Cesium potentiates epileptiform activities induced by bicuculline methiodide in rat neocortex maintained in vitro

Heterogeneous firing behavior during ictal-like epileptiform activity in vitro

Seizure activity in vivo is caused by populations of neurons displaying a high degree of variability in activity pattern during the attack. The reason for this variability is not well understood. Here we show in an in vitro preparation that hippocampal CA1 pyramidal cells display four types of afterdischarge behavior during stimulus-induced ictal-like events in the presence of Cs(+) (5 mM): type I (43.7%) consisting of high-frequency firing riding on a plateau potential; type II (28.2%) consisting of low-frequency firing with no plateau potential; type III (18.3%) consisting of high-frequency firing with each action potential preceded by a transient hyperpolarization and time-locked to population activity, no plateau potential; "passive" (9.9%) typified by no afterdischarge. Type I behavior was blocked by TTX (0.2 μM) and intracellular injection of QX314 (12.5-25 mM). TTX (0.2 μM) or phenytoin (50 μM) terminated ictal-like events, suggesting that the persistent Na(+) current (I(NaP)) is pivotal for type I behavior. Type I behavior was not correlated to intrinsic bursting capability. Blockade of the M current (I(M)) with linopirdine (10 μM) increased the ratio of type I neurons to 100%, whereas enhancing I(M) with retigabine (50-100 μM) greatly reduced the epileptiform activity. These results suggest an important role of I(M) in determining afterdischarge behavior through control of I(NaP) expression. We propose that type I neurons act as pacemakers, which, through synchronization, leads to recruitment of type III neurons. Together, they provide the "critical mass" necessary for ictogenesis to become regenerative.

Electrophysiological and structural aspects in the frontal cortex after the bee (Apis mellifera) venom experimental treatment

The aim of this study is to evaluate the bioelectrical and structural-functional changes in frontal cortex after the bee venom (BV) experimental treatments simulating both an acute envenomation and a subchronic BV therapy. Wistar rats were subcutaneously injected once with three different BV doses: 700 μg/kg (T(1) group), 2100 μg/kg (T(3) group), and 62 mg/kg (sublethal dose-in T(SL) group), and repeated for 30 days with the lowest dose (700 μg/kg-in T(S) group). BV effects were assessed by electrophysiological, histological, histochemical, and ultrastructural methods. Single BV doses produced discharges of negative and biphasic sharp waves, and epileptiform spike-wave complexes. The increasing frequency of these elements suggested a dose-dependent neuronal hyperexcitation or irritation. As compared to the lower doses, the sublethal dose was responsible for a pronounced toxic effect, confirmed by ultrastructural data in both neurons and glial cells that underwent extensive, irreversible changes, triggering the cellular death. Subchronic BV treatment in T(S) group resulted in a slower frequency and increased amplitude of cortical activity suggesting neuronal loss. However, neurons were still stimulated by the last BV dose. Structural-functional data showed a reduced cellular density in frontal cortex of animals in this group, while the remaining neurons displayed both specific (stimulation of neuronal activity) and unspecific modifications (moderate alterations to necrotic phenomena). Molecular mechanisms involved in BV interactions with the nervous tissue are also discussed. We consider all these data very important for clinicians who manage patients with multiple bee stings, or who intend to set an appropriate BV therapy.

Noradrenaline unmasks novel self-reinforcing motor circuits within the mammalian spinal cord

Spiking activity in motor axons represents the final central coding for muscle contraction. Recurrent collaterals in spinal cord from these same axons are known to offer a negative feedback control of motor output via a class of interposed inhibitory interneurons. Here we demonstrate that, during noradrenergic drive, a previously unknown recurrent excitatory pathway is unmasked and expressed. These excitatory projections are shown to have broad bilateral actions within and between hindlimb spinal segments and can alter ongoing pattern-generating motor behaviors. Thus, motor output strength is controlled via central positive and negative feedback loops, undoubtedly to provide a greater flexibility and dynamic range of control. That this novel function is regulated by a descending neuromodulatory transmitter indicates a conditional recruitment during certain behavioral states as part of the central noradrenergic arousal apparatus.

Methodological approaches to exploring epileptic disorders in the human brain in vitro

Brain surgery, and in particular epilepsy surgery, offers the unique opportunity to study viable human central nervous tissue in vitro. This does not only open a window to address the basic mechanisms underlying human disease, such as epilepsy, but it allows to venture into investigating neurophysiological functions per se. In the present paper, we describe the most commonly used methods in the electrophysiological (and, at least to some extent, also histochemical and molecular) analysis of human tissue in vitro. In addition, we consider the pitfalls and limitations of such studies, in particular regarding the issue of tissue sampling procedures and control experiments.

Postnatal development of a new type of epileptiform activity in the rat hippocampus

Long-term application of Cs(+) (5 mM) induces an epileptiform field potential (Cs-FP) in area CA1 of the rat hippocampus, which is independent of N-methyl-D-aspartate (NMDA) and non-NMDA glutamate receptors and gamma-aminobutyric acid (GABA)(A) receptors. To gain insight into possible mechanisms for the induction of the Cs-FP, we investigated the postnatal development of the response. In brain slices prepared from rats at different ages, the Cs-FP was evoked by stimulation of the Schaffer-collateral-commisural pathway. We found that expression of this potential was clearly dependent on the postnatal age. Thus, the Cs-FP was completely absent at 1 week of age. By 2 weeks, a reduced form of the response was observed, whereas slices taken from 3-week-old rats, displayed full Cs-FP, which were indistinguishable in size and shape from the adult form. In the presence of 4-aminopyridine, potentials resembling the Cs-FP were evoked. These potentials showed a similar age-dependency as the Cs-FP. The Na(+)/K(+) pump inhibitor dihydroouabain (DHO), when present during wash-in of Cs(+), gave a partial block of the Cs-FP in adult slices. This effect was not seen when DHO was applied after development of the Cs-FP. The data indicate that the processes necessary for expression of the Cs-PF are absent at birth and develop during the second postnatal week. We propose that the Cs-FP depends on Cs(+) entry into presynaptic neurons, and that the Na(+)/K(+) pump contributes to this transport of Cs(+). The observed age-dependency could therefore, in part, reflect the delayed development of the Na(+)/K(+) pump.

New Type of Synaptically Mediated Epileptiform Activity Independent of Known Glutamate and GABA Receptors

It is well known that excitatory synaptic transmission at the hippocampal CA3–CA1 synapse depends on the binding of released glutamate to ionotropic receptors. Here we report that during long-term application of Cs+ (5 mM), stimulation of the Schaffer collateral-commisural pathway evokes an epileptic field potential (Cs-FP) in area CA1 of the rat hippocampal slice, which is resistant to antagonists of ionotropic glutamate and GABAA receptors. The Cs-FP was blocked by N-type but not L-type Ca2+ channel antagonists and was attenuated by adenosine (0.5 mM), as expected for a synaptically mediated response. These properties make the Cs-FP fundamentally different from other types of Cs+-induced epileptiform activity. Replacement of Cs+ with antagonists of the hyperpolarization-activated nonselective cation current Ih and inwardly rectifying potassium channels (KIR) or partial inhibition of the Na+/K+ pump did not cause Cs-FP–like potentials, which indicates that such actions of Cs+ were not responsible for the Cs-FP. The effect of Cs+ was partly mimicked by 4-aminopyridine (4-AP; 2 mM), suggesting that an increase in transmitter release is involved. The group I metabotropic glutamate receptor (mGluR) agonist ( RS)-3,5-dihydroxyphenylglycine (DHPG) attenuated the Cs-FP. This effect was not, however, antagonized by group I mGluR antagonists. Selective and nonselective mGluR antagonists did not attenuate the Cs-FP. We conclude that long-term exposure to Cs+ induces a state where excitatory synaptic transmission can exist between area CA3 and CA1 in the hippocampus, independent of ionotropic and metabotropic glutamate receptors and GABAA receptors.

A potential role for astrocytes in mediating the antiepileptic actions of furosemide in vitro

Epileptic seizures are characterized by abnormal electrical discharge. In previous studies we established a powerful antiepileptic action for a commonly used diuretic (furosemide). However, it remains unclear precisely how furosemide terminates abnormal electrical discharges. To address this issue, we performed in vitro experiments to examine conditions where furosemide exerts antiepileptic activity and patch-clamp studies to analyze the effect of furosemide on neuronal membrane properties, synaptic function and inward potassium current. Furosemide was not found to alter synaptic field responses, excitatory postsynaptic currents or intrinsic membrane properties of principal hippocampal neurons. Our in vitro studies indicate that furosemide does not abolish spontaneous epileptiform bursting during co-application of Ba2+ or Cs+ ions (to block inwardly rectifying potassium channels). Our patch-clamp data indicate that furosemide enhances the function of astrocytic, but not neuronal, inward potassium channels and that this modulation may be required for its antiepileptic activity. Although a variety of antiepileptic drugs are already available, none of these compounds selectively target astrocytes while preserving synaptic/neuronal function. Thus, furosemide-mediated modulation of inward potassium current (on astrocytes) represents a new target for control of abnormal electrical discharge in the CNS.

Cesium induces spontaneous epileptiform activity without changing extracellular potassium regulation in rat hippocampus

Cesium has been widely used to study the roles of the hyperpolarization-activated (I(h)) and inwardly rectifying potassium (K(IR)) 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 K(IR) 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 I(h) blocker, 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 I(h). 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.

Reduction of K+ uptake in glia prevents long-term depression maintenance and causes epileptiform activity

Extracellular cesium causes synchronous, interictal-like bursting and prevents maintenance of long-term depression (LTD) in the CA1 hippocampal region. We have investigated the cellular mechanisms underlying cesium actions. Whole-cell recordings showed that brief (2 min) bath exposures to cesium caused pyramidal cell hyperpolarization associated with decreased membrane conductance attributable to blockade of an inward h-type current. After prolonged (>2 min) exposures, a late depolarizing response was observed; this effect was not associated with changes in cell membrane conductance. Recordings from interneurons revealed that Ih is expressed in a subpopulation of cells and that cesium effects on interneurons expressing Ih are comparable to those observed in pyramidal cells. Consistent with this effect, cesium decreased the early component of the IPSP recorded in pyramidal cells. Interneurons lacking Ih were not affected by cesium but developed a depolarizing response when drug applications were paired to orthodromic stimulation. We concluded that cesium actions on LTD and cesium-induced epileptiform activity were not attributable exclusively to its direct effects on neurons. Recordings from hippocampal slice astrocytes revealed that cesium interfered with glial electrical responses during LTD induction. Cesium blocked glial inwardly rectifying potassium channels and increased the amplitude and duration of stimulation-evoked [K+]out increases. Thus, the effects of cesium on CA1 synchronization and synaptic plasticity appear to be mediated predominantly by blockade of glial voltage-dependent potassium uptake.