Increased neurons containing neuronal nitric oxide synthase in the brain of a hypoxic-ischemic neonatal rat model

Mechanisms of acute brain injury after subarachnoid hemorrhage

Brain injury after subarachnoid hemorrhage (SAH) is a biphasic event with an acute ischemic insult at the time of the initial bleed and secondary events such as cerebral vasospasm 3 to 7 days later. Although much has been learned about the delayed effects of SAH, less is known about the mechanisms of acute SAH-induced injury. Distribution of blood in the subarachnoid space, elevation of intracranial pressure, reduced cerebral perfusion and cerebral blood flow (CBF) initiates the acute injury cascade. Together they lead to direct microvascular injury, plugging of vessels and release of vasoactive substances by platelet aggregates, alterations in the nitric oxide (NO)/nitric oxide synthase (NOS) pathways and lipid peroxidation. This review will summarize some of these mechanisms that contribute to acute cerebral injury after SAH.

The importance of early brain injury after subarachnoid hemorrhage

Aneurysmal subarachnoid hemorrhage (aSAH) is a medical emergency that accounts for 5% of all stroke cases. Individuals affected are typically in the prime of their lives (mean age 50 years). Approximately 12% of patients die before receiving medical attention, 33% within 48 h and 50% within 30 days of aSAH. Of the survivors 50% suffer from permanent disability with an estimated lifetime cost more than double that of an ischemic stroke. Traditionally, spasm that develops in large cerebral arteries 3-7 days after aneurysm rupture is considered the most important determinant of brain injury and outcome after aSAH. However, recent studies show that prevention of delayed vasospasm does not improve outcome in aSAH patients. This finding has finally brought in focus the influence of early brain injury on outcome of aSAH. A substantial amount of evidence indicates that brain injury begins at the aneurysm rupture, evolves with time and plays an important role in patients' outcome. In this manuscript we review early brain injury after aSAH. Due to the early nature, most of the information on this injury comes from animals and few only from autopsy of patients who died within days after aSAH. Consequently, we began with a review of animal models of early brain injury, next we review the mechanisms of brain injury according to the sequence of their temporal appearance and finally we discuss the failure of clinical translation of therapies successful in animal models of aSAH.

(-)-Epigallocatechin gallate attenuates NADPH-d/nNOS expression in motor neurons of rats following peripheral nerve injury

Background

Oxidative stress and large amounts of nitric oxide (NO) have been implicated in the pathophysiology of neuronal injury and neurodegenerative disease. Recent studies have shown that (-)-epigallocatechin gallate (EGCG), one of the green tea polyphenols, has potent antioxidant effects against free radical-mediated lipid peroxidation in ischemia-induced neuronal damage. The purpose of this study was to examine whether EGCG would attenuate neuronal expression of NADPH-d/nNOS in the motor neurons of the lower brainstem following peripheral nerve crush. Thus, young adult rats were treated with EGCG (10, 25, or 50 mg/kg, i.p.) 30 min prior to crushing their hypoglossal and vagus nerves for 30 seconds (left side, at the cervical level). The treatment (pre-crush doses of EGCG) was continued from day 1 to day 6, and the animals were sacrificed on days 3, 7, 14 and 28. Nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) histochemistry and neuronal nitric oxide synthase (nNOS) immunohistochemistry were used to assess neuronal NADPH-d/nNOS expression in the hypoglossal nucleus and dorsal motor nucleus of the vagus.

Results

In rats treated with high dosages of EGCG (25 or 50 mg/kg), NADPH-d/nNOS reactivity and cell death of the motor neurons were significantly decreased.

Conclusions

The present evidence indicated that EGCG can reduce NADPH-d/nNOS reactivity and thus may enhance motor neuron survival time following peripheral nerve injury.

Nitric oxide in early brain injury after subarachnoid hemorrhage

Nitric Oxide (NO) is the major regulator of cerebral blood flow. In addition, it inhibits platelet adherence and aggregation, reduces adherence of leukocytes to the endothelium, and suppresses vessel injury. NO is produced on demand by nitric oxide synthase and has a very short half life. Hence maintenance of its cerebral level is crucial for normal vascular physiology. Time dependent alterations in cerebral NO level and the enzymes responsible for its synthesis are found after subarachnoid hemorrhage (SAH). Cerebral NO level decreases, recovers and increases within the first 24 h after SAH. Each change in cerebral NO level elicits a different pathological response form already compromised brain. These response range from constriction, platelet aggregation and vascular injury that occurs during the early hours and delayed occurring vasospasm, neuronal and axonal damage. This review summarizes the underlying mechanism and the consequence of alteration in cerebral NO level on brain during the first 72 h after SAH.

Microglia in close vicinity of glioma cells: correlation between phenotype and metabolic alterations

Microglia are immune cells within the central nervous system. In brain-developing tumors, gliomas are able to silence the defense and immune functions of microglia, a phenomenon which strongly contributes to tumor progression and treatment resistance. Being activated and highly motile, microglia infiltrate tumors and secrete macrophagic chemoattractant factors. Thereafter, the tumor cells shut down their immune properties and stimulate the microglia to release tumor growth-promoting factors. The result of such modulation is that a kind of symbiosis occurs between microglia and tumor cells, in favor of tumor growth. However, little is known about microglial phenotype and metabolic modifications in a tumoral environment. Co-cultures were performed using CHME5 microglia cells grown on collagen beads or on coverslips and placed on monolayer of C6 cells, limiting cell/cell contacts. Phagocytic behavior and expression of macrophagic and cytoskeleton markers were monitored. Respiratory properties and energetic metabolism were also studied with regard to the activated phenotype of microglia. In co-cultures, transitory modifications of microglial morphology and metabolism were observed linked to a concomitant transitory increase of phagocytic properties. Therefore, after 1 h of co-culture, microglia were activated but when longer in contact with tumor cells, phagocytic properties appear silenced. Like the behavior of the phenotype, microglial respiration showed a transitory readjustment although the mitochondria maintained their perinuclear relocation. Nevertheless, the energetic metabolism of the microglia was altered, suggesting a new energetic steady state. The results clearly indicate that like the depressed immune properties, the macrophagic and metabolic status of the microglia is quickly driven by the glioma environment, despite short initial phagocytic activation. Such findings question the possible contribution of diffusible tumor factors to the microglial metabolism.

Mild hypoxic preconditioning attenuates injury-induced NADPH-d/nNOS expression in brainstem motor neurons of adult rats

Excessive production of nitric oxide (NO) might have detrimental effects on the hypoxia-related neuropathology. This study aimed to test if mild hypoxic preconditioning (MHPC) would attenuate the pathological changes in the brainstem motoneurons having a different functional component after peripheral nerve crush injury (PNCI). Prior to PNCI treatment, young adult rats were caged in the mild hypoxic altitude chamber with 79Torr of the partial oxygen concentration ( pO(2)) (i.e., 0.5atm at 5500m in height) for 4 weeks to adapt the environmental changes. After that, all the animals having successfully crushed both the hypoglossal and vagus nerves (left-side) were allowed to survive for 3, 7, 14, 30 and 60 successive days in normoxic condition. Nicotinamine adenine dinucleotide phosphate-diaphorase (NADPH-d) histochemistry and neuronal nitric oxide synthase (nNOS) immunohistochemistry revealed that MHPC reduces NADPH-d/nNOS expression in the hypoglossal nucleus (HN) and the dorsal motor nucleus of the vagus (DMN) at different time points after PNCI. The morphological findings were further ascertained by Western blot analysis of nNOS and nitrite assay for NO production. Both the morphological and quantitative results peaked at 7 days in HN, whereas for those in DMN were progressively increased up to 60 days following PNCI. The staining intensity of NADPH-d/nNOS(+) neurons, expression of nNOS protein, NO production levels as well as the neuronal loss in HN and DMN of MHPC rats following PNCI were attenuated, especially for those having a longer survival period over 14 days. The MHPC treatment might induce minute amounts of NO to alter the state of milieu of the experimental animals to protect against the PNCI.

Effect of ischemia in vivo and oxygen-glucose deprivation in vitro on NOS pools in the spinal cord: comparative study

1. This study was performed to compare both the Ca(2+)-dependent nitric oxide synthase (NOS) activity and the neuronal nitric oxide synthase immunoreactivity (nNOS-IR) in the rabbit lumbosacral spinal cord after 15 min abdominal aorta occlusion (ischemia in vivo) and oxygen-glucose deprivation of the spinal cord slices for 45 and 60 min (ischemia in vitro). All ischemic periods were followed by 15, 30 and 60 min reoxygenation in vitro. 2. Catalytic nitric oxide synthase activity was determined by the conversion of (L)-[(14)C]arginine to (L)-[(14)C]citrulline. Neuronal nitric oxide synthase immunoreactivity in the spinal cord was detected by incubation of sections with polyclonal sheep-nNOS-primary antibody and biotinylated anti-sheep secondary antibody. 3. Our results show that ischemia in vivo and the oxygen-glucose deprivation of spinal cord slices in vitro result in a time-dependent loss of constitutive NOS activity with a partial restoration of enzyme activity during 15 and 45 min ischemia followed by 30 min of reoxygenation. A significant decrease of enzyme activity was found during 60 min ischemia alone, which persisted up to 1 h of oxygen-glucose restoration. The upregulation of neuronal nitric oxide synthase was observed in the ventral horn motoneurons after all ischemic periods. The remarkable changes in optical density of neuronal nitric oxide synthase immunoreactive motoneurons were observed after 45 and 60 min ischemia in vitro followed by 30 and 60 min reoxygenation. 4. Our results suggest that the oxygen-glucose deprivation followed by reoxygenation in the spinal cord is adequately sensitive to monitor ischemia/reperfusion changes. It seems that 15 min ischemia in vivo and 45 min ischemia in vitro cause reversible changes, while the decline of Ca(2+)-dependent nitric oxide synthase activity after 60 min ischemic insult suggests irreversible alterations.

Effects of acute hypobaric hypoxia on the nitric oxide system of the rat cerebral cortex: Protective role of nitric oxide inhibitors

Exposure to hypobaric hypoxia produces neuropsychological disorders. The brain nitrergic system was investigated following hypobaric hypoxia in the presence or absence of nitric oxide synthase (NOS) inhibitors. Adult rats were exposed to a simulated altitude of 8325 m (27,000 ft) for 7 h and killed after 0, 1, 3, 5, and 10 days of recovery. In addition to normobaric controls, three experimental groups were studied: i) subjected to hypobaric hypoxia without inhibitors; ii) subjected to hypobaric hypoxia and treated with 7-nitroindazole; iii) subjected to hypobaric hypoxia and treated with N(omega)-nitro-l-arginine methyl ester (l-NAME). Cerebral cortex was assayed by immunohistochemistry, Western blotting, and enzymatic assays. In animals subjected to hypobaric hypoxia without inhibitors, there was an increase in neuronal nitric oxide synthase (nNOS) immunoreactivity and Ca(2+)-dependent NOS activity from 0 to 1 days of reoxygenation. In these animals, inducible nitric oxide synthase (iNOS) expression and Ca(2+)-independent activity were undetectable, but nitrotyrosine immunoreactivity was found in some neurons. Administration of either inhibitor prevented the increase in nNOS immunoreactivity and enzymatic activity provoked by hypobaric hypoxia. Concomitantly, nitrotyrosine immunoreactivity decreased progressively. In conclusion, activation of the nitrergic system constitutes a cortical response to hypobaric hypoxia and the administration of NOS inhibitors could provide new therapeutic avenues to prevent and/or treat the symptoms produced by hypobaric hypoxia.

Effects of acupuncturing Tsusanli (ST36) on expression of nitric oxide synthase in hypothalamus and adrenal gland in rats with cold stress ulcer

AIM: To study the protective effect of acupuncturing Tsusanli (S_T36) on cold stress ulcer, and the expression of nitric oxide synthase (NOS) in hypothalamus and adrenal gland.

METHODS: Ulcer index in rats and RT-PCR were used to study the protective effect of acupuncture on cold stress ulcer, and the expression of NOS in hypothalamus and adrenal gland. Images were analyzed with semi-quantitative method.

RESULTS: The ulcer index significantly decreased in rats with stress ulcer. Plasma cortisol concentration was up regulated during cold stress, which could be depressed by pre-acupuncture. The expression of NOS1 in hypothalamus increased after acupuncture. The increased expression of NOS2 was related with stress ulcer, which could be decreased by acupuncture. The expression of NOS3 in hypothalamus was similar to NOS2, but the effect of acupuncture was limited. The expression of NOS2 and NOS3 in adrenal gland increased after cold stress, only the expression of NOS1 could be repressed with acupuncture. There was no NOS2 expression in adrenal gland in rats with stress ulcer.

CONCLUSION: The protective effect of acupuncturing Tsusanli (S_T36) on the expression of NOS in hypothalamus and adrenal gland can be achieved.

Neuronal NADPH-d/NOS expression in the nodose ganglion of severe hypoxic rats with or without mild hypoxic preconditioning

This study aimed to test the hypothesis that mild hypoxic preconditioning (MHPC)-induced NOS expression would attenuate the neuropathological changes in the nodose ganglion (NG) of severe hypoxic exposure (SHE) rats. Thus, the young adult rats were caged in the altitude chamber for 4 weeks prior to SHE for 4 h to gain hypoxic preconditioning. The altitude chamber was used to set the height at the level from 5500 m (0.50 atm; pO2=79 Torr) to 10,000 m (0.27 atm; pO2=43 Torr) for MHPC and SHE, respectively. The experimental animals were allowed to survive for 0, 7, 14, 30 and 60 successive days, respectively. Nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) histochemistry and neuronal nitric oxide synthase (nNOS) immunohistochemistry were used to detect NADPH-d/nNOS reactivity in the NG at various time points following hypoxic exposure. The present results showed that about 38% of the neurons in the NG displayed NADPH-d/nNOS positive [NADPH-d/nNOS(+)] in normoxic rats. In SHE rats, a peak in the percentage (71%) and staining intensity (230%) of NADPH-d/nNOS(+) nodose neurons at 0 day, which then gradually decreased at 7-60 days. About 25% of the nodose neurons died 60 days after SHE. However, in MHPC rats subjected to SHE, NADPH-d/nNOS(+) neurons peaked in the percentage (51%) and staining intensity (171%) at 0 day, which then decreased at 7-60 days. In addition, neuronal survival was markedly increased by MHPC. These results suggested that MHPC might have a neuroprotective effect that reduces the susceptibility of the nodose neurons to NOS mediated neuropathy subsequent to SHE.

Nitric Oxide Synthase in Acute Alteration of Nitric Oxide Levels after Subarachnoid Hemorrhage

OBJECTIVE

Subarachnoid hemorrhage (SAH) is associated with acute decreases and subsequent recovery of cerebral nitric oxide (NO) levels, but the mechanisms of these alterations are not known. In this study, we measured NO synthase (NOS) protein and kinetics to determine its involvement in the alterations of cerebral NO levels after SAH.

METHODS

The endovascular rat model of SAH was used. The number of NOS-1 (neuronal) and NOS-2 (inducible)-positive cells (0-96 h) was determined by counting immunoreactive cells in 8-microm cryostat sections. The tissue content of active NOS and its kinetic parameters were studied with an enzymatic l-citrulline assay.

RESULTS

The number of NOS-1-positive cells increased between 1 and 3 hours after SAH, decreased to and below control values at 6 and 72 hours after SAH, and increased to control values 96 hours after SAH. The number of NOS-2-positive cells increased 1 hour after SAH, decreased to control values at 24 hours, and increased above control values 96 hours after SAH. The Michaelis-Menten kinetic parameters (V(max), K(m), slope) of NOS remained unchanged at 10 and 90 minutes after SAH.

CONCLUSION

NOS-1 and -2 proteins undergo a triphasic alteration after SAH, whereas the amount of active NOS and its kinetic parameters remain unchanged during the first 90 minutes after SAH. Depletion of NOS is not involved in the acute alterations of cerebral NO levels after SAH.

Green tea polyphenol (−)-epigallocatechin gallate attenuates the neuronal NADPH-d/nNOS expression in the nodose ganglion of acute hypoxic rats

Recent studies have shown that (-)-epigallocatechin gallate (EGCG), one of the green tea polyphenols, has a potent antioxidant property. Nitric oxide (NO) plays an important role in the neuropathogenesis induced by brain ischemia/reperfusion and hypoxia. This study aimed to explore the potential neuroprotective effect of EGCG on the ganglionic neurons of the nodose ganglion (NG) in acute hypoxic rats. Thus, the young adult rats were pretreated with EGCG (10, 25, or 50 mg/kg, i.p.) 30 min before they were exposed to the altitude chamber at 10,000 m with the partial pressure of oxygen set at the level of 0.27 atm (pO2=43 Torr) for 4 h. All the animals examined were allowed to survive for 3, 7, and 14 successive days, respectively, except for those animals sacrificed immediately following hypoxic exposure. Nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) histochemistry and neuronal nitric oxide synthase (nNOS) immunohistochemistry were carried out to detect the neuronal NADPH-d/nNOS expression in the NG. The present results show a significant increase in the expression of NADPH-d/nNOS reactivity in neurons of the NG at various time intervals following hypoxia. However, the hypoxia-induced increase in NADPH-d/nNOS expression was significantly depressed only in the hypoxic rats treated with high dosages of EGCG (25 or 50 mg/kg). These data suggest that EGCG may attenuate the oxidative stress following acute hypoxia.

The role of NADPH oxidase and neuronal nitric oxide synthase in zinc-induced poly(ADP-ribose) polymerase activation and cell death in cortical culture

In the present study, we examined the role and the mechanism of poly(ADP-ribose) polymerase (PARP) and poly(ADP-ribose) glycohydrolase (PARG) activation in zinc-induced cell death in cortical culture. After brief exposure to 400 microM zinc, cortical cells exhibited DNA fragmentation, increased poly(ADP-ribosyl)ation, and decreased levels of nicotinamide adenine dinucleotide (NAD) and ATP and subsequently underwent cell death. Inhibitors of PARP/PARG attenuated both zinc-induced NAD/ATP depletion and cell death, thereby implicating the PARP/PARG cascade in these processes. The zinc-inducible enzymes NADPH oxidase and neuronal nitric oxide synthase (nNOS) contributed to PARP activation as their inhibitors attenuated zinc-induced poly(ADP-ribosyl)ation. Levels of nitric oxide and nitrites increased following zinc exposure, consistent with NOS activation. In addition, Western blots and RT-PCR analysis revealed that protein and mRNA levels of nNOS specifically increased following zinc exposure in a manner similar to that of NADPH oxidase. The present study demonstrates that induction of NADPH oxidase and nNOS actively contributes to PARP/PARG-mediated NAD/ATP depletion and cell death induced by zinc in cortical culture.

Nitric oxide and nitric oxide synthases in the fetal cerebral cortex of rats following transient uteroplacental ischemia

The effect of transient uteroplacental ischemia on nitric oxide (NO) levels, enzymatic activity, and expression of NO synthase (NOS) isoforms was studied in fetal rat brains. Fetuses were subjected to ischemia by clamping the uterine arteries for 5 min on gestational day 17 (GD17). At different times after ischemia, fetuses were delivered by Cesarean section under anesthesia to obtain the brains. Transient uteroplacental ischemia produced a time dependent increase in nitrite levels in the brain, reaching a maximum value (300 +/- 25% of baseline) 24 h after uterine artery occlusion and remaining elevated as long as 48 h. Significantly increased nitrite levels were found in the cerebral cortex but not in the mesencephalon and cerebellum. The ischemia-induced increment in nitrite levels was totally blocked by either L-NAME (10 mg/kg) or AMT (0.65 mg/kg) administered i.p. 1 h before uterine artery occlusion. Both Ca(2+)-dependent and Ca(2+)-independent NOS activities in the cerebral cortex remained significantly increased with respect to controls after 24 h following the ischemia. Reverse transcriptase-polymerase chain reaction showed augmented levels of mRNAs for both nNOS and iNOS when compared with controls at 8 h after ischemia. At 36 h, nNOS mRNA returned to basal levels whereas eNOS mRNA levels increased and iNOS mRNA remained elevated. Our results show that the three NOS isoforms participate in increasing NO levels after transient ischemia and suggest a biphasic and differential regulation of the expression of constitutive NOS isoforms in the rat cerebral cortex.

Nitric oxide synthase expression after human brain contusion

OBJECTIVE

Nitric oxide is a universal mediator of biological effects in the brain, and it has been implicated in the pathophysiological processes of traumatic brain injury. Experimental studies have indicated posttraumatic up-regulation of the three different isoforms of nitric oxide synthase (NOS) (i.e., inducible NOS [iNOS], endothelial NOS, and neuronal NOS) after brain trauma. This study was undertaken to investigate the cellular sources and tissue compartments of nitric oxide produced by human patients undergoing surgical treatment for contusional brain injuries.

METHODS

Contused brain tissue specimens from eight consecutive patients who underwent surgical treatment for brain contusions 3 hours to 5 days after trauma were evaluated in immunohistochemical analyses. Double-staining assays were used to define which cells produced the different isoforms.

RESULTS

Increases in iNOS-positive cells were detectable within 6 hours after trauma, with a peak at 8 to 23 hours. Expression of iNOS after trauma was detected in neurons, macrophages, neutrophils, astrocytes, and oligodendrocytes. The cellular sources of iNOS differed at different times after trauma. No detectable difference in the expression of the neuronal or endothelial isoforms was observed for trauma patients, compared with control subjects.

CONCLUSION

iNOS expression was up-regulated in a time-dependent manner in human brain tissue obtained from patients undergoing surgical treatment for contusional trauma. Our human data largely parallel experimental findings in rats, indicating that such trauma models are relevant for experimental studies and treatment trials.

Preconditioning-mediated neuroprotection: role of nitric oxide, cGMP, and new protein expression

Preconditioning adaptation induced by transient ischemia can increase brain tolerance to oxidative stress, but the underlying neuroprotective mechanisms are not fully understood. Recently, we developed a human brain-derived cell model to investigate preconditioning mechanism in SH-SY5Y neuroblastoma cells.(1) Our results demonstrate that a non-lethal serum deprivation-stress for 2 h (preconditioning stress) enhanced the tolerance to a subsequent lethal oxidative stress (24 h serum deprivation) and also to 1-methyl-4-phenyl-pyridinium (MPP(+)).(2) Two-hour non-lethal preconditioning stress increased the expression of neuronal nitric oxide (NOS1/nNOS) mRNA, Fos, Ref-1, NOS protein, and then nitric oxide (*NO) production. As well as MnSOD expression, the *NO-cGMP-PKG pathway mediated the preconditioning-induced upregulation of antiapoptotic protein Bcl-2 and the downregulation of adaptor protein p66(shc). We also propose that cGMP-mediated preconditioning-induced adaptation against oxidative stress may be due to the synthesis of a new protein, such as thioredoxin (Trx) since the protective effect can be blocked by Trx reductase inhibitor.(3) The antioxidative potency of Trx was approximately 100 and 1,000 times greater than GSNO and GSH, respectively. These results suggest that *NO-cGMP-PKG signaling pathway plays an important role in the preconditioning-induced neuroprotection, and perhaps cardioprotection, against oxidative stress.

Neuroprotective MK801 is associated with nitric oxide synthase during hypoxia/reoxygenation in rat cortical cell cultures

The neuroprotective effect of MK801 against hypoxia and/or reoxygenation-induced neuronal cell injury and its relationship to neuronal nitric oxide synthetase (nNOS) expression were examined in cultured rat cortical cells. Treatment of cortical neuronal cells with hypoxia (95% N(2)/5% CO(2)) for 2 h followed by reoxygenation for 24 h induced a release of lactate dehydrogenase (LDH) into the medium, and reduced the protein level of MAP-2 as well. MK801 attenuated the release of LDH and the reduction of the MAP-2 protein by hypoxia, suggesting a neuroprotective role of MK801. MK801 also diminished the number of nuclear condensation by hypoxia/reoxygenation. The NOS inhibitors 7-nitroindazole (7-NI) and N (G)-nitro-L-arginine methyl ester (L-NAME), as well as the Ca(2+) channel blocker nimodipine, reduced hypoxia-induced LDH, suggesting that nitric oxide (NO) and calcium homeostasis contribute to hypoxia and/or the reoxygenation-induced cell injury. The levels of nNOS immunoactivities and mRNA by RT-PCR were enhanced by hypoxia with time and, down regulated following 24 h reoxygenation after hypoxia, and were attenuated by MK801. In addition, the reduction of nNOS mRNA levels by hypoxia/reoxygenation was also diminished by MK801. Further delineation of the mechanisms of NO production and nNOS regulation are needed and may lead to additional strategies to protect neuronal cells against hypoxic/reoxygenation insults.

Melatonin attenuates the neuronal NADPH-d/NOS expression in the nodose ganglion of acute hypoxic rats

Excessive production of nitric oxide (NO) may play a detrimental role in the process of hypoxia-related neuropathology. This study explored whether treatment with melatonin would attenuate the neuropathological changes in the vagal ganglia following a severe hypoxic insult. Thirty minutes prior to hypoxia treatment, young adult rats were pre-treated with melatonin at 5. 25 or 100 mg/kg injected intraperitoneally. Hypoxia was achieved by subjecting the rats to a barometric pressure of 0.2 atm (PO2 = 43 Torr) for 4 hr in an altitude chamber. Nicotinamine adenine dinucleotide phosphatediaphorase (NADPH-d) histochemistry combined with the neuronal nitric oxide synthase (nNOS) immunohistochemistry were used to detect the NADPH-d/nNOS reactivity in the nodose ganglion (NG) at various time points following the hypoxic exposure. In normal untreated rats, about 43% of the neurons in the NG displayed NADPH-d/nNOS reactivity. Following hypoxic exposure, both the percentage and the staining intensity of NADPH-d/nNOS positive neurons in the NG were markedly increased, but these were reduced in longer surviving animals. Quantitative analysis of cell counts revealed that about 17% of the neurons died at 14 days after hypoxia treatment. However, in hypoxic rats given different doses of melatonin pretreatment, neuronal death as well as the frequency and staining intensity of NADPH-d/nNOS reactivity of the nodose neurons were significantly decreased. The effect of melatonin on neuronal survival and NADPH-d/ nNOS expression was dose-dependent. It is therefore suggested that melatonin exerts a neuroprotective effect and may serve as a potential therapeutic strategy for prevention and/or reducing the susceptibility of nodose neurons to NO-mediated hypoxic neuropathy.

Nitric oxide synthase-I containing cortical interneurons co-express antioxidative enzymes and anti-apoptotic Bcl-2 following focal ischemia: evidence for direct and indirect mechanisms towards their resistance to neuropathology

Neuronal nitric oxide-I is constitutively expressed in approximately 2% of cortical interneurons and is co-localized with gamma-amino butric acid, somatostatin or neuropeptide Y. These interneurons additionally express high amounts of glutamate receptors which mediate the glutamate-induced hyperexcitation following cerebral injury, under these conditions nitric oxide production increases contributing to a potentiation of oxidative stress. However, perilesional nitric oxide synthase-I containing neurons are known to be resistant to ischemic and excitotoxic injury. In vitro studies show that nitrosonium and nitroxyl ions inactivate N-methyl-D-aspartate receptors, resulting in neuroprotection. The question remains of how these cells are protected against their own high intracellular nitric oxide production after activation. In this study, we investigated immunocytochemically nitric oxide synthase-I containing cortical neurons in rats after unilateral, cortical photothrombosis. In this model of focal ischemia, perilesional, constitutively nitric oxide synthase-I containing neurons survived and co-expressed antioxidative enzymes, such as manganese- and copper-zinc-dependent superoxide dismutases, heme oxygenase-2 and cytosolic glutathione peroxidase. This enhanced antioxidant expression was accompanied by a strong perinuclear presence of the antiapoptotic Bcl-2 protein. No colocalization was detectable with upregulated heme oxygenase-1 in glia and the superoxide and prostaglandin G(2)-producing cyclooxygenase-2 in neurons. These results suggest that nitric oxide synthase-I containing interneurons are protected against intracellular oxidative damage and apoptosis by Bcl-2 and several potent antioxidative enzymes. Since nitric oxide synthase-I positive neurons do not express superoxide-producing enzymes such as cyclooxygenase-1, xanthine oxidase and cyclooxygenase-2 in response to injury, this may additionally contribute to their resistance by reducing their internal peroxynitrite, H(2)O(2)-formation and caspase activation.

Prolonged suppression of brain nitric oxide synthase activity by 7-nitroindazole protects against cerebral hypoxic-ischemic injury in neonatal rat

Nitric oxide mediates glutamate-induced excitotoxicity associated with cerebral hypoxia-ischemia through production in the brain by several isoforms of nitric oxide synthase (NOS). We examined the influence of the selective neuronal NOS inhibitor, 7-nitroindazole (7-NI), on brain NOS activity and its neuroprotective effects against cerebral hypoxic-ischemic injury in the postnatal day (PND) 7 rat. In the first set of experiments, 7-NI (50 mg/kg) administered intraperitoneally (i.p.) transiently inhibited NOS activity to 40% below the vehicle control level at 1 h after injection (P<0.001, analysis of variance (ANOVA)). In contrast, 7-NI (100 mg/kg, i.p.) inhibited NOS activity to 56% below the control level at 1 h with prolonged suppression of NOS activity at 3, 6, 9 and 12 h after injection. Two-factor ANOVA revealed an overall effect on NOS activity of 7-NI treatment (P<0.001) and time after injection (P<0.001). In the second set of experiments, 7-NI (50, 100 mg/kg) or an equal volume of vehicle was administered after unilateral carotid artery ligation, but 30 min before hypoxia in PND 7 rats. 7-NI (100 mg/kg) significantly protected against cerebral hypoxic-ischemic injury (100 mg/kg of 7-NI, 1.7+/-1.0% damage; control, 8.7+/-1.6%,P<0.05). 7-NI administered 15 min after cerebral hypoxia-ischemia was not neuroprotective. The data suggest that the protective effect of 7-NI is dose dependent, and is related to the duration of suppressed NOS activity.

Effect of auditory cortex lesions on NADPH-diaphorase staining in the inferior colliculus of rat

Projections from the auditory cortex (AC) in the rat terminate in the dorsal cortex (DC) and in the external cortex (EC) of the inferior colliculus (IC), areas which exhibit a moderate number of nicotinamide-adenine dinucleotide phosphate-diaphorase (NADPH-d) positive neurons. NADPH-d co-localizes with nitric oxide synthase, which is responsible for the production of the transcellular messenger, nitric oxide. Changes in NADPH-d staining in the IC were found after unilateral lesions of the AC. Lesions resulted in a reduction in NADPH-d staining in neurons and neuropil within the ipsilateral DC and EC with the maximum reduction occurring 3-4 days after lesion. The reduction in NADPH-d staining in the contralateral IC was less pronounced. Lesions affecting auditory areas Te 1 and Te 3 produced the largest decrease in NADPH-d staining in neurons and neuropil. This finding may be related to the abolition of the influence of glutamatergic corticocollicular and commissural pathways.

Rapid expression of neuronal and inducible nitric oxide synthases during post-ischemic reperfusion in rat brain

OBJECTIVE

To determine whether neuronal and inducible nitric oxide synthase (nNOS and iNOS) isoforms are expressed within cortical neurons during early reperfusion after focal cerebral ischemia.

METHODS

Male spontaneously hypertensive rats underwent occlusion of the left middle cerebral artery for 2 h. Coronal brain sections with normal and ischemic cortex were obtained after 15 min or 1, 6 or 24 h of reperfusion. Immunohistochemical and double-label immunofluorescent techniques were used to confirm cellular identity and localize nNOS and iNOS.

RESULTS

Immunoreactive nNOS was identified within isolated neurons in layer V of normal cortex. However, the number of nNOS-immunoreactive neurons in ischemic cortex rose markedly at 15 min and persisted for 24 h (P< or =0.001 at each time point when compared to normal cortex). Cells that were immunoreactive for nNOS appeared in perivascular clusters within ischemic brain at all sampling times. Immunoreactive iNOS was also expressed within neurons in ischemic cortex, peaking after 15 min of reperfusion (P< or =0.01). Although nNOS-immunoreactive neurons were observed in random numbers within normal tissue throughout reperfusion, iNOS-immunoreactive neurons increased steadily in the same region (P< or =0.05).

CONCLUSIONS

Ischemic neurons become immunoreactive for both nNOS and iNOS during early reperfusion. Expression of iNOS immunoreactivity in unaffected neurons may reflect transcription of immediate early genes in response to stimulatory neurotransmission from ischemic cortex.

Delayed increase in neuronal nitric oxide synthase immunoreactivity in thalamus and other brain regions after hypoxic-ischemic injury in neonatal rats

We examined the response of neuronal nitric oxide synthase (nNOS)-containing CNS neurons in rats exposed to a unilateral hypoxic-ischemic insult at 7 days of age. Animals were sacrificed at several time points after the injury, up to and including 7 days (Postnatal Day 14). Brain regions ipsilateral to the injury (including cerebral cortex, caudate-putamen, and thalamus) exhibited delayed, focal increases in nNOS immunoreactivity. The increase in nNOS immunoreactive fiber staining was prominent in areas adjacent to severe neuronal damage, especially in the cortex and the thalamus, regions that are also heavily and focally injured in term human neonates with hypoxic-ischemic encephalopathy. In cerebral cortex, these increases occurred despite modest declines in nNOS catalytic activity and protein levels. Proliferation of surviving nNOS immunoreactive fibers highlights regions of selective vulnerability to hypoxic-ischemic insult in the neonatal brain and may also contribute to plasticity of neuronal circuitry during recovery.

Perinatal brain injury

Perinatal brain damage in the mature fetus is usually brought about by severe intrauterine asphyxia following an acute reduction of the uterine or umbilical circulation. The areas most heavily affected are the parasagittal region of the cerebral cortex and the basal ganglia. The fetus reacts to a severe lack of oxygen with activation of the sympathetic-adrenergic nervous system and a redistribution of cardiac output in favor of the central organs (brain, heart and adrenals). If the asphyxic insult persists, the fetus is unable to maintain circulatory centralization, and the cardiac output and extent of cerebral perfusion fall. Owing to the acute reduction in oxygen supply, oxidative phosphorylation in the brain comes to a standstill. The Na+/K+ pump at the cell membrane has no more energy to maintain the ionic gradients. In the absence of a membrane potential, large amounts of calcium ions flow through the voltage-dependent ion channels, down an extreme extra-/intracellular concentration gradient, into the cell. Current research suggests that the excessive increase in levels of intracellular calcium, so-called calcium overload, leads to cell damage through the activation of proteases, lipases and endonucleases. During ischemia, besides the influx of calcium ions into the cells via voltage-dependent calcium channels, more calcium enters the cells through glutamate-regulated ion channels. Glutamate, an excitatory neurotransmitter, is released from presynaptic vesicles during ischemia following anoxic cell depolarization. The acute lack of cellular energy arising during ischemia induces almost complete inhibition of cerebral protein biosynthesis. Once the ischemic period is over, protein biosynthesis returns to preischemic levels in non-vulnerable regions of the brain, while in more vulnerable areas it remains inhibited. The inhibition of protein synthesis, therefore, appears to be an early indicator of subsequent neuronal cell death. A second wave of neuronal cell damage occurs during the reperfusion phase. This cell damage is thought to be caused by the postischemic release of oxygen radicals, synthesis of nitric oxide (NO), inflammatory reactions and an imbalance between the excitatory and inhibitory neurotransmitter systems. Part of the secondary neuronal cell damage may be caused by induction of a kind of cellular suicide programme known as apoptosis. Interestingly, there is increasing evidence from recent clinical studies that perinatal brain damage is closely associated with ascending intrauterine infection before or during birth. However, a major part of this damage is likely to be of hypoxic-ischemic nature due to LPS-induced effects on fetal cerebral circulation. Knowledge of these pathophysiological mechanisms has enabled scientists to develop new therapeutic strategies with successful results in animal experiments. The potential of such therapies is discussed here, particularly the promising effects of intravenous administration of magnesium or postischemic induction of cerebral hypothermia.

Immature brain injury via peroxynitrite production induced by inducible nitric oxide synthase after hypoxia-ischemia in rats

OBJECTIVE

To determine whether, and if so how, iNOS expresses and affects brain injury induced by hypoxia-ischemia in an immature brain.

MATERIAL AND METHODS

Seven-day-old Wistar rat pups were exposed to right common carotid artery ligation followed by 1.5 hours of hypoxia. The time course of iNOS mRNA expression, enzymatic activity, and protein production in the cerebral cortex were determined. The extent of the infarct area in the cerebral cortex and the production of 3-nitrotyrosine (a biomarker of peroxynitrite) were compared between the control pups and pups treated with S-methyl-isothiourea (a selective iNOS inhibitor).

RESULTS

In the cortex ipsilateral to carotid ligation, iNOS mRNA appeared from 6 hours to 24 hours after hypoxia-ischemia and disappeared at 48 hours. The iNOS protein and its activity also increased at 12 hours and reached a maximum level at 48 hours after the insult. The percentage of damage in the cerebral cortex was significantly higher in the control pups than in treated pups (31.9 vs 10.6%). Tri-nitrotyrosine following iNOS expression-positive cells were located predominantly at the infarct and peri-infarct regions.

CONCLUSIONS

iNOS expression might be an important determinant of ischemic immature brain injury.

Intracellular signals coupled to muscarinic acetylcholine receptor activation in cerebral frontal cortex from hypoxic mice

1. The aim of the present work was to determine hypoxia-induced modifications in the cascade of intracellular events coupled to muscarinic acetylcholine receptor (mAChR) activation in brain. For this purpose, enzymatic activities were measured on normoxically incubated frontal cortical slices from mice exposed to hypobaric hypoxia for 72 hr. 2. We found that hypoxia induced alterations in several cerebral enzymatic basal activities: it increased nitric oxide synthase (NOS), but it decreased both membrane protein kinase C (PKC) and phospholipase C activities. 3. The mAChR agonist carbachol was found to increase phosphoinositide hydrolysis to greater values in hypoxic tissues than those found in normoxic conditions. Furthermore, a greater translocation of PKC in response to carbachol was observed in hypoxic tissues than in normoxic ones. 4. Besides, carbachol induced a drastic reduction of NOS activity in hypoxic brains, at concentrations that stimulated this enzyme activity in normoxic preparations. In the latter, inhibition is obtained only with high concentrations of the cholinergic muscarinic agonist. 5. These results pointed to a carbachol-mediated mAChR hyperactivity induced by hypoxic insult. 6. The possibility that these effects would account for a compensatory mechanism to diminish NOS hyperactivity, probably protecting for NO neurotoxic action in hypoxic brain, is also discussed.

N-tosyl-L-phenylalanyl-chloromethylketone reduces hypoxic-ischemic brain injury in rat pups

N-tosyl-L-phenylalanyl-chloromethylketone (TPCK) in vitro blocks apoptotic pathways leading to cell death. We wished to see if TPCK would reduce brain injury in vivo. Seven-day-old rat pups had the right carotid artery ligated and then received either vehicle or TPCK (5 to 100 mg/kg i.p.). They were then given 8% oxygen for 2.25 h. Twenty-two days later, the cerebral hemispheres were weighed to determine the reduction in size in the right hemisphere. TPCK decreased the reduction in right hemisphere weight from 15+/-3% (vehicle, n=20), to 4+/-2% (10 mg/kg, n = 19, P<0.01). TPCK reduced the number of cells staining for DNA breaks 3 days after injury from 1729+/-275 mm(-2) (vehicle, n = 8) to 550+/-236 mm(-2) (10 mg/kg TPCK, n = 9, P<0.01), decreased the amount of DNA fragmentation 3 days after injury by gel electrophoreses (20 mg/kg, n = 16, P<0.01) and eliminated the increase in nitric oxide metabolites 6 h after injury (vehicle 1.5+/-0.4, n = 10; and 20 mg/kg TPCK 0.0+/-0.1 nM/mg protein, n = 10, P<0.001). TPCK pretreatment in the newborn rat model of hypoxic-ischemic brain injury reduces DNA fragmentation, nitric oxide production and brain injury.

Dopamine transporter and nitric oxide synthase in hypoxic-ischemic brain

Changes in dopamine transporter and neuronal nitric oxide synthase (nNOS) were investigated by immunohistochemistry in 18 cases of hypoxic-ischemic basal ganglia necrosis. Neuropil dopamine transporter immunostaining in the striatum was increased in seven cases, with relatively mild basal ganglia necrosis, and decreased in four cases, with marked basal ganglia necrosis, compared with age-matched control subjects. Correspondingly, some striatal neurons had increased immunoreactivity to dopamine transporter in the cases of increased immunostaining in the neuropil. nNOS-positive neurons did not obviously change in cases of basal ganglia necrosis within 2 days after birth and then decreased or were not detectable in cases of basal ganglia necrosis at more than 3 days after birth. The results suggest that the synthesis of dopamine transporter is up-regulated in relatively mild basal ganglia necrosis to compensate for the uptake of increased dopamine, that this compensative ability is lost in marked basal ganglia necrosis, and that nNOS-containing neurons in the striatum are relatively resistant to hypoxic ischemia. We speculate that glutamate excitotoxicity mediated by glutamate receptors 1, 2/3, and 4 and excessive dopaminergic excitatory activity may play important roles in hypoxic-ischemic basal ganglia necrosis and that nNOS does not contribute to that condition.

Constitutive and inducible nitric oxide synthases after dynorphin-induced spinal cord injury

It has recently been demonstrated that selective inhibition of both neuronal constitutive and inducible nitric oxide synthases (ncNOS and iNOS) is neuroprotective in a model of dynorphin (Dyn) A(1-17)-induced spinal cord injury. In the present study, various methods including the conversion of 3H-L-arginine to 3H-citrulline, immunohistochemistry and in situ hybridization are employed to determine the temporal profiles of the enzymatic activities, immunoreactivities, and mRNA expression for both ncNOS and iNOS after intrathecal injection of a neurotoxic dose (20 nmol) of Dyn A(1-17). The expression of ncNOS immunoreactivity and mRNA increased as early as 30 min after injection and persisted for 1-4 h. At 24-48 h, the number of ncNOS positive cells remained elevated while most neurons died. The cNOS enzymatic activity in the ventral spinal cord also significantly increased at 30 min 48 h, but no significant changes in the dorsal spinal cord were observed. However, iNOS mRNA expression increased later at 2 h, iNOS immunoreactivity and enzymatic activity increased later at 4 h and persisted for 24-48 h after injection of 20 nmol Dyn A(1-17). These results indicate that both ncNOS and iNOS are associated with Dyn-induced spinal cord injury, with ncNOS predominantly involved at an early stage and iNOS at a later stage.

Protective effect of aminoguanidine on hypoxic-ischemic brain damage and temporal profile of brain nitric oxide in neonatal rat

Nitric oxide (NO) produced by inducible NO synthase contributes to ischemic brain damage. However, the role of inducible NO synthase-derived NO on neonatal hypoxic-ischemic encephalopathy has not been clarified. We demonstrate here that aminoguanidine, a relatively selective inhibitor of inducible NO synthase, ameliorated neonatal hypoxic-ischemic brain damage and that temporal profiles of NO correlated with the neuroprotective effect of aminoguanidine. Seven-day-old Wister rat pups were subjected to left carotid artery occlusion followed by 2.5 h of hypoxic exposure (8% oxygen). Infarct volumes (cortical and striatal) were assessed 72 h after the onset of hypoxia-ischemia by planimetric analysis of coronal brain slices stained with hematoxylin-eosin. Aminoguanidine (300 mg/kg i.p.), administered once before the onset of hypoxia-ischemia and then three times daily, significantly ameliorated infarct volume (89% reduction in the cerebral cortex and 90% in the striatum; p<0.001). NO metabolites were measured by means of chemiluminescence using an NO analyzer. In controls, there was a significant biphasic increase in NO metabolites in the ligated side at 1 h (during hypoxia) and at 72 h after the onset of hypoxia (p<0.05). Aminoguanidine did not suppress the first peak but significantly reduced the second one (p<0.05), and markedly reduced infarct size in a neonatal ischemic rat model. Suppression of NO production after reperfusion is a likely mechanism of this neuroprotection.

Induction of neuronal and inducible nitric oxide synthase in the motoneurons of spinal cord following transient abdominal aorta occlusion in rats

BACKGROUND

Motoneurons in the spinal cord are especially vulnerable to ischemic injury and selectively destroyed after transient ischemia. Nitric oxide (NO) has been implicated in both neurodegneration and neuroprotection to ischemic insult. To evaluate the role of NO in pathophysiology to spinal cord ischemia, the expression of neuronal and inducible nitric oxide synthase (n-NOS and i-NOS) and nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) in the motoneurons of the lumbosacral spinal cord was examined in a rat model with transient abdominal aorta (TAA) occlusion.

MATERIALS AND METHODS

Male Sprague-Dawley rats were divided into sham-operated (n = 12) and TAA occlusion (n = 24) groups. TAA occlusion was induced by placement of a microvascular clamp around the abdominal aorta for 20 min. Three sham-operated and six TAA occlusion animals were sacrificed at each time interval at 4, 24, and 48 h and 7 days after operation. Tissue sections obtained from the lumbosacral spinal cord were processed for n-NOS, i-NOS, NADPH-d, and hematoxylin-eosin (HE) staining. Histological changes of motoneurons in ventral horn were assessed by HE staining.

RESULTS

In sham-operated control animals, n-NOS-, i-NOS-, and NADPH-d-positive neurons were barely detectable in the ventral horn of the spinal cord. At 4 h after TTA occlusion, n-NOS and NADPH-d expression became evident in the motoneurons and was markedly enhanced at 24 and 48 h. i-NOS expression was also induced in the ventral horn motoneurons of the lumbosacral spinal cord at the same time points. Enzymatic expression in the motoneurons was diminished 7 days after operation. Hyperchromatic neurons indicative of cell death were observed in HE-stained specimens 7 days following TAA occlusion.

CONCLUSIONS

The rapid induction of n-NOS, i-NOS, and NADPH-d in the motoneurons of ventral horn suggests that NO may be involved in the selective and delayed neuronal death in the spinal cord to the ischemic insult.

Core and penumbral nitric oxide synthase activity during cerebral ischemia and reperfusion in the rat pup

Our studies examined the hypothesis that the distribution of cerebral injury after a focal ischemic insult in the immature rat pup is associated with the regional distribution of nitric oxide synthase (NOS) activity and that differences in the vulnerability to ischemia between pup and adult might be related to differences in cofactor availability. We measured NOS activity in well-defined regions prone to become either core or penumbra in controls and at different times (end of occlusion, 0.5 h, and 24 h reperfusion) after middle cerebral artery occlusion (MCAO) from the right and left hemispheres in a 14- to 18-day-old rat pup filament model. Three groups of corresponding isoflurane sham controls were also included. "Core" NOS activity for combined right and left hemispheres ranged from 113% to 217% more than "penumbral" regions in control and sham groups. In the three MCAO groups, marked decreases in ischemic core and penumbral NOS activity were seen; however, core NOS remained higher than penumbral regions bilaterally. The effects of cofactor addition (10 microM tetrahydrobiopterin, 3 microM flavin adenine dinucleotide, and 3 microM flavin mononucleotide) on NOS activity were similar in "core" and "penumbral" regions in control and sham groups. However, after 24 h MCAO, cofactor addition preferentially increased NOS activity in the ischemic hemisphere. Co-factor addition in the pup also had a greater effect on enhancing NOS activity in all regions compared with the adult. Greater NOS activity in core regions in the rat pup, as in the adult, could in part, explain the increased vulnerability of that region to ischemia. NOS activity also can be influenced by the availability of cofactors and this effect may be greater in the immature animal.

Pathophysiology of perinatal brain damage

Perinatal brain damage in the mature fetus is usually brought about by severe intrauterine asphyxia following an acute reduction of the uterine or umbilical circulation. The areas most heavily affected are the parasagittal region of the cerebral cortex and the basal ganglia. The fetus reacts to a severe lack of oxygen with activation of the sympathetic-adrenergic nervous system and a redistribution of cardiac output in favour of the central organs (brain, heart and adrenals). If the asphyxic insult persists, the fetus is unable to maintain circulatory centralisation, and the cardiac output and extent of cerebral perfusion fall. Owing to the acute reduction in oxygen supply, oxidative phosphorylation in the brain comes to a standstill. The Na(+)/K(+) pump at the cell membrane has no more energy to maintain the ionic gradients. In the absence of a membrane potential, large amounts of calcium ions flow through the voltage-dependent ion channel, down an extreme extra-/intracellular concentration gradient, into the cell. Current research suggests that the excessive increase in levels of intracellular calcium, so-called calcium overload, leads to cell damage through the activation of proteases, lipases and endonucleases. During ischemia, besides the influx of calcium ions into the cells via voltage-dependent calcium channels, more calcium enters the cells through glutamate-regulated ion channels. Glutamate, an excitatory neurotransmitter, is released from presynaptic vesicles during ischemia following anoxic cell depolarisation. The acute lack of cellular energy arising during ischemia induces almost complete inhibition of cerebral protein biosynthesis. Once the ischemic period is over, protein biosynthesis returns to pre-ischemic levels in non-vulnerable regions of the brain, while in more vulnerable areas it remains inhibited. The inhibition of protein synthesis, therefore, appears to be an early indicator of subsequent neuronal cell death. A second wave of neuronal cell damage occurs during the reperfusion phase. This cell damage is thought to be caused by the post-ischemic release of oxygen radicals, synthesis of nitric oxide (NO), inflammatory reactions and an imbalance between the excitatory and inhibitory neurotransmitter systems. Part of the secondary neuronal cell damage may be caused by induction of a kind of cellular suicide programme known as apoptosis. Knowledge of these pathophysiological mechanisms has enabled scientists to develop new therapeutic strategies with successful results in animal experiments. The potential of such therapies is discussed here, particularly the promising effects of i.v. administration of magnesium or post-ischemic induction of cerebral hypothermia.

Increased nitric oxide synthesis is not involved in delayed cerebral energy failure following focal hypoxic-ischemic injury to the developing brain

This study addressed the hypothesis that the delayed impairment in cerebral energy metabolism that develops 10-24 h after transient hypoxia-ischemia in the developing brain is mediated by induction of increased nitric oxide synthesis. Four groups of 14-d-old Wistar rat pups were studied. Group 1 was subjected to unilateral carotid artery ligation and hypoxia followed immediately by treatment with the nitric oxide synthase (NOS) inhibitor, Nomega-nitro-L-arginine methyl ester (L-NAME, 30 mg/kg). Group 2 underwent hypoxia-ischemia but received saline vehicle. Group 3 received L-NAME without hypoxia-ischemia, and group 4, saline vehicle alone. At defined times after insult, the expression of neuronal and inducible NOS were determined and calcium-dependent and -independent NOS activities measured. Cerebral energy metabolism was observed using 31P magnetic resonance spectroscopy. At 48 h after insult, the expression of inducible NOS increased, whereas neuronal NOS at 24 h decreased on the infarcted side. Calcium-dependent NOS activity was higher than calcium-independent NOS activity, but did not increase within 36 h after insult, and was significantly inhibited by the administration of L-NAME. However, L-NAME did not prevent delayed impairment of cerebral energy metabolism or ameliorate infarct size. These results suggest that the delayed decline in cerebral energy metabolism after hypoxia-ischemia in the 14-d-old rat brain is not mediated by increased nitric oxide synthesis.

Roles of nitric oxide in brain hypoxia-ischemia

A large body of evidence has appeared over the last 6 years suggesting that nitric oxide biosynthesis is a key factor in the pathophysiological response of the brain to hypoxia-ischemia. Whilst studies on the influence of nitric oxide in this phenomenon initially offered conflicting conclusions, the use of better biochemical tools, such as selective inhibition of nitric oxide synthase (NOS) isoforms or transgenic animals, is progressively clarifying the precise role of nitric oxide in brain ischemia. Brain ischemia triggers a cascade of events, possibly mediated by excitatory amino acids, yielding the activation of the Ca2+-dependent NOS isoforms, i.e. neuronal NOS (nNOS) and endothelial NOS (eNOS). However, whereas the selective inhibition of nNOS is neuroprotective, selective inhibition of eNOS is neurotoxic. Furthermore, mainly in glial cells, delayed ischemia or reperfusion after an ischemic episode induces the expression of Ca2+-independent inducible NOS (iNOS), and its selective inhibition is neuroprotective. In conclusion, it appears that activation of nNOS or induction of iNOS mediates ischemic brain damage, possibly by mitochondrial dysfunction and energy depletion. However, there is a simultaneous compensatory response through eNOS activation within the endothelium of blood vessels, which mediates vasodilation and hence increases blood flow to the damaged brain area.

Effect of chronic hypoxemia on the regulation of nitric-oxide synthase in the fetal sheep brain

We tested the hypothesis that chronic hypoxemia modulates NO production of the fetal brain by altering its gene and protein expression. Chronically instrumented preterm fetal sheep were made hypoxemic by placental embolization for 21 days. Fetal oxygen content was measured to determine the level of hypoxemia. The expression of both eNOS and nNOS proteins and mRNA and enzyme activities of fetal sheep cerebrum were measured and compared between normoxic and hypoxemic animals. Our results show that in utero hypoxemia downregulates both Ca2+ dependent NOS activity and expression of eNOS protein and mRNA in the fetal sheep brain. In contrast, hypoxemia increased nNOS protein and mRNA levels in the cerebrum. This suggests that chronic hypoxemia has an opposing effect on eNOS and nNOS gene regulation. We propose that increased nNOS activity during chronic hypoxemia may excessively stimulate the neurons and contribute to fetal brain injury. On the other hand, downregulation of eNOS activity and expression may compromise the neuroprotective effect of eNOS and, therefore, further exacerbate brain injury.

Expression of neuronal nitric oxide synthase during embryonic development of the rat cerebral cortex

The expression of neuronal nitric oxide synthase (nNOS) during the development of the rat cerebral cortex from embryonic day (E) 13 to postnatal day (P) 0 was analyzed by immunocytochemical procedures using a specific antibody against rat brain nNOS. Expression of nNOS was first seen on E14 in cells of Cajal-Retzius morphology located in the marginal zone. Neuronal NOS immunoreactivity persisted in this layer throughout the embryonic period and only began to decrease on E20, when neuronal migration is coming to an end. From E17 onwards, migrating neurons expressing nNOS were observed in the intermediate zone with their leading processes directed towards the cortical plate. At the same time, efferent nNOS-immunoreactive axons originating from cortical plate cells entered the intermediate zone. From E19 onwards, cells expressing nNOS and with the morphological characteristics of migrating cells were observed in and near the subventricular zone. Confocal analysis of double immunostaining for nNOS and glial fibrillary acidic protein or nestin showed no coexpression of nNOS and glial markers in these cells, suggesting that nNOS-positive cells leaving the subventricular zone were not glial cells. Commissural, callosal and fimbrial fibers were seen to express nNOS on E18 and E19. This expression decreased from E20 and was very weak on E21 and P0. The observations suggest that nitric oxide is synthesized during embryonic life in relation to maturational processes such as the organization of cerebral lamination, and is involved in controlling migrational processes and fiber ingrowth.

Developmental characteristics of neuronal nitric oxide synthase (nNOS) immunoreactive neurons in fetal to adolescent human brains

The developmental characteristics of the neuronal nitric oxide synthase (nNOS) immunoreactive neurons in the human brain were studied. In the frontal lobe, nNOS immunoreactive cells appeared as early as 18 gestational weeks (GW) in the subcortical plate and then increased predominantly in the subcortical white matter during the fetal period, while weakly immunoreactive neurons were found in the cortical II-IV layers after 26 GW. In the basal ganglia, immunoreactive neurons could be detected in the striatum as early as 13 GW, and then showed a transient increase with peaks at 23-24 GW and 33-36 GW in the putamen and caudate nucleus, respectively. In the cerebellum, immunoreactivity was detected in the Purkinje and basket cells after 23 GW and 31 GW, respectively. The immunoreactivity of internal granule cells was constantly weak. In the brain stem, constant and intense immunoreactive neurons were found in the central gray, pedunculopontine tegmental nucleus, solitary tract nucleus, and lateral reticular nucleus. The immunoreactivity in the neurons of the pontine nucleus and inferior olivary nucleus was transiently increased, with peaks at 38-40 GW and 23-24 GW, respectively. This characteristic nNOS development suggests that transient nNOS hyperproduction may contribute to neuron maturation as well as vulnerability in each period and region, and NO may play an important role in the basic development of human brain functions.

Intrauterine hypoxia-ischemia alters nitric oxide synthase expression and activity in fetal and neonatal rat brains

The effects of intrauterine hypoxia-ischemia (HI) on nitric oxide synthase (NOS) activity and on expression of NOS isoforms were investigated in fetal and neonatal rat brains. Rat fetuses were subjected to either a 30-min intrauterine HI insult or a sham operation (SH) on gestational day 17 (G17). NOS activity in the homogenate of the rat brain was detectable on G17 and increased with age. NOS activity in the HI group was 20-30% higher than in the SH group from 6 to 48 h after the HI, but was 30% lower than in the SH group from postnatal day 8 to 14. Expression of the inducible NOS (iNOS) mRNA, as examined by RT-PCR, was increased as compared to the SH group from 6 to 24 h after the HI surgery. Expression of the constitutive neuronal NOS (nNOS) mRNA was reduced in the HI group from 24 h after the HI surgery up to postnatal day 14. Immunoblotting data have shown that alterations in NOS isoform protein expression caused by the intrauterine HI were consistent with the mRNA expression data. The overall results indicate that prenatal HI has long-lasting effects on function and expression of NOS in fetal and neonatal rat brains and that the altered NOS activity may be associated with prenatal HI-induced neurological abnormalities.

Nitric oxide mediates brain mitochondrial damage during perinatal anoxia

The possible role of nitric oxide (.NO) in brain energy metabolism during perinatal asphyxia in the rat was studied. Exposure of early neonates to 5 min of anoxia significantly inhibited brain mitochondrial complex II-III activity by 25%, without affecting complex I, complex IV or citrate synthase activities. This insult was accompanied by ATP depletion (54%) and increased concentration of nitrites plus nitrates (1.4-fold), suggesting enhanced .NO synthesis. Administration of Nomega-nitro-L-arginine monomethyl ester (L-NAME) to the mothers inhibited neonatal brain .NO synthase activity, as reflected by the decreased (23%) cyclic GMP concentration. These L-NAME-treated neonates showed complete resistance to anoxic-mediated brain mitochondrial complex II-III damage. Our results suggest that brain mitochondrial dysfunction leading to energy deficiency during perinatal asphyxia is a .NO-mediated process.

Increase in nitric oxide in the hypoxic-ischemic neonatal rat brain and suppression by 7-nitroindazole and aminoguanidine

We measured the changes in nitric oxide (NO) metabolites in the brains of neonatal rats with hypoxic-ischemic damage. There were two peaks of NO metabolites in the lesioned side of the cortex without treatment: one during hypoxia and the other during the re-oxygenation period. Prehypoxic treatment with 7-nitroindazole, a selective neuronal NO synthase inhibitor, suppressed both peaks of NO metabolites, whereas prehypoxic treatment with aminoguanidine, a selective inducible NO synthase inhibitor, partially suppressed only the peak in the re-oxygenation period. These data suggest different roles of neuronal and inducible NO synthases in the pathogenesis of hypoxic-ischemic encephalopathy.

Nitric oxide in developing brain

The role of NO in the neonatal brain, particularly during hypoxia and ischaemia has been studied extensively in animal models of focal and global ischaemia. The n-NOS and i-NOS activation have been found to be harmful whereas e-NOS activation has a neuroprotective effect in focal ischaemia (Fig. 1). The findings following global ischaemia are somewhat more controversial. Although all these studies clearly demonstrate that NO has an important role in the neonatal brain, it may be difficult to apply the results to humans for it is not clear when and which isoform of NOS gets activated following ischaemia in newborn infants. Also it is hard to determine the timing of intervention to inhibit or stimulate the production of NO in humans since we still do not know all the details about protective mechanisms of the human body. Some interventions may have a deleterious effect on some of those mechanisms. In addition, the relatively selective NOS inhibitors which are now used in animal experiments are not appropriate for human studies. New studies regarding the production of NO following ischaemia will be needed in newborns, together with the development of selective NOS inhibitors which can be used in humans. If the NO production follows the same pattern in humans as in animals at least the effects of i-NOS may be prevented either by selective inhibitors or by neuroprotective agents.