMG), an inhibitor of cholesterol side-chain cleavage cytochrome P450, attenuated acute retinal cell damage. The inhibitory nature of AMG on NMDA-stimulated neurosteroidogenesis was confirmed in the observation that drug treatment reduced pregnenolone content and did not affect the bindings of [3H] MK-801 and [3H] muscimol. The results demonstrate that NMDA receptors regulate neurosteroidogenesis through a transneuronal mechanism, which implies GABAA receptor activation. The early NMDA-mediated stimulation of neurosteroid synthesis seems to play a critical role in acute excitotoxicity; consequently, its inhibition is likely to delay neuronal cell death.

ATP-independent membrane depolarization with ischemia in the oxygen-ventilated isolated rat lung

We hypothesize that lung ischemic injury is related to cessation of flow leading to endothelial cell membrane depolarization and activation of oxidant-generating systems. Cell membrane potential was assessed in isolated, oxygen ventilated, Krebs-Ringer bicarbonate buffer-dextran-perfused rat lungs by lung surface fluorescence after infusion of bis-oxonol or 5,5',6,6'-tetrachloro-1, 1',3,3'-tetraethylbenzimidazolyl-carbocyanine iodide (JC-1), voltage-sensitive dyes. Surface fluorometry showed increased bis-oxonol fluorescence (34.7 +/- 3.3% above baseline) and decreased JC-1 fluorescence (24.5 +/- 4.5% below baseline) with ischemia, compatible with membrane depolarization. Fluorescence change was initiated within 1-2 min of the onset of ischemia and was rapidly reversible with reperfusion. Fluorescence changes varied with perfusion flow rate; maximal increase occurred with the transition from 1.8 ml/min to zero flow. Elevation of static intravascular pressure resulted in only a minor increase of bis-oxonol fluorescence. In situ subpleural fluorescence microscopy showed that endothelial cells are the major site of the increased bis-oxonol fluorescence signal with ischemia. These results indicate that endothelial cell membrane depolarization represents an early event with lung ischemia. Since the adenosine triphosphate content of lung was unchanged with ischemia in the O2-ventilated lungs, we postulate that membrane depolarization results from elimination of shear stress, possibly via inactivation of flow-sensitive K+-channels.

Effects of hyperbaric oxygen on neurologic outcome for cerebral ischemia in rats

OBJECTIVE

To evaluate the effect of hyperbaric oxygen (HBO) therapy on neurologic outcome (assessed by the quantal bioassay approach) following acute focal cerebral ischemia in rats.

METHODS

Two separate experimental trials were conducted. Trial 1, a nonblinded experiment, used 38 rats. Trial 2, a blinded experiment, used 59 rats. Focal cerebral ischemia was induced with a surgically placed intraluminal occlusion of the left middle cerebral artery; subsequent removal allowed reperfusion. Arterial occlusion times were varied from 5 to 90 minutes in trial 1, and from 3 to 45 minutes in trial 2. The control groups were maintained at ambient pressure for the duration of each trial. The trial 1 treatment group received a single 30-minute HBO treatment at 2.0 atmospheres absolute (ATA) on the initial day of ischemia. The trial 2 treatment group received 30-minute HBO treatments at 2.0 ATA daily for 4 consecutive days. The animals underwent daily 5-point neurologic examinations. A computerized quantal bioassay was used to determine the ET50--the occlusion time required to cause a neurologic abnormality in half of the animals. The control and treatment ET50 values were compared in each trial using a 2-tailed t-test. An increased ET50 for the treatment vs the control group implied a beneficial effect of HBO; a decrease, the opposite. The study had a power of 80% to detect a difference of 11.4 minutes in the ET50 for a 2-sided alpha = 0.05.

RESULTS

For trial 1: the HBO ET50 was 18.1 +/- 21.9 minutes and the control ET50 was 22.8 +/- 25.0 minutes (p > 0.2). For trial 2: the HBO ET50 was 9.49 +/- 17.4 minutes and the control ET50 was 14.9 +/- 14.2 minutes (p < 0.2).

CONCLUSION

HBO therapy showed no apparent benefit in a rat model as a treatment modality for acute cerebral ischemia with reperfusion.

The attenuation of kainate-induced neurotoxicity by chlormethiazole and its enhancement by dizocilpine, muscimol, and adenosine receptor agonists

Systemically administered kainate (10 mg.kg-1) caused neuronal loss in both the hippocampus and the entorhinal regions of the rat brain. This resulted in a loss of 68.3 +/- 13.8 and 53.3 +/- 12.8% of pyramidal neurones in the hippocampal CA1 and CA3a regions, respectively. Chlormethiazole attenuated the loss of neurones in the hippocampal cell layers CA1 (cell loss 10 +/- 3.2%) and CA3a (cell loss 10 +/- 7.7%). The neuroprotective activity of chlormethiazole was apparent in the presence or absence of a low dose of clonazepam (200 micrograms.kg-1 i.p.). The kainate-induced damage could also be measured by the increase in binding of the peripheral benzodiazepine ligand ([3H]PK11195) in the hippocampus. In kainate-treated rats there was a 350-500% increase in binding indicative of reactive gliosis. Chlormethiazole prevented this elevation in a dose- and time-dependent manner, with an ED50 of 10.64 mg.kg-1 and an effective therapeutic window from 1 to 4 h posttreatment. Dizocilpine also attenuated damage significantly. The GABAA agonist muscimol was also able to attenuate the increase in [3H]PK11195 binding in a dose-dependent manner, with an ED50 of approximately 0.1 mg.kg-1. If muscimol, dizocilpine, or the adenosine A1 receptor agonist R-N6-phenylisopropyl-adenosine were administered together with chlormethiazole at their respective ED25 doses, a potentiation was apparent in the degree of neuroprotection. It is concluded that the combination of neuroprotective agents with different mechanisms of action can lead to a synergistic protection against excitotoxicity.

GABA and glutamate levels in the substantia nigra reticulata following repetitive cerebral ischemia in gerbils

Repetitive cerebral ischemia produces more severe damage than a similar single duration insult. We have previously shown that, in gerbils, damage in the substantia nigra reticulata (SNr) is seen with repetitive insults rather than a single insult. We have also shown that there is a progressive decrease in the extracellular GABA in the striatum in the days preceding such damage, speculating that a loss of GABA may be in part responsible for this damage. This study evaluates the GABA levels in the SNr in animals exposed to repetitive ischemic insults. Each animal received a total of three ischemic insults of 3-min duration at hourly intervals. In vivo microdialysis was carried out to analyze the GABA and glutamate dialysate levels on Days 1, 3, 5, 7, and 14 following the ischemic insult. In the control and treated (ischemic) animals, there was a significant increase in the GABA levels with the introduction of nipecotic acid on Days 1, 3, 5, and 14. However, on Day 7 there was a significant attenuation in the GABA response to nipecotic acid in the treated animals in comparison to the controls. The glutamate levels in the treated animals were similar to the control animals on Days 1, 3, 5, and 7. However, on Day 14 the glutamate levels were significantly lower than on previous days. Our experiments for the first time measure extracellular glutamate and GABA responses in the SNr in animals exposed to repetitive ischemic insults. Our experiments show that there is a significant decrease in the GABA concentrations at a time when ischemic damage is developing in this region. This confirms our hypothesis that a decrease in GABA may be one factor contributing to neuronal damage during the period following repetitive ischemic insults. Further, the rebound increase in GABA levels on Day 14 with a concomitant fall in glutamate levels would indicate that reparative processes are still active in the 2 weeks following the insult.

High dose baclofen is neuroprotective but also causes intracerebral hemorrhage: a quantal bioassay study using the intraluminal suture occlusion method

Agonists of the GABA-A receptor are neuroprotective after experimental stroke, but studies of GABA-B agonists have contradicted each other. To further investigate whether GABA-B agonists may be neuroprotective, we devised a quantal bioassay using the intraluminal occlusion method of inducing reversible cerebral ischemia. Subjects underwent middle cerebral artery occlusion for varying amounts of time, ranging from 5 to 90 min. Behavioral outcome was measured 48 h later with a quantal observational scale: score of abnormal given for any one of asymmetric forepaw flexion on tail lift, asymmetric grip, circling, reduced exploration, seizures, or death. To the grouped response data the logistic equation was used to find the ED50, the duration of occlusion that caused one-half of the subjects to be abnormal. To find the potency ratio for each drug, we divided the ED50 for treatment by that for vehicle. We administered baclofen, a GABA-B agonist, intraperitoneally 5 min after the onset ofischemia. Baclofen (20 mg/kg) was neuroprotective (potency ratio of 3.0, P < 0.05), but a lower dose (10 mg/kg) was not. However, both doses of baclofen caused significantly more intracerebral hemorrhages than control. In awake animals, both baclofen doses caused significant increases in mean arterial pressure, but no changes in other cardiorespiratory variables. The glutamate antagonist MK-801, the GABA-A agonist muscimol, and hypothermia were all protective using the bioassay (potency ratios ranging from 1.5 to 3.0). We conclude that although baclofen (20 mg/kg) may be neuroprotective, its utility is complicated by postischemic hypertension and cerebral hemorrhages.

GABAergic regulation of cerebral microvascular tone in the rat

The role of GABA in regulating cerebral microvessels was examined in the parenchyma of the hippocampus and the surface of the neocortex. Microvessels were monitored in in vitro slices using computer-assisted videomicroscopy, and synaptically evoked field responses were simultaneously recorded. gamma-Aminobutyric acid (GABA) and the GABAA receptor agonist, muscimol, elicited vasodilation in hippocampal microvessels, whereas the GABAB receptor agonist, baclofen, elicited constriction. The muscimol-induced dilation persisted in the presence of the nitric oxide synthase inhibitor, N-nitro-L-arginine, indicating that this response is not mediated by nitric oxide. Inhibition of neuronal discharge activity with tetrodotoxin did not alter this dilation, but it fully blocked the constrictor response to baclofen. These data suggest that GABAB-mediated, but not GABAA-mediated, responses are dependent on action potential generation. The GABAA receptor antagonists, bicuculline and picrotoxin, elicited constriction, suggesting a tonic dilatory influence by endogenous GABA. Bicuculline-induced constriction was not attenuated by tetrodotoxin. In contrast, these vessels were unresponsive to the GABAB receptor antagonist, 2-hydroxysaclofen. Hippocampal microvessels dilated in response to moderate hypoxia, and this response persisted in the presence of bicuculline, indicating that the hypoxia-induced dilation is not mediated by an action at GABAA receptors. In arterioles located on the surface of the neocortex (i.e., not embedded in the parenchyma of the brain), muscimol elicited vasodilation, whereas bicuculline was ineffective. These results suggest that although these vessels are responsive to GABA, the local concentration of endogenous GABA is insufficient to elicit a tonic effect at rest. These findings raise the possibility that GABA plays a role in local neurovascular signaling in the parenchyma of the brain.

Effect of ischemic cerebral volume changes on behavior

Ischemia causes long-term effects on brain volume and neurologic function but the relationship between the two is poorly characterized. We studied the relationships between brain volume and three measures of rodent behavior after cerebral ischemia was induced by injecting several thousand microspheres into the internal carotid arteries of rats. Forty eight hours later, each subject was rated using a global neurologic rating scale. Several weeks later, the subjects were tested for open field activity and visual spatial learning. Post-mortem we measured the volume of the cerebral hemispheres and estimated the volume densities of cortex, white matter, hippocampus, basal ganglia, thalamus, ventricle, and visible infarction. Ischemia caused significant impairment, as measured by the global rating scale; the probability of an abnormal rating was correlated with the number of microspheres trapped in the brains. Visual spatial learning was significantly impaired by ischemia, but this deficit was independent of the count of microspheres, whether the subject was abnormal at 48 h, and whether the left or right hemisphere was embolized. Cerebral hemisphere volume was reduced from 430 mm3 to 376 mm3 (P < 0.05). The cortex was reduced from 22 to 19% of cerebrum (P < 0.05) and the white matter compartment was reduced to similar degree. The lesion volume was 6% of cerebrum, comparable to that seen with other ischemia methods. The global outcome rating was significantly related to total cerebral volume, but not to volume changes in any single compartment. On the other hand, visual spatial learning was significantly influenced by volume changes in the cortex and white matter, but not by the topography of the visible infarctions. Open field activity was not altered by infarction. Our data suggests that the total volume of brain tissue lost to infarction may partially determine global neurological rating independently of the topography of the volume loss. Integrative functions such as learning may depend more on the integrity of specific compartments and less on the total volume of intact brain. The volume of visible cystic infarction was not related to long term behavioral outcome. These results should be confirmed using another method of inducing ischemia.

MK-801-induced neuronal damage in rats

The non-competitive N-methyl-D-aspartate antagonist MK-801 has been frequently used to attenuate neurotoxicity mediated by excessive release of glutamate. However, doses of MK-801, effective to prevent cell loss in some areas have been reported to induce pathological changes in retrosplenial cortex [32]. In the present study, we examined the extent of the MK-801-induced damage. Silver staining techniques were used to label damaged neurons, axon terminals and activated microglia. In addition to the retrosplenial cortex, we observed silver-impregnated neurons in the pyriform, and entorhinal cortices, in amygdala in tenia tecti, and in the temporal two thirds of the dentate gyrus. With the exception of the dentate gyrus, signs of early degeneration appeared in the first 4 days in all observed regions. Activated microglia have been found 1 and 3 weeks after the lesion in the same areas. The time course and dose dependence of the damage was also investigated. The distribution of labeled neurons resembled the pattern observed after certain epileptic states. Our data suggest that irreversible cell damage occurred in the affected regions. These findings confirm and extend previous suggestions that, besides its protective effect, MK-801 may lead to neuronal degeneration.

GABAA receptor activation attenuates excitotoxicity but exacerbates oxygen-glucose deprivation-induced neuronal injury in vitro

We examined the effects of GABA receptor stimulation on the neuronal death induced by exogenously added excitatory amino acids or combined oxygen-glucose deprivation in mouse cortical cell cultures. Death induced by exposure to NMDA, AMPA, or kainate was attenuated by addition of GABA or the GABAA receptor agonist, muscimol, but not by the GABAB receptor agonist, baclofen. The antiexcitotoxic effect of GABAA receptor agonists was blocked by bicuculline or picrotoxin. In contrast, GABA or muscimol, but not baclofen, markedly increased the neuronal death induced by oxygen-glucose deprivation. Muscimol potentiation of neuronal death was associated with increased glutamate efflux to the bathing medium, and increased cellular 45Ca2+ accumulation; it was blocked by MK-801, but not NBQX, suggesting mediation by NMDA receptors. Bicuculline only weakly attenuated muscimol potentiation of oxygen-glucose deprivation-induced neuronal death, probably because it itself increased this death. Present results raise a note of caution in the proposed use of GABAA receptor stimulation to limit ischemic brain damage in vivo.

Neurochemical mechanisms in brain injury and treatment: a review

This article reviews cellular energy transformation processes and neurochemical events that take place at the time of brain injury and shortly thereafter emphasizing hypoxia-ischemia, cerebrovascular accident, and traumatic brain injury. New interpretations of established concepts, such as diffuse axonal injury, are discussed; specific events, such as free radical production, excess production of excitatory amino acids, and disruption of calcium homeostasis, are reviewed. Neurochemically-based interventions are also presented: calcium channel blockers, excitatory amino acid antagonists, free radical scavengers, and hypothermia treatment. Concluding remarks focus on the role of clinical neuropsychologists in validation of treatment interventions.

Effect of the novel high-affinity glycine-site N-methyl-D-aspartate antagonist ACEA-1021 on 125I-MK-801 binding after subdural hematoma in the rat: an in vivo autoradiographic study

Acute subdural hematoma (SDH) complicates 20% of severe human head injuries and causes death or severe disability in 60% of these cases, due to brain swelling and high intracranial pressure. Although the mechanisms for these phenomena are unknown, previous studies have implicated excitatory amino acid-mediated mechanisms in both humans and animal models. The authors therefore performed in vivo autoradiography using 125I-MK-801, a high-affinity noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist, as a tracer to evaluate NMDA ion channel activation spatially and temporally as a factor causing cytotoxic swelling. Acute SDH was induced in 16 anesthetized rats using 0.4 ml autologous venous blood. Fifty microcuries of 125I-MK-801 was injected via an aortic arch cannula 30 minutes after onset of SDH. The effect of a new putatively neuroprotective drug, ACEA-1021, a glycine-specific binding site NMDA antagonist, on 125I-MK-801 binding was tested on five animals "Nonspecific" 125I-MK-801 binding in the rat brain was assessed by pretreatment with "cold" (nonradiolabeled) MK-801 in five more animals. Four hours later the animals were sacrificed and brain sections were apposed to radiation-detecting high-sensitivity photographic film with precalibrated plastic standards for 4 weeks. A striking and highly significant 1.7- to 4.8-fold increase in 125I-MK-801 binding was seen in the penumbra of viable tissue surrounding the ischemic zone beneath the acute SDH, when compared to contralateral hemisphere binding (p < 0.001). The MK-801 pretreatment markedly reduced 125I-MK-801 uptake in this penumbral zone (4.73 +/- 0.36 nCi/mg control vs. 2.85 +/- 0.08 nCi/mg cold MK-801; p < 0.0001), indicating that the increased binding in the penumbra of the lesion was due to NMDA ion channel activation. Pretreatment with ACEA-1021 reduced 125I-MK-801 uptake by 28% (3.41 +/- 0.26 nCi/mg vs. 4.73 +/- 0.36 nCi/mg; p < 0.05), indicating that this agent prevents opening of the NMDA ion channel and, thus, exposure of its receptor for MK-801 binding. These studies show intense foci of penumbral NMDA receptor-mediated ion channel activation after onset of SDH, which is markedly reduced by an NMDA antagonist. Such agents are thus likely to reduce cell swelling after SDH occurs.

Spermidine/spermine N1-acetyltransferase mRNA levels show marked and region-specific changes in the early phase after transient forebrain ischemia

Considerable evidence points to an involvement of natural polyamines (putrescine, spermidine and spermine) in trophic regulation of brain tissue. Spermidine/spermine N1-acetyltransferase is the key enzyme in the interconversion pathway which leads to the formation of spermidine and putrescine from spermine and spermidine, respectively. In the present paper we have studied using in situ hybridization histochemistry the levels of spermidine/spermine N1-acetyltransferase mRNA in the rat central nervous system after transient forebrain ischemia. In the first hours after the insult, a modest increase in spermidine/spermine N1-acetyltransferase mRNA levels was observed in ependymal cells and other non-neuronal cells of all telencephalic and diencephalic regions. In addition, major increases in spermidine/spermine N1-acetyltransferase mRNA levels were observed in regions selectively vulnerable to the ischemic insult, such as striatum, hippocampus and cerebral cortex, during the first day post-reperfusion. The time course and extent of labelling increase were subregion- and cell-specific. At the cellular level, the labelling appeared markedly increased in neurons (8-10 fold in ventromedial striatum and CA1 region) and, to a lesser extent, in non-neuronal cells. The increase in SSAT mRNA levels was not directly related to cell degeneration, as it was detected in both some vulnerable and some resistant cell populations. However, the peak increase of SSAT labelling was precocious in resistant neurons (such as those of ventromedial striatum and dentate gyrus granular layer) and delayed or very limited in vulnerable neurons (such as those of CA1 pyramidal layer and dorsolateral striatum). The increase in spermidine/spermine N1-acetyltransferase may contribute to the increase in putrescine and decrease in spermidine levels observed after ischemia and gives further support to the notion that polyamine metabolism in the early phase after lesion is oriented towards putrescine production. This phenomenon could be relevant in determining the prevalence of neurotrophic vs. neurotoxic effects of polyamines.

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

Enhancing inhibitory mechanisms has been shown to improve neuronal survival after transient focal or global ischemia. In most studies, histological evaluations have been confined to the CA1 region of the hippocampus up to 7 days after an ischemic insult. We have previously shown that continuous intra-ventricular infusion of gamma-vinyl GABA (GVG) results in significant protection after cerebral ischemia. This present study was designed to assess histological and behavioral function at 7 and 28 days after a single 5 min ischemic episode in gerbils. One set of animals received the medication 30 min before the insult and the other set at 1 h after the insult. Evaluation at 7 days showed significant protection in most regions of the brain in both the pre- and post-ischemic treated animals in comparison to the controls. Delayed evaluation at 28 days showed significant protection only in the pre-ischemic treated animals. Behavioral testing with Morris water maze showed no differences in either pre- or post-ischemic treated animals when compared to saline-treated ischemic controls. Our study clearly demonstrates the usefulness of delayed evaluation in the assessment of 'true' neuronal protection. Pre-ischemic treated animals showed persistent and true neuronal protection, in contrast to a temporary protection as seen at 7 days in the post-ischemic treated animals. The lack of behavioral improvement in the pre- and post-ischemic treated animals suggests that morphological protection alone cannot be considered as the sole criterion for successful outcome.

Neurobehavioral consequences of induced spreading depression following photothrombotic middle cerebral artery occlusion

In a model of experimental focal cerebral ischemia, we have recently reported a strong correlation between the magnitude of ischemic depolarizations in the peri-infarct borderzone and the extent of histological injury. In the present study, we assessed the neurobehavioral consequences of spontaneously occurring and induced ischemic depolarizations in rats following middle cerebral artery (MCA) occlusion, as well as the effects of induced spreading depression (SD) in intact animals. Halothane-anesthetized, artificially ventilated Sprague-Dawley rats underwent photothrombotic MCA occlusion coupled with ipsilateral common carotid artery (CCA) occlusion. The electroencephalogram and direct current (DC) potential were recorded in the parietal infarct borderzone-corresponding to the cortical forelimb area-for 3 h following MCA occlusion. Group 1 rats (n = 9) received MCA/CCA occlusion, and the spontaneously occurring negative DC shifts were recorded in the ischemic borderzone. In Group 2 animals (n = 9), the (non-ischemic) frontal pole of the ipsilateral hemisphere was electrically stimulated in order to double the frequency of peri-infarct DC shifts occurring over the initial 3 h postocclusion. Group 3 consisted of intact rats (n = 3) in which SD was repeatedly evoked in the frontal pole. Four animals served as sham-operated controls. A battery of sensorimotor behavioral tests, consisting of beam balance, postural reflex and elicited forelimb placing, was applied in a blinded fashion. Sham controls and animals of Groups 1 and 2 were tested 24 h after surgery, and Group 3 rats were tested 2, 6 and 24 h after generation of SDs. A cumulative neurobehavioral index, ranging from 0 to 144, was calculated by adding the individual test results. Brains were perfusion-fixed 24 h following surgery for calculation of volumes of infarction and scattered neuronal injury. Functional outcome at 24 h was significantly worse in Group 2 animals (spontaneous plus induced ischemic depolarizations) (neurobehavior index 43 +/- 19, mean +/- S.D.) compared to Group 1 rats, in which only spontaneous depolarizations occurred (neurobehavior index 24 +/- 19, P < 0.05). The cumulative neurobehavioral index of Group 1 and 2 animals correlated positively with the volume of total ischemic injury (r = 0.765, P < 0.001) and with the frequency of ischemic depolarizations (r = 0.474, P < 0.05). Correlations between severe forelimb placing deficits and severe degrees of histological injury (necrosis or ischemic cell change) in the corresponding primary sensorimotor cortical region FR1 were significant in these rats. Group 3 rats showed severe neurobehavioral deficits at 2 and 6 h following SD stimulation (index 57 +/- 1 and 39 +/- 1, respectively) but returned to normal at 24 h (4 +/- 0). The findings indicate that cortical spreading depression is accompanied by transient neurobehavioral deterioration and that SD in the ischemic hemisphere of animals subjected to MCA occlusion worsened functional outcome 24 h after surgery.

Protection of the brain after aneurysmal rupture

The majority of patients survive the first dangerous hours after an aneurysmal rupture. However, many subsequently succumb as a result of a variety of lethal complications. The most important of these develop as sequelae of the initial ischemia, rebleeding and the delayed onset of vasospasm. Some of these deleterious cascades can be aborted. Since the delayed complications such as vasospastic infarction can be accurately predicted, this is one of rare "strokes" that can have pharmacological pre-treatment. The natural history of rebleeding and vasospasm are described as well as their effects on blood flow, oxygen delivery and metabolism. Strategies to ameliorate acute and delayed ischemia and hypoxia are discussed. Finally, potential pharmacotherapies are detailed.

Etomidate reduces ischemia-induced glutamate release in the hippocampus in rats subjected to incomplete forebrain ischemia

Etomidate and thiopental reduce ischemic neuronal injury but the mechanism by which they do so is not clear. Ischemia-induced release of the excitatory neurotransmitters glutamate and glycine is thought to play a major role in the pathophysiology of ischemic injury. To determine how etomidate and thiopental modulate excitatory transmitter release, their effect on the release of glycine and glutamate during ischemia was evaluated by microdialysis in the hippocampus and cortex of rats. Three groups of Wistar-Kyoto rats (n = 5/group) were studied. In the etomidate and thiopental groups, electroencephalogram (EEG) burst-suppression was achieved and maintained by a continuous infusion of either etomidate (0.6 mg.kg-1.min-1) or thiopental (3 mg.kg-1.min-1) 40 min prior to ischemia. Halothane anesthetized (1 minimum alveolar anesthetic concentration [MAC]) rats served as controls. Ischemia was induced in all three groups by bilateral carotid artery occlusion with simultaneous hypotension to 35 mm Hg for 10 min. Pericranial temperature was controlled at 38 degrees C. Dialysate was collected before, during, and after ischemia. The levels of glutamate and glycine in the dialysate were measured by high-performance liquid chromatography. Within the hippocampus, both glutamate and glycine levels increased significantly in the thiopental and control groups. By contrast, in the etomidate group, glutamate and glycine levels did not increase during ischemia, and peak levels were significantly less than those in the thiopental group. Peak glutamate levels in the thiopental group were significantly larger than in the control group, whereas the peak glycine levels were not different among the groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Combination chemotherapy extends the therapeutic window to 60 minutes after stroke

We sought to extend the therapeutic window for acute stroke therapy using the combination of a glutamate antagonist and a GABA agonist, which in prior studies was effective if given 5 min after stroke. We used a quantal bioassay to measure neuroprotective potency after injection of several thousand microspheres into the cerebral circulation of rats. The GABA-A agonist muscimol, but not MK-801, was effective if given 30, 45, or 60 min after embolization (potency ratio compared with saline of 3.0, 2.3, 1.8, respectively). If muscimol was combined with MK-801 at lower doses of each drug, the combination was neuroprotective (potency ratio of 4.2). Agonists of GABA-A, but not GABA-B, receptors blocked the toxic vacuolization seen in the cingulate and retrosplenial cortex after MK-801 treatment. Combination chemotherapy appears to extend the time window for acute stroke therapy in rats to 1 h and to result in fewer side effects.

[Pharmacologic brain protection: specific agents]

Dysfunctional sodium influx is the first step in the ischaemic cascade. It has been recently demonstrated that reducing ionic flux through voltagegated Na channels shortens the NMDA receptor activity of cultured hippocampal slices in which oxidative phosphorylation and glycolysis have been blocked. The implication of this finding is that blocking initial events in the ischaemic cascade, events which do not directly cause neuronal damage, will reduce the damage done by downstream events. It also seems intuitively reasonable to suppose that truncating initial steps of the ischaemic cascade, as distinct from blocking glutamate receptors and scavening free radicals, will reduce the probability of interfering with endogenous mechanisms of repair. Clinically useful, substantive, prophylactic, pharmacological cerebral protection will come from drugs that work upstream. And for pharmacological protection that can only be initiated subsequent to an ischaemic event, the more we learn about endogenous repair, or genetic pharmacology, the closer we will come to maximizing the benefits and minimizing the costs of downstream intervention.

Effect of gamma-aminobutyric acid modulation on neuronal ischemia in rabbits

BACKGROUND AND PURPOSE

Antagonists of excitatory neurotransmitters are effective in limiting ischemic damage to the brain and spinal cord, but use in clinical stroke may be limited by side effects. Agonists of inhibitory neurotransmitters, such as gamma-aminobutyric acid (GABA), may provide similar neuroprotection with less severe side effects. This study examines the effect of an agonist and antagonist of the GABA-A receptor on neuronal ischemic damage.

METHODS

Either muscimol (a GABA-A agonist) or bicuculline (a GABA-A antagonist) was administered intravenously to groups of rabbits exposed to reversible spinal cord ischemia induced by temporary occlusion of the infrarenal aorta. The duration of occlusion for individual animals was varied, providing a range of ischemia for each experimental group. The group P50 represents the duration (in minutes) associated with 50% probability of resultant permanent paraplegia. Neuroprotection was demonstrated if a drug prolonged the P50 compared with the control group.

RESULTS

The P50 of the control group was 26.3 +/- 2.0 minutes. Treatment with intravenous muscimol at 5 mg/kg significantly prolonged the P50 (32.4 +/- 1.3; P = .01). Treatment with intravenous bicuculline at 0.1 mg/kg significantly shortened the P50 (20.6 +/- 1.5; P = .03). The physiological and apparent behavioral effects of the drugs at these doses did not appear substantial.

CONCLUSIONS

Pharmacological manipulation of the GABA-A receptor may offer another avenue of therapy for central nervous system ischemia, possibly with less severe associated physiological side effects than other effective drugs.

GABAA receptor stimulation enhances NMDA-induced Ca2+ influx in mouse cerebral cortical neurons in primary culture

The effect of GABAA receptor stimulation on N-methyl-D-aspartate(NMDA)-induced [45Ca2+]influx has been examined using primary cultured cerebral cortical neurons. NMDA induced a dose-dependent increase in [45Ca2+]influx, which was blocked by MK-801 in a dose-dependent manner. GABAA receptor agonists significantly enhanced the NMDA-induced [45Ca2+]influx, and this enhancement was dose-dependently inhibited by bicuculline, although picrotoxin and tert-butyl-bicyclo[2.2.2]phosphoro-thionate (TBPS) exhibited no alterations in this stimulatory action of GABAA receptor agonists. Blockers of L-type voltage-dependent calcium channels significantly reduced the NMDA-induced [45Ca2+]influx. The increased [45Ca2+]influx by both NMDA and GABAA receptor agonists was also reduced by verapamil and nifedipine. These results suggest that the enhancement of NMDA-induced [45Ca2+]influx by GABAA receptor stimulation in immature cerebral cortical neurons may be due to the increased opening of voltage-dependent calcium channel by synergestic actions between NMDA and GABAA receptors.