1Department of Physiology, Wakayama Medical College, Wakayama 641-0012, Japan; and 2The Rockefeller University, New York, New York 10021
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ABSTRACT |
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Kimura, Akihisa and Constantine Pavlides. Long-Term Potentiation/Depotentiation Are Accompanied by Complex Changes in Spontaneous Unit Activity in the Hippocampus. J. Neurophysiol. 84: 1894-1906, 2000. Typically, long-term potentiation (LTP) has been assessed as long-lasting changes in field potentials or intracellularly recorded postsynaptic potentials evoked by activation of a set of afferents. In the present experiment, we determined changes in spontaneous unit activity in the dentate gyrus (DG) following high-frequency (HFS) or low-frequency stimulation (LFS) of the medial perforant pathway. Experiments were performed in anesthetized rats. Field potentials and unit recordings were obtained alternatively from the same recording electrode. Of 39 single units isolated (from 25 independent sessions), the spontaneous discharges of 13 units (33%) increased, while 7 units (18%) decreased their discharges following HFS that induced significant LTP of the field potentials. Such opposing modulations of unit discharges following HFS were observed on simultaneously recorded units. LFS applied following HFS also induced bi-directional effects on unit discharges. Of 20 single units isolated from a subset of recordings (12 experiments) to which LFS was applied, 6 units increased and 4 units decreased their discharges. LFS produced a long-lasting (>20 min) depotentiation, to the baseline level, on field potentials in four recording cases. The autocorrelation functions indicated that the isolated unit discharges were comparable to those of the putative DG granule cells and interneurons, shown in previous studies. The results suggest that changes in synaptic efficacy following HFS or LFS produce rather dynamic changes in cell activity in the DG.
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INTRODUCTION |
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Changes in synaptic weights
within a neuronal network and the subsequent alteration of cell
activity have been hypothesized as a neuronal substrate for learning
and memory. In the hippocampus, discharge patterns of an individual
neuron or an ensemble of neurons encode sensory and/or contextual
information related to learning and memory (Deadwyler et al.
1996; Eichenbaum et al. 1987
; Sakurai 1994
, 1996
; Vidyasagar et al. 1991
;
Wilson and McNaughton 1993
; Young et al.
1994
). A number of previous studies have shown modulation of
hippocampal cell discharges in the course of learning and memory formation (Berger and Thompson 1978
; Deadwyler et
al. 1979
; Segal and Olds 1972
; West et
al. 1981
). The most putative cellular mechanism underlying
learning and memory is long-term potentiation (LTP) and depression
(LTD), which represent activity-dependent changes in synaptic efficacy
(Collingridge and Bliss 1995
; Linden and Connor
1995
). In the hippocampus, various forms of synaptic plasticity have been revealed in both the well studied "tri-synaptic" circuit and local synaptic connections involving both principal neurons and
interneurons (Bear and Abraham 1996
; Bliss and
Collingridge 1993
; Grunze et al. 1996
;
Maccaferri and McBain 1996
; Xie and Lewis
1995
). Plasticity in the hippocampus is expressed on both excitatory and inhibitory synaptic connections, and
subsequent cell interactions on polysynaptic connectivity are also
likely to be affected by the primary change of synaptic efficacy
(Buzsáki 1988
; Mott et al. 1993
;
Xie and Lewis 1995
; Yeckel and Berger 1998
). Such ubiquitous and diverse plasticity leads to a view that the modulation of hippocampal cell activity is very dynamic in the
presumed information processing related to learning and memory.
However, despite the great number of studies and wealth of information
concerning the mechanisms underlying synaptic plasticity, it is less
clear how LTP induction may affect individual cell firing in a highly
complex network, such as exists in the hippocampus.
The most common measure of neuronal plasticity has been assessed as
long-lasting changes in field potentials or intracellularly recorded
postsynaptic potentials evoked by activation of a set of afferents.
Thus far, very little attention has been focused on possible dynamic
modulations of cell discharges. It has been shown that LTP of a given
set of excitatory afferents in the hippocampus is accompanied with an
increase of cell discharge probability to a subsequent afferent
activation by electrical stimulation (Andersen et al.
1980; Buzsáki and Eidelberg 1982
).
The modulation of cell discharges driven by naturally incoming synaptic
inputs, as in actual information processing, has yet to be examined.
Activity-dependent modification of synaptic connectivity has been
implied, for example, in the reactivation of specific cell interactions
during sleep after waking experience (Pavlides and Winson
1988; Skaggs and McNaughton 1996
; Wilson
and McNaughton 1994
). Of particular interest is whether the
conditioning of a given set of inputs, which induces changes in
synaptic efficacy, would also bring about modifications in connectivity
and cell interactions that would produce specific patterns of cell
activities as may be seen in actual information processing.
It has previously been reported that tonic activation of the entorhinal
cortex induces a long-lasting increase of spontaneous discharges of
hippocampal cells (Deadwyler et al. 1976). It has been
postulated that the overall spontaneous discharge probability of a
given cell is determined by spatio-temporal interactions of synaptic
inputs converging on the cell. Therefore possible modifications of cell
interactions brought about by the induction of LTP could have
significant effects on spontaneous cell activity. Given that the
induction of LTP should produce plasticity of both mono- and
poly-synaptic connections, it would be predicted that a diverse
modulation of cell activity must take place. We recorded both field
potentials and spontaneous cell discharges in the dentate gyrus (DG)
and examined the effect of high-frequency stimulation (HFS) of the
perforant path. HFS induced LTP of field potentials evoked by the
perforant path stimulation and, in the majority of cases, a
long-lasting increase of spontaneous cell discharges, concomitantly. A
subgroup of cells, however, decreased their spontaneous firings despite
the induction of LTP of field potentials. In a subset of experiments,
low-frequency stimulation (LFS) was applied following LTP induction.
Similar to the findings with HFS, LFS induced either decrement or an
enhancement of spontaneous cell discharges.
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METHODS |
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Thirty-two adult albino rats (6 Sprague Dawley and 26 Wistar) of
both sexes weighing 125-340 g were anesthetized with Chloropent (15 mg/100 g body wt ip) and placed on a stereotaxic apparatus. Stainless
steel screws were placed on the skull and were used for stimulation and
recording reference and ground. A stimulating electrode
(tungsten-in-glass, 0.2-0.5 M impedance) was placed in the medial
perforant pathway (coordinates: AP:
+7.9 mm; ML: 4.0-5.0 mm). A
recording electrode (tungsten-in-glass or glass capillary filled with
4% biocytin in saline, impedance 1-3 M
) was aimed at the granular
cell/hilar area in the dentate gyrus (coordinates: AP:
+3.9 mm; ML:
2.3 mm). Electrode placement was aided by monitoring the depth profile
of evoked field potentials, and the final positions of the stimulating
and recording electrodes were adjusted to produce maximum field
potentials. The intensity of test stimulation (25-500 µA) was
adjusted to evoke field potentials ~30% of the maximum on the basis
of an input/output function. A test stimulus was delivered five times,
once every 10 s, to obtain an averaged field potential. Field
potentials were band-pass filtered (3 Hz to 3 kHz), digitized (10 kHz;
MIO-16X, National Instruments) and averaged (5) on-line using LabVIEW
(National Instruments) custom-built software. Both the excitatory
postsynaptic potential slope (measured in the initial positive rise)
and population spike amplitude of each averaged field potential were
measured. Recordings of field potentials evoked by the perforant path
stimulation were performed every 2-7 min, while spontaneous unit
discharges were continuously recorded through the same recording
electrode and stored on video tape, via a digital recorder (VR-100A,
Instrutech) for future off-line analysis. The unit signals were
filtered (300 Hz to 3 kHz), digitized (20 kHz), and stored on disk.
Using LabVIEW programs, single units (each consisting of
negative-positive spikes) were isolated from the digitized data on the
basis of the following four parameters: amplitude of negative peak,
time from onset to negative peak, amplitude difference between negative
peak and positive peak, and time from negative peak to positive peak
(Kimura et al. 1996
). To characterize an isolated single
unit, we assessed the autocorrelation function of unit discharges
during baseline recordings and the spike width, which was defined as
the average time from negative to positive peak.
After recording a stable baseline of field potentials and spontaneous
unit activity for ~10 min, high-frequency stimulation (HFS, 400 Hz,
10-50 pulses, 5 or 10 trains, 10 s apart) was delivered to the
perforant path at the same intensity as test stimulation. The effect of
HFS on spontaneous cell discharges was assessed by counting the number
of discharges in blocks of 10 s of a continuous recording
(duration, 90-180 s). The recording was then interrupted for the
recording of field potentials. In a subset of experiments, after
recording for 25 min following HFS, LFS (1 Hz for 5 or 10 min) was
applied, and the effect of LFS was assessed in the same way.
Statistical significance in the alterations of unit discharges and
field potentials was examined by a Mann-Whitney rank-sum test.
Statistical significance was set at P < 0.001 and P < 0.05, for unit activity and field potentials, respectively.
Some of the recording sites were verified histologically (8 animals) by
injecting biocytin iontophoretically (anodal 5-10 µA, 7 s
ON-OFF, 20-30 min) after the recordings (King et
al. 1989). Following completion of the injection, the electrode
was removed, and the wound was closed. The animals were administered
antibiotics and allowed to recover. After a survival period of 1-2
days, the animals were anesthetized deeply with an overdose of
pentobarbital sodium (100 mg/kg ip) and perfused transcardially with
saline followed by phosphate buffered 4% paraformaldehyde. After
postfixing overnight and storing at 4°C in 30% sucrose-phosphate
buffer solution for 3 days, the brains were cut at 60-µm coronal
sections. The sections were processed with biotin-avidin complex (ABC
kit, Vector Labs) to visualize the biocytin-labeled cells by
horseradish peroxidase histochemical reaction (Horikawa and
Armstrong 1988
). The sections were counterstained with neutral
red and examined under a light microscope. The injection sites were
located in the upper blade of stratum granulosum (Fig. 6) or the border
between the hilus and the upper blade of stratum granulosum. Some were
observed in the crest of stratum granulosum.
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RESULTS |
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Data analysis was carried out on 25 recording sessions. Only units adhering strictly to the isolation criteria were included in the final analysis. Signals of units adhering to the four parameters (defined in the preceding text) were monitored off-line throughout the blocks of continuous recordings (90-180 s). Units showing fluctuations in the four parameters were discarded as their recordings could be affected by electrode drift even though the field potentials were judged to be stable. The total number of single units obtained from 25 recording sessions was 39. Of these, simultaneous isolations of two to three units were made for analysis in 12 recording sessions. HFS was applied in all the recording sessions. LFS was also applied following HFS in 12 cases that involved 20 units. Furthermore a second HFS was applied in seven recording cases that involved 14 units (Table 1).
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Using our HFS parameters, significant potentiation of field potentials was evidenced more consistently in population spike amplitudes (mean, 191%, maximum, 572% of baseline levels) than in excitatory postsynaptic potential (EPSP) slopes (mean, 107%, maximum, 125%), as measured at 20 min after conditioning to the end of recordings or the next conditioning (maximum 70 min). HFS induced significant potentiation (P < 0.05) of both population spike amplitudes and EPSP slopes in 20 cases (Table 1). In three cases (recording sessions 5, 7, and 12), a significant potentiation of population spike amplitudes was observed, but not of the EPSP slopes. In the other two cases, HFS did not induce significant changes (recording sessions 10 and 20). In contrast to HFS, LFS produced a long-lasting (>20 min) depotentiation, to the baseline level (i.e., prior to HFS) in four cases (Fig. 7). A long-lasting decrease in field potentials (although not a complete depotentiation to baseline levels), was induced in six cases (Fig. 5). In the other two cases, no significant long-lasting changes were observed (Table 1).
Spike characteristics
Autocorrelation functions of spontaneous unit discharges were assessed in baseline recordings. They indicated that the isolated units were heterogeneous in discharge pattern as well as in spike width. A subgroup of units had relatively wide spike widths (>300 µs) and a clear propensity to fire in bursts with short interspike intervals, as indicated by a tall peak in the center of the autocorrelation function (Figs. 1, 3, 5, and 7, unit B). In contrast, units with relatively short spike widths (<300 µs) did not have a propensity to fire in bursts (Fig. 7, unit A; except 1 case, unit 2, in Table 1). The spike widths of units with a propensity to fire in bursts (n = 16) were significantly wider (P < 0.05, Mann-Whitney rank-sum test) than those of the other units (n = 23). There were two cases in which two different types of units, one with a long spike width and a propensity to fire in bursts and the other with a short spike width and no propensity to fire in bursts, were obtained simultaneously (Fig. 7). Further, we recorded from a unit that fired rhythmically as indicated by a clear oscillation (4-5 Hz) in the autocorrelation function (Fig. 2). Discharge rates in baseline recordings ranged from 0.03 to 5.99 Hz (Table 1). There was no significant difference in discharge rate between the units with a propensity to fire in bursts and the nonbursting units.
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Effects of HFS on spontaneous unit discharges
Of particular interest was the finding that the effects of HFS were bi-directional, i.e., either an increase or a decrease of discharges, following conditioning stimulation. In the two cases presented in Figs. 1 and 2 (recording sessions 6 and 1, Table 1), an increase of discharges of a single unit was induced immediately following HFS, which lasted until the end of the recording (~25 and 55 min, respectively). The two units isolated in these recordings had characteristic firing patterns. The example shown in Fig. 1 was a "typical" type of unit that had a relatively long spike width (381 µs, negative to positive peak) and fired mainly in bursts. A burst of discharges consisted of two to six spikes over a 5- to 30-ms duration (Fig. 1C). In contrast, the unit in Fig. 2 had a short spike width (287 µs) and fired rhythmically as indicated by a clear oscillation (4-5 Hz) in the autocorrelation function (Fig. 2C). In the other case (recording session 4, Table 1), an intense HFS (400 Hz, 50 pulses, 10 trains) induced a depression of spontaneous unit discharges (Fig. 3). The spike width of the unit was relatively wide (365 µs), and the unit had a propensity to fire in short bursts (Fig. 3C), which were mainly a train of two to three spikes. Interestingly, a transient stagnation of population spike amplitudes preceded a long-lasting potentiation, while the potentiation of the EPSP slope was transient in this case.
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Bi-directional changes in unit discharges were observed following HFS, on different units that were recorded simultaneously. In the units presented in Fig. 4 (recording session 11, Table 1), the discharges of three units (units A-C) were altered differently by HFS, which was applied twice. The first HFS (400 Hz, 50 pulses, 5 trains) increased the discharges of unit C, whereas it depressed the discharges of unit B. The second more intense HFS (400 Hz, 50 pulses, 10 trains) increased the discharges of unit A, which had not been affected by the first HFS, whereas it depressed the discharges of units B and C. All three units had wide spike widths (>350 µs). Unit A had a propensity to fire in bursts, but the other two did not have characteristic firing patterns. A cluster of granule cells labeled by biocytin indicated that the recording site was in the middle of stratum granulosum (Fig. 6A).
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Thus the effects of HFS on spontaneous unit discharges were diverse and bi-directional as summarized in Table 1. An increase in discharges was induced in 13 of the 39 units. A second more intense HFS was required to produce an increase in three units. A long-lasting increase of discharges (>30 min) was observed on 11 units, and a potentiation of field potentials accompanied all these cases, except one (unit 28, Table 1) in which no significant change of field potentials was induced. The other two units (units 27 and 31) showed a transient increase of discharges lasting <10 min; however, there was a long-lasting potentiation of field potentials. Despite potentiation of field potentials, seven units showed a decrease of discharges following HFS. In one of the units, a transient increase of discharges preceded a long-lasting depression (unit 34, Table 1). A further decrease of discharges was induced by the second more intense HFS in two of these units. In the cases in which HFS was applied twice, both increases and decreases in discharges were induced in five units. HFS did not induce significant changes in the other 14 units.
Effects of LFS on spontaneous unit discharges
LFS also produced bi-directional effects on unit discharges. The effects of LFS were observed on unit discharges that had been either altered, or had been unaffected, by the preceding HFS (Table 1). Figure 5 (recording session 17, Table 1) shows depotentiation of unit discharges by LFS (1 Hz, 5 min) that accompanied depotentiation of field potentials. In this case the change in field potentials was long-lasting but was not a complete depotentiation to the baseline level, while the spontaneous activity was depressed further below baseline levels. The unit had a propensity to fire in bursts (Fig. 5C) and was recorded from the upper blade of stratum granulosum as shown by a cluster of granule cells labeled by biocytin (Fig. 6B).
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Figure 7 depicts a case (recording session 14, Table 1) in which the opposing effects of LFS were observed on a pair of units (units A and B), which were of different types, in terms of spike width and firing patterns (Fig. 7C). HFS (400 Hz, 30 pulses, 5 trains) induced potentiation of field potentials and an increase of discharges of unit A, while the discharges of unit B were not affected. The enhanced discharges of unit A were further augmented by LFS (1 Hz, 10 min), which depotentiated field potentials to the baseline level. On the other hand, the discharges of unit B were depressed by LFS. The second more intense HFS (400 Hz, 50 pulses, 5 trains), applied after LFS, potentiated field potentials again, but it depressed the enhanced discharges of unit A. No significant effects were observed on the suppressed discharges of unit B. A noteworthy observation was that the potentiation of field potentials following the second HFS was preceded by a transient stagnation of field potentials as in the above-mentioned case (Fig. 3).
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Thus the effects of LFS on spontaneous unit discharges were also diverse and bi-directional as summarized in Table 1. Twenty units were obtained from the subset of recordings to which LFS was applied. LFS induced a further increase of discharges in three units, which had been augmented by the preceding HFS and an increase of discharges in three units that had been depressed by the preceding HFS. A decrease of discharges was observed following LFS in four units; two of them showed a decrease from the enhanced activity induced by the preceding HFS. The discharges of one unit had been depressed transiently by the preceding HFS and the other units had not been affected by the preceding HFS. No significant effect by LFS was observed on the other 10 units. Opposing effects on a pair of units isolated simultaneously were also observed in two recording sessions.
Unit discharges during conditioning
Unit discharges were recorded continuously during HFS. In the case
shown in Fig. 7, the discharges of unit A were activated at
~200 ms following each of the five trains of stimuli in the second
HFS (Fig. 8), although the first HFS did
not produce a significant effect on the unit discharges. The discharges
of unit B (white arrows in Fig. 8B) were not
affected by HFS. Such an activation of discharges during HFS was
observed in 11 units (Table 1). The activation took place with a time
lag [100-300 ms, except unit 31 (10 ms) and unit
21 (500 ms)] and an increase of discharge probability lasted for
200 to 1,500 ms (mostly <500 ms). A transient suppression of
discharges followed the activation in this case but subsequent
repetitive paroxysmal discharges or epileptic afterdischarges (Bragin et al. 1997; Somjen et al. 1985
)
were not observed. In six units, the HFS that activated unit
discharges, however, did not produce any significant effect on
spontaneous activity afterward. These results suggest that there is no
significant relation between the activation of unit discharges during
HFS and the alterations of spontaneous activity. Of significance was
the observation that only two of these units activated during HFS were
the type of units showing a propensity to fire in bursts. It, therefore
appeared that the type of units without a propensity to fire in bursts were more susceptible to activation during HFS.
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DISCUSSION |
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The results show that cell activity in the DG could be modulated in either direction, i.e., either potentiation or depression, by HFS of the perforant path. LFS, which was applied after HFS in the present study, was also potent in modulating cell activity in opposite directions. The most striking finding is that the direction of the long-lasting change in discharge probability of a given cell in the DG was not always in accordance with the direction of a change in field potentials; a potentiation of field potentials by HFS could be accompanied with a depression of discharges and a depotentiation of field potentials by LFS could occur coincidentally with a potentiation of discharges. Changes of population spike amplitudes represent overall changes of discharge probability of a group of units in response to synaptic activation of a set of afferents. In contrast, changes in spontaneous unit discharges are thought to represent changes of spatial and temporal interactions of synaptic inputs driving discharges of a given cell in the group, which were shown to not always be similar as those of field potentials. The results indicate that the conditioning of a given set of afferents is in fact potent in reorganizing spatio-temporal characteristics of the neuronal circuit that determines cell activity in the DG. It is, further, suggested that activity-dependent changes in synaptic efficacy modify the DG neuronal circuit to produce specific patterns of cell interactions and activities in actual information processing.
Effects of HFS on spontaneous unit discharges
The increase of cell discharge probability, following the
induction of LTP by HFS, most likely consists of two components: a
potentiation of EPSPs and a decrease of firing threshold to a given
synaptic input [an alteration of the EPSP/spike (E-S) relationship,
i.e., E-S potentiation] (Abraham et al. 1985, 1987
; Andersen et al. 1980
; Bliss and Lømo
1973
; Chavez-Noriega et al. 1990
). Accordingly,
a previous study has documented that tonic activation of the entorhinal
cortex produces a long-lasting increase of spontaneous discharges in
the DG, although this study did not confirm potentiation of synaptic
efficacy in field potentials (Deadwyler et al. 1976
). In
the present study, potentiation of field potentials was in fact
accompanied with an increase in spontaneous discharges in the majority
of cases.
The observation of a decrease in spontaneous discharges, however, was
inconsistent with the changes in homosynaptic LTP of excitatory inputs.
Other changes in mono- and poly-synaptic connections, especially those
involving the modification of inhibitory synaptic inputs, could be
assumed to take place. Various interneurons in the DG [which are
mainly inhibitory GABAergic neurons (Obenaus et al.
1993; Seress and Ribak 1983
; Sloviter and
Nilaver 1987
)] are recipients of perforant path synaptic
inputs (Deller et al. 1996
) and mediate feed-forward
inhibitory connections (Buzsáki and Eidelberg
1982
; Scharfman 1991
). The E-S potentiation as a component of homosynaptic LTP by HFS is thought to involve a
concomitant modulation of the feed-forward inhibitory inputs
(Tomasulo and Ramirez 1993
; Tomasulo and Steward
1996
; Tomasulo et al. 1991
; Wilson
1981
; Wilson et al. 1981
) along with possible
changes of firing properties of postsynaptic cells (Taube and
Schwartzkroin 1988a
,b
; Wathey et al. 1992
).
While in the DG a decrease of feed-forward inhibition has been shown
(Kanda et al. 1989
; Tomasulo and Ramirez 1993
), HFS of the perforant path induces homo- and
heterosynaptic potentiation of interneuron activity
(Buzsáki and Eidelberg 1982
; Tomasulo and
Steward 1996
) and, consequently, an increase in feed-forward inhibition on granule cells (Kairiss et al. 1987
;
Tomasulo and Ramirez 1993
; Xie and Lewis
1995
). A positive net change of synaptic weight in response to
an artificial activation of the perforant path by electrical
stimulation would augment the discharge probability of granule cells
and interneurons (Andersen et al. 1980
;
Buzsáki and Eidelberg 1982
). For the spontaneous
activity of a given cell, however, it is also possible that depending
on the route and timing of incoming synaptic inputs in the neuronal
circuit, potentiated inhibition would lead to a reduction of the
overall discharge probability that could not be assessed in the
discharges elicited by an activation of a specific group of synaptic
inputs by electrical stimulation. Of interest is the observation that
in the cases in which a depression of discharges was seen, a transient
stagnation of field potentials, especially in the population spike
amplitude, occurred following relatively intense HFS (Figs. 3 and 7).
According to Tomasulo and Ramirez (1993)
, high-intensity
conditioning of the perforant path is liable to potentiate feed-forward
inhibition. Therefore a speculative hypothesis is that the transient
stagnation of population spike amplitude, which was originally reported
by Bliss and Lømo (1973)
, is suggestive of a
significant recruitment of feed-forward inhibition by strong
conditioning and the consequently augmented inhibition might depress
tonically the activity of a given cell. On the other hand, tetanic
stimulation of the perforant path depresses feed-back inhibition
(Maru 1989
) that is mediated by various interneurons
receiving mossy fiber inputs (Acsády et al. 1998
)
and controls the excitability of DG cells (Halasy and Somogyi
1993
; Han et al. 1993
; Sik et al.
1997
) along with feed-back excitation (Scharfman 1995
,
1996
). The modulation of inhibitory synaptic weights in
feed-forward and -back circuits is thus postulated to be pivotal for
the bi-directional alterations of discharge probability. In 11 units,
trains of stimuli during HFS provoked a transient activation of
discharges (Fig. 8), suggesting some recruitment of excitatory driving
force in these circuits. The detailed mechanism of this activation is
unclear at this moment because there was no consistent relation between
the activation and the change of spontaneous activity. It is indicated,
however, that HFS, including that which was subthreshold to drive
neurons, (like the first HFS in the case shown in Fig. 7), could have
significant effects on the neuronal circuits.
Finally, a reduction of spontaneous discharges could follow HFS due to
an induction of heterosynaptic LTD (Abraham and Goddard 1983; Abraham et al. 1985
; Christie et
al. 1995
; Krug et al. 1985
; White et al.
1988
, 1990
; Zhang and Levy 1993
) or
heterosynaptic depotentiation (Doyère et al. 1997
;
Levy and Steward 1979
), if the main driving force of
spontaneous discharges of a given cell is provided by an unconditioned
excitatory synaptic input. Then, homo- and heterosynaptic plasticity
produced by associative interactions of conditioned and unconditioned
afferents (Tomasulo et al. 1993
; White et al.
1990
) might bring about diverse modulations of overall discharge probability, which could be further complicated due to the
modulation of the lateral inhibition arising from association pathways
(Sloviter and Brisman 1995
). Using the present
techniques, it would not be possible to determine the source of the
changes in neuronal excitability. It is noteworthy to state, however, that the heterogeneity in the modifications of synaptic connectivity was observed even in adjacent neurons isolated with the same electrode (Figs. 4 and 7). Furthermore the overall discharge probability of each
cell was consistently stabilized at a new static level. This implies
that the neuronal circuit in the DG consists of the spatio-temporal
structure that is modifiable but also consistently stabilized, in terms
of each cell activity, which might be a fundamental property of the
neuronal circuit to preserve information in specific patterns of cell activities.
Effects of LFS on spontaneous unit discharges
It has been controversial whether LFS of the perforant
path induces homosynaptic LTD or depotentiation in the DG (Bear
and Abraham 1996), despite being a relatively reliable
paradigm in the CA1 field (Barrionuevo et al. 1980
;
Dudek and Bear 1993
; Errington et al.
1995
; Fujii et al. 1991
; Mulkey and
Malenka 1992
; Staubli and Lynch 1990
). There
have been several in vitro studies documenting homosynaptic LTD
induction by LFS in the DG (O'Mara et al. 1995
; Wang et al. 1997
); however, in vivo, it is thought that
LFS is not capable of inducing LTD or depotentiation (Abraham et
al. 1996
; Errington et al. 1995
) except under
certain conditions when afterdischarges follow LFS (Abraham et
al. 1996
; Bramham and Srebro 1987
) or LFS is
applied within a short time (<2 min) after the HFS (Martin
1998
). Nonetheless homosynaptic LTD or depotentiation itself
can be produced in the DG in vivo by HFS applied on the negative phase
of theta rhythm (Pavlides et al. 1988
) or in conjunction with the activation of adrenal steroid receptors (Pavlides et al. 1995
). It is therefore pertinent to consider tentatively
that the DG contains at least some neuronal properties that diminish or
reset synaptic efficacy of excitatory inputs (Rick and Milgram 1996
). In the present study, LFS (1 Hz) induced a
depotentiation of field potentials to the baseline level lasting
20
min, in 4 of 12 cases where afterdischarges were not observed following LFS. Since this observation was limited in time and number of cases, we
cannot conclude that the depotentiation observed in the present study
was truly a long-term effect of LFS. However, it should be noted that
spontaneous cell discharges were also affected by LFS as long as some
effect was exerted on the field potentials. This observation
substantiates the potential effect of LFS on synaptic efficacy in the DG.
As has been suggested for the mechanisms underlying E-S potentiation,
the concomitant modulation of synaptic efficacy in inhibitory connections is considered to play a pivotal role in molding the effect
of LFS as well (Wagner and Alger 1996). Homosynaptic LTD and depotentiation by LFS in the CA1 are accompanied by E-S
potentiation (Bernard and Wheal 1995
), and it was
reported in a study examining habituation of responses to repeated
stimuli that LFS (1 Hz) of the perforant path also resulted in E-S
potentiation (Abraham and Bliss 1985
). The neuronal
mechanisms of depotentiation are also considered to involve the
GABAergic system that is modulated by the prior conditioning
(Wagner and Alger 1995
). It is, therefore possible that
in the spontaneous cell activity the concomitant modulation of
excitatory and inhibitory synaptic weights by LFS results in the
bi-directional alterations of overall discharge probability of a given
cell, depending on the route and timing of incoming inputs.
Identity of isolated units
A question that arises concerns the identity of neurons in which
spontaneous activity was modulated in the present study. Previous
extracellular recording studies categorized units in the DG into those
of putative granule cells and inhibitory interneurons. A number of
studies reported that the putative granule cells have electrophysiological characteristics similar to those of theta cells
that fire rhythmically at relatively high rates and do not exhibit
bursts of discharges (Buzsáki et al. 1983;
Foster et al. 1987
; Rose et al. 1983
).
Other studies, however, suggest that the putative granule cells exhibit
both rhythmic as well as burst firing (Bland et al.
1980
; Suzuki and Smith 1985
) or categorized the
putative granule cells and interneurons as nontheta and theta cells,
respectively (Fox and Ranck 1975
). Recently Jung
and McNaughton (1993)
defined the putative granule cells as
those that fire in bursts at extremely low rates, while the putative
inhibitory interneurons as those that fire rhythmically at high rates.
They further suggested that the putative granule cells and inhibitory
interneurons have relatively wide and short spike widths, respectively,
although the difference was not statistically significant. Their
definitions were derived from the criteria of Mizumori et al.
(1989)
based on electrophysiological characteristics such as
the response latency to the perforant path stimulation and the effect
of paired pulse inhibition. However, according to Mizumori et
al. (1989)
, the majority of cells recorded in stratum
granulosum do not have characteristic firing patterns under
pentobarbital sodium anesthesia.
In the present study, we obtained a subgroup of units that fired mostly
in bursts with short interspike intervals and had a relatively wide
spike width (Table 1). In line with the classification by Jung
and McNaughton (1993), those units with an especially low
firing rate are likely to be granule cells although the overall discharge rates we observed for this subgroup were relatively higher
and were not distinct from the discharge rates of the other units. An
example of such a unit, with a propensity to fire in bursts and with a
relatively long spike width, was recorded in the middle of the granule
cell layer (Fig. 6). A single unit exhibiting a clear theta oscillation
in firing (Fig. 2) had a short spike width (280 µs) and high firing
rates (~6 Hz during baseline and 10 Hz following HFS), which are
characteristics similar to those of the putative interneurons. Other
units, in the present study, with relatively short spike widths (<300
µs) did not exhibit burst discharges (Table 1), suggesting that they
may constitute another major subgroup. Other units, however, lacked
characteristic features and could not be categorized. Taken together,
the present result suggest that our neuronal population consisted of
both granule cells and interneurons.
Functional considerations
Because the synaptic and biophysical bases of spontaneous cell
activity are not well understood, spontaneous cell activity is usually
regarded as noise, unrelated to information processing. Yet it has been
shown in the sensory system that spontaneous cell activity signifies
the functional connectivity and its reorganization for information
processing (deCharms and Merzenich 1996; Dinse et
al. 1993
; Johnson and Alloway 1996
). Another
view is that spontaneous cell activity itself is functionally
significant ongoing network dynamics that has a major influence on
sensory processing in its specific interactions with the activity
evoked by sensory inputs (Arieli et al. 1995
,
1996
). In this sense, the modifiability of the neuronal circuit
indicated by the alterations of spontaneous cell activities could be
fundamental also for cell activities that encode actual information.
However, because of the unpredictable diversity in the results, the
activity-dependent modifiability of cell discharges shown in the
present study does not appear feasible as a neuronal basis for the
functionally rational reorganization of cell activities in actual
information processing. As mentioned in the preceding text, the diverse
results could be ascribed to the ambiguity inherent to the experimental
paradigm using an artificial activation of synaptic inputs that must
have consequently involved many factors affecting cell activities in a
highly complex neuronal circuit. In actual information processing, it
is postulated that subtle activation of specific synaptic inputs in a
spatial frame (Moser 1996
) and probably in a specific
time sequence of activation (Christie and Abraham 1992
;
Otani and Connor 1995
; Thiels et al. 1994
,
1996
) implements a functionally rational reorganization of
synaptic connectivity and a specific alteration of cell activity on the
basis of the modifiability shown in the present study.
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ACKNOWLEDGMENTS |
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The authors thank Professors H. Asanuma, B. S. McEwen, and Y. Tamai for continued support.
This work was supported by a Grant-in-Aid for Scientific Research (A) from the Ministry of Education of Japan and a grant for medical research of Wakayama prefecture to A. Kimura and by a Whitehall Foundation grant to C. Pavlides.
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FOOTNOTES |
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Address for reprint requests: A. Kimura, Dept. of Physiology, Wakayama Medical College, Kimiidera 811-1, Wakayama 641-0012, Japan (E-mail: akimura{at}wakayama-med.ac.jp).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 26 August 1999; accepted in final form 10 July 2000.
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REFERENCES |
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