Alexander N. Moskvin

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

Hyperbaric oxygen (HBO2) increases oxygen tension (PO2) in blood but reduces blood flow by means of O2-induced vasoconstriction. Here we report the first quantitative evaluation of these opposing effects on tissue PO2 in brain, using anesthetized rats exposed to HBO2 at 2 to 6 atmospheres absolute (ATA). We assessed the contribution of regional cerebral blood flow (rCBF) to brain PO2 as inspired PO2 (PiO2) exceeds 1 ATA. We measured rCBF and local PO2 simultaneously in striatum using collocated platinum electrodes. Cerebral blood flow was computed from H2 clearance curves in vivo and PO2 from electrodes calibrated in vitro, before and after insertion. Arterial PCO2 was controlled, and body temperature, blood pressure, and EEG were monitored. Scatter plots of rCBF versus pO2 were nonlinear (R2 = 0.75) for rats breathing room air but nearly linear (R2 = 0.88–0.91) for O2 at 2 to 6 ATA. The contribution of rCBF to brain PO2 was estimated at constant inspired PO2, 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 PO2 increased 7- to 33-fold at 2 to 6 ATA, compared with room air. A doubling of rCBF increased striatal PO2 not quite two-fold in rats breathing room air but 13- to 64-fold in those breathing HBO2 at 2 to 6 ATA. These findings support our hypothesis that HBO2 increases PO2 in brain in direct proportion to rCBF.

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. Experiments were performed on both anesthetized and conscious rats to differentiate responses to HBO(2) from the effects of anesthesia. EEG spikes, markers of CNS toxicity in anesthetized animals, were approximately four times as likely to develop in control rats than in animals with central NOS inhibition. In inhibitor-treated animals, autonomic discharges, cardiovascular pressures, catecholamine release, and cerebral blood flow all remained below baseline throughout exposure to HBO(2). In control animals, however, initial declines in these parameters were followed by significant increases above their baselines. In awake animals, central NOS inhibition significantly decreased the incidence of clonic-tonic convulsions or delayed their onset, compared with controls. The novel findings of this study are that NO produced by nNOS in the periventricular regions of the brain plays a critical role in the events leading to both CNS toxicity in HBO(2) and to the associated sympathetic hyperactivation involved in pulmonary injury.

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. Until now, however, central hemodynamic parameters and autonomic activity have not been studied concurrently in HBO₂, so any hypothetical connections between the two have remained untested. Therefore, we performed experiments using rats in which cerebral blood flow, electroencephalographic activity, cardiopulmonary hemodynamics, and autonomic traffic were measured in HBO₂ at 5 and 6 atmospheres absolute. In some animals, autonomic pathways were disrupted pharmacologically or surgically. Our findings indicate that pulmonary damage in HBO₂ is caused by an abrupt and significant increase in pulmonary vascular pressure, sufficient to produce barotrauma in capillaries. Specifically, extreme HBO₂ exposures produce massive sympathetic outflow from the central nervous system that depresses left ventricular function, resulting in acute left atrial and pulmonary hypertension. We attribute these effects on the heart and on the pulmonary vasculature to HBO₂-mediated central sympathetic excitation and catecholamine release that disturbs the normal equilibrium between excitatory and inhibitory activity in the autonomic nervous system.

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. Cardiovascular and autonomic responses to HBO2 in these animals were compared with those in intact animals at 2.5 ATA for conscious rats and at 3 ATA for anesthetized rats. During O2 compression, hypertension was greater after aortic or carotid baroreceptor deafferentation and was significantly more severe if these procedures were combined. Similarly, the hyperoxic bradycardia observed in intact animals was diminished after aortic or carotid baroreceptor deafferentation and replaced by a slight tachycardia after complete baroreceptor deafferentation. We found that hypertension, bradycardia, and reduced CO--the initial cardiovascular responses to moderate levels of HBO2--are coordinated through a baroreflex-mediated mechanism initiated by HBO2-induced vasoconstriction. Furthermore, we have shown that baroreceptor activation in HBO2 inhibits sympathetic outflow and can partially reverse an O2-dependent increase in arterial pressure.

Involvement of Extracellular Superoxide Dismutase in Regulating Brain Blood Flow

The physiological role of extracellular superoxide dismutase (SOD3) has received insufficient study. We investigated the hypothesis that SOD3, which neutralizes superoxide anions (O2–) in the intercellular space of the brain, prevents the inactivation of nitric oxide (NO) and is thus involved in regulating cerebral vascular tone. Local brain blood flow was measured in the striatum of anesthetized rats during administration of various combinations of a SOD mimetic, a SOD inhibitor, an NO donor, and an NOS inhibitor into the striatum using a Hamilton syringe. In normal conditions, SOD3 was found to minimize O2– levels, protecting endogenously produced NO at a sufficient level to maintain cerebral vascular tone and reactivity. SOD3 was found to increase the vasodilatory effect of endogenously produced NO in the brain. SOD3 was found to neutralize superoxide anions produced in the brain during respiration of 100% O2 and to maintain basal NO levels and its vasodilatory potential in normobaric hyperoxia.

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. The first phase, which conferred protection against potentially neurotoxic doses of oxygen, was concurrent with an increase in baroreflex sensitivity and included decreases in cerebral blood flow, heart rate, cardiac output, and sympathetic drive. The second phase was characterized by baroreflex impairment, cerebral hyperemia, spiking on the electroencephalogram, increased sympathetic drive, parasympatholysis, and pulmonary injury. Complete arterial baroreceptor deafferentation abolished the initial protective response, whereas electrical stimulation of intact arterial baroreceptor afferents prolonged it. We concluded that increased afferent traffic attributable to arterial baroreflex activation delays the development of excessive central excitation and seizures. Baroreflex inactivation or impairment removes this protection, and seizures may follow. Finally, electrical stimulation of intact baroreceptor afferents extends the normal delay in seizure development. These findings reveal that the autonomic nervous system is a powerful determinant of susceptibility to sympathetic hyperactivation and seizures in hyperbaric oxygen and the ensuing neurogenic pulmonary injury.

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. The drugs varied in efficacy at the doses used. Of the five tested Na+-channel antagonists, carbamazepine and lamotrigine more than tripled seizure latency compared to values seen in vehicle controls. Primidone, zonisamide and oxcarbazepine were less effective. Of the four GABA reuptake inhibitors, tiagabine and vigabatrin also increased seizure latency by more than three times control values; valproic acid was less effective, and the GABA synthesis promoter gabapentin was intermediate in effectiveness. We infer that Na+-channel function and GABA neurotransmission may be critical targets in the pathophysiology of CNS O2 toxicity. Because these essential components of neuronal excitation and inhibition are also implicated in the pathogenesis of other seizure disorders, including generalized epilepsy, we propose that, at some level, common pathways are involved in these pathologies, although the initiating insults differ. Furthermore, hyperoxic exposures are not known to cause the spontaneously-recurring seizures that characterize true clinical epilepsy. Nonetheless, experimental studies of hyperbaric oxygen toxicity could provide new insights into molecular mechanisms of seizure disorders of various etiologies. In addition, the neuropathology of hyperbaric oxygen is particularly relevant to the hypothesis held by some investigators that oxidative stress is an etiological factor in clinical epilepsies.

Effects of hyperbaric oxygenation on subependymal microglia of the rat brain

not yet well understood. We only know that HBO enhances production of reactive oxygen species (ROS) that oxidize cell structures and produce neurochemical changes that lead to neurotoxicity [2]. Microglia is one of the most important effector cells of the nervous tissue, mediating the damaging effects of various environmental factors. For example, low lead concentrations resulted in a decrease of the number of microglial cells [3], while during ischemia the proliferation and migration of these cells is observed [4]. This fact may indicate the oxygen sensitivity of the microglial cells. However , there are no data concerning the reaction of microglia to the impact of HBO. The goal of this work was to study effect of HBO on the state of subependymal microglia—an essential element of the cerebrospinal fluid–brain barrier. The study was performed on male Wistar rats weighing 200–250 g (n = 12). The HBO séance for 6 animals was carried out in a hyperbaric chamber of 107-l-volume at an oxygen pressure of 0.3 MPa for 4 h. During this time period the chro-noconcentration effect of oxygen did not lead Hyperbaric oxygenation (HBO) is a method of oxygen saturation of an organism under high pressure , which is used in medical and diving practice in preventive and curative purposes [1]. Application of HBO is accompanied by positive effects that are related to a possibility of compensating the oxygen debt of the organism in insufficient external respiration or oxygen-binding function of blood, deficiency of regional and general supply , etc. HBO promotes deceleration and deeper breathing, a slowing of the heart rate, decreases the cardiac output and volume of the organ blood flow, increases the peripheral vascular resistance, improves myocardial contractility, and increases its functional reserve. Also, HBO increases essentially the level of regenerative processes. However, if the concentration of hyperbaric oxygen exceeds the therapeutic level, a neurotoxic effect is evident , which is manifested in development of oxygen convulsions. The HBO-induced physiological and pathological reactions of the central nervous system have long been known phenomenologi-cally, but the molecular mechanisms of oxygen toxicity and the occurring cellular responses are

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.

Electrical Stimulation of the Vagus Nerve Modulates the Development of Oxygen Epilepsy in Rabbits

We report here our studies of the effects of electrical stimulation of the vagus nerve on the development of oxygen convulsions and cardiovascular reactions in hyperbaric oxygen (HBO2). The motor signs of the neurotoxic action of HBO2 were identified in conscious rabbits exposed to O2 at 5 atm, along with EEG, ECG, external respiration, and brain blood filling in conditions of stimulation of the cervical component of the vagus nerve. Oxygen convulsions were found to be preceded by increased sympathetic tone, with a specific EEG pattern (synchronization of slow waves), changes in the ECG (tachycardia) and respiration (hyperventilation). Vagal stimulation enhanced the parasympathetic component of the autonomic system and significantly delayed the prodromal signs of oxygen toxicity and the onset of convulsions. Afferent spike activity from the visceral organs is regarded as a new target for countering the neurotoxic effects of hyperbaric oxygen.