Comparison of two neonatal ischemic injury models using magnetic resonance imaging

Early diffusion-weighted MRI as a predictor of caspase-3 activation after hypoxic-ischemic insult in neonatal rodents

BACKGROUND AND PURPOSE

Neonatal encephalopathy in human babies is a serious condition associated with permanent neurological deficits. Diffusion-weighted MRI (DWI) is increasingly used for early diagnosis of brain injury in human babies. The relationship between the presence of DWI abnormalities and cellular injury, including apoptosis, during the neonatal period are not well understood. We asked whether the extent of injury depicted on DWI can predict the presence of caspase-3 activation, a quantitative marker of apoptotic injury, after hypoxia-ischemia (H-I) in postnatal day 7 rats.

METHODS

Injury volume was determined by DWI at 2 hours, 24 hours, and 7 days after H-I and compared with histology. Caspase-3 activation and microgliosis were determined at 24 hours post-H-I.

RESULTS

DWI-defined lesions (eg, decreased apparent diffusion coefficient) at 24 hours post-H-I correlated with a major increase in caspase-3 activity in the injured hemisphere and predicted injury. A modest but significant increase in caspase-3 activity occurred in the cortex of rats that had no apparent diffusion coefficient decrease in the injured hemisphere but had unilaterally enlarged regions of high apparent diffusion coefficient at the ipsilateral ventricle/white matter interface. Caspase-3 activity was similar in both hemispheres in pups with unchanged DWI.

CONCLUSIONS

Abnormal DWI signal at 24 hours post-H-I is predictive of caspase-3 activation and can be used as an indicator that injury involving an apoptotic-like mechanism is present. Our data also suggest that the presence of an enlarged unilateral region with high apparent diffusion coefficient at the ventricle/white matter interface without significant apparent diffusion coefficient decrease in the cortex is a sign of modest caspase-3 activation after H-I.

Application of magnetic resonance imaging in animal models of perinatal hypoxic-ischemic cerebral injury

Brain injury occurring in the perinatal period is an important etiology of subsequent neurodevelopmental disabilities. Magnetic resonance imaging (MRI) is a tool that is used to evaluate the nature of brain injury in the human infant. MRI techniques have also been applied to various animal models of perinatal injury. The most commonly used model is the immature rat, but there have also been imaging studies in mice, rabbit kits and piglets. The studies have been carried out using MR systems of various magnetic field strengths, ranging from 1.5 to 11.7tesla (T), with applications for quantification of infarct volume, T1 measurements, T2 measurements, proton and phosphorus spectroscopy and diffusion imaging. The MR findings are then related to histopathology and, in a few cases, behavioral evaluations. There is also a growing number of studies utilizing MRI in evaluating the efficacy of neuroprotective treatments, such as hypothermia.

The cannabinoid agonist WIN55212 reduces brain damage in an in vivo model of hypoxic-ischemic encephalopathy in newborn rats

Neonatal hypoxic-ischemic encephalopathy (NHIE) is a devastating condition for which effective therapeutic treatments are still unavailable. Cannabinoids emerge as neuroprotective substances in adult animal studies; therefore, we aimed herein to test whether cannabinoids might reduce brain damage induced by hypoxiaischemia (HI) in newborn rats. Thus, 7-d-old Wistar rats (P7) were exposed to 8% O2 for 120 min after left carotid artery ligature, then received s.c. vehicle (VEH) (HI+VEH), the cannabinoid agonist WIN55212 (WIN) (0.1 mg/kg), or WIN with the CB1 or CB2 receptor antagonist SR141617 (SR1) (3 mg/kg) or SR141588 (SR2) (2 mg/kg). Brain damage was assessed by magnetic resonance imaging (MRI) at 1, 3, and 7 d after the insult. At the end of the experiment, MRI findings were corroborated by histology (Nissl staining). HI+VEH showed an area of cytotoxic and vasogenic edema at 24 h after the insult, then evolving to necrosis. HI+WIN showed a similar damaged area at 24 h after the insult, but the final necrotic area was reduced by 66%. Coadministration of either SR1 or SR2 reversed the effects of WIN. In conclusion, likely by activating CB1 and CB2 receptors, WIN afforded robust neuroprotection in newborn rats after HI.

Temporal and regional evolution of aquaporin-4 expression and magnetic resonance imaging in a rat pup model of neonatal stroke

Edema formation can be observed using magnetic resonance imaging (MRI) in patients with stroke. Recent studies have shown that aquaporin-4 (AQP4), a water channel, is induced early after stroke and potentially participates in the development of brain edema. We studied whether induction of AQP4 correlated with edema formation in a rat pup filament stroke model using high field (11.7-Tesla) MRI followed by immunohistochemical investigation of AQP4 protein expression. At 24 h, we observed increased T2 values and decreased apparent diffusion coefficients (ADC) within injured cortical and striatal regions that reflected the edema formation. Coincident with these MR changes were significant increases in AQP4 expression on astrocytic end-feet in the border regions of injured tissues. Striatal imaging findings were still present at 72 h with a slow normalization of AQP4 expression in the border regions. At 28 d, AQP4 expression normalized in the border while in this region ADC values increased. We show that induction of AQP4 is increased during the period of active edema formation in the border region without regional correlation with edema. Finally, induction of AQP4 on astrocyte end-feet could participate in tissue preservation after ischemia in the immature rat brain.