Quantitative evaluation of inhibitory effects of epileptic spikes on theta rhythms in the network of hippocampal CA3 and entorhinal cortex in patients with temporal lobe epilepsy / 生理学报

Epileptic spike is an indicator of hyper-excitability and hyper-synchrony in the neural networks. The inhibitory effects of spikes on theta rhythms (4-8 Hz) might be helpful to understand the mechanism of epileptic damage on the cognitive functions. To quantitatively evaluate the inhibitory effects of spikes on theta rhythms, intracerebral electroencephalogram (EEG) recordings with both sporadic spikes (SSs) and spike-free transient period between adjacent spikes were selected in 4 patients in the status of rapid eyes movement (REM) sleep with temporal lobe epilepsy (TLE) under the pre-surgical monitoring. The electrodes of hippocampal CA3 and entorhinal cortex (EC) were employed, since CA3 and EC built up one of key loops to investigate cognition and epilepsy. These SSs occurred only in CA3, only in EC, or in both CA3 and EC synchronously. Theta power was respectively estimated around SSs and during the spike-free transient period by Gabor wavelet transform and Hilbert transform. The intermittent extent was then estimated to represent for the loss of theta rhythms during the spike-free transient period. The following findings were obtained: (1) The prominent rhythms were in theta frequency band; (2) The spikes could transiently reduce theta power, and the inhibitory effect was severer around SSs in both CA3 and EC synchronously than that around either SSs only in EC or SSs only in CA3; (3) During the spike-free transient period, theta rhythms were interrupted with the intermittent theta rhythms left and theta power level continued dropping, implying the inhibitory effect was sustained. Additionally, the intermittent extent of theta rhythms was converged to the inhibitory extent around SSs; (4) The average theta power level during the spike-free transient period might not be in line with the inhibitory extent of theta rhythms around SSs. It was concluded that the SSs had negative effects on theta rhythms transiently and directly, the inhibitory effects aroused by SSs sustained during the spike-free transient period and were directly related to the intermittent extent. It was indicated that the loss of theta rhythms might qualify exactly the sustained inhibitory effects on theta rhythms aroused by spikes in EEG. The work provided an argumentation about the relationship between the transient negative impact of interictal spike and the loss of theta rhythms during spike-free activity for the first time, offered an intuitive methodology to estimate the inhibitory effect of spikes by EEG, and might be helpful to the analysis of EEG rhythms based on local field potentials (LFPs) in deep brain.

Inhibitory effects of epileptic spikes on theta rhythm in rat pilocarpine model of temporal lobe epilepsy / 生理学报

Epilepsy clinically has an inhibitory impact on cognitive function, but whether it is associated with epileptogenesis is unclear. Since the epileptic spike characterizes temporal lobe epilepsy (TLE), the present study was aimed to analyze the transient effects of sporadic spikes (SSs) on theta rhythm during epileptogenesis. The local field potentials (LFPs) were recorded in CA1 area in four rats with the pilocarpine injections during exploration, and theta phase stability and power were globally estimated around SSs, also during prolonged period without SS (both as experiments) as well as pre-injections (control). Finally, the LFPs were simulated by changing the average excitatory and inhibitory synaptic gain values (including slow and fast inhibition loops) with the help of simplified dynamical model of CA1 networks, and then theta phase stability was evaluated in several cases. It was found that the SSs could have negative impacts on theta rhythm both transiently and persistently, which may be dependent on the temporal courses leading to epilepsy, being acuter in early stage than later stage, but even in latent stage, theta power was strong. The simulations partly demonstrated that the synaptic imbalance concomitant with the occurrence of SSs might be related to the dynamics of theta phase stability. The results indicate that the SSs might have persistent negative impacts on the cognition rhythm, and the effects might alter during epileptogenesis, leading to the cognitive dysfunction.