The lateral spread of ictal discharges in neocortical brain slices

Anticonvulsant drug effects on spontaneous thalamocortical rhythms in vitro: ethosuximide, trimethadione, and dimethadione

Spontaneous generalized epileptiform discharges were elicited in rodent thalamocortical slices by perfusion with a medium containing no added Mg2+. In multiple-channel extracellular field potential recordings in thalamus and cortex, several distinct types of discharges were recorded, with two principal variants bearing marked similarity to spike-wave and generalized tonic-clonic seizure discharges recorded in patients with generalized seizure disorders. These discharges were termed sTBCs and cTBCs, respectively, for simple and complex thalamocortical burst complexes. The sensitivity of these discharges to the generalized absence anticonvulsants ethosuximide, trimethadione and dimethadione (the active metabolite of trimethadione) was studied. sTBCs were reduced or blocked by ethosuximide and dimethadione, when these drugs were applied in clinically relevant concentrations. The order of effectiveness of these agents was dimethadione > or = ethosuximide >> trimethadione. This paralleled the relative efficacy of these drugs in blocking T current in thalamic neurons. cTBCs were unaffected or exacerbated by these drugs. Structural control drugs including succinimide, the behaviorally inactive ring base of ethosuximide, and alpha, alpha-dimethyl-beta-methylsuccinimide, a convulsant succinimide, were inactive or exacerbated either sTBCs or cTBCs, respectively. These spontaneous generalized thalamocortical discharges in rodent thalamocortical slices may represent a potentially valuable in vitro model of generalized seizure discharges, with marked pharmacological and physiological similarities to various forms of clinical epileptic seizure activity.

An in vitro model of persistent epileptiform activity in neocortex

An in vitro model of persistent epileptiform activity was developed to study the mechanisms involved in epileptogenesis. Extracellular recordings were obtained from rat neocortical slices exposed to magnesium-free solution for 2 h. During exposure to magnesium-free solution spontaneous epileptiform activity consisting of interictal bursting and ictal-like discharges were observed. Interestingly, this activity persisted for hours after the slices were returned to magnesium-containing control solution. The N-methyl-D-aspartate (NMDA) receptor antagonist CPP prevented the development of the epileptiform activity, while the non-NMDA receptor antagonist CNQX abolished the epileptiform discharge that persisted after slices were returned to control solution. These findings suggest there are two distinct phases in the development of epileptic activity in this model, namely, induction (mediated by NMDA receptor activity) and maintenance (supported largely by non-NMDA receptor activity). The similarities and possible parallels between the mechanisms underlying this epileptogenesis and other forms of use-dependent modification of synaptic excitation, such as long-term potentiation, are discussed. This in vitro model of neocortical epileptogenesis may provide insights into the events underlying the development of clinical partial epilepsy.

Epileptiform activity in the guinea-pig neocortical slice spreads preferentially along supragranular layers--recordings with voltage-sensitive dyes

The spread of epileptiform activity was monitored in guinea-pig neocortical slices by the use of a voltage-sensitive dye (RH795) and a fast optical recording technique. Epileptiform activity induced by bicuculline methiodide (10-20 microM) and single-pulse stimulation spread from the stimulation site in layer I or in the white matter across most of the slice. Different lesions were made in the slice in order to specify the neuronal connections used for spread in the horizontal direction. In the slice, intracortical connections are necessary for the spread of epileptiform activity, as shown by vertical cuts through all cortical layers but sparing the white matter. Horizontal connections were interrupted by cuts parallel to the axis of pyramidal neurons through either supragranular or infragranular layers. Vertical connections were interrupted by cuts perpendicular to the axis of pyramidal neurons separating supragranular and infragranular layers. Spread of epileptiform activity in the horizontal direction was not hindered by horizontal cuts. Vertical cuts through infragranular layers also did not hinder the spread of epileptiform activity. In contrast, vertical cuts through supragranular layers either abolished completely (nine slices) or delayed significantly (ten slices) the spread of epileptiform activity. The mean delay at the supragranular lesion was 44 ms in layer III and 30 ms in layer V; at the infragranular lesion the mean delay was 2 ms in layer III and 6 ms in layer V. Also, with horizontal cuts, in three out of five slices the velocity of spread was significantly lower in infragranular as compared to supragranular layers. It is concluded that both supra- and infragranular layers if isolated possess the ability to initiate and propagate epileptiform activity independently. However, in the intact slice the influence of the supragranular networks on initiation and propagation of epileptiform activity appears to dominate.

Pharmacology and electrophysiology of a synchronous GABA-mediated potential in the human neocortex

Spontaneous synchronous field potentials of negative polarity (duration = 200-700 ms, inter-event interval = 9.1 +/- 2.9 s; n = 27 slices) were recorded, during application of 4-aminopyridine (50 microM), from the superficial/middle layers of slices of human neocortex obtained in the course of neurosurgery for the relief of intractable seizures. The negative-going field potential corresponded to an intracellular long-lasting (duration = 200-1600 ms) depolarization that could be preceded by preceded by an excitatory postsynaptic potential-hyperpolarizing inhibitory postsynaptic potential sequence and followed by a long-lasting hyperpolarization. This synchronous activity continued to occur following blockade of excitatory synaptic transmission by excitatory amino acid receptor antagonists, but was greatly reduced and eventually disappeared during application of the GABAA receptor antagonist bicuculline methiodide. Simultaneous extracellular recordings from three sites in the slice located along an axis parallel to the pia showed that successive synchronous field potentials could originate from any of the three areas. They invaded the other two sites in c. 35.5% of the cases, while propagation to another site only or no propagation at all was observed, respectively, in 44.4% and 20% of instances. The velocity of lateral propagation of the synchronous field potential was 7.9 +/- 2.5 mm/s (range = 4.5-11.8 mm/s, n = 6). The modalities of origin and propagation remained the same after blockade of excitatory amino acid receptors. Under these conditions, however, there was a higher incidence of non-propagation and the velocity was significantly lower than in control (5.6 +/- 1.9 mm/s; range = 2.8-7.7 mm/s, n = 6). These data indicate that, in the human neocortex, 4-aminopyridine can reveal a synchronous field potential that correlates with an intracellular long-lasting depolarization and is mainly due to the activation of postsynaptic GABAA receptors. The action of excitatory amino acid receptors is not necessary for the generation and propagation of these GABA-mediated potentials. We propose that this potential represents a novel mechanism for synchronization and spread of neuronal activity, including seizure-like discharges in the human neocortex.

Lowering of extracellular pH suppresses low-Mg(2+)-induces seizures in combined entorhinal cortex-hippocampal slices

Lowering [Mg2+]o induces epileptiform bursting in hippocampus and entorhinal cortex (EC), presumably by activation of N-methyl-D-aspartate (NMDA) receptors. Since increasing [H+]o has been shown to reduce NMDA receptor activation, we hypothesized that this could contribute to anticonvulsant actions of acidic pH. To test this, we studied the effects of raising extracellular PCO2 (20.6%, pH = 6.7) or lowering extracellular pH (6.7 or 6.2) on low-Mg(2+)-induced epileptiform discharges. Lowering the pH to 6.7 by either means increased the interval between seizure-like events (SLEs), decreased the maximal amplitude of SLEs, and if the site of seizure generation was at a distance from the recording site, acidification slowed the rate of seizure propagation. In contrast, the duration of SLEs was unaffected by acidic pH or high PCO2. Raising PCO2 or lowering pH to 6.7 also blocked early (8-10 min) but not late (> 20 min) phases of status-like discharges. All effects of the extracellular pH changes were fully reversible. Further lowering of extracellular pH to 6.2 completely and reversibly blocked both SLEs and status-like discharges. Our data show that the effects of high PCO2 and low pH on seizures in the EC in vitro may be dose-dependent and consistent with induction by proton blockade of NMDA receptors. Thus, blockade of NMDA currents by protons may be an important component of the anticonvulsant action of extracellular acidosis. The results also suggest that acidosis may be a desirable property for new antiepileptic treatments.

Safety of rapid-rate transcranial magnetic stimulation in normal volunteers

In 9 normal volunteers, we studied the safety of rapid-rate transcranial magnetic stimulation (rTMS) applied to different scalp positions at various frequencies and intensities. Pure tone threshold audiometry showed temporary threshold shifts in 3 subjects. In the subject stimulated at the highest intensity, rTMS induced a focal, secondarily generalized seizure despite the absence of definite risk factors for seizures. Rapid-rate TMS did not result in any important changes in the neurological examination findings, cognitive performance, electroencephalogram, electrocardiogram, and hormone levels (prolactin, adrenocorticotropic hormone, thyroid-stimulating hormone, luteinizing hormone, and follicle-stimulating hormone). In 10 additional subjects, the electromyographic activity in several contralateral muscles showed that trains of rTMS applied to the motor cortex induced a spread of cortical excitability. The spread of excitability depended on the intensity and frequency of the stimuli and probably constituted an early epileptogenic effect of rTMS. Guidelines for preventing the undesirable side effects of rTMS are offered.

Seizure circuit analysis with voltage sensitive dye

Voltage sensitive dye was used to produce a map of average membrane polarization for the purpose of analysing the circuitry involved in seizures. Dorsal hippocampus, ventral hippocampus, entorhinal cortex, substantia nigra and occipital cortex were selected to calculate the relative changes in polarization. Rats were induced with bicuculline to have convulsive seizures and mild limbic seizures with kainic acid. A 20 second sample of these seizures were recorded using the voltage sensitive dye. Control animals showed a relatively uniform polarization state in the five brain areas. The bicuculline seizure produced hyperpolarization in all five areas. The magnitude of the hyperpolarization varied among the regions to produce a distinctive pattern. The kainic acid seizure produced depolarization in the four limbic areas. The magnitude of the depolarization also varied, producing a different pattern compared with bicuculline or control. Future applications of this technique in animal models could help identify those areas in the brain which regulate seizure propagation, and the anatomical loci in which antiepileptic drugs interfere with this propagation. Ultimately, human applications would include linking voltage sensitive dyes with paramagnetic or positron emitting traces so that epileptic processes could be visualized using magnetic resonance imaging or positron emission computed tomography.

Regional and time dependent variations of low Mg2+ induced epileptiform activity in rat temporal cortex slices

In order to study spatial interactions during low magnesium induced epileptiform activity, changes in extracellular potassium concentration ([K+]0) and associated slow field potentials (f.p.'s) were recorded in thin rat temporal cortex slices (400 microns) containing the neocortical temporal area 3 (Te3), the entorhinal cortex (EC) and the hippocampal formation with the dentate gyrus, area CA3 and CA1 and the subiculum (Sub). The epileptiform activity was characterized by short recurrent epileptiform discharges (40 to 80 ms, 20/min) in areas CA3 and CA1 and by interictal discharges and tonic and clonic seizure like events (SLE's) (13-88s) in the EC, Te3 and Sub. While interictal discharges occurred independent of each other in the different subfields, the three areas became synchronized during the course of a SLE. The EC, Te3 and Sub all could represent the "focus" for generation of the SLE's. This initiation site for SLE's sometimes changed from one area to another. The characteristics of the rises in [K+]0 and subsequent undershoots were comparable to previous observations in in vivo preparations. Interestingly, rises in [K+]0 could start before actual onset of seizure like activity in secondarily recruited areas. The epileptiform activity could change its characteristics to either a state of recurrent tonic discharge episodes or to a continuous clonic discharge state reminiscent of various forms of status epilepticus. We did not observe, in any of these states, active participation by area CA3 in the epileptiform activity of the EC in spite of clear projected activity to the dentate gyrus. Even after application of picrotoxin (20 microM), area CA3 did not actively participate in the SLE's generated in the entorhinal cortex. When baclofen (2 microM) was added to the picrotoxin containing medium, SLE's occurred both in the entorhinal cortex and in area CA3, suggesting that inhibition of inhibitory interneurons by baclofen could overcome the "filtering" of projected activity from the entorhinal cortex to the hippocampus.