Strain-related brain injury in neonatal mice subjected to hypoxia-ischemia

Sustained neocortical neurogenesis after neonatal hypoxic/ischemic injury

OBJECTIVE

Neocortical neurons are sensitive to hypoxic-ischemic (H-I) injuries at term and their demise contributes to neurological disorders. Here we tested the hypothesis that the subventricular zone of the immature brain regenerates neocortical neurons, and that this response is sustained.

METHODS

Systemic injections of 5-bromo-2'-deoxyuridine (BrdU) and intraventricular injections of replication-deficient retroviruses were used to label newly born cells, and confocal microscopy after immunofluorescence was used to phenotype the new cells from several days to several months after perinatal H-I in the postnatal day 6 rat. Quantitative polymerase chain reaction was used to evaluate chemoattractants, growth factors, and receptors.

RESULTS

Robust production of new neocortical neurons after perinatal H-I occurs. These new neurons are descendants of the subventricular zone, and they colonize the cell-sparse columns produced by the injury to the neocortex. These columns are populated by reactive astrocytes and microglia. Surprisingly, this neuronogenesis is sustained for months. Molecular analyses demonstrated increased neocortical production of insulin-like growth factor-1 and monocyte chemoattractant factor-1 (but statistically insignificant production of erythropoietin, brain-derived neurotrophic factor, glial-derived neurotrophic factor, and transforming growth factor-alpha).

INTERPRETATION

The young nervous system has long been known to possess a greater capacity to recover from injury than the adult system. Our data indicate that H-I injury in the neonatal brain initiates an enduring regenerative response from the subventricular zone. These data suggest that additional mechanisms than those previously surmised contribute to the remarkable ability of the immature brain to recover from injury.

Hypoxic preconditioning reverses protection after neonatal hypoxia-ischemia in glutathione peroxidase transgenic murine brain

The effect of hypoxic preconditioning (PC) on hypoxic-ischemic (HI) injury was explored in glutathione peroxidase (GPx)-overexpressing mice (human GPx-transgenic [hGPx-tg]) mice. Six-day-old hGPx-tg mice and wild-type (Wt) littermates were pre-conditioned with hypoxia for 30 min and subjected to the Vannucci procedure of HI 24 h after the PC stimulus. Histopathological injury was determined 5 d later (P12). Additional animals were killed 2 h or 24 h after HI and ipsilateral cerebral cortices assayed for GPx activity, glutathione (GSH), and hydrogen peroxide (H2O2). In line with previous studies, hypoxic PC reduced injury in the Wt brain. Preconditioned Wt brain had increased GPx activity, but reduced GSH, relative to naive 24 h after HI. Hypoxic PC did not reduce injury to hGPx-tg brain and even reversed the protection previously reported in the hGPx-tg. GPx activity and GSH in hGPx-tg cortices did not change. Without PC, hGPx-tg cortex had less H2O2 accumulation than Wt at both 2 h and 24 h. With PC, H2O2 remained low in hGPx-tg compared with Wt at 2 h, but at 24 h, there was no longer a difference between hGPx-tg and Wt cortices. Accumulation of H2O2 may be a mediator of injury, but may also induce protective mechanisms.

Prostaglandin D2 protects neonatal mouse brain from hypoxic ischemic injury

Prostaglandin D2 (PGD) is synthesized by hematopoietic PGD synthase (HPGDS) or lipocalin-type PGDS (L-PGDS), depending on the organ in which it is produced, and binds specifically to either DP1 or DP2 receptors. We investigated the role of PGD2 in the pathogenesis of hypoxic-ischemic encephalopathy (HIE) in neonatal mice at postnatal day 7. In wild-type mice, hypoxia-ischemia increased PGD2 production in the brain up to 90-fold compared with the level in sham-operated brains at 10 min after cessation of hypoxia. Whereas the size of the infarct was not changed in L-PGDS or DP2 knock-out mouse brains compared with that in the wild-type HIE brains, it was significantly increased in HPGDS-L-PGDS double knock-out or DP1 knock-out mice. The PGD2 level in L-PGDS, HPGDS, and HPGDS-L-PGDS knock-out mice at 10 min of reoxygenation was 46, 7, and 1%, respectively, of that in the wild-type ones, indicating the infarct size to be in inverse relation to the amount of PGD2 production. DP1 receptors were exclusively expressed in endothelial cells after 1 h of reoxygenation, and cerebral blood flow decreased more rapidly after the onset of hypoxia and did not return to the baseline level after reoxygenation in HPGDS-L-PGDS knock-out mice. Endothelial cells were severely damaged in HPGDS-L-PGDS and DP1 knock-out mice after 1 h of reoxygenation. In the human neonatal HIE brain, HPGDS-positive microglia were increased in number. In conclusion, it is probable that PGD2 protected the neonatal brain from hypoxic-ischemic injury mainly via DP1 receptors by preventing endothelial cell degeneration.

Early induction of neuronal lipocalin-type prostaglandin D synthase after hypoxic-ischemic injury in developing brains

Lipocalin-type prostaglandin (PG) D synthase (L-PGDS) is up-regulated in oligodendrocytes (OLs) in mouse models for genetic neurological disorders including globoid cell leukodystrophy (twitcher) and GM1 and GM2 gangliosidoses and in the brain of patients with multiple sclerosis. Since L-PGDS-deficient twitcher mice undergo extensive neuronal death, we concluded that L-PGDS functions protectively against neuronal degeneration. In this study, we investigated whether L-PGDS is also up-regulated in acute and massive brain injury resulting from neonatal hypoxic-ischemic encephalopathy (HIE). Analysis of brains from human neonates who had died from HIE disclosed that the surviving neurons in the infarcted lesions expressed L-PGDS. Mouse models for neonatal HIE were made on postnatal day (PND) 7. Global infarction in the ipsilateral hemisphere was evident at 24h after reoxygenation in this model. Intense L-PGDS immunoreactivity was already observed at 10 min after reoxygenation in apparently normal neurons in the cortex, and thereafter, in neurons adjacent to the infarcted area. Quantitative RT-PCR revealed that the L-PGDS mRNA level of the infarcted hemisphere was 33-fold higher than that of the sham-operated mouse brains at 1h after reoxygenation and that it decreased to the normal level by 24h thereafter. Furthermore, in both human and mouse brains, many of L-PGDS-positive cells were also immunoreactive for p53; and some of these expressed cleaved caspase-3. The expression of L-PGDS in degenerating neurons implies that L-PGDS functions as an early stress protein to protect against neuronal death in the HIE brain.

Brief update on animal models of hypoxic-ischemic encephalopathy and neonatal stroke

The discovery of safe and effective therapies for perinatal hypoxia ischemia (HI) and stroke remains an unmet goal of neonatal-perinatal medicine. Because of the many developmental and functional differences between the neonatal brain and the adult brain, the ability to extrapolate adult data to the neonatal condition is very limited. For this reason, it is incumbent on scientists in the field of neonatal brain injury to address the questions of therapeutic efficacy of an array of potential therapies in a developmentally appropriate model. Toward that end, a number of new models of neonatal HI and stroke have been introduced recently. Additionally, some of the established models have been adapted to different species and different ages, giving scientists a greater choice of models for the study of neonatal HI and stroke. Many of these models are now also being used for functional and behavioral testing, an absolute necessity for preclinical therapeutic trials. This review focuses primarily on the newly developed models, recent adaptations to established models, and the studies of functional outcome that have been published since 2000.

Adult neurological function following neonatal hypoxia-ischemia in a mouse model of the term neonate: water maze performance is dependent on separable cognitive and motor components

BACKGROUND AND PURPOSE

Hypoxic-ischemic injury in term neonates remains a significant cause of long-term neurological morbidity. The post-natal day 10 (P10) mouse is accepted as a model for the term human. This study was designed to assess the relationships between the duration of hypoxia-ischemia (HI) on P10 and the structural and functional neurological deficits that appear in the adult mouse as a consequence.

METHODS

Post-natal day 10 129T2xC57Bl/6 F1 hybrid mice were subjected to 0, 45, 60 or 75 min of hypoxia-ischemia using the Rice-Vannucci model. Beginning on P50 these mice were tested over the next 8 weeks using zero maze, locomotor activity, novel object recognition, cued, hidden and reduced Morris water mazes, delayed probe trials and response to apomorphine injection. Brain weights and histology were obtained at the end of testing.

RESULTS

The degree of structural and behavioral abnormalities in adult mice correlated with the duration of hypoxia-ischemia on P10. Useful behavioral tests for separating adult mice according to duration of hypoxia-ischemia on P10 include locomotor activity, the Morris water mazes and response to apomorphine. We found cued "learning" persisted, although latencies increased, with increasing HI time while spatial learning decayed as a function of HI time. Severe HI injury involving the ventral hippocampus resulted in excessive locomotor activity.

CONCLUSIONS

After correcting for motor deficits, there is evidence for persistence of "cued" learning but not spatial learning with increasing hypoxia-ischemia time on P10 in this model system.

Selective involvement of BH3-only Bcl-2 family members Bim and Bad in neonatal hypoxia–ischemia

Perinatal hypoxic-ischemic injury is a common cause of neurologic disability mediated in part by Bcl-2 family-regulated neuronal apoptosis. The Bcl-2 protein family consists of both pro- (e.g. Bax, Bad, Bid, Bim) and anti-apoptotic (e.g. Bcl-2, Bcl-X(L)) proteins that regulate mitochondrial outer membrane integrity, cytochrome c release and caspase activation. Previous studies have implicated Bax as an important mediator of neuronal death in several models of brain injury, including neonatal hypoxia-ischemia (HI). In this study, we assessed the roles of several members of the pro-apoptotic BH3 domain-only Bcl-2 sub-family in an in vivo mouse model of neonatal HI. Seven-day old control and gene-disrupted mice underwent unilateral left carotid ligation followed by 45 min exposure to 8% oxygen and the extent of brain injury was assessed 2 days later. Following HI, mice deficient in Bad or Bim exhibited reduced activated caspase-3 and glial fibrillary acidic protein immunostaining in their brains compared to similarly treated control animals. Measurement of hippocampal area showed decreased parenchymal loss in both Bad- and Bim-deficient mice versus control animals. In contrast, loss of Bid, another BH3-only protein, provided no protection from neonatal HI brain injury. These results indicate that Bad and Bim are selectively involved in neuron death following neonatal HI and may be targets for therapeutic intervention.

Effects of oxidative challenge and calcium on ATP-levels in neuronal cells

BACKGROUND

Neurocellular overload with hydrogen peroxide (H2O2) induces oxidative stress and may initiate a cascade of intracellular toxic events leading to energy failure, increased lipid peroxidation and subsequently cell death. Studies suggest that hippocampal neurons may be more vulnerable to oxidative stress than cortical cells pointing to a differential vulnerability of neuronal cells. Since disturbed ATP- and calcium (Ca2+)-metabolism may be involved in this process, we here evaluated the effects of H2O2-induced oxidative stress and the involvement of Ca2+-regulation on neuronal energy metabolism.

METHODS

Using primary cortical and hippocampal neurons as well as immortalized hippocampal HT22 cells, we determined ATP-levels and accompanying cell death after oxidative challenge with H2O2. Additionally, the combined effects of H2O2 and alterations in Ca2+-concentrations were pharmacologically characterized in more detail.

RESULTS

H2O2-incubation decreased ATP-levels in a dose- and time-dependent manner in all neuronal cell systems tested. Such effects were most pronounced in primary hippocampal neurons. In cortical cells, increased ATP-levels were notable under low H2O2-concentrations. A dose-dependent decrease in ATP-concentrations was observed after treatment with Ca2+, which was further enhanced by additional H2O2-challenge.

CONCLUSIONS

Our data underline that both, H2O2- and Ca2+-treatment, are able to disturb intracellular energy metabolism. Out of the different systems studied, the ATP-decrease is most pronounced in hippocampal primary neurons, suggesting that this mechanism contributes to the selective neuronal vulnerability to oxidative stress in these neurons.

Neural stem/progenitor cells participate in the regenerative response to perinatal hypoxia/ischemia

Perinatal hypoxia/ischemia (H/I) is the leading cause of neurologic injury resulting from birth complications. Recent advances in critical care have dramatically improved the survival rate of infants suffering this insult, but approximately 50% of survivors will develop neurologic sequelae such as cerebral palsy, epilepsy or cognitive deficits. Here we demonstrate that tripotential neural stem/progenitor cells (NSPs) participate in the regenerative response to perinatal H/I as their numbers increase 100% by 3 d and that they alter their intrinsic properties to divide using expansive symmetrical cell divisions. We further show that production of new striatal neurons follows the expansion of NSPs. Increased proliferation within the NSP niche occurs at 2 d after perinatal H/I, and the proliferating cells express nestin. Of those stem-cell related genes that change, the membrane receptors Notch1, gp-130, and the epidermal growth factor receptor, as well as the downstream transcription factor Hes5, which stimulate NSP proliferation and regulate stem cellness are induced before NSP expansion. The mechanisms for the reactive expansion of the NSPs reported here reveal potential therapeutic targets that could be exploited to amplify this response, thus enabling endogenous precursors to restore a normal pattern of brain development after perinatal H/I.

Acute hypoxia-ischemia results in hydrogen peroxide accumulation in neonatal but not adult mouse brain

The neonatal brain responds differently to hypoxic-ischemic injury and may be more vulnerable than the mature brain due to a greater susceptibility to oxidative stress. As a measure of oxidative stress, the immature brain should accumulate more hydrogen peroxide (H2O2) than the mature brain after a similar hypoxic-ischemic insult. To test this hypothesis, H2O2 accumulation was measured in postnatal day 7 (P7, neonatal) and P42 (adult) CD1 mouse brain regionally after inducing HI by carotid ligation followed by systemic hypoxia. H2O2 accumulation was quantified at 2, 12, 24, and 120 h after HI using the aminotriazole (AT)-mediated inhibition of catalase spectrophotometric method. Histologic injury was determined by an established scoring system, and infarction volume was determined. P7 and P42 animals were subjected to different durations of hypoxia to create a similar degree of brain injury. Despite similar injury, significantly less H2O2 accumulated in P42 mouse cortex compared with P7 at 2, 12, and 24 h after HI. In addition, less H2O2 accumulated in P42 mouse hippocampus compared with P7 hippocampus at 2 h. Since immature neurons are more vulnerable to the toxic effects of H2O2 than mature neurons, this increased accumulation in the immature brain may explain why the neonatal brain may be more devastated, even after a milder degree of acute hypoxic-ischemic injury.

Mammalian stem cells

Stem cells are quickly coming into focus of much biomedical research eventually aiming at the therapeutic applications for various disorders and trauma. It is important, however, to keep in mind the difference between the embryonic stem cells, somatic stem cells and somatic precursor cells when considering potential clinical applications. Here we provide the review of the current status of stem cell field and discuss the potential of therapeutic applications for blood and Immune system disorders, multiple sclerosis, hypoxic-ischemic brain injury and brain tumors. For the complimentary information about various stem cells and their properties we recommend consulting the National Institutes of Health stem cell resources (http://stemcells.nih.gov/info/basics).

Hypoxia/ischemia expands the regenerative capacity of progenitors in the perinatal subventricular zone

Neurons and oligodendrocyte progenitors are highly sensitive to perinatal hypoxic-ischemic injury. As accumulating evidence suggests that many insults to the human infant occur in utero, and preventing brain damage to infants in utero will prove difficult, there is strong rationale to pursue regenerative strategies to reduce the morbidity associated with developmental brain injuries. The purpose of this study was to determine whether a hypoxic-ischemic insult stimulates the neural stem/progenitor cells in the subventricular zone to generate new neurons and oligodendrocytes. Hypoxia-ischemia was induced using the Vannucci rat model on postnatal day-6 pups. Injections of 5'-bromo-2'-deoxyuridine to label cells undergoing DNA synthesis after hypoxia-ischemia revealed that there is a robust proliferative response within the subventricular zone of the injured hemisphere that continues for at least 1 week after the hypoxic-ischemic episode. Using the neurosphere assay to quantify the number of neural stem/progenitor cells in the subventricular zone, we find that there are twice as many neural stem/progenitor cells in the affected dorsolateral subventricular zone at 1 week of recovery and that these cells generate larger spheres in response to growth factors compared with controls. Precursors from the injured hemisphere generate three times as many neurons in vitro and more than twice as many oligodendroglia compared with controls. Hypoxia-ischemia also increases neurogenesis in vivo. Doublecortin positive cells with migratory profiles were observed streaming from the ipsilateral subventricular zone to the striatum and neocortex, whereas, few doublecortin positive cells were found in the contralateral hemisphere after hypoxia-ischemia. These observations provide evidence that the somatic neural progenitors of the subventricular zone participate in the production of new brain cells lost after hypoxia-ischemia.

The Physiologic Effects of Isoflurane Anesthesia in Neonatal Mice

In neonatal rodents, isoflurane has been shown to confer neurological protection during hypoxia-ischemia and to precipitate neurodegeneration after prolonged exposure. Whether neuroprotection or neurotoxicity result from a direct effect of isoflurane on the brain or an indirect effect through hemodynamic or metabolic changes remains unknown. We recorded arterial blood pressure, heart rate, blood gases, and glucose in 10-day-old mice during 60 min of isoflurane anesthesia with spontaneous or mechanical ventilation, as well as during 60 min of hypoxia-ischemia with isoflurane anesthesia or without anesthesia. During isoflurane anesthesia, hypoglycemia and metabolic acidosis occurred with spontaneous and mechanical ventilation. During hypoxia-ischemia, isoflurane was fatal with spontaneous breathing but survivable with mechanical ventilation, with arterial blood pressure and heart rate being similar to that observed in unanesthetized animals. Minimum alveolar concentration (MAC) was 2.3% in 10-day-old mice. In summary, isoflurane anesthesia precipitated hypoglycemia, which may have contributed to the neurodegeneration observed in neonatal rodents. Use of 0.8 MAC isoflurane for evaluation of neuroprotection during hypoxia-ischemia requires mechanical ventilation and glucose supplementation in this model.

Immature mouse unilateral carotid ligation model of stroke

Cerebral palsy in humans results from a diverse group of disorders that produce nonprogressive motor impairments in the developing brain. Stroke is an important cause of hemiparetic cerebral palsy in neonates and young children. We recently developed a new immature mouse model of stroke that demonstrates seizures, the severity of which correlates with brain injury. This model has strengths compared with other immature rodent models of ischemic injury, such as relative technical ease and the presence of seizures, as is seen in humans. This model also has relative weaknesses, such as the inability to titrate the severity of the injury with different periods of hypoxia. In addition, more work is needed to delineate the long-term consequences of the insult in this new model.

Strain variability, injury distribution, and seizure onset in a mouse model of stroke in the immature brain

Neonatal stroke is an important cause of neurologic morbidity and cerebral palsy. Recently, we have determined that in postnatal day 12 CD1 mice unilateral carotid ligation alone results in seizures and brain injury. We have shown that, in this model, seizure scores correlate with brain injury scores. We have applied this model to another strain of mice to assess strain-related differences in vulnerability to seizures and brain injury after unilateral carotid ligation. Under isoflurane anesthesia, unilateral right-sided carotid ligation was performed in postnatal day 12 C3HeB/FeJ mice followed by a 4-hour period of observation in a 35 degrees C incubator. Seizure scores and brain jury scores were assigned and compared to scores in mice receiving sham surgery. Timing of seizure onset and regional distribution of brain injury were compared in the CD1 and C3HeB/FeJ mice. Unilateral carotid ligation in postnatal day 12 C3HeB/FeJ mice resulted in seizure behavior and brain injury in some animals, with similar time to seizure onset and regional injury distribution, but affected a significantly smaller percentage of C3HeB/FeJ pups than that observed in postnatal day 12 CD1 mice, indicating strain-related vulnerability in this model.

Combined deficiency of IL-1beta18, but not IL-1alphabeta, reduces susceptibility to hypoxia-ischemia in the immature brain

Interleukin (IL)-1 and IL-18 belong to the IL-1 family. IL-18 deficiency has been shown to confer moderate protection after hypoxia-ischemia (HI) in the immature brain, while the contribution of the two isoforms of IL-1 (IL-1alpha and IL-1beta) in neonatal HI brain injury has not been investigated previously. The aim of this study was to examine the contribution of the different members of the IL-1 family to neonatal HI damage. Unilateral HI was induced at postnatal day 9 in IL-1beta, IL-1beta18, and IL-1alphabeta knockout and wild-type mice and brain injury was evaluated 1 week later. IL-1beta18-deficient mice showed 17% reduction in brain injury, while no significant reduction in injury was detected between any of the other groups. These results indicate that IL-18, but not IL-1beta, or the combination of IL-1alpha and IL-1beta, is a contributor to HI injury in the immature brain.

Overexpression of glutathione peroxidase protects immature murine neurons from oxidative stress

Neuronal enzyme systems involved in free radical detoxification are developmentally regulated such that intracellular glutathione peroxidase (GPx-1) activity is low in the newborn mouse brain. We hypothesized that neurons expressing a higher level of GPx-1 will be more resistant to hydrogen peroxide (H(2)O(2)) exposure. We show a dose-dependent protection against H(2)O(2) in primary neuronal cultures from fetuses overexpressing human GPx-1 compared to wild types of the same genetic background. Exogenous antioxidants completely protected neurons, even at extremely high H(2)O(2 )concentrations and regardless of the genotype. Specific depletion of glutathione with buthionine sulfoximine increased cell death in transgenic cultures exposed to 200 microM H(2)O(2), reducing protection afforded by increased GPx-1 activity. Increased GPx-1 expression in immature cortical neurons confers protection from oxidative stress, but availability of reducing equivalents determines susceptibility to oxidative cell death.

Hsp70 overexpression sequesters AIF and reduces neonatal hypoxic/ischemic brain injury

Apoptosis is implicated in neonatal hypoxic/ischemic (H/I) brain injury among various forms of cell death. Here we investigate whether overexpression of heat shock protein (Hsp) 70, an antiapoptotic protein, protects the neonatal brain from H/I injury and the pathways involved in the protection. Postnatal day 7 (P7) transgenic mice overexpressing rat Hsp70 (Tg) and their wild-type littermates (Wt) underwent unilateral common carotid artery ligation followed by 30 mins exposure to 8% O(2). Significant neuroprotection was observed in Tg versus Wt mice on both P12 and P21, correlating with a high level of constitutive but not inducible Hsp70 in the Tg. More prominent injury was observed in Wt and Tg mice on P21, suggesting its continuous evolution after P12. Western blot analysis showed that translocation of cytochrome c, but not the second mitochondria-derived activator of caspase (Smac)/DIABLO and apoptosis-inducing factor (AIF), from mitochondria into cytosol was significantly reduced in Tg 24 h after H/I compared with Wt mice. Coimmunoprecipitation detected more Hsp70 bound to AIF in Tg than Wt mice 24 h after H/I, inversely correlating with the amount of nuclear, but not cytosolic, AIF translocation. Our results suggest that interaction between Hsp70 and AIF might have reduced downstream events leading to cell death, including the reduction of nuclear AIF translocation in the neonatal brains of Hsp70 Tg mice after H/I.

Perinatal hypoxia/ischemia damages and depletes progenitors from the mouse subventricular zone

Hypoxia-ischemia (H/I) as a result of asphyxia at term remains a major cause of neurologic disability. Our previous studies in the P7 rat model of perinatal H/I have shown that progenitors within the subventricular zone (SVZ) are vulnerable to this insult. Since many investigators are using transgenic and knockout mice to determine the importance of specific molecules in the evolution of damage after a stroke, there is a need to perform comparative studies on the relative vulnerability of the mouse SVZ. Here we assess damage to the SVZ of 5-, 7- and 10-day-old C57BL/6 mice after unilateral common carotid artery cauterization followed by 70 min of H/I (10% O2). Whereas 5- and 7-day-old mice sustained little SVZ damage as assessed by hematoxylin and eosin staining, there was a 16% reduction of cellularity in 10-day-old animals by 18 h of recovery. Additionally, swollen cells were observed in the medial region of the SVZ of 10-day-old mice. However, few caspase-3+ and TUNEL+ cells were observed in this region, which contains the putative neural stem cells. Rather, the majority of the dying cells were situated in the mediolateral and lateral tail of the SVZ. At 18 h of recovery, there was a 2-fold increase in the frequency of TUNEL+ cells in the ipsilateral SVZ as well as a 3-fold increase in the frequency of active-caspase-3+ cells. We conclude that progenitors within the neonatal mouse SVZ are vulnerable to hypoxic/ischemic insult. The demise of these early progenitors likely leads to depletion of neuronal and late oligodendrocyte progenitors, contributing to cerebral dysgenesis.

Susceptibility to excitotoxic and metabolic striatal neurodegeneration in the mouse is genotype dependent

Previously, we had reported that hippocampal susceptibility to the neurotoxic effects of excitotoxin administration is strain dependent [Schauwecker and Steward, Proc. Natl. Acad. Sci. U.S.A. 94 (1997) 4103]. However, it has been unclear whether strain-related gene products may play a similar role in providing protection against drugs that produce striatal lesions. The present series of experiments sought to elucidate whether genetic background alters neuronal viability within the striatum following metabolic or excitotoxic injury. Thus, we have examined the effect of mouse strain on susceptibility to striatal injury using well-characterized animal models of Huntington's disease by examining whether C57BL/6 mice, previously identified as resistant to excitotoxin-induced hippocampal cell death, are resistant to quinolinate, malonate, and 3-nitropropionic acid (3-NP). Intrastriatal injection of either malonate or quinolinate and systemic administration of 3-NP resulted in significantly smaller striatal lesions in C57BL/6 mice as compared to FVB/N mice, previously identified as susceptible to hippocampal excitotoxic injury. The existence of an animal strain with decreased resistance to striatal lesions suggests that there are mediating factors involved in the preferential vulnerability of the striatum to neurotoxic lesioning. The identification of these factors could provide strategies for therapeutic intervention in Huntington's disease.

BrdU-positive cells in the neonatal mouse hippocampus following hypoxic-ischemic brain injury

Background

Mechanisms that affect recovery from fetal and neonatal hypoxic-ischemic (H-I) brain injury have not been fully elucidated. The incidence of intrapartum asphyxia is approximately 2.5%, but the occurrence of adverse clinical outcome is much lower. One of the factors which may account for this relatively good outcome is the process of neurogenesis, which has been described in adult animals. We used a neonatal mouse model to assess new cells in the hippocampus after H-I injury.

Results

Neonatal mice underwent permanent unilateral carotid ligation on the seventh postnatal day followed by exposure to 8% hypoxia for 75 minutes. The presence of new cells was determined by bromodeoxyuridine (BrdU) incorporation into cells with sacrifice of the animals at intervals. Brain sections were stained for BrdU in combination with neuronal, glial, endothelial and microglial stains.

We found a significant increase in BrdU-positive cells in the neonatal mouse hippocampus in the injured area compared to the non-injured area, most prominent in the dentate gyrus (DG) (154.5 ± 59.6 v. 92.9 ± 32.7 at 3 days after injury; 68.9 ± 23.4 v. 52.4 ± 17.1 at 35 days after injury, p < 0.0011). Among the cells which showed differentiation, those which were stained as either microglial or endothelial cells showed a peak increase at three days after the injury in the DG, injured versus non-injured side (30.5 ± 17.8 v. 2.7 ± 2.6, p < 0.0002). As in the adult animal, neurogenesis was significantly increased in the DG with injury (15.0 ± 4.6 v. 5.2 ± 1.6 at 35 days after injury, p < 0.0002), and this increase was subsequent to the appearance of the other dividing cells. Numbers of new oligodendrocytes were significantly higher in the DG on the non-injured side (7.0 ± 24.2 v. 0.1 ± 0.3, p < 0.0002), suggesting that oligodendrocyte synthesis was reduced in the injured hippocampus.

Conclusion

These findings demonstrate that the neonatal animal responds to brain injury with neurogenesis, much like the adult animal. In addition, H-I insult leads to more neurogenesis than hypoxia alone. This process may play a role in the recovery of the neonatal animal from H-I insult, and if so, enhancement of the process may improve recovery.

The influence of age on apoptotic and other mechanisms of cell death after cerebral hypoxia-ischemia

Unilateral hypoxia-ischemia (HI) was induced in C57/BL6 male mice on postnatal day (P) 5, 9, 21 and 60, corresponding developmentally to premature, term, juvenile and adult human brains, respectively. HI duration was adjusted to obtain a similar extent of brain injury at all ages. Apoptotic mechanisms (nuclear translocation of apoptosis-inducing factor, cytochrome c release and caspase-3 activation) were several-fold more pronounced in immature than in juvenile and adult brains. Necrosis-related calpain activation was similar at all ages. The CA1 subfield shifted from apoptosis-related neuronal death at P5 and P9 to necrosis-related calpain activation at P21 and P60. Oxidative stress (nitrotyrosine formation) was also similar at all ages. Autophagy, as judged by the autophagosome-related marker LC-3 II, was more pronounced in adult brains. To our knowledge, this is the first report demonstrating developmental regulation of AIF-mediated cell death as well as involvement of autophagy in a model of brain injury.

Metallothionein I and II mitigate age-dependent secondary brain injury

Both the immediate insult and delayed apoptosis contribute to functional deficits after brain injury. Secondary, delayed apoptotic death is more rapid in immature than in adult CNS neurons, suggesting the presence of age-dependent protective factors. To understand the molecular pathobiology of secondary injury in the context of brain development, we identified changes in expression of oxidative stress response genes during postnatal development and target deprivation-induced neurodegeneration. The antioxidants metallothionein I and II (MT I/II) were increased markedly in the thalamus of adult C57BL/6 mice compared to mice <15 days old. Target deprivation generates reactive oxygen species that mediate neuronal apoptosis in the central nervous system; thus the more rapid apoptosis observed in the immature brain might be due to lower levels of MT I/II. We tested this hypothesis by documenting neuronal loss after target-deprivation injury. MT I/II-deficient adult mice experienced greater thalamic neuron loss at 96 hr after cortical injury compared to that in controls (80 +/- 2% vs. 57 +/- 4%, P < 0.01), but not greater overall neuronal loss (84 +/- 4% vs. 79 +/- 3%, MT I/II-deficient vs. controls). Ten-day-old MT I/II-deficient mice, however, experienced both faster onset of secondary neuronal death (30 vs. 48 hr) and greater overall neuronal loss (88 +/- 2% vs. 69 +/- 4%, P = 0.02). MT I/II are thus inhibitors of age-dependent secondary brain injury, and the low levels of MT I/II in immature brains explains, in part, the enhanced susceptibility of the young brain to neuronal loss after injury. These findings have implications for the development of age-specific therapeutic strategies to enhance recovery after brain injury.

Hypoxia-ischemia in the immature brain

The immature brain has long been considered to be resistant to the damaging effects of hypoxia and hypoxia-ischemia (H/I). However, it is now appreciated that there are specific periods of increased vulnerability, which relate to the developmental stage at the time of the insult. Although much of our knowledge of the pathophysiology of cerebral H/I is based on extensive experimental studies in adult animal models, it is important to appreciate the major differences in the immature brain that impact on its response to, and recovery from, H/I. Normal maturation of the mammalian brain is characterized by periods of limitations in glucose transport capacity and increased use of alternative cerebral metabolic fuels such as lactate and ketone bodies, all of which are important during H/I and influence the development of energy failure. Cell death following H/I is mediated by glutamate excitotoxicity and oxidative stress, as well as other events that lead to delayed apoptotic death. The immature brain differs from the adult in its sensitivity to all of these processes. Finally, the ultimate outcome of H/I in the immature brain is determined by the impact on the ensuing cerebral maturation. A hypoxic-ischemic insult of insufficient severity to result in rapid cell death and infarction can lead to prolonged evolution of tissue damage.

Neonatal mice lacking functional Fas death receptors are resistant to hypoxic-ischemic brain injury

Neonatal hypoxia-ischemia (HI) upregulates Fas death receptor expression in the brain, and alterations in expression and activity of Fas signaling intermediates occur in neonatal brain injury. B6.MRL-Tnfrsf6(lpr) mice lacking functional Fas death receptors are protected from HI brain damage in cortex, striatum, and thalamus compared to wild-type mice. Expression of Fas death receptor and active caspases increase in the cortex after HI. In wild-type mice, the hippocampus is most severely injured, and the hippocampus is the only region not protected in the B6.MRL-Tnfrsf6(lpr) mice. The selective vulnerability of the hippocampus to injury correlates with (1) lower basal expression of [Fas-associated death-domain-like IL-1beta-converting enzyme]-inhibitory protein (FLIP), (2) increased degradation of spectrin to its 145 or 150 kDa breakdown product, and (3) a higher percentage of non-apoptotic cell death following neonatal HI. We conclude that Fas signaling via both extrinsic and intrinsic caspase cascades causes brain injury following neonatal HI in a region-dependent manner. Basal levels of endogenous decoy proteins may modulate the response to Fas death receptor signaling and provide a novel approach to understanding mechanisms of neonatal brain injury.

Chronic neonatal hypoxia leads to long term decreases in the volume and cell number of the rat cerebral cortex

Preterm birth results in significant neurodevelopmental disability. The neonatal rodent model of chronic sublethal hypoxia faithfully mimics the effect of preterm birth on the developing brain. We employed this model to test the hypothesis that the hypoxia that accompanies preterm birth results in inappropriate signaling of apoptotic mechanisms in developing brain. We performed cortical cell counts, determinations of neuronal size and Western analyses of the apoptosis related proteins, Bcl-2 and Bax, in rat pups who were raised in chronic hypoxia (FiO2 9.5%) beginning on postnatal day 3 (P3) and extending for either 10 (P13) or 30 (P33) days. A third group of animals was exposed to 30 days of hypoxia followed by an additional 30 days in a normoxic environment (P63) to assess the potential for recovery from the initial effects of hypoxia. Age matched control pups were raised in room air throughout the experimental time period. Assessment of cortical cell number revealed a 25% reduction (P < 0.01) in total cell number following 30 days of hypoxic rearing. Glia were significantly reduced by 34% and 41% after 10 and 30 days of hypoxia, respectively, while neuron numbers were only significantly reduced (14%) after 30 days of hypoxia. Animals exposed to a hypoxic environment for 30 days followed by 30 days in a normoxic environment revealed some recovery of glial cell numbers, but no significant recovery of neuronal cell numbers. Measurement of cell size at both P13 and P33 revealed that neurons of layer III were significantly smaller in cross-sectional area in hypoxic compared with control rats (P < 0.01). However, no significant difference was noted in neuronal size following 30 days of normoxic recovery. Western blot analyses of Bcl-2 and Bax protein levels demonstrated a ratio favorable to Bax at multiple time points during the period of hypoxic exposure. These data suggest that chronic exposure to hypoxia during the perinatal period alters the production and maintenance of glial and neuronal cells and that glia and neurons demonstrate differential patterns of vulnerability and recovery following subsequent periods of normoxic exposure. It is hypothesized that the mechanisms responsible for these alterations in cortical cell number may depend on the state of differentiation of the different cell types at the time of hypoxic exposure.

Global gene expression in the immature brain after hypoxia-ischemia

Ischemia induces a complex response of differentially expressed genes in the brain. In order to understand the specific mechanisms of injury in the developing brain, it is important to obtain information on global changes in the transcriptome after neonatal hypoxia-ischemia. In this study, oligonucleotide arrays were used to investigate genomic changes at 2, 8, 24, and 72 hours after neonatal hypoxia-ischemia, which was induced in 9-day-old mice by left carotid artery ligation followed by hypoxia (10% O2). In total, 343 genes were differentially expressed in cortex, hippocampus, thalamus, and striatum 2 to 72 hours after hypoxia-ischemia, when comparing ipsilateral with contralateral hemispheres and with controls, using the significance analysis for microarrays. A total of 283 genes were upregulated and 60 were downregulated, and 94% of the genes had not previously been shown after neonatal hypoxia-ischemia. Genes related to transcription factors and metabolism had mostly upregulated transcripts, whereas most downregulated genes belonged to the categories of ion and vesicular transport and signal transduction. Genes involved in transcription, stress, and apoptosis were induced early after the insult, and many new genes that may play important roles in the pathophysiology of neonatal hypoxia-ischemia were identified.

Mechanisms of hypoxic-ischemic injury in the term infant

The pathogenesis of hypoxic-ischemic brain injury in the term infant is multifactorial and complex. Over the past decade the investigative emphasis has turned to cellular and molecular mechanisms of injury, and it has been increasingly recognized that the neonatal brain differs vastly from the adult brain in terms of response to hypoxia-ischemia. This review will discuss the initiation and evolution of brain injury in the term neonate, and the inherent biochemical and physiologic qualities of the neonatal brain that make its response to hypoxia-ischemia unique. Attention will be given to specific areas of investigation including excitotoxicity, oxidative stress, and inflammation. The coalescence of these entities to a final common pathway of hypoxic-ischemic brain injury will be emphasized.

Differential vulnerability of immature murine neurons to oxygen-glucose deprivation

In vivo studies support selective neuronal vulnerability to hypoxia-ischemia (HI) in the developing brain. Since differences in intrinsic properties of neurons might be responsible, pure cultures containing immature neurons (6-8 days in vitro) isolated from mouse cortex and hippocampus, regions chosen for their marked vulnerability to oxidative stress, were studied under in vitro ischemic conditions-oxygen-glucose deprivation (OGD). Twenty-four hours of reoxygenation after 2.5 h of OGD induced significantly greater cell death in hippocampal than in cortical neurons (67.8% vs. 33.4%, P = 0.0068). The expression of neuronal nitric oxide synthase (nNOS) protein, production of nitric oxide (NO), and reactive oxygen species (ROS), as well as glutathione peroxidase (GPx) activity and intracellular levels of reduced glutathione (GSH), were measured as indicators of oxidative stress. Hippocampal neurons had markedly higher nNOS expression than cortical neurons by 24 h of reoxygenation, which coincided with an increase in NO production, and significantly greater ROS accumulation. GPx activity declined significantly in hippocampal but not in cortical neurons at 4 and 24 h after OGD. The decrease in GSH level in hippocampal neurons correlated with the decline of GPx activity. Our data suggest that developing hippocampal neurons are more sensitive to OGD than cortical neurons. This finding supports our in vivo studies showing that mouse hippocampus is more vulnerable than cortex after neonatal HI. An imbalance between excess prooxidant production (increased nNOS expression, and NO and ROS production) and insufficient antioxidant defenses created by reduced GPx activity and GSH levels may, in part, explain the higher susceptibility to OGD of immature hippocampal neurons.

A new model of stroke and ischemic seizures in the immature mouse

Ischemic brain injury from stroke is an important cause of disability in infants and children, but current experimental models for the disorder are complex. These preparations require occlusion of small intracerebral vessels or common carotid artery ligation combined with exposure to reduced levels of oxygen. Unilateral carotid artery ligation alone was sufficient to cause brain injury in more than 70% of 12-day-old CD1 mice. Using a blinded behavioral rating scale of seizure activity in mice, a direct, highly significant correlation between the severity of seizures over the 4-hour period after ligation and the severity of histologic brain injury 7 days later (Spearman's rho = 0.835, P < 0.001) was documented. This study presents the first model of stroke in immature mice produced by unilateral carotid artery ligation alone, and the first to demonstrate a clear correlation between acute ischemia-induced seizures and brain injury. This new model should be useful for examining the pathogenesis of stroke in the immature brain and the potential contribution of seizures to final outcome.

Neonatal hypoxic-ischemic injury increases forebrain subventricular zone neurogenesis in the mouse

Neurogenesis persists throughout life in the rodent subventricular zone (SVZ)-olfactory bulb pathway and increases in the adult after brain insults. The influence of neonatal injury on SVZ neural precursors is unknown. We examined the effects of hypoxia-ischemia (HI) on neonatal mouse SVZ cell proliferation and neurogenesis. Postnatal day 10 (P10) mice underwent right carotid artery ligation followed by 10% O2 exposure for 45 min. The SVZ area and hemispheric injury were quantified morphometrically 1-3 weeks later. Bromodeoxyuridine (BrdU) was used to label proliferating cells, and cell phenotypes of the progeny were identified by immunohistochemistry. HI significantly enlarged the ipsilateral SVZ at P18, P24, and P31, and increases in the SVZ area correlated directly with the degree of hemispheric damage. HI also stimulated cell proliferation and neurogenesis in the SVZ and peri-infarct striatum. Some newborn cells expressed a neuronal phenotype at P24, but not at P31, indicating that neurogenesis was short-lived. These results suggest that augmenting SVZ neuroblast recruitment and survival may improve neural repair after neonatal brain injury.

New oligodendrocytes are generated after neonatal hypoxic-ischemic brain injury in rodents

Neonatal hypoxic-ischemic (HI) white matter injury is a major contributor to chronic neurological dysfunction. Immature oligodendrocytes (OLGs) are highly vulnerable to HI injury. As little is known about in vivo OLG repair mechanisms in neonates, we studied whether new OLGs are generated after HI injury in P7 rats. Rats received daily BrdU injections at P12-14 or P21-22 and sacrificed at P14 to study the level of cell proliferation or at P35 to permit dividing OLG precursors to differentiate. In P14 HI-injured animals, the number of BrdU+ cells in the injured hemisphere is consistently greater than controls. At P35, sections were double-labeled for BrdU and markers for OLGs, astrocytes, and microglia. Double-labeled BrdU+/myelin basic protein+ and BrdU+/carbonic anhydrase+ OLGs are abundant in the injured striatum, corpus callosum, and the infarct core. Quantitative studies show four times as many OLGs are generated from P21-35 in HI corpora callosa than controls. Surprisingly, the infarct core contains many newly generated OLGs in addition to hypertrophied astrocytes and activated microglia. These glia and non-CNS cells may stimulate OLG progenitor proliferation or induce their migration. At P35, astrogliosis and microgliosis are dramatic ipsilaterally but only a few microglia and some astrocytes are BrdU+. This finding indicates microglial and astrocytic hyperplasia occurs shortly after HI but before the P21 BrdU injections. Although the neonatal brain undergoes massive cell death and atrophy the first week after injury, it retains the potential to generate new OLGs up to 4 weeks after injury within and surrounding the infarct.

Strain dependence of receptor regulation on chemical preconditioning in mice hippocampus

While one current focus for studying mechanisms of disease is investigation of transgenic mice confounding effects of the background strain often are neglected. We investigated mRNA expression of known markers of hypoxic tolerance by a semiquantitative RT-PCR (adenosine receptors (A1 and A3), nitric oxide synthases (eNOS and nNOS), APP production, progesterone receptor, and estrogen receptors alpha and beta) in CD-1, C3H, and B6 mice. We found differences in the baseline mRNA expression of adenosine A3 receptors in C3H mice and neuronal NOS in B6 mice as well as a distinct regulation of adenosine A3 receptors and estrogen receptor beta (no changes in C3H and B6 compared to upregulation in CD-1) on treatment of animals with a low dosage of 3-nitropropionate (20mg/kg body weight, i.p.). We conclude that the choice of background strain may confound interpretation of the effects of specific transgens in the study of the mechanisms of primary and induced hypoxic tolerance.

Brain injury and neurofunctional deficit in neonatal mice with hypoxic-ischemic encephalopathy

Birth asphyxia accounts for the majority of developmental motor and cognitive deficits. Studies were undertaken to develop a reproducible murine model of perinatal hypoxic-ischemic encephalopathy (HIE) which would permit both anatomic and neurofunctional quantification of injury. Short-term neurofunctional outcomes consisted of three developmental reflexes (righting, cliff aversion and geotaxis) assessed in 7-day-old mouse pups 24 h after unilateral carotid artery ligation followed by inhalation of 8% oxygen. Cerebral infarct volume was dependent on duration of hypoxia, being approximately 2.5-fold greater with longer (60 min) versus shorter (30 min) hypoxia exposure (P=0.001). All three sensorimotor neonatal reflexes assessed at 24 h after HIE injury correlated significantly with long-term neurofunction evaluated using a water-maze test of navigational learning and memory assessed 8 weeks later in the same animals. Cerebral atrophy, a delayed consequence of HIE in this model, also correlated strongly with water-maze performance (r=0.86, P=0.002). These data demonstrate for the first time that murine neonatal sensorimotor reflex performance can be rigorously quantified in the acute phase of perinatal HIE and has strong predictive value not only for anatomic extent of cerebral injury, but also for long-term neurofunctional outcome.

Neonatal Hypoxia-Ischemia Differentially Upregulates MAGUKs and Associated Proteins in PSD-93–Deficient Mouse Brain

Background and Purpose— Postsynaptic density (PSD)-93 and PSD-95 are the major membrane-associated guanylate kinases (MAGUKs) at excitatory synapses of the brain linking the N -methyl- d -aspartate receptor (NMDAR) with neuronal nitric oxide synthase (nNOS), which contributes to cell death after neonatal hypoxia-ischemia (HI). We investigated whether deletion of PSD-93 would dissociate the NMDAR from nNOS and be neuroprotective.

Methods— Postnatal day 7 wild-type (+/+), heterozygous (+/−), and homozygous (−/−) PSD-93 knockout mice were subjected to HI by permanent ligation of the right carotid artery, followed by exposure to 8% O 2 /92% N 2 for 1 hour. Brains were scored 5 days later for damage with cresyl violet and iron stains. Western blot and coimmunoprecipitation were used to determine the expression and association of the major PSD proteins.

Results— There was no significant difference between PSD-93 (−/−) and (+/+) mice in mortality or degree of brain injury. In the absence of PSD-93, PSD-95 still interacted with NR2B and nNOS. Under physiological conditions, PSD-95, nNOS, NR2A, and NR2B were unaltered in the (−/−) pups. However, at 24 hours after HI, protein expression of PSD-95, nNOS, and NR2A but not NR2B was markedly higher in the (−/−) than in the (+/+) pups. In (+/+) pups, HI resulted in decreased expression of NR2A but not NR2B in cortex and decreased NR2A and NR2B expression in hippocampus, but this reduction was not observed in (−/−) pups.

Conclusions— PSD-93 is not essential for baseline synaptic function but may participate in regulation of NMDAR-associated signaling pathways after HI injury. Deletion of PSD-93 alone does not provide neuroprotection after neonatal HI, possibly a result, in part, of upregulation of PSD-95. MAGUKs may substitute for one another, allowing normal NMDAR function in the postnatal period.

Quantitative analysis of perinatal rodent oligodendrocyte lineage progression and its correlation with human

The development of a rodent model in the perinatal rat or mouse that reproduces the principal features of human perinatal white matter injury (periventricular leukomalacia) has been hampered by uncertainty about the developmental window in the rodent that coincides temporally with cerebral white matter development in the premature infant. We recently determined oligodendrocyte (OL) lineage progression in human cerebral white matter and found that the late OL progenitor (preOL) predominates throughout the high-risk period for periventricular leukomalacia [J. Neurosci. 21(2001), 1302-1312]. Here, we determined in the perinatal rat and mouse when each species displays a distribution of OL stages that is similar to the premature human cerebral white matter. PreOLs are abundant in the rat and mouse at P2. By P7, extensive OL maturation occurs in both species and coincides with the onset of early myelination. PreOLs and immature OLs mature in the P2 white matter along a medial to lateral gradient. This may provide an explanation for regional variation in the susceptibility of perinatal white matter to injury. We propose that the sequence of OL lineage progression is a useful means to estimate developmental windows of white matter maturation in perinatal rodents that coincide with those of developing human cerebral white matter. These studies support that the vulnerable period for white matter injury in the rodent is centered around P2 and should decline thereafter, coincident with the onset of myelination.

Selective vulnerability of subplate neurons after early neonatal hypoxia-ischemia

Neonatal hypoxia-ischemia in the preterm human leads to selective injury to the subcortical developing white matter, which results in periventricular leukomalacia (PVL), a condition associated with abnormal neurodevelopment. Maturation-dependent vulnerability of late oligodendrocyte progenitors is thought to account for the cellular basis of this condition. A high frequency of cognitive and sensory deficits with decreasing gestational age suggests pervasive abnormalities of cortical development. In a neonatal rat model of hypoxic-ischemic injury that produces the characteristic pattern of subcortical injury associated with human PVL, selective subplate neuron death is seen. The premature subplate neuron death occurs after thalamic axons have reached their targets in cortex. Thus, as expected, thalamocortical connections form normally, including patterned connections to somatosensory cortex. However, deficits in motor function still occur, as in babies with PVL. Subplate neuron cell death in PVL provides another mechanism for abnormal neurodevelopment after neonatal hypoxia-ischemia.

Aggravated brain damage after hypoxic ischemia in immature adenosine A2A knockout mice

BACKGROUND AND PURPOSE

Cerebral hypoxic ischemia (HI) is an important cause of brain injury in the newborn infant. Adenosine is believed to protect against HI brain damage. However, the roles of the different adenosine receptors are unclear, particularly in young animals. We examined the role of adenosine A2A receptors (A2AR) using 7-day-old A2A knockout (A2AR(-/-)) mice in a model of HI.

METHODS

HI was induced in 7-day-old CD1 mice by exposure to 8% oxygen for 30 minutes after occlusion of the left common carotid artery. The resulting unilateral focal lesion was evaluated with the use of histopathological scoring and measurements of residual brain areas at 5 days, 3 weeks, and 3 months after HI. Behavioral evaluation of brain injury by locomotor activity, rotarod, and beam-walking test was made 3 weeks and 3 months after HI. Cortical cerebral blood flow, assessed by laser-Doppler flowmetry, and rectal temperature were measured during HI.

RESULTS

Reduction in cortical cerebral blood flow during HI and rectal temperature did not differ between wild-type (A2AR(+/+)) and knockout mice. In the A2AR(-/-) animals, brain injury was aggravated compared with wild-type mice. The A2AR(-/-) mice subjected to HI displayed increased forward locomotion and impaired rotarod performance in adulthood compared with A2AR(+/+) mice subjected to HI, whereas beam-walking performance was similarly defective in both groups.

CONCLUSIONS

These results suggest that, in contrast to the situation in adult animals, A2AR play an important protective role in neonatal HI brain injury.

Pathogenesis of hypoxic-ischemic cerebral injury in the term infant: current concepts

Multiple, biochemical cascades contribute to the pathogenesis of neonatal hypoxic-ischemic brain injury. This article summarizes experimental evidence that supports the role of excitatory amino acids, calcium, free radicals, nitric oxide, proinflammatory cytokines, and bioactive lipids. Specific vulnerabilities that distinguish the response of the immature brain from that of the mature brain are highlighted. These include increased susceptibility to excitotoxicity and free radical injury, greater tendency to apoptotic death, and heightened vulnerability of developing oligodendrocytes. Available supportive evidence from human studies is also included. Implications for clinical neuroprotective strategies are discussed.

Strain-dependent neurodevelopmental abnormalities in caspase-3-deficient mice

Targeted gene disruptions have revealed significant roles for caspase family members in the regulation of neuronal programmed cell death. Both caspase-3- and caspase-9-deficient mice exhibit a variably severe neurodevelopmental phenotype that may include marked ventricular zone expansion, exencephaly, and ectopic neuronal structures. Our previous studies of caspase-3- and caspase-9-deficient mice were performed using mice on mixed genetic backgrounds, raising the possibility that strain-specific generic factors influence the effects of caspase deficiency on nervous system development. To directly test this hypothesis. we backcrossed the caspase-3 mutation for 7-10 generations onto pure C57BL/6J and 129X1/SvJ genetic backgrounds. Caspase-3-deficient 129X1/SvJ mice were uniformly and severely affected. These mice died during the perinatal period and exhibited marked neural precursor cell expansion and exencephaly. In contrast, caspase-3-deficient C57BL/6J mice reached adulthood, were fertile and showed minimal brain pathology. Intercrosses of C57BL/6J and 129X1/SvJ mutants revealed that the vast majority of caspase-3-/- F1 mice displayed the severe 129X1/SvJ-"like" phenotype. These findings are consistent with an incompletely penetrant strain-dependent genetic modifier (or modifiers) that alters the neurodevelopmental consequences of caspase-3 deficiency. Since caspase-9- and Apaf-1-deficient mice also display variably severe developmental neuropathology, this strain-dependent modifier(s) may be involved in the activation of a caspase-independent death pathway; alternatively, strain-dependent compensatory caspase activation and/or its inhibition may influence the severity of the caspase-3-deficient neuronal phenotype.

MRI evaluation and functional assessment of brain injury after hypoxic ischemia in neonatal mice

BACKGROUND AND PURPOSE

Severe perinatal asphyxia is an important cause of brain injury in the newborn infant. We examined early events after hypoxic ischemia (HI) in the 7-day-old mouse brain by MRI and related them to long-term functional effects and histopathology in the same animals at 4 to 5 weeks of age.

METHODS

HI was induced in 7-day-old CD1 mice by exposure to 8% oxygen for 30 minutes after occlusion of the left common carotid artery. The resulting unilateral focal lesion was evaluated in vivo by MRI (T2 maps and apparent diffusion coefficient maps) at 3, 6, and 24 hours and 5 days after hypoxia. Locomotion and sensorimotor function were analyzed after 3 weeks. Four weeks after HI, the mice were killed, and cresyl violet-stained brain sections were examined morphologically.

RESULTS

A decrease in apparent diffusion coefficient values in cortex on the affected side was found at 3 hours after HI. T2 values were significantly increased after 6 hours and remained so for 5 days. Maximal size of the lesion was attained at 3 to 6 hours after HI and declined thereafter. Animals with MRI-detected lesions had decreased forward locomotion, performed worse than controls in the beam-walking test, and showed a unilateral hypotrophy in the cresyl violet-stained brain sections 4 weeks later.

CONCLUSIONS

The temporal progression of the damage after HI in 7-day-old mice differs from that of the adult brain as judged by MRI. The early lesions detected by MRI were related to functional impairments for these mice in near-adult life.

Delayed cell death in neonatal mouse hippocampus from hypoxia-ischemia is neither apoptotic nor necrotic

Hypoxic-ischemic (HI) injury in neonatal mice is associated with significant cell loss in hippocampus, striatum and deep layers of the cortex. The pattern of cell death in hippocampus after a moderate focal ischemic-global hypoxic insult is studied through morphologic changes in dying neurons at both the light and ultrastructural levels. Light microscopy at 24 h showed a number of injured neurons, as evidenced by dark, round, condensed nuclei, primarily in CA1 through CA3. Nuclei appeared punctate and cytoplasm vacuolated. Electron microscopy revealed that the punctate appearance of the nuclei corresponded to clumped chromatin. At 7 days after HI, injured neurons were shrunken and had a uniformly dark, angular appearance. While dying cells had an appearance consistent with apoptosis on light microscopy, cells were neither necrotic nor apoptotic at the ultrastructural level.

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.

Nitric oxide synthase activity and inhibition after neonatal hypoxia ischemia in the mouse brain

Despite the emergence of therapies for hypoxic-ischemic injury to the mature nervous system, there have been no proven efficacious therapies for the developing nervous system. Recent studies have shown that pharmacological blockade of neuronal nitric oxide synthase (nNOS) activity can ameliorate damage after ischemia in the mature rodent. We have previously shown that elimination of nNOS neurons, either by targeted disruption of the gene or by pharmacological depletion with intraparenchymal quisqualate, can decrease injury after hypoxia-ischemia. Using a simpler pharmacological approach, we studied the efficacy of a systemically administered NOS inhibitor, 7-nitroindazole, a relatively selective inhibitor of nNOS activity. Using multiple doses and concentrations administered after the insult, we found that there was only a trend for protection with higher doses of the drug. A significant decrease in NOS activity was seen at 18 h and 5 days in the cortex, and at 2 h and 18 h in the hippocampus after the hypoxia-ischemia. nNOS expression decreased and remained depressed for at least 18 h after the insult. When nNOS expression was normalized to MAP2 expression, a decrease was seen at 18 h in the cortex and at 2 and 18 h in the hippocampus. These data suggest that further inhibition of NOS activity at early timepoints may not provide substantial benefit. At 5 days after the insult, however, NOS activity and normalized nNOS expression returned to baseline or higher in the hippocampus, the region showing the most damage. These data suggest that delayed administration of nNOS inhibitor after hypoxic-ischemic injury might be beneficial.

Evidence disputing the importance of excitotoxicity in hippocampal neuron death after experimental traumatic brain injury

The hippocampus is selectively vulnerable to experimental traumatic brain injury (TBI). Beneficial effects of glutamate receptor antagonists and increased extracellular levels of glutamate have suggested that glutamate-mediated excitotoxicity may be responsible for this selective damage. In order to clarify this important issue, we applied a severe parasagittal fluid percussion injury (FPI) to strains of mice shown to be susceptible and resistant to kainic acid (KA)-induced excitotoxic hippocampal damage. Dystrophic neurons were present by 10 min after FPI in the hippocampi of both strains. Damaged hippocampal neurons were absent at 4 days and 7 days. Additionally, there was no significant difference (p = 1.00) in CA3 neuron survival between KA-susceptible and -resistant mice at 4 days. In conclusion, excitotoxicity does not significantly contribute to hippocampal neuron loss after FPI and, in contrast to classic studies of excitotoxicity in vivo, the pattern of hippocampal cell death after TBI is extremely acute.

Genetic susceptibility to renal ischemia reperfusion injury revealed in a murine model

BACKGROUND

The development of genetically engineered mice has led to increased use of mouse models to study renal ischemic reperfusion injury (IRI). We hypothesized that susceptibility to IRI could result from strain differences due to genetic factors.

METHODS

Our study compared recovery subsequent to renal IRI in NIH Swiss, C57BL/6, and BALB/c mice. Serum creatinine (SCr) and blood urea nitrogen (BUN) levels were evaluated postischemia. We also conducted reverse transcriptase-polymerase chain reaction (RT-PCR) analyses of renal cytokines and adhesion molecules postischemia.

RESULTS

At 48 hr postischemia, renal dysfunction in NIH Swiss mice was significantly reduced, compared with other groups (P<0.01). BUN measurements confirmed renal protection at 48 hr in the NIH Swiss group. RT-PCR analysis of mRNA postischemia demonstrated that, between strains, there was little difference in mRNA expression for renal cytokines and adhesion molecules.

CONCLUSIONS

NIH Swiss mice appear to be resistant in susceptibility to renal IRI. Early expression of pro-inflammatory genes was not associated with resistance to IRI, thus genetic factors could be important in outcome after renal IRI.