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Persistent thalamic and cortical barbiturate spindle activity after ablation of the orbital cortex in cats
Electroencephalography and Clinical Neurophysiology Elsevier Publishing Company, Amsterdam - Printed in The Netherlands
485
PERSISTENT T H A L A M I C A N D CORTICAL B A R B I T U R A T E SPINDLE ACTIVITY A F T E R A B L A T I O N OF THE ORBITAL CORTEX IN CATS EVA DAHL, LEIF I. GJERSTAD AND KNUT KR. SKREDE hTstitute o[' Neurophysiology, University of Oslo, Karl Johansgt. 47, Oslo l (Norway)
(Accepted for publication: April 28, 1972)
Several investigations indicate that the pacemaker for thalamic and cortical barbiturate spindles is located in the thalamus (see Andersen and Andersson 1968). In contrast to earlier findings (Dempsey and Morison 1942; Morison et al. 1943; Jasper 1949), Andersen and Sears (1964) found that all major thalamic nuclei were able to produce spontaneous rhythmic discharges. Andersen et al. (1967a) showed that ablation of the midline and the intralaminar nuclei did not abolish spontaneous barbiturate spindle activity either in the lateral part of the thalamus or in the cortex. Successive ablation of the lateral part, however, was reported to abolish normal spindle activity in the pericruciate region. A major part of the rhythmic activity was therefore assumed to be transmitted directly to the cortex in a distinct topographical pattern in accordance with the distribution of the specific thalamo-cortical projections. On the other hand, Velasco and Lindsley (1965) and Velasco et al. (1968) reported that ablation of the orbital cortex completely suppressed cortical recruiting waves and spindle activity in both the thalamus and the cortex. This effect was explained by assuming that connections between the thalamic nuclei and the orbital cortex regulate the rhythmic activity in both the thalamus and cortex. The system investigated by Velasco et al. (1968) was thought to belong to the non-specific system described by Morison and Dempsey (1942) and by Jasper (1949). Anatomical evidence for such a system has been found by Nauta and Whitlock (1954) and by Scheibel and Scheibel (1967). According to the hypothesis of Andersen et al. (1967b), however, the intactness of the orbital cortex should be of
less importance for the thalamic and cortical rhythmic activity than suggested by Velasco and Lindsley (1965) and Velasco et al. (1968). The present study is an effort to clarify the discrepancy reported above, by recording thalamic and cortical spontaneous activity in barbiturate anaesthetized cats before and after selective ablations of the orbital cortices. The investigation shows that intactness of the orbitofrontal cortex is of minor importance for the occurrence of rhythmic barbiturate activity in the thalamus and in the cortex. These findings are supported by the parallel and independent investigation by Robertson and Lynch (1971) showing persistent cortical rhythmic activity following orbital ablations. METHODS
Operative procedure
Adult cats anaesthetized by intraperitone~tl injection of sodium pentobarbital (30 mg/kg body weight) were used. During the time of recording, additional doses were given intravenously to maintain a constant level of anaesthesia at which withdrawal of the leg to pinching was just abolished. The head was fixed in a holder and extensive craniotomy was performed. In most experiments the frontal sinuses were opened and the roof of the orbit removed to obtain a good approach to the orbital cortex. For the same reason bilateral enucleation of the eyes was performed in 7 experiments. In most cats the neocortex and the hippocampus overlying the thalamus were removed by suction. This operation was restricted rostrally, so that thalamo-cortical fibres to the somatosensory cortex were left intact. Electroenceph. clin. Neurophysiol., 1972, 33:485-~96
486 Ablation of the orbital cortex Ablation of the orbital cortex was performed by aspiration under visual guidance. In most experiments the lesions were made on both sides at the same time. The ablated area usually included the anterior sylvian convolution, the prepiriform area and the olfactory tubercle (Velasco and Lindsley 1965). In 4 experiments the lesions were restricted to a smaller area lying around the presylvian gyrus (Skinner and Lindsley 1967). After an experiment the brain was removed and fixed in 10~, formalin. The areas with lesions were studied macroscopically both before and after fixation. In some experiments histological sections were also prepared, to ascertain that complete removal of the intended part of the cortex had been performed. The blood supply to the brain appeared to be critical for the cortical rhythmic activity. Therefore, the colour and the consistency of the cortex during the experiments were observed. In 3 cats India ink or vinyl acetate was injected in the carotid artery after the lesions were made to study whether the cortical blood supply was altered by the ablations. Identical injection was performed on 2 cats without orbito-frontal lesions; these cats served as controls. Recordin9 and stimulation In cats in which the thalamus was exposed, thalamic recordings were made with coarse glass micropipettes (tip resistance 1-2 M~) filled with 4 M NaC1. In 5 experiments in which the neocortex was left intact thalamic records were made by electrolytically sharpened tungsten micro-electrodes (Hubel 1957), which penetrated the cortex without causing any significant damage to it. The signals from the microelectrodes were fed through a cathode followeramplifier system and displayed on an oscilloscope and ink writer. Records from the cortex were obtained by silver ball electrodes resting lightly on the pial surface. The indifferent electrode was usually situated on the bone or in the surrounding connective tissue. The somatosensory cortex and the ventrobasal complex of the thalamus were identified by recording the evoked potentials following electrical stimulation of the nerves of the contra-
E. DAHL et al. lateral foreleg. In addition, the ventro-basal complex of the thalamus could be identified by the response to tactile stimulation of the skin, or movement of a joint. Spindle measurements To obtain a quantitative expression of the thalamic and cortical rhythmic activity the following parameters were measured before and after cortical ablation: (1) the duration of the spindles; (2) the intra-spindle frequency; (3) the spindle amplitude; and (4) the interval between successive spindles. The data were generally obtained from 30 consecutive spindles. In the thalamus as well as the cortex the spindle duration was measured from the start of the first to the end of the last positive wave. In the cortical records the intra-spindle frequency was counted as the number of waves per second. In the thalamus the interval between consecutive bursts of spikes within the spindle and the number of P waves (positive waves following unitary discharges) per second were measured. The amplitude of the cortical spindle was taken as its peak-to-peak amplitude after smoothing; i.e., deleting waves with exceptionally large amplitudes. In the thalamus the spindle amplitude was taken as the maximal P wave amplitude of the smoothed spindle. Since it proved difficult to record from the same thalamic units before and following the orbital ablations, no effort was made to count the number of single units participating in a spindle. The spindle interval was defined as the time from the start of one spindle to the start of the following one. Since neither thalamic nor cortical spindles usually showed an idealized appearance, it often proved difficult to apply the criteria listed above. In order to counteract any bias by the observers, three observers measured the same data independently. The bias was probably small, since the histograms obtained by the three investigators were practically identical. RESULTS Effect QJ lesions on thalamic spindle activity To see whether orbital ablations affected thalamic spindle activity, the orbital cortex was Electroenceph. olin. Neurophysiol., 1972, 33:485 406
487
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Fig. 1. Thalamic spindle activity recorded from the same site within the ventral postero-lateral nucleus (VPL) before (B) and after (D) extensive bilateral ablation of the orbital cortex, illustrated in A. The peripherally evoked potential (C, E) was not changed by the lesion. The spindle duration, intra-spindle frequency, P wave amplitude and spindle occurrence were also unaffected (F I).
removed in l0 cats. In barbiturate anaesthetized animals thalamic spindles could still be recorded following both small and extensive unilateral or bilateral ablations of the orbital cortex. In one of the experiments in which extensive bilateral lesions of the orbital cortices were made by suction (Fig. 1, A), spontaneous thalamic activity was recorded with micro-electrodes from the ventral postero-lateral (VPL) nucleus before and after the lesions (Fig. 1, B and D). Attempts were made to place the electrode at the same site within the VPL after the ablations, guided by the maximal response to peripheral stimulation (Fig. 1, C and E). Both before and after the surgery, the activity showed the characteristic spindle appearance with episodes of unitary discharges appearing at regular intervals, separated by periods of relative silence. Each episode of activity lasted 2-3 sec and consisted of series of bursts of spikes superimposed on negative waves. The bursts were separated by positive waves, P waves, lasting approximately 100 msec. The thalamic somatosensory evoked potentials were unchanged after removal of the
orbital cortices. The duration of the spindles, the intraspindle frequency, the amplitude of the P waves and the spindle intervals were measured from 30 successive spindles before and after the lesion and plotted as histograms (Fig. 1, F-I). There were no clear changes in either of the parameters after the ablation (hatched columns) compared with the activity before it (open columns). In most experiments the investigation was confined to the VPL nucleus, which can easily be identified by electrophysiological means. To study whether other parts of the thalamus also presented spindle activity after removal of the orbital cortex, the thalamic complex was systematically investigated in 3 cats after extensive bilateral lesions. In the experiment shown in Fig. 2 the VPL nucleus was identified electrophysiologically after the lesion was made (Fig. 2, A). Seven penetrations were subsequently made through different parts of the thalamus on both sides, using the VPL as a reference point. Along each of the tracks spindle activity was recorded from several positions. Examples of the activity Electroenceph. clin. Neurophysiol., 1972, 33:485 496
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recorded from the VPL, the ventro-lateral and the central median nuclei are presented in Fig. 2, B D. In this experiment the cortical postcruciate spindle activity and evoked potential disappeared following the ablation. Recordings from the thalamus were mostly made after removal of the overlying cortex (the g
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lateral, the suprasylvian and the ectosylvian gyri). To avoid a systematic error due to this procedure, which has been reported to have a facilitatory effect on spindle activity (Velasco et al. 1968), the neocortex was left intact in 5 experiments. The results obtained in the latter experiments did not differ from those in which
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Electroenceph. clin. Neurop,Sysiol.. 1972, 33:485~-96
489
SPINDI.ES AND ORBITAL CORTEX
thalamic exposure was performed, either with respect to the thalamic or to the cortical spindle activity. In two experiments only was it impossible to record thalamic spindle activity after severing the orbital cortex, probably because the whole brain rapidly deteriorated due to excessive bleeding or oedema.
Ej]ect of lesions on cortical spindle activity The effect of orbital ablation on rhythmic cortical activity was in some contrast to the effect on the thalamic activity, the cortical spindles being more easily influenced by the lesions. The cortical activity could disappear completely while typical spindle activity was recorded in the thalamus. In some experiments distinct cortical spindles were recorded immediately after the lesions were made, but disappea-
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red during the following 10 min, simultaneously with the development of a pronounced oedema. In most experiments, however, the rhythmic cortical activity was not abolished by orbital lesions, but only decreased in amplitude. The degree of amplitude decrease depended both on the depth and extent of the lesion and on the distance of the cortical electrode from it. A restricted bilateral orbital lesion was earlier reported to reduce spindle activity in animals under Flaxedil (Skinner and Lindsley 1967). The effect of such a lesion in our barbiturate anaesthetized preparations is illustrated in Fig. 3. A slight reduction of the spindle amplitude occurred in the left postcruciate area but with an unaltered spindle activity in both the right postcruciate cortex (Fig. 3, C-F) and the posterior parts of the cortex (suprasylvian gyrus). The thalamic activity was unaltered by the operation
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Fig. 4. After a unilateral lesion of the left orbital cortex the spindle amplitude of the left postcruciate cortex was reduced (C). The amplitude was only slightly further decreased when the right orbital surface was removed (D). The results are expressed graphically in E and F. The spindle amplitude of the right postcruciate cortex was unaltered after the bilateral lesion. A : Extent of the lesion,~
Electroenceph. clin. Neurophysiol., 1972, 33:485 496
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According to Velasco et al. (1968) unilateral ablation attenuated the cortical spindles, while after bilateral lesions they were abolished. In this investigation it was found that unilateral lesions sometimes gave a reduction of pericruciate spindle amplitude on the affected side, while the unoperated side was unaltered. Subsequent ablation on the second side usually caused no further decrease of the spindle amplitude on the initially operated side. In such an experiment the total lesion was as shown in Fig. 4, A. After removal of the orbital surface on the left side, the spindles recorded from the ipsilateral pericruciate region were reduced in amplitude (Fig. 4, B, C and E). Bilateral lesion caused only a slight further decrease of the amplitude (Fig. 4, D and F). On the right side the spindle amplitude was not reduced at all (Fig. 4, G) after the bilateral lesion. With extensive orbital lesions a marked difference between rostral (near the ablated area) and caudal cortical areas (lateral, suprasylvian and ectosylvian gyri) was often observed. In Fig. 5 the activity in the postcruciate area is compared with the activity in the middle suprasylvian gyrus of the same hemisphere. Fig. 5, A gives the lesion. The activity in the postcruciate cortex (Fig. 5, B and C) was affected by the surgery, while no obvious change can be seen in
the activity of the suprasylvian gyrus (Fig. 5, D and E). The evoked response recorded in the somatosensory cortex was greatly depressed after the ablation (Fig. 5, G). To obtain a quantitative expression of the changes in spindle activity in this experiment the spindle duration, the intra-spindle frequency, the spindle amplitude and the spindle intervals were measured from 25 successive spindles before and after the lesion of the orbital cortex. The histograms (Fig. 6, A D) show that the spindle amplitude was markedly reduced in the postcruciate cortex (Fig. 6, C), while the other parameters were unaltered (Fig. 6, A, B and D). Measurements from the precruciate cortex gave similar results, although the spindle amplitude was reduced even further. In the suprasylvian gyrus, however, no deterioration of spindle activity was observed (Fig. 6, E-H). On the contrary, the duration of the spindles increased (Fig. 6, E). The intra-spindle frequency and the spindle amplitudes were unchanged (Fig. 6, F and G) whereas the spindle intervals increased (Fig. 6, H). The latter observation is partly explained by the increased spindle duration, but may also be partly due to a change in the depth of anaesthesia. In most of our experiments spindle activity was present in the posterior parts of the cortex Electroenceph. clin. Neurophysio/., 1972, 33:485 4 9 6
491
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following bilateral lesions and disappeared only in those few experiments where serious damage to the cortex was observed, such as following impairment of cortical blood flow or pronounced oedema. Usually the spindle pattern recorded after ablations was similar to the control activity. We conclude, therefore, that the amplitude was the only aspect of the cortical spindles which was affected by orbital lesions and that this change was localized to the rostral parts of the cortex; in some experiments the spindles in the frontal cortex were completely abolished by the ablations, while the lateral, the suprasylvian and the ectosylvian gyri still showed well developed and virtually unchanged spindles.
Cortical evoked potentials and spindle amplitudes The localized decrease of spindle amplitude near the removed orbital cortex suggested that this effect might be due either to damage to thalamo-cortical fibres or to the cells surrounding the lesion. The evoked potential in the postcruciate cortex induced by stimulation of a peripheral nerve gives a measure of the integrity of the thalamo-cortical fibres as well as of the condition of the neurones in the primary somato-
sensory cortex. Therefore, the postcruciate cortical evoked potential to peripheral nerve stimulation was measured before and after the orbital ablations. In all the experiments there was reduction of the evoked potential (Fig. 5, F and G) whenever the amplitude of the postcruciate spindles decreased after an ablation (Fig. 5, B and C). The evoked potential of the VPL nucleus of the thalamus was not affected by the lesions, indicating that the peripherally induced volley passed unaltered up to and through this relay nucleus. A parallel decrease of the evoked potentials and the spindle amplitude was observed after symmetrical bilateral orbital lesions and the decrease was of the same magnitude in the two hemispheres. When the bilateral lesions gave an asymmetrical reduction of the spindle amplitude, a corresponding asymmetry in the decrease of the evoked potentials was most often observed. Unilateral lesions made the evoked potential and the spindle amplitude decrease in parallel on the operated side, while both were unaltered on the unoperated side. The connection between spindle amplitude Electroenceph. clin. Neurophysiol., 1972, 33:485M96
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Fig. 7. Data from the postcruciate cortex collected from all experiments. Spindle duration (A), intra-spindle frequency (B), spindle amplitude (C) and spindle interval (D) following the lesions are calculated as percentages of the control values and plotted against the amplitude of the cortical evoked potential, calculated in the same way. The spindle amplitude (C) is the only parameter correlated with the size of the evoked potential. In 8 hemispheres (4 cats) no postcruciate spindles or ew)ked potentials were recorded after the lesion (D).
and evoked potential was studied by construction of a correlation plot (Fig. 7, C). This was done in all experiments in which postcruciate spindles were present following orbital lesions. Each experiment is represented by one or two data points in Fig. 7, C (one data point represents one hemisphere). In Fig. 7, C the average spindle amplitude from each experiment after ablation was calculated as percentage of the control amplitude and plotted against the amplitude of the maximal cortical evoked response, expressed as percentage of the amplitude before the lesion. For comparison, the duration, the intra-spindle
frequency and the interval between spindles were plotted against the evoked potential size, all data expressed as percentages of the appropriate control values (Fig. 7, A, B and D). The amplitudes of the spindle waves and of the evoked potential were positively correlated, while the other parameters showed no connection with the evoked potentials. The amplitude of the evoked potential might have zero value although spindles were still present, albeit reduced, which may suggest that the postcruciate spindles are not entirely dependent on the integrity of the specific thalamo-cortical projections. Electroenceph. olin. Neurophysiol., 1972, 33:485 496
SPINDI.ES A N D O R B I T A L C O R T E X
I)IS('USSION
The results demonstrate that none of the aspects of the thalamic barbiturate spindle activity we have investigated is critically dependent on an intact orbital cortex, since neither small nor extensive ablations altered the spindles in any of the thalamic nuclei studied. This finding is in accordance with the hypothesis that the ability to generate rhythmic discharges is an inherent property of the thalamus (Andersen and Andersson 1968; Junge and Sveen 1968). The hypothesis is supported by the observations that thalamic spindles can be recorded after decortication both in barbiturateanaesthetized(Morison et al. 1943: Morison and Basset 1945) and in unanaesthetized cats (Andersson and Manson 1971). It is also supported by the present investigation, showing that thalamic spindles could be observed without any detectable cortical activity. The present experiments further suggest that the orbital cortex is of limited or no importance for the cortical barbiturate spindles. This view is based upon the observations that : (1)the spindle activity in the occipital, parietal and temporal parts of the cortex were unaltered by the lesions ; (2) the reduction of spindle amplitude near the lesion seemed to be parallelled by local damage to the cortical tissue or to the thalamo-cortical fibres. The fact that attenuation of the cortical spindle amplitude often was observed in one hemisphere only further supports this hypothesis. The frontal cortex in general proved to be highly sensitive to trauma to the basal frontal area, which often caused macroscopically visible cortical oedema or a decreased blood flow. Some pilot experiments, using intravascular injections of a plastic material, indicated that the blood supply to the pre- and postcruciate areas (and possibly to the thalamo-cortical irradiation) could easily be damaged by orbital cortical ablation. This interpretation is strengthened by the findings that the cortical evoked potentials were reduced in parallel with a reduction of the postcruciate spindle amplitude, while the evoked potentials in the ventro-basal complex of the thalamus were unaltered. The observation that the spindle amplitude was the only parameter of the cortical activity to be affected supports the
493 theory that the cortical rhythmic activity is generated in the thalamus and transmitted to the cortex by specific thalamo-cortical fibres (Andersen et al. 1967a, b). According to this theory, only the electromotive forces of the cortical rhythmic waves are provided by the cortical neurones. On this basis a selective reduction of cortical spindle amplitude with maintained duration, frequency and occurrence of the cortical spindles would be expected following localized damage to thalam0-cortical fibres or cortical tissue. The observation in some experiments of completely abolished cortical evoked potentials while low voltage spindles were still present in the postcruciate cortex (Fig. 7, C) may, however, indicate that rhythmic activity in this area is not entirely dependent on the thalamo-cortical relay cells. It is tempting to suggest that the so-called non-specific thalarno-cortical system (Jasper 1949; Scheibel and Scheibel 1967) may operate in addition to the specific somatosensory system in the generation of cortical spindle activity. Alternatively, specific thalamo-cortical afferents, other than those tested, may have been spared by the lesion. The results of the present investigation are in contrast with the observation that orbito-frontal ablations abolish thalamic and cortical spindle activity (Velasco and Lindsley 1965: Skinner and Lindsley 1967; Velasco et al. 1968). On the basis of our results a possible explanation of this discrepancy can be found in the fact that damage to the cortical circulation, or to thalamo-cortical fibres, can easily be made by lesions of this type. An additional factor is the anaesthesia employed. In the above reports animals under Flaxedil with mesencephalic lesions were used, while in our experiments barbiturate anaesthesia was employed. This opens the possibility that persistence of thalamic and cortical rhythmic activity might require barbiturate anaesthesia. The possibility is supported by the recent findings of Robertson and Lynch (1971) that removal of the orbital cortex abolished cortical spindle activity in unanaesthetized cats only. In these cats, however, the rhythmic activity returned when the cat was left alone and got drowsy. This was explained by postulating that ablation of the orbital cortex disturbed the Electroenceph. olin. Neurophysiol., 1972, 33 : 485 4 9 6
494 arousal level of the animals. Another possibility is that the thalamus, in order to produce rhythmic activity in the preparation with a mesencephalic lesion, needs an excitatory drive which may come from the olfactory bulb. The ablation of Velasco and Lindsley (1965), which we have attempted to copy, destroyed this input, but it was probably spared in the preparation of Robertson and Lynch (1971). The random drive which may be necessary for the occurrence of thalamic spindle activity may, however, also derive from within the thalamic complex itself (Andersen and Andersson 1968). The persistence of barbiturate rhythmic activity which was also found by Robertson and Lynch (1971) may be explained in two ways. Barbiturates have a well known depressant effect on excitatory functions (Sharpeless 1970). Accordingly, the excitatory input to the thalamus, or any spontaneous excitatory thalamic activity, is reduced during barbiturate anaesthesia, and subsequently the inherent tendency to rhythmicity will dominate. Totally unanaesthetized animals show a marked decrease of spontaneous thalamic unit activity and a corresponding increase of rhythmic discharges following administration of a barbiturate (Gjerstad and Skrede 1970 and unpublished observations). Another possibility is that the barbiturate exerts a direct effect on postsynaptic inhibition, which has been shown to be of great importance for the generation of rhythmic discharges in the thalamus (Andersen and Eccles 1962: Andersen and Sears 1964). This would be in accordance with the increase in thalamic inhibition found during barbiturate anaesthesia compared with the unanaesthetized state (Poggio and Mountcastle 1963; Aitkin et al. 1966). These two proposed mechanisms for the barbiturate effect may possibly operate together. SUMMARY
1. The orbital cortex was ablated in barbiturate anaesthetized cats in order to investigate a possible role of this part of the brain in the generation of spontaneous thalamic and cortical rhythmic discharges. Thalamic and cortical activity was recorded before and following the
E. DAHLet al. lesions, using micro-electrodes and silver ball electrodes, respectively. 2. Spindle duration, intra-spindle frequency, P wave amplitude and the occurrence of spindles were measured in the ventral postero-lateral (VPL) nucleus of the thalamus and found to be identical before and after the ablation. Spindles could be recorded from all part of the thalamic complex following both small and extensive orbital lesions. 3. Spindles recorded from the lateral, the suprasylvian and the ectosylvian cortices were generally in all respects unaffected by orbital ablations. Spindles recorded near the lesion (from the pre- and postcruciate cortices) often showed a reduced amplitude while the other parameters were unchanged. In a few experiments, in which cortical oedema or disturbed blood supply was observed, the cortical spindles disappeared completely. 4. Following orbital lesions the potentials evoked in the postcruciate cortices by stimulation of peripheral nerves decreased in parallel with the amplitude of the spindles recorded from the same areas. Evoked potentials of the VPL nucleus were unaffected by the lesions. Therefore, the amplitude reduction was probably due to damage to thalamo-cortical fibres or to the pericruciate cortex. 5. The findings are in accordance with the hypothesis that rhythmic barbiturate activity is generated in the thalamus and transmitted to the cortex by the specific thalamic relay cells. It is, however, suggested that a non-specific system may operate in addition to the specific system. 6. Differences between previous and present observations are discussed. RESUME ACTIVITE THALAMIQUE ET CORTICALE DE SPINDLES BARBITURIQUES PERSISTANT APRES ABLATION DU CORTEX ORBITAIRE CHEZ LE CHAT
1. Le cortex orbitaire a 6t6 enlev6 chez des chats sous anesth6sie barbiturique afin de rechercher le r61e possible de cette partie du cerveau dans la g6n6se des d6charges rythmiques spontan6es thalamiques et corticales. L'activit6 thalamique et corticale est enregistr6e avant et apr6s les 16sions, /~ l'aide, respectivement, de microElectroenceph. clin. Neurophysiol., 1972, 33:485-496
SPINDLES AND ORBITAL CORTEX
electrodes et d'electrodes spheriques d'argent. 2. La duree des spindles, la frequence intraspindles, l'amplitude des andes P e t la survenue des spindles, mesurees dans le noyau VPL du thalamus se revelent identiques avant et apr+s ablation. Les spindles peuvent etre enregistres au niveau de toutes les regions du complexe thalamique awes lesion orbitaire restreinte ou &endue. 3. Les spindles enregistres au niveau des cortex lateral, suprasylvien et ectosylvien ne sont generalement nullement affectes par les ablations orbitaires. Les spindles enregistres pres de la lesion (au niveau des cortex pr6- et post-crucies) sont souvent d'amplitude reduite alors que les autres parametres sont inchanges. Dans un petit nombre d'experiences, dans lesquelles on a pu observer un oedeme cortical ou une perturbation de la suppleance sanguine, les spindles corticaux disparaissent completement. 4. Apres lesions orbitaires, les potentiels 6voques dans les cortex post-crucies par stimulation des nerfs peripheriques diminuent parallelement 'a l'amplitude des spindles enregistres au niveau des m6mes aires. Les potentiels evoques du noyau VPL ne sont pas affectes par les lesions. Ainsi la reduction d'amplitude est probablement due ;tune atteinte soit des fibres thalamo-corticales soit du cortex peri-crucie. 5. Ces donnees sont en accord avec l'hypothese suivant laquelle l'activite barbiturique rythmique prend son origine dans le thalamus et se transmet au cortex par les cellules de relai thalamique specifiques. Il est cependant possible qu'un systeme non specifique puisse agir en plus du systeme specifique. 6. Les differences entre les observations anterieures et actuelles sont discutees.
We wish to thank Professor Per Andersen for his valuable help and criticism since the start of this work. We are also grateful to Professor Birger K a a d a for reviewing the manuscript. The investigation was supported in part by the Norwegian Research Council for Science and the Humanities.
REFERENCES AITKIN, L. M., DUNLOP, C. W. and WEBSTER, W. R. Clickevoked response patterns of single units in the medial geniculate body of the cat. J. Neurophysiol., 1966, 29:
495 109 123. ANDERSEN, P. and ANDERSSON, S. A. Physiological hasis qf the alpha rhythm. Neuroscience Series No. I, Appleton Century Crofts, New York, 1968, 235 p. ANDERSEN, P. and ECCLES, J. Inhibitory phasing of neuronal discharge. Nature (Land.), 1962, 196:645 647. ANDERSEN, P. and SEARS, T. A. The role of inhibition in the phasing of spontaneous thalamo-cortical discharge. J. Physiol. (Land.), 1964, 173: 459~480. ANDERSEN, P., ANDERSSON, S. A. and LaMa, T. Some factors involved in the thalamic control of spontaneous barbiturate spindles. J. Physiol. (Land.), 1967a, 192:257 281. ANDERSEN, P., ANDERSSON, S. A. and LOMO, T. Nature of thalamo-cortical relations during spontaneous barbiturate spindle activity. J. Physiol. (Land.), 1967b, 192: 283 307. ANDERSSDN, S. A. and MANSON, J. R. Rhythmic activity in the thalamus of the unanaesthetized decorticate cat. Electroenceph. clin. Neur.ophysiol., 1971, 31:21 34. DEMPSEY, E. W. and MORISON, R. S. The interaction of certain spontaneous and induced cortical potentials. Amer. J. Physiol., 1942, 135:301 308. GJERSTAD, L. I. and SKREDE, K. K. Rhythmic thalamic unit activity in the unanaesthetized cat. Acta physiol, scand., 1970, 79: 34A-35A. HUBEL, D. H. Tungsten microelectrode for recording from single units. Science, 1957, 125: 549. JASPER, H. Diffuse projection systems: the integrative action of the thalamic reticular system. Electroeneeph. olin. Neurophysiol., 1949, l: 4 0 5 4 2 0 . JUNGE, K. and SVEEN, O. Exclusive thalamic location of subcortical spontaneous barbiturate spindles. A eta physial. scand., 1968, 73:22 31. MORISON, R. S. and BASSET, D. L. Electrical activity of the thalamus and basal ganglia in decorticate cats. J. Neurophysiol., 1945, 8:309 314. MORISON, R. S. and DEMPSEY, E. W. A study ofthalamo-cortical relations. Amer. J. Physiol., 1942, 135:281 292. MORISON, R. S., FINLEY, K. H. and LOTHROP, G. N. Spontaneous electrical activity of the thalamus and other forebrain structures. J. Neurophysiol., 1943, 6:243 254. NAUTA, W. J. H. and WHITLOCK, D. G. An anatomical analysis of the nonspecific thalamic projection system. In J. F. DELAFRESNAYE (Ed.), Brain mechanisms and consciousness. Thomas, Springfield, Ill., 1954:81 116. POGGrO. G. F. and MOUNTCASTLE, V. B. The functional properties of ventrobasal thalamic neurons studied in unanaesthetized monkeys. J. Neurophysiol., 1963, 26: 775 806. ROBERTSDN, R. T. and LYNCH. G. S. Orbitofrontal modulation of LEG-spindles. Brain Res. 1971, 28:562 566. SCHEmEL, M. E. and SCHEmEL, A. B. Structural organization of non-specific thalamic nuclei and their projection toward cortex. Brain Res., 1967, 6: 60--94. SHARPELESS, S. K. Hypnotics and sedatives. I. The barbiturates. In L. S. GOODMAN and A. GlLLMAN (Eds.), The pharmacok~Jical basis q/' therapeutics. Macmillan, New York, 1970:98 120. SKINNER, J. E. and LINDSLEY, D. B. Electrophysiological and behavioral effects of blockade of,the nonspecific thalamocortical system. Brain Res., 1967, 6:95 118.
Electroenceph. olin. Neurophysiol., 1972, 33:485~496
E. DAHL e t a / .
496 VELASCO, M. and LINDSLEY,D. B. Role of orbital cortex in regulation of thalamocortical electrical activity. Science, 1965,145 : 1375-1377.
VELASCO, M., SKINNER, J. E., ASARO, K. D. and LINDSLEY, D. B. Thalamo-cortical systems regulating spindle bursts and recruiting responses. I. Effect of cortical ablations. Electroenceph. c/in. Neurophysiol., 1968, 25: 4 6 3 4 7 0 .
Rejerence." DAHL, E., GJERSTAD,L. I. and SKREDE,K. K. Persistent thalamic and cortical barbiturate spindle activity after ablation of the orbital cortex in cats. Electroenceph. clin. Neurophysiol., 1972, 33:485 496.
485
PERSISTENT T H A L A M I C A N D CORTICAL B A R B I T U R A T E SPINDLE ACTIVITY A F T E R A B L A T I O N OF THE ORBITAL CORTEX IN CATS EVA DAHL, LEIF I. GJERSTAD AND KNUT KR. SKREDE hTstitute o[' Neurophysiology, University of Oslo, Karl Johansgt. 47, Oslo l (Norway)
(Accepted for publication: April 28, 1972)
Several investigations indicate that the pacemaker for thalamic and cortical barbiturate spindles is located in the thalamus (see Andersen and Andersson 1968). In contrast to earlier findings (Dempsey and Morison 1942; Morison et al. 1943; Jasper 1949), Andersen and Sears (1964) found that all major thalamic nuclei were able to produce spontaneous rhythmic discharges. Andersen et al. (1967a) showed that ablation of the midline and the intralaminar nuclei did not abolish spontaneous barbiturate spindle activity either in the lateral part of the thalamus or in the cortex. Successive ablation of the lateral part, however, was reported to abolish normal spindle activity in the pericruciate region. A major part of the rhythmic activity was therefore assumed to be transmitted directly to the cortex in a distinct topographical pattern in accordance with the distribution of the specific thalamo-cortical projections. On the other hand, Velasco and Lindsley (1965) and Velasco et al. (1968) reported that ablation of the orbital cortex completely suppressed cortical recruiting waves and spindle activity in both the thalamus and the cortex. This effect was explained by assuming that connections between the thalamic nuclei and the orbital cortex regulate the rhythmic activity in both the thalamus and cortex. The system investigated by Velasco et al. (1968) was thought to belong to the non-specific system described by Morison and Dempsey (1942) and by Jasper (1949). Anatomical evidence for such a system has been found by Nauta and Whitlock (1954) and by Scheibel and Scheibel (1967). According to the hypothesis of Andersen et al. (1967b), however, the intactness of the orbital cortex should be of
less importance for the thalamic and cortical rhythmic activity than suggested by Velasco and Lindsley (1965) and Velasco et al. (1968). The present study is an effort to clarify the discrepancy reported above, by recording thalamic and cortical spontaneous activity in barbiturate anaesthetized cats before and after selective ablations of the orbital cortices. The investigation shows that intactness of the orbitofrontal cortex is of minor importance for the occurrence of rhythmic barbiturate activity in the thalamus and in the cortex. These findings are supported by the parallel and independent investigation by Robertson and Lynch (1971) showing persistent cortical rhythmic activity following orbital ablations. METHODS
Operative procedure
Adult cats anaesthetized by intraperitone~tl injection of sodium pentobarbital (30 mg/kg body weight) were used. During the time of recording, additional doses were given intravenously to maintain a constant level of anaesthesia at which withdrawal of the leg to pinching was just abolished. The head was fixed in a holder and extensive craniotomy was performed. In most experiments the frontal sinuses were opened and the roof of the orbit removed to obtain a good approach to the orbital cortex. For the same reason bilateral enucleation of the eyes was performed in 7 experiments. In most cats the neocortex and the hippocampus overlying the thalamus were removed by suction. This operation was restricted rostrally, so that thalamo-cortical fibres to the somatosensory cortex were left intact. Electroenceph. clin. Neurophysiol., 1972, 33:485-~96
486 Ablation of the orbital cortex Ablation of the orbital cortex was performed by aspiration under visual guidance. In most experiments the lesions were made on both sides at the same time. The ablated area usually included the anterior sylvian convolution, the prepiriform area and the olfactory tubercle (Velasco and Lindsley 1965). In 4 experiments the lesions were restricted to a smaller area lying around the presylvian gyrus (Skinner and Lindsley 1967). After an experiment the brain was removed and fixed in 10~, formalin. The areas with lesions were studied macroscopically both before and after fixation. In some experiments histological sections were also prepared, to ascertain that complete removal of the intended part of the cortex had been performed. The blood supply to the brain appeared to be critical for the cortical rhythmic activity. Therefore, the colour and the consistency of the cortex during the experiments were observed. In 3 cats India ink or vinyl acetate was injected in the carotid artery after the lesions were made to study whether the cortical blood supply was altered by the ablations. Identical injection was performed on 2 cats without orbito-frontal lesions; these cats served as controls. Recordin9 and stimulation In cats in which the thalamus was exposed, thalamic recordings were made with coarse glass micropipettes (tip resistance 1-2 M~) filled with 4 M NaC1. In 5 experiments in which the neocortex was left intact thalamic records were made by electrolytically sharpened tungsten micro-electrodes (Hubel 1957), which penetrated the cortex without causing any significant damage to it. The signals from the microelectrodes were fed through a cathode followeramplifier system and displayed on an oscilloscope and ink writer. Records from the cortex were obtained by silver ball electrodes resting lightly on the pial surface. The indifferent electrode was usually situated on the bone or in the surrounding connective tissue. The somatosensory cortex and the ventrobasal complex of the thalamus were identified by recording the evoked potentials following electrical stimulation of the nerves of the contra-
E. DAHL et al. lateral foreleg. In addition, the ventro-basal complex of the thalamus could be identified by the response to tactile stimulation of the skin, or movement of a joint. Spindle measurements To obtain a quantitative expression of the thalamic and cortical rhythmic activity the following parameters were measured before and after cortical ablation: (1) the duration of the spindles; (2) the intra-spindle frequency; (3) the spindle amplitude; and (4) the interval between successive spindles. The data were generally obtained from 30 consecutive spindles. In the thalamus as well as the cortex the spindle duration was measured from the start of the first to the end of the last positive wave. In the cortical records the intra-spindle frequency was counted as the number of waves per second. In the thalamus the interval between consecutive bursts of spikes within the spindle and the number of P waves (positive waves following unitary discharges) per second were measured. The amplitude of the cortical spindle was taken as its peak-to-peak amplitude after smoothing; i.e., deleting waves with exceptionally large amplitudes. In the thalamus the spindle amplitude was taken as the maximal P wave amplitude of the smoothed spindle. Since it proved difficult to record from the same thalamic units before and following the orbital ablations, no effort was made to count the number of single units participating in a spindle. The spindle interval was defined as the time from the start of one spindle to the start of the following one. Since neither thalamic nor cortical spindles usually showed an idealized appearance, it often proved difficult to apply the criteria listed above. In order to counteract any bias by the observers, three observers measured the same data independently. The bias was probably small, since the histograms obtained by the three investigators were practically identical. RESULTS Effect QJ lesions on thalamic spindle activity To see whether orbital ablations affected thalamic spindle activity, the orbital cortex was Electroenceph. olin. Neurophysiol., 1972, 33:485 406
487
SPINDLES AND ORBITAL CORTEX
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removed in l0 cats. In barbiturate anaesthetized animals thalamic spindles could still be recorded following both small and extensive unilateral or bilateral ablations of the orbital cortex. In one of the experiments in which extensive bilateral lesions of the orbital cortices were made by suction (Fig. 1, A), spontaneous thalamic activity was recorded with micro-electrodes from the ventral postero-lateral (VPL) nucleus before and after the lesions (Fig. 1, B and D). Attempts were made to place the electrode at the same site within the VPL after the ablations, guided by the maximal response to peripheral stimulation (Fig. 1, C and E). Both before and after the surgery, the activity showed the characteristic spindle appearance with episodes of unitary discharges appearing at regular intervals, separated by periods of relative silence. Each episode of activity lasted 2-3 sec and consisted of series of bursts of spikes superimposed on negative waves. The bursts were separated by positive waves, P waves, lasting approximately 100 msec. The thalamic somatosensory evoked potentials were unchanged after removal of the
orbital cortices. The duration of the spindles, the intraspindle frequency, the amplitude of the P waves and the spindle intervals were measured from 30 successive spindles before and after the lesion and plotted as histograms (Fig. 1, F-I). There were no clear changes in either of the parameters after the ablation (hatched columns) compared with the activity before it (open columns). In most experiments the investigation was confined to the VPL nucleus, which can easily be identified by electrophysiological means. To study whether other parts of the thalamus also presented spindle activity after removal of the orbital cortex, the thalamic complex was systematically investigated in 3 cats after extensive bilateral lesions. In the experiment shown in Fig. 2 the VPL nucleus was identified electrophysiologically after the lesion was made (Fig. 2, A). Seven penetrations were subsequently made through different parts of the thalamus on both sides, using the VPL as a reference point. Along each of the tracks spindle activity was recorded from several positions. Examples of the activity Electroenceph. clin. Neurophysiol., 1972, 33:485 496
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recorded from the VPL, the ventro-lateral and the central median nuclei are presented in Fig. 2, B D. In this experiment the cortical postcruciate spindle activity and evoked potential disappeared following the ablation. Recordings from the thalamus were mostly made after removal of the overlying cortex (the g
Right
lateral, the suprasylvian and the ectosylvian gyri). To avoid a systematic error due to this procedure, which has been reported to have a facilitatory effect on spindle activity (Velasco et al. 1968), the neocortex was left intact in 5 experiments. The results obtained in the latter experiments did not differ from those in which
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Electroenceph. clin. Neurop,Sysiol.. 1972, 33:485~-96
489
SPINDI.ES AND ORBITAL CORTEX
thalamic exposure was performed, either with respect to the thalamic or to the cortical spindle activity. In two experiments only was it impossible to record thalamic spindle activity after severing the orbital cortex, probably because the whole brain rapidly deteriorated due to excessive bleeding or oedema.
Ej]ect of lesions on cortical spindle activity The effect of orbital ablation on rhythmic cortical activity was in some contrast to the effect on the thalamic activity, the cortical spindles being more easily influenced by the lesions. The cortical activity could disappear completely while typical spindle activity was recorded in the thalamus. In some experiments distinct cortical spindles were recorded immediately after the lesions were made, but disappea-
A
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red during the following 10 min, simultaneously with the development of a pronounced oedema. In most experiments, however, the rhythmic cortical activity was not abolished by orbital lesions, but only decreased in amplitude. The degree of amplitude decrease depended both on the depth and extent of the lesion and on the distance of the cortical electrode from it. A restricted bilateral orbital lesion was earlier reported to reduce spindle activity in animals under Flaxedil (Skinner and Lindsley 1967). The effect of such a lesion in our barbiturate anaesthetized preparations is illustrated in Fig. 3. A slight reduction of the spindle amplitude occurred in the left postcruciate area but with an unaltered spindle activity in both the right postcruciate cortex (Fig. 3, C-F) and the posterior parts of the cortex (suprasylvian gyrus). The thalamic activity was unaltered by the operation
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Electroenceph. clin. Neurophysiol., 1972, 33:485 496
490
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According to Velasco et al. (1968) unilateral ablation attenuated the cortical spindles, while after bilateral lesions they were abolished. In this investigation it was found that unilateral lesions sometimes gave a reduction of pericruciate spindle amplitude on the affected side, while the unoperated side was unaltered. Subsequent ablation on the second side usually caused no further decrease of the spindle amplitude on the initially operated side. In such an experiment the total lesion was as shown in Fig. 4, A. After removal of the orbital surface on the left side, the spindles recorded from the ipsilateral pericruciate region were reduced in amplitude (Fig. 4, B, C and E). Bilateral lesion caused only a slight further decrease of the amplitude (Fig. 4, D and F). On the right side the spindle amplitude was not reduced at all (Fig. 4, G) after the bilateral lesion. With extensive orbital lesions a marked difference between rostral (near the ablated area) and caudal cortical areas (lateral, suprasylvian and ectosylvian gyri) was often observed. In Fig. 5 the activity in the postcruciate area is compared with the activity in the middle suprasylvian gyrus of the same hemisphere. Fig. 5, A gives the lesion. The activity in the postcruciate cortex (Fig. 5, B and C) was affected by the surgery, while no obvious change can be seen in
the activity of the suprasylvian gyrus (Fig. 5, D and E). The evoked response recorded in the somatosensory cortex was greatly depressed after the ablation (Fig. 5, G). To obtain a quantitative expression of the changes in spindle activity in this experiment the spindle duration, the intra-spindle frequency, the spindle amplitude and the spindle intervals were measured from 25 successive spindles before and after the lesion of the orbital cortex. The histograms (Fig. 6, A D) show that the spindle amplitude was markedly reduced in the postcruciate cortex (Fig. 6, C), while the other parameters were unaltered (Fig. 6, A, B and D). Measurements from the precruciate cortex gave similar results, although the spindle amplitude was reduced even further. In the suprasylvian gyrus, however, no deterioration of spindle activity was observed (Fig. 6, E-H). On the contrary, the duration of the spindles increased (Fig. 6, E). The intra-spindle frequency and the spindle amplitudes were unchanged (Fig. 6, F and G) whereas the spindle intervals increased (Fig. 6, H). The latter observation is partly explained by the increased spindle duration, but may also be partly due to a change in the depth of anaesthesia. In most of our experiments spindle activity was present in the posterior parts of the cortex Electroenceph. clin. Neurophysio/., 1972, 33:485 4 9 6
491
SPINDLES AND ORBITAL CORTEX
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Fig. 6. Histograms from the data obtained in the experiment described in Fig. 5. In the postcruciate cortex the spindle duration, the intra-spindle frequency and spindle occurrence were unaltered by the lesion (A, B, D). The spindle amplitude (C) was markedly reduced. The plots from the right suprasylvian gyrus (E-H) show that the spindles there were almost unchanged by the ablation.
following bilateral lesions and disappeared only in those few experiments where serious damage to the cortex was observed, such as following impairment of cortical blood flow or pronounced oedema. Usually the spindle pattern recorded after ablations was similar to the control activity. We conclude, therefore, that the amplitude was the only aspect of the cortical spindles which was affected by orbital lesions and that this change was localized to the rostral parts of the cortex; in some experiments the spindles in the frontal cortex were completely abolished by the ablations, while the lateral, the suprasylvian and the ectosylvian gyri still showed well developed and virtually unchanged spindles.
Cortical evoked potentials and spindle amplitudes The localized decrease of spindle amplitude near the removed orbital cortex suggested that this effect might be due either to damage to thalamo-cortical fibres or to the cells surrounding the lesion. The evoked potential in the postcruciate cortex induced by stimulation of a peripheral nerve gives a measure of the integrity of the thalamo-cortical fibres as well as of the condition of the neurones in the primary somato-
sensory cortex. Therefore, the postcruciate cortical evoked potential to peripheral nerve stimulation was measured before and after the orbital ablations. In all the experiments there was reduction of the evoked potential (Fig. 5, F and G) whenever the amplitude of the postcruciate spindles decreased after an ablation (Fig. 5, B and C). The evoked potential of the VPL nucleus of the thalamus was not affected by the lesions, indicating that the peripherally induced volley passed unaltered up to and through this relay nucleus. A parallel decrease of the evoked potentials and the spindle amplitude was observed after symmetrical bilateral orbital lesions and the decrease was of the same magnitude in the two hemispheres. When the bilateral lesions gave an asymmetrical reduction of the spindle amplitude, a corresponding asymmetry in the decrease of the evoked potentials was most often observed. Unilateral lesions made the evoked potential and the spindle amplitude decrease in parallel on the operated side, while both were unaltered on the unoperated side. The connection between spindle amplitude Electroenceph. clin. Neurophysiol., 1972, 33:485M96
492
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Fig. 7. Data from the postcruciate cortex collected from all experiments. Spindle duration (A), intra-spindle frequency (B), spindle amplitude (C) and spindle interval (D) following the lesions are calculated as percentages of the control values and plotted against the amplitude of the cortical evoked potential, calculated in the same way. The spindle amplitude (C) is the only parameter correlated with the size of the evoked potential. In 8 hemispheres (4 cats) no postcruciate spindles or ew)ked potentials were recorded after the lesion (D).
and evoked potential was studied by construction of a correlation plot (Fig. 7, C). This was done in all experiments in which postcruciate spindles were present following orbital lesions. Each experiment is represented by one or two data points in Fig. 7, C (one data point represents one hemisphere). In Fig. 7, C the average spindle amplitude from each experiment after ablation was calculated as percentage of the control amplitude and plotted against the amplitude of the maximal cortical evoked response, expressed as percentage of the amplitude before the lesion. For comparison, the duration, the intra-spindle
frequency and the interval between spindles were plotted against the evoked potential size, all data expressed as percentages of the appropriate control values (Fig. 7, A, B and D). The amplitudes of the spindle waves and of the evoked potential were positively correlated, while the other parameters showed no connection with the evoked potentials. The amplitude of the evoked potential might have zero value although spindles were still present, albeit reduced, which may suggest that the postcruciate spindles are not entirely dependent on the integrity of the specific thalamo-cortical projections. Electroenceph. olin. Neurophysiol., 1972, 33:485 496
SPINDI.ES A N D O R B I T A L C O R T E X
I)IS('USSION
The results demonstrate that none of the aspects of the thalamic barbiturate spindle activity we have investigated is critically dependent on an intact orbital cortex, since neither small nor extensive ablations altered the spindles in any of the thalamic nuclei studied. This finding is in accordance with the hypothesis that the ability to generate rhythmic discharges is an inherent property of the thalamus (Andersen and Andersson 1968; Junge and Sveen 1968). The hypothesis is supported by the observations that thalamic spindles can be recorded after decortication both in barbiturateanaesthetized(Morison et al. 1943: Morison and Basset 1945) and in unanaesthetized cats (Andersson and Manson 1971). It is also supported by the present investigation, showing that thalamic spindles could be observed without any detectable cortical activity. The present experiments further suggest that the orbital cortex is of limited or no importance for the cortical barbiturate spindles. This view is based upon the observations that : (1)the spindle activity in the occipital, parietal and temporal parts of the cortex were unaltered by the lesions ; (2) the reduction of spindle amplitude near the lesion seemed to be parallelled by local damage to the cortical tissue or to the thalamo-cortical fibres. The fact that attenuation of the cortical spindle amplitude often was observed in one hemisphere only further supports this hypothesis. The frontal cortex in general proved to be highly sensitive to trauma to the basal frontal area, which often caused macroscopically visible cortical oedema or a decreased blood flow. Some pilot experiments, using intravascular injections of a plastic material, indicated that the blood supply to the pre- and postcruciate areas (and possibly to the thalamo-cortical irradiation) could easily be damaged by orbital cortical ablation. This interpretation is strengthened by the findings that the cortical evoked potentials were reduced in parallel with a reduction of the postcruciate spindle amplitude, while the evoked potentials in the ventro-basal complex of the thalamus were unaltered. The observation that the spindle amplitude was the only parameter of the cortical activity to be affected supports the
493 theory that the cortical rhythmic activity is generated in the thalamus and transmitted to the cortex by specific thalamo-cortical fibres (Andersen et al. 1967a, b). According to this theory, only the electromotive forces of the cortical rhythmic waves are provided by the cortical neurones. On this basis a selective reduction of cortical spindle amplitude with maintained duration, frequency and occurrence of the cortical spindles would be expected following localized damage to thalam0-cortical fibres or cortical tissue. The observation in some experiments of completely abolished cortical evoked potentials while low voltage spindles were still present in the postcruciate cortex (Fig. 7, C) may, however, indicate that rhythmic activity in this area is not entirely dependent on the thalamo-cortical relay cells. It is tempting to suggest that the so-called non-specific thalarno-cortical system (Jasper 1949; Scheibel and Scheibel 1967) may operate in addition to the specific somatosensory system in the generation of cortical spindle activity. Alternatively, specific thalamo-cortical afferents, other than those tested, may have been spared by the lesion. The results of the present investigation are in contrast with the observation that orbito-frontal ablations abolish thalamic and cortical spindle activity (Velasco and Lindsley 1965: Skinner and Lindsley 1967; Velasco et al. 1968). On the basis of our results a possible explanation of this discrepancy can be found in the fact that damage to the cortical circulation, or to thalamo-cortical fibres, can easily be made by lesions of this type. An additional factor is the anaesthesia employed. In the above reports animals under Flaxedil with mesencephalic lesions were used, while in our experiments barbiturate anaesthesia was employed. This opens the possibility that persistence of thalamic and cortical rhythmic activity might require barbiturate anaesthesia. The possibility is supported by the recent findings of Robertson and Lynch (1971) that removal of the orbital cortex abolished cortical spindle activity in unanaesthetized cats only. In these cats, however, the rhythmic activity returned when the cat was left alone and got drowsy. This was explained by postulating that ablation of the orbital cortex disturbed the Electroenceph. olin. Neurophysiol., 1972, 33 : 485 4 9 6
494 arousal level of the animals. Another possibility is that the thalamus, in order to produce rhythmic activity in the preparation with a mesencephalic lesion, needs an excitatory drive which may come from the olfactory bulb. The ablation of Velasco and Lindsley (1965), which we have attempted to copy, destroyed this input, but it was probably spared in the preparation of Robertson and Lynch (1971). The random drive which may be necessary for the occurrence of thalamic spindle activity may, however, also derive from within the thalamic complex itself (Andersen and Andersson 1968). The persistence of barbiturate rhythmic activity which was also found by Robertson and Lynch (1971) may be explained in two ways. Barbiturates have a well known depressant effect on excitatory functions (Sharpeless 1970). Accordingly, the excitatory input to the thalamus, or any spontaneous excitatory thalamic activity, is reduced during barbiturate anaesthesia, and subsequently the inherent tendency to rhythmicity will dominate. Totally unanaesthetized animals show a marked decrease of spontaneous thalamic unit activity and a corresponding increase of rhythmic discharges following administration of a barbiturate (Gjerstad and Skrede 1970 and unpublished observations). Another possibility is that the barbiturate exerts a direct effect on postsynaptic inhibition, which has been shown to be of great importance for the generation of rhythmic discharges in the thalamus (Andersen and Eccles 1962: Andersen and Sears 1964). This would be in accordance with the increase in thalamic inhibition found during barbiturate anaesthesia compared with the unanaesthetized state (Poggio and Mountcastle 1963; Aitkin et al. 1966). These two proposed mechanisms for the barbiturate effect may possibly operate together. SUMMARY
1. The orbital cortex was ablated in barbiturate anaesthetized cats in order to investigate a possible role of this part of the brain in the generation of spontaneous thalamic and cortical rhythmic discharges. Thalamic and cortical activity was recorded before and following the
E. DAHLet al. lesions, using micro-electrodes and silver ball electrodes, respectively. 2. Spindle duration, intra-spindle frequency, P wave amplitude and the occurrence of spindles were measured in the ventral postero-lateral (VPL) nucleus of the thalamus and found to be identical before and after the ablation. Spindles could be recorded from all part of the thalamic complex following both small and extensive orbital lesions. 3. Spindles recorded from the lateral, the suprasylvian and the ectosylvian cortices were generally in all respects unaffected by orbital ablations. Spindles recorded near the lesion (from the pre- and postcruciate cortices) often showed a reduced amplitude while the other parameters were unchanged. In a few experiments, in which cortical oedema or disturbed blood supply was observed, the cortical spindles disappeared completely. 4. Following orbital lesions the potentials evoked in the postcruciate cortices by stimulation of peripheral nerves decreased in parallel with the amplitude of the spindles recorded from the same areas. Evoked potentials of the VPL nucleus were unaffected by the lesions. Therefore, the amplitude reduction was probably due to damage to thalamo-cortical fibres or to the pericruciate cortex. 5. The findings are in accordance with the hypothesis that rhythmic barbiturate activity is generated in the thalamus and transmitted to the cortex by the specific thalamic relay cells. It is, however, suggested that a non-specific system may operate in addition to the specific system. 6. Differences between previous and present observations are discussed. RESUME ACTIVITE THALAMIQUE ET CORTICALE DE SPINDLES BARBITURIQUES PERSISTANT APRES ABLATION DU CORTEX ORBITAIRE CHEZ LE CHAT
1. Le cortex orbitaire a 6t6 enlev6 chez des chats sous anesth6sie barbiturique afin de rechercher le r61e possible de cette partie du cerveau dans la g6n6se des d6charges rythmiques spontan6es thalamiques et corticales. L'activit6 thalamique et corticale est enregistr6e avant et apr6s les 16sions, /~ l'aide, respectivement, de microElectroenceph. clin. Neurophysiol., 1972, 33:485-496
SPINDLES AND ORBITAL CORTEX
electrodes et d'electrodes spheriques d'argent. 2. La duree des spindles, la frequence intraspindles, l'amplitude des andes P e t la survenue des spindles, mesurees dans le noyau VPL du thalamus se revelent identiques avant et apr+s ablation. Les spindles peuvent etre enregistres au niveau de toutes les regions du complexe thalamique awes lesion orbitaire restreinte ou &endue. 3. Les spindles enregistres au niveau des cortex lateral, suprasylvien et ectosylvien ne sont generalement nullement affectes par les ablations orbitaires. Les spindles enregistres pres de la lesion (au niveau des cortex pr6- et post-crucies) sont souvent d'amplitude reduite alors que les autres parametres sont inchanges. Dans un petit nombre d'experiences, dans lesquelles on a pu observer un oedeme cortical ou une perturbation de la suppleance sanguine, les spindles corticaux disparaissent completement. 4. Apres lesions orbitaires, les potentiels 6voques dans les cortex post-crucies par stimulation des nerfs peripheriques diminuent parallelement 'a l'amplitude des spindles enregistres au niveau des m6mes aires. Les potentiels evoques du noyau VPL ne sont pas affectes par les lesions. Ainsi la reduction d'amplitude est probablement due ;tune atteinte soit des fibres thalamo-corticales soit du cortex peri-crucie. 5. Ces donnees sont en accord avec l'hypothese suivant laquelle l'activite barbiturique rythmique prend son origine dans le thalamus et se transmet au cortex par les cellules de relai thalamique specifiques. Il est cependant possible qu'un systeme non specifique puisse agir en plus du systeme specifique. 6. Les differences entre les observations anterieures et actuelles sont discutees.
We wish to thank Professor Per Andersen for his valuable help and criticism since the start of this work. We are also grateful to Professor Birger K a a d a for reviewing the manuscript. The investigation was supported in part by the Norwegian Research Council for Science and the Humanities.
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