Developmental changes in the sensitivity of the neonatal rat brain to hypoxic/ischemic injury

Focal Ischemic Injury to the Early Neonatal Rat Brain Models Cognitive and Motor Deficits with Associated Histopathological Outcomes Relevant to Human Neonatal Brain Injury

Neonatal arterial ischemic stroke is one of the more severe birth complications. The injury can result in extensive neurological damage and is robustly associated with later diagnoses of cerebral palsy (CP). An important part of efforts to develop new therapies include the on-going refinement and understanding of animal models that capture relevant clinical features of neonatal brain injury leading to CP. The potent vasoconstrictor peptide, Endothelin-1 (ET-1), has previously been utilised in animal models to reduce local blood flow to levels that mimic ischemic stroke. Our previous work in this area has shown that it is an effective and technically simple approach for modelling ischemic injury at very early neonatal ages, resulting in stable deficits in motor function. Here, we aimed to extend this model to also examine the impact on cognitive function. We show that focal delivery of ET-1 to the cortex of Sprague Dawley rats on postnatal day 0 (P0) resulted in impaired learning in a touchscreen-based test of visual discrimination and correlated with important clinical features of CP including damage to large white matter structures.

Strain-Related Differences in Mouse Neonatal Hypoxia-Ischemia

Neonatal hypoxic-ischemic brain injury is commonly studied by means of the Vannucci procedure in mice or rats (unilateral common carotid artery occlusion followed by hypoxia). Previously, we modified the postnatal day 7 (P7) rat procedure for use in mice, and later demonstrated that genetic strain strongly influences the degree of brain injury in the P7 mouse model of hypoxia-ischemia (HI). Recently, the P9 or P10 mouse brain was recognized as the developmental equivalent of a term neonatal human brain, rather than P7. Consequently, the Vannucci procedure has again been modified, and a commonly used protocol employs 10% oxygen for 50 min in C57Bl/6 mice. Strain differences have yet to be described for the P9/P10 mouse model. In order to determine if the strain differences we previously reported in the P7 mouse model are present in the P9 model, we compared 2 commonly used strains, CD1 and C57Bl/6J, in both the P7 (carotid ligation [in this case, right] followed by exposure to 8% oxygen for 30 min) and P9 (carotid ligation [in this case left] followed by exposure to 10% oxygen) models of HI. Experiments using the P7 model were performed in 2001-2012 and those using the P9 model were performed in 2012-2016. Five to seven days after the HI procedure, mice were perfused with 4% paraformaldehyde, their brains were sectioned on a Vibratome (50 µm) and alternate sections were stained with Perl's iron stain or cresyl violet. Brain sections were examined microscopically and scored for the degree of injury. Since brains in the P7 group had been scored previously with a slightly different system, they were reanalyzed using our current scoring system which scores injury in 11 regions: the anterior, middle, and posterior cortex; the anterior, middle, and posterior striatum; CA1, CA2, CA3, and the dentate gyrus of the hippocampus and thalamus, on a scale from 0 (none) to 3 (cystic infarct) for a total score of 0-33. Brains in the P9 group were scored with the same system. Given the same insult, the P7 CD1 mice had greater injury than the C57Bl/6J mice, which agrees with our previous findings. The P9 CD1 mice also had greater injury than the C57Bl/6J mice. This study confirms that CD1 mice are more susceptible to injury than C57Bl/6J mice and that strain selection is important when using mouse models of HI.

Local Injection of Endothelin-1 in the Early Neonatal Rat Brain Models Ischemic Damage Associated with Motor Impairment and Diffuse Loss in Brain Volume

Cerebral palsy is an irreversible movement disorder resulting from cerebral damage sustained during prenatal or neonatal brain development. As survival outcomes for preterm injury improve, there is increasing need to model ischemic injury at earlier neonatal time-points to better understand the subsequent pathological consequences. Here we demonstrate a novel neonatal ischemic model using focal administration of the potent vasoconstrictor peptide, endothelin-1 (ET-1), in newborn rats. The functional and histopathological outcomes compare favourably to those reported following the widely used hypoxic ischemia (HI) model. These include a robust motor deficit sustained into adulthood and recapitulation of hallmark features of preterm human brain injury, including atrophy of subcortical white matter and periventricular fiber bundles. Compared to procedures involving carotid artery manipulation and periods of hypoxia, the ET-1 ischemia model represents a rapid and technically simplified model more amenable to larger cohorts and with the potential to direct the locus of ischemic damage to specific brain areas.

Models of hypoxia and ischemia-induced seizures

Despite greater understanding and improved management, seizures continue to be a major problem in childhood. Neonatal seizures are often refractory to conventional antiepileptic drugs, and can result in later life epilepsy and cognitive deficits, conditions for which there are no specific treatments. Hypoxic and/or ischemic encephalopathy (HIE) is the most common cause for neonatal seizures, and accounts for more than two-thirds of neonatal seizure cases. A better understanding of the cellular and molecular mechanisms is essential for identifying new therapeutic strategies that control the neonatal seizures and its cognitive consequences. This heavily relies on animal models that play a critical role in discovering novel mechanisms underlying both epileptogenesis and associated cognitive impairments. To date, a number of animal models have provided a tremendous amount of information regarding the pathophysiology of HIE-induced neonatal seizures. This review provides an overview on the most important features of the main animal models of HIE-induced seizures. In particular, we focus on the methodology of seizure induction and the characterizations of post-HIE injury consequences. These aspects of HIE-induced seizure models are discussed in the light of the suitability of these models in studying human HIE-induced seizures.

Sexual dimorphism and brain lateralization impact behavioral and histological outcomes following hypoxia-ischemia in P3 and P7 rats

Neonatal cerebral hypoxia-ischemia (HI) is a major cause of neurological disorders and the most common cause of death and permanent disability worldwide, affecting 1-2/1000 live term births and up to 60% of preterm births. The Levine-Rice is the main experimental HI model; however, critical variables such as the age of animals, sex and hemisphere damaged still receive little attention in experimental design. We here investigated the influence of sex and hemisphere injured on the functional outcomes and tissue damage following early (hypoxia-ischemia performed at postnatal day 3 (HIP3)) and late (hypoxia-ischemia performed at postnatalday 7 (HIP7)) HI injury in rats. Male and female 3- (P3) or 7-day-old (P7) Wistar rats had their right or left common carotid artery occluded and exposed to 8% O2 for 1.5h. Sham animals had their carotids exposed but not occluded nor submitted to the hypoxic atmosphere. Behavioral impairments were assessed in the open field arena, in the Morris water maze and in the inhibitory avoidance task; volumetric extent of tissue damage was assessed using cresyl violet staining at adult age, after completing behavioral assessment. The overall results demonstrate that: (1) HI performed at the two distinct ages cause different behavioral impairments and histological damage in adult rats (2) behavioral deficits following neonatal HIP3 and HIP7 are task-specific and dependent on sex and hemisphere injured (3) HIP7 animals presented the expected motor and cognitive deficits (4) HIP3 animals displayed discrete but significant cognitive impairments in the left hemisphere-injured females (5) HI brain injury and its consequences are determined by animal's sex and the damaged hemisphere, markedly in HIP3-injured animals.

Long term creatine monohydrate supplementation, following neonatal hypoxic ischemic insult, improves neuromuscular coordination and spatial learning in male albino mouse

Creatine is known to rescue animals following brain damage. Present study was designed to demonstrate the effect of long term (15 week) supplementation of 2% creatine monohydrate (Cr), following neonatal hypoxic ischemic insult, on learning and memory formation in male albino mouse. Albino mice pups were subjected to right common carotid artery ligation followed by 8% hypoxia for 25 minutes. Following weaning, animals were separated and grouped on the basis of dietry supplementation for 15 weeks followed by a battery of neurological tests including Morris water maze, open field and rota rod. It was observed that HI mice fed on 2% Cr for 15 weeks performed better than their littermates mice on normal rodent diet during water maze (learning and memory) and rotating rod (neuro-muscular coordination and balance) test while the results of open field test remained unaffected. It was also observed that Cr treated animals had a reduced brain infarct volume than untreated but this difference did not reached statistical significance. We have also observed an overall increase in body weight in Cr treated mice during the study. Over all our results are indicating that long term Cr supplementation is beneficial for male albino following hypoxic ischemic insult.

Behavioral and histological outcomes following neonatal HI injury in a preterm (P3) and term (P7) rodent model

Hypoxia-ischemia (HI) occurs when blood and/or oxygen delivery to the brain is compromised. HI injuries can occur in infants born prematurely (<37 weeks gestational age) or at very low birth weight (<1500 g), as well as in term infants with birth complications. In both preterm and term HI populations, brain injury is associated with subsequent behavioral deficits. Neonatal HI injury can be modeled in rodents (e.g., the Rice-Vannucci method, via cautery of right carotid followed by hypoxia). When this injury is induced early in life (between postnatal day (P)1-5), neuropathologies typical of human preterm HI are modeled. When injury is induced later (P7-12), neuropathologies typical of those seen in HI term infants are modeled. The current study sought to characterize the similarities/differences between outcomes following early (P3) and late (P7) HI injury in rats. Male rats with HI injury on P3 or P7, as well as sham controls, were tested on a variety of behavioral tasks in both juvenile and adult periods. Results showed that P7 HI rats displayed deficits on motor learning, rapid auditory processing (RAP), and other learning/memory tasks, as well as a reduction in volume in various neuroanatomical structures. P3 HI animals showed only transient deficits on RAP tasks in the juvenile period (but not in adulthood), yet robust deficits on a visual attention task in adulthood. P3 HI animals did not show any significant reductions in brain volume that we could detect. These data suggest that: (1) behavioral deficits following neonatal HI are task-specific depending on timing of injury; (2) P3 HI rats showed transient deficits on RAP tasks; (3) the more pervasive behavioral deficits seen following P7 HI injury were associated with substantial global tissue loss; and (4) persistent deficits in attention in P3 HI subjects might be linked to neural connectivity disturbances rather than a global loss of brain volume, given that no such pathology was found. These combined findings can be applied to our understanding of differing long-term outcomes following neonatal HI injury in premature versus term infants.

Quantification of cysteinyl S-nitrosylation by fluorescence in unbiased proteomic studies

Cysteinyl S-nitrosylation has emerged as an important post-translational modification affecting protein function in health and disease. Great emphasis has been placed on global, unbiased quantification of S-nitrosylated proteins because of physiologic and oxidative stimuli. However, current strategies have been hampered by sample loss and altered protein electrophoretic mobility. Here, we describe a novel quantitative approach that uses accurate, sensitive fluorescence modification of cysteine S-nitrosylation that leaves electrophoretic mobility unaffected (SNOFlo) and introduce unique concepts for measuring changes in S-nitrosylation status relative to protein abundance. Its efficacy in defining the functional S-nitrosoproteome is demonstrated in two diverse biological applications: an in vivo rat hypoxia-ischemia/reperfusion model and antimicrobial S-nitrosoglutathione-driven transnitrosylation of an enteric microbial pathogen. The suitability of this approach for investigating endogenous S-nitrosylation is further demonstrated using Ingenuity Pathways analysis that identified nervous system and cellular development networks as the top two networks. Functional analysis of differentially S-nitrosylated proteins indicated their involvement in apoptosis, branching morphogenesis of axons, cortical neurons, and sympathetic neurites, neurogenesis, and calcium signaling. Major abundance changes were also observed for fibrillar proteins known to be stress-responsive in neurons and glia. Thus, both examples demonstrate the technique's power in confirming the widespread involvement of S-nitrosylation in hypoxia-ischemia/reperfusion injury and in antimicrobial host responses.

Short- and long-term behavioral effects of exposure to 21%, 40% and 100% oxygen after perinatal hypoxia-ischemia in the rat

Until recently, supplementation with 100% oxygen was standard therapy for newborns who required resuscitation at birth or suffered later hypoxic-ischemic events. Exposure to high concentrations of oxygen, however, may worsen oxidative stress induced by ischemic injury. In this study we investigated the short- and long-term behavioral outcomes in rats that had undergone hypoxic-ischemic brain injury on postnatal day 7, followed by 2h exposure to 21%, 40%, or 100% oxygen, compared to normal controls. There were no differences in the development of walking, head lifting and righting reflexes from postnatal days 9 to 15. Cliff avoidance showed some abnormal responses in the H21 animals. From postnatal days 28 to 56, three tests of sensorimotor coordination were performed weekly: ledged tapered beam, cylinder, and bilateral tactile stimulation. The ledged tapered beam test without prior training of animals was sensitive to injury, but did not distinguish between treatment groups. The cylinder test showed a greater use of the unimpaired limb in female 21% and 40% oxygen groups compared to controls. Performance in both cylinder and the beam tests showed a correlation with the degree of brain injury. The bilateral tactile stimulation test showed that the male 21% oxygen groups had worse sensory asymmetry than male 40% or 100% oxygen groups, but was not statistically significantly different from controls. We thus found a minor benefit to post-hypoxia-ischemic treatment with 100% and 40% oxygen compared to 21% in one test of early motor skills. Our results for long-term sensorimotor behavior, however, showed conflicting results, however, as males treated with 40% or 100% oxygen had less sensory asymmetry (better performance) in the bilateral tactile stimulation test than males treated with 21% oxygen, while females had impaired motor performance in the cylinder test with both 21% and 40% oxygen.

Oxygen resuscitation does not ameliorate neonatal hypoxia/ischemia-induced cerebral edema

Neonatal hypoxia/ischemia (HI) is a common cause of cognitive and behavioral deficits in children with hyperoxia treatment (HHI) being the current therapy for newborn resuscitation. HI induces cerebral edema that is associated with poor neurological outcomes. Our objective was to characterize cerebral edema after HI and determine the consequences of HHI (40% or 100% O(2)). Dry weight analyses showed cerebral edema 1 to 21 days after HI in the ipsilateral cortex; and 3 to 21 days after HI in the contralateral cortex. Furthermore, HI increased blood-brain barrier (BBB) permeability 1 to 7 days after HI, leading to bilateral cortical vasogenic edema. HHI failed to prevent HI-induced increase in BBB permeability and edema development. At the molecular level, HI increased ipsilateral, but not contralateral, AQP4 cortical levels at 3 and up to 21 days after HI. HHI treatment did not further affect HI-induced changes in AQP4. In addition, we observed developmental increases of AQP4 accompanied by significant reduction in water content and increase permeability of the BBB. Our results suggest that the ipsilateral HI-induced increase in AQP4 may be beneficial and that its absence in the contralateral cortex may account for edema formation after HI. Finally, we showed that HI induced impaired motor coordination 21 days after the insult and HHI did not ameliorate this behavioral outcome. We conclude that HHI treatment is effective as a resuscitating therapy, but does not ameliorate HI-induced cerebral edema and impaired motor coordination.

CD133+ cells from human umbilical cord blood reduce cortical damage and promote axonal growth in neonatal rat organ co-cultures exposed to hypoxia

To evaluate the effect of CD133(+) cells (endothelial progenitor cells) on the hypoxia-induced suppression of axonal growth of cortical neurons and the destruction of blood vessels (endothelial cells), we used anterograde axonal tracing and immunofluorescence in organ co-cultures of the cortex and the spinal cord from 3-day-old neonatal rats. CD133(+) cells prepared from human umbilical cord blood were added to the organ co-cultures after hypoxic insult, and axonal growth, vascular damage and apoptosis were evaluated. Anterograde axonal tracing with 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate was used to analyze axonal projections from the cortex to the spinal cord. Immunolabeling co-cultured tissues of the cortex and the spinal cord were used to investigate the effect of CD133(+) cells on the survival of blood vessels and apoptosis in the brain cortex. Hypoxia remarkably suppressed axonal growth in organ co-cultures of the cortex and the spinal cord, and this suppression was significantly restored by the addition of CD133(+) cells. CD133(+) cells also reduced the hypoxia-induced destruction of the cortical blood vessels and apoptosis. CD133(+) cells had protective effects on hypoxia-induced injury of neurons and blood vessels of the brain cortex in vitro. These results suggest that CD133(+) cell transplantation may be a possible therapeutic intervention for perinatal hypoxia-induced brain injury.

Normobaric hyperoximia increases hypoxia-induced cerebral injury: DTI study in rats

Perinatal hypoxia affects normal neurological development and can lead to motor, behavioral and cognitive deficits. A common acute treatment for perinatal hypoxia is oxygen resuscitation (hyperoximia), a controversial treatment. Magnetic resonance imaging (MRI), including diffusion tensor imaging (DTI), was performed in a P7 rat model of perinatal hypoxia to determine the effect of hyperoximia. These studies were performed on two groups of animals: 1) animals which were subjected to ischemia followed by hypoxia (HI), and 2) HI followed by hyperoximic treatment (HHI). Lesion volumes on high resolution MRI and DTI derived measures, fractional anisotropy (FA), mean diffusivity (MD), and axial and radial diffusivities (lambda(l) and lambda(t), respectively) were measured in vivo one day, one week, and three weeks after injury. Most significant differences in the MRI and DTI measures were found at three weeks after injury. Specifically, three weeks after HHI injury resulted in significantly larger hyperintense lesion volumes (95.26 +/- 50.42 mm(3)) compared to HI (22.25 +/- 17.62 mm(3)). The radial diffusivity lambda(t) of the genu of corpus callosum was significantly larger in HHI (681 +/- 330 x 10(-6) mm(2)/sec) than in HI (486 +/- 96 x 10(-6) mm(2)/sec). Over all, most significant differences in all the DTI metrics (FA, MD, lambda(t), lambda(l)) at all time points were found in the corpus callosum. Our results suggest that treatment of perinatal hypoxia with normobaric oxygen does not ameliorate, but exacerbates damage.

Brain development and susceptibility to damage; ion levels and movements

Responses of immature brains to physiological and pathological stimuli often differ from those in the adult. Because CNS function critically depends on ion movements, this chapter evaluates ion levels and gradients during ontogeny and their alterations in response to adverse conditions. Total brain Na(+) and Cl(-) content decreases during development, but K(+) content rises, reflecting shrinkage of the extracellular and increase in the intracellular water spaces and a reduction in total brain water volume. Unexpectedly, [K(+)](i) seems to fall during the first postnatal week, which should reduce [K(+)](i)/ [K(+)](e) and result in a lower V(m), consistent with experimental observations. Neuronal [Cl(-)](i) is high during early postnatal development, hence the opening of Cl(-) conduction pathways may lead to plasma membrane depolarization. Equivalent loss of K(+)(i) into a relatively large extracellular space leads to a smaller increase in [K(+)](e) in immature animals, while the larger reservoir of Ca(2+)(e) may result in a greater [Ca(2+)](i) rise. In vivo and in vitro studies show that compared with adult, developing brains are more resistant to hypoxic/ischemic ion leakage: increases in [K(+)](e) and decreases in [Ca(2+)](e) are slower and smaller, consistent with the known low level of energy utilization and better maintenance of [ATP]. Severe hypoxia/ischemia may, however, lead to large Ca(2+)(i) overload. Rises in [K(+)](e) during epileptogenesis in vivo are smaller and take longer to manifest themselves in immature brains, although the rate of K(+) clearance is slower. By contrast, in vitro studies suggest the existence of a period of enhanced vulnerability sometime during the developmental period. This chapter concludes that there is a great need for more information on ion changes during ontogeny and poses the question whether the rat is the most appropriate model for investigation of mechanisms of pathological changes in human neonates.

Neuroprotective effect of memantine combined with topiramate in hypoxic-ischemic brain injury

Glutamate receptor-mediated neurotoxicity is a major mechanism contributing to hypoxic-ischemic brain injury (HIBI). Memantine is a safe non-competitive NMDA receptor blocker characterized by its low affinity and fast unblocking kinetics. Topiramate is an AMPA/KA receptor blocker and use-dependent sodium channel blocker with several other neuroprotective actions and little neurotoxicity. We hypothesized that the coadministration of memantine and topiramate would be highly effective to attenuate HIBI in neonatal rats. Seven-day-old Sprague-Dawley rat pups were subjected to right common carotid artery ligation and hypoxia for 2 h, and then were randomly and blindly assigned to one of four groups: vehicle, memantine, topiramate and combination group. Brain injury was evaluated by gross damage and weight deficit of the right hemisphere at 22d after hypoxic-ischemia (HI) and by neurofunctional assessment (foot-fault test) at 21d post-HI. Acute neuronal injury was also evaluated by microscopic damage grading at 72 h post-HI. Results showed the combination of memantine and topiramate improved both pathological outcome and performance significantly. The drug-induced apoptotic neurodegeneration was assessed by TUNEL staining at 48 h post-HI and the result showed no elevated apoptosis in all observed areas. The result of the experiment indicates the combination therapy is safe and highly effective to reduce brain damage after HIBI.

Developmental shift of cyclophilin D contribution to hypoxic-ischemic brain injury

Cyclophilin D (CypD), a regulator of the mitochondrial membrane permeability transition pore (PTP), enhances Ca(2+)-induced mitochondrial permeabilization and cell death in the brain. However, the role of CypD in hypoxic-ischemic (HI) brain injury at different developmental ages is unknown. At postnatal day (P) 9 or P60, littermates of CypD-deficient [knock-out (KO)], wild-type (WT), and heterozygous mice were subjected to HI, and brain injury was evaluated 7 d after HI. CypD deficiency resulted in a significant reduction of HI brain injury at P60 but worsened injury at P9. After HI, caspase-dependent and -independent cell death pathways were more induced in P9 CypD KO mice than in WT controls, and apoptotic activation was minimal at P60. The PTP had a considerably higher induction threshold and lower sensitivity to cyclosporin A in neonatal versus adult mice. On the contrary, Bax inhibition markedly reduced caspase activation and brain injury in immature mice but was ineffective in the adult brain. Our findings suggest that CypD/PTP is critical for the development of brain injury in the adult, whereas Bax-dependent mechanisms prevail in the immature brain. The role of CypD in HI shifts from a predominantly prosurvival protein in the immature to a cell death mediator in the adult brain.

Postnatal age influences hypoglycemia-induced neuronal injury in the rat brain

Acute hypoglycemia is associated with neuronal injury in the mature human and rodent brains. Even though hypoglycemia is a common metabolic problem during development, its effects on the developing brain are not well understood. To characterize the severity of regional brain injury, postnatal day (P) 7, P14, P28 (N=20-30/age) and adult rats (N=8-12) were subjected to acute hypoglycemia of equivalent severity and duration (mean blood glucose concentration: 30.0+/-0.1 mg/dL for 210 min). Neuronal injury in the cerebral cortex, striatum, hippocampus and hypothalamus was assessed 24 h, 72 h and 1 wk later by determining the number of degenerating cells positive for Fluoro-Jade B (FJB+) in the region. Compared with age-matched control, greater number of FJB+ cells was present per brain section of P14, P28 and adult hypoglycemia groups (p<0.005, each). The cerebral cortex was more vulnerable than hippocampus and striatum at all three ages (p<0.01). Compared with P28 (131+/-21) and adult (171+/-21) rats, fewer FJB+ cells (39+/-6) per brain section were present in P14 hypoglycemic rats (p<0.01, each). Hypoglycemia was not associated with cell injury in P7 rats. FJB+ cells were absent in the hypothalamus in all four ages. Similar results were present 24 h post-hypoglycemia, whereas analysis at 1 wk demonstrated efficient clearing of FJB+ cells in the brain regions of developing rats. Varying the duration of fasting did not alter the severity of regional cell injury. These results suggest that postnatal age influences the regional vulnerability to hypoglycemia-induced neuronal death in the rat brain.

Long-term histological outcome after post-hypoxic treatment with 100% or 40% oxygen in a model of perinatal hypoxic-ischemic brain injury

Hypoxic newborns have traditionally been given supplemental oxygen, and until recently, guidelines for neonatal resuscitation recommended that 100% oxygen be used. Exposure to 100% oxygen after hypoxic injury, however, may exacerbate oxidative stress. The current study evaluated the effect of exposure to 100, 40 or 21% oxygen after neonatal hypoxic-ischemic injury on the severity of brain injury after long-term survival. The severity of histological brain injury was not different in animals exposed to 100% oxygen versus room air. Male animals treated with 40% oxygen post-hypoxia had the lowest mean total histology scores, but this was not statistically significant due to the large variation in injury within each treatment group. These results support the growing number of studies in human infants and experimental animals that show no benefit of 100% oxygen over room air for neonatal resuscitation. Our results suggest that post-hypoxia treatment with 40% oxygen may be beneficial, particularly in males. Further studies of the effects of different concentrations of oxygen on brain injury are warranted and should have sufficient power to examine sex differences.

Neuroprotective effect of nipradilol, an NO donor, on hypoxic-ischemic brain injury of neonatal rats

Hypoxia-ischemia is a common cause of neonatal brain injuries. Nitric oxide (NO) is upregulated in the brain after hypoxia-ischemia and generally believed to exert a paradoxical effect on neurons, neurodestruction and neuroprotection, but it has not been demonstrated that NO is actually neuroprotective in neonatal hypoxic-ischemic encephalopathy. We evaluated the effect of intracerebroventricular administration of nipradilol (3,4-dihydro-8-(2-hydroxy-3-isopropylamino)-propoxy-3-nitroxy-2H-1-benzopyran), a potent NO donor, at various concentrations (0.1 muM to 1 mM in 5 mul PBS/brain) to neonatal rats with hypoxic-ischemic treatment. The extent of the infarct area in the brain was significantly reduced by injection of the 1 muM nipradilol solution. However, denitro-nipradilol (3,4-dihydro-8-(2-hydroxy-3-isopropylamino)-propoxy-3-hydroxy-2H-1-benzopyran), that does not release NO, did not show the neuroprotective effect, suggesting that NO released from nipradilol exerts a neuroprotective effect on neonatal neurons.

Impaired expression of postsynaptic density proteins in the hippocampal CA1 region of rats following perinatal hypoxia

Perinatal hypoxia is an important cause of brain injury amongst the newborn, such injury often resulting in an increased risk of impaired performance as regards learning and memory in later life for the affected individual. The postsynaptic density 95 (PSD-95) protein is a cytoskeletal specialization involved in the anchoring of N-methyl-d-aspartate (NMDA) receptors in postsynaptic neurons and has been reported to serve several important functions (e.g., synaptogenesis, synaptic plasticity and learning and memory performance) for the mammalian brain. Herein we investigated the long-term effects of perinatal hypoxia upon the complex of PSD-95 with NMDAR subunits by means of downstream signalling cAMP response element binding protein (CREB) phosphorylation at the Serine-133 locus (CREB(Ser-133) phosphorylation) within the hippocampal CA1 area (an essential integration area for mammalian learning and memory) within test-rat brains, as well as the effects upon afflicted-individual long-term learning and memory performance. We also assessed the therapeutic efficacy of dopamine D1/D5 receptor (D1/D5R) activation for such study animals. Perinatal hypoxia on postnatal day ten (P10) led to impaired performance as regards long-term spatial learning and memory (as determined on P45) associated with decreases in the level of CREB(Ser-133) phosphorylation and decreases in the expression of the complex of PSD-95 with NMDAR subunits (NR1, NR2A, and NR2B). In addition, activation of the D1/D5R via A68930 (a selective, CNS-permeable agonist of D1/D5Rs) administration (2 mg/kg/day, P17-23 inclusively) markedly attenuated the hypoxia-induced deleterious effects, suggesting an effective therapeutic efficacy for A68930. Our results demonstrate the long-term effects of perinatal hypoxia upon the developing brain and provide additional insights into the relative vulnerability of postsynaptic density (PSD) proteins to such insult, as well as the impairment of downstream transcription signalling CREB(Ser-133) phosphorylation following perinatal hypoxia. More importantly, D1/D5R activation following perinatal hypoxia may be an alternative therapeutic strategy to that which is currently available and may offer significant clinical potential for hypoxia sufferers.

Gender-specific neuroprotection by 2-iminobiotin after hypoxia-ischemia in the neonatal rat via a nitric oxide independent pathway

We have shown earlier that 2-iminobiotin (2-IB) reduces hypoxia-ischemia (HI)-induced brain damage in neonatal rats, and presumed that inhibition of nitric oxide synthases (NOS) was the underlying mechanism. We now investigated the effect of 2-IB treatment in P7 rat pups to determine the role of gender and the neuroprotective mechanism. Pups were subjected to HI (occlusion of right carotid artery and 120 mins FiO(2) 0.08) and received subcutaneous (s.c.) 10 mg/kg 2-IB at 0, 12 and 24 h after hypoxia. After 6 weeks, neuronal damage was assessed histologically. We determined cerebral nitrite and nitrate (NO(x)) and nitrotyrosine, heat-shock protein 70, cytosolic cytochrome c, cleaved caspase 3, nuclear translocation of apoptosis-inducing factor (AIF) and the effect of 2-IB on NOS activity in cultured cells. 2-Iminobiotin treatment reduced long-term brain damage in female but not male rats. Unexpectedly, 2-IB treatment did not reduce cerebral NO(x) or nitrotyrosine levels, and did not inhibit NOS activity in vitro. The gender-dependent neuroprotective effect of 2-IB was reflected in inhibition of the HI-induced increase in cytosolic cytochrome c and cleaved caspase 3 in females only. Hypoxia-ischemia-induced activation of AIF was observed in males only and was not affected by 2-IB. Post-HI treatment with 2-IB provides gender-specific long- and short-term neuroprotection in female P7 rats via inhibition of the cytochrome c-caspase 3 neuronal death pathway. 2-Iminobiotin did not alter cerebral NO(x) nor inhibited NOS in intact cells. Therefore, we conclude that it is highly unlikely that the neuroprotective effect of 2-IB involves NOS inhibition.

Developmental regulation of alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptor subunit expression in forebrain and relationship to regional susceptibility to hypoxic/ischemic injury. I. Rodent cerebral white matter and cortex

This is the first part of a two-part study to investigate the cellular distribution and temporal regulation of alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptor (AMPAR) subunits in the developing white matter and cortex in rat (part I) and human (part II). Western blot and immunocytochemistry were used to evaluate the differential expression of AMPAR subunits on glial and neuronal subtypes during the first 3 postnatal weeks in the Long Evans and Sprague Dawley rat strains. In Long Evans rats during the first postnatal week, GluR2-lacking AMPARs were expressed predominantly on white matter cells, including radial glia, premyelinating oligodendrocytes, and subplate neurons, whereas, during the second postnatal week, these AMPARs were highly expressed on cortical neurons, coincident with decreased expression on white matter cells. Immunocytochemical analysis revealed that cell-specific developmental changes in AMPAR expression occurred 2-3 days earlier by chronological age in Sprague Dawley rats compared with Long Evans rats, despite overall similar temporal sequencing. In both white and gray matter, the periods of high GluR2 deficiency correspond to those of regional susceptibility to hypoxic/ischemic injury in each of the two rat strains, supporting prior studies suggesting a critical role for Ca2+-permeable AMPARs in excitotoxic cellular injury and epileptogenesis. The developmental regulation of these receptor subunits strongly suggests that Ca2+ influx through GluR2-lacking AMPARs may play an important role in neuronal and glial development and injury in the immature brain. Moreover, as demonstrated in part II, there are striking similarities between rat and human in the regional and temporal maturational regulation of neuronal and glial AMPAR expression.

Brain Injury from Cardiac Arrest in Children

This article reviews the important differences between children and adults suffering brain injury following cardiac arrest. The differences in etiology, pathophysiology, neuronal vulnerability, and repair in the context of the developing brain are reviewed. The available clinical data are reviewed, and selected treatment priori-ties are declared. The article includes a discussion of knowledge gaps and future directions.

Inflammatory responses of the substantia nigra after acute hypoxia in neonatal rats

The neocortex and the striatum are the brain regions most known to be particularly vulnerable to acute insults like hypoxia or ischemia. In this work, we assess the possibility of cellular damage to the substantia nigra (SN) after hypoxia-reoxygenation in the new born rat. The aim of the present paper was to evaluate the expression of growth factor IGF-I, and growth factor binding proteins IGFBP-3 and IGFBP-5 genes and induction of NOS family members (nNOS, eNOS and iNOS) and TNF-alpha genes together with glia activation, in the SN at 5 and 48 h after severe hypoxia in the 7 day-old rat, a model for the term human fetus. At early time, while IGFs remain unchanged, we found a transient increase in eNOS and nNOS. Two days after the injury, nNOS expression remained high, iNOS and TNF-alpha increased and also GFAP protein expression was observed together with a profusion of reactive astrocytes distributed throughout the SN. This study on the acute effects of hypoxia on the developing brain provides additional insights into the vulnerability of the SN, a brain region involved in neurodegenerative pathologies.

Biphasic changes in the levels of poly(ADP-ribose) polymerase-1 and caspase 3 in the immature brain following hypoxia-ischemia

Poly(ADP-ribose) polymerase-1 (PARP-1) is a DNA repair-associated enzyme that has multiple roles in cell death. This study examined the involvement of PARP-1 in ischemic brain injury in the 7-day old rat, 0.5-48 h after unilateral carotid artery ligation and 2 h of 7.8% oxygen. This experimental paradigm produced a mild to moderate injury; 40-67% of animals in the ligated groups had histological evidence of neuronal death. Ipsilateral cortical injury was seen at all survival times, while mild contralateral cortical injury was seen only at the 1h survival time. Hippocampal injury was delayed relative to the cortex and did not show a biphasic pattern. Immunohistochemical staining for PARP showed bilateral increased staining as early as 1 h post-hypoxia. PARP staining at early time periods was most intense in layer V of cortex, but did not demonstrate a pattern of cell clusters or columns. Ipsilateral PARP-1 levels quantified by western blotting showed a biphasic pattern of elevation with peaks at 0.5 and 12 h post-hypoxia. Contralateral PARP-1 levels were also elevated at 0.5 and 24 h. PARP activity as determined by immunoreactivity for poly(ADP-ribose) (PAR) was increased ipsilaterally at 0.5, 2 and 12 h survival times. Cortical caspase 3-activity was increased ipsilaterally at 6, 12, and 24 h and contralaterally at 0.5, 1, 2 and 6 h post-hypoxia. There are three main findings in this study. First, changes in the distribution and amount of cell death correlate well with measured PARP-1 levels after hypoxia-ischemia, and both display biphasic characteristics. Second, there are significant early, transient morphological and biochemical changes in the contralateral cortex after neonatal hypoxia-ischemia due to unilateral permanent occlusion of a carotid artery followed by 2 h of systemic hypoxia. Third, variability in the responses of individual pups to hypoxia-ischemia suggests the presence of unidentified confounding factors.

Differential effects of hypoxia-ischemia on phosphorylation of the N-methyl-D-aspartate receptor in one- and three-week-old rats

The effects of transient cerebral hypoxia-ischemia (HI) on phosphorylation of the NR1 subunit of the N-methyl-D-aspartate (NMDA) receptor were investigated in 7 (P7)- and 21 (P21)-day-old rats. Unilateral HI was induced by ligation of the right common carotid artery and exposure to 8% O(2)/92% N(2) for 120 (P7) or 90 (P21) min. Phosphorylation by protein kinase A (PKA; S897) and PKC (S896 and S890) was depressed in the ipsilateral hemisphere relative to both naïve controls and the contralateral hemisphere immediately following HI at both ages. At P7, but not P21, reperfusion resulted in an initial recovery to control phosphorylation levels at all 3 sites followed by a secondary decline. At both ages, pS896 was less than control values after 24 h of recovery, whereas pS890 had returned to control levels by this time. pS897 recovered to control levels by 24 h in P21 animals but not in P7 animals. Differential effects of HI on phosphorylation of the NMDA receptor at P7 and P21 may contribute to age-related changes in sensitivity to HI.

Perinatal Hypoxic-Ischemic Brain Damage: Evolution of an Animal Model

Early research in the Vannucci laboratory prior to 1981 focused largely on brain energy metabolism in the developing rat. At that time, there was no experimental model to study the effects of perinatal hypoxia-ischemia in the rodent, despite the tremendous need to investigate the pathophysiology of perinatal asphyxial brain damage in infants. Accordingly, we developed such a model in the postnatal day 7 rat, using a modification of the Levine preparation in the adult rat. Rat pups underwent unilateral common carotid artery ligation followed by exposure to systemic hypoxia (8% oxygen) at a constant temperature of 37 degrees C. Brain damage, seen histologically, was generally confined to the cerebral hemisphere ipsilateral to the arterial occlusion, and consisted of selective neuronal death or infarction, depending on the duration of the systemic hypoxia. Tissue injury was observed in the cerebral cortex, hippocampus, striatum, and thalamus. Subcortical and periventricular white matter injury was also observed. This model was originally described in the Annals of Neurology in 1981, and during the more than 20 years since that publication numerous investigations utilizing the model have been conducted in our laboratories as well as laboratories around the world. Cerebral blood flow and metabolic correlates have been fully characterized. Physiologic and pharmacologic manipulations have been applied to the model in search of neuroprotective strategies. More recently, molecular biologic alterations during and following the hypoxic-ischemic stress have been ascertained and the model has been adapted to the immature mouse for specific use in genetically altered animals. As predicted in the original article, the model has proven useful for the study of the short- and long-term effects of hypoxic-ischemic brain damage on motor activity, behavior, seizure incidence, and the process of maturation in the brain and other organ systems.

Neurological reflexes and early motor behavior in rats subjected to neonatal hypoxic-ischemic injury

Severe perinatal hypoxia-ischemia is an important cause of brain injury in both full-term and premature newborns, with a high risk of future behavioral and neurological deficits. The most commonly used animal model of neonatal hypoxia-ischemia is the unilateral ligation of the common carotid artery followed by exposure to hypoxia in 7-day-old rats. In spite of the wide use of this model, lot of contradictions and discrepancies exist between the results obtained by different laboratories regarding behavioral deficits and there are no data regarding the possible delay of the appearance of neurological reflexes and the time-course of reflex performances following neonatal hypoxic-ischemic injury in rats. In the present study we showed that neonatal hypoxia-ischemia retarded the development of somatic growth and several neurological reflexes (ear twitch, grasping, gait and negative geotaxis). Hypoxic animals also displayed retarded performance in righting, geotaxis and gait reflexes. Although hypoxic pups performed worse in most tests for motor coordination, they reached normal levels by 5 weeks of age except in the footfault test. In the open-field, hypoxic animals were generally more active, except at 3 weeks, when activity of normal pups increased enormously as well. Brain areas were significantly reduced in hypoxic animals, but no close correlation was found with behavioral deficits.

Activation of nuclear factor-kappaB signaling pathway by interleukin-1 after hypoxia/ischemia in neonatal rat hippocampus and cortex

Perinatal hypoxia/ischemia (HI) is a common cause of neurological deficits in children. Interleukin-1 (IL-1) activity has been implicated in HI-induced brain damage. However, the mechanisms underlying its action in HI have not been characterized. We used a 7-day-old rat model to elucidate the role of nuclear factor-kappaB (NF-kappaB) activation in HI stimulation of IL-1 signaling. HI was induced by permanent ligation of the left carotid artery followed by 90 min of hypoxia (7.8% O(2)). Using ELISA assays, we observed increased cell death and caspase 3 activity in hippocampus and cortex 3, 6, 12, 24 and 48 h post-HI. IL-1beta protein expression increased, beginning at 3 h after HI and lasting until 24 h post-HI in hippocampus and 12 h post-HI in cortex. Intracerebroventricular injection of 2 microg IL-1 receptor antagonist (IL-1Ra) 2 h after HI significantly reduced cell death and caspase 3 activity. Electrophoretic mobility shift assay analyses of hippocampus and cortex after HI for NF-kappaB activity showed increased p65/p50 DNA-binding activity at 24 h post-HI. Western blot analyses showed significant nuclear translocation of p65. Protein expression levels of two known inflammatory agents, inducible nitric oxide synthase and cycloxygenase 2, known to be transcriptionally regulated by NF-kappaB, also increased at 24 h after HI. All these HI-induced changes were reversed by IL-1Ra blockade of IL-1 signaling, consistent with IL-1 triggering of inflammatory apoptotic outcomes via NF-kappaB transcriptional activation. The observed increase in cytoplasmic phosphorylated inhibitor kappaBalpha (IkappaBalpha) and nuclear translocation of Bcl-3 24 h after HI was also significantly attenuated by IL-1Ra blockade, suggesting that HI-induced IL-1 activation of NF-kappaB is via both the degradation of IkappaBalpha and the nuclear translocation of Bcl-3.

Effects of NF-kappaB oligonucleotide "decoys" on gene expression in P7 rat hippocampus after hypoxia/ischemia

"Decoy" oligonucleotides can be used as gene-specific nuclear factor (NF-kappaB) inhibitors to regulate gene expression. We applied two different decoy oligonucleotides that contained the NF-kappaB binding consensus sequences present in the immunoglobulin G (IgG)-kappaB and Bcl-x promoter into 7-day-old (P7) rat lateral ventricles before hypoxia/ischemia (HI) and compared their effects on gene expression in hippocampi to saline-treated, scrambled decoy-treated, or untreated hippocampi exposed to HI. Left hippocampi were collected at 12 hr after HI. Electrophoretic mobility shift assays (EMSAs) showed that the two decoy treatments had different effects on NF-kappaB binding to the IgG-kappaB and Bcl-x promoter-specific consensus sequences, respectively. We assessed the decoys' effects on gene expression 12 hr after HI using ribonuclease protection assays (RPAs) and Affymetrix DNA microarrays. RPAs showed that both decoys significantly decreased interleukin (IL)-1alpha mRNA levels but had no impact on IL-1beta, IL-6, and IL-10 mRNA levels. IgG-kappaB decoys significantly decreased tumor necrosis factor (TNF)-alpha and TNF-beta mRNA levels compared to minimal changes after treatment with Bcl-x decoys. DNA microarray analyses showed that Bcl-x decoy treatment significantly decreased Bcl-x(L) mRNA levels. The decreased Bcl-x(L) mRNA levels after Bcl-x decoy treatment was confirmed by RPA analysis. DNA microarray data also indicated that several other genes were affected by both decoys. Our results suggest that different NF-kappaB decoy treatments could differentially regulate transcriptional responses to central nervous system trauma. Careful design of decoy sequences, however, is essential to acquire selective effects on cell death outcome.

Neonatal hypoxia triggers transient apoptosis followed by neurogenesis in the rat CA1 hippocampus

Continuous generation of new neurons has been demonstrated in the adult mammalian brain, and this process was shown to be stimulated by various pathologic conditions, including cerebral ischemia. Because brain oxygen deprivation is particularly frequent in neonates and represents the primary event of asphyxia, we analyzed long-term consequences of transient hypoxia in the newborn rat. Within 24 h after birth, animals were exposed to 100% N(2) for 20 min at 36 degrees C, and temporal changes in the vulnerable CA1 hippocampus were monitored. Cell density measurements revealed delayed cell death in the pyramidal cell layer reflecting apoptosis, as shown by characteristic nuclear morphology and expression levels of Bcl-2, Bax, and caspase-3. Neuronal loss was confirmed by reduced density of neuron-specific enolase (NSE)-labeled cells, and peaked by 1 wk post insult, to reach 27% of total cells. A gradual recovery then occurred, and no significant difference in cell density could be detected between controls and hypoxic rats at postnatal d 21. Repeated injections of bromodeoxyuridine (50 mg/kg) showed that newly divided cells expressing neuronal markers increased by 225% in the germinative subventricular zone, and they tended to migrate along the posterior periventricle toward the hippocampus. Therefore, transient hypoxia in the newborn rat triggered apoptosis in the CA1 hippocampus followed by increased neurogenesis and apparent anatomical recovery, suggesting that the developing brain may have a high capacity for self-repair.

Tactile stimulation and maternal separation prevent hippocampal damage in rats submitted to neonatal hypoxia–ischemia

Unilateral neonatal hypoxia-ischemia causes important damage to the hippocampus of the hemisphere ipsilateral to carotid artery occlusion; two forms of neonatal handling, tactile stimulation and maternal separation for a short period, have been shown to produce functional/behavioral protection in distinct models of CNS challenge. In this paper we investigated whether neonatal handling could alter the hippocampal damage caused by neonatal hypoxia-ischemia (HI) in the Wistar rat. Pups at postnatal day 7, P7, received HI (8% O(2)-92% N(2)) for 90 min and were submitted to neonatal handling, tactile stimulation of maternal separation daily, from P8 to P21, for 10 min. On adulthood, hippocampal volume was analyzed by stereological techniques, along with measures of cortical thickness and hemispheric area at the level -3.30 mm from bregma. HI caused a reduction of volume of whole hippocampus, of Amon's horn and of dentate gyrus, with no effect on cortical and hemispheric measures; neonatal handling prevented such effect. This is the first report showing that both tactile stimulation and neonatal handling exert a morphological neuroprotective action for HI-induced damage to the hippocampus.

Bcl‐2 family members make different contributions to cell death in hypoxia and/or hyperoxia in rat cerebral cortex

Hypoxic brain injury during fetal or neonatal development leads to damaged immature neurons and can result in cognitive or behavioral dysfunction. Hyperoxia therapy (treatment with oxygen) is commonly applied to infants with signs of perinatal hypoxia-anoxia. Both hypoxia and hyperoxia have been shown to result in apoptosis in the brains of rats in several animal models. One determinant of cellular commitment to cell death is the differential expression of the Bcl-2 family of proteins in response to trauma. Here, we characterize cell death and the expression of Bcl-2 homologous proteins in 7-day-old neonatal rat cerebral cortex after hypoxia (5% O(2) for 40 min) and/or hyperoxia (>95% O(2) for 2 h after hypoxia). The expression of Bcl-2 and Bcl-X(L), two anti-apoptotic proteins, decreased at 24 h after hypoxia. Bcl-X(L) increased after either hyperoxia or hypoxia+hyperoxia. We did not detect significant changes in the cytoplasmic levels of pro-apoptotic protein Bax after any of these three treatments. Using cell death ELISA and DNA FragEL assays, we observed increased cell death at 24h after hypoxia, hyperoxia or hypoxia+hyperoxia treatments. At 24 h after either hypoxia, hyperoxia or hypoxia+hyperoxia, caspase 3 activity also increased significantly. Our results suggest that both hypoxia and hyperoxia alone can induce cell death. The Bcl-2 --> cytochrome c --> caspase 3 pathway played a role in hypoxia-induced cell death, while other pathways may be involved in hyperoxia-induced cell death.

Differential neuronal fates in the CA1 hippocampus after hypoxia in newborn and 7-day-old rats: Effects of pre-treatment with MK-801

The brain displays an age-dependent sensitivity to ischemic insults. However, the consequences of oxygen deprivation per se in the developing brain remain unclear, and the role of glutamate excitotoxicity via N-methyl-D-aspartate (NMDA) receptors is controversial. To gain a better understanding of the mechanisms involved in the cerebral response to severe hypoxia, cell damage was temporally monitored in the CA1 hippocampus of rat pups transiently exposed to in vivo hypoxia (100% N2) at either 24 h or 7 days of age. Also, the influence of a pre-treatment with the NMDA receptor antagonist MK-801 (5 mg/kg, i.p.) was examined. At both ages, morphometric analyses and cell counts showed hypoxia-induced significant neuronal loss (30-35%) in the pyramidal layer, with injury appearing more rapidly in rats exposed at 7 days. Morphological alterations of 4,6-diamidino-2-phenylindole (DAPI)-labeled nuclei, DNA fragmentation patterns on agarose gels, as well as expression profiles of the apoptosis-related regulatory proteins Bax and Bcl-2 showed that apoptosis was prevalent in younger animals, whereas only necrosis was detected in hippocampi of rats treated at 7 days. Moreover, pre-treatment with MK-801 was ineffective in protecting hippocampal neurons from hypoxic injury in newborn rats, but significantly reduced necrosis in older subjects. These data confirm that hypoxia alone may trigger neuronal death in vivo, and the type of cell death is strongly influenced by the degree of brain maturity. Finally, NMDA receptors are not involved in the apoptotic consequences of hypoxia in the newborn rat brain, but they were found to mediate necrosis at 7 days of age.

Postnatal hypobaric hypoxia in rats impairs water maze learning and the morphology of neurones and macroglia in cortex and hippocampus

Newborn rats were exposed to intermittent hypobaric hypoxia from birth until the age of 19 days. Spatial memory was tested in a Morris water maze from postnatal day (P) 23 to P32 and from P100 to P109. From P24 to P27 and on days P100 and P101, the escape latencies of hypoxic animals were longer than those of controls. At P24, the number of neuronal bodies increased in cortical layer II of the somatosensory, motor, and auditory areas, and in layer V of the motor area, but the number of neuronal bodies throughout the whole cortical thickness was unchanged. Decreases in the immunostaining density for neurofilaments (anti-NF 160), astrocytes (anti-GFAP), and oligodendrocytes (RIP) were found in the hippocampus, and the typical parallel organisation of neuronal and macroglial processes was lost. Decreases in immunostaining for neurofilaments and oligodendrocytes were also found in the somatosensory cortex and motor cortex. In adult hypoxic rats, at P114-P240, the number of neuronal bodies and the immunostaining density for neurofilaments, astrocytes, and oligodendrocytes in the examined areas were similar to adult controls; however, in the hippocampus we found hypertrophy of fine astrocytic processes and a decreased number of oligodendrocytic processes. We conclude that the neonatal brain damage induced by hypobaric hypoxia impairs spatial memory in infant as well as adult rats. Hypobaric hypoxia delays the maturation of neurones and substantially affects macroglia in the cortex and hippocampus.

Differential effects of hypoxia-ischemia on subunit expression and tyrosine phosphorylation of the NMDA receptor in 7- and 21-day-old rats

The effect of cerebral hypoxia-ischemia (HI) on levels and tyrosine phosphorylation of the NMDA receptor was examined in 7- (P7) and 21 (P21)-day-old rats. Unilateral HI was administered by ligation of the right common carotid artery and exposure to an atmosphere of 8% O2/92% N2 for 2 (P7) or 1.5 (P21) h. This duration of HI produces significant infarction in nearly all of the survivors with damage being largely restricted to the cortex, striatum, and hippocampus of the hemisphere ipsilateral to the carotid artery ligation. NR2A levels in the right hemisphere of P7 pups were markedly reduced after 24 h of recovery, while NR1 and NR2B remained unchanged. In contrast, NR2B, but not NR2A, was reduced after HI at P21. At both ages, HI resulted in a transient increase in tyrosine phosphorylation of a number of forebrain proteins that peaked between 1 and 6 h of recovery. At both P7 and P21, tyrosine phosphorylation of NR2B was enhanced 1 h after HI and had returned to basal levels by 24 h. HI induced an increase in tyrosine phosphorylation of NR2A in 21 day, but not in 7-day-old animals. The differential effects of HI on the NMDA receptor at different post-natal ages may contribute to changing sensitivity to hypoxia-ischemia.

Neuroprotective role of delta-opioid receptors in cortical neurons

We recently demonstrated that delta-opioid receptor (DOR) activation protects cortical neurons against glutamate-induced injury. Because glutamate is a mediator of hypoxic injury in neurons, we hypothesized that DOR is involved in neuroprotection during O2 deprivation and that its activation/inhibition may alter neuronal susceptibility to hypoxic stress. In this work, we tested the effect of opioid receptor activation and inhibition on cultured cortical neurons in hypoxia (1% O2). Cell injury was assessed by lactate dehydrogenase release, morphology-based quantification, and live/dead staining. Our results show that 1) immature neurons (days 4 and 6) were not significantly injured by hypoxia until 72 h of exposure, whereas day 8 neurons were injured after only 24-h hypoxia; 2) DOR inhibition (naltrindole) caused neuronal injury in both day 4 and day 8 normoxic cultures and further augmented hypoxic injury in these neurons; 3) DOR activation ([D-Ala2,D-Leu5]enkephalin) reduced neuronal injury in day 8 cultures after 24 h of normoxic or hypoxic exposure and attenuated naltrindole-induced injury with prolonged exposure; and 4) mu- or kappa-opioid receptor inhibition (beta-funaltrexamine or nor-binaltorphimine) had little effect on neurons in either normoxic or hypoxic conditions. Collectively, these data suggest that DOR plays a crucial role in neuroprotection in normoxic and hypoxic environments.

Heat shock protein 72 expression and microtubule-associated protein 2 disappearance after hypoxia-ischemia in the developing rat brain

OBJECTIVE

This study was intended to investigate the temporal changes in heat shock protein 72 expression and microtubule-associated protein 2 disappearance in rat brain at 2 different ages after hypoxic-ischemic insult.

STUDY DESIGN

Both 5-day-old and 14-day-old Wistar rats were subjected to unilateral common carotid artery ligation and hypoxia in 8% oxygen for 2 hours at 33 degrees C. Brain sections were examined sequentially for heat shock protein 72 expression at 0.5, 3, 6, 12, 24, 48, and 72 hours of recovery after hypoxia-ischemia and for microtubule-associated protein 2 disappearance at 0, 24, 48, and 72 hours of recovery and at 7 days of recovery after hypoxia-ischemia. Results of immunohistochemical staining for heat shock protein 72 and microtubule-associated protein 2 were used as markers for detection of early hypoxic-ischemic brain damage. Permanent neuronal damage was assessed with hematoxylin and eosin staining at 7 days after hypoxia.

RESULTS

In 5-day-old rats microtubule-associated protein 2 expression was lost as early as 0 hours after hypoxia-ischemia in the cerebral cortex and hippocampus, with a peak at 48 hours after which expression recovered. Expression of heat shock protein 72 was detected in the ligated hemisphere at 0.5 hours after hypoxia-ischemia and peaked at 6 to 24 hours of recovery. In 14-day-old rats microtubule-associated protein 2 was stained in the cortex at 0 hours after hypoxia-ischemia but gradually disappeared in the cerebral cortex and hippocampus after 24 hours of recovery. The expression of heat shock protein 72 was not detected by 6 hours of recovery in the cerebral cortex and by 3 to 12 hours of recovery in the hippocampus, but heat shock protein 72 was persistently expressed in the cortex and hippocampus after 48 hours of recovery. Neuronal damage was significantly less in 5-day-old rats than in 14-day-old rats.

CONCLUSION

In 5-day-old rats hypoxia-ischemia causes earlier changes in heat shock protein 72 and microtubule-associated protein 2 immunostaining results and causes less severe brain damage than in 14-day-old rats.

Metabolite changes in neonatal rat brain during and after cerebral hypoxia-ischemia: a magnetic resonance spectroscopic imaging study

Cerebral metabolite concentrations were measured in infant rats using proton magnetic resonance spectroscopic imaging. Measurements were made prior to, during and after exposure of rats (6- and 7-day-old) to unilateral cerebral hypoxia-ischemia (right carotid artery occlusion +2h 8% oxygen). Data clustered according to age and outcome-6-day-old animals with no infarct and 7-day-old animals with infarct. In 6-day-old animals, cerebral lactate concentration increased during hypoxia-ischemia, particularly ipsilateral to the occlusion, and returned to normal soon after the end of hypoxia. There were no major changes in N-acetyl-aspartate levels (NAA) in this group and no regions of hyperintensity on T2 or DW weighted images at 24 h. In the 7-day-old animals, lactate increased during hypoxia-ischemia and remained elevated in the first hour after reperfusion. Furthermore, lactate remained at 258+/-117% and 233+/-56% of pre-hypoxic levels, 24 and 48 h post-hypoxia, respectively. NAA concentrations ipsilateral to the occlusion decreased to 55+/-14% during hypoxia, recovered early post-hypoxia and again decreased to 61+/-25% and 41+/-28% at 24 and 48 h post-hypoxia-ischemia, respectively. The infarct volumes measured by diffusion weighted and T2 weighted MRI at 48 h post-hypoxia were 152+/-40 mm3 and 172+/-35 mm3, respectively. Thus, irreversible damage correlated well with measured in vivo lactate and NAA changes. Those animals in which NAA was unaltered and lactate recovered soon after hypoxia did not show long-term damage (6-day-old animals), whereas those animals in which NAA decreased and lactate remained elevated went on to infarction (7-day-old animals).

Magnetic resonance imaging during cerebral hypoxia-ischemia: T2 increases in 2-week-old but not 4-week-old rats

We investigated whether the changes detectable with magnetic resonance imaging techniques during and after an episode of cerebral hypoxia-ischemia differ in immature and older brain. Diffusion weighted (DW) and T2-weighted (T2W) images were repeatedly acquired before, during, and after an episode of cerebral hypoxia-ischemia (unilateral carotid artery occlusion plus hypoxia) in 2- and 4-wk-old rats lightly anesthetized with isoflurane. Areas of increased brightness were detected in DW images from both 2- and 4-wk-old rats by 10-20 min after the start of hypoxia. These hyperintense areas increased during hypoxia, comprising 60.8+/-4.9% and 30.5+/-2.7% of the brain image at the level of the thalamus in 2-wk-old and 4-wk-old animals, respectively (p < 0.003). Hyperintense areas (e.g. 27.0+/-8.3%) also appeared in T2W images during hypoxia-ischemia in 2-wk-old animals, but these did not occur in 4-wk-old animals (p < 0.02). This observation was reflected in T2, which increased during hypoxia-ischemia in the 2-wk-old but not the 4-wk-old group. By 60 min after the termination of hypoxia-ischemia in either age group, areas of hyperintensity resolved and then reappeared 24 h later on both DW and T2W images. Thus, irrespective of age, magnetic resonance imaging changes during transient hypoxia-ischemia generally recover with a delayed or secondary increase in DW and T2W hyperintensity hours later. Immature brain differs from older brain primarily with respect to some combination of hypoxic/ischemic cellular or biochemical changes, that are detectable as increases in T2 within 2-wk-old but not 4-wk-old animals.

Neuronal degeneration and microglial reaction in the fetal and postnatal rat brain after transient maternal hypoxia

This study examined the neuropathological changes in different areas of the brain of fetal and postnatal rats after transient maternal hypoxia. At different time intervals following hypoxia, reactive microglia as determined immunohistochemically with the antibody OX-42 that recognizes complement type three (CR3) receptors, responded vigorously to the hypoxic stress. Microglial activation was particularly evident in the cingulate cortex and the corpus callosum between 3 h and 14 days after hypoxia. Massive cell degeneration as determined ultrastructurally and significant neuronal loss as evaluated by cell counts were observed in the cingulate cortex at 1 and 3 days after hypoxic insults; thereafter, however, the neuronal density was restored to normal levels. Present results suggest that the cingulate cortex is most vulnerable to the hypoxic injury probably due to a redistribution of cerebral blood flow and/or metabolic changes. Besides being involved in the phagocytosis of cellular debris, it is suggested that the reactive microglial cells may have both neurotoxic and neurotrophic functions.

Tirilazad widens the therapeutic window for riluzole-induced attenuation of progressive cortical degeneration in an infant rat model of the shaken baby syndrome

Our infant rat model of traumatic subarchnoid hemorrhage combines violent shaking and hypoxia to produce subdural hemorrhaging and progressive cortical degeneration similar to that seen in victims of the shaken baby syndrome. Anesthetized, 6-day-old male rats were subjected to one episode of shaking under hypoxic conditions. Brain histologies revealed moderate-to-severe cortical hemorrhaging at 48 h postinjury and progressive cortical degeneration, as indicated by a 15.3% and 20.2% reduction in cortical wet weight, at 7 and 14 days postinjury, respectively. The purpose of the present study was to assess the effects of two antioxidant lipid peroxidation inhibitors (tirilazad mesylate and PNU-101033E), and the glutamate release inhibitor (riluzole), upon the brain pathology seen in this model. A significant, 54.3-75.3%, reduction in cortical hemorrhaging was observed in rats that were treated with a total of three doses of tirilazad (10 mg/kg, i.p.): 10 min before or 5-30 min after injury, and again at 2 and 24 h postinjury (p < 0.01 vs. vehicle). However, treatment with tirilazad or the more potent, brain-penetrating pyrrolopyrimidine, PNU-101033E (10 min before plus 2, 24, 48, and 72 h after), did not attenuate the progressive cortical degeneration typically seen at 14 days postinjury. These results suggest that free radicals play an important role in the pathophysiology of secondary brain hemorrhaging due to shaking + hypoxia, but may not be critical in the mediation of the subsequent neurodegeneration. Rather, glutamate neurotoxicity may be a key factor here. This is suggested by our observation that the glutamate release inhibitor, riluzole, significantly reduced cortical degeneration when it was administered up to 1 h postinjury in the present model. Specifically, the cortical wet weights of rats treated with 8 mg/kg riluzole (i.p.) 10 min before or 1 h after shaking + hypoxia (and again at 24 h postinjury) were 95.3% and 97.4% of noninjured controls, respectively, at 14 days postinjury (p < 0.02 vs. vehicle). Riluzole treatment beyond 1 h (e.g., 2 or 4 h postinjury) did not reduce the neurodegeneration. Lastly, we attempted to demonstrate that the therapeutic window for riluzole-induced attenuation of cortical degeneration could be extended beyond 1 h through the use of combination therapy. In this experiment, rat pups were treated with 10 mg/kg tirilazad (i.p.) at 30 min postinjury followed by 8 mg/kg riluzole (i.p.) at 4 and 24 h postinjury. At 14 days postinjury, the cortical wet weights of these rats were 94.5% of noninjured controls, thus demonstrating significant neuroprotection (p < 0.05 vs. vehicle) and a widening of the therapeutic window from 1 to 4 h in length. These results suggest that early attenuation of free radical-induced lipid peroxidation may slow down the biochemical cascade of events related to glutamate-induced excitotoxicity and, in doing so, prolong the time during which a glutamate release inhibitor, such as riluzole, is effective.

Infant rat model of the shaken baby syndrome: preliminary characterization and evidence for the role of free radicals in cortical hemorrhaging and progressive neuronal degeneration

Infants subjected to repeated episodes of violent shaking develop brain damage characterized by intracranial hemorrhage and progressive cortical atrophy. We have developed an animal model that mimics this pathological state and investigated its etiology and treatment. Anesthetized male rats, 6 days of age, were subjected to one episode of shaking per day for 3 consecutive days. Separate groups of rats were sacrificed 1 h postinjury on the third day of shaking for HPLC quantification of cortical .OH and vitamin E levels, and histological assessment of cortical hemorrhaging. Additional groups were sacrificed 7 or 14 days postinjury to demonstrate progressive neuronal degeneration via cortical wet weight comparisons. In comparison to noninjured shams, the results indicated that cortical vitamin E and .OH levels rose 53.7% (p < 0.005) and 457.1% (p < 0.001), respectively, in shaken infant rats. Brain histologies revealed a moderate-to-severe degree of cortical hemorrhaging in these animals 1 h postinjury. By 7 and 14 days postinjury, there was a 13.3% and 28.7% (p < 0.0001 vs. sham) loss of cortical tissue in shaken infants, respectively, indicating progressive neuronal degeneration. Treatment with 10 mg/kg (ip) of the 21-aminosteroid antioxidant, tirilazad mesylate, 10 min before and 2 h after each episode of shaking, resulted in a 53.1% attenuation of cortical .OH levels and a 34.9% decrease in brain hemorrhaging (p < 0.05 vs. vehicle). Tirilazad treatment did not, however, significantly effect cortical vitamin E concentrations at 1 h postinjury or the extent of progressive neuronal degeneration at either 7 or 14 days postinjury. The present animal model mimics the brain pathology seen in abused children. Our observation that tirilazad mesylate, an antioxidant-lipid peroxidation inhibitor, significantly reduces cortical .OH levels and brain hemorrhaging in shaken infant rats supports a role for oxygen radicals in the pathophysiology of this type of CNS injury. The failure of tirilazad to block progressive cortical degeneration suggests that mechanisms other than free radicals may be of prime importance in the mediation of this aspect of the pathology.

Glutamate in neurologic diseases

Excitotoxicity has been implicated as a mechanism of neuronal death in acute and chronic neurologic diseases. Cerebral ischemia, head and spinal cord injury, and prolonged seizure activity are associated with excessive release of glutamate into the extracellular space and subsequent neurotoxicity. Accumulating evidence suggests that impairment of intracellular energy metabolism increases neuronal vulnerability to glutamate which, even when present at physiologic concentrations, can damage neurons. This mechanism of slow excitotoxicity may be involved in neuronal death in chronic neurodegenerative diseases such as the mitochondrial encephalomyopathies, Huntington's disease, spinocerebellar degeneration syndromes, and motor neuron diseases. If so, glutamate antagonists in combination with agents that selectively inhibit the multiple steps downstream of the excitotoxic cascade or help improve intracellular energy metabolism may slow the neurodegenerative process and offer a therapeutic approach to treat these disorders.

Hypoxia-induced dysfunction in developing rat neocortex

Neocortical slices from young [postnatal day (P) 5-8], juvenile (P14-18), and adult (>P28) rats were exposed to long periods of hypoxia. Field potential (FP) responses to orthodromic synaptic stimulation, the extracellular DC potential, and the extracellular Ca2+ concentration ([Ca2+]o] were measured simultaneously in layers II/III of primary somatosensory cortex. Hypoxia caused a 42 and 55% decrease in the FP response in juvenile and adult cortex, respectively. FP responses recorded in slices from young animals were significantly more resistant to oxygen deprivation as compared with the juvenile (P < 0.01) and adult age group (P < 0.001) and declined by only 3% in amplitude. In adult cortex, hypoxia elicited, after 7 +/- 4.5 min (mean +/- SD), a sudden anoxic depolarization (AD) with an amplitude of 14 +/- 6 mV and a duration of 0.89 +/- 0.28 min at half-maximal amplitude. Although the AD onset latency was significantly longer in P5-8 (12.5 +/- 4.9 min, P < 0.001) and P14-18 (8.7 +/- 3.2 min, P < 0.002) cortex, the amplitude and duration of the AD was larger in young (45.7 +/- 7.6 mV, 2.19 +/- 0.71 min, both P < 0.001) and juvenile animals (29.9 +/- 9.1 mV, P < 0.001, 0.96 +/- 0.26 min, P > 0.05) when compared with the adults. The hypoxia-induced [Ca2+]o decrease was significantly (P < 0.002) larger in young cortex (1,115 +/- 50 microM) as compared with the adult (926 +/- 107 microM). Prolongation of hypoxia after AD onset for >5 min elicited in young and juvenile cortex a long-lasting AD with an amplitude of 40.5 mV associated with a decrease in [Ca2+]o by >1 mM. On reoxygenation, only slices from these age groups showed spontaneous repetitive spreading depression in 3 out of 26 cases. In adults, the same protocol caused a significantly (P < 0.05) smaller and shorter AD and never a spreading depression. However, recovery in synaptic transmission after this long-term hypoxia was better in young and juvenile cortex, indicating a prolonged or even irreversible deficiency in synaptic function in mature animals. Application of ketamine caused a 49% reduction in the initial amplitude of the AD in juvenile cortex but did not significantly affect the AD in slices from adult animals. These data indicate that the young and juvenile cortex tolerates much longer periods of oxygen deprivation as compared with the adult, but that a sufficiently long hypoxia causes severe pathophysiological activity in the immature cortex. This enhanced sensitivity of the immature cortex is at least partially mediated by activation of N-methyl-D-aspartate receptors.

Prognostic value of neonatal electroencephalography in premature newborns less than 33 weeks of gestational age

In a prospective study of 417 premature neonates born before 33 weeks' gestational age, neonatal tracings were reviewed to evaluate the use of EEG in prognosis of neurological injuries. The population was divided into two groups: Group 1, infants who died before the age of 1, and Group 2, survivors in which two categories of motor development were considered. Category A, were abnormal, and Category B, were always normal. Positive rolandic sharp waves (PRSW), which reflect white matter injury, occurred equally in both groups, indicating a similar incidence of white matter damage in Groups 1 and 2. In Group 2, there was a significant correlation of PRSW with developmental motor sequelae (Category A). A frequency of PRSW above 2/min (suggesting more severe periventricular white matter injury) and seizures were significantly more prevalent in Group 1 than in Group 2 and in Category A of Group 2 than in Category B. Background abnormalities occurred equally in both subgroups of extremely premature infants (< or = 28 weeks' gestation) they were significantly more numerous in the subgroup of very premature infants (between 28 and 33 weeks' gestation) who died, than in the subgroup of very premature infants who survived. This study shows the potential utility of using neonatal EEG in association with transfontanellar ultrasonography in anticipating the neurological development of very (> 28 weeks' gestation) and extremely (< or = 28 weeks' gestation) premature newborns.