Michail Samoilov

Mild hypoxia preconditioning prevents impairment of passive avoidance learning and suppression of brain NGFI-A expression induced by severe hypoxia.

The aim of this work was to study effects of mild preconditioning hypobaric hypoxia (380 Torr for 2 h, repeated 3 or 6 times spaced at 24 h) on brain NGFI-A immunoreactivity and passive avoidance (PA) behavior in rats exposed to severe hypoxia (160 Torr for 3 h). Severe hypobaric hypoxia produced extensive neuronal loss in hippocampal CA1, while the preceding hypoxic preconditioning had clear protective effect on neuronal viability of vulnerable hippocampal cells. Besides, the hypoxic preconditioning prevented impairment of acquisition and retention of PA caused by severe hypoxia. The six-trial hypobaric preconditioning was more effective in protection against PA learning deficits in severe hypoxia exposed rats than the three-trial preconditioning. The preconditioning up-regulated severe hypoxia-suppressed neocortical and hippocampal expression of NGFI-A, suggesting a possible role for NGFI-A in the neuroprotective mechanisms activated by hypoxic preconditioning.

The preconditioning modified neuronal expression of apoptosis-related proteins of Bcl-2 superfamily following severe hypobaric hypoxia in rats.

The patterns of expression of the Bcl-2, Bax, and Bcl-xL proteins were examined immunocytochemically in rat hippocampus and neocortex after severe hypobaric hypoxia (180 Torr for 3 h) and severe hypoxia preconditioned by intermittent mild hypoxia (360 Torr for 2 h daily, for 3 consecutive days, 24 h prior to severe hypoxia). As revealed by TUNEL assay, severe hypobaric hypoxia produced extensive apoptotic damage to the neurons of hippocampal CA1-CA4 and the neocortex but not the dentate gyrus granule cells. Remarkable posthypoxic up-regulation of Bax expression maximal at 24 h was detected in the CA1-CA4 areas of hippocampus and neocortex 3-72 h after severe hypoxia. The preconditioning to severe hypoxia protected neurons from the posthypoxic apoptotic transformations, the up-regulation of Bax expression, and resulted in persistent overexpression of Bcl-2 and Bcl-xL. We conclude that the protective action of hypoxic preconditioning is at least in part mediated by shifting of neuronal Bax/Bcl-2-Bcl-xL ratio to a favor of antiapoptotic proteins Bcl-2 and Bcl-xL.

Preconditioning enhances the expression of mitochondrial antioxidant thioredoxin-2 in the forebrain of rats exposed to severe hypobaric hypoxia

The impact of severe hypoxia and preconditioning on the expression of the mitochondrial antioxidant thioredoxin‐2 (Trx‐2) in rat hippocampus (CA1, CA2, CA3 fields, and dentate gyrus) and neocortex was studied by immunocytochemistry. The preconditioning consisted of three trials of mild hypobaric hypoxia (360 Torr, 2 hr) spaced at 24 hr. The last trial was followed by severe hypobaric hypoxia (180 Torr, 3 hr) 24 hr later. Both in hippocampus and in neocortex, severe hypobaric hypoxia resulted in enhanced Trx‐2 expression at 3 hr, followed by a slight decline in Trx‐2 levels, which nevertheless remained increased at 24 hr elsewhere except for the CA1 region. The preconditioning considerably augmented severe hypoxia‐induced Trx‐2 immunoreactivity, affecting both the number of immunoreactive cells and the intensity of immunostaining. The findings suggest a role for Trx‐2 in the formation of brain hypoxic/ischemic tolerance accomplished by the preconditioning.

Preconditioning induces prolonged expression of transcription factors pCREB and NF‐κB in the neocortex of rats before and following severe hypobaric hypoxia

Preconditioning using mild repetitive hypobaric hypoxia is known to increase a tolerance of brain neurons to severe hypoxia and other injurious exposures. In the present study, the effects of mild hypoxic preconditioning on the expression of transcription factors NF‐κB and phosphorylated CREB (pCREB) has been studied in the neocortex of rats exposed to severe hypobaric hypoxia. As revealed by quantitative immunocytochemistry, the injurious severe hypobaric hypoxia (180u2003Torr, 3u2003h) remarkably reduced the neocortical levels of pCREB and NF‐κB. The three‐trial hypoxic preconditioning (360u2003Torr, 2u2003h, 3u2003days) induced persistent up‐regulation of pCREB and NF‐κB expression in the neocortex of rats 3–24u2003h following the severe hypoxia. In addition, the preconditioning alone which was not followed by the severe hypoxia, considerably increased neocortical pCREB and NF‐κB levels. The findings suggest a role for transcription factors cAMP response element‐binding protein and NF‐κB in the neuroprotective mechanisms activated by the hypoxic preconditioning.

The augmentation of brain thioredoxin-1 expression after severe hypobaric hypoxia by the preconditioning in rats

Induction of endogenous antioxidants is one of the key molecular mechanisms of cell resistance to hypoxia/ischemia. The effect of severe hypoxia on the expression of cytosolic antioxidant thioredoxin-1 (Trx) in hippocampus and neocortex was studied in preconditioned and non-preconditioned rats. The preconditioning consisted of three trials of mild hypobaric hypoxia (360 Torr, 2 h) spaced at 24 h. Twenty-four hours after the last trial rats were subjected to severe hypobaric hypoxia (180 Torr, 3 h). Trx expression was studied by immunocytochemistry. In hippocampus severe hypobaric hypoxia rapidly induced Trx expression, which remained elevated still at 24 h. In neocortex the enhanced expression appeared only at 24 h. The preconditioning significantly augmented severe hypoxia-induced Trx-immunoreactivity at 3 h but not at 24 h. These findings point out that Trx contributes to mechanisms of brain tolerance to hypobaric hypoxia, especially in early periods after the exposure.

Mild hypobaric hypoxia preconditioning up-regulates expression of transcription factors c-Fos and NGFI-A in rat neocortex and hippocampus

Transcription factors c-Fos and NGFI-A encoded by immediate early genes largely participate in the biochemical cascade leading to genomically driven lasting adaptation by neurons to injurious exposures including hypoxia/ischemia. Present study was designed to examine the involvement of c-Fos and NGFI-A in the development of brain hypoxic tolerance induced by mild hypoxic preconditioning. Earlier we have reported that preconditioning by repetitive mild hypobaric hypoxia (MHH) considerably increases neuronal resistance to subsequent severe injurious exposures. Herein, changes of c-Fos and NGFI-A expression in vulnerable rat brain areas (hippocampus, neocortex) in response to preconditioning MHH itself were studied using quantitative immunocytochemistry. Exposure to MHH differentially enhanced c-Fos and NGFI-A expression in neocortex and hippocampal fields 3-24h following the last MHH trial. The c-Fos up-regulation was the most pronounced in neocortex, CA1, and dentate gyrus, but it was twice lower in CA3/CA4. The up-regulation of NGFI-A in CA1, dentate gyrus and neocortex was 1.5-2-fold lower than that of c-Fos; but in CA3 and CA4 the rates of the c-Fos and NGFI-A induction were comparable. The present findings indicate that cooperative but differential activation of c-Fos and NGFI-A expression in vulnerable brain areas contribute to the development of tolerance achieved by MHH preconditioning.

Antidepressant-like effects of mild hypoxia preconditioning in the learned helplessness model in rats.

The effects of preconditioning using mild repetitive hypobaric hypoxia (360 Torr for 2 h each of 3 days) have been studied in the learned helplessness model of depression in rats. Male Wistar rats displayed persistent depressive symptoms (depressive-like behaviour in open field, increased anxiety levels in elevated plus maze, ahedonia, elevated plasma glucocorticoids and impaired dexamethasone test) following the exposure to unpredictable and inescapable footshock in the learned helplessness paradigm. Antidepressant treatment (ludiomil, 5 mg/kg i.p.) augmented the development of the depressive state. The hypoxic preconditioning had a clear antidepressive action returning the behavioural and hormonal parameters to the control values and was equally effective in terms of our study as the antidepressant. The findings suggest hypoxic preconditioning as an effective tool for the prophylaxis of post-stress affective pathologies in humans.

Mild preconditioning hypoxia modifies nerve growth factor-induced gene A messenger RNA expression in the rat brain induced by severe hypoxia

The effect of preconditioning (PC) on the changes of nerve growth factor-induced gene A (NGFI-A) expression induced by severe hypobaric hypoxia was studied by in situ hybridization. A PC consisted of three trials of mild hypobaric hypoxia (360 Torr, 2 h) spaced at 24 h. The last trial was followed by severe hypoxia (SH; 180 Torr, 3 h) 24 h later. The PC hypoxia prevented the NGFI-A messenger RNA (mRNA) increase in the cortex, neostriatum, piriform cortex, amygdala and hippocampus detected 3 h after SH. The preconditioned SH caused a peak in NGFI-A mRNA expression at the 24 h time-point and thus abolished the dramatic decrease of the mRNA in vulnerable areas seen by 24 and 72 h after SH. The findings suggest a role of brain NGFI-A in the protective effect of hypoxic/ischemic PC.

Involvement of the hypothalamic-pituitary-adrenal axis in the antidepressant-like effects of mild hypoxic preconditioning in rats

The preconditioning (PC) by using mild intermittent hypobaric hypoxia (PC) increases a resistance of the brain to severe hypoxia/ischemia and various stresses. Recently, potent antidepressant-like effects of PC have been described in animal models of depression. In the present study, the impact of PC on the activity and feedback regulation of the hypothalamic-pituitary-adrenal axis (HPA) impaired in depression has been studied in the model of shock-induced depression in rats. PC completely prevented depressive-like behavior (54% reduction in ambulance, 59% reduction in rearing in the open field, 654% increase of the anxiety level in the elevated plus maze), the HPA hyperactivity and the impairment of HPA feedback regulation that appeared in response to the inescapable footshock. Not affecting basal HPA activity, PC remarkably enhanced the HPA reactivity to stresses and substantially up-regulated the expression of glucocorticoid receptors in the ventral hippocampus following footshock that apparently contributes to the mechanisms responsible for the antidepressant-like action of PC.

Expression of glucocorticoid and mineralocorticoid receptors in hippocampus of rats exposed to various modes of hypobaric hypoxia: Putative role in hypoxic preconditioning.

Effects of mild (preconditioning) and severe injurious hypobaric hypoxia (SH), as well as of their combination on hippocampal expression of glucocorticoid (GR) and mineralocorticoid (MR) receptors and HPA axis activity have been examined in rats. As revealed by quantitative immunocytochemistry, three-trial exposure to mild hypoxia produced robust GR and MR overexpression located mainly in the neuronal nuclei in the dentate gyrus (DG) but only MR overexpression was observed in the CA1. SH induced sharp reduction of MR levels and enhanced GR expression in the CA1, suggesting that the unbalance of GR and MR observed might be at the bottom of the extensive neuronal loss seen in this area in response to SH. Contrastingly, SH in tolerant (preconditioned) rats failed to imbalance GR and MR expression in CA1 and up-regulated GR levels in DG. Radioimmunoassay of serum corticosterone showed that both preconditioning hypoxia itself and SH in tolerant rats produced moderate activation of HPA axis followed by its proper inactivation. In the non-preconditioned rats, HPA axis response to SH was impaired. Taken together, these novel results suggest that modifications of the hippocampal expression of GR and MR produced by preconditioning may contribute to the molecular and neuroendocrine mechanisms of tolerance to severe hypoxic stress.