The survival of young rats in nitrogen

Effects of antenatal dexamethasone and hyperglycemia on cardiovascular adaptation to asphyxia in preterm fetal sheep

Antenatal glucocorticoids improve outcomes among premature infants but are associated with hyperglycemia, which can exacerbate hypoxic-ischemic injury. It is still unclear how antenatal glucocorticoids or hyperglycemia modulate fetal cardiovascular adaptations to severe asphyxia. In this study, preterm fetal sheep received either saline or 12 mg im maternal dexamethasone, followed 4 h later by complete umbilical cord occlusion (UCO) for 25 min. An additional cohort of fetuses received titrated glucose infusions followed 4 h later by UCO to control for the possibility that hyperglycemia contributed to the cardiovascular effects of dexamethasone. Fetuses were studied for 7 days after UCO. Maternal dexamethasone was associated with fetal hyperglycemia (P < 0.001), increased arterial pressure (P < 0.001), and reduced femoral (P < 0.005) and carotid (P < 0.05) vascular conductance before UCO. UCO was associated with bradycardia, femoral vasoconstriction, and transient hypertension. For the first 5 min of UCO, fetal blood pressure in the dexamethasone-asphyxia group was greater than saline-asphyxia (P < 0.001). However, the relative increase in arterial pressure was not different from saline-asphyxia. Fetal heart rate and femoral vascular conductance fell to similar nadirs in both saline and dexamethasone-asphyxia groups. Dexamethasone did not affect the progressive decline in femoral vascular tone or arterial pressure during continuing UCO. By contrast, there were no effects of glucose infusions on the response to UCO. In summary, maternal dexamethasone but not fetal hyperglycemia increased fetal arterial pressure before and for the first 5 min of prolonged UCO but did not augment the cardiovascular adaptations to acute asphyxia.

A Physiology-Based Pharmacokinetic Framework to Support Drug Development and Dose Precision During Therapeutic Hypothermia in Neonates

Therapeutic hypothermia (TH) is standard treatment for neonates (≥36 weeks) with perinatal asphyxia (PA) and hypoxic-ischemic encephalopathy. TH reduces mortality and neurodevelopmental disability due to reduced metabolic rate and decreased neuronal apoptosis. Since both hypothermia and PA influence physiology, they are expected to alter pharmacokinetics (PK). Tools for personalized dosing in this setting are lacking. A neonatal hypothermia physiology-based PK (PBPK) framework would enable precision dosing in the clinic. In this literature review, the stepwise approach, benefits and challenges to develop such a PBPK framework are covered. It hereby contributes to explore the impact of non-maturational PK covariates. First, the current evidence as well as knowledge gaps on the impact of PA and TH on drug absorption, distribution, metabolism and excretion in neonates is summarized. While reduced renal drug elimination is well-documented in neonates with PA undergoing hypothermia, knowledge of the impact on drug metabolism is limited. Second, a multidisciplinary approach to develop a neonatal hypothermia PBPK framework is presented. Insights on the effect of hypothermia on hepatic drug elimination can partly be generated from in vitro (human/animal) profiling of hepatic drug metabolizing enzymes and transporters. Also, endogenous biomarkers may be evaluated as surrogate for metabolic activity. To distinguish the impact of PA versus hypothermia on drug metabolism, in vivo neonatal animal data are needed. The conventional pig is a well-established model for PA and the neonatal Göttingen minipig should be further explored for PA under hypothermia conditions, as it is the most commonly used pig strain in nonclinical drug development. Finally, a strategy is proposed for establishing and fine-tuning compound-specific PBPK models for this application. Besides improvement of clinical exposure predictions of drugs used during hypothermia, the developed PBPK models can be applied in drug development. Add-on pharmacotherapies to further improve outcome in neonates undergoing hypothermia are under investigation, all in need for dosing guidance. Furthermore, the hypothermia PBPK framework can be used to develop temperature-driven PBPK models for other populations or indications. The applicability of the proposed workflow and the challenges in the development of the PBPK framework are illustrated for midazolam as model drug.

Adaptive sugar provisioning controls survival of C. elegans embryos in adverse environments

The ability to adapt to changing environmental conditions is essential to the fitness of organisms. In some cases, adaptation of the parent alters the offspring's phenotype [1-10]. Such parental effects are adaptive for the offspring if the future environment is similar to the current one but can be maladaptive otherwise [11]. One mechanism by which adaptation occurs is altered provisioning of embryos by the parent [12-16]. Here we show that exposing adult Caenorhabditis elegans to hyperosmotic conditions protects their offspring from these conditions but causes sensitivity to anoxia exposure. We show that this alteration of survival is correlated with changes in the sugar content of adults and embryos. In addition, mutations in gene products that alter sugar homeostasis also alter the ability of embryos to survive in hyperosmotic and anoxic conditions and engage in the adaptive parental effect. Our results indicate that there is a physiological trade-off between the presence of glycerol, which protects animals from hyperosmotic conditions, and glycogen, which is consumed during anoxia. These two metabolites play an essential role in the survival of worms in these adverse environments, and the adaptive parental effect we describe is mediated by the provisioning of these metabolites to the embryo.

Preterminal gasping during hypoxic cardiac arrest increases cardiac function in immature rats

Newborn animals are more resistant to anoxia than older animals, partly due to an increased tolerance of the immature heart to anoxia. Newborn animals also have a more robust preterminal gasp. We investigated the relationship between gasping and cardiac function in immature and maturing rats exposed to anoxia. Immature postnatal day 7 (PND7) rats (n = 13) and maturing PND17 rats (n = 13) were exposed to 100% nitrogen (anoxia) for 10 min. Echocardiography was used to calculate cardiac contractility (CC) by left ventricular shortening fraction and cardiac output (CO) from Doppler velocity recordings of pulmonary artery blood flow. In a separate group of PND7 rats, CC and CO were recorded after the paralytic agent pancuronium was used to prevent gasping. Anoxia decreased CC and CO in PND7 and PND17 rats, followed by a partial and transient recovery. Gasping preceded recovery of CO and was required to sustain CO. Gasping in PND7 rats lasted longer (541 s versus 351 s, p < 0.01) and resulted in a greater recovery of CC and CO. Anoxia-induced gasping and the associated recovery of cardiac function were abolished by paralysis. Thus, anoxia-induced gasping transiently improves cardiac function, and more robust gasping in immature rats is associated with increased cardiac anoxic tolerance.

Effect of glycemic state in rats submitted to status epilepticus during development

The effect of glycemic state on status epilepticus (SE) development was studied in animals of different ages, submitted to pilocarpine model of epilepsy. Groups: I- Rats with 9-day-old (P9): IA. Submitted to 1SE; IB. Saline-treated; IC. Induced- hyperglycemia; ID. Induced- hyperglycemia+SE; II- Rats submitted to three consecutive episodes of SE at P7, P8 and P9; III- Rats submitted to 1SE at P17; IV- Rats submitted to 1SE at P21. Hippocampal cell death and the expression of glucose transporter GLUT3 were analyzed in group I. The results demonstrated normoglycemia in the groups IA, IB and II, hypoglycemia in group III and hyperglycemia in group IV, showing that the glycemia during SE is age dependent. Induced hyperglycemia during SE in P9 protected the hippocampal neurons from death and both groups IC and ID presented increased GLUT3 expression, showing high glucose consumption by the hippocampus.

Prior exposure to hypoxic-induced apnea impairs protective responses of newborn rats in an exposure-dependent fashion: influence of normoxic recovery time

Experiments were carried out to determine whether prior exposure to hypoxic-induced apnea impairs protective responses of newborn rats. Ninety-five, 5- to 6-day-old rat pups were instrumented for respiratory measurements and placed prone in a metabolic chamber regulated to 37.0°C. The time to first and last gasp as well as the number of gasps were determined on exposure to unrelenting hypoxia after each pup had experienced 0, 1, 2, 3, 4, 9, or 14 hypoxic-induced apnea/autoresuscitation cycles (HIA/AR) at 5-min intervals. Prior exposure to HIA/AR did not significantly alter the time to first gasp, but it decreased the time to last gasp after two HIA/AR and the number of gasps after three HIA/AR on exposure to unrelenting hypoxia. When the normoxic recovery time after 9 HIA/AR was varied from 5 to 120 min, the time to last gasp as well as the total number of gasps increased on exposure to unrelenting hypoxia but only at 120 min (i.e., the number of gasps was similar but the time to last gasp was still decreased compared with that observed in naive animals exposed to unrelenting hypoxia). Thus prior exposure to hypoxic-induced apnea as may occur during obstructive sleep apnea or positional asphyxia decreases the number and duration of potential autoresuscitation producing gasps on exposure to unrelenting hypoxia for a period of up to and exceeding 120 min, respectively. The mechanism by which prior exposure to hypoxic-induced apnea influences the duration and number of hypoxic-induced gasps is unknown.

Protective responses of the newborn to hypoxia

In human infants, spontaneous recovery from sleep related apnea or positional asphyxia can occur early with or without behavioral and/or cortical arousal or later as a result of autoresuscitation from "asphyxial coma" by hypoxic gasping. Because it occurs when early defense mechanisms are absent or fail to resolve apnea or positional asphyxia, autoresuscitation serves as a backup mechanism and is considered to be the last operative mechanism used by mammals to ensure survival during exposure to severe hypoxia. In this short review, factors will be considered that influence the onset, duration and number of potential autoresuscitation producing gasps as well as the integrated physiology of successful autoresuscitation and pathophysiology of failed autoresuscitation from hypoxic-induced apnea.

Utilization of juvenile animal studies to determine the human effects and risks of environmental toxicants during postnatal developmental stages

BACKGROUND

Toxicology studies utilizing animals and in vitro cellular or tissue preparations have been used to study the toxic effects and mechanism of action of drugs and chemicals and to determine the effective and safe dose of drugs in humans and the risk of toxicity from chemical exposures. Testing in animals could be improved if animal dosing using the mg/kg basis was abandoned and drugs and chemicals were administered to compare the effects of pharmacokinetically and toxicokinetically equivalent serum levels in the animal model and human. Because alert physicians or epidemiology studies, not animal studies, have discovered most human teratogens and toxicities in children, animal studies play a minor role in discovering teratogens and agents that are deleterious to infants and children. In vitro studies play even a less important role, although they are helpful in describing the cellular or tissue effects of the drugs or chemicals and their mechanism of action. One cannot determine the magnitude of human risks from in vitro studies when they are the only source of toxicology data.

METHODS

Toxicology studies on adult animals is carried out by pharmaceutical companies, chemical companies, the Food and Drug Administration (FDA), many laboratories at the National Institutes of Health, and scientific investigators in laboratories throughout the world. Although there is a vast amount of animal toxicology studies carried out on pregnant animals and adult animals, there is a paucity of animal studies utilizing newborn, infant, and juvenile animals. This deficiency is compounded by the fact that there are very few toxicology studies carried out in children. That is one reason why pregnant women and children are referred to as "therapeutic orphans."

RESULTS

When animal studies are carried out with newborn and developing animals, the results demonstrate that generalizations are less applicable and less predictable than the toxicology studies in pregnant animals. Although many studies show that infants and developing animals may have difficulty in metabolizing drugs and are more vulnerable to the toxic effects of environmental chemicals, there are exceptions that indicate that infants and developing animals may be less vulnerable and more resilient to some drugs and chemicals. In other words, the generalization indicating that developing animals are always more sensitive to environmental toxicants is not valid. For animal toxicology studies to be useful, animal studies have to utilize modern concepts of pharmacokinetics and toxicokinetics, as well as "mechanism of action" (MOA) studies to determine whether animal data can be utilized for determining human risk. One example is the inability to determine carcinogenic risks in humans for some drugs and chemicals that produce tumors in rodents, When the oncogenesis is the result of peroxisome proliferation, a reaction that is of diminished importance in humans.

CONCLUSIONS

Scientists can utilize animal studies to study the toxicokinetic and toxicodynamic aspects of drugs and environmental toxicants. But they have to be carried out with the most modern techniques and interpreted with the highest level of scholarship and objectivity. Threshold exposures, no-adverse-effect level (NOAEL) exposures, and toxic effects can be determined in animals, but have to be interpreted with caution when applying them to the human. Adult problems in growth, endocrine dysfunction, neurobehavioral abnormalities, and oncogenesis may be related to exposures to drugs, chemicals, and physical agents during development and may be fruitful areas for investigation. Maximum permissible exposures have to be based on data, not on generalizations that are applied to all drugs and chemicals. Epidemiology studies are still the best methodology for determining the human risk and the effects of environmental toxicants. Carrying out these focused studies in developing humans will be difficult. Animal studies may be our only alternative for answering many questions with regard to specific postnatal developmental vulnerabilities.

Apoptosis and neurogenesis after transient hypoxia in the developing rat brain

Perinatal brain damage following a hypoxic-ischemic episode has been considered for a long time as an irreversible phenomenon. However, recent studies have shown that various insults may induce de novo neurogenesis in the adult rodent brain. The present study tested the hypothesis that acute hypoxia may trigger neurogenesis in the developing brain. In vitro, the influence of transient hypoxia was analyzed on the outcome of embryonic rat neurons in culture. In vivo, the temporal profile of brain damage was monitored at the level of the CA1 layer of the hippocampus after the exposure to hypoxia of 1-day-old rats. The extent of cell loss and regeneration was evaluated after staining with DAPI. The characterization of newly generated cells was performed in the subventricular zone at 20 days postexposure by immunohistochemistry. Following hypoxia for 6 hours, neuronal viability in the culture dishes was reduced by 36% at 96 hours, with a significant number of cell nuclei showing apoptosis features. In contrast, a 3-hour hypoxia apparently did not damage cultured neurons whose number increased by 14%. The Bax/Bcl-2 ratio tended to increase after 6-hour hypoxia and to decrease after 3-hour hypoxia. In vivo, hypoxia induced cell damage in the CA1 subfield of the hippocampus, where the total number of cells was reduced by 27% at days 6-7 postreoxygenation, with histopathological hallmarks of apoptosis. This cell deficit was followed by a gradual recovery observable from day 20, suggesting a repair mechanism. Brain incorporation of BrdU in the subventricular zone revealed an accumulation of proliferating cells expressing the neuronal marker NeuroD. The present data demonstrate that a posthypoxic neurogenesis does occur during development and may account for brain protection.

Characterization of successful and failed autoresuscitation in human infants, including those dying of SIDS

Our purpose was to identify and further characterize physiologic mechanisms relevant to autoresuscitation from hypoxic apnea in infants dying suddenly and unexpectedly. We studied cardiorespiratory recordings of 24 infants (age range, 0.8-21 months) who died suddenly while being monitored at home. These recordings were analyzed for features indicated by studies in animal models to be characteristic of hypoxic gasping, and of recovery from bradycardia and apnea associated with gasping (e.g., autoresuscitation). Findings in 5 infants diagnosed as having sudden infant death syndrome were compared with 6 non-SIDS infants whose deaths resulted from other conditions. Additionally, we studied 15 healthy infants during sleep, using home monitor and other respiratory recording techniques, in order to obtain comparison data. We found in recordings from 23 of 24 subjects that hypoxic gasps with characteristic features occurred immediately preceding death. A unique pattern of complex, closely spaced gasps ("double" or "triple" gasps) was present in many subjects. Evidence of partially successful autoresuscitation closely following one or more gasps occurred in 11 subjects, while another 4 had evidence of complete autoresuscitation with return of normal heart rate and resolution of apnea on one or more occasions. Significant differences between SIDS infants and those dying from other causes included increased occurrence of complex gasps and decreased occurrence of partial or complete autoresuscitation in the SIDS infants. The non-SIDS cases were different from the SIDS cases in that only one had "double" gasps (n = 7), while none had "triple" gasps, as compared with 4 out of 5 SIDS cases with these patterns (P < 0.05, chi-square). Also, in contrast with the SIDS cases, more of the cases with specific postmortem diagnoses had evidence of partial (5 out of 6 cases) or complete (1 out of 6 cases) autoresuscitation (P < 0.05, chi-square). We conclude that partial or complete autoresuscitation by gasping is not uncommon in moribund infants during the first year of life. Failure of autoresuscitation mechanisms other than failure to initiate gasping may be characteristic of infants dying of SIDS. Some SIDS infants appear to be different from infants dying with other diagnoses with respect to efficacy and characteristics of hypoxic gasping.

Hypoglycemic brain injury

Hypoglycemia frequently occurs in newborn infants who previously have suffered asphyxia, who are offspring of diabetic mothers, or who are low birthweight for gestational age (IUGR). Many infants who are hypoglycemic do not exhibit clinical manifestations, while others are symptomatic and at risk for the occurrence of permanent brain damage. This review emphasizes the clinical, neuropathologic, and neuro-imaging features of hypoglycemia in newborn infants, especially those who are symptomatic. Neurologic morbidity occurs particularly in those infants who have suffered severe, protracted, or recurrent symptomatic hypoglycemia. Experimental observations emphasize the resistance of the immature brain to the damaging effect of hypoglycemia; such resistance occurs as a consequence of compensatory increases in cerebral blood flow, lower energy requirements, higher endogenous carbohydrate stores, and an ability to incorporate and consume alternative organic substrates to spare glucose for energy production. Hypoglycemia combined with hypoxia-ischemia (asphyxia) is more deleterious to the immature brain than either condition alone.

Postnatal age influences the ability of rats to autoresuscitate from hypoxic-induced apnea

Failure to autoresuscitate from apnea by gasping has been suggested to have a role in sudden infant death. Little is known, however, about the factors that influence the ability of gasping to sustain life during acute hypoxia in the newborn. The present experiments were carried out on 105 rat pups to investigate the influence of postnatal age on the time to last gasp during a single hypoxic exposure and on the ability to autoresuscitate from primary apnea during repeated hypoxic exposures. On days 1-2, 5-6, 10-11, 15-16, and 19-20 postpartum, each pup was placed into a temperature-controlled chamber regulated to 37 +/- 1 degrees C and was exposed either to a single period of hypoxia produced by breathing an anoxic gas mixture (97% N(2)-3% CO(2)), and the time to last gasp was determined, or repeated exposure to hypoxia was performed, and the ability to autoresuscitate from primary apnea was determined. Increases in postnatal age decreased the time to last gasp following a single hypoxic exposure and decreased the number of successful autoresuscitations following repeated hypoxic exposures. Thus our data provide evidence that postnatal age influences protective responses that may prevent death during hypoxia as may occur during episodes of prolonged sleep apnea.

Influence of core temperature on autoresuscitation during repeated exposure to hypoxia in normal rat pups

Failure to autoresuscitate by hypoxic gasping during prolonged sleep apnea has been suggested to play a role in sudden infant death. Furthermore, thermal stress brought about by a contribution of infection, overwrapping, or excessive environmental heating has been shown to be associated with an increased risk of sudden infant death, particularly in prone sleeping infants. The present experiments were carried out on newborn rat pups to investigate the influence of "forced" changes in core temperature on their time to last gasp during a single hypoxic exposure and on their ability to autoresuscitate during repeated exposure to hypoxia. On day 5 or 6 postpartum the pups were placed in a temperature-controlled chamber regulated to 33, 35, 37, 39, or 41 degrees C and exposed to a single period of hypoxia (97% N(2)-3% CO(2)) and their time to last gasp was determined, or they were exposed repeatedly to hypoxia and their ability to autoresuscitate from primary apnea was determined. Increases in core temperature brought about by changes in ambient temperature from 33 to 41 degrees C decreased the time to last gasp after a single hypoxic exposure and decreased the number of successful autoresuscitations after repeated hypoxic exposures. Thus our data support the hypothesis that forced changes in core temperature brought about by changes in ambient temperature influence protective responses in newborns that may prevent death during hypoxia, as may occur during single or repeated episodes of prolonged sleep apnea.

Neonatal tolerance to hypoxia: a comparative-physiological approach

Newborn mammals exhibit a number of physiological reactions which differ from normal adult physiology and are often regarded as signs of immaturity. However, when looked upon from a comparative point of view, it becomes obvious that some of these 'physiological peculiarities' bear striking similarity to adaptation mechanisms known from hypoxia-tolerant animals and may thus contribute to the well-established, yet poorly understood, phenomenon of neonatal hypoxia tolerance. As the mammalian fetus lives at oxygen partial pressures corresponding to 8000 m altitude, the first line of perinatal hypoxia defense consists of long-term adaptations to limited intrauterine oxygen supply: (1) improved O2 transport by fetal acclimatization to high altitude, (2) reduced metabolic rate by hibernation-like deviation from metabolic size allometry, (3) diminished cerebral vulnerability by functional analogies to diving turtle brain, and (4) enhanced metabolic flexibility by optional repartitioning of energy supply from growth to maintenance metabolism. In the case of birth asphyxia, these background mechanisms are complemented by short-term responses to acute oxygen lack: (1) reduction of body temperature as in natural torpor, (2) reduction of heart rate and redistribution of circulation as in diving mammals, (3) reduction of respiration rate typical of 'hypoxic hypometabolism', and (4) reduction of blood pH according to the concept of 'acidotic torpidity'. Although anaerobic metabolism is improved in neonatal mammals by increased glycogen stores, reduced metabolic demands, and sustained wash-out of acid metabolites, neonatal hypoxia tolerance seems to be primarily based on the ability to maintain tissue aerobiosis as long as possible. This is even reflected by isoenzyme patterns which do not consistently favour anaerobic glycolysis and, thus, are reminiscent of the 'lactate paradox' found in high altitude adaptation. Altogether, from a biological point of view, the perinatal period appears as a source of adaptive mechanisms that can be refound, in varying combinations, in many survival strategies. From a clinical point of view, the interplay of long- and short-term mechanisms offers a novel approach to estimation of the newborn's ability to withstand temporary oxygen lack. However, most of these mechanisms are not unambiguous and, above all, not unlimited in their protective effect so that they do not release obstetricians or neonatologists from their obligation to counteract fetal or neonatal hypoxia without delay.

Failure of autoresuscitation in weanling mice: significance of cardiac glycogen and heart rate regulation

"Autoresuscitation" (AR) is the spontaneous recovery from hypoxic apnea by gasping. We examined aspects of heart function in two situations: 1) the maturationally acquired failure of AR that is characteristic of SWR, but not BALB/c, weanling mice and 2) AR failure in BALB/c mice induced by repeated exposures to anoxia. We determined maturational changes in heart and liver glycogen. Unlike liver glycogen levels, heart glycogen levels in SWR mice differed from those in BALB/c mice. They were consistently much lower throughout maturation and reached a nadir during the brief period when SWR weanling mice are vulnerable to AR failure. Also, rate of cardiac glycogen utilization in vulnerable SWR mice was lower than that of same-aged BALB/c mice and was nil during the latter one-half of the gasping stage when heart function is critical for AR success. Therefore, because glycogen utilization reflects cardiac work, heart failure could explain AR failure in SWR weanlings. Additionally, the increase in hypoxic heart rate that occurs with maturation is developmentally delayed in SWR mice, and this may contribute to their AR failure. Cardiac glycogen was not fully depleted in BALB/c mice during repeated anoxic exposures, indicating other reasons for AR failure. We view these findings as a potential model for the age-related peak in incidence of sudden infant death syndrome.

Maturation of the mammalian respiratory system

In this review, the maturational changes occurring in the mammalian respiratory network from fetal to adult ages are analyzed. Most of the data presented were obtained on rodents using in vitro approaches. In gestational day 18 (E18) fetuses, this network functions but is not yet able to sustain a stable respiratory activity, and most of the neonatal modulatory processes are not yet efficient. Respiratory motoneurons undergo relatively little cell death, and even if not yet fully mature at E18, they are capable of firing sustained bursts of potentials. Endogenous serotonin exerts a potent facilitation on the network and appears to be necessary for the respiratory rhythm to be expressed. In E20 fetuses and neonates, the respiratory activity has become quite stable. Inhibitory processes are not yet necessary for respiratory rhythmogenesis, and the rostral ventrolateral medulla (RVLM) contains inspiratory bursting pacemaker neurons that seem to constitute the kernel of the network. The activity of the network depends on CO2 and pH levels, via cholinergic relays, as well as being modulated at both the RVLM and motoneuronal levels by endogenous serotonin, substance P, and catecholamine mechanisms. In adults, the inhibitory processes become more important, but the RVLM is still a crucial area. The neonatal modulatory processes are likely to continue during adulthood, but they are difficult to investigate in vivo. In conclusion, 1) serotonin, which greatly facilitates the activity of the respiratory network at all developmental ages, may at least partly define its maturation; 2) the RVLM bursting pacemaker neurons may be the kernel of the network from E20 to adulthood, but their existence and their role in vivo need to be further confirmed in both neonatal and adult mammals.

Perinatal nicotine exposure impairs ability of newborn rats to autoresuscitate from apnea during hypoxia

Failure to autoresuscitate by hypoxic gasping during prolonged sleep apnea has been suggested to play a role in sudden infant death. Furthermore, maternal smoking has been repeatedly shown to be a risk factor for sudden infant death. The present experiments were carried out on newborn rat pups to investigate the influence of perinatal exposure to nicotine (the primary pharmacological and addictive agent in tobacco) on their time to last gasp during a single hypoxic exposure and on their ability to autoresuscitate during repeated exposure to hypoxia. Pregnant rats received either nicotine (6 mg. kg-1. 24 h-1) or vehicle continuously from day 6 of gestation to days 5 or 6 postpartum via an osmotic minipump. On days 5 or 6 postpartum, pups were exposed either to a single period of hypoxia (97% N2-3% CO2) and their time to last gasp was determined, or they were exposed repeatedly to hypoxia and their ability to autoresuscitate from primary apnea was determined. Perinatal exposure to nicotine did not alter the time to last gasp, but it did impair the ability of pups to autoresuscitate from primary apnea. After vehicle, the pups were able to autoresuscitate from 18 +/- 1 (SD) periods of hypoxia, whereas, after nicotine, the pups were able to autoresuscitate from only 12 +/- 2 periods (P < 0.001) of hypoxia. Thus our data provide evidence that perinatal exposure to nicotine impairs the ability of newborn rats to autoresuscitate from primary apnea during repeated exposure to hypoxia, such as may occur during episodes of prolonged sleep apnea.

Maternal cocaine alters eupneic ventilation but not gasping of neonatal rats

In severe hypoxia, normal eupneic respiration is replaced by gasping. Gasping provides for 'autoresuscitation' such that, if air is available, normal breathing returns. As maternal use of cocaine may increase the incidence of the Sudden Infant Death Syndrome (SIDS), prenatal exposure to cocaine was hypothesized to result in a failure of gasping in the newborn. Cocaine was administered daily to pregnant rats. In the newborn, no impairment of gasping as a mechanism of autoresuscitation was detected. However, on the first and second days after birth, ventilation in hypoxia was less in newborns having exposure to cocaine than in the control rats. Maternal use of cocaine may retard the development of the ventilatory control system. However, gasping mechanisms are not included in this retardation.

Increasing vulnerability of astrocytes to oxidative injury with age despite constant antioxidant defenses

This paper investigates the vulnerability of astrocytes to oxidative injury as a function of age in culture in mice. Primary murine cortical astrocyte cultures of different ages were exposed to H2O2, combined oxygen-glucose deprivation or glucose deprivation. Astrocytes became more vulnerable to damage from each injury paradigm with age, showing transitions between 15 and 22 days. Both the antioxidant glutathione and superoxide dismutase activity increased after 30 days in culture, while catalase activity did not change up to 34 days. When the decrease in glutathione with injury was measured, young cells showed no change with H2O2 and decreases of < 20% after oxygen-glucose deprivation or glucose deprivation, while older cultures lost > 50% of their glutathione with the same insults. Since iron can be a catalyst for hydroxyl radical formation, we stained cultures and found both iron staining and ferritin immunoreactivity increased with age. Increased iron correlated with protection by deferoxamine against H2O2 injury. The three injury paradigms each had a unique pattern of protection by antioxidants. Dimethylthiourea, a hydrophilic antioxidant, protected from all three insults. Trolox, a lipophilic antioxidant, protected older astrocytes from oxygen-glucose deprivation and glucose deprivation. Deferoxamine provided near complete protection from H2O2, partial protection from oxygen-glucose deprivation and no protection from glucose deprivation. As evidence of increasing oxidative stress, lipid peroxidation resulting from oxygen-glucose deprivation increased with age, assessed with cis-parinaric acid. The increasing sensitivity of ageing astrocytes to oxidative injury occurs while antioxidant defenses are maintained. Increased sensitivity to H2O2 or oxygen-glucose deprivation correlates with iron accumulation.

Maturation of anoxia-induced gasping in the rat: potential role for N-methyl-D-aspartate glutamate receptors

After anoxia-induced apnea, gasping remains the last operative mechanism for survival. In developing rats, the gasping response to anoxia exhibits triphasic characteristics. Because anoxia is associated with enhanced release of glutamate, we hypothesized that N-methyl-D-aspartate (NMDA) glutamate receptors may underlie components of the gasping response. Rat pups aged 2 d (n = 50), 5 d (n = 43), 10 d (n = 42), and 15 d (n = 45) underwent anoxic challenges with 100% N2 in a whole body plethysmograph, 30 min after intraperitoneal administration of MK801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate; dizocilpine] (3 mg/kg), a noncompetitive NMDA glutamate receptor channel antagonist, or normal saline. In control pups, after primary apnea onset, a triphasic gasping pattern was apparent at all postnatal ages and included two distinct types of gasps (I and II). In 2- and 5-d MK801-treated animals, phase 1 and type I gasps were absent, leading to marked prolongations of the gasp latency and phase 2, the latter displaying type II gasps only. In addition, phase 3 duration was also prolonged with increased type II gasp frequencies. In contrast, in some 10-d-old (40%) and in all 15-d-old MK801-treated pups, although overall gasping duration was prolonged, the triphasic gasping pattern seen in matched controls was also present. We conclude that NMDA glutamate receptors mediate particular phasic components of the gasping response during early postnatal life but not at later stages of development. We speculate that developmental changes occur in both function and expression of NMDA and other neurotransmitters within brainstem regions underlying the neural substrate for gasp generation.

The cardiorespiratory response to anoxia: normal development and the effect of nicotine

Maternal smoking increases the risk of the sudden infant death syndrome (SIDS) 2-4-fold. The mechanism is unknown but may be related to hypoxia responses. Recovery from hypoxic apnea by young mammals depends on gasping and bradycardia. We asked whether prenatal nicotine exposure, reported to reduce hypoxic survival in 2 day old rat pups, acted by impairing gasping or bradycardia. Pregnant rats were infused throughout gestation and 1 week postnatally with nicotine tartrate (NIC) 12 mg/kg per day or saline (CON). Maternal plasma nicotine was 134.4 +/- 42 ng/ml, significantly reducing pup body weight. Pups at 3-28 days were exposed to anoxia (97% N2/3% CO2) until gasping ceased, while breathing and heart rate were recorded. NIC and CON groups were not significantly different at any age, in baseline heart rate, respiratory rate, the time course for bradycardia, time to gasp onset, duration of gasping, or number of gasps, although most of these variables declined significantly with age. We conclude that responses to anoxia are not affected by prenatal high-dose nicotine.

Medullary regions for neurogenesis of gasping: noeud vital or noeuds vitals?

Gasping is a critical mechanism for survival in that it serves as a mechanism for autoresuscitation when eupnea fails. Eupnea and gasping are separable patterns of automatic ventilatory activity in all mammalian species from the day of birth. The neurogenesis of the gasp is dependent on the discharge of neurons in the rostroventral medulla. This gasping center overlaps a region termed "the pre-Bötzinger complex." Neuronal activities of this complex, characterized in an in vitro brain stem spinal cord preparation of the neonatal rat, have been hypothesized to underlie respiratory rhythm generation. Yet, the rhythmic activity of this in vitro preparation is markedly different from eupnea but identical with gasping in vivo. In eupnea, medullary neuronal activities generating the gasp and the identical rhythm of the in vitro preparation are incorporated into a portion of the pontomedullary circuit defining eupneic ventilatory activity. However, these medullary neuronal activities do not appear critical for the neurogenesis of eupnea, per se.

The effect of hyperglycemia on cerebral metabolism during hypoxia-ischemia in the immature rat

Unlike adults, hyperglycemia with circulating glucose concentrations of 25-35 mM/L protects the immature brain from hypoxic-ischemic damage. To ascertain the effect of hyperglycemia on cerebral oxidative metabolism during the course of hypoxia-ischemia, 7-day postnatal rats underwent unilateral common carotid artery ligation followed by exposure to 8% O2 for 2 h at 37 degrees C. Experimental animals received 0.2 cc s.c. 50% glucose at the onset of hypoxia-ischemia, and 0.15 cc 25% glucose 1 h later to maintain blood glucose concentrations at 20-25 mM/L for 2 h. Control rat pups received equivalent concentrations or volumes of either mannitol or 1 N saline at the same intervals. The cerebral metabolic rate for glucose (CMRglc) increased from 7.1 (control) to 20.2 mumol 100 g-1 min-1 in hyperglycemic rats during the first hour of hypoxia-ischemia, 79 and 35% greater than the rates for saline-and mannitol-injected animals at the same interval, respectively (p < 0.01). Brain intracellular glucose concentrations were 5.2 and 3.0 mM/kg in the hyperglycemic rat pups at 1 and 2 h of hypoxia-ischemia, respectively; glucose levels were near negligible in mannitol- and saline-treated animals at the same intervals. Brain intracellular lactate concentrations averaged 13.4 and 23.3 mM/kg in hyperglycemic animals at 1 and 2 h of hypoxia-ischemia, respectively, more than twice the concentrations estimated for the saline- and mannitol-treated littermates. Phosphocreatine (PCr) and ATP decreased in all three experimental groups, but were preserved to the greatest extent in hyperglycemic animals. Results indicate that anaerobic glycolytic flux is increased to a greater extent in hyperglycemic immature rats than in normoglycemic littermates subjected to cerebral hypoxia-ischemia, and that the enhanced glycolysis leads to greater intracellular lactate accumulation. Despite cerebral lactosis, energy reserves were better preserved in hyperglycemic animals than in saline-treated controls, thus accounting for the greater resistance of hyperglycemic animals to hypoxic-ischemic brain damage.

Animal models of perinatal asphyxia: contributions, contradictions, clinical relevance

Animal models have contributed immensely to our understanding of hypoxic ischemic encephalopathy in the newborn. A number of animal models have been used, including both primate and subprimate species. Although the Rhesus monkey model has a dramatically similar pathological distribution of brain injury when compared with the human, other pathologic processes secondary to asphyxia may be more appropriately assessed in other species. The maxim that because primates are closer on the phylogenetic tree to humans than are subprimates all observations in the primate are applicable to the human is simply not true. Understanding of the neurochemical consequences of asphyxia in the past decade have arisen from experiments primarily in the neonatal rat. We have come to understand that not only is the hypoxic event of major significance, but that, once reperfused, reoxygenation causes further injury. Free-radical generation following reperfusion may be massive and may further contribute to cell membrane injury. These observations have lead to rational theoretic approaches to the treatment of hypoxic ischemic brain injury. On the other hand, previously used treatments such as osmotic agents and glucocorticoids would appear to be not only inefficacious but hazardous in the treatment of hypoxic ischemic brain injury. The role of nitric oxide (NO) in the pathogenesis of brain injury is yet uncertain, but there is little doubt that it plays a significant role. Although survival of the immature animal subjected to hypoxic environment is longer than in the mature animal, the central nervous system of the immature animal is more sensitive to glutamate and N-Methyl-D-aspartate (NMDA) receptor-mediated injury.

Medullary loci critical for expression of gasping in adult rats

1. Our purpose was to define whether a region of medulla could be identified that is critical for the expression of gasping. 2. Decerebrate, vagotomized, paralysed and ventilated adult rats were used. The pattern of phrenic activity was reversibly altered from eupnoea to gasping by exposure to hypoxia or anoxia. 3. Gasping was irreversibly eliminated following unilateral electrolytic lesions of the lateral tegmental field of the medulla. The eupnoeic rhythm continued after these lesions. 4. Injections of kainic acid into the lateral tegmental field also eliminated gasping. Phrenic activity in eupnoea was not altered. 5. Lesions outside the lateral tegmental field caused marked changes in the eupnoeic rhythm, including expiratory apnoea. Upon exposure to hypoxia or anoxia, gasping was still induced. 6. This region for the neurogenesis of gasping in rats is identical to the region that serves a comparable function in cats. Moreover, it overlaps with the 'pre-Bötzinger' complex which has been described for the in vitro brainstem preparation of the neonatal rat. Our results raise doubts that this complex plays a role in the neurogenesis of eupnoea.

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.

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

The quantitative autoradiographic 2-[14C]-deoxyglucose technique (2-DG) was applied to measure the effects of an acute hypoxic exposure on local cerebral metabolic rates for glucose (LCMRglcs) 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 mixture (21% O2/79% N2) for 20 min before the initiation and for the duration of the 2-DG procedure. Lumped constants were not affected by hypoxia at any age. At P10, the exposure to the hypoxic gas mixture induced a generalized increase in LCMRglc which affected 41 structures of the 45 studied. At P14, average cerebral glucose utilization was similar in hypoxic and control rats. LCMRglc increased in 5 areas and decreased in 11 regions, mainly brainstem and respiratory areas in hypoxic rats. Finally, at P21, LCMRglc decreased in 11 structures of hypoxic rats. The increase in LCMRglc in the hypoxic 10-day-old rat likely reflects stimulation of anaerobic glycolysis. Conversely, at P14 and P21, when the brain has become more dependent upon oxygen supply for its energy metabolism, levels of LCMRglc are similar in both groups of animals or decreased in a few structures of hypoxic compared to normoxic rats. The results of the present study show that the immature brain responds to an acute hypoxic insult in a specific way according to its maturational state. They are also in good accordance with the higher resistance of the immature animal to oxygen deprivation.

Glucose, lactic acid, and perinatal hypoxic-ischemic brain damage

Investigations suggest that hyperglycemia, superimposed on hypoxia-ischemia or cerebral ischemia, accentuates brain damage in adult experimental animals and humans, but not in immature animals. Fundamental differences in the immature and adult brain, which account for the age-specific paradox, are discussed. Based on currently available data, we recommend that glucose supplementation not be curtailed during labor and delivery of asphyxiated human infants; on the contrary, glucose therapy may substantially reduce hypoxic-ischemic brain damage.

Cerebral ischemia revisited: new insights as revealed using in vitro brain slice preparations

The elucidation of the pathophysiological mechanisms of cerebral ischemia/hypoxia dictates the use of experimental models which mimic this disabling brain condition. In vivo experimental models have been available for many decades and are responsible for the bulk of, though incomplete, knowledge we have about these mechanisms. Since study in isolation of each postulated mechanism is impossible in vivo, the need for an in vitro experimental model has intensified in recent years. Consequently, rat and guinea pig hippocampal slice preparations have emerged as the models of choice. This review attempts to highlight some of the results obtained using brain slices in the study of cerebral ischemia/hypoxia and compare them to those obtained in vivo. Both the biochemical and the physiological correlates of energy metabolism, ion homeostasis, neurotransmission and neuromodulation of this brain condition are reviewed. The agreements, and especially the disagreements, between the in vivo and in vitro findings are emphasized. Details are given of the possible roles of both lactic acid, Ca2+ and excitotoxins in the neuronal damage inflicted by cerebral ischemia/hypoxia. Recent attempts to protect brain slices against experimental cerebral ischemic/hypoxic damage are also reviewed here briefly.

The mechanism of neuronal resistance and adaptation to hypoxia

In this work we provide a theoretical explanation for the observations that: (i) young animals are more resistant to hypoxia than adult ones and (ii) repeated exposure to a hypoxic insult increases the tolerance of young animals and isolated brain tissue to that insult. Considered here is the role of taurine, a putative Ca2+ transport modulator, in attenuating Ca2+ influx and overload in brain tissue upon hypoxia. It is proposed that the higher resistance of young animals to hypoxia stems from their higher brain content of taurine as compared with adults. The increased resistance to lack of oxygen upon re-exposure to hypoxia may occur as a result of protein and coenzyme A (CoA) breakdown which leads to the accumulation of products like cystine, cysteine, cysteamine and other sulfur-containing compounds. Upon reoxygenation, these compounds are oxidized to form taurine, which in turn attenuates neuronal Ca2+ accumulation. The sulfur-containing compounds are considered to be natural scavengers of oxygen-derived free radicals which are formed upon reoxygenation and have been implicated as a major component in the process leading to ischemic/hypoxic brain damage. Repeated hypoxic insults bring about the formation of higher levels of taurine and hence the observed adaptation to oxygen lack. The hypothesis presented here is supported by experimental observations in our laboratory and those of others.

Taurine improves the recovery of neuronal function following cerebral hypoxia: an in vitro study

Rat hippocampal slices were used in the present study to assess the effect of a pretreatment with the amino acid taurine on their ability to recover synaptic function following a standardized hypoxic insult. After 10 min hypoxia, 47% of all control (untreated) slices exhibited recovery of synaptic function (orthodromically evoked CA1 population spike). Of slices pretreated with 0.5, 1.0 or 2.0 mM taurine, 63, 88 and 97% recovered from the same hypoxic insult. This dose-dependent protective effect was biphasic, as 5.0 mM taurine produced no protection. When hypoxia was extended to 15 min, only 20% of the untreated slices recovered, while 88% of slices treated with 1.0 mM taurine recovered their population spike. The same pretreatment attenuated the fall in the population spike amplitude upon Ca2+ depletion. We hypothesize that taurine plays an important role in an endogenous antihypoxic mechanism through the attenuation of Ca2+ movement across the neuronal membrane.

Adaptation of adult brain tissue to anoxia and hypoxia in vitro

The rat hippocampal slice preparation was used in the present study to demonstrate the ability of adult brain tissue to adapt to anoxic and hypoxic conditions. Adaptation was induced by pre-exposure of hippocampal slices to a short (5 min) anoxic episode. The evoked electrical activity of pre-exposed slices recovered from a subsequent, longer anoxic insult, while that of controls (without pre-exposure), receiving the same insult, did not. The adaptation process is time-dependent; an interval of 0.5 h between the pre-exposure and the subsequent anoxic insult allowed slices to resist anoxic periods of 13 +/- 2 min while after an interval of 2 h an anoxic period of 16 +/- 2 min could be tolerated. Evoked electrical activity persisted in adapted slices during exposure to hypoxia while their non-adapted controls exhibited synaptic silence under hypoxic conditions.

Similarities between lethal asphyxia in postneonatal rats and the terminal episode in SIDS

The age-related decrease in asphyxia tolerance was examined in young rats under conditions that mimic prolonged apnea. A precise end point of asphyxia tolerance was determined by calculation of LD50 values for exposed groups. Daily exposure to episodes of sublethal asphyxia did not alter the age-related decrease in asphyxia tolerance. These results permitted further studies in which groups of 10-day-old rats were exposed to 10 min of asphyxia resulting in partial mortality of the group. The postmortem findings in these animals were compared to figures for unexposed littermates. The age-related changes in response to asphyxia and the postmortem findings in asphyxiated animals were compared to the age distribution of sudden infant death syndrome (SIDS) and the postmortem findings in these infants. The comparison showed that the terminal event in SIDS shares many features with lethal asphyxia.

Perinatal hypoxia and subsequent development of seizures

Perinatal asphyxia has been implicated in the pathogenesis of persistent convulsive disorders later in life. Whether epilepsy is the result of oxygen deficiency alone or is due to the combined effect of hypoxia and ischemia is not known. In this report we studied the role of perinatal hypoxia alone on the development of epilepsy. One day and ten days old rat pups were exposed to prolonged hypoxia (6% O2). The subsequent susceptibility to focally elicited (kindled) or generalized (flurothyl) seizures was determined in the fourth week of age. Rats exposed to hypoxia were not more susceptible to the development of either type of seizures when compared to controls. Since the equivalent degree of hypoxia used for the 1 day old rat has previously been shown to result in lasting neurochemical and behavioral alterations and the degree of hypoxia used for the 10 day rat was lethal in over 35% of the animals, it is suggested that oxygen deficiency in and of itself may not be sufficient to lead to the development of epilepsy.

Oxygen consumption and survival prediction in neonatal rats exposed to prenatal hypervitaminosis A

Fetal exposure to excess vitamin A results in a highly variable degree of lung pathology and high neonatal mortality in the Long-Evans rat. The present study evaluated O2 consumption in newborn of vitamin A-treated, vehicle-treated, and untreated pregnancies on five consecutive postnatal days beginning with the day of delivery (D0). Pregnant female rats were treated by gavage with 160,000 USP units of retinyl acetate dissolved in 0.5 ml corn oil on days 15 through 19 of gestation. Vehicle and undisturbed controls were run concurrently. All animals delivered spontaneously, and the pups were tattooed and individually tested in a closed system consisting of three chambers submerged within a thermostatically controlled water bath at 33 degrees C. Vitamin A-exposed pups, as a group, have significantly lower QO2 (ml O2 consumed/min/kg body weight) values than controls through postnatal day 2 (p less than 0.05). By days 3 and 4 of age, the mean QO2 values of surviving vitamin A-treated pups were similar to those of controls. A QO2 of 30 or greater on day 0 appears to be critical for early neonatal survival of vitamin A-exposed pups, as 87% of the pups with initial QO2 less than 30 died prior to day 4. Oxygen consumption rates in teratogen-exposed pups exhibiting low QO2 on day 0 rarely reached normal levels. In contrast, the occasional control pup with such low initial levels were well within normal limits (means +/- 1 SD) by the following day.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of age on the metabolic and electrical responses to decapitation in the awake and the anesthetized rat brain

The effects of age on the responses of the brain to decapitation (complete ischemia) were tested in the awake and anesthetized rat. The metabolic and reflected light activities were evaluated by the use of a DC fluorometer/reflectometer (one or two hemispheres) and the electrical activity was measured by electrocorticogram. In all 81 animals used in these experiments the above-mentioned parameters were measured, to enable comparison between the age groups. The rats were divided into six age groups ranging from 2 to 120 weeks and each age group was subdivided into awake and anesthetized animals. The results can be summarized as follows. (1) The rate of energy consumption was lower in the young (2 weeks) and the old rats (120 weeks) compared to the adult rats. (2) The effect of anesthetic was more pronounced in the young and the old rats than in the adult rats. (3) The electrical activity was in good correlation with the energy availability in all animals of various ages as well as in the awake and anesthetized rats.

Cerebral carbohydrate metabolism during hypoglycemia and anoxia in newborn rats

The cerebral metabolic responses to perinatal hypoglycemia and anoxia were studied in newborn rats given regular insulin (30 units per kilogram of body weight). Animals were observed for up to 2 hours with no apparent ill effects in spite of blood glucose concentrations of 0.75 mmol per liter. When exposed to 100% nitrogen at 37 degrees C, hypoglycemic animals survived only one-tenth as long as littermate controls with normal blood glucose levels (4.7 mmol/L). Pretreatment of hypoglycemic rats with glucose (10 mmol/kg) 10 and 30 minutes prior to nitrogen exposure nearly completely reversed the anoxic vulnerability. Hypoglycemia led to progressive reductions in cerebral glycogen and glucose; however, only glucose reverted to normal levels 20 minutes after systemic glucose administration. The glycolytic intermediates glucose 6-phosphate and lactate were also lower during hypoglycemia. Brain glucose levels below 0.1 mmol per kilogram were associated with a disrupted cerebral energy state, reflected by declines in phosphocreatine (33%) and adenosine triphosphate (ATP) (10%). Cerebral energy utilization (metabolic rate) was minimally reduced (-7.2%) by hypoglycemia and returned to the control value (2.36 mmol approximately P/kg/min) with glucose treatment. The cerebral energy reserves ATP, adenosine diphosphate, and phosphocreatine declined more rapidly and to a lower level in hypoglycemic rats subjected to 2 1/2 minutes of anoxia than in normoglycemic animals rendered similarly hypoxic. The findings suggest that decreased anoxic resistance of hypoglycemic newborn rats is not primarily a function of reduced brain glycogen or altered cerebral metabolic rate. The presence of endogenous cerebral glucose stores combined with continued circulating glucose (cerebrovascular perfusion) appear to be critical factors for maintaining perinatal hypoxic survival.