The cause of the imbalance in the neuronal network leading to seizure activity can be predicted by the electrographic pattern of the seizure onset

Abstract

This study investigates the temporal dynamics of ictal electrical activity induced by injection of the GABA(A) receptor antagonist bicuculline, and the glutamate agonist kainic acid, into the CA3 area of hippocampus. Experiments were conducted in freely moving adult Wistar rats implanted with microelectrodes in multiple brain areas. Wide-band electrical activity (0.1-3000 Hz) was recorded, and the latency of seizure onset as well as the pattern of electrical activity were investigated for each drug. The latencies between injection and the occurrence of first epileptiform events were 3.93 +/- 2.76 (+/-STD) min for bicuculline and 6.37 +/- 7.66 min for kainic acid, suggesting the existence of powerful seizure-suppressive mechanisms in the brain. Bicuculline evoked high-amplitude rhythmic epileptiform events at the site of injection which resembled interictal EEG spikes and rapidly propagated to adjacent and remote brain areas. Kainic acid evoked a completely different pattern with a gradual increase in the amplitude of 30-80 Hz activity. Whereas there was strong temporal correlation between EEG events at the site of bicuculline injection and discharges in distant areas, much less correlation was seen with kainic acid injection. Both patterns were followed by generalized ictal EEG discharges and behavioral seizures. Our results illustrate that the same area of the brain can trigger seizures with different electrographic patterns. The knowledge of the network mechanisms underlying these two distinct electrographic patterns might be helpful in designing differential strategies for preventing seizure occurrence.

Figure 1.

A schematic representation of the location of the area of drug injection in right posterior CA3 and the recording sites in the adjacent and remote areas of the brain. LaDG and RaDG, Left and right anterior dentate gyrus; LEC, left entorhinal cortex. RPir is not in the plane of the schematic.

Figure 2.

Patterns of seizure onsets in the dentate gyrus induced by bicuculline injection into ipsilateral CA3 area of hippocampus. Numbers on the left indicate rat identification. Bracket above each record outlines the electrographic phase followed by the clinical phase of the seizure with the behavioral component.

Figure 3.

Involvement of brain areas in the epileptiform discharge evoked by injection of bicuculline into CA3. A, A sequential occurrence of epileptiform events at the point of injection (CA3) and in the adjacent dentate gyrus (as shown in Fig. 1). Numbers on the left indicate sequential number of the epileptiform event from the moment of occurrence. B, A graph of amplitude of epileptiform events during the transition period in the area of bicuculline injection (RCA3), in areas receiving monosynaptic or bisynaptic projections from CA3, LpDG, RpDG, and REC, and areas remote from the point of bicuculline injection [left entorhinal cortex (LEC), LaDG, right anterior dentate gyrus (RaDG), and RPir].

Figure 4.

A, Examples of epileptiform events recorded in different brain areas during the electrographic phase of a seizure evoked by bicuculline injection. Events were collected and superimposed within a period of 20 s before the clinical phase of the seizure using the peak of the event generated in the right dentate gyrus as a zero point. Red lines represent average of these potentials. Dashed lines outline a 40 ms window when all events reach the maximum of amplitude. B, Examples of 14 epileptiform events from the same recording sites within a period of 20 s before the clinical phase of a seizure evoked by kainic acid injection. See schematic of electrode montage in Figure 1. The calibration bar on the right of each line is 1.0 mV.

Figure 5.

Development of a seizure within the CA1–dentate gyrus axis after bicuculline injection into ipsilateral CA3. The location of the silicon probe is illustrated on the left side of the figure (A). The characters “p” and “g” indicate pyramidal and granular layers. The numbers indicate the recording sites from the top to the bottom. The schematic on the left indicates the location of the synaptic inputs to CA1 and DG at the area of the penetration of the silicon probe. com, Commissural; bc, basket cells; sch, Shaffer collaterals; pp, perforant path. B, Voltage-depth profiles of evoked potentials (EP) to the perforant path stimulation (arrow). The color illustrates the current-source density map, where the “hot” color indicates source and the “cold” indicates sink. C, D, Voltage-depth profiles and current-source density map of epileptiform events correspondingly 12 and 2 s before the seizure. E, Seizure onset. For details, see Results.

Figure 6.

Field potentials and unit discharges during the transition period after bicuculline injection. A, Field potential (top) recorded from CA3 and multiunit activity recorded in different areas of the brain (for details, see schematic in Fig. 1). Blue lines mark areas of the brain where neuronal discharges show strong correlation with the onset of epileptiform events. B, Perievent time histograms of multiunit discharges in relation to epileptiform field potentials. The peak of the field potential is taken as the zero point. Only units located in CA3, ipsilateral posterior dentate gyrus, and entorhinal cortex show responses during onset of the epileptiform event. Other brain areas became involved in the epileptiform event later. Arrow indicates the peak of the initial wave of the epileptiform event that was used as a triggering point for construction of perievent histograms.

Figure 7.

A, B, Development of seizures after injection into CA3 of kainic acid (A) and bicuculline (B). To adjust similar time for behavioral seizure occurrence, a 30 s break was used in the tracings with kainic acid injection. In the case of kainic acid injection, the most significant changes occurred outside the area of injection, whereas after bicuculline injection, the most dramatic changes occurred within the area of injection and immediate surrounding areas: RpDG and REC. LaDG and RaDG, Left and right anterior dentate gyrus, respectively; LEC, left entorhinal cortex.

Figure 8.

Development of a seizure after kainic acid injection. A, Color-coded power spectral maps calculated with a Fast Fourier Transform using a sliding window of 10 s with 1 s overlap. The change in the color is coded in SDs with the scale on the right side of the plot. The hot color indicates an increase in the power of specific frequency bands. B, Increase in amplitude during the transition period on the records of raw data (0.1 Hz–3 kHz) and multiunit activity (hp 400 Hz). Below, Extended fragments of records from different times during the transition period.

Figure 9.

Transition to seizure after intra-CA3 kainic acid injection recorded by silicon probe implanted in CA1–dentate gyrus. Eight recording sites were selected for demonstration. Epileptiform activity first appeared in the dentate gyrus and much later in CA1. The histological section with the track of the silicon probe (white arrows) is shown on the left. or., Oriens; pyr, pyramidale; rad, radiatum; lac-mol, lacunose-moleculare; fis, fissure; gr, granular; hil, hilus.

Figure 10.

Correlation of multiunit discharges with local field potentials within dentate gyrus during the transition period to seizure evoked by kainic acid injection into ipsilateral CA3. A, Six-minute-long record of the transition period and seizure. B, Thirty-second-long record at the beginning of the transition period. B1, Spike-triggered average of 151 field potentials. B2, Autocorrellogram of multiunit discharges. C, A field potential record during the last 30 s of the transition period before the seizure onset. C1, Spike-triggered average of 163 field potentials. C2, Autocorrellogram of multiunit discharges.