Glutamatergic pre-ictal discharges emerge at the transition to seizure in human epilepsy

Journal name:
Nature Neuroscience
Year published:
Published online


The mechanisms involved in the transition to an epileptic seizure remain unclear. To examine them, we used tissue slices from human subjects with mesial temporal lobe epilepsies. Ictal-like discharges were induced in the subiculum by increasing excitability along with alkalinization or low Mg2+. During the transition, distinct pre-ictal discharges emerged concurrently with interictal events. Intracranial recordings from the mesial temporal cortex of subjects with epilepsy revealed that similar discharges before seizures were restricted to seizure onset sites. In vitro, pre-ictal events spread faster and had larger amplitudes than interictal discharges and had a distinct initiation site. These events depended on glutamatergic mechanisms and were preceded by pyramidal cell firing, whereas interneuron firing preceded interictal events that depended on both glutamatergic and depolarizing GABAergic transmission. Once established, recurrence of these pre-ictal discharges triggered seizures. Thus, the subiculum supports seizure generation, and the transition to seizure involves an emergent glutamatergic population activity.

At a glance


  1. Ictal discharges generated in the human subiculum.
    Figure 1: Ictal discharges generated in the human subiculum.

    Convulsants produced patterned ictal discharges restricted to the subiculum. (a) Multiple extracellular recordings of an ictal event in a slice containing the hippocampus, subiculum and entorhinal cortex. Electrode locations: 1, dentate gyrus; 2, CA2; 3, CA1; 4–5–6, subiculum; 7, presubiculum; 8, entorhinal cortex. (b) Ictal event structure. An ictal discharge recorded in a 65 mM HCO3 and 8 mM K+ solution. Top, pre-ictal discharges (larger events, filled circles) recurred before a fast low-voltage activity (gray line) at seizure onset followed by rythmic bursts (black line). Seizure onset is indicated by the dotted arrow. Middle, time frequency representation of the local field potential (LFP). Bottom, multi-unit activity (MUA) revealed recurring PIDs of short duration followed by the rapid unpatterned action potential discharges at seizure onset and then by oscillatory bursts.

  2. IIDs, PIDs and ictal discharges in intracranial recordings.
    Figure 2: IIDs, PIDs and ictal discharges in intracranial recordings.

    (a) Stereo EEG recording (electrode HipAg1) showing activity of the subiculum and head of the hippocampus at seizure onset. The seizure was preceded by recurring IIDs (blue circles) and PIDs (pink circles). It began (arrow) with fast low-voltage activity and continued with oscillatory rhythmic bursts. (b) Recordings from multiple electrode contacts on a referential montage. Traces from contacts with no epileptic activity are shown in green, those with isolated IIDs in blue, and those recording both IIDs and PIDs in pink. Left, IID sample (blue circle). Middle, PID sample (pink circle). Right, seizure onset (thick arrow). The thin arrow indicates an expanded trace from the HipAg1 contact in a. Seizure onset is highlighted in yellow. Electrodes are identified according to the recorded area (amyg, amygdala; Hip, hippocampus; TB, temporo-basal; T2, second temporal gyrus; OrFr, orbito-frontal), location (A, anterior; M, median; P, posterior; AP, antero-posterior) and hemisphere (g, left; d, right). The first contact is at the tip of the electrode and the last as it emerges from the cranium. (c) Amplitude distribution for all field potentials preceding a single ictal event by 30 min showing the distinct amplitudes of IIDs (25–125 μV) and PIDs (150–500 μV). (d) Three-dimensional reconstruction of electrode contacts from post-implantation magnetic resonance imaging showing sites where PIDs (pink), IIDs (blue) or no epileptic events (green) were recorded. An arrow highlighted in yellow indicates the site of seizure onset. The name of each electrode is shown at the site of emergence from the skull.

  3. PIDs emerge during the transition to ictal-like activity in vitro.
    Figure 3: PIDs emerge during the transition to ictal-like activity in vitro.

    (a) Extracellular recordings of the transition to seizure-like activity induced by increased external HCO3 (85 mM) and K+ (8 mM). E1 and E2 are recordings from two subicular electrodes. MUA frequency (upper trace) and the extracellular signal from E2 (lower trace) are shown. (b) Amplitude measurements for all field potentials recorded by electrode E2 during the transition show the emergence of larger PIDs, whereas the amplitude of inter-ictal events did not change. (c) Dual extracellular recordings showing IIDs (open circles, left) before convulsant application and coexpression of PIDs (filled circles, right) with IIDs during the transition. (d) Amplitude distribution for all field potentials during the 35-min transition period showed IIDs of amplitude 10–50 μV and PIDs of amplitude 125–175 μV. (e) Mean and s.d. of the amplitude (black), duration (red) and propagation speed (blue) of IIDs and PIDs at steady state. Amplitudes and propagation speed, but not durations, were significantly different (line with asterisk). (f) Propagation of IIDs and PIDs. Triple extracellular recording (E1, E2, E3; E1–E2 distance, 1 mm; E2–E3 distance, 0.7 mm; E1–E3 distance, 1.4 mm) during the transition to ictal-like events. Field potential (FP) amplitude is plotted from signals from each electrode and the propagation speed of fields during the transition (between E1 and E3) is shown above. Top traces show initial IIDs (left), emerging PIDs (middle) and fully developed PIDs (right). Gray lines link FP peaks.

  4. PIDs depend on glutamatergic signaling.
    Figure 4: PIDs depend on glutamatergic signaling.

    (a) Intracellular recording from a subicular pyramidal cell (I) with a local extracellular recording (E). This cell received hyperpolarizing synaptic inputs during IIDs (open circle) both before and after convulsants were added. In contrast, it was depolarized during PIDs (filled circle) in the presence of convulsants. Intracellular action potentials were cut. (b) IIDs and PIDs coexisted during the transition to ictal discharges induced by 0.25 mM Mg2+ and 8 mM K+ (left trace). Bicuculline (bic) blocked IIDs, but not PIDs (middle left). The NMDA receptor blocker D,L-AP5 (100 μM) did not change PIDs (middle right), but the AMPA receptor antagonist NBQX (10 μM) suppressed them (right). (c) Distinct reversal potentials for synaptic events associated with IIDs and PIDs. The amplitude of postsynaptic potentials associated with IIDs (open circles) and PIDs (filled circles) is plotted against membrane potential during the transition. The data shown are from three different cells (black, blue and red). Inset, postsynaptic potentials associated with IIDs and PIDs at different membrane potentials. Intracellular (upper) and extracellular (lower). The mean reversal potential of IIDs was −59.1 ± 3.8 mV and the estimated reversal potential of PIDs was −15.1 ± 5.9 mV. (d) PIDs were induced by increasing excitability with high K+. Field potentials revealed that PIDs (filled circle) and IIDs (open circle) coexisted in 10 mM K+.

  5. NMDA receptor signaling is involved in seizure generation and in the emergence of PIDs, but not their maintenance.
    Figure 5: NMDA receptor signaling is involved in seizure generation and in the emergence of PIDs, but not their maintenance.

    All extracellular recordings were made from the same site in the subiculum. (a) Left, spontaneous IIDs (open circles). D,L-AP5 did not affect IIDs (middle), but the convulsant solution (10 mM K+ / 0.25 mM Mg2+) did not induce PIDs or ictal-like events (right trace) when D,L-AP5 was present. (b) Left, after D,L-AP5 washout, the convulsant solution induced PIDs (filled circles) during the transition period (middle left) to ictal discharges (gray line, middle right). Application of D,L-AP5 after PIDs had emerged did not change them, but suppressed the initiation of ictal-like events (right).

  6. IIDs and PIDs are generated by distinct networks.
    Figure 6: IIDs and PIDs are generated by distinct networks.

    (a,b) Firing of putative pyramidal cells (a) and interneurons (b) recorded juxtacellularly during IIDs (blue) and PIDs (red). Cell type was determined from action potential duration (upper traces) measured from the positive to the negative peak (pyramidal cells > 0.7 ms, interneurons < 0.7 ms). Spike timing for pyramidal cells and interneurons (shown as dots in the box), spike probability (histograms) and spike frequency (lower traces) are shown with respect to IID and PID field potentials (upper trace). The dotted line with an arrow tail indicates the onset of field potentials and the dotted line with an arrow head indicates its peak. Most pyramidal cells were inhibited during IIDs and fired during PIDs (left), whereas a subset of pyramidal cells (7 of 39) fired both during IIDs and before PIDs onset (right) (a). All interneurons fired before IID onset and after PID initiation (left), but some interneurons (2 of 6) fired before both IID and PID onset (b).

  7. Dynamics of population activity during the transition to ictal events.
    Figure 7: Dynamics of population activity during the transition to ictal events.

    (a) Dual extracellular recordings made during the interictal period (white line), the pre-ictal period (black line) and the onset of an ictal-like event (green line). Interictal events (open circle) and pre-ictal events (filled circle) during the transition are shown below on an expanded timescale. Time frequency analysis of the extracellular signal is shown below. (b) Cross-correlation index versus time lag for IIDs (empty circles) and PIDs (filled circles). (c) Three-dimensional plot of the cross-correlation index, time lag between sites, and the amplitude of the field potential for IIDs, PIDs and initial ictal discharges. (d) Histograms of the amplitude, time lag and cross-correlation index for each type of event. Error bars indicate s.d. Lines with an asterisk indicate a significant difference (P < 0.05).

  8. Repeated PIDs trigger seizure-like events.
    Figure 8: Repeated PIDs trigger seizure-like events.

    (a) Dual extracellular recordings of an ictal event preceded by PIDs. The lower black trace was recorded at the site of onset of PIDs and ictal events. The upper gray record was made from a follower region. Right, expanded traces from the period indicated with the asterisk. (b) Electrical stimulation (2 Hz, 2 s, black sign) near the site of PID initiation elicited PIDs (expanded at right), which induced a seizure-like event. (c) Identical electrical stimulation (gray sign) in a region of PID propagation did not trigger seizure-like events. (d) High-intensity bipolar electrical stimulation (large gray sign) in the region of PID propagation generated PIDs at their initiation site. A seizure-like event followed. The double-headed arrow indicates the delay between stimulation and back-propagated PID onset. (e) Probability of triggering an ictal-like event by moderate intensity stimuli at the PID focus (black), at a follower site (white) or by strong stimuli at the follower site generating back-propagated PIDs (gray). *P < 10−6, **P < 10−12.


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Author information


  1. Cortex and Epilepsy, Centre de Recherche de l'Institut du Cerveau et de la Moelle Epinière, INSERM UMRS975, Centre National de la Recherche Scientifique (CNRS) UMR7225, Université Pierre et Marie Curie (UPMC), Paris, France.

    • Gilles Huberfeld,
    • Liset Menendez de la Prida,
    • Johan Pallud,
    • Ivan Cohen,
    • Stéphane Clemenceau,
    • Michel Baulac &
    • Richard Miles
  2. Unité d'Epileptologie, Centre Hospitalo-Universitaire Pitié-Salpêtrière, Assistance Publique - Hôpitaux de Paris (AP-HP), Paris, France.

    • Gilles Huberfeld,
    • Claude Adam,
    • Stéphane Clemenceau &
    • Michel Baulac
  3. Département de Neurophysiologie, UPMC, Centre Hospitalo-Universitaire Pitié-Salpêtrière, Paris, France.

    • Gilles Huberfeld
  4. Laboratorio de Circuitos Neuronales, Instituto Cajal, Consejo Superior de Investigaciones Científicas, Madrid, Spain.

    • Liset Menendez de la Prida
  5. Service de Neurochirurgie, Centre Hospitalier Ste Anne, Université Paris Descartes, France.

    • Johan Pallud
  6. Network Dynamics and Cellular Excitability, Centre de Recherche de l'Institut du Cerveau et de la Moelle Epinière, INSERM UMRS975, CNRS UMR7225, UPMC, Paris, France.

    • Michel Le Van Quyen
  7. Service de Neurochirurgie, Centre Hospitalo-Universitaire Pitié-Salpêtrière, AP-HP, Paris, France.

    • Stéphane Clemenceau


G.H., L.M.d.l.P. and R.M. designed the study. G.H., L.M.d.l.P. and R.M. performed the in vitro experiments. G.H., S.C., J.P., C.A. and M.B. performed the in vivo work and analysis. G.H., L.M.d.l.P., J.P., I.C., M.L.V.Q. and R.M. contributed to data analysis. G.H. and R.M. wrote the paper.

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