Early hypoxia modulates the phenotype of dopaminergic cells in rat di- and mesencephalic cell cultures and induces a higher vulnerability of non-dopaminergic neurons to a second hypoxic exposure

Episodic neonatal hypoxia evokes executive dysfunction and regionally specific alterations in markers of dopamine signaling

Perinatal ischemic-anoxic and prolonged anoxic insults lead to impaired dopaminergic signaling and are hypothesized to contribute, at least in part, to the pathogenesis of disorders of minimal brain dysfunction such as attention-deficit hyperactivity disorder. We hypothesized that subtle intermittent hypoxic insults, occurring during a period of critical brain development, are also pathogenic to dopaminergic signaling, thereby contributing to behavioral and executive dysfunction. Between postnatal days 7 and 11, rat pups were exposed to either 20-s bursts of isocapnic hypoxic gas, compressed air, or were left undisturbed with the dam. On postnatal days 23 pups were instrumented with electroencephalographic/electromyographic electrodes and sleep-wake architecture was characterized. Locomotor activity was assessed between postnatal days 35 and 38, learning, and working memory evaluated between postnatal days 53 and 64. Rats were killed on postnatal day 80 and tyrosine hydroxylase, vesicular monoamine transporter, dopamine transporter, and dopamine D1 receptors were quantified in the prefrontal cortex, primary sensorimotor cortex, and precommissural striatum by Western blot analyses. Post-hypoxic pups spent less time awake and more time in rapid-eye-movement sleep during the lights-on phase of the circadian cycle, were hyperlocomotive, and expressed impaired working memory. Striatal expression of vesicular monoamine transporter and D1 receptor proteins were increased in post-hypoxic rats, consistent with depressed dopaminergic signaling. These observations lead to the intriguing hypothesis that intermittent hypoxia occurring during a period of critical brain development evokes behavioral and neurochemical alterations that are long lasting, and consistent with disorders of minimal brain dysfunction.

Developmental differences in cortical and hippocampal vulnerability to intermittent hypoxia in the rat

Obstructive sleep apnea is characterized by intermittent hypoxic events during sleep, and is associated with substantial neurocognitive morbidity, particularly in children. Intermittent hypoxia (IH) leads to increases in apoptosis in the cortex and hippocampus of the adult rat, peaking at 48 h of exposure. To examine whether the susceptibility to IH exhibits developmental differences, rats were exposed to 48 h of IH at ages 2, 5, 10, 15, 20, 25, 30, 60, and 120-day postnatally, and apoptosis was determined by terminal deoxy-nucleotidyl transferase-mediated in situ end labeling and immunohistochemical staining for single-stranded DNA. Although IH induced apoptosis at all postnatal ages, smaller increases were apparent in 2 and 5-day old (P < 0.01 vs. any other age) while peak apoptosis occurred at 10-25 days (P < 0.001 vs. 30, 60, and 120 days). We conclude that a unique window of vulnerability to IH is present in the cortex and hippocampus during post-natal maturation, and may underlie the high frequency of neurobehavioral deficits associated with obstructive sleep apnea in children.