Effect of preceding in vivo sublethal ischemia on the evoked potentials during secondary in vitro hypoxia evaluated with gerbil hippocampal slices

[Isoflurane provides neuroprotection in neonatal hypoxic ischemic brain injury by suppressing apoptosis]

BACKGROUND AND OBJECTIVES

Isoflurane is halogenated volatile ether used for inhalational anesthesia. It is widely used in clinics as an inhalational anesthetic. Neonatal hypoxic ischemia injury ensues in the immature brain that results in delayed cell death via excitotoxicity and oxidative stress. Isoflurane has shown neuroprotective properties that make a beneficial basis of using isoflurane in both cell culture and animal models, including various models of brain injury. We aimed to determine the neuroprotective effect of isoflurane on hypoxic brain injury and elucidated the underlying mechanism.

METHODS

A hippocampal slice, in artificial cerebrospinal fluid with glucose and oxygen deprivation, was used as an in vitro model for brain hypoxia. The orthodromic population spike and hypoxic injury potential were recorded in the CA1 and CA3 regions. Amino acid neurotransmitters concentration in perfusion solution of hippocampal slices was measured.

RESULTS

Isoflurane treatment caused delayed elimination of population spike and improved the recovery of population spike; decreased frequency of hypoxic injury potential, postponed the onset of hypoxic injury potential and increased the duration of hypoxic injury potential. Isoflurane treatment also decreased the hypoxia-induced release of amino acid neurotransmitters such as aspartate, glutamate and glycine induced by hypoxia, but the levels of γ-aminobutyric acid were elevated. Morphological studies showed that isoflurane treatment attenuated edema of pyramid neurons in the CA1 region. It also reduced apoptosis as evident by lowered expression of caspase-3 and PARP genes.

CONCLUSIONS

Isoflurane showed a neuro-protective effect on hippocampal neuron injury induced by hypoxia through suppression of apoptosis.

Isoflurane provides neuroprotection in neonatal hypoxic ischemic brain injury by suppressing apoptosis

BACKGROUND AND OBJECTIVES

Isoflurane is halogenated volatile ether used for inhalational anesthesia. It is widely used in clinics as an inhalational anesthetic. Neonatal hypoxic ischemia injury ensues in the immature brain that results in delayed cell death via excitotoxicity and oxidative stress. Isoflurane has shown neuroprotective properties that make a beneficial basis of using isoflurane in both cell culture and animal models, including various models of brain injury. We aimed to determine the neuroprotective effect of isoflurane on hypoxic brain injury and elucidated the underlying mechanism.

METHODS

A hippocampal slice, in artificial cerebrospinal fluid with glucose and oxygen deprivation, was used as an in vitro model for brain hypoxia. The orthodromic population spike and hypoxic injury potential were recorded in the CA1 and CA3 regions. Amino acid neurotransmitters concentration in perfusion solution of hippocampal slices was measured.

RESULTS

Isoflurane treatment caused delayed elimination of population spike and improved the recovery of population spike; decreased frequency of hypoxic injury potential, postponed the onset of hypoxic injury potential and increased the duration of hypoxic injury potential. Isoflurane treatment also decreased the hypoxia-induced release of amino acid neurotransmitters such as aspartate, glutamate and glycine induced by hypoxia, but the levels of γ-aminobutyric acid were elevated. Morphological studies showed that isoflurane treatment attenuated edema of pyramid neurons in the CA1 region. It also reduced apoptosis as evident by lowered expression of caspase-3 and PARP genes.

CONCLUSIONS

Isoflurane showed a neuro-protective effect on hippocampal neuron injury induced by hypoxia through suppression of apoptosis.

Detection of electrophysiological indicators of neurotoxicity in human and rat brain slices by a three-dimensional microelectrode array

Electrophysiological techniques for the assessment of in vitro neurotoxicology have several advantages over other currently-used methods (for example, morphological techniques), including the ability to detect damage at a very early stage. Novel recording techniques based on microelectrode arrays are available, and could improve recording power. In this study, we investigated how a three-dimensional microelectrode array detects the electrophysiological endpoints of neurotoxicity. We conclude that electrophysiology sensitively reveals neurotoxic actions, and that three-dimensional microelectrode arrays could be proposed for use in neurotoxicology as recording tools that allow easy and sensitive multisite recording, from both rodent and human brain tissue.

Changes in Extracellular Action Potential Detect Kainic Acid and Trimethyltin Toxicity in Hippocampal Slice Preparations Earlier than do MAP2 Density Measurements

In vitro electrophysiological techniques for the assessment of neurotoxicity could have several advantages over other methods in current use, including the ability to detect damage at a very early stage, and could further assist in replacing animal experimentation in vivo. We investigated how an electrophysiological parameter, the extracellularly-recorded compound action potential (“population spike”, PS) could be used as a marker of in vitro neurotoxicity in the case of two well-known toxic compounds, kainic acid (KA) and trimethyltin (TMT). We compared the use of this electrophysiological endpoint with changes in immunoreactivity for microtubule-associated protein 2 (MAP2), a standard histological test for neurotoxicity. We found that both toxic compounds reliably caused disappearance of the PS, and that such disappearance occurred after only 1 hour of exposure to the drug. By contrast, densitometric measurements of MAP2 immunoreactivity were unaffected by both KA and TMT after such a short exposure time. We conclude that, in the case of KA and TMT, the extracellular PS was abolished at a very early time-point, when MAP2 immunoreactivity levels were still comparable to those of the untreated controls. Electrophysiology could be a reliable and early indicator of neurotoxicity, which could improve our ability to test for neurotoxicity in vitro, thus further replacing the need for in vivo experimentation.

On the role of Ca2+ in cerebral ischemic preconditioning

Cerebral ischemic preconditioning (IPC) represents a potent endogenous method of inducing tolerance to otherwise lethal ischemia, both in in vivo and in vitro models. Investigation into the mechanism of this phenomenon has yet again transformed the way that neuroscientists view Ca2+. Generally viewed as an agent of neuronal death, particularly within an excitotoxic setting of cerebral ischemia, Ca2+ is now regarded as a key mediator of IPC. Classification of the role of Ca2+ in IPC defies simple description, but seems to possess a stimulatory role during the tolerance-inducing ischemia and an inhibitory or modulatory role during or following the second normally lethal ischemia.

Brief, repeated, oxygen-glucose deprivation episodes protect neurotransmission from a longer ischemic episode in the in vitro hippocampus: role of adenosine receptors

1. Ischemic preconditioning in the brain consists of reducing the sensitivity of neuronal tissue to further, more severe, ischemic insults. We recorded field epsps (fepsps) extracellularly from hippocampal slices to develop a model of in vitro ischemic preconditioning and to evaluate the role of A1, A2A and A3 adenosine receptors in this phenomenon. 2. The application of an ischemic insult, obtained by glucose and oxygen deprivation for 7 min, produced an irreversible depression of synaptic transmission. Ischemic preconditioning was induced by four ischemic insults (2 min each) separated by 13 min of normoxic conditions. After 30 min, an ischemic insult of 7 min was applied. This protocol substantially protected the tissue from the irreversible depression of synaptic activity. 3. The selective adenosine A1 receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 100 nm), completely prevented the protective effect of preconditioning. The selective adenosine A2A receptor antagonist 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol (ZM 241385, 100 nm) did not modify the magnitude of fepsp recovery compared to control slices. The selective A3 adenosine receptor antagonists, 3-propyl-6-ethyl-5[ethyl(thio)carbonyl]-2-phenyl-4-propyl-3-pyridinecarboxylate (MRS 1523, 100 nm) significantly improved the recovery of fepsps after 7 min of ischemia. 4. Our results show that in vitro ischemic preconditioning allows CA1 hippocampal neurons to become resistant to prolonged exposure to ischemia. Adenosine, by stimulating A1 receptors, plays a crucial role in eliciting the cell mechanisms underlying preconditioning; A2A receptors are not involved in this phenomenon, whereas A3 receptor activation is harmful to ischemic preconditioning.

Cross-tolerance to otherwise lethal N-methyl-D-aspartate and oxygen-glucose deprivation in preconditioned cortical cultures

In vitro ischemic preconditioning induced by subjecting rat cortical cultures to nonlethal oxygen-glucose deprivation protects against a subsequent exposure to otherwise lethal oxygen-glucose deprivation. We provide evidence that attenuation of the postsynaptic N-methyl-D-aspartate (NMDA) receptor- and Ca(2+)-dependent neurotoxicity underlies oxygen-glucose deprivation tolerance. It is demonstrated that extended tolerance to otherwise lethal NMDA or oxygen-glucose deprivation can be induced by either of their sublethal forms of preconditioning. These four pathways are linked, since NMDA receptor blockade during preconditioning by oxygen-glucose deprivation eliminates tolerance. These results suggest that NMDA tolerance, induced by nonlethal activation of these receptors during oxygen-glucose deprivation preconditioning, underlies oxygen-glucose deprivation tolerance. Several neurotoxic downstream Ca(2+)-dependent signaling events specifically linked to NMDA receptor activation are attenuated during otherwise lethal oxygen-glucose deprivation in preconditioned cultures. Specifically, calpain activation, as well as degradation of microtubule-associated protein-2 and postsynaptic density-95, are attenuated 2 h following otherwise lethal NMDA treatment alone or oxygen-glucose deprivation in preconditioned cultures. Formation of microtubule-associated protein-2-labeled dendritic varicosities is also attenuated in preconditioned cultures within 1 h of lethal oxygen-glucose deprivation or NMDA application. Intracellular Ca(2+) levels, measured using the high- or low-affinity dyes Fluo-4 (K(d) approximately equal 345 nM) or Fluo-4FF (K(d) approximately equal 9.7 microM) respectively, are markedly attenuated during lethal oxygen-glucose deprivation in preconditioned cultures.Collectively, the results suggest the attenuation of the postsynaptic NMDA-mediated component of otherwise lethal oxygen-glucose deprivation through the suppression of Ca(2+)-dependent neurotoxic signaling, a mechanism that is initially induced by transient nonlethal activation of this receptor during ischemic preconditioning.

ATP-dependent potassium channel mediates neuroprotection by chemical preconditioning with 3-nitropropionic acid in gerbil hippocampus

Chemical preconditioning with low dose of 3-nitropropionic acid (3-NPA) prolongs the latency to hypoxic depolarization (HD), which triggers cell death, and also restores the synaptic transmission which disappears during hypoxia in gerbil hippocampal slices. Here we show that these neuroprotective effects are mediated by the activation of K(ATP) channels. Diazoxide, a K(ATP) channel opener, prolonged the latency to HD dose-dependently to the same extent as that of the chemical preconditioning with 3-NPA. Glibenclamide, a K(ATP) channel blocker, abrogated the prolongation of HD with 3-NPA. The hypoxic tolerance of synaptic transmission with 3-NPA was also abolished by glibenclamide. Diazoxide also induced the hypoxic tolerance of synaptic transmission. Theses results suggest that K(ATP) channel is involved in the neuroprotection afforded by the chemical preconditioning.

Chemical preconditioning with 3-nitropropionic acid in gerbil hippocampal slices: therapeutic window and the participation of adenosine receptor

Ischemic tolerance induced by a subtoxic dose of neurotoxin, 3-nitropropionic acid (3-NPA), was recently reported as "chemical preconditioning." We electrophysiologically investigated the therapeutic window and the effect via action at the adenosine receptor using a gerbil hippocampal slice model of the tolerance phenomenon. 3-NPA at the dose of 4 mg/kg was administered intraperitoneally at 3, 24, and 72 h prior to slice preparation. Prolonged delay to hypoxic depolarization (HD) and improvement of the field excitatory postsynaptic potential recovery following a fixed period of hypoxia (8 min) were observed in the groups pretreated at 3 and 24 h compared with the control (P < 0.05). Correlation between the delay to HD and the recovery was highly significant (r = 0.37, P < 0.001). These effects were completely reversed by administration of theophylline (20 mg/kg), an adenosine receptor blocker. These findings indicate that chemical preconditioning with 3-NPA induces early onset (3 h) and long-lasting (24 h) tolerance of hypoxic damage to excitatory synaptic mechanisms in the hippocampus by delayed calcium entry, and the activation of adenosine receptor contributes to this neuroprotective effect.

Ischemic tolerance preserves propagation of membrane depolarization

The functional integrity of the synaptic connections within the hippocampus in gerbils that had acquired ischemic tolerance was investigated. The propagation of membrane depolarization across the hippocampus in response to electrical stimulation of CA1 was monitored with the use of a high speed optical recording technique. In comparison to control slices, propagation was significantly depressed and depolarization was shortened in slices from gerbils subjected to 5 min of ischemia. Hippocampal slices from gerbils who were preconditioned with prior sublethal ischemia demonstrated only a slight reduction in propagation. The duration of depolarization was longer than that of ischemia group. These findings suggest that ischemia induces a functional disturbance of synaptic transmission and membrane depolarization. Ischemic preconditioning significantly reduced the extent of this functional disturbance.

Preconditioning the Brain and Heart: Implications for Cardiac Surgery

Despite many recent advances in emboli detection, aortic imaging, myocardial preservation, and perfusion equipment, ischemic injury to the heart and brain remains a serious complications after cardiac surgery. Hypoperfusion (particularly in the heart) and microem boli (particularly in the brain) during cardiopulmonary bypass constitute the etiology of ischemia. Although hypothermia has traditionally been the mainstay for systemic protection from transient ischemia, there has been a general trend to accept warmer heart and core temperatures during bypass, which increases the poten tial for ischemic injury to various organs. This article discusses recent advances in the understanding of myocardial and brain preconditioning and their poten tial role to provide additional protection during cardiac surgery.

Effects of acidosis on the post-hypoxic recovery of synaptic transmission in gerbil hippocampal slices

We investigated the effects of acidosis on the hypoxic neuronal damage using gerbil hippocampal slices. Acidosis has delayed the onset of harmful hypoxic depolarization, resulting in a decrease in the total hypoxic period and the hypoxic depolarization. This effect has been considered to be protective. However, the synaptic recovery after reoxygenation was attenuated when acidosis (pH: 6.2-6.9) was sustained. Conversely, the synaptic recovery was potentiated when the acidosis was restored to the physiological milieu during the reoxygenation period. These results suggest that acidosis plays a protective effect against the hypoxic neuronal damage only when rapid appreciable pH recovery is achieved during reoxygenation.

Hypothermia-induced ischemic tolerance

Delayed resistance to ischemic injury can be induced by a variety of conditioning stimuli. This phenomenon, known as delayed ischemic tolerance, is initiated over several hours or a day, and can persist for up to a week or more. The present paper describes recent experiments in which transient hypothermia was used as a conditioning stimulus to induce ischemic tolerance. A brief period of hypothermia administered 6 to 48 hours prior to focal ischemia reduces subsequent cerebral infarction. Hypothermia-induced ischemic tolerance is reversed by 7 days postconditioning, and is blocked by the protein synthesis inhibitor anisomycin. Electrophysiological studies utilizing in vitro brain slices demonstrate that hypoxic damage to synaptic responses is reduced in slices prepared from hypothermia-preconditioned animals. Taken together, these findings indicate that transient hypothermia induces tolerance in the brain parenchyma, and that increased expression of one or more gene products contributes to this phenomenon. Inasmuch as hypothermia is already an approved clinical procedure for intraischemic and postischemic therapy, it is possible that hypothermia could provide a clinically useful conditioning stimulus for limiting injury elicited by anticipated periods of ischemia.

Nitric oxide is involved in anoxic preconditioning neuroprotection in rat hippocampal slices

Sublethal anoxia/ischemia protects against subsequent damaging insults in intact brain or hippocampal slices. To help further understand mechanisms underlying anoxic/ischemic preconditioning, we tested three hypotheses which were that: (a) anoxic preconditioning (APC) improves electrical recovery in rat hippocampal slices; (b) anoxic preconditioning requires nitric oxide (NO); and (c) anoxic preconditioning blocks mitochondrial dysfunction that occurs following re-oxygenation after anoxia. Control hippocampal slices underwent a single 'test' anoxic insult. Experimental slices were preconditioned by 3 short anoxic insults prior to the 'test' insult. Evoked potentials (EPs), and NADH redox status were recorded prior to, during and after preconditioning and/or 'test' anoxic insults. To examine the role of NO, studies sought to determine whether APC could be produced by the NO donor, DEA/NO, and whether APC could be inhibited by NO synthase (NOS) inhibitor (7-nitroindazole). EP amplitudes recovered significantly better after reoxygenation in preconditioned slices and after NO-emulated preconditioning (90.0+/-17.7% and 90.0+/-21.3%, respectively, n=9, ** p<0.01, vs. 17.0+/-7.9%, n=9, in control slices). Inhibition of NOS blocked APC protection (6.8+/-6.8%, n=9). The intensity of NADH hyperoxidation was not significantly different among groups following 'test' anoxia. These data confirm that preconditioning by anoxia improves electrical recovery after anoxia in hippocampal slices. Evidence supports that NO from constitutive hippocampal NOS may be involved in the neuroprotection afforded by preconditioning by a mechanism that does not change the apparent mitochondrial hyperoxidation after anoxia.

Focal ischemic preconditioning induces rapid tolerance to middle cerebral artery occlusion in mice

In a process called ischemic preconditioning, a brief, sublethal ischemic insult protects tissue from subsequent, more severe injury. There have been no reports of rapidly induced ischemic preconditioning. The authors sought to develop a model of cerebral ischemic preconditioning in the mouse that can be applied to transgenic and knockout animals. They found that brief middle cerebral artery (MCA) occlusion only minutes before a severe ischemic insult can induce protection from that insult. Here the investigators describe a mouse model of preconditioning using intraluminal MCA occlusion as both the conditioning and the test stimulus. One or three 5-minute episodes of ischemia given 30 minutes before MCA occlusion for 1 or 24 hours (permanent occlusion) confer significant protection as assessed by infarct volume measurements 24 hours later.