Antiepileptic drugs prevent seizures in hyperbaric oxygen: A novel model of epileptiform activity.

Breathing oxygen at sufficiently elevated pressures can trigger epileptiform seizures. Therefore, we tested the hypothesis that pre-treatment with FDA-approved antiepileptic drugs could prevent seizure onset in hyperoxia at 5 atmospheres absolute. We selected drugs from two putative functional categories, Na-channel antagonists and GABA enhancers, each administered intraperitoneally at four doses in separate groups of C57BL/6 mice.

Effects of striatal nitric oxide production on regional cerebral blood flow and seizure development in rats exposed to extreme hyperoxia.

The endogenous vasodilator and signaling molecule nitric oxide has been implicated in cerebral hyperemia, sympathoexcitation, and seizures induced by hyperbaric oxygen (HBO2) at or above 3 atmospheres absolute (ATA). It is unknown whether these events in the onset of central nervous system oxygen toxicity originate within specific brain structures and whether blood flow is diverted to the brain from peripheral organs with high basal flow, such as the kidney. To explore these questions, total and regional cerebral blood flow (CBF) were measured in brain structures of the central autonomic network in anesthetized rats in HBO2 at 6 ATA.

Baroreceptor afferents modulate brain excitation and influence susceptibility to toxic effects of hyperbaric oxygen.

Unexplained adjustments in baroreflex sensitivity occur in conjunction with exposures to potentially toxic levels of hyperbaric oxygen. To investigate this, we monitored central nervous system, autonomic and cardiovascular responses in conscious and anesthetized rats exposed to hyperbaric oxygen at 5 and 6 atmospheres absolute, respectively. We observed two contrasting phases associated with time-dependent alterations in the functional state of the arterial baroreflex.

Baroreflex-mediated cardiovascular responses to hyperbaric oxygen.

The cardiovascular system responds to hyperbaric hyperoxia (HBO2) with vasoconstriction, hypertension, bradycardia, and reduced cardiac output (CO). We tested the hypothesis that these responses are linked by a common mechanism-activation of the arterial baroreflex. Baroreflex function in HBO2 was assessed in anesthetized and conscious rats after deafferentation of aortic or carotid baroreceptors or both.

Brain oxygenation and CNS oxygen toxicity after infusion of perfluorocarbon emulsion.

Intravenous perfluorocarbon (PFC) emulsions, administered with supplemental inspired O(2), are being evaluated for their ability to eliminate N(2) from blood and tissue prior to submarine escape, but these agents can increase the incidence of central nervous system (CNS) O(2) toxicity, perhaps by enhancing O(2) delivery to the brain. To assess this, we infused a PFC emulsion (Oxycyte, 6 ml/kg iv) into anesthetized rats and measured cerebral Po(2) and regional cerebral blood flow (rCBF) in cortex, hippocampus, hypothalamus, and striatum with 100% O(2) at 1, 3, or 5 atmospheres absolute (ATA). At 1 ATA, brain Po(2) stabilized at >20 mmHg higher in animals infused with PFC emulsion than in control animals infused with saline, and rCBF fell by ~10%.

Nitric oxide-mediated central sympathetic excitation promotes CNS and pulmonary O₂ toxicity.

In hyperbaric oxygen (HBO(2)) at or above 3 atmospheres absolute (ATA), autonomic pathways link central nervous system (CNS) oxygen toxicity to pulmonary damage, possibly through a paradoxical and poorly characterized relationship between central nitric oxide production and sympathetic outflow. To investigate this possibility, we assessed sympathetic discharges, catecholamine release, cardiopulmonary hemodynamics, and lung damage in rats exposed to oxygen at 5 or 6 ATA. Before HBO(2) exposure, either a selective inhibitor of neuronal nitric oxide synthase (NOS) or a nonselective NOS inhibitor was injected directly into the cerebral ventricles to minimize effects on the lung, heart, and peripheral circulation.

Autonomic activation links CNS oxygen toxicity to acute cardiogenic pulmonary injury.

Breathing hyperbaric oxygen (HBO₂), particularly at pressures above 3 atmospheres absolute, can cause acute pulmonary injury that is more severe if signs of central nervous system toxicity occur. This is consistent with the activation of an autonomic link between the brain and the lung, leading to acute pulmonary oxygen toxicity. This pulmonary damage is characterized by leakage of fluid, protein, and red blood cells into the alveoli, compatible with hydrostatic injury due to pulmonary hypertension, left atrial hypertension, or both.

Hemoglobin, nitric oxide and molecular mechanisms of hypoxic vasodilation.

The protected transport of nitric oxide (NO) by hemoglobin (Hb) links the metabolic activity of working tissue to the regulation of its local blood supply through hypoxic vasodilation. This physiologic mechanism is allosterically coupled to the O(2) saturation of Hb and involves the covalent binding of NO to a cysteine residue in the beta-chain of Hb (Cys beta93) to form S-nitrosohemoglobin (SNO-Hb). Subsequent S-transnitrosation, the transfer of NO groups to thiols on the RBC membrane and then in the plasma, preserves NO vasodilator activity for delivery to the vascular endothelium.

Phosphodiesterase-5 inhibitors oppose hyperoxic vasoconstriction and accelerate seizure development in rats exposed to hyperbaric oxygen.

Oxygen is a potent cerebral vasoconstrictor, but excessive exposure to hyperbaric oxygen (HBO(2)) can reverse this vasoconstriction by stimulating brain nitric oxide (NO) production, which increases cerebral blood flow (CBF)-a predictor of O(2) convulsions. We tested the hypothesis that phosphodiesterase (PDE)-5 blockers, specifically sildenafil and tadalafil, increase CBF in HBO(2) and accelerate seizure development. To estimate changes in cerebrovascular responses to hyperoxia, CBF was measured by hydrogen clearance in anesthetized rats, either control animals or those pretreated with one of these blockers, with the NO inhibitor N(omega)-nitro-l-arginine methyl ester (l-NAME), with the NO donor S-nitroso-N-acetylpenicillamine (SNAP), or with a blocker combined with l-NAME.

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.

Contributions of nitric oxide synthase isoforms to pulmonary oxygen toxicity, local vs. mediated effects.

Reactive species of oxygen and nitrogen have been collectively implicated in pulmonary oxygen toxicity, but the contributions of specific molecules are unknown. Therefore, we assessed the roles of several reactive species, particularly nitric oxide, in pulmonary injury by exposing wild-type mice and seven groups of genetically altered mice to >98% O2 at 1, 3, or 4 atmospheres absolute. Genetically altered animals included knockouts lacking either neuronal nitric oxide synthase (nNOS(-/-)), endothelial nitric oxide synthase (eNOS(-/-)), inducible nitric oxide synthase (iNOS(-/-)), extracellular superoxide dismutase (SOD3(-/-)), or glutathione peroxidase 1 (GPx1(-/-)), as well as two transgenic variants (S1179A and S1179D) having altered eNOS activities.

Similar but not the same: normobaric and hyperbaric pulmonary oxygen toxicity, the role of nitric oxide.

Pulmonary manifestations of oxygen toxicity were studied and quantified in rats breathing >98% O(2) at 1, 1.5, 2, 2.5, and 3 ATA to test our hypothesis that different patterns of pulmonary injury would emerge, reflecting a role for central nervous system (CNS) excitation by hyperbaric oxygen.

How do red blood cells cause hypoxic vasodilation? The SNO-hemoglobin paradigm.

One of the most intriguing areas of research in erythrocyte physiology is the interaction of hemoglobin with nitric oxide (NO). These two molecules independently fulfill diverse and complex physiological roles, while together they subtly modulate microvascular perfusion in response to second-by-second changes in local metabolic demand, contributing to hypoxic vasodilation. It is through an appreciation of the temporal and structural constraints of the microcirculation that the principal requirements of the physiological interplay between NO and hemoglobin are revealed, elucidating the role of the erythrocyte in hypoxic vasodilation.

Electrochemical detection of nitric oxide in biological fluids.

The challenges that must be overcome in order to detect nitric oxide (NO) in biological fluids include its low physiological concentration (1-nM) and its short half-life (a few seconds or less). Electrochemistry is capable of making such measurements, if certain principles, both biological and electrochemical, are kept in mind. We discuss these principles and demonstrate an example of practical measurement by detecting NO release in a drop of blood suspended within the reference electrode of an electrochemical cell.

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.

Electrochemical activation of electrodes for amperometric detection of nitric oxide.

An open question in the literature of nitric oxide detection was investigated: does electrochemical activation account for the enhanced properties of certain presumed chemically-modified electrodes? Uniform electrodes of graphite, iridium, palladium, platinum, and ruthenium were exposed to potential cycling and then tested for amperometric response to nitric oxide to identify principles that govern electrochemical activation of nitric oxide electrodes. These electrodes were compared to similar electrodes that were not cycled. Only cycled graphite and ruthenium showed significantly increased responses.

Electrochemical detection of physiological nitric oxide: materials and methods.

Advances in the electroanalytical technology of NO detection make it possible to detect the release of robust concentrations of NO from living systems under pathological or pharmacological conditions. However, technical improvements should enable the construction of research instruments with one or two orders of magnitude improvement in both detection limit and temporal resolution. Such instruments would be capable of revealing physiological NO production and could help quantify the correlations between NO levels and health or disease, ultimately leading to important applications in biomedical research and clinical medicine.