1Departments of Neurology and 2Anatomy, and 3The Neuroscience Training Program, University of Wisconsin, and the 4William S. Middleton Veterans Administration Hospital, Madison, WI 53792
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ABSTRACT |
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Sayin, Ümit,
Paul Rutecki, and
Thomas Sutula.
NMDA-dependent currents in granule cells of the dentate gyrus
contribute to induction but not permanence of kindling.
Single-electrode voltage-clamp techniques and bath application of the
N-methyl-D-aspartate (NMDA) receptor antagonist
2-amino-5-phosphonovaleric acid (APV) were used to study the time
course of seizure-induced alterations in NMDA-dependent synaptic
currents in granule cells of the dentate gyrus in hippocampal slices
from kindled and normal rats. In agreement with previous studies,
granule cells from kindled rats examined within 1 wk after the last of
3 or 30-35 generalized tonic-clonic (class V) seizures demonstrated an
increase in the NMDA receptor-dependent component of the perforant
path-evoked synaptic current. Within 1 wk of the last kindled seizure,
NMDA-dependent charge transfer underlying the perforant path-evoked
current was increased by 63-111% at a holding potential of 30 mV.
In contrast, the NMDA-dependent component of the perforant-evoked
current in granule cells examined at 2.5-3 mo after the last of 3 or
90-120 class V seizures did not differ from age-matched controls.
Because the seizure-induced increases in NMDA-dependent synaptic
currents declined toward control values during a time course of 2.5-3
mo, increases in NMDA-dependent synaptic transmission cannot account
for the permanent susceptibility to evoked and spontaneous seizures
induced by kindling. The increase in NMDA receptor-dependent
transmission was associated with the induction of kindling but was not
responsible for the maintenance of the kindled state. The time course
of alterations in NMDA-dependent synaptic current and the dependence of
the progression of kindling and kindling-induced mossy fiber sprouting
on repeated NMDA receptor activation are consistent with the
possibility that the NMDA receptor is part of a transmembrane signaling
pathway that induces long-term cellular alterations and circuit
remodeling in response to repeated seizures, but is not required for
permanent seizure susceptibility in circuitry altered by
kindling.
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INTRODUCTION |
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Seizures induce long-term cellular alterations in
the hippocampus that include neuronal loss and mossy fiber sprouting in the dentate gyrus accompanied by alterations in the sequence of synaptic activation (Golarai and Sutula 1996b),
in excitatory connectivity (Wuarin and Dudek 1996
),
evolving changes in inhibition (Buhl et al. 1996
;
Otis et al. 1994
; Rutecki et al.
1996
), increased susceptibility to recurrent seizures
(kindling) (Goddard et al. 1969
), and development of
memory deficits in behavioral tasks that depend on the integrity of
hippocampal pathways (Sutula et al. 1995
). Recent
studies have demonstrated that the induction of mossy fiber sprouting
and the progression of kindling in response to repeated seizures are
dependent on repeated activation of
N-methyl-D-aspartate (NMDA) receptors
(Sutula et al. 1996
). Because NMDA receptors play a critical role in the induction of these seizure-induced cellular
alterations in response to kindling, and NMDA-dependent gene expression
may play a role in the development of long-term structural and
functional alterations induced in hippocampal circuitry (Sprengel et al. 1998
), it was of interest to examine in
further detail how NMDA receptor-dependent synaptic transmission
contributes to the induction and permanence of the kindled state.
NMDA and non-NMDA receptors play distinct roles in synaptic
transmission and in the induction of a variety of forms of synaptic and
circuit plasticity in the CNS. The NMDA receptor-gated channels typically contribute only minimally to synaptic transmission at membrane potentials in the range of 60 to
75 mV because of
voltage-dependent block of the channel by Mg2+. With
depolarization of the membrane to
30 to
40 mV, the Mg2+
block is relieved, and NMDA-gated channels become permeable to Ca2+, Na+, and K+, which results in
a slowly developing, long-duration synaptic current. The influx of
Ca2+ into the postsynaptic cell triggers a variety of
second-messenger pathways, which can activate transcription factors
such as c-fos, and may induce changes in gene expression that could
play a role in the induction of long-term seizure-induced cellular
alterations in the dentate gyrus (McNamara 1995
).
Repeated activation of the NMDA receptor during kindling plays a
critical role in the progression of kindling and the induction of mossy
fiber sprouting (Sutula et al. 1996
).
As in many pathways of the CNS, fast synaptic transmission between axon
terminals of the perforant path and the granule cells of the dentate
gyrus is mediated primarily by
-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)
receptors, but there is also a significant contribution by NMDA
receptors under normal conditions (Keller et al. 1991
; Lambert and Jones 1990
). Previous studies have
demonstrated that the initial high-frequency stimulation that induces
kindled seizures increases the NMDA-dependent component of synaptic
transmission in granule cells (Mody et al. 1988
). The
increase in NMDA-dependent synaptic transmission persists for at least
1 mo after stimulation (Mody et al. 1988
), but it has
not been clear whether the permanent increase in hippocampal
excitability induced by kindling is due to a permanent increase in
NMDA-dependent synaptic transmission, or might be caused by permanent
seizure-induced cellular alterations such as neuronal loss or sprouting
(Bengzon et al. 1997
; Cavazos and Sutula
1990
; Cavazos et al. 1991
, 1994
;
Sutula et al. 1988
). The aim of this study was to
address these possibilities by examining NMDA receptor-dependent
currents in granule cells of the dentate gyrus during the induction of
kindling, and at long intervals after the last evoked seizure in the
fully kindled state. A preliminary report has been presented in
abstract form (Sayin et al. 1996
).
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METHODS |
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Kindling procedures
Male Sprague Dawley rats (250-300 g) were anesthetized with a
combination of ketamine (80 mg/kg im) and xylazine (10 mg/kg im) and
were stereotaxically implanted with chronic electrodes for stimulation
and recording in the olfactory bulb (9 mm anterior, 1.2 mm lateral, 1.8 mm ventral to bregma), amygdala (1.5 mm posterior, 4.2 mm lateral, 8.8 mm ventral to bregma), or angular bundle (8.1 mm posterior, 4.4 mm
lateral, and 3.5 mm ventral to bregma). The electrodes were fixed to
the skull with screws and dental acrylic. After a 2-wk recovery period
following electrode placement, the unrestrained awake animals in the
kindling group received twice daily kindling stimulation (5 days per
week) with a 1-s train of 62-Hz biphasic constant-current 1.0-ms
square-wave pulses. The stimulation was delivered at the lowest
intensity that evoked an afterdischarge (AD) according to standard
procedures (Cavazos et al. 1994). The
electroencephalogram and AD were recorded from the bipolar electrode,
which could be switched to the stimulator by a digital circuit for the
delivery of the kindling stimulation. The evoked behavioral seizures
were classified according to standard criteria (Sutula and
Steward, 1986
). Rats were studied within 1 wk of 3 or
30-35 evoked class V seizures, or at 2.5-3 mo after 3 or >90 evoked
generalized tonic clonic (class V) seizures.
Preparation of hippocampal slices
The rats were anesthetized with pentobarbital sodium (60 mg/kg
ip), and the brain was rapidly removed and transferred to iced normal
saline. The hippocampus was dissected and cut in the transverse plane
at a thickness of 400-450 µm using a McIllwain tissue chopper. The
slices were maintained in interface chamber superfused with oxygenated
(95% O2-5% CO2) artificial cerebrospinal
fluid (ACSF) at 32-36°C. The ACSF composition (in mM) was 124 NaCl,
2.5 KCl, 1.25 NaH2PO4, 2 CaCl2, 2 MgSO4, 26 NaHCO3, and 10 glucose. The slices
were allowed to equilibrate in ACSF for 2 h before recording. Recordings were obtained in 10 µM bicuculline methiodide (Sigma) to
block -aminobutyric acid-A (GABAA) receptor-mediated
inhibition.
Stimulation and recording procedures
Intracellular recordings in granule cells of the dentate gyrus
were evoked by stimulation of perforant path fibers in the outer
molecular layer of the dentate gyrus in hippocampal slices from kindled
rats and age-matched controls. The recordings were obtained using glass
microelectrodes (impedances of 25-40 M) filled with cesium acetate
to block outward K+ currents and QX-314 to block
action-potential generation. The outer molecular layer was stimulated
every 20 s by a stainless steel bipolar electrode that delivered
constant-current pulses of 0.05 ms duration at a range of intensities
from 40 to 200 µA. Recordings were obtained with an Axoclamp 2B
amplifier in the single-electrode voltage-clamp mode at a switching
frequency of 3-5 kHz. A separate oscilloscope was used to adjust
capacitance compensation and sampling rate. Synaptic currents were
recorded at holding potentials from
70 to +30 mV in 10-mV steps in
response to a perforant path stimulus. The stimulus intensity was
adjusted to evoke a synaptic current of at least 0.5 nA with an
apparent monoexponential decay at a holding potential of
70 mV.
Recordings that demonstrated voltage-dependent contaminants such as
regenerative currents indicated by escape voltage transients, failure
to reverse near 0 mV, or clear polysynaptic components were excluded.
The evoked intracellular potentials or currents were amplified and displayed on an oscilloscope. The evoked currents were recorded, stored, and analyzed using a DIGIDATA 1200 AD converter (Axon Instruments) and PCLAMP 6.02. Clampex 6.02 was used for stimulus generation and data collection. Clampfit 6.02 was used for analysis of
10-90% rise time constants, decay time constants, and signal analysis.
Data analysis and statistics
Voltage-clamp recordings were made from 44 granule cells in normal rats and 66 granule cells in kindled rats (Table 1). Because not every cell was subjected to the same sequence of measurements and bathing conditions, the number of cells for specific observations are provided separately in the text and figures. Charge transfer was measured as the integrated area under the curve of evoked synaptic current from the stimulus to 100 ms, when the inward current had typically decayed nearly to baseline (see Fig. 3 for additional details of the calculation). Differences in current amplitude, charge transfer, and current-voltage (I-V) relationships were evaluated for statistical significance with the Student's t-test when the data were normally distributed, or the Mann-Whitney rank sign test. When multiple comparisons were required, one-way analysis of variance (ANOVA) followed by Dunnett's test or the Newman-Keuls test were used post hoc for paired comparisons.
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RESULTS |
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Synaptic currents recorded from granule cells in the dentate gyrus (DGCs) of hippocampal slices obtained from kindled rats within 1-7 days after 3 or 30-35 class V seizures (n = 33), and at 2.5-3.0 mo after 3 or 90-120 class V seizures (n = 33), were compared with currents in age-matched, normal controls (n = 44, see Table 1). There were no significant differences in stimulus intensities required to evoke currents in kindled and control groups, but there was a trend toward slightly lower stimulation intensities required to evoke currents in the granule cells from kindled rats as compared with age-matched controls (Table 1).
Perforant path-evoked currents in granule cells from normal rats
In DGCs from normal rats at a holding potential of 70 mV, inward
current was evoked by a perforant path stimulus at a latency of 1-2
ms. The rise time and decay time of the evoked inward current at
holding potentials from
70 to 0 mV (see Table
2) were comparable with previously
published studies for granule cells from rats (Keller et al.
1991
) and resected human dentate gyrus (Isokawa et al.
1997
). The duration of the inward current was longer at holding
potentials positive to
50 mV (see Fig.
1A). The reversal potential of
the evoked inward current was 1.1 ± 0.56 (SE) mV and did
not differ from kindled groups (see Table 2). In agreement with
previous studies (Isokawa et al. 1997
; Keller et
al. 1991
; Lambert and Jones 1990
), a component
of the inward current evoked by perforant path stimulation was blocked
by 50 µM d-2-amino-5-phosphovalerate (APV) and was therefore NMDA
receptor dependent (Fig. 1A).
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I-V plots and calculation of charge transfer before and
after bath application of 50 µM APV were used to characterize the timing and voltage dependence of the NMDA component of the perforant path-evoked inward current in DGCs from normal rats. For the granule cell from the normal rat illustrated in Fig. 1A, the
I-V relationship measured at 20 ms after the stimulus (Fig.
1B) and evoked charge transfer (Fig. 1C)
demonstrated an APV-sensitive inward current recorded at holding
potentials of 40 to
30 mV. Synaptic charge transfer was calculated
before and after application of 50 µM APV (see Fig. 3B for
additional details of the calculation). The APV-sensitive component of
evoked synaptic charge transfer increased at holding potentials
positive to
50 mV, reached a maximum at
30 to
20 mV, and was not
detected at 0 mV (Fig. 1C).
Perforant path-evoked currents in granule cells from kindled rats
DGCs from kindled rats (n = 22) studied within 1 wk after the last of three generalized tonic clonic (class V) seizures
evoked by kindling stimulation of the olfactory bulb demonstrated
larger inward synaptic current evoked by perforant path stimulation
compared with DGCs from normal rats (n = 14, compare
Figs. 1A and 2A). These results confirmed
previous observations that kindling induced by stimulation of a variety
of limbic sites, including the amygdala and hippocampal commissures
(Kohr et al. 1993; Kohr and Mody
1994
; Mody et al. 1988
), increases the
NMDA-dependent component of synaptic transmission in DGCs. Except for
an increase in the 10-90% rise time of the inward current evoked at
30 mV in granule cells from kindled rats studied at 1-7 days after
the last seizure, there were no significant differences in rise times
or decay times between kindled and control groups (see Table 2).
The I-V plot at 20 ms for the DGC from a kindled rat in Fig.
2A demonstrated a prominent
APV-dependent region of negative slope conductance between 50 and
10 mV compared with the DGC from a normal rat (compare Figs.
1B and 2B). Calculation of evoked charge transfer
also revealed a prominent APV-sensitive, voltage-dependent increase in
charge transfer between
40 and
10 mV (compare Figs. 1C
and 2C). These results were consistent with an increase in the NMDA-dependent component of the inward current evoked in DGCs from
kindled rats and confirmed previous observations (Mody et al.
1988
). Within 1 wk after the last of 30-35 class V seizures in
DGCs (n = 11) from kindled rats, there were also
significant increases in the amplitude and duration of the inward
synaptic currents and the NMDA-dependent component of the inward
current evoked at 20 ms and a holding potential of
30 mV compared
with DGCs from normal rats (n = 8, data not shown).
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Because the NMDA receptor-dependent current evoked in granule cells by
perforant path stimulation has a long duration and is voltage
dependent, the ratio of the inward current at 20 ms to peak inward
current at a holding potential of 30 mV has been used as a measure of
the NMDA component of the inward evoked current (Keller et al.
1991
). The ratio of the inward current at 20 ms to peak inward
current at a holding potential of
30 mV was also used in this study
to compare seizure-induced alterations in NMDA-dependent synaptic
currents in normal and kindled rats (Fig.
3A). This measure was compared
with the NMDA receptor-dependent charge transfer in DGCs from kindled
and normal rats (Fig. 3, B and C). The area under
the inward synaptic current waveform from the time of stimulus to 100 ms was measured before and after application of 50 µM APV, and the
calculated difference was the NMDA-dependent charge transfer (Fig.
3B). In DGCs from both normal and kindled rats, there was a
significant correlation between the ratio of the inward current at 20 ms to the peak inward current and the NMDA receptor-dependent charge
transfer at a holding potential of
30 mV (r = 0.686, P < 0.0001, Fig. 3C). There were also
linear relationships between the ratio of inward current at 20 ms to
peak inward current (expressed as a percent) and the NMDA-dependent
charge transfer in granule cells from normal rats (r = 0.425, P < 0.05,
, Fig. 3C) and kindled rats (r = 0.8, P < 0.0001,
, Fig.
3C).
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There was a significant increase in the ratio of the inward
current at 20 ms to peak inward current at a holding potential of 30
mV within 1-7 days after the last of 3 or 30-35 class V seizures
compared with controls (n = 33, P < 0.001, Fig. 4, A and
B). Significant increases in the ratio of the inward current at 20 ms to peak inward current at a holding potential of +20 mV were
also observed within 1 wk after the last of 3 class V seizures
(P < 0.05, Fig. 4A) and 30-35 class V
seizures (P < 0.001, Fig. 4B). Significant
increases in NMDA-dependent charge transfer were observed in DGCs at
holding potentials of
40,
30, and
20 mV within 1 wk after the
last of 3 or 30-35 class V seizures (n = 14) compared
with DGCs from normal controls (n = 12, Fig. 4, C and D).
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Perforant path-evoked currents in granule cells at 2.5-3.0 mo after kindled seizures
The increase in the NMDA-dependent component of the inward current
of granule cells from kindled rats was not permanent. The amplitude,
duration, voltage sensitivity, and APV sensitivity of the inward
currents evoked in granule cells by perforant path stimulation in
hippocampal slices obtained at 2.5-3 mo after the last of 3 or 90-120
class V seizures did not differ from granule cells in age-matched
normal rats (Fig. 5, A and
B). This effect was not dependent on the site of kindling
stimulation and was observed in rats that received kindling stimulation
delivered to the olfactory bulb (n = 11), perforant
path (n = 4), and amygdala (n = 1).
There were no significant differences in the ratio of the inward
current at 20 ms to peak inward current at a holding potential of 30
or +20 mV in granule cells from kindled rats examined at 2.5-3 mo
after the last of 3 or 90-120 class V seizures (Fig.
6, A and B). This
effect was not dependent on the site of kindling, as the ratio of
inward current at 20 ms to peak inward current in hippocampal slices
obtained from rats kindled in the olfactory bulb (62.5 ± 9.9%)
or the perforant path (59.8 ± 7.3%) did not differ from the
ratio in age-matched controls (66.8 ± 4.5%, not significant).
There was also no difference in NMDA-dependent charge transfer at
2.5-3 mo after 3 or 90-120 class V seizures compared with DGCs from
age-matched normal controls (Fig. 6, C and D).
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DISCUSSION |
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Single-electrode voltage-clamp techniques and bath application of
the NMDA antagonist APV were used to evaluate the time course of
seizure-induced alterations in the perforant path-evoked synaptic current in granule cells of the dentate gyrus from kindled rats. The
results confirmed the following observations from previous studies:
1) the presence of an NMDA-dependent component in the synaptic current evoked by perforant path stimulation in the granule cells of normal rats (Keller et al. 1991; Lambert
and Jones 1990
), and 2) an increase in the
NMDA-dependent component of the synaptic current evoked by a perforant
path stimulus in granule cells from rats kindled by olfactory bulb
stimulation, as observed previously in granule cells from kindled rats
that received amygdala and hippocampal commissural stimulation
(Kohr et al. 1993
; Kohr and Mody 1994
;
Mody et al. 1988
). In addition, the experiments provided new quantitative details about seizure-induced increases in the NMDA-dependent component of synaptic currents at perforant
path-granule cell synapses, and revealed that the kindling-induced
increase in the NMDA-dependent component of the evoked synaptic
current, which was detected as long as 4-6 wk after the last seizure
in previous studies (Kohr et al. 1993
; Kohr and
Mody 1994
; Mody et al. 1988
), was no longer
apparent at 2.5-3 mo after the last kindled seizure. At this interval
after the last evoked seizure, the NMDA-dependent component of the
current evoked in granule cells by a perforant path stimulus in slices
from rats kindled by stimulation of the olfactory bulb and perforant
path was indistinguishable from age-matched controls.
Although it might be of interest to consider the possibility that
the initial increase and evolving alterations in the NMDA-dependent component of the evoked granule cell current varied as function of the
site of kindling stimulation, this study was not specifically designed
to address this question. Our results and previous studies (Kohr
et al. 1993; Kohr and Mody 1994
; Mody et
al. 1988
), however, clearly demonstrate that the NMDA-dependent
current evoked in granule cells by activation of the perforant path is
increased by kindling of a variety of sites and is not limited only to
kindling of the direct monosynaptic input from the entorhinal cortex.
These results confirm long-standing observations that the changes
induced by kindling of the perforant path are transynaptic in the
hippocampus and are not restricted to the pathway of stimulation
(Goddard et al. 1969
; Messenheimer et al.
1979
). These previous observations and our results are also
consistent with other studies which demonstrated that afferent input to
the rodent hippocampal formation is bilaterally propagated in the
dentate gyrus (Golarai and Sutula 1996
), and that the
initial ADs are likewise bilaterally propagated in limbic pathways
(Stringer and Lothman 1992
). Our results furthermore provided no evidence suggesting that the evolving alteration in the
NMDA-dependent current, which decreased and was comparable with
controls at 2.5-3 mo after the last seizures, was dependent on the
site of stimulation. Because the NMDA-dependent current at 2.5-3 mo
after the last seizure evoked by kindling of the perforant path did not
differ from age-matched controls or from rats that received kindling
stimulation of the olfactory bulb, it seems unlikely that there are
significant differences in the persistence of seizure-induced
alterations in the NMDA-dependent current in rats on the basis of
kindling of the monosynaptic perforant pathway or polysynaptic pathways
to granule cells from the olfactory bulb or amygdala.
The time course and APV sensitivity of the seizure-induced
alterations in the perforant path-evoked synaptic current suggest that
NMDA-dependent synaptic transmission in granule cells could contribute
to abnormal hippocampal excitability in the early stages of kindling,
but not at long intervals after the last evoked seizure. Furthermore,
previous studies have demonstrated that repeated activation of the NMDA
receptor plays a critical role in the induction of mossy fiber
sprouting and the progression of kindling (Sutula et al.
1996). These observations implicate the NMDA receptor
in the induction of kindling and the long-term structural and
functional alterations in the dentate gyrus associated with repeated
kindled seizures, but also demonstrate that increases in NMDA
receptor-dependent synaptic transmission in granule cells cannot
account for the permanence of the kindling effect.
NMDA-dependent synaptic currents in granule cells of normal rats
The synaptic current evoked in granule cells by perforant path
stimulation in normal rats displayed voltage dependence, time course,
and APV sensitivity that were consistent with NMDA receptor-dependent synaptic transmission, as demonstrated in previous current-clamp and
voltage-clamp studies (Keller et al. 1991;
Lambert and Jones 1990
). The cell bodies of granule
cells are compact, but it is unlikely that adequate space clamp can be
achieved by single-electrode voltage-clamp or other whole cell clamp
methods throughout the extent of the granule cell dendrites. Because
the enhanced APV-sensitive component of the synaptic current was
apparent not only at a holding potential of
30 mV but was also
observed at +20 mV, it is unlikely that the inward current can be
explained by a Ca2+ current or other dendritic
voltage-dependent currents. Despite the possibility of some
contribution of voltage-dependent calcium currents in dendrites to the
evoked inward currents, the results of this study and previous whole
cell voltage-clamp recordings are consistent with the presence of a
substantial NMDA-dependent component of perforant path synaptic
transmission in granule cells of normal rats, which confirms earlier
studies (Keller et al. 1991
; Lambert and Jones
1990
).
Time course of increases in NMDA-dependent synaptic currents induced by kindling
The present experiments also confirmed previous observations that
kindled seizures increase NMDA receptor-dependent synaptic transmission in granule cells of the dentate gyrus (Mody et al. 1988). The increase is rapidly detected in granule cells after kindling stimulation and is accompanied by increasing sensitivity to
iontophoresis of NMDA, by alteration of activation/inactivation kinetics of calcium currents, and by changes in the properties of NMDA
receptor-gated channels including alterations in Mg2+
sensitivity (Kohr et al. 1993
; Kohr and Mody
1994
; Mody et al. 1988
). These altered
properties may reflect underlying molecular alterations or changes in
subunit composition of the NMDA receptor channel complex (Kraus
et al. 1994
, 1996
). Previous studies
have detected alterations in NMDA receptor channel properties at 24-72 h after the last seizure. In this study, increases in the
voltage-dependent NMDA-dependent currents were also observed within
1-7 days after the last seizure. Seizures rapidly induce alterations
in single channel properties of the NMDA receptor (Kohr et al.
1993
; Kohr and Mody 1994
), which persist for as
long as 4-6 wk after kindled seizures. At 2.5-3 mo after the last of
either 3 or 90-120 class V seizures, perforant path-evoked synaptic
currents in granule cells, and the NMDA-dependent component of these
currents, could not be distinguished from age-matched normal controls.
Repeated seizures evoked by kindling stimulation induce permanent
increases in susceptibility to seizures, but the increase in the NMDA
component of synaptic transmission induced in granule cells by kindling is clearly not permanent.
Role of the NMDA receptor in seizure-induced plasticity
The increase in NMDA-dependent synaptic current evoked in
granule cells of kindled rats by perforant path stimulation is not permanent, but recent studies have provided evidence that the NMDA
receptor plays a critical role in the induction of permanent structural
and functional alterations induced by repeated kindled seizures.
Administration of the NMDA antagonist MK801 before each kindling
stimulation, which prolongs stimulation-induced seizures relative to
untreated controls and increases the duration and number of
electrographic seizures required to evoke class V seizures, not only
prevents the behavioral progression of kindling, but also prevents the
development and progression of seizure-induced mossy fiber sprouting
(Sutula et al. 1996). The intracellular domain of the
NR2 subunit of the NMDA receptor also plays a role in the progression
of kindling and mossy fiber sprouting (Sprengel et al.
1998
). The NMDA receptor has also been implicated in the induction of mossy fiber sprouting after seizures evoked by kainic acid
(Cantallops and Routtenberg 1996
; McNamara and
Routtenberg 1995
). These observations suggest that the NMDA
receptor could be a critical link in a signal transduction pathway that
translates acute effects of seizures into long-term structural and
functional alterations in hippocampal circuitry.
The present results are consistent with the possibility that repeated
activation of the NMDA receptor during seizures increases NMDA-dependent synaptic transmission, and initiates a series of molecular and cellular alterations that initially contribute to enhanced excitability in hippocampal pathways and eventually induce long-term structural and functional alterations in the dentate gyrus.
In support of this possibility, NMDA receptor antagonists such as MK801
have pronounced effects on the progression of kindling and
epileptogenesis in chronic experimental models of epilepsy, but are for
the most part weak anticonvulsants against generalized seizures or
fully kindled class V seizures epilepsy (Loscher and Honack
1991; McNamara 1988
; Yoshida et
al. 1997
).
These experiments provided evidence that increases in
NMDA-dependent synaptic transmission are distinct from the long-term effects of repeated NMDA receptor activation on structural and functional alterations in the circuitry of the dentate gyrus. Although
anticonvulsant potency of NMDA antagonists is relatively weak, NMDA
receptor antagonism may be potentially valuable as treatment for the
long-term effects of seizures because of the effects on the induction
of sprouting and other kindling-induced cellular processes that may
reduce seizure susceptibility (Sutula et al. 1996). For
this therapeutic purpose, development of NMDA antagonists with
acceptable toxicity profiles and weak anticonvulsant activity
(Loscher 1998
) may still be worthwhile to prevent the undesirable long-term consequences of repeated seizures, which include
increases in susceptibility to seizures and memory dysfunction. Brief
administration of drugs targeting the NMDA receptor or signal transduction pathways and genes activated by the NMDA receptor may be
useful to modify induction processes that lead to long-term cellular
alterations in hippocampal circuitry, epilepsy, and memory disturbances.
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ACKNOWLEDGMENTS |
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This research was supported by National Institutes of Neurological Disorders and Stroke NS-25020, and a Klingenstein Fellowship (to T. Sutula).
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FOOTNOTES |
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Address for reprint requests: T. Sutula, Dept. of Neurology, H6/570, University of Wisconsin, Madison, WI 53792.
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 16 March 1998; accepted in final form 9 October 1998.
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REFERENCES |
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