EXPERIMENTAL

NEUROLOGY

Single-Unit

65, 16-28 (1979)

Discharges Behaving

in the Dorsolateral Cats: lctal Activity

Thalamus

of

J. C. HIRSCH AND A. FOURMENT’ Unitk

de Recherches

Received

Neurophysiologiqaes 75634 Paris Ociober

de I’lnserm (c/3), Cedex 13. France

26, 1978; revision

received

47 Boulevard

December

de

/‘HGpital,

27, 1978

A new model of experimental epilepsy was designed to study in undrugged, behaving cats the involvement of the thalamus during a cortical epileptiform afterdischarge (EAD). EADs were induced by electrical repetitive stimulation applied to the posterior region of an isolated suprasylvian gyrus and allowed to invade the thalamus through a pedicle of white matter connecting areas 5 and 7 of the gyrus to their thalamic target, the dorsolateral nuclei. Extracellular recordings were obtained from 49 dorsolateral cells. Projection to the thalamus of the slowly propagating cortical EAD resulted in a characteristic sequence of events in 88% of the cells. First there occurred an arrest of cellular discharges upon invasion of the anterior suprasylvian cortex (phase I). During phase II, the cell fired slow-wave sleep (SWS)-like bursts strictly timed with surface events. Later (phase III), the cell generated long trains of action potentials on a one-to-one basis with cortical waves; in most cells, partial spike inactivation was seen within each burst. A statistical study indicated that the mean intraburst interspike intervals of SWS and phase II were not significantly different. Surprisingly, a significant increase (P < 0.0005) in the mean intraburst interspike interval was found between SWS (4.4 2 0.4 ms) and phase III (6.0 + 0.6 ms). Arguments are presented which suggest that the long intraburst interspike intervals are orthodromically generated.

INTRODUCTION Focal cortical epileptogenesis is known to involve other regions of the brain through two main types of propagation. One is a slow, creeping invasion through the cortical feltwork; the other is represented by projection of Abbreviations: EAD-epileptiform afterdischarge, ECoG-electrocorticogram, SWSslow-wave sleep, ISI-interspike interval, DL-dorsolateral thalamus. ’ The authors would like to thank Mrs. M. E. Marc for assistance with the physiological experiments, Mr. J. R. Teilhac for the illustrations, and Mrs. J. Nicolet for typing the manuscript. 16 0014-4886/79/070016-13$02.00/O Copyright 0 1979 by Academic Press, Inc. All rights of reproduction in any form reserved.

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the primary process through axonal pathways (16). The object of this study was to observe the progressive involvement of a distant region of the brain when a seizure was occurring in functionally related cortical areas. In a previous experimental work the propagation and development of a focal seizure in chronically isolated cortex were shown to follow a stereotyped pattern (14). The same model of experimental epilepsy, “long-duration epileptiform afterdischarges (EADs) induced by electrical stimulation of isolated cortex,” was utilized on a slightly modified surgical preparation. EADs were elicited in the posterior region of an isolated suprasylvian gyrus and allowed to invade the anterior region which still retained connections with subcortical structures and particularly with its main target, the dorsolateral nuclei of the thalamus (4, 7, 13). The electrocorticogram (ECoG) and simultaneously recorded slow-wave activity and response from single neurons at the thalamic level were compared during the development and propagation of a cortical EAD in chronic cats. METHODS Aseptic techniques were used to carry out three-stage experiments in six adult male cats: the first two were done under Nembutal anesthesia and included the isolation procedures, and were followed 3 weeks later by implantation of the electrodes. As the final step, the freely behaving animals were recorded three times a week. Surgery. An operating microscope was used throughout surgery. The left suprasylvian gyrus was completely isolated from the surrounding cerebral cortex, by a subpial aspiration technique (15) which respected the anatomy and vascularization of the isolated region (Fig. 1A). All connections with

FIG. 1. (A) Peroperative macroscopic aspect of an isolated suprasylvian gyrus. Arrows point to the anterior (on the right) and posterior limits of isolation. Note intact vessels over removed cortical regions. (B) Macroscopic view of a sagittal section ofa brain cut through the suprasylvian gyrus (upper part of the picture is dorsal). Arrows point to the anterior (on the right) and posterior limits of the pedicle. Black area underneath gyrus indicates extent of undercutting. Thai-thalamus, NC-caudate nucleus.

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HIRSCH AND FOURMENT

. not involved

. involved

FIG. 2. Sites of the 49 cells recorded during an epileptiform afterdischarge in stereotaxic planes from Reinoso-Suarez (21). Abbreviations of nuclei: CM-centrum medianum, GMgeniculatus mediahs, LA-lateralis anterior, LP-lateralis posterior, M-medialis dorsalis, Pul-pulvinar, Ret-reticularis, SG-suprageniculatus, VLA-ventralis, pars lateralis, VPM-ventralis posteromedialis.

the subcortex were severed except for a pedicle of white matter under the anterior suprasylvian gyrus [area 5 and part of area 7 of Hassler and MuhsClement (12)]. The pedicle extended approximately from coordinates A 20 to 10 (Fig. 1B). On termination of the operation, the dura mater was replaced by a sheet of amniotic membrane, the bone defect was repaired with acrylic resin, and the scalp was sutured. Techniques for recording the ECoG and thalamic slow-wave and unit activities in behaving animals are described in (6). The cortical EADs were elicited by electrically stimulating one pair of transcortical electrodes through a constant current stimulator (parameters: trains of 1 to 2 s; pulse duration, 0.5 ms; frequency, 40 Hz). Intensity was increased until the threshold for an EAD was attained. Analysis of the data was carried out off-line on a wired computer. The spikes were counted after sampling by an amplitude impulse discriminator. For each unit the following measures were made: interspike interval (ISI) histograms of(i) all spikes during a given period of activity, and (ii) for the different intervals within a burst. A burst was arbitrarily defined as a group of at least two spikes separated by less than 9.9 ms. In all cases, differences between mean values were assessed using Student’s r-test. RESULTS Epileptiform

Stimulation

Afterdischarge

applied

in Semiisolated and Isolated

to the posterior

region of the semiisolated

Cortex.

cortex

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elicited long-duration EADs (over 2 min) with ECoG characteristics similar to those described previously for completely isolated cortex (14). Near the site of stimulation, the bipolar transcortical record disclosed sharp surface-negative waves (Fig. 3, Cx2) followed later and until the end of the EAD by surface positive waves.* The same pattern was seen to occur under every transcortical electrode pair, including those in the anterior suprasylvian gyrus still connected to the thalamus (Fig. 3, Cxl). The velocity of propagation of this phenomenon remained constant throughout the entire gyrus and ranged from 7 to 20 mm/min. Thus, an EAD induced in the deafferented region of the suprasylvian gyrus spread unaltered into a cortical region still connected to subcortical structures. This cortical region (areas 5 and 7) exhibited physiological, spontaneous activities between seizures [see the companion paper (6) for a detailed analysis of ECoG activities]. Slow- Wave Activity in the Dorsolateral Nuclei. Epileptic activity spread to the thalamus as soon as the anterior suprasylvian cortex was invaded. Figure 4 shows an example of the field potentials generated in the thalamus during propagation of a cortical EAD. As long as only surfacenegative waves were recorded in the cortex no paroxysmal activity was present in the thalamus (Fig. 4A, 153 s). Later, when a surface-positive wave occurred in the cortex a negative wave with superimposed spikes was recorded in the lateralis posterior nucleus [Fig. 4A; compare activities in that nucleus when the cortex showed an early surface-positive wave (b) and when it did not (c)l. The thalamic potentials became increasingly regular as the cortex shifted from surface-negative to surface-positive waves (Fig. 4A, 161 s). During each clonic surface discharge, large negative potentials with superimposed unit activities were recorded in the thalamus (Fig. 4B). Subsequent to the development of the EAD in the cortex, the slow surface-positive wave always led the corresponding thalamic slow wave on a one to one basis. Paroxysmal activity always stopped simultaneously in the two structures. No secondary generalization of the seizure occurred. One cat displayed clonic eye movements at the end of some particularly long EADs. Organization of Cell Discharges in the Dorsolateral Nuclei. Extracellular recordings were obtained from 49 dorsolateral nuclei neurons (their respective loci are indicated in Fig. 2) during the course of 63 * A laminar study in chronically completely isolated cortex had shown that this passage from surface-negative to surface-positive waves on transcortical records indicated a radial propagation of the zone of maximal depolarisation of paroxysmal activities from the cortical surface downward (14). This interpretation was further confirmed by the fact that the emergence of surface-positive waves coincided with the occurrence of a massive cellular discharge and a large DC shift.

t

1Osec

FIG. 3. Simultaneous record of cortex and lateralis posterior (LP) nucleus during an ictal episode. Insert indicates, respectively, the positions ofstimulating (St) and transcortical electrodes (Cx 1, Cx2) over the suprasylvian gyrus (the unhatched area corresponds to the cortex overlying the thalamocortical pedicle). Arrows point to the occurrence of surface-negative waves in Cx2 then in Cxl and to the invasion of the LP nucleus. Polarity: cortex, positivity down; LP, positivity up.

LP Ml h .

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A l2wJ

a b c

a

cx ---J-b4

b

C

/L-----lI5oopv

LP

200 mscc FIG. 4. Simultaneous record of cortex and lateralis posterior (LP) nucleus during an ictal episode. Numbers refer to the time (in seconds) elapsed since stimulation. A-invasion of cortex (Cx) overlying the thalamocortical pedicle by the epileptiform afterdischarge is indicated by the occurrence of surface-negative waves; no activity in LP (153 s). Note in expanded sweep segment (a, b, c) that a negative wave with superimposed spikes(b) occurred in LP as soon as the cortex displayed surface-positive waves. B-large depolarizing waves with superimposed unit activities in LP are strictly timed with the surface-positive waves of the cortical record. Polarity: LP, positivity up; cortex, positivity down.

cortical EADs. Only those cells that generated action potentials without signs of injury for a long period of recording were analysed. Because this study was coupled with another one centered on spontaneous activities of the same nuclei, most of the 49 neurons were recorded during at least two different stages of the sleep-waking cycle; all EADs were elicited during a period of slow-wave sleep (SWS) when the cell generated small bursts of action potentials intermingled with isolated spikes (Fig. 5, top segment). Modifications of cellular activities during an EAD followed a stereotyped pattern for most dorsolateral nuclei cells and could be divided in

HIRSCH AND FOURMENT

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b i

I

1

I

1

35

1 set FIG. 5. Simultaneous record of cortex (Cx) and unit activities of a lateralis posterior (LP) nucleus neuron during slow-wave sleep and an epileptiform afterdischarge. Numbers refer to the time elapsed (in seconds) since stimulation (t). Underlined segments are shown at an expanded sweep speed in Fig. 6. Polarity for cortex as in Fig. 4.

three phases. After stimulation, and before the seizure reached the thalamus, the same pattern of discharge as that of SWS was observed (Fig. 5, second segment). At the onset of the ictal episode, spike generation was either suppressed or considerably decreased in frequency (Fig. 5, 35 s); during that period, if the cell fired, it generated only isolated spikes unrelated to cortical events. That period, during which cellular discharges were arrested, will be referred to as phase I. The pause was followed by a

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1OOmsec FIG. 6. Expanded sweeps ofthe corresponding segments from Fig. 5. Note the similar short interspike intervals bursts in a, b, and c and the attenuation of spike amplitude already visible in d. Spikes have been retouched for clarity. Polarity for cortex as in Fig. 4.

period (phase II) during which the cell resumed firing in small bursts of spikes similar to those of SWS, but strictly timed with cortical epileptic activities (Fig. 5, third segment; Fig. 6, compare a and c). Later, the cell began to fire longer and longer trains of action potentials (Fig. 5, fourth segment). As the number of spikes in a burst increased, the amplitude of the action potentials within the burst decreased and a typical “butterfly” pattern was obtained (Figs. 5, and 6e); this period will be referred to as phase III. Most often, toward the end of the ictal episode, the cell was inactivated so rapidly during the bursts that only one or two spikes remained distinguishable as such at the beginning of each clonic burst (Fig. 6f). A complete arrest of discharge lasting 3 s to 5 min was observed at the end of the EAD; as soon as the cell resumed firing the action potentials regained at once their preseizure amplitude. Of the 49 cells recorded during an ictal episode, 43 (88%) were activated; phases I and III were always present but phase II could be either very short or missing. The remaining six cells (12%) which were not involved were all situated near the medial boundaries of the LP nucleus (Fig. 2). To compare the short bursts of spikes of phase II and the long trains of action potentials generated during phase III, a statistical analysis of the

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HIRSCH AND FOURMENT

intraburst ISIS was carried out. Furthermore, the similitude seen on the records between the short bursts of phase II and those of SWS (Fig. 6a, c) was also tested. An example of such an analysis is presented in Fig. 7. The intraburst IS1 histograms were computed during a period of SWS and during phases II and III of the EAD, for all the IS1 (Fig. 7, part A) and then for the first and second intervals of the bursts (Fig. 7, part B). Comparison of the means calculated for all intervals and separately for the first and second intraburst interval (Fig. 7, part B, ISI, and ISI*) indicated that there was no significant difference between the mean ISIS of SWS and phase II. The increased means during phase III compared to either SWS or phase II were highly significant (P < 0.005). A similar analysis was carried out for 26 cells; however, because for some cells phase II was absent or its duration too short to allow a statistical analysis, only the intraburst IS1 histograms of SWS and phase III were compared. For each cell, the histogram of all the intraburst ISIS during SWS differed significantly from that of phase III

N:b, x:3.2 S0:O.S

c 0

2

4

6

8

10

nsec

0

2

4

b

tl msec

0

1

4

b

8

nlsl!c

FIG. 7. Interspike interval histograms during slow-wave sleep (SWS) and the epileptiform afterdischarge (EAD) (II-period of short bursts at beginning of EAD. III-period of long clonic bursts). Same cell as in Figs. 5 and 6. A-IS1 histogram of all the bursts. Note the complete disappearance in III of the mode between 2 and 3 ms common to SWS and II. B-IS1 histogram of the first two intraburst ISIS of SWS, II, and III. Both first (ISI,) and second (ISI,) ISIS have, respectively, a similar distribution in SWS and II. Increased mode for both ISIS during III compared to SWS or II. Bin width for all histograms: 1 ms. Symbols: N, number of ISI; X, mean IS1 in ms; SD, standard deviation of mean.

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(Kolmogorov-Smirnov test: levels of probability,

Single-unit discharges in the dorsolateral thalamus of behaving cats: ictal activity.

EXPERIMENTAL NEUROLOGY Single-Unit 65, 16-28 (1979) Discharges Behaving in the Dorsolateral Cats: lctal Activity Thalamus of J. C. HIRSCH AND...
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