Dexamethasone prevents hypoxia/ischemia-induced reductions in cerebral glucose utilization and high-energy phosphate metabolites in immature brain

Bidirectional Crosstalk Between Hypoxia Inducible Factors and Glucocorticoid Signalling in Health and Disease

Glucocorticoid-induced (GC) and hypoxia-induced transcriptional responses play an important role in tissue homeostasis and in the regulation of cellular responses to stress and inflammation. Evidence exists that there is an important crosstalk between both GC and hypoxia effects. Hypoxia is a pathophysiological condition to which cells respond quickly in order to prevent metabolic shutdown and death. The hypoxia inducible factors (HIFs) are the master regulators of oxygen homeostasis and are responsible for the ability of cells to cope with low oxygen levels. Maladaptive responses of HIFs contribute to a variety of pathological conditions including acute mountain sickness (AMS), inflammation and neonatal hypoxia-induced brain injury. Synthetic GCs which are analogous to the naturally occurring steroid hormones (cortisol in humans, corticosterone in rodents), have been used for decades as anti-inflammatory drugs for treating pathological conditions which are linked to hypoxia (i.e. asthma, ischemic injury). In this review, we investigate the crosstalk between the glucocorticoid receptor (GR), and HIFs. We discuss possible mechanisms by which GR and HIF influence one another, in vitro and in vivo, and the therapeutic effects of GCs on HIF-mediated diseases.

Glucocorticoids, metabolism and brain activity

The review integrates different experimental approaches including biochemistry, c-Fos expression, microdialysis (glutamate, GABA, noradrenaline and serotonin), electrophysiology and fMRI to better understand the effect of elevated level of glucocorticoids on the brain activity and metabolism. The available data indicate that glucocorticoids alter the dynamics of neuronal activity leading to context-specific changes including both excitation and inhibition and these effects are expected to support the task-related responses. Glucocorticoids also lead to diversification of available sources of energy due to elevated levels of glucose, lactate, pyruvate, mannose and hydroxybutyrate (ketone bodies), which can be used to fuel brain, and facilitate storage and utilization of brain carbohydrate reserves formed by glycogen. However, the mismatch between carbohydrate supply and utilization that is most likely to occur in situations not requiring energy-consuming activities lead to metabolic stress due to elevated brain levels of glucose. Excessive doses of glucocorticoids also impair the production of energy (ATP) and mitochondrial oxidation. Therefore, glucocorticoids have both adaptive and maladaptive effects consistently with the concept of allostatic load and overload.

Huntingtin-Associated Protein 1 in Mouse Hypothalamus Stabilizes Glucocorticoid Receptor in Stress Response

Huntingtin-associated protein 1 (Hap1) was initially identified as a brain-enriched protein that binds to the Huntington’s disease protein, huntingtin. Unlike huntingtin that is ubiquitously expressed in the brain, Hap1 is enriched in the brain with the highest expression level in the hypothalamus. The selective enrichment of Hap1 in the hypothalamus suggests that Hap1 may play a specific role in hypothalamic function that can regulate metabolism and stress response. Here we report that Hap1 is colocalized and interacts with the glucocorticoid receptor (GR) in mouse hypothalamic neurons. Genetic depletion of Hap1 reduced the expression level of GR in the hypothalamus. Dexamethasone, a GR agonist, treatment or fasting of mice induced stress, resulting in increased expression of Hap1 in the hypothalamus. However, when Hap1 was absent, these treatments promoted GR reduction in the hypothalamus. In cultured cells, loss of Hap1 shortened the half-life of GR. These findings suggest that Hap1 stabilizes GR in the cytoplasm and that Hap1 dysfunction or deficiency may alter animal’s stress response.

The fetus at the tipping point: modifying the outcome of fetal asphyxia

Brain injury around birth is associated with nearly half of all cases of cerebral palsy. Although brain injury is multifactorial, particularly after preterm birth, acute hypoxia-ischaemia is a major contributor to injury. It is now well established that the severity of injury after hypoxia-ischaemia is determined by a dynamic balance between injurious and protective processes. In addition, mothers who are at risk of premature delivery have high rates of diabetes and antepartum infection/inflammation and are almost universally given treatments such as antenatal glucocorticoids and magnesium sulphate to reduce the risk of death and complications after preterm birth. We review evidence that these common factors affect responses to fetal asphyxia, often in unexpected ways. For example, glucocorticoid exposure dramatically increases delayed cell loss after acute hypoxia-ischaemia, largely through secondary hyperglycaemia. This critical new information is important to understand the effects of clinical treatments of women whose fetuses are at risk of perinatal asphyxia.

Corticosteroids and perinatal hypoxic-ischemic brain injury

Perinatal hypoxic-ischemic (HI) brain injury is the major cause of neonatal mortality and severe long-term neurological morbidity. Yet, the effective therapeutic interventions currently available are extremely limited. Corticosteroids act on both mineralocorticoid (MR) and glucocorticoid (GR) receptors and modulate inflammation and apoptosis in the brain. Neuroinflammatory response to acute cerebral HI is a major contributor to the pathophysiology of perinatal brain injury. Here, we give an overview of current knowledge of corticosteroid-mediated modulations of inflammation and apoptosis in the neonatal brain, focusing on key regulatory cells of the innate and adaptive immune response. In addition, we provide new insights into targets of MR and GR in potential therapeutic strategies that could be beneficial for the treatment of infants with HI brain injury.

Glucocorticoids, genes and brain function

The identification of key genes in transcriptomic data constitutes a huge challenge. Our review of microarray reports revealed 88 genes whose transcription is consistently regulated by glucocorticoids (GCs), such as cortisol, corticosterone and dexamethasone, in the brain. Replicable transcriptomic data were combined with biochemical and physiological data to create an integrated view of the effects induced by GCs. The most frequently reported genes were Errfi1 and Ddit4. Their up-regulation was associated with the altered transcription of genes regulating growth factor and mTORC1 signaling (Gab1, Tsc22d3, Dusp1, Ndrg2, Ppp5c and Sesn1) and progression of the cell cycle (Ccnd1, Cdkn1a and Cables1). The GC-induced reprogramming of cell function involves changes in the mRNA level of genes responsible for the regulation of transcription (Klf9, Bcl6, Klf15, Tle3, Cxxc5, Litaf, Tle4, Jun, Sox4, Sox2, Sox9, Irf1, Sall2, Nfkbia and Id1) and the selective degradation of mRNA (Tob2). Other genes are involved in the regulation of metabolism (Gpd1, Aldoc and Pdk4), actin cytoskeleton (Myh2, Nedd9, Mical2, Rhou, Arl4d, Osbpl3, Arhgef3, Sdc4, Rdx, Wipf3, Chst1 and Hepacam), autophagy (Eva1a and Plekhf1), vesicular transport (Rhob, Ehd3, Vps37b and Scamp2), gap junctions (Gjb6), immune response (Tiparp, Mertk, Lyve1 and Il6r), signaling mediated by thyroid hormones (Thra and Sult1a1), calcium (Calm2), adrenaline/noradrenaline (Adcy9 and Adra1d), neuropeptide Y (Npy1r) and histamine (Hdc). GCs also affected genes involved in the synthesis of polyamines (Azin1) and taurine (Cdo1). The actions of GCs are restrained by feedback mechanisms depending on the transcription of Sgk1, Fkbp5 and Nr3c1. A side effect induced by GCs is increased production of reactive oxygen species. Available data show that the brain's response to GCs is part of an emergency mode characterized by inactivation of non-core activities, restrained inflammation, restriction of investments (growth), improved efficiency of energy production and the removal of unnecessary or malfunctioning cellular components to conserve energy and maintain nutrient supply during the stress response.

Dexamethasone-induced neuroprotection in hypoxic-ischemic brain injury in newborn rats is partly mediated via Akt activation

Prior treatment with dexamethasone (Dex) provides neuroprotection against hypoxia ischemia (HI) in newborn rats. Recent studies have shown that the phosphatidylinositol-3-kinase/Akt (PI3K/Akt) pathway plays an important role in the neuroprotection. The objective of this study is to evaluate the role of the PI3K/Akt pathway in the Dex-induced neuroprotection against subsequent HI brain injury. Seven-day-old rat pups had the right carotid artery permanently ligated followed by 160min of hypoxia (8% oxygen). Rat pups received i.p. injection of either saline or Dex (0.25mg/kg) at 24 and 4h before HI exposure. To quantify the effects of a PI3K/Akt inhibitor, wortmannin (1μl of 1μg/μl) or vehicle was injected intracerebroventricularly in the right hemisphere on postnatal day 6 at 30min prior to the first dose of Dex or saline treatment. Dex pretreatment significantly reduced the brain injury following HI which was quantified by the decrease in cleaved caspase-3 protein as well as cleaved caspase-3 and TUNEL positive cells at 24h and percent loss of ipsilateral hemisphere weight at 22d after HI, while wortmannin partially reversed these effects. We conclude that Dex provides robust neuroprotection against subsequent HI in newborn rats in part via activation of PI3/Akt pathway.

Fetal hypoxia increases vulnerability of hypoxic-ischemic brain injury in neonatal rats: role of glucocorticoid receptors

Gestational hypoxia is a common stress to the fetal development and increases the risk of neonatal morbidity. The present study tested the hypothesis that fetal hypoxia results in heightened brain vulnerability to hypoxic-ischemic (HI) injury in neonatal rats via down-regulation of glucocorticoid receptor (GR) in the developing brain. Time-dated pregnant rats were exposed to hypoxia (10.5% O2) from days 15 to 21 of gestation. Brain HI injury was determined in day 10 pups. Maternal hypoxia resulted in asymmetric intrauterine growth restriction in the fetus. The brain HI injury was significantly increased in maternal hypoxia-treated pups as compared with the normoxia control in both males and females. Activation of brain GR by dexamethasone injection into the right lateral ventricle produced a concentration-dependent reduction of HI-induced brain injury in control pups. Maternal hypoxia significantly decreased GR mRNA and protein abundance in the fetal brain and neonatal hippocampus and abolished the dexamethasone-mediated neuroprotective effect in pup brains. This decreased GR expression was resulted from increased DNA methylation, decreased binding of transcription factors Egr-1 and Sp1 to GR gene exon 17 and 111 promoters, and reduced expression of GR exon 17 and 111 mRNA variants. The results demonstrate that gestational hypoxia causes epigenetic repression of GR gene expression in the developing brain resulting in the heightened brain vulnerability to HI injury in neonatal rats.

Glucocorticoids and preterm hypoxic-ischemic brain injury: the good and the bad

Fetuses at risk of premature delivery are now routinely exposed to maternal treatment with synthetic glucocorticoids. In randomized clinical trials, these substantially reduce acute neonatal systemic morbidity, and mortality, after premature birth and reduce intraventricular hemorrhage. However, the overall neurodevelopmental impact is surprisingly unclear; worryingly, postnatal glucocorticoids are consistently associated with impaired brain development. We review the clinical and experimental evidence on how glucocorticoids may affect the developing brain and highlight the need for systematic research.

Dexamethasone pre-treatment protects brain against hypoxic-ischemic injury partially through up-regulation of vascular endothelial growth factor A in neonatal rats

Dexamethasone (Dex) provides neuroprotection against subsequent hypoxia ischemia (HI) in newborn rats, but the mechanism of this neuroprotection is not well understood. It is known that vascular endothelial growth factor A (VEGF) has neuroprotective effects. The objective of this study was to evaluate the role of the VEGF signaling pathway in the Dex-induced neuroprotection in newborn rats. Seven-day-old rat pups had the right carotid artery permanently ligated followed by 140 or 160 min of hypoxia (8% oxygen). Rat pups received two i.p. injections of either saline or Dex (0.25 mg/kg) at 24 and 4 h before HI exposure. To quantify the effects of a glucocorticoid receptor (GR) blocker, on postnatal day (PD) 6 and 15 min prior to Dex treatment rat pups received s.c. vehicle or RU486 (GR blocker, 60 mg/kg). After 24 h at PD 7, all rat pups had HI as described earlier. To quantify the effects of a VEGFR 2 blocker, at 24 h after Dex/Veh treatment (PD7), SU5416, a VEGFR 2 inhibitor or vehicle was injected intracerebroventricularly in the right hemisphere at 30 min before and 2 h after HI. Dex pre-treatment reduced brain injury and enhanced the HI-induced brain VEGF protein while a GR blocker inhibited these effects. Treatment with VEGFR 2 blocker decreased Dex-induced neuroprotection also. Dex pre-treatment enhanced the HI-induced increase in mRNA expression of VEGF splice variants and decreased the HI-induced reduction of Akt phosphorylation. Additionally, it also decreased HI-induced increase of caspase-3 activity and DNA fragments in neonatal rat brain. We conclude that Dex provides robust neuroprotection against subsequent HI in newborn rats via GR likely with the partial involvement of VEGF signaling pathway.

Effects of antenatal glucocorticoids on cerebral substrate metabolism in the preterm ovine fetus

OBJECTIVE

Although the benefits of antenatal glucocorticoids are well known for infants who are born preterm, there is increasing evidence of adverse effects on brain development, which may relate to altered metabolic activity. We have determined the effect of maternal glucocorticoid administration at doses that are used clinically on cerebral substrate metabolism in the preterm ovine fetus.

STUDY DESIGN

Chronically instrumented pregnant sheep at 0.85 gestation received 2 intramuscular injections of betamethasone at 170 microg/kg maternal weight (n = 13) or saline (n = 10) 24 hours apart together with a continuous infusion of L-[1-(13)C] leucine to the fetus. Fetal cerebral substrate arteriovenous differences (O2, glucose, leucine, leucine enrichment) and blood flow (fluorescent microspheres) were measured at baseline, 24 hours after the first betamethasone/saline injection (late beta/saline 1), and 4 hours after the second betamethasone/saline injection (early beta/saline 2) to obtain substrate deliveries and fractional extractions.

RESULTS

Fetal pH, blood gases, and metabolites were little changed in either group over the course of the study, except for glucose values in the betamethasone animals, which increased 1.4- and 1.9-fold, measured late beta 1 and early beta 2, respectively (both P < .01). Cerebral blood flow, although little changed in the control group or at late beta 1, was decreased at early beta 2 by approximately 30% (P < .05). As such, early beta 2 animals showed a decrease in cerebral O2 delivery of approximately 20% (P = .06) and conversely an increase in cerebral glucose delivery of 1.4- and 1.3-fold at late beta 1 (P < .05) and early beta 2 (P = .08), respectively. Fractional extraction values for these substrates were not changed significantly, which resulted in corresponding decreases in estimated O2 uptake and increases in estimated glucose uptake, such that the glucose/oxygen quotient (as an index of glucose oxidative metabolism) measured 1.6 at early beta 2, which was considerably greater than baseline values at 1.1 (P < .05). Fractional extraction values for leucine and leucine enrichment averaged 2%-3%; although somewhat higher in the betamethasone animals, none of the between or within group differences were significant.

CONCLUSION

Fetal cerebral metabolism in the preterm ovine fetus is altered by antenatal glucocorticoid administration, which is comparable with that used in human pregnancy, and includes an acute decrease in cerebral blood flow and a probable increase in anaerobic glucose metabolism. Although likely of short duration in conjunction with peak glucocorticoid levels, these metabolic effects may place the developing brain at added risk for superimposed hypoxic injury.

Lack of effect of antenatal glucocorticoid therapy in the fetal baboon on cerebral cortical glucose transporter proteins

BACKGROUND

Maternal antenatal glucocorticoid therapy is used to accelerate lung maturation of immature babies at risk of preterm delivery. It acutely affects brain activity of the human fetus and reduces the immunoreactivity of neurocytoskeletal and synaptic proteins in the fetal baboon brain. These effects might be based on cerebral energy failure due to a decreased neuronal glucose uptake that has been shown in vitro.

METHODS

Glucose uptake into the brain is selectively facilitated by GLUT1 expressed in the blood-brain barrier and GLUT3 expressed in the neuronal membrane. Immunohistochemical distribution of GLUT1 and GLUT3 were examined in the frontal neocortex of the fetal baboon brain at 0.73 gestation (i.e. similar to 28 weeks of human gestation) after maternal betamethasone administration, mimicking the clinical dose regimen.

RESULTS

Betamethasone did not alter GLUT1 and GLUT3 immunoreactivity.

CONCLUSIONS

The results suggest that inhibition of glucose uptake is not the mechanism for the cerebral effects of antenatal glucocorticoids.

Glucocorticoid receptor expression in the cortex of the neonatal rat brain with and without focal cerebral ischemia

BACKGROUND/AIMS

Glucocorticoid receptors (GR) mediate cellular processes which may be neuroprotective and/or neurotoxic to the neonatal rat brain. Our aim was to describe GR ontogeny in the developing rat brain cortex and changes in GR expression after permanent neonatal focal cerebral ischemia (FCI).

METHODS

GR Western blots and immunohistochemical stains were performed on neonatal rat cortices on P1, P3, P7, P10, P15, and P30 and on P7 at 1 h, 3 h, 6 h, 12 h, 24 h, and 72 h after FCI or sham-operation (S-O), 8 per group. Nissl staining was performed on FCI or S-O P7 cortical samples.

RESULTS

Cortical GR expression was increased by 65.2% at P7, 110.1% at P15, and 87.0% at P30, compared to P1. On P7, GR expression decreased in the ischemic cortex after 6 h and in the non-ischemic cortex after 24 h of FCI (p < 0.05). Cortical GR expression was not altered in S-O P7 rats. Immunohistochemistry supported Western blot findings. Nissl staining revealed no gross decrease in neuronal number in non-ischemic cortices after 24 h of FCI, compared to baseline.

CONCLUSIONS

Neonatal rat cortical GR expression increases during P1 to P30, peaking at P15. At P7, cortical GR expression appears downregulated in the ischemic cortex after 6 h and in the non-ischemic cortex after 24 h of FCI. Thus, cortical GR may play important roles in normal brain development and neonatal brain injury responses.

Glucose transporter proteins GLUT1 and GLUT3 like immunoreactivities in the fetal sheep brain are not reduced by maternal betamethasone treatment

Synthetic glucocorticoids administered to accelerate fetal lung maturation in threatened preterm delivery change electrocortical brain activity in the human and sheep fetus and alter structural neuronal proteins in fetal baboon and sheep. We hypothesized that these changes are due to a decreased amount of glucose transporter proteins (GLUT). Glucose uptake into cerebral neurons is selectively facilitated by glucose transporter protein GLUT1 in the blood brain barrier and GLUT3 in neuronal membranes. GLUT1 and GLUT3 immunoreactivity was examined in fetal sheep brain sections of the frontal neocortex, caudate putamen and hippocampus at 0.73 gestation after fetal exposure to betamethasone by direct fetal intravenous infusion or maternal intramuscular injections at the clinically relevant dosage. Betamethasone did not alter GLUT1 and GLUT3 immunoreactivity in any of the brain regions investigated, independently of the dose and route of administration. These data indicate that alteration of GLUT expression is unlikely to explain the cerebral functional effects of antenatal glucocorticoids.

Dexamethasone downregulates chemokine receptor CXCR4 and exerts neuroprotection against hypoxia/ischemia-induced brain injury in neonatal rats

OBJECTIVE

Hypoxia/ischemia (H/I) induces rapid and massive brain damage in neonatal rat brain, resulting in long-term consequences on structural and functional maturation of the central nervous system. Inflammatory mediators contribute to these permanent pathological changes, which are sensitive to corticoid treatments. Since the chemokine receptor CXCR4, specific for the SDF-1 alpha/CXCL12 ligand, regulates both apoptotic and neuroregeneration processes, this receptor was quantified 2 days following H/I in neonatal rat brain in relation with dexamethasone (DEX) treatment.

METHODS

Seven-day-old male rats were exposed to a 90-min hypoxia following unilateral carotid ligation (H/I) and were sacrificed 48 h later. Glucocorticoid-pretreated animals were injected subcutaneously 5 h prior to hypoxia with 0.5 microg/g DEX. Glial fibrillary acidic protein and cresyl violet staining were used for assessing the extent of brain lesion subdivided into necrotic and penumbra-like areas. The density of CXCR4 receptors was determined by quantitative autoradiography using [(125)I]SDF-1 alpha as a ligand.

RESULTS

The H/I resulted in a massive lesion ipsilateral to the carotid ligation, which was extended to cortical, striatal, hippocampal and thalamic areas, while the contralateral hemisphere remained apparently unaffected. DEX decreased the lesion size by reducing mainly the necrotic area. H/I induced a marked increase in CXCR4 receptor binding in the penumbra-like areas. DEX pretreatment decreased CXCR4 receptor density in the penumbra and attenuated astrocytosis. Furthermore, DEX strongly lowered mortality rate and reduced functional recovery time right after hypoxia.

CONCLUSION

The rapid enhancement in CXCR4 chemokine receptor binding in the affected brain areas suggests that SDF-1 alpha/CXCR4 may play a role in the hypoxia-induced inflammatory reaction in the neonatal brain. Attenuation of CXCR4 expression and astrogliosis could contribute to the neuroprotective effect of DEX pretreatment via influencing the inflammatory cascade induced by H/I in the neonatal brain.

Corticosteroids in the prevention and management of bronchopulmonary dysplasia

Corticosteroids were first prescribed for preterm neonates to treat respiratory distress syndrome, but they were found to have no beneficial effect in this disorder. About 20 years ago, dexamethasone was first used to treat infants with bronchopulmonary dysplasia who were ventilator dependent after the age of 3 weeks. There were short-term benefits, with an improvement in lung function and a facilitation of endotracheal extubation. During the 1990s, corticosteroid treatment, mainly with dexamethasone in relatively high doses, became very common in neonatal intensive care units. Towards the end of the decade, however, follow-up studies provided evidence of abnormal neurodevelopment, especially in infants treated early (<4 days) with dexamethasone. The precise cause of these neurodevelopmental problems is unclear, but until further evidence has been obtained, the early use of dexamethasone cannot be recommended for preterm infants. This review attempts to provide evidence-based guidelines for postnatal steroid therapy in the management of chronic lung disease.

Dexamethasone reduces energy utilization in ischemic gerbil brain

Glucocorticoids have been reported to aggravate ischemic neuronal damage. Because energy failure is a crucial factor in the development of ischemic neuronal injury, the effects of dexamethasone on histologic outcome and energy metabolism were investigated in gerbil brain. Dexamethasone (3 microg, i.c.v.) was administered 1 h prior to ischemia, and its effect on delayed neuronal death caused by 2 min of bilateral common carotid artery occlusion was observed in hippocampal CA1 pyramidal neurons. The brain concentration of ATP after various durations of decapitation ischemia was determined, and the effect of dexamethasone (3 microg, i.c.v.) was examined. Na+,K+-activated adenosine triphosphatase (Na+,K+-ATPase) activity was evaluated after the administration of the agent. Forebrain ischemia for 2 min produced neuronal damage in animals pretreated with dexamethasone, although neuronal damage was not observed in vehicle-injected animals. Decapitation ischemia for 0.5 and 1 min reduced the brain ATP concentration to 44% and 15% of the basal level, respectively. Dexamethasone attenuated the ischemia-induced reduction in ATP, and the values were 58% and 25% of the basal level, respectively. Na+,K+-ATPase activity at pH 6.7 was suppressed to 47% by dexamethasone treatment (3 microg, i.c.v.), whereas the activity at pH 7.4 was not influenced by the agent. The results show that a contributing factor to the aggravation of ischemic neuronal damage may be a disturbance in Na+,K+-ATPase despite adequate levels of ATP.

Low glucocorticoid concentrations decrease oxidative phosphorylation of isolated rat brain mitochondria: an additional effect of dexamethasone

The effects of hydrocortisone, triamcinolone, prednisolone and dexamethasone have been investigated in vitro using mitochondria isolated from rat brain. Respiratory control ratio (RCR), oxygen consumption, ATP synthesis, enzymatic activities of involved complexes and superoxide anion generation have been measured to assess the effects of these drugs. Our data showed that the decrease of RCR induced by glucocorticoids was due to a common inhibition of oxidative phosphorylation (State 3) and of complex V activity and a modification of the proton-fluxes through the mitochondrial inner membrane. These effects were quantitatively limited, since they occurred at concentrations lower than 2 nM. Dexamethasone was the only one able to induce a specific inhibition of complex I activity and to decrease the superoxide anion radical generation. Inhibition of complex V and partial reversion of uncoupling seem to be common properties of glucocorticoids. The theoretical consequence of these inhibitions could be the modulation of the mitochondrial function, oxygen consumption rate, ATP synthase activity and superoxide anion radical production, involved in many patho-physiological phenomena.

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

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

Dexamethasone effects on cerebral protein synthesis prior to and following hypoxia-ischemia in immature rat

We hypothesized that the neuroprotection against cerebral hypoxic-ischemic damage observed with dexamethasone treatment in immature rats is related to a change in cerebral protein synthesis. Six-day-old Wistar rats were injected with either vehicle (10 ml/kg) or dexamethasone (0.1 mg/kg) 24 h prior to cerebral hypoxia-ischemia. Local cerebral protein synthesis (incorporation of 14C-leucine into proteins) was measured in 7-day-old rats during normoxia, during hypoxia-ischemia, and after hypoxia-ischemia which was produced with right carotid artery ligation and 2-h exposure to 8% O2. In normoxic controls, cerebral protein synthesis was similar in dexamethasone and vehicle-treated animals. During hypoxia-ischemia, local cerebral protein synthesis decreased markedly (p < 0.0001) in ischemic regions ipsilateral to the occlusion, irrespective of treatment. After hypoxia-ischemia, protein synthesis declined even further in vehicle-treated animals. Reductions in protein synthesis were substantially more severe in vehicle- than dexamethasone-treated animals, particularly after hypoxia-ischemia (p < 0.0001). Thus, neuroprotection with dexamethasone is not related to a reduction in basal levels of cerebral protein synthesis, but is associated with an improved protein synthesis during and following hypoxia-ischemia.