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

NGF, TrkA-P and neuroprotection after a hypoxic event in the developing central nervous system

A decrease in the concentration of oxygen in the blood and tissues (hypoxia) produces important, sometimes irreversible, damages in the central nervous system (CNS) both during development and also postnatally. The present work aims at analyzing the expression of nerve growth factor (NGF) and p75 and the activation of TrkA in response to an acute normobaric hypoxic event and to evaluate the possible protective role of exogenous NGF. The developing chick optic tectum (OT), a recognized model of corticogenesis, was used as experimental system by means of in vivo and in vitro studies. Based on identification of the period of highest sensitivity of developmental programmed cell death (ED15) we show that hypoxia has a mild but reproducible effect that consist of a temporal increase of cell death 6 h after the end of a hypoxic treatment. Cell death was preceded by a significant early increase in the expression of Nerve Growth Factor (NGF) and its membrane receptor p75. In addition, we found a biphasic response of TrkA activation: a decrease during hypoxia followed by an increase -4 h later- that temporally coincide with the interval of NGF overexpression. To test the NGF - NGF receptors role in hypoxic cell death, we quantified, in primary neuronal cultures derived from ED15 OT, the levels of TrkA activation after an acute hypoxic treatment. A significant decline in the level of TrkA activation was observed during hypoxia followed, 24 h later, by significant cell death. Interestingly, this cell death can be reverted if TrkA inactivation during hypoxia is suppressed by the addition of NGF. Our results suggest that TrkA activation may play an important role in the survival of OT neurons subjected to acute hypoxia. The role of TrkA in neuronal survival after injury may be advantageously used for the generation of neuroprotective strategies to improve prenatal insult outcomes.

Hypobaric Preconditioning Modifies Group I mGluRs Signaling in Brain Cortex

The study assessed involvement of Ca(2+) signaling mediated by the metabotropic glutamate receptors mGluR1/5 in brain tolerance induced by hypoxic preconditioning. Acute slices of rat piriform cortex were tested 1 day after exposure of adult rats to mild hypobaric hypoxia for 2 h at a pressure of 480 hPa once a day for three consecutive days. We detected 44.1 ± 11.6 % suppression of in vitro anoxia-induced increases of intracellular Ca(2+) levels and a fivefold increase in Ca(2+) transients evoked by selective mGluR1/5 agonist, DHPG. Western blot analysis of cortical homogenates demonstrated a 11 ± 4 % decrease in mGluR1 immunoreactivity (IR), and in the nuclei-enriched fraction a 12 ± 3 % increase in IR of phospholipase Cβ1 (PLCβ1), which is a major mediator of mGluR1/5 signaling. Immunocytochemical analysis of the cortex revealed increase in the mGluR1/5 and PLCβ1 IR in perikarya, and a decrease in IR of the neuronal inositol trisphosphate receptors (IP3Rs). We suggest that enhanced expression of mGluR5 and PLCβ1 and potentiation of Ca(2+) signaling may represent pro-survival upregulation of Ca(2+)-dependent genomic processes, while decrease in mGluR1 and IP3R IR may be attributed to a feedback mechanism preventing excessive intracellular Ca(2+) release.

Genetics and genomics of ischemic tolerance: focus on cardiac and cerebral ischemic preconditioning

A subthreshold ischemic insult applied to an organ such as the heart and/or brain may help to reduce damage caused by subsequent ischemic episodes. This phenomenon is known as ischemic tolerance mediated by ischemic preconditioning (IPC) and represents the most powerful endogenous mechanism against ischemic injury. Various molecular pathways have been implicated in IPC, and several compounds have been proposed as activators or mediators of IPC. Recently, it has been established that the protective phenotype in response to ischemia depends on a coordinated response at the genomic, molecular, cellular and tissue levels by introducing the concept of 'genomic reprogramming' following IPC. In this article, we sought to review the genetic expression profiles found in cardiac and cerebral IPC studies, describe the differences between young and aged organs in IPC-mediated protection, and discuss the potential therapeutic application of IPC and pharmacological preconditioning based on the genomic response.

Postconditioning by mild hypoxic exposures reduces rat brain injury caused by severe hypoxia

A potent neuroprotective effect of ischemic postconditioning has previously been described using cerebral artery occlusion but this is not a practical therapeutic option. The present study has been performed to determine whether postconditioning by mild episodes of hypobaric hypoxia (hypoxic postconditioning, HP) can reduce post-hypoxic brain injury in rats. Male Wistar rats were submitted to severe hypobaric hypoxia (180 Torr, 3 h) followed by HP (360 Torr, 2 h, 3 trials spaced at 24 h) starting either 3h (early HP) or 24 h (delayed HP) after severe hypoxia. The structural and functional brain injury was assessed by a complex of histological techniques, behavioral methods, and by testing the functions of the hypothalamic-pituitary-adrenal axis (HPA). It was found that early and delayed HP considerably attenuated post-hypoxic injury, reducing pyknosis, hyperchromatosis, and interstitial brain edema, as well as the rates of neuronal loss in hippocampus and neocortex. Delayed HP produced prominent anxiolytic effect on rat behavior, preventing development of post-hypoxic anxiety. Both modes of HP had beneficial effect on the functioning of HPA, but only delayed HP normalized completely the baseline HPA activity and its reactivity to stress. The results obtained demonstrate that postconditioning by using repetitive episodes of mild hypobaric hypoxia may provide a powerful neuroprotective procedure that can be easily adopted for clinical practice and recommended as a research tool for identification of endogenous mechanisms involved in post-ischemic neuroprotection.

Differential expression of ADAM15 and ADAM17 metalloproteases in the rat brain after severe hypobaric hypoxia and hypoxic preconditioning

The ADAMs (a disintegrin and metalloprotease) are a family of membrane-anchored glycoproteins capable of shedding a multitude of proteins from the cell surface. Although ADAMs are being considered as crucial modulators of physiological and pathophysiological processes, their roles in neuronal death/survival are largely unexplored. In the present study, changes in brain expression of ADAM15 and ADAM17 (TACE) have been quantitatively examined in rats in response to injurious severe hypoxia (SH) and in animals which acquired hypoxic tolerance through preconditioning to mild hypoxia prior SH. SH persistently up-regulated ADAM15 mRNA and protein levels in hippocampus and neocortex but not in thalamus or hypothalamus. This effect was not observed in the preconditioned rats tolerant to SH. In contrast, hippocampal levels of ADAM17 mRNA and neocortical levels of ADAM17 mRNA and protein were largely reduced following SH in non-preconditioned rats. Hypoxic preconditioning prevented down-regulation of the adam17 gene and considerably enhanced ADAM17 protein expression in hippocampus and neocortex in response to SH. The present findings implicate ADAM15 in the processes of neuronal hypoxic injury. On the other hand, these results also provide evidence for a pro-survival neuroprotective role of ADAM17 and its engagement in the process of preconditioning-induced hypoxic tolerance. The analysis of the protein levels of soluble and membrane-bound forms of APP in the neocortex and hippocampus of rats subjected to SH and SH with preconditioning has demonstrated that an increased ADAM17 expression in preconditioned animals 24h after hypoxia corresponded to a higher level of soluble form of APP and a reduction of the membrane bound fraction which reflects the role of ADAM17 in APP shedding.

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.

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.

The possible use of hypoxic preconditioning for the prophylaxis of post-stress depressive episodes

The protective effects of hypoxic preconditioning on the development of depressive states in rat models were studied. Three episodes of intermittent preconditioning using hypobaric hypoxia (360 mmHg, 2 h) prevented the onset of depressive behavioral reactions, hyperfunction of the hypophyseal-adrenal system, and impairments in its suppression in the dexamethasone test in rats following unavoidable aversive stress in a model of endogenous depression. The anxiolytic and antidepressant actions of hypoxic preconditioning in experiments on rats were no less marked than those of the tetracyclic antidepressant ludiomil. The results obtained here provide evidence that preconditioning with intermittent hypobaric hypoxia increases resistance to psychoemotional stresses, has marked anxiolytic and antidepressant effects, and can be used for the prophylaxis of depressive episodes.

Preconditioning induces prolonged expression of transcription factors pCREB and NF-kappa B 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-kappaB 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 (180 Torr, 3 h) remarkably reduced the neocortical levels of pCREB and NF-kappaB. The three-trial hypoxic preconditioning (360 Torr, 2 h, 3 days) induced persistent up-regulation of pCREB and NF-kappaB expression in the neocortex of rats 3-24 h following the severe hypoxia. In addition, the preconditioning alone which was not followed by the severe hypoxia, considerably increased neocortical pCREB and NF-kappaB levels. The findings suggest a role for transcription factors cAMP response element-binding protein and NF-kappaB in the neuroprotective mechanisms activated by the hypoxic preconditioning.

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.

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.

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.

Vagus Nerve Stimulation Induces Concomitant Respiratory Alterations and a Decrease in SaO2 in Children

PURPOSE

To analyze respiratory alterations and effects on SaO(2) caused by vagus nerve stimulation (VNS) in children with epilepsy.

METHODS

Polysomnographic recordings, including electroencephalography, thoracoabdominal distention, nasal airflow, SaO(2), and VNS artifact were evaluated in 10 children with pharmacoresistant epilepsy treated with VNS.

RESULTS

Each VNS caused a significant increase in respiratory frequency (p < 0.05) throughout the stimulation period and a decrease in thoracoabdominal-distention amplitude (p < 0.05), especially at the beginning of the stimulation. These respiratory alterations induced a decrease in SaO(2) from 1 to 5%. The effects of VNS on respiration differed significantly between rapid-eye-movement (REM) and non-REM (NREM) sleep states.

CONCLUSIONS

VNS caused a pronounced change in respiration in children with epilepsy, and this induced a decrease in SaO(2). It is possible that VNS has a neuroprotective effect, and this possibility calls for further investigation.

Expression of early gene proteins, structural changes in brain neurons in hypobaric hypoxia, and the correcting effects of preconditioning

The Nissl method and immunocytochemistry were used to study the effects of severe hypobaric hypoxia and its actions in combination with the preconditioning actions of moderate hypoxia on the expression of the early gene proteins c-Fos and NGFI-A as well as structural changes in hippocampal and neocortical neurons in the rat brain. Severe hypoxia was found to suppress c-Fos and NGFI-A synthesis (3-24 h after exposure) and to induce delayed (days 3-7) structural damage to neurons, of the "light" and predominantly the "dark" types, which appear to reflect the development of necrotic and apoptotic processes respectively. Preconditioning with the regime used here corrected these derangements, resulting in increases in the expression of early gene proteins and significant reductions in structural damage to neurons after severe hypoxia.

Expression of immediate early genes and vasopressin heteronuclear RNA in the paraventricular and supraoptic nuclei of rats after acute osmotic stimulus

Monitoring the expression of immediate early genes (IEGs) is useful for following stress-induced cellular responses in the neuroendocrine system. We have examined the transcriptional activities of four IEGs (c-fos, junB, NGFI-A and NGFI-B) and of the arginine vasopressin (AVP) gene in the hypothalamic paraventicular (PVN) and supraoptic nuclei (SON) of rats after acute osmotic stimuli, using in situ hybridization histochemistry. After intraperitoneal (i.p.) administration of hypertonic saline (2% body weight, 900 mOsm/kg), the expression levels of all IEG mRNAs were increased significantly both in the PVN and SON at as early as 10 min, peaked at 30 min and remained elevated until 60 min. The expression of AVP heteronuclear (hn)RNA also peaked at 30 min, and remained elevated until 180 min. Thirty min after i.p. administration of hypertonic saline (600 mOsm/kg), the expression levels of all IEG mRNAs in the PVN and SON were significantly increased in comparison with those after i.p. administration of isotonic saline (290 mOsm/kg). Regression analysis revealed that expression levels of the IEG mRNAs and AVP hnRNA were positively correlated with the plasma concentration of sodium, and the rates of increase of the expression levels of all IEG mRNAs were similar. The expression levels of all IEG mRNAs examined are useful markers for following the changes of the AVP gene transcription in the PVN and SON after acute osmotic stimuli in rats.

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.

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.

Mediators of ischemic preconditioning identified by microarray analysis of rat spinal cord

Spinal ischemia is a frequent cause of paralysis. Here we explore the biological basis of ischemic preconditioning (IPC), the phenomenon in which a brief period of ischemia can confer protection against subsequent longer and normally injurious ischemia, to identify mediators of endogenous neuroprotection. Using microarrays, we examined gene expression changes induced by brief spinal ischemia using a rat balloon occlusion model. Among the nearly 5000 genes assayed, relatively few showed two-fold changes, and three groups stood out prominently. The first group codes for heat shock protein 70, which is induced selectively and robustly at 30 min after brief ischemia, with increases up to 100-fold. A second group encodes metallothioneins 1 and 2. These mRNAs are increased at 6 and 12 h after ischemia, up to 12-fold. The third group codes for a group of immediate-early genes not previously associated with spinal ischemia: B-cell translocation gene 2 (BTG2), the transcription factors early growth response 1 (egr-1) and nerve growth factor inducible B (NGFI-B), and a mitogen-activated protein kinase phosphatase, ptpn16, an important cell signaling regulator. These mRNAs peak at 30 min and return to baseline or are decreased 6 h after ischemia. Several other potentially protective genes cluster with these induced mRNAs, including small heat shock proteins, and many have not been previously associated with IPC. These results provide both putative mediators of IPC and molecular targets for testing preconditioning therapies.