Central nervous system toxicity of hyperbaric oxygen--effects of light, norepinephrine depletion and beta-adrenergic blockade

Adrenoceptor blockade modifies regional cerebral blood flow responses to hyperbaric hyperoxia: Protection against CNS oxygen toxicity

Exposure to extreme-hyperbaric oxygen (HBO2), > 5-6 atmospheres absolute (ATA), produces baroreflex impairment, sympathetic hyperactivation, hypertension, tachycardia, and cerebral hyperemia, known as Phase II, culminating in seizures. We hypothesized that attenuation of the effects of high sympathetic outflow would preserve regional cerebral blood flow (rCBF) and protect against HBO2-induced seizures. To explore this possibility, we tested four adrenoceptor antagonists in conscious and anesthetized rats exposed to HBO2 at 5 and 6 ATA, respectively: phentolamine (nonselective α1 and 2), prazosin (selective α1), propranolol (nonselective β1 and 2) and atenolol (selective β1). In conscious rats, 4 drug-doses were administered to rats prior to HBO2 exposures, and seizure latencies were recorded. Drug-doses that provided similar protection against seizures were administered before HBO2 exposures in anesthetized rats to determine the effects of adrenoceptor blockade on mean arterial pressure, heart rate, rCBF and EEG spikes. All four drugs modified cardiovascular and rCBF responses in HBO2 that aligned with epileptiform discharges, but only phentolamine and propranolol effectively increased EEG spike latencies by ~20 and 36 min, respectively. When phentolamine and propranolol were delivered during HBO2 at the onset of phase II, only propranolol led to sustained reductions in heart rate and rCBF, preventing the appearance of epileptiform discharges. The enhanced effectiveness of propranolol may extend beyond β-adrenoceptor blockade, i.e. membrane stability and reduced metabolic activity. These results indicate that adrenoceptor drug pre-treatment will minimize the effects of excessive sympathetic outflow on rCBF and extend HBO2 exposure time.

Central nervous system glucose utilization rate during oxygen-induced respiratory changes at 2 atmospheres oxygen in the rat

Hyperbaric oxygenation (HBO) at pressures higher than 3 atmospheres absolute (ATA) primarily affects the CNS, while at lower pressures, the respiratory functions are predominantly changed. Due to the outstanding respiratory manifestations of HBO at pressures lower than 3 ATA O2, the possible overlapping neurological effects of oxygen toxicity may not be easily identified. However, rats exposed for 1 and 4 h to 2 ATA O2 have shown increases in the regional cerebral metabolic rate for glucose (rCMRgl) when neither visible signs of respiratory nor nervous distress were observed. The purpose of the present study was to differentiate between the CNS and the respiratory effects of HBO at 2 ATA during development of the early signs of the respiratory distress. Changes in the rCMRgl measured by [14C]-2-deoxyglucose (2-DG) autoradiographic technique and respiratory frequency (Rf) were used as criteria for determination of CNS and respiratory effects of HBO, respectively. The results demonstrate no differences in the rCMRgl (28 major structures examined) among 3 groups of conscious rats exposed to 2 ATA O2 and normoxia at 1 and 2 ATA. At the same time a significant reduction was found in the Rf of the oxygen-exposed rats. In conclusion the respiratory system at 2 ATA O2 is apparently affected earlier than, and independent from the CNS. However, due to limited resolution power of the [14C]2-DG technique, the effect of HBO on certain undetected central respiratory control centers cannot yet be ruled out.

Hyperbaric oxygen induced experimental epilepsy: the age factor

Two methods of inducing convulsions were used in male Swiss albino mice of different ages: exposure to hyperbaric oxygen and pentylenetetrazole treatment. Hyperbaric oxygen proved to be a valid model of experimental epilepsy with an age-dependent trend. The youngest mice presented a much longer convulsion latency time than the adult mice but the curve of distribution of latency time versus age showed progressively increasing sensitivity. Pentylenetetrazole induced convulsions, apart from the youngest subgroup, without variations by age. Hyperbaric oxygen convulsions provide an interesting model for the study not only of the neurochemistry of convulsions but also of the membrane changes that occur in the course of cerebral maturation and aging.

Effect of pentobarbital anesthesia on regional cerebral metabolic rate for glucose during hyperbaric oxygenation in rat

Hyperbaric oxygenation (HBO) at different pressures and durations of exposure produces an increase in regional cerebral metabolic rate for glucose (rCMRgl) in conscious rats. Pentobarbital anesthesia is known to significantly reduce rCMRgl in rats during air breathing. To test if pentobarbital anesthesia is also effective in reducing HBO-induced increases in rCMRgl, the combined effect of pentobarbital anesthesia and HBO on rCMRgl was autoradiographically measured in 28 neuroanatomical structures. Two groups of rats (11 each) were chronically cannulated in one femoral artery and vein 3 days prior to the experiments. Thirty minutes before the rCMRgl measurements the rats were anesthetized with intravenous injection of 50 mg/kg pentobarbital and exposed to either 1-h air breathing at atmospheric pressure or oxygen at 2 atm absolute. No differences in rCMRgl between oxygen-exposed and air-exposed anesthetized rats were observed in 27 of the 28 neuroanatomical structures examined. The superior olivary nucleus was the only one of the 28 structures showing a significant reduction in rCMRgl following anesthesia and HBO. The possible factors involved in the elimination by anesthesia of the previously observed increases in rCMRgl in conscious rats exposed to HBO are discussed.

Influence of rat brain superoxide dismutase inhibition by diethyldithiocarbamate upon the rate of development of central nervous system oxygen toxicity

Exposure to oxygen at pressures greater than 2.8 ATA (OHP) results in central nervous system toxicity seen as grand mal seizures. The time to onset of seizures (ts) is related to the pO2 above the 2.8 ATA threshold. The components of the endogenous antioxidant defense mechanism, superoxide dismutase (SOD), glutathione measured here as nonprotein sulfhydryl content (NPSH), glucose-6-phosphate dehydrogenase (G-6-PD), glutathione reductase (GR), and glutathione peroxidase (GPx) occur in brain. Their role in OHP-induced CNS toxicity is not clear. This study examined the effect of inhibition of SOD by diethyldithiocarbamate (DDC) on ts at 4 ATA O2. Antioxidant components (SOD, NPSH, G-6-PD, GR, and GPx) were measured in male Sprague-Dawley rats pretreated with 250, 500, and 1000 mg/kg DDC ip, 2 hr prior to termination in room air. SOD activity was inhibited 11, 31, and 49%, respectively, when compared with control values. Among the other antioxidant components, only GPx showed a significant loss of activity of 24% at 1000 mg/kg DDC. Rats were also pretreated 2 hr prior to exposure to hyperbaric oxygen with either 250, 500, or 1000 mg/kg DDC. Ts for the treated animals was significantly shortened by 12, 55, and 75%, respectively, compared to the saline-treated, oxygen-exposed control animals. These studies demonstrated that the rate of onset of CNS oxygen toxicity was increased by inhibition of SOD by DDC. These data suggested that SOD plays a role as part of an endogenous antioxidant defense mechanism in the brain.

The toxicology of molecular oxygen

Molecular oxygen, itself not very reactive, can be converted by photosensitization to electronically excited singlet states, and by partial reduction to the superoxide and hydroxyl free radicals and to hydrogen peroxide. The very considerable toxicity of oxygen, which is due primarily to the properties of these derivatives, is ordinarily overlooked because aerobes have evolved an elaborate system of defenses which is reasonably adequate under ambient conditions. This toxicity becomes all too apparent when these defenses are overwhelmed at elevated pO2 or through the action of compounds which increase the conversion of oxygen to its more reactive derivatives. We will here consider the threat posed by oxygen and the defenses which make aerobic life possible.

Beta adrenergic receptors and glucagon in seizures from exposure to oxygen at high pressure (OHP)

Previous studies have shown that sympathetic factors and blood glucose are of importance in the development of seizures and lung damage from OHP. In the present study we examined the influence of beta sympathetic agonists and blocking agents and glucagon on OHP toxicity. Rats were exposed to 6 ATA OHP and examined for time-to-seizure and lung damage. Pretreatment with propranolol increased the time-to-seizure by 70% and practolol by 50% without altering gross lung appearance or lung wet wt/dry wt. Propranolol and practolol also prevented brain glycogen depletion prior to seizure which otherwise occurred in subconvulsive exposure to OHP. Isoproterenol and glucagon pretreatment had no effect on time-to-seizure but isoproterenol did increase lung injury. Both practolol and propranolol block the beta-receptor influence on adenyl cyclase-stimulated second messenger production, while both isoproterenol and glucagon activate adenyl cyclase to produce second messenger. Our results may suggest a possible role for second messenger in mediating some of the acute toxic effects of OHP on the CNS.