Cholinergic drug studies of generalized penicillin epilepsy in the cat

Quantitative spatio-temporal characterization of epileptic spikes using high density EEG: Differences between NREM sleep and REM sleep

In this study, we applied high-density EEG recordings (HD-EEG) to quantitatively characterize the fine-grained spatiotemporal distribution of inter-ictal epileptiform discharges (IEDs) across different sleep stages. We quantified differences in spatial extent and duration of IEDs at the scalp and cortical levels using HD-EEG source-localization, during non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep, in six medication-refractory focal epilepsy patients during epilepsy monitoring unit admission. Statistical analyses were performed at single subject level and group level across different sleep stages for duration and distribution of IEDs. Tests were corrected for multiple comparisons across all channels and time points. Compared to NREM sleep, IEDs during REM sleep were of significantly shorter duration and spatially more restricted. Compared to NREM sleep, IEDs location in REM sleep also showed a higher concordance with electrographic ictal onset zone from scalp EEG recording. This study supports the localizing value of REM IEDs over NREM IEDs and suggests that HD-EEG may be of clinical utility in epilepsy surgery work-up.

Heightened Background Cortical Synchrony in Patients With Epilepsy: EEG Phase Synchrony Analysis During Awake and Sleep Stages Using Novel Ensemble Measure

INTRODUCTION

Excessive cortical synchrony within neural ensembles has been implicated as an important mechanism driving epileptiform activity. The current study measures and compares background electroencephalographic (EEG) phase synchronization in patients having various types of epilepsies and healthy controls during awake and sleep stages.

METHODS

A total of 120 patients with epilepsy (PWE) subdivided into 3 groups (juvenile myoclonic epilepsy [JME], temporal lobe epilepsy [TLE], and extra-temporal lobe epilepsy [Ex-TLE]; n = 40 in each group) and 40 healthy controls were subjected to overnight polysomnography. EEG phase synchronization (SI) between the 8 EEG channels was assessed for delta, theta, alpha, sigma, and high beta frequency bands using ensemble measure on 10-second representative time windows and compared between patients and controls and also between awake and sleep stages. Mean ± SD of SI was compared using 2-way analysis of variance followed by pairwise comparison ( P ≤ .05).

RESULTS

In both delta and theta bands, the SI was significantly higher in patients with JME, TLE, and Ex-TLE compared with controls, whereas in alpha, sigma, and high beta bands, SI was comparable between the groups. On comparison of SI between sleep stages, delta band: progressive increase in SI from wake ⇒ N1 ⇒ N2 ⇒ N3, whereas REM (rapid eye movement) was comparable to wake; theta band: decreased SI during N2 and increase during N3; alpha band: SI was highest in wake and lower in N1, N2, N3, and REM; and sigma and high beta bands: progressive increase in SI from wake ⇒ N1 ⇒ N2 ⇒ N3; however, sigma band showed lower SI during REM.

CONCLUSION

This study found an increased background cortical synchronization in PWE compared with healthy controls in delta and theta bands during wake and sleep. This background hypersynchrony may be an important property of epileptogenic brain circuitry in PWE, which enables them to effortlessly generate a paroxysmal EEG depolarization shift.

The relationship between sleep and epilepsy

The occurrence of seizures in the sleep state is observed in nearly one third of patients. This is caused by an intimate relationship between the physiological state of sleep and the pathological process underlying epileptic seizures. Both sleep and sleep deprivation influence the frequency of epileptiform discharges on electroencephalograms as well as the occurrence of clinical seizures, typically during nonrapid eye movement sleep. The relationship of epileptiform activity to nonrapid eye movement sleep is vividly shown in the syndrome of continuous spikes in slow-wave sleep and the Landau-Kleffner syndrome. Seizure semiology can also be influenced by sleep and sleep deprivation. Sleep disorders may influence seizure control, and effective treatment of sleep disorders can improve seizure control. Seizures, antiepileptic drugs, ketogenic diet, and vagus nerve stimulation all influence sleep quality, daytime alertness, and neurocognitive function.

The alpha2 adrenoreceptor agonist clonidine suppresses evoked and spontaneous seizures, whereas the alpha2 adrenoreceptor antagonist idazoxan promotes seizures in amygdala-kindled kittens

Microinfusion of alpha2 adrenoreceptor agonists and antagonists into amygdala has contrasting effects on evoked and spontaneous seizure susceptibility in amygdala-kindled kittens. Subjects were 14 preadolescent kittens between 3 and 4 months old at the beginning of kindling. The same protocol was followed except that half the kittens received microinfusions (1 mul) of the alpha2 agonist clonidine (CLON; 1.32 nmol), and half received the alpha2 antagonist idazoxan (IDA; 0.33 nmol). Infusions were made over 1 min through needles inserted into cannulae adjacent to stimulating electrodes in the kindled amygdala, and evoked seizures were tested 10-12 min later. The results were: (1) CLON elevated seizure thresholds obtained once at the beginning and end of kindling, but only when compared to sham control values (needle insertion only) in the same animals; IDA significantly reduced thresholds. (2) CLON retarded and IDA accelerated kindling rate, defined as the number of afterdischarges (ADs) required to achieve the first stage 6 seizure or generalized tonic-clonic convulsion (GTC). These effects were most pronounced on the emergence of seizure "generalization" stages (3-6) from "focal" seizure stages (1-2). (3) CLON prevented onset of spontaneous seizures, whereas IDA precipitated onset of spontaneous seizures in 100% of the animals before or during the 5-week post-kindling follow-up during which seizures were evoked once each work day. The study confirms previous findings in kindled rodents to show that CLON and IDA can have opposing effects on kindling development in kittens and is the first report to show contrasting effects on spontaneous epileptogenesis in kindled animals as well.

The relationship between sleep and epilepsy

Seizures occur extensively during sleep or on awakening in a substantial proportion of patients with epilepsy. Interictal epileptiform discharges are also influenced by sleep and sleep deprivation. Continuous spike-waves in slow-wave sleep are the hallmark of Landau-Kleffner syndrome and ESES (Electrical Status in Slow Sleep). Sleep deprivation is known to influence not only the occurrence but also the symptomatology of epileptic seizures. Sleep architecture and daytime alertness are influenced by seizures and antiepileptic medications. This review examines the clinical and basic science aspects of this relationship between sleep and epilepsy.

Pathophysiological mechanisms of genetic absence epilepsy in the rat

Generalized non-convulsive absence seizures are characterized by the occurrence of synchronous and bilateral spike and wave discharges (SWDs) on the electroencephalogram, that are concomitant with a behavioral arrest. Many similarities between rodent and human absence seizures support the use of genetic rodent models, in which spontaneous SWDs occur. This review summarizes data obtained on the neurophysiological and neurochemical mechanisms of absence seizures with special emphasis on the Genetic Absence Epilepsy Rats from Strasbourg (GAERS). EEG recordings from various brain regions and lesion experiments showed that the cortex, the reticular nucleus and the relay nuclei of the thalamus play a predominant role in the development of SWDs. Neither the cortex, nor the thalamus alone can sustain SWDs, indicating that both structures are intimely involved in the genesis of SWDs. Pharmacological data confirmed that both inhibitory and excitatory neurotransmissions are involved in the genesis and control of absence seizures. Whether the generation of SWDs is the result of an excessive cortical excitability, due to an unbalance between inhibition and excitation, or excessive thalamic oscillations, due to abnormal intrinsic neuronal properties under the control of inhibitory GABAergic mechanisms, remains controversial. The thalamo-cortical activity is regulated by several monoaminergic and cholinergic projections. An alteration of the activity of these different ascending inputs may induce a temporary inadequation of the functional state between the cortex and the thalamus and thus promote SWDs. The experimental data are discussed in view of these possible pathophysiological mechanisms.

The alpha 2-adrenoreceptor agonist clonidine suppresses seizures, whereas the alpha 2-adrenoreceptor antagonist idazoxan promotes seizures in amygdala-kindled kittens: a comparison of amygdala and pontine microinfusion effects

PURPOSE

We sought to determine whether local, in vivo microinfusion of an alpha 2-adrenoreceptor agonist and antagonist into either the amygdala or the pons (locus ceruleus, LC) would have contrasting effects on evoked amygdala-kindled seizure susceptibility.

METHODS

The study population consisted of 6 amygdala-kindled kittens, each undergoing the same protocol, in which the amygdala microinfusion paradigm preceded the pontine microinfusion series. Microinfusions (1 microliter) of the alpha 2-agonist clonidine (CLON) and the alpha 2-antagonist idazoxan (IDA) were made over 1 min through cannulas adjacent to stimulating electrodes in the kindled amygdala or through cannulas adjacent to recording electrodes in the ipsilateral LC. Order of administered drugs (CLON vs. IDA) and dosages (n = 3 each) was partly counterbalanced. Focal and convulsive seizure thresholds were evaluated 10-12 min postinfusion and compared to thresholds obtained during two interspersed control conditions (vehicle control = 1 microliter microinfusion of sterile saline; sham control = needle insertion only).

RESULTS

CLON significantly increased focal and generalized seizure thresholds, whereas IDA significantly reduced seizure thresholds when compared to controls. Magnitude of effects was dose dependent and more potent after pontine than amygdala microinfusion.

CONCLUSIONS

Our results confirm and extent findings of previous researchers who used unlocalized in vivo manipulations to show that norepinephrine (NE) is a highly antiepileptic agent in the amygdala kindling preparation. With further investigation, the results may ultimately lead to development of microinfusion techniques as an alternative treatment option for limbic epilepsy.

Mesopontine cholinergic control over generalized non-convulsive seizures in a genetic model of absence epilepsy in the rat

Pharmacological data have shown that the cholinergic transmission participates in the control of spike-and-wave discharges in rats with genetic absence epilepsy. The corticothalamic circuitry which generates spontaneous spike-and-wave discharges, the electroencephalographic expression of absence seizures, receives important cholinergic inputs from two distinct sources: (i) the nucleus basalis projecting mainly to the cortex and (ii) the pedunculopontine and laterodorsal tegmental nuclei providing cholinergic afferents to the thalamus. In the present study, the involvement of the cholinergic mesopontothalamic projections in the control of spike-and-wave discharges was investigated. Activation of cell bodies in the pedunculopontine and laterodorsal tegmental nuclei, by local microinjections of non-toxic doses of kainate (20 pmol/side) or picrotoxin (66 pmol/side), suppressed spike-and-wave discharges. Similar effects were produced by direct cholinergic activation of the ventrolateral part of the thalamus: intrathalamic microinjections of carbachol (0.7-2.8 pmol/side), a cholinergic receptor agonist, resulted in a dose-dependent suppression of spike-and-wave discharges. This suppression was partially reversed by a simultaneous microinjection of an equimolar dose of scopolamine, a muscarinic receptor antagonist. Electrolytic or neuroexcitotoxic lesions of the pedunculopontine and laterodorsal tegmental nuclei did not modify spike-and-wave discharges. These results suggest that the cholinergic mesopontine projection to the thalamus exerts a phasic inhibitory control of generalized non-convulsive epileptic seizures.

[Sleep and intra-ictal epileptic electroencephalographic activities]

From this review it appears that the slow wave sleep (SWS) increases the mean density of electroencephalographic paroxysmal activities (PA) whatever the epileptic syndrome. This pattern is not marked according to a bell curve among the epileptic population: about half the patients exhibit few or no PA during SWS, 20% show an increase during waking and another 20% during SWS. Begnin epilepsy with centro-temporal spikes is associated with an important sleep PA increase. In partial epilepsy, stage 3 and 4 sleep should increase the PA transmission. In children, a large increase in PA during SWS defines the continuous spike-wave during sleep syndrome, which is also observed in the syndrome of acquired aphasia with epilepsy of Landau-Kleffner; both conditions raise the issue of the neuropsychological consequences of the sleep PA. The sleep effect on the various epileptic models is analysed, showing a mean increase in PA during SWS and during transition between sleep and waking. This evidence is in agreement with the fact that during light sleep thalamocortical loops are functioning with an oscillatory pattern which facilitates PA expression. More hypothetic is the effect of sleep on the discharge rate of epileptic focus and on the cortical diffusion of the epileptic discharges. Gaba certainly participates in the thalamic influence, but its role on PA by the cortical and brain stem inhibition is speculative. Noradrenaline and acetylcholine, implicated in waking, reduce PA activity. Interindividual variations suggest that each epileptic has his own pathological neuronal organisation in which cortex and thalamocortical connexions are variously sensitive to the neurotransmitters implicated in sleep and waking.

Nucleus basalis lesions suppress spike and wave discharges in rats with spontaneous absence-epilepsy

Cholinergic drugs were shown to affect spike and wave discharges in a selected strain of Wistar rats with generalized non-convulsive absence epilepsy, named GAERS (Genetic Absence Epilepsy Rats from Strasbourg). The involvement of cholinergic transmission from the nucleus basalis in the control of absence seizures in GAERS was investigated in the present study, by examining the effects of unilateral excitotoxic lesions of this nucleus on the occurrence of spike-wave discharges. Ibotenate (0.01 M) and quisqualate (0.03 and 0.06 M)-induced lesions of the nucleus basalis suppressed spike-wave discharges in the cortex ipsilateral to the lesion. The suppression was associated with a disappearance of both acetylcholinesterase-fibres in the cerebral cortex and choline acetyltransferase immunopositive neurons within the nucleus basalis. Concomitantly, the background electroencephalographic activity was slowed. These results suggest that cholinergic innervation of the cerebral cortex by the nucleus basalis is involved in the occurrence of generalized non-convulsive seizures, in relation to the control of cortical activation.

Effects of cholinergic drugs on genetic absence seizures in rats

Wistar rats of a selected strain show spontaneous generalized non-convulsive seizures with bilateral synchronous spike-wave discharges on the cortical electroencephalograph (EEG). The 7 to 9 c/s spike-wave discharges occur predominantly in waking states of inactivity. The effects of cholinergic drugs on the cumulated duration of spike-wave discharges were investigated in this rat model of absence epilepsy. I.p. injections of drugs which potentiate cholinergic neurotransmission, namely the acetylcholinesterase inhibitor, physostigmine (0.1-0.5 mg/kg), the muscarinic receptor agonists, oxotremorine (0.25-1 mg/kg) and pilocarpine (0.125-2 mg/kg), and the nicotinic receptor agonist, nicotine (0.062-2 mg/kg), suppressed discharges in a dose-dependent manner and induced an arousal-like cortical EEG. The muscarinic receptor antagonist, scopolamine, increased the spike-wave discharges at doses below 0.05 mg/kg; at higher doses (0.05-1 mg/kg) it decreased discharges and induced a sleep-like EEG. The nicotinic receptor antagonist, mecamylamine (0.5-6 mg/kg), had no effect on spike-wave discharges or the EEG. These results suggest that cholinergic activity accounts for the preferential occurrence of absence seizures in states of reduced arousal.

Genetic and phenotypic heterogeneity of inherited spike-wave epilepsy: two mutant gene loci with independent cerebral excitability defects

Two recessive gene loci controlling cerebral excitability in the mouse (tg, chr 8 and stg, chr 15) share generalized neocortical spike-wave seizures as a common mutant phenotype. Although the primary molecular defects are unknown, homozygous tg mutants display a gene-linked hyperplasia of central noradrenergic axons originating in the locus ceruleus, and early selective lesions of these fibers correct the epileptic phenotype in the adult. In contrast, we find that stg homozygotes, despite a more severe seizure disorder, show no alterations in regional noradrenergic fiber innervation, and seizure frequency is unaffected by neonatal noradrenergic depletion. These mutations demonstrate that excessive synchronous neuronal discharges alone are insufficient to trigger abnormal growth of locus ceruleus fibers, and reveal the existence of two distinct intervening brain neuromodulatory mechanisms, norepinephrine (NE)-dependent and NE-independent, underlying the inheritance of this common pattern of epilepsy.

The role of the laterodorsal tegmental nucleus of the rat in experimental seizures

This study determined the effects of discrete microinjections of GABA agonists in the cholinergic nuclei of the pontomesencephalic tegmentum on spontaneous behavior and seizures induced by intravenous pentylenetetrazol, bicuculline or strychnine, in the rat. Injections of both the GABAA agonist piperidine-4-sulfonic acid and the GABAB agonist (-)baclofen in the laterodorsal tegmental nucleus produced a dose-dependent suppression of behavioral arousal and a reduction in the threshold of myoclonic and clonic but not tonic seizures induced by bicuculline and pentylenetetrazol. There were no significant effects on any type of strychnine seizure. Injections in the surrounding brainstem structures, including the pedunculopontine tegmental nucleus, had little effect on spontaneous behavior and did not significantly alter the thresholds of pentylenetetrazol-induced seizures. We have previously demonstrated that injections of GABA agonists in the central medial intralaminar nucleus of the thalamus have similar effects on behavior and seizures. Since the central medial nucleus receives important direct cholinergic projections from the laterodorsal tegmental nucleus, these two nuclei form a discrete ascending system which regulates seizure threshold.

Mechanisms of seizure suppression during rapid-eye-movement (REM) sleep in cats

REM sleep is the most antiepileptic state in the sleep-wake cycle for human generalized epilepsy, yet the neural mechanism is unknown. This study verified the antiepileptic properties of REM sleep in feline generalized epilepsy and also isolated the responsible factors. Conclusions are based on 20 cats evaluated for generalized EEG and motor seizure susceptibility before and after dissociation of specific REM sleep components. Bilateral electrolytic lesions of the medial-lateral pontine tegmentum created a syndrome of REM sleep without atonia. Systemic atropine created a syndrome of REM sleep without thalamocortical EEG desynchronization. Identical results were obtained in two seizure models, systemic penicillin epilepsy and electroconvulsive shock. (1) Normal REM sleep retarded the spread of EEG seizure discharges and had even more potent anticonvulsant effects. (2) Selective loss of 'sleep paralysis' (skeletal muscle atonia) during REM abolished REM sleep protection against myoclonus and convulsions without affecting generalized EEG paroxysms. (3) Conversely, selective loss of thalamocortical EEG desychronization abolished REM sleep protection against generalized EEG seizures without affecting clinical motor accompaniment. These results suggest that the descending brainstem pathways which mediate lower motor neuron inhibition also protect against generalized motor seizures during REM sleep. Protection against spread of EEG paroxysms is governed by a separate mechanism, presumably the ascending brainstem pathways mediating intense thalamocortical EEG desynchronization during REM sleep.

Modulation of generalized spike-and-wave discharges during sleep by cyclic alternating pattern

Because arousal plays a critical role in activation of epileptic phenomena, we analyzed the behavior of interictal generalized spike-and-waves (S-W) during the two recently identified modalities of arousal control during NREM sleep: (a) the cyclic alternating pattern (CAP), expressed by successive biphasic (phase A and phase B) fluctuations of arousal; and (b) non-CAP (NCAP), characterized by prolonged stable periods of EEG and arousal level. Within the single NREM stages, phase A consists of clusters of phasic events associated with transient arousal activation, whereas phase B is represented by the periodic reappearance of the EEG background and reflects a rebound inhibitory condition. The polysomnograms of 10 subjects with a wide age range and affected by primary generalized epilepsy displayed significant differences (all at p less than 0.01) between the spike indexes (S-W/min) of CAP (2.9) and NCAP (1.3), of phase A (7.4) and phase B (0.3), and of NCAP and phase B. For distribution of S-W, a significant prevalence of EEG paroxysms was detected during CAP as compared with NCAP (68 vs. 32%, p less than 0.001), and 93% of all the interictal bursts that occurred in CAP occurred in phase A (p less than 0.001). Our data stress the arousal-dependent influence of CAP and NCAP on modulation of generalized epileptic mechanisms during sleep.

Suppression of penicillin-induced epileptiform activity by noxious stimulation: mediation by 5-hydroxytryptamine

Noxious stimulation has a well known suppressant effect on epileptiform activity in both laboratory animals and man. To study this phenomenon in an animal model, focal epileptiform activity was induced in anaesthetized rats by applying penicillin from one barrel of a micropipette while recording intracortical electrical activity from another barrel. Penicillin produced either focal epileptiform activity or focal penicillin spikes, depending somewhat on the rate of penicillin release. Focal epileptiform activity was suppressed by noxious stimulation, both somatic and olfactory, whereas non-noxious stimulation was ineffective in this regard. Focal penicillin spikes were only rarely suppressed by noxious stimulation. Reserpine blocked the suppressant effect of noxious stimulation as did p-chlorophenylalanine, a more selective depletor of 5-hydroxytryptamine. L-5-Hydroxytryptophan, a 5-hydroxytryptamine precursor, restored the suppressant effect of noxious stimulation blocked by reserpine and p-chlorophenylalanine. These results suggest that the suppression of SW by noxious stimulation is mediated by 5-hydroxytryptamine. Data from experiments employing pharmacological antagonists suggest the suppression of spike and wave activity by noxious stimulation not to be mediated by activation of dopamine receptors, alpha 2 or beta adrenoceptors, or muscarinic cholinoceptors. Prazosin, a selective alpha 1 adrenoceptor antagonist, did block the suppressant effect of noxious stimulation but only at a very high dose (2.4 mumols/kg). This likely reflects a known analgesic action of prazosin or weak binding to the 5-HT receptor. Methysergide, a 5-hydroxytryptamine antagonist, failed to antagonize the suppressant effect of noxious stimulation, however, many inhibitory actions of 5-hydroxytryptamine are not blocked by methysergide. It is concluded that suppression of focal epileptiform activity by noxious stimulation is mediated, at least in part, by 5-hydroxytryptamine.

Suppression of penicillin-induced focal epileptiform activity by locus ceruleus stimulation: mediation by an alpha 1-adrenoceptor

Application of penicillin to the cerebral cortex of anesthetized rats by pressure ejection from a micropipette resulted in the appearance of focal epileptiform activity with low rates of penicillin release and focal penicillin spikes with higher rates. Electrical stimulation of the locus ceruleus (LC), a major norepinephrine-containing nucleus in the brainstem, or of its axons projecting to the forebrain, the dorsal noradrenergic bundle, suppressed penicillin-induced focal epileptiform activity but was less effective in suppressing focal penicillin spikes. Depletion of monoamines with reserpine blocked the suppressant effect of LC stimulation. Neither the selective depletion of 5-hydroxytryptamine with p-chlorophenylalanine nor administration of methysergide reduced the effectiveness of LC stimulation, suggesting that 5-hydroxytryptamine probably does not mediate the suppression. Pimozide partially antagonized the suppression of focal epileptiform activity induced by LC stimulation, which is consistent with antagonism of alpha-adrenoceptors but not dopamine receptors. beta-Receptor antagonists did not block the suppression of focal epileptiform activity by LC stimulation, suggesting that beta-receptors are not important in the observed suppression. Prazosin, a selective alpha 1-antagonist, at low doses blocked the suppression of focal epileptiform activity by LC stimulation whereas yohimbine, an alpha 2-antagonist enhanced the stimulation-induced suppression. Taken together, the data are consistent with LC and dorsal bundle stimulation releasing norepinephrine, which in turn suppresses focal epileptiform activity by an action mediated by an alpha 1-adrenoceptor.

Sleep and epilepsy

Epileptic mechanisms in the brain are subject to long-duration, time-ordered neuromodulatory processes controlled by endogenous oscillators which are responsible for appropriately phased modulation of various normal physiological processes, including the 24-h sleep/wakefulness cycle and the ultradian 100-min cycle of rapid eye movement/non-rapid eye movement sleep. Both focal and generalized types of epileptiform activity in humans are subject to biorhythmic modulation, and the various modulation patterns observed are in accord with a model which explains these patterns as a consequence of the interaction of two endogenous modulatory processes: one with a period of about 24 h, the other with a period of about 100 min. Differences in the phase angle between the two cyclic processes, determined by time of sleep onset, explain the various modulatory patterns observed. The mechanisms involved in the genesis and elaboration of electrical epileptiform activity in animal models are examined in relation to known processes involved in the physiology of sleep, and compared with data derived from long-term studies of the time distribution of epileptic events in humans. In infantile spasms, clinical seizure activity and the ictal and interictal EEG patterns in relationship to the phases of the sleep cycle, the significant defects in the quality and quantity of sleep in this disorder, and the changes that take place in all of these when seizures are abolished by effective treatment, suggest that pontine mechanisms responsible for the sleep cycle may be involved in the elaboration of infantile spasms and hypsarrhythmia.

Behavioral alterations associated with generalized spike and wave discharges in the EEG of the cat

Instrumental conditioning procedures demonstrated that in feline generalized penicillin epilepsy (FGPE) the cat's ability to respond to sensory (visual or auditory) stimuli was selectively impaired during penicillin-induced generalized spike and wave (SW) discharge. Responsiveness between SW bursts remained unimpaired. Most often the performance deficit consisted of a total absence of a learned response to stimuli presented during SW bursts or, if such a response occurred, reaction times were on the average significantly longer than to stimuli presented between SW bursts. Stimuli falling in the middle of a SW burst were associated with the highest likelihood of response failure. Spontaneous motor performance which was not contingent on any stimulus was also impaired during SW discharge. Response failure during SW bursts is either attributable to a cognitive defect or to motor impairment associated with temporary amnesia. Impairment of motor performance unassociated with amnesia or a cognitive defect was sometimes present during SW discharge, as evidenced by failure to carry out a motor response or to complete it until the SW burst was over. These deficits are similar to those seen in human absence attacks associated with generalized SW discharge. These observations thus support the validity of FGPE as an acceptable model of human primary generalized epilepsy.

The perspective of GABA replenishment therapy in the epilepsies: a critical evaluation of hopes and concerns

Impaired GABA-mediated inhibition is probably one of the cellular abnormalities leading to Focal Epilepsy. The role of GABA in generalized seizures, particularly of Petit Mal type, is unknown. Various approaches are available to potentiate GABA function. Merits and flaws of each one of them are critically evaluated. In some forms of epilepsy, GABA agonists may replenish depleted pools, and in some others may nonspecifically raise the general excitability threshold of the brain, yet in other forms they may exert a glutamate/aspartate antagonistic effect. The available experimental evidence suggests that in bilaterally synchronous spike and wave epilepsies, GABA agonists are either ineffective or pejorative.

Cholinergic effects on arousal and cocaine-induced olfactory-amygdala spindling and seizures in cats

Effects of intravenous cholinergic and dopaminergic agents on pre-cocaine olfactory bulb (OB) spindling and behavioral arousal, and on cocaine-induced OB-amygdala spindling and behavioral seizures were evaluated in seven cats with stereotaxically implanted electrodes. Spindle data were computer analyzed using a special program for spindle detection and averaging. The number, duration and amplitude of pre-cocaine OB spindles were increased by physostigmine and decreased by atropine. Physostigmine augmented pre-cocaine behavioral arousal levels and this effect was associated with the absence of cocaine seizures. Cocaine-induced amygdala spindle number was increased by physostigmine. Changes in seizure duration following cholinergic drugs suggest cholinergic inhibitory effects. These data, in accord with previous studies showing cholinergic effects on reticulocortical arousal and seizures, suggest a cholinergic mechanism which is excitatory on OB-amygdala arousal spindling and inhibitory on cocaine-induced seizures.

The effects of transient functional depression of the thalamus on spindles and on bilateral synchronous epileptic discharges of feline generalized penicillin epilepsy

A transient functional depression of thalamic activity (TFDTA) was induced in acute experiments in cats by the microinjection of 25% KCl into the thalamus. Spontaneous and evoked thalamic electrical activity was markedly depressed at the site of KCl microinjection. Spread of this depression to other thalamic areas often occurred, mainly when KCl was injected into the midline thalamus. In normal cats both spontaneous and evoked cortical spindle bursts as well as other evoked thalamocortical responses were reduced or abolished during the KCl-induced TFDTA. The generalized spike-and-wave discharges of feline generalized epilepsy were also suppressed for the duration of TFDTA, while incidental focal cortical interictal and ictal epileptic discharges, as well as generalized tonic-clonic seizure discharge, remained unaffected. The same effects were observed in animals with lesions of the mesencephalic reticular formation, indicating that the suppression of spindles and spike-and-wave discharges cannot be attributed to a release of the activity of the reticular formation by the TFDTA. An unexplained occurrence of generalized tonic-clonic EEG seizure was observed in most cases late after thalamic KCl microinjection, usually after the spike-and-wave discharges had recovered. These data are consistent with the hypothesis that the spontaneous bilaterally synchronous epileptic bursts of feline generalized penicillin epilepsy are not only closely related to spindles but are crucially dependent on thalamic inputs to the cerebral cortex.

Activating effects of homotaurine and taurine on corticoreticular epilepsy

Homotaurine and taurine are two powerful inhibitory aminoacids with anticonvulsant properties against various experimental models of focal epilepsy. This study reports on their effects in the feline model of corticoreticular epilepsy induced by parenteral administration of large amounts of penicillin. Both aminoacids, but particularly homotaurine, remarkably potentiate epileptiform discharges in cats. Brainstem transection at the precollicular level does not modify the activation, thus ruling out the intervention of mesoromboencephalic structures in the observed effect. The opposing action of these two amino acids on focal epilepsy as compared to corticoreticular epilepsy suggests that the two types of epileptiform activity stem from very different pathophysiological mechanisms. Homotaurine is a powerful GABA agonist that exerts a central action upon parenteral administration. Other GABA analogs such as muscimol, imidazole acetic acid, and gamma-hydroxybutyrate have been reported to potentiate experimental models of spike and wave epilepsy. Thus, the activating effects of homotaurine in this epilepsy model are in keeping with the demonstrated GABAmimetic properties of the compound.

Effects of changes in cortical excitability upon the epileptic bursts in generalized penicillin epilepsy of the cat

Previous studies had suggested that the epileptic bursts of feline generalized penicillin epilepsy represent the response of hyperexcitable cortex to thalamocortical volleys normally evoking spindles. If this were the case, it should be possible to convert the epileptic bursts of generalized penicillin epilepsy into spindles by decreasing the excitability of cortical neurons. In cats exhibiting the EEG signs of feline generalized penicillin epilepsy cortical excitability was decreased by hypoxia, by the topical application to the cortex of KCl (inducing spreading depression), barbiturates, GABA, AMP or noradrenaline. During generalized penicillin epilepsy, hypoxia and KCl-induced spreading depression abolished epileptic bursts which were replaced by spindles. When spindles and epileptic complexes occurring in the same animal were compared, a direct correlation between the frequencies of these two rhythms could be demonstrated, that of the epileptic complexes being about half that of the spindle waves. These observations support the hypothesis that the epileptic bursts of feline generalized penicillin epilepsy are induced by thalamocortical volleys normally involved in spindle genesis. Topical cortical applications of barbiturates, GABA, AMP and noradrenaline reduced or inverted the negative spikes of the spike and wave complexes, while augmenting the negative slow waves, or revealing them clearly in instances in which they had been poorly developed. This effect is interpreted as being due to a selective inactivation of the superficial cortical layers. That topical cortical application of barbiturates, GABA, AMP and noradrenaline was capable of transforming into typical spike and wave complex epileptic bursts, which had not previously conformed to this pattern, indicates that the intracortical electrophysiological events of typical and atypical epileptic bursts in feline generalized penicillin epilepsy are fundamentally the same and reflect an alternation between excitatory and inhibitory sequences.

Neuronal firing patterns during generalized penicillin epilepsy in the awake cat

Single unit activity was recorded in the neocortex of awake cats during spontaneous spike wave discharges. Ongoing activity was interrupted during paroxysmal events and was replaced by (1) bursts of action potentials coincident with the EEG spike and cessation of firing during the wave, or (2) cessation of firing for the duration of the paroxysm. Other units which displayed little or no background activity were recruited to fire during the EEG spike. Surface cerebellar stimulation at high frequencies led to decreased neuronal activity while single pulse shocks resulted in short latency activation. These results were compared to studies of spike wave activity elicited by electrical stimulation.

Pathophysiology of generalized penicillin epilepsy in the cat: the role of cortical and subcortical structures. I. Systemic application of penicillin

The mechanism of precipitation of generalized epileptiform discharges in feline generalized penicillin epilepsy, a model of human generalized corticoreticular ('centrencephalic') epilepsy, was studied in acute and chronic experiments in cats with implanted skull and intracerebral electrodes. Single shock and low frequency repetitive stimulation of subcortical sites from which prior to penicillin administration spindle activity and recruiting responses could be elicited, readily triggered epileptiform discharges in the same animals after penicillin. These structures comprised the intralaminar and midline thalamic nuclei, the neostriatum, and some posterior thalamic association nuclei (Pulvinar and nucleus lateralis posterior). Subcortical and cortical structures which prior to penicillin elicited neither spindle activity nor recruiting responses were significantly less effective in triggering generalized epileptic bursts after penicillin injection. The probability with which such bursts were elicited from these structures was still, however, in many instances above chance level. It is concluded that the generalized epileptiform discharges in feline generalized penicillin epilepsy can be triggered from a large number of brain sites, but most reliably so from subcortical nuclei involved in spindle generation and recruiting responses. The experimental evidence presented still does not allow one to determine whether epileptic alteration of neuronal function in this form of epilepsy primarily resides in cortical or subcortical nerve cells or in both.

The results of penicillin G Administration of chronic unrestrained cats: electrographic and behavioral observations

Intramuscular administration of penicillin G was carried out in a series of 27 chronic cat preparations. In addition to the EEG, the high frequency components of cerebral electrical activity were also recorded from cortical and deep structures. Videotape recordings using split-screen technique allowed correlations of the animal's clinical state with the EEG. The results showed that 'spike-wave' type activity occurred earliest in the cortex. The discharges were also best formed in cortical and thalamic structures. The other deep structures showed characteristic bursting, but not of the 'spikewave' type variety. The cerebellum showed also early participation. Clinical expressivity of the bursts depended upon the state of alertness of the animal, the height of the spike components and the extent of depth participation. The high frequency recordings revealed characteristic alternation of decrease and increase in activity mostly in cortical structures and to a lesser extent in thalamus. Pontine and medullary reticular formation areas showed usually no changes in the high frequency records during the bursts. When major seizures were induced, they were always of focal onset rather than of the primary generalized type as one sees with pentylenetetrazol. Different cortical or deep structures served as the initiating site in different animals. It was concluded that systemic penicillin adminstration does not lead to a truly primary generalized form of epilepsy, but produces its effects by multifocal activation of various cerebral structures, with cortex and cerebullum usually showing earliest involvement.

Parenteral penicillin in rats: an experimental model of multifocal epilepsy

Parenterally injected penicillin in rats induces a peculiar pattern of multifocal epilepsy. The effective amount is higher than that required in cats. The epileptiform activity initially appears on the cortical mantle of one hemisphere; after a variable delay, contralateral cortical spikes arise in a completely asynchronous way. Spontaneous independent firing is observed, with a further delay in subcortical structures. Although cortical spiking shows a tendency to become bilaterally synchronous, such synchrony has been only occasionally observed between the various subcortical structures. Formation of generalized spike-and-wave complexes, as reported in cats, or other features reminiscent of the human petit mal did not occur. This study stresses the differences in pathophysiological responses to the same epileptogenic model according to species variation.