High-frequency network activity, global increase in neuronal activity, and synchrony expansion precede epileptic seizures in vitro.

How seizures start is a major question in epilepsy research. Preictal EEG changes occur in both human patients and animal models, but their underlying mechanisms and relationship with seizure initiation remain unknown. Here we demonstrate the existence, in the hippocampal CA1 region, of a preictal state characterized by the progressive and global increase in neuronal activity associated with a widespread buildup of low-amplitude high-frequency activity (HFA) (>100 Hz) and reduction in system complexity. HFA is generated by the firing of neurons, mainly pyramidal cells, at much lower frequencies. Individual cycles of HFA are generated by the near-synchronous (within approximately 5 ms) firing of small numbers of pyramidal cells. The presence of HFA in the low-calcium model implicates nonsynaptic synchronization; the presence of very similar HFA in the high-potassium model shows that it does not depend on an absence of synaptic transmission. Immediately before seizure onset, CA1 is in a state of high sensitivity in which weak depolarizing or synchronizing perturbations can trigger seizures. Transition to seizure is characterized by a rapid expansion and fusion of the neuronal populations responsible for HFA, associated with a progressive slowing of HFA, leading to a single, massive, hypersynchronous cluster generating the high-amplitude low-frequency activity of the seizure.

Synchronization measurement of multiple neuronal populations.

The purpose of the present paper is to develop a method, based on equal-time correlation, correlation matrix analysis and surrogate resampling, that is able to quantify and describe properties of synchronization of population neuronal activity recorded simultaneously from multiple sites. Initially, Lorenz-type oscillators were used to model multiple time series with different patterns of synchronization. Eigenvalue and eigenvector decomposition was then applied to identify "clusters" of locally synchronized activity and to calculate a "global synchronization index." This method was then applied to multichannel data recorded from an in vitro model of epileptic seizures. The results demonstrate that this novel method can be successfully used to analyze synchronization between multiple neuronal population series.

The effect of neuronal population size on the development of epileptiform discharges in the low calcium model of epilepsy.

The CA1 region of the rat hippocampal slice generates spontaneous electrographic seizures (field bursts) when exposed to ACSF containing < or = 0.2 mM calcium. It has been proposed that, particularly during the early part of a field burst, synchronised activity in small independent aggregates of neurons results in low amplitude irregular population spikes and subsequent fusion of aggregates generates high amplitude, regular discharging spikes. In the present experiments, we have tested the hypothesis that progression from aggregate formation to aggregate fusion requires a critical mass of participating neurons. We found that isolated CA1 segments >2 mm are still able to generate high amplitude, regular discharging population spikes, but when segment length is reduced to 1-2 mm, only 29% generate spikes with these characteristics; in the remainder, the field burst shows a DC shift+/-low amplitude irregular population spikes. No field bursts were seen in segments < 0.7 mm or in 50% of those 0.7-1 mm in length (in the remaining 50%, only the DC component of the field burst was present). Exposing 1-2 mm segments to hypo-osmolar perfusate induced a return of high amplitude rhythmic discharging population spikes in the field burst. We interpret these observations by indicating that progression from aggregate formation to aggregate fusion requires a critical neuronal mass and can be enhanced by reducing osmolarity of the perfusate.

Tissue resistance changes and the profile of synchronized neuronal activity during ictal events in the low-calcium model of epilepsy.

Population spikes vary in size during prolonged epileptic ("ictal") discharges, indicating variations in neuronal synchronization. Here we investigate the role of changes in tissue electrical resistivity in this process. We used the rat hippocampal slice, low-Ca(2+) model of epilepsy and measured changes in pyramidal layer extracellular resistance during the course of electrographic seizures. During each burst, population spike frequency decreased, whereas amplitude and spatial synchronization increased; after the main discharge, there could be brief secondary discharges that, in contrast with those in the primary discharge, started with high-amplitude population spikes. Mean resistivity increased from 1,231 Omega.cm immediately before the burst to a maximum of 1,507 Omega.cm during the burst. There was no significant increase during the first 0.5-1 s of the field burst, but resistance then increased (tau approximately 5 s), reaching its peak at the end of the burst, and then decayed slowly (tau approximately 10 s). In further experiments, we modulated the efficacy of electrical field effects by changing perfusate osmolarity. Reducing osmolarity by 40-70 mOsm increased preburst resistivity by 19%; it reduced minimum population spike frequency (x0.6-0.7) and increased both maximum population spike amplitude (x1.5-2.3) and spatial synchronization (x1.4-2.5, cross-correlation over 0.5 mm) during bursts. Increasing osmolarity by 20-40 mOsm had the opposite effects. These results suggest that, during each field burst, field effects between neurons gradually become more effective as cells swell, thereby modulating burst dynamics and facilitating the rapid synchronization of secondary discharges.

Effects of uniform extracellular DC electric fields on excitability in rat hippocampal slices in vitro.

The effects of uniform steady state (DC) extracellular electric fields on neuronal excitability were characterized in rat hippocampal slices using field, intracellular and voltage-sensitive dye recordings. Small electric fields (1 s) changes in neuronal excitability. Electric fields perpendicular to the apical-dendritic axis did not induce somatic polarization, but did modulate orthodromic responses, indicating an effect on afferents. These results demonstrate that DC fields can modulate neuronal excitability in a time-dependent manner, with no clear threshold, as a result of interactions between neuronal compartments, the non-linear properties of the cell membrane, and effects on afferents.

Depolarization block of neurons during maintenance of electrographic seizures.

Epileptic seizures are associated with neuronal hyperactivity. Here, however, we investigated whether continuous neuronal firing is necessary to maintain electrographic seizures. We studied a class of "low-Ca2+" ictal epileptiform bursts, induced in rat hippocampal slices, that are characterized by prolonged (2-15 s) interruptions in population spike generation. We found that, during these interruptions, neuronal firing was suppressed rather than desynchronized. Intracellular current injection, application of extracellular uniform electric fields, and antidromic stimulation showed that the source of action potential disruption was depolarization block. The duration of the extracellular potassium transients associated with each ictal burst was not affected by disruptions in neuronal firing. Application of phenytoin or veratridine indicated a critical role for the persistent sodium current in maintaining depolarization block. Our results show that continuous neuronal firing is not necessary for the maintenance of experimental electrographic seizures.

Neuronal aggregate formation underlies spatiotemporal dynamics of nonsynaptic seizure initiation.

High-frequency activity often precedes seizure onset. We found that electrographic seizures, induced in vitro using the low-Ca(2+) model, start with high-frequency (>150 Hz) activity that then decreases in frequency while increasing in amplitude. Multichannel and unit recordings showed that the mechanism of this transition was the progressive formation of larger neuronal aggregates. Thus the apparent high-frequency activity, at seizure onset, can reflect the simultaneous recording of several slower firing aggregates. Aggregate formation rate can be accelerated by reducing osmolarity. Because synaptic transmission is blocked when extracellular Ca(2+) is reduced, nonsynaptic mechanisms (gap junctions, field effects) must be sufficient for aggregate formation and recruitment.