Epileptic high-frequency network activity in a model of non-lesional temporal lobe epilepsy

Abstract

High-frequency cortical activity, particularly in the 250-600 Hz (fast ripple) band, has been implicated in playing a crucial role in epileptogenesis and seizure generation. Fast ripples are highly specific for the seizure initiation zone. However, evidence for the association of fast ripples with epileptic foci depends on animal models and human cases with substantial lesions in the form of hippocampal sclerosis, which suggests that neuronal loss may be required for fast ripples. In the present work, we tested whether cell loss is a necessary prerequisite for the generation of fast ripples, using a non-lesional model of temporal lobe epilepsy that lacks hippocampal sclerosis. The model is induced by unilateral intrahippocampal injection of tetanus toxin. Recordings from the hippocampi of freely-moving epileptic rats revealed high-frequency activity (>100 Hz), including fast ripples. High-frequency activity was present both during interictal discharges and seizure onset. Interictal fast ripples proved a significantly more reliable marker of the primary epileptogenic zone than the presence of either interictal discharges or ripples (100-250 Hz). These results suggest that fast ripple activity should be considered for its potential value in the pre-surgical workup of non-lesional temporal lobe epilepsy.

Figure 1

Interictal discharges in the intrahippocampal tetanus toxin model of epilepsy. (A) Unilateral discharges originating in ipsilateral CA3, spreading to ipsilateral CA1. (B) Bilateral synchronous discharge, originating in ipsilateral CA3. In the ipsilateral hippocampus HFA is superimposed on interictal discharges. (C) Contralateral independent CA3 interictal discharges. (D) Bilateral synchronous discharges originating in contralateral CA1 with propagation to ipsilateral hippocampus. HFA is an integral part of the ipsi- and contralateral CA1 discharges. All panels are of raw data (0.5 Hz to 1 kHz).

Figure 2

Different types of high-frequency activity in the tetanus toxin model of temporal lobe epilepsy. High-frequency activity is superimposed on interictal discharges, most often during their peaks, but in some cases earlier or later. HFA of different frequencies can be observed (A–F), in some cases during individual interictal discharges (D, E). The marked frequency values are the peak frequency of the power spectra. (F) Repeated epileptiform discharges with frequency ∼5 Hz: HFA is present throughout its course, but varies in amplitude and frequency. In each panel the top trace consists of raw data and the bottom is band-pass filtered 100–600 Hz.

Figure 3

Properties of HFA. (A) Averaged interictal discharge waveform superimposed on cross-correlation between HFA and interictal discharge shows that HFA can be observed throughout the entire course of interictal discharge with maximal probability of occurrence during the peak of interictal discharge. (B) Proportion of interictal discharges with superimposed HFA in ipsilateral (filled bar) and contralateral (open bar) hippocampi. (C) Epoch lasting 0.5 s including an interictal discharge, with expansion of band-pass filtered HFA below, recorded from ipsilateral CA3. (D) Corresponding power spectrum for the band passed 0.5 s interictal epoch shown in C (dashed line shows first moment at 359 Hz). (E) Comparison of first spectral moment in ipsilateral and contralateral hippocampi. (F) First spectral moments in subregions of ipsilateral and contralateral hippocampi. (G) Comparison of ratio of fast ripple (FR) to ripple (R) power in ipsi- and contralateral hippocampi (i.e. the summated HFA powers below and above 250 Hz for all interictal events in all epileptic rats). (H) Ratios of fast ripple to ripple power in subregions of ispilateral and contralateral hippocampi. (I) Intrahippocampal (filled symbols) and interhippocampal (open symbols) coherence. (J) Mean values of intra- and interhippocampal coherence for ripple band (filled and open bars, respectively). (K) Mean values of coherence for fast ripple band. (Error bars represent SEM, significance values are ***P < 0.001). DG = dentate gyrus; HIP = hippocampi.

Figure 4

Interspike interval and cycle width and waveform shape contribute to spectral profile of epileptic HFA. (A) Interictal discharge with superimposed HFA of varying frequencies. (B) Band-passed filtered data (100–1000 Hz) from A showing isolated HFA. (C) Instantaneous frequency plot derived from the interspike interval between minima of successive HFA cycles from B. (D) Instantaneous frequency derived from the width of individual spikes from B, measured between maxima either side of the spikes. (E) Waveform of fast HFA. Note the sinusoidal shape of the waveform. (F) Waveform of HFA at a lower frequency, which has a more pronounced triangular (spike-like) shape: individual spikes start and stop at local maxima that result in the shorter periods (and faster instantaneous frequency) in the later segment of panel D. (G) Wavelet spectrogram corresponding to data in panel A, with the corresponding power spectrum projected to the right to illustrate how the dynamic changes revealed in the spectrogram are represented in conventional power spectra. In G the full false colour scale spans –10 to –2 log(mV²).

Figure 5

Ripples and fast ripples detected by spike thresholding. (A) Typical ripple event from contralateral hippocampus. (B) Typical fast ripple event from ipsilateral hippocampus. (C) The incidence of ripples per 10 min did not differ between hippocampi. (D) The incidence of fast ripples per 10 min was significantly greater in the ipsilateral hippocampus [P < 0.05, Mann–Whitney test, (*)]. (E) in all cases the incidence of fast ripples was greater in the ipsilateral hippocampus (R), and in 5 of 10 animals was zero in the contralateral (L).

Figure 6

Ictal HFA. (A) Typical seizure activity observed in tetanus toxin model of epilepsy: example of a seizure with ‘hypersynchronous’ onset. (B) Seizure onset occurs suddenly and is characterized by bilateral synchronous slow discharges that were first recorded in ipsilateral CA3 and propagated to ipsilateral CA1 and to the contralateral hippocampus. The HFA superimposed on this discharge was initially 440 Hz and later decreased to 270 Hz. HFA was also present in contralateral CA1. (C) HFA observed during ictal discharges. (D) Comparison of first spectral moment of ictal onset HFA between ipsi- and contralateral hippocampus. (E) Regional values of the first spectral moments. (F) Fast ripple (FR)/ripple (R) power ratio at ictal onset in the two hippocampi. (G) Regional differences of the ratios of fast ripples/ripples. Significance values are *P < 0.05, **P < 0.01, ***P < 0.001. Only raw data are shown.

Figure 7

Histology from hippocampus exhibiting interictal discharges with HFA. (A) Interictal discharge recorded in the ipsilateral CA3 with superimposed HFA progressing from 333 to 187 Hz. (B) Interictal discharge from the ipsilateral dentate gyrus (DG) with superimposed HFA at 362 Hz. (C) Histology of ipsilateral hippocampus showing electrode track (Nissl stain). (D) Detail of dentate gyrus histology. (E) Detail of CA3 histology. (F) Detail of CA1 histology. In A and B, top trace is raw data and the bottom is band-pass filtered 100–600 Hz. Note absence of hippocampal sclerosis and substantial neuronal loss. s.g. = stratum granulosum; s.p. = stratum pyramidale; s.o. = stratum oriens; s.r. = stratum radiatum. See also Jefferys et al. (1992).