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

Ectopic burst induced by blockade of axonal potassium channels on the mouse hippocampal mossy fiber

A potassium channel blocker 4-AP has been shown to exert pronounced convulsive action to generate burst firings when applied to hippocampal slices. However, it remains unclear how the blockade of potassium channels leads to the generation of burst firings. One possibility is ectopic spiking from the sites different from those for physiological spike initiation at the axon initial segment, as suggested for several experimental models of epileptogenesis in vitro. To test for possible ectopic spiking at the distal axon by 4-AP application, direct recordings from large mossy fiber terminals were made with the loose-patch clamp technique in mouse hippocampal slices. To localize the action of 4-AP on the distal axon, focal perfusion, as well as micro-cut to disconnect soma and distal axons, were adopted. Focal application of 4-AP on the distal portion of mossy fibers reliably induced burst discharges of the mossy fiber terminals. Photochemical blockade of potassium channels at distal axons, by the application of RuBi-4-AP, a visible wavelength blue light-sensitive caged compound, and the illumination of blue light caused robust bursting activity originating from distal axons. Computer simulation suggested that local blockade of axonal potassium channels prolongs the duration of action potentials and thereby causes reverberating spiking activities at distal axons and subsequent antidromic propagation toward the soma. Taken together, it was suggested that local blockade of voltage-dependent potassium channels in distal axons by application of 4-AP is sufficient to cause a hyperexcitable state of hippocampal mossy fiber axons.

Diverse nature of interictal oscillations: EEG-based biomarkers in epilepsy

Interictal electroencephalogram (EEG) patterns, including high-frequency oscillations (HFOs), interictal spikes (ISs), and slow wave activities (SWAs), are defined as specific oscillations between seizure events. These interictal oscillations reflect specific dynamic changes in network excitability and play various roles in epilepsy. In this review, we briefly describe the electrographic characteristics of HFOs, ISs, and SWAs in the interictal state, and discuss the underlying cellular and network mechanisms. We also summarize representative evidence from experimental and clinical epilepsy to address their critical roles in ictogenesis and epileptogenesis, indicating their potential as electrophysiological biomarkers of epilepsy. Importantly, we put forwards some perspectives for further research in the field.

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.

Physiological sharp wave-ripples and interictal events in vitro: what's the difference?

Sharp wave-ripples and interictal events are physiological and pathological forms of transient high activity in the hippocampus with similar features. Sharp wave-ripples have been shown to be essential in memory consolidation, whereas epileptiform (interictal) events are thought to be damaging. It is essential to grasp the difference between physiological sharp wave-ripples and pathological interictal events to understand the failure of control mechanisms in the latter case. We investigated the dynamics of activity generated intrinsically in the Cornu Ammonis region 3 of the mouse hippocampus in vitro, using four different types of intervention to induce epileptiform activity. As a result, sharp wave-ripples spontaneously occurring in Cornu Ammonis region 3 disappeared, and following an asynchronous transitory phase, activity reorganized into a new form of pathological synchrony. During epileptiform events, all neurons increased their firing rate compared to sharp wave-ripples. Different cell types showed complementary firing: parvalbumin-positive basket cells and some axo-axonic cells stopped firing as a result of a depolarization block at the climax of the events in high potassium, 4-aminopyridine and zero magnesium models, but not in the gabazine model. In contrast, pyramidal cells began firing maximally at this stage. To understand the underlying mechanism we measured changes of intrinsic neuronal and transmission parameters in the high potassium model. We found that the cellular excitability increased and excitatory transmission was enhanced, whereas inhibitory transmission was compromised. We observed a strong short-term depression in parvalbumin-positive basket cell to pyramidal cell transmission. Thus, the collapse of pyramidal cell perisomatic inhibition appears to be a crucial factor in the emergence of epileptiform events.

Reduced ictogenic potential of 4-aminopyridine in the hippocampal region in the pilocarpine model of epilepsy

It was previously shown that the ictogenic potential of 4-aminopyridine (4-AP) was reduced in the parahippocampal region of kainate treated chronic epileptic rats. In the actual study we investigated the potential of 4-aminopyridine (50 and 100μM) to induce seizure like events (SLEs) in combined entorhinal cortex hippocampal slices from Wistar rats following pilocarpine induced status epilepticus. The potential of 4-AP to induce SLEs in the entorhinal cortex was reduced in the latent period and in slices of chronic epileptic animals with a high seizure incidence in vivo (>2seizures/24h). 4-AP induced SLEs in slices from animals with a low incidence of seizures in vivo (<2seizures/24h) in a similar manner as compared to controls. The hippocampal formation displayed no SLEs, instead short recurrent epileptiform discharges (REDs) were evoked by application of 4-AP in areas CA3 and CA1. The incidence of REDs was largest in slices from control animals. This study shows that the reduced ictogenic potential of 4-AP is not restricted to kainate treated chronic epileptic animals as it can be found in the pilocarpine model as well. The underlying mechanisms may relate to altered expression and editing of voltage gated potassium channels.

NKCC1 and KCC2 prevent hyperexcitability in the mouse hippocampus

During postnatal development of the central nervous system (CNS), the response of GABA(A) receptors to its agonist undergoes maturation from depolarizing to hyperpolarizing. This switch in polarity is due to the developmental decrease of the intracellular Cl concentration in neurons. Here we show that absence of NKCC1 in P9-P13 CA3 pyramidal neurons, through genetic manipulation or through bumetanide inhibition, results in a significant increase in cell excitability. Furthermore, the pro-convulsant agent 4-aminopyridine induces seizure-like events in NKCC1-null mice but not in wild-type mice. Measurements of muscimol responses in the presence and absence of NKCC1 shows that the Na-K-2Cl cotransporter only marginally affects intracellular Cl(-) in P9-P13 CA3 principal neurons. However, large increases in intracellular Cl(-) are observed in CA3 pyramidal neurons following increased hyperexcitability, indicating that P9-P13 CA3 pyramidal neurons lack robust mechanisms to regulate intracellular Cl(-) during high synaptic activity. This increase in the Cl(-) concentration is network-driven and activity-dependent, as it is blocked by the non-NMDA glutamate receptor antagonist DNQX. We also show that expression of the outward K-Cl cotransporter, KCC2, prevents the development of hyperexcitability, as a reduction of KCC2 expression by half results in increased susceptibility to seizure under control and 4-AP conditions.

Functional Pharmacology in Human Brain

Most neurological and psychiatric disorders involve selective or preferential impairments of neurotransmitter systems. Therefore, studies of functional transmitter pathophysiology in human brain are of unique importance in view of the development of effective, mechanism-based, therapeutic modalities. It is well known that central nervous system functional proteins, including receptors, transporters, ion channels, and enzymes, can exhibit high heterogeneity in terms of structure, function, and pharmacological profile. If the existence of types and subtypes of functional proteins amplifies the possibility of developing selective drugs, such heterogeneity certainly increases the likelihood of interspecies differences. It is therefore essential, before choosing animal models to be used in preclinical pharmacology experimentation, to establish whether functionally corresponding proteins in men and animals also display identical pharmacological profiles. Because of evidence that scaffolding proteins, trafficking between plasma membrane and intracellular pools, phosphorylation and allosteric modulators can affect the function of receptors and transporters, experiments with human clones expressed in host cells where the environment of native receptors is rarely reproduced should be interpreted with caution. Thus, the use of neurosurgically removed fresh human brain tissue samples in which receptors, transporters, ion channels, and enzymes essentially retain their natural environment represents a unique experimental approach to enlarge our understanding of human brain processes and to help in the choice of appropriate animal models. Using this experimental approach, many human brain functional proteins, in particular transmitter receptors, have been characterized in terms of localization, function, and pharmacological properties.

Ectopic action potential generation in cortical interneurons during synchronized GABA responses

In the presence of 4-aminopyridine and excitatory amino acid receptor antagonists, individual neurons in brain slice preparations exhibit large gamma aminobutyric acid (GABA)-mediated responses as a consequence of synchronous GABA release from a network of interneurons. These synchronized GABA responses are frequently associated with ectopic action potentials (EAPs), which are thought to be action potentials initiated in distal axon terminals which subsequently travel antidromically toward the soma. Ectopic action potentials feature prominently in some models of epilepsy. Neocortical synchronized GABA responses propagate across the cortex, predominantly in superficial layers. The role that EAPs may play in contributing to laminar differences in the synchronized GABA response has not been addressed. Here we examined the occurrence of EAPs during synchronized GABA responses in neurons within layers I and II/III. EAPs occurred in 78% of layer I interneurons and in 25% of layer II/III interneurons (including chandelier cells). EAPs were not observed in layer II/III pyramidal neurons. The prevalence of EAPs in layer I interneurons provides a mechanism by which layer I can support both the initiation and propagation of synchronized GABA responses. Thus, layer I interneurons are a critical component of a network capable of synchronizing a propagating wave of GABA release across the neocortex.

Synaptic and non-synaptic mechanisms of amygdala recruitment into temporolimbic epileptiform activities

Lateral amygdala (LA) activity during synchronized-epileptiform discharges in temporolimbic circuits was investigated in rat horizontal slices containing the amygdala, hippocampus (Hip), perirhinal (Prh) and lateral entorhinal (LEnt) cortex, through multiple-site extra- and intracellular recording techniques and measurement of the extracellular K+ concentration. Application of 4-aminopyridine (50 microm) induced epileptiform discharges in all regions under study. Slow interictal-like burst discharges persisted in the Prh/LEnt/LA after disconnection of the Hip, seemed to originate in the Prh as shown from time delay analyses, and often preceded the onset of ictal-like activity. Disconnection of the amygdala resulted in de-synchronization of epileptiform discharges in the LA from those in the Prh/LEnt. Interictal-like activity was intracellularly reflected in LA projection neurons as gamma-aminobutyric acid (GABA)A/B receptor-mediated synaptic responses, and depolarizing electrogenic events (spikelets) residing on the initial phase of the GABA response. Spikelets were considered antidromically conducted ectopic action potentials generated at axon terminals, as they were graded in amplitude, were not abolished through hyperpolarizing membrane responses (which effectively blocked evoked orthodromic action potentials), lacked a clear prepotential or synaptic potential, were not affected through blockers of gap junctions, and were blocked through remote application of tetrodotoxin at putative target areas of LA projection neurons. Remote application of a GABAB receptor antagonist facilitated spikelet generation. A transient elevation in the extracellular K+ level averaging 3 mm above baseline occurred in conjunction with interictal-like activity in all areas under study. We conclude that interictal-like discharges in the LA/LEnt/Prh spread in a predictable manner through the synaptic network with the Prh playing a leading role. The rise in extracellular K+ may provide a depolarizing mechanism for recruitment of interneurons and generation of ectopic action potentials at axon terminals of LA projection neurons. Antidromically conducted ectopic action potentials may provide a spreading mechanism of seizure activity mediated by diffuse axonal projections of LA neurons.

Spontaneous, low frequency (approximately 2-3 Hz) field activity generated in rat ventral hippocampal slices perfused with normal medium

This study demonstrates that transverse slices taken from the ventral hippocampus of adult rats perfused with a medium of normal ionic composition sustain spontaneous periodic field potentials due to the synchronous activity of a population of neurons. This ventral hippocampus spontaneous synchronous activity (VHSSA) in CA1 stratum pyramidale consisted of positive potentials (approximately 0.12 mV, 55 ms) occurring at a frequency of 2.8 +/- 0.2 Hz for hours without interruption. VHSSA was most frequently observed in slices taken 1-3 mm from the ventral end of hippocampus, and was absent in slices taken from tissue more than 4.5 mm away from it. Stimulation of Schaffer collaterals primed the appearance of potentials, which were similar to VHSSA and clearly distinguishable from excitatory postsynaptic potentials. In view of the known relative proneness of ventral hippocampus to epilepsy, we perfused ventral slices with high-[K(+)](o) medium (8 mM). Albeit reduced in amplitude, VHSSA persisted during the high-[K(+)](o) induced interictal-like epileptiform activity. We could not document any temporal relationship between the two phenomena. Low concentrations of the antagonist of gamma-amino-butyric acid receptors, type A, bicuculline (2-3 microM), which enhanced the high-[K(+)](o) induced epileptiform activity, reversibly blocked the VHSSA. We conclude that under standard in vitro conditions small circuits in the ventral hippocampus are most often and for long periods of time engaged in synchronous quasi-rhythmic low-frequency activity, generated locally by mechanisms substantially differing from those supporting epileptiform discharges.

Seizure-like events in disinhibited ventral slices of adult rat hippocampus

Epileptic discharges lasting 2-90 s, were studied in vitro in slices from the ventral hippocampus of adult rats, in which inhibition was blocked acutely with bicuculline methiodide (BMI, 5-30 microM) and potassium ([K(+)](o)) raised to 5 mM. These seizure-like events (SLEs) comprised three distinct phases, called here primary, secondary, and tertiary bursts. Primary bursts lasted 90-150 ms. Secondary bursts lasted a further 70-250 ms, comprising a short series of afterdischarges riding on the same depolarization as the primary burst. Finally a train of tertiary bursts started with a peak frequency of 5-10 Hz and could last >1 min. Slices from the ventral hippocampus showed significantly higher susceptibility to SLEs than did dorsal slices. SLEs proved sensitive to alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonists. They were insensitive to N-methyl-D-aspartate (NMDA) receptor antagonists; 50 microl D-2-amino-5-phosphonopentanoic acid (D-AP5) did block the transient secondary bursts selectively. SLEs were restricted to the hippocampus proper even if the entorhinal cortex was present. Entorhinal bursts could last <2 s and were only coupled with hippocampal bursts in a minority of slices. Reentry of epileptic bursts occasionally occurred during interictal discharges, but not during the later stages of SLEs. Full-length SLEs always started in CA3 region and could be recorded in minislices containing CA3 plus dentate hilus. Ion-sensitive microelectrodes revealed that interictal discharges were followed by short (2-3 s) [K(+)](o) waves, peaking at approximately 7.5 mM. SLEs were always accompanied by increases in [K(+)](o) reaching approximately 8.5 mM at the start of tertiary bursts; [K(+)](o) then increased more slowly to a ceiling of 11-12 mM. After the end of each SLE, [K(+)](o) fell back to baseline within 10-15 s. SLEs were accompanied by significant increase in synaptic activity, compared with baseline and/or interictal activity, estimated by the variance of the intracellular signal in the absence of epileptic bursts and action potentials (0. 38 mV(2), compared with 0.13 mV(2), and 0.1 mV(2), respectively). No significant increases were observed in the interval preceding spontaneous interictal activity. These studies show that focal assemblies of hippocampal neurons, without long reentrant loops, are sufficient for the generation of SLEs. We propose that a key factor in the transition from interictal activity to SLEs is an increase in axonal and terminal excitability, resulting, at least in part, from elevations in [K(+)](o).

Regulation of extracellular potassium in the developing hippocampus

It has long been felt that one of the reasons that the immature brain has an increased propensity to seize is due to an inability to regulate the extracellular potassium ([K+]o). However, the data supporting this hypothesis is controversial. Here, we tested the regulation of [K+]o in urethane-anesthetized juvenile and adult rats during and after synchronized epileptiform activity induced by 20 Hz stimulus trains to the CA3 region. The regulation of [K+]o in CA3 was compared to the same measurements in CA1. Across all age groups, there was no difference between CA1 and CA3. There was a slight decrease in the peak level of [K+]o reached during stimulation only in the youngest age groups tested (PN9-11). There was an age-dependent change in the rate of recovery of [K+]o from the elevated levels. This recovery was slowest in the PN9-11 age group. In all the animals in the PN9-11 group and 3/7 animals in the PN14-15 group, there was a secondary increase in the [K+]o during the afterdischarge. This secondary rise was never observed in animals over 15 days of age. These data confirm that there is altered regulation of [K+]o in the developing brain that takes two forms: (1) the ability to recover from elevated [K+]o levels, suggesting maturation of regulatory, or uptake, mechanisms; and (2) that which is related to mechanisms of synchronization and initiation of afterdischarges and the level of [K+]o that is produced during the discharge.

Ionic mechanisms of spontaneous GABAergic events in rat hippocampal slices exposed to 4-aminopyridine

Ionic mechanisms of spontaneous GABAergic events in rat hippocampal slices exposed to 4-aminopyridine. J. Neurophysiol. 78: 2582-2591, 1997. Ion-selective (H+ and K+) microelectrode techniques as well as conventional extra- and intracellular recordings were used to study the ionic mechanisms of propagating spontaneous GABAergic events (SGEs) in rat hippocampal slices exposed to 4-aminopyridine (4-AP, 50-100 mu M). All experiments were made in the presence of antagonists of ionotropic glutamate receptors [10 mu M 6-nitro-7-sulphamoylbenzoquinoxaline-2,3-dione (NBQX) and 40 mu M -2-amino-5-phosphonopentanoic acid (AP5)]. The SGEs were composed of a negative-going change in field potential with a temporally coincident increase (0.7 +/- 0.3 mM; mean +/- SE) in extracellular K+ ([K+]o) and an alkaline transient (0.01-0.08 units) in extracellular pH (pHo) in stratum radiatum of the area CA1. Simultaneous intracellular recordings showed a triphasic hyperpolarization-depolarization-late hyperpolarization response in pyramidal cells. Application of pentobarbital sodium (PB, 100 mu M) decreased the interval between SGEs from a mean value of 35 to approximately 20 s and shortened the period of refractoriness of stimulus-evoked propagating events. This was accompanied by an increase in the amplitude of the field potential response of the [K+]o and the pHo shifts and of the depolarizing phase of the pyramidal-cell response. The SGEs were completely blocked by the gamma-aminobutyric acid-A (GABAA) receptor antagonist, picrotoxin (PiTX; 100 mu M). The amplitudes of the negative-going field potential and of the depolarizing phase of the pyramidal-cell response as well as the ionic shifts associated with SGEs were strongly suppressed in the nominal absence of CO2/HCO-3. There was a five-fold increase in the interevent interval, and propagating SGEs could not be evoked by stimuli given at intervals shorter than approximately 2-3 min. Exposure to inhibitors of carbonic anhydrase, benzolamide (BA; 10 micro M) or ethoxyzolamide (EZA; 50 mu M) fully blocked the alkaline pHo transients and turned them into acid shifts. The poorly membrane-permeant BA had no discernible effect on the other components of the SGEs, but application of EZA had effects reminiscent to those of CO2/HCO-3-free medium. Addition of the GABAA receptor-permeant weak-acid anion, formate (20 mM) reestablished the SGEs that were first suppressed by exposure to the CO2/HCO-3-free medium. No SGEs were seen in the presence of a similar concentration of the GABAA receptor-impermeant anion propionate. Unlike the alkaline transients associated with HCO-3-driven SGEs, those supported by formate were not blocked by BA. The present data suggest that an inward current carried by bicarbonate is necessary for the generation of SGEs and that the GABAA receptor-mediated excitatory coupling among GABAergic interneurons is essentially dependent on the availability of intracellular bicarbonate.

Activation of brain nitric oxide synthase in depolarized human temporal cortex slices: differential role of voltage-sensitive calcium channels

1. Nitric oxide (NO) synthase activity was studied in slices of human temporal cortex samples obtained in neurosurgery by measuring the conversion of L-[3H]-arginine to L-[3H]-citrulline. 2. Elevation of extracellular K+ to 20, 35 or 60 mM concentration-dependently augmented L-[3H]-citrulline production. The response to 35 mM KCl was abolished by N(G)-nitro-L-arginine (100 microM) demonstrating NO synthase specific conversion of L-arginine to L-citrulline. Increasing extracellular MgCl2 concentration up to 10 mM also prevented the K+ (35 mM)-induced NO synthase activation, suggesting the absolute requirement of external calcium ions for enzyme activity. 3. However, the effect of high K+ (35 mM) on citrulline synthesis was insensitive to the antagonists of ionotropic and metabotropic glutamate receptors dizocilpine (MK-801), 6-nitro-7-sulphamoylbenzo(f)-quinoxaline-2-3-dione (NBQX) or L-2-amino-3-phosphonopropionic acid (L-AP3) as well as to the nicotinic receptor antagonist, mecamylamine. 4. The 35 mM K+ response was insensitive to omega-conotoxin GVIA (1 microM) and nifedipine (100 microM), but could be prevented in part by omega-agatoxin IVA (0.1 and 1 microM). The inhibition caused by 0.1 microM omega-agatoxin IVA (approximately 30%) was enhanced by adding omega-conotoxin GVIA (1 microM) or nifedipine (100 microM). Further inhibition (up to above 70%) could be observed when the three Ca2+ channel blockers were added together. Similarly, synthetic FTX 3.3 arginine polyamine (sFTX) prevented (50% at 100 microM) the K+-evoked NO synthase activation. This effect of sFTX was further enhanced (up to 70%) by adding 1 microM omega-conotoxin GVIA plus 100 microM nifedipine. No further inhibition could be observed upon addition of MK-801 or/and NBQX. 5. It was concluded that elevation of extracellular [K+] causes NO synthase activation by external Ca2+ entering cells mainly through channels of the P/Q-type. Other Ca2+ channels (L- and N-type) appear to contribute when P/Q-channels are blocked.

Posttetanic excitation mediated by GABA(A) receptors in rat CA1 pyramidal neurons

The contributions of gamma-aminobutyric acid (GABA) receptors to posttetanic excitation of CA1 pyramidal neurons in rat hippocampal slices were studied using extracellular and intracellular recording techniques. Synaptic responses were evoked on tetanic stimulation (100-200 Hz, 40-100 pulses) applied in stratum radiatum close (300-600 microm) to the recording site. Under control conditions, tetanic stimulation resulted in a triphasic depolarization/hyperpolarization/sustained depolarization sequence in area CA1 pyramidal cells. The late depolarization usually gave rise to a prolonged (< or = 3 s) spike firing. The late depolarization and the associated spike firing were blocked both specifically and completely (within a time window of 3-6 min starting from picrotoxin application) by the GABA(A) receptor antagonist picrotoxin (PiTX, 100 microM). Paradoxically, at this early stage of PiTX application, overall neuronal firing was attenuated to a higher degree than what was achieved by ionotropic glutamate antagonists. Complete block of ionotropic glutamate receptors by the antagonists D-2-amino-5-phosphonopentoate (AP5, 80 microM), 6-nitro-7-sulphamoylbenzo[f]quinoxaline-2,3-dione (NBQX, 10 microM), and ketamine (50 microM) blocked the initial fast depolarization and suppressed the late one. Exposure to a permeable inhibitor of carbonic anhydrase, ethoxyzolamide (EZA, 50 microM) inhibited the late, apparently GABA-mediated depolarization. It is concluded that GABA can provide the main posttetanic excitatory drive in the adult hippocampus. The present results suggest that intense activation of GABAergic interneurons may accentuate the excitation of principal neurons and, hence, play an important facilitatory role in the induction of long-term potentiation (LTP) and epileptogenesis.

Altered glutamatergic transmission in neurological disorders: from high extracellular glutamate to excessive synaptic efficacy

This review is a critical appraisal of the widespread assumption that high extracellular glutamate, resulting from enhanced pre-synaptic release superimposed on deficient uptake and/or cytosolic efflux, is the key to excessive glutamate-mediated excitation in neurological disorders. Indeed, high extracellular glutamate levels do not consistently correlate with, nor necessarily produce, neuronal dysfunction and death in vivo. Furthermore, we exemplify with spreading depression that the sensitivity of an experimental or pathological event to glutamate receptor antagonists does not imply involvement of high extracellular glutamate levels in the genesis of this event. We propose an extension to the current, oversimplified concept of excitotoxicity associated with neurological disorders, to include alternative abnormalities of glutamatergic transmission which may contribute to the pathology, and lead to excitotoxic injury. These may include the following: (i) increased density of glutamate receptors; (ii) altered ionic selectivity of ionotropic glutamate receptors; (iii) abnormalities in their sensitivity and modulation; (iv) enhancement of glutamate-mediated synaptic efficacy (i.e. a pathological form of long-term potentiation); (v) phenomena such as spreading depression which require activation of glutamate receptors and can be detrimental to the survival of neurons. Such an extension would take into account the diversity of glutamate-receptor-mediated processes, match the complexity of neurological disorders pathogenesis and pathophysiology, and ultimately provide a more elaborate scientific basis for the development of innovative treatments.

GABA-mediated synchronous potentials and seizure generation

This article summarizes findings related to a synchronous, GABA-mediated potential that may contribute to the initiation and spread of epileptiform discharges within the brain. This phenomenon is observed in cortical structures such as the hippocampus, the entorhinal cortex, and the neocortex during application of low concentrations of 4-aminopyridine and is characterized at the intracellular level by a long-lasting membrane depolarization. The synchronous, GABA-mediated potential continues to occur after blockade of excitatory synaptic transmission and relays on the synchronous firing of inhibitory interneurons and consequent activation of postsynaptic (mainly type A) GABA receptors leading to a transient elevation of [K+]O. Studies performed in young rat hippocampus indicate that the synchronous, GABA-mediated potential may play a role in initiating ictal discharges under normal conditions (i.e., when excitatory amino acid receptors are operant). Moreover, a similar phenomenon may also occur in adult rat entorhinal cortex. These findings therefore indicate a novel role that is played by GABAA receptors in limbic structures. The ability of this synchronous GABA-mediated potential to propagate in the absence of excitatory synaptic transmission may also be relevant for the propagation of synchronous activity outside conventional neuronal-synapse dependent pathways. This condition may occur in brain structures with neuronal loss and consequent disruption of normal excitatory synaptic connections such as mesial limbic structures of temporal lobe epilepsy patients with Ammon's horn sclerosis.

Synchronization of rat hippocampal neurons in the absence of excitatory amino acid-mediated transmission

Extracellular and intracellular recordings and measurements of extracellular K+ concentration ([K+]o) were performed in the adult rat hippocampus in an in vitro slice preparation. Excitatory amino acid receptor antagonists, as well as the K(+)-channel blockers 4-aminopyridine (4AP, 50 microM) and/or tetraethylammonium (TEA, 5 mM), were added to the bath. Synchronous, negative-going field potentials were recorded in the CA3 stratum radiatum during application of 4AP and excitatory amino acid receptor antagonists. Each of these events was associated with an intracellular long-lasting depolarization and a concomitant rise in [K+]o that attained peak values of 4.3 +/- 0.1 mM (mean +/- S.E.M., n = 6 slices) and lasted 29 +/- 3 s. These field potentials were still recorded in CA3 stratum radiatum after addition of TEA. Under these conditions, prolonged field potentials (40.2 +/- 4.5 s, n = 18) characterized by a prominent positive component; discharge of population spikes also occurred. [K+]o increases associated with these prolonged field-potential discharges had a considerable variability in magnitude (peak value = 3.8-14.1 mM, 6.1 +/- 0.7 mM, n = 5) and duration (14-210 s; 48 +/- 13 s, n = 5). In 8% of the cases spreading depression-like episodes were observed. [K+]o increases during spreading depression-like episodes attained peak values of 11-27 mM (22.8 +/- 0.2 mM, n = 2) and had a duration of 160-396 s (244 +/- 29 s, n = 2). All types of synchronous activity were abolished by the GABAA-receptor antagonist bicuculline methiodide (10 microM) (n = 11). A similar effect was obtained by applying Ca(2+)-free/high-Mg2+ medium (n = 5). Simultaneous field-potential recordings in CA3, CA1, dentate area and subiculum demonstrated that negative-going potentials and prolonged field-potential discharges occurred in all areas in a synchronous fashion. Spreading depression-like episodes were more frequently recorded in the CA1 than in the CA3 area and were not seen in the subiculum or dentate area. These experiments indicate that a glutamatergic-independent, synchronous GABA-mediated potential which is elicited by 4AP in the adult rat hippocampus continues to occur in the presence of TEA. In addition, concomitant application of these K(+)-channel blockers induces a novel type of prolonged field-potential discharge as well as spreading depression-like episodes. Since all synchronous potentials (including spreading depression-like episodes) were abolished by bicuculline methiodide, we conclude that their occurrence is presumably dependent upon the post-synaptic activation of GABAA receptors located on neuronal and glial elements. As excitatory synaptic transmission was nominally blocked under our experimental conditions, we also propose that rises in [K+]o and consequent redistribution processes are per se sufficient to make all types of synchronous activity propagate.

On the structure of ictal events in vitro

PURPOSE

To analyze the cellular and network mechanisms of sustained seizures, we reviewed the literature and present new data on in vitro epileptiform events. We considered single and recurring synchronized population bursts occurring on a time scale from tens of milliseconds to 1 min.

METHODS

We used intracellular and field potential recordings, together with computer network simulations, derived from three types of experimental epileptogenesis: gamma-aminobutyric-acidA (GABAA) blockade, low extracellular [Mg2+]o, and 4-aminopyridine (4-AP).

RESULTS

In all three models, sustained depolarizing synaptic currents developed, either through N-methyl-D-aspartate (NMDA) receptors, depolarizing GABAA receptors, or both. Ectopic action potentials (APs), probably originating in axonal structures, occurred in 4-AP and (as shown by other researchers) after tetanic stimulation; ectopic APs, occurring at sufficient frequency, should also depolarize dendrites, by synaptic excitation, enough to trigger bursts.

CONCLUSIONS

Ictal-like events appear to arise from two basic mechanisms. The first mechanism consists of sustained dendritic depolarization driving a series of dendritic bursts. The second mechanism consists of an increase in axonal and presynaptic terminal excitability driving a series of bursts analogous to interictal spikes.

Synchronous GABA-mediated potentials and epileptiform discharges in the rat limbic system in vitro

Application of 4-aminopyridine (4AP, 50 microM) to combined slices of adult rat hippocampus-entorhinal cortex-induced ictal and interictal epileptiform discharges, as well as slow field potentials that were abolished by the mu-opioid agonist [D-Ala2,N-Me-Phe4,Gly-ol5] enkephalin (DAGO, 10 microM) or the GABAA receptor antagonist bicuculline methiodide (BMI, 10 microM); hence, they represented synchronous GABA-mediated potentials. Ictal discharges originated in the entorhinal cortex and propagated to the hippocampus, whereas interictal activity of CA3 origin was usually recorded in the hippocampus. The GABA-mediated potentials had no fixed site of origin or modality of propagation; they closely preceded (0.2-5 sec) and thus appeared to initiate ictal discharges. Only ictal discharges were blocked by the antagonist of the NMDA receptor 3,3-(2-carboxypiperazine-4-yl)propyl-1-phosphonate (CPP, 10 microM), whereas the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM) abolished all epileptiform activities. The GABA-mediated potentials continued to occur synchronously in all regions even after concomitant application of CNQX and CPP. [K+]o elevations were recorded in the entorhinal cortex during the ictal discharge (peak values = 13.9 +/- 0.9 mM) and the synchronous GABA-mediated potentials (peak values = 4.2 +/- 0.1 mM); the latter increases were presumably attributable to postsynaptic GABAa-receptor activation because they were abolished by DAGO or BMI. Their role in initiating ictal activity was demonstrated by using DAGO, which abolished both GABA-mediated synchronous potentials and ictal discharges. These data indicate that NMDA-mediated ictal discharges induced by 4AP originate in the entorhinal cortex; such a conclusion is in line with clinical evidence obtained in temporal lobe epilepsy patients. 4AP also induces GABA-mediated potentials that spread within the limbic system when excitatory transmission is blocked and may play a role in initiating ictal discharge by increasing [K+]o.

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

1. Field potential recordings and measurements of the extracellular concentration of free K+ ([K+]o) and Ca2+ ([Ca2+]o) were made during application of 4-aminopyridine (4-AP, 50 microM) in hippocampal slices that were obtained from 11- to 32-day-old rats. 2. Spontaneous field potentials recorded under this experimental condition in the CA3 stratum radiatum of slices from rats < 23 days old consisted of interictal (duration, 0.2-1.4 s; intervals of occurrence, 0.9-3.4 s) and ictal epileptiform discharges (duration, 5-46 s; intervals of occurrence, 22-259 s) and negative-going potentials that often preceded the onset of ictal discharge. Ictal activity became rare in slices from rats > 25 days old. 3. The negative-going potential (which also corresponded to the ictal discharge onset) was associated with [K+]o increases to 9.4 +/- 3.6 mM (mean +/- S.D.) from 3.25 mM baseline (n = 11 slices). [K+]o remained elevated at 5-6 mM throughout the ictal event. Decreases in [Ca2+]o (from 1.8 mM baseline to 1.3 +/- 0.1 mM, n = 7) were observed during the ictal discharge. 4. Interictal and ictal discharges were abolished by the non-N-methyl-D-aspartate (NMDA) receptor antagonist 6-cyano-7-nitroquinoxaline-2, 3-dione (CNQX, 10 microM). CNQX and the NMDA receptor antagonist 3-((+/-)-2-carboxypiperazine-4-yl)propyl-1-phosphonic acid (CPP) did not influence negative-going potentials or the associated [K+]o increases (peak values were 8.7 +/- 3.2 mM, n = 8), that were blocked, however, by bicuculline methiodide (BMI, 10 microM). 5. The mu-opioid receptor agonist (D-Ala2,N-Me-Phe4,Gly5-ol)-enkephalin (DAGO, 10 microM) which inhibits GABA release from interneurons, prevented the occurrence of both GABA-mediated synchronous potentials and subsequent ictal discharges (n = 6) as well as the [K+]o elevations. DAGO effects were antagonized by naloxone (10 microM; n = 4). 6. The GABA-mediated [K+]o elevations changed as a function of age. In hippocampal slices obtained from 11- to 17-day-old rats, peak values of 10.6 +/- 2.0 mM (n = 10) and half-width durations of 8.7 +/- 1.3 s (n = 7) were observed. In slices obtained from 25- to 32-day-old animals these parameters were 5.2 +/- 0.5 mM (n = 13) and 4.6 +/- 1.1 s (n = 4), respectively. 7. This study shows that, in the juvenile rat hippocampus, 4-AP induces a glutamatergic independent synchronous potential that is due to GABA released from inhibitory terminals and is associated with an increase in [K+]o. This [K+]o elevation undergoes age-dependent changes, and is instrumental in synchronizing neurons thus initiating prolonged epileptiform discharges.

Developmental features of 4-aminopyridine induced epileptogenesis

4-Aminopyridine (4-AP, 50 microM), perfused in rat hippocampal slices from postnatal days 2-30 (P2-P30), induced in the CA3 area the appearance of spontaneous epileptiform discharges, short (interictal-like) and sustained (ictal-like), as well as slow potential. The duration of epileptiform discharges decreased and their rate of occurrence (frequency) increased with maturation: their duration during the 1st postnatal week was 4-6 times longer and their frequency 5 times lower, compared to those of the 4th postnatal week. Ictal discharges gradually disappeared at the end of the 4th postnatal week. Spontaneous synchronous activity-as a whole-often appeared in clusters separated by equal or longer length inactive periods, during the first two postnatal weeks. At the same period, ictal discharges were often followed by repetitive afterdischarges, forming sequences which lasted 0.7-1.5 min. Sectioning experiments showed that epileptiform discharges were generated in CA3, and their presence in CA1 depended on the integrity of CA1-CA3 synaptic connections. In conclusion, these findings demonstrate that (i) immature CA3 can generate synchronous epileptiform discharges as early as P2, (ii) such discharges are longer lasting and more complex during the early developmental stages and (iii) there are two time points (end of 2nd, end of 4th postnatal weeks), when maturational changes alter the epileptogenic properties of immature hippocampus.

Extracellular K+ accumulations and synchronous GABA-mediated potentials evoked by 4-aminopyridine in the adult rat hippocampus

Transient changes in extracellular potassium concentration ([K+]o) and field potentials were evoked by 4-aminopyridine (4-AP; 50-100 microM) and recorded with ion-selective microelectrodes in CA1b, CA3b and dentate sectors of adult rat hippocampal slices. Long-lasting field potentials recurred at a frequency of approximately 1/60 s (0.016 +/- 0.003 Hz) in association with increases in [K+]o which were largest and most sustained in the dendritic regions where afferent fibers terminate (dentate > CA1 > CA3) and in the hilus. In stratum radiatum of CA1 or stratum moleculare of the dentate these fields had a peak amplitude of 1.4 +/- 0.29 mV, duration 8.3 +/- 1.6 s, and were accompanied by increases in [K+]o of 1.8 +/- 0.22 mM that lasted 32 +/- 5.5 s (n = 17 slices). Interictal epilentiform potentials, which were brief (< 0.2 s) and more frequent at approximately 1/3 s (0.30 +/- 0.02Hz) were also present in CA1, CA3 and the hilus and associated with small increases in [K+]o (< or = 0.5 mM, duration < or = 2 s). Interictal activity was blocked by 6-cyano-7-nitroquinoxalone-2,3-dione (CNQX; 5-20 microM); the slow, less frequent potentials were resistant to both CNQX and DL-2-amino-5-phosphonovaleric acid (APV; 50 microM) and reversibly blocked (or attenuated by approximately 80%) by bicuculline methiodide (BMI) (25-100 microM). The BMI-sensitive potentials were also abolished by baclofen (100 microM), an effect which was reversed by 2-OH-saclofen (100 microM). Focal application of KCI or GABA in the absence of 4-AP evoked long-lasting field and [K+]o potentials which were similar to those evoked by 4-AP but more sustained. The proportional relationship between the amplitudes of field and K+ potentials with GABA closely resembled that observed for 4-AP; in contrast the slope of KC1-evoked responses was lower. Our results demonstrate that in the adult rat hippocampus 4-AP induces in many different regions accumulations of [K+]o in synchrony with the long-lasting field potentials, which are known to correspond to an intracellular long-lasting depolarization of the pyramidal cells. These changes are smaller than those which occur in the immature rat hippocampus--which may be related to differences in Na-K-ATPase and susceptibility to seizures. These events involve the activation of GABAA receptors, are under the modulatory control of GABAB receptors, and likely arise from the activity of GABAergic interneuron and/or afferent terminals. The long-lasting field potentials appear to reflect mainly the direct depolarizing actions of GABA and to much more limited extent the associated accumulation of [K+]o.

During afterdischarges in the young rat in vivo extracellular potassium is not elevated above adult levels

Studies have suggested that in the immature brain an unusually high ceiling level for potassium may lead to an increased propensity for seizures. In these experiments, the peak levels of extracellular potassium in the hippocampus in vivo were recorded in immature rats 10-27 days old and compared to levels reached in adults. There was no difference in the peak level of potassium attained during an afterdischarge in any of the age groups tested.

Extracellular potassium elevations in the hippocampus of rats with long-term pilocarpine seizures

Pilocarpine injection into rodents leads to the development of chronic limbic seizures that follow an initial status epilepticus and a seizure-free interval. It has been proposed that a decreased efficacy of the mechanisms that buffer the extracellular concentration of K+ ([K+]o) leads to an increase in seizure susceptibility. Therefore, we analyzed the changes in [K+]o associated with the synchronous activity induced by 4-aminopyridine (4AP) in hippocampal slices obtained from control and pilocarpine-treated rats. At all recording sites (i.e. stratum radiatum of the CA1 and CA3 subfields, and hilus of the dentate gyrus), the amplitude of GABA-mediated synchronous field potentials induced by 4AP, as well as the associated [K+]o increases, were significantly reduced in slices obtained from the pilocarpine-treated rats. In the control group, the field-potential amplitudes reached 1 mV (i.e. 1.7 +/- 0.3 mV in CA1, 0.93 +/- 0.2 mV in CA3, and 1.03 +/- 0.12 mV in the hilus; mean +/- SEM), while the accompanying rises in [K+]o exceeded 4 mM (i.e. 4.17 +/- 0.15 mM in CA1, 4.04 +/- 0.12 mM in CA3, 4.04 +/- 0.11 mM in the hilus) from a baseline of 3.25 mM. The corresponding values in slices from the pilocarpine-treated group were rarely greater than 0.4 mV (i.e. 0.3 +/- 0.09 mV in CA1, 0.27 +/- 0.03 mV in CA3 and 0.38 +/- 0.06 mV in the hilus), and larger than 3.6 mM (i.e. 3.63 +/- 0.04 mM in CA1, 3.64 +/- 0.03 mM in CA3 and 3.60 +/- 0.04 mM in the hilus) from a similar baseline value. With pilocarpine, the rate of occurrence of the GABA-mediated potential significantly decreased from 0.035 to 0.016 s-1. Since the rises in [K+]o decreased rather than increased and their overall duration was unchanged (possibly reflecting cell loss), we conclude that a modification of [K+]o buffering capacity is unlikely to account for the appearance of in vivo seizures in the pilocarpine model of epilepsy.

Synchronous potentials and elevations in [K+]o in the adult rat entorhinal cortex maintained in vitro

Extracellular field-potential recordings and measurements of the extracellular concentration of potassium ([K+]o) were made in layers II and III of the adult rat entorhinal cortex in a slice preparation. Two types of spontaneous, synchronous potentials were induced by the convulsant drug 4-aminopyridine (4AP, 50 microM). The first type was seen in all slices (n = 19) and consisted of a negative-going field potential that lasted 0.5-3.5 s and occurred at rates of 0.013-0.13 Hz. This event was accompanied by an elevation in [K+]o that attained peak values of 4.0-7.6 mM. The second type was reminiscent of ictal epileptiform discharges and was recorded in 6 of 19 slices; it lasted 21-190 s, recurred at 0.001-0.003 Hz and was associated with [K+]o increases that had peak values of 14-17 mM. Whenever such an ictal discharge occurred, it was closely preceded and thus appeared to be initiated by the first type of field potential. Perfusion with N-methyl-D-aspartate (NMDA) receptor antagonist (+/-)-3-(2-carboxy-piperazine-4-yl)propyl-1-phosphonate (CPP; 10 microM) abolished the ictal discharge (n = 4). This pharmacological procedure did not abolish the negative-going potentials that continued to occur during further application of the non-NMDA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 10 microM; n = 4). These glutamatergic-independent potentials were, however, blocked by the GABBAA-receptor antagonist bicuculline methiodide (10 microM, n= 3). Thus, as in hippocampus, 4AP can induce in the entorhinal cortex a synchronous GABA-mediated potential that is resistant to excitatory amino acid antagonists.(ABSTRACT TRUNCATED AT 250 WORDS)