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Depth electrode studies and intracellular dentate granule cell recordings in temporal lobe epilepsy.

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Depth Electrode Studies and Intracellular
Dentate Granule Cell Recordings in
Temporal Lobe Epilepsy
Anne Williamson, PhD," Susan S. Spencer, MD,t and Dennis D. Spencer, MD"
Hippocampal depth electrodes are often used to localize seizure onset in patients who may have temporal lobe epilepsy
(TLE). A number of features of the spontaneous seizures and of their ictal onset patterns can be analyzed from these
recordings. We compared a number of the typical electroencephalographic (EEG) changes at seizure onset with several
cellular parameters recorded in dentate granule cells from the same 14 patients diagnosed with medial temporal
sclerosis (MTS) to examine the pathophysiological correlates of this spontaneous EEG activity in this form of TLE.
The intracellularly recorded parameters include the propensity to fire evoked epileptiform bursts, the absence of
evoked inhibitory potentials, the presence of polysynaptic excitatory postsynaptic potentials, and the presence of
spontaneous excitatory activity. We noted several correlations between the EEG data and the intracellular recordings.
The absence of synaptically evoked bursts was correlated with the presence of Iow-voltage fast activity at seizure
onset. In addition, the loss of inhibitory postsynaptic potentials was correlated with the presence of periodic spiking
pre-ictally. Several other correlations were also noted. These data indicate that EEG findings may be predictive of
anatomical and cellular pathological changes and provide clues to the physiological mechanisms involved in this form
of epilepsy.
Williamson A, Spencer SS, Spencer DD. Depth electrode studies and intracellular dentate granule cell
recordings in temporal lobe epilepsy. A n n Neurol 1995;38:778-787
The cellular mechanisms that characterize the transition from the interictal to the ictal state are not understood in either chronic animal models of epilepsy or
in the human disease. This transition is often observed
in temporal lobe epilepsy using hippocampal depth
electrodes and/or medial temporal subdural electrodes.
These electrodes are implanted to allow accurate localization of seizure onset in patients with medically intractable epilepsy of possible temporal lobe origin
El, 21.
Two common and distinct electroencephalographic
(EEG) patterns of ictal onset have been described from
intracranial EEG recordings in medial temporal lobe
seizures, periodic spiking and low-voltage fast activity.
The spikes are characteristic of medial temporal lobe
epilepsy due to mesial temporal sclerosis [3, 41. These
events occur at frequencies of 1 to 2 Hz, have large
amplitudes, are predominantly hippocampal, and precede any low-voltage fast activity, subsequent seizure
propagation, and clinical expression by 5 to 200 seconds. In addition, they have been correlated with a
reduction in CA1 neuronal density [5}. By contrast,
the low-voltage fast activity occurs at 10 to 15 Hz, has
a greater tendency to propagate, and is seen with or
without periodic spikes { 3 , 4, 61.
T h e ictal spiking activity has been correlated with an
increase in inhibitory drive based on unit recordings in
depth electrode studies in vivo 17, 81. This view was
also taken by Engel [3,7].In the view of these authors,
the periodic spike activity relies on inhibition to synchronize populations of hippocampal neurons. By contrast, the low-voltage fast activity is believed to represent periods of disinhibition [3].
The true cellular basis of the EEG, however, remains
poorly understood. In kindled rat hippocampi, spikes
were associated with action potentials followed by prolonged hyperpolarizations but not paroxysmal depolarizations [ 101. In cortical models of epilepsy, interictal
spikes are associated with PDSs, which then merge to
trigger a seizure 111. Therefore, in these animal models of epilepsy, EEG spikes appear to correlate with
excitatory, rather than inhibitory, events. Whether the
preictal or transitional spikes are physiologically akm
to the true "interictal" spike to which they bear striking
morphological similarity is not clear.
W e have had the opportunity to correlate the intra-
From the "Secrion of Neurosurgery and tDeparrment of Neurology,
Yale University School of Medicine, New Haven. CT.
Address correspondence to Dr Williamson, Secrion of Neurosurgery, 333 Cedar Sr, PO Box 208039, New Haven. CT 06520-8039.
Received J u n 6, 1994, and in revised form Feb 28 and Jul 5 , 1995.
Accepted for publication Jul 12. 1995.
778 Copyright 0 1995 by the American Neurological Association
cranially derived EEG recordings performed during
long-term monitoring for seizure localization with intracellular recordings in dentate granule cells performed o n t h e same hippocampi following resection of
the epileptogenic temporal lobe. This represents an
increasingly rare opportunity, as noninvasive techniques such as magnetic resonance imaging (MRI), positron emission tomographic, and single-photon emission computed tomographic scans in conjunction with
neuropsychological assessments have greatly reduced
the n u m b e r of patients being implanted with intracranial electrodes. The goal of this study was to determine
t h e relationships between the EEG data obtained from
spontaneous seizures in these patients and the cellular
characteristics of t h e granule cells.
In cases of mesial temporal sclerosis (the most comm o n form of nonlesional temporal lobe epilepsy), the
granule cells are of special interest for several reasons.
First, t h e granule cells provide much of the synaptic
input into t h e hippocampus and thus act as a gate for
incoming entorhinal input El2, 131. Second, t h e dentate gyrus undergoes profound synaptic reorganization
in temporal lobe sclerosis, including mossy fiber
sprouting, interneuronal cell loss, and extensive neurochemical remodeling [ 14-16}. Finally, with t h e exception of t h e relatively small CA2 pyramidal cell layer,
the granule cells are t h e only principal cell type that
survives in significant numbers [ 151. Therefore, t h e
hippocampal EEG recorded with d e p t h electrodes is
likely t o be recording granule cell-dependent events.
Patients with medically refractory partial seizures whose noninvasive EEG and functional and structural imaging studies
do not provide sufficient concordant data to localize the region of seizure onset may undergo chronic EEG recording
with implanted depth and/or subdural electrodes for more
precise localization [ 17). Those selected for this analysis included 14 consecutive patients who fulfilled the following
criteria: ( 1 ) spontaneous seizures of medial temporal onset
were recorded with hippocampal depth electrodes and/or
medial temporal subdural electrodes; (2) subsequent anteromedial temporal lobectomy including en bloc resection
of the hippocampus (from which seizure onset was documented) was performed; ( 3 ) intracellular recordings from
dentate granule cells in the resected hippocampus were obtained; ( 4 ) a diagnosis of medial temporal sclerosis (MTS)
was made based on quantitative cell counts and immunocytochemical analyses. At least three spontaneous seizures were
recorded from each patient; when some recorded seizures
arose in a different location, only those with onset in the
resected hippocampus were used for this analysis. Recordings
of spontaneous seizures sometimes required withdrawal of
phenytoin, valproic acid, or carbamazepine; but no other activarion procedures were used.
1~ i t r u r riul
a ~ Electrodes
Implanted electrodes were multicontact flexible depth electrodes stereotactically inserted from the posterior with a target area of the anterior hippocampus. Subdural electrodes
were niulticontact flexible electrodes inserted to sample lateral and medial inferior temporal structures through burr
holes. Anatomical locations of recording contacts were verified with postimplantation MRI.
Sixty-four channels of EEG were recorded continuously with
audiovisual split screen on a Telefactor Beehive/Beekeeper
system and stored referentially to allow reformatting and
montage selection for review of relevant channels. Spontaneous seizures were detected by event markers activated by
patients, nurses, or other individuals present, or by an automatic seizure detection system (Stellate Systems, 1983). Seizures were considered localized for subsequent surgery when
a rhythmic alteration in EEG activity in a restricted and consistent area preceded typical clinical seizure activity. Subclinical electrographic alterations consistent with this definition
but unaccompanied by clinical seizure activity were not included in this analysis.
Seizure Analysis
For this study, all spontaneous seizures arising in subsequently resected temporal lobe were analyzed for morphology at seizure onset, time to hrst propagation of seizure activity outside the hippocampus, duration of hippocampal
electrical seizure discharge, total electrical seizure duration,
and interhemispheric propagation time. Ictal onset was defined as a localized, sustained rhythmic electrical discharge
greater than 2 H z in frequency not due to changes in arousal
and associated with subsequent clinical seizure activity [GI.
For onset morphology, division into patterns of preictal spiking and low-voltage fast onset were made. Preictal spikes
were defined as focal hippocampal high-amplitude spikes at
0- to 2-Hz frequency lasting at least 5 seconds before seizure
onset (Fig 1) [ 5 ] . The low-voltage fast pattern of onset was
defined as 10- to 15-Hz low-amplitude rhythmic discharge
without other superimposed slow or spike activity (Fig 1).
Since each patient had multiple seizures recorded, and some
variability in pattern was often seen, we then assessed the
percentage of seizures in each individual that began with the
preictal spike or low-voltage fast activity onset pattern. In
some patients, in a variable number of their seizures, both
onset types were observed, with the periodic spiking preceding the low-voltage activity. Occasional seizures had other
types of onset (e.g., rhythmic slow discharge). A similar definition was applied to seizure onset in other sampled cortical
areas, from which time to initial and interhemispheric propagation was determined. Time from onset to complete cessation of all electrical seizure activity was designated total
electrical seizure duration. Finally, time from onset to termination of seizure activity in the hippocampus of onset (often,
but not always the same as total duration) was designated
hippocampal duration. Not all seizures recorded in an individual patient propagated to a similar degree; i.e., some seizures might begin in hippocampal electrode contacts and
propagate only to neocortex ipsilaterally, while others would
Williamson et al: TLE: In Vivo/In Vitro Physiology
LPT 1-3
RPT 1-2
LERC 1-2
LFT 1-3
RFT 1-3
LFPO 1-3
RFPO 1-3
LFO 1-3
RFO 1-3
F i g I . 0met of spontaneous seizure in 1 of the 14 patients reported. The periodic. ictal spike pattern of seizure onjet is seen
in LPT 3-5 and 5-6. and giws ulay t o the luziwoltagefast pattern (arrow) in the same location. Some seizures in this (and
other) patie?its began u.dth the low-voltagefast pattern without
the periodic spikes: others had the ictal spikes with a subsequent
gradual increuse in frequency ujrthout the louwoltage fast pat-
tern. Each dirision = 1 second: full scale marker = 750 FV.
LPT, RPT = lefr, right hippocampal depth electrodes; LERC
= leJt entorhinal cortex depth elertrode: LFT. RFT = frontal
t o temporal subdural strip electrodes;LFPo. RFPo = frontopolar
svbdural strips; LFO, RFO = orbital frontal subdural d-trips.
Electrode contacts are numbered deep ( I I to superficial.
780 Annals of Neurology Vol 38 No 5 November 1995
subsequently propagate to contralateral hippocampus. In
those situations, only seizures representing the measurement
of concern were used for mean calculations.
Slice Physiology
The hippocampi of these patients
were resected as described by Spencer [IS] and a 5- to
10-mm slab was placed in cold (4°C) artificial cerebrospinal
fluid (ACSF) oxygenated with 9597 OJS% C 0 2 within 5
minutes following the resection. W e usually obtained a section of the anterior body of the hippocampus.
The tissue was transported to the laboratory where
400-pm slices were prepared using either a Vibratome
(Lancer) or a Vibroslice (WPI) and placed in the recording
chamber 15 to 20 minutes following removal. The slices were
maintained at the gas-liquid interface in an interface type
recording chamber (Fine Science Tools) at 35 ? 1°C and
were allowed to recover for at least 2 hours prior to recording. The slices were constantly perfused at 1 mlimin with
oxygenated ACSF. The ACSF contained (in mM) NaCl 124,
KCI 3.5, MgSO, 2, NaH,PO., 1.2, N a H C 0 3 26, CaCI, 2.0,
and dextrose 10, and was maintained at a p H of 7.4.
ELEcTRoPmSioLocx. Recordings were made with an Axoclamp I1 amplifier (Axon Instruments) using microelectrodes formed on a Brown-Flaming type electrode puller
(Sutter Instruments). The electrodes were filled with 4 M
K-acetate and had resistances of 40 to 80 M a . Only cells
with membrane potentials hyperpolarized below - 55 mV
and with input resistances greater than 20 MR were analyzed.
Synaptic stimuli were delivered to the perforant path using
a monopolar tungsten stimulating electrode. The electrode
was placed at the outer edge of the molecular layer to activate
the incoming perforant path fibers. To reduce variability in
synaptic transmission between trials, three to four responses
were averaged for a given stimulus intensity. Synaptic stimuli
were delivered at a number of different intensities so that
both the excitatory postsynaptic potentials (EPSPs) and
events superthreshold to action potential generation could
be examined. Synaptic intensity ranged from 0.03 to 1.5 mA
with a pulse duration of 10 ksec. In addition, inhibitory postsynaptic potentials (IPSPs) were studied by delivering synaptic stimuli while the membrane potential was held at a number of potentials with D C current injection. In most cases,
stimuli were delivered while the cell was held at potentials
ranging from - 55 to - 90 mV. The investigator was blinded
to the history and clinical findings of the patients during the
data collection.
Intracelhlar Data Analysis
The data were collected using pClamp software and analyzed
off-line using either pClamp or Axograph software (Axon
Instruments). The data were also recorded on a strip chart
recorder (Gould). For the purposes of this study, the following four parameters were examined: the ability of the cells
to fire evoked epileptiform bursts, the absence of evoked
inhibitory potentials, and the presence of either polysynaptic
excitatory potentials or spontaneous activity.
The degree of bursting for a given cell was rated as the
sum of the number of action potentials generated by stimuli
delivered at 0.3, 0.5, and 0.7 mA. The stimuli used in these
analyses were given from the resting membrane potential
(mean -68.5 t 7.9 mV).
A simpler scale was used to measure the degree of IPSP
loss. Cells were ranked such that 0 = normal inhibition was
present following synaptic stimulation; 1 = some inhibitory
potentials were present, but they were unusually small or
were of an unusual morphology; and 2 = no inhibition could
be evoked at any stimulus intensity. We chose this method
because in many cases, the evoked responses were followed
by indistinct IPSPs or no IPSPs at all.
We assessed the degree of polysynaptic activity by counting the number of presumed polysynaptic events occurring
on the falling phase of 32 consecutive EPSPs. In a number
of cases there were up to four of these events associated with
a single synaptic response. Finally, spontaneous activity was
determined by counting the number of spontaneous events
occurring in a 10-second period. These data are expressed
in hertz units (Hz).
All the cellular and EEG data for each patient were averaged. A Spearman rank correlation coefficient analysis was
performed comparing each of the cellular parameters (except
for the IPSPs) and the EEG data for a total of 15 comparisons. This analysis was not valid for the IPSP scores because
it is not a continuous variable. These data were divided into
high and low groups based on the mean IPSP score and a
Mann-Whitney U test was performed. Statistical significance
was set at 50.05 and all averaged data are shown as the mean
? standard deviation.
The data described here are from a subset of 54 consecutive patients who underwent temporal lobectomies, 24 of
whom also had depth electrode studies. The EEG and intracellular data were analyzed by different investigators prior to
the pathological classification of the tissue into four distinct
groups. For clarity, only the MTS patients are included in
this study. Thus, the data were analyzed blind as to whether
a given patient would be included in this study.
All patients involved in this study had given their informed
consent and the slice experiments were approved by the Yale
Human Investigation Committee.
The data presented in this study are from a total of 14
consecutive patients diagnosed with MTS w h o were
implanted with depth electrodes t o localize seizure onset and whose hippocampi were subsequently resected
and made available for electrophysiological study. T h e
anatomical changes characteristic of MTS seen in these
hippocampi included cell loss in the hilus, CA1, and
CA3 in conjunction with sprouting of t h e mossy fibers
and of neuropeptide Y, substance P, and somatostatinimmunoreactive fibers [16]. From these 14 cases, w e
recorded 81 seizures of which 70 were localized t o t h e
medial temporal structures. A total of 34 intracellular
recordings were made from dentate granule cells in
these hippocampi. We studied between o n e and five
cells per patient.
Williamson et al: TLE: In Vivo/In Vitro Physiology
There was a great deal of variability in the degree of
bursting noted in these cells. However, granule cells
from sclerotic hippocampi could usually be induced to
fire epileptiform bursts. Bursts consisting of at least
three spikes could be evoked in 17 of 34 cells studied
from sclerotic tissue and doublets could be triggered
in nine of the remaining cells. The mean burst score
for the MTS cells was 5.4 i 4.6 with a range of 2 to
Figure 2A shows the responses to stimuli at 0.3, 0.5,
and 0.7 nA in three different cells from three different
patients. The data shown in the left panel is from a cell
in which we could not evoke more than a single spike.
The center panel shows a cell from a patient with MTS
that was moderately excitable. In this cell a doublet
could be easily elicited at both 0.5 and 0.7 nA. The
last trace shows the response from a very excitable cell
from a different hippocampus in which we could evoke
four spikes with a 0.7-nA stimulus.
Part B of Figure 2 illustrates the differences between
the characteristics of IPSPs used in these rankings. In
7 of 34 cells studied here, the IPSPs appeared comparable with those seen in rodent granule cells and in the
granule cells of patients with temporal lobe tumors in
which there is no evidence of hyperexcitability 1191.
By contrast, the middle trace shows an example of a 1
ranking in which only the slow IPSP is evident; 9 of
34 cells were ranked this way. Finally, a cell ranked as
a 2 is shown in the right hand panel of Figure 3B. We
observed this pattern of activity in 18 of 34 cells. Thus,
we saw evidence for disinhibition in 27 of 34 cells
included in this study. It is unclear from these data
whether there is specific loss of either the fast or slow
IPSPs; however, a slow component was often preserved.
Figure 3 shows examples of the other two parameters
used to classify this population of human granule cells,
i.e., polysynaptic EPSPs and spontaneous excitatory activity. The majority of cells (30 of 34) studied had
evidence of some polysynaptic activity; however there
was a wide range of values in this population (4 to 42
events in 32 trials). In 10 of 34 of these cells, fewer
than 10 polysynaptic events were seen. Between 10
and 20 events were seen in 13 of 34 cells; and in 11
of 34 cells, more than 20 polysynaptic EPSPs were
noted. The mean number of polysynaptic EPSPs occurring in 32 trials for these cells was 19.2 ? 15.6
events. Most, but not all of these components occurred
on the falling phase of the potential. Figure 3A shows
three examples of evoked EPSPs that demonstrate the
variability present in the population. Note that in the
Annals of Neurology
Vol 38
No 5
second and third traces, there are multiple components
to the EPSPs that are not seen in the left-hand recording. In addition, note that evoked EPSPs that had numerous polysynaptic events riding on them were prolonged relative to those with more normal kinetics.
Spontaneous Actizity
Figure 3B shows examples of the degrees of spontaneous activity that were noted in these cells. This spontaneous activity occurred at frequencies ranging from
0.04 to 4.3 Hz. The majority of cells studied (18 of
34) had low levels of spontaneous activity (<1.5 Hz)
as shown in the left panel of 4B. Only 5 of 34 had
very high levels (>3 Hz) of activity as shown in the
right panel of the figure. The remainder of the cells
studied exhibited intermediate levels of spontaneous
activity. The recorded cells received spontaneous activity at a mean frequency of 1.9 Hz. These spontaneous
potentials are probably excitatory, since they did not
reverse near the resting membrane potential (as would
be expected for y-aminobutyric acid type A [GABA,Imediated events) and their amplitude increased upon
hyperpolarization. We rarely noted spontaneous IPSPs
as have been described in rodent granule cells, probably because we used K-acetate rather than KCI in the
intracellular electrodes in this study.
EEG Parameten
Each of the 14 patients studied here had between 3 and
10 spontaneous seizures with the initial electrographic
change occurring in the hippocampus subsequently resected and studied electrophysiologically.
When the number of action potentials generated by
increasing stimulus intensities was compared with the
EEG data, we found a significant correlation with seizure onsets that began with low-voltage fast activity
and the abssmce of bursting activity. However, there was
no correlation between the presence of synaptically
evoked bursts and periodic spiking activity. These data
are shown in the Table.
When the absence of inhibition was examined with
respect to the EEG parameters, there was a significant
correlation ( p = 0.03) between the lack of synaptically
evoked IPSPs in the granule cells studied and the
Fig 2. Examples of the scoring rlsrd for bursting and for the
presence of iiihibitov3,posts.yaptic potentials (1PSP.o. I n A . examples of cels scored as 2, 5 . and 9 are shouw. The three stimub s inteiisitie.r zisedfor this scoring are dmm o n the right in
milliamperes ImA). B shouts the ziariation in the degree of s-yrzaptic inhibition seen in three celh ranked aj a 0. 1 . and 2. The
membrane potentials at ufhiihthe cetlj u'ere held are shown at
the ldt of each trace.
November 1995
Burst Score = 2
Burst Score =5
Burst Score =9
50 msec
IPSP Score = 0
IPSP Score = 1
IPSP Score = 2
Williamson et al: TLE: In Vivo/In Vitro Physiology
EPSP Score = 2
EPSP Score = 17
120 m~
50 msec
EPSP Score = 35
Spont. Activity Score = 0.8
Spont. Activity Score = 1.8
Spont. Activity Score = 3.9
200 msec
Fig 3. Examples of the ranking usedfor polysynaptic excitatory
postsynapticpotentials tEPSPs1 and spontaneous ai-tidy.Part A
shou1.s synaptic responses to prvforant path stinidation for three
cellsfrom diflerent patients. The ref1 in the left panel appears nornul;there is no widencefor niultipfe components in either the
EPSP or in the EPSPhllouing an action potential. In the center
panel. note that the J.econd spike arosejroni a discrete secondary depolarization. The right panel~hoic.?r
an example of extensivepoly-
synaptic actiaity that acts to prolong the duration of both the suband superthreshold responses. The scores gioen for each group of
traces represents the totalscorefor that cell (number of etments seen
in 32 traces).The number.r at the left of each trare indicate the
stimulus intensity in millianiperes (niA).Part B shows examp/es
of spontaneous actiziit-yfrom cells from three different patients.
Each panelshous 4 of the 10 seconds rrsed to .scorefor the frequencq' (in H z ) of the spontaneous actiiitj.
Correlation Between Intrucel~ulurPhysiology and
I nrracranial
Evoked bursts
Loss of IPSPS
Polysynaptic EPSPs
Periodic spikes
Seizure duration
0.029 Z = -2.2
0.038Z = -2.1
0.05 Z = 2.0
Spontaneous activity
0.05 Z = 1.9
This table shows the correlations between each of the intracellular
variables and the electroencephalographic parameters for those patients diagnosed with medial temporal sclerosis. Only those values
reaching significance of 50.05 are shown.
LVF = low-voltage fast; IPSPs = inhibitory postsynaptic potentials;
EPSPs = excitatory postsynaptic potentials.
presence of periodic EEG spikes. The mean high score
was 1.81 0.6 for 7 patients and the low score was 1.09
2 0.2 for the remaining 7 patients. No other significant correlations were noted.
The third variable studied here was the presence of
polysynaptic EPSPs. We found that the number of
EPSPs was correlated with both a longer delay to interhemispheric spread and with the seizure duration. It
was interesting that the presence of polysynaptic EPSPs
did not correlate with the hippocampal duration.
Finally, a strong, but not significant, correlation was
seen between the presence of spontaneous activity and
low-voltage fast activity ( p = 0.06) in the 14 patients
with MTS.
These studies represent the first attempt to correlate
electrophysiological data obtained at two very different
levels of resolution in the same tissue, i.e., intracranial
depth electrode recordings and intracellular recordings
in the brain slice preparation. Our initial hypothesis
was that the pattern of activity originating in the medial
temporal lobe shown to be epileptogenic should be
correlated with the pattern of excitability seen in the
cell population most resistant to cell loss, the dentate
granule cells. Our data indicate that this is the case
and may provide some insights into the physiological
alterations that give rise to certain EEG patterns in
human medial temporal lobe epilepsy.
Cowelations with EEG Activity
The relationship between the loss of synaptically
evoked inhibition and the presence of periodic EEG
spiking at seizure initiation was the most surprising
aspect of the present study. Since the periodic spikes
prior to ictal onset are a hallmark of temporal lobe
epilepsy, granule cell disinhibition may prove to be a
critical feature of this disease and in the synchronization mechanism that underlies these spikes.
The classical concept of the inrerictal spike and wave
complex in both partial and primary generalized epilep-
sies and in the ictal petit mal discharge is that it reflects
paroxysmal depolarization shifts (spikes) followed by
afterhyperpolaritations (AHPs). The A H P in those circumstances is thought to be due, at least in some part,
to enhanced GABA-mediated synaptic inhibition. This
conclusion is partly based on the finding that stimulation of the anterior hippocampus stopped the activity
of spontaneously active cells in single unit recordings
recorded on the epileptogenic side [7}. This mechanism has been suggested to explain the “hypersynchronous” spiking onset in some medial temporal lobe seizures. However, our data suggest that, despite the
roughly similar appearance, periodic spikes of ictal onset in medial temporal lobe epilepsy as recorded directly from hippocampal tissue prior to its removal are
not due to similar synaptic inhibitory mechanisms since
they were correlated with decreased inhibition.
There are several possible explanations for the discordance between our data and prior hypotheses. The
hypothesis proposed by Sloviter [20) provides one explanation for the differences in our data. He proposes
that granule cell activation produces lateral inhibition
in adjacent hippocampal laminae and that in cases of
MTS the relative size of a lamina is enhanced so that
a larger group of cells fires together. However, within
a given lamina, the inhibitory cells are deafferented and
thus cannot contribute to synaptic inhibition. These
longitudinal connections will have been disrupted in
the slices where we are studying only a few of these
functional units. Thus the experiments performed by
Isokawa and colleagues in vivo demonstrated this proposed lateral inhibition. The current observations support the data from a number of laboratories that demonstrate that both the granule cells and pyramidal cells
in hippocampi from patients with temporal lobe sclerosis appear disinhibited when studied in the slice preparation 121-231.
Alternatively, the unit studies may have been performed on putative pyramidal cells rather than on granule cells 17). In many of these cases, there is substantial
cell loss in the C A l and CA3 pyramidal cell layers
while CA2 is relatively spared 114, 241. Therefore,
many of the in vivo recordings may have been from
the CA2 pyramidal cell population. Our intracellular
recordings from CA2 pyramidal cells from patients
with mesial temporal sclerosis indicate that these cells
are not hyperexcitable and are resistant to firing bursts
and, thus, appear to be well inhibited {21) and may,
therefore, account for the observation that the hippocampi of these patients receive robust inhibition. Finally, many of these studies report findings that can be
interpreted as either due to increased inhibition [ 3 ] or
to the cell loss that is known to occur in these hippocampi [14, 241.
It was especially interesting that the ability of cells
to fire bursts was negatively correlated with the pres-
Williamson et al: TLE: In Vivo/In Vitro Physiology
ence of low-voltage fast activity. These data suggest
that the generation of low-voltage fast activity may not
be due to granule cell hyperexcitability. Alternatively,
since those patients whose seizures began with lowvoltage fast activity tended to have briefer synaptic responses, the granule cells would be able to follow at
the higher frequencies that characterize low-voltage
fast activity.
The presence of polysynaptic EPSPs was significantly
correlated with the total seizure duration. Studies in
animal models of temporal lobe epilepsy suggest that
single sprouted mossy fibers can synapse on distant
granule cells within the same lamina [25]. We propose
that perforant path stimulation, which activates a number of granule cells will, through the sprouted mossy
fibers, send secondary synaptic events throughout the
dentate, which will vary in their delay. We hypothesize
that the presence of these recurrent pathways serves
to prolong the duration of the seizures, partially by
prolonging the duration of individual EPSPs.
Another feature that we noted in the hippocampi
of patients with MTS is the presence of spontaneous
excitatory activity. Unlike the other variables that we
studied, this did not correlate significantly with any of
the measured EEG parameters. While there may be
spontaneous GABA,-mediated IPSPs present, as has
been shown in rodent granule cells [26), the spontaneous activity seen in the human material did not reverse
its polarity depolarized to --70 (EC, in these studies)
and did not disappear when bicuculline was applied
(Williamson A, unpublished 'results) and is thus likely
to be due to spontaneous glutamate release. However,
there was a correlation between the presence of spontaneous activity and low-voltage fast activity that approached significance ( p = 0.062). These data suggest,
therefore, that this type of activity may be involved in
the generation of seizure activity in these patients.
The possible source for this type of activity is still
unclear. The primary afferent input into the granule
cells arises from the upper layer entorhinal cortical stellate cells 113). However, in these slices, the bulk of
the entorhinal cortex was removed. The mossy cells
also provide excitatory input to the granule cells [ 2 7 )
in normal rodent hippocampus; however, one of the
features of temporal lobe sclerosis is the loss of significant numbers of both of these cell populations. It is
possible that some of the observed spontaneous excitatory input is due to activity in the sprouted mossy
fibers as described above. The sprouted mossy fibers
primarily contact the granule cell dendrites in the inner
molecular layer; therefore, single synaptic events might
be measurable at the soma. At the onset of a seizure,
this increase in the level of baseline spontaneous activity may allow for the eventual synchronization and
propagation of seizures.
These studies are a preliminary attempt to under-
786 Annals of Neurology
Vol 38 No 5
stand physiology of medial temporal lobe epilepsy by
examining the relationship between intracellular physiology and the EEG in patients with MTS. The correlations reported here suggest that the patterns of EEG
changes associated with seizures reflect specific anatomical and physiological alterations in the dentate
granule cells.
Supported by NIH grant NS30012
NS30613 to D.D.S. and S.S.S.
A.W. and NIH grant
We thank Dr Nihal de Lanerolle for providing us with the information on the anatomical changes seen in this tissue. In addition, we
thank the patients whose consent mllkes these studies possible.
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electrode, depth, granules, intracellular, dentate, temporal, studies, recording, epilepsy, cells, lobel
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