Intrauterine hypoxia-ischemia alters nitric oxide synthase expression and activity in fetal and neonatal rat brains

Current Therapies for Neonatal Hypoxic-Ischaemic and Infection-Sensitised Hypoxic-Ischaemic Brain Damage

Neonatal hypoxic-ischaemic brain damage is a leading cause of child mortality and morbidity, including cerebral palsy, epilepsy, and cognitive disabilities. The majority of neonatal hypoxic-ischaemic cases arise as a result of impaired cerebral perfusion to the foetus attributed to uterine, placental, or umbilical cord compromise prior to or during delivery. Bacterial infection is a factor contributing to the damage and is recorded in more than half of preterm births. Exposure to infection exacerbates neuronal hypoxic-ischaemic damage thus leading to a phenomenon called infection-sensitised hypoxic-ischaemic brain injury. Models of neonatal hypoxia-ischaemia (HI) have been developed in different animals. Both human and animal studies show that the developmental stage and the severity of the HI insult affect the selective regional vulnerability of the brain to damage, as well as the subsequent clinical manifestations. Therapeutic hypothermia (TH) is the only clinically approved treatment for neonatal HI. However, the number of HI infants needed to treat with TH for one to be saved from death or disability at age of 18-22 months, is approximately 6-7, which highlights the need for additional or alternative treatments to replace TH or increase its efficiency. In this review we discuss the mechanisms of HI injury to the immature brain and the new experimental treatments studied for neonatal HI and infection-sensitised neonatal HI.

Taurine Increases Zinc Preconditioning-Induced Prevention of Nitrosative Stress, Metabolic Alterations, and Motor Deficits in Young Rats following Intrauterine Ischemia

Oxygen deprivation in newborns leads to hypoxic-ischemic encephalopathy, whose hallmarks are oxidative/nitrosative stress, energetic metabolism alterations, nutrient deficiency, and motor behavior disability. Zinc and taurine are known to protect against hypoxic-ischemic brain damage in adults and neonates. However, the combined effect of prophylactic zinc administration and therapeutic taurine treatment on intrauterine ischemia- (IUI-) induced cerebral damage remains unknown. The present work evaluated this issue in male pups subjected to transient IUI (10 min) at E17 and whose mothers received zinc from E1 to E16 and taurine from E17 to postnatal day 15 (PND15) via drinking water. We assessed motor alterations, nitrosative stress, lipid peroxidation, and the antioxidant system comprised of superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx). Enzymes of neuronal energetic pathways, such as aspartate aminotransferase (AST), alanine aminotransferase (ALT), and lactate dehydrogenase (LDH), were also evaluated. The hierarchization score of the protective effect of pharmacological strategies (HSPEPS) was used to select the most effective treatment. Compared with the IUI group, zinc, alone or combined with taurine, improved motor behavior and reduced nitrosative stress by increasing SOD, CAT, and GPx activities and decreasing the GSSG/GSH ratio in the cerebral cortex and hippocampus. Taurine alone increased the AST/ALT, LDH/ALT, and AST/LDH ratios in the cerebral cortex, showing improvement of the neural bioenergetics system. This result suggests that taurine improves pyruvate, lactate, and glutamate metabolism, thus decreasing IUI-caused cerebral damage and relieving motor behavior impairment. Our results showed that taurine alone or in combination with zinc provides neuroprotection in the IUI rat model.

Oxygen radical disease in the newborn, revisited: Oxidative stress and disease in the newborn period

Thirty years ago, there was an emerging appreciation for the significance of oxidative stress in newborn disease. This prompted a renewed interest in the impact of oxygen therapy for the newborn in the delivery room and beyond, especially in premature infants. Today, the complexity of oxidative stress both in normal regulation and pathology is better understood, especially as it relates to neonatal mitochondrial oxidative stress responses to hyperoxia. Mitochondria are recipients of oxidative damage and have a propensity for oxidative self-injury that has been implicated in the pathogenesis of neonatal lung diseases. Similarly, both intrauterine growth restriction (IUGR) and macrosomia are associated with mitochondrial dysfunction and oxidative stress. Additionally, reoxygenation with 100% O₂ in a hypoxic-ischemic newborn lamb model increased the production of pro-inflammatory cytokines in the brain. Moreover, the interplay between inflammation and oxidative stress in the newborn is better understood because of animal studies. Transcriptomic analyses have found a number of genes to be differentially expressed in murine models of bronchopulmonary dysplasia (BPD). Epigenetic changes have also been detected both in animal models of BPD and premature infants exposed to oxygen. Antioxidant therapy to prevent newborn disease has not been very successful; however, new therapeutic principles, like melatonin, are under investigation.

Role of Prenatal Hypoxia in Brain Development, Cognitive Functions, and Neurodegeneration

This review focuses on the role of prenatal hypoxia in the development of brain functions in the postnatal period and subsequent increased risk of neurodegenerative disorders in later life. Accumulating evidence suggests that prenatal hypoxia in critical periods of brain formation results in significant changes in development of cognitive functions at various stages of postnatal life which correlate with morphological changes in brain structures involved in learning and memory. Prenatal hypoxia also leads to a decrease in brain adaptive potential and plasticity due to the disturbance in the process of formation of new contacts between cells and propagation of neuronal stimuli, especially in the cortex and hippocampus. On the other hand, prenatal hypoxia has a significant impact on expression and processing of a variety of genes involved in normal brain function and their epigenetic regulation. This results in changes in the patterns of mRNA and protein expression and their post-translational modifications, including protein misfolding and clearance. Among proteins affected by prenatal hypoxia are a key enzyme of the cholinergic system-acetylcholinesterase, and the amyloid precursor protein (APP), both of which have important roles in brain function. Disruption of their expression and metabolism caused by prenatal hypoxia can also result, apart from early cognitive dysfunctions, in development of neurodegeneration in later life. Another group of enzymes affected by prenatal hypoxia are peptidases involved in catabolism of neuropeptides, including amyloid-β peptide (Aβ). The decrease in the activity of neprilysin and other amyloid-degrading enzymes observed after prenatal hypoxia could result over the years in an Aβ clearance deficit and accumulation of its toxic species which cause neuronal cell death and development of neurodegeneration. Applying various approaches to restore expression of neuronal genes disrupted by prenatal hypoxia during postnatal development opens an avenue for therapeutic compensation of cognitive dysfunctions and prevention of Aβ accumulation in the aging brain and the model of prenatal hypoxia in rodents can be used as a reliable tool for assessment of their efficacy.

Anti-IL-6 neutralizing antibody modulates blood-brain barrier function in the ovine fetus

Impaired blood-brain barrier function represents an important component of hypoxic-ischemic brain injury in the perinatal period. Proinflammatory cytokines could contribute to ischemia-related blood-brain barrier dysfunction. IL-6 increases vascular endothelial cell monolayer permeability in vitro. However, contributions of IL-6 to blood-brain barrier abnormalities have not been examined in the immature brain in vivo. We generated pharmacologic quantities of ovine-specific neutralizing anti-IL-6 mAbs and systemically infused mAbs into fetal sheep at 126 days of gestation after exposure to brain ischemia. Anti-IL-6 mAbs were measured by ELISA in fetal plasma, cerebral cortex, and cerebrospinal fluid, blood-brain barrier permeability was quantified using the blood-to-brain transfer constant in brain regions, and IL-6, tight junction proteins, and plasmalemma vesicle protein (PLVAP) were detected by Western immunoblot. Anti-IL-6 mAb infusions resulted in increases in mAb (P < 0.05) in plasma, brain parenchyma, and cerebrospinal fluid and decreases in brain IL-6 protein. Twenty-four hours after ischemia, anti-IL-6 mAb infusions attenuated ischemia-related increases in blood-brain barrier permeability and modulated tight junction and PLVAP protein expression in fetal brain. We conclude that inhibiting the effects of IL-6 protein with systemic infusions of neutralizing antibodies attenuates ischemia-related increases in blood-brain barrier permeability by inhibiting IL-6 and modulates tight junction proteins after ischemia.

Towards improved animal models of neonatal white matter injury associated with cerebral palsy

Newborn neurological injuries are the leading cause of intellectual and motor disabilities that are associated with cerebral palsy. Cerebral white matter injury is a common feature in hypoxic-ischemic encephalopathy (HIE), which affects full-term infants, and in periventricular leukomalacia (PVL), which affects preterm infants. This article discusses recent efforts to model neonatal white matter injury using mammalian systems. We emphasize that a comprehensive understanding of oligodendrocyte development and physiology is crucial for obtaining new insights into the pathobiology of HIE and PVL as well as for the generation of more sophisticated and faithful animal models.

Magnesium sulfate treatment alters fetal cerebellar gene expression responses to hypoxia

Prenatal perturbation of brain circulation and oxygenation is a leading cause of perinatal brain damage affecting about 0.3-0.9% of births. Hypoxia-ischemia (HI) in preterm human infants at gestational week 23-32 results in neurodevelopmental abnormalities in childhood, presenting as learning disability, seizure activity, motor impairment and in the most severe cases, death. Here, we examined the potential of MgSO4 treatment, prior to foetal hypoxia, to attenuate hypoxia induced damage in a murine model of maternal hypoxia. We studied the time course of maternal hypoxia and MgSO4 pre-treatment effects on cerebellar tissue by means of DNA microarray analyses. Mild hypoxia induced minor expression changes in most genes. However, there were 5 gene sets which were down-regulated by maternal hypoxia. MgSO4 pre-treatment abrogated these decreases in gene. A cell cycle gene set which responded immediately (2 h) to hypoxia, showed a delayed response (24 h) when MgSO4 pre-treatment was given. Similar proportions of cell death were observed in all groups before P7, where combined hypoxia and MgSO4 treatment increased cell death in the internal granule layer. There were a higher number of BrdU positive cells at the end of hypoxic episodes and a down-regulation of Reelin signaling, compared to control. MgSO4 pre-treatment prevented the enhancement of cell proliferation due to hypoxia and increased Reelin levels. Altogether, MgSO4 pre-treatment both reduced the number of genes differentially affected by hypoxia and delayed the responses to hypoxia. In addition, MgSO4 pre-treatment modified the nature of the transcriptional response; while hypoxia induced down-regulation of gene sets, MgSO4 pre-treatment mostly up-regulated them. The dual reaction to the MgSO4 treatment may be the source of the ambiguity in observations reported for affected newborns.

Quantitative immunohistochemical analysis of nitric oxide synthases and apoptosis regulator proteins in the fetal rat brain following maternal uterine artery ligation

The purpose of this study was to determine the relation between nitric oxide synthases (calcium-independent iNOS and calcium-dependent eNOS) and apoptosis regulator proteins (anti-apoptotic Bcl-2, pro-apoptotic p53) of fetal rat brain in experimental intrauterine growth retardation (IUGR) model via quantitative immunohistochemistry. Cortical zone of parietal cerebral cortex and ventricular zone of third ventricle were studied following bilateral uterine artery ligation on gestational day 18. Significant increase in iNOS immunoreactivity was determined in parietal cerebral cortex and ventricular zones as eNOS immunoreactivity increased in ventricular zone of IUGR group. Bcl-2 expression was significantly decreased in ventricular zone; whereas cortical zone of IUGR group expressed p53 immunoreactivity.

Prenatal hypoxia preconditioning improves hypoxic ventilatory response and reduces mortality in neonatal rats

OBJECTIVE

Severe hypoxia/ischemia is a major cause of neonatal cardiorespiratory dysfunction and mortality. We tested whether prenatal hypoxia preconditioning would augment hypoxic and hypercapnic ventilatory responses, and thereby reduce neonatal mortality.

METHODS

Pregnant rats at 19 days' gestation were exposed to six episodes of intermittent hypoxia (10-min of 15% O(2) followed by 10-min of normoxia/episode, PPC), or room air (CON) per day until delivery. The ventilatory responses to 1 min of 10% O(2) and 10% CO(2), and 5 min of 5% O(2) were performed in anesthetized pups. The conscious pups were exposed to 5% O(2) for approximately 105 min, and their mortality and dry/wet weight of the lung and brain were evaluated.

RESULTS

We found that augmented ventilatory responses to 1 min of 10% O(2) and 10% CO(2) were similar in the two groups (P>0.05). In contrast, 5 min of 5% O(2) initially caused a ventilatory peak response followed by a decline that was markedly diminished (35%, P=0.013) by PPC. PPC also significantly decreased neonatal mortality by 22% (P=0.044) as compared with CON.

CONCLUSION

We conclude that prenatal hypoxia preconditioning reduces neonatal mortality apparently by improving the severe hypoxic ventilatory response.

Vascular endothelial growth factor and nitric oxide production in response to hypoxia in the choroid plexus in neonatal brain

Damage to the choroid plexus in 1-day-old Wistar rats subjected to hypoxia was investigated. The mRNA and protein expression of hypoxia-inducible factor-1alpha (HIF-1alpha), endothelial, neuronal, inducible nitric oxide synthase (eNOS, nNOS, iNOS), and vascular endothelial growth factor (VEGF) along with nitric oxide (NO) production and VEGF concentration was up-regulated significantly in hypoxic rats. Ultrastructurally, the choroid plexus epithelial cells showed massive accumulation of glycogen. A striking feature was the extrusion of cytoplasmic fragments from the apical cell surfaces into the ventricular lumen following the hypoxic insult. Intraventricular macrophages showed increased expression of complement type 3 receptors, major histocompatibility complex class I and II antigens, and ED1 antigens. Following an intravenous injection of horseradish peroxidase (HRP), a large number of intraventricular macrophages were labeled suggesting enhanced leakage of the tracer from the blood vessels in the choroid plexus connective tissue stroma into the ventricular lumen. We suggest that increased production of NO in hypoxia is linked to the structural alteration of the choroid plexus, and along with VEGF, may lead to increased vascular permeability. Melatonin treatment reduced VEGF and NO levels as well as leakage of HRP suggesting its potential value in ameliorating damage in choroid plexus pathologies.

A nitric oxide donor reduces brain injury and enhances recovery of cerebral blood flow after hypoxia-ischemia in the newborn rat

Nitric oxide (NO) released in response to hypoxia-ischemia (HI) in the newborn brain may mediate both protective and pathologic responses. We sought to determine whether pharmacologic increase of NO using an NO donor would reduce neurologic injury resulting from HI in the postnatal day 7 rat. We measured NO levels and CBF in the presence of either a NOS inhibitor, N-nitro-l-arginine methyl ester (L-NAME) or an NO donor (Z)-1-[N-(2-amino-ethyl)-N-(2-ammonio-ethyl)amino]diazen-1-ium-1,2-diolate (DETANONOate). Both inhibition of NOS and administration of an NO donor reduced neuropathologic injury after 7-day recovery. NO levels decreased in both ischemic and contralateral hemispheres during HI. This response was prevented by treatment with DETANONOate. Despite the decrease in NO, CBF increased during ischemia in the contralateral hemisphere but decreased when combined with brief hypoxia. Treatment with L-NAME abolished these increases, which were not altered by DETANONOate. Reduction of cellular metabolism by mild hypothermia also reduced both NO and CBF. Following prolonged HI, CBF remained decreased in the ischemic hemisphere up to 24-h recovery. This decrease was prevented by treatment with DETANONOate. These data show that administration of an NO donor reduces neurologic injury following HI in the newborn rat. This mechanism of this protection, in part, is due to an increase in the rate of recovery of CBF compared to vehicle-treated animals. Augmentation of NO-dependent increases in CBF may serve to improve neurologic outcome after perinatal asphyxia.

The effect of prenatal hypoxia on brain development: short- and long-term consequences demonstrated in rodent models

Hypoxia (H) and hypoxia-ischemia (HI) are major causes of foetal brain damage with long-lasting behavioral implications. The effect of hypoxia has been widely studied in human and a variety of animal models. In the present review, we summarize the latest studies testing the behavioral outcomes following prenatal hypoxia/hypoxia-ischemia in rodent models. Delayed development of sensory and motor reflexes during the first postnatal month of rodent life was observed by various groups. Impairment of motor function, learning and memory was evident in the adult animals. Activation of the signaling leading to cell death was detected as early as three hours following H/HI. An increase in the counts of apoptotic cells appeared approximately three days after the insult and peaked about seven days later. Around 14-20 days following the H/HI, the amount of cell death observed in the tissue returned to its basal levels and cell loss was apparent in the brain tissue. The study of the molecular mechanism leading to brain damage in animal models following prenatal hypoxia adds valuable insight to our knowledge of the central events that account for the morphological and functional outcomes. This understanding provides the starting point for the development and improvement of efficient treatment and intervention strategies.

Influence of nitric oxide on morphine-induced amnesia and interactions with dopaminergic receptor agents

The interactions of dopaminergic receptors and nitric oxide (NO) with morphine-induced memory of passive avoidance have been investigated in mice. Pre-training administration of morphine (1, 3 and 5 mg/kg, s.c.) dose-dependently decreased the learning of a one-trial passive avoidance task. Pre-training administration of L-arginine, a nitric oxide precursor (50, 100 and 200 mg/kg, i.p.), alone did not affect memory formation. The drug (100 and 200 mg/kg) decreased significantly amnesia induced by pre-training morphine (5 mg/kg). Pre-training administration of L-NAME (N(G)-nitro-L-arginine methyl ester), a nitric oxide synthase (NOS) inhibitor (20 and 30 mg/kg, i.p.), dose-dependently impaired memory formation. In addition, co-pretreatment of different doses of L-NAME (10, 20 and 30 mg/kg) with lower dose of morphine (1 mg/kg), which did not induce amnesia by itself, caused inhibition of memory formation. Pre-training administration of apomorphine, a dopaminergic receptor agonist (0.25, 0.5 and 1 mg/kg, i.p.), alone also did not affect memory formation, but morphine-induced amnesia was significantly inhibited by pretreatment with apomorphine (0.5 and 1 mg/kg, 5 min, i.p.). On the other hand, the inhibition of morphine-induced amnesia by L-arginine (200 mg/kg, i.p.) was significantly decreased by pretreatment with different doses of dopamine D1 receptor antagonist, SCH 23390 (0.001, 0.01 and 0.1 mg/kg, i.p.) or D2 receptor antagonist, sulpiride (12.5, 25, 50 and 100 mg/kg, i.p.). However, the dopamine receptor antagonists could not affect memory formation by themselves. It may be concluded that the morphine-induced impairment of memory formation can be prevented by nitric oxide donor and, in this effect, dopaminergic mechanism is involved.

Hypoxic regulation of the fetal cerebral circulation

Fetal cerebrovascular responses to acute hypoxia are fundamentally different from those observed in the adult cerebral circulation. The magnitude of hypoxic vasodilatation in the fetal brain increases with postnatal age although fetal cerebrovascular responses to acute hypoxia can be complicated by age-dependent depressions of blood pressure and ventilation. Acute hypoxia promotes adenosine release, which depresses fetal cerebral oxygen consumption through action of adenosine on neuronal A1 receptors and vasodilatation through activation of A2 receptors on cerebral arteries. The vascular effect of adenosine can account for approximately half the vasodilatation observed in response to hypoxia. Hypoxia-induced release of nitric oxide and opioids can account for much of the adenosine-independent cerebral vasodilatation observed in response to hypoxia in the fetus. Direct effects of hypoxia on cerebral arteries account for the remaining fraction, although the vascular endothelium contributes relatively little to hypoxic vasodilatation in the immature cerebral circulation. In contrast to acute hypoxia, fetal cerebral blood flow tends to normalize during acclimatization to chronic hypoxia even though cardiac output is depressed. However, uncompensated chronic hypoxia in the fetus can produce significant changes in brain structure and function, alteration of respiratory drive and fluid balance, and increased incidence of intracranial hemorrhage and periventricular leukomalacia. At the level of the fetal cerebral arteries, chronic hypoxia increases protein content and depresses norepinephrine release, contractility, and receptor densities associated with contraction but also attenuates endothelial vasodilator capacity and decreases the ability of ATP-sensitive and calcium-sensitive potassium channels to promote vasorelaxation. Overall, fetal cerebrovascular adaptations to chronic hypoxia appear prioritized to conserve energy while preserving basic contractility. Many gaps remain in our understanding of how the effects of acute and chronic hypoxia are mediated in fetal cerebral arteries, but studies of adult cerebral arteries have produced many powerful pharmacological and molecular tools that are simply awaiting application in studies of fetal cerebral artery responses to hypoxia.

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.

Developmental expression of endothelial nitric oxide synthase (eNOS) in the rat liver

Transition from fetal to postnatal life requires significant changes in cardiac, pulmonary, and hepatic blood flow. As such, there must be changes in vascular control in these vascular systems. Vascular resistance, a major contributor to blood flow, is mediated in the ductus arteriosus and pulmonary vasculature by endothelial nitric oxide synthase (eNOS). This study was conducted to determine the ontogeny of hepatic eNOS expression and activity. Additionally, the expression and activity of inducible nitric oxide synthase (iNOS) was measured to determine whether perinatal hypoxia resulted in detectable levels. NOS mRNA and proteins were determined by reverse transcription PCR assay and semiquantitative Western blot analysis, respectively. NOS activity was measured by the formation of [14C]-citrulline from [14C]-arginine. Localization of eNOS within the liver was determined by immunohistochemistry. eNOS mRNA was detectable at low levels at 18-d gestation and increased after birth, reaching a maximum level (4.5-fold increase) at 20 d of life. Parallel patterns for eNOS protein and activity were seen, with 6.9-fold and 16.1-fold increases, respectively. In the prenatal rat, eNOS was localized to areas of extramedullary hematopoiesis, with little signal in the sinusoids. Postnatally, there was a decrease in staining in the hematopoietic cells and a gradual increase in the staining of the endothelium of the sinusoids and central veins. iNOS mRNA and protein could not be detected at any age. eNOS expression and activity are developmentally regulated, increasing after birth coincident with an initial relative hypoxia and an increase in shear forces upon closure of the ductus venosus.

Early neuroprotective effect of nitric oxide in developing rat brain irradiated in utero

Pregnant Wistar rats were exposed on day 17 of gestation to 1 Gy gamma irradiation from a Co(60) source. Even though it is established that gamma radiation-dependent damage is mainly due to free radical generation neither the ascorbyl radical/ascorbate ratio nor the lipid radical content in developing rat brain were affected by prenatal irradiation. A distinctive EPR signal for the adduct NO-Fe-MGD (g=2.03 and a(N)=12.5 G) was detected in brain homogenates prepared from irradiated rats. Nitric oxide (NO)-dependent EPR signal increased in a time-dependent manner up to 2h post-irradiation. NO concentration in unirradiated brains was 37+/-4 pmol/g brain and 45+/-2, 77+/-5 and 216+/-6 after 30, 60 and 120 min post-irradiation, respectively. Total nitric oxide synthase activity was increased by 77 and 51% after 30 and 60 min post-irradiation, respectively, and returned to control values after 120 min. Thus, increased NO steady-state concentration could be ascribed to an increase in NOS activity. Taken as a whole, these results suggest that NO might act to protect the developing brain from the cytotoxicity of reactive species.

Nitric oxide and nitric oxide synthases in the fetal cerebral cortex of rats following transient uteroplacental ischemia

The effect of transient uteroplacental ischemia on nitric oxide (NO) levels, enzymatic activity, and expression of NO synthase (NOS) isoforms was studied in fetal rat brains. Fetuses were subjected to ischemia by clamping the uterine arteries for 5 min on gestational day 17 (GD17). At different times after ischemia, fetuses were delivered by Cesarean section under anesthesia to obtain the brains. Transient uteroplacental ischemia produced a time dependent increase in nitrite levels in the brain, reaching a maximum value (300 +/- 25% of baseline) 24 h after uterine artery occlusion and remaining elevated as long as 48 h. Significantly increased nitrite levels were found in the cerebral cortex but not in the mesencephalon and cerebellum. The ischemia-induced increment in nitrite levels was totally blocked by either L-NAME (10 mg/kg) or AMT (0.65 mg/kg) administered i.p. 1 h before uterine artery occlusion. Both Ca(2+)-dependent and Ca(2+)-independent NOS activities in the cerebral cortex remained significantly increased with respect to controls after 24 h following the ischemia. Reverse transcriptase-polymerase chain reaction showed augmented levels of mRNAs for both nNOS and iNOS when compared with controls at 8 h after ischemia. At 36 h, nNOS mRNA returned to basal levels whereas eNOS mRNA levels increased and iNOS mRNA remained elevated. Our results show that the three NOS isoforms participate in increasing NO levels after transient ischemia and suggest a biphasic and differential regulation of the expression of constitutive NOS isoforms in the rat cerebral cortex.

Intrauterine hypoxia-ischemia alters expression of the NMDA receptor in the young rat brain

Effects of intrauterine hypoxia-ischemia (HI) on expression of the NMDA receptor subunits as well as on [3H]MK-801 binding of the NMDA receptor were studied in 1-day to 30-day old rat brain. Intrauterine HI conditions were achieved on gestation day 17 by clamping the uterine vasculature for 30 min followed by removal of the clamps to permit reperfusion. As determined by reverse-transcriptase polymerase chain reaction, prenatal HI significantly reduced mRNA expression of the NRI subunit of the NMDA receptor in the hippocampus of 4, 8, and 30-day old rat brains. NR2A and NR2B subunit mRNAs were expressed in the hippocampus and the cortex of both the control and the prenatal HI rat brains. Intrauterine HI did not significantly affect expression of either the NR2A or NR2B subunit mRNA. Consistent with the RT-PCR data, protein expression of the NRI subunit in the hippocampus, but not the cortex, of 21-day old prenatal HI rat brains was significantly decreased as compared to the control rat brain. Intrauterine HI also significantly reduced binding affinity, but not the number of binding sites, of the NMDA receptor to [3H]MK-801, a noncompetitive antagonist of the NMDA receptor, in the hippocampus of 21-day old rat brain. The overall results suggest that prenatal HI-induced reduction of NRI expression and the altered binding ability of the NMDA receptor in the young rat brain may contribute to other long-lasting effects of intrauterine HI that we reported previously.

Chronic ischemia preferentially causes white matter injury in the neonatal rat brain

Chronic ischemic brain injuries were studied in 7- and 14-day-old rat pups, which were subjected to bilateral carotid artery occlusion (BCAO) on postnatal day 1. BCAO preferentially injured white matter in the corpus callosum, subcortex and internal capsule areas while largely spared cortical neurons. White matter rarefaction in the corpus callosum was observed in 12 out of the 17 BCAO rat brains and significantly enlarged lateral ventricles were found in five out of seven P14 BCAO rat brains. These white matter changes were similar to injuries found in newborn infants with periventricular leukomalacia (PVL). White matter injuries in the 7-day-old BCAO rat brain were accompanied with increased activation of microglia/macrophages, as indicated by ED1 and OX42 positive immunostaining. Immature oligodendrocytes in the 7-day-old BCAO rat brain, as indicated by O4+/O1+ staining, were much fewer than in the sham-operated rat brain. Immunostaining for myelin basic protein (MBP) at the fimbria hippocampus and the internal capsule areas in the 7-day-old BACO rat brain was also much less than in the control rat brain. Consistent with the immunostaining data, MBP mRNA expression in the 7-day-old, but not in the 14-day-old, BCAO rat brain was significantly less than in the control rat brain. The overall results suggest that pre-oligodendrocytes and immature oligodendrocytes might be major targets for chronic ischemic insults and activated microglia/macrophages are possibly involved in the process of white matter injury.

Transient increase in rab 3A and synaptobrevin immunoreactivity after mild hypoxia in neonatal rats

1. In the present work we describe the short term effects of mild neonatal hypoxia on the synapse as assessed by the immunoreactivity (IR) of two synaptic proteins: rab 3A and synaptobrevin (VAMP). 2. Using the sensitive methodology of immunoblotting, we measured rab 3A and VAMP-IR in homogenates from the cerebral cortex, hippocampus, and corpus striatum of control (breathing room air) and hypoxiated (breathing 95.5% N2-6.5% O2 for 70 min) 4-day-old rats at 1, 2, and 6 h after the end of the hypoxia. Immunostaining with examination by light microscopy was performed using the synaptic protein-specific antibodies on fixed brain sections from animals belonging to the same litter and submitted to hypoxia. 3. A transient increase of VAMP-IR was observed in the hippocampus and corpus striatum, and for rab 3A in the striatum, 1 h after initiating reoxygenation. At the following time points the values returned to control levels. This effect was less clearly observed in the immunostained sections. 4. Mild hypoxia has an effect on sensitive brain regions, eliciting an increase in the IR of at least two proteins involved in the synaptic vesicle cycle. The transient nature of this effect possibly indicates the activation of endogenous neuroprotective mechanisms.

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.

Immature brain injury via peroxynitrite production induced by inducible nitric oxide synthase after hypoxia-ischemia in rats

OBJECTIVE

To determine whether, and if so how, iNOS expresses and affects brain injury induced by hypoxia-ischemia in an immature brain.

MATERIAL AND METHODS

Seven-day-old Wistar rat pups were exposed to right common carotid artery ligation followed by 1.5 hours of hypoxia. The time course of iNOS mRNA expression, enzymatic activity, and protein production in the cerebral cortex were determined. The extent of the infarct area in the cerebral cortex and the production of 3-nitrotyrosine (a biomarker of peroxynitrite) were compared between the control pups and pups treated with S-methyl-isothiourea (a selective iNOS inhibitor).

RESULTS

In the cortex ipsilateral to carotid ligation, iNOS mRNA appeared from 6 hours to 24 hours after hypoxia-ischemia and disappeared at 48 hours. The iNOS protein and its activity also increased at 12 hours and reached a maximum level at 48 hours after the insult. The percentage of damage in the cerebral cortex was significantly higher in the control pups than in treated pups (31.9 vs 10.6%). Tri-nitrotyrosine following iNOS expression-positive cells were located predominantly at the infarct and peri-infarct regions.

CONCLUSIONS

iNOS expression might be an important determinant of ischemic immature brain injury.

Reduced nitric oxide is involved in prenatal ischemia-induced tolerance to neonatal hypoxic-ischemic brain injury in rats

To explore the role of nitric oxide (NO) in the hypoxic-ischemic (HI) tolerance phenomenon, NO production and brain injury following neonatal hypoxia-ischemia (induced by unilateral common carotid artery ligation followed by hypoxic exposure) were assessed in rat pups with or without HI preconditioning. A previously demonstrated prenatal HI rat model of preconditioning was used in this study. On G17, rat fetuses were subjected to either HI in utero (PreHI) for 30 min or a sham operation (SH). The PreHI treatment provided significant protection against neonatal HI-induced brain injury, as indicated by decreased ipsilateral brain weight reduction, less severe tissue damage, and decreased activation of caspase-3. Concomitant with the protective effect of prenatal HI preconditioning, elevation of nitrite/nitrate content in the ipsilateral cortex of the brain, as an indirect measure of NO production, was significantly lower in the PreHI group than in the SH group following neonatal HI. The protective effect of prenatal HI preconditioning could be reversed by sodium nitroprusside (SNP), a spontaneous NO donor, while SNP had no effect on neonatal HI-induced brain injury in the SH group. Intraperitoneal administration of SNP to pups from the PreHI group (2 mg/kg, 24 and 1.5 h before neonatal HI) increased neonatal HI-induced brain injury similar to that observed in the SH group. On the other hand, L-N(G)-nitro-arginine (2 mg/kg, i.p., 1.5 h before the hypoxic exposure), an NO synthase inhibitor, significantly attenuated neonatal HI-induced brain injury in the SH group. The overall results indicate that reduced NO production in the preconditioned rat brain contributes to prenatal HI-induced tolerance to neonatal HI brain injury.

Cytokine induction in fetal rat brains and brain injury in neonatal rats after maternal lipopolysaccharide administration

Induction of proinflammatory cytokines has been proposed to be a link between prenatal maternal intrauterine infection and neonatal brain damage. It is known that the endotoxin, lipopolysaccharide (LPS), released during bacterial infection crosses the placenta. Cytokine induction in the fetal rat brain after maternal administration of LPS was determined by reverse transcriptase-polymerase chain reaction method. LPS suspension in pyrogen-free saline was administered (i.p.) to pregnant rats at 18 d of gestation. The control group was treated with pyrogen-free saline. Expression of the proinflammatory cytokines, tumor necrosis factor-alpha and IL-1beta mRNA, in the fetal rat brain was increased in a dose-dependent manner at 1 h after LPS administration. The great increase in expression of IL-1beta mRNA was only observed at 1 h after injection of LPS (4 mg/kg), whereas the increased expression of tumor necrosis factor-alpha was still detectable from 4 to 24 h after LPS administration. Brain injuries were examined by immunohistochemistry in 8-d-old rat pups born to the dams that were consecutively treated with LPS (500 microg/kg) or pyrogen-free saline on gestation d 18 and 19. No apparent necrotic tissue damage was found in either the LPS group or the control group. Myelin basic protein staining, as a marker of myelin, was clearly observed in the internal capsule and the fimbria hippocampus in the rat brain from the control group. Myelin basic protein staining was much less and weaker in the brains of the LPS-treated group. Glial fibrillary acidic protein-positive astrocytes were observed in both the control and the LPS-treated groups. The LPS-treated group appeared to have more glial fibrillary acidic protein-positive astrocytes in the hippocampal and the cortex areas of the brain than the control group. Immunoblotting data showed that glial fibrillary acidic protein content in the cortex or the hippocampus of the LPS-treated rat brain was higher than in the control group. OX-42-positive staining (a marker of the type 3 complement receptors) of microglial cells was greatly reduced in the 8-d-old rat brain after maternal LPS administration. However, histochemistry with tomato lectin showed that staining of both amoeboid and ramified microglial cells in the LPS-treated rat brain was similar to that in the control group. The overall results indicate that maternal LPS administration induces an increased expression of IL-1beta and tumor necrosis factor-alpha mRNA in the fetal brain. Maternal LPS administration also increases glial fibrillary acidic protein-positive astrocytes, decreases myelin basic protein and alters immunoreactivity of microglia in the brain of offspring. Although results from the current study do not provide direct evidence linking LPS-induced cytokines with the abnormalities in the neonatal rat brain, our animal model may be appropriate for exploring the mechanisms involved in the effects of maternal infection on glial cells in the brains of offspring.

Protective effect of aminoguanidine on hypoxic-ischemic brain damage and temporal profile of brain nitric oxide in neonatal rat

Nitric oxide (NO) produced by inducible NO synthase contributes to ischemic brain damage. However, the role of inducible NO synthase-derived NO on neonatal hypoxic-ischemic encephalopathy has not been clarified. We demonstrate here that aminoguanidine, a relatively selective inhibitor of inducible NO synthase, ameliorated neonatal hypoxic-ischemic brain damage and that temporal profiles of NO correlated with the neuroprotective effect of aminoguanidine. Seven-day-old Wister rat pups were subjected to left carotid artery occlusion followed by 2.5 h of hypoxic exposure (8% oxygen). Infarct volumes (cortical and striatal) were assessed 72 h after the onset of hypoxia-ischemia by planimetric analysis of coronal brain slices stained with hematoxylin-eosin. Aminoguanidine (300 mg/kg i.p.), administered once before the onset of hypoxia-ischemia and then three times daily, significantly ameliorated infarct volume (89% reduction in the cerebral cortex and 90% in the striatum; p<0.001). NO metabolites were measured by means of chemiluminescence using an NO analyzer. In controls, there was a significant biphasic increase in NO metabolites in the ligated side at 1 h (during hypoxia) and at 72 h after the onset of hypoxia (p<0.05). Aminoguanidine did not suppress the first peak but significantly reduced the second one (p<0.05), and markedly reduced infarct size in a neonatal ischemic rat model. Suppression of NO production after reperfusion is a likely mechanism of this neuroprotection.

Prenatal hypoxia-ischemia alters expression and activity of nitric oxide synthase in the young rat brain and causes learning deficits

Inhibition of nitric oxide synthase (NOS) is known to possibly impair learning and memory. Our previous studies have demonstrated that prenatal hypoxia-ischemia (HI) decreases NOS expression and NOS activity in the neonatal rat brain. To investigate whether effects of prenatal HI on NOS expression continue and whether prenatal HI affects learning and memory in young rats, NOS expression and NOS activity were determined in the hippocampus of rat brains at 28 days of age following a prenatal HI insult on G17. Performances in the passive avoidance test and the Morris water maze test were also studied in these young rats prior to sampling. Rat fetuses were subjected to either a 30-min prenatal HI insult or a sham operation (SH) on gestation day 17 and rat pups were delivered naturally. Increased locomotor activity was observed in the prenatal HI rats as compared to the SH rats on postnatal days 13 and 15, but not on postnatal days 20 and 30. Prenatal HI affected learning ability in these young rats at 28 days of age, as indicated by a delayed acquisition of passive avoidance and by longer escape latency in the Morris water maze test as compared to the SH group. Prenatal HI did not affect retention of passive avoidance and spatial memory. Concomitant with these learning deficits, expression of neuronal NOS and endothelial NOS mRNAs as well as Ca2(+)-dependent NOS activity in the hippocampus of the prenatal HI rat brain were significantly decreased as compared to the SH group. These results suggest that a 30-min prenatal HI insult on gestation day 17 in rats has long-lasting effects on NOS expression and NOS activity in the offspring brain and on learning ability of these young rats. The learning deficit in offspring is possibly associated with the reduction in expression of NOS mRNA and NOS activity in the hippocampus of these animals.