Electroencephalography and Clinical Neurophysiology, 1977, 43: 707--724

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© Elsevier/North-Holland Scientific Publishers, Ltd.

S T A T U S EPILEPTICUS: A NEW R O D E N T MODEL KATHERINE H. TABER **, JOHN J. McNAMARA and STEVEN F. ZORNETZER *

Department of Neuroscience, College of Medicine, University of Florida, Gainesville, Fla. 32610 (U.S.A.) (Accepted for publication: April 8, 1977)

The p h e n o m e n o n of lowering the seizure discharge threshold in brain by administering repetitive low intensity electrical stimulation was described first by Delgado (Delgado and Sevillano 1961). Goddard expanded this observation b y sampling a wide range of brain structures (Goddard 1967) and b y carrying o u t a series of parametric tests in the amygdala designed to identify the o p t i m u m stimulus parameters for the development o f brain seizure activity and subsequent behavioral convulsions (Goddard et al. 1969). Goddard reported that 24 h was the most efficient interstimulus interval for the eventual develo p m e n t of convulsions in rats. Current intensity could be below the initial afterdischarge (AD) threshold (50--100 pA) and still result in eventual convulsions. Goddard also demonstrated that the residual effect of such repetitive brain stimulation was persistent (up to at least 12 weeks) once a behavioral convulsion developed. Thus, electrical stimulation following a long period of no stimulation immediately elicited the convulsive behavior previously seen. Goddard named this m e t h o d of producing a persistently seizure-prone brain region 'kindling'. Racine (1973) duplicated and extended Goddard's parametric study (Goddard et al. 1969) and concluded that the interstimulus interval could be reduced to one hour in some * This research was partially supported by the Alfred P. Sloan Foundation. ** Present address: Dept. of Neurobiology and Anatomy, University of Texas Medical School at Houston, P.O. Box 20708, Houston, Texas 77025, U.S.A.

strains of rat w i t h o u t a significant increase in the number of ADs needed to produce a convulsion. With interstimulus intervals shorter than one hour, however, an increasing number of ADs were needed before a convulsion was produced (Racine et al. 1973). Leech (1976), using an interstimulus interval of one minute for a series of 50 stimuli delivered to the amygdala, found that convulsions could be produced in some mice. Most animals did n o t develop convulsive activity. Complete information concerning AD activity is unavailable from this study since EEG activity was only monitored in some animals. Kindling as a model o f epilepsy Kindling has been primarily considered as a model for epilepsy. F r o m this point of view, that area of the brain rendered seizure-prone after periods of repetitive stimulation is analogous to the primary epileptic focus found in pathophysiological states. Similarly, brain areas which become seizure-prone due to the propagated activity of the primary kindled focus are analogous to the so-called 'mirror foci' (Hughes, 1966). Kindling as a model for epilepsy has several advantages over previously developed models such as chemical irritants implanted within brain tissue to create an epileptic focus or the systemic infusion of a convulsant drug to create brain seizures and m o t o r convulsions (Purpura et al. 1972). First, seizure activity induced in the primary kindled focus can be constantly monitored via the stimulating electrode, providing detailed information concern-

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ing the development and characteristics of electrophysiological patterns of brain seizure. Second, stimulus parameters during kindling can be greatly varied, resulting in a broad range of possible stimulus configurations thus providing a high degree of flexibility with regard to altering the developmental sequence of the establishment of the kindled focus. Finally, and perhaps most importantly, the epileptic focus is easy to establish in any susceptible brain area and can be precisely characterized anatomically, chemically and neurophysiologically without the confounding presence of extrinsic chemical contaminants.

Kindling as a model for neural plasticity Researchers interested in mechanisms of neural plasticity have suggested that kindling may provide a workable model for learning and m e m o r y (Gaito 1974; Goddard and Douglas 1975). Since kindling results in the production of a long-lasting seizure-prone brain area, it can be argued that the kindled tissue has 'learned', as a result of repeated experience, to respond to electrical stimulation with prolonged seizure activity. In support of this suggestion there are a number of intriguing parallels between kindling and learning. For example, both the rate of learning (Lorge 1930) and the rate of kindling (Leech 1976) are more rapid when distributed rather than massed trials are used during acquisition. Similarly, greater retention (suggesting better learning) is seen following distributed trials for both kindling (Leech 1976) and learning (Cain and Wiley 1939). Kindling, like conventional learning paradigms, shows both proactive and retroactive interference effects. Thus, if one brain area is kindled (e.g., the amygdala), and then the homologous contralateral structure is electrically stimulated, two effects are seen: First, the contralateral side will require more stimuli to kindle than norreal if stimulation is begun within two weeks of primary site kindling. This is similar to proactive interference (Postman 1971). Second, after the contralateral site is kindled it is necessary to apply several stimulations to the pri-

K.H. TABER ET AL.

mary site before it will again be active (McIntyre and Goddard 1973). This is similar to retroactive interference (Postman 1971). Thus, if the animal had had a rest period of equal time with no secondary site kindling, the primary site would have been immediately responsive since normally a previously kindled brain region develops AD in response to the first stimulation after a prolonged rest period. Racine et al. (1972c) and more recently Goddard and Douglas (1975) demonstrated that kindling causes enduring transsynap~ic changes. Learning probably also involves enduring changes in synaptic transmission in selected neural pathways subsequent to activation. These changes presumably lead to facilitation of neural transmission due to alteration of synaptic conductance over widespread areas of the brain. All of the above similarities support the hypothesis that kindling is a model of neural plasticity worthy of additional investigation. Recently we have found, using a variation on the kindling paradigm, that it is possible to induce, in a relatively brief period of time, long-lasting self-sustained epileptif0rm activity in mice via hippocampal stimulation. This seizure activity appears to have many similarities to status epilepticus, a brain disorder for which no adequate experimental models have been developed. A previous report by Pinel (1975) indicated that status epilepticus could be produced in rats when kindling stimuli were continued for several months. The present study was done to determine the stimulus parameters most effective in eliciting status epilepticus, and to characterize the phenomenon electrophysiologically and behaviorally. Materials and methods

Male Swiss/ICR mice (Flow labs, Inc.) weighing between 25 and 40 g were used for all experiments. Prior to surgery mice were housed in group cages (7--8/cage). Following surgery they were housed individually.

Surgical procedure Animals were anesthetized with Nembutal

STATUS EPILEPTICUS: A NEW RODENT MODEL (50 mg/kg, i.p.) followed b y administration of atropine sulfate (0.05 cc) to minimize respiratory complications. Bipolar twisted nichrome wire (125 pro) electrodes insulated with enamel were stereotaxically implanted bilaterally into the dorsal hippocampus (+2.5 mm AP from lambda, 1.6 mm M L , - - 1 . 6 mm DV) and cemented in place with dental acrylic. The electrodes were uninsulated only at the cross sections of the cut ends. A stainless steel anchor screw with a connector pin was attached to the skull overlying the nasal sinus and cemented in place. Connector pins from the electrodes and from the grounding screw were inserted into an ITT Cannon plastic strip connector and cemented in place along the midline of the skull. Mice were allowed a 7--10 day post-operative recovery period.

Experimental apparatus The EEG recording chamber consisted of a t w o ~ o m p a r t m e n t clear plexiglas b o x (15 cm × 2 7 c m ) allowing behavioral observation. Water and food pellets were freely available throughout the testing period. Mice were connected to a Grass Model 7D polygraph through a flexible low-noise cable attached to the headplug via a freely movable counterbalanced arm. The recording cable connected to a relay b o x which automatically switched the animal's electrode connections b e t w e e n the polygraph and the stimulator. The operation of the relay b o x was controlled b y a,digital logic board programmed to deliver stimulation at a preset interval. Hippocampal EEG was monitored throughout the testing session. A testing session was defined as either the time required to deliver a minimum o f ninety stimulations, or as the time required for the development and subsequent termination of self-sustained seizure (SSS) activity. Stimulus parameters Brain stimulation was delivered unilaterally at one minute intervals. A constant current stimulator delivered l m s e c biphasic square wave pulses at 6 0 c / s e c for I sec (1 train/trial). Current intensity for different animals ranged

709 between 100 #A and 800 #A. The EEG was analyzed for the per cent after discharge occurring during each one-minute interval. Groups were statistically compared using the Mann--Whitney U test. All testing sessions were begun between 8 and 9 a.m. in order to avoid possible circadian complications. The mice were maintained on a 7 a.m.--7 p.m. light cycle.

Histology After all testing was completed brains were removed and stored in formalin for later histological analyses. Alternate 30 #m sections stained with either cresyl violet or h e m o t o x i n and eosin were obtained from all brains. The extent of gross tissue damage (i.e. abnormal pyramidal or granule cell bodies) around the stimulated and unstimulated electrode sites was compared for all animals using a paired t-test. Histological analysis was done without reference to either behavioral or EEG data.

Expedment 1 This experiment was done in order to explore the possibility that long-lasting seizure activity could be produced in mice via repetitive hippocampal stimulation.

Materials and methods Nine mice were used in this initial experiment. Mice were prepared as described previously and stimulated unilaterally with either 300/~A or 600/~A. Results Six of the 9 subjects eventually developed massive bilateral long-lasting SSS. Within this group t w o distinctly different patterns of seizure development were seen: ( 1 ) N o n - L e t h a l SSS and (2) Lethal SSS. Non-Lethal SSS. A group made up of 5 of the 6 mice exhibited a slow development of seizure activity occurring over a period of 90-120 stimulations (Non-Lethal SSS). A characteristic electroencephalographic developmen-

710 tal sequence was seen in these mice (see Fig. 1). A brief afterdischarge (AD) was generally seen in response to the first, and occasionally the second, stimulation. This AD activity was followed by depression of normal hippocampal activity as indicated by isopotential EEG records (see Fig. 1A,B). During this period of depression the mouse showed no EEG response to electrical stimulation. Hippocampal activity gradually returned to normal over a period of 6--10 stimulations (see Fig. 1B). Following this initial hypoexcitabte block to AD activity, mice again became responsive to electrical stimulation. AD was typically induced b y the next 1--3 stimulations (see Fig. 1C). This second period of AD activity was followed by a second phase of EEG depression and concomitant resistance to AD (see Fig. 1C}. When the mouse once again became responsive to brain stimulation the resulting AD tended to be more severe than that seen earlier (see Fig. 1D). Oscillations between AD in response to electrical stimulation and severe post-stimulation depression occurred for several trials (see Fig. 1E). This period of alternating depression and AD lasted from 1 0 - 5 0 stimulations. Toward the end of this oscillatory phase a new and unusual form of AD developed. This new form of seizure activity began several seconds after the termination of the brain stimulation, often continuing until the next scheduled brain stimulation was administered, and usually persisted through that stimulation (see Fig. 1F). At this advanced time in the testing session those ADs that began i~nmediately after the brain stimulation became prolonged enough to continue until the next scheduled brain stimulation was administered. The first few times this prolonged AD activity occurred the AD was terminated by the onset of the next electrical stimulation. After several such trials the AD activity persisted through the next electrical stimulation and in some cases was augmented by its occurrence. Once this stage was achieved the hippocampus was in almost constant seizure. For blocks of 3--5 trials the seizure would continue unabated b y the successive brain stimulations, although the

K.H. TABER ET AL. form of the seizure often changed (see Fig. 1E,F). At this stage in the testing session a characteristic waveform consisting of high amplitude (up to 5 mV peak to peak) regular spikes appeared. These high amplitude spikes phased in and out of the EEG for several trials before eventually dominating most of the record (see Fig. 1G). Once this pattern of regular, high amplitude spiking was reliably established (after 24--59 min of AD overall) brain stimulation could be discontinued and the mouse would remain in SSS activity for some time (see Fig. 1H--K). All five of the mice in this group remained in continuous SSS for 165-195 min after the cessation of brain stimulation. Eventual recovery and return to a normal EEG pattern was rapid, in all cases occurring within 10--15 min of the first break in the SSS (see Fig. 1L). The sequence of behavioral changes occurring during the development of the SSS activity was not as distinct as the electrophysiological sequence of changes described above. AD-bound behavioral arrest was the response seen during the early portion of the testing session. Mice typically returned to normal behavioral activities at the termination of the stimulation or upon cessation of AD activity. Stimulation and seizure-bound behavioral arrest was seen throughout the testing session, b u t as the AD activity became more intense other types of behavioral abnormalities appeared. Behavioral automatisms such as repetitive grooming, chewing, whisker-twitching, and burrowing developed by the end of the second period of EEG hypoexcitability. These stereotyped behaviors were seizurebound, with mice otherwise remaining in behavioral arrest. As the duration of AD activity lengthened toward the second half of the testing session "preconvulsive" actions such as clonic jerking (usually synchronous with EEG spiking), rapid forepaw movement ("fanning"), and ipsilateral (to the side of stimulation) forepaw elevation were seen. Mild convulsions occurred very infrequently. Througho u t the later phase of the testing session such periods of automatic and preconvulsive behav-

S T A T U S EPILEPTICUS: A NEW RODENT MODEL

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mice alternated between periods of behavioral arrest and periods of the previously described relatively normal activity. The time spent in either behavioral state varied considerably between individual mice. Interspersed between these two types of behavior were episodes of stereotyped and preconvulsive actions. These episodes coincided with sessions of higher frequency lower amplitude EEG spiking (see Fig. II,K). In some mice these episodes recurred at regular intervals. Occasionally convulsions developed during these episodes of stereotyped and preconvulsive activity. This varied among mice, with some mice never displaying

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Fig. 1. EEG recording from a typical " N o n - L e t h a l " SSS mouse showing the gradual development of persistent seizure activity during the stimulation session. Calibration marks on A (A--J) and K ( K - - L ) are 5 sec and 500 pV. L and R indicate left and right hippoeampi. The right hippocampus was stimulated. The large deflections seen in most panels are stimulus artifact. The number of minutes following the start of stimulation are marked on each panel. (A) Brief bilateral afterdischarge (BAD) followed by post-ictal depression (PID) followed the first stimulation. (B) Recovery of normal EEG by stimulations 9 and 10. (C) Longer duration BAD followed by PID followed stimulation 17. (D) Longer duration BAD recurred as the second period of hyporeactivity (see text) ended with stimulation 73. (E) At stimulation 115 BAD continued through the stimulation for the first time. (F) By stimulation 125 BAD was augmented by the occurrence of the stimulation. (G) The regular high amplitude spikes characteristic of SSS appeared after stimulation 145. (H) The regularity of the spikes was well established 15 min after the onset of SSS (minute 166 of trial). (I) The paper speed was increased briefly, 30 min after the onset of SSS (minute 181 of trial), to 50 mm/sec (from 5 mm/sec) to show the spike waveform more clearly. (J) 60 rain after the onset of SSS (minute 211 of trial). (K) 120 min after the onset of SSS (rrdnute 271 of trial); note the short bursts of high frequency and amplitude spikes. (L) 1 , 1 8 0 rain after the onset of 88S (minute 331 of trial) the spikes become irregular. 2, 10 rain later the EEG is normal.

convulsive activity while others manifested as many as 24 separate convulsions during the course o f the SSS. When convulsions were

seen they generally occurred in the first one to two hours of the SSS discharge. Toward the end of the SSS activity mice returned to

STATUS EPILEPTICUS: A NEW RODENT MODEL

the previously described pattern of alternating behavioral arrest and relatively normal activity. With several mice in this group an a t t e m p t was made to reestablish SSS activity after an extended recovery period had elapsed. The same stimulation parameters originally used to produce the SSS activity in each animal were used. With a recovery period of one week or less virtually no AD of any t y p e was elicited b y hippocampal stimulation. When a longer recovery period was used (1--2 weeks) some AD was produced b y the stimulation, b u t considerably less than originally seen. No prolongation of AD was observed. One mouse was allowed to recover for 5 weeks, at which time considerable AD occurred in response to the stimulation. This animal rapidly developed unilateral regular high amplitude spiking (within t w e n t y stimulations) with a delayed contralateral spread (forty stimulations). The data from this animal suggest it may be possible to reestablish SSS activity with fewer stimulations after a prolonged rest period. Longer recovery periods were not tried. Lethal SSS. The sixth animal to develop SSS activity exhibited a very different pattern and rate of AD development (Lethal SSS). This mouse developed AD to the second electrical stimulation (see Fig. 2A). This AD was followed by mild EEG depression and simultaneous resistance to the stimulation as described above. This period of hypoexcitability lasted for only 5 stimulations, at which time AD occurred again briefly {see Fig. 2B). A second period of hypoexcitability lasted for 7 stimulations and then waned, allowing more prolonged AD activity in response to stimulations (Fig. 2C,D). The AD increased rapidly in amp l i t u d e and duration, and by the thirtieth stimulation was continuing through several stimulations (Fig. 2C). This is very different from the pattern of SSS development in the group previously described, in which animals were still in the middle of the second period of hypoexcitability at a comparable time in the testing session. The AD continued to increase in severity and by the forty-sixth

713 stimulation the characteristic high amplitude, regular spikes described in the earlier group appeared (Fig. 2F). Within a few stimulations these became completely regular and hippocampal stimulation was terminated. During the period of SSS activity there were frequent bursts of faster spiking accompanied by violent convulsive activity (Fig. 2G). The animal experienced six prolonged convulsions within 55 min before dying during convulsion (Fig. 2H). Behaviorally this mouse differed from mice in the previously described group in that preconvulsive activity was more prevalent throughout the testing session. In addition, the convulsive activity occurring during the SSS activity was much more intense than that seen in the previous group. Tonic-clonic convulsions were frequent, as were violent running attacks. The animal was seldom still, showing neither the arrest behavior nor the relatively normal activity described earlier. Three of the mice in Experiment 1 did not develop SSS activity (Non~SSS group). Two of these animals showed little AD in response to the stimulation. The AD activity which did occur showed no tendency to increase in either amplitude or intensity (see Fig. 3). Stimulations were carried o u t to 130 trials w i t h o u t success in an effort to induce more violent seizure activity with an increased number of stimulations. Fig. 4A shows the difference between the Non.SSS group and the Non-Lethal SSS group in occurrence and duration of AD during the testing session. Histology

Mice developing Non-Lethal SSS (n = 5) had electrode tip placements in various hippocampal and dentate gyrus subfields (see Fig. 5A for electrode tip placements). Histology was n o t available on the one Lethal SSS animal stimulated at 600/~A. Of the three mice that did n o t develop SSS (Non~SSS} t w o had electrode tip placements in the hippocampal fissure and one in the corpus callosum. No significant differences were seen in the extent of damage surrounding the stimulated and un-

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The results of Experiment 2 suggest that the administration of a brief (1.0 sec) electrical stimulus to the hippocampal formation is sufficient to cause a long-lasting ( > 3 days and 7 days) decrease in the reactivity of the hippocampus to subsequent electrical stimulation. This hypoexcitability was such that the subsequent stimulus intensity had to be doubled {from 400 to 800 #A) to induce SS$ when compared to mice given a 7 day rest between

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720 stimulation sessions. This finding extends the observation of others (Herberg and Watkins 1966) that an induced AD can temporarily elevate the seizure threshold for a period of several hours. Our data are also consistent with Pinel's recent report (1976) that high intensity electrical stimulation can elevate the AD threshold for as long as a week.

Experiment 3 Electrophysiologicai data from the previous experiment indicated a long-term (at least 3 days) hippocampal malfunction resulting from electrical stimulation. To explore the nature and extent of such a hippocampal malfunction we evaluated the behavior of mice in a testing situation sensitive to hippocampal dysfunction (Isaacson and Pribram 1976).

Methods Twenty-three mice were surgically prepared as previously described. Seven to ten days postoperatively each mouse was given a series of 400 pA stimulations at one minute intervals as described in Experiment 2. All animals were stimulated for a minimum of 90 trials or until SSS activity appeared. Twenty-four or forty~eight hours after the testing session ended mice were trained on a one-trial inhibitory avoidance task. For complete description of the task and apparatus see Jarvik and Kopp (1967). Training consisted of placing the mouse in the outer compartment of a two c o m p a r t m e n t apparatus and measuring the latency to step through into the inner compartment (initial step through latency = ISTL). Once in the inner compartment the mouse automatically receivdd footshock (300 ttA) until it escaped back to the outer compartment. The animal was then returned to its home cage and tested for retention of the avoidance response 24 h later. Retention testing consisted of placing the mouse in the outer compartment and again measuring the latency to stepping into the inner c o m p a r t m e n t (test step through

K.H. TABER ET AL. latency = TSTL). The trial was terminated either when the mouse stepped through or when it had remained in the outer compartment for a total of 300 sec plus its original step through latency. The step through latency difference score ( T S T L - ISTL = ASTL) was taken as an indicator of the a m o u n t of retention of the learning experience. Thus, a large difference score indicated good learning and m e m o r y of the avoidance response, while a small difference score indicated poor learning and/or memory of the avoidance response. The Man'n--Whitney U test was used for statistical comparison of ASTL scores between groups.

Results Electrophysiological. Of the 23 mice used in this experiment 9 developed self-sustained seizure activity. Of t h e 1 4 mice that did not develop SSS, 9 were shown to have their stimulating electrode in the hippocampal fissure and 2 in the corpus caUosum (see Fig. 5C). Histology was not available on 3 animals of this group. None of the Non-SSS mice responded to the electrical stimulation with prolonged AD at any time during the testing session. Although occasional AD was seen in response to stimulation, it was of short duration (see Fig. 3). Behaviorally, Non-SSS mice (n = 14) showed arrest during brain stimulation, with occasional automatic actions such as repetitive grooming and chewing. At no time were preconvulsive or convulsive behaviors observed. As mentioned above, 9 animals developed SSS activity. Six of these developed the previously described Non-Lethal SSS. Three of the mice developing SSS in this experiment exhibited the lethal form described ~previously There were no apparent differences between Lethal SSS mice in this and previous experiments in the number of stimulations to the onset of SSS, in the mean % duration of AD/10 min interval, in the number of convulsions before the onset of SSS, or in the duration of SSS prior to death. As a result of the consistency found in all measures examined

S T A T U S EPILEPTICUS: A N E W R O D E N T M O D E L

across the 3 experiments reported here all Lethal SSS mice (n = 5) and all Non-Lethal SSS mice (n = 17) were grouped and compared. They were found to be significantly d i f f e r e n t in the number of stimulations needed to elicit SSS (P < 0.002) and in the mean % duration of A D / 1 0 min interval (P < 0.001) during the first 200 min of the testing session (see Fig. 4D). Behavioral. Of the 6 mice developing NonLethal SSS, 4 were trained and tested on the one trial inhibitory avoidance task as previously described. Twelve of the Non~SSS mice were trained and tested. Initially, SSS animals showed a significantly longer ISTL (P < 0.02) than Non-SSS mice. Retention testing indicated that SSS mice were deficit when compared to Non-SSS mice (U = 1; P < 0.01; Mann-Whitney U test). Thus Non-Lethal SSS mice had a median ASTL of 58 sec while the Non-SSS mice had a median ASTL of 300 sec, indicating good learning and m e m o r y of the inhibitory avoidance response.

Discussion We have described two distinctly different forms of SSS activity. Non-Lethal SSS is characterized by slow development (over a session of 100--130 stimulations) in both duration and intensity of AD and b y the presence of automatic or stereotyped behaviors. Convulsive activity is seldom seen. Once the SS activity is established EEG spikes are regular and of high amplitude, with a 2--3 c/sec domeshaped waveform reminiscent of the waveform seen in p s y c h o m o t o r epilepsy. This type of SSS is proposed as a possible model for p s y c h o m o t o r status epilepticus. The second t y p e of SSS activity (Lethal SSS) is characterized b y a rapid development (over a session of 40--50 stimulations) of AD in b o t h duration and intensity and by the presence of frequent preconvulsive and convulsive behaviors. Once SSS activity is established the EEG spikes are more frequent {6--8 c/sec) and of lower amplitude than in the previous group. Periods of intense seizure activity are seen regularly and are correlated with the occurrence of convul-

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sire activity. The waveform varies, b u t generally a spike and d o m e form is seen. By definition, Lethal SSS invariably ends in death. This form of SSS is proposed as a model for convulsive or major status epilepticus, the most dangerous form of status epilepticus in humans. Estimates of human mortality due directly to this form of seizure range up to 21% (Rowan and Scott 1970; Gordon 1972). In order to determine the value of these proposed clinical models, their responses to the drugs currently used in the management of human convulsive and p s y c h o m o t o r status epilepticus must be evaluated. If clinically established treatments are also effective (or ineffective, in the case of drugs which have proven n o t useful clinically) when used to block the experimentally-induced status epilepticus, the proposed animal models for convulsive and p s y c h o m o t o r status epilepticus should prove valuable for future screening of new drugs and drug combinations as well as for testing of supportive treatments. Additionally, these t w o proposed experimental models of status epilepticus should provide investigators with an experimental paradigm in which two forms of the disorder can be compared on various measures in order to identify underlying mechanistic differences. As models for status epilepticus in general, both Lethal SSS and Non-Lethal SSS have several advantages over previously developed animal models. The two most c o m m o n l y used methods of experimentally inducing status epilepticus are: (1) administration of serial electroconvulsive shock (ECS); and (2) systemic infusion of bicucuUine. Serial ECS (usually at short intervals) has been used successfully for the study of the metabolic and biochemical changes occurring during prolonged seizure activity (Duffy et al. 1975; Wasterlain 1972; Wasterlain 1974). While a great deal of information has been gathered in this manner, the ECS model has an inherent weakness which prevents generalization of the results to human epilepsy; the EC8 used creates a massive, diffuse seizure throughout the brain leading to extreme m o t o r convulsions. Serial ECS

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is thus not an accurate model for human status epilepticus in which there are probably regional differences in seizure activity that serial ECS cannot even begin to duplicate. Systemic infusion of bicuculline has also been used to mimic the widespread seizure activity characteristic of convulsive status epilepticus. Metdrum (Meldrum and Horton 1973; Meldrum and Brierly 1973) has used this technique extensively in his studies on the physiological, morphological, and neurochemical changes accompanying prolonged seizure activity in primates. An unfortunate complication of any such biochemical study is the use of a chemical to create the seizures under study. There is the ever-present danger in such studies that the direct effects of the exogenously administered agent may effect the endogenous process under study. The controlled and predictable onset of SSS induced by electrical brain stimulation should make many types of experiments easier to perform. For example, one proposed mechanism for the prolongation of seizure activity is that changes in extracellular potassium concentration lead to changes in the neuronal resting membrane potential (Izquierdo et al. 1970; Dichter et al. 1972; Hotson et al. 1973). Such a change would make the neuron hyperexcitable. The relatively short time period involved in the development of SSS would make the evaluation of extracellular ion changes with a potassium-sensitive electrode relatively easy. Finally, although epilepsy is an important area of study, it is not the only area of brain plasticity to which this model can be applied. Another potentially useful application is to the study of processes involved in learning and memory. Thus, as demonstrated in Experiment 3, mice that developed SSS and were subsequently trained on the one-trial inhibitory avoidance task were deficit in performance of that task. The data indicate alterations in the acquisition of the task {learning) and/or in its retention (memory). Interestingly, animals that received the stimulation but did not develop SSS (Non-SSS) showed no impairment

K.H. T A B E R ET AL.

upon testing. This finding indicates that the electrical stimulation by itself was not sufficient to create a deficit in the task. Rather, the brain response to the electrical stimulation was the significant variable correlated with the subsequent behavioral deficit. This deficit suggests that hippocampal SSS may create prolonged hippocampal dysfunction. Deficit performance on this type of learning task is characteristic of the hippocampal syndrome (Isaacson and Pribram 1976). Our histological data suggest that there are regional differences within the hippocampus related to differing forms of the SSS. Thus when the stimulating electrode was placed in the regio inferior of dorsal hippocampus (CA~--CA4) or in the dentate gyrus the form of the seizures was Non-Lethal SSS. All the Lethal SSS mice proved to have regio superior (CA1) placements. In view of the morphological and electrophysiological differences that have been found between these two regions of dorsal hippocampus (Vinogradova 1976) this finding is particularly interesting. We intend to pursue the study of these anatomical differences.

Summary Kindling, the phenomenon of producing a persistently seizure-prone area of brain by administering repetitive low intensity electrical stimulation, has recently undergone intensive investigation. The most c o m m o n l y used interstimulus interval in these experiments has been 24 h. We have found, using an interstimulus interval of one minute, that it is possible to produce long-term self-sustained seizures (SSS) in mice via hippocampal electrode placements. These results suggest an animal model for the clinically-defined syndrome of status epilepticus. Effective stimulus parameters for the production of SSS were a 1.0 sec, 400/~A (constant current) train of 60 c/sec 1 msec biphasic square wave pulses. The intertrain interval was 60 sec. With stimulating electrodes located in

S T A T U S EPILEPTICUS: A NEW RODENT MODEL

hippocampal subfields CA2, CA3, CA4 or in the dentate g y m s a slowly developing form of SSS (Non-Lethal SSS) was seen. This form o f seizure was characterized by a slow (100--130 stimulation) increase in the intensity and duration of afterdischarge. Behaviorally the major manifestations of Non-Lethal SSS were automatisms or stereotyped behaviors (repetitive grooming, chewing, etc.) and behavioral arrest. These animals displayed performance deficits to an inhibitory avoidance task 24-48 h after the cessation of SSS. Electrode placements in subfield CA~ of hippocampus generally resulted in a more quickly (40--50 stimulations) developing form of SSS (Lethal SSS). Behaviorally these mice exhibited b o t h preconvulsive and convulsive m o t o r activities. All CAl-stimulated animals died in convulsion within 1 h of the onset of SSS. Non-Lethal SSS is discussed as a model for p s y c h o m o t o r status epilepticus. Lethal SSS is discussed as a model for convulsive status epilepticus.

R~sumd Etat de mal dpileptique: un nouveau moddle chez le rongeur Le phdnomAne de kindling, consistant produire une zone cdrdbrale susceptible en permanence de faire des crises par administration rdpdtitive de stimulations dlectriques de basse intensitd, a dt~ r~cemment l'objet d'investigations intensives. L'intervalle interstimulus le plus habituellement utilisd dans ces expdriences est de 24 h. Les auteurs ont trouv~ qu'il est possible, par un intervalle inter-stimulus d'une minute, de produire des crises auto-entretenues de longue dur~e chez la souris (SSS), en mettant des dlectrodes dans l~ippocampe. Ces rdsultats sugg~rent un module animal pour le syndrome ddfini cliniquement c o m m e "~tat de mal dpileptique". Les param~tres de stimulation efficace pour la production des SSS consistent en un train du-

723

rant une seconde et ~ 400 pA (courant continu) d'impulsions carries biphasiques de I msec 60 c/sec. L'intervalle inter-train est de 60 sec. Lorsque l'~lectrode de stimulation est localisde dans les sub-champs hippocampiques CA2, CA3, CA4 ou dans le g y m s dentel~ on observe une forme de SSS ~ ddveloppement lent (SSS Non Lethal). Cette forme se catactdrise par une augmentation lente (100 ~ 130 stimulations) de l'intensitd et de la dur~e de la post-ddcharge. Du point de vue comportemental, la manifestation principale due SSS Non Lethal consiste en automatismes ou en comportements stdrdotypds (grognements r~pdtds, mastication, etc.) et arr~ts comportementaux. 28 ~ 48 h apr~s la cessation du SSS ces animaux m o n t r e n t des ddficits ddficiences ~ une tache d'~vitement inhibiteur. La localisation des dlectrodes dans le subchamp hippocampique CA~, provoque gdndralement une forme ~ d~veloppement plus rapide (40 A 50 stimulations) de SSS (SSS Lethal). Du point de vue comportemental, les souris montrent des activitds motrices ~ la fois prd-convulsives et convulsives. Tous les animaux stimulds en CA1 sont morts en dtat convulsif dans l'heure qui a suivie le ddmarrage du SSS. Le SSS Non Lethal est discutd c o m m e moddle de l'dtat de mal dpileptique psychomoteur. Le SSS Lethal est discutd c o m m e un module de l'dtat de mal convulsif.

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