Prenatal Hypoxia Induces Premature Aging Accompanied by Impaired Function of the Glutamatergic System in Rat Hippocampus

Prenatal hypoxia is among leading causes of progressive brain pathologies in postnatal life. This study aimed to analyze the characteristics of the hippocampal glutamatergic system and behavior of rats in early (2 weeks), adult (3 months) and advanced (18 months) postnatal ontogenesis after exposure to prenatal severe hypoxia (PSH, 180 Torr, 5% O2, 3 h) during the critical period in the formation of the hippocampus (days 14-16 of gestation). We have shown an age-dependent progressive decrease in the hippocampal glutamate levels, a decrease of the neuronal cell number in the CA1 hippocampal region, as well as impairment of spatial long-term memory in the Morris water navigation task. The gradual decrease of glutamate was accompanied by decreased expression of the genes that mediate glutamate metabolism and recycling in the hippocampus. That deficiency apparently correlated with an increase of the metabotropic glutamate receptor type 1 (mGluR1) and synaptophysin expression. Generation of the lipid peroxidation products in the hippocampus of adult rats subjected to prenatal severe hypoxia (PSH rats) was not increased compared to the control animals when tested in a model of glutamate excitotoxicity induced by severe hypoxia. This demonstrates that excessive glutamate sensitivity in PSH rats does not compensate for glutamate deficiency. Our results show a significant contribution of the glutamate system dysfunction to age-associated decrease of this mediator, cognitive decline, and early neuronal loss in PSH rats.

Impact of perinatal hypoxia on the developing brain

Perinatal hypoxia is still one of the greatest threats to the newborn child, even in developed countries. However, there is a lack of works which summarize up-to-date information about that huge topic. Our review covers a broader spectrum of recent results from studies on mechanisms leading to hypoxia-induced injury. It also resumes possible primary causes and observed behavioral outcomes of perinatal hypoxia. In this review, we recognize two types of hypoxia, according to the localization of its primary cause: environmental and placental. Later we analyze possible pathways of prenatal hypoxia-induced injury including gene expression changes, glutaminergic excitatory damage (and a role of NMDA receptors in it), oxidative stress with ROS and RNS production, inflammation and apoptosis. Moreover, we focus on the impact of these pathophysiological changes on the structure and development of the brain, especially on its regions: corpus striatum and hippocampus. These brain changes of the offspring lead to impairments in their postnatal growth and sensorimotor development, and in their motor functions, activity, emotionality and learning ability in adulthood. Later we compare various animal models used to investigate the impact of prenatal and postnatal injury (hypoxic, ischemic or combinatory) on living organisms, and show their advantages and limitations.

Role of Prenatal Hypoxia in Brain Development, Cognitive Functions, and Neurodegeneration

This review focuses on the role of prenatal hypoxia in the development of brain functions in the postnatal period and subsequent increased risk of neurodegenerative disorders in later life. Accumulating evidence suggests that prenatal hypoxia in critical periods of brain formation results in significant changes in development of cognitive functions at various stages of postnatal life which correlate with morphological changes in brain structures involved in learning and memory. Prenatal hypoxia also leads to a decrease in brain adaptive potential and plasticity due to the disturbance in the process of formation of new contacts between cells and propagation of neuronal stimuli, especially in the cortex and hippocampus. On the other hand, prenatal hypoxia has a significant impact on expression and processing of a variety of genes involved in normal brain function and their epigenetic regulation. This results in changes in the patterns of mRNA and protein expression and their post-translational modifications, including protein misfolding and clearance. Among proteins affected by prenatal hypoxia are a key enzyme of the cholinergic system-acetylcholinesterase, and the amyloid precursor protein (APP), both of which have important roles in brain function. Disruption of their expression and metabolism caused by prenatal hypoxia can also result, apart from early cognitive dysfunctions, in development of neurodegeneration in later life. Another group of enzymes affected by prenatal hypoxia are peptidases involved in catabolism of neuropeptides, including amyloid-β peptide (Aβ). The decrease in the activity of neprilysin and other amyloid-degrading enzymes observed after prenatal hypoxia could result over the years in an Aβ clearance deficit and accumulation of its toxic species which cause neuronal cell death and development of neurodegeneration. Applying various approaches to restore expression of neuronal genes disrupted by prenatal hypoxia during postnatal development opens an avenue for therapeutic compensation of cognitive dysfunctions and prevention of Aβ accumulation in the aging brain and the model of prenatal hypoxia in rodents can be used as a reliable tool for assessment of their efficacy.