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

Iron mediated toxicity and programmed cell death: A review and a re-examination of existing paradigms

Iron is an essential micronutrient that is problematic for biological systems since it is toxic as it generates free radicals by interconverting between ferrous (Fe²⁺) and ferric (Fe³⁺) forms. Additionally, even though iron is abundant, it is largely insoluble so cells must treat biologically available iron as a valuable commodity. Thus elaborate mechanisms have evolved to absorb, re-cycle and store iron while minimizing toxicity. Focusing on rarely encountered situations, most of the existing literature suggests that iron toxicity is common. A more nuanced examination clearly demonstrates that existing regulatory processes are more than adequate to limit the toxicity of iron even in response to iron overload. Only under pathological or artificially harsh situations of exposure to excess iron does it become problematic. Here we review iron metabolism and its toxicity as well as the literature demonstrating that intracellular iron is not toxic but a stress responsive programmed cell death-inducing second messenger.

KCC2 expression changes in Diazepam-treated neonatal rats with hypoxia-ischaemia brain damage

Hypoxia-ischaemia brain damage (HIBD) is a major type of perinatal brain injury in newborns. In this study, we investigate the short- and long-term neuroprotective effects of Diazepam on neonatal rats with HIBD and the potential mechanisms underlying its protective effects. Seven-day-old Sprague-Dawley rats were subjected to left carotid artery ligation followed by a 2-h exposure to 8% oxygen and 92% nitrogen. Diazepam was administered immediately via intraperitoneal (i.p.) injection after inducing HIBD at a dose of 10 mg kg(-1)8h(-1) for three consecutive days. Three days after HIBD, rats were decapitated, and the extent of brain injury was evaluated using 2,3,5-triphenyltetrazolium chloride (TTC) staining. Additionally, the expression of Potassium-chloride cotransporter-2 (KCC2) was analysed using real-time PCR, Western blot analysis and immunohistochemistry. Three weeks after HIBD, rats were subjected to the Morris water maze (MWM) test and the locomotor activity test to determine the long-term therapeutic effects of Diazepam. We observed that the volume of infarction in the Diazepam group was significantly less (P<0.01) compared with the HIBD group. We also observed that the learning and memory abilities of the Diazepam rats improved significantly compared with the untreated rats (P<0.05) and that the decrease in KCC2 expression was prevented (P<0.01). Early treatment with Diazepam appears to attenuate HIBD and can efficiently improve the long-term learning and memory capabilities of the animal. A potential mechanism underlying these effects may involve preventing the decrease in KCC2 expression.

Diazepam blocks striatal lipid peroxidation and improves stereotyped activity in a rat model of acute stress

In this work, the effect of a single dose of diazepam was tested on different markers of oxidative damage in the striatum of rats in an acute model of immobilization (restraint) stress. In addition, the locomotor activity was measured at the end of the restraint period. Immobilization was induced to animals for 24 hr, and then, lipid peroxidation, superoxide dismutase activity and content, and mitochondrial function were all estimated in striatal tissue samples. Corticosterone levels were measured in serum. Diazepam was given to rats as a pre-treatment (1 mg/kg, i.p.) 20 min. before the initiation of stress. Our results indicate that acute stress produced enhanced striatal levels of lipid peroxidation (73% above the control), decreased superoxide dismutase activity (54% below the control), reduced levels of mitochondrial function (35% below the control) and increased corticosterone serum levels (86% above the control). Pre-treatment of stressed rats with diazepam decreased the striatal lipid peroxidation levels (68% below the stress group) and improved mitochondrial function (18% above the stress group), but only mild preservation of superoxide dismutase activity was detected (17% above the stress group). In regard to the motor assessment, only the stereotyped activity was increased in the stress group with respect to control (46% above the control), and this effect was prevented by diazepam administration (30% below the stress group). The preventive actions of diazepam in this acute model of stress suggest that drugs exhibiting anxiolytic and antioxidant properties might be useful for the design of therapies against early acute phases of physic stress.

Expression of tissue-type transglutaminase (tTG) and the effect of tTG inhibitor on the hippocampal CA1 region after transient ischemia in gerbils

Chronological changes of tissue-type transglutaminase (tTG) were observed in the hippocampal CA1 region after transient forebrain ischemia in gerbils. In the sham-operated group, tTG immunoreactivity was weakly detected in blood vessels which were immunostained with platelet endothelial cell adhesion molecule-1 (PECAM-1), and tTG immunoreactivity in blood vessels was highest 5 days after ischemia/reperfusion. In addition, tTG immunoreaction was expressed in microglia which were immunostained with Iba-1 at 4 days post-ischemia, and tTG immunoreactivity in the microglia was also highest at 5 days post-ischemia. In Western blot analysis, tTG protein levels in the CA1 region after ischemia/reperfusion began to increase 3 days after ischemia/reperfusion and peaked 5 days after ischemia/reperfusion. The expression of tTG in PECAM-1-immunoreactive blood vessels may be associated with integrin regulation or transendothelial migration of leukocytes in the ischemic CA1 region. In this study, we also observed the effect of cystamine, a tTG inhibitor, against ischemic damage. Administration of cystamine protected in certain degree neuronal damage from ischemic damage in the CA1 region. These results suggest that tTG may be associated with neuronal death in the hippocampal CA1 region induced by ischemia/reperfusion.

Diazepam attenuates the post-traumatic hyperactivity of excitatory synapses in rat hippocampal CA1 neurons

The effect of diazepam, a benzodiazepine derivative, on the post-traumatic hyperactivity of excitatory synaptic transmission was examined in rat hippocampal CA1 area. Optical recordings showed that the activity of hippocampal neurons was enhanced in rats treated with fluid percussion injury (FPI) as compared with that of sham-operated rats. The optical response was characterized by fast and slow components. FPI did not affect the fast component that reflects presynaptic action potentials, but enhanced the slow component that reflects excitatory synaptic responses. Intracellular recordings showed that the amplitude and duration of the excitatory postsynaptic potential (EPSP) were increased after FPI. However, FPI did not affect the resting membrane potential and action potentials of hippocampal neurons. Intraperitoneal (i.p.) administration of diazepam (30 and 90 min after FPI) attenuated the post-traumatic hyperactivity of the slow optical response. The slope of input-to-output relation of excitatory synapses was decreased by acute administration of diazepam to FPI rats, but not by delayed administration of diazepam (4 and 5 h after FPI). The fast optical responses were not affected by either FPI or i.p. administration of diazepam. These results suggest that administration of diazepam at early post-traumatic period prevents the FPI-induced delayed enhancement of excitatory synaptic transmission in rat hippocampal CA1 neurons.

Neuroprotection afforded by diazepam against oxygen/glucose deprivation-induced injury in rat cortical brain slices

The aim of the present investigation was to assess neuroprotection exerted by diazepam (0.1-25 microM) in rat cortical brain slices subjected to oxygen-glucose deprivation and reoxygenation. Neuronal injury and neuroprotection were assessed by measuring the release of glutamate and lactate dehydrogenase and tissue water content. Results demonstrate that diazepam exerted neuroprotective effects according to a "U-shaped", hormetic-like, concentration-response curve, with an efficacy window of 0.5-5 microM concentration. Flumazenil (20 microM) fully antagonised neuroprotection afforded by 5 microM diazepam. In conclusion, the hormetic response of diazepam should be taken into consideration when designing experiments aimed at assessing diazepam neuroprotection against ischemia/reoxygenation injury.

Anesthetic-mediated protection/preconditioning during cerebral ischemia

Cerebral ischemia is a multi-faceted neurodegenerative pathology that causes cellular injury to neurons within the central nervous system. In light of the underlying mechanisms being elucidated, clinical trials to find possible neuroprotectants to date have failed, thus highlighting the need for new putative targets to offer protection. Recent evidence has clearly shown that anesthetics can confer significant protection and or induce a preconditioning effect against cerebral ischemia-induced injury. This review will focus on the putative protection/preconditioning that is afforded by anesthetics, their possible interaction with GABA(A) and glutamate receptors and two-pore potassium channels. In addition, the interaction with inflammatory, apoptotic and underlying molecular (particularly immediately early genes and inducible nitric oxide synthase etc) pathways, the activation of K(ATP) channels and the ability to provide lasting protection will also be addressed.

A simple method to induce focal brain hypothermia in rats

Hypothermia reduces cell death and promotes recovery in models of cerebral ischemia, intracerebral hemorrhage and trauma. Clinical studies report significant benefit for treating cardiac arrest and studies are investigating hypothermia for stroke and related conditions. Both local (head) and generalized hypothermia have been used. However, selective brain cooling has fewer side effects than systemic cooling. In this study, we developed a method to induce local (hemispheric) brain hypothermia in rats. The method involves using a small metal coil implanted between the Temporalis muscle and adjacent skull. This coil is then cooled by flushing it with cold water. In our first experiment, we tested whether this method induces focal brain hypothermia in anesthetized rats. Brain temperature was assessed in the ipsilateral cortex and striatum, and contralateral striatum, while body temperature was kept normothermic. Focal, ipsilateral cooling was successfully produced, while the other locations remained normothermic. In the second experiment, we implanted the coil, and brain and body temperature telemetry probes. The coil was connected via overhead swivel to a cold-water source. Brain hypothermia was produced for 24 h, while body temperature remained normothermic. A third experiment measured brain and body temperature along with heart rate and blood pressure. Brain cooling was produced for 24 h without significant alterations in pressure, heart rate or body temperature. In summary, our simple method allows for focal brain hypothermia to be safely induced in anesthetized or conscious rats, and is, therefore, ideally suited to stroke and trauma studies.

Functional imaging of dolphin brain metabolism and blood flow

This report documents the first use of magnetic resonance images (MRIs) of living dolphins to register functional brain scans, allowing for the exploration of potential mechanisms of unihemispheric sleep. Diazepam has been shown to induce unihemispheric slow waves (USW), therefore we used functional imaging of dolphins with and without diazepam to observe hemispheric differences in brain metabolism and blood flow. MRIs were used to register functional brain scans with single photon emission computed tomography (SPECT) and positron emission tomography (PET) in trained dolphins. Scans using SPECT revealed unihemispheric blood flow reduction following diazepam doses greater than 0.55 mg kg(-1) for these 180-200 kg animals. Scans using PET revealed hemispheric differences in brain glucose consumption when scans with and without diazepam were compared. The findings suggest that unihemispheric reduction in blood flow and glucose metabolism in the hemisphere showing USW are important features of unihemispheric sleep. Functional scans may also help to elucidate the degree of hemispheric laterality of sensory and motor systems as well as in neurotransmitter or molecular mechanisms of unihemispheric sleep in delphinoid cetaceans. The findings also demonstrate the potential value of functional scans to explore other aspects of dolphin brain physiology as well as pathology.

Antioxidant activity and neuroprotective effects of zolpidem and several synthesis intermediates

Structural relationship between the antioxidant melatonin and the non-benzodiazepine hypnotic zolpidem (ZPD) suggests possible direct antioxidant and neuroprotective properties of this compound. In the present work, these effects were analyzed for zolpidem and four of its synthesis intermediates. In vitro assays include lipid peroxidation and protein oxidation studies in liver and brain homogenates. Intracellular antioxidant effects were analyzed by evaluation of free radical formation prevention in HT-22 hippocampal cells treated with glutamate 10mM and measured by flow cytometer DCF fluorescence. The neuroprotective effect of these compounds was evaluated as neuronal death prevention of HT-22 cells treated with the same concentration of glutamate. Zolpidem was found to prevent induced lipid peroxidation in rat liver and brain homogenates showing figures similar to melatonin, although it failed to prevent protein oxidation. ZPD-I was the most effective out of the several zolpidem intermediates studied as it prevented lipid peroxidation with an efficiency higher than melatonin or zolpidem and with an effectiveness similar to estradiol and trolox. ZPD-I prevents protein oxidation, which trolox is known to be unable to prevent. When cellular experiments were undertaken, ZPD-I prevented totally the increase of intracellular free radicals induced by glutamate 10mM in culture medium for 12h, while zolpidem and ZPD-III partially prevented this increase. Also the three compounds protected hippocampal neurons from glutamate-induced death in the same conditions, being their comparative efficacy, ZPD-III > ZPD-I = ZPD.

Dual effect of diazepam on cGMP levels in rat brain slices

The effect of diazepam on NO-mediated cGMP synthesis was studied in rat brain slices. It was found that diazepam dose-dependently decreased cGMP synthesis in cerebellar slices, with an inhibition of 90% at 1 mM diazepam. cGMP levels in the presence of diazepam were not restored to control levels by the addition of 0.1 mM sodium nitroprusside, whereas the decrease in cerebellar cGMP levels induced by 0.1 mM L-NAME was restored by the simultaneous application of NO-donors. In addition to the decrease of cGMP levels in neuronal structures induced by 1 mM diazepam, we observed increased cGMP immunoreactivity in glial cells in the cerebellum, the hippocampus, and the cerebral cortex. The significance of this observation is discussed.

Post-ischemic diazepam does not reduce hippocampal CA1 injury and does not improve hypothermic neuroprotection after forebrain ischemia in gerbils

The hippocampal CA1 sector is especially vulnerable to brief forebrain ischemia. Excitotoxicity is widely thought to contribute to this cell death. Accordingly, drugs that presumably counteract excitotoxicity, such as GABAergic agonists, have been repeatedly tested and found to reduce CA1 cell loss. Post-ischemic diazepam reduces CA1 injury. However, diazepam also causes hypothermia, which by itself is neuroprotective. Most studies fail to adequately control for this confound. In this study, we tested whether diazepam reduces injury in temperature controlled gerbils subjected to brief forebrain ischemia. Furthermore, we tested whether diazepam augments hypothermic neuroprotection. All gerbils were implanted with a core temperature telemetry probe and a cannula for the subsequent insertion of a thermocouple probe to measure ischemic brain temperature. Subsequently, they were given a 5-min normothermic ischemic insult. In Experiment 1, two groups of gerbils were given 10 mg/kg doses of diazepam (i.p.) at both 30 and 90 min post-ischemia. Temperature was maintained in one group by heating lamps. Another group was administered saline. Diazepam reduced cell death at 7 days post-ischemia when the drug-induced hypothermia was permitted, but not when it was prevented. In Experiment 2, four groups of ischemic gerbils were treated starting at 12 h post-ischemia with prolonged hypothermia, diazepam and the combination or saline treatment. Hypothermia, but not diazepam, provided partial neuroprotection and diazepam did not augment hypothermic neuroprotection. Thus, neuroprotection with diazepam is solely due to hypothermia. These data do not support the clinical use of diazepam as a neuroprotectant after global ischemia.

Intravenous administration of magnesium is only neuroprotective following transient global ischemia when present with post-ischemic mild hypothermia

We hypothesized that post-ischemic hypothermia plays an important role in magnesium mediated neuroprotection following global cerebral ischemia. To test this hypothesis, we subjected rats to 8 min of global cerebral ischemia and magnesium treatment with and without post-ischemic body temperature maintenance. In Group 1, rats received an intravenously administered loading dose (LD) of 360 micromol/kg MgSO4 immediately before ischemia followed by a 48-h intravenous infusion (IVI) at either 60, 120 or 240 micromol/kg/h. Animal body temperature was kept at 37+/-0.2 degrees C during ischemia and between 36.6 and 37.8 degrees C for 6 h after ischemia. In Group 2, rats received a 360 micromol/kg MgSO4 LD followed by a 48-h IVI of either 120 or 240 micromol/kg/h MgSO4. In this group, body temperature following ischemia was monitored but not regulated. Control animals in Groups 1 and 2 received normal saline. Seven days after ischemia, hippocampal CA1 neurons were histologically examined. All Group 1 MgSO4-treated and control animals demonstrated less than 6% hippocampal CA1 neuronal survival. In Group 2, the rectal temperature of MgSO4-treated and control animals spontaneously dropped as low as 35.4 degrees C during the 6-h post-ischemia monitoring period. In addition, Group 2 animals that received the LD followed by an IVI of 120 or 240 micromol/kg/h MgSO4 demonstrated 34% (p<0.05) and 20% (p=0.936) CA1 neuronal survival, respectively. The CA1 neuronal survival in saline-treated control animals in both groups was less than 6%. Our data demonstrate only the combination of MgSO4 treatment and post-ischemic mild hypothermia is neuroprotective following global ischemia.

Magnesium sulfate fails to reduce infarct volume following transient focal cerebral ischemia in rats

Studies on the neuroprotective effect of magnesium treatment in animal models of focal and global cerebral ischemia have produced inconsistent results. Nevertheless, two magnesium acute stroke phase III trials (IMAGES and FAST-MAG) have either been completed or are planned. Therefore, we decided to re-evaluate the efficacy of magnesium following focal cerebral ischaemia in rats. Two experiments were carried out in two independent laboratories based in Australia. Both used the intraluminal thread method to induce focal cerebral ischemia in the rat. In the Perth study the middle cerebral artery (MCA) was occluded for 45 min and body temperature was controlled during and after ischemia. In the Canberra laboratory the MCA was occluded for 2 h and body temperature was only controlled during surgery. Three different doses (180, 360, or 720 micromol/kg) of MgSO4 in the Perth study and two different MgSO4 doses (370 or 740 micromol/kg) in the Canberra study were intravenously or intra-arterially administered immediately before ischemia. Control animals were given an equal volume of normal saline just before ischemia in both studies. Twenty-four or 72 h post-ischemia, infarct volume was determined following 2',3',5'-triphenyl-2H-tetrazolium chloride (TTC) staining. No significant differences (P > 0.05) in total, cortical and striatal infarct volumes between saline and MgSO4 treated animals were observed in either study. We conclude MgSO4 does not reduce infarct volume when administered before focal cerebral ischemia in rats.

Long-term effects of clomethiazole in a model of global ischemia

The failure of neuroprotective drugs in clinical trials has raised questions about the predictive value of animal models. To address this issue we reexamined the efficacy of clomethiazole using functional and histological outcome measures in combination with long-term survival times. Gerbils were exposed to 5 min of global ischemia and received 400 mg/ml clomethiazole (via osmotic minipump) plus a bolus injection (60 mg/kg) 30 min after ischemia. Brain temperature was maintained at approximately 36.5 degrees C during ischemia and for the first 30 min after ischemia, and was monitored in all groups for 24 h. Subgroups of clomethiazole-treated gerbils had their temperatures regulated in the normothermic range while in other animals temperature was not controlled. Open-field habituation tests were conducted 5, 10, 30, and 60 days after occlusion. CA1 cell counts and CA1 slice recordings were done at the conclusion of behavioral testing. Clomethiazole significantly attenuated CA1 cell loss at 10-, 30-, and 60-day survival. A modest reduction in habituation deficits was evident only on Day 10 (P < 0.05). Similarly, field potential amplitude was not maintained in the rostral CA1 region. Clomethiazole produced mild hypothermia that developed over several hours. Based on short-term CA1 cell counts, clomethiazole provided significant histological protection with limited functional preservation. Neuroprotection disappeared when longer survival times (60 day) were employed and temperature confounds eliminated. These data demonstrate the necessity of utilizing more clinically relevant survival times and carefully monitoring/regulating postischemic temperature when assessing potential neuroprotective compounds.

Diazepam and chlormethiazole attenuate the development of hyperthermia in an animal model of the serotonin syndrome

The serotonin (5-HT) syndrome is the most serious toxic interaction of antidepressants, but no pharmacotherapy has yet been established. In the present study, we created an animal model of the 5-HT syndrome by intraperitoneally injecting rats with clorgyline (2 mg/kg) and 5-hydroxy-L-tryptophan (5-HTP) (100 mg/kg) and evaluated the effectiveness of potent 5-HT(2A) receptor antagonists and GABA-enhancing drugs, including diazepam and chlormethiazole. The rectal temperature of the rats was measured, and the noradrenaline (NA) and 5-HT levels in the anterior hypothalamus were measured by microdialysis. In the group pre-treated with saline, the rectal temperature increased to more than 40 degrees C, and all of the animals died within 90 min after administration. Pre-treatment with potent 5-HT(2A) receptor antagonists prevented the development of hyperthermia and death in the rats. Pre-treatment with diazepam, 10 and 20mg/kg, and chlormethiazole, 50 and 100mg/kg, attenuated the development of hyperthermia. Although neither of these drugs completely prevented the rats from dying, they prolonged their survival time. Regardless of the type of therapeutic agents, the concentration of 5-HT increased to about 1100-fold the pre-administration level. The NA levels in the saline group increased to about 16-fold the pre-administration levels, but the increase was significantly lower in the rats that survived as a result of drug therapy. These results suggest that GABA-mimetic drugs may be effective against the 5-HT syndrome, although they have a somewhat weaker effect than the potent 5-HT(2A) receptor blockers, and that not only is 5-HT activity increased in the brain in the 5-HT syndrome, but the NA system is also enhanced.

Tacrolimus (FK506) reduces ischemia-induced hippocampal damage in rats: a 7- and 30-day study

The neuroprotective effect of the immunosuppressant agent FK506 was evaluated in rats after brain ischemia induced for 15 min in the 4-vessel occlusion model. In the first experimental series, single doses of 1.0, 3.0 or 6.0 mg FK506/kg were given intravenously (iv) immediately after ischemia. In the second series, FK506 (1.0 mg/kg) was given iv at the beginning of reperfusion, followed by doses applied intraperitoneally (ip) 6, 24, 48, and 72 h post-ischemia. The same protocol was used in the third series except that all 5 doses were given iv. Damage to the hippocampal field CA1 was assessed 7 or 30 days post-ischemia on three different stereotaxic planes along the septotemporal axis of the hippocampus. Ischemia caused marked neurodegeneration on all planes (P<0.001). FK506 failed to provide neuroprotection to CA1 both when applied iv as a single dose of 1.0, 3.0 or 6.0 mg/kg (experiment 1), and after five iv injections of 1.0 mg/kg (experiment 3). In contrast, the repeated administration of FK506 combining iv plus ip administration reduced CA1 cell death on all stereotaxic planes both 7 and 30 days post-ischemia (experiment 2; P<=0.01). Compared to vehicle alone, FK506 reduced rectal temperature in a dose-dependent manner (P<=0.05); however, this effect did not alter normothermia (37 C). FK506 reduced ischemic brain damage, an effect sustained over time and apparently dependent on repeated doses and on delivery route. The present data extend previous findings on the rat 4-vessel occlusion model, further supporting the possible use of FK506 in the treatment of ischemic brain damage.

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.

Diazepam does not reduce infarct size in rats subjected to transient occlusion of the middle cerebral artery when normothermia is maintained

Activation of the gamma-amino butyric acid (GABA)-ergic system might protect against the damage that occurs after cerebral ischaemia. We examined this hypothesis by administering diazepam to rats subjected to transient middle cerebral artery occlusion (MCAO) using the intraluminal thread method. Diffusion MRI (DWI) and perfusion imaging (PI) were acquired during MCAO to assess brain tissue status and haemodynamics, respectively. Rats were intraperitoneally injected with either 10 mg kg(-1) diazepam (n = 5) or vehicle (n = 5) both 30 min and 90 min after the onset of MCAO. To exclude the possibility that neuroprotection was due to the hypothermic action of the drug, body temperature was maintained at 37-38 degrees C for up to 7 h postischaemia with a feed-back controlled thermoregulatory unit. Infarct volumes quantified 2 days after MCAO from T(2)-weighted images were similar in ischaemic control rats and in ischaemic rats treated with diazepam. We conclude that diazepam-induced enhancement of GABA(A) activity does not effectively protect against neuronal damage that occurs after transient MCAO in normothermic rats.

[3H]muscimol binding to gamma-aminobutyric acid(A) receptors is upregulated in CA1 neurons of the gerbil hippocampus in the ischemia-tolerant state

BACKGROUND AND PURPOSE

Excitotoxic activation of glutamate receptors is currently thought to play a pivotal role in delayed neuronal death (DND) of highly vulnerable CA1 neurons in the gerbil hippocampus after transient global ischemia. Postischemic degeneration of these neurons can be prevented by "preconditioning" with a short sublethal ischemic stimulus. The present study was designed to test whether ischemic preconditioning is associated with specific alterations of ligand binding to excitatory glutamate and/or inhibitory gamma-aminobutyric acid (GABA)A receptors compared with ischemia severe enough to induce DND.

METHODS

With the use of quantitative receptor autoradiography, postischemic ligand binding of [3H]MK-801 and [3H]alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) to excitatory N-methyl-D-aspartate (NMDA) and AMPA receptors as well as [3H]muscimol to inhibitory GABA(A) receptors in hippocampal subfields CA1, CA3, and the dentate gyrus were analyzed in 2 experimental paradigms. Gerbils were subjected to (1) a 5-minute ischemic period resulting in DND of CA1 neurons and (2) a 2.5-minute period of ischemia mediating tolerance induction.

RESULTS

[3H]MK-801 and [3H]AMPA binding values to excitatory NMDA and AMPA receptors showed a delayed decrease in relatively ischemia-resistant CA3 and dentate gyrus despite maintained neuronal cell density. [3H]Muscimol binding to GABA(A) receptors in CA1 neurons was transiently but significantly increased after preconditioning but not after global ischemia with consecutive neuronal death.

CONCLUSIONS

Downregulation of ligand binding to glutamate receptors in relatively ischemia-resistant CA3 and dentate gyrus neurons destined to survive suggests marked synaptic reorganization processes despite maintained structural integrity. More importantly, upregulation of binding to inhibitory GABA(A) receptors in the hippocampus indicates a relative shift between inhibitory and excitatory neurotransmission that we suggest may participate in endogenous postischemic neuroprotection.

Inhibition of cyclin-dependent kinases improves CA1 neuronal survival and behavioral performance after global ischemia in the rat

Increasing evidence suggests that cyclin-dependent kinases participate in neuronal death induced by multiple stresses in vitro. However, their role in cell death paradigms in vivo is not well characterized. Accordingly, the authors examined whether cyclin-dependent kinase inhibition resulted in functionally relevant and sustained neuroprotection in a model of global ischemia. Intracerebroventricular administration of the cyclin-dependent kinase inhibitor flavopiridol, immediately or at 4 hours postreperfusion after a global insult, reduced injury in the CA1 of the hippocampus when examined 7 days after reperfusion. No significant protection was observed when flavopiridol was administered 8 hours after reperfusion. The tumor-suppressor retinoblastoma protein, a substrate of cyclin-dependent kinase, was phosphorylated on a cyclin-dependent kinase consensus site after the global insult; this phosphorylation was inhibited by flavopiridol administration. Importantly, flavopiridol had no effect on core body temperature, suggesting that the mechanism of neuroprotection was through cyclin-dependent kinase inhibition but not through hypothermia. Furthermore, inhibition of cyclin-dependent kinases improved spatial learning behavior as assessed by the Morris water maze 7 to 9 days after reperfusion. However, the histologic protection observed at day 7 was absent 28 days after reperfusion. These results indicate that cyclin-dependent kinase inhibition provides an extended period of morphologic and functional neuroprotection that may allow time for other neuroprotective modalities to be introduced.

Effect of NXY-059 on infarct volume after transient or permanent middle cerebral artery occlusion in the rat; studies on dose, plasma concentration and therapeutic time window

1. The efficacy of the free radical trapping agent NXY-059 in reducing the infarct volume following both transient and permanent focal ischaemia has been examined in rats. 2. In the transient ischaemia model, rats were subjected to a 2 h occlusion of the middle cerebral artery (MCA). Intravenous infusion of NXY-059 (1, 10 and 30 mg kg(-1) h) for 21.75 h starting 2.25 h after the occlusion, produced a dose-dependent decrease in both neurological impairment and the histologically measured infarct volume (a mean 59% decrease at 10 mg kg(-1) h). 3. In the permanent ischaemia model, animals were injected (s.c.) with a loading dose of NXY-059 of 32.5, 53.8 or 75.4 mg kg(-1) and osmotic minipumps were implanted which had been primed to deliver respectively 30, 50 or 70 mg kg(-1) h. When treatment was initiated 5 min after MCA occlusion there was a dose dependent protection of both cortical and sub-cortical tissue (cortex: 63% at the mid-range dose). Protection was related linearly to plasma concentration (plasma unbound NXY-059 concentration at 1 h: 37+/-16 micromol l(-1) at the mid-range dose). 4. When the mid range dose was administered between 5 min - 4 h after MCA occlusion, a marked and statistically significant protection was seen at all time points (44% protection in cortex at 4 h). 5. These data demonstrate the substantial neuroprotective efficacy of NXY-059 at plasma concentrations that can be achieved clinically and indicate that NXY-059 also has a therapeutic window of opportunity that is clinically relevant.

Neuroprotection with anaesthetic agents

The term 'neuroprotection' is used to refer to any prophylactic measure that is initiated during the peri-ischaemic period in order to improve neuronal survival. Cell death after ischaemia has an immediate, necrotic and a delayed, apoptotic origin. The major biochemical mechanisms that are involved in this process include transmembrane ionic fluxes and intracellular calcium increase, excitotoxicity, free radical formation, peroxynitrite production, release of inflammatory mediators, mitochondrial dysfunction, cytochrome c release, and activation of caspases and transcription factors. Strategies of neuroprotection essentially impact on those biochemical pathways. The label 'neuroprotectant' requires that the therapy has basic properties that are consistent with potential mechanisms of neuroprotection, and that conclusive results are available from animal studies that can be converted into clinical benefit. The present review focuses on neuroprotective effects of anaesthetics and is based on the most recently published reports.

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.

Therapeutic implications of hypothermic and hyperthermic temperature conditions in stroke patients

Brain temperature is an important variable in determining the outcome of cerebral ischemia; increases in core temperature escalate neural damage whereas decreases in core temperature reduce damage. Fever induction often occurs in patients prior to or as a direct or indirect result of the ischemic insult, with a worsened stroke outcome, compared with non-febrile ischemic patients. Most importantly, post-ischemic hypothermia reduces long term neural damage and associated behavioral deficits in animals studied for up to a year after the ischemic insult. This review discusses the importance of monitoring the brain temperature of stroke patients and implemention of therapeutic thermoregulatory strategies to reduce the temperature of ischemic patients.Key words: hypothermia, neuroprotection, fever, neural and behavioral outcomes.

Benzodiazepines protect hippocampal neurons from degeneration after transient cerebral ischemia: an ultrastructural study

The ability of full and partial benzodiazepine receptor agonists to prevent DNA fragmentation and neuronal death after transient cerebral ischemia was investigated in the Mongolian gerbil. Diazepam (10mg/kg, i.p.) or the partial agonist imidazenil (3mg/kg, i.p.) was administered 30 and 90min after transient forebrain ischemia produced by occlusion of the carotid arteries for 5min. Treatment with diazepam completely protected CA1b hippocampal pyramidal neurons in 94% of the animals and partially protected pyramidal neurons in 6% of the animals, as assessed with a standard Nissl stain three and four days after ischemia. DNA fragmentation was examined by the terminal dUTP nick-end labeling (TUNEL) reaction. Prior to cell death, there were no TUNEL-positive neurons in area CA1b. By three days after ischemia, when neuronal degeneration was nearly complete, 14 out of 16 gerbils exhibited a positive TUNEL reaction throughout area CA1b stratum pyramidale. In 13 out of 14 gerbils treated with diazepam, no TUNEL-positive neurons were observed in this region. Imidazenil was less effective than diazepam with respect to both neuroprotection and prevention of DNA fragmentation. Three days after ischemia, six out of eight gerbils treated with imidazenil showed partial to complete neuroprotection. Imidazenil completely prevented DNA fragmentation in only one of the animals; varying degrees of TUNEL reaction persisted in the remainder. To determine whether the neurons protected by diazepam had a normal ultrastructure, gerbils were killed two to 30 days after ischemia and the hippocampal neurons in area CA1b were examined by electron microscopy. Within the first 48h after ischemia, early cytoplasmic changes of varying degrees (e.g., vacuolation, rough endoplasmic reticulum stacking, swollen mitochondria) and electron-dense dendrites were observed in gerbils not treated with diazepam. Degeneration was nearly complete by three days after ischemia. In contrast, pyramidal neuron ultrastructure appeared normal in gerbils that exhibited complete area CA1b neuroprotection (defined at the light microscope level) by diazepam when studied two, seven or 30 days after ischemia. In gerbils with partial protection of area CA1b, most of the remaining neurons exhibited varying degrees of necrosis when studied 30 days after ischemia. No apoptotic bodies were observed. We conclude that: (i) diazepam can fully protect CA1 pyramidal cells from the toxic effects of transient cerebral ischemia; (ii) when diazepam affords only partial neuroprotection, the residual CA1 pyramidal cells exhibit ultrastructural abnormalities consistent with necrotic damage; and (iii) diazepam is a more efficacious neuroprotectant than the partial benzodiazepine receptor agonist, imidazenil.

Early microglial reaction following mild forebrain ischemia induced by common carotid artery occlusion in rats

Early microglial reaction following mild ischemic injury caused by bilateral common carotid artery occlusion has been investigated in rats. The ischemic insults lasted for 10, 15 and 20 min without recirculation, and with several reperfusion intervals from 1 h to 3 days. The resting and activated microglial cells were visualized with immunohistochemistry using monoclonal antibodies raised against the CR3 complement receptor, the MHC class I and class II antigens, the macrophage common antigen and with Bandeiiraea simplicifolia lectin-histochemistry. The neuroprotective effect of hypothermia on the early microglial activation was also studied. Ten minutes bilateral common carotid artery occlusion in hypothermic rats without reperfusion caused a mild microglial reaction in the hippocampus. Strong reaction was seen following 20 min insult without reperfusion. Ischemia followed by recirculation caused milder reaction than without reperfusion. Our results suggest that the microglial cells are very sensitive indicators of a mild, transient ischemic insult that does not result in neuronal cell death.