Contributions of endothelial and neuronal nitric oxide synthases to cerebrovascular responses to hyperoxia

Cerebrovascular injury in premature infants: current understanding and challenges for future prevention

Cerebrovascular insults are a leading cause of brain injury in premature infants, contributing to the high prevalence of motor, cognitive, and behavioral deficits. Understanding the complex pathways linking circulatory immaturity to brain injury in premature infants remains incomplete. These mechanisms are significantly different from those causing injury in the mature brain. The gaps in knowledge of normal and disturbed cerebral vasoregulation need to be addressed. This article reviews current understanding of cerebral perfusion, in the sick premature infant in particular, and discusses challenges that lie ahead.

Two faces of nitric oxide: implications for cellular mechanisms of oxygen toxicity

Recent investigations have elucidated some of the diverse roles played by reactive oxygen and nitrogen species in events that lead to oxygen toxicity and defend against it. The focus of this review is on toxic and protective mechanisms in hyperoxia that have been investigated in our laboratories, with an emphasis on interactions of nitric oxide (NO) with other endogenous chemical species and with different physiological systems. It is now emerging from these studies that the anatomical localization of NO release, which depends, in part, on whether the oxygen exposure is normobaric or hyperbaric, strongly influences whether toxicity emerges and what form it takes, for example, acute lung injury, central nervous system excitation, or both. Spatial effects also contribute to differences in the susceptibility of different cells in organs at risk from hyperoxia, especially in the brain and lungs. As additional nodes are identified in this interactive network of toxic and protective responses, future advances may open up the possibility of novel pharmacological interventions to extend both the time and partial pressures of oxygen exposures that can be safely tolerated. The implications of a better understanding of the mechanisms by which NO contributes to central nervous system oxygen toxicity may include new insights into the pathogenesis of seizures of diverse etiologies. Likewise, improved knowledge of NO-based mechanisms of pulmonary oxygen toxicity may enhance our understanding of other types of lung injury associated with oxidative or nitrosative stress.

Hyperbaric oxygen treatment improves GFR in rats with ischaemia/reperfusion renal injury: a possible role for the antioxidant/oxidant balance in the ischaemic kidney

BACKGROUND

Ischaemic kidney injury continues to play a dominant role in the pathogenesis of acute renal failure (ARF) in many surgical and medical settings. A major event in the induction of renal injury is related to the generation of oxygen-free radicals. Hyperbaric oxygen therapy (HBO) is indicated for treatment of many ischaemic events but not for ARF. Therefore, the present study examined the effects of HBO on kidney function and renal haemodynamics in rats with ischaemic ARF.

METHODS

Renal ischaemia was induced by unilateral renal artery clamping (45 min) in rats. Within 24 h following ischaemia, rats were treated twice with HBO of 100% O(2) at 2.5 absolute atmospheres for 90 min each (+HBO). Untreated rats (-HBO) served as a control. Forty-eight hours later, GFR, RBF and endothelial-dependent vasorelaxation were measured. In addition, the immunoreactive staining of 4-hydroxy-2-noneal (4-HNE), a major product of endogenous lipid peroxidation, and superoxide dismutase (SOD) were assessed.

RESULTS

In the -HBO group, GFR was reduced by 94% compared with the untouched normal kidney (ischaemic: 0.06 +/- 0.03 ml/min, normal: 1.02 +/- 0.13 ml). In contrast, in the +HBO group, GFR of the ischaemic kidney (0.36 +/- 0.07 ml/min) was reduced only by 68% compared with the contralateral normal kidney (1.12 +/- 0.12 ml/min). In line with these findings, HBO improved the vasodilatory response to ACh as expressed in enhancement of both total and regional renal blood flow. In addition, HBO reduced the formation of 4-HNE by 33% and 76% and increased SOD by 30% and 70% in the cortex and outer stripe region of the medulla of the ischaemic kidney, respectively.

CONCLUSION

HBO attenuates the decline in GFR following renal ischaemia, and improves endothelial-dependent vasorelaxation, suggesting that treatment with HBO may be beneficial in the setting of ischaemic ARF.

Strategies for molecular imaging dementia and neurodegenerative diseases

Dementia represents a heterogeneous term that has evolved to describe the behavioral syndromes associated with a variety of clinical and neuropathological changes during continuing degenerative disease of the brain. As such, there lacks a clear consensus regarding the neuropsychological and other constituent characteristics associated with various cerebrovascular changes in this disease process. But increasing this knowledge has given more insights into memory deterioration in patients suffering from Alzheimer's disease and other subtypes of dementia. The author reviews current knowledge of the physiological coupling between cerebral blood flow and metabolism in the light of state-of-the-art-imaging methods and its changes in dementia with special reference to Alzheimer's disease. Different imaging techniques are discussed with respect to their visualizing effect of biochemical, cellular, and/or structural changes in dementia. The pathophysiology of dementia in advanced age is becoming increasingly understood by revealing the underlying basis of neuropsychological changes with current imaging techniques, genetic and pathological features, which suggests that alterations of (neuro) vascular regulatory mechanisms may lead to brain dysfunction and disease. The current view is that cerebrovascular deregulation is seen as a contributor to cerebrovascular pathologies, such as stroke, but also to neurodegenerative conditions, such as Alzheimer's disease. The better understanding of these (patho) physiological mechanisms may open an approach to new interventional strategies in dementia to enhance neurovascular repair and to protect neurovascular coupling.

Hyperoxia reduces basal release of nitric oxide and contracts porcine coronary arteries

AIM

The purpose of the present study was to investigate whether changes in nitric oxide (NO) concentration is involved in hyperoxia-induced vasoconstriction in porcine conduit coronary arteries.

METHODS

The effect of hyperoxia on NO release and vasoconstriction was evaluated by tension recording, microsensor measurements, and immunoblotting in porcine conduit coronary arteries contracted with U46619 or 5-hydroxytryptamine.

RESULTS

In endothelium-intact segments exchanging 20% O2, 5% CO2, 75% N2 (normoxia) for 95% O2, 5% CO2 (hyperoxia) increased contraction. In segments without endothelium hyperoxia-evoked contraction was abolished, but restored by an encircling donor segment with endothelium. An inhibitor of NOS, asymmetric dimethylarginine (ADMA, 300 mum), reduced hyperoxic contraction and basal NO concentration by, respectively, 38 +/- 12% and 46 +/- 3% (P < 0.05, n = 9). A NO donor, S-nitroso-N-acetylpenicillamine (SNAP), increased NO concentration and evoked relaxation to the same levels in normoxic and hyperoxic conditions. beta-actin and endothelial NO synthase (eNOS) protein expression was similar in normoxic and hyperoxic arterial segments. Phosphorylation of eNOS was unaltered in normoxia vs. hyperoxia, but phosphorylation of eNOS-Ser(1177) was increased and phosphorylation of eNOS-Thr(495) decreased by U46619. Blockers of ATP-sensitive, voltage-dependent and calcium-activated K+ channels did not change hyperoxic contraction. However, high extracellular K+ concentration or a second and third exposure to hyperoxia decreased contraction.

CONCLUSION

The present study provides direct evidence that hyperoxia reduces basal release of NO leading to depletable endothelium-dependent vasoconstriction in porcine coronary arteries independent of changes in eNOS phosphorylation.

Reliability of the microdialysis pump CMA 107 under hyperbaric conditions

OBJECTIVE

Microdialysis measurements of extracellular substances under hyperbaric conditions were manifold used in several investigations. However, to our knowledge there is no analysis, which verified the applicability of microdialysis pumps under hyperbaric conditions. Thus, a goal of this study was to investigate the reliability of the microdialysis pump (MDP) CMA 107 in a hyperbaric environment up to 2.4bar absolute pressure.

METHODS

The CMA 107 with a perfusion rate of 2microL/min was stored in a decompression chamber. The ambient pressure was increased from 1 to 2.4bar absolute within 15min, maintained for 90min and then decreased to 1bar within 15min. The vials were changed every 15min, weighed before as well as after collecting the sample volume and the absolute recovery of glutamate, pyruvate, lactate, glucose and glycerol was determined. The same setup was performed under normobaric conditions.

RESULTS

The pumping capacity was 1.7% greater than expected under normobaric conditions, 36.5% less than expected in the compression phase, 10.5% less than expected in the isopression phase and 26.3% greater than expected in the decompression phase under hyperbaric conditions. The absolute recoveries under hyperbaric conditions were affected during the isopression phase with a deviation from -6 to +20% compared to normobaric environments.

CONCLUSION

The study demonstrated that an absolute ambient pressure up to 2.4bar did influence the pumping capacity and the reliability of the absolute recovery. These results need to be taken into consideration when interpreting microdialysis studies performed under hyperbaric conditions.

Multi-parameter in vivo cardiac magnetic resonance imaging demonstrates normal perfusion reserve despite severely attenuated beta-adrenergic functional response in neuronal nitric oxide synthase knockout mice

AIMS

The role of neuronal nitric oxide synthase (nNOS) in regulating contractile function remains controversial, and in regulating myocardial perfusion is uninvestigated. We used magnetic resonance imaging (MRI) to phenotype nNOS(-/-) and wild-type (WT) mice regarding left ventricular (LV) structure, baseline function, beta-adrenergic responsiveness, and perfusion reserve.

METHODS AND RESULTS

Cine MRI showed higher LV mass to end-diastolic volume ratio (2.3 +/- 0.2 mg/microL nNOS(-/-) vs. 1.7 +/- 0.1 mg/microL WT; P=0.032) and LV ejection fraction (64.9 +/- 2.1% nNOS(-/-) vs. 55.8 +/- 1.1% WT; P = 0.003) in nNOS(-/-). Myocardial tagging demonstrated similar baseline systolic circumferential strain (Ecc) in nNOS(-/-) and WT. With dobutamine, the normal change in Ecc was nearly absent in nNOS(-/-) (-0.5 +/- 0.3% nNOS(-/-) vs. -2.2 +/- 0.3% WT; P = 0.001), and the systolic strain rate (dEcc/dt) response to dobutamine seen in WT was reduced in nNOS(-/-) (-29 +/- 13%/s nNOS(-/-) vs. -106+/-16%/s WT; P = 0.001). Diastolic strain rate increased significantly with dobutamine only in WT. Arterial spin labelling showed that baseline perfusion and perfusion reserve with either dobutamine or an adenosine receptor agonist are normal in nNOS(-/-).

CONCLUSION

MRI provides non-invasive in vivo evidence that nNOS does not play a role in basal contractile function or myocardial perfusion, but is required for increasing cardiac inotropy and lusitropy upon beta-adrenergic stimulation.

Hyperbaric oxygen and cerebral physiology

Hyperbaric oxygen (HBO) therapy is defined by the Undersea and Hyperbaric Medical Society (UHMS) as a treatment in which a patient intermittingly breathes 100% oxygen under a pressure that is greater than the pressure at sea level [a pressure greater than 1 atmosphere absolute (ATA)]. HBO has been shown to be a potent means to increase the oxygen content of blood and has been advocated for the treatment of various ailments, including air embolism, carbon monoxide poisoning, wound healing and ischemic stroke. However, definitive established mechanisms of action are still lacking. This has led to uncertainty among clinicians, who have understandingly become hesitant in regard to using HBO therapy, even in situations where it could prove beneficial. Therefore, this review will summarize the literature regarding the effects of HBO on brain oxygenation, cerebral blood flow and intracranial pressure in both the healthy and injured brains, as well as discuss how changes in these three factors can impart protection.

Oxygen as a neonatal health hazard: call for détente in clinical practice

Education in oxygenation and in how oxygen is given to newborns needs to increase. Treatment with oxygen should no longer be considered proverbial and customary, regardless of our 'past experience' or consensus recommendations in clinical guidelines, since oxygen may lead to acute or chronic health effects.

CONCLUSION

Inappropriate oxygen use is a neonatal health hazard associated with aging, DNA damage and cancer, retinopathy of prematurity, injury to the developing brain, infection and others. Neonatal exposure to pure O2, even if brief, or to pulse oximetry >95% when breathing supplemental O2 must be avoided as much as possible.

Time-dependent effects of hyperoxia on the BOLD fMRI signal in primate visual cortex and LGN

Hyperoxia is present in many anaesthesia protocols used in animal blood oxygen level-dependent (BOLD) functional magnetic resonance imaging (fMRI) studies. However, little data exist on the influence of hyperoxia on the magnitude of stimulus-induced relative changes in BOLD fMRI signal (DeltaBOLD%). No study to date has investigated these effects in a time-resolved manner, although cerebral vasoregulation offers sites for a time-dependent interaction of hyperoxia and DeltaBOLD%. Here we investigated time-dependent effects of an inspiratory oxygen fraction of 90%. We tightly clamped end tidal CO(2) and body temperature and recorded physiological parameters relevant to rCBF in (fentanyl/isoflurane) anaesthetized monkeys while using visual stimulation to elicit DeltaBOLD%. To clarify whether changes in DeltaBOLD% arose from changes in baseline blood oxygenation or rather altered neuronal or vascular reactivity, we directly measured changes in rCBV using monocrystalline ion oxide nanoparticles (MION) as contrast agent. In visual cortex we found a biphasic modulation of stimulus-induced DeltaBOLD% under hyperoxia: We observed first a significant decrease in DeltaBOLD% by -24% for data averaged over the time interval of 0-180 min post onset of hyperoxia followed by a subsequent recovery to baseline. rCBV response amplitudes were decreased by 21% in the same time interval (0-180 min). In the LGN, we neither found a significant modulation of DeltaBOLD% nor of MION response amplitude. The cerebrovascular effects of hyperoxia may, therefore, be regionally specific and cannot be explained by a deoxyhemoglobin dilution model accounting for plasma oxygenation without assuming altered neuronal activity or altered neurovascular coupling.

Role of astrocytes in matching blood flow to neuronal activity

The brain is critically dependent on oxygen and glucose supply for normal function. Various neurovascular control mechanisms assure that the blood supply of the brain is adequate to meet the energy needs of its components. Emerging evidence shows that neuronal activity can control microcirculation using astrocytes as a mediator. Astrocytes can sense neuronal activity and are involved in signal transmission. Synaptic activity triggers an increase in the intracellular calcium concentration [Ca(2+)]i of adjacent astrocytes, stimulating the release of adenosine triphosphate (ATP) and glutamate. The released ATP mediates the propagation of Ca(2+) waves between neighboring astrocytes, thereby recruiting them to mediate adequate cerebrovascular response to neuronal activation. Simultaneously, sodium-dependent glutamate uptake in astrocytes generates Na(+) waves and subsequently increases glucose uptake and metabolism that leads to the formation of lactate, which is then delivered to neurons as an energy substrate. Further, astrocytic Ca(2+) elevations can lead to secretion of vasodilatory substances from perivascular endfeet, such as epoxyeicosatrienoic acid (EETs), adenosine, nitric oxide (NO), and cyclooxygenase-2 (COX-2) metabolites, resulting in increased local blood flow. Thus, astrocytes by releasing vasoactive molecules mediate the neuron-astrocyte-endothelial signaling pathway and play a profound role in coupling blood flow to neuronal activity.

Hyperoxic exposure leads to nitrative stress and ensuing microvascular degeneration and diminished brain mass and function in the immature subject

BACKGROUND AND PURPOSE

Neonates that survive very preterm birth have a high prevalence of cognitive impairment in later life. A common factor detected in premature infants is their postnatal exposure to high oxygen tension relative to that in utero. Hyperoxia is known to elicit injury to premature lung and retina. Because data on the exposure of the brain to hyperoxia are limited, we studied the effects of high oxygen on this tissue.

METHODS

Rat pups were exposed from birth until day 6 to 21% or 80% O(2). Cerebral vascular density was quantified by lectin immunohistochemistry. Immunoblots for several proteins were performed on brain extracts. We assessed cerebral functional deficits by visual evoked potentials.

RESULTS

Exposure of pups to hyperoxia leads to cerebral microvascular degeneration, diminished brain mass, and cerebral functional deficits. These effects are preceded by an upregulation of endothelial nitric oxide synthase (eNOS) in cerebral capillaries and a downregulation of Cu/Zn superoxide dismutase (SOD). The imbalance in nitric oxide (NO) production and antioxidant defenses favors the formation of nitrating agents in the microvessels revealed by increased nitrotyrosine (3-nt) immunoreactivity and decreased expression of NF-kappaB and the dependent vascular endothelial growth factor receptor 2. NOS inhibitors and eNOS deletion as well as an SOD mimetic (CuDIPS) restore vascular endothelial growth factor receptor-2 levels and nearly abolish the vasoobliteration. NOS inhibitors and SOD mimetic also prevent O(2)-induced diminished brain mass and functional deficit.

CONCLUSIONS

Data identify NO and nitrating agents as major mediators of cerebral microvascular damage, ensuing impaired brain development and function in immature subjects exposed to hyperoxia.

Role of endothelial nitric oxide synthase in cerebral blood flow changes during kainate seizures: A genetic approach using knockout mice

The role of endothelial nitric oxide (NO) in the cerebrovascular response to partial seizures was investigated in mice deleted for the endothelial NO synthase gene (eNOS-/-) and in their paired wild-type (WT) congeners. Local cerebral blood flow (LCBF, quantitative [14C]iodoantipyrine method) was measured 3-6 h after unilateral kainate (KA) injection in the dorsal hippocampus; controls received saline. In WT mice, KA seizures induced a 22 to 50% LCBF increase restricted to the ipsilateral hippocampus, while significant LCBF decreases (15-33%) were noticed in 22% of the contralateral areas, i.e., the parietal cortex, amygdala and three basal ganglia areas, compared to saline-injected WT mice. In eNOS-/- mice, no LCBF increases were recorded within the epileptic focus and generalized contralateral LCBF decreases (22-46%) were noticed in 2/3 of the brain areas, compared to saline-injected eNOS-/- mice. Thus, endothelial NO is the mediator of the cerebrovascular response within the epileptic focus and participates in the maintenance of LCBF in distant areas.

In vivo X-ray angiography in the mouse brain using synchrotron radiation

BACKGROUND AND PURPOSE

We, for the first time, performed in vivo x-ray angiography in the mouse brain using SPring-8, a third-generation synchrotron radiation facility.

METHODS

A thin PE-50 tube was placed in the unilateral external carotid artery in adult male C57BL/6J mice. While maintaining the blood flow in the internal carotid artery, 33 muL of contrast agent was injected and then selective angiography of the hemisphere was performed.

RESULTS

The average diameters of cerebral artery were as follows: 142.5+/-7.90 microm in middle cerebral artery, 138.3+/-9.35 microm in anterior cerebral artery, 120.5+/-5.53 microm in posterior cerebral artery, and 162.6+/-10.87 microm in internal carotid artery (n=5). To demonstrate the changes in diameter, we induced hypercapnia and detected the dilatation of the vessels between 121% and 124% of the original diameters (n=5). We also repeated angiography in the mice before and after intracarotid injection of vasodilatation drugs papaverine hydrochloride, ATP disodium, and fasudil hydrochloride hydrate and demonstrated the chronological changes in the diameters in each artery at 1, 5, 15, and 30 minutes after injection (n=1 for each drug).

CONCLUSIONS

Using only a minimum volume of the contrast agent, synchrotron radiation enables us to study x-ray angiography in the mouse brain. The morphology of the vessels can be clearly observed under physiological conditions. The diameters and their changes can also be successfully studied in vivo.

Vascular reactivity and endothelial NOS activity in rat thoracic aorta during and after hyperbaric oxygen exposure

Accumulating evidence suggests that hyperbaric oxygen (HBO) stimulates neuronal nitric oxide (NO) synthase (NOS) activity, but the influence on endothelial NOS (eNOS) activity and vascular NO bioavailability remains unclear. We used a bioassay employing rat aortic rings to evaluate vascular NO bioavailability. HBO exposure to 2.8 atm absolute (ATA) in vitro decreased ACh relaxation. This effect remained unchanged, despite treatment with SOD-polyethylene glycol and catalase-polyethylene glycol, suggesting that the reduction in endothelium-derived NO bioavailability was independent of superoxide production. In vitro HBO induced contraction of resting aortic rings with and without endothelium, and these contractions were reduced by the NOS inhibitor N(omega)-nitro-l-arginine. In addition, in vitro HBO attenuated the vascular contraction produced by norepinephrine, and this effect was reversed by N(omega)-nitro-l-arginine, but not by endothelial denudation. These findings indicate stimulation of extraendothelial NO production during HBO exposure. A radiochemical assay was used to assess NOS activity in rat aortic endothelial cells. Catalytic activity of eNOS in cell homogenates was not decreased by HBO, and in vivo HBO exposure to 2.8 ATA was without effect on eNOS activity and/or vascular NO bioavailability in vitro. We conclude that HBO reduces endothelium-derived NO bioavailability independent of superoxide production, and this effect seems to be unrelated to a decrease in eNOS catalytic activity. In addition, HBO increases the resting tone of rat aortic rings and attenuates the contractile response to norepinephrine by endothelium-independent mechanisms that involve extraendothelial NO production.

Extension of brain tolerance to hyperbaric O2 by intermittent air breaks is related to the time of CBF increase

Intermittent air breaks during hyperbaric oxygen (HBO2) exposures protect against pulmonary and central nervous system (CNS) toxicity. The mechanisms of this beneficial effect from intermittency are not known. In this study, we examined if release of vasoconstriction during HBO2 exposure indicates a threshold for toxic dose of HBO2 and how it may be related to tolerance by intermittency. Awake rats instrumented for EEG and cerebral blood flow (CBF) measurement were exposed to 100% O2 at 6 ATA (absolute pressure). Air breaks of 3 or 10 min were given at different times after CBF increase. Following the air break, animals were exposed to 100% O2 until seizure and total O2 time was used to calculate benefit/toxicity. The most beneficial schedule was then used to assess the role of the multiple air breaks in extension of HBO2 tolerance. A significant increase in seizure latency was observed in animals with a single 3- or 10 min air break given 5-10 min after CBF increase. No change in seizure latency was observed when air breaks were given beyond (>10 and <5 min) this window. The duration of total O2 time to seizures was doubled with multiple 3 min air breaks, and quadrupled with 10 min air breaks compared with continuous HBO2 exposures. With more time spent on O2, the duration of air breaks was not sufficient for recovery from O2 toxicity and for CBF to return to baseline. Results show that an "optimal window" of HBO2 exposure is required for benefits by intermittent exposure to air.

Endothelial influences on cerebrovascular tone

The cerebrovascular endothelium exerts a profound influence on cerebral vessels and cerebral blood flow. This review summarizes current knowledge of various dilator and constrictor mechanisms intrinsic to the cerebrovascular endothelium. The endothelium contributes to the resting tone of cerebral arteries and arterioles by tonically releasing nitric oxide (NO•). Dilations can occur by stimulated release of NO•, endothelium-derived hyperpolarization factor, or prostanoids. During pathological conditions, the dilator influence of the endothelium can turn to that of constriction by a variety of mechanisms, including decreased NO• bioavailability and release of endothelin-1. The endothelium may participate in neurovascular coupling by conducting local dilations to upstream arteries. Further study of the cerebrovascular endothelium is critical for understanding the pathogenesis of a number of pathological conditions, including stroke, traumatic brain injury, and subarachnoid hemorrhage.

Cerebral blood flow and brain oxygenation in rats breathing oxygen under pressure

Hyperbaric oxygen (HBO(2)) increases oxygen tension (PO(2)) in blood but reduces blood flow by means of O(2)-induced vasoconstriction. Here we report the first quantitative evaluation of these opposing effects on tissue PO(2) in brain, using anesthetized rats exposed to HBO(2) at 2 to 6 atmospheres absolute (ATA). We assessed the contribution of regional cerebral blood flow (rCBF) to brain PO(2) as inspired PO(2) (PiO(2)) exceeds 1 ATA. We measured rCBF and local PO(2) simultaneously in striatum using collocated platinum electrodes. Cerebral blood flow was computed from H(2) clearance curves in vivo and PO(2) from electrodes calibrated in vitro, before and after insertion. Arterial PCO(2) was controlled, and body temperature, blood pressure, and EEG were monitored. Scatter plots of rCBF versus PO(2) were nonlinear (R(2)=0.75) for rats breathing room air but nearly linear (R(2)=0.88-0.91) for O(2) at 2 to 6 ATA. The contribution of rCBF to brain PO(2) was estimated at constant inspired PO(2), by increasing rCBF with acetazolamide (AZA) or decreasing it with N-nitro-L-arginine methyl ester (L-NAME). At basal rCBF (78 mL/100 g min), local PO(2) increased 7- to 33-fold at 2 to 6 ATA, compared with room air. A doubling of rCBF increased striatal PO(2) not quite two-fold in rats breathing room air but 13- to 64-fold in those breathing HBO(2) at 2 to 6 ATA. These findings support our hypothesis that HBO(2) increases PO(2) in brain in direct proportion to rCBF.

Effects of 7-nitroindazole and N-nitro-l-arginine methyl ester on changes in cerebral blood flow and nitric oxide production preceding development of hyperbaric oxygen-induced seizures in rats

Hyperbaric oxygen (HBO(2)) exposure induces increases in cerebral blood flow (CBF) and extracellular concentrations of nitric oxide (NO) that precede the appearance of central nervous system toxicity, which may manifest as convulsions. To elucidate the origins of NO production during HBO(2) exposure, we examined the effects of the selective neuronal NO synthase (NOS) inhibitor, 7-nitroindazole (7-NI), and the non-selective NOS inhibitor, N-nitro-l-arginine methyl ester (l-NAME), on changes in CBF and NO metabolites (NO(x), nitrite and nitrate) using a laser Doppler flow probe and in vivo microdialysis techniques, respectively. Rats were anesthetized, artificially ventilated, and pressurized to 5 atmosphere absolute (ATA) with pure oxygen for 60 min. In rats treated with vehicle, CBF and NO(x) levels in the cortex increased to 201% and 239% of basal levels, respectively, before the onset of electrical discharges, measured by electroencephalogram. The increase in CBF and NO(x) was completely inhibited by 7-NI and l-NAME. Both drugs also inhibited the appearance of electrical discharges for 60 min. Dynamic changes in CBF and NO(x) were not significantly different between 7-NI and l-NAME. These findings suggest that neuronal NOS is the main mediator of NO production associated with increase in CBF leading to the appearance of electrical discharge during HBO(2) exposure.

Partial Seizure Provoked by Hyperbaric Oxygen Therapy: Possible Mechanisms and Implications

Hyperbaric oxygen treatment (HBO2) is used commonly for treatment of bone and soft-tissue radiation necrosis. It may be a potential therapy for radiation necrosis seen after brain irradiation. HBO2 risks include generalized tonic-clonic convulsions. We report a patient after resection of anaplastic astrocytoma and 5,580 cGy of total external-beam radiation treatments with brain radiation necrosis who underwent HBO2 therapy and developed a partial seizure during treatment. Mechanisms and implications are discussed.

Role of nitric oxide in cerebral blood flow changes during kainate seizures in mice: genetic and pharmacological approaches

The role of neuronal nitric oxide (NO) in the cerebrovascular response to partial seizures induced by intrahippocampal injection of kainate (KA) was investigated in mice deleted for the neuronal NO synthase gene (nNOS-/-) and in wild-type controls (WT). A second group of WT mice received the nNOS inhibitor, 7-nitroindazole (WT-7NI). Local cerebral blood flow (LCBF) was measured using the quantitative (14)C-iodoantipyrine method. Within the epileptic focus, all three groups of seizing mice (WT, WT-7NI, and nNOS-/-) showed significant 26-88% LCBF increases in ipsilateral hippocampus, compared to saline-injected mice. Contralaterally to the epileptic focus, KA seizures induced a 21-47% LCBF decreases in hippocampus and limbic cortex of WT mice and in most contralateral brain structures of nNOS-/- mice, while WT-7NI mice showed no contralateral CBF change. Neuronal NO appears to be not involved in the cerebrovascular response within the epileptic focus, but may rather have a role in the maintenance of distant LCBF regulation during seizures.

Redox-dependent effects of nitric oxide on microvascular integrity in oxygen-induced retinopathy

Opposing effects have been ascribed to nitric oxide (NO) on retinal microvascular survival. We investigated whether changes in the redox state may contribute to explain apparent conflicting actions of NO in a model of oxygen-induced retinal vasoobliteration. Retinal microvascular obliteration was induced by exposing 7-day-old rat pups (P7) for 2 or 5 days to 80% O(2). The redox state of the retina was assessed by measuring reduced glutathione and oxidative and nitrosative products malondialdehyde and nitrotyrosine. The role of NO on vasoobliteration was evaluated by treating animals with nitric oxide synthase (NOS) inhibitors (N-nitro-l-arginine; L-NA) and by determining NOS isoform expression and activity; the contribution of nitrosative stress was also determined in animals treated with the degradation catalyst of peroxynitrite FeTPPS or with the superoxide dismutase mimetic CuDIPS. eNOS, but not nNOS or iNOS, expression and activity were increased throughout the exposure to hyperoxia. These changes were associated with an early (2 days hyperoxia) decrease in reduced glutathione and increases in malondialdehyde and nitrotyrosine. CuDIPS, FeTPPS, and L-NA treatments for these 2 days of hyperoxia nearly abolished the vasoobliteration. In contrast, during 5 days exposure to hyperoxia when the redox state rebalanced, L-NA treatment aggravated the vasoobliteration. Interestingly, VEGFR-2 expression was respectively increased by NOS inhibition after short-term (2 days) exposure to hyperoxia and decreased during the longer hyperoxia exposure. Data disclose that the dual effects of NO on newborn retinal microvascular integrity in response to hyperoxia in vivo depend on the redox state and seem mediated at least in part by VEGFR-2.