Acute hypoxia induces specific changes in local cerebral glucose utilization at different postnatal ages in the rat

Hypoxic-ischemic insult decreases glutamate uptake by hippocampal slices from neonatal rats: prevention by guanosine

Brain injury secondary to hypoxic-ischemic disease is the predominant form of damage encountered in the perinatal period. The impact of neonatal hypoxia-ischemia (HI) in 7-day-old pups on the high-affinity [3H] glutamate uptake into hippocampal slices at different times after insult was examined. Immediately following, and 1 day after the insult there was no effect. But at 3 to 5 days after the HI insult, glutamate uptake into the hippocampus was markedly reduced; however, after 30 or 60 days the glutamate uptake into hippocampal slices returned to control levels. Also, this study demonstrated the effect of the nucleoside guanosine (Guo) on the [3H] glutamate uptake in neonatal HI injury, maintaining the [3H] glutamate uptake at control levels when injected before and after insult HI. We conclude that neonatal HI influences glutamate uptake a few days following insult, and that guanosine prevents this action.

Long-term effects of acute and of chronic hypoxia on behavior and on hippocampal histology in the developing brain

Ten-day-old rat pups (P10) subjected to acute hypoxia (down to 4% O2) had as adults increased aggression (handling test), memory impairment (water maze test), and decreased CA1 cell counts. Pups subjected to chronic hypoxia (10% O2 from P0 to P21) had increased aggression, hyperactivity (open-field test), and decreased CA1 cell counts. Chronic hypoxia with superimposed acute hypoxia resulted in consequences that were not different from those of chronic hypoxia.

Neonatal hypoxia-ischemia reduces ganglioside, phospholipid and cholesterol contents in the rat hippocampus

Hypoxia-ischemia is a common cause of neonatal brain damage producing serious impact on cerebral maturation. This report demonstrates that rats submitted to hypoxia-ischemia present a marked decrease in hippocampal gangliosides, phospholipids and cholesterol contents as from 7 days after the injury. Although chromatographic profiles of the different ganglioside species (GM1, GD1a, GD1b, and GT1b) from the hippocampus of hypoxic-ischemic hippocampi groups (HI) were apparently unaffected, as compared with controls, there were quantitative absolute reductions in HI. The phospholipid patterns were altered in HI as from the 14th to the 30th day after the injury, where phosphatidylcholine (PC) quantities were higher than phosphatidylethanolamine (PE); additionally, the cardiolipin band was detected only in hippocampi of control adult rats. In general, the absolute quantities of phospholipids were lower in HI than in correspondent controls since 7th day after the injury. Considering that reported effects were maintained, we suggest they express a late biochemical response triggered by the neonatal hypoxic-ischemic episode; the consequences would be cell death and a delay on brain development, expressed by a reduction on synaptogenesis and myelinogenesis processes.

Effects of neonatal hypoxia/ischemia on ganglioside expression in the rat hippocampus

Neonatal Hypoxia-Ischemia (HI) triggers a cascade of biochemical events that result in neuronal injury, but the mechanisms underlying these processes are not completely understood, and information regarding the effect of HI on the synthesis of brain glycoconjugates is lacking. The present work evaluates the effects of neonatal HI on hippocampal ganglioside synthesis. Seven-day-old rat pups were exposed to HI for 2.5 h according to the modified Levine model and samples from hyppocampus were obtained at 30 min as well as at 1, 2 and 4 days later. The activity for synthesis of gangliosides was evaluated by determining the incorporation of N-acetyl [3H]neuraminc acid ([3H]NeuAc) into the endogenous gangliosides of Golgi membranes and by determining the activity of Sial-T2 (GD3 synthase) and GalNAc-T (GM2 synthase), the two enzymes acting on sialyllactosylceramide (GM3) at the branching point of synthesis of a- and b-ganglioside pathway. Northern blot experiments were also conducted to determine transcription levels of the mRNAs specific for these transferases. Neonatal HI caused a relative increase of in vitro [3H]NeuAc incorporation into endogenous lactosylceramide, which was most noticeable at 30 min and I day post-event and disappeared by day 2 and 4. The transient accumulation of [3H]GM3 correlated with decreases in the activities of GD3- and GM2 synthase measured at 30 min and at 1 day after the HI insult. No significant variations in the expression of the genes for these enzymes were observed. Results suggest that transient accumulation of GM3 may be due to post-translational events negatively modulating both GD3- and GM2 synthase activities.

Bilirubin exerts additional toxic effects in hypoxic cultured neurons from the developing rat brain by the recruitment of glutamate neurotoxicity

Both hypoxia and bilirubin are common risk factors in newborns, which may act synergistically to produce anatomical and functional disturbances of the CNS. Using primary cultures of neurons from the fetal rat brain, it was recently reported that neuronal apoptosis accounts for the deleterious consequences of these two insults. To investigate the influence of hypoxia, bilirubin, or their combination on the outcome of neuronal cells of the immature brain, and delineate cellular mechanisms involved, 6-d-old cultured neurons were submitted to either hypoxia (6 h), unconjugated bilirubin (0.5 microM), or to combined conditions. Within 96 h, cell viability was reduced by 22.7% and 24.5% by hypoxia and bilirubin, respectively, whereas combined treatments decreased vital score by 34%. Nuclear morphology revealed 13.4% of apoptotic cells after hypoxia, 16.2% after bilirubin, and 22.6% after both treatments. Bilirubin action was specifically blocked by the glutamate receptor antagonist MK-801, which was without effect on the consequences of hypoxia. Temporal changes in [(3)H]leucine incorporation rates as well as beneficial effects of cycloheximide reflected a programmed phenomenon dependent upon synthesis of selective proteins. The presence of bilirubin reduced hypoxia-induced alterations of cell energy metabolism, as reflected by 2-D-[(3)H]deoxyglucose incorporation, raising the question of free radical scavenging. Measurements of intracellular radical generation, however, failed to confirm the antioxidant role of bilirubin. Taken together, our data suggest that low levels of bilirubin may enhance hypoxia effects in immature neurons by facilitating glutamate-mediated apoptosis through the activation of N:-methyl-D-aspartate receptors.

Perinatal human hypoxia-ischemia vulnerability correlates with brain calcification

Deregulation of intracellular calcium homeostasis is widely considered as one of the underlying pathophysiological mechanisms of hypoxic-ischemic brain injury. Whether this alteration can result in cerebral calcification was investigated in basal ganglia, cerebral cortex, and hippocampus of human premature and term neonates together with glial reaction. In all samples nonarteriosclerotic calcifications were observed, their number and size were area-specific and increased in term neonates. Basal ganglia always presented the highest degree of calcification and hippocampus the lowest, located mainly in the CA1 subfield. In all cases, neuronal damage was associated with astroglial reaction and calcium precipitates, with microglial reaction only in basal ganglia and cerebral cortex, and argues for the participation of excitatory amino acid receptors in hypoxia-ischemia damage. These data correlate with hypoxia-ischemia vulnerability in the perinatal period. The clinical relevance of these precipitates and the neuroprotective interest of non-NMDA receptor manipulation are discussed in the light of our results.

Respiratory network function in the isolated brainstem-spinal cord of newborn rats

The in vitro brainstem-spinal cord preparation of newborn rats is an established model for the analysis of respiratory network functions. Respiratory activity is generated by interneurons, bilaterally distributed in the ventrolateral medulla. In particular non-NMDA type glutamate receptors constitute excitatory synaptic connectivity between respiratory neurons. Respiratory activity is modulated by a diversity of neuroactive substances such as serotonin, adenosine or norepinephrine. Cl(-)-mediated IPSPs provide a characteristic pattern of membrane potential fluctuations and elevation of the interstitial concentration of (endogenous) GABA or glycine leads to hyperpolarisation-related suppression of respiratory activity. Respiratory rhythm is not blocked upon inhibition of IPSPs with bicuculline, strychnine and saclofen. This indicates that GABA- and glycine-mediated mutual synaptic inhibition is not crucial for in vitro respiratory activity. The primary oscillatory activity is generated by neurons of a respiratory rhythm generator. In these cells, a set of intrinsic conductances such as P-type Ca2+ channels, persistent Na+ channels and G(i/o) protein-coupled K+ conductances mediates conditional bursting. The respiratory rhythm generator shapes the activity of an inspiratory pattern generator that provides the motor output recorded from cranial and spinal nerve rootlets in the preparation. Burst activity appears to be maintained by an excitatory drive due to tonic synaptic activity in concert with chemostimulation by H+. Evoked anoxia leads to a sustained decrease of respiratory frequency, related to K+ channel-mediated hyperpolarisation, whereas opiates or prostaglandins cause longlasting apnea due to a fall of cellular cAMP. The latter observations show that this in vitro model is also suited for analysis of clinically relevant disturbances of respiratory network function.

Effects of neonatal cerebral hypoxia-ischemia on the in vitro phosphorylation of synapsin 1 in rat synaptosomes

Synapsins are phosphoproteins related to the anchorage of synaptic vesicles to the actin skeleton. Hypoxia-ischemia causes an increased calcium influx into neurons through ionic channels gated by activation of glutamate receptors. In this work seven-day-old Wistar rats were submitted to hypoxia-ischemia and sacrificed after 21 hours, 7, 30, or 90 days. Synaptosomal fractions were obtained by Percoll gradients and incubated with 32P (10 microCi/g). Proteins were analysed by SDS-PAGE and radioactivity incorporated into synapsin 1 was counted by liquid scintillation. Twenty-one hours after hypoxia-ischemia we observed a reduction on the in vitro phosphorylation of synapsin 1, mainly due to hypoxia, rather than to ischemia; this effect was reversed at day 7 after the insult. There was another decrease in phosphorylation 30 days after the event interpreted as a late effect of hypoxia-ischemia. No changes were observed at day 90. Our results suggest that decreased phosphorylation of synapsin 1 could be related to neuronal death that follows hypoxia-ischemia.

Effect of chronic hypoxia on glucose transporters in heart and skeletal muscle of immature and adult rats

Glucose transporter (GLUT) modulation can be an important mechanism that contributes to adaptation to hypoxic stress, but little is known about GLUT modulation in heart and skeletal muscle with prolonged hypoxia. In this work, the effect of chronic hypoxia on GLUT-4 and GLUT-1 mRNA and protein was studied in these two tissues in the adult and during development. Hypoxia (fractional inspired O2 = 9 +/- 0.5%) was administered to two groups, i.e., an immature group exposed from 3 to 30 days of age and an adult group exposed from 90 to 120 days of age. Rats were then killed and their heart and skeletal muscles were sampled for measurements of GLUT mRNA and protein with Northern and Western blots. In the adult, chronic hypoxia significantly decreased cardiac GLUT mRNA level by > 25% of control (P < 0.05), but had little effect on GLUT protein. A very different hypoxic effect was seen in the immature rat heart with a major increase in protein and no appreciable change in mRNA density. Adult skeletal muscle had no change in GLUT mRNA level but GLUT protein increased (15-20%, P < 0.05) while both GLUT mRNA and protein were significantly increased in the immature skeletal muscles (60-90% over control). We conclude that during chronic O2 deprivation, GLUT-1 and GLUT-4 expressions show a similar pattern but greatly depend on tissue type and age. These differences in GLUT regulation may be due to different strategies for coping with prolonged O2 deprivation in both immature and adult animals.

Developmental changes in the hypoxic response of the hypoglossus respiratory motor output in vitro

The transverse brain stem slice of mice containing the pre-Bötzinger complex (PBC), a region essential for respiratory rhythm generation in vitro, was used to study developmental changes of the response of the in vitro respiratory network to severe hypoxia (anoxia). This preparation generates, at different postnatal stages [postnatal day (P)0-22], spontaneous rhythmic activity in hypoglossal (XII) rootlets that are known to occur in synchrony with periodic bursts of neurons in the PBC. It is assumed that this rhythmic activity reflects respiratory rhythmic activity. At all examined stages anoxia led to a biphasic response: the frequency of rhythmic XII activity initially increased ("primary augmentation") and then decreased ("secondary depression"). In neonates (P0-7), anoxia did not significantly affect the amplitude of integrated XII bursts. Secondary depression never led to a cessation of rhythmic activity. In mice older than P7, augmentation was accompanied by a significant increase in the amplitude of XII bursts. A significant decrease of the amplitude of XII bursts occurred during secondary depression. This depression led always to cessation of rhythmic activity in XII rootlets. The anoxia-induced response of the respiratory rhythmic XII motor output is biphasic and changes during development in a similar way to the in vivo respiratory network. Whether this biphasic response is due to a biphasic response of the respiratory rhythm generator and/or to a biphasic modulation of the XII motor nucleus remains unresolved and needs further cellular analysis. We propose that the transverse slice is a useful model system for examination of the mechanisms underlying the hypoxic response.

Interstitial PCO2 and pH, and their role as chemostimulants in the isolated respiratory network of neonatal rats

1. CO2-H(+)-sensitive microelectrodes were used for simultaneous measurements of the partial pressure of CO2 (PCO2) and extracellular pH (pHo) in the ventral respiratory group (VRG) of the isolated brainstem-spinal cord of neonatal rats. Some of the data were analysed using diffusion equations. 2. With increasing recording depth within the boundaries of the VRG (300-600 microns below the tissue surface), PCO2 increased from 77 to 95 mmHg and pHo fell from 7.0 to 6.8 at steady state in standard saline equilibrated with 5% CO2 and 95% O2. 3. Elevating bath CO2 from 5 to 10-12.5% produced a mean increase in PCO2 of 18 mmHg, a fall in pHo of 0.13 pH units, and a 50-250% increase in the frequency of respiration-related spinal (C2) nerve bursts. Similar effects on C2 activity and pHo were observed upon lowering bath [HCO3-] from 25 to 10 mM, leading to a mean decrease in PCO2 of 4.4 mmHg in the VRG. 4. Raising bath [HCO3-] to 50 mM produced a substantial frequency decrease, a rise in pHo of 0.24 pH units and an elevation in PCO2 of 9.3 mmHg. C2 activity was not profoundly affected upon doubling the CO2-HCO3- content, leading to a mean increase in pHo of 0.13 pH units and elevation of PCO2 by 30 mmHg. 5. In a CO2-HCO3(-)-free, Hepes-buffered solution, PCO2 decreased to 18 mmHg in the VRG and pHo fell by 0.15 pH units with no major effect on rhythmic activity. Subsequent anoxic exposure for more than 15 min produced a further fall in PCO2 to below 1 mmHg, a decrease in pHo of 0.55 pH units, and blockade of respiration-related activity. In three out of the six preparations tested, C2 activity could be restored by reapplication of CO2-HCO3- in the absence of O2. 6. C2 activity persisted at a reduced frequency, even up to 30 min, during anoxia in the CO2-HCO(-)-buffered saline,leading to an elevation in PCO2 of 15 mmHg and a fall in pHo of 0.18 pH units. 7. The diffusion coefficient of CO2 in the tissue was found to be equal to that in saline. Two mean estimates for anoxic tissue of the function lambda 2/ alpha of tortuosity (lambda) and extracellular volume fraction (alpha), affecting extracellular diffusion of bicarbonate, were 4.7 and 4.1. The mean rate of acid production by anoxic tissue was 1.1 mequiv 1-1 min-1. 8. The results suggest that extracellular H+ is the primary stimulating factor in central chemosensitivity, which may often mask the less evident effects of CO2. A model of diffusion of acid equivalents in brain tissue is proposed.

Topography of hypoxic injury proved by argyrophilia in postnatal rat brain

The argyrophil III method, a new esterification-silver staining approach, was used to elucidate regional differences in the susceptibility of developing brain to hypoxic-ischemic (H-I) injury. We created a unilateral common carotid artery-ligation model with hypoxia (8% oxygen) in postnatal day (P) 7, P14 and P21 rats. The argyrophil (i.e., deteriorated) neurons were apparent in the ipsilateral hippocampus, cortex, and striatum in each age group. Argyrophil neurons exhibited some morphological signs of the "early phase" of injury preceding the loss of structure and/or cell death in the "late phase," as indicated by hematoxylin-eosin (H-E) staining. The argyrophil neurons were apparent as early as 12 hours after the insult, whereas the histological changes revealed by H-E staining were subtle. The early phase and late phase histological changes had a stereotyped pattern of appearance in all ages studied. However, the duration of H-I situation required to produce argyrophil cells differed according to age. The most resistive age was P14 (P14 > P7 > P21) in this observation. Therefore, argyrophil III staining is feasible for H-I brain damage model in neonates. The results suggest that both the early phase and the late phase pathological processes after H-I injury have a characteristic topographical vulnerability that does not change during development but have a differing susceptibility according to age.

The model of pentylenetetrazol-induced status epilepticus in the immature rat: short- and long-term effects

In order to assess acute, short and long-term effects of seizures in the immature rat brain, we studied the metabolic, circulatory and histopathological changes induced by pentylenetetrazol (PTZ) given at postnatal day 10 (P10) or 21 (P21). Seizures were induced by repetitive subconvulsive injections of PTZ given as a first dose of 40 mg/kg followed 10 min later by 20 mg/kg. Thereafter, rats received every 10 min additional injections of PTZ 10 mg/kg until the onset of status epilepticus. Local cerebral metabolic rates for glucose (LCMRglc) were measured both during the seizures in P10 and P21 rats and in the young adult animal at P60 by means of the quantitative 2-deoxyglucose technique. Rates of local cerebral blood flow (LCBF) were determined during the seizures by the iodoantipyrine technique. Short-term histological changes were assessed by acid fuchsin and hematoxylin-eosin staining and by HSP72 immunohistochemistry. At P10, LCMRglcs uniformly increased (38-400%) over control values during seizures. At P21, metabolic increases (39-181%) occurred only in 20% of the structures while LCMRglcs decreased in most cortical, hippocampal and sensory areas as well as in mammillary body, discrete thalamic nuclei and white matter areas. At P10, LCBF rose (32-184%) in all brain structures whereas, at P21, LCBF decreased in cortical, hippocampal and sensory regions and increased in most other areas. At P60, in animals having seized at either age, significant long-term decreases in LCMRglcs were recorded in hippocampus, auditory and piriform cortex, medial geniculate body and mammillary body. In P60 animals exposed to PTZ at P10, LCMRglcs were also decreased in 3 other sensory areas. In P60 animals exposed to seizures at P21, LCMRglcs were additionally decreased in sensory regions, cortices, thalamic and hypothalamic regions. Neuronal cells were transiently stained with acid fuchsin, with a peak occurring at 24 h after the seizures. The stain was visible in all regions of cerebral cortex and hippocampus and in some thalamic and hypothalamic nuclei. This transient staining was not accompanied by cell degeneration as assessed by hematoxylin-eosin histology. No HSP72 expression could be detected 24 h after the seizures, neither at P10 nor at P21. The present study shows that the immature rat neurons undergo altered metabolic rates and local circulatory decreases in the acute phase, a change in the affinity of acid fuchsin as a short-term effect and long-term metabolic decreases. All these changes are located in the same regions, i.e., cerebral cortex, hippocampus, sensory regions as well as scattered thalamic and hypothalamic nuclei. Thus, short- and long-term metabolic changes induced by seizures can be used as an index of cell stress in the immature rat brain. Since all these changes occur in the absence of visible neuronal death, they might be related to changes in the final arborization and synaptic organization of the developing brain.

NMDA Receptor-dependent increase of cerebral glucose utilization after hypoxia-ischemia in the immature rat

Post-treatment with the N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 reduces hypoxic-ischemic brain injury in immature animals. To elucidate possible mechanisms, cerebral glucose utilization (CMRglc) and cerebral blood flow (CBF) were measured 1-5 h after hypoxia-ischemia and administration of MK-801 in 7-day-old rats. After 100 min of unilateral hypoxia-ischemia, half of the pups were injected with MK-801. CMRglc was assessed by the [14C]deoxyglucose (2-DG) method. The brains were analyzed either by autoradiography or for energy metabolites and chromatographic separation of 2-DG-6-phosphate and 2-DG. CBF was measured by the autoradiographic [14C]iodoantipyrine method. Mean CMRglc in the cerebral cortex was increased ipsilaterally after hypoxia-ischemia to 15 +/- 3.3 mumol 100 g-1 min-1 (p < 0.01) and areas with CMRglc > 20 mumol 100 g-1 min-1 amounted to 8.0 +/- 7.7 mm2 in the ipsilateral hemisphere compared with 1.2 +/- 1.6 mm2 contralaterally (p < 0.001). Treatment with MK-801 decreased CMRglc bilaterally (p < 0.05) and reduced ipsilateral areas with increased CMRglc by 64% (p < 0.01). CBF was unaltered after hypoxia-ischemia and by MK-801 treatment. In conclusion, regional glucose hyperutilization in the parietal cortex after hypoxia-ischemia was attenuated by MK-801; this may have relevance to the neuroprotective effect of NMDA-receptor antagonists in this model.

Functional relevance of anaerobic metabolism in the isolated respiratory network of newborn rats

Respiratory (C3-C5) activity and extracellular K+, pH and Ca2+ (aKe, pHe, [Ca]e, respectively) in the ventral respiratory group (VRG) were measured in vitro. In brainstem-spinal cord preparations from 0- to 1-day-old rats, lowering of bath glucose content from 30 to 10 mM for 1 h did not affect aKe or rhythmic activity. In preparations from 2- to 3-day-old animals, however, an aKe rise by about 1 mM and disturbance of rhythm occurred after a delay of 50 min. Glucose-free saline resulted, after about 30 min, in reversible blockade of respiratory rhythm and an aKe rise by more than 8 mM, whereas pHe remained unaffected. Exposure to anoxia for 30 min after 1 h of pre-incubation in 10 mM glucose led to a progressive rise of aKe, and a fall of [Ca]e. The concomitant suppression of rhythm was irreversible in preparations from 2- to 3-day-old animals. Similar effects on aKe and [Ca]e and irreversible blockade of rhythm were revealed during anoxia in glucose-free solution, or by addition of 2-5 mM iodoacetate to oxygenated or hypoxic solutions. Iodoacetate led to a slow increase of pHe by more than 0.2 pH units, which was accelerated by anoxia. Our findings show that normal respiratory network functions in the en bloc medulla, in particular from rats older than 1 day, depend on high bath glucose levels, necessary for effective utilization of anaerobic metabolism.

Hypoxia and brain development

Hypoxia threatens brain function during the entire life-span starting from early fetal age up to senescence. This review compares the short-term, long-term and life-spanning effects of fetal chronic hypoxia and neonatal anoxia on several behavioural paradigms including novelty-induced spontaneous and learning behaviours. Furthermore, it reveals that perinatal hypoxia is an additional threat to neurodegeneration and decline of cognitive and other behaviours during the aging process. Prenatal hypoxia evokes a temporary delay of ingrowth of cholinergic and serotonergic fibres into the hippocampus and neocortex, and causes an enhanced neurodegeneration of 5-HT-ir axons during aging. Neonatal anoxia suppresses hippocampal ChAT activity and up-regulates muscarinic receptor sites for 3H-QNB and 3H-pirenzepine binding in the hippocampus in the early postnatal age. The altered development of axonal arborization and pre- and postsynaptic cholinergic functions may be an important underlying mechanism to explain the behavioural deficits. As far as the cellular mechanisms of perinatal hypoxia is concerned, our primary aim was to study the putative importance of Ca2+ homeostasis of developing neurons by means of pharmacological interventions and by measuring the development of immunoexpression of Ca(2+)-binding proteins. We assessed that nimodipine, an L-type calcium channel blocker, prevented or attenuated the adverse behavioural and neurochemical effects of perinatal hypoxias, while it enhanced the early postnatal development of ir-Ca(2+)-binding proteins. The results are discussed in the context of different related research areas on brain development and hypoxia and ischaemia.

Influence of post-hypoxia reoxygenation conditions on energy metabolism and superoxide production in cultured neurons from the rat forebrain

Brain reperfusion and/or reoxygenation may be of particular importance in the etiology of neuronal damage after hypoxic-ischemic insult in neonates, especially with reference to the generation of free radicals. To investigate this issue, the influence of either standard reoxygenation or transient hyperoxia was studied on the consequences of severe hypoxia in a model of cultured neurons isolated from the fetal rat brain. Culture dishes were exposed for 6 h to hypoxia (95% N2/5% CO2). They were then placed under normoxia (95% air/5% CO2) or hyperoxia (95% O2/5% CO2) for 3 h, and finally returned to normoxia. Control cultures were kept under normoxic conditions. Cell morphology, protein concentrations, lactate dehydrogenase leakage, energy metabolism, as reflected by specific transport and incorporation of 2-D-[3H]deoxyglucose, as well as superoxide radical formation were analyzed as a function of time. Po2 values in the cell incubating medium were decreased by 78% by hypoxia and increased by 221% by hyperoxia. No morphologic alteration could be noticed before 72 h posthypoxia, when cell degeneration became apparent, with a concomitant reduction in protein contents. Hypoxia-reoxygenation induced a transient cellular hypermetabolism, as shown by a 36% increase in 2-D-[3H]deoxyglucose uptake 24 h after hypoxia, and then a 23% decrease below control values at 72 h. It also led to a sharp increase in the formation of superoxide radicals (+108%). Transient hyperoxia during reoxygenation did not exacerbate these events, and thus would not enhance their deterimental effects on cell integrity.

Ontogenetic differences in energy metabolism and inhibition of protein synthesis in hippocampal slices during in vitro ischemia and 24 h of recovery

The present study was designed to clarify whether ontogenetic differences in the vulnerability of the brain towards hypoxic-ischemic insults are only caused by the low cerebral energy demand of immature animals or whether there are additional mechanisms, such as protein synthesis (PSR), that may be involved in this phenomenon. We therefore measured tissue levels of adenylates and PSR in hippocampal slices from immature (E40) and mature (E60) guinea pigs fetuses and from adult guinea pigs during in vitro ischemia and 24 h of recovery using a recently modified method. Hippocampal slices were incubated in a temperature controlled flow-through chamber, gassed with 95% O2/5% CO2. In vitro ischemia was induced by transferring slices to a glucose-free artificial cerebrospinal fluid (aCSF) equilibrated with 95% N2/5% CO2. The duration of ischemia ranged from 10 to 40 min. Adenylates were measured by HPLC after extraction with perchloric acid. PSR was evaluated as the incorporation rate of [14C]leucine into proteins. Under control conditions, tissue levels in adenylates did not change, whereas PSR increased slightly in hippocampal slices from mature fetuses and adult animals during a 24-h control incubation period. In slices from immature fetuses ATP levels were only maintained for 2 h. During in vitro ischemia the decline in ATP, total adenylate pool, and adenylate energy charge was much slower in slices from immature fetuses than in slices from mature fetuses or adults. After in vitro ischemia, ATP and the total adenylate pool did not completely recover in mature fetuses and adults, whereas adenylate energy charge almost returned to control values independently of the developmental stage. Two hours after in vitro ischemia PSR was undisturbed in slices from immature fetuses, but severely inhibited in slices from mature fetuses and adults. With ongoing recovery, PSR in mature fetuses returned to control values, while in adults it was still inhibited even 24 h after in vitro ischemia. From these results we conclude that hippocampal slices prepared from mature guinea pig fetuses as well as from adult guinea pigs can be held metabolically stable during long-term incubation using a recently modified technique. However, in slices from immature fetuses a stable energy state could not be maintained for more than 2 h. We further conclude that postischemic disturbances in PSR closely reflect the ontogenetic changes in the vulnerability of the brain to ischemia and that low energy metabolism is certainly not the only cause of the increased vulnerability of the fetal brain to ischemia.

Autoradiographic mapping of local cerebral permeability to bilirubin in immature rats: effects of hyperbilirubinemia

Kernicterus is characterized by the accumulation of bilirubin mainly into subcortical brainstem nuclei. Inasmuch as premature infants are more susceptible to kernicterus, we hypothesized that the cerebral permeability to bilirubin could vary by cerebral region and with age. Therefore, in the present study, we measured the blood-to-brain transfer constant (Ki) of [3H]bilirubin in 6-8 rats at postnatal age 10 (P10) or 21 d (P21) in basal conditions and after a bilirubin perfusion to explore age-related and bilirubin-induced changes in the cerebral permeability to the dye. Blood-to-brain transfer of [3H]bilirubin was measured in 39 brain regions by quantitative autoradiography in 15-min experiments. Rats exposed to unlabeled bilirubin received a loading dose of 160 mg/kg over 15 min followed by a 90-min bilirubin perfusion at a speed of 64 mg/kg/h. At P10, cerebral permeability to bilirubin ranged from 0.07 to 0.12 microL/g/min, except in the auditory nerve, dentate nucleus, hypothalamus, and thalamus where it reached 0.41-0.47 microL/g/min. At P21, Ki of bilirubin was significantly lower than at P10 and ranged from 0.03-0.06 microL/g/min in most brain areas. In P10 bilirubin-exposed rats, permeability to bilirubin significantly increased over control levels in all brain regions but three. The largest increases (> 350%) were recorded in the sensory regions, most limbic areas, hypothalamus, and thalamus. At P21, hyperbilirubinemia induced increases in blood-to-brain transfer of bilirubin of 50-200% in 16 brain areas, except in the hippocampus, sensory-motor cortex, and thalamic nuclei where they reached 200-433%. Thus, it appears that the immature rat brain (P10) is very permeable to bilirubin. The increased permeability with preexposure to the dye, especially in brain regions which are affected in infants with kernicterus, could be related either to the large decrease in the value of the albumin:bilirubin ratio between control (15-16) and hyperbilirubinemic conditions (1.7-1.8) and/or to an increased permeability to bilirubin.

Long-term metabolic effects of pentylenetetrazol-induced status epilepticus in the immature rat

The present study was devoted to the long-term effects of seizures induced by pentylenetetrazol in immature rats on cerebral metabolic rates in young adult animals. Seizures were induced by repetitive intraperitoneal injections of subconvulsive doses of pentylenetetrazol either in 10- (P10) or in 21- (P21) day-old rats. The long-term metabolic effects of the seizures were studied at P60 in 54 cerebral structures by means of the [14C]deoxyglucose method. At P60, metabolic activity was decreased in 10 brain regions of rats exposed to pentylenetetrazol at P10 and in 29 structures in rats exposed to seizures at P21. Among the structures whose metabolic activity was reduced at P60 by seizures occurring either at P10 or at P21 were mainly sensory, cortical and hippocampal regions plus mammillary body, i.e. all the structures metabolically characterized as most vulnerable to pentylenetetrazol-induced status epilepticus in our previous study [Pereira de Vasconcelos A. et al. (1992) Devl Brain Res. 69, 243-259]. In the animals exposed to seizures at P21, metabolic activity was also reduced at P60 in additional sensory and cortical regions, as well as in limbic, thalamic and hypothalamic nuclei, also considered as highly sensitive to short-term pentylenetetrazol-induced seizures [Pereira de Vasconcelos A. et. al. (1992)]. Rates of glucose utilization were also reduced in a few additional areas such as the monoaminergic cell groupings. In conclusion, there are some parallels between the structures metabolically most sensitive during pentylenetetrazol-induced status epilepticus in immature rats and the long-term regional metabolic decreases recorded at P60. Our data also confirm the well-known higher sensitivity to seizures during the third postnatal week in rodents.

Chronic hypoxia causes opposite effects on glucose transporter 1 mRNA in mature versus immature rat brain

We have shown previously that chronic hypoxia can regulate the expression of membrane proteins. Since there are virtually no glucose stores in the brain and glucose transport can be rate-limiting during stress, the role of glucose transporters becomes crucial for cell survival under stress. In the present study, we asked whether mRNA levels for glucose transporter 1 (GT1), which is expressed in a variety of cells in the brain, especially in the microvessels for glucose transport from blood vessels to brain, change in response to chronic hypoxia. Because major developmental changes occur in the rat CNS in-utero and in the first few weeks postnatally, we studied brain GT1 mRNA using Northern blot analysis at different ages after exposure of fetuses (from embryonic day 10 to birth), developing rats (from birth to 30 day old) or adult rats (from 90 to 120 day old) to hypoxia (Fractional inspired O2 9%). Our data show that (i) GT1 mRNA level was much lower in the newborn than in the adult and increased with age; (ii) chronic hypoxia caused a decrease of approximately 65% in GT1 mRNA in adult brain but induced an increase in fetal (more than 50%) and developing (approximately 80%) rats and (iii) the response of housekeeping gene (glyceraldehyde 3-phosphate dehydrogenase) was not similar to that of GT1, suggesting that the changes of GT1 mRNA are specific to glucose transporter.(ABSTRACT TRUNCATED AT 250 WORDS)

Correlation of local changes in cerebral blood flow, capillary density, and cytochrome oxidase during development

Although elevations in cerebral metabolic demand during development may induce angiogenesis, the correlation among ontogenic changes in local cerebral blood flow, cytochrome oxidase activity (an index of oxidative capacity) and capillary density have not been examined previously. We measured these parameters in selected regions of the brains of anesthetized rabbits of various ages. Increases in all three parameters occurred postnatally within the cerebral cortex and striatum, whereas in the medulla, values at birth were similar to those in adults. In newborns, the pattern of distribution of blood flow within the parietal cortex was such that levels were maximal in the outer layers and declined in deeper layers. This distribution correlated closely with that of capillary density, whereas cytochrome oxidase activity was maximal at levels deeper in the cortex. By postnatal day 17, the distribution for all three parameters was similar to that of cytochrome oxidase activity in young animals. A regression analysis of the regional values demonstrated a positive correlation between capillary density and blood flow in young (< or = postnatal day 8) and old (> or = postnatal day 17) animals. In contrast, cytochrome oxidase activity and capillary density were poorly correlated in young animals but positively correlated in older animals, with the slopes being markedly different (P < 0.005). The results suggest that early in postnatal development, the pattern of cytochrome oxidase activity is relatively mature compared with that of capillary density. By postnatal day 17, microvascular anatomy is closely associated to oxidative capacity, likely reflecting a steady state regulation of capillary density to metabolic requirements.

Regional cerebral blood flow response to acute hypoxia changes with postnatal age in the rat

The quantitative autoradiographic [14C]iodoantipyrine technique was applied to measure the effects of an acute hypoxic exposure on rates of local cerebral blood flow (LCBF) in the 10 (P10)-, 14 (P14)- and 21 (P21)-day-old rat. The animals were exposed to hypoxic (7% O2/93% N2) or control gas mixtures (21% O2/79% N2) for 40 min before the initiation of the 1-min LCBF measurement. At P10, hypoxia induced a 142-415% increase in LCBF over control levels, which affected the 45 structures studied. The highest increases in LCBF were noticed in posterior midbrain and brainstem regions. These increases are in good accordance with hypoxia-induced increases in LCBF recorded during acute hypoxia exposure in both newborn and adult animals. At P14 and P21, rates of LCBF decreased with hypoxia. These decreases were significant in 23 and 21 brain regions, respectively, belonging to all systems studied. These changes in LCBF are in quite good correlation with our previous data on the effects of acute hypoxia exposure on cerebral glucose utilization but the decrease in LCBF is of higher amplitude than the one in cerebral glucose utilization translating into a relative hypoperfusion at a constant metabolic level at P14 and P21. However, arterial blood pressure was reduced by 16 mmHg and arterial pCO2 was significantly decreased at the two latter ages in hypoxic animals compared to controls. These two systemic factors, and mainly hypocapnia, are rather responsible for the cerebral hypoperfusion recorded at P14 and P21 in hypoxic rats whereas the circulatory response seems to be predominantly hypoxic at P10.

Effects of bilirubin infusion on local cerebral glucose utilization in the immature rat

The clinical features of kernicterus have been extensively described. However, there are still no data available on a possible correlation between the areas which appear to preferentially accumulate bilirubin and regional changes in cerebral functional activity. Therefore, we applied the quantitative autoradiographic [14C]2-deoxyglucose method to the measurement of local cerebral metabolic rates for glucose (LCMRglc) in immature rats receiving a bilirubin infusion. A loading dose of 160 mg/kg bilirubin in a buffered serum albumin solution was first given to the rats over 15 min. Thereafter, bilirubin was infused at a reduced rate, 64 mg/kg/h. Bilirubin infusion lasted from 2 to 3 h according to the age of the animal, in order to obtain a plasma concentration of bilirubin ranging from 200 to 300 mumol/l over the experimental period. Bilirubin entered the brain without any sign of blood-brain barrier alteration. The [14C]2-deoxyglucose was injected to the animals 45 min before the end of bilirubin infusion. Rats were studied at 3 postnatal ages, 10 (P10), 14 (P14) and 21 days (P21). Hyperbilirubinemia induced widespread decreases in LCMRglc's in all brain areas and at all ages. These decreases were mostly prominent in sensory areas, auditory and visual, as well as in hypothalamic and thalamic regions. Especially at P10, the distribution of LCMRglc's was strikingly heterogeneous in both cerebral cortex and caudate nucleus, appearing as alternate dark and white columns or as alternate dark and light dots, respectively. The data of the present study are in agreement with clinical observations reporting that bilirubin mostly accumulates in the striatum and cranial nerves and that the neurological sequelae of kernicterus are very often hearing loss as well as motor problems.

Relationship between ion gradients and neurotransmitter release in the newborn rat striatum during anoxia

It has been well documented that mammalian newborns are more resistant to hypoxia than adults. The mechanisms for this tolerance has attracted considerable attention due to its clinical implications. Recently, there has been great interest in comparing the mechanisms involved in such tolerance with those of turtle brain, which has shown a remarkable tolerance to anoxia. In the latter, much attention has been paid to the role of neurotransmitters in regulating brain metabolic rate. In order to investigate this phenomenon in the mammalian neonate the pattern of neurotransmitter release with respect to pre- and postdepolarization stages was determined. Microdialysis was used to ascertain levels of neurotransmitters in the striatum of 5-day-old rats. Ion homeostasis was determined with a potassium-selective microelectrode. We report here that during anoxia at the predepolarization stage purines (inosine, hypoxanthine, xanthine and adenosine) were significantly released. However, amino acids (glutamate, gamma-amino butyric acid (GABA), aspartate and taurine) remained low during the first 30 min, but were released during anoxic depolarization. It was concluded that mammalian neonate brain differs from that of the turtle in hypoxic adaptations, which may be consequence of its comparatively undifferentiated state.

An experimental model of generalized seizures for the measurement of local cerebral glucose utilization in the immature rat. II. Mapping of brain metabolism using the quantitative [14C]2-deoxyglucose technique

The quantitative autoradiographic [14C]2-deoxyglucose technique (2DG) was applied to measure the effects of pentylenetetrazol (PTZ)-induced status epilepticus (SE) on local cerebral metabolic rates for glucose (LCMRglc) in 10 (P10)-, 14 (P14)-, 17 (P17)- and 21 (P21)-day-old rats. To produce long-lasting SE (55 min), the animals received repetitive, timed intraperitoneal injections of subconvulsive doses of PTZ until SE was reached. At P10 and P14, SE induced a marked increase in LCMRglc which affected 66 of the 76 structures studied. Increases were especially high (200-400%) in limbic and motor cortices at P10 and in some brainstem areas at these 2 ages. At P17 and P21, average brain glucose utilization was similar in seizing and control rats, but in PTZ-treated rats reflected a redistribution in local metabolic rates with increases in brainstem, midbrain, hypothalamus and septum, decreases in cortex, hippocampus, some sensory areas and white matter and no change in many motor and limbic structures. In a few cerebral regions, such as hippocampus, dentate gyrus and mammillary body, LCMRglc did not increase at P10 and P14 and decreased at P17 and P21 in PTZ- vs. saline-treated rats. The results of the present study show that the immature brain responds to sustained seizure activity in a specific way according to its maturational state. Moreover, these data allow the mapping of the vulnerability of cerebral structures to seizures, according to their metabolic response to convulsions.

An experimental model of generalized seizures for the measurement of local cerebral glucose utilization in the immature rat. I. Behavioral characterization and determination of lumped constant

An experimental model of status epilepticus has been developed in the immature rat by administration of pentylenetetrazol (PTZ) using repetitive, timed intraperitoneal injections of subconvulsive doses. The pattern of behavioral signs has been well characterized in each age group, i.e. 10 (P10), 14 (P14), 17 (P17) and 21 postnatal days (P21). In this model, the dose of convulsant could be adjusted as a function of interindividual sensitivity and status epilepticus lated for quite a long duration to allow the measurement of local cerebral metabolic rates for glucose (LCMRglc) by means of the [14C]2-deoxyglucose method [J. Neurochem., 28 (1977) 897-916]. To estimate LCMRglc during status epilepticus, the lumped constant (LC) was re-calculated in controls and PTZ-treated rats. The control LC was 0.54 at P10 and 0.50-0.51 at the three older ages studied (P14, P17 and P21). During status epilepticus, it increased to 0.64 in P10 rats and decreased to 0.42 and 0.40, respectively, in P17 and P21 animals. At P14, LC was not affected by seizures. The measurements of brain lactate levels showed a large 4.5-10-fold increase in PTZ-treated rats as compared to controls at all ages. The results of the present study show that the immature brain responds to sustained seizure activity in a specific way according to its postnatal age. Moreover, our results underscore the necessity of re-calculation of LC to the quantification of LCMRglc in such pathological states, particularly in immature animals.

Effects of pentylenetetrazol-induced seizures on dopamine and norepinephrine levels and on glucose utilization in various brain regions of the developing rat

Levels of dopamine and norepinephrine were measured in seven brain areas after 60 min of sustained seizure activity induced by intraperitoneal repetitive timed administrations of pentylenetetrazol in rats at 10, 14, 17 and 21 days of postnatal life. The tissue levels of norepinephrine were markedly reduced in the majority of brain structures, except for striatum at 10 and 14 days. Conversely, dopamine concentrations increased in many areas and at various ages, except in cerebral cortex at 10 and 14 days and in midbrain between 14 and 21 days. PTZ seizures induced marked increases over control levels in the rates of glucose utilization, measured by the quantitative autoradiographic [14C]2-deoxyglucose method, in all dopamine- and norepinephrine-innervated areas studied at 10 and 14 days, except in cerebellar cortex at both ages and in frontal cortex and anteroventral thalamus at 14 days. At 17 and 21 days, glucose utilization remained increased over control levels in some areas, mainly in catecholaminergic cell groupings such as substantia nigra, ventral tegmental area and locus coeruleus, but was significantly reduced in cortex, caudate nucleus and thalamus, and similar to control rates in other regions. The present results suggest that pentylenetetrazol-induced seizures lead to a simultaneous increase in functional activity of norepinephrine neurons and an inhibition of dopaminergic-mediated neurons. They also confirm the maturation of connections, of metabolic activity and of neurotransmitter interaction within the brain, occurring mainly during the third week of postnatal life, paralleled by an increased selective vulnerability of some regions to this kind of insult.