Spectrum of Short- and Long-Term Brain Pathology and Long-Term Behavioral Deficits in Male Repeated Hypoxic Rats Closely Resembling Human Extreme Prematurity

Abstract

Brain injury in the premature infant is associated with a high risk of neurodevelopmental disability. Previous small-animal models of brain injury attributable to extreme prematurity typically fail to generate a spectrum of pathology and behavior that closely resembles that observed in humans, although they provide initial answers to numerous cellular, molecular, and therapeutic questions. We tested the hypothesis that exposure of rats to repeated hypoxia from postnatal day 1 (P1) to P3 models the characteristic white matter neuropathological injury, gray matter volume loss, and memory deficits seen in children born extremely prematurely. Male Sprague Dawley rats were exposed to repeated hypoxia or repeated normoxia from P1 to P3. The absolute number of pre-oligodendrocytes and mature oligodendrocytes, the surface area and g-ratio of myelin, the absolute volume of cerebral white and gray matter, and the absolute number of cerebral neurons were quantified stereologically. Spatial memory was investigated on a radial arm maze. Rats exposed to repeated hypoxia had a significant loss of (1) pre-oligodendrocytes at P4, (2) cerebral white matter volume and myelin at P14, (3) cerebral cortical and striatal gray matter volume without neuronal loss at P14, and (4) cerebral myelin and memory deficits in adulthood. Decreased myelin was correlated with increased attention deficit hyperactivity disorder-like hyperactivity. This new small-animal model of extreme prematurity generates a spectrum of short-and long-term pathology and behavior that closely resembles that observed in humans. This new rat model provides a clinically relevant tool to investigate numerous cellular, molecular, and therapeutic questions on brain injury attributable to extreme prematurity.

Mild intermittent hypoxemia in neonatal mice causes permanent neurofunctional deficit and white matter hypomyelination

“…There are several reports exploring the role of experimental intermittent hypoxemia in neurologic morbidity associated with adult sleep apnea and apnea of prematurity (Cai et al, 2011(Cai et al, , 2012Douglas et al, 2010;Oorschot et al, 2013;Xu et al, 2004). Similar to our results, studies on neonatal animals demonstrated poorer WM myelination in mice and rats exposed to IH compared to their naïve controls.…”

“…In this paradigm the cumulative duration of hypoxic exposure reached 25% of each 24 h time-period. In another report hypoxic exposure was relatively short (three days), but very severe (1.5% O 2 for 15 min, seven events a day) (Oorschot et al, 2013). This may explain a significant loss of precursors of myelin-producing cells observed in the above referenced model and hippocampal and cortical neuronal loss in the IH-models by Douglas et al (2010) and Gozal et al (2001).…”

“…This may explain a significant loss of precursors of myelin-producing cells observed in the above referenced model and hippocampal and cortical neuronal loss in the IH-models by Douglas et al (2010) and Gozal et al (2001). Furthermore, in the protocol reported by Cai et al (2012) IH stress resulted in bradycardia and the protocol by Oorschot et al (2013) was not controlled for changes in the CBF or the presence of bradycardia. Thus, an ischemic damage to the brain in the above referenced IH paradigms cannot be excluded.…”

“…It is possible that sensitivity of the same modification of the Morris water maze test is not sufficient to detect memory deficit in the absence of neurodegeneration. Of note, Oorschot et al, using the eight-arm radial maze test have reported spatial memory impairment in the absence of neuronal cell loss, but associated with a death of oligodendrocyte precursors in adult rats exposed to IH from p1 to p3 (Oorschot et al, 2013).…”

“…To test this hypothesis we developed a mouse model where WMI was induced by non-lethal IH stress and the phenotype was characterized by cerebral hypomyelination associated with permanent neurofunctional deficit, without cell degeneration in the brain. In contrast to reported models of WMI induced by IH mimicking severe hypoxemic event associated with apnea of prematurity (Cai et al, 2012;Oorschot et al, 2013), our paradigm of IH simulates only mild, non-bradycardia-associated, repetitive hypoxemic episodes, the IH-pattern of unknown neurological significance, yet the most commonly seen in human premature infants.…”