Arthur A. Ward

Human cortical neurons in epileptogenic foci: Comparison of inter-ictal firing patterns to those of “epileptic” neurons in animals

Abstract 1. 1. Extracellular recordings of the spontaneous firing patterns of cortical neurons were obtained in patients undergoing craniotomy for the surgical excision of an epileptogenic focus. 2. 2. Because many kinds of experimental “epileptic” foci in animals exhibit cells with high frequency (200–500/sec) bursts of action potentials, lasting 5–50 msec and recurring many times per second, we explored the electrocorticographically defined epileptogenic focus in humans in search of bursting firing patterns. 3. 3. In addition to normal firing patterns, many cells near the focus exhibited “epileptic” bursting firing patterns. Sometimes, normal and bursting cells could be recorded simultaneously, indicating that cells in proximity to one another are not uniform in such firing properties. 4. 4. Such high frequency bursts are not typically the result of artifact such as micro-electrode pressure upon a cell, or heartbeat or respiratory movements of the cortex. Most bursting cells were obtained under operating conditions involving local anesthesia, but similar results were seen under general anesthesia. 5. 5. Bursts were not necessarily synchronized with the EEG sharp waves, nor with bursts from other simultaneously recorded neurons. 6. 6. Attempts were made to modify the burst patterns by arousing a sleeping patient and by electrical stimulation of the adjacent cortical surface. While some modifications in the rate of recurrence of the bursts could be obtained, the timing patterns of the first few spikes within a burst did not change readily. 7. 7. High frequency tonic firing was also seen. Some such activity could be observed to undergo spontaneous changes from silence to high frequency tonic firing and then to bursts. 8. 8. Within a burst, the timing of spikes may be very repeatable (stereotyped bursts) in some cases. In a few cases, the structured bursts reported in chronic monkey foci have been observed, where there seems to be a characteristic pause in the firing after the first one or two spikes and then a resumption of high frequency firing in a manner identical to the stereotyped bursts. These structured timing patterns have been considered a clue towards the identification of primarily dysfunctional epileptic neurons (in contrast to normal cells recruited into bursting firing patterns by an abnormally large synaptic input). 9. 9. We would conclude that there is a good correspondence between the chronic alumina “epileptic” foci in animals and the human disease, insofar as the inter-ictal firing patterns of neurons near the focus is concerned.

Firing patterns of epileptic and normal neurons in the chronic alumina focus in undrugged monkeys during different behavioral states.

This communication summarizes data from a series of experiments on the activity of single units in chronic epileptogenic alumina foci in precentral cortex of undrugged monkeys. The foci contained a mixture of normal and epileptic cells, which differed consistently in their spontaneous firing patterns under various behavioral conditions, and in their responses to electrical stimulation: (1) during restful waking the spontaneous activity of normal precentral cells rarely exhibited intervals less than 10 msec, whereas the activity of epileptic cells included high frequency bursts with intervals less than 5 msec. The percentage of total activity in bursts was defined as the ‘burst index’; (2) responses evoked antidromically by pyramidal tract stimulation and orthodromically by stimulation of center median of thalamus consisted of single action potentials in normal cells and bursts in epileptic cells; the probability of evoking a burst in epileptic cells was proportional to the burst index; (3) bidirectional operant conditioning of firing rates was most readily successful in normal cells and appeared to be increasingly difficult in epileptic cells in proportion to their burst index and (4) during sleep, epileptic cells fired in longer and higher frequency bursts than normal cells. To the extent that both types of cells receive similar inputs, these observations suggest that many epileptic cells in the alumina focus are intrinsically hyperexcitable,viz. they respond abnormally to normal inputs rather than responding normally to abnormally intense inputs. These hyperexcitable neurons may drive other cells in the focus, but activity of both may be operantly controlled.

Structured timing patterns within bursts from epileptic neurons in undrugged monkey cortex

Abstract While normal cortical neurons often show ill-defined, labile bursts during spontaneous firing, stereotyped high-frequency bursts every 50–200 msec are characteristic of the firing patterns of neurons near chronically alumina-induced epileptic foci in sensorimotor cortex of awake, undrugged rhesus monkeys. Computer-assisted analysis of the timing patterns within bursts revealed an unusually long interval between the first and second spikes of these epileptic bursts, with the later spikes of the burst time-locked to the second spike, not the first spike. In some neurons, this stereotyped “remainder” of the burst would begin at a highly variable time following the first spike. In other neurons, this first interval was bimodal, with the remainder of the burst starting either 3 or 4 msec following the first spike. In a third class of epileptic neuron, the first interval was unusually long and remarkably lacking in variability (8.6 ± 0.2 msec). Other neurons, especially those located further away from the epileptogenic focus, showed less stereotyped bursts without the long first intervals. One explanation considered for these phenomena is that the neuron is being stimulated antidromically (first spike) and responds repetitively (remainder of the burst).

Extracellular potassium concentration changes during propagated seizures in neocortex.

Cortical surface and intracortical, extracellular K+ selective microelectrode recording was carried out in the pericruciate cortex of cats during propagated seizures produced by repetitive stimulation of the surface of the contralateral homotopic neocortex. The K+ selective microelectrode consistently recorded a potential change which corresponded to an increase in [K+]0 which was directly related to the seizure amplitude × duration. The interictal [K+]0 in neocortical extracellular fluid was determined to be 4 meq/liter. The maximum increase of [K+]0 during propagated seizures in these experiments was 7 meq/liter which correlates lates well with increases in [K+]0 calculated from neuroglial depolarizations during similar propagated seizures.

The epileptic neurone.

Since seizure activity can be induced in neuronal populations in a variety of ways, we have concentrated the efforts in our laboratories on seizure activity induced in the monkey by the intracortical injection of alumina because we have felt that this experimental preparation most closely approximated human epilepsy. Such epileptogenic foci were induced by a modification of the Kopeloff technique previously described (10). Under sterile conditions, commercial aluminum hydroxide gel was injected intracortically into the sensorimotor cortex. In such animals, spontaneous clinical seizures appeared in 30-60 days. After maturation of the epileptogenic focus, the activity of single cells in the epileptogenic focus was studied using relatively large micro-electrodes of 4-7 p . Rather extensive studies of the extracellular activity of epileptic neurones in the monkey have been carried out in this fashion over the past seven years in collaboration with others in our laboratory including, in particular, Dr. L. B. Thomas and Dr. Richard P. Schmidt. More recently intracellular recording has been undertaken by Dr. William Kelly. The spontaneous activity of single neurons in the epileptogenic focus varies considerably and a wide diversity of pathological patterns of discharge have been recorded. In Fig. 1 random bursts of moderately high frequency autonomous discharge are seen. Such spontaneous bursts of high frequency discharge occur without consistent relation to the slow wave activity. In addition to the factors involved in the genesis of the high frequency discharge, abnormalities of those cortical properties responsible for slow rhythmic activity also appear to be present in the epileptogenic cortex. In Fig. 2, representative samples are presented of the activity of a single neuron at various times during the hour or so that this neuron was under observation. It will be seen that each burst of four discharge have an almost constant pattern and that the long interval between bursts is extraordinarily constant. In addition to the random bursts of autonomous discharge, intermittent “spikes” are seen (Fig. 3A, 3B) during

Effects of lesions in ventral anterior thalamus on experimental focal epilepsy

Abstract The effect of stereotaxic lesions of the ventral anterior thalamus, or the adjacent inferior thalamic peduncle, on experimental models of focal cortical epilepsy was studied. Acute epileptic foci in cat sensorimotor cortex were made by injection of tungstic acid gel. Following ipsilateral lesions of ventral anterior thalamus or the adjacent inferior thalamic peduncle in these animals there was a highly significant reduction in electrographic seizure frequency and duration compared to prelesion control periods. Interictal activity at the focus was not altered. The frequency and duration of spontaneous clinical seizures in five rhesus monkeys with chronic alumina cream foci in motor strip was continuously monitored in activity chairs. Both seizure frequency and duration decreased in all animals in the 4-week period after ipsilateral ventral anterior thalamic lesions as compared to the 4-week control period. Sham lesions did not have these effects. The thalamic lesions did not discernably alter behavior or neurologic function in these primates. Thus ventral anterior thalamic lesions decrease seizure frequency and duration in both acute and chronic experimental models of focal cortical epilepsy. These findings indicate that pathways originating in or passing through the ventral anterior thalamus play a role in the generalization of focal cortical seizures. These lesions in ventral anterior thalamus may be useful in the treatment of medically intractable seizures secondary to foci inaccessible to direct excision.

Unidentified neuroglia potentials during propagated seizures in neocortex

Cortical surface and intracellular recording of silent cells (neuroglia) was carried out in the pericruciate cortex of cats during propagated seizures produced by repetitive stimulation of the surface of the opposite homotopic neocortex. The membrane characteristics of these cells were similar to neuroglial cells studied in leech, amphibian, and rat optic nerves, tissue culture, and mammalian cerebral cortex. By varying the parameters of transcallosal stimulation, it was possible to obtain either minor or major propagated seizures. All cells with resting membrane potentials (RMP) greater than 30 mv recorded during minor propagated seizures exhibited a depolarizing response (5–14 mv) during the seizure episode followed by a postictal hyperpolarizing response (1–9 mv) and a slow return to the original resting level. The peak amplitude of the depolarizing response was proportional to the cells RMP and the amplitude of the seizure waves in the EEG. During major propagated seizures, an augmentation of the depolarizing response to 16–30 mv and the hyperpolarizing response to 10–15 mv was noted. A membrane conductance change during these events was not observed. During major propagated seizures, an increase in [K+]o over the resting [K+]o was calculated to be 10 meq/liter. However, the level of [K+]o reached in the extracellular clefts was probably much higher than this calculated value for reasons which are discussed. A model for seizure propagation is presented. The postictal hyperpolarization most likely represents the effect of a K+-sensitive electrogenic pump in the glial membrane.

Changes in extracellular potassium activity during neocortical propagated seizures

Abstract Direct measurements of extracellular potassium concentration [(K + )o] using a potassium-sensitive microelectrode were carried out in the pericruciate cortex of cats during propagated seizures initiated by repetitive stimulation of the surface of the opposite homotopic neocortex. (K + )o increases during either subthreshold stimulus trains or ictal episodes were dependent on cortical depth and distance from the homotopic point. Seizures could be classified by typical increases in (K + )o: 0.5± 0.06m m /1 with minor and 5.9 ± 0.8 m m /1 above the steady state value with major seizures. The upper limit for (K + )o during ictal events was 10.2 m m /1. A postictal undershoot of (K + )o below steady state was observed. The elevation of (K + )o to a critical value during stimulation was associated with a “threshold” for the initiation of propagated seizures. That value was 0.5 m m /1 for minor seizures and 1.8 m m /1 for major seizures. Both minor and major seizures were found to act as “all or none” phenomena of variable morphology. The role of (K + )o in the modulation of CNS activity and the pathogenesis of ictal phenomena is discussed.

Spontaneous firing patterns of epileptic neurons in the monkey motor cortex

Abstract We recorded the activity of single cells in an alumina-induced epileptogenic focus in precentral cortex of an awake monkey. The firing patterns of 12 units exhibiting the “long first interval” burst pattern were studied in detail. When the monkey was quiescent, the spontaneous activity of these cells consisted of repeated bursts in which the initial spike was followed by a long first interval and then by a high-frequency afterburst. If the interval between the long first interval burst and the preceding activity was less than 100 msec, an inverse relationship between the duration of afterburst and the interburst interval was observed. When cell activity increased sufficiently (sometimes accompanied by overt motor activity) these cells fired at a tonic rate characteristic of normal precentral cells. In all cells, single pyramidal tract shocks elicited a short latency (0.6–1 msec) antidromic spike which could be followed after a long first interval by an afterburst. In most cells the afterburst spikes were slightly larger and longer than the initial spikes (and the spikes occurring during tonic activity). In two cells the afterburst spikes were clearly compounded of two parts, which varied independently as a function of electrode depth and probably represented activation of two regions of the cell. These observations suggest that the afterburst may be triggered by a pacemaker mechanism in a remote region of the same cell.

Single-unit analysis of propagated seizures in neocortex

Abstract Cortical-surface, extracellular, and intracellular recording was carried out in the pericruciate cortex anesthetized and unanesthetized cats during propagated seizures produced by surface repetitive stimulation of the opposite homotopic neocortex. Well developed trans-synaptically initiated seizures could be easily elicited in the unanesthetized animals. Almost all cells observed were found to be overtly involved in the seizure sequence and responded in a relatively uniform manner. Neurons were classified on the basis of their behavior as types I and II (active) and type III (passive). In active neurons, changes in the membrane polarization and spike discharges were observed which characterized the various phases of the seizure. The afterhyperpolarization following either spontaneous or antidromically evoked action potentials was lost and replaced by an after depolarization at the onset of the seizure. The tonic phase was characterized by excessive membrane depolarization and spike discharge with resultant inactivation of A and B spike generators. The clonic phase corresponded to slow membrane repolarization, recovery of the A followed by the B spike generators, and large depolarizing waves exhibiting afterdepolarizations. Hyperpolarization characterized the postictal electrical silence. In passive neurons, recurring depolarizing waves with high-frequency bursts of spikes occurred throughout the tonic and clonic phoses without inactivation of the spike generator followed by a slight postictal hyperpolarization. Light barbiturate anesthesia profoundly suppressed the trans-synaptic elicitation of seizures. Intense tetanizing current produced weak unit and surface seizure activity. The majority of cells simply ceased firing or were not overtly involved in the seizure sequence recorded from the cortical surface.