Cerebral energy metabolism during hypoxia-ischemia and early recovery in immature rats

Mitochondrial Dysfunction and Permeability Transition in Neonatal Brain and Lung Injuries

This review discusses the potential mechanistic role of abnormally elevated mitochondrial proton leak and mitochondrial bioenergetic dysfunction in the pathogenesis of neonatal brain and lung injuries associated with premature birth. Providing supporting evidence, we hypothesized that mitochondrial dysfunction contributes to postnatal alveolar developmental arrest in bronchopulmonary dysplasia (BPD) and cerebral myelination failure in diffuse white matter injury (WMI). This review also analyzes data on mitochondrial dysfunction triggered by activation of mitochondrial permeability transition pore(s) (mPTP) during the evolution of perinatal hypoxic-ischemic encephalopathy. While the still cryptic molecular identity of mPTP continues to be a subject for extensive basic science research efforts, the translational significance of mitochondrial proton leak received less scientific attention, especially in diseases of the developing organs. This review is focused on the potential mechanistic relevance of mPTP and mitochondrial dysfunction to neonatal diseases driven by developmental failure of organ maturation or by acute ischemia-reperfusion insult during development.

Neuronal Circuit Activity during Neonatal Hypoxic-Ischemic Seizures in Mice

OBJECTIVE

To identify circuits active during neonatal hypoxic-ischemic (HI) seizures and seizure propagation using electroencephalography (EEG), behavior, and whole-brain neuronal activity mapping.

METHODS

Mice were exposed to HI on postnatal day 10 using unilateral carotid ligation and global hypoxia. EEG and video were recorded for the duration of the experiment. Using immediate early gene reporter mice, active cells expressing cfos were permanently tagged with reporter protein tdTomato during a 90-minute window. After 1 week, allowing maximal expression of the reporter protein, whole brains were processed, lipid cleared, and imaged with confocal microscopy. Whole-brain reconstruction and analysis of active neurons (colocalized tdTomato/NeuN) were performed.

RESULTS

HI resulted in seizure behaviors that were bilateral or unilateral tonic-clonic and nonconvulsive in this model. Mice exhibited characteristic EEG background patterns such as burst suppression and suppression. Neuronal activity mapping revealed bilateral motor cortex and unilateral, ischemic somatosensory cortex, lateral thalamus, and hippocampal circuit activation. Immunohistochemical analysis revealed regional differences in myelination, which coincide with these activity patterns. Astrocytes and blood vessel endothelial cells also expressed cfos during HI.

INTERPRETATION

Using a combination of EEG, seizure semiology analysis, and whole-brain neuronal activity mapping, we suggest that this rodent model of neonatal HI results in EEG patterns similar to those observed in human neonates. Activation patterns revealed in this study help explain complex seizure behaviors and EEG patterns observed in neonatal HI injury. This pattern may be, in part, secondary to regional differences in development in the neonatal brain. ANN NEUROL 2019;86:927-938.

Dichloroacetate treatment improves mitochondrial metabolism and reduces brain injury in neonatal mice

The purpose of this study was to evaluate the effect of dichloroacetate (DCA) treatment for brain injury in neonatal mice after hypoxia ischemia (HI) and the possible molecular mechanisms behind this effect. Postnatal day 9 male mouse pups were subjected to unilateral HI, DCA was injected intraperitoneally immediately after HI, and an additional two doses were administered at 24 h intervals. The pups were sacrificed 72 h after HI. Brain injury, as indicated by infarction volume, was reduced by 54.2% from 10.8 ± 1.9 mm³ in the vehicle-only control group to 5.0 ± 1.0 mm³ in the DCA-treated group at 72 h after HI (p = 0.008). DCA treatment also significantly reduced subcortical white matter injury as indicated by myelin basic protein staining (p = 0.018). Apoptotic cell death in the cortex, as indicated by counting the cells that were positive for apoptosis-inducing factor (p = 0.018) and active caspase-3 (p = 0.021), was significantly reduced after DCA treatment. The pyruvate dehydrogenase activity and the amount of acetyl-CoA in mitochondria was significantly higher after DCA treatment and HI (p = 0.039, p = 0.024). In conclusion, DCA treatment reduced neonatal mouse brain injury after HI, and this appears to be related to the elevated activation of pyruvate dehydrogenase and subsequent increase in mitochondrial metabolism as well as reduced apoptotic cell death.

In Vivo Neurochemical Characterization of Developing Guinea Pigs and the Effect of Chronic Fetal Hypoxia

The guinea pig is a frequently used animal model for human pregnancy complications, such as oxygen deprivation or hypoxia, which result in altered brain development. To investigate the impact of in utero chronic hypoxia on brain development, pregnant guinea pigs underwent either normoxic or hypoxic conditions at about 70 % of 65-day term gestation. After delivery, neurochemical profiles consisting of 19 metabolites and macromolecules were obtained from the neonatal cortex, hippocampus, and striatum from birth to 12 weeks postpartum using in vivo (1)H MR spectroscopy at 9.4 T. The effects of chronic fetal hypoxia on the neurochemical profiles were particularly significant at birth. However, the overall developmental trends of neurochemical concentration changes were similar between normoxic and hypoxic animals. Alterations of neurochemicals including N-acetylaspartate (NAA), phosphorylethanolamine, creatine, phosphocreatine, and myo-inositol indicate neuronal loss, delayed myelination, and altered brain energetics due to chronic fetal hypoxia. These observed neurochemical alterations in the developing brain may provide insights into hypoxia-induced brain pathology, neurodevelopmental compromise, and potential neuroprotective measures.

Physiology-based kinetic modeling of neuronal energy metabolism unravels the molecular basis of NAD(P)H fluorescence transients

Imaging of the cellular fluorescence of the reduced form of nicotinamide adenine dinucleotide (phosphate) (NAD(P)H) is one of the few metabolic readouts that enable noninvasive and time-resolved monitoring of the functional status of mitochondria in neuronal tissues. Stimulation-induced transient changes in NAD(P)H fluorescence intensity frequently display a biphasic characteristic that is influenced by various molecular processes, e.g., intracellular calcium dynamics, tricarboxylic acid cycle activity, the malate-aspartate shuttle, the glycerol-3-phosphate shuttle, oxygen supply or adenosine triphosphate (ATP) demand. To evaluate the relative impact of these processes, we developed and validated a detailed physiologic mathematical model of the energy metabolism of neuronal cells and used the model to simulate metabolic changes of single cells and tissue slices under different settings of stimulus-induced activity and varying nutritional supply of glucose, pyruvate or lactate. Notably, all experimentally determined NAD(P)H responses could be reproduced with one and the same generic cellular model. Our computations reveal that (1) cells with quite different metabolic status may generate almost identical NAD(P)H responses and (2) cells of the same type may quite differently contribute to aggregate NAD(P)H responses recorded in brain slices, depending on the spatial location within the tissue. Our computational approach reconciles different and sometimes even controversial experimental findings and improves our mechanistic understanding of the metabolic changes underlying live-cell NAD(P)H fluorescence transients.

Therapeutic hypothermia achieves neuroprotection via a decrease in acetylcholine with a concurrent increase in carnitine in the neonatal hypoxia-ischemia

Although therapeutic hypothermia is known to improve neurologic outcomes after perinatal cerebral hypoxia-ischemia, etiology remains unknown. To decipher the mechanisms whereby hypothermia regulates metabolic dynamics in different brain regions, we used a two-step approach: a metabolomics to target metabolic pathways responding to cooling, and a quantitative imaging mass spectrometry to reveal spatial alterations in targeted metabolites in the brain. Seven-day postnatal rats underwent the permanent ligation of the left common carotid artery followed by exposure to 8% O2 for 2.5 hours. The pups were returned to normoxic conditions at either 38 °C or 30 °C for 3 hours. The brain metabolic states were rapidly fixed using in situ freezing. The profiling of 107 metabolites showed that hypothermia diminishes the carbon biomass related to acetyl moieties, such as pyruvate and acetyl-CoA; conversely, it increases deacetylated metabolites, such as carnitine and choline. Quantitative imaging mass spectrometry demarcated that hypothermia diminishes the acetylcholine contents specifically in hippocampus and amygdala. Such decreases were associated with an inverse increase in carnitine in the same anatomic regions. These findings imply that hypothermia achieves its neuroprotective effects by mediating the cellular acetylation status through a coordinated suppression of acetyl-CoA, which resides in metabolic junctions of glycolysis, amino-acid catabolism, and ketolysis.

The pentose phosphate pathway and pyruvate carboxylation after neonatal hypoxic-ischemic brain injury

The neonatal brain is vulnerable to oxidative stress, and the pentose phosphate pathway (PPP) may be of particular importance to limit the injury. Furthermore, in the neonatal brain, neurons depend on de novo synthesis of neurotransmitters via pyruvate carboxylase (PC) in astrocytes to increase neurotransmitter pools. In the adult brain, PPP activity increases in response to various injuries while pyruvate carboxylation is reduced after ischemia. However, little is known about the response of these pathways after neonatal hypoxia-ischemia (HI). To this end, 7-day-old rats were subjected to unilateral carotid artery ligation followed by hypoxia. Animals were injected with [1,2-(13)C]glucose during the recovery phase and extracts of cerebral hemispheres ipsi- and contralateral to the operation were analyzed using (1)H- and (13)C-NMR (nuclear magnetic resonance) spectroscopy and high-performance liquid chromatography (HPLC). After HI, glucose levels were increased and there was evidence of mitochondrial hypometabolism in both hemispheres. Moreover, metabolism via PPP was reduced bilaterally. Ipsilateral glucose metabolism via PC was reduced, but PC activity was relatively preserved compared with glucose metabolism via pyruvate dehydrogenase. The observed reduction in PPP activity after HI may contribute to the increased susceptibility of the neonatal brain to oxidative stress.

2-vessel occlusion/hypotension: a rat model of global brain ischemia

Cardiac arrest followed by resuscitation often results in dramatic brain damage caused by ischemia and subsequent reperfusion of the brain. Global brain ischemia produces damage to specific brain regions shown to be highly sensitive to ischemia (1). Hippocampal neurons have higher sensitivity to ischemic insults compared to other cell populations, and specifically, the CA1 region of the hippocampus is particularly vulnerable to ischemia/reperfusion (2). The design of therapeutic interventions, or study of mechanisms involved in cerebral damage, requires a model that produces damage similar to the clinical condition and in a reproducible manner. Bilateral carotid vessel occlusion with hypotension (2VOH) is a model that produces reversible forebrain ischemia, emulating the cerebral events that can occur during cardiac arrest and resuscitation. We describe a model modified from Smith et al. (1984) (2), as first presented in its current form in Sanderson, et al. (2008) (3), which produces reproducible injury to selectively vulnerable brain regions (3-6). The reliability of this model is dictated by precise control of systemic blood pressure during applied hypotension, the duration of ischemia, close temperature control, a specific anesthesia regimen, and diligent post-operative care. An 8-minute ischemic insult produces cell death of CA1 hippocampal neurons that progresses over the course of 6 to 24 hr of reperfusion, while less vulnerable brain regions are spared. This progressive cell death is easily quantified after 7-14 days of reperfusion, as a near complete loss of CA1 neurons is evident at this time. In addition to this brain injury model, we present a method for CA1 damage quantification using a simple, yet thorough, methodology. Importantly, quantification can be accomplished using a simple camera-mounted microscope, and a free ImageJ (NIH) software plugin, obviating the need for cost-prohibitive stereology software programs and a motorized microscopic stage for damage assessment.

A rat pup model of cerebral palsy induced by prenatal inflammation and hypoxia

Animal models of cerebral palsy established by simple infection or the hypoxia/ischemia method cannot effectively simulate the brain injury of a premature infant. Healthy 17-day-pregnant Wistar rats were intraperitoneally injected with lipopolysaccharide then subjected to hypoxia. The pups were used for this study at 4 weeks of age. Simultaneously, a hypoxia/ischemia group and a control group were used for comparison. The results of the footprint test, the balance beam test, the water maze test, neuroelectrophysiological examination and neuropathological examination demonstrated that, at 4 weeks after birth, footprint repeat space became larger between the forelimbs and hindlimbs of the rats, the latency period on the balance beam and in the Morris water maze was longer, place navigation and ability were poorer, and the stimulus intensity that induced the maximal wave amplitude of the compound muscle action potential was greater in the lipopolysaccharide/hypoxia and hypoxia/ischemia groups than in the control group. We observed irregular cells around the periventricular area, periventricular leukomalacia and breakage of the nuclear membrane in the lipopolysaccharide/hypoxia and hypoxia/ischemia groups. These results indicate that we successfully established a Wistar rat pup model of cerebral palsy by intraperitoneal injection of lipopolysaccharide and hypoxia.

Using the endocannabinoid system as a neuroprotective strategy in perinatal hypoxic-ischemic brain injury

One of the most important causes of brain injury in the neonatal period is a perinatal hypoxic-ischemic event. This devastating condition can lead to long-term neurological deficits or even death. After hypoxic-ischemic brain injury, a variety of specific cellular mechanisms are set in motion, triggering cell damage and finally producing cell death. Effective therapeutic treatments against this phenomenon are still unavailable because of complex molecular mechanisms underlying hypoxic-ischemic brain injury. After a thorough understanding of the mechanism underlying neural plasticity following hypoxic-ischemic brain injury, various neuroprotective therapies have been developed for alleviating brain injury and improving long-term outcomes. Among them, the endocannabinoid system emerges as a natural system of neuroprotection. The endocannabinoid system modulates a wide range of physiological processes in mammals and has demonstrated neuroprotective effects in different paradigms of acute brain injury, acting as a natural neuroprotectant. The aim of this review is to study the use of different therapies to induce long-term therapeutic effects after hypoxic-ischemic brain injury, and analyze the important role of the endocannabinoid system as a new neuroprotective strategy against perinatal hypoxic-ischemic brain injury.

Neuroprotective therapies after perinatal hypoxic-ischemic brain injury

Hypoxic-ischemic (HI) brain injury is one of the main causes of disabilities in term-born infants. It is the result of a deprivation of oxygen and glucose in the neural tissue. As one of the most important causes of brain damage in the newborn period, the neonatal HI event is a devastating condition that can lead to long-term neurological deficits or even death. The pattern of this injury occurs in two phases, the first one is a primary energy failure related to the HI event and the second phase is an energy failure that takes place some hours later. Injuries that occur in response to these events are often manifested as severe cognitive and motor disturbances over time. Due to difficulties regarding the early diagnosis and treatment of HI injury, there is an increasing need to find effective therapies as new opportunities for the reduction of brain damage and its long term effects. Some of these therapies are focused on prevention of the production of reactive oxygen species, anti-inflammatory effects, anti-apoptotic interventions and in a later stage, the stimulation of neurotrophic properties in the neonatal brain which could be targeted to promote neuronal and oligodendrocyte regeneration.

A maternal diet supplemented with creatine from mid-pregnancy protects the newborn spiny mouse brain from birth hypoxia

The creatine-phosphocreatine shuttle is essential for the maintenance of cellular ATP, particularly under hypoxic conditions when respiration may become anaerobic. Using a model of intrapartum hypoxia in the precocial spiny mouse (Acomys cahirinus), the present study assessed the potential for maternal creatine supplementation during pregnancy to protect the developing brain from the effects of birth hypoxia. On day 38 of gestation (term is 39 days), the pregnant uterus was isolated and placed in a saline bath for 7.5 min, inducing global hypoxia. The pups were then removed, resuscitated, and cross-fostered to a nursing dam. Control offspring were delivered by caesarean section and recovered immediately after release from the uterus. At 24 h after birth hypoxia, the brains of offspring from dams fed a normal diet showed significant increases in lipid peroxidation as measured by the amount of malondialdehyde. In the cortical subplate, thalamus and piriform cortex there were significant increases in cellular expression of the pro-apoptotic protein BAX, cytoplasmic cytochrome c and caspase-3. When pregnant dams were fed the creatine supplemented diet, the increase in malondialdehyde, BAX, cytochrome c and caspase 3 were almost completely prevented, such that they were not different from control (caesarean-delivered) neonates. This study provides evidence that the neuroprotective capacity of creatine in the hypoxic perinatal brain involves abrogation of lipid peroxidation and apoptosis, possibly through the maintenance of mitochondrial function. Further investigation into these mechanisms of protection, and the long-term development and behavioural outcomes of such neonates is warranted.

Oxygen treatment after experimental hypoxia-ischemia in neonatal rats alters the expression of HIF-1alpha and its downstream target genes

Recently, mounting evidence has emerged to suggest that hyperbaric oxygenation (HBOT)-induced neuroprotection after experimental global ischemia and subarachnoid hemorrhage entails a decrease in the expression of hypoxia-inducible factor-1alpha (HIF-1alpha). Therefore, the purpose of this study was to test the hypothesis that oxygen-induced neuroprotection after neonatal hypoxia-ischemia involves alterations in the expression of HIF-1alpha. Seven-day-old rat pups were subjected to unilateral carotid artery ligation followed by 2 h of hypoxia (8% O(2) at 37 degrees C). Pups were then treated with HBOT (2.5 ATA) or normobaric oxygenation treatment (NBOT) for 2 h. The expression and phosphorylation status of HIF-1alpha was evaluated at intervals up to 24 h after the insult, as was the expression of glucose transporter (GLUT)-1, GLUT-3, lactate dehydrogenase (LDH), aldolase (Ald), and p53. The protein-protein interaction of HIF-1alpha and p53 was also examined. An elevated expression of HIF-1alpha, GLUT-1, GLUT-3, Ald, and LDH was observed after the insult. An increase in the dephosphorylated form of HIF-1alpha was followed by an increase in the association of HIF-1alpha with p53 and an increase in p53 levels. Both HBOT and NBOT reduced the elevated expression of HIF-1alpha and decreased its dephosphorylated form. Furthermore, both treatments promoted a transient increase in the expression of GLUT-1, GLUT-3, LDH, and Ald, while decreasing the HIF-1alpha-p53 interaction and decreasing the expression of p53. Therefore, the alteration of the HIF-1alpha phenotype by a single oxygen treatment may be one of the underlying mechanisms for the observed oxygen-induced neuroprotection seen when oxygen is administered after a neonatal hypoxic-ischemic insult.

Oxidative metabolism, apoptosis and perinatal brain injury

Perinatal hypoxic-ischaemic injury (HII) is a significant cause of neurodevelopmental impairment and disability. Studies employing 31P magnetic resonance spectroscopy to measure phosphorus metabolites in situ in the brains of newborn infants and animals have demonstrated that transient hypoxia-ischaemia leads to a delayed disruption in cerebral energy metabolism, the magnitude of which correlates with the subsequent neurodevelopmental impairment. Prominent among the biochemical features of HII is the loss of cellular ATP, resulting in increased intracellular Na+ and Ca2+, and decreased intracellular K+. These ionic imbalances, together with a breakdown in cellular defence systems following HII, can contribute to oxidative stress with a net increase in reactive oxygen species. Subsequent damage to lipids, proteins, and DNA and inactivation of key cellular enzymes leads ultimately to cell death. Although the precise mechanisms of neuronal loss are unclear, it is now clear both apoptosis and necrosis are the significant components of cell death following HII. A number of different factors influence whether a cell will undergo apoptosis or necrosis, including the stage of development, cell type, severity of mitochondrial injury and the availability of ATP for apoptotic execution. This review will focus on some pathological mechanisms of cell death in which there is a disruption to oxidative metabolism. The first sections will discuss the process of damage to oxidative metabolism, covering the data collected both from human infants and from animal models. Following sections will deal with the molecular mechanisms that may underlie cerebral energy failure and cell death in this form of brain injury, with a particular emphasis on the role of apoptosis and mitochondria.

The current etiologic profile and neurodevelopmental outcome of seizures in term newborn infants

OBJECTIVES

The objectives of this study were to delineate the etiologic profile and neurodevelopmental outcome of neonatal seizures in the current era of neonatal intensive care and to identify predictors of neurodevelopmental outcome in survivors.

METHODS

Eighty-nine term infants with clinical neonatal seizures underwent neurologic examination, electroencephalography (EEG), neuroimaging, and extensive diagnostic tests in the newborn period. After discharge, all infants underwent regular neurologic evaluations and, at 12 to 18 months, formal neurodevelopmental testing. We tested the prognostic value of seizure etiology, neurologic examination, EEG, and neuroimaging.

RESULTS

Etiology was found in 77 infants. Global cerebral hypoxia-ischemia, focal cerebral hypoxia-ischemia, and intracranial hemorrhage were most common. Neonatal mortality was 7%; 28% of the survivors had poor long-term outcome. Association between seizure etiology and outcome was strong, with cerebral dysgenesis and global hypoxia-ischemia associated with poor outcome. Normal neonatal period/early infancy neurologic examination was associated with uniformly favorable outcome at 12 to 18 months; abnormal examination lacked specificity. Normal/mildly abnormal neonatal EEG had favorable outcome, particularly if neonatal neuroimaging was normal. Moderate/severely abnormal EEG, and multifocal/diffuse cortical or primarily deep gray matter lesions, had a worse outcome.

CONCLUSIONS

Mortality associated with neonatal seizures has declined although long-term neurodevelopmental morbidity remains unchanged. Seizure etiology and background EEG patterns remain powerful prognostic factors. Diagnostic advances have changed the etiologic distribution for neonatal seizures and improved accuracy of outcome prediction. Global cerebral hypoxia-ischemia, the most common etiology, is responsible for the large majority of infants with poor long-term outcome.

Hypoxia-ischemia in the immature brain

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

N-ethylmaleimide causes aquaporin-2 trafficking in the renal inner medullary collecting duct by direct activation of protein kinase A

The antidiuretic hormone arginine vasopressin increases the osmotic water permeability of the renal collecting ducts by inducing the shuttling of aquaporin-2 (AQP2) water channels from intracellular vesicles to the apical plasma membrane of the principal cells. This process has been demonstrated to be dependent on the cytoskeleton and protein kinase A (PKA). Previous studies in the toad urinary bladder, a functional homologue of the renal collecting duct, have demonstrated that the sulfhydryl reagent N-ethylmaleimide (NEM) is also able to activate the vasopressin-sensitive water permeability pathway in this tissue. The aim of the present study was to investigate the effects of NEM on AQP2 trafficking in a mammalian system. We show that NEM causes translocation of AQP2 from the cytosol to the plasma membrane in rat inner medullary collecting ducts; like the response to arginine vasopressin, this action was also dependent on an intact cytoskeleton and PKA. This effect is not mediated by cAMP but results from direct activation of PKA by NEM.

Plasticity of purine release during cerebral ischemia: clinical implications?

Adenosine is a powerful modulator of neuronal function in the mammalian central nervous system. During a variety of insults to the brain, adenosine is released in large quantities and exerts a neuroprotective influence largely via the A(1) receptor, which inhibits glutamate release and neuronal activity. Using novel enzyme-based adenosine sensors, which allow high spatial and temporal resolution recordings of adenosine release in real time, we have investigated the release of adenosine during hypoxia/ischemia in the in vitro hippocampus. Our data reveal that during the early stages of hypoxia adenosine is likely released per se and not as a precursor such as cAMP or an adenine nucleotide. In addition, repeated hypoxia results in reduced production of extracellular adenosine and this may underlie the increased vulnerability of the mammalian brain to repetitive or secondary hypoxia/ischemia.

Transient hypoxia-ischemia in rats: changes in diffusion-sensitive MR imaging findings, extracellular space, and Na+-K+ -adenosine triphosphatase and cytochrome oxidase activity

PURPOSE

To investigate the correlation between diffusion-weighted (DW) magnetic resonance (MR) image changes with alterations in extracellular volume and changes in cytochrome oxidase and Na(+)-K(+)-adenosine triphosphatase (ATPase) activity at various times during and after cerebral hypoxia-ischemia in neonatal and juvenile rats.

MATERIALS AND METHODS

One- and 4-week-old rats were randomly assigned to control or transient cerebral hypoxia-ischemia (ie, right carotid artery occlusion plus exposure to 8% oxygen) groups. Hypoxic-ischemic changes compared with normal ipsilateral brain tissue on DW images and the apparent diffusion coefficient of water were measured during and at 1 and 24 hours after hypoxia-ischemia ended. Hypoxic-ischemic changes in extracellular space and ipsilateral versus contralateral differences in Na(+)-K(+)-ATPase and cytochrome oxidase activity were measured.

RESULTS

Hyperintensities on DW images obtained during hypoxia-ischemia correlated well (P <.05) with extracellular space reductions, which occurred 15 minutes earlier in the brains of 4-week-old rats than in the brains of 1-week-old rats. Similarly, within 1 hour after hypoxia-ischemia ended, DW image and extracellular space changes normalized. In contrast, Na(+)-K(+)-ATPase and cytochrome oxidase activity decreased in some regions during hypoxia-ischemia and remained reduced 1 hour after the end of hypoxia-ischemia. Twenty-four hours after signal intensity normalization, hyperintense areas reappeared on DW images, and Na(+)-K(+)-ATPase and cytochrome oxidase activity remained decreased.

CONCLUSION

Signal intensity alterations with diffusion-sensitive MR imaging during and after transient hypoxia-ischemia are closely associated with a corresponding shrinkage and reexpansion of the extracellular space, irrespective of age. Mechanisms other than Na(+)-K(+)-ATPase changes may induce the early cell volume changes detected with diffusion-sensitive MR imaging.

Abnormal cerebral haemodynamics in perinatally asphyxiated neonates related to outcome

AIM

To measure changes in cerebral haemodynamics during the first 24 hours of life following perinatal asphyxia, and relate them to outcome.

METHODS

Cerebral blood volume (CBV), its response (CBVR) to changes in arterial carbon dioxide tension (PaCO(2)), and cerebral blood flow (CBF) were measured using near infrared spectroscopy (NIRS) in 27 term newborn infants with clinical and/or biochemical evidence consistent with perinatal asphyxia.

RESULTS

Both CBF and CBV were higher on the first day of life in the infants with adverse outcomes, and a CBV outside the normal range had a sensitivity of 86% for predicting death or disability. The mean (SD) CBVR on the first day of life was 0.13 (0.12) ml/100 g/1/kPa, which, in 71% of infants, was below the lower 95% confidence limit for normal subjects.

CONCLUSION

An increase in CBV on the first day of life is a sensitive predictor of adverse outcome. A reduction in CBVR is almost universally seen following asphyxia, but is not significantly correlated with severity of adverse outcome.

Diffusion- and T2-weighted increases in magnetic resonance images of immature brain during hypoxia-ischemia: transient reversal posthypoxia

Hypoxic-ischemic changes in brain are detected earlier with diffusion-weighted (DW) than with T2-weighted magnetic resonance (MR) imaging techniques in adults, whereas the response in immature brain is not known. We investigated MR imaging changes prior to, during, and/or after 2 h of hypoxia-ischemia (right carotid artery occlusion + 2 h of hypoxia) in 7-day-old rats anesthetized with isoflurane. In general, within the first 45 min of hypoxia-ischemia there were no changes in the DW or T2-weighted images. By the second hour of hypoxia-ischemia there were marked areas of increased intensity in both the T2 and the DW images, with cortex and striatum being affected prior to thalamus and hippocampus. The area of DW exceeded that of T2 hyperintensities. In the first hour after hypoxia-ischemia there was a transient recovery of hyperintensities on both T2 and DW images. Between 24 and 72 h the hyperintense area on DW images decreased, whereas that on T2-weighted images increased. The distribution of pathological damage assessed histologically correlated with the areas of hyperintensity on the MR images. In contrast to adult brain, early hypoxic-ischemic injury in immature brain is detected as an increase in intensity in both diffusion- and T2-weighted images, indicating a unique alteration in brain water dynamics in this neonatal model of hypoxia-ischemia. These imaging changes and alterations in brain water can rapidly but transiently reverse upon the start of normoxia and reperfusion, suggestive of secondary energy failure or delayed neuronal death.

Relation between delayed impairment of cerebral energy metabolism and infarction following transient focal hypoxia-ischaemia in the developing brain

Phosphorus magnetic resonance spectroscopy (31P MRS) was used to determine whether focal cerebral injury caused by unilateral carotid artery occlusion and graded hypoxia in developing rats led to a delayed impairment of cerebral energy metabolism and whether the impairment was related to the magnitude of cerebral infarction. Forty-two 14-day-old Wistar rats were subjected to right carotid artery ligation, followed by 8% oxygen for 90 min. Using a 7T MRS system. 31P brain spectra were collected during the period from before until 48 h after hypoxia-ischaemia. Twenty-eight control animals were studied similarly. In controls, the ratio of the concentration of phosphocreatine ([PCr]) to inorganic orthophosphate ([Pi]) was 1.75 (SD 0.34) and nucleotide triphosphate (NTP) to total exchangeable phosphate pool (EPP) was 0.20 (SD 0.04): both remained constant. In animals subjected to hypoxia-ischaemia, [PCr] to [Pi] and [NTP] to [EPP] were lower in the 0- to 3-h period immediately following the insult: 0.87 (0.48) and 0.13 (0.04), respectively. Values then returned to baseline level, but subsequently declined again: [PCr] to [Pi] at -0.02 h-1 (P < 0.0001). [PCr] to [Pi] attained a minimum of 1.00 (0.33) and [NTP] to [EPP] a minimum of 0.14 (0.05) at 30-40 h. Both ratios returned towards baseline between 40 and 48 h. The late declines in high-energy phosphates were not associated with a fall in pHi. There was a significant relation between the extent of the delayed impairment of energy metabolism and the magnitude of the cerebral infarction (P < 0.001). Transient focal hypoxia-ischaemia in the 14-day-old rat thus leads to a biphasic disruption of cerebral energy metabolism, with a period of recovery after the insult being followed by a secondary impairment some hours later.

Cerebral Carbohydrate and Energy Metabolism in Perinatal Hypoxic-Ischemic Brain Damage

Cerebral hypoxia-ischemia remains a major cause of acute perinatal brain injury. Research in experimental animals over the past decade has greatly expanded our knowledge of those oxidative events which occur during a hypoxic-ischemic insult to the brain, as well as those metabolic alterations which evolve during the recovery period following resuscitation. The available evidence suggests that hypoxia alone does not lead to brain damage, but rather a combination of hypoxia-ischemia or isolated cerebral ischemia is a necessary prerequisite for tissue injury to occur. Furthermore, hypoxia-ischemia severe enough to produce irreversible tissue injury is always associated with major perturbations in the energy status of the perinatal brain which persists well into the recovery period. The lingering energy depletion sets in motion a cascade of biochemical alterations that are initiated during the course of the insult and proceed well into the recovery period to culminate in either neuronal necrosis or infarction. Unlike the adult, where glucose supplementation prior to or during hypoxia-ischemia accentuates tissue injury, glucose treatment of perinatal animals subjected to a similar insult substantially reduces the extent of tissue injury. The mechanism for the age-specific effect of glucose on hypoxic-ischemic brain damage is discussed in relation to pathogenetic mechanisms responsible for the occurrence of permanent brain damage.