Ictal-like Epileptiform Discharges Research Articles

Modulation of epileptiform activity by three subgroups of GABAergic interneurons in mouse somatosensory cortex

4-Aminopyridine (4-AP) induces ictal-like epileptiform discharges in a variety of brain regions. These events are associated with enhanced inhibitory and excitatory synaptic neurotransmission. The relative contribution of specific subclasses of GABAergic interneurons (INs) to epileptiform activity in the 4-AP model is not well characterized. We have used genetically encoded channelrhodopsin (ChR) and Archaerhodopsin (Arch) expression in parvalbumin (PV), somatostatin (SST) and vasoactive intestinal polypeptide (VIP) INs to investigate the role of interneuron subclasses in 4-AP-induced epileptiform discharges. Whole-cell patch-clamp recordings were obtained from L5 pyramidal cells (PYRs) in somatosensory cortex of 30-to-70-day old mice. In the presence of 100 µM 4-AP, photostimulation of ChR in PV and SST, but not VIP INs, evoked epileptiform discharges similar to spontaneous and electrically evoked events. Light activation of Arch in PV INs was more effective in reducing epileptiform activity compared to SST and VIP INs. Epileptiform discharges were evoked at offset of Arch induced hyperpolarizations in PV and SST interneurons but not VIP INs. PV and SST INs could both initiate and inhibit 4-AP-induced epileptiform activity in L5 PYRs. VIP INs did not contribute significantly to eliciting or inhibiting epileptiform discharges. These results suggest that subclasses of INs contribute differently to the initiation and modulation of epileptiform discharges in cortical networks.

KCC2 Chloride Transport Contributes to the Termination of Ictal Epileptiform Activity.

Recurrent seizures intensely activate GABAA receptors (GABAA-Rs), which induces transient neuronal chloride ([Cl-]i) elevations and depolarizing GABA responses that contribute to the failure of inhibition that engenders further seizures and anticonvulsant resistance. The K+-Cl- cotransporter KCC2 is responsible for Cl- extrusion and restoration of [Cl-]i equilibrium (ECl) after synaptic activity, but at the cost of increased extracellular potassium which may retard K+-Cl- extrusion, depolarize neurons, and potentiate seizures. Thus, KCC2 may either diminish or facilitate seizure activity, and both proconvulsant and anticonvulsant effects of KCC2 inhibition have been reported. It is now necessary to identify the loci of these divergent responses by assaying both the electrographic effects and the ionic effects of KCC2 manipulation. We therefore determined the net effects of KCC2 transport activity on cytoplasmic chloride elevation and Cl- extrusion rates during spontaneous recurrent ictal-like epileptiform discharges (ILDs) in organotypic hippocampal slices in vitro, as well as the correlation between ionic and electrographic effects. We found that the KCC2 antagonist VU0463271 reduced Cl- extrusion rates, increased ictal [Cl-]i elevation, increased ILD duration, and induced status epilepticus (SE). In contrast, the putative KCC2 upregulator CLP257 improved chloride homeostasis and reduced the duration and frequency of ILDs in a concentration-dependent manner. Our results demonstrate that measuring both the ionic and electrographic effects of KCC2 transport clarify the impact of KCC2 modulation in specific models of epileptiform activity. Anticonvulsant effects predominate when KCC2-mediated chloride transport rather than potassium buffering is the rate-limiting step in restoring ECl and the efficacy of GABAergic inhibition during recurrent ILDs.

Blockade of in vitro ictogenesis by low-frequency stimulation coincides with increased epileptiform response latency.

Low-frequency stimulation, delivered through transcranial magnetic or deep-brain electrical procedures, reduces seizures in patients with pharmacoresistant epilepsy. A similar control of ictallike discharges is exerted by low-frequency electrical stimulation in rodent brain slices maintained in vitro during convulsant treatment. By employing field and "sharp" intracellular recordings, we analyzed here the effects of stimuli delivered at 0.1 or 1 Hz in the lateral nucleus of the amygdala on ictallike epileptiform discharges induced by the K(+) channel blocker 4-aminopyridine in the perirhinal cortex, in a rat brain slice preparation. We found that 1) ictal events were nominally abolished when the stimulus rate was brought from 0.1 to 1 Hz; 2) this effect was associated with an increased latency of the epileptiform responses recorded in perirhinal cortex following each stimulus; and 3) both changes recovered to control values following arrest of the 1-Hz stimulation protocol. The control of ictal activity by 1-Hz stimulation and the concomitant latency increase were significantly reduced by GABAB receptor antagonism. We propose that this frequency-dependent increase in latency represents a short-lasting, GABAB receptor-dependent adaptive mechanism that contributes to decrease epileptiform synchronization, thus blocking seizures in epileptic patients and animal models.

Acute and chronic efficacy of bumetanide in an in vitro model of posttraumatic epileptogenesis.

Seizures triggered by acute injuries to the developing brain respond poorly to first-line medications that target the inhibitory chloride-permeable GABAA receptor. Neuronal injury is associated with profound increases in cytoplasmic chloride ([Cl(-)]i) resulting in depolarizing GABA signaling, higher seizure propensity and limited efficacy of GABAergic anticonvulsants. The Na(+)-K(+)-2Cl(-) (NKCC1) cotransporter blocker bumetanide reduces [Cl(-)]i and causes more negative GABA equilibrium potential in injured neurons. We therefore tested both the acute and chronic efficacy of bumetanide on early posttraumatic ictal-like epileptiform discharges and epileptogenesis. Acute hippocampal slices were used as a model of severe traumatic brain injury and posttraumatic epileptogenesis. Hippocampal slices were then incubated for 3 weeks. After a 1-week latent period, slice cultures developed chronic spontaneous ictal-like discharges. The anticonvulsant and anti-epileptogenic efficacy of bumetanide, phenobarbital, and the combination of these drugs was studied. Bumetanide reduced the frequency and power of early posttraumatic ictal-like discharges in vitro and enhanced the anticonvulsant efficacy of phenobarbital. Continuous 2-3 weeks administration of bumetanide as well as phenobarbital in combination with bumetanide failed to prevent posttraumatic ictal-like discharges and epileptogenesis. Our data demonstrate a persistent contribution of NKCC1 cotransport in posttraumatic ictal-like activity, presumably as a consequence of chronic alterations in neuronal chloride homeostasis and GABA-mediated inhibition. New strategies for more effective reduction in posttraumatic and seizure-induced [Cl(-)]i accumulation could provide the basis for effective treatments for posttraumatic epileptogenesis and the resultant seizures.

On the ictogenic properties of the piriform cortex in vitro

The piriform cortex (PC) is known to be epileptic-prone and it may be involved in the manifestation of limbic seizures. Herein, we have characterized some electrophysiologic and pharmacologic properties of the spontaneous epileptiform activity generated by PC networks maintained in vitro. We performed field potential recordings from the PC in coronal or sagittal rat brain slices along with pharmacologic manipulations of γ-aminobutyric acid (GABA)ergic and glutamatergic signaling during application of the convulsant drug 4-aminopyridine (4AP, 50 μm). Coronal and sagittal preparations generated interictal-like and ictal-like epileptiform discharges with similar duration and frequency. Ictal-like discharges in sagittal slices were initiated mostly in the PC anterior subregion, whereas interictal activity did not have any preferential site of origin. In sagittal slices, high frequency oscillations (HFOs) at 80-200 Hz were detected mainly at the beginning of the ictal discharge in both posterior and anterior subregions. N-Methyl-d-aspartate (NMDA) receptor antagonism abolished ictal discharges, but failed to influence interictal activity. In the absence of ionotropic glutamatergic transmission, PC networks generated slow, GABA receptor-dependent events. Finally, GABA(A) receptor antagonism during application of 4AP only, abolished ictal discharges and disclosed recurrent interictal activity. Our findings demonstrate that PC networks can sustain in vitro epileptiform activity induced by 4AP. HFOs, which emerge at the onset of ictal activity, may be involved in PC ictogenesis. As reported in several cortical structures, ionotropic glutamatergic neurotransmission is necessary but not sufficient for ictal discharge generation, a process that also requires operative GABA(A) receptor-mediated signaling.

Zn2+ Slows Down CaV3.3 Gating Kinetics: Implications for Thalamocortical Activity

We employed whole cell patch-clamp recordings to establish the effect of Zn(2+) on the gating the brain specific, T-type channel isoform Ca(V)3.3 expressed in HEK-293 cells. Zn(2+) (300 microM) modified the gating kinetics of this channel without influencing its steady-state properties. When inward Ca(2+) currents were elicited by step depolarizations at voltages above the threshold for channel opening, current inactivation was significantly slowed down while current activation was moderately affected. In addition, Zn(2+) slowed down channel deactivation but channel recovery from inactivation was only modestly changed. Zn(2+) also decreased whole cell Ca(2+) permeability to 45% of control values. In the presence of Zn(2+), Ca(2+) currents evoked by mock action potentials were more persistent than in its absence. Furthermore, computer simulation of action potential generation in thalamic reticular cells performed to model the gating effect of Zn(2+) on T-type channels (while leaving the kinetic parameters of voltage-gated Na(+) and K(+) unchanged) revealed that Zn(2+) increased the frequency and the duration of burst firing, which is known to depend on T-type channel activity. In line with this finding, we discovered that chelation of endogenous Zn(2+) decreased the frequency of occurrence of ictal-like epileptiform discharges in rat thalamocortical slices perfused with medium containing the convulsant 4-aminopyridine (50 microM). These data demonstrate that Zn(2+) modulates Ca(V)3.3 channel gating thus leading to increased neuronal excitability. We also propose that endogenous Zn(2+) may have a role in controlling thalamocortical oscillations.

Rat subicular networks gate hippocampal output activity in an in vitro model of limbic seizures

Evidence obtained from human epileptic tissue maintained in vitro indicates that the subiculum may play a crucial role in initiating epileptiform discharges in patients with mesial temporal lobe epilepsy. Hence, we used rat hippocampus-entorhinal cortex (EC) slices to identify the role of subiculum in epileptiform synchronization during bath application of 4-aminopyridine (4AP, 50 microM). In these slices, fast CA3-driven interictal-like events were restricted to the hippocampal CA3/CA1 areas and failed to propagate to the EC where slow interictal-like and ictal-like epileptiform discharges were recorded. However, antagonizing GABA(A) receptors with picrotoxin (50 microM) made CA3-driven interictal activity spread to EC. Sequential field potential analysis along the CA3-CA1-subiculum axis revealed that the amplitude of CA3-driven interictal discharges recorded in the presence of 4AP only diminished within the subiculum. Furthermore, CA1 electrical stimulation under control conditions elicited little or no subicular activation and never any response in EC; in contrast, robust subicular discharges that spread to EC could be evoked after picrotoxin. Intracellular recordings indicated that potentiation by picrotoxin was associated with blockade of hyperpolarizing IPSPs in subicular cells. Finally, when surgically isolated from adjacent structures, the subiculum generated low-amplitude synchronous discharges that corresponded to an intracellular hyperpolarization-depolarization sequence, were resistant to glutamatergic antagonists, and represented the activity of synchronized interneuronal networks. Bath application of picrotoxin abolished these 4AP-induced events and in their place robust network bursting occurred. In conclusion, our study demonstrates that the subiculum plays a powerful gating role on hippocampal output activity. This function depends on GABA(A) receptor-mediated inhibition and controls hippocampal-parahippocampal interactions that are known to modulate limbic seizures.

Thalamocortical connectivity in a rat brain slice preparation: participation of the ventrobasal complex to synchronous activities

We studied the synchronous cortical and thalamic activities induced by low (0.5–1 mM) and high (50–100 mM) concentrations of the K+ channel blocker 4-aminopyridine (4AP) in a rat thalamocortical preparation. The presence of reciprocal thalamocortical connectivity was documented by diffusion of the fluorescent tracer Di-IC18 between the somatosensory cortex and the ventrobasal complex (VB) of the thalamus in vitro. Functional reciprocal connectivity was also demonstrated by stimulating the cortical middle-deep layers (which elicited orthodromic responses in VB) or the VB (which induced orthodromic and antidromic responses in the cortex). Spontaneous field potentials were not recorded in either the thalamus or cortex in control medium. Low concentrations of 4AP produced local spindle-like rhythmic oscillations in cortex and VB (duration = 0.4–3.5 s; frequency = 9–16 Hz). In contrast, high concentrations of 4AP induced widespread ictal-like epileptiform discharges (duration = 8–45 s) characterised by lsquo ‘tonic’ component followed by a period of ‘clonic’ discharges in both cortex and VB. Spindle-like activity was abolished in cortex and thalamus by applying the excitatory amino acid receptor antagonist kynurenic acid in VB. In contrast, the same procedure exacerbated ictal-like discharges, while depressing VB activity. Our results indicate that thalamus and cortex can produce synchronous activities in this in vitro thalamocortical network: spindle-like rhythmic oscillations are generated at the thalamic level and imposed upon the cortical network whereas ictal-like discharges have a cortical origin and are modulated by the thalamic network activity. In addition, we have shown that it is possible to preserve reciprocal projections between cortex and thalamus in an in vitro rat slice preparation that could be a valuable tool to study epileptic-prone rat strains.

Thalamocortical connectivity in a rat brain slice preparation: participation of the ventrobasal complex to synchronous activities

We studied the synchronous cortical and thalamic activities induced by low (0.5–1 mM) and high (50–100 mM) concentrations of the K + channel blocker 4-aminopyridine (4AP) in a rat thalamocortical preparation. The presence of reciprocal thalamocortical connectivity was documented by diffusion of the fluorescent tracer Di-IC 18 between the somatosensory cortex and the ventrobasal complex (VB) of the thalamus in vitro. Functional reciprocal connectivity was also demonstrated by stimulating the cortical middle-deep layers (which elicited orthodromic responses in VB) or the VB (which induced orthodromic and antidromic responses in the cortex). Spontaneous field potentials were not recorded in either the thalamus or cortex in control medium. Low concentrations of 4AP produced local spindle-like rhythmic oscillations in cortex and VB (duration=0.4–3.5 s; frequency=9–16 Hz). In contrast, high concentrations of 4AP induced widespread ictal-like epileptiform discharges (duration=8–45 s) characterised by a ‘tonic’ component followed by a period of ‘clonic’ discharges in both cortex and VB. Spindle-like activity was abolished in cortex and thalamus by applying the excitatory amino acid receptor antagonist kynurenic acid in VB. In contrast, the same procedure exacerbated ictal-like discharges, while depressing VB activity. Our results indicate that thalamus and cortex can produce synchronous activities in this in vitro thalamocortical network: spindle-like rhythmic oscillations are generated at the thalamic level and imposed upon the cortical network whereas ictal-like discharges have a cortical origin and are modulated by the thalamic network activity. In addition, we have shown that it is possible to preserve reciprocal projections between cortex and thalamus in an in vitro rat slice preparation that could be a valuable tool to study epileptic-prone rat strains.

Epileptiform discharges in the human dysplastic neocortex: in vitro physiology and pharmacology.

Field potential and intracellular recordings were made in slices of human neocortical tissue obtained during surgery for the treatment of seizures associated with focal cortical dysplasia. Ictal-like epileptiform discharges, along with isolated field potentials, were induced by bath application of 4-aminopyridine (50-100 microM). Some of the isolated field potentials were associated with fast transients representing population spikes. Field potential profile analysis indicated that both types of synchronous activity had maximal negative values at 1,400 to 1,600 microm from the pia. The intracellular counterpart of the ictal-like discharge was a prolonged membrane depolarization capped by repetitive action potential burst firing. By contrast, the isolated field potentials were mirrored by long-lasting depolarizations with minimal action potential firing; only when population spikes occurred, the isolated field potentials were associated with epileptiform action potential bursting. Ictal-like discharges were abolished by either N-methyl-D-aspartate or non-N-methyl-D-aspartate receptor antagonists. In contrast, the isolated field potentials continued to occur synchronously during excitatory transmission blockade (although they lacked fast transients) but were abolished by the gamma-aminobutyric acid(A) receptor antagonist bicuculline methiodide (n = 2 slices). Our study demonstrates that focal cortical dysplasia tissue maintained in vitro has an intrinsic ability to generate ictal-like epileptiform events when challenged with 4-aminopyridine. These discharges depend on excitatory amino acid receptor-mediated mechanisms. Our results also show the presence in focal cortical dysplasia tissue of glutamatergic-independent synchronous potentials that are mainly contributed by gamma-aminobutyric acid(A) receptor-mediated conductances.

Activity-Dependent pH Shifts and Periodic Recurrence of Spontaneous Interictal Spikes in a Model of Focal Epileptogenesis

The mechanisms that control the periodicity of spontaneous epileptiform cortical potentials were investigated in the in vitro isolated guinea pig brain preparation. A brief intracortical application of bicuculline in the piriform cortex induced spontaneous interictal spikes (sISs) that recurred with high periodicity (8.5 +/- 3.1 sec, mean +/- SD). Intracellular recordings from principal neurons showed that the early phase of the inter-sIS period is caused by a GABAb receptor-mediated inhibitory potential. The late component of the interspike period correlated to a slowly decaying depolarization abolished at membrane potentials positive to -32.1 +/- 5.3 mV and was not associated with membrane conductance changes. Specific pharmacological tests excluded the contribution of synaptic and intrinsic conductances to the late inter-sIS interval. Recordings with ion-sensitive electrodes demonstrated that sISs determined both a rapid increase in extracellular K+ concentration (0.5-1 mM) and an extracellular alkalinization (0.05-0.08 pH units) that slowly decayed during the inter-sIS period and returned to control values just before a subsequent sIS was generated. These observations were not congruous with the presence of a silent period, because both extracellular increase in K+ and alkalinization are commonly associated with an increase in neuronal excitability. Extracellular alkalinization could be correlated to an sIS-induced intracellular acidification, a phenomenon that reduces cell coupling by impairing gap junction function. When intracellular acidification was transiently prevented by arterial perfusion with NH4Cl (10-20 mM), spontaneous ictal-like epileptiform discharges were induced. In addition, the gap junction blockers octanol (0.2-2 mM) and 18-alpha-glycyrrethinic acid (20 microM) applied either via the arterial system or locally in the cortex completely and reversibly abolished the sIS. The results reported here suggest that the massive cell discharge associated with an sIS induce a strong inhibition, possibly secondary to a pH-dependent uncoupling of gap junctions, that regulates sIS periodicity.

Participation of GABAA-mediated inhibition in ictallike discharges in the rat entorhinal cortex.

The spontaneous, synchronous activity induced by 4-aminopyridine (4AP, 50 microM) in the adult rat entorhinal cortex was analyzed with simultaneous field potential and intracellular recordings in an in vitro slice preparation. Four-AP induced isolated negative-going field potentials (interval of occurrence = 27.6 +/- 9.9 (SD) s; n = 27 slices) that corresponded to intracellular long-lasting depolarizations (LLDs), and ictallike epileptiform discharges (interval of occurrence = 10.4 +/- 5.7 min; n = 27 slices) that were initiated by the negative field potentials. LLDs recorded with K-acetate-filled microelectrodes triggered few action potentials of variable amplitude and had a duration of 1.7 +/- 0.8 s (n = 26 neurons), a peak amplitude of 11.8 +/- 5.0 mV (n = 26 neurons) and a reversal potential of -66.2 +/- 3.9 mV (n = 17 neurons). The ictal discharges studied with K-acetate microelectrodes consisted of prolonged depolarizations (duration = 72.9 +/- 44.3 s; peak amplitude = 29.2 +/- 11.4 mV; n = 25 neurons) with action-potential firing during both the tonic and the clonic phase. These depolarizations had a reversal potential of -45.3 +/- 3.8 mV (n = 4 neurons). Intracellular Cl- diffusion from KCl-filled microelectrodes made both LLDs and ictal depolarizations increase in amplitude (30.5 +/- 8.2 mV, n = 8 and 41.8 +/- 9.8 mV, n = 6 neurons, respectively). LLDs recorded with KCl and 2-(trimethyl-amino)N-(2, 6-dimethylphenyl)-acetamide (QX-314) microelectrodesreached an amplitude of 36.3 +/- 5.2 mV, lasted 12.5 +/- 6.5 s, and had a reversal potential of -31.3 +/- 2.5 mV (n = 4 neurons); under these recording procedures the ictal discharge amplitude was 41.5 +/- 5.0 mV and the reversal potential -24.0 +/- 7.0 mV (n = 4 neurons). The N-methyl-D-aspartate (NMDA) receptor antagonist 3,3-(2-carboxy-piperazine-4-yl)-pro-pyl-l-phosphonate (10 microM, n = 5 neurons) alone or concomitant with the nonNMDA receptor antagonist 6-cyano-7-nitro-quinoxaline-2,3-dione (10 microM, n = 4 neurons) abolished ictal discharges, without influencing LLDs. LLDs were blocked by the gamma-aminobutyric acid-A (GABAA) receptor antagonist bicuculline methiodide (BMI, 10 microM, n = 6 neurons) or the mu-opioid receptor agonist (-Ala2-N-Me-Phe, Gly-ol) enkephalin (DAGO, 10 microM, n = 2 neurons). Application of BMI (n = 4 neurons) or DAGO (n = 2 neurons) to control the medium abolished LLDs and ictal discharges but disclosed a novel type of epileptiform depolarization that lasted 3.5 +/- 1.2 s and occurred every 5.2 +/- 2.6 s (n = 6 neurons). Our data indicate that 4AP induces in the rat entorhinal cortex a synchronous, GABA-mediated potential that is instrumental in initiating NMDA-dependent, ictal discharges. Moreover we present evidence for an active role played by GABAA-mediated potentials in the maintenance and termination of these prolonged epileptiform events.

Extracellular free potassium during synchronous activity induced by 4-aminopyridine in the juvenile rat hippocampus

Field potential recordings and measurements of the extracellular concentration of free K + ([K + 0]) were made in the stratum radiatum of the CA3 subfield of hippocampal slices that were obtained from 12- to 17-day-old rats. Spontaneous, synchronous field potentials were recorded in the presence of the convulsant drug 4-aminopyridine (4AP, 50 μM). They consisted of interictal- (duration = 0.2−1.2 s; rate of occurrence = 0.3−1.3 Hz) and ictal-like epileptiform discharges (8–40 s; 4−38·10 −3 Hz), as well as large amplitude, negative-going potentials that preceded the onset of the ictal-like event. Such a temporal correlation suggested that the negative-going potential might facilitate the onset of ictal-like activity. Interictal- and ictal-like discharges were abolished by the AMPA/kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 μM), while the negative-going potential was selectively blocked by bicuculline methiodide (BMI, 10 μM). Hence it was presumably due to the activation of GABA A receptors. [K +] 0 increased up to 12.5 mM (7.9 ± 2.7 mM, mean ± S.D.) from a resting value of 3.25 mM during the BMI-sensitive potentials (which also corresponded to the onset of ictal-like events), and after a decline to approximately 5 mM it remained elevated throughout the ictal event. Small, transient increases in [K +] 0 (up to 3.7 mM) could be seen during each interictal-like event. Following blockade of interictal- and ictal-like discharges by CNQX increases in [K +] 0 (up to 11 mM; 7.3 ± 2.1; half-width = 7.2 ± 2.3 s) still accompanied the BMI-sensitive negative-going potentials. Our findings indicate that the negative-going synchronous potential induced by 4AP is associated with an increase in [K +] 0 that is presumably due to the activation of GABA A receptors. Such a mechanism might be instrumental in synchronizing neurons and thus facilitate the onset of prolonged epileptiform discharges.

Excitatory synaptic involvement in epileptiform bursting in the immature rat neocortex

1. Neocortical brain slices were prepared from animals 8-15 days of age and maintained in vitro. Intracellular recordings were obtained from neurons in cortical layers 2-3. The role of synaptic activity and excitatory amino acid receptors in generation of picrotoxin-induced ictal-like epileptiform activity in the immature neocortex was investigated. D-2-amino-5-phosphonovaleric acid (D-APV) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) were used as selective antagonists of N-methyl-D-aspartate (NMDA) and non-NMDA receptors, respectively. 2. Ictal-like epileptiform discharges were induced by bath application of the GABAA-receptor antagonist picrotoxin. Paroxysmal discharges, 7-25 s in duration, occurred spontaneously or could be evoked by electrical stimulation. These events consisted of an initial paroxysmal depolarizing shift (PDS) followed by a long-duration depolarization (LLD) with superimposed late PDSs. 3. The amplitudes of the initial PDS, LLD, and late PDSs were linearly dependent on membrane potential, increasing with hyperpolarization and diminishing on depolarization. All responses reversed polarity near 0 mV. Under voltage-clamp conditions, both transient and sustained currents were observed, coincident with PDSs and the LLD, respectively. The duration of the ictal-like events was similar under current- and voltage-clamp conditions, suggesting activation of intrinsic membrane currents did not significantly prolong epileptiform discharges. 4. Bath application of D-APV (20 microM) decreased the amplitude and duration of both the initial PDS and LLD without affecting the time-to-onset of epileptiform activity.(ABSTRACT TRUNCATED AT 250 WORDS)