Search

We studied the effects of different modes of hypobaric hypoxia on the content of epigenetic factors acH3K24, meH3K9, and meDNA modulating conformational characteristics of chromatin and gene expression in neurons of associative complex of rat parietal neocortex. Severe destructive hypoxia dramatically reduced the level of acH3K24 in 3 h after the end of exposure and increased meH3K9 and meDNA content. By contrast, 3-fold (but not single) adaptive exposure to moderate hypobaric hypoxia that produced a neuroprotective effect enhanced neuronal acH3K24 expression and decreased both meH3K9 and meDNA levels. Elevated acH3K24 content facilitates, while increased content of meH3K9 hampers binding of transcription factors to the target genes. At the same time, increased expression of meDNA suppresses transcription. The role of modification of epigenetic mechanisms in the regulation of proadaptive genes under the effects of hypoxic exposure according to various protocols is discussed.

In 5 groups of rats (6 animals in each), the changes of neurons in hippocampal fields CA1 and CA4 were studied 7 days after severe hypobaric hypoxia (180 mm Hg, for 3 h) preceded by various numbers (1, 3 and 6) of sessions of preconditioning (PC) by mild hypobaric hypoxia (360 mm Hg, for 2 h, 24 h prior to severe hypoxia). It was found that a single session of PC did not prevent the damage to the structure of neurons and their death after exposure to severe hypoxia. Meanwhile, 6, and especially 3 sessions of PC induced protective mechanisms of neuronal damage prevention. In rats after 6 sessions of PC, unlike those exposed to 3 sessions, mild chromatolysis of hippocampal neurons was demonstrated. This could result from prolonged hypermetabolic activity of neurons and indicate their functional overloading.

Using immunocytochemical method, the level of expression of Bcl-2 antiapoptotic factor was studied in neurons of the neocortex and hippocampus in 72 male Wistar rats exposed to damaging severe hypoxia (SH), moderate hypobaric hypoxia (MHH), as well as their combination. After SH (180 mmHg) Bcl-2 expression in the neurons of the brain regions examined was reduced or. unchanged. The effect of preconditioning with one trial of MHH (360 mmHg) on Bcl-2 expression was similar to that seen after SH. In contrast, preconditioning with repeated exposures to MHH significantly up-regulated Bcl-2 expression levels 3-24 h after SH that apparently protected neurons from SH-induced injury. MHH alone, not followed by SH, significantly increased Bcl-2 expression only after multiple (three or six) exposures whereas single MHH exposure had no effect on Bcl content. Hence, up-regulation of Bcl-2 seen in response to multiple MHH trials appears to be important for the formation of the mechanisms of brain neuronal tolerance to damaging factors.

Using quantitative immunohistochemistry, modifications of HIF-1alpha expression in neocortex of rats exposed to various modes of mild hypobaric hypoxia (MHH) (1,3 and 6 episodes) differed in their neuroprotective efficacy have been studied. It has been shown that three-trial MHH being the most effective neuroprotective mode when used as a preconditioning produces most considerable changes in HIF-1 by substantial up-regulation of its regulatory alpha-subunit expression in the rat neocortex. Present findings support the hypothesis on important roles of HIF-1 in the mechanisms of brain hypoxic tolerance induced by the hypoxic preconditioning with three-trial MHH.

Transcription factor NF-kappaB plays a pivotal role in mechanisms of brain neuron survival and degeneration under injurious stimuli, first of all different types of hypoxia. In the present work, using quantitative immunohystochemistry, we provide analysis of expression of different subunits of NF-kappaB (p65 and c-Rel) in the rat neocortex in response to severe injurious hypobaric hypoxia (HH) or after a single or multiple sessions of mild protective HH. Severe hypoxia (SH), resulting in loss of brain neurons, has no effect on the level of expression of p65 but suppresses expression of c-Rel. Multiple (but not single one) trials of preconditioning using mild HH which reduce neuronal damage promote p65 expression and prevent suppression of c-Rel level after SH. Triple session of mild HH itself when applied as a preconditioning stimulus upregulate expression of both subunits, while single administration or sixfold trials has no effect on the level of immunoreactivity of both subunits. The revealed peculiarities of the expression of p65 and c-Rcl implies that these subunits of NF-kappaB appear to contribute to the mechanisms of brain tolerance to SH.

Gluco- and mineralocorticoid receptors are believed to play important roles in mechanisms of the hypothalamic-pituitary-adrenal axis (HPA) regulation, neuronal death/survival, as well as learning and memory processes. Imbalanced levels of MR and GR result in impairment of HPA activity and can promote neuronal injury and loss following exposures to extreme factors. In the present study, using quantitative immunohistochemistry, the comparative analysis of the effects of hypobaric hypoxia in several modes on expression profiles of GR and MR in dorsal (CA1) and ventral (dentate gyrus) hippocampus was performed. According to the data obtained, severe injurious hypoxia induced prominent disturbances of GR and MR expression in the cells of CA1 and dentate gyrus that correlated to the remarkable neuronal injury/loss in CA1 and dysregulated HPA activity. Sets of three- or six-trial (but not one-trial) preconditioning using mild hypoxia prior to severe hypoxia prevented these abnormalities.

In the review, results of the long-standing authors'studies and literature data concerning one of the underresearched aspects of actual problem of induced brain tolerance to injurious factors - "preventive" signal function of the hypoxic preconditioning, as well as molecular and hormonal mechanisms underlying its protective effects are presented. Hypoxic preconditioning by using of mild hypobaric hypoxia in special mode mobilizes evolutionary acquired genome determined defense mechanisms of brain neurons and whole organism. This process involves an activation of multiple intracellular components, as well as hypothalamic-pituitary-adrenal axis. Cascade mechanisms of intracellular signaling including receptors, mitochondrial respiratory chain, key intracellular regulatory systems, early genes, superfamilies of the inducible and activation transcription factors are sequentially engaged in the processes of initiation, induction and expression of hypoxic tolerance. The determination of optimal modes of hypoxic preconditioning appears to be of significant importance to assure the effective activation of protective signal mechanisms.

Hypobaric hypoxia may have either detrimental or adaptive effect on structural and functional characteristics of brain neurons. In this study, the effect of different regimes of hypobaric hypoxia on the structural and functional characteristics of hippocampal and neocortical neurons was examined in rats (n = 30). It was shown that severe hypoxia (induced by pressure in the pressure chamber equal to 180 Torr) caused structural neuronal damage both in the fronto-parietal neocortex and dorsal and ventral hippocampus 3 days after the exposure. The preconditioning using mild hypobaric hypoxia (pressure equal to 360 Torr) had varied effect on the morphological characteristics of brain neurons of rats, subjected to severe hypoxia. Multiple (three-trial or six-trial) preconditioning prevents structural damage of neurons induced by subsequent severe hypoxia. On the contrary, single preconditioning trial of mild hypoxia was ineffective in terms of neuroprotection.

Preconditioning using three sessions of moderate hypobaric hypoxia, i.e., hypoxic preconditioning (HP), increased the tolerance of susceptible brain neurons to severe hypoxia and other harmful factors. The study addressed changes in the expression of transcription factors NF-kappaB (nuclear factor kappa B) and CREB (cAMP response element binding protein) in the hippocampus of rats preconditioned with moderate hypoxia. Immunocytochemical methods demonstrated that HP increased immunoreactivity for NF-kappaB and phosphorylated CREB (pCREB) in hippocampal fields CA1-CA4 and the dentate gyrus and promoted increases in the expression of these transcription factors in the hippocampus of preconditioned rats 3-24 h after severe hypobaric hypoxia. These data provide evidence that NF-kappaB and CREB are involved in the mechanisms forming HP-induced tolerance of the brain.

Preconditioning using threefold mild hypobaric hypoxia (HH) is known to increase the tolerance of vulnerable brain neurons to severe hypoxia and other damaging factors. In the present study, the changes of the expression of transcription factors NF-kappaB (nuclear factor kappa B) and CREB (cAMP response element binding protein) were studied in the hippocampus of rats preconditioned by mild hypoxia. Using immunocytochemical method, it was demonstrated that HH increased NF-kappaB and phosphorylated CREB (pCREB) immunoreactivity in CA1-CA4 fields of the hippocampus and gyrus dentatus. It also contributed to the up-regulation of the expression of these transcription factors in the hippocampus of preconditioned rats 3-24 h following severe HH. These findings suggest that NF-kappaB and CREB are involved in HH-activated mechanisms of brain tolerance development.