Departments of Neurology and Physiology, Reed Neurological Research Center, UCLA School of Medicine, Los Angeles, California 90095-1769
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ABSTRACT |
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Behr, J., K. J. Lyson, and I. Mody. Enhanced propagation of epileptiform activity through the kindled dentate gyrus. J. Neurophysiol. 79: 1726-1732, 1998. Extracellular recordings were performed in combined hippocampal-entorhinal cortex (HC-EC) slices obtained from control and commissural kindled rats to investigate the propagation of epileptiform activity from the entorhinal cortex (EC) to the hippocampus (HC) after chronic epilepsy. Lowering extracellular Mg2+ concentration in control slices induced epileptiform activity consisting of spontaneous epileptiform bursts in area CA3 and of electrographic seizures in the EC. In contrast, the CA3 region of HC-EC slices obtained from kindled rats displayed significantly longer lasting epileptiform bursts and electrographic seizures. The electrographic seizures that were absent in controls propagated from the EC because disconnecting the HC from the EC stopped their occurrence in the CA3, whereas epileptiform bursts persisted with an unaltered pattern and frequency. Thus the area CA3 is affected by kindling and contributes to the spread of epileptiform activity within the EC-HC complex. We developed a method to induce focal epileptiform activity in the EC by locally perfusing the -aminobutyric acid-A (GABA) antagonist bicuculline (50 mM) in 10 mM KCl containing artificial cerebrospinal fluid. This method enabled us to investigate the propagation of epileptiform discharges from the disinhibited EC to the DG without affecting the DG with the epileptogenic medium. We show here that kindling facilitates the propagation of epileptiform activity through the DG. These data are consistent with the normal function of the DG as a filter limiting the spread of epileptiform activity within the HC-EC complex. This gating mechanism breaks down after chronic epilepsy induced by kindling.
Numerous studies investigated alterations in the dentate gyrus (DG) during epileptogenesis (for reviews, see Ribak et al. 1992 The experiments were performed on 21 kindled and 21 control horizontal slices containing the EC, the subiculum, and the hippocampal formation obtained from 30 implanted 450- to 600-g adult Wistar rats. Bipolar stainless steel electrodes were implanted under pentobarbital sodium anesthesia (75 mg/kg ip) into the midline hippocampal commissures (AP,
Lowering the extracellular Mg2+ concentration in the ACSF induced different patterns of epileptiform activity in HC-EC slices. In the EC of control slices, 30-60 min after lowering Mg2+, spontaneous electrographic seizures could be observed lasting between 5 and 67 s (36 ± 6.8 s, n = 10; Fig. 2A, top left). In area CA3, the electrical activity consisted of epileptiform bursts with a mean duration of1.1 ± 0.2 s and a frequency of 0.05-0.27 s
We investigated the impact of kindling-induced alterations in the DG and area CA3 on the propagation of epileptiform activity in combined HC-EC slices. After chronic epilepsy, the epileptiform activity generated in the EC propagated more easily to the DG and area CA3.
INTRODUCTION
Abstract
Introduction
Methods
Results
Discussion
References
; Schwartzkroin 1993
). These alterations seem to play a major role in the propagation of epileptiform activity from the entorhinal cortex (EC) to the hippocampus (HC).
; Jones and Heinemann 1988
; Walther et al. 1986
). Areas CA3 and CA1 exhibit epileptiform bursts that are quite stable over time (Leschinger et al. 1993
; Mody et al. 1987
). Longer lasting events cannot be observed in 400- to 450-µm-thick slices (Dreier and Heinemann 1991
), although such events have been reported to occur occasionally in 625-µm slices with decreasing frequency in older animals (Anderson et al. 1986
). In contrast, the electrical activity induced by low Mg2+ in the EC and the subiculum consists of electrographic seizures (Jones and Heinemann 1988
; Walther et al. 1986
), which develop into epileptiform bursts (Dreier and Heinemann 1991
). In spite of the well-preserved fiber connectivity between the EC and the DG in slice preparations, electrographic seizures generated in the EC do not readily recruit the DG (Heinemann et al. 1990
; Jones and Lambert 1990
; Lambert and Jones 1989
; Walther et al. 1986
) and are not transmitted to hippocampal areas CA3 and CA1 (Dreier and Heinemann 1991
). Thus the DG was suggested to function as a filter preventing the spread of epileptiform activity to the HC (Heinemann et al. 1992
; Lothman et al. 1992
). In contrast to controls, in HC-EC slices obtained from kindled rats, electrographic seizures generated in the EC spread to the DG and to areas CA3 and CA1 (Behr et al. 1996
). Kindling seems to facilitate the propagation of epileptiform activity through the HC-EC slice and appears to alter the filtering function of the DG. However, all previous in vitro studies have resorted to perfusing the epileptogenic medium over the whole extent of the slice preparation. Therefore the contribution of the DG to seizure propagation always was assessed in the presence of an epileptogenic medium, precluding the measurement of its unperturbed function. In superfused brain slice preparations where many neurons are connected synaptically, it is difficult to single out the effect of a particular treatment on a specific neuronal population. Local perfusion systems (Engert and Bonhoeffer 1997
; Veselovsky et al. 1996
; West et al. 1992
) have been used successfully to modify the extracellular environment of a restricted slice region. Yet no studies exist on the effects of various compounds administered locally on a relatively distant synaptically connected brain area. To examine the spread of epileptiform activity, we developed a local perfusion system that allowed us to induce focal epileptiform activity in the EC without altering the physiology of the DG. We monitored the propagation of stimulus-evoked responses and of spontaneous epileptiform events from the disinhibited EC to the DG in control and kindled rat slices. Moreover, we investigated the contribution of the hippocampal area CA3 to the spread of epileptiform activity induced by the EC in kindled animals.
METHODS
Abstract
Introduction
Methods
Results
Discussion
References
1.8; L, 0.0; V, 4.2) (Paxinos and Watson 1986
). Commissural stimulation was employed to accelerate the uniform and bilateral development of stage 5 seizures. After a postsurgical recovery period of 7 days, 16 animals were stimulated daily through the implanted electrode with a train of biphasic 150-µA pulses at 60 Hz for 1 s. The behavioral changes induced by the kindling process were scored according to the scale of Racine (1972)
. The remaining 14 unstimulated animals were used as controls (sham-controls). Animals were stimulated until
15 consecutive stage 5 seizures were obtained. Three weeks after the last seizure, the animals were decapitated under pentobarbital sodium (75 mg/kg ip) anesthesia, and the brains were removed. The brains were cooled for
1 min in 4°C artificial cerebrospinal fluid (ACSF) containing (in mM) 126 NaCl, 1.25 NaH2PO4, 26 NaHCO3, 2.5 KCl, 2 CaCl2, 2 MgCl2, and 10 D-glucose at a pH of 7.4. Horizontal 400-µm-thick slices were prepared using a Lancer Series 1000 vibratome (Staley et al. 1992
). Before recordings, the slices were incubated at 32°C for 1-6 h in a 300 ml storage chamber filled with ACSF and continuously bubbled with a mixture of 95% O2-5% CO2. To induce epileptiform activity, either MgCl2 was omitted from the ACSF or a restricted region of the slice was perfused with 50 µM bicuculline (BMI) in 10 mM KCl containing ACSF. Preliminary experiments have shown that local perfusion of BMI or KCl alone were not as reliable as both agents together in altering the activity in the EC.
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FIG. 1.
Schematic diagram of the local entorhinal cortex perfusion in the combined hippocampal-entorhinal cortex slice. One cannula each was inserted into the 2 barrels of a theta-glass capillary. Cannulae were connected to a push-pull pump. Tip of the capillary was lowered into layer III of the medial entorhinal cortex (mEC) in the vicinity of a recording electrode (R1). A relatively high flow rate (2-3 ml/min) in the direction indicated [ , away from the dentate gyrus(DG)] was necessary to prevent diffusion of the perfusate to the DG where a 2nd recording electrode (R2) was used to monitor electrical activity. A stimulating electrode (S) was placed in the perforant path to antidromically and orthodromically activate entorhinal cortext (EC) and DG neurons, respectively. Occasionaly, the extent of the local flow was monitored by including a dye (phenol red or methylene blue) in the local perfusate. pp, perforant path; mf, mossy fibers; Sch, Schaffer collaterals; alv, alveus; CA3, area CA3; CA1, area CA1; SUB, subiculum. Bottom: in each recording session, before any experimental manipulations were started, the restricted nature of the local EC perfusion was ensured by first perfusing an artificial cerebrospinal fluid (ACSF) containing 5 mM kynurenic acid for 10 min. If this treatment did not reduce the evoked excitatory postsynaptic potentials (EPSPs) recorded in the DG, the local perfusion was judged to be restricted to the EC.
2-test for determining differences between the fraction of total slices. Significance level was set to P < 0.05.
RESULTS
Abstract
Introduction
Methods
Results
Discussion
References
1 (n = 11) (Fig. 2A, top right). After lowering the Mg2+, the epileptiform bursts started at the same time as the electrographic seizures in the EC. In 4 of 13 control HC-EC slices, electrographic seizures also occurred in the DG and the CA3 region with a duration of 42.5 ± 4.9 s.
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FIG. 2.
Extracellular recordings of spontaneous early epileptiform activity in combined hippocampal-entorhinal cortex slices of control and kindled rats after lowering extracellular Mg2+. A: electrographic seizures were recorded in layer III of the EC of kindled and control animals. In area CA3 of controls, the epileptiform activity consisted of spontaneous epileptiform bursts. In contrast, area CA3 of kindled animals displayed significantly longer lasting epileptiform bursts (Note the presence of secondary discharges). B: schematic diagram of the transverse cuts done to examine whether electrographic seizures recorded in area CA3 propagated from the EC. R, recording electrode. Right: electrographic seizures recorded in area CA3 in 2 different slices obtained from kindled animals. In contrast to controls, the area CA3 of intact kindled slices displayed electrographic seizures. Disconnecting the EC from the DG (EC-DG) immediately stopped electrographic seizures in area CA3, but cutting the HC-EC loop between area CA1 and the subiculum (CA1-SUB) did not alter the propagation of electrographic seizures from the EC to area CA3. Calibration bars in A (for ECIII) also apply to recordings in B.
-aminobutyric acid (GABA) antagonists (Schwartzkroin and Prince 1977
, 1978
), K+-channel blockers (Schwartzkroin and Prince 1980
), low extracellular Ca2+ (Jefferys and Haas 1982
; Konnerth et al. 1986
; Yaari et al. 1983
) or Mg2+ (Anderson et al. 1986
; Mody et al. 1987
) or high extracellular K+ (Chamberlin and Dingledine 1988
; Traynelis and Dingledine 1988
). In all of these models, the whole brain slice is exposed to the agent that induces the epileptiform activity. Such experimental arrangement would preclude establishing whether the kindled DG may prevent the propagation of epileptiform events from the EC to the HC under physiological conditions. To avoid any change in the physiology of the DG while inducing epileptiform activity in the EC, a local perfusion system was developed to investigate the propagation of epileptiform activity through the control and kindled DG (Fig. 1; see METHODS).
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FIG. 3.
A: superimposed traces of stimulus evoked responses recorded in the granule cell layer of a control and kindled DG during local (EC) perfusion with ACSF and with the -aminobutyric acid-A (GABAA) receptor antagonist bicuculline (BMI; 50 µM) dissolved in 10 mM KCl containing ACSF. Local perfusion of the disinhibitory medium in slices obtained from implanted and sham-stimulated control animals showed only small increases in evoked activity in the DG. In contrast, in kindled slices, it resulted in a significant increase in epileptiform activity localized to the DG. B: we measured the "coastline index" (the total dot-to-dot length of the digitized traces during a 50-ms period after the stimulus) to quantitate the BMI + KCl-induced epileptiform activity in extracellular recordings. Histogram demonstrates changes in the recorded coastline index in control and kindled preparations. Ordinate shows the ratio obtained by dividing the value of the index obtained in BMI + KCl by that recorded in ACSF (i.e., the enhancement of the index by local perfusion of BMI + KCl in the EC). Note the large increase in the DG ratios in the kindled preparations. Significant difference in the EC ratios of kindled and control animals might result from the already enhanced EC responses recorded in ACSF in kindled slices. Thus addition of BMI + KCl led to no further increases in the excitability of the kindled EC. Ratios of the changes in the DG vs. those in the EC are shown in the DG/EC bars. The relative change was significantly larger in kindled preparations. Significant differences between kindled and control preparations are marked (* n = 4-6 per bar; P < 0.05).
. To obtain the coastline index, the total (dot-to-dot) length of the digitized traces was measured for 50 ms after the stimulus. To assess the enhancement of this index in the EC as well as in the DG after local disinhibition of the EC, we divided the numerical value of each index obtained inBMI + KCl by that recorded in ACSF. The ratios then were compared between kindled and control animals (Fig. 3B). The ratios observed in the DG showed a significantly larger increase in kindled slices, in spite of the lack of diffusion of BMI + KCl to this region. Although in control animals the local BMI + KCl increased the coastline of the antidromically evoked EC responses (ratio ~1.8), there was no significant change in the EC of kindled preparations (ratio ~1.1). This difference might be attributed to the already prolonged EC responses recorded in ACSF in kindled slices. Thus application of BMI + KCl lead to no further increase in the excitability of the kindled EC. It is also possible that extracellular recordings do not provide the required resolution to measure this small enhancement. The "throughput index" of the DG, calculated by dividing the ratios for the DG with those in the EC was 100% larger in kindled slices (n = 4-6 per bar; P < 0.05). These data demonstrate facilitated propagation of epileptiform activity through the DG in kindled preparations.
2-test). The activity consisted of epileptiform bursts, but electrographic seizures comparable with those after lowering extracellular Mg2+ in the whole slice could not be recorded.
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FIG. 4.
Propagation of epileptiform activity from the EC to DG in slices prepared from control and kindled animals. A: spontaneous epileptiform activity was induced in the EC of 3 of 8 control and 9 of 9 kindled slices after prolonged (>10 min) local (EC) perfusion of BMI + KCl (see text for details). Although in control HC-EC slices no propagated activity was recorded in the DG, in 7 of 9 kindled slices, the DG participated in interictal discharges. B: simultaneous recordings of spontaneous epileptiform activity in the EC and the DG of kindled slices after local (EC) perfusion of BMI + KCl. Left: boxed traces shown on an expanded time scale on the right to demonstrate the earlier timing of spontaneous events in the EC relative to the DG.
DISCUSSION
Abstract
Introduction
Methods
Results
Discussion
References
; Leschinger et al. 1993
; Mody et al. 1987
). Similarly, a low Mg2+ concentration results in repetitive firing of CA3 pyramidal cells in response to synaptic activation (Coan and Collingridge 1985
; Herron et al. 1985
) attributed to the abundant recurrent excitatory circuits (Johnston and Brown 1981
; MacVicar and Dudek 1980
; Miles and Wong 1986
; Miles et al. 1984
). Moreover, the ability of the CA3 pyramidal cells to generate intrinsic bursts may support epileptiform activity in this region (Miles and Wong 1983
; Miles et al. 1991
). Kindling significantly increased the duration of the epileptiform bursts. Furthermore, electrographic seizures restricted to the subiculum and the EC (Behr et al. 1996
; Dreier and Heinemann 1991
; Leschinger et al. 1993
) in controls, were recorded in area CA3 of kindled preparations. The sensitivity of electrographic seizures in area CA3 to disconnecting the HC from the EC are consistent with the idea that electrographic seizures were propagated from the EC. Thus the hypothesis that kindling enables area CA3, at least in the slice preparation, to generate electrographic seizures without entorhinal epileptiform input can be ruled out. However, area CA3 appears to be affected by local alterations after kindling as epileptiform bursts were lengthened significantly compared with controls. Therefore, electrographic seizures that pass through the DG apparently spread to an altered and epilepsy-prone CA3 area that may support their propagation through the HC.
; Lothman et al. 1992
). Both single cellular and neuronal network alterations may be responsible for the loss of this filter function, leading to a larger ease of seizure propagation through the kindled DG.
; Slevin et al. 1986
). Kamphuis et al. (1994)
showed kindling to be accompanied by an increased expression of the flip variant of GluR-A,-B, and -C mRNAs in DG cells 24 h after the last seizure. However, in the long term, only the GluR-A flip mRNA was enhanced. The NMDA subtype of glutamate receptor contributes significantly to epileptogenesis. Kindling-induced epilepsy and the associated mossy fiber sprouting depend on the activation of this receptor type (McNamara et al. 1990
; Sutula et al. 1996
). Moreover, the activation characteristics of NMDA receptors are altered (Köhr et al. 1993
; Mody and Heinemann 1987
; Mody et al. 1988
), resulting in persistent changes in the excitability of individual kindled neurons. Paradoxically, the function of the GABAergic system seems to be increased in the DG after kindling (Oliver and Miller 1985
; Tuff et al. 1983
). This may stem from an increased excitatory input onto GABAergic neurons, an increased quantal size of inhibitory postsynaptic currents, and a reduced presynaptic autoinhibition of GABA release (Buhl et al. 1996
; Otis and Mody 1993
). Yet, the enhanced activity of GABAergic synapses should not be considered synonymous with an augmented inhibition. Enlarged IPSPs may actually underlie an increased synchrony of the principal cells (Cobb et al. 1995
).
; Mody et al. 1988
). There is some support for the hypothesis that mossy fiber sprouting after kindling may facilitate DG throughput (Golarai and Sutula 1996
; Houser et al. 1990
; Isokawa et al. 1993
; Sutula et al. 1989
). For example, after kainate-induced mossy fiber sprouting, glutamate microstimulation in the granule cell layer evoked EPSPs in neighboring granule cells (Wuarin and Dudek 1996
). Mossy fibers therefore may synapse on granule cells, resulting in feed-back excitation. Okazaki et al. (1995)
showed that mossy fibers develop recurrent collaterals after pilocarpine-induced status epilepticus. These collaterals formed synapses in the dentate molecular layer, but the authors did not provide an estimate for the proportion of recurrent mossy fiber boutons innervating granule cells versus other neurons. An inhibitory rather than an excitatory function of the reorganized dentate mossy fibers also has been proposed (Ribak and Peterson 1991
; Sloviter 1992
). Alternatively, sprouting, albeit common, may not be a prerequisite of epilepsy. A recent report (Longo and Mello 1997
) demonstrated that cycloheximide blocked pilocarpine- and kainate-induced mossy fiber sprouting without preventing the development of spontaneous epileptic seizures.
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ACKNOWLEDGEMENTS |
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We thank B. K. Oyama and M. T. Kim for excellent technical assistance.
This work was supported by a grant from the Schering Forschungsgesellschaft mbH to J. Behr and National Institute of Neurological Disorders and Stroke Grant NS-36142 to I. Mody.
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FOOTNOTES |
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Present address of K. J. Lyson: Dept. of Internal Medicine, Division of Nephrology, The University of Iowa, 200 Hawkins Dr., Iowa City, IA 52242-1081.
Address for reprint requests: I. Mody, Depts. of Neurology and Physiology, Reed Neurological Research Center, UCLA School of Medicine, 710 Westwood Plaza, Los Angeles, CA 90095-1769.
Received 2 September 1997; accepted in final form 23 December 1997.
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REFERENCES |
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