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Models of SSSE
The first model of SSSE was derived from the observation that, when rats were paralyzed, ventilated with oxygen, and kept in good metabolic balance, repetitive application of electroconvulsive shocks (ECS) for more than 25 minutes resulted in seizures that continued after stimulation stopped (Figure 17.1). The duration and severity of these self-sustaining seizures directly correlated with the duration of stimulation (71, 72). Animals exposed to repeated ECS for 25 minutes had self-sustaining seizures for a few minutes, whereas animals subjected to ECS for 50 minutes continued to seize for up to an hour, and rats stimulated for 100 minutes remained in SSSE for hours and usually died, even though their oxygenation, acid-base balance, and other metabolic parameters remained stable.
Figure 17.1.
Repeated electroconvulsive shock induces SSSE in rats. Representative electrographic recordings from skull screw electrodes in paralyzed and oxygen-ventilated rats. Animals shocked repeatedly for 25 minutes (50 shocks) or longer showed self-sustaining seizure activity after the end of electrical stimulation. Increasing the number of shocks resulted in longer lasting self-sustaining seizures (Reprinted with permission from Wasterlain [71].)
Following the discovery of the kindling phenomenon by Goddard et al. (15), Taber et al. (61) and de Campos and Cavalheiro (8) modified the method of stimulation to obtain SE. McIntyre et al. (43, 44) showed that continuous stimulation (for 60 minutes) of the basolateral amygdala of kindled animals with high-frequency current induced SSSE in about 60% of animals, thus demonstrating that the kindled state predisposes to the development of SSSE. Buterbaugh et al. (1982) showed that small amounts of pilocarpine would also induce SSSE in kindled animals. Seeking a model in which SE could be induced in naive animals, Turski et al. (65) developed the pilocarpine model of SE. Buterbaugh et al. (6) and Morrisset et al. (46) showed that, in these chemical models, seizures become independent of the initial trigger and self-sustaining, as they do in electrical stimulation models. Later it was shown that both continuous high- (45) and low-frequency (7) stimulation of limbic structures can induce SSSE. Inoue et al. (23, 24) reproduced SSSE in naive rats by electrical stimulation of prepiriform cortex. Handforth and Ackerman (18, 19) used a similar approach, with continuous high-frequency stimulation of hippocampus or amygdala, and analyzed the functional anatomy of SSSE, correlating metabolic (14C-deoxyglucose) mapping with the behavioral pattern of seizures. They delineated several types of SSSE of differing severity, ranging from a very restricted limbic pattern with regional involvement around the site of stimulation and mild behavioral manifestations (motor arrest) to bilateral extensive involvement of limbic and extralimbic structures accompanied by widespread clonic seizures. This approach was later used by a number of investigators, notably Lothman and colleagues (32), who showed that stimulation of dorsal hippocampus for 60 minutes with high-frequency trains with very short intertrain intervals, a protocol they called continuous hippocampal stimulation (CHS), resulted in the development, in many animals, of SSSE characterized by nonconvulsive or mild convulsive seizures that lasted for hours after the end of CHS (29). Metabolic activity was increased in many brain structures (maximal in CA1, dentate gyrus, presubiculum, and subiculum) (67); these seizures led to loss of GABAergic hippocampal inhibition, to hippocampal interictal spiking, and to delayed (1 month after CHS) spontaneous seizures (31, 33).
Vicedomini and Nadler (68, 69) reevaluated the importance of the site of stimulation for induction of SSSE. Repeated application of high-frequency trains to the perforant path, which induced afterdischarges from the dentate gyrus, a key structure in the propagation of seizure activity (21), resulted in the build-up of self-sustaining seizure activity, which lasted for hours. SSSE developed in each animal that showed 10 consecutive afterdischarges.
Morissett et al. (46) modified the pilocarpine model of SE by administering atropine sulfate, which removed the cholinergic stimulus. Although it was effective in blocking SE when given before the onset of behavioral seizures, administration of atropine sulfate after overt seizures occurred was unable to stop seizures. This finding clearly showed that different mechanisms are responsible for initiation and maintenance of SE, and that SSSE can be triggered by chemical as well as by electrical stimulation. These results were extended to juvenile animals by Suchomelova et al. (60).
We used a protocol derived from those of Vicedomini and Nadler (68, 69) and Sloviter (57). We stimulated the perforant path in awake rats with 10-second, 20-Hz trains (1-msec square wave, 20V) delivered every minute, and with 2-Hz continuous stimulation. Recording was made from the dentate gyrus (42). Nissinen et al. (50) developed a similar model based on amygdala stimulation. Space limitations preclude discussion here of the very nice work done with the amygdala model or with variations of the perforant path model (16, 66).
The Perforant Path Stimulation Model of SSSE
Threshold for initiation of SSSE
Our initial studies derived from the need to eliminate the potentially confounding effect of anesthesia in our studies of seizure-induced brain damage in immature animals (62). In free-running adult rats, intermittent perforant path stimulation (PPS) easily triggered SSSE. With 7 minutes of PPS, animals (n = 5) showed epileptiform discharges in response to stimulation, but none of the animals showed any seizures after the end of stimulation. During 15-minute PPS, four of six rats showed ictal behavior, which stopped when stimulation ended. However, the two remaining rats displayed prolonged seizures during PPS (total time, 900 and 700 seconds), and after the end of stimulation they developed SSSE, which lasted for 2 and 16 hours, respectively. The animal with 6-hour SSSE died during SSSE. During 30-minute PPS, discrete spikes or spike-and-wave complexes or organized electrographic seizures appeared independently of stimuli, after 7–20 minutes of stimulation (Figure 17.1). As PPS continued, seizure duration increased, and the interval between the seizures decreased. After 20–25 minutes of PPS, all animals were displaying repetitive stage 3 to 4 seizures. The time spent in seizures during PPS was 12 ± 1.4 minutes (mean ± SEM). After the end of PPS, seizures continued in all animals (Figure 17.1). The severity of seizures varied from stage 1 (facial clonus) to stage 5 (clonus, rearing and falling). Electrographically, either discrete spikes or spike-and-wave complexes (recognized as spikes by the software), associated with stage 1–3 convulsions, or organized seizures lasting for up to 2 minutes and accompanied by stage 4–5 convulsions were observed (Figure 17.1). They merged into nearly continuous polyspike activity, which later became interrupted by postictal low-voltage periods. The time spent in seizure after 30-minute PPS averaged 490 ± 128 minutes in this series of animals. As SSSE continued, the incidence of overt seizures decreased (Figure 17.2). Starting at 3–6 hours, postictal episodes lengthened, and ictal activity evolved toward a pattern of periodic epileptiform discharges (PEDs), with a frequency of 1.5Hz, accompanied by stage 3 seizures. In the final hours of SSSE the amplitude of PEDs decreased and slow waves appeared. However, interictal spikes occurring at irregular intervals (sometimes 1 per minute) were observed even 24 hours after PPS. After 60-minute PPS, seizures occurred with higher frequency, and the duration of individual seizures was longer. Indeed, although the time of the last seizure after 30-minute and 60-minute PPS did not differ significantly (644 ± 134 and 730 ± 147 minutes, respectively), 60-minute PPS resulted in a significantly longer time spent in seizures (510 ± 71 vs. 352 ± 801 minutes, P < 0.05).
Figure 17.2.
EEG during SSSE induced by 30 minutes of perforant path stimulation (PPS). (A) Representative course of spikes. (B) The 24-hour distribution of seizures (black bars). PPS is indicated by the gray bar at top. Each line represents 2 hours of monitoring. (C) Sample electrographic activity in the dentate gyrus during SSSE. (Modified with permission from Mazarati et al. [42]. © 1998 by Elsevier Publishing.)
These results show that SSSE has a threshold for duration of epileptogenic stimulation that is critical for transition from stimulus-bound to self-sustaining seizures, and that this threshold lies between 7 and 30 minutes, which is close to the time needed to induce 10 afterdischarges in the studies of Vicedomini and Nadler (68). In this paradigm, the duration of seizure-like stimulation needed to induce SSSE was very short: 5 minutes of high-frequency trains (30-minute PPS) induced SSSE in 100% of rats, and behavioral and electroencephalographic (EEG) patterns were stereotypical from animal to animal.
SSSE maintained by an underlying change in excitability not dependent on continuous seizure activity
Perihilar injection of the AMPA/kainate receptor blocker CNQX 10 minutes after 30-minute PPS produced only transient effects: there was strong suppression of both electrographic (Figure 17.3E) and behavioral seizures. However, 4–5 hours after injection of CNQX, electrographic spikes and seizures reappeared, and soon after that behavioral convulsions recurred. Despite the effective seizure suppression for hours, total time spent in seizures over 24 hours (253 + 60 minutes vs. 352 + 80 minutes in controls) and the time of occurrence of the last seizure (627 + 40 minutes vs. 644 + 95 minutes in controls) did not significantly differ from controls (41). It seems that the change in excitability triggered by SE outlasted the drug and did not depend on continuous seizure activity in limbic circuits. We discuss some possible substrates for this change in excitability.
Figure 17.3.
The effects of NMDA (A–D) and AMPA/kainate (E) receptor blockers on SSSE induced by 60-minute PPS (A–D) or 30-minute PPS (E). Each graph shows the frequency of spikes (number of spikes per 30-minute epoch) plotted against time during the course of SSSE. PPS is indicated by the dotted gray bar on each graph. Representative time course of seizures detected by the software is shown next to the graphs. Each line represents 2 hours of monitoring, and each seizure is indicated by a black bar. Arrows indicate time of drug administration. NMDA receptor blockers MK-801 (0.5mg/kg IP), 2,5-DCK (10nmol into the hilus), and ketamine (10mg/kg IP) administered 10 minutes after the end of PPS irreversibly aborted SSSE soon after injection. CNQX (10nmol into the hilus) injected after 30-minute PPS induced only transient suppression of seizures, which reappeared within 2–4 hours after CNQX injection. (Modified with permission from Mazarati et al. [41]. © 1998 by Elsevier Publishing.)
The two phases of SE are pharmacologically distinct
Pharmacologically, a large number of agents are able to induce SSSE (Table 17.1), suggesting that the circuit that maintains self-sustaining seizures has many potential points of entry. However, pharmacologic responsiveness during initiation of SSSE and during established SSSE are strikingly different. Minute amounts of many agents that enhance inhibitory transmission or reduce excitatory transmission easily block the development of SSSE (Table 17.1), suggesting that brain circuits are biased against it and that all systems must be “go” in order for the phenomenon to develop. This is hardly surprising, insofar as SSSE is a rare, life-threatening event. However, once seizures are self-sustaining, few agents are effective in terminating them, and they usually work only in large concentration. The most efficacious agents are blockers of NMDA synapses, or presynaptic inhibitors of glutamate release (Table 17.1).
Table 17.1 : Agents capable of initiating and blocking self-sustaining status epilepticus
| Initiators |
Blockers of initiation phase |
Blockers of maintenance phase |
| Low Nao+, high Ko+ |
Na+ channel blockers |
NMDA antagonists |
| GABAA antagonists |
GABAA agonists |
Tachykinin antagonists |
| Glutamate agonists: NMDA, AMPA, kainate, low Mgo2+, low Cao2+, stimulation of glutamatergic pathways |
NMDA antagonists, high |
Galanin |
| Mgo2+ |
Dynorphin |
| AMPA/kainate antagonists |
|
| Cholinergic muscarinic antagonists |
|
| Cholinergic muscarinic agonists, stimulation of muscarinic pathways |
SP, neurokinin B antagonists |
|
| Galanin |
|
| Tachykinins (SP, NKB) |
Somatostatin |
|
| Galanin antagonists |
NPY |
|
| Opiate delta agonists |
Opiate δ antagonists |
|
| Opiate kappa antagonists |
Dynorphin (κ agonist) |
|
Initiation is accompanied by a loss of GABA inhibition
Prolonged loss of paired-pulse inhibition occurs after brief (<5 minutes) perforant path stimulation in vitro (in hippocampal slices), as well as in vivo, with the paired-pulse population-spike amplitude ratio (P2/P1) increasing from a baseline of 0.53 ± 0.29 to 1.17 ± 0.09 after PPS (P < 0.05). After perfusion with the GABAA antagonist bicuculline, the P2/P1 ratio increases from a baseline of 0.52 ± 0.16 to 1.15 ± 0.26 (P < 0.05). After 1–2 minutes of PPS, a 22% ± 6% (P < 0.05) decrease occurs in the P2/P1 amplitude ratio of paired-pulse-evoked inhibitory postsynaptic currents, consistent with the involvement of GABAA synaptic receptors. The findings suggest that loss of inhibition at GABAA synapses may be an important early event in the initiation of SE.
Maintenance of SSSE depends on the activation of NMDA receptors
Intraperitoneal administration of the NMDA receptor blocker MK801 (0.5mg/kg) after 60-minute PPS effectively aborted SSSE. Suppression of electrographic seizure activity was reflected in a decrease in spike frequency (Figure 17.3), in the total time spent in seizures (9.8 + 3 minutes vs. 510 + 70 minutes in controls), and in the time of occurrence of the last seizure (28 + 6 minutes vs. 730 + 148 minutes in controls). Another NMDA receptor blocker, 5,7-dichlorokinurenic acid (10 nmoles injected into the hilus of the dentate gyrus), quickly and irreversibly stopped both electrographic and behavioral manifestations of SSSE (Figure 17.3) without inducing behavioral depression. Ketamine (10mg/kg IP) and felbamate, two NMDA antagonists available for intravenous use in the U.S. Pharmacopeia, when administered after either 30- or 60-minute PPS, stopped both behavioral and electrographic seizures within 10 minutes after drug injection.
Time-dependent development of pharmacoresistance
Diazepam and phenytoin (or their analogues, lorazepam and fosphenytoin) are the two anticonvulsants most often used for treatment of SE in humans (34, 35, 59, 70). We examined the effects of these two antiepileptic drugs with regard to dose and time of injection. Pretreatment with diazepam in doses of 0.5mg/kg, 5mg/kg (not shown), 10mg/kg (Figure 17.4), or phenytoin (50mg/kg), before initiation of stimulation, effectively prevented the development of SSSE. Even for the lower doses, the cumulative time spent in seizures after the end of PPS did not exceed 5 minutes, and the last seizure occurred within 10 minutes after treatment. When administered 10 minutes after the end of 30-minute PPS, diazepam in doses of 0.5 and 5mg/kg had no seizure-protective effects. After injection of 10mg/kg, motor seizures stopped within 10 minutes. At this dose, diazepam induced strong muscle relaxation and ataxia. However, despite the absence or mild character of behavioral seizures, electrographic seizures continued. Total time spent in seizures was 95 ± 22 minutes, and the last seizure was observed at 140 ± 32 minutes after the administration of diazepam, a significantly longer period than in the group pretreated with diazepam before PPS. Phenytoin (50mg/kg) effectively aborted SSSE when injected 10 minutes after 30-minute PPS: animals spent 6.3 ± 2.5 minutes in seizures, and the last seizure occurred within 30 minutes after drug administration (Figure 17.4). When injected 10 minutes after 60-minute PPS, both diazepam (10mg/kg, Figure 17.4) and phenytoin (50mg/kg) failed to stop established SSSE. Time spent in seizures was 204 ± 30 and 216 ± 27 minutes, and the last seizure occurred 366 ± 65 and 300 ± 93 minutes after injection of diazepam and phenytoin, respectively (P < 0.05 vs. control). All of these measurements were significantly higher than in the pretreatment protocol (36). In other words, the same dose that was very effective when given as pretreatment failed when administered after SSSE was established. The reduction through endocytosis of the number of GABAA receptors available at the synapse may explain the loss of benzodiazepine potency: the clathrin-binding site, which is the mediator of endocytosis, is located on the benzodiazepine-binding γ2 subunit of GABAA receptors, and this could make that subunit particularly prone to internalization.
Figure 17.4.
Time-dependent development of pharmacoresistance in SSSE induced by 60-minute PPS. (A) When administered before PPS, both diazepam (DZP) and phenytoin (PHT) prevented the development of SSSE. (B) When injected 10 minutes after the end of PPS, neither diazepam nor phenytoin aborted SSSE, although they shortened its duration (*P < 0.05 vs. control; #P < 0.05 vs. diazepam and phenytoin as shown in A [pretreatment]). Open bars indicate cumulative seizure time; solid bars indicate the duration of SSSE (time of occurrence of the last seizure during SSSE). (C–E) Representative time course of seizures in a control animal (C), an animal pretreated with diazepam (D), or an animal with diazepam injected 10 minutes after PPS (E). Each line represents 2 hours of EEG monitoring. Each software-recognized seizure is shown by a small black bar. PPS is indicated by gray dotted bars at the top of each graph. Time of injection of diazepam is indicated by an arrow in D and E. In the control animal, self-sustaining seizures were observed for 17 hours. In diazepam-pretreated rats, seizures occurred during PPS, but only a few seizures were observed after PPS and only within the first 20 minutes. In the diazepam-post-treated animal, self-sustaining seizures continued for 8 hours. (Modified with permission from Mazarati et al. [36]. © 1998 by Elsevier Publishing.)
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