Effects of Different Modes of Hypobaric Hypoxia on the Content of Epigenetic Factors in the Rat in Neurons of Rat Neocortex.

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.

Cerebral Mechanisms of Hypoxic/Ischemic Postconditioning.

This review analyzes recent data on mechanisms of cerebral hypoxia and the protective methods of hypoxic and ischemic postconditioning, as well as their interrelationship with the key mechanisms responsible for neuroprotection and neuroplasticity. Upregulation of expression of antiapoptotic factors and neurotrophins and modulation of activity of several protein kinases and transcription factors such as hypoxia-inducible factor-1 (HIF-1) are considered as the most important aspects in the neuroprotective potential of postconditioning. The presented information indicates substantial transformative promise of the noninvasive techniques of hypoxic postconditioning as well as significant similarity between the adaptive pathways activated by various postconditioning methods, which are far from being fully understood.

Effects of Hypobaric Hypoxia in Various Modes on Expression of Neurogenesis Marker NeuroD2 in the Dentate Gyrus of Rats Hippocampus.

The expression of neurogenesis marker--NeuroD2 transcription factor--in the hippocampal dentate gyrus was studied in rats exposed to severe destructive hypoxia, a single or three episodes of moderate hypobaric hypoxia, preconditioned severe hypoxia, and severe hypoxia followed by 3 sessions of postconditioning by moderate hypobaric hypoxia. All the studied hypoxic exposure modes led to an increase of NeuroD2 level. Three-fold moderate hypoxia per se and in the preconditioning mode (followed by exposure to severe hypoxia) produced most pronounced up-regulatory effect on NeuroD2 expression. The results indicated that stimulation of neurogenesis processes seemed to be one of the aspects of the neuroprotective effect of three-fold preconditioning moderate hypoxia, but not of hypoxic postconditioning.

Current Theory on the Cerebral Mechanisms of Hypoxic PRE- and Postconditioning.

An exposure of the organism to several episodes of mild hypoxia results in the development of brain hypoxic/ischemic tolerance, as well as cross-tolerance to the stressful factors of psychoemotional nature. Such kind of preconditioning by mild hypoxia functions as "alarm signalization" by I.P. Pavlov, preparing the organism and, in particularly, brain to the forthcoming harmful event. Dose-dependent action of hypoxia on the brain can be considered as one particular case of the general phenomenon termed hormesis, or neurohormesis. Endogenous defense processes launched by the hypoxic preconditioning and leading to the development of cerebral tolerance are associated with activation of intracellular signal cascades, transcriptional factors, regulatory proteins and expression of pro-adaptive genes and their products in the susceptible brain regions. Important mechanism of systemic adaptation induced by hypoxic preconditioning includes modifications of pituitary-adrenal axis aimed at enhancement of its adaptive resources. All these components are involved in the neuroprotective processes in three sequential phases - initiation, induction, and expression. Important role belongs also to epigenetic mechanisms controlling the activity of pro-adaptive genes. In contrast to the preconditioning, hypoxic postconditioning is comparatively novel phenomenon and therefore its mechanisms are less studied. The involvement of hypoxia-inducible factor HIF-1, and non-specific protective processes as up-regulation of anti-apoptotic factors and neurotrophines.

[MORPHOLOGICAL DIFFERENCES BETWEEN THE EFFECTS OF VARIOUS MODES OF PRECONDITIONING AIMED AT CORRECTING THE DAMAGE TO THE HIPPOCAMPAL NEURONS BY SEVERE HYPOBARIC HYPOXIA].

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.

[Effect of hypoxic postconditioning on the expression of antiapoptotic protein Bcl-2 and neurotrophin BDNF in CA1 hippocampal field of rats surviving severe hypoxia].

Using the method of quantitative immunohistochemistry, the expression of antiapoptotic protein Bcl-2 and neurotrophin BDNF was studied in CA1 hippocampal field of rats that survived severe hypoxia (SH), the damaging effects of which were compensated by subsequent three postconditioning (PC) sessions of mild hypobaric hypoxia (360 mm Hg, 2 hours, three times with 24 hour intervals). It was shown that the expression of the proteins studied was decreased in rat hippocampus after SH. Hypoxic postconditioning which improved the structural and functional rehabilitation after SH, was shown to up-regulate the expression of Bcl-2 and BDNF in hippocampal CA1 neurons in rats that survived SH. These results suggest the involvement of Bcl-2 and BDNF in processes of adaptation to SH and compensation of its damaging effects.

[Changes in the expression of Bcl-2 antiapoptotic protein in rat neocortex and hippocampus under the influence of various modes of hypobaric hypoxia].

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.

[A comparison of a neuroprotective effects of hypoxic postconditioning and cerebrolysin in the experimental model].

Hypoxic postconditioning using episodes of mild hypobaric hypoxia is a new neuroprotective technique. We compared the neuroprotective efficacy of hypoxic postconditioning and cerebrolysin in a model of posthypoxic pathology in rats. Animals that survived the severe hypoxia (180 Torr, 3 h) were exposed to hypoxic postconditioning or received cerebrolysin. Postconditioning prevented the injury and loss of hippocampal (fields CA1, CA4) and neocortical neurons whereas cerebrolysin was protective only for CA4 and the neocortex. Besides that, postconditioning, unlike cerebrolysin, led to the complete functional rehabilitation from the severe hypoxia by normalizing the level of anxiety and the pituitary-adrenal axis activity. The findings demonstrate that the elaborated postconditioning technique might provide useful tool for therapy of posthypoxic pathology and stroke.

[Hypoxic preconditioning modifies the activity of pro- and antioxidant systems in rat hippocampus].

The effects of repetitive mild hypobaric hypoxic preconditioning upon pro- and antioxidant systems in rat hippocampus were studied. It was found that three-trial preconditioning by mild hypobaric hypoxia (360 mm Hg, 2 h) induced moderate oxidative stress immediately after the last preconditioning trial. In addition, it down-regualted the levels of peptide antioxidants (Trx-1, Trx-2, Cu,Zn-SOD) and several lipid peroxidation products 24 h later.

[Pattern of neuronal expression of transcription factors NF-kappaB under different modes of hypobaric hypoxia].

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.

[Signal molecular and hormonal mechanisms of formation of the hypoxic preconditioning protective effects].

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.

[Participation of metabotropic glutamate receptors of the brain in mechanizms of hypoxic signaling].

Group I of metabotropic glutamate receptors (ImGluRs) are a family of G-protein-coupled receptors which activate a multitude of signaling pathways important for modulating neuronal excitability and synaptic plasticity as well as anti- and prosurvival pathways initiated by hypoxia. However these functions are still not complete and sometimes controversial. The present work is a review of data concerning involvement of ImGluRs in mechanisms of cell response to hypoxia. We also present original data demonstrating their participation in forming pathogenic and adaptogenic intracellular events, appearing in rat neocortex during a day after severe or moderate hypobaric hypoxia, respectively. Ca2+ responses to ImGluRs stimulation in survival cortical slices and expression of ImGluRs, IP3Rs and PLCbeta1 in immunolabelled cortical preparations were estimated for these two different hypoxic models.

[Hypoxic postconditioning corrects behavioral abnormalities in a model of post-traumatic stress disorder in rats].

Protective effects of the novel technique of hypoxic postconditioning with a hypobaric hypoxia paradigm were studied in "stress-restress" model ofposttraumatic stress disorder in rats. It was shown that repeated (3 times) exposure of rats that survived after severe traumatic stress to mild hypobaric hypoxia (postconditioning mode) efficiently abolished the development of stress-induced anxiety state. Postconditioning had a clear anxiolytic effect both when it was delivered after traumatic stress and after restress, but the intensity of this effect depended on the period ofpathogenesis of the posttraumatic stress disorder, when postconditioning was given. The results indicate that suggested postconditioning model with repetitive mild hypobaric hypoxia exerts potent anxiolytic and stress-protective action.

Antidepressant effect of hypoxic preconditioning is associated with modification of expression of transcription factor c-Fos in rat brain in response to unavoidable stress.

Development of post-stress depression in rats was accompanied by long-term moderate activation of the expression of transcription factor c-Fos in the neocortex, hippocampus, and paraventricular nucleus of the hypothalamus. Hypoxic preconditioning preventing depressive state in rats under conditions of unavoidable stress considerably enhanced c-Fos expression in the studied brain regions during the early stages of stress response (days 1-5) and promoted its normalization at later terms (10 days). Disturbances in the wavy dynamics of c-Fos expression can contribute to the pathogenic mechanisms of depression, in particular and induce hyperproduction of hypothalamic neurohormone corticoliberin, whereas potentiation of early expression of this factor in response to stress is obviously necessary for prevention of post-stress disorders.

[Changes in hippocampal and neocortical rat neurons induced by different regimes of hypobaric hypoxia].

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.

[Changes in intensity of hypoxic brain damage in rats induced by hypoxic postconditioning].

The present study has been aimed to estimate a neuroprotective effect of postconditioning (PostC) by using mild hypobaric hypoxia (360 mm Hg, 2 h) in a model of severe hypoxic brain injury (180 mm Hg, 3 h) in rats. PostC was performed by three trials of mild hypoxia with 24 h intervals, according to two different protocols--PostC was started 3 h (early PostC) or 24 h (delayed PostC) following severe hypoxia. Using histological methods and computer image analysis, loss of neurons in hippocampus and neocortex was analyzed 7 days after severe hypoxia. Severe hypoxia caused loss of 24% of neurons in layer V of the neocortex, 26% of neurons in CA1 region of hippocampus and 22% of neurons in CA4 region. Early PostC prevented loss of neurons in CA1 region of hippocampus and significantly reduced loss of neurons in neocortex (to 13%) and in CA4 region (to 10%). Delayed PostC fully prevented neuronal damage in CA4 region of hippocampus and neocortex and was to a large extent but not completely protective in CA1 region (12% of neurons were lost). The results show that PostC performed by hypobaric hypoxia has a pronounced neuroprotective effect, reducing the loss of neurons in vulnerable structures of brain (hippocampus and neocortex). The efficacy of neuroprotection depends upon the time of presentation of the first PostC session.

[Molecular-cellular and hormonal mechanisms of induced brain tolerance of extreme factors].

This review includes results of own studies and literature data on the topical problem of neurobiology and medicine: discovery of the mechanisms of increased brain resistance to extreme exposures. The emphasis is made on the molecular-cellular and hormonal mechanisms of hypoxic preconditioning-induced brain tolerance to injurious hypoxia, psychoemotional and traumatic stress. A role of basic hormonal and intracellular cascade pro-adaptive processes mediating the neuroprotective action of hypoxic preconditioning is reviewed. A dynamics of the mechanisms of development of induced susceptible brain areas (hippocampus, neocortex) tolerance which includes phases of induction, transformation and expression, is presented. New data on preconditioning-induced cross-tolerance providing increased brain resistance not only to hypoxia but also to other stresses are reported. For the first time neuroprotective effects of hypoxic postconditioning are described.

[Effects of several modes of hypobaric hypoxia on expression profiles of corticosteroid receptors in rat hippocampus].

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.

[Comparison of effects of a single and thrice-repeated moderate hypobaric hypoxia upon expression of thioredoxine-1 in the rat hippocampus].

We have previously shown that severe acute hypobaric hypoxia (SH) increases the expression of several endogenous antioxidants including thioredoxin-1 (Trx-1) in hippocampal neurons of rats. Preconditioning by three sessions of mild hypobaric hypoxia (MH) significantly augments this increase at the early period after subsequent SH, but MH itself without subsequent SH, in contrast, decreases expression of Trx-1. The dynamics of Trx-1 expression between the first and the last (third) sessions of preconditioning remains, however, unclear. In the present work, the previously studied Trx-1 expression in different areas of the hippocampus at hours 3 and 24 after thrice-repeated MH is compared to its expression at hours 3 and 24 after a single MH. It is shown that both a single and a thrice-repeated MH have similar effects on the Trx-1 expression. Since their neuroprotective effects in subsequent SH significantly differ, it is possible to conclude that hypoxic tolerance of neurons is determined not by the "background" level of antioxidants expression itself but has rather more complex regulatory mechanisms. These mechanisms may be associated with the wave-like oscillations of Trx-1 expression during preconditioning which was described in the present study.

[Changes in Mn-superoxide dismutase expression in rat hippocampus induced by single and triple exposure to moderate hypobaric hypoxia].

The purpose of this work was to study the dynamics of expression of mitochondrial Mn-dependent superoxide dismutase (Mn-SOD) 3 and 24 hours after single and triple exposure to mild hypoxia. The investigation was conducted in 18 male Wistar rats using immunocytochemical method. It was shown that in various hippocampal areas the effects of single and triple hypoxia exposure on the Mn-SOD expression could be different or largely similar. The expression dynamics at four time points studied (3 and 24 hours after the first exposure, 3 and 24 hours after the third one) had a wave character which may be important for the development of moderate hypoxia-induced tolerance to subsequent more severe exposures.