Fetal cerebral blood flow and metabolism during oligemia and early postoligemic reperfusion

The instrumented fetal sheep as a model of cerebral white matter injury in the premature infant

Despite advances in neonatal intensive care, survivors of premature birth remain highly susceptible to unique patterns of developmental brain injury that manifest as cerebral palsy and cognitive-learning disabilities. The developing brain is particularly susceptible to cerebral white matter injury related to hypoxia-ischemia. Cerebral white matter development in fetal sheep shares many anatomical and physiological similarities with humans. Thus, the fetal sheep has provided unique experimental access to the complex pathophysiological processes that contribute to injury to the human brain during successive periods in development. Recent refinements have resulted in models that replicate major features of acute and chronic human cerebral injury and have provided access to complex clinically relevant studies of cerebral blood flow and neuroimaging that are not feasible in smaller laboratory animals. Here, we focus on emerging insights and methodologies from studies in fetal sheep that have begun to define cellular and vascular factors that contribute to white matter injury. Recent advances include spatially defined measurements of cerebral blood flow in utero, the definition of cellular maturational factors that define the topography of injury and the application of high-field magnetic resonance imaging to define novel neuroimaging signatures for specific types of chronic white matter injury. Despite the higher costs and technical challenges of instrumented preterm fetal sheep models, they provide powerful access to clinically relevant studies that provide a more integrated analysis of the spectrum of insults that appear to contribute to cerebral injury in human preterm infants.

Cerebral blood flow heterogeneity in preterm sheep: lack of physiologic support for vascular boundary zones in fetal cerebral white matter

Periventricular white matter (PVWM) injury is the leading cause of neurologic disability in survivors of prematurity. To address the role of ischemia in PVWM and cerebral cortical injury, we hypothesized that immaturity of spatially distal vascular 'end zones' or 'border zones' predisposes PVWM to greater decreases in cerebral blood flow (CBF) than more proximal structures. We quantified regional CBF with fluorescently labeled microspheres in 0.65 gestation fetal sheep in histopathologically defined three-dimensional regions by post hoc digital dissection and coregistration algorithms. Basal flow in PVWM was significantly lower than in gyral white matter and cortex, but was equivalent in superficial, middle, and deep PVWM. Absolute and relative CBF (expressed as percentage of basal) did not differ significantly during ischemia or reperfusion between PVWM, gyral white matter, or cortex. Moreover, CBF during ischemia-reperfusion was equivalent in three adjacent PVWM levels and was not consistent with the magnitude of severity of PVWM injury, defined by TUNEL (terminal deoxynucleotidyltransferase-mediated dUPT nick end labeling) staining. However, the magnitude of ischemia was predicted by the severity of discrete cortical lesions. Hence, unlike cerebral cortex, unique CBF disturbances did not account for the distribution of PVWM injury. Previously defined cellular maturational factors, thus, appear to have a greater influence on PVWM vulnerability to ischemic injury than the presence of immature vascular boundary zones.

Role of mitochondrial permeability transition in the immature brain following intrauterine ischemia

Recirculation following 30 minutes of intrauterine ischemia due to uterine artery occlusion has previously been found to be accompanied by delayed deterioration of the cellular bioenergetic state and of mitochondrial function in the fetal rat brain. The objective of this study was to assess whether the delayed deterioration is due to the activation of mitochondrial permeability transition (MPT), which is observed ultrastructurally as mitochondrial swelling. The respiratory activities and ultrastructure of isolated mitochondria and the cellular bioenergetic state in the fetal rat brain were examined at the end of 30 minutes of intrauterine ischemia and after 1, 2, 3 or 4 hours of recirculation. Cyclosporin A (CsA), a potent and specific MPT blocker, or vehicle was given 1 hour after recirculation. In the vehicle-treated animals, the transient ischemia was associated with a delayed deterioration of the cellular bioenergetic state and mitochondrial activities 4 hours of recirculation. The number of swollen mitochondria increased markedly after 4 hours of recirculation. Both the deterioration and swelling were prevented by CsA. The present study indicates that treatment with CsA improves recovery of energy metabolism and inhibits mitochondrial swelling following transient intrauterine ischemia in the fetal brain. The results suggest that mitochondria and MPT may be involved in the development of ischemic brain damage in the immature rat.

Spatial heterogeneity in oligodendrocyte lineage maturation and not cerebral blood flow predicts fetal ovine periventricular white matter injury

Although periventricular white matter injury (PWMI) is the leading cause of chronic neurological disability and cerebral palsy in survivors of premature birth, the cellular-molecular mechanisms by which ischemia-reperfusion contributes to the pathogenesis of PWMI are not well defined. To define pathophysiologic relationships among ischemia, acute cerebral white matter damage, and vulnerable target populations, we used a global cerebral ischemia-reperfusion model in the instrumented 0.65 gestation fetal sheep. We developed a novel method to make repeated measurements of cerebral blood flow using fluorescently labeled microspheres to resolve the spatial heterogeneity of flow in situ in three-dimensional space. Basal flow in the periventricular white matter (PVWM) was significantly lower than in the cerebral cortex. During global cerebral ischemia induced by carotid occlusion, flow to all regions was reduced by nearly 90%. Ischemia of 30 or 37 min duration generated selective graded injury to frontal and parietal PVWM, two regions of predilection for human PWMI. Injury was proportional to the duration of ischemia and increased markedly with 45 min of ischemia to extensively damage cortical and subcortical gray matter. Surprisingly, the distribution of PVWM damage was not uniform and not explained by heterogeneity in the degree of white matter ischemia. Rather, the extent of white matter damage coincided with the presence of a susceptible population of late oligodendrocyte progenitors. These data support that although ischemia is necessary to generate PWMI, the presence of susceptible populations of oligodendrocyte progenitors underlies regional predilection to injury.

Effects of antenatal steroids on ischemic brain injury in near-term ovine fetuses

BACKGROUND

Hypoxia/ischemia in utero can result in brain damage to the fetus and newborn. Antenatal steroids are a routine part of the management of women who develop premature labor. Pretreatment of young postnatal rats with dexamethasone before hypoxic/ischemic insults has been reported to attenuate brain injury. However, the effects of antenatal steroids on ischemic brain injury in fetuses have not been investigated.

OBJECTIVE

We examined the effects of maternally administered antenatal corticosteroids on ischemic brain injury in near-term ovine fetuses.

METHODS

Chronically instrumented fetuses at 122 days of gestation were studied 12 h after the last of four 4 mg dexamethasone, or placebo injections were given over 48 h to the ewes. Groups were dexamethasone/ischemic, placebo/ischemic and sham-treated control. Fetuses were exposed to 30 min of carotid occlusion (ischemia) or no occlusion (control) and 72 h of reperfusion. Whole brain coronal sections stained with Luxol fast blue-hematoxylin-eosin were scored for white matter and cerebral cortical lesions. Both areas received pathological scores of 0 to 5 reflecting the degree of injury (0=0%, 1=1-10%, 2=11-50%, 3=51-90%, 4=91-99% and 5=100%). Bilateral carotid blood flow also was measured before, during and after brain ischemia in the dexamethasone/ischemic and placebo/ischemic groups.

RESULTS

White matter (WM) and cerebral cortical scores did not differ between the dexamethasone/ischemic and placebo/ischemic (WM: 3.0+/-1.9 and 2.9+/-1.7; cortex: 3.1+/-1.7 and 2.6+/-1.8, mean+/-S.D.) groups. White matter and cerebral cortical scores were higher in the dexamethasone/ischemic (WM: 3.0+/-1.9, P<0.02; cortex: 3.1+/-1.7, P<0.005) and placebo/ischemic (WM: 2.9+/-1.7, P<0.006; cortex: 2.6+/-1.8, P<0.007) than control (WM: 0.2+/-0.4; cortex: 0.2+/-0.4) group. Carotid blood flow was relatively higher (P<0.05) after 24, 48 and 72 h of reperfusion in the dexamethasone/ischemic than placebo/ischemic group.

CONCLUSIONS

We conclude that maternal pretreatment with antenatal dexamethasone did not attenuate ischemic brain injury in the fetus, and that carotid blood flow was higher during reperfusion in fetuses of dexamethasone than placebo-treated ewes, most likely secondary to decreases in arterial oxygen tension.

Short therapeutic window for nifedipine in transient intrauterine ischemia in fetal rat brain

The aim of this study was to explore whether nifedipine influences the secondary deterioration of cerebral mitochondrial function after transient intrauterine ischemia in fetal rats. Intrauterine ischemia was induced by a 30-min occlusion of the right uterine artery at 20 days of gestation in Wistar rats. Nifedipine (1 mg kg(-1)) or vehicle was injected subcutaneously before the onset of ischemia or 1 h after the start of recirculation. Fetuses were delivered by cesarean section at the end of ischemia (n=6 with vehicle; n=6 with nifedipine pretreatment) or at 4 h of recirculation (n=6 with vehicle; n=6 with nifedipine pretreatment; n=6 with nifedipine posttreatment), and the cerebral mitochondrial respiration was measured polarographically. Tissue oxygen tension was evaluated in placental and fetal cerebral tissues (n=5 with vehicle; n=5 with nifedipine pretreatment). The vehicle treated animals showed a significant decrease in mitochondrial activities at the end of ischemia and 4 h of recirculation. Nifedipine attenuates the secondary deterioration at 4 h of recirculation when given just prior to ischemia, but had no neuroprotective activity when given 1 h after the start of recirculation. Nifedipine pretreatment had no influence on oxygen delivery in placenta and fetal cerebrum during and after ischemia. Despite the short therapeutic window, the treatment of nifedipine attenuates the secondary deterioration of cerebral mitochondrial function after transient intrauterine ischemia in fetal rats when given just prior to ischemia.

Effect of the immunosuppressant drug FK506 on neonatal cerebral mitochondrial function and energy metabolism after transient intrauterine ischemia in rats

Mitochondrial respiratory activities and energy metabolism were measured in neonatal rat brains to evaluate the influence of transient intrauterine ischemia on the near-term fetus and to assess the effect of the immunosuppressant drug FK506 treatment. Transient intrauterine ischemia was induced by 30 min of right uterine artery occlusion at 17 days of gestation in Wistar rats. The vehicle or 1.0 mg/kg of FK506 was administered after 1 h of recirculation. All of the pups were delivered by cesarean section at 21 days of gestation and samples of cerebral cortical tissue were obtained from pups at 1 h after birth. The mitochondrial respiration was measured polarographically in homogenates. For the analysis of ATP, ADP, and AMP, neonatal brains were frozen in situ and fluorometric enzymatic techniques were used. In the neonatal cortical tissue exposed to ischemia, mitochondrial respiratory activities and ATP concentrations decreased significantly to approximately 59 and 67% of those in normoxic controls, respectively. The deterioration of both mitochondrial respiratory activities and high-energy phosphates was prevented by FK506, given 1 h after the start of recirculation. The present results indicate that transient intrauterine ischemia is accompanied by mitochondrial dysfunction and cellular bioenergetic failure in the neonatal rat brain and suggest that treatment with FK506 prevents the deterioration, even when administered after the ischemic periods.

Secondary mitochondrial dysfunction after transient intrauterine ischemia in the fetal rat brain

OBJECTIVE

Recirculation following transient intrauterine ischemia has previously been found to cause partial recovery and secondary deterioration of cellular bioenergetic states in the fetal rat brain. Our objective was to assess whether secondary bioenergetic failure is due to mitochondrial dysfunction.

STUDY DESIGN

Cerebral cortical tissues were obtained from 20-day-old fetal rats at the end of 30 minutes of intrauterine ischemia (n = 6) and after 1 hour (n = 6), 2 hours (n = 6), and 4 hours (n = 6) of recirculation. Mitochondrial respiratory activities were measured polarographically using homogenates.

RESULTS

Mitochondrial activities decreased to 45% of sham-operated controls at the end of ischemia. Recirculation (1 hour) brought about partial recovery, but continued reflow (2 hours and 4 hours) was associated with a secondary deterioration of mitochondrial activities.

CONCLUSION

The results indicate that although during the early time period after ischemia mitochondrial respiratory capacity is restored, secondary mitochondrial dysfunction develops in the immature rat brain.

Effect of alpha-phenyl-N-tert-butyl nitrone (PBN) on fetal cerebral energy metabolism during intrauterine ischemia and reperfusion in rats

The objective of the present study was to explore whether a free radical spin trap agent, alpha-phenyl-N-tert-butyl nitrone (PBN), influences bioenergetic failure induced in the 20-day-old fetal brain by 30 min of intrauterine ischemia in Wistar rats. Fetal brains were frozen in situ at the end of ischemia and after 1, 2, and 4 h of recirculation for analysis of ATP, ADP, AMP, and lactate. PBN or vehicle was given 1 h after recirculation. Tissue oxygen tension was evaluated in placental and fetal cerebral tissues throughout the whole periods of 30 min of ischemia and 4 h of recirculation. Ischemia was associated with a decrease in ATP concentration and an increase in lactate concentration (p < 0.001). Recirculation (1 and 2 h) led to a recovery of ATP concentration, but continued reflow (4 h) was associated with a secondary deterioration of high-energy phosphates (p < 0.01). Lactate concentration increased during this recovery period. This deterioration was prevented by PBN (p < 0.05). After 30 min of ischemia, tissue oxygen tension in placenta and fetal brain decreased to about 30% and 50% of control, respectively. However, recirculation brought about a recovery of oxygen delivery. The results indicate that although during the early time period after ischemia fetal cerebral energy metabolism is maintained by an acceleration of the anaerobic glycolytic rate, secondary deterioration of cellular bioenergetic state develops in the immature fetal brain. This deterioration may be due to mitochondrial dysfunction, which may be induced by oxygen-derived free radicals, and not by compromised microcirculation.