London, N.W. 7. Tubocurarine perfused through the cerebral ventricles of cats produces

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1 J. Physiol. (1962), 164, pp With 7 text-figures Printed in Great Britain THE RELATION BETWEEN SEIZURE DISCHARGE AND MYOCLONUS DURING PERFUSION OF THE CEREBRAL VENTRICLES WITH TUBOCURARINE BY E. A. CARMICHAEL, W. FELDBERG AND K. FLEISCHHAUER* From the National Institute for Medical Research, Mill Hill, London, N.W. 7 (Received 28 May 1962) Tubocurarine perfused through the cerebral ventricles of cats produces a number of effects which depend on the sites reached from different parts of the ventricular system. Experiments on anaesthetized cats have shown that tremor originates from an action of tubocurarine on the hypothalamus reached from the ventral part of the third ventricle (Carmichael, Feldberg, & Fleischhauer, 1962). On the other hand, the abnormal discharge in the e.e.g. which resembles the seizure discharge of epilepsy must be attributed to an action on the hippocampus and/or amygdala, since it occurs only when tubocurarine enters the posterior half of the lateral ventricle (Feldberg & Fleischhauer, 1962). Another effect is myoclonus, which appears to be a remnant of the convulsive activity seen in the unanaesthetized cat on intraventricular injection of tubocurarine. What is the association between the abnormal discharge and the myoclonus, and what is the effect of anaesthetics on these two phenomena: are they affected similarly or differently? These problems have been investigated in the present experiments which include observations on respiratory changes and tremor, and have revealed an additional pattern of motor activity produced by intraventricular tubocurarine-walking or running movements. METHODS The experiments were carried out on anaesthetized cats weighing between 2-5 and 4-5 kg. Anaesthesia was effected either with intravenous chloralose (60 mg/kg) or with intraperitoneal pentobarbitone sodium (36 mg/kg). The left femoral vein was cannulated. For the chloralose injections this was done under anaesthesia induced with ethyl chloride and ether. The trachea was cannulated and respiration was recorded on a smoked drum through a a side tube in the tracheal cannula, connected by rubber tubing to a Marey tambour. In some experiments the left femoral artery was cannulated for recording blood pressure. With the cat lying on its belly the head was fixed to the ear bars and mouth-piece of a head holder, similar to that of the Horsley Clark stereotaxic instrument. * Permanent address: Anatomisches Institut, Martinistrasse, 52, Hamburg 20, Germany. 20 Physiol. 164

2 302 E. A. CARMICHAEL AND OTHERS Myoclonus of the legs was recorded on a smoked drum by means of tension levers attached to the foot of the right hind leg, and to the wrist of the right foreleg. Before the threads from the foot and wrist were connected to the tension levers the cat was lifted and placed with its belly on a wooden block, 15 cm high, so as to allow free movement of the limbs. The method of perfusion of the cerebral ventricles was the same as described previously (Feldberg & Fleischhauer, 1962). If not otherwise stated the tubocurarine was perfused from both lateral ventricles to aqueduct with separate injectors, each delivering fluid at a rate of 01 ml./min. The method of recording the e.e.g. epidurally with frontal and occipital monopolar leads on both sides was also the same as that previously described. In the present experiments, however, vigorous movements of the animals which occurred on prolonged perfusion with tubocurarine sometimes caused movements of the indifferent electrode and introduced artifacts in the e.e.g. WVhen this happened the e.e.g. was recorded bipolarly from the right frontal to the right occipital, and from the left frontal to the left occipital electrodes. RESULTS Experiments under chloralose anaesthesia Perfusion of tubocurarine 1/5000 from both lateral ventricles to aqueduct produces in the e.e.g. an abnormal discharge, which is similar to that previously obtained in experiments under pentobarbitone sodium anaesthesia on perfusion with stronger concentrations of tubocurarine (Feldberg & Fleischhauer, 1962). The discharge begins a few minutes after the onset of tubocurarine perfusion with a rhythmic discharge of large surfacenegative spikes, which become multiple as perfusion progresses, increase in frequency and are interrupted from time to time by episodes, i.e. short periods of fast synchronous high-voltage activity or of multiple spikes occurring in quick succession. An episode is often followed by a period of electrical silence. The abnormal discharge appears first in the occipital and then in the frontal leads, but at a lower voltage. In addition, a new feature is observed. Under chloralose anaesthesia positive spikes are recorded synchronously in all four leads. They appear spontaneously and as a result of extraneous stimuli, for instance, following an auditory stimulus such as clapping the hands. The positive spikes, both the spontaneous and the evoked ones, become increasingly greater during the perfusion with tubocurarine. This effect begins with, or shortly before, the onset of the abnormal discharge of large surface-negative spikes. Later, when these spikes have become multiple, the response to a 'clap' consists of a large positive spike followed by a large negative spike which is multiple and indistinguishable from that of the abnormal discharge. On repeated clapping at a rate faster than about 1/sec, some claps fail to produce a response in the e.e.g. The motor effects produced by the perfusion of tubocurarine 1/5000 from both lateral ventricles to aqueduct are tremor and myoclonus. Tremor is one of the earliest effects and, as described previously, develops

3 '303 SEIZURE DISCHARGE AND MYOCLONUS simultaneously with an increase in depth and frequency of respiration. Its onset is associated with a general increase in muscle tension. Myoclonus appears somewhat later, first in the muscles of respiration, then in those of the neck and shoulder girdle, next in those of the forelegs, and finally in those of the hind legs. Myoclonic contractions also occur in the facial and, at a late stage, in the extraocular muscles. The myoclonus of the respiratory muscles, which results in short inspiratory muscular efforts and is superimposed on the regular rhythm of respiration, usually appears a few minutes after the onset of tremor. As seen from the respiratory records at B and E of Fig. 1, their effectiveness depends on the phase of the respiratory cycle at which they occur. They are more effective during inspiration or expiration than during an apnoeic phase. The inspiratory muscular efforts increase in strength and frequency, and finally the rhythmic phases of respiration are no longer discernible: ventilation is then maintained by a quick succession of strong myoclonic contractions of the inspiratory muscles. A few minutes after the onset of the inspiratory muscular efforts the myoclonus involves the muscles of the neck and shoulder girdle, and each myoclonus causes a short, abrupt elevation of the scapulae. Again a few minutes later, the myoclonus spreads to the forelegs, causing weak contractions which increase in strength and frequency. Several minutes elapse before myoclonic jerks are recorded from the hind legs, although a weak contraction, too slight to deflect the tension lever, occasionally occurs during this time in association with a strong myoclonus in the forelegs. As the myoclonus develops some jerks occur which are extremely vigorous. At first they appear only from time to time, either singly or in small groups. Later more and more jerks show this increase in vigour, until finally all jerks have become extremely vigorous and follow each other in quick succession. The myoclonus also spreads to the muscles supplied by the cranial nerves. Twitching of the lower lips usually occurs first and is often followed by sudden movements of the vibrissae and then by champing and gaping of the jaws. The champing occurs at a higher frequency than the myoclonus elsewhere, but is later interrupted by gapi-ng movements which synchronize with the myoclonus. The gaping movements increase in vigour and frequency along with the myoclonus and eventually replace the champing. The contractions of the extraocular muscles result in sudden downward movements of the eyeballs. These movements synchronize with the myoclonus elsewhere. Initially they are small, but gradually they increase in range. The increased excitability to extraneous stimuli of cats anaesthetized 20-2

4 304 E. A. CARMICHAEL AND OTHERS A B J.LLLLt C ---A ---L 1* E Fig. 1. Simultaneous records of respiration (left) and e.e.g. (right) in a cat anaesthetized with chloralose. A before, B-E 10, 13, 24 and 45 min after beginning of perfusion with tubocurarine 1/5000 from both lateral ventricles to aqueduct. In B and C the white dots above the respiratory record indicate the inspiratory muscular efforts; in B-E the abnormal spikes are signalled below the respiratory record, a large signal for a large, and a small for a small spike. In E myoclonus in the right foreleg is recorded above the respiratory tracing. Time marker on the left, 10 sec. E.e.g. record from left frontal (LF) and left occipital (LO) leads. Calibration 600 pav, negativity upwards; time marker on the right, seconds.

5 SEIZURE DISCHARGE AND MYOCLONUS 305 with chloralose is well known; for example, the jerks which occur on tapping the table on which the animal is lying. This increased excitability is accentuated by the perfusion with tubocurarine. An auditory stimulus such as that produced by clapping the hands, which before perfusion with tubocurarine does not cause muscular contractions, elicits a jerk of the legs after tubocurarine has been perfused for some time. Such evoked jerks are obtainable before the spontaneous myoclonus occurs in the legs; once this begins the evoked jerks quickly increase in vigour, so that they become more vigorous than most of the spontaneous jerks. Later, when all spontaneous jerks have become extremely vigorous and follow each other in quick succession, it is no longer possible to be certain whether a jerk occurs in response to a clap or spontaneously. Correlation between motor effects and abnormal discharge on the e.e.g. It has been shown that tremor is a motor effect which occurs before the abnormal discharge and that there is no correlation between these two (Carmichael et al. 1962). There is, however, a correlation between the abnormal discharge and the various forms of myoclonus. The abrupt inspiratory muscular efforts begin either shortly before or with the onset of the abnormal spike discharge in the occipital leads. At first there are fewer spikes, but most though not all coincide with an inspiratory effort. Later, when the abnormal discharge develops from large single to large multiple spikes with smaller ones in between, the association becomes closer and often absolute, so that each spike, whether large or small, coincides with an inspiratory effort and each inspiratory effort with a spike. This development is illustrated in Fig. 1, in which e.e.g. records from the frontal and occipital leads are presented alongside respiratory records taken simultaneously on a smoked drum. In addition, each spike as it occurs on the e.e.g. is signalled on the smoked drum, a large spike by a large and a small by a small stroke. In Fig. 1 A, taken before perfusion with tubocurarine, respiration is uninterrupted by abrupt inspiratory efforts and in the e.e.g. record there are no abnormal spikes. In Fig. 1 B, taken 10 min after the beginning of the perfusion with tubocurarine, six inspiratory efforts, each marked by a dot, are seen superimposed on the regular respiratory rhythm. Of the six, five are small, occurring during apnoea, and one large during expiration. On the e.e.g. six large spikes are recorded, the first three are single and the remaining multiple. Each of the multiple spikes, but only the second of the three single spikes, coincides with an abrupt inspiratory effort, and there are two such efforts without a coinciding spike. In C, taken 3 min later, there are seventeen inspiratory efforts but only four large spikes. These are multiple and coincide with inspiratory efforts. In D, taken 11 min after C, the frequency of the spikes increased, as small spikes have appeared

6 306 E. A. CARMICHAEL AND OTHERS between the large ones, and the relation between spikes and inspiratory efforts has become 1 to 1. But the depth of the inspiratory movements is not related to the size of the spikes. As the inspiratory muscular efforts increase in strength, and as the myoclonus spreads to the muscles of the limbs and face, the close association with the spikes is maintained, but again their size bears no relation to the vigour of the myoclonus. This is illustrated in experiment Fig. 1 at E, which is taken 20 min after D and includes a record of the myoclonus in the right foreleg. By this time myoclonus has spread to the forelegs but not to the hind legs. Every myoclonus and each inspiratory effort except one coincides with a spike. The association with the episodes is as follows. The early episodes, which consist of fast synchronous high-voltage activity, occur usually before the onset of myoclonus. In contrast to the result obtained in the experiments under pentobarbitone sodium anaesthesia (see p. 311) they are not reflected on the respiratory record. In one experiment in which episodes of this type occurred after the onset of myoclonus the jerks became more frequent but less vigorous during the episodes, and ceased during the subsequent period of electrical silence. When episodes consist of a train of multiple spikes in quick succession, each spike coincides with a myoclonus and during the subsequent period of electrical silence jerks are again absent. There is also an association between the myoclonus evoked by clapping the hands and the discharge elicited in the e.e.g. This happens when the perfusion with tubocurarine has continued for some time and the e.e.g. response to clapping no longer consists of a positive spike but of a positive followed by a negative multiple spike. The multiple spike of this response coincides with a myoclonic jerk in the legs. Pentobarbitone sodium. An intravenous injection of a small dose of pentobarbitone sodium, given when all the motor effects associated with the abnormal discharge are fully developed, has a striking result. It quickly abolishes or greatly reduces the motor effects, but hardly influences the spike discharge. An injection of 12 mg is sufficient to abolish, within sec, tremor, eye movements, facial twitches, jaw movements and myoclonic jerks of the legs. Only the myoclonic inspiratory efforts persist, but they are greatly reduced in strength. The rhythmic discharge of multiple spikes, however, continues unchanged or, if affected at all, at a slower rate with little or no reduction in voltage. A decrease in the rate of the rhythmic discharge is associated with a corresponding decrease in frequency of the reduced inspiratory muscular efforts. These changes outlast the transient fall in arterial blood pressure produced by the injection. Within min they begin to wear off and by the end of 1 or 2 hr

7 SEIZURE DISCHARGE AND MYOCLONUS 307 the motor effects have returned, except tremor (which returns occasionally) and the motor response to a clap which does not reappear. The first sign of recovery is an increase in vigour of the inspiratory muscular efforts. Then the facial contractions return; next myoclonus begins in both forelegs and gradually increases in strength; during this time jaw movements reappear, and the last motor effects to return are the myoclonus of the hind legs and of the extraocular muscles. Thus the motor effects return in the order in which they appear after the beginning of a perfusion with tubocurarine. Any effects produced on the frequency and voltage of the abnormal discharge have also disappeared within 2 hr. A B C D - Fig. 2. Tension record from right hind (upper tracing) and foreleg (middle tracing) and record of respiration (lower tracing) in a cat anaesthetized with chloralose, during perfusion of tubocurarine 1/5000 from both lateral ventricles to aqueduct. A-D obtained 54, 95, 109 and 117 min after onset of tubocurarine perfusions. At the arrows (4) intravenous injections of 12 mg pentobarbitone sodium. Time marker, 10 sec. For details see text. Figures 2 and 3 illustrate some of the effects of intravenous pentobarbitone sodium. The rapid abolition of myoclonus in the legs and the reduction in vigour of the inspiratory muscular efforts produced by 12 mg are seen in Fig. 2A. In B, taken 40 min later at a time when the inspiratory muscular efforts have regained their previous strength, the myoclonus in the legs has begun to reappear; in the hind leg it is just perceptible. Ten minutes later, at the beginning of C, the myoclonus in the legs has fully returned and has become even greater than it had previously been. An additional effect is illustrated in C and D following a second injection.

8 308 E. A. CARMICHAEL AND OTHERS Besides the disappearance of the myoclonus in the legs and the reduction in vigour of the inspiratory muscular efforts, there is also a decrease in their frequency which becomes fully developed within a few minutes. This decrease coincides with a reduction in frequency of the spike discharge, as illustrated in Fig. 3, in which the e.e.g. recorded from the left occipital lead is shown below the respiratory record. Each spike is associated with an inspiratory muscular effort, and in A with a myoclonus in the foreleg. Spikes and inspiratory muscular efforts are equally reduced in frequency after the pentobarbitone sodium injection (B). A B Fig. 3. Same experiment as that of Fig. 2. Records taken shortly before (A) and after (B) record C of Fig. 2. From above downwards: tension record from right foreleg; respiration; e.e.g. from left occipital lead; time marker, 10 sec. Calibration of e.e.g. 600,V, negativity upwards. For details see text. There is an additional effect on the e.e.g. which is irreversible and obtained after a first intravenous injection of 12 mg of pentobarbitone sodium. The background activity in the e.e.g. is diminished, the spontaneous positive spikes are abolished (see Fig. 4) and so is the response to a clapping stimulus. Doses as small as 1-3 mg intravenous pentobarbitone sodium are still able to reduce the myoclonus produced by the perfused tubocurarine and sometimes even to abolish the myoclonic jerks in the hind legs. The effect

9 SEIZURE DISCHARGE AND MYOCLONUS 309 begins to wear off within a few minutes, and it takes about 30 min for the myoclonus fully to return. In two experiments in which pentobarbitone sodium was injected intraperitoneally in a dose of 12 and 20 mg/kg, respectively, the effects on myoclonus and spike discharge were essentially the same as those described for the intravenous injections, but with this difference that they began later, developed more slowly and lasted longer. It took several 1X~~~~l Fig. 4. Bipolar e.e.g. recorded from left frontal to left occipital electrode in a cat anaesthetized with chloralose during perfusion with tubocurarine 1/5000 from both lateral ventricles to aqueduct. At the arrow (i), 28 min after the onset of the tubocurarine perfusion, intravenous injection of 12 mg pentobarbitone sodium. Calibration 600 1V, negativity upwards; time marker, seconds. minutes for the injection to become effective. The first signs were the diminution of background activity in the e.e.g., the disappearance of the positive spikes, the abolition of the e.e.g. and motor response to the clapping stimulus, and a reduction in strength of the most vigorous myoclonic jerks. It took an hour before the myoclonus disappeared in the forelegs and another hour before it began to return. Experiments under pentobarbitone sodium anaesthesia Under pentobarbitone sodium anaesthesia a stronger concentration of tubocurarine has to be perfused through the cerebral ventricles, in order to elicit the abnormal discharge in the e.e.g., than under chloralose (Carmichael et al. 1962). In the present experiments perfusion with tubocurarine 1/2000 produces approximately the same changes in the e.e.g. as does 1/5000 under chloralose. Tremor and myoclonus also occur, but an additional pattern of motor effects is encountered-alternating movements of the legs resembling walking or running. They will be referred to throughout as 'walking movements'.

10 310 E. A. CARMICHAEL AND OTHERS As under chloralose, tremor develops, or if already present, increases in strength within a few minutes of the beginning of the tubocurarine perfusion. At the same time there is an increase in depth and frequency of respiration. Within the next few minutes walking movements occur which involve all four legs, and finally myoclonus begins with abrupt inspiratory muscular efforts, and spreads first to the forelegs and then to the hind legs. The myoclonic jerks in the legs are superimposed on the walking movements. Unlike the experiments under chloralose, myoclonus does not appear shortly after the onset of tremor, but after an interval which may be as long as 30 min. In addition, the following differences are noted. The myoclonus remains less vigorous, spreads more slowly to the leg muscles and does not involve the extraocular muscles although twitching of facial muscles is occasionally seen. The walking movements occur only when the tubocurarine enters the lateral ventricle. This was shown in experiments in which artificial cerebrospinal fluid was perfused from one, and tubocurarine 1/1000 from the other implanted cannula, the tip of which rested in the third ventricle. Under this condition, in which no tubocurarine enters either lateral ventricle (Feldberg & Fleischhauer, 1962), no walking movements occurred. However, when in the same experiment 30 min later the tubocurarine was perfused through the cannula whose tip rested in the lateral ventricle, the walking movements appeared within a few minutes. Correlation with the abnormal discharge. The walking movements, like tremor, are independent of the abnormal discharge. Usually they begin and increase in strength and frequency before the abnormal spikes are recorded in the occipital leads, and they do not change in character with the development of the abnormal discharge. At no time is there evidence of an association between the movements and the abnormal spikes or episodes. The myoclonus, however, shows an association with the abnormal discharge. This association develops gradually and in essentially the same way as in the corresponding experiments under chloralose. But there is this difference: the abnormal spikes appear and may continue for up to 30 min before there are signs of myoclonus. There then follows a period in which spikes are more frequent than the inspiratory muscular efforts, and although most of these are accompanied by a spike, some are not. This condition remains after the myoclonus has spread to the forelegs, but eventually the ratio between spikes and myoclonus becomes 1 to 1. The association with the episodes is as follows: The initial episodes, which consist of fast synchronous high-voltage activity and occur before the onset of myoclonus, are reflected on the respiratory record by a decrease in depth and frequency of the respiratory movements (see Fig. 5). Later,

11 SEIZURE DISCHARGE AND MYOCLONUS 311 when episodes consist of a train of multiple spikes in quick succession, each spike coincides with a myoclonus, and during the period of electrical silence following an episode there are no myoclonic jerks, but the walking movements continue. Fig. 5. Simultaneous records of respiration (upper tracing) and e.e.g. from left occipital electrode (lower tracing) in a cat anaesthetized with pentobarbitone sodium taken 14 min after beginning of perfusion with tubocurarine 1/2000 from both lateral ventricles to aqueduct. Calibration 600 pav, negativity upwards; time marker, 10 sec. Additional intravenous pentobarbitone sodium. An intravenous injection of 12 mg pentobarbitone sodium has essentially the same action as in the corresponding experiments under chloralose anaesthesia. But the effect is more transient and myoclonus is less affected. In addition, walking movements, which are not observed in the chloralose experiments, are abolished. At an early stage of the tubocurarine perfusion, when depth and frequency of respiration have increased and when tremor and walking movements are present, such an injection abolishes the motor effects and decreases depth and frequency of respiration (Fig. 6), thus reversing the changes produced by the tubocurarine. At a later stage, when myoclonus has developed, it is reduced by the injection. The effect is less pronounced than in the experiments under chloralose, where the myoclonic jerks of the legs are abolished. Under pentobarbitone sodium they are seldom abolished by the injection, even though they are less vigorous. The effect on the abnormal discharge in the e.e.g., if present, is similar to but of shorter duration than that obtained under chloralose.

12 312 E. A. CARMICHAEL AND OTHERS Intravenous chloralose. An intravenous injection of 20 mg chloralose/kg abolishes the walking movements but enhances the myoclonus without reducing its frequency or that of the spike discharge. The effect is long lasting. Recovery has not been observed during the following hours. Figure 7 illustrates the effect on the tension record obtained from the right foreleg. The myoclonic jerks produce sharp upward deflexions, which in record A, taken 3 min before the injection, are superimposed on slow Fig. 6. Respiratory record in a cat anaesthetized with pentobarbitone sodium. Lower tracing continuation of upper one after an interval of 27 min. The arrow (t) applies to both tracings; it indicates beginning of perfusion with tubocurarine 1/2000 from both lateral ventricles to aqueduct in upper, and intravenous injection of 12 mg pentobarbitone sodium in lower tracing. Time marker, 10 sec. Fig. 7. Tension record from the right foreleg of a cat anaesthetized with pentobarbitone sodium during perfusion of tubocurarine 1/2000 from both lateral ventricles to aqueduct. A taken 110 min after beginning of the tubocurarine perfusion; B taken 13 min later and 10 min after an intravenous injection of 50 mg chloralose. Time marker, 10 sec. (For details see text.)

13 SEIZURE DISCHARGE AND MYOCLONUS 313 irregular waves resulting from the walking movements. In record B, taken 10 min after the injection, these waves have disappeared and the size of the recorded jerks has increased. DISCUSSION The main problem investigated in the present experiments is the association between the abnormal discharge in the e.e.g., which resembles the seizure discharge of epilepsy, and the various other effects obtained by tubocurarine perfused through the cerebral ventricles. Tremor, which is not part of the symptomatology of epilepsy, has recently been shown to be unrelated to the abnormal discharge and this has been confirmed in the present experiments. Since tremor results from an action of the tubocurarine on the hypothalamus reached from the third ventricle, and the abnormal discharge from an action on structures reached from the posterior half of the lateral ventricle, i.e. the hippocampus and/or amygdala, this lack of association between the two phenomena is not surprising. Since recent experiments, not yet published, strongly point to the hippocampus as the site of origin, we shall in the following paragraphs refer to this structure as the site of origin of the abnormal discharge. As far as the changes in depth and frequency of respiration are concerned they occur before the onset of the abnormal discharge and remain independent of it. In the experiments under pentobarbitone sodium anaesthesia the initial episodes, however, are reflected by a decrease in depth and frequency of respiration. This association does not justify the conclusion that the increase in depth and frequency produced by the tubocurarine results from its action on the hippocampus, although this possibility cannot be excluded since electrical stimulation of this structure, as well as of the amygdala, may result in similar changes of respiration (for references see Green, 1960; Gloor, 1960). No experiments have so far been done to establish the site where tubocurarine acts when producing these respiratory changes. The site may well be in the wall, not ofthe lateral, but of the third ventricle. If so, this would explain why they begin with the onset of tremor and why all drugs which were found to abolish tremor on intraventricular administration also antagonized the respiratory changes (Carmichael et al. 1962). The fact that in these experiments the tubocurarine had been perfused from one, but the antagonizing drug from the other, lateral ventricle is perhaps the most convincing indirect evidence for an action of tubocurarine on structures in the wall of the third ventricle when causing the increase in depth and frequency of respiration. The walking movements have no association with the abnormal dis-

14 314 E. A. CARMICHAEL AND OTHERS charge, although they appear to originate from an action on structures reached not from the third but from the lateral ventricle. It is not known at which site in the wall of the lateral ventricle the tubocurarine acts when eliciting these movements. The fact that they are independent of the abnormal discharge, however, would suggest that the structures acted upon are other than the hippocampus. The present and previous experiments have shown that on perfusion with tubocurarine, myoclonic jerks may occur without abnormal spikes, and spikes without jerks, but that for long periods the association may be absolute, each spike coinciding with a jerk, and each jerk with a spike. These findings suggest that tubocurarine when producing myoclonic jerks acts on structures other than the hippocampus and that these structures come under the influence of the hippocampal discharge once it has fully developed, with the result that every jerk coincides with a spike. Further, the finding that myoclonus may be eliminated by intravenous pentobarbitone sodium without affecting the abnormal discharge at all or only slightly indicates that the tubocurarine-induced activity in these structures, or its projection to the motor neurones, is more sensitive to the depressant action of pentobarbitone sodium than the abnormal hippocampal activity and its projection to the cerebral cortex. It has been mentioned that the myoclonus appears to be a remnant of the convulsive activity seen in the unanaesthetized cat after an intraventricular injection of tubocurarine. In the present experiments it is shown that even this remnant can be eliminated by intravenous pentobarbitone without affecting the abnormal discharge, or at most only slightly. Thus the pentobarbitone sodium has a much weaker depressant effect on the abnormal hippocampal activity and on its projection to the cerebral cortex than it has on those structures through which the myoclonic discharge is mediated. A depressant action on mid-brain structure would explain the paradox that, in cats anaesthetized with chloralose, intravenous pentobarbitone abolishes not only the auditory myoclonus which can be elicited during the tubocurarine perfusion but also the multiple spike associated with each evoked myoclonus, although the pentobarbitone sodium injection does not affect the multiple spikes of the abnormal discharge. Blocking of the auditory response in the mid-brain, from where it would otherwise spread to the hippocampus and to the cerebral cortex, would have this effect and would also explain why the positive spike which precedes the multiple negative spike is abolished as well. Similarly, the abolition of the positive spikes that occur spontaneously in this condition could also result from a block at mid-brain structures of incoming impulses from other unknown regions.

15 SEIZURE DISCHARGE AND MYOCLONUS 315 On the other hand, the accentuation of the myoclonic jerks following an intravenous injection of chloralose during perfusion of tubocurarine in cats anaesthetized with pentobarbitone sodium is best explained by an excitatory action of chloralose on mid-brain structures and not on the hippocampus itself, since the projection of its activity to the cerebral cortex, causing the abnormal discharge, remains unaffected. Chloralose is known to have some of the properties of a convulsant and to produce over-excitability (Adrian & Moruzzi, 1938). This action is apparently on structures lining the ventricular cavities, since over-excitability occurs also when small amounts of chloralose are injected intraventricularly (Feldberg, 1959). The accentuation of myoclonus produced by intravenous chloralose goes hand in hand with an abolition of the walking movements. This shows that chloralose affects the various patterns of drug-induced motor activity differently, and has thus an excitatory action on some and a depressant action on other parts of the central nervous system. The abolition of the walking movements recalls a similar effect exerted by chloralose on another pattern of motor activity, the abolition of scratching movements elicited on topical application of tubocurarine to the dorsal surface of the upper cervical cord (Feldberg & Fleischhauer, 1960). In this instance the depressant action of chloralose is on the spinal cord. Whether chloralose also abolishes the walking movements by its depressant action on the cord or whether it has in addition a depressant action on those structures in the wall of the lateral ventricles on which the tubocurarine acts when producing these movements, has not been examined. SUMMARY 1. In cats anaesthetized with either chloralose or pentobarbitone sodium, tubocurarine was perfused from both lateral ventricles to aqueduct whilst the e.e.g. was recorded from occipital and frontal epidural electrodes. The association between the changes in the e.e.g. and the motor effects produced by the tubocurarine has been investigated and a comparison made of the influence of the two anaesthetics on the various effects. 2. Under chloralose anaesthesia perfusion of the cerebral ventricles with tubocurarine 1/5000 produces an abnormal discharge in the e.e.g., an increase in voltage of the positive spikes which occur spontaneously or are evoked by a 'clap', an increase in depth and frequency of respiration, tremor and myoclonus. 3. The abnormal discharge resembles the seizure discharge in epilepsy and consists of large surface-negative spikes which later become multiple and are interrupted from time to time by episodes, i.e. short periods of fast synchronous activity.

16 316 E. A. CARMICHAEL AND OTHERS 4. The changes in respiration and tremor are early effects of the tubocurarine perfusion and show no correlation with the abnormal discharge. 5. The myoclonus begins with small inspiratory muscular efforts which increase in strength and frequency, spreads to the muscles of the neck, shoulder, face and legs and finally to the extraocular muscles; it occurs synchronously in all these muscles. A myoclonic jerk is also evoked by an extraneous stimulus, such as a clap, which has been ineffective before the tubocurarine perfusion. 6. The inspiratory muscular efforts begin shortly before the onset of the abnormal discharge. When the spikes appear some, but not all, coincide with an inspiratory effort, yet there are many more inspiratory efforts than spikes. As myoclonus increases the association with the abnormal discharge becomes closer, and finally when the spikes have become multiple each spike coincides with a jerk and each jerk with a spike. The auditory myoclonus also is associated with a negative multiple spike which follows the intitial positive spike. 7. Deepening the chloralose anaesthesia with an intravenous injection of a small amount of pentobarbitone sodium has little effect on the abnormal discharge in the e.e.g., but greatly reduces and often abolishes myoclonus for long periods. The auditory myoclonus is abolished with its associated response in the e.e.g. Spontaneous positive spikes and tremor are also abolished. 8. Under pentobarbitone sodium anaesthesia perfusion of the cerebral ventricles with tubocurarine 1/2000 is required to produce approximately the same changes in the e.e.g. as does a 1/5000 solution under chloralose. But there are no positive spikes before or during the tubocurarine perfusion and a clap does not elicit a response on the e.e.g. 9. The changes in respiration are the same as those described in the chloralose experiments. Tremor and myoclonus occur. Myoclonus, however, begins later, spreads more slowly, does not involve the extraocular muscles, remains less vigorous and is not evoked by clapping. Further, there are walking or running movements which are not seen under chloralose anaesthesia. 10. The association of the abnormal discharge with tremor and myoclonus is similar to that described in the experiments with chloralose anaesthesia. There is no association with the walking movements. A decrease in depth and frequency of respiration has been observed during initial episodes. 11. Deepening the pentobarbitone sodium anaesthesia with intravenous injection of additional small amounts of pentobarbitone sodium abolishes walking movements and tremor, decreases depth and frequency of respiration. The injection has little effect on the abnormal discharge but reduces

17 SEIZURE DISCHARGE AND MYOCLONUS 317 and sometimes abolishes myoclonus. The effect is of shorter duration than in the corresponding experiments under chloralose anaesthesia. 12. Deepening the pentobarbitone sodium anaesthesia with intravenous injection of a small dose of intravenous chloralose abolishes the walking movements. The injection has little effect on the abnormal discharge but increases the vigour of the myoclonic jerks. The effect is long lasting. REFERENCES ADRIAN, E. D. & MoRuzzI, G. (1939). Impulses in the pyramidal tract. J. Physiol. 97, CARMICHAEL, E. A., FELDBERG, W. & FLEISCHEAUER, K. (1962). The site of origin of the tremor produced by tubocurarine acting from the cerebral ventricles. J. Physiol. 162, FELDBERG, W. (1959). A physiological approach to the problem of general anaesthesia and of loss of consciousness. Brit. med. J. ii, FELDBERG, W. & FLEISCHHAUER, K. (1960). Scratching movements evoked by drugs applied to the upper cervical cord. J. Physiol. 151, FELDBERG, W. & FLEISCHHAUER, K. (1962). The site of origin of the seizure discharge produced by tubocurarine acting from the cerebral ventricles. J. Physiol. 160, GLOOR, P. (1960). Amygdala. In Handbook of Physiology. Section I. Neurophysiology, Vol. 2. ed. FIELD, J. Washington, D.C.: American Physiological Society. GREEN, J. D. (1960). The hippocampus. In Handbook of Physiology. Section I. Neurophysiology. Vol. 2. ed. FIELD, J. Washington, D.C.: American Physiological Society. '' k~ 2i1 Physiol. 164