Changes in purine metabolism and production of oxygen free radicals by intermittent partial umbilical cord occlusion in chronically instrumented fetal lambs

Antenatal allopurinol reduces hippocampal brain damage after acute birth asphyxia in late gestation fetal sheep

Free radical-induced reperfusion injury is a recognized cause of brain damage in the newborn after birth asphyxia. The xanthine oxidase inhibitor allopurinol reduces free radical synthesis and crosses the placenta easily. Therefore, allopurinol is a promising therapeutic candidate. This study tested the hypothesis that maternal treatment with allopurinol during fetal asphyxia limits ischemia-reperfusion (I/R) damage to the fetal brain in ovine pregnancy. The I/R challenge was induced by 5 repeated measured compressions of the umbilical cord, each lasting 10 minutes, in chronically instrumented fetal sheep at 0.8 of gestation. Relative to control fetal brains, the I/R challenge induced significant neuronal damage in the fetal hippocampal cornu ammonis zones 3 and 4. Maternal treatment with allopurinol during the I/R challenge restored the fetal neuronal damage toward control scores. Maternal treatment with allopurinol offers potential neuroprotection to the fetal brain in the clinical management of perinatal asphyxia.

MCI-186 administered to the maternal circulation inhibits fetal brain injury resulting from total umbilical cord occlusion in the chronically instrumented fetal lamb

OBJECTIVE

To evaluate the transplacental effects of MCI-186 (edaravone), a potent hydroxyl radical scavenger, administered to the maternal circulation to inhibit fetal brain injury caused by umbilical cord occlusion.

METHODS

Nine chronically instrumented lambs were prepared. In three cases, 10-min persistent total umbilical cord occlusion (group A) was performed. Another three cases underwent occlusion and were administered 60 mg of MCI-186 through the maternal femoral vein prior to the end of occlusion (group B). The remaining three cases underwent sham operation (group C). On day 3 after insult, fetal brains were extirpated. Paraffin-embedded brain tissue sections were stained with hematoxylin and eosin, Bodian, Kluver-Barrera, and TUNEL. Neuronal cellular damage was evaluated by two pathologists blinded to the experimental conditions.

RESULTS

Group A displayed numerous cells with eosinophilic condensation of nuclear chromatin and proliferation of microglia in the hippocampus and basal ganglia. TUNEL-positive cells were observed in the periventricular area. Group B showed microglial proliferations, but no marked changes. No pathological changes were apparent in group C.

CONCLUSIONS

MCI-186 administered to the maternal circulation could inhibit fetal brain injury resulting from hypoxia-reperfusion induced by umbilical cord occlusion.

The production of hydroxyl radicals in the fetal lamb brain resulting from occlusion of the umbilical circulation and the transplacental effect of MCI-186 to inhibit hydroxyl radical production

The present study evaluated hydroxyl radical production in fetal lamb brain during and after umbilical cord occlusion and examined the effects of injecting MCI-186 (3-metyl-1-phenyl-2-pyrazolin-5-one; Edaravone), a hydroxyl radical scavenger, into the maternal circulation. In 11 chronically instrumented lambs, intermittent total umbilical cord occlusions 1 min out of every 3 min for 1 h and 10-min persistent total umbilical cord occlusion were performed with brain microdialysis using 5 mM of sodium salicylate. In the remaining four lambs, 60 mg of MCI-186 was administered into the maternal circulation from shortly before the end of 10-min persistent total umbilical cord occlusion. Concentrations of 2,3-dihydroxy-benzoic acid (2,3-DHBA), produced by hydroxyl radical reactions with sodium salicylate, were measured in perfusate by HPLC. Concentration of 2,3-DHBA in perfusate was 23.05 +/- 10.95 nM before umbilical cord occlusion. Levels of 2,3-DHBA tended to increase slightly during and after intermittent umbilical cord occlusion, and were significantly increased by the end of 10-min occlusion (40.06 +/- 21.36 nM) and after occlusion (93.74 +/- 29.17 nM). Infusion of MCI-186 suppressed 2,3-DHBA concentration to 29.35 +/- 14.95 nM after occlusion. Administration of MCI-186 into the maternal circulation reduces hydroxyl radical production induced by umbilical cord occlusion in the fetal lamb brain.

Transplacental effects of allopurinol on suppression of oxygen free radical production in chronically instrumented fetal lamb brains during intermittent umbilical cord occlusion

OBJECTIVE

To evaluate the transplacental effect of allopurinol, which acts as a xanthine oxidase inhibitor and free radical scavenger, on inhibiting the production of superoxides during intermittent partial umbilical cord occlusion.

METHODS

Using four chronically instrumented fetal lambs, ewes received 400 mg allopurinol over a period of two hours. Concentrations of allopurinol and oxypurinol in blood samples from mothers and fetuses and fetal brain microdialysis perfusate were measured by HPLC. In another three cases the production of superoxide during intermittent umbilical cord occlusion was studied by measurement of chemiluminescence in perfusate before and after administration of Allopurinol.

RESULTS

(i) Allopurinol concentration in mothers had reached equilibrium by 30 min after starting administration and maintained a concentration about 6 mug/ml. Allopurinol concentration in fetuses increased gradually and reached 2.25 +/- 0.54 microg/ml at 120 min; (ii) Oxypurinol concentration in both mothers and fetuses increased during administration of allopurinol; (iii) Concentrations of allopurinol and oxypurinol in the perfusates reached 0.32 +/- 0.12 microg/ml, 0.53 +/- 0.22 microg/ml at 120 min respectively; and (iv) Administration of allopurinol significantly suppressed superoxide production during intermittent partial umbilical cord occlusion.

CONCLUSION

These results demonstrated a good transfer of allopurinol from mother to fetus and suggested the possibility of intrauterine treatment to inhibit fetal brain damage resulting from increased oxygen free radicals.

Melatonin provides neuroprotection in the late-gestation fetal sheep brain in response to umbilical cord occlusion

Oxygen free radicals, including the highly toxic hydroxyl radical (*OH), initiate lipid peroxidation and DNA/RNA fragmentation and damage cells. The pineal hormone melatonin is an antioxidant and powerful scavenger of *OH. We hypothesized that maternally administered melatonin could reduce *OH formation, lipid peroxidation, and DNA/RNA damage in the fetal brain in response to asphyxia. In 15 fetal sheep, extracellular *OH was measured by microdialysis in white and gray matter of the parasagittal cortex. In 10 fetuses, asphyxia was induced by umbilical cord occlusion for 10 min using an inflatable cuff - the ewes of these fetuses received either intravenous melatonin (1 mg bolus, then 1 mg/h for 2 h; n = 5) or vehicle (1% ethanol in saline; n = 5), and results were compared to fetuses with sham cord occlusion and vehicle-infused ewes (n = 5). Hypoxemia, acidemia, hypertension and bradycardia produced by cord occlusion was similar in the melatonin- and vehicle-treated groups. In the vehicle-treated group, cord occlusion resulted in a significant increase in *OH in gray matter at 8-9.5 h after occlusion (p < 0.05); in contrast, there was no *OH change in the melatonin-treated group. After cord occlusion, lipid peroxidation (4-hydroxynonenal immunoreactivity) found throughout the brain of vehicle-infused ewes was significantly less in the melatonin-infused group. Melatonin had no significant effect on the distribution of DNA/RNA fragmentation, as shown by 8-hydroxydeoxyguanosine immunoreactivity. Thus, brief asphyxia results in significant and delayed entry of *OH into the extracellular space of cortical gray matter in the fetal sheep brain, and melatonin given to the mother at the time of the insult abrogates this increase. Melatonin, in reducing O2 free radical production, may be an effective neuroprotective treatment for the fetus.

Purine and pyrimidine metabolism and electrocortical brain activity during hypoxemia in near-term lambs

Insufficient cerebral O(2) supply leads to brain cell damage and loss of brain cell function. The relationship between the severity of hypoxemic brain cell damage and the loss of electrocortical brain activity (ECBA), as measure of brain cell function, is not yet fully elucidated in near-term newborns. We hypothesized that there is a strong relationship between cerebral purine and pyrimidine metabolism, as measures of brain cell damage, and brain cell function during hypoxemia. Nine near-term lambs (term, 147 d) were delivered at 131 (range, 120-141) d of gestation. After a stabilization period, prolonged hypoxemia (fraction of inspired oxygen, 0.10; duration, 2.5 h) was induced. Mean values of carotid artery blood flow, as a measure of cerebral blood flow, and ECBA were calculated over the last 3 min of hypoxemia. At the end of the hypoxemic period, cerebral arterial and venous blood gases were determined and CSF was obtained. CSF from 11 normoxemic siblings was used for baseline values. HPLC was used to determine purine and pyrimidine metabolites in CSF, as measures of brain cell damage. Concentrations of purine and pyrimidine metabolites were significantly higher in hypoxemic lambs than in their siblings, whereas ECBA was lower in hypoxemic lambs. Significant negative linear relationships were found between purine and pyrimidine metabolite concentrations and, respectively, cerebral O(2) supply, cerebral O(2) consumption, and ECBA. We conclude that brain cell function is related to concentrations of purine and pyrimidine metabolites in the CSF. Reduction of ECBA indeed reflects the measure of brain damage due to hypoxemia in near-term newborn lambs.

Lipid peroxidation, caspase-3 immunoreactivity, and pyknosis in late-gestation fetal sheep brain after umbilical cord occlusion

Umbilical cord occlusion (UCO), a known risk factor for perinatal brain damage, causes severe fetal asphyxia leading to oxidative stress, lipid peroxidation, and cell death. We have determined the effects of two 10-min UCO on the distribution of the lipid peroxidation marker 4-hydroxynonenal (4-HNE) and the activated form of the apoptosis marker caspase-3 in the brains of late-gestation fetal sheep. UCO caused asphyxia, hypertension, and bradycardia, but these parameters normalized 2 h after the occlusion. At postmortem, 48 h after the second UCO there were significantly higher numbers of 4-HNE-positive cells in all layers of the hippocampus and cerebellum, the parietal cortex, substantia nigra, caudate nucleus, putamen, and thalamus compared with control brains. 4-HNE immunoreactivity was also found in white matter tracts of the subcallosal bundle, external medullary lamina, reticular thalamic nucleus, and cerebellar fiber tracts only in UCO brains. Double-labeling identified these cells as predominantly neurons and astrocytes, with oligodendrocytes showing lower levels of 4-HNE immunoreactivity. After UCO, the number of caspase-3-immunopositive cells was increased significantly in the hippocampal CA1, molecular layer and dentate gyrus, ventrolateral thalamic nucleus, substantia nigra, putamen, and cerebellar granular and molecular layers compared with controls. Double-labeling revealed caspase-3 immunoreactivity was mainly in neurons, and to lesser extent in astrocytes and oligodendrocytes. Pyknotic cell numbers were significantly increased in hippocampal CA1 and CA3, parietal cortex, caudate nucleus, putamen, and cerebellar Purkinje cells after UCO. These data indicate that brief asphyxia induces widespread lipid peroxidation involving all cell types of the fetal brain and apoptosis in both neurons and glia.

Effects of maternal oxygen supplementation on fetal oxygenation and lipid peroxidation following a single umbilical cord occlusion in fetal goats

Maternal oxygen supplementation is commonly performed to improve fetal oxygenation and acid-base balance during fetal asphyxia. The efficiency of this treatment is controversial, which may be associated with the production of oxygen free radicals and lipid peroxidation. However, only a few studies have been performed to evaluate these issues. To clarify them, we investigated the effects of maternal oxygen supplementation on fetal oxygenation and lipid peroxidation following fetal asphyxia in late gestation goats. We measured fetal blood gases, pH and plasma malondialdehyde (MDA), one of the endproducts of lipid peroxidation, before, during and after fetal asphyxia with and without maternal oxygenation in late gestation goats. Fetal asphyxia was induced by a single total umbilical cord occlusion of 3 minutes' duration, and maternal oxygenation was initiated at 20 min before the cord occlusion and terminated at 20 min after the release of cord occlusion. Maternal oxygen supplementation resulted in a significant increase in fetal PaO(2) before and after the cord occlusion (p<0.05). During the cord occlusion, the extents of hypoxia and acidemia were not changed by maternal oxygen supplementation. Fetal plasma MDA levels before maternal oxygen supplementation averaged 0.80+/-0.04 micromol/L, significantly increased after the initiation of maternal oxygen supplementation (1.11+/-0.07 micromol/L), and further increased following fetal asphyxia (1.28+/-0.06 micromol/L), and after the release of the cord occlusion (1.58+/-0.7 micromol/L)(p<0.05). These values were significantly higher than those in fetuses without oxygenation. We conclude that maternal oxygen supplementation increased fetal oxygenation but caused a concomitant increase in lipid peroxidation in the fetus.

Oxidative stress in the fetal lamb brain following intermittent umbilical cord occlusion: a path analysis

OBJECTIVE

To evaluate the relative contribution of cord occlusion length intervals between occlusions and experimental duration on oxidative stress in the fetal lamb brain.

DESIGN

Acute, partially exteriorised fetal lambs with intermittent total cord occlusion.

SETTING

The Vivarium of Westmead Hospital, University of Sydney, Australia and The Chinese University of Hong Kong.

MAIN OUTCOME MEASURES

Arterio-venous differences in the concentration of organic hydroperoxides, measured in paired samples of carotid arterial and jugular venous blood, as an index of oxidative stress in the brain.

METHODS

Thirty-two fetal lambs were exposed to graded hypoxia, induced by intermittent total umbilical cord compression of 30 seconds, 60 seconds and 90 seconds duration, occurring every minute for a total of 27 occlusions over 81 minutes. Three sham experiments were also performed. In addition to organic hydroperoxides, carotid arterial blood samples were also assayed in 15 animals (two sham) for oxygen saturation, pH, hypoxanthine, xanthine and urate concentrations. A causal model for oxidative stress was defined: occlusions leading to hypoxia with a rise in hypoxanthine; reperfusion during intervals between occlusions leading to the accelerated production of xanthine and uric acid and the generation of oxygen free radicals, which in turn, are responsible for the rise in lipid peroxidation. Path analysis was performed to assess the strength of the relationships between these variables and the cord occlusion length, the interval between occlusions and the duration of the experiment.

RESULTS

Sham experiments showed no change in organic hydroperoxide production. Following 30-second umbilical cord occlusions a sixfold drop in mean organic hydroperoxides was observed between carotid arterial and jugular venous levels. In contrast, following occlusions of 60 seconds duration (or longer) a median 20-fold increase in organic hydroperoxide production was observed. Path analysis revealed a strong indirect pathway from occlusion length --> hypoxanthine --> urate and weak positive pathways from oxygen saturation --> urate and from interval between occlusions --> urate. After accounting for these pathways reflecting oxidative stress, a strong direct path remained from time from first occlusion --> organic hydroperoxide production.

CONCLUSIONS

Peroxidation of lipids in the brain occurs under conditions of severe hypoxia and reperfusion associated with intermittent umbilical cord occlusions of 60 seconds or longer. The path analysis supported the causal model as originally defined, with the exception that the indirect pathway via pH was found to be trivial.