Prolonged field potentials evoked by 1 Hz stimulation in the dentate gyrus of temporal lobe epileptic human brain slices

BRAIN RESEARCH ELSEVIER

Brain Research 721 (1996) 132-139

Research report

Prolonged field potentials evoked by 1 Hz stimulation in the dentate gyms of temporal lobe epileptic human brain slices Leona M . M a s u k a w a

a, *,

Haiwei Wang a, Michael J. O'Connor b, Katsuhisa Uruno a

a Department of Neurology, The Graduate Hospital Research Center and UniversiO, of Pennsylvania Medical School, Philadelphia, PA 19146, USA b Department of Surgeo', The Graduate Hospital Research Center and University of Pennsylvania Medical School, Philadelphia, PA 19146, USA

Accepted 23 January 1996

Abstract

An abnormal electrophysiological response in brain slices of the dentate gyms from biopsy material from patients surgicially treated for intractable epilepsy (46/57), exhibited characteristics similar to the physiological hallmark of epilepsy, the paroxysmal discharge, a prolonged (30-600 ms) and often large amplitude field potential. The most striking feature of the prolonged response to a single perforant path stimulus was a predominantly biphasic field potential (23/46 cases). The biphasic response was characterized by a negative field potential of substantial duration exceeding 180 ms which followed an initial shorter duration positive field potential. Multiple population spikes occurred during both phases of the response. During a 1 Hz stimulus train applied to the perforant path, the magnitude and duration of the negative component of the field response was significantly increased. Approximately half of the cases (Group 1; 30/57) exhibited potentiation of the biphasic response, while the remaining cases (Group 2; 27/57) exhibited no negative field component during 1 Hz stimulation trains. This repetitive stimulation, in general, increased the area of the field response in a large majority of cases (44/57) regardless of the sign of the field potential. The number of population spikes following 1 Hz stimulation increased significantly for cases in both groups, although the increase was greater for those in Group 1 than in Group 2. Paired pulse depression (20 ms ISI) was reduced in cases that exhibited potentiated biphasic responses during 1 Hz stimulation (Group 1) in comparison to cases that exhibited no negative field potentials (Group 2). Paired pulse depression at a 200 ms ISI was not significantly different between the groups. During a single stimulus, bicuculline disinhibition (20 /xM) resulted in either a prolonged positive or biphasic field potential. Intracellularly recorded responses to single perforant path stimuli also exhibited prolonged and large depolarizations that were comparable in time course to the duration of field potentials recorded in the same area whether generated in the absence or presence of bicuculline. The prolonged field potential after bicuculline was reduced by APV (20 /xM). We suggest that the prolonged field response, whether biphasic or monophasic when generated by either 1 Hz stimulation or bicuculline disinhibition, may be due directly or indirectly to an increase in membrane depolarization mediated by activation of the NMDA receptor. Keywords: Paroxysmal discharge; Population spike; Paired pulse depression; 2-Amino-5-phosphonovalerate;Excitability; Disinhibition

1. I n t r o d u c t i o n

Paroxysmal discharges are considered one of the primary electrophysiological hallmarks of epilepsy and are synonymous with the interictal spike recorded in the electroencephalograms of epileptic patients [8,9,19,56,57]. This neuronal population response has been described as unusually large in amplitude and prolonged in duration. In animal models of epilepsy, a similar field potential is e x h i b i t e d under a variety o f adverse conditions

* Corresponding author. Fax: (l) (215) 893-4178. 0006-8993/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved Pll S0006-8993(96)00153-9

[ 1,3,4,30,31,33,34,40,41,43 -45,47,50,58]. The paroxysmal discharge reflects an increase in hyperexcitability in part due to its association with a prolonged intracellularly recorded depolarization [10,26]. In brain slices from epileptic patients, such prolonged potentials have been reported during intracellular and field recordings but required pharmacological disinhibition [5,6,13] or lowering of extracellular magnesium [7] and have not been observed with regularity in field recordings under control conditions [1,7,18,20-24,32,36-39,46,48,51-54,56]. W e previously described hyperexcitable field responses in brain slices from the dentate gyrus of temporal lobe epileptic patients [23]. Using a 1 Hz stimulation protocol, we reported a potentiation of the orthodromic response that

L.M. Masukawa et al. / Brain Research 721 (1996) 132-139

was reflected by an increase in the number of population spikes. We have since recognized that perforant path evoked field responses, in some cases, exhibited biphasic characteristics that were prolonged [25]. In the present study, we examined: (1) the likelihood of generating prolonged discharges during 1 Hz stimulation; (2) the relationship between paired pulse depression and changes in population spike number with potentiation of the prolonged biphasic response; (3) the effect of NMDA receptor antagonism on the time course of the paroxysmal discharge; and (4) the intracellular correlate of the prolonged response.

2. Materials and methods

We examined the perforant path evoked response in slices from the dentate gyrus of 57 patients who underwent temporal lobectomies for the treatment of medically intractable epilepsy [42] by using conventional brain slice techniques as we previously described [24,52,53]. Briefly, biopsy tissue of the hippocampus was obtained from the operating room immediately after its surgical removal and was returned to the laboratory in ice-cold low C a / h i g h Mg solution where it was cut transversely on a vibratome into 450-500 /xm thick slices. Biopsy material was located 1.5-3.0 cm from the tip of the hippocampus. Slices were held in an interface chamber for a period of 3 - 4 h and were superfused with oxygenated (95% 0 2 - 5 % CO2), normal physiological solution at 33-35°C before recording began. The components of the control physiological solution were as follows: (mM) NaC1 124, KC1 5.0, NaHzPO 4 1.25, NaHCO 3 26, MgSO 4 1.3, CaC12 2.0, D-glucose 10. In the low calcium/high magnesium physiological solution, all concentrations remained unchanged except that CaC12 was decreased (0.2 raM) and MgSO 4 was increased (5.0 mM). Field potential recordings from the granule cell layer were carried out with approximately 1 MY2 glass electrodes filled with physiological solution as previously described [24,52,53]. Perforant path stimulation was applied to the molecular layer using a pair of sharpened tungsten electrodes. Orthodromic stimulation pulses of 100/xs duration were delivered at a frequency of 0.05 to 0.1 Hz to generate field potentials. Stimulation trains composed of pulses delivered at a 1 Hz frequency were applied for 10-20 s using an intensity to produce 60% of the maximum response. The first pulse in the train was considered the control response or the response to a single stimulus, and a response during the train that represented the maximum change in response area or the 20th response, whichever occurred first, was used for the ' 1 Hz response'. Paired stimulus pulses were applied to the perforant path (0.05 Hz) to examine paired pulse depression using 20 ms and 200 ms interstimulus intervals (ISI) at 60% maximal stimulus intensity. Paired pulse measurements were recorded under control conditions. Bicuculline disinhibi-

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tion was carried out in a subset of cases that were tested with 1 Hz stimulation trains. Intracellular recordings were carried out using glass microelectrodes of 90-120 MY2 tip resistances filled with 4 M potassium acetate. Tissue responses remained stable for up to 15 h which reflected the viability of the biopsy brain slices. Data were collected using a P D P l l / 7 3 (DEC) computer with a customized data acquisition program. Waveform area was measured by integrating the area of the evoked field response above (positive component) and below (negative component) the baseline potential for a period of 200 ms using a customized analysis program. Corrections of the waveform area (mV * ms) were made to compensate for the occurrence of population spikes. The area of the population spike above the baseline was determined by joining the points before and after the return of the potential from the negative excursion of the population spike. Negative waveform areas were used in the analysis if the average amplitude was greater than 0.25 mV. The duration of the positive and negative field potentials were also determined by computer analysis. Durations were measured from the beginning of the positive wave to the time of return to baseline or end of the epoch, whichever occurred first. The negative duration was measured from the time when the field response crossed below the baseline potential to the time the response returned to the baseline potential or the end of the epoch. Negative excursions did not often return to baseline during the epoch which indicated a much longer duration or area than the measurements permitted. In a few cases, longer epochs were measured which reflected the maximum possible duration (see Fig. 4A). Paired pulse depression was quantified by the ratio of the population spike amplitude of the second response relative to the first response (PS2/PS 1) as described previously [53]. Cases were divided into two groups based on the presence (Group 1) or absence (Group 2) of a negative field component during 1 Hz stimulation trains. The data were analyzed using a 2 (group) × 2 (train or condition) analysis of variance. Significant effects were analyzed further using simple effects tests or orthogonal mean constraints, as appropriate. Data were expressed as the mean + standard error.

Bicuculline methiodide (20 /xM; Sigma) and APV (D2-amino-5°phosphonovalerate, 20 /zM; Cambridge Research) were added to the perfusion solution.

3. Results

In the granule cell layer of the dentate gyrus of brain slices from a vast majority of temporal lobe epileptic patients under control conditions (46/57), orthodromic field potentials evoked by a single perforant path stimulus were prolonged postsynaptic potentials of a duration greater than 30 ms and could exceed 600 ms. Responses varied

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a m o n g cases from a short duration positive field potential ( < 3 0 ms; Fig. 1A; n = 1 1 / 5 7 ) to more prolonged responses ( > 3 0 ms; Fig. 1B and C; n = 4 6 / 5 7 ) . The prolonged responses were either principally monophasic, positive potentials (Fig. 1B; n -- 1 9 / 5 7 ) or biphasic potentials (Fig. 1C; n = 2 7 / 5 7 ) . Biphasic responses were characterized by an initial positive potential followed by a negative potential (Fig. 1C). The negative c o m p o n e n t often lasted d u r i n g a major portion of the 200 ms epoch (mean: 148.41 _+ 5.19 ms; n = 24) and could reach an amplitude of 3 m V (range: 0.250 m V to 3.0 m V ; m e a n 0.5 + 0.98 mV). The r e m a i n i n g responses were m o n o p h a s i c and were on average of shorter duration than the biphasic responses (mean: 42.06 _+ 5.89 ms; n = 33; Fig. 1A and B). Multiple population spikes were sometimes present ( 2 7 / 5 7 ) and w h e n the response was biphasic, population spikes were observed during both phases of the potential (Fig. 1C). The n u m b e r of population spikes ranged from n o n e to ten (mean: 1.88 _+ 0.25; n = 57). Previously, we reported a potentiation of m o n o p h a s i c positive orthodromic field responses to 1 Hz stimulation [23]. In the present study, we observed that during 1 Hz

CONTROL

1HZ

A

B

C

A Fig. 2. Changes in the orthodromic response during l Hz stimulation trains (10-20 pulses). 1 Hz perforant path stimulation produced a variety of responses that were usually altered from the first response in the train (Control). Some cases showed small changes of the orthodromic response to 1 Hz stimulation (A), while other cases showed relatively large changes (B) and (C). These changes were exhibited as increases in the number of population spikes as well as an increase in duration and magnitude of the field potential. 1 Hz stimulation produced increases in the orthodromic response area for individual cases that were either monophasic (B) or biphasic (C) potentials, although a significant group change was observed only for responses from Group 1 (C) and not from Group 2 (A and B). Negative field potentials can be greatly potentiated during 1 Hz stimulation (C). Calibration: A and C (1 mV and 40 ms); B (2 mV and 40 ms).

C

rN l

~

Fig. 1. Variation of orthodromic field responses of the dentate granule cell layer of brain slices of biopsy material from temporal lobe epileptic patients to single perforant path stimuli. Field responses varied from narrow monophasic field potentials with a superimposed single population spike (A), to prolonged monophasic positive field potentials with multiple population spikes (B), and biphasic responses that lasted longer than 200 ms and generated multiple population spikes (C). Biphasic potentials were composed of an initial positive field which was followed by a negative component. Population spikes were present on both the positive and negative field components. Calibration: 2 mV and 20 ms.

perforant path stimulation trains, biphasic field potentials were also potentiated in the dentate granule cell layer of 53% of the cases. In order to e x a m i n e the relationships b e t w e e n other evoked response parameters and the potentiated negative field response, cases were divided into two groups based on the presence of a negative field potential during 1 Hz stimulation (Group 1, n = 30; Fig. 2C) or the absence of a negative c o m p o n e n t during 1 Hz stimulation (Group 2, n = 27; Fig. 2 A and B). The total response area that i n c l u d e d both negative and positive c o m p o n e n t s of the field in Group 1 increased substantially on average from 136.016 _+ 17.25 to 223.926 _+ 29.99 m V * m s ( P < 0.00001), representing an increase of 76%. The change was greatest for the negative field c o m p o n e n t which significantly increased by 108% (77.843 ± 16.679 to 165.214 _+ 29.31 m V * m s ; P < 0.00001). The duration of the entire response generally was longer than the 200 ms epoch. The

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Table 1 Effects on field responses during 1 Hz stimulation PS GROUP 1 Mean S.E. n Group 2 Mean S.E. n

PPD (ISI)

Positive area

Negative area

Control

1 Hz

20 ms

200 ms

Control

1 Hz

Control

1 Hz

2.233 0.403 30

7.533 a.b 1.041 30

0.716 a 0.111 18

0.879 0.050 13

47.699 6.082 30

58.712 9.737

77.843 16.679

165.214 b 29.309

1.731 0.276 26

2.423 b 0.352 26

0.450 0.075 20

0.803 0.078 17

42.782 5.382 27

40.677 4.487

5.655 3.899

0.000 0.000

Group 1: negative field during 1 Hz stimulation; Group 2: no negative field during 1 Hz stimulation. a Between groups: P < 0.05. b Between conditions: P < 0.05. PPD: paired pulse depression (PS2/PSI) under control conditions. Interstimulus interval (ISI). area (mV* ms). PS: number of population spikes.

m a x i m u m duration o f the n e g a t i v e field potential r e c o r d e d f r o m G r o u p 1 was not m e a s u r e d in m o s t cases due to the limited r e c o r d e d e p o c h w h i c h lasted for 200 ms, Therefore, increases in area f o l l o w i n g 1 H z stimulation e x c e e d those reported here. In G r o u p 2 (those cases that did not exhibit a n e g a t i v e field c o m p o n e n t during 1 H z stimulation), no significant c h a n g e in the area of the positive c o m p o n e n t was o b s e r v e d during 1 H z stimulation on average ( 4 2 . 7 8 2 _ 5.382 to 40.677 + 4.487 m V * m s , respectively; P > 0.05; T a b l e 1; Fig. 2A). A l t h o u g h G r o u p 2 on a v e r a g e e x h i b i t e d no c h a n g e in the area o f the positive c o m p o n e n t o f the field response on

average, individual cases that r e m a i n e d m o n o p h a s i c during 1 H z stimulation, w e r e s o m e t i m e s potentiated (Fig. 2B). This response was identical to those described p r e v i o u s l y [231. The n u m b e r o f e v o k e d population spikes increased significantly on average f r o m 2 to 7 spikes in G r o u p 1 during 1 H z stimulation (range during 1 H z stimulation: to 20 spikes; T a b l e 1). In contrast, G r o u p 2 also exhibited a significant increase in the n u m b e r o f population spikes during 1 H z stimulation, although it was a m u c h smaller increase (1 to 2 population spikes; T a b l e 1). Paired pulse depression exhibited to 20 ms interstimulus

1

A

Control

Bicuculline

\ Bicuculline ~h ........................................ APV "" ~W ~'r"r'~'

~ '

~l"~'~r"r'ml'~""

"¢ "

Fig. 3. Alterations in orthodromic field potential responses to single perforant path stimuli during exposure to bicuculline (20 /xM) and APV (20 /xM). Before drug application (Control) the orthodromic field response of both examples was monophasic (A and B). After bicuculline exposure, both (A) a biphasic potential with a substantial negative component (see arrow) or (B) a predominantly positive orthodromic field potential (see arrow) were exhibited with superimposed multiple population spikes. Exposure to APV (bicuculline and APV) reduced the duration and amplitude of the prolonged phase of both responses generated during bicuculline regardless of the direction of the field change. Calibration: 2 mV and 20 ms.

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intervals was greater in Group 2 than in Group 1 ( P S 2 / P S I : 0.45 + 0.08 vs. 0.77 ___0.12 respectively; P = 0.05). There was no significant difference between the two groups for paired pulse depression at the 200 ms interstimulus interval.

4. Effect o f b i c u c u l l i n e

Bicuculline disinhibition also led to prolonged field potentials evoked by single stimuli. During bicuculline (20 /xM) disinhibition (n = 18), prolonged potentials that were either predominantly negative (Fig. 3A and Fig. 4A) or positive (Fig. 3B) were observed along with an additional increase in the number of population spikes. These responses were similar in characteristics to those generated during 1 Hz stimulation (see Fig. 2B and C). When N M D A receptors were blocked by A P V (20 /zM) during continued exposure to bicuculline, the positive field response duration and amplitude, as well as the negative component of the field response were reduced (Fig. 3). This effect was in agreement with our previous observation that A P V blocked the potentiation of the positive field response evoked by 1 Hz stimulation trains [22]. When intracellular recordings were carried out in the granule cell layer in tissue which exhibited a prolonged field response either in the absence or presence of bicuA

.a.,l B

I Fig. 4. During exposure to bicuculline (20 /xM), the orthodromic field potential increased in duration and was sometimes biphasic (A and see also Fig. 3A). The slow time base illustrates the long duration of at least 500 ms that disinhibited field responses can achieve. (B) An intracellular recording from the same area in the dentate granule cell layer exhibited a prolonged and large postsynaptic potential to perforant path stimulation. The time course of the field and intracellular responses were comparable. Calibration: A (1 mV and 50 ms) and B (10 mV and 50 ms).

Table 2 Association between intracellular EPSPs and field potential duration Intracellular EPSP Prolonged 29 Normal 13

Prolonged fields ( > 60 ms) Biphasic

Positive

Normal ( < 60 ms)

17

7

5

1

4

8

culline, prolonged ( > 60 ms) depolarizing potentials were observed to single orthodromic stimuli (n = 2 4 / 2 9 ; Table 2). Similarly, short duration EPSPs were more often observed in tissue that exhibited short duration field responses ( < 60 ms; n = 8 / 1 3 ; Table 2). A majority of the field responses that were biphasic also exhibited EPSPs that were prolonged ( > 60 ms; n = 1 7 / 2 4 ; Table 2). An example of an intracellular response, which exhibited the same prolonged time course and large magnitude as the biphasic field potential under bicuculline disinhibition, is illustrated in Fig. 4. Prolonged monophasic field responses were also associated with prolonged EPSPs (n = 8 / 1 I). Intracellular recordings were not performed during 1 Hz stimulation.

5. D i s c u s s i o n

The biphasic orthodromically evoked field potential generated either by single or repetitive (1 Hz) stimulation in the dentate gyrus of biopsy material from temporal lobe epileptic patients exhibited characteristics of a paroxysmal discharge, a prolonged and sometimes large magnitude potential with superimposed population spikes. An increase in the negative field component was the predominant effect of 1 Hz stimulation. The similarity of the duration of the field negativity and the intracellularly recorded depolarization, along with the reduction of the field potential by APV, suggests that prolonged depolarizing potentials may underlie the biphasic field potential. A similar prolonged potential has been named the paroxysm al discharge in animal models [1,4,15,26,30,33,34,40,43,45,47] and is often longer in duration than an intrinsic burst observed intracellularly from pyramidal cells of the hippocampus in animal models [55]. Differences in both excitation, exhibited by increases in the number of population spikes and response area, as well as in inhibition, reflected in reduction of paired pulse depression, were associated with potentiation of the negative component of the field response during 1 Hz stimulation. The reduction of the prolonged, disinhibited orthodromic field potential by APV that we observed is consistent with the suggestion that the prolonged response is due at least in part to N M D A receptor activation [4,5,17,22,35], regardless of the biphasic character of the response. The positive field potential recorded in the granule cell body

L.M. Masukawa et al./Brain Research 721 (1996) 132-139

layer during orthodromic stimulation has been attributed to excitatory input along the outer 2 / 3 of the granule cell dendrites [2]. In contrast, a negative field recorded at the layer of the cell bodies, if due solely to excitatory synaptic input, would suggest that glutamate receptors may be localized at or near the cell body. The presence of an increased excitatory synaptic input in epileptic human tissue is supported by studies that report an increase in glutamate receptors in hippocampal biopsy material from epileptic patients [14,16,28]. The large intracellular depolarization that occurs in neurons from tissue exhibiting biphasic responses raises the possibility that activation of glutamate receptors may underlie the generation of the negativity or lead to events that produce a negative field potential. Depolarization due to activation of NMDA receptors is one mechanism to explain the field negativity. Glutamate receptor mediated depolarization may also lead to activation of voltage-dependent conductances, therefore, a summation of currents could lead to a field whose sign would be determined by the site of origin and ion cartier for each conductance. The presence of a substantial voltage-gated inward current which would enlarge a glutamate receptor mediated depolarization, however, has never been described in dentate granule cells in animal or human tissue [10,13,18,20,34,37]. The significant increase in number of population spikes during the 1 Hz stimulus in cases from Group 1 suggests that greater potentiation of spike activation is associated with potentiation of the negative field generated by 1 Hz stimulation. Similarly, the significantly weaker paired pulse depression (20 ms interstimulus interval) of Group 1 in comparison to Group 2 is consistent with a reduction of inhibition. These results support our earlier observation that potentiation during 1 Hz stimulus trains may be due to a reduction of GABAA-associated inhibition [23]. Others have shown that 1 Hz stimulation can reduce synaptic inhibition [11,12,27,29,49], therefore, 1 Hz stimulation in human slices may further erode an already weakened synaptic inhibition resulting in greater excitability. This erosion would occur more readily in tissue from Group 1 where paired pulse depression is initially weaker. A reduction of inhibition through a variety of mechanisms may contribute to the generation of the paroxysmal discharge in the epileptic human. An increase in receptor density, a rearrangement of excitatory synaptic input or altered conductances, however, are not necessary for the expression of a negative field potential or paroxysmal discharge. Observations in animal brain slices and in in vivo animal models demonstrate that paroxysmal discharges or prolonged negative responses can be generated in normal animals under altered extracellular conditions, e.g., elevated potassium [31,33,47,50,58], 0 Ca ++ or 0 Mg -+ [1,3,30,40,58], pharmacological disinhibition [15,55] and tetanic stimulation [4,34,41,43-45]. These findings suggest that the paroxysmal discharge can

137

be a response to a diversity of abnormal conditions in animal models. It may be that the same cellular mechanisms participate in the generation of abnormal physiology displayed by tissue from epileptic patients examined under control conditions and during relatively low frequencies of repetitive stimulation, and may not be due solely to alterations in excitatory synaptic input. The paroxysmal discharge also indicates a relatively longer term effect of 1 Hz stimulation on excitability than an increase in population spike generation, an effect which can outlast the period of synchronized spike activity for periods of more than 500 ms in some cases. The mechanistic explanation for the prolonged and potentiated negativity of the field response is yet to be determined. The variety of responses to 1 Hz stimulation across cases is consistent with our observations from three previous studies using non-overlapping patient groups that excitability can vary widely across patient cases [24,52,53]. The lack of potentiation of both population spikes and field potentials to 1 Hz stimulation in some cases suggests that not all tissue may be altered in the same way by repetitive stimulation either due to the diverse etiologies of epileptic patients or due to variation in the location of focal seizure activity. The observation that prolonged orthodromic responses occurred in most of the cases (46/57) in response to single stimuli and that a large increase in the prolonged response, especially the negative component of the field response, during 1 Hz stimulation suggests that physiological abnormality expressed as a paroxysmal discharge may be a more common feature of tissue from temporal lobe patients than has been previously reported. The paroxysmal discharge in the epileptic human dentate gyrus is especially unusual in light of the generally accepted inexcitability of the dentate gyrus in animal models under control conditions and 1 Hz stimulation [2,23,27,34,40,54].

Acknowledgements We are grateful for the technical help provided by T. Vandegrift, W.M. O'Connor, and J. Lynott. We are also indebted to Dr. L. Burdette for valuable discussions. This research was supported by NIH Grant # NS-23077 to L.M.M. and was carried out with the approval of the Institution Review Board for Human Studies of The Graduate Hospital.

References [1] Agopyan, N. and Avoli, M., Synaptic and non-synaptic mechanisms underlying low calcium bursts in the in vitro hippocampal slice, Exp. Brain Res., 73 (1988) 533-540. [2] Andersen, P., Holmqvist, B. and Voorhoeve, P.E., Entorhinal activation of dentate granule cells, Acta Physiol. Scand., 66 (1966) 448-460.

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[3] Anderson, W.W., Stasheff, S.F., Swartzwelder, H.S. and Wilson, W.A., Regenerative, all-or-none eleetrographic seizures in the rat hippocampal slice in Mg-free and physiological medium, Brain Res., 532 (1990) 288-298. [4] Anderson, W.W., Swartzwelder, H.S., Wilson, W.A., The NMDA receptor antagonist 2-amino-5-phosphonovalerate blocks stimulus train-induced epileptogenesis but not epileptiform bursting in the rat hippocampal slice, .L Neurophysiol., 57 (1987) 1-21. [5] Avoli, M., and Olivier, A., Bursting in human epileptogenic neocortex is depressed by an N-methyl-D-aspartate antagonist, Neurosci. Lett., 76 (1987) 249-254. [6] Avoli M, and Olivier, A., Electrophysiological properties and synaptic responses in the deep layers of the human epileptogenie neocortex in vitro, J. Neurophysiol., 61 (1989) 589-606. [7] Avoli, M., Louvel, J., Pumain, R. and Olivier, A., Seizure-like discharges induced by lowering [Mg2+] ° in the human epileptogenic neocortex maintained in vitro, Brain Res., 417 (1987) 199 203. [8] Brazier, M.A.B., Regional activities within the human hippocampus and hippocampal gyms, Exp. Neurol., 26 (1970) 354-368. [9] Calvin, W.H., Ojemann, G.A. and Ward, A.A., Jr., Human cortical neurons in epileptogenic foci: comparison of inter-ictal firing patterns to those of "epileptic" neurons in animals, Electroenc. Clin. Neurophysiol., 34 (1973) 337-381. [10] Cronin, J., Obenaus, A., Houser, C.R. and Dudek, F.E., Electrophysiology of dentate granule cells after kainate-induced synaptic reorganization of the mossy fibers, Brain Res., 573 (1992) 305-310. [11] Davies, C.H., Davies, S.N. and Collingridge, G.L., Paired pulse depression of monosynaptic GABA-mediated inhibitory postsynaptic responses in rat hippocampus, J. Physiol., 424 (1990) 513-531. [12] Deisz, R.A. and Prince, D.A., Frequency-dependent depression of inhibition in guinea-pig neocortex in vitro by GABA b receptor feed-back on GABA release, J. Physiol., 412 (1989) 513-541. [13] Franck, J.E., Pokorny, J., Kunkel, D.D. and Scbwartzkroin, P.A., Physiological and morphological characteristics of granule cell circuitry in human epileptic hippocampus, Epilepsia, 36 (1995) 543538. [14] Geddes, J.W., Cahan, L.D., Cooper, S.M., Kim, R.C., Choi, B.H. and Cotman, C.W., Altered distribution of excitatory amino acid receptors in temporal lobe epilepsy, Exp. Neurol., 108 (1990) 214220. [15] Hablitz, J.J., Picrotoxin-induced epileptiform activity in hippocampus: role of endogenous versus synaptic factors, J. Neurophysiol., 51 (1984) 1011-1027. [16] Hosford, D.A., Crain, B.J., Cao, Z., Bonhaus, D.W., Friedman, A.H., Okazaki, M.M., Nadler, J.V. and McNamara, J.O., Increased AMPA-sensitive quisqualate receptor binding and reduced NMDA receptor binding in epileptic human hippocampus, J. Neurosci., 11 (1991 ) 428-434. [17] Isokawa, M. and Levesque, M.F., Increased NMDA responses and dendritic degeneration in human epileptic hippocampal neurons in slices, Neurosci. Lett., 132 (1991) 212-216. [18] Isokawa, M., Avanzini, G., Finch, D.M., Babb, T.L. and Levesque, M.F., Physiologic properties of human dentate granule cells in slices prepared from epileptic patients, Epilepsy Res., 9 (1991) 242-250. [19] King, W.D. and Ajemone-Marsan, C., Clinical features and ictal patterns in epileptic patients with EEG temporal lobe foci, Ann. Neurol., 2 (1977) 138-147. [20] Knowles, W.D.. Awad, I.A. and Nayel, M.H., Differences of in vitro electrophysiology of hippocampal neurons from epileptic patients with mesiotemporal sclerosis versus structural lesions, Epilepsia, 33 (1992) 601-609. [21] Lorenzon, N.M. and Foehring, R.C., Relationship between repetitive firing and afterhyperpolarizations in human neocortical neurons, J. Neurophysiol., 67 (1992) 350-363. [22] Masukawa, L.M., Higashima, M., Hart, G.J., Spencer, D.D. and O'Connor, M.J., NMDA receptor activation during epileptiform

[23]

[24]

[25]

[26]

[27]

[28]

[29]

[30]

[31]

[32] [33]

[34]

[35]

[36]

[37]

[38] [39]

[40]

[41]

[42]

responses in the dentate gyrus of epileptic patients, Brain Res., 562 (1991) 176-180. Masukawa, L.M., Higashima, M., Kim, J.H. and Spencer, D.D., Epileptiform discharges evoked in hippocampal brain slices from epileptic patients, Brain Res., 493 (1989) 168-174. Masukawa, L.M., Uruno, K., Sperling, M., O'Connor, M.J. and Burdette, L.J., The functional relationship between antidromically evoked field responses of the dentate gyrus and mossy fiber reorganization in temporal lobe epileptic patients, Brain Res., 579 (1992) 119 127. Masukawa, L.M., Uruno, K., Wang. H., O'Connor, W. and O'Connor M.J., Repetitive abnormal electrical discharges in brain slices of the dentate gyrus from epileptic patients, Soc. Neurosci. Abst.~ 20 (1994) 1669. Matsumoto, H. and Ajmone Marsan, C., Cortical cellular phenomena in experimental epilepsy: interictal manifestations, Exp. Neurol., 9 (1964) 286-304. McCarren. M. and Alger, B.E., Use-dependent depression of IPSPs in rat hippocampal pyramidal cells in vitro, J. Neurophysiol., 53 (1985) 557-571. McDonald, J.W., Garofalo, E.A., Hood, T., Sackellares, J.C., Gilman, S., McKeever, P.E., Troncoso, J.C. and Johnston, M.V., Altered excitatory and inhibitory amino acid receptor binding in hippocampus of patients with temporal lobe epilepsy, Ann. Neurol., 29 (1991) 529-541. Misgeld, U., Muller, W. and Brunner, H., Effects of (-)baclogen in inhibitory neurons in the guinea pig hippocampal slice, Pfiug. Arch. Eur. J. Physiol., 414 (1989) 139-144. Mody, l., Lambert, J.D.C. and Heinemann, U., Low extracellular magnesium induces epileptiform activity and spreading depression in rat hippocampal slices, J. Neuroph~'siol., 57 (1987) 869-888. O'Connor, M.J. and Lewis, D.V., Recurrent seizures induced by potassium in the penicillin treated hippocampus, Eleetroenceph. Clin. Neurophysiol., 36 (1974) 337-345. Prince, D.A. and Wong, R.K.S., Human epileptic neurons studied in vitro, Brain Res., 210 (1981) 323-333. Rutecki, P.A., Lebeda, F.J. and Johnston, D., Epileptiform activity induced by changes in extracellular potassium in hippocampus, J. Neurophysiol., 54 (1985) 1363-1374. Scharfman, H.E. and Schwartzkroin, P.A., Responses of cells of the rat fascia dentate to prolonged stimulation of the perforant path: sensitivity of hilar cells and changes in granule cell excitability, Neuroscience, 35 (1990)491-504. Schneiderman, J.H. and MacDonald, J.F.. Excitatory amino acid blockers differentially affect bursting of in vitro hippocampal neurons in two pharmacological models of epilepsy, Neuroscienee, 31 (1989) 593-6(/3. Schwartkroin, P.A. and Haglund, M.M., Spontaneous rhythmic synchronous activity in epileptic human and normal monkey temporal lobe, Epilepsia, 27 (1986) 523-533. Schwartzkroin, P.A. and Knowles, W.D., Intracellular study of human epileptic cortex: in vitro maintenance of epileptiform activity? Science, 223 (1984) 709-712. Schwartzkroin, P.A. and Prince, D.A., Microphysiology of human cerebral cortex studied in vitro, Brain Res., 115 (1976) 497-500. Schwartzkroin, P.A.. Turner, D.A., Knowles, W.D. and Wyler, A.R., Studies of human and monkey "epileptic" neocortex in the in vitro slice preparation, Ann. Neurol., 13 (1983) 249-257. Schweitzer, J.S., Patrylo, P.R. and Dudek, F.E., Prolonged field bursts in the dentate gyms: dependence on low calcium, high potassium, and nonsynaptic mechanisms, J. Neurophysiol., 68 (1992) 2016-225. Somjen, G.G., Aitken, P.G., Giacchino, J.L. and McNamara, J.O., Sustained potential shifts and paroxysmal discharges in hippocampal formation, J. Neurophysiol., 53 (1985) 1079-1097. Sperling, M.R., O'Connor, M.J., Saykin, A.J., Phillip, A., Morrell, M.J., Bridgman, P.A., French, J.A. and Gonatas, N., A non-invasive

L.M. Masukawa et aL /Brain Research 721 (1996) 132 139

[43]

[44]

[45]

[46]

[47]

[48]

[49]

[50]

protocol for anterior temporal lobectomy, Neurology, 42 (1992) 416-422. Stasheff, S.F., Bragdon, A.C. and Wilson, W.A., Induction of epileptiform activity in hippocampal slices by trains of electrical stimuli, Brain Res., 344 (1985) 296-302. Stringer, J.L. and Lothman, E.W., Epileptiform discharges induced by altering extracellular potassium and calcium in the rat hippocampal slice, Exp. Neurol., 101 (1988) 147-157. Stringer, J.L., Williamson, J.M. and Lothman, E.W., Induction of paroxysmal discharges in the dentate gyrus: Frequency dependence and relationship to afterdischarge production, J. Neurophysiol., 62 (1989) 126-135. Strowbridge, B.W., Masukawa, L.M., Spencer, D.D. and Shepherd, G.M., Hyperexcitability associated with localizable lesions in epileptic patients, Brain Res., 587 (1992) 158-163. Tancredi, V. and Avoli, M., Control of spontaneous epileptiform discharges by extracellular potassium: an "in vitro" study in the CAI subfield of the hippocampal slice, Exp. Brain Res., 67 (1987) 363-372. Tasker, J.G., Peacock, W.J. and Dudek, F.E., Local synaptic circuits and epileptiform activity in slices of neocortex from children with intractable epilepsy, J. Neurophysiol., 67 (1992) 496-507. Thompson, S.M. and Gahwiler, B.H., Activity-dependent disinhibition I. Repetitive stimulation reduces IPSP driving forces and conductance in the hippocampus in vitro, J. Neurophysiol., 61 (1989) 501-511. Traynelis, S.F. and Dingledine, R., Potassium-induced spontaneous

[51]

[52]

[53]

[54]

[55] [56]

[57]

[58]

139

electrographic seizures in the rat hippocampal slice, J. Neurophysiol., 59 (1988) 259-276. Urban, L., Aitken, P.G., Crain, B.J., Friedman, A.H. and Somjen, G.G., Correlation between function and structure in "epileptic" human hippocampal tissue maintained in vitro, Epilepsia, 34 (1993) 54-60. Uruno, K., O'Connor, M.J. and Masukawa, L.M., Alterations of inhibitory synaptic responses in the dentate gyrus of temporal lobe epileptic patients, Hippocampus, 4 (1994) 583-593. Uruno, K., O'Connor, M.J. and Masukawa, L.M. Effects of bicuculline and baclofen on paired-pulse depression in dentate gyrus of epileptic patients, Brain Res., 695 (1995) 163-172. Williamson, A., Spencer, D.D. and Shepherd, G.M., Comparison between the membrane and synaptic properties of human and rodent dentate granule cells, Brain Res., 622 (1993) 194-202. Wong, R.K., Traub, R.D. and Miles, R., Cellular basis of neuronal synchrony in epilepsy, Adv. Neurol., 44 (1986) 583-592. Wuarin, J., Kim, Y.I., Cepeda, C., Tasker, J.G., Walsh, J,P., Peacock, W.J., Buchwald, N.A. and Dudek, F.E., Synaptic transmission in human neocortex removed for treatment of intractable epilepsy in children, Ann. Neurol., 28 (1990) 503-511. Wyler, A.R., Ojemann, G.A. and Ward, A.A., Jr., Neurons in human epileptic cortex: correlation between unit and EEG activity, Ann. Neurol., 11 (1982) 301-308. Yaari, Y., Konnerth, A. and Heinemann, U., Nonsynaptic epileptogenesis in the mammalian hippocampus in vitro II role of extracellular potassium, J. Neurophysiol., 56 (1986) 424-438.