Block of Glutamate-Glutamine Cycle Between Astrocytes and Neurons Inhibits Epileptiform Activity in Hippocampus

2002 ◽  
Vol 88 (5) ◽  
pp. 2302-2310 ◽  
Author(s):  
Alberto Bacci ◽  
Giulio Sancini ◽  
Claudia Verderio ◽  
Simona Armano ◽  
Elena Pravettoni ◽  
...  
Keyword(s):  
Epileptiform Activity ◽  
Hippocampal Slices ◽  
Brain Slices ◽  
Rhythmic Activity ◽  
Pharmacological Treatments ◽  
Metabolic Interaction ◽  
Epileptiform Discharges ◽  
Gaba Inhibition ◽  
Hippocampal Cultures ◽  
Glutamine Uptake

Recurrent epileptiform activity occurs spontaneously in cultured CNS neurons and in brain slices in which GABA inhibition has been blocked. We demonstrate here that pharmacological treatments resulting in either the block of glutamine production by astrocytes or the inhibition of glutamine uptake by neurons suppress or markedly decrease the frequency of spontaneous epileptiform discharges both in primary hippocampal cultures and in disinhibited hippocampal slices. These data point to an important role for the neuron-astrocyte metabolic interaction in sustaining episodes of intense rhythmic activity in the CNS, and thereby reveal a new potential target for antiepileptic treatments.

scholarly journals Epileptiform activity in mouse hippocampal slices induced by moderate changes in extracellular Mg2+, Ca2+, and K+

2021 ◽  
Vol 22 (1) ◽  
Author(s):  
Haiyu Liu ◽  
Sai Zhang ◽  
Liang Zhang
Keyword(s):  
Prolonged Exposure ◽  
Epileptiform Activity ◽  
Hippocampal Slices ◽  
Brain Slices ◽  
Experimental Investigations ◽  
High Frequency Stimulation ◽  
Field Potentials ◽  
Epileptiform Discharges ◽  
Frequency Stimulation

Abstract Background Rodent brain slices—particularly hippocampal slices—are widely used in experimental investigations of epileptiform activity. Oxygenated artificial cerebrospinal fluid (ACSF) is used to maintain slices in vitro. Physiological or standard ACSF containing 3–3.5 mM K+, 1–2 mM Mg2+, and 1–3 mM Ca2+ generally does not induce population epileptiform activity, which can be induced by ACSF with high K+ (8–10 mM), low Mg2+, or low Ca2+ alone or in combination. While low-Mg2+ ACSF without intentionally added Mg salt but with contaminating Mg2+ (≤ 50–80 µM) from other salts can induce robust epileptiform activity in slices, it is unclear whether such epileptiform activity can be achieved using ACSF with moderately decreased Mg2+. To explore this issue, we examined the effects of moderately modified (m)ACSF with 0.8 mM Mg2+, 1.3 mM Ca2+, and 5.7 mM K+ on induction of epileptiform discharges in mouse hippocampal slices. Results Hippocampal slices were prepared from young (21–28 days old), middle-aged (13–14 months old), and aged (24–26 months old) C57/BL6 mice. Conventional thin (0.4 mm) and thick (0.6 mm) slices were obtained using a vibratome and pretreated with mACSF at 35–36 °C for 1 h prior to recordings. During perfusion with mACSF at 35–36 °C, spontaneous or self-sustained epileptiform field potentials following high-frequency stimulation were frequently recorded in slices pretreated with mACSF but not in those without the pretreatment. Seizure-like ictal discharges were more common in thick slices than in thin slices. Conclusions Prolonged exposure to mACSF by pretreatment and subsequent perfusion can induce epileptiform field potentials in mouse hippocampal slices.

scholarly journals Mecamylamine modulates epileptiform discharges in low-Mg2+ model of epilepsy

2021 ◽  
Vol 67 (1) ◽  
pp. 11-15
Author(s):  
O.S. Zapukhliak ◽  
◽  
D.S. Isaev ◽  
Keyword(s):  
Nicotinic Acetylcholine Receptors ◽  
Acetylcholine Receptors ◽  
Epileptiform Activity ◽  
Hippocampal Slices ◽  
Brain Slices ◽  
Network Activity ◽  
Therapeutic Effects ◽  
Nicotinic Acetylcholine ◽  
Beneficial Effects ◽  
Epileptiform Discharges

Mecamylamine is a nonselective antagonist of nicotinic acetylcholine receptors that was developed as an antihypertensive medication and is now being studied for its beneficial effects in several pathological conditions, such as substance abuse, depression, anxiety and epilepsy. In this work, we investigate the effect of mecamylamine on the manifestations of seizure-like activity evoked by perfusion of hippocampal slices with low-Mg2+ solution of artificial cerebrospinal fluid. Reducing Mg2+ concentration in extracellular solution induced two distinct types of epileptiform activity: recurring seizure-like activity and continuous discharges. Application of mecamylamine significantly increased internal frequency of recurring seizurelike activity and significantly decreased inter-event intervals between continuous discharges. We also show that mecamylamine significantly decreased internal frequency of continuous epileptiform discharges. The results of our work show that mecamylamine exerts modulatory effect on the low-Mg2+ epileptiform activity induced in hippocampal acute rat brain slices. Additionally, obtained results indicate the role of nicotinic acetylcholine receptors in the modulation of hippocampal network activity, which might explain some of the therapeutic effects of mecamylamine in CNS.

Enhanced NMDAR-dependent epileptiform activity is controlled by oxidizing agents in a chronic model of temporal lobe epilepsy

1996 ◽  
Vol 76 (6) ◽  
pp. 4185-4189 ◽  
Author(s):  
J. C. Hirsch ◽  
O. Quesada ◽  
M. Esclapez ◽  
H. Gozlan ◽  
Y. Ben-Ari ◽  
...  
Keyword(s):  
Synaptic Transmission ◽  
Ex Vivo ◽  
Epileptiform Activity ◽  
Hippocampal Slices ◽  
Nmda Antagonist ◽  
Pyramidal Cells ◽  
Resting Membrane Potential ◽  
Postsynaptic Potentials ◽  
Epileptiform Discharges ◽  
Late Part

1. Graded N-methyl-D-aspartate receptor (NMDAR)-dependent epileptiform discharges were recorded from ex vivo hippocampal slices obtained from rats injected a week earlier with an intracerebroventricular dose of kainic acid. Intracellular recordings from pyramidal cells of the CA1 area showed that glutamate NMDAR actively participated in synaptic transmission, even at resting membrane potential. When NMDAR were pharmacologically isolated, graded burst discharges could still be evoked. 2. The oxidizing reagent 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB, 200 microM, 15 min) suppressed the late part of the epileptiform burst that did not recover after wash but could be reinstated by the reducing agent tris (2-carboxyethyl) phosphine (TCEP, 200 microM, 15 min) and again abolished with the NMDA antagonist D-2-amino-5-phosphonovaleric acid (D-APV). 3. Pharmacologically isolated NMDAR-mediated responses were decreased by DTNB (56 +/- 10%, mean +/- SD, n = 6), an effect reversed by TCEP. 4. When only the fast glutamateric synaptic component was blocked, NMDA-dependent excitatory postsynaptic potentials (EPSPs) could be evoked despite the presence of underlying fast and slow inhibitory postsynaptic potentials (IPSPs). DTNB decreased EPSPs to 48 +/- 12% (n = 5) of control. 5. Since a decrease of the NMDAR-mediated response by +/- 50% is sufficient to suppress the late part of the burst, we suggest that epileptiform activity can be controlled by manipulation of the redox sites of NMDAR. Our observations raise the possibility of developing new anticonvulsant drugs that would spare alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-R (AMPAR)-mediated synaptic responses and decrease NMDAR-mediated synaptic transmission without blocking it completely.

scholarly journals Epileptiform Discharges With In-Vivo-Like Features in Slices of Rat Piriform Cortex With Longitudinal Association Fibers

2001 ◽  
Vol 86 (5) ◽  
pp. 2445-2460 ◽  
Author(s):  
Rezan Demir ◽  
Lewis B. Haberly ◽  
Meyer B. Jackson
Keyword(s):  
Seizure Activity ◽  
Epileptiform Activity ◽  
Piriform Cortex ◽  
Brain Slices ◽  
Epileptiform Discharges ◽  
Local Circuitry ◽  
Association Fibers ◽  
Discharge Propagation

Brain slices serve as useful models for the investigation of epilepsy. However, the preparation of brain slices disrupts circuitry and severs axons, thus complicating efforts to relate epileptiform activity in vitro to seizure activity in vivo. This issue is relevant to studies in transverse slices of the piriform cortex (PC), the preparation of which disrupts extensive rostrocaudal fiber systems. In these slices, epileptiform discharges propagate slowly and in a wavelike manner, whereas such discharges in vivo propagate more rapidly and jump abruptly between layers. The objective of the present study was to identify fiber systems responsible for these differences. PC slices were prepared by cutting along three different nearly orthogonal planes (transverse, parasagittal, and longitudinal), and epileptiform discharges were imaged with a voltage-sensitive fluorescent dye. Interictal-like epileptiform activity was enabled by either a kindling-like induction process or disinhibition with bicuculline. The pattern of discharge onset was very similar in slices cut in different planes. As described previously in transverse PC slices, discharges were initiated in the endopiriform nucleus (En) and adjoining regions in a two-stage process, starting with low-amplitude “plateau activity” at one site and leading to an accelerating depolarization and discharge onset at another nearby site. The similar pattern of onset in slices of various orientations indicates that the local circuitry and neuronal properties in and around the En, rather than long-range fibers, assume dominant roles in the initiation of epileptiform activity. Subtle variations in the onset site indicate that interneurons can fine tune the site of discharge onset. In contrast to the mode of onset, discharge propagation showed striking variations. In longitudinal slices, where rostrocaudal association fibers are best preserved, discharge propagation resembled in vivo seizure activity in the following respects: propagation was as rapid as in vivo and about two to three times faster than in other slices; discharges jumped abruptly between the En and PC; and discharges had large amplitudes in superficial layers of the PC. Cuts in longitudinal slices that partially separated the PC from the En eliminated these unique features. These results help clarify why epileptiform activity differs between in vitro and in vivo experiments and suggest that rostrocaudal pyramidal cell association fibers play a major role in the propagation of discharges in the intact brain. The longitudinal PC slice, which best preserves these fibers, is ideally suited for the study their role.

Spontaneous interictal-like activity originates in multiple areas of the CA2-CA3 region of hippocampal slices

1994 ◽  
Vol 71 (4) ◽  
pp. 1574-1585 ◽  
Author(s):  
L. V. Colom ◽  
P. Saggau
Keyword(s):  
Guinea Pig ◽  
Epileptiform Activity ◽  
Hippocampal Slices ◽  
Optical Recording ◽  
Objective Lens ◽  
Gamma Aminobutyric Acid ◽  
Subsequent Increase ◽  
Epileptiform Discharges ◽  
High K ◽  
Ca3 Region

1. The sites of origin of spontaneous interictal-like epileptiform activity in hippocampal slices from guinea pig, mouse, and rat were determined. A multisite fast optical recording technique using voltage-sensitive dyes and an array of 100 photodiodes was employed. The use of a low-magnification objective lens allowed the visualization of almost the entire transverse hippocampal slice. Three in vitro models of epilepsy were employed, utilizing different manipulations of the bath perfusion medium to induce epileptiform activity: 1) raising the external potassium (K+) concentration, 2) adding the potassium channel blocker 4-aminopyridine (4-AP), and 3) adding antagonists of gamma-aminobutyric acid-A (GABAA) receptors (bicuculline and picrotoxin, BIC-PTX). 2. Spontaneous epileptiform discharges were detected in each subfield of cornu ammonis (CA) but not in the dentate gyrus (DG) of each studied species. Preliminary experiments confirmed that interictal-like epileptiform activity originated in the CA2-CA3 region. Ictal-like activity was never observed in our experiments. 3. In the guinea pig, when GABAA antagonists were employed, the site of origin of spontaneous epileptiform discharges was consistently located in the CA2-CA3a region. When high K+ or 4-AP was used, this region was the most frequent site of origin. Subsequent epileptiform discharges with similar sites of origin occasionally invaded different areas of the CA2-CA3 region, revealing a variable area of occupance of epileptiform discharges. 4. In the mouse and rat, the site of origin of spontaneous discharges was invariably located in the CA3b-CA3c region independent of the epilepsy model. 5. In both the guinea pig and rat, when the CA2-CA3a region was surgically separated from the CA3b-CA3c region, independent discharges were observed in both regions. Areas that could generate discharges only under certain epileptogenic conditions were found in these species (potential sites of origin). Two independent sites of origin with different propagation patterns and area of occupance were occasionally observed within the CA2-CA3a region. 6. In the guinea pig, such lesions demonstrated that both regions can independently generate epileptiform discharges at different frequencies. When high K+ or 4-AP was employed, epileptiform activity was observed in both regions. Although BIC-PTX only generated discharges in the CA2-CA3a region, a subsequent increase in K+ induced additional discharges in the CA3b-CA3c region, revealing a potential site of origin. 7. In rat hippocampal slices with such lesions, spontaneous epileptiform discharges were observed in both CA2-CA3a and CA3b-CA3c region when 4-AP was employed.(ABSTRACT TRUNCATED AT 400 WORDS)

scholarly journals SYNCHRONIZATION OF EPILEPTIFORM ACTIVITY BETWEEN CA1 AND CA3 HIPPOCAMPAL FIELDS UNDER SYNAPTIC AND NON-SYNAPTIC CONDITIONS IN RAT BRAIN SLICES

2020 ◽  
Vol 16 (1) ◽  
pp. 3-7
Author(s):  
O.S. Zapukhliak ◽  
O.V. Netsyk ◽  
D.S. Isaev
Keyword(s):  
Epileptiform Activity ◽  
Hippocampal Slices ◽  
Brain Slices ◽  
Neural Net ◽  
Cellular Interactions ◽  
Analog To Digital ◽  
Cross Correlation Analysis ◽  
Low Pass ◽  
Hippocampal Ca3 ◽  
High Level

Relevance. Over-synchronization of neuronal activity results in epileptic-like discharges that can lead to seizures and status epilepticus. Understanding mechanisms of neural net synchronization could provide new insights into the treatment of epileptic disorders. Objective: to compare the levels of synchronization between CA3 and CA1 hippocampal zones during epileptiform activity induced under synaptic and non-synaptic conditions. Materials and Methods. Transverse brain slices were obtained from 12-14 days old rats. For induction of epileptiform activity common pro-epileptic agents were used: bicuculline and 4-aminopiridine. Nonsynaptic epileptiform activity was induced by perfusion brain slices with low-Ca2+ and Cd2+-containing artificial cerebrospinal fluid (aCSF). Simultaneous extracellular recordings of field potentials were obtained from the CA3 and CA1 pyramidal cell layer with extracellular glass microelectrodes (2–3 MΩ). Signals were then low-pass filtered (kHz), amplified using a 2-channel differential amplifier M1800, digitized at 10 kHz using analog-to-digital converter. The level of synchronization between CA3 and CA1 was evaluated using cross-correlation analysis.  Results: Perfusion hippocampal slices with bicuculline and 4-aminopyridine induced epileptiform activity with high level of synchronization between CA3 and CA1 hippocampal zones. Removing Ca2+ from extracellular solution as well as adding CdCl2 to the perfusion aCSF induced epileptiform activity that was not synchronized between hippocampal CA3 and CA1 fields.  Conclusions: Synaptic interaction account for high level of CA3-CA1 synchronization induced by pro-epileptic agents bicuculline and 4-aminopiridine. Under non-synaptic conditions, local cellular interactions induce epileptiform activity with no synchronization between CA3 and CA1 hippocampal zones.

Cesium Induces Spontaneous Epileptiform Activity Without Changing Extracellular Potassium Regulation in Rat Hippocampus

1999 ◽  
Vol 82 (6) ◽  
pp. 3339-3346 ◽  
Author(s):  
Zhi-Qi Xiong ◽  
Janet L. Stringer
Keyword(s):  
Nmda Receptor ◽  
Dentate Gyrus ◽  
Extracellular Space ◽  
Hippocampal Neurons ◽  
Epileptiform Activity ◽  
Hippocampal Slices ◽  
Nmda Receptor Antagonist ◽  
Extracellular Potassium ◽  
Epileptiform Discharges ◽  
Cultured Hippocampal Neurons

Cesium has been widely used to study the roles of the hyperpolarization-activated (Ih) and inwardly rectifying potassium (KIR) channels in many neuronal and nonneuronal cell types. Recently, extracellular application of cesium has been shown to produce epileptiform activity in brain slices, but the mechanisms for this are not known. It has been proposed that cesium blocks the KIR in glia, resulting in an abnormal accumulation of potassium in the extracellular space and inducing epileptiform activity. This hypothesis has been tested in hippocampal slices and cultured hippocampal neurons using potassium-sensitive microelectrodes. In the present study, application of cesium produced spontaneous epileptiform discharges at physiological extracellular potassium concentration ([K+]o) in the CA1 and CA3 regions of hippocampal slices. This epileptiform activity was not mimicked by increasing the [K+]o. The epileptiform discharges induced by cesium were not blocked by the N-methyl-d- aspartate (NMDA) receptor antagonist AP-5, but were blocked by the non-NMDA receptor antagonist CNQX. In the dentate gyrus, cesium induced the appearance of spontaneous nonsynaptic field bursts in 0 added calcium and 3 mM potassium. Moreover, cesium increased the frequency of field bursts already present. In contrast, ZD-7288, a specific Ihblocker, did not cause spontaneous epileptiform activity in CA1 and CA3, nor did it affect the field bursts in the dentate gyrus, suggesting that cesium induced epileptiform activity is not directly related to blockade of the Ih. When potassium-sensitive microelectrodes were used to measure [K+]o, there was no significant increase in [K+]o in CA1 and CA3 after cesium application. In the dentate gyrus, cesium did not change the baseline level of [K+]o or the rate of [K+]o clearance after the field bursts. In cultured hippocampal neurons, which have a large and relatively unrestricted extracellular space, cesium also produced cellular burst activity without significantly changing the resting membrane potential, which might indicate an increase in [K+]o. Our results suggest that cesium causes epileptiform activity by a mechanism unrelated to an alteration in [K+]o regulation.

scholarly journals Excitatory GABAergic signalling is associated with acquired benzodiazepine resistance in status epilepticus

2018 ◽  
Author(s):  
Richard J. Burman ◽  
Joshua S. Selfe ◽  
John Hamin Lee ◽  
Maurits van den Burg ◽  
Alexandru Calin ◽  
...  
Keyword(s):  
Status Epilepticus ◽  
Epileptiform Activity ◽  
Brain Slices ◽  
Medical Emergency ◽  
Dependent Manner ◽  
High Concentration ◽  
Cellular Mechanisms ◽  
Epileptiform Discharges ◽  
Threatening Condition ◽  
Eeg Recordings

AbstractStatus epilepticus (SE) is defined as a state of unrelenting seizure activity. Generalised convulsive SE is associated with a rapidly rising mortality rate, and thus constitutes a medical emergency. Benzodiazepines, which act as positive modulators of chloride (Cl-) permeable GABAA receptors, are indicated as first-line treatment, but this is ineffective in many cases. We found that 48% of children presenting with SE were unresponsive to benzodiazepine treatment, and critically, that the duration of SE at the time of treatment is an important predictor of non-responsiveness. We therefore investigated the cellular mechanisms that underlie acquired benzodiazepine resistance, using rodent organotypic and acute brain slices. Removing Mg2+ ions leads to an evolving pattern of epileptiform activity, and eventually to a persistent state of repetitive discharges that strongly resembles clinical EEG recordings of SE. We found that diazepam loses its antiseizure efficacy and conversely exacerbates epileptiform activity during this stage of SE-like activity. Interestingly, a low concentration of the barbiturate phenobarbital had a similar exacerbating effect on SE-like activity, whilst a high concentration of phenobarbital was effective at reducing or preventing epileptiform discharges. We then show that the persistent SE-like activity is associated with a reduction in GABAA receptor conductance and Cl- extrusion capability. We explored the effect on intraneuronal Cl- using both gramicidin, perforated-patch clamp recordings and Cl- imaging. This showed that during SE-like activity, reduced Cl- extrusion capacity was further exacerbated by activity-dependent Cl- loading, resulting in a persistently high intraneuronal Cl-. Consistent with these results, we found that optogenetic stimulation of GABAergic interneurons in the SE-like state, actually enhanced epileptiform activity in a GABAAR dependent manner. Together our findings describe a novel potential mechanism underlying benzodiazepine-resistant SE, with relevance to how this life-threatening condition should be managed in the clinic.

Burst characteristics of dentate gyrus granule cells: evidence for endogenous and nonsynaptic properties

1996 ◽  
Vol 75 (1) ◽  
pp. 124-132 ◽  
Author(s):  
E. Pan ◽  
J. L. Stringer
Keyword(s):  
Dentate Gyrus ◽  
Action Potentials ◽  
Granule Cells ◽  
Epileptiform Activity ◽  
Hippocampal Slices ◽  
Current Injection ◽  
Single Action ◽  
Epileptiform Discharges

1. Hippocampal slices bathed in 8 mM potassium and 0-added calcium exhibited spontaneous epileptiform activity in the dentate gyrus. Extracellular recording revealed recurrent prolonged bursts of population spikes and an associated negative DC shift. These episodes were very similar to the in vivo phenomenon termed maximal dentate activation (MDA). Therefore this in vitro activity will be referred to as MDA-like activity or events. 2. During the MDA-like activity, the individual granule cells exhibited a sustained depolarization that matched the duration of the negative extracellular DC shift. At the beginning of the MDA-like activity, there was a burst of action potentials. After the burst, most granule cells either continued to fire action potentials regularly or in bursts. Some cells exhibited this initial burst of activity and then a dramatic reduction in firing rate. This reduction in rate was followed by a gradual increase in the amplitude and frequency of the epileptiform activity recorded during the remainder of the MDA-like event. 3. Before and between MDA-like events, spontaneous cellular activity consisted of single action potentials and bursts of action potentials on a depolarizing envelope. In addition, depolarizing potentials, up to 13 mV, were recorded. There were no extracellular field potentials associated with these intracellularly recorded potentials. 4. In the 8 mM potassium, 0-added calcium test solution, the membrane potential threshold for burst production was significantly lower than in normal potassium and calcium medium. 5. The effect of depolarizing and hyperpolarizing current injections on the amplitude and frequency of the epileptiform activity was tested. Current injection had no effect on the frequency of the epileptiform activity recorded during the MDA-like events. However, the frequency of the cellular bursts between MDA-like events was very sensitive to current injection. Depolarizing current increased the frequency, and hyperpolarizing current decreased the frequency of the spontaneous activity. 6. This study has shown that in 8 mM potassium and 0-added calcium the granule cells of the dentate gyrus are capable of generating spontaneous bursts that appear to be mediated by endogenous mechanisms. In addition, synchronized epileptiform discharges were recorded from the granule cells at regular intervals that appear were recorded from the granule cells at regular intervals that appear to be mediated by exogenous nonsynaptic mechanisms.

Ictal Epileptiform Activity in the CA3 Region of Hippocampal Slices Produced by Pilocarpine

1998 ◽  
Vol 79 (6) ◽  
pp. 3019-3029 ◽  
Author(s):  
Paul A. Rutecki ◽  
Yili Yang
Keyword(s):  
Epileptiform Activity ◽  
Hippocampal Slices ◽  
Reversal Potential ◽  
Extracellular Potassium ◽  
Muscarinic Agonist ◽  
Epileptiform Discharges ◽  
Artificial Cerebrospinal Fluid ◽  
Discharge Duration ◽  
Interictal Discharges ◽  
Ca3 Region

Rutecki, Paul A. and Yili Yang. Ictal epileptiform activity in the CA3 region of hippocampal slices produced by pilocarpine. J. Neurophysiol. 79: 3019–3029, 1998. Pilocarpine, a muscarinic agonist, produces status epilepticus that is associated with the later development of chronic recurrent seizures. When applied to rat hippocampal slices, pilocarpine (10 μM) produced brief (<200 ms) epileptiform discharges that resembled interictal activity that occurs between seizures, as well as more prolonged synchronous neuronal activation that lasted seconds (3–20 s), and was comparable to ictal or seizures-like discharges. We assessed the factors that favored ictal patterns of activity and determined the biophysical properties of the ictal discharge. The probability of observing ictal discharges was increased when extracellular potassium ([K+]o) was increased from 5 to 7.5 mM. Raising [K+]o to 10 mM resulted in loss of ictal patterns and, in 20 of 34 slices, desynchronization of epileptiform activity. Making the artificial cerebrospinal fluid (ACSF) hyposmotic favored ictal discharges at 5 mM [K+]o, but shifted 7.5 mM [K+]o ACSF patterns to interictal discharges or desynchronized activity. Conversely, increasing osmolality suppressed ictal patterns. The pilocarpine-induced ictal discharges were blocked by atropine (1 μM, n = 5), a muscarinic antagonist, and pirenzepine (1 μM, n = 6), a selective M1 receptor antagonist. Kainate/α-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid receptor blockade stopped all epileptiform activity ( n = 8). The N-methyl-d-aspartate antagonist d,l-2-amino-5-phosphonovaleric acid (100 μM, n = 34) did not change the pattern of epileptiform activity but significantly increased the rate of interictal discharges and prolonged the duration of ictal discharges. The ictal discharge was characterized intracellularly by a depolarization that was associated with action potential generation and persisted as a membrane oscillation of 4–10 Hz. The ictal oscillations reversed in polarity at −22.7 ± 2.2 mV ( n = 11) with current-clamp recordings and −20.9 ± 3.1 mV ( n = 7) with voltage-clamp recordings. The reversal potential of the ictal discharge in the presence of the γ-aminobutyric acid-A blocker bicuculline (10 μM, n = 6) was −2.2 ± 2.6 mV and was significantly different from that measured without bicuculline. Bicuculline added to 7.5 mM [K+]o and 10 μM pilocarpine did not cause epileptiform activity to change pattern but significantly increased the rate of interictal discharges and prolonged the ictal discharge duration. Both synaptic and nonsynaptic mechanisms are important for the generation of ictal patterns of epileptiform activity. Although the synchronous epileptiform activity produced by pilocarpine required fast glutamate-mediated synaptic transmission, the transition from an interictal to ictal pattern of activity depended on [K+]o and could be influenced by extracellular space.

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