Brain tissue responses to ischemia

Neutrophil-derived glutamate regulates vascular endothelial barrier function

Endothelial barrier function is altered by the release of soluble polymorphonuclear leukocyte (PMN)-derived mediators during inflammatory states. However, endogenous pathways to describe such changes are only recently appreciated. Using an in vitro endothelial paracellular permeability model, cell-free supernatants from formylmethionylleucylphenylalanine-stimulated PMNs were observed to significantly alter endothelial permeability. Biophysical and biochemical analysis of PMN supernatants identified PMN-derived glutamate in modulating endothelial permeability. Furthermore, novel expression of metabotropic glutamate receptor 1 (mGluR1), mGluR4, and mGluR5 by human brain and dermal microvascular endothelial cells was demonstrated by reverse transcription-PCR, in situ hybridization, immunofluorescence, and Western blot analysis. Treatment of human brain endothelia with glutamate or selective, mGluR group I or III agonists resulted in a time-dependent loss of phosphorylated vasodilator-stimulated phosphoprotein (VASP) and significantly increased endothelial permeability. Glutamate-induced decreases in brain endothelial barrier function and phosphorylated VASP were significantly attenuated by pretreatment of human brain endothelia with selective mGluR antagonists. These observations were extended to an in vivo hypoxic mouse model in which pretreatment with mGluR antagonists significantly decreased fluorescein isothiocyanate-dextran flux across the blood-brain barrier. We conclude that activated human PMNs release glutamate and that endothelial expression of group I or III mGluRs function to decrease human brain endothelial VASP phosphorylation and barrier function. These results identify a novel pathway by which PMN-derived glutamate may regulate human endothelial barrier function.

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.

Fas (CD95/Apo-1)/Fas ligand expression in neonates with pontosubicular neuron necrosis

Pontosubicular neuron necrosis (PSN) represents an age-specific response to severe hypoxic-ischemic injury occurring in human neonates but not in older children or adults. Histologically, PSN is characterized by acute neuronal death in the pontine nuclei and the hippocampal subiculum bearing the hallmarks of apoptosis. In animal models of hypoxic-ischemic injury, induction of neuronal apoptosis can be triggered by Fas (CD95/Apo-1), a cell surface receptor of the tumor necrosis factor-alpha superfamily, which transduces apoptotic death signals when cross-linked by its natural ligand. Here, we have investigated the expression of Fas/Fas ligand in human autopsy material consisting of 13 PSN cases and 10 age-matched cases without PSN. Terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling, immunohistochemistry, and double labeling for Fas/Fas ligand and the astrocyte marker glial fibrillary acid protein, the microglia/macrophage specific marker KiM1P, and the neuronal marker NeuN were performed on formalin-fixed brain specimens. Although mainly neurons of both PSN and controls expressed Fas receptor, expression was significantly increased (p = 0.001) in PSN cases in which predominantly degenerating cells with signs of early apoptosis showed Fas expression. In contrast, Fas ligand expression was found mainly on astrocytes and microglial cells. There was no significant difference between cases with and without PSN. We conclude that in the developing human brain, cells expressing the Fas receptor may be susceptible to undergoing apoptosis in response to hypoxic-ischemic injury.

Extradural compression of sensorimotor cortex: a useful model for studies on ischemic brain damage and neuroprotection

Behavioral and morphological changes were examined for up to 9 days after moderate cerebral ischemia caused by slow compression of a specific brain area in the sensorimotor cortex of Sprague-Dawley rats. Functional deficits after the cerebral ischemia were assessed by daily beam-walking tests, whereas morphological changes were verified using Nissl staining on day 1, 2, 3, 5, and 9, respectively. Rats exposed to cerebral ischemia displayed impaired beam walking performance. Mild hypothermia prevented both the compression-produced functional deficits and the brain damage. Younger (5 weeks) animals showed less neurological deficits than older (9 weeks) animals. Histological examination revealed a pronounced increase in the number of injured pyramidal neurons from day 1 to day 3 in the primarily damaged brain region. Between day 3 and day 5, the number of injured cells remained constant, whereafter there was a slow decline of thionin-positive neurons as examined on day 9. The noncompetitive NMDA receptor antagonist, dizocilpine (MK-801; 3 mg/kg, i.p.), did not alter the neurological impairment on day 1, but improved thereafter the rate of functional recovery and reduced the number of damaged cells. The AMPA receptor antagonist, LY326325 (15 or 30 mg/kg; i.p.), dose-dependently diminished the neurological deficits on day 1, enhanced the rate of recovery, and reduced the number of injured neurons over time. Our data suggest that short-lasting extradural compression of a well-defined brain area in the sensorimotor cortex is a highly reproducible model with a high success rate for the study of functional and morphological consequences after cerebral ischemia as well as for the evaluation of the therapeutic potential of novel, neuroprotective pharmacological agents.

Selective mGluR5 receptor antagonist or agonist provides neuroprotection in a rat model of focal cerebral ischemia

Activation of group I metabotropic glutamate receptors (mGluR) has been implicated in the pathophysiology of acute central nervous system injury. However, the relative roles of the two group I subtypes, mGluR1 or mGluR5, in such injury has not been well examined. We compared the effects of treatment with the newly developed, selective mGluR5 antagonist 2-methyl-6-phenylethynylpyridine (MPEP) and the selective mGluR5 agonist (R,S)-2-chloro-5-hydroxyphenylglycine (CHPG) in a rat intraluminal filament model of temporary middle cerebral artery occlusion (MCAo). Rats were administered MPEP or CHPG i.c.v. beginning 15 or 135 min after induction of ischemia for 2 h. Infarct size was measured after either 22 or 70 h of reperfusion, and neurological function was quantified at 2, 24, 48 and 72 h. Treatment with MPEP or CHPG at 15 min reduced 24 h infarct volume by 61 and 44%, respectively. The neuroprotective effects were dose dependent. Delaying MPEP treatment until 135 min eliminated the neuroprotective effects. In other studies, using early MPEP treatment (15 min) at optimal doses, infarct volume was reduced by 44% at 72 h and this was correlated with significant neurological recovery. These data suggest that both MPEP and CHPG are neuroprotective when administered after focal cerebral ischemia. In separate, recent studies we found that although MPEP does act as an mGluR5 antagonist and blocks agonist induced phosphoinositide hydrolysis, it also serves as a non-competitive NMDA antagonist; in contrast, other results indicate that CHPG mediated neuroprotection may reflect anti-apoptotic activity. Therefore, both types of compounds may prove to have therapeutic potential for the treatment of stroke.

Increased susceptibility to ischemic brain injury in cyclooxygenase-1-deficient mice

Cyclooxygenase-1 (COX-1), a rate-limiting enzyme in the synthesis of prostanoids, is involved in selected vasodilatatory responses of the cerebral circulation. Cyclooxygenase-1-null mice were used to determine whether COX-1 influences cerebral ischemic damage. The middle cerebral artery was occluded in COX-1 -/- and +/+ mice (n = 9/group), and lesion volume was determined in thionin-stained sections 24 or 96 hours later. Middle cerebral artery occlusion produced larger infarcts in COX-1 -/- mice, both at 24 (35 +/- 17%; P < 0.05) and 96 hours (41 +/- 16%; P < 0.05) after ischemia. The enlargement was not due to increased susceptibility to glutamate excitotoxicity, because microinjection of N-methyl-D-aspartate or kainate in the parietal cortex produced comparable lesions in COX-1 +/+ and -/- mice ( P > 0.05; n = 8/group). To examine the contribution of hemodynamic factors to the enlargement of the infarct, cerebral blood flow was monitored by laser-Doppler flowmetry in the ischemic territory (n = 6/group). Although the reduction in cerebral blood flow was comparable in the ischemic core ( P > 0.05), at the periphery of the ischemic territory the reduction was greater in COX-1 -/- mice (-58 +/- 4%) than in COX-1 +/+ mice (-34 +/- 5%; P < 0.05). It is concluded that mice lacking COX-1 are more susceptible to focal cerebral ischemia, an effect that can be attributed to a more severe cerebral blood flow reduction in vulnerable regions at the periphery of the ischemic territory. Thus, the vascular effects of COX-1 may contribute to maintain cerebral blood flow in the postischemic brain and, as such, play a protective role in ischemic brain injury.

Neuroprotective adaptations in hibernation: therapeutic implications for ischemia-reperfusion, traumatic brain injury and neurodegenerative diseases

Brains of hibernating mammals are protected against a variety of insults that are detrimental to humans and other nonhibernating species. Such protection is associated with a number of physiological adaptations including hypothermia, increased antioxidant defense, metabolic arrest, leukocytopenia, immunosuppression, and hypocoagulation. It is intriguing that similar manipulations provide considerable protection as experimental treatments for central nervous system injury. This review focuses on neuroprotective mechanisms employed during hibernation that may offer novel approaches in the treatment of stroke, traumatic brain injury, and neurodegenerative diseases in humans.

Immunohistochemical study on the distribution of voltage-gated K(+) channels in rat brain following transient focal ischemia

The present study examined the spatial localizations of voltage-gated K(+) (Kv) channels in the rat brain following transient focal ischemia, using immunohistochemistry. Increased expression of Kv1.2 was obvious in the cerebral cortex, dentate gyrus, amygdala, and hypothalamic areas at 3 days following ischemic insults. There was a significant increase in Kv1.2 immunoreactivity in several cortical regions, including cingulate cortex, infralimbic cortex, dorsal peduncular cortex and piriform cortex. On the contrary, Kv1.2 immunoreactivity had not significantly increased in the hippocampal CA1-3 regions although moderate Kv1.2 immunoreactivity was found in the cell bodies and processes of some neurons. Potentially the first demonstration of spatial changes in Kv1.2 channel expression could provide important molecular basis for altered neuronal excitability after ischemic brain injury.

Enhanced spontaneous transmitter release is the earliest consequence of neocortical hypoxia that can explain the disruption of normal circuit function

After the onset of an acute episode of arrested circulation to the brain and consequent cerebral hypoxia, EEG changes and modifications of consciousness ensue within seconds. This in part reflects the rapid effect of hypoxia on the neocortex, where oxygen deprivation leads to impaired neuronal excitability and abnormal synaptic transmission. To identify the cellular mechanisms responsible for the earliest changes in neocortical function and to determine their time course, we have used patch-in-slice recording techniques to investigate the effects of acute hypoxia on the synaptic and intrinsic properties of layer 5 neurons. Coronal slices of mouse somatosensory cortex were maintained at 37 degrees C and challenged with episodes of hypoxia (3-4 min of exposure to 95% N(2), 5% CO(2)). In recordings with cell-attached patch electrodes, activation of ATP-sensitive potassium channels first became detectable 211 +/- 11 sec (range, 185-240 sec; n = 6 patches) after the onset of hypoxia. Similar recording techniques revealed no alterations in the properties of Na(+) currents in the first 4 min after the onset of hypoxia. The earliest hypoxia-induced disturbance was a marked increase in the frequency of spontaneous EPSCs and IPSCs, which began within 15-30 sec of the removal of oxygen. This rapid synaptic effect was not sensitive to TTX and was present in Ca(2+)-free perfusate, indicating that the hypoxia had a direct influence on the vesicular release mechanisms. The incoherent, massive increase in miniature PSCs would be expected to deplete the readily releasable pool of vesicles in cortical terminals, and to thereby markedly distort the neuronal interactions that underlie normal circuit function.

Enhanced spontaneous transmitter release is the earliest consequence of neocortical hypoxia that can explain the disruption of normal circuit function

After the onset of an acute episode of arrested circulation to the brain and consequent cerebral hypoxia, EEG changes and modifications of consciousness ensue within seconds. This in part reflects the rapid effect of hypoxia on the neocortex, where oxygen deprivation leads to impaired neuronal excitability and abnormal synaptic transmission. To identify the cellular mechanisms responsible for the earliest changes in neocortical function and to determine their time course, we have used patch-in-slice recording techniques to investigate the effects of acute hypoxia on the synaptic and intrinsic properties of layer 5 neurons. Coronal slices of mouse somatosensory cortex were maintained at 37 degrees C and challenged with episodes of hypoxia (3-4 min of exposure to 95% N(2), 5% CO(2)). In recordings with cell-attached patch electrodes, activation of ATP-sensitive potassium channels first became detectable 211 +/- 11 sec (range, 185-240 sec; n = 6 patches) after the onset of hypoxia. Similar recording techniques revealed no alterations in the properties of Na(+) currents in the first 4 min after the onset of hypoxia. The earliest hypoxia-induced disturbance was a marked increase in the frequency of spontaneous EPSCs and IPSCs, which began within 15-30 sec of the removal of oxygen. This rapid synaptic effect was not sensitive to TTX and was present in Ca(2+)-free perfusate, indicating that the hypoxia had a direct influence on the vesicular release mechanisms. The incoherent, massive increase in miniature PSCs would be expected to deplete the readily releasable pool of vesicles in cortical terminals, and to thereby markedly distort the neuronal interactions that underlie normal circuit function.

Hypoxia induces the activation of the phosphatidylinositol 3-kinase/Akt cell survival pathway in PC12 cells: protective role in apoptosis

Hypoxia is a common environmental stress that influences signaling pathways and cell function. Several cell types, including neuroendocrine chromaffin cells, have evolved to sense oxygen levels and initiate specific adaptive responses to hypoxia. Here we report that under hypoxic conditions, rat pheochromocytoma PC12 cells are resistant to apoptosis induced by serum withdrawal and chemotherapy treatment. This effect is also observed after treatment with deferoxamine, a compound that mimics many of the effects of hypoxia. The hypoxia-dependent protection from apoptosis correlates with activation of the phosphatidylinositol 3-kinase (PI3K)/Akt pathway, which is detected after 3-4 h of hypoxic or deferoxamine treatment and is sustained while hypoxic conditions are maintained. Hypoxia-induced Akt activation can be prevented by treatment with cycloheximide or actinomycin D, suggesting that de novo protein synthesis is required. Finally, inhibition of PI3K impairs both the protection against apoptosis and the activation of Akt in response to hypoxia, suggesting a functional link between these two phenomena. Thus, reduced oxygen tension regulates apoptosis in PC12 cells through activation of the PI3K/Akt survival pathway.

Molecular and biochemical mechanisms of perinatal brain injury

Hypoxic-ischemic injury to the prenatal and perinatal brain is a major contributor to morbidity and mortality to infants, often leading to mental retardation, seizures, and cerebral palsy. The susceptibility of the immature CNS to hypoxia-ischemia is largely dependent on the temporal and regional status of critical developmental processes, as well as on the regulation of cerebral blood flow and metabolism. The molecular and biochemical mechanisms of acute injury to the neonatal brain in experimental rodent and murine models of hypoxic-ischemic and ischemic injury, including disturbances of intracellular homeostasis, role of glutamate receptors, free radicals and transitional ions, as well as the modifying role of gene expression to cell death/survival will be reviewed in this chapter.

Neurobiology of disease

Advances in technology and basic sciences this past decade have transformed neurobiological research. Practitioners looking prospectively in 1990 could hardly have hoped for the diagnostics and rational therapeutics that have become part of regular practice today. Here, we discuss three areas that have had great impact: genetics, cell death, and stem cell/gene therapy research.