Termination of epileptic afterdischarge in the hippocampus

Abstract

The mechanism of afterdischarge termination in the various hippocampal regions was examined in the rat. Stimulation of the perforant path or the commissural system was used to elicit afterdischarges. Combination of multiple site recordings with silicon probes, current source density analysis, and unit recordings in the awake animal allowed for a high spatial resolution of the field events. Interpretation of the field observations was aided by intracellular recordings from anesthetized rats. Irrespective of the evoking conditions, afterdischarges always terminated first in the CA1 region. Termination of the afterdischarge was heralded by a large DC shift initiated in dendritic layers associated with a low amplitude "afterdischarge termination oscillation" (ATO) at 40 to 80 Hz in the cell body layer. ATOs were also observed in the CA3 region and the dentate gyrus. The DC shift spread at the same velocity (0. 1-0.2 mm/sec) in all directions and could cross the hippocampal fissure. All but 1 of the 25 putative interneurons in the CA1 and dentate regions ceased to fire before the onset of ATO. Intracellularly, ATO and the emerging DC potential were associated with fast depolarizing potentials and firing of pyramidal cells and depolarization block of spike initiation, respectively. Both field ATO and the intracellular depolarization shift were replicated by focal microinjection of potassium. We hypothesize that [K+]o lost by the intensely discharging neurons during the afterdischarge triggers propagating waves of depolarization in the astrocytic network. In turn, astrocytes release potassium, which induces a depolarization block of spike generation in neurons, resulting in "postictal depression" of the EEG.

Fig. 1.

Main patterns of the afterdischarge in the hippocampus evoked by COM stimulation (200 Hz). The continuous voltage traces from 16 recording sites (1–2000 Hz bandpass) were used to calculate the CSD derivatives. Only three selected CSD traces from the CA1 pyramidal layer (p), stratum radiatum (r), and the supragranular layer (m/g) are shown here. Sinks are up. Arrows, Onset of the sustained potential (SP) shifts recorded by the AC-coupled amplifiers; pID, postictal depression.ATO, Afterdischarge termination oscillation. The primary afterdischarge (pad) shown here is omitted from the illustrations in Figures 2, 3, 4.

Fig. 2.

Sustained potentials associated with afterdischarge termination. A, Sixteen-site recording of an afterdischarge in the CA1-dentate gyrus axis in the awake rat. The early part of the pAD is omitted (see example in Fig. 1) to provide a better time resolution. Arrows above traces indicate the onset of large DC shifts recorded by AC-coupled amplifiers (0.1–200 Hz). ATO is not visible at this magnification and low sampling rate (200 Hz). The propagation speed of the DC front was 0.28 mm/sec in the CA1 region (open arrows) and 0.12 mm/sec in the dentate gyrus (filled arrows). Asterisk, A second wave of DC shift in the absence of neuronal activity.B, Evoked potentials in response to perforant path stimulation at the same recording position as A.C, Silicon probe in situ at its final recording position as seen during vibratome sectioning (different depth from A and B). D, The same section as shown in C after Nissl staining. Recording sites during the afterdischarge shown in A are marked by1 to 16. The exact recording position of each site is determined from the laminar profile of the evoked potentials (B). o, CA1 stratum oriens;p, CA1 pyramidal layer; r, stratum radiatum; hf, hippocampal fissure; m, molecular layer; g, granule cell layer;h, hilus.

Fig. 3.

ATO and associated sustained potentials in the CA1 region. Only the end of the primary afterdischarge, induced by COM stimulation, is shown. Traces 1 to 6 are CSD derivatives of band-passed filtered EEG traces (0.3 Hz to 2 kHz). Note the onset of the sustained potential in stratum lacunosum-moleculare and its slow (0.15 mm/sec) spread toward the pyramidal layer (open arrows). Strata pyramidale and oriens were invaded after a 3 sec delay (filled arrows). Fast sinks of ATO waves are largest in the pyramidal layer, surrounded by sources in the strata radiatum and oriens. o, CA1 stratum oriens;p, CA1 pyramidal layer; r, stratum radiatum; lm, stratum lacunosum-moleculare.

Fig. 4.

ATO in different hippocampal regions.A, CSD traces of ATO (1–2000 Hz) in the CA1 region.o, Stratum oriens; p, pyramidal layer;r, stratum radiatum. Open andfilled arrowheads indicate slow upward movement of the sources and sinks, respectively. Sinks are up.B, CSD trace in the granule cell layer.C, CSD trace in the CA3c pyramidal layer. The ATOs shown in CA1 (A), granule cell layer (B), and CA3 pyramidal layer (C) occurred 19, 25, and 41 sec after afterdischarge onset in the same rat, respectively. The expanded sweeps reveal the fast field oscillations with and without population spikes. Histograms, autocorrelograms of ATO.

Fig. 5.

Longitudinal spread of the ATO in the CA1 region (A) and dentate hilar region (B).A, Simultaneous recordings from four shanks in the pyramidal layer (voltage traces). Population spikes are clipped. B, Simultaneous recordings from four hilar sites. C, Approximate recording sites in the CA1 region (white arrows) and hilar region (black arrows) indicated on a section cut parallel to the septotemporal axis of the hippocampus. D, Evoked potentials in the hilar region in response to perforant path stimulation. Note slow spread of ATO (0.1 mm/sec in CA1; 0.2 mm/sec in hilus) in the temporoseptal direction (A, B) but synchronously occurring evoked population spikes (D).p, Pyramidal layer; g, granule cell layer.

Fig. 6.

Interneuronal activity during the ATO. Top trace, Voltage trace recorded from the CA1 pyramidal layer (wide band; 1 Hz to 5 kHz). Negative peaks were clipped.Arrow, Slow wave reflecting the onset of the extracellular DC shift. Shorter epochs of the wide band trace (middle) and their high-pass-filtered (0.5–5 kHz;bottom) derivatives are shown at faster speedbelow. Note relatively rhythmic firing of the interneuron at the beginning of ATO (left) and its complete silence on the waning phase (right).

Fig. 7.

Intracellular correlates of the ATO and sustained potential shift in a CA1 pyramidal neuron. A, COM-induced afterdischarge in the urethane anesthetized rat recorded intracellularly (top trace) and field activity (bottom trace) recorded <0.5 mm posterior to the micropipette. Thirty seconds are omitted between traces (30 s). Note fast spike burst and depolarization block of the cell and associated with extracellular ATO and the onset of the extracellular DC potential shift (arrow above field trace). Extracellular trace was wide-band-filtered (1 Hz to 5 kHz). Resting membrane potential was restored after 102 sec (last trace segment). B, Details of records in A (arrows) at faster speed. Note fast depolarizing potentials and action potentials during ATO.C, Depolarization block could be mimicked by extracellular injection of KCl. Recovery occurred after 192 sec. In other experiments, KCl induced fast field oscillations (ATO) and a 5–30 mV negative DC shift in the pyramidal cell layer (not shown).

Fig. 8.

Intracellular correlates of the ATO in a pyramidal neuron at the CA1-subicular border. COM-induced afterdischarge recorded intracellularly (upper trace) and field activity (bottom trace) recorded ∼1 mm posterior to the micropipette in the CA1 pyramidal layer. Ten seconds (10 s) were omitted between the traces. Note fast membrane oscillation (open triangle), spike burst, and large depolarization ∼10 sec after the onset of the extracellular ATO (black triangle). Note also a secondary depolarization wave (double arrow). Whereas actions potentials reoccurred on the descending part of the first depolarization wave, only a single action potential was observed on the falling phase of the second wave. The extracellular trace was wide-band-filtered (1 Hz to 5 kHz). Arrow above field trace, Slow wave indicating the beginning of extracellular DC potential shift.