Adaptive brain shut-down counteracts neuroinflammation in the near-term ovine fetus

Abstract

Objective: Repetitive umbilical cord occlusions (UCOs) in ovine fetus leading to severe acidemia result in adaptive shut-down of electrocortical activity [electrocorticogram (ECoG)] as well as systemic and brain inflammation. We hypothesized that the fetuses with earlier ECoG shut-down as a neuroprotective mechanism in response to repetitive UCOs will show less brain inflammation and, moreover, that chronic hypoxia will impact this relationship.

Methods: Near-term fetal sheep were chronically instrumented with ECoG leads, vascular catheters, and a cord occluder and then underwent repetitive UCOs for up to 4 h or until fetal arterial pH was <7.00. Eight animals, hypoxic prior to the UCOs (SaO2 <55%), were allowed to recover 24 h post insult, while 14 animals, 5 of whom also were chronically hypoxic, were allowed to recover 48 h post insult, after which brains were perfusion-fixed. Time of ECoG shut-down and corresponding pH were noted, as well as time to then reach pH <7.00 (ΔT). Microglia (MG) were counted as a measure of inflammation in gray matter layers 4-6 (GM4-6) where most ECoG activity is generated. RESULTS are reported as mean ± SEM for p < 0.05.

Results: Repetitive UCOs resulted in worsening acidosis over 3-4 h with arterial pH decreasing to 6.97 ± 0.02 all UCO groups' animals, recovering to baseline by 24 h. ECoG shut-down occurred 52 ± 7 min before reaching pH <7.00 at pH 7.23 ± 0.02 across the animal groups. MG counts were inversely correlated to ΔT in 24 h recovery animals (R = -0.84), as expected. This was not the case in normoxic 48 h recovery animals, and, surprisingly, in hypoxic 48 h recovery animals, this relationship was reversed (R = 0.90).

Conclusion: Adaptive brain shut-down during labor-like worsening acidemia counteracts neuroinflammation in a hypoxia- and time-dependent manner.

Keywords: ECoG; EEG; acidemia; fetus; hypoxia; labor; microglia; sheep.

Figures

Figure 1

Example of an individual electrocorticogram (ECoG) response to repetitive umbilical cord occlusions (UCOs). (A) A complete 5 h recording is shown with baseline, mild, moderate, and severe UCOs. UCOs occurrences are indicated by the UCP channel: umbilical contraction pressure increases correspond to an occlusion of the umbilical cord. Note emergence of the adaptive brain shut-down pattern visible in ECoG- and fetal heart rate (FHR) synchronization and the accompanying changes in arterial blood pressure (ABP). (B) Ten minutes zoomed-in window of this synchronized ECoG–FHR pattern.

Figure 2

Fetal brain weights’ correlations to the amplitudes of electrocorticogram (ECoG) at baseline, i.e., prior to commencing with umbilical cord occlusions. (A) Correlation across all three groups showing increasing fetal ECoG amplitude with increasing brain weight. (B) Group-specific correlations still hold true despite lower brain weights in the H/UCO 24 h group compared to the N/UCO 48 h group.

Figure 3

Neuroinflammation in gray matter layers 4–6 (GM4–6) assessed as microglia (MG) counts per high-power field (HPF) at 24 and 48 h post insult in normoxic (N/UCO) and hypoxic (H/UCO) groups versus respective control groups. Cf. Figure 3 for the inter-group comparison. H/UCO 24 data reproduced with kind permission from Elsevier.

Figure 4

(A) Neuroinflammation in gray matter layers 4–6 (GM4–6) assessed as microglia (MG) counts per high-power field (HPF) at 24 and 48 h post insult, normalized by average MG counts per HPF in respective control groups’ brain regions (to allow for inter-group comparison, see “Materials and Methods” for details). Mean ± SEM. *p = 0.03 for N/UCO 48 h versus H/UCO 24 h group; $p = 0.02 for H/UCO 48 h versus H/UCO 24 h group. (B) Correlation to adaptive brain shut-down timing expressed by ΔT as the difference between the individual times of fetal adaptive brain shut-down onset (observed at the start of the synchronized ECoG/FHR change) and the time to then reach the target pH <7.00. Spearman correlation coefficients R = −0.84 for H/UCO 24 h group and R = 0.90 for H/UCO 48 h group (both p = 0.04); no significant correlation for N/UCO 48 h group (R = −0.05, p = 0.91). Due to artifacts in ECoG, ΔT was missing in two out of eight H/UCO 24 h group animals and in one animal from the N/UCO 48 h group.