The effect of vigabatrin on central nervous system oxygen toxicity in rats

CNS function and dysfunction during exposure to hyperbaric oxygen in operational and clinical settings

Hyperbaric oxygen (HBO2) is breathed during hyperbaric oxygen therapy and during certain undersea pursuits in diving and submarine operations. What limits exposure to HBO2 in these situations is the acute onset of central nervous system oxygen toxicity (CNS-OT) following a latent period of safe oxygen breathing. CNS-OT presents as various non-convulsive signs and symptoms, many of which appear to be of brainstem origin involving cranial nerve nuclei and autonomic and cardiorespiratory centers, which ultimately spread to higher cortical centers and terminate as generalized tonic-clonic seizures. The initial safe latent period makes the use of HBO2 practical in hyperbaric and undersea medicine; however, the latent period is highly variable between individuals and within the same individual on different days, making it difficult to predict onset of toxic indications. Consequently, currently accepted guidelines for safe HBO2 exposure are highly conservative. This review examines the disorder of CNS-OT and summarizes current ideas on its underlying pathophysiology, including specific areas of the CNS and fundamental neural and redox signaling mechanisms that are thought to be involved in seizure genesis and propagation. In addition, conditions that accelerate the onset of seizures are discussed, as are current mitigation strategies under investigation for neuroprotection against redox stress while breathing HBO2 that extend the latent period, thus enabling safer and longer exposures for diving and medical therapies.

Increased Antiseizure Effectiveness with Tiagabine Combined with Sodium Channel Antagonists in Mice Exposed to Hyperbaric Oxygen

Hyperbaric oxygen (HBO₂) is acutely toxic to the central nervous system, culminating in EEG spikes and tonic-clonic convulsions. GABA enhancers and sodium channel antagonists improve seizure latencies in HBO₂ when administered individually, while combining antiepileptic drugs from different functional classes can provide greater seizure latency. We examined the combined effectiveness of GABA enhancers (tiagabine and gabapentin) with sodium channel antagonists (carbamazepine and lamotrigine) in delaying HBO₂-induced seizures. A series of experiments in C57BL/6 mice exposed to 100% oxygen at 5 atmospheres absolute (ATA) were performed. We predicted equally effective doses from individual drug-dose response curves, and the combinations of tiagabine + carbamazepine or lamotrigine were tested to determine the maximally effective combined doses to be used in subsequent experiments designed to identify the type of pharmacodynamic interaction for three fixed-ratio combinations (1:3, 1:1, and 3:1) using isobolographic analysis. For both combinations, the maximally effective combined doses increased seizure latency over controls > 5-fold and were determined to interact synergistically for fixed ratios 1:1 and 3:1, additive for 1:3. These results led us to explore whether the benefits of these drug combinations could be extended to the lungs, since a centrally mediated mechanism is believed to mediate hyperoxic-induced cardiogenic lung injury. Indeed, both combinations attenuated bronchoalveolar lavage protein content by ~ 50%. Combining tiagabine with carbamazepine or lamotrigine not only affords greater antiseizure protection in HBO₂ but also allows for lower doses to be used, minimizing side effects, and attenuating acute lung injury.

Delaying latency to hyperbaric oxygen-induced CNS oxygen toxicity seizures by combinations of exogenous ketone supplements

Central nervous system oxygen toxicity (CNS-OT) manifests as tonic-clonic seizures and is a limitation of hyperbaric oxygen therapy (HBOT), as well as of recreational and technical diving associated with elevated partial pressure of oxygen. A previous study showed that ketone ester (1,3-butanediol acetoacetate diester, KE) administration delayed latency to seizures (LS) in 3-month-old Sprague-Dawley (SD) rats. This study explores the effect of exogenous ketone supplements in additional dosages and formulations on CNS-OT seizures in 18 months old SD rats, an age group correlating to human middle age. Ketogenic agents were given orally 60 min prior to exposure to hyperbaric oxygen and included control (water), KE (10 g/kg), KE/2 (KE 5 g/kg + water 5 g/kg), KE + medium-chain triglycerides (KE 5 g/kg + MCT 5 g/kg), and ketone salt (Na+ /K+ βHB, KS) + MCT (KS 5 g/kg + MCT 5 g/kg). Rats were exposed to 100% oxygen at 5 atmospheres absolute (ATA). Upon seizure presentation (tonic-clonic movements) experiments were immediately terminated and blood was tested for glucose and D-beta-hydroxybutyrate (D-βHB) levels. While blood D-βHB levels were significantly elevated post-dive in all treatment groups, LS was significantly delayed only in KE (P = 0.0003), KE/2 (P = 0.023), and KE + MCT (P = 0.028) groups. In these groups, the severity of seizures appeared to be reduced, although these changes were significant only in KE-treated animals (P = 0.015). Acetoacetate (AcAc) levels were also significantly elevated in KE-treated animals. The LS in 18-month-old rats was delayed by 179% in KE, 219% in KE + MCT, and 55% in KE/2 groups, while only by 29% in KS + MCT. In conclusion, KE supplementation given alone and in combination with MCT elevated both βHB and AcAc, and delayed CNS-OT seizures.

S-nitrosylation of GAD65 is implicated in decreased GAD activity and oxygen-induced seizures

Breathing oxygen at partial pressures ≥2.5 atmospheres absolute, which can occur in diving and hyperbaric oxygen (HBO₂) therapy, can rapidly become toxic to the central nervous system (CNS). This neurotoxicity culminates in generalized EEG epileptiform discharges, tonic-clonic convulsions and ultimately death. Increased production of neuronal nitric oxide (NO) has been implicated in eliciting hyperoxic seizures by altering the equilibrium between glutamatergic and GABAergic synaptic transmission. Inhibition of glutamic acid decarboxylase (GAD) activity in HBO₂ promotes this imbalance; however, the mechanisms by which this occurs is unknown. Therefore, we conducted a series of experiments using mice, a species that is highly susceptible to CNS oxygen toxicity, to explore the possibility that NO modulates GABA metabolism. Mice were exposed to 100% oxygen at 4 ATA for various durations, and brain GAD and GABA transaminase (GABA-T) activity, as well as S-nitrosylation of GAD65 and GAD67 were determined. HBO₂ inhibited GAD activity by 50% and this was negatively correlated with S-nitrosylation of GAD65, whereas GABA-T activity and S-nitrosylation of GAD67 were unaltered. These results suggest a new mechanism by which NO alters GABA metabolism, leading to neuroexcitation and seizures in HBO₂.

Central Nervous System Oxygen Toxicity and Hyperbaric Oxygen Seizures

INTRODUCTION

The use of hyperbaric oxygen (O2) as a therapeutic agent carries with it the risk of central nervous system (CNS) O2 toxicity.

METHODS

To further the understanding of this risk and the nature of its molecular mechanism, a review was conducted on the literature from various fields.

RESULTS

Numerous physiological changes are produced by increased partial pressures of oxygen (Po2), which may ultimately result in CNS O2 toxicity. The human body has several equilibrated safeguards that minimize effects of reactive species on neural networks, believed to play a primary role in CNS O2 toxicity. Increased partial pressure of oxygen (Po2) appears to saturate protective enzymes and unfavorably shift protective reactions in the direction of neural network overstimulation. Certain regions of the CNS appear more susceptible than others to these effects. Failure to decrease the elevated Po2 can result in a tonic-clonic seizure and death. Randomized, controlled studies in human populations would require a multicenter trial over a long period of time with numerous endpoints used to identify O2 toxicity.

CONCLUSIONS

The mounting scientific evidence and apparent increase in the number of hyperbaric O2 treatments demonstrate a need for further study in the near future.

Vigabatrin prevents seizure in swine subjected to hyperbaric hyperoxia

Oxygen is the most widely used therapeutic strategy to prevent and treat decompression sickness (DCS). Oxygen prebreathe (OPB) eliminated DCS in 20-kg swine after rapid decompression from saturation at 60 feet of seawater (fsw). However, hyperbaric oxygen (HBO) has risks. As oxygen partial pressure increases, so do its toxic effects. Central nervous system (CNS) oxygen toxicity is the most severe side effect, manifesting as seizure. An adjunctive therapeutic is needed to extend OPB strategies to deeper depths and prevent/delay seizure onset. The Food and Drug Administration-approved anti-epileptic vigabatrin has prevented HBO-induced seizures in rats up to 132 fsw. This study aimed to confirm the rat findings in a higher animal model and determine whether acute high-dose vigabatrin evokes retinotoxicity symptoms seen with chronic use clinically in humans. Vigabatrin dose escalation studies were conducted 20-kg swine exposed to HBO at 132 or 165 fsw. The saline group had seizure latencies of 7 and 11 min at 165 and 132 fsw, respectively. Vigabatrin at 180 mg/kg significantly increased latency (13 and 27 min at 165 and 132 fsw, respectively); 250 mg/kg abolished seizure activity at all depths. Functional electroretinogram and histology of the retinas showed no signs of retinal toxicity in any of the vigabatrin=treated animals. In the 250 mg/kg group there was no evidence of CNS oxygen toxicity; however, pulmonary oxygen toxicity limited HBO exposure. Together, the findings from this study show that vigabatrin therapy is efficacious at preventing CNS oxygen toxicity in swine, and a single dose is not acutely associated with retinotoxicity.

Therapeutic ketosis with ketone ester delays central nervous system oxygen toxicity seizures in rats

Central nervous system oxygen toxicity (CNS-OT) seizures occur with little or no warning, and no effective mitigation strategy has been identified. Ketogenic diets (KD) elevate blood ketones and have successfully treated drug-resistant epilepsy. We hypothesized that a ketone ester given orally as R,S-1,3-butanediol acetoacetate diester (BD-AcAc(2)) would delay CNS-OT seizures in rats breathing hyperbaric oxygen (HBO(2)). Adult male rats (n = 60) were implanted with radiotelemetry units to measure electroencephalogram (EEG). One week postsurgery, rats were administered a single oral dose of BD-AcAc(2), 1,3-butanediol (BD), or water 30 min before being placed into a hyperbaric chamber and pressurized to 5 atmospheres absolute (ATA) O2. Latency to seizure (LS) was measured from the time maximum pressure was reached until the onset of increased EEG activity and tonic-clonic contractions. Blood was drawn at room pressure from an arterial catheter in an additional 18 animals that were administered the same compounds, and levels of glucose, pH, Po(2), Pco(2), β-hydroxybutyrate (BHB), acetoacetate (AcAc), and acetone were analyzed. BD-AcAc(2) caused a rapid (30 min) and sustained (>4 h) elevation of BHB (>3 mM) and AcAc (>3 mM), which exceeded values reported with a KD or starvation. BD-AcAc(2) increased LS by 574 ± 116% compared with control (water) and was due to the effect of AcAc and acetone but not BHB. BD produced ketosis in rats by elevating BHB (>5 mM), but AcAc and acetone remained low or undetectable. BD did not increase LS. In conclusion, acute oral administration of BD-AcAc(2) produced sustained ketosis and significantly delayed CNS-OT seizures by elevating AcAc and acetone.

Hyperoxic stimulation of synchronous orthodromic activity and induction of neural plasticity does not require changes in excitatory synaptic transmission

The first study, described in the companion article, reports that acute exposure of rat hippocampal slices to either hyperbaric oxygen (HBO: 2.84 and 4.54 atmospheres absolute, ATA) or normobaric reoxygenation (NBOreox; i.e., normobaric hyperoxia: 0.6 or 0.0→0.95 ATA) stimulates synchronous orthodromic activity in CA1 neurons, which includes activation of O2-induced potentiation (OxIP) and, in some cases, hyperexcitability (secondary population spikes, sPS). In this second study we tested the hypothesis that HBO and NBOreox increase orthodromic activity of CA1 neurons (oPS, orthodromic population spike) and OxIP via a combination of both increased excitatory synaptic transmission (field excitatory postsynaptic potential, fEPSP) and intrinsic excitability (antidromic population spike, aPS). HBO and NBOreox increased the oPS but rarely increased or potentiated the fEPSP. HBO exposure produced epileptiform antidromic activity, which was abolished during inhibition of fast GABAergic and glutamatergic synaptic transmission. Decreasing O2 from 0.95 ATA (control) to 0.6 ATA (intermediate O2) or 0.0 ATA (hypoxia) reversibly abolished the fEPSP, and reoxygenation rarely induced potentiation of the fEPSP or aPS. Intracellular recordings and antidromic field potential recordings, however, revealed that synaptic transmission and neuronal excitability were preserved, albeit at lower levels, in 0.60 ATA O2. Together, these data indicate that 1) the changes in excitatory postsynaptic activity are not required for stimulation of the oPS during and HBO/NBOreox or for activation of OxIP, suggesting the latter is a form of intrinsic plasticity; 2) HBO disinhibits spontaneous synaptic transmission to induce epileptiform activity; and 3) although synchronous synaptic activation of the CA1 neuronal population requires hyperoxia (i.e., 0.95 ATA O2), synaptic activation of individual CA1 neurons does not.

Rapid Decrease of GAD 67 Content Before the Convulsion Induced by Hyperbaric Oxygen Exposure

Exposure to hyperbaric oxygen (HBO) can lead to seizures, the etiology of which is not completely understood. Glutamic acid decarboxylase (GAD) plays a very important role in maintaining excitatory-inhibitory balance of the central nervous system (CNS). In the present study we investigated the effects of HBO on the activity and content of GAD in vivo and in primarily cultured neurons to probe in detail its effect on the formation of convulsion induced by HBO exposure. The results obtained from in vivo and in vitro experiments were identical. In the latent period before the onset of seizure, the GAD activity followed a rise-and-fall pattern with the prolongation of HBO exposure. At the time of the onset of seizure, GAD activity descended to the normal level. Besides, in the latent period, GAD content also reduced. Such reduction came from a GAD subtype, GAD67, while the content of another GAD subtype, GAD65, remained almost unchanged. Our investigations indicated that GAD is indeed an enzyme highly sensitive to the effect of HBO exposure. The rapid reduction in GAD67 content may be very closely related to seizures induced by HBO exposure.

Hyperbaric oxygen therapy in orthopedic conditions: an evaluation of safety

BACKGROUND

Because of the documented cellular and biochemical benefits of hyperbaric oxygen (HBO), HBO therapy is applied now with increasing frequency to various orthopedic conditions. Many traumatologists and orthopedic surgeons might refer their patients for adjuvant HBO therapy. However, the potential risks and risk-benefit ratio have often been underemphasized in therapeutic trials.

METHODS

From October 2002 to September 2004, 240 patients with a total of 4,638 treatments received HBO therapy at the hyperbaric medicine center of our institution on an identical treatment protocol. HBO therapy patient treatment logs were reviewed to analyze the incidence of complications during HBO treatment.

RESULTS

The overall incidence of complications was 1.83%. Over 94% of treatment complications were mild to moderate and designated as minor complications; fewer than 6% were severe or life threatening and designated as major complications. The incidence of major complications (central nervous system [CNS] oxygen toxicity in this series) was 0.109%. There was no mortality. Two patients with unusual presentation of CNS oxygen toxicity were observed during the study period.

CONCLUSIONS

HBO therapy in orthopedic conditions is considered as a safe treatment because of a very low complication rate; however, analysis of patients with CNS oxygen toxicity revealed its unpredictability and inevitability. Although it is common sense that patients who develop a seizure in the hospital need help from the medical staff, it cannot be done in a monoplace hyperbaric chamber because of pressure unequalization. Therefore, a multiplace chamber equipped with an antechamber for medical contingency is possibly the better facility in consideration of safety.

GABA shunt deficiencies and accumulation of reactive oxygen intermediates: insight fromArabidopsismutants

In plants, succinic semialdehyde dehydrogenase (SSADH)-deficiency results in the accumulation of reactive oxygen intermediates (ROI), necrotic lesions, dwarfism, and hypersensitivity to environmental stresses. We report that Arabidopsis ssadh knockout mutants contain five times the normal level of gamma-hydroxybutyrate (GHB), which in SSADH-deficient mammals accounts for phenotypic abnormalities. Moreover, the level of GHB in Arabidopsis is light dependent. Treatment with gamma-vinyl-gamma-aminobutyrate, a specific gamma-aminobutyrate (GABA)-transaminase inhibitor, prevents the accumulation of ROI and GHB in ssadh mutants, inhibits cell death, and improves growth. These results provide novel evidence for the relationship between the GABA shunt and ROI, which may, in part, explain the phenotype of SSADH-deficient plants and animals.

Attenuation of Cerebral Oxygen Toxicity by Sound Conditioning

HYPOTHESIS

Sound conditioning might reduce cerebral oxygen toxicity.

BACKGROUND

Cerebral oxygen toxicity is related to high levels of reactive oxygen species. Noise-induced hearing loss has been shown to result from ischemia-reperfusion, in which reactive oxygen species play a major role. Repeated exposure to loud noise at levels below that which produces permanent threshold shift prevented noise-induced hearing loss and was associated with significant elevation of the antioxidant enzymes measured in the inner ear. We tested the hypothesis that sound conditioning might reduce cerebral oxygen toxicity.

METHODS

Forty-five guinea pigs were prepared for electroencephalography and auditory brainstem recording. The auditory brainstem recording detection threshold was determined to confirm baseline normal hearing. The animals were divided into three equal groups and subjected to the following procedures: Group 1, electroencephalography electrode implantation and auditory brainstem recording only; Group 2, exposure to oxygen at 608 kPa (the latency to the first electrical discharge in the electroencephalogram preceding the appearance of seizures was measured); and Group 3, sound conditioning followed by oxygen exposure. The animals were killed, and the brains were excised and homogenized. Brain levels of superoxide dismutase, catalase, glutathione peroxidase, glutathione transferase, glutathione reductase, glucose-6-phosphate dehydrogenase, and thiobarbituric acid reactive substances were compared among the groups.

RESULTS

Latency to the first electrical discharge was compared between Groups 2 and 3, and was found to be significantly longer in Group 3 (27.9 +/- 11 versus 20.4 +/- 7.6 min, p < 0.03). No significant changes were found in brain levels of superoxide dismutase, catalase, glutathione peroxidase, glutathione transferase, glutathione reductase, glucose-6-phosphate dehydrogenase, or thiobarbituric acid reactive substances.

CONCLUSION

Our data show that sound conditioning prolongs the latency to oxygen-induced convulsions. This effect was not accompanied by significant changes in whole-brain antioxidant enzyme activity or the magnitude of lipid peroxidation.

Eubaric hyperoxemia and experimental cerebral infarction

We explore three questions concerning arterial hyperoxygenation and focal ischemia. (1) Does greater benefit accrue with higher levels of arterial hyperoxemia? (2) Is the net effect of continuous (intraischemic plus postischemic) oxygen therapy toxic, or beneficial to middle cerebral artery infarction? (3) In view of free radical theories of reperfusion injury, does hyperoxia isolated to the reperfusion period damage tissue? Rats subjected to transient, focal, normothermic, normoglycemic ischemia were assessed at 2 weeks' survival. Arterial hyperoxygenation from 98.9 +/- 4.0 to 312.2 +/- 48.4mm Hg during ischemia improved (p < 0.05) neurological function, as did isolated reperfusion hyperoxemia, but treatment with continuous hyperoxemia both during and after ischemia yielded greatest benefit (p < 0.001). Cortical infarcts constituted 6.5 +/- 1.8% of the hemisphere at normoxia, but 2.3 +/- 0.9% at hyperoxic levels (p < 0.01). Hyperoxia isolated to the reperfusion period also reduced cortical necrosis, from 6.5% to 2.7 +/- 1.2%. However, continuous intraischemic and reperfusion hyperoxemia led to only 0.2 +/- 0.1% cortical necrosis (p = 0.0005). Increasing the degree of hyperoxemia did not augment the benefit. We conclude that (1) eubaric hyperoxemia improves neurological and neuropathological outcome, (2) continuous oxygen therapy offers the greatest benefit, and (3) reperfusion hyperoxemia is beneficial. The findings should allay clinical concerns regarding oxygen-induced reperfusion injury, and, by obviating hyperbaric chambers, encourage clinical trials studying arterial hyperoxemia in treating stroke.

Hyperbaric oxygenation prevents delayed neuronal death following transient ischaemia in the gerbil hippocampus

The mechanism of the neuroprotective effect of hyperbaric oxygenation remains unclear although its clinical benefits have been well recognized for human ischaemic neuronal disease. The preventive effect of hyperbaric oxygenation against delayed neuronal death was investigated in the gerbil following transient forebrain ischaemia. Delayed neuronal death in the gerbil was produced by clips on both the common carotid arteries (10 min). Morphological examination was carried out after several protocols of hyperbaric oxygenation, modified from the protocols for human ischaemic neuronal disease. Neurons in the hippocampal CA1 were well preserved in the gerbils treated with hyperbaric oxygenation, more so than in the gerbils with no hyperbaric oxygenation. Moreover, more neurons were preserved in the CA1 treated with hyperbaric oxygenation within 6 h of the ischaemia, than when the hyperbaric oxygenation was started 24 h after the ischaemia. The induction of heat shock proteins (HSP72 and HSP27) became weaker in the gerbils with hyperbaric oxygenation than in those without hyperbaric oxygenation, as seen immunohistochemically. We also observed an increase in dense bodies, that were shown to be lysosomes and myelinoid structures in the cytoplasm of the neurons ultrastructurally, in the hippocampus with hyperbaric oxygenation. However, no oxygen toxicity to the neurons was detected, up to at least two atmospheres absolute. This experimental system was useful to investigate the preventive mechanism of hyperbaric oxygenation against delayed neuronal death in the gerbil, and to determine the clinical indications and the most effective protocol for hyperbaric oxygenation for ischaemic neuronal damage in the human brain.

Caffeine attenuates CNS oxygen toxicity in rats

We tested the effect of caffeine, on hyperoxia-induced seizures. Thirty-seven rats with chronic cortical electrodes were injected i.p. with caffeine (1.25, 2.5, and 10 mg/kg) or vehicle before exposure to 0.5 MPa oxygen and 17 rats to oxygen with 5% CO2 at 0.5 MPa. EEG monitoring and spectral analysis of EEG activity were carried out. Caffeine significantly prolonged the latent period to the onset of seizures (P < 0.05 in ANOVA), in a dose-related manner. Our results suggest that caffeine may be used in low doses for protection against hyperoxia-induced seizures.