Blockade of hyperbaric oxygen induced seizures by excitatory amino acid antagonists

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.

Dopamine, Neurochemical Processes, and Oxygen Toxicity at Pressure

All mammals, including man, exposed to breathing gas mixtures at high pressures exhibit central nervous system disturbances, which differ according to the gas used. With the use of compressed air, the increased oxygen partial pressure induces hyperoxic disturbances that consist of epileptic seizures that occur, on average, after 30 min exposure to 2.8 ATA in man or to 5 ATA in rats. Increased oxygen partial pressure induces reactive oxygen species and reactive nitrogen species production that could be related to neurotransmitter changes reported for the preepileptic phase or at pressures that produce epileptic seizures. In rats, oxygen pressures lower than 5 ATA induce a decrease of dopamine release in the stratum that could be due to disturbances of neurotransmitter regulatory processes that are different from those implicated for hyperbaric oxygen-induced epileptic seizures. © 2016 American Physiological Society. Compr Physiol 6:1339-1344, 2016.

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.

Alteration of striatal dopamine levels under various partial pressure of oxygen in pre-convulsive and convulsive phases in freely-moving rats

The purpose of this study was to investigate the change in the striatal dopamine (DA) level in freely-moving rat exposed to different partial pressure of oxygen (from 1 to 5 ATA). Some works have suggested that DA release by the substantia nigra pars compacta (SNc) neurons in the striatum could be disturbed by hyperbaric oxygen (HBO) exposure, altering therefore the basal ganglia activity. Such changes could result in a change in glutamatergic and GABAergic control of the dopaminergic neurons into the SNc. Such alterations could provide more information about the oxygen-induced seizures observed at 5 ATA in rat. DA-sensitive electrodes were implanted into the striatum under general anesthesia. After 1 week rest, awaked rats were exposed to oxygen-nitrogen mixture at a partial pressure of oxygen of 1, 2, 3, 4 and 5 ATA. DA level was monitored continuously (every 3 min) by in vivo voltammetry before and during HBO exposure. HBO induced a decrease in DA level in relationship to the increase in partial pressure of oxygen from 1 ATA to 4 ATA (-15 % at 1 ATA, -30 % at 2 ATA, -40 % at 3 ATA, -45 % at 4 ATA), without signs of oxygen toxicity. At 5 ATA, DA level strongly decreases (-75 %) before seizure which occurred after 27 min ± 7 HBO exposure. After the epileptic seizure the decrease in DA level disappeared. These changes and the biphasic effect of HBO were discussed in function of HBO action on neurochemical regulations of the nigro striatal pathway.

Reactive astrocytes contribute to increased epileptic susceptibility induced by subthreshold dose of pilocarpine

Seizures may influence epileptogenesis, but it is not yet clearly established whether subthreshold stimulations that are not sufficient to induce visible behavioral seizures change epileptic susceptibility, and the possible underlying mechanisms have not been completely understood. We assessed the susceptibility to epilepsy after subthreshold dose of pilocarpine, as well as glial fibrillary acidic protein (GFAP) expression using immunohistochemistry. An increase in the susceptibility to pentylenetetrazole (PTZ)-induced seizures was observed in rats previously subjected to subthreshold dose of pilocarpine. The immunoreactivity of GFAP was also increased, indicating that astrocytes became reactive in some brain subfields. The increased epileptic susceptibility was significantly reduced by L-alpha-aminoadipic acid (L-AAA), an inhibitor of astrocytic function. Our results suggest that subthreshold stimulation may increase the susceptibility to subsequent development of epilepsy, and reactive astrocytes might be an important contributor to this process. Adequate inhibition of astrocytic function may be a potential preventive approach against epileptogenesis.

Prolonged exposure to hyperbaric oxygen induces neuronal damage in primary rat cortical cultures

While seizure attack is one of the serious complications during the hyperbaric oxygen (HBO) therapy, there is still no direct evidence showing that HBO can induce neuronal damage in the brain. The objective of this study was first to investigate whether HBO would lead to neurotoxicity in the primary rat cortical culture. Second, since alterations in neurotransmitters have been suggested in the pathophysiology of central nervous system (CNS) oxygen toxicity, the protective effects of the N-methyl-D-aspartate (NMDA) receptor antagonism and nitric oxide (NO) synthase inhibition on the HBO-induced neuronal damage were examined. The results showed that HBO exposure to 6 atmosphere absolute pressure (ATA) for 30, 60, and 90 min increased the lactate dehydrogenase (LDH) activity in the culture medium in a time-dependent manner. Accordingly, the cell survival, measured by the 3,(4,5-dimethyl-2-thiazolyl)2, 5-diphenyl-tetrazolium bromide (MTT) assay, was decreased after HBO exposure. Pretreatment with the NMDA antagonist MK-801 protected the cells against the HBO-induced damage. The protective effect was also noted in the cells pretreated with L-N(G)-nitro-arginine methyl ester, an NO synthase inhibitor. Thus, our results suggest that activation of NMDA receptors and production of NO play a role in the neurotoxicity produced by hyperbaric oxygen exposure.

Effect of MK-801 on seizures induced by exposure to hyperbaric oxygen: comparison with AP-7

The effect of the noncompetitive N-methyl-d-aspartate (NMDA)-receptor antagonist MK-801 on seizures induced by hyperbaric oxygen in relation to changes in cerebral blood flow (CBF) was investigated. Rats were injected with MK-801 (0.005-8 mg/kg) 30 min before exposure to 100% O2 at 5 atm (gauge pressure). MK-801 administration resulted in a biphasic response in seizure latency. Doses of 0.1-4 mg/kg significantly decreased time to EEG and motor seizures, while 8 mg/kg had no effect on seizure latency. MK-801 had no effect on seizure duration. In a dose range 0.1-8 mg/kg MK-801 increased CBF in awake animals, which might be responsible for the decreased seizure latency. The gradual increase in seizure latency with increasing MK-801 doses suggests involvement of an additional factor probably related to the drug's anticonvulsive effect. Unlike MK-801, a competitive NMDA receptor antagonist, AP-7, at a dose 250 mg/kg had no effect on latency to seizures or CBF.

Role of cerebral blood flow in seizures from hyperbaric oxygen exposure

Hyperbaric O2 exposure causes seizures by an unknown mechanism. Cerebral blood flow (CBF) may affect seizure latency, although no studies have demonstrated a direct relationship. Awake rats (male, Sprague-Dawley, 350-450 g), instrumented for measuring electroencephalographic activity (EEG) and CBF (laser-Doppler flowmetry), were exposed to 100% O2 at 4 or 5 atm (gauge pressure) until EEG seizures. Compression with O2 caused vasoconstriction to about 70% of control flow that was maintained for various times. CBF then suddenly, but transiently, increased at a time that was reliably related to seizure latency (r=0.8, p<0.01). Additional animals were treated with agents that have diverse pharmacology and their effects on CBF and latency were measured. Glutamate receptor antagonists MK-801 (1 or 4 mg/kg) and ketamine (20-100 mg/kg) significantly increased CBF by 60-80% and decreased seizure latency from about 17+/-8 min (+/-S.D.) in controls to 5+/-1 and 6+/-2 min, respectively. In opposite, a nitric oxide synthase (NOS) inhibitor, N-nitro-L-arginine (NNA)(25 mg/kg) decreased CBF by about 25% and increased time to seizure to 60+/-16 min. If these effects occur in humans, non-invasive measurement of CBF could potentially improve the safety and reliability of hyperbaric O2 usage in clinical and diving applications. It also appears that the effect of drugs on seizure latency can be explained, at least in part, by their effect on CBF.

Amino acids and ammonia in the cerebral cortex, the corpus striatum and the brain stem of the mouse prior to the onset and after a seizure induced by hyperbaric oxygen

The contents of amino acids (AA) and ammonia (NH3) were measured in corpus striatum, brain stem and cerebral cortex of two strains of mice exposed to hyperbaric oxygen (HBO). Mice of the HBO-sensitive strain (CD1) were exposed to 600 kPa O2 for 24 min versus 90 min for mice of the normal C57 strain, so that 50% of the mice in both strains developed a generalized convulsion. In the cortex of exposed but unconvulsed (EXUN) C57 mice, the contents of taurine, glutamine and NH3 increased while that of GABA decreased when compared to control mice. In the CD1 mice, NH3 content was increased while that of Asp decreased. After a convulsion, NH3 was increased in both strains, the AA contents returned to normal in C57 but Asp remained low in CD1 mice. Somewhat similar changes occurred in the striatum except that NH3 levels were less affected while GABA ones were significantly decreased in the CD1 mice exposed to HBO, whether convulsed or not. In the EXUN brain stem, Asp and Glu contents decreased. These decreases were greater in C57 on a percentage basis than in CD1 mice. GABA content was decreased in the C57 strain. After a convulsion, Asp and Glu levels remained low and NH3 accumulated in CD1 whereas in C57 only the Glu level was decreased. The cortical and striatal changes may indicate a lesser GABA supply in C57 strain and some Asp release in CD1 strain. In the brain stem of both strains, Asp and Glu release is possible in addition to GABA in C57 strain.(ABSTRACT TRUNCATED AT 250 WORDS)

Extracellular superoxide dismutase, nitric oxide, and central nervous system O2 toxicity

Although reactive O2 species appear to participate in central nervous system (CNS) O2 toxicity, the exact roles of different reactive O2 species are undetermined. To study the contribution of extracellular superoxide anion (O2-) to CNS O2 toxicity we constructed transgenic mice overexpressing human extracellular superoxide dismutase (ECSOD; superoxide:superoxide oxidoreductase, EC 1.15.1.1) in the brain. Remarkably, when exposed to 6 atm (1 atm = 101.3 kPA) of hyperbaric oxygen for 25 min, transgenic mice demonstrated higher mortality (83%) than nontransgenic litter-mates (33%; P < 0.017). Pretreatment with diethyldithiocarbamate, which inhibits both ECSOD and Cu/Zn superoxide dismutase (Cu/Zn SOD) activity, increased resistance to CNS O2 toxicity, in terms of both survival (100% in transgenics and 93% in nontransgenics) and resistance to seizures (4-fold increase in seizure latency in both transgenic and nontransgenic mice; P < 0.05). Thus, O2- apparently protects against CNS O2 toxicity. We hypothesized that O2- decreased toxicity by inactivating nitric oxide (NO.). To test this, we inhibited NO. synthase (EC 1.14.23) with N omega-nitro-L-arginine to determine whether NO. contributes to enhanced CNS O2 toxicity in transgenic mice. N omega-nitro-L-arginine protected both transgenic and nontransgenic mice against CNS O2 toxicity (100% survival and a 4-fold delay in time to first seizure; P < 0.05), as well as abolishing the difference in sensitivity to CNS O2 toxicity between transgenic and nontransgenic mice. These results implicate NO. as an important mediator in CNS O2 toxicity and suggest that ECSOD increases CNS O2 toxicity by inhibiting O2(-)-mediated inactivation of NO.

Spinal seizures evoked by sudden cooling of amphibian isolated spinal cords: involvement of excitatory amino acids

Sudden cooling of the isolated spinal cord of frogs results in characteristic seizure-like activity in the hind legs. In the present investigation, these spinal seizures induced by sudden cooling (SSSC) were studied to determine whether excitatory amino acids (EAAs) are involved in the mediation of this activity. The nonspecific EAA antagonist, L-glutamic acid diethyl ester and cis-2,3-piperidine dicarboxylic acid inhibited the clonic and tonic phase of SSSC after intralymphatic or intrathecal administration. The antagonist gamma-D-glutamylaminomethylsulfonic acid and gamma-D-glutamyltaurine also suppressed both phases after intrathecal injections. The NMDA receptor antagonist DL-2-amino-5-phosphonovaleric acid, DL-2-amino-7-phosphonoheptanoic acid, and 3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid were effective inhibitors of the tonic phase and actually prolonged the duration of the clonic phase, an effect similar to that observed after low doses of gamma-D-glutamylglycine. SSSC were resistant to spinal perfusion to tetrodotoxin (1 microM). The concentrations of glutamate, aspartate, and glycine were increased in the Ringer's solution surrounding rapidly cooled spinal cord slices, but only in cords from species that elicited some magnitude of SSSC, not in cords from species resistant to induction of SSSC. Our data support the hypothesis that EAAs play a role in SSSC via activation of quisqualate receptors.

Effect of N-acetyl-cysteine, D-penicillamine and buthionine sulfoximine on glutathione levels and CNS oxygen toxicity in rats

The effect of glutathione (GSH) synthesis modulators - L-buthionine sulfoximine (BSO), N-acetyl cysteine (NAC) and D-penicillamine (DPA) - on the susceptibility of rat CNS to O2 toxicity was investigated. The animals were given 5% sucrose or 40 mM solutions of BSO, NAC or DPA in 5% sucrose as drinking water for one week and sacrificed prior to or after exposure to 4.5 ATA O2. The GSH content in brain, liver, lung and blood, and the activity of glutathione peroxidase (GSH-Px), glutathione reductase (GSSG-R), glucose-6-phosphate dehydrogenase (G-6-PD) and superoxide dismutase (SOD) in brain and lungs were measured. The brain GSH content and the enzyme activities were not changed by any of the drugs. BSO decreased the GSH content in all the other tissues; NAC and DPA treatments increased the GSH content in lungs, blood and/or liver. The CNS toxicity threshold as measured by the time of appearance of first electrical discharge (FED) on ECoG recording was not changed by NAC or DPA, but BSO brought about a significant delay in FED time. It is suggested that increased extracerebral GSH levels do not protect against CNS oxygen toxicity, and that BSO provides some protection, probably via a glutathione-independent mechanism.

Proline and proline derivatives as anticonvulsants

1. The anticonvulsant properties of L-proline, of proline derivatives (trans-4-hydroxy-L-proline, cis-4-hydroxy-D-proline, 3,4-dehydro-D,L-proline) and of D- and L-pipecolic acid were studied alone and in combination with vigabatrin (R/S-4-aminohex-5-enoic acid). 3-Mercaptopropionic acid and pentylenetetrazol-induced convulsions in mice were used as animal models of epilepsy. 2. Proline and proline derivatives are weak anticonvulsants if given alone in doses up to 10 mmol/kg, however, they are capable of potentiating the anticonvulsant effects of vigabatrin, in a manner similar to that reported previously for glycine, and some glycine derivatives. Among the compounds tested, trans-4-hydroxy-L-proline was the most potent anticonvulsant in combination with the indirect GABA agonist vigabatrin. 3. A potential explanation for the synergistic anticonvulsant effect of the combination of the GABA agonist and proline is the presumed role of proline as inhibitory neurotransmitter, and/or its glutamate antagonistic effects. 4. The current study points out the lack of basic knowledge on the neurochemistry and pharmacology of proline and hydroxyproline.

The glycine-prodrug, milacemide, increases the seizure threshold due to hyperbaric oxygen; prevention by 1-deprenyl

The novel glycine-prodrug anticonvulsant, milacemide (2-N-pentylaminoacetamide) (500 mg/kg), significantly increased (greater than 400% the seizure threshold induced by hyperbaric oxygen (4.5 atmosphere). This effect was significantly reduced by the selective inhibition of monoamine oxidase B by 1-deprenyl (2.0 mg/kg). 1-Deprenyl alone hardly affected the seizure threshold. These results suggest that, in the brain, milacemide is oxidized to glycine and that this reaction is mediated primarily by monoamine oxidase B. However, the interaction of milacemide metabolites (glycine amide, pentanoate and glycine) as antagonists of receptors of the glutamate NMDA (N-methyl-D-aspartate) subtype cannot be excluded.

Changes in synaptic transmission produced by hydrogen peroxide

The effect of hydrogen peroxide (H2O2) on excitatory and inhibitory synaptic transmission was studied at the lobster neuromuscular junction. H2O2 produced a dose dependent decrease in the amplitude of the junction potential (Vejp). This decrease was due to changes in both presynaptic transmitter release and the postsynaptic response to the neurotransmitter. Observed presynaptic changes due to exposure to H2O2 were a decrease in the amount of transmitter released, that is, quantal content, as well as a decrease in the fast facilitation, that is, the amplitude increase of successive excitatory junction potentials at a rate of 3 Hz. To discern postsynaptic changes, glutamate, the putative excitatory neurotransmitter for this preparation was applied directly to the bathing medium in order to bypass the presynaptic release process. H2O2 produced a decreased response of the glutamate receptor/ionophore. The action of H2O2 was not selective to excitatory (glutamate-mediated) transmission because inhibitory (GABA-mediated) transmission was also depressed by H2O2. This effect was primarily presynaptic since H2O2 produced no change in the postsynaptic response to applied GABA.