Increase in nitric oxide in the hypoxic-ischemic neonatal rat brain and suppression by 7-nitroindazole and aminoguanidine

We measured the changes in nitric oxide (NO) metabolites in the brains of neonatal rats with hypoxic-ischemic damage. There were two peaks of NO metabolites in the lesioned side of the cortex without treatment: one during hypoxia and the other during the re-oxygenation period. Prehypoxic treatment with 7-nitroindazole, a selective neuronal NO synthase inhibitor, suppressed both peaks of NO metabolites, whereas prehypoxic treatment with aminoguanidine, a selective inducible NO synthase inhibitor, partially suppressed only the peak in the re-oxygenation period. These data suggest different roles of neuronal and inducible NO synthases in the pathogenesis of hypoxic-ischemic encephalopathy.

Reduction of tyrosine kinase activity and protein tyrosine dephosphorylation by anoxic stimulation in vitro

Tyrosine-specific protein phosphorylation has been recently implicated in mediating pathological changes associated with cerebral ischemia. In the present study, acute hypoxia/ischemia (anoxia) was simulated in vitro by incubating rat hippocampal slices in glucose-free artificial cerebrospinal fluid saturated with 95% N2/5% CO2. A marked decrease in the level of tyrosine phosphorylation of many protein bands compared with the control was observed. Immunoprecipitation and western blot confirmed that the NR2A/2B subunits of the N-methyl-D-aspartate receptors are among the dephosphorylated proteins. Maximal dephosphorylation of bands corresponding to NR2A/2B was reached after 10 min, and no recovery was observed even after 1 h in normal, oxygenated artificial cerebrospinal fluid. The effect was partially blocked by dephostatin, a membrane-permeable inhibitor of protein tyrosine phosphatases, but was not affected by the presence of glutamate receptor antagonists, or by removing extracellular Ca2+ or chelating intracellular Ca2+. Enzyme assay showed that anoxic stimulation resulted in a selective reduction in protein tyrosine kinase activity without affecting protein tyrosine phosphatase activity. Thus the present work suggests that anoxic stimulation produces a selective inhibition of protein tyrosine kinase activity leading to tyrosine-dephosphorylation of several proteins including the N-methyl-D-aspartate receptors. The underlying mechanism may involve a novel signal transduction pathway, which may protect neurons from degeneration during ischemic stress.

Modest hypothermia preserves cerebral energy metabolism during hypoxia-ischemia and correlates with brain damage: a 31P nuclear magnetic resonance study in unanesthetized neonatal rats

Recent studies have shown that mild to moderate (modest) hypothermia decreases the damage resulting from hypoxic-ischemic insult (HI) in the immature rat. To determine whether suppression of oxidative metabolism during HI is central to the mechanism of neuroprotection, 31P nuclear magnetic resonance (NMR) spectroscopy was used to measure high energy metabolites in 7-d postnatal rats under conditions of modest hypothermia during the HI. The rats underwent unilateral common carotid artery ligation followed by exposure to hypoxia in 8% oxygen for 3 h. Environmental temperature was decreased by 3 or 6 degrees C from the control temperature, 37 degrees C, which reliably produces hemispheric damage in over 90% of pups. The metabolite parameters and tissue swelling (edema) at 42 h recovery varied very significantly with the three temperatures. Tissue swelling was 26.9, 5.3, and 0.3% at 37, 34, and 31 degrees C, respectively. Core temperature and swelling were also measured, with similar results, in parallel experiments in glass jars. Multislice magnetic resonance imaging, histology, and triphenyltetrazolium chloride staining confirmed the fairly uniform damage, confined to the hemisphere ipsilateral to the ligation. The NMR metabolite levels were integrated over the last 2.0 h out of 3.0 h of HI, and were normalized to their baseline for all surviving animals (n = 25). ATP was 47.9, 69.0, and 83.0% of normal, whereas the estimator of phosphorylation potential (phosphocreatinine/inorganic phosphorus) was 16.9, 27.8, and 42.6% of normal at 37, 34, and 31 degrees C, respectively. There was a significant correlation of both phosphocreatinine/inorganic phosphorus (p < 0.0001) and ATP levels (p < 0.0001) with brain swelling. Abnormal brain swelling and thus damage can be reliably predicted from a threshold of these metabolite levels (p < 0.0001). Thus for all three temperatures, a large change in integrated high energy metabolism during HI is a prerequisite for brain damage. With a moderate hypothermia change of 6 degrees C, where there is an insufficient change in metabolites, there is no subsequent HI brain damage. In general, treatment for HI in our 7-d-old rat model should be aimed at preserving energy metabolism.

Prostaglandin H synthase-2 and cytosolic phospholipase A2 in the hypoxic-ischemic brain: role in neuronal death or survival?

The breakdown of membrane phospholipids and subsequent arachidonic acid metabolism to prostanoids is a well-documented brain response to cerebral ischemia. To further elucidate the components of this signal transduction pathway, immunocytochemistry was used to determine the levels of two potentially important enzymes, cytosolic phospholipase A2 (cPLA2) and prostaglandin H synthase-2 (PGHS-2), in the immature rat brain following moderate unilateral hypoxic-ischemia (HI). The CA1 pyramidal cells of the hippocampus which undergo delayed neuronal death on the injured side following HI demonstrated a significant induction of PGHS-2 immunoreactivity 48 h post-insult. However, a consistent increase in PGHS-2 was also evident in the resistant dentate granule cells at an earlier time point. Although PGHS-2 is present in both susceptible and resistant cell populations following HI, the possibility remains that divergence further down-stream in the pathway is responsible for selective vulnerability. In contrast to the neuronal PGHS-2 expression, cPLA2 immunoreactivity appears to be of glial origin with increases in and around the CAI-2 pyramidal cell layer at the 72-168-h time points. These results suggest that prostanoids are likely to serve important roles in HI brain damage and repair in infant brain.

In vivo hypoxia-induced neuronal damage with an enhancement of neuronal nitric oxide synthase immunoreactivity in hippocampus

Although it is well known that brain ischemia is dominantly caused by hypoxia and hypoglycemia, it is still unclear how hypoxia participates in ischemia. We studied the changes in neuronal nitric oxide synthase (nNOS) and the effect of the NOS inhibitor NG-nitro-L-arginine (NNA) on hypoxia. In vivo hypoxia (5% O2/95% N2 for 30 min) induced mild degenerative neuronal changes (shrunken and eosinophilic somata with picnotic nuclei) in neurons of the CA3, the hilus of the dentate gyrus (DG) and the DG, but not in the CA1. At 3 and 7 days after hypoxia, levels of nNOS protein were significantly enhanced to 153 and 209%, but iNOS protein could not be detected. The numbers of nNOS-immunopositive neurons were significantly enhanced to 145 and 191% in the CA3, 145 and 178% in the hilus of the DG, and 243 and 387% in the DG after 3 and 7 days, respectively. In contrast, no statistical difference was determined in the CA1. We further examined the effect of NNA administered at 5 min and 3, 6, and 24 h after hypoxia. Administration of NNA (0.1 and 1 mg/kg, i.p.) significantly decreased the number of damaged neurons in the hilus of the DG and the DG. However, higher doses of NNA (10 mg/kg, i.p.) did not prevent damage. These results suggest that hypoxia alone induces enhancement of nNOS protein and nNOS immunoreactivity in neurons of the hippocampus and that NNA has biphasic effects against hypoxia-induced neuronal damage in the hilus of the DG and the DG.

Adenosine A1 receptor activation preferentially protects cultured cerebellar neurons versus astrocytes against hypoxia-induced death

Administration of adenosine A1 receptor agonists in vivo is neuroprotective in various stroke models. Experiments using either mixed cultures of neurons and astrocytes or brain slices, in which several cell types are present, have demonstrated that activation of A1 receptors also id protective against hypoxia and/or hypoglycemia in vitro. In this study, we have examined the effect of the A1 agonist cyclopentyladenosine (CPA) on cellular damage, measured by efflux of lactate dehydrogenase (LDH), in highly enriched primary cultures of either neurons of astrocytes exposed to different metabolic insults. CPA reduced neuronal LDH release induced by a combination of hypoxia and substrate deprivation ("simulated ischemia"; IC50 = 28 nM) of by hypoxia alone (IC50 = 170 nM). In contrast, CPA had no effect on neuronal damage induced by substrate deprivation alone, not did it affect ischemic death to astrocytes. The neuroprotective effect of CPA during simulated ischemia and hypoxia were reversed by the A1 antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX). These data demonstrate that activation of an adenosine A1 receptor on neurons, but not astrocytes, is protective against cellular damage of death induced specifically by hypoxia as opposed to other metabolic insults such as hypoglycemia.

Cytochrome oxidase is decreased in piglet hippocampus following hypoxia-ischemia

We assessed cytochrome oxidase (CytOx) staining in sham-operated control piglets and in piglets subjected to 30 minutes of cerebral hypoxia-ischemia (H-I) plus 4 hours of reperfusion (REP). The 1-day-old piglets were sedated, anesthetized, and ventilated. Cerebral blood flows (CBFs) were quantitated using microspheres. H-I was induced by a combination of phlebotomy and cervical tourniquet; the brain was reperfused for four hours after 30 minutes of H-I. CBF was reduced during ischemia in experimental animals from 42 + 13 to 12 + 5 ml/min/100g. CytOx staining of hippocampal sections from 3 control and 3 experimental animals was compared. The staining of the stratum pyramidale neurons of the same portion of the CA1 sector in a single high power field was assessed in a blinded fashion in 4 corresponding sections from each animal, and graded from 0 = no staining to 3 = heavy staining. The results were compared using one-way analysis of variance. Cells with grade 3 staining were significantly more numerous in controls compared to H-I/REP animals (p = 0.03). There were significantly more cells with no CytOx staining in the experimental animals (p = 0.01). These findings suggest that CytOx staining in newborn piglet CA1 is a reliable method of assessing cell dysfunction after H-I.

Ubiquitin-mediated stress response in a rat model of brain transient ischemia/hypoxia

Ubiquitin (Ub) is a small 76-residue protein, involved in intracellular protein degradation through a specific ATP-dependent system, which uses Ub as a tag to label proteins committed to be hydrolyzed by a specific 26 S protease. PGP-9.5 is another important component of the Ub system, i.e. a neuron-specific carboxyl-terminal hydrolase, which recycles Ub from Ub-polypeptide complexes. We have investigated the expression of Ub and PGP-9.5 in rat hippocampal neurons in an early phase of reperfusion in a model of transient global brain ischemia/hypoxia (bilateral occlusion of common carotid arteries for 10 min accompanied by mild hypoxia-15% O2-for 20 min), by means of immunohistochemical methods using light and electron microscopy. The intensity of Ub and PGP-9.5 immunoreactivity was evaluated by image analysis. We have detected a marked increase of Ub immunoreactivity (UIR) in neurons of CA1, CA2, CA3, CA4, and dentate gyrus subfields 1 hr after ischemia/hypoxia (but not after hypoxia only), statistically significant as confirmed by image analysis. Such increase in immunoreactivity in ischemic/hypoxic rats was localized essentially in the nuclei of hippocampal neurons. There were no changes in PGP-9.5 immunoreactivity. The data suggest that in the present model of rat brain ischemia/hypoxia Ub is involved in the neuronal stress response.

Induction of cyclooxygenase-2 following anoxic stress in piglet cerebral arteries

OBJECTIVE

Ischemic stress causes damage to cerebrovascular endothelium and alters arteriolar responses to prostanoid-dependent stimuli. However, effects of ischemic stress on cyclooxygenase (COX) levels in endothelium are unclear. We examined the effect of ischemia and reperfusion and asphyxia and reventilation on production of COX isoforms in cerebral vascular endothelium.

METHODS

Neonatal pigs were exposed to global ischemia (n = 4) or asphyxia (n = 3) for 5-10 min. Following 2-6 h of recovery, the animals were killed, and the cerebral arteries and arterioles were removed. Cerebral arteries and arterioles were also removed from untreated control animals (n = 1) and from time control animals (n = 3). Cerebral vessels were fixed in 4% formalin and paraffin embedded, and constitutive and inducible COX (COX-1 and COX-2, respectively) levels were assessed using indirect immunofluorescence.

RESULTS

Hemotoxylin and eosin staining indicated that anoxic stress leads to enlargement of endothelial cells. Immunofluorescence for COX-1 in endothelium was minimal in cerebral arteries and arterioles from control animals and did not show an increase in animals exposed to anoxic stress. Similarly, cerebral vessels from control animals showed little immunostaining for COX-2. In contrast, immunofluorescence for COX-2 was greatly increased in cerebral arteries and arterioles from animals exposed to asphyxia or ischemia.

CONCLUSIONS

We concluded that anoxic stress increases COX-2 but not COX-1 levels in cerebral endothelium.

Increased neurons containing neuronal nitric oxide synthase in the brain of a hypoxic-ischemic neonatal rat model

We evaluated the temporal profile of the number of neurons containing neuronal nitric oxide synthase (nNOS neurons) in the brain of a neonatal hypoxic-ischemic rat model. Hypoxic-ischemic insults were produced in the brains of 7-day-old rat pups using a combination of unilateral carotid artery ligation and hypoxic (8% oxygen) exposure. Sections of brain from rats killed at 0-24 h after the onset of hypoxia were stained immunohistochemically using a polyclonal anti-nNOS antibody. Histological changes of neuronal injury were evaluated in the adjacent Nissl stained sections. The number of nNOS neurons in the hemisphere ipsilateral to the carotid ligation was significantly increased (P < 0.05) at 3 h, when the neuronal injury consisted of clusters of degenerating hyperchromic neurons. Neuronal degeneration and an increased number of nNOS neurons were seen only in the ipsilateral hemisphere and the increase was most prominent in the dorsolateral area of the striatum. The increase in the number of nNOS neurons continued at 6 h, when the area of neuronal injury continued to expand. At 24 h, the neuronal injury was diffuse, and the number of nNOS neurons on the ipsilateral side significantly decreased. The increase of the number of nNOS neurons in the early phase of neonatal neuronal injury suggests its possible involvement in the hypoxic-ischemic injury. The delineation of its role in neuronal injury may lead to an improvement in managing neonatal hypoxic-ischemic brain injury.

Paradoxical mitochondrial oxidation in perinatal hypoxic-ischemic brain damage

Measurements of cytoplasmic and mitochondrial markers of the oxidation-reduction (redox) state of brain tissue were conducted in a perinatal animal model of cerebral hypoxia-ischemia to ascertain underlying biochemical mechanisms whereby ischemia (reduced oxygen and substrate supply) causes brain damage. Seven-day postnatal rats underwent unilateral common carotid artery ligation followed by exposure to 8% oxygen at 37 degrees C for 3 h. During the course of hypoxia-ischemia, the rat pups were quick frozen in liquid nitrogen and their brains processed for the enzymatic, fluorometric measurement of cerebral metabolites necessary for the calculation of intracellular pH and cytoplasmic and mitochondrial redox states. The results showed an early mitochondrial reduction followed by re-oxidation during the course of hypoxia-ischemia. The oxidation reflected a partial depletion in accumulated reducing equivalents and coincides temporally with the duration of hypoxia-ischemia required to convert selective neuronal necrosis into cerebral infarction. The findings suggest that perinatal cerebral hypoxia-ischemia is characterized more by a limitation of substrate than of oxygen supply to the brain, which may explain why glucose supplementation of the immature animal improves neuropathologic outcome, in contrast to adults.

Cytosolic and membrane-bound cerebral nitric oxide synthase activity during hypoxia in cortical tissue of newborn piglets

To determine the role of nitric oxide production during hypoxia, the presence of two forms of neuronal nitric oxide synthase, cytosolic (cNOS) and membrane-bound (memNOS), in cortical tissue of newborn piglets and the effects of hypoxia on the activity of these enzymes were studied. Experiments were performed in 12 anesthetized and ventilated Yorkshire piglets, 2-4 days of age. Hypoxia was induced by decreasing the FiO2 to 0.07. The control group was ventilated maintaining normoxia. After 1 h of normoxic or hypoxic ventilation brain tissue was removed and frozen immediately in liquid nitrogen. Tissue hypoxia was confirmed by analysis of adenosine triphosphate (ATP) and phosphocreatine (PCr): ATP was reduced to 52% and PCr to 28% of control values. cNOS activity was 35.3 +/- 13.7 pmol/mg protein per min in the control group and 28.3 +/- 7.0 in the hypoxia group; memNOS activity was 10.5 +/- 4.5 and 12.0 +/- 3.9 pmol/mg protein per min in the control and hypoxia groups, respectively. Differences in cNOS and memNOS activity between control and hypoxic animals were not significant. The results indicate that both cNOS and memNOS are present in cortical tissue of newborn piglets and that the activity is unaffected by 1 h of tissue hypoxia. We suggest that production of nitric oxide and its derivative peroxynitrite during hypoxia may therefore be a potential mechanism for hypoxia-induced brain cell membrane lipid peroxidation.

Effect of hypoxia and reoxygenation on regional activity of nitric oxide synthase in brain of newborn piglets

The activity of nitric oxide synthase (NOS) was measured in homogenates from cortex, striatum, hippocampus, cerebellum, pons, thalamus and midbrain of the brain of newborn piglets and the effects of hypoxia and posthypoxic period on this activity was evaluated. The control activities were 19.7, 31.5, 26.8, 16.7, 33.6, 19.3 and 39.4 pmol/mg protein per min, respectively. A 1 h period of hypoxia (an FiO2 of 7%) resulted in statistically significant decreases in the activity of NOS in every region of the brain except for the cortex, where the activity was not significantly altered compared to control. By 2 h of reoxygenation following such a hypoxic episode, the NOS activities increased to above control levels in all regions of the brain, but this increase was statistically significant compared to control only in thalamus. Since hypoxia induced the greatest decrease in NOS activity in the cerebellum, the kinetic constants of the enzyme were measured in homogenates from this region of brain. The decreased activity following the hypoxic episode was associated with an approximately four-fold increase in the apparent affinity (KM) for arginine with no significant change in the maximal rate of reaction (Vmax). The decrease in NOS activity subsequent to a hypoxic episode may contribute to the disturbances in cellular metabolism in the immature brain induced by episodes of hypoxia-reoxygenation.

Neonatal mice lacking neuronal nitric oxide synthase are less vulnerable to hypoxic-ischemic injury

We hypothesized that elimination of neuronal nitric oxide synthase (nNOS) by targeted disruption of the nNOS gene would result in amelioration of damage seen after hypoxia-ischemia in the developing brain since nitric oxide (NO) has been implicated in glutamate-mediated neurotoxicity after ischemia. Both wildtype and nNOS-deficient pups were subjected to focal ischemia followed by 1.5 h of hypoxia at Postnatal Day 7. Seven days later, brains of surviving animals were analyzed for damage. The nNOS-deficient pups (n = 17) had less histopathologic evidence of injury in both the hippocampus (P = 0.008) and the cortex (P = 0.0008) than the wildtype (n = 30) mice. When injured, the nNOS-deficient mice had damage that was limited to the hippocampus. These results support a role for neuronally produced NO in injury after perinatal hypoxia-ischemia.

Is the Mg(2+)-ATP-dependent proton pumping activity of the synaptic vesicles a factor involved in the cerebral hypoxia?

The changes in the Mg(2+)-dependent V-type ATPase activity and the Mg(2+)-ATP-dependent H+ pumping activity of the synaptic vesicles from the cerebral cortex of rats submitted to intermittent chronic (4 weeks) mild or severe hypoxia were evaluated. The adaptation to the chronic severe hypoxia increases both the ATPase and the H+ pumping activities which are inhibited by NEM with an exponential relationship between the IC(50) values and the in vivo O2 concentration. The Mg(2+)-dependent increase in H+ pumping activity of synaptic vesicles from the rats subjected to in vivo chronic hypoxia may be antagonized by nigericin (dissipating delta pH) and by FCCP (dissipating delta pH and delta psi SV). In contrast, valinomycin (dissipating the delta psi SV) and facilitating an enhancement in delta pH) increases in vitro the H+ pumping activity that is inhibited by the addition of high concentration of K gluconate (reducing the rate of K+ efflux). The preincubation of vesicles from hypoxic rats with FCCP, but not with nigericin, inhibits the valinomycin-increased H+ pumping activity. L-glutamate increases the H+ pumping activity in synaptic vesicles from the cerebral cortex of chronic hypoxic rats, whereas other amino acids (i.e., L-aspartate and L-homocysteate) and glutamate analogs (i.e., quisqualate and ibotenate) are ineffective. The adaptation to both chronic intermittent severe hypoxia and in vivo treatment with posatireline causes a decrease in the Mg(2+)-ATPase activity consistent with the decrease in the H+ pumping one of the synaptic vesicles. The addition of nigericin into incubation medium magnifies the decrease in the H+ pumping activity, while the addition of FCCP is ineffective, suggesting that the treatment with posatireline interferes with the delta psi SV component in the delta mu H+ of the synaptic vesicles from rats submitted to chronic hypoxia. The results of the in vivo and in vitro experiments suggest that in the synaptic vesicles from hypoxic rats the delta psi SV component in delta mu H+ may be most effective in increasing the Mg(2+)-ATP-dependent H+ pumping activity.

Mechanisms of 1S,3R-ACPD-induced neuroprotection in rat hippocampal slices subjected to oxygen and glucose deprivation

The efficacy and mechanisms of 1-amino-cyclopentyl-1S,3R-dicarboxylate (1S,3R-ACPD)-induced neuroprotection were investigated in rat hippocampal slices subjected to 10 min of oxygen and glucose deprivation. Neuronal viability was assessed by measuring both the amplitude of evoked population spike in the CA1 pyramidale and by imaging CA1 neurons using a live/dead fluorescence assay with confocal microscopy. CA1 pyramidal neurons in oxygen-glucose deprived slices remained viable for up to 120 min following the insult but were dead by 240 min. Pretreatment with 1S,3R-ACPD significantly protected the oxygen-glucose deprived slices in a concentration-dependent fashion. Oxygen-glucose deprived slices pretreated for the same period with the protein kinase C (PKC) activation phorbol 12-myristate 13-acetate (PMA; 1 microM) were significantly protected whereas oxygen-glucose deprived slices treated with the adenylyl cyclase activator, forskolin (30 microM) were not. Oxygen-glucose deprivation induced a rapid and persistent decrease (approximately 50%) in PKC activity and a > 6 fold increase in cyclic adenosine monophosphate (cAMP) levels in whole hippocampal slices. While 1S,3R-ACPD did not stimulate PKC activity and had no effect on basal cAMP in whole slices, it significantly enhanced the rate of return of cAMP to basal levels following reperfusion. Consistent with this observation, the 1S,3R-ACPD-induced neuroprotection was inhibited by forskolin (30 microM). These results suggest that in vitro neuroprotection of CA1 neurons by 1S,3R-ACPD involves metabotropic glutamate receptors negatively linked to cAMP and possibly those which increase PKC activity.

Ornithine decarboxylase activity in vitro in response to acute hypoxia: a novel use of newborn rat brain slices

In fetal as well as newborn rats, acute hypoxic exposure results in significantly elevated brain ornithine decarboxylase (ODC) activity, polyamine concentrations, and ODC mRNA. The interpretations of these in vivo hypoxic-induced changes, however, are complicated by maternal confounding effects. To test the hypothesis that acute hypoxia will also increase ODC activity in vitro, we developed a brain slice preparation which eliminates such maternal effects. Sections of whole cerebrum, approximately 300-500 microns thick, were made from 3- to 4-day old Sprague-Dawley rat pups. The slices were equilibrated for 1 h in artificial cerebrospinal fluid (ACSF) continuously bubbled with 95% O2/5% CO2, prior to induction of hypoxia. We induced hypoxia by changing the oxygen concentration to 40%, 30%, 21%, 15%, 10%, or 0% O2, all with 5% CO2 and balance N2. In the normoxic control brain slices, low but stable basal ODC activity persisted for up to 5 h post-sacrifice. Slices in ACSF treated with bovine serum albumin (BSA), or both BSA and fetal bovine serum (FBS), however, showed stable ODC activity values 2- to 3-fold higher than slices in ACSF alone, for up to 5 h. In response to acute hypoxia (i.e., 15, 21, and 30% O2), ODC activity was elevated 1.5- to 2-fold above control values between 1 and 2 h after initiation of hypoxia. Qualitative light and electron microscopic examination of the neonatal brain slices following 2 h hypoxic exposure suggested that the great majority of cells did not show severe hypoxic damage or necrosis. It was concluded that: (1) in neonatal rat brain slices in vitro, stable ODC activity values approximating the whole brain ODC activity seen at sacrifice, can be maintained for several hours; (2) the in vivo hypoxic-induced increase in ODC activity can be approximated in vitro; (3) the neonatal rat brain slice preparation may be an alternative to other methods for studying hypoxic-induced ODC enzyme kinetics, or other brain enzymes, without maternal confounding effects; and (4) ODC activity may be an indicator of active metabolism within the newborn brain slice both in normoxia and hypoxia.

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)

Delayed increase of tyrosine hydroxylation in the rat A2 medullary neurons upon long-term hypoxia

In vivo and in vitro activity of tyrosine hydroxylase (TH) was estimated in the catecholaminergic A2 cell group of the nucleus tractus solitarius (NTS) in rats exposed to normobaric hypoxia (10% O2 in nitrogen) for 2 h, 3, 7, 14 or 21 days. The A2 cell group was subdivided into two subgroups. In the caudal A2 subgroup located caudal to the calamus scriptorius, long-term but not acute hypoxia elicited an increase of in vivo tyrosine hydroxylation rate after 7 days of exposure (+60% above normoxic controls). The increase of in vivo TH activity was maintained at the same level at the end of hypoxic exposure. In vitro TH activity was increased transiently after 7 days of hypoxia (+92% above normoxic (controls). In thr rostral A2 subgroup, hypoxia elicited a significant increase of in vivo tyrosine hydroxylation at 7 days (+38%) but did not alter in vitro TH activity throughout the whole exposure. Hypoxia produced no detectable change in TH activity in other noradrenergic cell groups of the brain stem (locus coeruleus, A5) except for a transient inhibition of in vivo TH activity in A5 after 2 h. Immunocytochemical analyses confirmed that the catecholaminergic neurons in the caudal A2 area are not only of a noradrenergic nature. The neurons were located in the commissural subnucleus of the NTS. On the other hand, the rostral A2 area contains noradrenergic neurons intermingled with a small number of adrenergic cell bodies.(ABSTRACT TRUNCATED AT 250 WORDS)

Reduction of cytochrome aa3 measured by near-infrared spectroscopy predicts cerebral energy loss in hypoxic piglets

Near-infrared spectroscopy is a noninvasive monitoring technique that allows quantitative measurement of changes in cerebral oxygenated Hb (HbO2), deoxygenated Hb (Hb), total Hb, and oxidized cytochrome aa3 (CytO2). Changes in cerebral Hb oxygenation and CytO2 have been measured in human neonates and infants under a variety of conditions. However, the association of these measurements with cerebral high-energy phosphate loss is not known. We studied simultaneous changes in cerebral HbO2, Hb, total Hb, and CytO2 by near-infrared spectroscopy and changes in nucleoside triphosphate (NTP, mostly ATP) and phosphocreatine (PC) concentrations and intracellular pH by in vivo 31P-labeled magnetic resonance spectroscopy. Four-wk-old piglets (n = 8) underwent sequential hypoxic episodes of increasing severity (inspired O2 concentration, 12, 8, 6, 4, and 0%). Animals were anesthetized and mechanically ventilated. At all levels of hypoxia, cerebral HbO2 decreased, and Hb increased. Loss of PC or NTP was not observed until inspired O2 concentration was decreased to less than 12%. With such severe hypoxia, hypotension, intracellular acidosis, and increasingly severe PC and NTP depletions occurred. Decreases in PC and NTP correlated closely with decreased CytO2 and arterial blood pressure (p < 0.0001) but not with changes in HbO2 and Hb. In conclusion, cerebral hypoxemia is readily detected by near-infrared spectroscopy as a decrease in HbO2 and an increase in Hb. However, relative changes in cerebral HbO2 and Hb have low predictive value for cerebral energy failure. Reduction of CytO2 is highly correlated with decreased brain energy state and may indicate impending cellular injury.

Quantitative immunofluorescence data suggest a permanently enhanced GAD67/GAD65 ratio in nerve endings in rat cerebral cortex damaged by early postnatal hypoxia-ischemia: a comparison between two computer-assisted procedures for quantification of confocal laser scanning microscopic immunofluorescence images

The aim of the present study was 2-fold: (1) to determine the ratio between the amount of GAD67 and GAD65 (two isoforms of the GABA synthetizing enzyme glutamic acid decarboxylase) in nerve endings in the mature rat cerebral cortex damaged by hypoxia-ischemia during early postnatal life; and (2) to compare two different computer-assisted procedures developed for quantitative analysis of immunofluorescence images obtained with a confocal laser scanning microscope (CLSM). One procedure was based on a program present in the standard Leica CLSM software packet for full-field analysis, the other on a specially written program for object-oriented analysis run on a Kontron IBAS-KAT image analysis system. To this end, rat pups were unilaterally exposed to hypoxic-ischemic conditions and, after a survival period of 6.5 months, sacrificed by perfusion fixation. After dissection of the brain and vibratome sectioning, three animals with substantial damage on one cortical side were selected. Sections of these animals were double-stained with primary antibodies against GAD67 and GAD65 and fluorophore-conjugated secondary antibodies and subsequently sampled with a CLSM. Analysis of the CLSM images with both computer-assisted procedures showed for all three animals a clear tendency to higher GAD67/GAD65 ratios in cortical GABAergic nerve endings on the hypoxia-damaged side than in matched areas on the contralateral side. This outcome led to the following conclusions. (1) The correspondence between the outcome of both analysis procedures indicates that both procedures are valid for quantification of immunofluorescence images of nerve endings obtained with a CLSM. (2) The outcome lends further support to our view that hypoxic-ischemic encephalopathy, sustained during early postnatal life, may result in an unstable cortical network generating abnormal synchronizations and oscillations which can be amplified and propagated as true epileptic discharges. In such a network both excitatory and inhibitory processes are tonically enhanced, the latter probably as a homeostatic reaction tending to keep abnormal excitation within physiological limits.

Acute disruption of cytochrome oxidase activity in brain in a perinatal rat stroke model

Cytochrome oxidase histochemistry in frozen brain sections provides an estimate of mitochondrial respiratory function and neuronal metabolic activity. We hypothesized that in a perinatal rodent stroke model acute selective disruption of cytochrome oxidase activity could identify neuronal populations susceptible to irreversible ischemic injury. In 7-d-old rats, focal ischemic injury was induced by right carotid ligation followed by 8% O2 exposure (2.75 h); this procedure elicits ipsilateral forebrain injury with prominent hippocampal lesions, often including dentate gyrus. Cytochrome oxidase activity was quantitated by densitometry in brain sections prepared from animals killed 1 h (n = 8), 24 h (n = 11), or 5 d (n = 3) posthypoxia. At 1 h posthypoxia, in six of eight there was diffuse ipsilateral suppression of cytochrome oxidase activity (p = 0.0001, two-way analysis of variance, compared with values in contralateral hemisphere), which was most marked in periventricular regions including dentate gyrus (-29%), medial habenula (-38%), and posterolateral thalamic nucleus (-42%). Nissl staining of adjacent sections was completely intact in four of eight, and four of eight had subtle focal reductions in staining. At 24 h and 5 d posthypoxia, heterogeneous suppression of cytochrome oxidase activity persisted in the lesioned hemisphere, with close anatomic correspondence between loss of cytochrome oxidase activity and loss of Nissl staining. Additional studies indicated that the threshold duration of 8% O2 exposure (after carotid ligation) for suppression of cytochrome oxidase activity was 1-2 h. Disruption of cytochrome oxidase activity represents one of the earliest detectable indicators of neuronal injury in this perinatal stroke model.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of hypoxia and reoxygenation on the activity of transglutaminase in brain of newborn piglets

The transglutaminase activity in five regions of the brain of newborn piglets was measured and the effects of hypoxia and posthypoxic period on this activity evaluated. Enzyme activity was measured in homogenates from cortex, hippocampus, striatum, thalamus and midbrain. The control activities were 7.2, 6.2, 6.0, 5.7 and 4.6 pmol/mg protein/min, respectively. The activities at the end of an 18 min period of hypoxia induced by an FiO2 of 9% were not significantly different from control activities. By 3 h after the hypoxic episode, however, the transglutaminase activities were significantly above control levels in all five regions of the brain. Measurements of the kinetic constants of tranglutaminase indicated that increases in enzyme activity were associated with an increase in Vmax with no significant change in the apparent affinity of the enzyme for the substrate, putrescine. The increased activity of transglutaminase during the posthypoxic period, with no changes immediately after hypoxia, suggest that the increases could be due to increased enzyme synthesis rather than activation of existing enzyme. The rise in transglutaminase activity subsequent to a hypoxic episode may contribute significantly to the long-term disturbances in cellular metabolism in the immature brain induced by hypoxic episodes.

Age-related alterations by chronic intermittent hypoxia on cerebral synaptosomal ATPase activities

The age-related alterations in the plasticity of synaptic energy-requiring ATPases [Na+,K(+)-ATPase, low- and high-affinity Ca(2+)-ATPase, Mg(2+)-ATPase, and Ca2+,Mg(2+)-ATPase] were assayed in synaptosomes and synaptosomal subfractions [namely, synaptosomal plasma membranes and synaptic vesicles] in the cerebral cortex from 3- and 24-month-old normoxic rats and rats subjected to either mild or severe chronic (four weeks) intermittent normobaric hypoxia. With the exception of the high-affinity Ca(2+)-ATPase, aging induced a decrease in the ATPase activities from normoxic rats. The adaptation to mild hypoxia was characterized by an increase in the activity of Mg(2+)-ATPase in 3-month-old rats, concomitant with a decrease in the activities of: (i) Na+,K(+)-ATPase and high-affinity Ca(2+)-ATPase in both 3- and 24-month-old rats, and (ii) Ca2+,Mg(2+)-ATPase in 3-month-old ones. The adaptation to chronic intermittent severe hypoxia was characterized by a decrease in the activities of: (i) Na+,K(+)-ATPase, Ca2+,Mg(2+)-ATPase and high-affinity Ca(2+)-ATPase in both 3- and 24-month-old rats, and (ii) low-affinity Ca(2+)-ATPase only in 24-month-old ones. The effect on Mg(2+)-ATPase activity was characterized by a decrease in the activity of the enzymatic form located in the synaptic plasma membranes [involved in ATP hydrolysis to adenosine production], concomitant with an increase in the activity of the form located in the synaptic vesicles [involved in the turnover of transmitters, e.g., glutamate].