Nonsynaptic Epileptiform Activity Research Articles

Differential role of oxidative stress in synaptic and nonsynaptic in vitro ictogenesis.

It is well established that dysfunctional glucose metabolism and in particular hypoglycemia can lead to hyperexcitability and exacerbate epileptic seizures. The precise mechanisms behind this form of hyperexcitability are still unresolved. The present study investigates to what extent oxidative stress can account for the acute proconvulsant effect of hypoglycemia. We used the glucose derivative 2-deoxy-d-glucose (2-DG) to mimic glucose deprivation in hippocampal slices during the extracellular recording of interictal-like (IED) and seizure-like (SLE) epileptic discharge in areas CA3 and CA1. After induction of IED in area CA3 by perfusion of Cs+ (3 mM), MK801 (10 µM), and bicuculline (10 µM), subsequent application of 2-DG (10 mM) resulted in the appearance of SLE in 78.3% of experiments. This effect was only observed in area CA3 and was reversibly blocked by tempol (2 mM), a scavenger of reactive oxygen species, in 60% of experiments. Preincubation with tempol reduced the incidence of 2-DG-induced SLE to 40%. Low-Mg2+-induced SLE in area CA3 and in the entorhinal cortex (EC) was also reduced by tempol. In contrast, to the above models, which depend on synaptic transmission, nonsynaptic epileptiform field bursts induced in area CA3 by a combination of Cs+ (5 mM) and Cd2+ (200 µM), or in area CA1 using the "low-Ca2+ model," was unaffected or even enhanced by tempol. These results indicate that oxidative stress significantly contributes to 2-DG-induced seizures in area CA3 and that the impact of oxidative stress differs between synaptic and nonsynaptic ictogenesis.NEW & NOTEWORTHY The main findings of the current study are that area CA3 but not area CA1 can support 2-DG-induced seizure activity, that oxidative stress significantly contributes to 2-DG-induced seizure activity in area CA3, and that the impact of oxidative stress differs between synaptic and nonsynaptic epileptiform activity. In in vitro models where ictogenesis depends on synaptic interactions, oxidative stress lowers the seizure threshold, whereas in nonsynaptic models seizure threshold is unchanged or even increased.

Nonsynaptic epileptiform activity in CA3-CA1 regions of the hippocampus in low-Ca2+ and Cd2+-containing milieu

Nonsynaptic epileptiform activity in CA3-CA1 regions of the hippocampus in low-Ca2+ and Cd2+-containing milieu

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

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.

Self-propagating, non-synaptic epileptiform activity recruits neurons by endogenous electric fields

It is well documented that synapses play a significant role in the transmission of information between neurons. However, in the absence of synaptic transmission, neural activity has been observed to continue to propagate. Previous studies have shown that propagation of epileptiform activity takes place in the absence of synaptic transmission and gap junctions and is outside the range of ionic diffusion and axonal conduction. Computer simulations indicate that electric field coupling could be responsible for the propagation of neural activity under pathological conditions such as epilepsy. Electric fields can modulate neuronal membrane voltage, but there is no experimental evidence suggesting that electric field coupling can mediate self-regenerating propagation of neural activity. Here we examine the role of electric field coupling by eliminating all forms of neural communications except electric field coupling with a cut through the neural tissue. We show that 4-AP induced activity generates an electric field capable of recruiting neurons on the distal side of the cut. Experiments also show that applied electric fields with amplitudes similar to endogenous values can induce propagating waves. Finally, we show that canceling the electrical field at a given point can block spontaneous propagation. The results from these in vitro electrophysiology experiments suggest that electric field coupling is a critical mechanism for non-synaptic neural propagation and therefore could contribute to the propagation of epileptic activity in the brain.

Celecoxib Ameliorates Seizure Susceptibility in Autosomal Dominant Lateral Temporal Epilepsy.

Autosomal dominant lateral temporal epilepsy (ADLTE) is an inherited syndrome caused by mutations in the leucine-rich glioma inactivated 1 (LGI1) gene. It is known that glutamatergic transmission is altered in LGI1 mutant mice, and seizures can be reduced by restoring LGI1 function. Yet, the mechanism underlying ADLTE is unclear. Here, we propose that seizures in male LGI1-/- mice are due to nonsynaptic epileptiform activity in cortical neurons. We examined the intrinsic excitability of pyramidal neurons in the temporal cortex of male LGI1-/- mice and found that the voltage-gated K+ channel Kv1.2 was significantly downregulated. We also found that cytosolic phospholipase A2 (cPLA2)-cyclooxygenase 2 (Cox2) signaling was enhanced in LGI1-/- mice. Interestingly, Cox2 inhibition effectively restored the dysregulated Kv1.2 and reduced the intrinsic excitability of pyramidal neurons. Moreover, in vivo injection of celecoxib, an FDA-approved nonsteroidal anti-inflammatory drug, rescued the defective Kv1.2 (an ∼1.9-fold increase), thereby alleviating the seizure susceptibility and extending the life of LGI1-/- mice by 5 d. In summary, we conclude that LGI1 deficiency dysregulates cPLA2-Cox2 signaling to cause hyperexcitability of cortical pyramidal neurons, and celecoxib is a potential agent to manage human ADLTE.SIGNIFICANCE STATEMENT Haploinsufficiency of the leucine-rich glioma inactivated 1 (LGI1) gene is the major pathogenic basis for ADLTE, an inherited syndrome with no cure to date. Existing studies suggest that altered glutamatergic transmission in the hippocampus causes this disease, but the data are paradoxical. We demonstrate that the loss of LGI1 decreases Kv1.2 expression, enhances intrinsic excitability, and thereby causes epilepsy. Interestingly, for the first time, we show that an FDA-approved drug, celecoxib, rescues the Kv1.2 defect and alleviates seizure susceptibility in LGI1-/- mice, as well as improving their survival. Thus, we suggest that celecoxib is a promising drug for the treatment of ADLTE patients.

Antiepileptičko dejstvo magnezijuma

Magnesium is a bioessential mineral. Magnesium ion is an important regulator of multiple ion channels' function. Magnesium has many neuroactive effects at cellular, network and whole body level. Magnesium shows stabilizing effects on electrophysiological properties and electrical activity of nerve cells and suppresses pathophysiological neuronal hyperexcitability. Magnesium deficiency has epileptogenic effect and its recompense - antiepileptic potential. Anticonvulsive action of MgSO4 infusions is clinically best known for the control of eclamptic convulsions. Under experimental conditions Mg2+ also exerts inhibitory effect on animal models of nonsynaptic epileptiform activity. Perspectives of future translational research on magnesium in epileptology point to potential benefit from magnesium therapy for better seizure control in refractory epilepsy.

Long-term alcohol exposure elicits hippocampal nonsynaptic epileptiform activity changes associated with expression and functional changes in NKCC1, KCC2 co-transporters and Na+/K+-ATPase

Nonsynaptic mechanism changes, particularly the enhancement of NKCC1 expression in the dentate gyrus (DG) after 4weeks of ethanol consumption, motivate the present work, in which rats were submitted to a period of chronic consumption (12weeks). Four groups of six animals (6-week-old male Wistar rats) were formed, including the control (C), ethanol 1 (E1), ethanol 2 (E2) and ethanol 3 (E3) groups. The rats in the E1, E2 and E3 groups were treated daily with a 30% v/v solution of ethanol, administered via oral gavage (1.0, 2.0 and 3.0g/kg, respectively). Nonsynaptic epileptiform activities (NEA) were induced by means of the zero-Ca2+ and high-K+ model using hippocampal slices and were recorded in the DG. The presence of NKCC1, KCC2, α1-Na+/K+-ATPase and GFAP immunoreactivity was analyzed. The results demonstrate that alcohol consumption changes NEA, and these changes are more prominent at the lower dosage. An increase in the DC shifts associated with epileptiform discharges was present with the low dose. This increase was correlated with the increment of NKCC1 expression. Confocal microscopy images indicate the NKCC1 increase was pronounced in the initial axonal segment of granule cells. The blockage of these cotransporters during NEA induction with bumetanide suppressed the DC shift increase and diminished all parameters of NEA that were quantified for all groups treated with ethanol. Therefore, the increase in NKCC1 expression and the effective activity of this cotransporter, which were observed in the treated groups, suggest that drugs that act for block NKCC1 represent promising strategies for diminishing the effects of alcohol damage on the brain.

Surface charge impact in nonsynaptic model of epilepsy in rat hippocampus.

Decreasing of surface charge screening near voltage-gated ion channels via reduction of extracellular cation divalent ions provide potent mechanism of altering cellular excitability and seizure threshold. Spontaneous field potentials were recorded from horizontal brain slices of young Wistar rats (postnatal day 10-12). Extracellular registrations wereobtained from CA1 and CA3 area of hippocampus. For induction of nonsynaptic epileptiform activity slices were perfused with artificial cerebrospinal fluid with omitted Ca2+and Mg2+ ions. Effect of different Mg2+ concentration (1, 2, and 3mmol/l) on initial stage of nonsynaptic epileptiform discharges was studied. Our results suggest that the change in Mg2+ concentration dramatically affects the probability of induction of low-Ca2+seizure-like activity (SLA), providing evidence that Mg2+ can alter cerebral excitability by affecting the surface charge and supporting the idea that surface charge could be a pharmacological target for anti-epileptic treatment.

PH during non-synaptic epileptiform activity—computational simulations

The excitability of neuronal networks is strongly modulated by changes in pH. The origin of these changes, however, is still under debate. The high complexity of neural systems justifies the use of computational simulation to investigate mechanisms that are possibly involved. Simulated neuronal activity includes non-synaptic epileptiform events (NEA) induced in hippocampal slices perfused with high-K+ and zero-Ca2+, therefore in the absence of the synaptic circuitry. A network of functional units composes the NEA model. Each functional unit represents one interface of neuronal/extracellular space/glial segments. Each interface contains transmembrane ionic transports, such as ionic channels, cotransporters, exchangers and pumps. Neuronal interconnections are mediated by gap-junctions, electric field effects and extracellular ionic fluctuations modulated by extracellular electrodiffusion. Mechanisms investigated are those that change intracellular and extracellular ionic concentrations and are able to affect [H+]. Our simulations suggest that the intense fluctuations in intra and extracellular concentrations of Na+, K+ and Cl− that accompany NEA are able to affect the combined action of the Na+/H+ exchanger (NHE), /Cl− exchanger (HCE), H+ pump and the catalytic activity of intra and extracellular carbonic anhydrase. Cellular volume changes and extracellular electrodiffusion are responsible for modulating pH.

Enhanced nonsynaptic epileptiform activity in the dentate gyrus after kainate-induced status epilepticus

Understanding the mechanisms that influence brain excitability and synchronization provides hope that epileptic seizures can be controlled. In this scenario, non-synaptic mechanisms have a critical role in seizure activity. The contribution of ion transporters to the regulation of seizure-like activity has not been extensively studied. Here, we examined how non-synaptic epileptiform activity (NEA) in the CA1 and dentate gyrus (DG) regions of the hippocampal formation were affected by kainic acid (KA) administration. NEA enhancement in the DG and suppression in area CA1 were associated with increased NKCC1 expression in neurons and severe neuronal loss accompanied by marked glial proliferation, respectively. Twenty-four hours after KA, the DG exhibited intense microglial activation that was associated with reduced cell density in the infra-pyramidal lamina; however, cellular density recovered 7days after KA. Intense Ki67 immunoreactivity was observed in the subgranular proliferative zone of the DG, which indicates new neuron incorporation into the granule layer. In addition, bumetanide, a selective inhibitor of neuronal Cl− uptake mediated by NKCC1, was used to confirm that the NKCC1 increase effectively contributed to NEA changes in the DG. Furthermore, 7days after KA, prominent NKCC1 staining was identified in the axon initial segments of granule cells, at the exact site where action potentials are preferentially initiated, which endowed these neurons with increased excitability. Taken together, our data suggest a key role of NKCC1 in NEA in the DG.

008 — (BEZ0143) Alcohol alters the nonsynaptic epileptiform activity in male offspring of rats subjected to the alcoholization regimen

(unpublished). It is known that the piriform cortex and amygdala are also activated during epileptic seizures. Because of TMT biological relevance when compared with toluene, coupled to the common circuitry activation in fear and seizures, TMT may represent an interesting alternative to study if the mechanisms involved in fear generation are capable of inhibiting seizures. In the present study, our goal and first step toward seizure inhibition protocol was to verify the appropriate concentration of TMT nanoemulsion that would produce behavioral effects in rats. Methods: Nine male Wistar rats were handled for 3 consecutive days. On the third day, animals received directly into the right nasal epithelium 10 μl of 0.9% saline (SAL), and on the fourth day, animals received 10 μl of vehicle (VEH) or TMT nanoemulsion — 38.7 nmol or 300 nmol. After the olfactory exposure, animals were recorded for 2 min for behavioral analyses — ethology assessment. All behaviors and their interactions were evaluated using ETHOMATIC software (chi-square analyses) [Ethics Committee Approval (172/2010)]. Results: There was an increase of interactions between exploratory behaviors as well as between grooming behaviors in VEH when compared with SAL (p b 0.05). The TMT 38.7 nmol promoted interactions between grooming behaviors that were not detected when SAL was applied, but exploratory items showed only slight differences. The TMT 300 nmol induced an expressive reduction in all interactions between behavioral items in relation to SAL. Discussion/conclusions: The results showed that the higher concentration of TMT was the most effective because there was a reduction of the behavioral activity when compared with VEH and TMT 38.7 nmol. This is in accordance with what is expected as part of fear reactions. On the other hand, VEH raised these activities, suggesting that the VEH is not innocuous to the animal but promotes a different response than the one induced by TMT. The next step is to increase the number of animals per group to confirm behavioral data and evaluate the higher TMT concentration in the WAR strain and amygdala rapid electrical kindling.

Alcohol Abuse Promotes Changes in Non-Synaptic Epileptiform Activity with Concomitant Expression Changes in Cotransporters and Glial Cells

Non-synaptic mechanisms are being considered the common factor of brain damage in status epilepticus and alcohol intoxication. The present work reports the influence of the chronic use of ethanol on epileptic processes sustained by non-synaptic mechanisms. Adult male Wistar rats administered with ethanol (1, 2 e 3 g/kg/d) during 28 days were compared with Control. Non-synaptic epileptiform activities (NEAs) were induced by means of the zero-calcium and high-potassium model using hippocampal slices. The observed involvement of the dentate gyrus (DG) on the neurodegeneration promoted by ethanol motivated the monitoring of the electrophysiological activity in this region. The DG regions were analyzed for the presence of NKCC1, KCC2, GFAP and CD11b immunoreactivity and cell density. The treated groups showed extracellular potential measured at the granular layer with increased DC shift and population spikes (PS), which was remarkable for the group E1. The latencies to the NEAs onset were more prominent also for the treated groups, being correlated with the neuronal loss. In line with these findings were the predispositions of the treated slices for neuronal edema after NEAs induction, suggesting that restrict inter-cell space counteracts the neuronal loss and subsists the hyper-synchronism. The significant increase of the expressions of NKCC1 and CD11b for the treated groups confirms the existence of conditions favorable to the observed edematous necrosis. The data suggest that the ethanol consumption promotes changes on the non-synaptic mechanisms modulating the NEAs. For the lower ethanol dosage the neurophysiological changes were more effective suggesting to be due to the less intense neurodegenertation.

Effect of co-transporter blockers on non-synaptic epileptiform activity—computational simulation

The important role of cation-chloride co-transporters in epilepsy is being supported by an increasing number of investigations. However, enormous complexity is involved since the action of these co-transporters has effects on the ionic homeostasis influencing directly the neuronal excitability and the tissue propensity to sustain seizure. To unravel the complex mechanisms involving the co-transporters action during seizure, this paper shows simulations of non-synaptic epileptiform activity and the effect of the blockage of the two different types of cation-chloride co-transporters present in the brain: Na, K and 2Cl co-transporter (NKCC) and K and Cl co-transporter (KCC). The simulations were performed with an electrochemical model representing the non-synaptic structure of the granule cell layer of the dentate gyrus (DG) of the rat hippocampus. The simulations suggest: (i) the potassium clearance is based on the systemic interplay between the Na/K pump and the NKCC co-transporters; (ii) the simultaneous blockage of the NKCC of the neurons and KCC of glial cells acts efficiently suppressing the epileptiform activities; and (iii) the simulations show that depending on the combined blockage of the co-transporters, the epileptiform activities may be suppressed or enhanced.

Firing patterns and synchronization in nonsynaptic epileptiform activity: the effect of gap junctions modulated by potassium accumulation

Several lines of evidence point to the modification of firing patterns and of synchronization due to gap junctions (GJs) as having a role in the establishment of epileptiform activity (EA). However, previous studies consider GJs as ohmic resistors, ignoring the effects of intense variations in ionic concentration known to occur during seizures. In addition to GJs, extracellular potassium is regarded as a further important factor involved in seizure initiation and sustainment. To analyze how these two mechanisms act together to shape firing and synchronization, we use a detailed computational model for in vitro high-K+ and low-Ca2+ nonsynaptic EA. The model permits us to explore the modulation of electrotonic interactions under ionic concentration changes caused by electrodiffusion in the extracellular space, altered by tortuosity. In addition, we investigate the special case of null GJ current. Increased electrotonic interaction alters bursts and action potential frequencies, favoring synchronization. The particularities of pattern changes depend on the tortuosity and array size. Extracellular potassium accumulation alone modifies firing and synchronization when the GJ coupling is null.

Mechanistic hypotheses for nonsynaptic epileptiform activity induction and its transition from the interictal to ictal state—Computational simulation

The aim of this work is to study, by means of computational simulations, the induction and sustaining of nonsynaptic epileptiform activity. The computational model consists of a network of cellular bodies of neurons and glial cells connected to a three-dimensional (3D) network of juxtaposed extracellular compartments. The extracellular electrodiffusion calculation was used to simulate the extracellular potential. Each cellular body was represented in terms of the transmembrane ionic transports (Na(+)/K(+) pumps, ionic channels, and cotransport mechanisms), the intercellular electrodiffusion through gap-junctions, and the neuronal interaction by electric field and the variation of cellular volume. The computational model allows simulating the nonsynaptic epileptiform activity and the extracellular potential captured the main feature of the experimental measurements. The simulations of the concomitant ionic fluxes and concentrations can be used to propose the basic mechanisms involved in the induction and sustaining of the activities. The simulations suggest: The bursting induction is mediated by the Cl(-) Nernst potential overcoming the transmembrane potential in response to the extracellular [K(+)] increase. The burst onset is characterized by a critical point defined by the instant when the Na(+) influx through its permeable ionic channels overcomes the Na(+)/K(+) pump electrogenic current. The burst finalization is defined by another critical point, when the electrogenic current of the Na(+)/K(+) pump overcomes its influx through the channels.

KCNQ/Kv7 Channel Regulation of Hippocampal Gamma-Frequency Firing in the Absence of Synaptic Transmission

Synchronous neuronal firing can be induced in hippocampal slices in the absence of synaptic transmission by lowering extracellular Ca2+ and raising extracellular K+. However, the ionic mechanisms underlying this nonsynaptic synchronous firing are not well understood. In this study we have investigated the role of KCNQ/Kv7 channels in regulating this form of nonsynaptic bursting activity. Incubation of rat hippocampal slices in reduced (<0.2 mM) [Ca2+]o and increased (6.3 mM) [K+]o, blocked synaptic transmission, increased neuronal firing, and led to the development of spontaneous periodic nonsynaptic epileptiform activity. This activity was recorded extracellularly as large (4.7 +/- 1.9 mV) depolarizing envelopes with superimposed high-frequency synchronous population spikes. These intraburst population spikes initially occurred at a high frequency (about 120 Hz), which decayed throughout the burst stabilizing in the gamma-frequency band (30-80 Hz). Further increasing [K+]o resulted in an increase in the interburst frequency without altering the intraburst population spike frequency. Application of retigabine (10 microM), a Kv7 channel modulator, completely abolished the bursts, in an XE-991-sensitive manner. Furthermore, application of the Kv7 channel blockers, linopirdine (10 microM) or XE-991 (10 microM) alone, abolished the gamma frequency, but not the higher-frequency population spike firing observed during low Ca2+/high K+ bursts. These data suggest that Kv7 channels are likely to play a role in the regulation of synchronous population firing activity.

Propagation of low calcium non-synaptic induced epileptiform activity to the contralateral hippocampus in vivo

Recent experiments show that non-synaptic epileptiform activity can be induced by high K + and low Ca 2+ solution in vivo in the hippocampal CA1 region when synaptic transmission is blocked. However, the ability of this type of epileptiform activity to propagate to other brain areas is unknown. Presumably, this epileptiform activity should propagate and project along the axons to remote brain areas. This hypothesis was tested in vivo by inducing non-synaptic seizures in the left hippocampus and by recording spontaneous and evoked field potentials in both left and right hippocampi. The results show that one type of non-synaptic epileptiform activity, late bursts, observed in the left exposed CA1 and CA3 regions could propagate to the contralateral intact CA1 and induce seizures with onsets of high-frequency rhythm. A cut of the commissural fibers near the midline of the brain prevented this propagation. In addition, the measurement of time delays between the exposed left CA3 and contralateral right CA1, as well as between the two recording electrodes in the right CA1, showed that the burst activity propagated through the commissural pathways. Experimental data also showed that these late bursts in the left hippocampus were first generated in the Schaffer collaterals of the CA1 region, traveled to the ipsilateral CA3 region and then propagated through the commissural fibers to the other side. These results suggest that non-synaptic epileptiform activity can propagate along axon projections to intact brain area causing seizure activity. This non-synaptic activity propagating through axonal pathway provides a possible mechanism for the generation of high-frequency low-amplitude onset activity observed commonly in human epileptic EEGs.

Role of potassium lateral diffusion in non-synaptic epilepsy: A computational study

An increase of extracellular potassium ion concentration can result in neuronal hyperexcitability, and thus contribute to non-synaptic epileptiform activity. It has been shown that potassium lateral diffusion alone is sufficient for synchronization in the low-calcium epilepsy in-vitro model. However, it is not yet known whether the lateral diffusion can, by itself, induce seizure activity. We hypothesize that spontaneous sustained neuronal activity can be generated by potassium coupling between neurons. To test this hypothesis, neuronal simulations with 2-cell or 4-cell models were used. Each model neuron was embedded in a bath of K + and surrounded by interstitial space. Interstitial potassium concentration was regulated by both K +-pump and glial buffer mechanisms. Simulations performed with two coupled neurons with parameter values within physiological range show that, without chemical and electrical synapses, potassium lateral diffusion alone can generate and synchronize zero-Ca 2+ non-synaptic epileptiform activity. Simulations performed with a network of four zero-Ca 2+ CA1 pyramidal neurons modeled in zero-calcium conditions also show that spontaneous sustained activity can propagate by potassium lateral diffusion alone with a velocity of ∼0.93 mm/sec. This diffusion model used for the simulations is based on physiological parameters, is robust for various kinetics, and is able to reproduce both the spontaneous triplet bursting of non-synaptic activity and speed of propagation in low-Ca 2+ non-synaptic epilepsy experiments. These simulations suggest that potassium lateral diffusion can play an important role in the synchronization and generation on non-synaptic epilepsy.

Suppression of excitatory synaptic transmission can facilitate low-calcium epileptiform activity in the hippocampus in vivo

It has been reported that the inhibitory postsynaptic potential (IPSP) is abolished before the excitatory postsynaptic potential (EPSP) when the extracellular concentration of Ca 2+ ([Ca 2+] o) is removed gradually in hippocampal slices. However, the low-Ca 2+ nonsynaptic epileptiform activity does not appear until the [Ca 2+] o is decreased to a level sufficient to depress the excitatory synaptic transmission. This suggests the hypothesis that the suppression of excitatory synaptic transmission itself could facilitate the generation of epileptiform activity. In the present study, we tested this hypothesis and developed a new model of nonsynaptic epileptiform activity by gradually raising the neuronal excitability and blocking the synaptic transmission with high K +, zero Ca 2+ and calcium chelator ethylene glycol-bis (β-aminoethyl ether)-N,N,N′,N′-tetraacetic acid (EGTA) in the CA1 region of hippocampus in vivo. The changes of synaptic transmission and recurrent inhibitory activity during this process were evaluated by measuring the amplitude of the population spikes (PS) in response to paired-pulse orthodromic stimulation. The results show that the epileptiform activity appeared only when the excitatory synaptic transmission was depressed by further lowering [Ca 2+] o with EGTA. Similar epileptiform activity could be induced when EGTA was replaced by the excitatory postsynaptic amino acid antagonists d-(−)-2-amino-5-phosphonopentanoic acid (APV) plus 6,7-dinitroquinoxaline-2,3-dione (DNQX) or APV alone but not DNQX alone. The combination application of APV and cadmium enhanced the epileptiform activity. These results suggest that the suppression of excitatory synaptic transmission can facilitate the appearance of epileptiform activity in solution with high K + and low Ca 2+ in vivo. These data provide new information to be considered in the development of antiepileptic drugs. They also suggest a possible mechanism to explain the fact that low-frequency electrical stimulation can suppress epileptiform activity.

Decrease in Synaptic Transmission Can Reverse the Propagation Direction of Epileptiform Activity in Hippocampus In Vivo

Most types of epileptiform activity with synaptic transmission have been shown to propagate from the CA3 to CA1 region in hippocampus. However, nonsynaptic epileptiform activity induced in vitro is known to propagate slowly from the caudal end of CA1 toward CA2/CA3. Understanding the propagation modes of epileptiform activity, and their causality is important to revealing the underlying mechanisms of epilepsy and developing new treatments. In this paper, the effect of the synaptic transmission suppression on the propagation of epilepsy in vivo was investigated by using multiple-channel recording probes in CA1. Nonsynaptic epileptiform activity was induced by calcium chelator EGTA with varied concentrations of potassium. For comparison, disinhibition synaptic epileptiform activity was induced by picrotoxin (PTX) with or without partial suppression of excitatory synaptic transmission. The propagation velocity was calculated by measuring the time delay between two electrodes separated by a known distance. The results show that in vivo nonsynaptic epileptiform activity propagates with a direction and velocity comparable to those observed in in vitro preparations. The direction of propagation for nonsynaptic activity is reversed from the PTX-induced synaptic activity. A reversal in propagation direction and change in velocity were also observed dynamically during the process of synaptic transmission suppression. Even a partial suppression of synaptic transmission was sufficient to significantly change the propagation direction and velocity of epileptiform activity. These results suggest the possibility that the measurement of propagation can provide important information about the synaptic mechanism underlying epileptic activity.