Protective effect of hypoxic preconditioning on hypoxic-ischemic injured newborn rats

Abstract

Brief episodes of cerebral hypoxia-ischemia cause transient ischemic tolerance to subsequent ischemic events that are otherwise lethal. This study was conducted to evaluate the protective effect of hypoxic preconditioning on hypoxic-ischemic injury in the neonatal rat and the persistence of a protective window after hypoxic preconditioning. The rats were preconditioned with hypoxia (8% oxygen, 92% nitrogen) for three hours, subjected to ischemia using ligation of the right common carotid artery, and then exposed to another three hours of hypoxia. Using proton magnetic resonance spectroscopy, terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end-labeling (TUNEL) staining, and morphologic scores, this study shows that hypoxic preconditioning 6-hr to 1-day before hypoxic-ischemic injury increases survival rates and has neuroprotective effects against subsequent hypoxic-ischemic injury. The mechanism of the protective effects of hypoxic preconditioning in the newborn rat brain may involve downregulation of apoptotic cell death.

Keywords: Hypoxia-Ischemia, Brain; Magnetic Resonance Spectroscopy.

Fig. 1

Schematic diagram illustrating the hypoxic-ischemic animal model. One day after hypoxic-ischemic injury, the number of TUNEL-positive cells and the lipid peak on ¹H MRS in the right brain markedly increase. Fourteen days after the insult, severe atrophy of the right hemisphere is observed.

Fig. 2

The effect of hypoxic preconditioning on survival rates after hypoxic-ischemic brain injury in newborn rats. The survival rates are higher in the Pre-1 day, Pre-12 hr, and Pre-6 hr groups than in the control group. The values are expressed as mean ± standard deviation.

Fig. 3

The ratios of lipid/N-acetyl aspartate (Lip/NAA) and lipid/creatine (Lip/Cr) on ¹H MRS in the control group and hypoxic preconditioned groups one day after hypoxic-ischemic brain injury in the newborn rat. The ratios of lipid/NAA and lipid/Cr at the hypoxic-ischemic injured right brain decrease significantly in the Pre-1 day, Pre-12 hr, and Pre-6 hr groups. The values are expressed as mean ± standard deviation. ^*P < 0.05.

Fig. 4

Histologic findings of TUNEL staining in the newborn rat brain obtained on the first day after hypoxic-ischemic injury. Increased TUNEL-positive cells are noted in the right hemisphere in the control, Pre-6 day, and Pre-3 day groups. Increased ¹H MRS lipid peaks were observed in the control, Pre-6 day, and Pre-3 day groups.

Fig. 5

The effect of hypoxic preconditioning on morphologic scores of the newborn rat brain fourteen days after hypoxic-ischemic brain injury. The morphologic scores are significantly lower in the Pre-1 day, Pre-12 hr, and Pre-6 hr groups than in the control group (^*P < 0.05). The values are expressed as mean ± standard deviation.

Fig. 6

Correlation between the morphologic scores and ¹H MRS findings. The high lipid/NAA and lipid/Cr ratios on the first day after hypoxic-ischemic injury were predictive of morphologic changes.