Dan Torbati

Correlation of brain glucose utilization and cortical electrical activity during development of brain oxygen toxicity

Central nervous system (CNS) oxygen toxicity in rats is characterized by the appearance of alterations in electrical cortical activity (ECoG), followed by the appearance of paroxysmal electrical discharges and finally the onset of clinical convulsions. The correlation between the changes in ECoG and the regional cerebral metabolic rate for glucose (rCMRgl) during progressive oxygen toxicity was studied. Cortical electrodes for ECoG recording and venal arterial cannula for autoradiographic measurement of rCMRgl were chronically implanted. Using [14C]2-deoxyglucose (2-DG), the rCMRgl was measured in conscious unrestrained rats during different periods of exposure to 5 atmospheres absolute oxygen as well as an equivalent normoxic high pressure, while ECoG was continuously recorded and analyzed. A statistically significant increase in rCMRgl in 13 out of 24 investigated brain structures was found during the pre-paroxysmal electrical discharge period. This increase was accompanied by an elevation in slow and a reduction in fast ECoG frequency bands. The largest increase in rCMRgl was found in cerebellar and cerebral cortices, limbic, auditory and visual structures. Following the appearance of the first paroxysmal electrical discharge (FED) some limbic structures and cerebellar cortex showed further increases in rCMRgl, while several auditory and visual structures exhibited a significant decrease. Five atmospheres normoxic pressure had no effect on rCMRgl in any of the brain structures examined. It is concluded that pre-paroxysmal electrical discharge ECoG changes and the onset of the FED during progressive oxygen toxicity are not due to inhibition of brain energy metabolism. The possible mechanisms leading to alterations in rCMRgl during hyperbaric oxygenation are discussed.

Regional cerebral metabolic rate for glucose during hyperbaric oxygen-induced convulsions.

Hyperbaric oxygen-induced convulsions in awake unrestrained rats are preceded by electrocorticographic changes including paroxysmal electrical discharges (PED). During oxygen induced convulsions, alterations in regional cerebral metabolic rate for glucose (rCMRgl) were autoradiographically measured and compared with rCMRgl results obtained during pre-convulsive periods in an earlier study. Statistically elevated rCMRgl during oxygen-induced convulsions were found in globus pallidus, substantia nigra, limbic structures, and cerebellar cortex. Significant reductions were found largely in auditory structures and cerebral cortex. This pattern of changes in rCMRgl resembles the pattern of changes during successive PED in the absence of overt convulsions. This similarity may indicate that a common sequence of biochemical changes leads to both oxygen-induced pre-convulsive as well as convulsive electrical discharges.

Regional cerebral glucose metabolic rate during thirty minutes hypoxia of 7% oxygen in adult conscious rats.

The effect of hypoxia on the regional cerebral metabolic rate for glucose (rCMRgl) was measured in 28 neuroanatomical structures of adult, conscious, unrestrained rats by the 2-[14C]deoxyglucose autoradiographic technique. Rats were cannulated in one femoral artery and vein 3 days before the experiments. The rCMRgl was measured in 15 rats during 30 min air breathing and in 13 rats during 30 min hypoxia of 7% O2 in N2. Statistically significant increases in rCMRgl in the order of 39-95% were observed in 25 of the 28 neuroanatomical structures examined. The highest increases in rCMRgl were observed in cerebral and cerebellar white matter (95% and 60%, respectively), as well as in limbic structures ranging from a 91% increase in the septal nuclei to a 55% increase in the hypothalamus. The superior olivary nucleus and inferior colliculus (auditory structures) were the only structures which did not show changes in glucose utilization. The present data are compared with previous studies during hypoxia in conscious or anesthetized animals. It is concluded that the degree of rCMRgl response to hypoxia is affected by anesthesia, age and species.

Regional cerebral glucose utilization rates in rats during asymptomatic period of exposure to 1, 2 and 3 atmospheres absolute of oxygen

A previous study has shown an increase in regional cerebral metabolic rate for glucose prior to the onset of central nervous system oxygen toxicity in rats exposed to 5 atmospheres absolute of oxygen. The present study was designed to measure regional cerebral glucose utilization rates at pressures used for oxygen therapy and prolonged exposures during which rats are known to be asymptomatic. The regional metabolic rate for glucose in 26 brain structures and in gray and white matter of the thoracic and lumbar spinal cord was autoradiographically measured in awake unrestrained rats using the autoradiographic [14C]2-deoxyglucose technique. Femoral artery and vein cannulae were inserted 3 days before the experiment. Rats were divided into four groups of 15: (a) air control; (b) 6 h at 1 atmosphere absolute oxygen; (c) 4 h at 2 atmospheres oxygen; and (d) 2 h at 3 atmospheres oxygen. Statistically significant increases in glucose utilization (p less than 0.05) are seen only in lateral thalamus at 3 atmospheres oxygen, in superior olivary nucleus and inferior colliculus at 2 atmospheres oxygen and in superior olivary nucleus at 1 atmosphere oxygen. The combination of our previous data at 5 atmospheres oxygen and the present results at prolonged and safe exposures to lower pressures indicated that increased glucose utilization in some neuronal structures precedes the onset of the central nervous system manifestations of oxygen toxicity.

Regional cerebral metabolic rate for glucose immediately following exposure to two atmospheres absolute oxygen in conscious rats.

Hyperbaric oxygenation (HBO) at different pressures produces significant increases in the regional cerebral metabolic rate for glucose (rCMRgl) of various neuroanatomical structures in conscious rats. These increases in rCMRgl precede the neurological signs of oxygen toxicity. In previous studies 1- and 4-h exposures to 2 atmospheres absolute oxygen (ATA O2) produced significant increases in rCMRgl of several neuroanatomical structures. These observations are now extended to include the rCMRgl changes produced during a brief 30-min exposure to 2 ATA O2. The purpose of the present study was to investigate whether HBO has an immediate effect on rCMRgl and to determine the neuroanatomical basis of changes in brain energy metabolism during an early stage of HBO. The rCMRgl was autoradiographically measured by 2-[14C]deoxyglucose technique in conscious unrestrained rats exposed either to air at atmospheric pressure or oxygen at 2 ATA O2 for 30 min. Statistically, no significant changes in rCMRgl were observed between air-exposed and oxygen-exposed groups in the 28 neuroanatomical structures examined. It is concluded that HBO requires a certain amount of time to elicit its effect on rCMRgl in a given pressure. However, the neuroanatomical origins of the previously observed increases in rCMRgl during prolonged HBO could not be detected in this early period of exposure to 2 ATA O2.

Effect of prolonged normobaric hyperoxia on regional cerebral metabolic rate for glucose in conscious rats

The CNS tolerance to prolonged normobaric hyperoxia (NH) was investigated using alterations in regional cerebral metabolic rate for glucose (rCMRgl) as a sensitive measure of brain oxygen poisoning. Conscious rats were continuously exposed either to air or to oxygen for 24 h at atmospheric pressure inside a closed and ventilated environmental chamber. The rCMRgl was measured during ongoing air or oxygen exposures by [14C]2-deoxyglucose (2-DG) autoradiographic technique. No significant difference in the rCMRgl of 29 neuroanatomical structures investigated was found between two groups of air- and oxygen-exposed rats. At the same time however, a significant reduction in the respiratory frequency (f) was observed only in the oxygen-exposed rats. It is suggested that brain energy metabolism is not affected at least up to 24 h NH in conscious rats. The NH-induced reduction in f on the other hand, may be due to alterations in afferent inputs from peripheral and central chemoreceptors or lung stretch receptors. Furthermore, since slight changes in rCMRgl of small neuroanatomical structures are not detectable by limited resolution power of [14C]2-DG autoradiographic technique, a subtle NH-induced damage to central respiratory control mechanisms cannot yet be ruled out.

Correlation of age and body weight with local cerebral glucose utilization in 8–14 week-old rats

The regional cerebral metabolic rate for glucose (rCMRgl) in 8-14 week-old, male, albino, Wistar rats, weighing 200-385 g was autoradiographically measured using the [14C]2-deoxyglucose (2-DG) technique. Three age-weight groups of rats were tested: (I) 8-9 weeks old (200-250 g, average 227 +/- 14); (II) 10-11 weeks old (255-300 g, average 280 +/- 15 g); (III) 12-14 weeks old (305-385 g; average 331 +/- 25 g). A gradual reduction in rCMRgl was observed in the older and heavier groups. The reductions were statistically significant in 18/28 investigated structures in group III as compared to group I. Since such significant variation in the investigated age-weight range may affect rCMRgl values obtained in different experiments, it is suggested to use narrow and uniform age-weight groups for experimentation concerning brain metabolic functions.

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.

Local cerebral glucose utilization rate following intermittent exposures to 2 atmosphere absolute oxygen.

Previous studies have shown significant increases in regional cerebral metabolic rate for glucose (rCMRgl) in 14 of 28 investigated brain structures in rats exposed to 1-h oxygen at 2 atmosphere absolute (ATA O2). Continuous 4-h exposure to 2 ATA O2 resulted in significant increases only in superior olivary nucleus and inferior colliculus. In the present study, the rCMRgl was autoradiographically measured by the [14C]2-deoxyglucose technique during the last 30 min of 4 intermittent 1-h exposures to either 2 ATA O2 or air at atmospheric pressure, with 3 h of breathing air outside the pressure chamber between each oxygen or air exposure. Statistically significant reductions in rCMRgl of the oxygen-exposed rats were observed in superior olivary nucleus and inferior colliculus, while no changes were observed in 26 other investigated structures. The previously observed increases in rCMRgl in a single 1- or 4-h exposure at 2 ATA O2 were reduced or reversed during the intermittent hyperbaric oxygen exposure. The relation of the observed changes in rCMRgl during single and intermittent hyperbaric oxygen exposures to the extension of tolerance to hyperbaric oxygenation is discussed.

Heparin effects during hyperbaric oxygenation in rats

The effects of heparin were studied concurrently with development of neurological and respiratory signs of oxygen toxicity in awake unrestrained rats exposed to 3 atmosphere absolute (ATA) oxygen. The modification of the early electrophysiological manifestations of CNS oxygen toxicity by heparin in the absence of obvious signs of pulmonary oxygen toxicity was also determined at 5 ATA oxygen by electrocorticographic recording. The femoral artery of all rats was cannulated two days before the exposures to hyperbaric oxygenation (HBO), and the effect of intraarterial injection of 10 U/100g/3h heparin or an equivalent volume of saline was studied in experimental and control rats, respectively. In rats exposed to 3 ATA oxygen, the latency of the onset of the first oxygen-induced convulsions, the time interval between the first convulsion and death, and the survival time were measured. Exposure to 5 ATA oxygen was continued until the onset of the first preconvulsive paroxysmal electrical discharges (FED), considered to be an early electrophysiological indicator of CNS oxygen toxicity. The onset of convulsions was slightly delayed in heparin-treated rats exposed to 3 ATA oxygen, and the time interval between the first convulsions and death was significantly reduced in heparinized rats. No difference in survival time between heparin- and saline-treated rats was observed. Heparin significantly delayed the time of onset of the FED during exposure to 5 ATA oxygen. Gross postmortem examination of the lungs and internal organs revealed only a bloody froth in the trachea of the heparin-treated rats exposed to 3 ATA oxygen. It is concluded that the heparin-hyperoxic interaction during development of pulmonary and CNS oxygen toxicity may be related to the anticoagulant effect of heparin and hyperoxic-induced pulmonary lesions.