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

Zhi-Qi Xiong and Janet L. Stringer

Department of Pharmacology and Division of Neuroscience, Baylor College of Medicine, Houston, Texas 77030


    ABSTRACT
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Xiong, Zhi-Qi and Janet L. Stringer. Cesium Induces Spontaneous Epileptiform Activity Without Changing Extracellular Potassium Regulation in Rat Hippocampus. J. Neurophysiol. 82: 3339-3346, 1999. Cesium has been widely used to study the roles of the hyperpolarization-activated (Ih) and inwardly rectifying potassium (KIR) channels in many neuronal and nonneuronal cell types. Recently, extracellular application of cesium has been shown to produce epileptiform activity in brain slices, but the mechanisms for this are not known. It has been proposed that cesium blocks the KIR in glia, resulting in an abnormal accumulation of potassium in the extracellular space and inducing epileptiform activity. This hypothesis has been tested in hippocampal slices and cultured hippocampal neurons using potassium-sensitive microelectrodes. In the present study, application of cesium produced spontaneous epileptiform discharges at physiological extracellular potassium concentration ([K+]o) in the CA1 and CA3 regions of hippocampal slices. This epileptiform activity was not mimicked by increasing the [K+]o. The epileptiform discharges induced by cesium were not blocked by the N-methyl-D- aspartate (NMDA) receptor antagonist AP-5, but were blocked by the non-NMDA receptor antagonist CNQX. In the dentate gyrus, cesium induced the appearance of spontaneous nonsynaptic field bursts in 0 added calcium and 3 mM potassium. Moreover, cesium increased the frequency of field bursts already present. In contrast, ZD-7288, a specific Ih 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 Ih. When potassium-sensitive microelectrodes were used to measure [K+]o, there was no significant increase in [K+]o in CA1 and CA3 after cesium application. In the dentate gyrus, cesium did not change the baseline level of [K+]o or the rate of [K+]o clearance after the field bursts. In cultured hippocampal neurons, which have a large and relatively unrestricted extracellular space, cesium also produced cellular burst activity without significantly changing the resting membrane potential, which might indicate an increase in [K+]o. Our results suggest that cesium causes epileptiform activity by a mechanism unrelated to an alteration in [K+]o regulation.


    INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Cesium is a potassium channel blocker that has been widely used to study the roles of certain potassium channels. Recently, cesium has been shown to have epileptogenic properties in neocortical and hippocampal slices (Hwa and Avoli 1991; Maccaferri et al. 1994). This epileptogenic effect has been suggested to be related to an alteration in extracellular potassium regulation by glia (D'Ambrosio et al. 1998; Janigro et al. 1997).

Extracellular application of cesium has been shown to cause a voltage-dependent block of both inwardly rectifying potassium (KIR) and H currents (Ih) (Maccaferri et al. 1993; Spain et al. 1987). It has been shown that only Ih is expressed in hippocampal CA1 neurons (Janigro et al. 1997; Maccaferri et al. 1993), whereas both KIR and Ih have been found in hippocampal astrocytes (Bayliss et al. 1994; Guatteo et al. 1996; Sontheimer and Waxman 1993). Neuronal Ih is thought to be responsible for control of resting membrane potential, and blockade of Ih hyperpolarizes neurons (Maccaferri et al. 1993). KIR is widely distributed in astrocytes and plays a dominant role in setting astroglial resting potentials (Duffy et al. 1995). The biophysical features of astrocyte KIR are consistent with those properties required for their proposed involvement in potassium spatial buffering. The channel has a high open probability at the resting potential, it increases conductance with increasing extracellular potassium concentration ([K+]o), and it shows rectification at positive potentials (Ransom and Sontheimer 1995). It has been suggested that clearance of [K+]o after neuronal activity is mediated primarily by the activity of these astrocytic KIR channels (Ballanyi et al. 1987; Berger et al. 1991; D'Ambrosio et al. 1998; Ransom and Sontheimer 1995; Soliven et al. 1988).

It has been concluded that the effects of cesium on epileptiform activity may be mediated predominantly by blockade of glial voltage-dependent potassium uptake (Janigro et al. 1997). This is based on several observations. First, cesium causes epileptiform activity in bicuculline-treated slices, but not when normal synaptic inhibition is present (Hwa and Avoli 1991), and cesium causes epileptiform discharges in hippocampal slices during 1-Hz stimulation, but not when paired with 0.1-Hz stimulation (Janigro et al. 1997). Stimulation at 1 Hz is sufficient to increase extracellular potassium (Janigro et al. 1997) and thus stimulate glial uptake, which would be blocked in the presence of cesium. Second, cesium causes an early hyperpolarization and late depolarization in CA1 pyramidal neurons, and the delayed depolarizing effects of cesium are not accompanied by further changes in input resistance (Janigro et al. 1997). This suggests that the late depolarization is due to an increase in [K+]o related to cesium block of potassium uptake. Third, the specific Ih channel blocker ZD-7288 does not cause epileptiform activity (Janigro et al. 1997), suggesting that cesium blockade of the KIR is responsible for the epileptiform activity. However, studies on the characterization of KIR in astrocytes show that cesium displays no blockade of KIR at potentials close to the resting potential but increases blockade with more negative potentials (Ransom and Sontheimer 1995). This suggests that addition of cesium would have no effect on channel conductance when the membrane has been depolarized, as with bicuculline or low-frequency stimulation.

The experiments presented in this article addressed the following questions: 1) Can cesium induce spontaneous epileptiform activity in hippocampal slices or in cultured hippocampal neurons in the absence of stimulation or blockade of inhibitory systems? 2) Does increasing the [K+]o mimic the effect of adding cesium? 3) Does addition of cesium change the regulation of [K+]o?


    METHODS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Hippocampal slices were prepared by conventional methods from Sprague-Dawley rats (100-200 g, both sexes). After the rats were anesthetized (25 mg/kg ketamine, 5 mg/kg xylazine, 0.8 mg/kg acepromazine, ip), the brains were removed. Transverse slices (400-500 µm) through the hippocampus were cut with a Vibratome (Technical Products). Slices were placed in an interface-type chamber and continuously perfused with artificial cerebrospinal fluid (ACSF) at 33°C under a stream of humidified 95% O2-5% CO2. Composition of the ACSF was (in mM) 127 NaCl, 2 KCl, 1.5 MgSO4, 1.1 KH2PO4, 26 NaHCO3, 2 CaCl2, and 10 glucose. All solutions were bubbled constantly with 95% O2-5% CO2. Thus, the total potassium concentration in normal ASCF was 3.1 mM. Slices were allowed to equilibrate for 1 h before electrophysiological recording was begun.

Recording electrodes were made of microfilament capillary thin-walled glass (0.9 mm ID, 1.2 mm OD; A-M Systems) pulled on a micropipette puller (P-87, Sutter Instruments). Intracellular electrodes were filled with 4 M potassium acetate and had impedances between 60 and 100 MOmega (tested with 20-nA current pulses). Extracellular electrodes were filled with 2 M NaCl and had impedances between 4 and 10 MOmega .

The spontaneously occurring epileptiform activities were induced by bath application of different concentration of CsCl (2-6 mM) in normal ACSF or by raising extracellular potassium concentration to 9-12 mM. Nonsynaptic epileptiform activity was induced in the dentate gyrus by changing to ACSF containing 0 added calcium and 8 mM potassium. This spontaneous epileptiform activity, termed field bursts, is associated with DC potential shifts and with population spikes that can measure in tens of millivolts. Field bursts can take a long time to develop (1-2 h), but once they appear, they remain stable for many hours (Pan and Stringer 1996).

DC-coupled recording of field potentials and [K+]o were measured with double-barreled, ion-sensitive electrodes (Stringer and Lothman 1989). One barrel was silanized with 15% tri-N-butylchlorosilane (Alfrebro, Monroe, OH) in chloroform, and the tip was filled with potassium-selective resin (Fluka Cocktail "B" or Corning 477317). The electrode was then backfilled with 1 M potassium acetate. The reference barrel was filled with 2 M NaCl. The reference and potassium signals were amplified and displayed on a chart recorder. The electrode was calibrated before each experiment in a series of standard solutions in ACSF (2, 3, 5, 10, and 20 mM K+) with or without 3 mM CsCl. Addition of cesium to the standard solutions altered the electrical potential measured but did not change the sensitivity of the electrode to the changes in potassium concentrations. During each experiment, the electrode was also calibrated by moving the electrode from the slice to the perfusion solution and comparing the potential recorded in the perfusing solution to that recorded from the tissue. Recordings were made 50-100 µm below the slice surface. The potassium microelectrode readings were allowed to stabilize at each locale for >= 2 min before the measurement was taken.

Hippocampal neurons were cultured at low density, as described elsewhere (Zarei and Dani 1994). Sprague-Dawley rats (5 d old) were anesthetized with halothane and decapitated. Under a dissection microscope, the area dentata of the hippocampus was dissected in ice-cold balanced salt solution (BSS) containing (in mM) 1.8 CaCl2, 0.81 MgSO4, 5.4 KCl, 140 NaCl, 5.55 D-glucose, and 5 HEPES with 0.01-0.1 gm/l phenol red, pH 7.3. The tissue was incubated in a sterile enzyme solution (BSS supplemented with 1.5 mM CaCl2, 0.5 mM EDTA, 0.2 mg/ml L-cysteine, and 20 U/ml papain) at 37°C in 5% CO2 for 30 min with gentle rocking. After incubation, tissue was washed and dissociated by triturating in MEM supplemented with 10% FCS (HyClone, Logan, UT), 10% horse serum (HS; Life Technologies, Gaithersburg, MD), 20 mM glucose, 1 µl/ml serum extender (Mito+; Collaborative Biomedical Products), 50 U/ml penicillin-streptomycin (Life Technologies), 2.5 mg/ml trypsin inhibitor, and 2.5 mg/ml BSA. Cells were plated at 70,000-450,000 cells/ml onto microislands that were prepared by coating coverslips with a thin layer of 0.15% agarose. The slips were sprayed with a mixture of 0.05 mg/ml poly-D-lysine and 0.25 mg/ml collagen, by using a glass microatomizer. Cells were kept at 37°C in 5% CO2 and fed 2 to 3 times per week with MEM supplemented with 5% HS, 20 mM glucose, 50 U/ml penicillin-streptomycin, 1 µl/ml serum extender, 10 mM MgCl2, 0-0.5 µM TTX, 0-1 nM methyllycaconitine, and 0-0.5% wt/vol BSA and were studied between days 15 and 25. Glial proliferation was inhibited on days 3-5 by 5 µM cytosine arabinofuroside.

Before electrical recording began, a microisland was transferred to a submerged chamber and continuously perfused with ACSF at room temperature. The cultured neurons were allowed to equilibrate for 1 h before electrophysiological recording began. The patch electrodes were pulled in two stages (PP-83, Narishige USA) using glass tubes (Garner Glass). The pipettes were then coated with Sylgard silicon elastomer; polished immediately before experiments, using a microforge (Narishige USA), to a final resistance of 2-4 MOmega ; and filled with a solution containing (in mM) 135 K-gluconate, 20 KCl, 10 HEPES, 0.6 EGTA, 4 Mg-ATP, 0.3 Na-GTP, and 7 creatine phosphate. The signals were amplified and filtered using an Axoclamp patch-clamp amplifier and were collected and analyzed using Axobasic and pClamp computer programs.

All compounds were purchased from Sigma (St Louis, MO), except Zeneca ZD-7288 (Tocris, Ballwin, MO). All chemicals were dissolved directly into the perfusing solution. For statistical analysis, a paired Student's t-test was used to compare measures before and after treatment. Significance level was set at P < 0.05. Data are expressed as means ± SE.


    RESULTS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Cesium caused spontaneous activity that was not mimicked by perfusion with high-potassium solutions

The first set of experiments was designed to test whether cesium can induce spontaneous epileptiform activity in the absence of stimulation or any other convulsant treatment. When a bath was applied to hippocampal slices, 2-6 mM CsCl produced spontaneously occurring epileptiform discharges at physiological [K+]o (3.1 mM) in the CA1 and CA3 regions of all slices (Fig. 1, n = 46). Simultaneous intracellular and extracellular recording showed that the duration and frequency of intracellular bursts were closely correlated with the field discharges (Fig. 1C, n = 7). The perfusion time needed to develop epileptiform activity depended on the cesium concentration. For example, CsCl at 2 mM needed >90 min to induce the spontaneous epileptiform activity, whereas CsCl at 6 mM caused spontaneous activity <= 30 min. The time needed to wash out cesium's effects also depended on the duration and concentration of exposure to cesium. For example, after perfusion of 3 mM cesium for 3 h, epileptiform discharges were still present after 1 h of washing. More than 90 min of washing was required before the spontaneous discharges stopped (n = 5, data not shown).



View larger version (27K):
[in this window]
[in a new window]
 
Fig. 1. Spontaneous epileptiform activity produced by 3 mM cesium. A: simultaneous chart recordings from the CA1 (top), CA3 (middle), and dentate gyrus (bottom) regions of a hippocampal slice show the epileptiform activity induced by 2 h of perfusion with 3 mM cesium. Extracellular potassium concentration was 3.1 mM and calcium was 2 mM. B: epileptiform activity illustrated in A shown on a faster time base. C: example of spontaneous epileptiform discharges recorded extra- and intracellularly in the CA1 region. Calibrations are indicated on the figure.

Because these epileptic actions of cesium could be attributed to the blockade of neuronal Ih, we used a specific Ih channel blocker ZD-7288 to test the contribution of Ih to cesium-induced epileptiform activity. In contrast to cesium, 20 µM ZD-7288, a dose that is 10 times higher than the apparent half-maximum concentration to block Ih (Harris and Constanti 1995), failed to cause spontaneous epileptiform activity in CA1 or CA3 after a 2-h perfusion (n = 3).

In 60% of slices treated with CsCl, a spreading depression occurred every 3-40 min (Fig. 2A). Although the time interval between spreading-depression episodes was different from slice to slice, it was also dose dependent. In 19 slices perfused with 3 mM CsCl, only 4 slices showed a spreading depression, and the interval between spreading depression episodes was ~30 min. In slices perfused with 6 mM CsCl, all (n = 6) had spreading depression after a 3-h perfusion. The interval between spreading-depression episodes was 4.5 min. Once spreading depression began, it spread throughout the whole hippocampus. In most cases, it occurred first in the dentate gyrus. During recovery from spreading depression, the dentate gyrus also showed epileptiform discharges. After washout of CsCl, the spreading depression stopped, and orthodromic stimulation was able to evoke fast excitatory postsynaptic potentials and population spikes of normal amplitude in the CA1 area of the slice.



View larger version (11K):
[in this window]
[in a new window]
 
Fig. 2. Comparison of spreading-depression episodes induced by cesium and high potassium in hippocampal slices. A spreading-depression episode recorded simultaneously in CA1 (top), CA3 (middle), and dentate gyrus (bottom) induced by 3 mM cesium (A) with [K+]o at 3 mM or by 10 mM potassium with no cesium (B) in different slices. Calibrations are indicated on the figure.

To compare the effects of cesium to the effects of high [K+]o in the CA1 and CA3 hippocampal regions, we perfused the slices with high [K+]o. Only 2 of 15 slices bathed in 9 mM [K+]o showed epileptiform activity after a 1-h perfusion. When perfused with 10 mM [K+]o, 10 of 21 slices showed spontaneous epileptiform activity in CA3 and CA1 (Fig. 3, A and D) after 45 min of perfusion. In contrast to the continuous epileptiform discharges induced by cesium, epileptiform activity induced by 9 or 10 mM potassium consisted of two phases: an initial discharge associated with a negative DC shift and later discharge not associated with a DC shift followed a silent period. This silent interval was regular within an individual slice but ranged between 30 s and 3 min between slices. Increasing [K+]o decreased this silent interval until continuous discharges occurred when the [K+]o was >= 12 mM (Fig. 3, B and C, n = 5 at each concentration).



View larger version (27K):
[in this window]
[in a new window]
 
Fig. 3. High-potassium induced epileptiform activity in CA1 and CA3. A: extracellular discharges in the CA1 and CA3 of a single hippocampal slice induced by 2 h of perfusion with 10 mM [K+]o. The second epileptiform burst, for which numbers appear above the recording, is shown on an expanded time base in D. B: as in A, except the potassium concentration in the perfusing solution was increased to 12 mM. C: as in A, except the potassium concentration in the perfusing solution was increased to 14 mM. D: expansion of the epileptiform activity in 10 mM [K+]o illustrated in A. Calibrations are indicated on the figure.

Spreading-depression episodes also occurred in 30% of slices that showed spontaneous epileptiform activity after prolonged perfusion with high-potassium solutions. For example, 2 of 10 slices showed spreading-depression episodes during epileptiform discharges induced by 10 mM potassium. The spreading depression induced by high potassium differed from that induced by cesium. The recovery phase was slower, and there was no epileptiform activity during and after the spreading depression in high [K+]o (Fig. 2B).

Because epileptogenesis could be attributable to an increase in excitatory transmission, we used the N-methyl-D-aspartate (NMDA) receptor antagonist AP-5 and the non-NMDA receptor antagonist CNQX to examine the roles played by these excitatory amino acid receptors in the spontaneous epileptiform discharges in CA1 and CA3 induced by extracellular application of cesium. The non-NMDA receptor antagonist CNQX (20 µM) abolished the epileptiform activity (Fig. 4A, n = 5). In contrast, blockade of NMDA receptors by AP-5 (0.1 mM) had no obvious effects on the epileptiform discharges induced by 3 mM cesium in CA1 and CA3 (Fig. 4B, n = 5).



View larger version (17K):
[in this window]
[in a new window]
 
Fig. 4. Effects of excitatory amino acid receptor antagonists on cesium-induced spontaneous epileptiform activity. A: chart recordings of extracellular spontaneous epileptiform activity in CA1 and CA3 after ~2 h of perfusion with 3 mM CsCl are presented (A1). Addition of CNQX (20 µM) to the bath medium blocked these spontaneous discharges after 10 min (A2). Spontaneous epileptiform activity recovered after washing for ~30 min (A3). B: as in A, except AP-5 (0.1 mM) was added.. AP-5 had no obvious effects on cesium-induced epileptiform activity. Calibrations are indicated on the figure.

Having demonstrated that cesium could cause spontaneous epileptiform discharges in the CA1 and CA3 areas that are dependent on synaptic transmission mediated through non-NMDA receptors, we investigated whether synaptic transmission is necessary for the effect of cesium by testing the effect of the cesium on nonsynaptic field bursts in the dentate gyrus. When hippocampal slices are bathed in 8 mM K+ and 0 added Ca2+, spontaneous regular field bursts developed in the dentate gyrus. These field bursts do not require synaptic transmission for their expression (Schweitzer et al. 1992). They can be prolonged, lasting a few seconds up to many tens of seconds.

After the appearance of recurrent field bursts in 8 mM [K+]o and 0 added calcium, addition of CsCl to the perfusing solution (1-3 mM) caused a decrease in the interval between bursts (an increase in burst frequency; Fig. 5A, n = 55). The effect of cesium could be quickly washed out, within 15 min. Moreover, cesium (3 mM) also induced spontaneous field bursts in the dentate gyrus perfused with 0 added Ca2+ and 3 mM K+ (data not shown, n = 4). In contrast to cesium, the Ih-specific blocker ZD-7288 (20 µM) did not affect this nonsynaptic field burst activity (Fig. 5B, n = 6).



View larger version (16K):
[in this window]
[in a new window]
 
Fig. 5. Effect of cesium on nonsynaptic epileptiform activity in the dentate gyrus. A: spontaneous field bursts were induced in the dentate gyrus by perfusing with 0 added calcium and 8 mM [K+]o A1: addition of 3 mM CsCl to the perfusing solution increased the frequency of the field bursts and decreased the time between bursts (A2). The tracings in A2 were continuous and began 2 min after the addition of CsCl to the bath medium. Washout of cesium is shown in A3. Chart recording was taken 15 min after switching to ACSF without cesium. B: as in A, after the induction of regular field bursts by a solution containing 0 added calcium and 8 mM [K+]o (B1), the Ih-specific blocker ZD-7288 (20 µM) was added to the bath medium (B2). In contrast to cesium, ZD-7288 did not affect the field bursts after 30 min of perfusion. Calibrations are indicated on the figure.

To rule out the possibility that cesium was affecting nonsynaptic field burst activity by release of glutamate from neurons or glial cells, we included AP-5 and CNQX in the perfusion medium. Bath application of AP-5 (0.1 mM) and CNQX (20 µM) did not modify the burst activity induced with 8 mM K+ and 0 added Ca2+ medium. However, addition of 2 mM cesium immediately increased the field burst frequency in the presence of AP-5 and CNQX (data not shown, n = 3).

Cesium does not interfere with potassium clearance during and after epileptiform activity

To test whether cesium was causing epileptiform activity by increasing [K+]o, we measured [K+]o in the CA1 and CA3 regions. Cesium did not change the baseline [K+]o in CA1 and CA3 when measured just before the onset of the spontaneous epileptiform discharges. During the epileptiform activity, extracellular potassium increased by only 0.5-2 mM (above baseline) in CA1 and CA3 (Fig. 6A, n = 10). Because the potassium-sensitive microelectrode is also sensitive to cesium, during each experiment, the electrode was calibrated by moving it from the slice to perfusion solution and comparing the electrical potential in the bath (known concentration) with the tissue (unknown concentration). It was assumed that the concentration of cesium was the same in the bath and tissue and that the difference in electrical potential recorded by the ion-sensitive electrode was the difference in [K+]o in the tissue compared with the bath.



View larger version (18K):
[in this window]
[in a new window]
 
Fig. 6. Effects of cesium of potassium regulation. A: chart recordings of the reference electrode recording of the extracellular field potentials in the CA3 region (f.p.) and differential recording of the extracellular potassium concentration ([K+]o). In a slice perfused with normal ACSF, the [K+]o is ~3.1 mM (A1). Addition of 3 mM cesium did not change the baseline potassium level before the occurrence of spontaneous epileptiform activity (A2). During cesium-induced epileptiform activity, the [K+]o fluctuated between 3.5 and 5 mM (A3). B: chart recordings of the extracellular field potentials in the dentate gyrus (f.p.) and extracellular potassium concentration ([K+]o). B1: recurrent field bursts in 0 added calcium and 8 mM [K+]o, along with the associated changes in [K+]o. Chart recordings in B2 were obtained 30 min after switching to a perfusing solution with only 5 mM [K+]o. Addition of 3 mM CsCl to the perfusion solution caused the reappearance of field bursts without a significant increase in baseline [K+]o. Chart recordings in B3 were obtained 30 min after addition of the cesium to the perfusing solution. The rate of recovery of [K+]o after a field burst was the same in B3 as that shown in B1. Calibrations are indicated on the figure.

To test whether cesium affects potassium clearance during and after epileptiform activity, we measured [K+]o before, during, and after the nonsynaptic field bursts in the dentate gyrus after cesium application (Fig. 6B). Because of the effect of cesium on burst frequency as shown in Fig. 5A, it was impossible to make an accurate measurement of baseline potassium level during cesium treatment in 8 mM [K+]o. Therefore, we lowered the [K+]o to test the effect of cesium on potassium regulation. After the induction of the field bursts in the dentate gyrus by 8 mM K+ and 0 added Ca2+, the perfusion solution was switched to 5 mM K+ and 0 added Ca2+ medium. Ten minutes later, the spontaneous field bursts stopped, and the [K+]o decreased from 8 to 5 mM. Addition of cesium to the perfusing solution (3 mM) caused the reappearance of field bursts. The midpoint [K+]o between two consecutive field bursts was taken as the baseline level. The half-time of [K+]o decline after field burst termination was used as an index of the rate of [K+]o recovery. Cesium did not significantly change the baseline potassium level (before cesium: 5.14 ± 0.11 mM; after 3 mM cesium: 5.09 ± 0.16 mM, n = 5) and did not slow the rate of clearance of potassium after field bursts (before cesium: 3.62 ± 0.2 s; after 3 mM cesium: 3.67 ± 0.31 s; n = 5).

To rule out the possibility that the epileptic action of cesium comes from a local potassium increase and that the potassium-sensitive microelectrode did not detect this localized potassium change, we tested the effects of cesium on dentate gyrus neurons (n = 5) in culture, where there is a large and relatively unrestricted extracellular space. Prolonged application (>20 min) of cesium at 5 mM produced bursting (Fig. 7) that occurred spontaneously (without stimulation or addition of convulsants). The effect of cesium could be washed out in ~1 h. Cesium at 5 mM did not significantly change the resting membrane potential after a 1-h perfusion (before cesium: -80.5 mV; after 5 mM cesium: -78.2 mV. n = 5).



View larger version (30K):
[in this window]
[in a new window]
 
Fig. 7. Cesium caused cellular bursting in cultured hippocampal neurons. Examples of spontaneous cellular bursts in two dentate gyrus neurons in culture induced by bath application of 5 mM cesium are shown. Before application of cesium (1) there was little spontaneous activity. After a 20-min exposure to 5 mM CsCl (2), cellular bursting developed at the resting potential. The recordings in 2 are continuous. One burst for each cell (*) is shown below on an expanded time scale. The effects of cesium reversed in 40 min of washing (3). Calibrations are indicated on the figure.


    DISCUSSION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

We demonstrated that extracellular cesium application could induce spontaneous epileptiform activity in a rat hippocampal slice without stimulation or bicuculline, and the epileptiform activity was not replicated by increasing the [K+]o. Measurements of the [K+]o indicated that cesium did not change the baseline level of [K+]o or the rate of recovery of [K+]o from elevated levels. Altogether, the data suggest that the bursting induced by cesium was not mediated simply by an increase in the extracellular potassium concentration.

Previous studies have shown that cesium can produce epileptiform activity in slices treated with bicuculline (Hwa and Avoli 1991) or paired with 1-Hz stimulation (Janigro et al. 1997), protocols that may be sufficient to increase the release of potassium from active neurons and stimulate glial uptake of potassium. In this study, we demonstrated that cesium could induce spontaneous epileptiform activity in CA1 and CA3 regions perfused with normal ACSF. It can be postulated that the neurons in the hippocampal slice perfused with normal ACSF are nominally at rest. At rest, it is believed that neurons release little potassium (Newman 1995) and that during low-frequency, unstimulated, and asynchronous neuronal activity, [K+]o is buffered independently from glial depolarization (Casullo and Krnjevic 1987). Therefore, the addition of a glial potassium uptake inhibitor would be predicted to have little or no effect. In addition, cesium (the presumed glial potassium uptake blocker) does not block glial KIR at the resting membrane potential (Ransom and Sontheimer 1995). These data suggest that the action of cesium in inducing epileptiform activity is independent of glial potassium uptake.

To further support our conclusion that the epileptogenic effect of cesium is independent of an alteration in [K+]o regulation, we compared the epileptiform discharges induced by cesium with those induced by high [K+]o. The epileptiform activity and spreading depression induced by cesium were different in morphology than those induced by increasing [K+]o. This suggests that the effect of cesium is not simply due to an increase in the [K+]o and suggests that cesium may have another action besides blocking the glial potassium uptake. In cultured hippocampal neurons, which have a large and relatively unrestricted extracellular space, cesium also induced spontaneous cellular bursting without significantly depolarizing the resting potential, which would result from an increase in [K+]o. Finally, direct measurement of [K+]o did not detect any significant [K+]o increase before cesium induced spontaneous discharges in CA1 and CA3. In the dentate gyrus, cesium did not alter the baseline potassium level between field bursts or rate of recovery of [K+]o. These data further suggest that extracellular potassium accumulation or changes in the regulation of [K+]o are not necessary for the epileptogenic action of cesium.

The results of the present study show that cesium caused spontaneous interictal discharges in the CA1 and CA3 regions of hippocampal slices mediated through glutamatergic transmission (blocked by CNQX). This effect may originate in the CA3 region, because the CA3 region is known to be necessary for the expression of interictal discharges in different in vitro seizure models. A previous study suggested that cesium-induced epileptiform activity originates in the CA3 region, because separation of the two regions obliterates bursting in CA1 but not in CA3 (D'Ambrosio et al. 1998). Studies in the low-Ca2+ model of epileptiform activity also have shown that cesium has no effect on CA1 field bursts (Bikson et al. 1998). We also found that cesium induced epileptiform activity in the dentate gyrus. These observations suggest that a cesium-sensitive conductance in CA3 and the dentate gyrus, not in CA1, plays an important role in controlling normal excitability in the mammalian hippocampus.

How might cesium be producing epileptiform activity, if not through a blockade of glial potassium uptake? It does not appear to be through a neuronal blockade of Ih, because the Ih-specific channel blocker ZD-7288 did not cause spontaneous epileptiform activity in the present experiments and in previous studies (Gasparini and Difrancesco 1997; Janigro et al. 1997). What cesium-sensitive conductance in CA3 and the dentate gyrus could account for the epileptogenic action of cesium? Several explanations could account for the cesium-induced hyperexcitability. Cesium may lead to a generalized blockade of potassium currents. If the A and K currents are reduced by cesium, enhanced transmitter release could occur. Indeed, extracellular application of cesium has been shown to prolong action potential duration (Hwa and Avoli 1989) and to increase the quantal release of excitatory transmitters (Kumamoto and Kuba 1985). Intracellular application of Cs+ is known to block a variety of potassium currents. If some cesium gets into the neurons and their axons during a long exposure time, a modest nonspecific blockade of potassium current may result. This may contribute to the abnormal epileptiform activity observed in the CA1 and CA3 areas.

However, the present studies in the dentate gyrus suggest that cesium can have a rapid effect, even in the absence of synaptic transmission. In addition to glial cells, many neurons also have KIR channels that are sensitive to cesium. KIR channels in neurons may be activated by neurotransmitters through G-proteins. For example, GABA, serotonin, and adenosine have been found to activate KIR current in hippocampal pyramidal neurons (Sodickson and Bean 1998). It has been reported that baclofen-induced GABAB inhibitory postsynaptic potentials are antagonized by cesium in CA3 neurons (Jarolimek et al. 1994). Serotonin-activated hypopolarizing currents are also blocked by cesium (Williams et al. 1988). Interestingly, recent studies have shown that pore mutation in Weaver mice KIR3.2 channels causes loss of potassium-dependent inhibition in the CA3 region of the hippocampus (Jarolimek et al. 1998) and KIR3.2-deficient mice exhibit spontaneous seizure activity (Signorini et al. 1997). On the other hand, overexpression of KIR channel genes driven by an inducible promoter inhibits evoked and spontaneous activity in cultured neurons. Blocking expressed channels reverses the suppression of excitability (Johns et al. 1999). Therefore, the cesium-induced epileptiform activity we describe may result from a reduction of neuronal KIR conductance.


    ACKNOWLEDGMENTS

The authors thank Dr. J. Dani and D. Ji for assistance with the experiments on cultured neurons.

This work was supported by National Institute of Neurological Disorders and Stroke Grant NS-01784 to J. S. Stringer.


    FOOTNOTES

Address for reprint requests: Dept. of Pharmacology, Baylor College of Medicine, One Baylor Plaza, Houston, TX 77030.

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

Received 26 April 1999; accepted in final form 30 August 1999.


    REFERENCES
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

0022-3077/99 $5.00 Copyright © 1999 The American Physiological Society