Penicillin Epileptogenic Foci Research Articles

Penicillin epileptogenic focus in the rat: Requisites for transcortical reflex triggering

Epileptiform discharges elicited by natural or electrical stimulations, proprioceptive or cutaneous, were studied in the rat with an experimental acute focus induced by penicillin application in the motor area. EEG paroxystic spikes were easily triggered with restricted foci (0.5 to 1 mm 2) located in the representation area of the stimulated region. However, despite the large overlap of sensory and motor cortical limb areas in the rat, EEG spikes, either spontaneous or triggered, were followed by muscular jerks only with much larger foci: at least 2 and 4 mm 2, respectively, for anterior and posterior limb areas. Cutaneous stimulations were the most efficient in discharge production; however, discharges were triggered indifferently by muscular or cutaneous afferent fibers in about three-fourths of the cases. The temporal relation between EEG spike and myoclonic jerk were very close. A latency analysis (delay between triggered EEG spike and EMG response, parallel latency fluctuation of both phenomena, delay between spontaneous EEG spike and jerk) supported the hypothesis that a transcortical reflex mechanism, rather than a spinal excitability rebound, was involved in the jerk genesis. Iontophoretic ejection of penicillin within layers III–IV resulted in the development of electroclinical paroxysms. However, similar penicillin ejection within layer V, did not allow efferent discharge production. It is concluded that the involvement of a large surface or volume of cortical tissue is required to produce efferent discharges following EEG paroxysms. This observation is likely related to the unexpectedly wide representation of individual muscles at the motor cortical level.

Cellular mechanisms underlying the inhibitory surround of penicillin epileptogenic foci

Penicillin applied locally to the surface of in vivo hippocampus or neocortex is known to produce not only periodic interictal spikes within a focus, but also altered cellular activities around the focus. In the zone adjacent to a hippocampal focus, for example, Dichter and Spencer recorded hyperpolarization-depolarizing burst-hyperpolarization sequences, while further from the focus they recorded only hyperpolarizations (presumably IPSPs). We use here our previously developed model of interictal spike generation to show that these observations can be explained by the following two assumptions: (1) there is a gradient of effectiveness of synaptic inhibition (small within the focus and increasing with distance from the focus) caused by the gradient of penicillin concentration; and (2) the radius for recurrent inhibition is larger than the radius for recurrent excitation. Our model best fits the experimental data if we assume further that recurrent excitation expends from any one small cortical region to only some — not all — of its neighboring regions.

Interictal-ictal transition in the feline penicillin epileptogenic focus

A study was undertaken to examine those EEG features which characterize interictal—ictal (II-I) transitions in the feline penicillin focus. The study describes certain complex changes in spike frequency and their statistical analysis, which appear to have predictive value in signaling II-I transitions. The significance of these data with respect to the conflicting results of other studies and their possible application to clinical epilepsy are discussed.

The effects of cerebellectomy on a penicillin epileptogenic focus in the cerebral cortex of the rat

1. (1) Total removal of the cerebellum from the rat anaesthetised with urethane was found to increase the duration of epileptiform foci produced by injection of 25 U of Penicillin into the cerebral cortex. 2. (2) This was not due to reduction in blood pressure nor to non-specific brain damage resulting from the cerebellectomy. 3. (3) The effects of total cerebellectomy are not due to the removal of the inhibitory influence of the Purkinje cells on the intracerebellar nuclei, which form part of an excitatory cortico-cortical circuit. 4. (4) The underlying mechanism is obscure, but it is possible that the cerebellum produces some inhibitory tone, which decreases the lifetime of penicillin foci.

Stereotyped and structured burst activity of single units in the penicillin epileptogenic focus in cats

Other authors have previously reported that stereotyped and structured (long-first-interval) bursts do not occur in the feline penicillin epileptogeneic focus. The purpose of this report is to detail our own contrary experience which indicates that long-first-interval bursts may be recorded in such foci. We estimate that only about 4% of the units in these foci fire in bursts with long first intervals. Possible reasons for the discrepancies between this study and other investigations are methodology, locus and intensity of foci, and sample size. The significance of such bursts for the application of data from animal models to human epilepsy is discussed.

Electroencephalographic and biochemical study of the antiepileptic action of taurine administered by cortical superfusion

After cortical superfusion with taurine, an electroencephalographic and biochemical study was performed in cats affected with unilateral and bilateral penicillin epileptogenic foci. Taurine-treated cats showed a depressed epileptic susceptibility of the cortex, which was more remarkable in the less severe epileptic picture produced by unilateral penicillin. Both in control and in epileptogenic cortex, at the end of taurine superfusion, the amino acid content in brain was altered only for taurine and glutamic acid, which were increased and decreased, respectively. The subsequent changes in the brain concentration of these two compounds were the opposite, i.e., taurine decreased and glutamate increased, and a significant negative correlation between the cortical concentrations of the two amino acids was calculated. The rapidity of the changes was greater in epileptogenic than in control cortex, and maximal in the severe epilepsy produced by bilateral penicillin. Furthermore, the rapidity of the changes decreased during and after the disappearance of spikes. The hypothesis is put forward that the changes in brain content of taurine and glutamate observed after taurine cortical loading share the responsibility for the depressed epileptic susceptibility of the cortex. The common enzyme involved in the metabolism of taurine and glutamate could be responsible for the negative correlation between the two compounds.

Glial cells and extracellular potassium: Their relationship in mammalian cortex

Simultaneous recordings were made of glial cell potentials and the extracellular potassium concentration ([K+]O) in cat cortex in an attempt to provide more quantitative information about the sensitivity of mammalian neuroglia to changes in [K+]O. A penicillin epileptogenic focus served to generate both transient and sustained elevations in [K+]O, thus allowing measurement of glial membrane potential (Vm) at both resting and increased [K+]O levels many times during the same experiment. Resting Vm averaged--92.6 +/- 10.9 mV for 33 glial cells. With each surface interictal spike, glial cells exhibited slow depolarizations averaging 18.4 +/- 6.5 mV which mirrored rises in [K+]O in many respects. Several discrepancies were found, however, between transient and focal rises in [K+]O and the associated glial cell depolarizations which made it difficult to determine accurately the effect of changes in [K+]O on glial Vm. For example, the amplitude of the glial depolarization caused by a single interictal discharge showed no constant relationship to depth below the cortical surface in contrast to the consistent laminar profile recorded by the K+ electrode. Thus, large glial membrane depolarizations could be recorded at times when there was little or no increase in measured [K+]O. Agreement between changes in [K+]O and glial cell depolarizations was closer to that predicted by the Nernst equation during sustained elevations in [K+]O such as occurred during ictal episodes ('seizures'). These findings may be related in part to methodology as a consequence of the different spatial relationships which exist between glial membrane, K+-electrode tip and released K+. In addition, though, they may indicate the presence of a functional glial syncytium.

Effects of Temperature on Interictal Discharge at Penicillin Epileptogenic Foci

The effects of local brain temperature on acute focal penicillin epilepsy in the exposed hippocampus of cat were studied. Results from anesthetized and from immobilized, unanesthetized animals were compared. Over the temperature range 26 to 43 degrees C (at the alveus), the interictal spike interval and duration of the spike discharge varied inversely with temperature. The former showed a Q10 of 2.4 and the latter a Q10 of 1.5, with no difference due to type of preparation. A significant trans-hippocampal thermal gradient may imply that these values are underestimates by 20% or more. The low Q10 of duration of the paroxysmal discharge was consistent with the known temperature dependence of impulse conduction velocity of intracortical neural networks. The high Q10 of the interictal interval, on the other hand, was consistent with the view that some slow endogenous, perhaps metabolic factor such as a NA,K-ATPase modulated excitability at the focus of penicillin spikes.

Extracellular potassium activity during epileptogenesis

Direct measurements of extracellular potassium concentration ([K +] 0) changes in penicillin epileptogenic foci of cat cortex were made using potassium-sensitive microelectrodes. Interictal EEG events were accompanied by increases in [K +] 0 lasting several seconds. The amplitudes and rise rates of these increases varied with cortical depth, distance from the center of the focus, and the [K +] 0 at which they occurred. During ictal events, [K +] 0 consistently reached 9–10 m m. The patterns of ictal [K +] 0 changes also showed variations with depth in the cortex and distance from the focus. There was an upper limit for [K +] 0 during both interictal and ictal epileptiform activity at about 10 m m. The role of these [K +] 0 changes in modulating neuronal excitability in the epileptogenic focus is discussed. The [K +] 0 level did not appear to be a critical factor in initiation or termination of ictal episodes.

Penicillin epilepsy in isolated islands of hippocampus

Incisions which isolated islands of hippocampal formation were made in 32 cats and penicillin epileptogenic foci were established in the isolated areas. Surface field potentials and extracellular unit and intracellular recordings demonstrated typical activity of penicillin foci. Analysis of the lesions established that neither the long extrahippocampal projection pathways, nor the intrahippocampal loop of dentate gyrus-CA3-4-CA1-2 of Ammon's horn-subiculum-entorhinal area, nor granule cells are necessary for the development of inter-ictal discharges or seizures. Thus, very local circuits in each region of the hippocampus can support epileptogenesis.

Burst activity of single units in the penicillin epileptogenic focus

Burst firing patterns of single units in the penicillin epileptogenic focus were studied in cat cerebral cortex. Based on an analysis of their inter-spike interval histograms, it is suggested that within a burst the spike intervals tend to be distributed logarithmic-normally. A possible model, based on logarithmic-normal theory, is proposed for ‘spontaneous’ unitary activity in epileptic and non-epileptic cortex. These data appear to be consistent with, and to support, the concept of the “epileptic-aggregate” as a basic mechanism underlying epileptogenesis.

Electrophysiology of “epileptic” neurons: Spike generation

1. 1. Intracellular recordings were obtained from 74 neurons in penicillin-induced epileptogenic foci on cat pericruciate cortex in order to study the mechanisms of spike generation in “epileptic” neurons. Spontaneous and evoked spike discharge and spiking associated with the large depolarization shifts (DSs) which occur in cells during inter-ictal epileptiform surface events were examined. 2. 2. Orthodromic and antidromic spike generation (14 PT cells) was generally normal in that the usual IS-SD spike components were demonstrated and spikes arose from stable firing levels. Direct depolarizing current pulses produced high frequency spiking without DS generation. 3. 3. Antidromic spikes elicited during a DS did not alter the DS amplitude or time course. Antidromic invasion failed during the peak of the DS in some cells, due to cathodal blockade and also during periods of high frequency DS spike generation, presumably because of refractoriness or collision in the axon. 4. 4. Peculiarities of spike generation occurred as follows: (1) In 19 cells the first DS spike sometimes arose from a membrane potential 2–10 mV below the usual firing level without a preceding slow pre-potential. (b) In 4 of these cells orthodromic spikes not associated with the DS also occasionally arose from below threshold and appeared to have more than one mode of onset. (c) In 3 neurons, intracellular negative or negative-positive spikes occurred during surface epileptiform discharges. (d) Partial spikes or “spiklets” in addition to spikes of normal amplitude were noted in 18 cells of the focus and 4 of 28 control cells from normal cortex. 5. 5. These pecularities of spike generation suggest that in proportion of neurons of the penicillin epileptogenic focus, impulses originate from trigger zones distant from the soma, presumably from dendrites. These data raise the possibility that dendritic spike generation may be a general property of “epileptic” neurons.

The depolarization shift in “epileptic” neurons

Intracellular recordings were obtained from neurons of penicillin epileptogenic foci in locally and lightly anesthetized cats, and the depolarization shifts (DS) which occurred in cells during each surface interictal epileptiform event were analyzed. Cells which generated DS had otherwise normal patterns of synaptic and spike activity. The slow envelope of the DS was not crucially related to all-or-none spike electrogenesis. Alterations in intensity and frequency of orthodromic stimuli had different effects on DS and EPSP. As the stimulus intensity increased, DS tended to occur in an all-or-none manner at a fixed amplitude but with decreasing latency, whereas EPSP increased in amplitude. Following a DS, a period of refractoriness for DS generation of at least 150 msec occurred. Failure of orthodromic stimuli to “trigger” DS occurred at times when normal patterns of PSP and spike generation could be evoked. Direct depolarization through the intracellular electrode never elicited a DS. Depolarization did decrease DS amplitude in some cells. In some cells hyperpolarizing current pulses appeared to have less effect upon evoked DS than upon short-latency EPSP, suggesting that the DS originated on membrane remote from the soma; however, in most neurons concurrent EPSP and IPSP made it difficult to assess the effects of current injection upon the DS. The data are most consistent with the interpretation that the DS is some form of “giant” synaptic potential. The extent to which membrane abnormalities contribute to this potential is not known. In some neurons the characteristics of the presynaptic drive appear to be important factors in DS genesis.

Inhibition in “epileptic” neurons

The hyperpolarizations which follow depolarization shifts (DS) in neurons of penicillin epileptogenic foci were investigated using techniques of intracellular recording and stimulation. The hyperpolarization following DS could be inverted by hyperpolarizing current pulses or by injection of chloride ions through the micropipette. These data support the conclusion that the hyperpolarizations represent inhibitory postsynaptic potentials (IPSP). The IPSP following DS were of higher amplitude and longer duration than other inhibitory potentials in the same cells. It is postulated that the high frequency spike discharges which occur in the majority of the cells of the focus during DS activate inhibitory circuits which in turn inhibit cells in the focus and surrounding cortex. The IPSP following DS were more effective in limiting the frequency of unit spike discharges than the frequency of orthodromically triggered DS. Full-sized DS could often be evoked during the IPSP following DS at times when suprathreshold depolarizing pulses failed to fire the cell because of membrane resistance changes. This evidence suggests that the DS is generated on dentrites remote from the sites of IPSP generation. Widespread IPSP generation in and around the focus could limit the frequency of DS by affecting presynaptic elements which provide the excitatory drive required for DS generation.