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

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.

Feline generalized penicillin epilepsy: changes of glutamic acid and taurine parallel the progressive increase in excitability of the cortex

A coinciding temporal sequence of electrophysiological and biochemical correlates of developing generalized penicillin epilepsy in cats may indicate a "cause and effect" relationship between the two phenomena. After intramuscular injection of penicillin, in the pre-epileptic state prior to the onset of spike-and-wave discharge, the cortical content of glutamic acid decreases. This change occurs when an increased amplitude of visual evoked potentials in association cortex heralds the approach of spike-and-wave activity. The decrease of glutamic acid and that of aspartic acid occur in parallel with an almost stoichiometric increase of glutamine, gamma-aminobutyric acid (GABA), or both, while taurine levels in the pre-epileptic state remain near normal. As the pre-epileptic progresses to the epileptic state, characterized by generalized 4-5 cycles/s spike-and-wave discharges, a failure of the glial capture mechanisms for taurine and glutamate appears to occur, since both amino acids are lost from the tissue and glutamine levels fall while GABA levels are maintained or become elevated but increasingly at the expense of aspartic acid. A presumed increase in interstitial glutamic acid concentration possibly in combination with subsequent failure of GABA inhibition appears the most plausible explanation for the increasing hyperexcitability during the development of feline generalized penicillin epilepsy.

Participation of corticothalamic cells in penicillin-induced generalized spike and wave discharges

Single unit extracellular recordings were performed in the cortex of awake painlessly immobilized unanesthetized cats during generalized spike and wave discharges (SW) induced by i.m. penicillin. Corticothalamic cells were identified in cortical areas 3a and 4 gamma by stimulating n. ventralis lateralis (VL) and in cortical areas 5 and 7 by stimulating n. lateralis posterior (LP). Twelve of 24 neurons antidromically invaded from VL were also pyramidal tract cells. Two of 11 neurons antidromically invaded from LP also displayed orthodromic responses. Corticothalamic cells fired bursts of action potentials in association with the 'spike' whereas a period of inhibition was associated with the 'wave' of the SW complex. The data suggest that in this experimental model the appearance of SW in the thalamus is due to secondary activation of thalamic neurons by volleys arising from the cortex and mediated through corticothalamic connections.

Potentiation and modification of recruiting responses precedes the appearance of spike and wave discharges in feline generalized penicillin epilepsy

Recruiting responses (RR) were evoked by stimulation of nucleus centralis medialis in awake and painlessly immobilized cats. Following the administration of sodium penicillin G (350,000 IU/kg i.m.) and at a time preceding the development of generalized spike-and-wave discharge we observed a strong potentiation of RR in 10 out of 15 experiments (50-200% increase in amplitude). The major feature of wave form modification consisted mainly of a development or increase of positive phases of individual recruiting waves. In between such large amplitude negative-positive recruiting waves a slow negative wave developed. One of every two recruiting waves was often diminished when the preceding recruiting wave had reached considerable amplitude. The changes in the RR were antagonized by barbiturates and by caffeine. In conjunction with previous evidence these results support the hypothesis that spikes of spike-and-wave discharges in FGPE are generated by similar thalamocortical volleys as those creating RR and spindles. They further suggest that the crucial neuronal mechanism underlying this effect of penicillin is a shift in the emphasis from distal apical dendritic thalamocortical synapses on cortical pyramidal neurons to more proximal ones.

Laminar analysis of spindles and of spikes of the spike and wave discharge of feline generalized penicillin epilepsy

Intracortical laminar profiles of spindles and spikes of spike and wave complexes in feline generalized penicillin epilepsy were studied using two methods: (i) sequential microelectrode recordings at various cortical depths, and simultaneous recordings at multiple cortical depths using a fine multi-contact electrode. Raw EEG data and EEG epochs averaged with respect to peaks of surface EEG waves were analyzed. Spindles and the spikes of the spike and wave complexes showed similar laminar profiles. This supports the hypothesis that the two are basically the same cortical electrophysiological phenomenon, the spike being a spindle wave enhanced and slightly altered because of the penicillin-induced increased cortical excitability. The latter causes the weight of the thalamic input to shift from superficial to more deep lying synapses. Both surface negative and surface positive phases of spindles and of spikes of spike and wave complexes show similar laminar profiles, those of the former suggesting activation of excitatory synapses in the superficial cortical layers, those of the latter suggesting activation of more deeply located excitatory synapses. The profiles generally conform to the dipole hypothesis of cortical electrogenesis and suggest that spindles and spikes of spike and wave complexes are generated by the same pyramidal neurons, probably through activation of the same sets of synapses. Some inconstant and relatively minor deviations of the laminar profiles from the pattern predicted by the dipole theory of cortical electrogenesis were encountered and are tentatively explained in the light of some of the complexities of the microanatomical organization of mammalian neocortex.

Effects of post-ictal depression on experimental spike and wave discharges

The effects of post-ictal depression on spike and wave (SW) discharges of feline generalized penicillin epilepsy (FGPE) were studied. After tonic-clonic seizures which are not uncommon in FGPE spindle bursts appeared during the post-ictal period. Upon recovery spontaneous and thalamically evoked SW discharges reappeared. Spindles before penicillin and during post-ictal depression showed a similar intraburst frequency (twice that of SW discharges) in the same animal. These findings add further evidence to the notion that any depression of cortical excitability in FGPE leads to replacement of SW by spindles and thus supports the hypothesis that SW discharges occur in hyperexcitable cortex in response to normally spindle-inducing thalamocortical volleys.

Comparative study of aminopyridine-induced seizure activities in primary and mirror foci of cat's cortex

Epileptogenic activity was induced by topical application of 3-Ap on the somatosensory cortex of anaesthetized cats. Surface epileptiform events were recorded and single cell activity was studied with intracellular electrodes both in the primary and in the mirror focus. Surface records showed that soon after the onset of paroxysmal activity in the primary focus, an epileptic mirror focus developed with variable delay. After MnCl2 treatment of the primary focus, the mirror focus generated independent epileptic events. In the primary and in the mirror focus 122 neurones were recorded intracellularly during seizure activity. Different firing patterns were observed. In the primary focus 31% and in the mirror focus 28% of the recorded neurones generated PDSs. The firing of the neurones in both foci was often phase-locked with surface waves. PDSs usually occurred asynchronously with the surface activity. Ionic mechanisms which may underlie the effect of 3-Ap are discussed.

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.

Anticonvulsant-induced status epilepticus in Lennox-Gastaut syndrome

A 22-year-old woman with the Lennox-Gastaut syndrome developed serial apnoeic-tonic seizures with concomitant bursts of repetitive spikes at 16-20/sec, after receiving therapeutic doses of chlormethiazole, clonazepam, and diazepam for the treatment of serial tonic-clonic generalized seizures. There was a direct relationship between dose of chlormethiazole and frequency of apnoeic-tonic seizures. Since these tonic seizures have been noted during natural sleep and after sleep induced by various sedative drugs in patients with slow spike-wave in EEG, it is likely that the reduction in level of consciousness induced by sedative drugs is the causative factor.

Power spectral analysis of EEG characteristics during sleep in epileptics

Power spectral analysis of spontaneous EEG activity recorded during non-rapid eye movement (REM) sleep was employed in an effort to identify characteristic EEG substrate abnormalities in epileptics. All-night polygraphic recordings were obtained from 10 poorly controlled subjects with mixed and generalized seizures. Similar data were obtained from 10 matched nonepileptic subjects. Continuous 10 min samples of non-REM sleep were identified at the beginning and end of these all-night recordings. EEG activity recorded during these periods from left sensorimotor and parieto-occipital bipolar leads was digitized, subjected to the fast Fourier transform, and then sorted into successive 4 Hz frequency bands between 0 and 23 Hz. Mean spectral density was determined for each frequency band from both samples in both groups. Comparison of these values disclosed both similarities and differences between the two groups. All subjects showed a significant "time-of-night effect," with power at 0-3, 4-7, and 8-11 Hz greater at the beginning of the night. This finding was related to a similar observation in developing infants and attributed to a circadian influence on neuronal generators. In group comparisons, epileptics showed selective differences in common when compared with nonepileptics. These differences were reduced when data from a small group of unmedicated epileptic subjects were partialled out. Medicated epileptics showed differences restricted to the early morning non-REM sleep sample, which included significantly greater power at 4-7 Hz and significantly lower power at 12-15 Hz. Unmedicated subjects showed these differences in both samples and also showed significantly greater power in the 0-3 Hz band. It was suggested that anticonvulsant medications may exert a general normalizing influence on EEG characteristics during sleep, particularly at the beginning of the night in proximity to their administration. The documentation of significant EEG deviations restricted to two basic rhythmic patterns during sleep suggests a disturbance of underlying thalamocortical mechanisms.

Cortical independence of type I and II barbiturate spindles

Two varieties of barbiturate-induced EEG spindles are recognized. Type I originates subcortically but recent data suggest that type II spindles are triggered by type I at the cortical level. This is based on a temporal relationship between the two spindle types in the gross electrocorticogram. In the present study no such relationship could be found except possibly in light anesthesia. These results fail to confirm the only data supporting a cortical mechanism underlying type II spindles.

The role of the corpus callosum in bilateral interhemispheric synchrony of spike and wave discharge in feline generalized penicillin epilepsy

In this study on interhemispheric synchrony of spike and wave discharges in feline generalized penicillin epilepsy, four groups of cats were treated in the following manner: Group A underwent complete section of the corpus callosum and anterior commissure; group B underwent division of the massa intermedia alone; group C underwent partial section of the corpus callosum; and in group D, a slab of the cortex on one side, comprising the middle parts of the lateral and suprasylvian gyri, was severed from all its subcortical inputs, without disturbing its connections with the opposite hemisphere through the corpus callosum. Two weeks after surgery or later the cats received an i.m. injection of penicillin. Bilateral synchrony of the epileptic discharges was abolished in group A, but not in group B. In group C, bilateral synchrony of the epileptic bursts was impaired, but not abolished. In group D, epileptic bursts synchronous with those occurring in the intact hemisphere continued to occur in the slab, but at lower amplitude. It is concluded that the corpus callosum is the main, if not the exclusive, pathway ensuring bilateral synchrony of the epileptic discharges of feline generalized penicillin epilepsy.

A multivariable evoked potential measure applied to human epilepsy

Based upon a three-dimensional plot (response plane) representing a condensation of human sensory evoked potential data obtained from a paired stimulus protocol, a study was undertaken to investigate electrophysiological changes following sodium valproate administration in subjects with photosensitive epilepsy. Initial results in two patients and two normal subjects suggest that the measure is reproducible and sensitive to electrophysiological changes accompanying anticonvulsant administration.

Acute FeCl3-induced epileptogenic foci in cats: electrophysiological analyses

Fifteen cats were rendered acutely epileptic via transpial iontophoresis of ferric ions. Extracellular electrical activity of individual cortical neurons was temporally correlated with ECoG epileptic spikes. Each unit's activity was characterized as excited, inhibited, or unaffected. The spatial coordinates of each neuron were logged, thereby generating a 3-dimensional activity map. Discharge patterns were further classified as bursting or non-bursting. A significantly increased population of inhibited cells was detected at radial distances greater than 3 mm from the iontophoretic site. This increase was at the expense of "unaffected-bursting" cells. Pathological discharge patterns, including units which fired in long-first-interval bursts, were common in the focus. A system was designed to electronically calculate single-unit firing probability before, during and after surface epileptiform spikes. An 8-12 Hz oscillation in firing probability level, time-locked to the epileptic spike, was observed in most excited-bursting units. Many of these findings in the ferric-induced epileptic focus accurately model the electrophysiological characteristics previously reported for human epileptic foci examined at neurosurgery.

Distinction between normal and epileptic rhythms in rodent sensorimotor cortex

Characteristics of spontaneous spindling in electrocorticograms from awake rodents are described. Comparisons are made between this spontaneous activity and epileptic activity generated in the sensorimotor cortex.