Jun Lian

Suppression of epileptiform activity by high frequency sinusoidal fields in rat hippocampal slices

1 Sinusoidal high frequency (20‐50 Hz) electric fields induced across rat hippocampal slices were found to suppress zero‐Ca2+, low‐Ca2+, picrotoxin, and high‐K+ epileptiform activity for the duration of the stimulus and for up to several minutes following the stimulus. 2 Suppression of spontaneous activity by high frequency stimulation was found to be frequency (< 500 Hz) but not orientation or waveform dependent. 3 Potassium‐sensitive microelectrodes showed that block of epileptiform activity was always coincident with a stimulus‐induced rise in extracellular potassium concentration during stimulation. Post‐stimulus inhibition was always associated with a decrease in extracellular potassium activity below baseline levels. 4 Intracellular recordings and optical imaging with voltage‐sensitive dyes showed that during suppression neurons were depolarized yet did not fire action potentials. 5 Direct injection of sinusoidal current into individual pyramidal cells did not result in a tonic depolarization. Injection of large direct current (DC) depolarized neurons and suppressed action potential generation. 6 These findings suggest that high frequency stimulation suppresses epileptiform activity by inducing potassium efflux and depolarization block.

Local suppression of epileptiform activity by electrical stimulation in rat hippocampus in vitro

High frequency electrical stimulation of deep brain structures (DBS) has been effective at controlling abnormal neuronal activity in Parkinsons patients and is now being applied for the treatment of pharmacologically intractable epilepsy. The mechanisms underlying the therapeutic effects of DBS are unknown. In particular, the effect of the electrical stimulation on neuronal firing remains poorly understood. Previous reports have showed that uniform electric fields with both AC (continuous sinusoidal) or DC waveforms could suppress epileptiform activity in vitro. In the present study, we tested the effects of monopolar electrode stimulation and low‐duty cycle AC stimulation protocols, which more closely approximate those used clinically, on three in vitro epilepsy models. Continuous sinusoidal stimulation, 50 % duty‐cycle sinusoidal stimulation, and low (1.68 %) duty‐cycle pulsed stimulation (120 μs, 140 Hz) could completely suppress spontaneous low‐Ca2+ epileptiform activity with average thresholds of 71.11 ± 26.16 μA, 93.33 ± 12.58 μA and 300 ± 100 μA, respectively. Continuous sinusoidal stimulation could also completely suppress picrotoxin‐ and high‐K+‐induced epileptiform activity with either uniform or localized fields. The suppression generated by the monopolar electrode was localized to a region surrounding the stimulation electrode. Potassium concentration and transmembrane potential recordings showed that AC stimulation was associated with an increase in extracellular potassium concentration and neuronal depolarization block; AC stimulation efficacy was not orientation‐selective. In contrast, DC stimulation blocked activity by membrane hyperpolarization and was orientation‐selective, but had a lower threshold for suppression.

Ionic mechanisms underlying spontaneous CA1 neuronal firing in Ca2+-free solution

Hippocampal CA1 neurons exposed to zero-[Ca(2+)] solutions can generate periodic spontaneous synchronized activity in the absence of synaptic function. Experiments using hippocampal slices showed that, after exposure to zero-[Ca(2+)](0) solution, CA1 pyramidal cells depolarized 5-10 mV and started firing spontaneous action potentials. Spontaneous single neuron activity appeared in singlets or was grouped into bursts of two or three action potentials. A 16-compartment, 23-variable cable model of a CA1 pyramidal neuron was developed to study mechanisms of spontaneous neuronal bursting in a calcium-free extracellular solution. In the model, five active currents (a fast sodium current, a persistent sodium current, an A-type transient potassium current, a delayed rectifier potassium current, and a muscarinic potassium current) are included in the somatic compartment. The model simulates the spontaneous bursting behavior of neurons in calcium-free solutions. The mechanisms underlying several aspects of bursting are studied, including the generation of triplet bursts, spike duration, burst termination, after-depolarization behavior, and the prolonged inactive period between bursts. We show that the small persistent sodium current can play a key role in spontaneous CA1 activity in zero-calcium solutions. In particular, it is necessary for the generation of an after-depolarizing potential and prolongs both individual bursts and the interburst interval.

Propagation of non‐synaptic epileptiform activity across a lesion in rat hippocampal slices

1 Spontaneous non‐synaptic epileptiform activity was induced by bathing rat hippocampal slices in low‐Ca2+ solution. Extracellular recordings from electrodes placed on both sides of a complete cut showed that non‐synaptic activity was synchronized across the lesion. 2 Ion‐selective electrode recordings showed that each event was accompanied by a transient increase in extracellular potassium that diffused across the lesion. The synchrony was destroyed when a thin film was inserted into the lesion site. 3 Local pressure ejection of KCl evoked an event that subsequently propagated across the lesion. 4 After a complete lesion was made, afterdischarges evoked on one half of a slice were not detected on the other half. 5 Voltage‐sensitive dye imaging methods showed that epileptic activity propagated across the mechanical lesion without significant attenuation or additional delays. The velocity of the activity was consistent with that of the slow diffusion of a potassium wave. 6 Since field effects were significantly attenuated across the lesion and all gap junctions and cell processes across the lesion would be cut, these data show that extracellular diffusion, most probably potassium, is sufficient to synchronize populations of neurons and propagate slow frequency epileptiform activity.

Control of phase synchronization of neuronal activity in the rat hippocampus

Analysis of the synchronization mechanisms of neural activity is crucial to the understanding of the generation, propagation and control of epileptiform activity. Recently, phase synchronization (PS) analysis was applied to quantify the partial synchrony that exists in complex chaotic or noisy systems. In a previous study, we have shown that neural activity between two remotely located sites can be synchronized through a complete cut of the tissue by endogenous non-synaptic signals. Therefore, it should be possible to apply signals to control PS. In this study, we test the hypothesis that stimulation amplitudes below excitation level (sub-threshold) can be used to control phase synchronization of two neural signals and we investigate the underlying mechanisms. PS of neuronal activity is first analysed in two coupled Rossler neuron models. Both synchronization and desynchronization could be generated with sub-threshold sinusoidal stimulation. Phase synchronization was then studied in in vitro brain slices. Neuronal activity between two sites was modulated by the application of small sinusoidal electric fields. PS between two remote sites could be achieved by the application of two identical waveforms while phase desynchronization of two close sites was generated by the application of a stimulus at a single site. These results show that sub-threshold stimuli are able to phase synchronize or desynchronize two networks and suggest that small signals could play an important role in normal neural activity and epilepsy.

Nonlinear dynamic properties of low calcium-induced epileptiform activity

The analysis of the dynamic properties of epileptiform activity in vitro has led to a better understanding of the time course of neural synchronization and seizure states. Nonlinear analysis is thus potentially useful for the prediction of seizure onset. We have used nonlinear analysis methods to investigate the development of activity in the low calcium model of epilepsy in brain slices. This model is particularly interesting since neurons synchronize in the absence of synaptic transmission. The dynamic properties calculated from extracellular recordings of activity were used to analyze the transition to synchronous firing and their relation to neuronal excitability. The global embedding dimension, local dimension and the Lyapunov exponent were calculated from time segments corresponding to the onset, transition and fully developed stages of activity. The analysis was repeated for recordings made in the presence of various levels of DC electric fields to modulate neuronal excitability. The global and local dimensions did not change once activity was first initiated, even in the presence of the electric field. The maximum Lyapunov exponents increased during the onset of activity but decreased when the applied hyperpolarizing electric field was large enough to partially suppress the activity. These findings establish a relationship between neuronal excitability and the maximum Lyapunov exponent, and suggest that the Lyapunov exponent may be used to distinguish between various states of the neural network and might be important in seizure prediction and control.

Nonlinear dynamic properties of low calcium induced epileptiform activity

The study of dynamic properties of epileptiform can help understand the mechanism of seizure initiation and synchronization. It would be a useful tool to predict the onset of seizure. In this project, we induced epileptiform activity in rat hippocampal brain slices by reducing [Ca/sup 2+/]/sub 0/. The low calcium model of epilepsy is particularly interesting since neurons can synchronize in the absence of synaptic transmission. DC electric fields were applied on the slice to modulate neuronal activity. Lyapunov exponent, global embedding dimension and the local dimension were calculated for two experiment conditions: (1) epileptiform activity formation course: from onset, transition state to completed developed state; (2) in the presence of various level DC electric fields. The global and local dimensions increased rapidly when low calcium induced epileptiform activity was initiated, but it did not change after that, even in the presence of the electric field. The Lyapunov exponent increased during the activity formation process and during application of small, hyperpolarizing electric fields, but decreased when the applied electric field is big enough to partially suppress the activity. These findings suggest that the maximum Lyapunov exponent can be a good method for distinguishing states of the systems and might be important in seizure prediction and control.