Changes in electrical activity of the cerebral cortex and of some subcortical centers in hyperbaric oxygen

Electroencephalography and Clinical Neurophysiology Elsevier Publishing Company, Amsterdam - Printed in The Netherlands

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CHANGES IN ELECTRICAL ACTIVITY OF THE CEREBRAL CORTEX AND OF SOME SUBCORTICAL CENTERS IN HYPERBARIC OXYGEN F. S. R u c o , M. L. GIRETTI1 AND M. LA ROCCA Istituto di Fisiologia Umana e Istituto di Clinica Chirurgica, Universitgt di Sassari, Sassari, Sardegna (Italy) (Accepted for publication: August 27, 1966)

The convulsive effects of an increase in O2 partial pressure are well known (literature in Lambertson 1965). Bert (1878) was the first to observe seizures caused by high 02 pressure. The electrocorticographic modifications induced by high 02 pressure were investigated by Cohn and Gersch (1945), Stein and Sonnenschein (1950), Sonnenschein and Stein (1953), Batini et al. (1954a, b) and by Stein (1955) on anesthetized, curarized or acute spinal animals. The present investigations were undertaken in order to study the changes in electrical activity of the cerebral cortex and of subcortical centers during the acute development of hyperoxic seizures in unanesthetized and unrestrained rats. No one hitherto has studied the behavior of the electrical activity of thalamic, mesencephalic and pontine structures during hyperoxic seizures in unanesthetized, unrestrained rats, in comparison with the electrocorticographic changes of the cerebral cortex. Only Batini et al. (1954a) recorded the electrical activity of the caudate nucleus and of the intralaminar nuclei of the thalamus under similar experimental conditions in the cat. This research is also justified by the fact that in recent years there has been a renewal of interest in the subcortical structures as the site of origin of generalized epileptic discharge (Gastaut and Fischer-Williams 1959; Ralston and Langer 1965). MATERIAL AND METHODS

Fifty-three male rats of the Sprague-Dawley 1 Fellowship of C.N.R.

strain, weighing 200-300 g, were used. They seemed to be more susceptible to hyperoxic seizures than rats belonging to the Wistar strain. Under ether anesthesia the animals were put into a stereotaxic apparatus. The scalp was incised along the midline and 2 or 4 extradural electrodes, consisting in iron screws (diameter: 1 mm), were embedded in the skulls of 47 animals and secured by means of dental cement. Twenty of these 47 rats had also been fitted with deep tungsten electrodes, wholly insulated except for the tips (diameter: 5-10 /z). These electrodes were introduced by micro-control, following the coordinates of the stereotaxic atlas of KSnig and Klippel (1963). There were 3 deep electrodes in almost all the rats, the anterior one at the level of the diencephalon, the middle one in the mesencephalon and posterior one in the pons. The exact positions of the recording tips of the electrodes are shown in Fig. 1. In a few animals, under surgical anesthesia, an additional electrode was placed in the biceps muscle of the right forelimb or in the posterior cervical muscles, in order to record the electromyogram (EMG). In six other animals the cortex was removed by suction, under ether anesthesia. Deep electrodes were placed in these rats also, an additional one having been inserted in the biceps muscle for EMG recording purposes. The cerebral electrical activity and the EMG were recorded by a Model 7, 4-channel ink-writing Grass polygraph. The electrocorticographic recordings (ECoG) were bipolar while the deep recordings were unipolar. The 47 rats with extradural electrodes were subjected to hyperbaric oxygenation on Eleetroenceph. olin. Neurophysiol., 1967, 22:231-238

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RESULTS

1. Changes in the electrical activity preceding the first epileptic seizure The first cortical seizure activity was frequent-

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A : Before hyperoxia. B: 40 min after the beginning of hyperoxia; note spindle-like waves. C: 64 min after beginning of hyperoxia. Generalized seizure. Calibrations: 5 sec and 150/~V.

Electroenceph. clin. Neurophysiol., 1967, 22:231-238

HYPEROXIC SEIZURES IN RAYS ly (23 out o f 33 rats) preceded by changes in the electrical activity o f the cerebral cortex and the subcortical centers, these changes consisting in spontaneous spindle-like waves and increase in the voltage and discharge rate (Fig. 2, B). The subcortical leads exhibited these changes in the great majority o f experiments, while sometimes they failed to appear in the cerebral cortex (Fig. 3, B). Voltage was higher in the diencephalic leads than in the mesencephalic, pontine and cortical ones. These changes could be inhibited by acoustic stimuli. Lastly, in five out o f 33 rats isolated spikes appeared in all the cortical and subcortical leads. In these instances short-du-

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ration outbursts o f low frequency spikes were visible in the cortical recordings and were followed by activation o f the subcortical structures, especially thalamic and mesencephalic. In a few instances only were isolated spikes seen in the cortical and thalamic recordings, followed by activation o f subcortical centers (Fig. 4, B, C). The above changes in electrical activity could not be related to possible drowsiness of the animals since they were not observed when normal rats were put in the hyperbaric chamber at normal pressure and breathing normal air. On the other hand such changes were not accompanied by the characteristic changes in the E M G of the

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I Fig. 3 Rat 17. Effect of hyperoxia on cortical and subcortical electrical activities. TF: transverse frontal lead, TV : unipolar record from right ventral nucleus of thalamus, RFM : unipolar record from left mesencephalic reticular formation, RFP: unipolar record from pontine reticular formation. A: Before hyperoxia. B: 20 min after beginning of hyperoxia. C: 23 rain after the beginning of hyperoxia: generalized seizure. Note spindle-like waves in the subcortical leads, particularly evident in TV before the onset of the first seizure. Calibrations : 6 sec and 150/~V. Electroenceph. clin. Neuroph)siol., 1967, 22:231-238

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Fig. 4 Rat 23. Effect of hyperoxia on the electrical activity of the cerebral cortex and of some subcortical structures. TF: transverse frontal lead, TV: unipolar record from left ventral nucleus of thalamus, R F P : unipolar record from pontine reticular formation, E M G : electromyogram from biceps of right forelimb. A: Before hyperbaric oxygen. B: 76 rain after beginning of hyperoxia. C: 118 rain after beginning of hyperoxia. D: 125 rain after beginning of hyperoxia. Isolated spikes are visible in the cortical and thalamic leads (B), followed by activation of subcortical leads (C), before the appearance of the first hyperoxic seizure (D). Calibrations: 6 sec and 150/~V.

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2. The first seizure Generalized seizures appeared in 33 out of 47 rats. In the other twelve animals no generalized attacks were observed, whereas only isolated spikes were noted in two animals. The greater incidence of initial seizures was observed during the first than in the following hours (19 out of 33). This fact shows the remarkable variability with which the first seizure may appear in relation to a different individual sensitivity. Two different patterns of seizure were observed: 1. In seventeen out of 33 animals the first seizure consisted in a typical "grand mal" attack, the total duration of which was usually over 2 min. The initial part was characterized by a high

discharge rate (15/sec) and by low voltage (100-150 /zV). The final part of the seizure usually began after a period of short duration without epileptic activity and was of low frequency (1/sec or less) and high discharge voltage. The latter part of the seizure often (twelve out of these seventeen rats) had opposite polarity as compared with the first part (Fig. 5, B). 2. In sixteen rats the seizure began after a short synchronization of the recording with low frequency and voltage (150-200 #V), followed by large waves (400-500 #V) at very low frequency. The total duration of these attacks was usually about 15-30 sec (Fig. 2, C, 4, D). The initial seizure always appeared simultaneously in all the cortical leads. The subcortical centers were fired at the same time as the cortex in eleven out of fifteen rats. In the remaining rats the first attack began twice in the subcortiElectroeneeph. cHn. Neurophysiol., 1967, 22:231-238

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cal leads and twice in the cortical leads. The seizure started in one with isolated spikes from the red nucleus and pontine reticular formation and in another with isolated spikes from the region of the peduncle of the mammillary body and from the pontine reticular formation (Fig. 6). The process of seizure extinction was always simultaneous in both cortical and subcortical leads. In some animals the seizure was only in the cerebral recordings and did not appear in the EMG.

3. Subsequent seizures The initial seizure was usually followed by other attacks. It was noticeable that the rats which exhibited early long-duration initial seizures showed milder subsequent attacks. Also in these cases the 2 different patterns of seizure described above were observed. The subsequent attacks always appeared at the same time in all

cortical leads. In the fifteen animals with deep electrodes the initial seizure was sometimes followed by short-duration outbursts in the thalamic and mesencephalic leads and spikes in all the recordings. The second seizure appeared simultaneously in the cortical and subcortical leads in nine out of twelve cases. The beginning was subcortical in one case and cortical in the two others. The third seizure appeared simultaneously in all the recordings in nine out of twelve cases and in three cases the subcortical centers were fired first. In the remaining seizures (48) all the leads were simultaneously fired. In two rats which showed seizures of type I the mesencephalic leads exhibited an initial high discharge rate whereas the cortical leads showed the latter part. In the course of subsequent attacks short-duration outbursts at very high frequency were sometimes observed (Fig. 7, A). Electroenceph. clin. Neurophysiol., 196% 22:231-238

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The process of seizure extinction was invariably simultaneous in the cortical and subcortical leads. Up to 13 seizures were observed in the same animal, even occurring outside the hyperbaric chamber. Reverse in polarity of the final part of a longduration seizure (at least 2 min) was observed in cortical (nineteen out of 32 rats) and subcortical deep leads (ten out of fifteen rats). It is to be noted that the change in polarity in subcortical recordings was predominant in the recordings from the mesencephalon (eight out of eleven animals) and from the pons (five out of six rats), while it was occasional when the recording was taken from the diencephalon (two out of nine rats) (Fig. 7). 4. D e c o r t i c a t e rats

Six decorticate rats were subjected to hyperbaric oxygen. Recordings were taken from the thalamus, the mesencephalon and the cerebellum, Three of these animals showed hyperoxic seizures. The duration of attacks usually exceeded 1 min. The beginning of these seizures was not always simultaneous in all the leads. On the

other hand the extinction of spiking was simultaneous. Experiments are under way to ascertain the beginning of hyperoxic seizures in the decorticate rats. DISCUSSION

The present investigations have shown that: 1. The onset ofhyperoxic seizures was simultaneous in all the cortical recordings. The subcortical leads were usually fired at the same time or earlieI than the cortex. When uncommon seizures began in the cortical recordings, a pre-seizure activity was always seen in the subcortica] centers. 2. The pre-seizure activity was more evident in the recordings from the subcortical centers. 3. The hyperoxic seizures were also observed in decorticate rats, in which the whole cerebral cortex had been removed. This fact supports the view that the cerebral cortex is not necessary for the starting and development of hyperoxic seizure. Divergent views have been held in regard to the beginning of hyperoxic seizures. According to Cohn and Gersh (1945) they had a cortical Eleetroeneeph. chn. Neurophysiol., 1967, 22:231-238

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onset; however, no subcortical records were taken in their experiments. Batini et al. (1954a), since they did not observe initial "foci" of the seizures in the cortex, found no evidence for a cortical beginning. On the other hand, the simultaneous firing of all the cortical recordings suggested that the beginning was in the subcortical centers. However, Batini et aL showed that the onset of seizures was elsewhere than in the nucleus caudatus or intralaminar nuclei of the thalamus, the only subcortical structures explored. Sonnenschein and Stein (1953) emphasized that the earlier appearance of a pre-seizure activity in the occipital recordings than in the parietal was indicative of a cerebellar genesis of hyperoxic seizures.

SUMMARY

The electrical activities of the cerebral cortex and of some subcortical structures were recorded during hyperoxic seizures, in order to ascertain the structures first fired. Unrestrained, unanesthetized rats were subjected to hyperbaric oxygen by being placed in a hyperbaric chamber at 4 arm abs. for 2-4 h. In some experiments E M G s from the biceps muscle of the right forelimb and from posterior cervical muscles were recorded for the duration of the hyperbaric oxygen. A greater incidence of initial seizures was observed during the first hour from the beginning of hyperoxia, The seizures were usually preceded by pre-seizure activity, more eviElectroenceph, clin. Neurophysiol., 1967~ 22:231-238

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dent in the subcortical centers, consisting in increase in voltage and discharge rate and in spindle-like waves. Two different patterns of seizure were observed characterized by different discharge rates and durations. The final part of long-duration seizures (type I) usually exhibited an opposite polarity compared with the initial one. The onset of hyperoxic seizures was simultaneous in all the cortical records. The subcortical leads were usually fired at the same time as the cortex and the process of seizure extinction was invariably simultaneous in the cortical and the subcortical leads. Hyperoxic seizures were also observed in decorticate rats in which the whole cerebral cortex had been removed by suction. This supports the view that the cerebral cortex is not necessary for starting and developing hyperoxic seizures. RI~SUMI~ MODIFICATIONS DE L'ACTIVITI~ I~LECTRIQUE DU CORTEX CI~RI~BRALET DE QUELQUES CENTRES SOUS-CORTICAUX SOUS L~EFFET DE L~OXYGENE HYPERBARE

Ce travail est effectu6 dans le but de mettre en 6vidence les structures qui d6chargent les premieres au cours des crises provoqu6es par l'oxyg~ne hyperbare. Des rats non anesth6si6s et libres de leurs mouvements sont plac6s darts une chambre hyperbarique pendant 2-4 h avec une pression d'Oz ~ 4 atm abs. En plus de l'activit6 61ectrique du cortex et des centres sous-corticaux on enregistre quelquefois I ' E M G du biceps de la patte ant6rieure droite et des muscles cervicaux post6rieurs. La plus grande incidence des crises initiales est observ6e pendant la premi6re heure apr6s le d6but de l'hyperoxie. Ces crises sont habituellement pr6c6d6es par une activit6 pr6critique plus 6vidente dans les centres souscorticaux, rev&ant l'aspect d'une augmentation d'amplitude et de fr6quence des d6charges et d'ondes dispos6es en fuseaux. Deux aspects diff6rents de crises sont observ6s suivant les caract6res et la dur6e de la d6charge. La partie terminale des crises de longue dur6e (type I) montre g6n6ralement une polarit6 oppos6e /a

celle du d6but. Le d6but des crises hyperoxiques est simultan6 dans t o u s l e s enregistrements corticaux. Les enregistrements sous-corticaux montrent que la d6charge ~ ce niveau est habituellement simultan6e par rapport ~ la d6charge corticale. Le processus d'extinction de la crise est toujours simultan6 dans les d6rivations corticales et sous-corticales. Les crises hyperoxiques sont aussi bien observ6es chez les rats enti6rement d6cortiqu6s par succion, ce qui d6montre que le cortex n'est pas n6cessaire pour le d6clenchement et l'entretien des crises hyperoxiques. We are indebted to Prof. V. Longo for his helpful criticisms. REFERENCES BATINI, C., PARMA, M., RICCI, G. F. e ZANCHETTI, A. Aspetti elettroencefalografici della sindrome convulsiva iperossica. Arch. Fisiol., 1954a, 53: 346-353. BATINI, C., PARMA, M., RtccI, G. F. e ZANCHETTI, A. Meccanismi piramidali ed extrapiramidali delle convulsioni iperossiche. Arch. Fisiol., 1954b, 53: 362-369. BERT, P. La pression baromdtrique. Recherches de physiologie expdrimentale. Masson, Paris, 1878, 744 p. COHN, R. and GERSCH, I. Changes in brain potentials during convulsions induced by oxygen under pressure. J. Neurophysiol., 1945, 8: 155-160. GASTAUT, H. and FiSCHER-Williams, M. The physiopathology of epileptic seizures. In J. FIELD et al. (Eds.), Handbook of physiology, Sect. I. Amer. Physiol. Soc., Washington, D.C., 1959, 1: 329-363. KONIG, J. F. R. and KLIPPEL, R. A. The rat brain. A stereotaxic atlas of the forebrain and lower parts of the brain stem. Williams and Wilkins, Baltimore, Md., 1963, 162 p. LAMBERTSON,C. J.tEffects of oxygen at high partial pressure. In W. O. FENN and H. RAHN (Eds.), Handbook of physiology, Sect. 3. Amer. Physiol. Soc., Washington, D.C., 1965, 3: 1027-1046. RALSTON, L. and LANGER, H. Experimental epilepsy of brain-stem origin. Electroenceph. clin. Neurophysiol., 1965, 18: 325-333. SONNENSCHEIN,R. R. and STEIN, S. N. Electrical activity of the brain in acute oxygen poisoning. Electroenceph. clin. Neurophysiol., 1953, 5: 521-523. STEIN, S. N. Neurophysiological effects of oxygen at high partial pressure. Proceeding of underwater physiolDRy symposium. Nat. Acad. Sci., Nat. Res. Council Publ., 1955, 377: 20-24. STERN, !S. N. and SONNENSCHEIN,R. R. Electrical activity and oxygen tension of brain during hyperoxic convulsions. J. aviat. Med., 1950, 21: 401M04.

Reference: RuccI, F. S., GIRETTI, M. L. and LA ROCCA, M. Changes in electrical activity of the cerebral cortex and of some subcortical centers in hyperbaric oxygen. Electroenceph. clin. Neurophysiol., 1967, 22:231-238.