Departments of 1Biology and 2Physics and Volen Center for Complex Systems, Brandeis University, Waltham, Massachusetts 02454
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ABSTRACT |
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Hempel, Chris M., Kenichi H. Hartman, X.-J. Wang, Gina G. Turrigiano, and Sacha B. Nelson. Multiple Forms of Short-Term Plasticity at Excitatory Synapses in Rat Medial Prefrontal Cortex. J. Neurophysiol. 83: 3031-3041, 2000. Short-term synaptic plasticity, in particular short-term depression and facilitation, strongly influences neuronal activity in cerebral cortical circuits. We investigated short-term plasticity at excitatory synapses onto layer V pyramidal cells in the rat medial prefrontal cortex, a region whose synaptic dynamic properties have not been systematically examined. Using intracellular and extracellular recordings of synaptic responses evoked by stimulation in layers II/III in vitro, we found that short-term depression and short-term facilitation are similar to those described previously in other regions of the cortex. In additition, synapses in the prefrontal cortex prominently express augmentation, a longer lasting form of short-term synaptic enhancement. This consists of a 40-60% enhancement of synaptic transmission which lasts seconds to minutes and which can be induced by stimulus trains of moderate duration and frequency. Synapses onto layer III neurons in the primary visual cortex express substantially less augmentation, indicating that this is a synapse-specific property. Intracellular recordings from connected pairs of layer V pyramidal cells in the prefrontal cortex suggest that augmentation is a property of individual synapses that does not require activation of multiple synaptic inputs or neuromodulatory fibers. We propose that synaptic augmentation could function to enhance the ability of a neuronal circuit to sustain persistent activity after a transient stimulus. This idea is explored using a computer simulation of a simplified recurrent cortical network.
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INTRODUCTION |
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Neurons in the mammalian cerebral cortex are
interconnected into networks by synapses whose strengths can change
rapidly as a function of recent activity. During repeated stimulation,
excitatory and inhibitory synapses in primary visual and somatosensory
cortices typically exhibit a mixture of facilitation and depression
(Buhl et al. 1997; Galarreta and Hestrin
1998a
; Markram and Tsodyks 1996
; Reyes et
al. 1998
; Tamas et al. 1998
;
Tarczy-Hornoch et al. 1998
; Thomson 1997
;
Thomson et al. 1993
, 1996
; Varela et al. 1997
). Recently we, along with others, have argued that
short-term plasticity at cortical synapses may strongly influence
network activity (Abbott et al. 1997
; Thomson and
Deuchars 1994
; Tsodyks and Markram
1997
). In particular, depression at excitatory synapses onto
pyramidal neurons and a shift in the relative balance between excitation and inhibition may tend to limit recurrent activity and make
cortical responses to sensory stimuli more transient (Chance et
al. 1998
; Galarreta and Hestrin
1998
; Thomson and Deuchars 1994
, Tsodyks and Markram 1997
;
Varela et al. 1999
).
Unlike the transient nature of the responses of many neurons in primary
visual and somatosensory cortices, neurons in many higher level
association areas in the temporal (Miyashita and Chang
1988), parietal (Gnadt and Andersen 1988
), and
frontal lobes (Funahashi et al. 1989
; Fuster
1973
; Schoenbaum and Eichenbaum 1995
) can
exhibit persistent activity that significantly outlasts the presence of
the initial stimulus. It is widely assumed that such activity reflects
reverberant activation of recurrent excitatory circuits (for review see
Amit 1995
). If this is the case, the expression of this
activity must depend critically on the steady-state balance between
recurrent excitation and inhibition, which in turn must depend on the
dynamic properties of synapses in these regions. Prior studies in
prefrontal cortex have focused primarily on long-term synaptic change
(Hirsch and Crepel 1990
; Nowicky and Bindman
1993
) but have also noted the existence of synaptic augmentation. Here we undertake a more systematic examination of
short-term plasticity at these synapses to begin to understand regional
differences in synaptic dynamics that may permit different forms of
activity to be expressed in different cortical regions. Understanding
synaptic dynamics in prefrontal cortex is of particular functional
interest considering the implication of this area in a wide variety of
integrative funcions including temporary memory, spatial orientation,
sequential organization of behaviors, sexual and social behaviors, and
behavioral flexibility in the rat (for review see Kolb
1990
) and working memory, attention, reasoning, and planning in
primates (for review see Grafman et al. 1995
, Uylings et al. 1990
).
Using a combination of field potential and single and dual whole cell recording, we find that depression and rapid facilitation of excitatory input to layer V neurons are similar to that previously observed in layer II/III of primary visual cortex and by others in layer V of somatosensory cortex. In addition, these experiments revealed prominent short-term synaptic enhancement lasting seconds to minutes. Paired recordings between layer V neurons revealed that the augmentation could be evoked at unitary connections within the prefrontal cortex and therefore did not require activation of afferents from other regions, release of extrinsic neuromodulators, or activation of a large network of neurons. These experiments suggest that brief periods of synaptic activity may be able to transiently shift a set of interconnected cortical neurons into a state in which recurrent excitation is sufficiently strong to support persistent activity. Incorporation of the observed synaptic dynamics into a computer simulation of a simplified recurrent cortical network supports this idea by showing that augmentation can help to sustain activity in response to a transient input.
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METHODS |
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Tissue preparation
Brain slices were obtained from Long Evans rats aged 15-19 days
postnatal (intracellular recordings) or 19-25 days (field-potential recordings). Rats of different ages were used because robust field potentials could be more reliably evoked in older animals whereas patch-clamp recordings were easier to obtain in slices from younger animals. Animals were deeply anesthetized with a 1:1 mixture of ketamine and acepromazine, decapitated, and their brains rapidly removed and placed in cold (<4°C) artificial cereberospinal fluid (ACSF). Coronal slices (400-500 µm thick) were cut on a vibratome at
the level of the frontal cortex or visual cortex and placed in a
room-temperature incubation chamber. All prefrontal cortex recordings
were made in medial frontal cortex, a region that has alternately been
referred to as anterior cingulate or prelimbic cortex. The anatomic
locations of these recordings were assessed with reference to
Paxinos and Watson (1986), using shape of the subcortical white matter as the primary landmark. The
anterior/posterior position of slices matched plates 9, 10, or 11 of
the atlas. Dorsal ventral position ranged from 1.5 to 3.5 mm ventral to
the dorsal pial surface. These coordinates placed the recording sites
primarily in areas designated Cg1 and
Cg3, with some sites near the border of
Cg2.
Electrophysiological recordings
Slices were transferred to a recording chamber and perfused at
1-2 ml/min with oxygenated ACSF (28-32°C). At least 30 min were
allowed for equilibration. During extracellular field-potential recordings slices were viewed at low magnification. Stimulation and
recording pipettes, both with tip diameters 30-50 µm, were filled
with ACSF and lowered 25-75 µm below the surface of the slice.
Stimulus isolation units delivered biphasic stimulus pulses (20-200
µA, 100 µs/phase with 100-µs interval). Field potentials were
filtered with a band-pass of 1-1000 Hz and amplified with a
differential amplifier (A & M Instruments). Visually guided whole
cell patch-clamp recordings were made using an Olympus BX50WI microscope equipped with infrared differential interference
contrast optics. Recording pipettes had a resistance of 3-5
Mohms when filled with recording solution. Voltage-clamp recordings
were made using an Axopatch 200B patch-clamp amplifier (Axon
Instruments), low-pass filtered at 2 kHz. Whole cell recordings were
rejected if they did not meet the following criteria: resting potential more negative than 50 mV and input resistance (measured at
70 mV
with
5 mV pulses) > 100 Mohm. Intracellularly recorded synaptic currents were evoked using smaller stimulating pipettes (~1-2 µm
diam) and correspondingly smaller stimulating currents (2-20 µA)
than those used to evoke synaptic field potentials. All intracellular and extracellular recordings in prefrontal cortex were made in layer V
(~600-800 µm from pial surface); extracellular recordings from
visual cortex were made in layer II/III (300-400 µm from pial
surface). Extracellular stimulation sites were in layer II/III (prefrontal cortex; 300-500 µm from pial surface) or layer IV (visual cortex; 400-600 µm from pial surface) and were always aligned with recording sites along the axis perpendicular to the pial surface.
Solutions
ACSF contained (in mM) 126 NaCl, 3 KCl, 1.25 NaH2PO4, 10 dextrose, 20 NaHCO3, 2 MgSO4, and 2.0 CaCl2. Osmolarity was 305-310 mOsm and pH 7.4 when equilibrated with 95% O2-5%
CO2. Whole cell patch pipettes were filled with a
solution with pH between 7.2 and 7.4 and containing (in mM) 130 potassium methylsulfonate, 10 KCl, 2 MgSO4, 10 HEPES, 0.1 or 0.5 EGTA, and 3 ATP (potassium salt). APV (+/) was
supplied by Research Biochemicals International.
Data analysis
All recordings were digitized at 5-10 kHz, stored, and analyzed
using IGOR software (Wavemetrics, Lake Oswego, OR). Synaptic efficacy
was determined from field postsynaptic potentials (fPSPs) using peak
amplitude measurements. Evoked field potentials consisted of two,
usually well-separated components (Fig. 2A): an early, biphasic component often overlapping with the stimulus artifact followed by a negative component of longer duration. Pharmacologic manipulations (Varela et al. 1997) identified the early
component as the field action potential (fAP) resulting from
synchronous action potentials in those neurons directly activated by
the stimulus, and the second component as the fPSP, in agreement with
Morris et al. (1999)
. The location of the fPSP peak was
found algorithmically within a user-specified window and response
amplitude was quantified as the mean amplitude of a 0.2-ms segment
centered on the peak location. The duration and location of the
user-specified window varied between recording sites but was confined
to a latency of <5 ms. In cases where later, presumably
polysynaptically activated synaptic potentials overlapped with the
initial peak, the user-specified window was placed before the point of
inflection separating the two components. Synaptic efficacy was
determined from intracellular postsynaptic currents (PSCs) using two
measurements: peak PSC amplitude and early PSC slope. Peak amplitude
was calculated as the mean amplitude of the evoked PSC waveform over a
1.0-ms window centered on the peak. Early slope was calculated using
linear regression applied to a 1-ms segment of the PSC waveform
starting 0.1-0.9 ms after onset. fPSP amplitude was monitored at a low frequency of 0.1 Hz to avoid short-term synaptic depression. For PSCs
whose response amplitudes were substantially more variable, a higher
monitoring frequency of 0.5 Hz was necessary to provide a better
estimate of synaptic efficacy.
Computer simulations
To explore the potential functional roles of short-term synaptic
enhancement, we simulated a network model of leaky integrate-and-fire neurons that are densely connected in an all-to-all fashion. These simulations were based on the model of persistent activity developed in
(Wang 1999a) with the addition of short-term synaptic
enhancement. Each model neuron obeys the following equation
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(1) |
Recurrent excitatory postsynaptic currents (EPSCs) consist of two
components, IAMPA and
INMDA. The AMPA receptor-mediated current IAMPA = gAMPAs(Vm Esyn), with
Esyn = 0 mV. To capture the rise and
decay kinetics of synaptic currents and their saturation, the gating
variable s (the fraction of open channels) is described by
two first-order kinetics
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(2) |
To implement short-term synaptic plasticity, the parameter
x is multiplied by DF, where
D and F describe depression and facilitation,
respectively. Short-term depression is assumed to be caused by
transmitter vesicle depletion at presynaptic terminals (Galarreta and Hestrin 1998b
; Stevens and Wang
1995
; Varela et al. 1997
). The depression factor
D is initially 1; it is reduced by a factor (1
pvF) for each spike and recovers
with time constant
D in the absence of
stimuli (Abbott et al. 1997
; Markram and Tsodyks
1996
). The parameter pv is the
release probability per vesicle in a simple model of short-term
depression by vesicle depletion (Wang 1999b
). We used
D = 300 ms and
pv = 0.60.
The facilitation factor F obeys the following dynamical
equation (Bertram et al. 1996; Wang
1999b
)
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In some simulations we blocked synaptic augmentation. In this case, the
parameter x is multiplied by the initial value
of F so that the synaptic conductance change
s(t) produced by a single spike remains the same.
In addition, the amount of short-term depression was preserved by
setting the initial release probability to 0.60.
Statistics
Sample means are reported ± SE throughout unless otherwise indicated. Statistically significant differences between population means are assessed using Student's t-tests with a confidence level of P < 0.05. Student's t-tests were two-tailed unless otherwise indicated.
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RESULTS |
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To characterize synaptic dynamics in prefrontal cortex we recorded intracellularly from visually identified layer V pyramidal cells. Stimuli delivered to layer III were adjusted in intensity to produce predominantly monosynaptic, excitatory synaptic currents between 30 and 200 pA in amplitude (mean 102 pA, n = 7; sample traces in Fig. 1A). A synaptic current was scored as monosynaptic if its onset latency (time to 5% of peak response) was constant at <4.0 ms (mean 2.6 ms) and the current rose to 95% of its peak value in <8.0 ms (mean 5.3 ms).
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To quantify the efficacy of synaptic transmission we measure peak PSC
amplitudes. To obtain a measure free from polysynaptic contamination,
we also measured early PSC slope (see METHODS); however,
these measurements were typically much noisier. Linear regression
analysis on pooled and normalized sets of recordings in which EPSCs
were varied over a range of ~50% by augmenting stimuli showed that
the two measures correlated well (R = 0.9; P = 2 × 1020). This
justifies the use of peak PSC amplitude as an index of monosynaptic transmission.
To establish the reversal potential of these monosynaptic responses, we
performed a set of experiments under conditions identical to those used
throughout the paper but with action potentials blocked by
intracellular QX-314 (10 mM). Because QX-314 had complicated effects on
synaptic transmission and plasticity it was not used routinely. These
experiments showed that the early portion of the response (<8 ms
relative to stimulus onset) reversed at 6.8 ± 4.9 (SE) mV
(n = 5; Fig. 1B). These results indicate
that the monosynaptic response reflects activation of primarily
excitatory glutamatergic synapses. In some recordings, later portions
of the response (>8 ms relative to stimulus onset) had more negative reversal potentials suggesting a contribution of a polysynaptically activated GABAergic, inhibitory current. This did not contaminate the
peak amplitude measurements which were made only on the early component
of the complex PSC.
Evoked EPSCs exhibited strong short-term depression during high-frequency stimulus trains (Fig. 1, B-D). Over the course of a 50-Hz train for example, EPSC amplitudes decreased to 40 ± 6% of their initial value (n = 7, Fig. 1D). In two of seven cells tested, however, short-term facilitation dominated the early part of the train, with EPSCs almost doubling in amplitude before dropping below baseline levels toward the end of the train. This between-cell variability in facilitation is reflected in Fig. 1D as an increase in the size of SE bars from pulse numbers 3-8 and as a deviation from the smooth, exponential decay profile expected from pure synaptic depression.
To further characterize short-term synaptic plasticity in the
prefrontal cortex we measured synaptic field potentials
(fPSPs). The long-term stability and high signal to noise ratio
of fPSP recordings make them a useful adjunct to intracellular
recording. In particular, they enable a systematic, parametric analysis
of regional synaptic properties. Therefore we used fPSP measurements to
determine the dependence of short-term facilitation and short-term depression on the frequency and duration of the conditioning tetanus. Amplitudes of fPSPs evoked by stimulation in layer III and recorded in
layer V were constant over extended periods of low frequency (0.1 Hz)
stimulation. As with intracellularly recorded EPSCs, however, higher
frequency (1-50 Hz) stimulation produced strongly depressing fPSPs
(Fig. 2, A and B).
This short-term depression was most pronounced at the end of a train
where field-potential amplitudes had reached a nearly steady state. To
quantify this process we calculated the "steady-state ratio," the
ratio of the last response to the first response. This quantity varied
as a function of the frequency of stimuli within the train (Fig.
2C). However, fPSP amplitude did not decay to steady-state
levels in a smooth fashion (Fig. 2B). Rather, as with EPSCs,
a second, facilitatory process appeared to exert an influence on fPSP
amplitude, most prominently at the beginning of the train. To quantify
the contribution of this process we calculated the "paired-pulse
ratio" as the amplitude of the second response divided by that of the
first. This index also varied as a function of the frequency of stimuli within the train (Fig. 2C), albeit with a profile different
from that of the steady-state ratio. These results are consistent with the presence of two competing short-term plasticity phenomena influencing synaptic transmission during a high-frequency train: short-term facilitation and short-term depression. Because their timecourses overlap, deconvolution of the two processes would require
either a pharmacologic separation or a more detailed quantitative analysis (Varela et al. 1997).
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The short-term changes in fPSP amplitude were accompanied by only
small changes in the amplitude of the presynaptic population action
potential (fAP), ruling out the possibility that these changes
could be attributed primarily to changes in presynaptic excitability.
During 50-Hz trains, fAP amplitude of the second and last responses
changed relative to the initial response on average by 7 ± 3%
and +2 ± 7% (mean ± SE, n = 4 slices). Similarly, during augmentation (see next paragraph)
fAP amplitude was changed by
5 ± 3% relative to baseline
(mean ± SE, n = 4). Thus changes in fPSP
amplitude reflected primarily a change in efficacy of synaptic transmission.
In addition to short-term facilitation and depression, occurring on a
timescale of tens to hundreds of milliseconds, we observed a
posttetanic enhancement of synaptic transmission occurring on the
timescale of seconds to tens of seconds (Fig.
3). Again, this was observed using both
intracellular and extracellular recordings. EPSCs evoked immediately
after a 50-Hz, 15-pulse tetanus were 55 ± 10% larger than those
sampled before the tetanus (Fig. 3C; n = 7).
The decay of this enhancement could be best fit with a sum of two
exponentials with fast and slow decay time constants of 7 and 71 s, respectively. Short-term enhancement on these time scales is
classically called augmentation and posttetanic potentiation for the
short and long phases, respectively (Zengel and Magleby 1982). However, because we had no further evidence of multiple processes we will refer to all phases of this enhancement collectively as "augmentation."
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In general, fPSPs corroborated the observations made with intracellular recordings (Fig. 3D). A 41 ± 10% (n = 5) augmentation was produced in layer V of the prefrontal cortex. Curiously, the apparent decay rate of augmentation depended on the assay, particularly the fast component of the decay. When measured using field-potential recordings, the timecourse of decay could be best fit with two exponentials with fast and slow decay constants of 19 and 74 s (fits performed on pooled data shown in Fig. 3D). Analogous fits to the whole cell data (Fig. 3C) yielded a similar slow decay constant of 71 s but a shorter fast decay constant of 7 s. This could reflect differences in the population of synapses stimulated, differences in the sampling frequency used (0.5 Hz for intracellular recordings vs. 0.1 Hz for the extracellular recordings), or differences in the age of animals used in the two recording configurations (see METHODS).
Augmentation was robust and reliably reproducible in layer V of the
prefrontal cortex. In contrast, EPSCs in visual cortical pyramidal
neurons exhibit little or no augmentation (Varela et al.
1997). We confirmed this result with a series of experiments carried out in visual cortex slices using the same recording
configuration used by Varela et al. (1997)
, namely fPSPs
elicited by extracellular stimulation in layer IV and recorded in layer
III. A 10 ± 1.5% increase in fPSP amplitude (n = 5) was produced by 50-Hz, 15-pulse tetanii compared with a 41 ± 10% increase (n = 5) under identical conditions in
prefrontal cortex. The difference between prefrontal and primary visual
cortices was significant (P = 0.0002). We did not
systematically test other pathways in either cortical area and
therefore cannot assess whether this difference in synaptic properties
reflects primarily differences within or between cortical areas.
We used field-potential recordings to quantify the dependence of augmentation in prefrontal cortex on the frequency and duration of the conditioning tetanus (Fig. 4). For trains of 15 stimuli, frequencies as low as 1-5 Hz produced augmentation, which appeared to saturate at frequencies of 50-100 Hz (Fig. 4, A and B). For trains at a fixed frequency of 20 Hz, as few as five stimuli produced augmentation, saturating with 15-30 stimuli (Fig. 4, C and D). These values are well within the physiological activity range of cortical neurons.
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We also used field-potential recordings to examine the role of the NMDA
receptor in the induction or expression of augmentation. Application of
the NMDA receptor antagonist APV (50 µM) did not prevent
augmentation; 15-stimulus, 50-Hz trains enhanced fPSPs by 56 ± 2% (n = 3; not shown). Thus NMDA-receptor activation
does not appear to be required for augmentation in the prefrontal
cortex in contrast to some forms of short-term enhancement that have been demonstated in the hippocampus (Malenka 1991) and
the somatosensory cortex (Castro-Alamancos and Connors
1996
).
The fPSPs and EPSCs described thus far were evoked by extracellular
stimulation. This does not permit the identification of activated
presynaptic neurons with respect to their precise location and cell
type. In addition, extracellular stimulation can excite neuromodulatory
fibers and/or generate polysynaptic network activity, both of which
could account for the induction of augmentation. To address these
problems and directly test if a train of presynaptic action potentials
in a single identified presynaptic neuron could produce synaptic
augmentation, we recorded intracellularly from synaptically connected
pairs of adjacent, visually identified layer V pyramidal neurons in the
prefrontal cortex (Fig. 5A). Approximately one of five tested pairs showed a one-way synaptic connection, which is a connection probability similar to that found for
these pairs in somatosensory cortex (Markram et al. 1997). Six pairs were held long enough to administer several
repetitions of an augmentation-inducing protocol. Pooled data from
these pairs revealed short-term depression (average of last 4 EPSCs was
33 ± 9% of initial EPSC amplitude; Fig. 5C) and
augmentation (first posttetanic pulse enhanced 56 ± 12% over
control; Fig. 5D). Although the variability in EPSC
amplitude was relatively large, both effects were statistically
significant. In the case of short-term depression the first EPSC in the
train was compared with the last five in an unpaired t-test
(P = 0.003). For augmentation, the last five pretetanic
EPSCs were compared with those of the first posttetanic EPSC in an
unpaired t-test (P = 0.002). Variability in
the paired-recording data precluded accurate measurement of the
augmentation decay timecourse.
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To explore the possible function of synaptic augmentation in the prefrontal cortex, we performed computer simulations of a simple network model for persistent activity. The model consists of 100 completely interconnected pyramidal neurons. In addition to recurrent input from other neurons in the network, each neuron receives random spontaneous input representing ongoing drive from extrinsic afferents. The details of the simulation are based on well-described properties of cortical networks such as the frequency-current relation of pyramidal neurons, time courses of the AMPA-receptor and NMDA-receptor mediated synaptic currents, recurrent excitation, and synaptic depression (see METHODS for details). We were interested in whether adding augmentation to such a network would endow it with the capability for persistent activity that outlasts a transient stimulus. The basic features of synaptic transmission within the model are illustrated in Fig. 6. A short current pulse delivered into a presynaptic cell produces a single presynaptic spike and an EPSP of 0.3 mV in a postsynaptic cell. A longer current pulse produces a train of action potentials (22 spikes at 50 Hz) in the presynaptic cell during which EPSPs first facilitate and then depress. A final short current pulse again produces a single action potential which elicits an enhanced EPSP (60% increase after a time interval of 3 s). Note that as a result of the slow decay of the residual presynaptic [Ca2+], the facilitation factor F is reduced only by a small amount between the tetanus and the second test stimulation.
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Figure 7 shows a simulation of the recurrently connected network in the presence of afferent input that causes the membrane potential of all neurons to fluctuate near their firing threshold. A transient increase of the rate of afferent input induced neuronal spike discharges. In the absence of short-term synaptic enhancement (Fig. 7A), the recurrent connections are weak and the activity dies out at the end of stimulation with the chosen set of model parameters. With short-term synaptic enhancement included in the model (Fig. 7B), recurrent synapses are temporarily strengthened during the neural response to the stimulus. As a result, network activity, sustained by sufficiently strong, recurrent synaptic excitation, now outlasts the transient input. The persistent network activity is terminated by a step decrease in the rate of afferent input.
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Because of short-term depression, the neuronal firing rate decreases
greatly during the transient stimulus (Fig. 7A). This effect
has been observed in recordings from prefrontal cortical neurons in
behaving animals (Funahashi et al. 1989; Romo et
al. 1999
). Moreover, persistent activity is sustained in the
network at relatively low firing rates (~50 Hz), which are within the observed range of rates for prefrontal cortical neurons in behaving animals (Funahashi et al. 1989
; Rainer et al.
1998
). This is worth noting because in a strongly recurrent
excitatory network, rate control is needed so that neurons are not
driven to saturation (firing at 100s of Hz) caused by the powerful
positive feedback. This is accomplished in the present model by
short-term depression of the recurrent synapses. If short-term
depression is not included in the simulation, model neurons fire at
>400 Hz in the persistent state (not shown). Although short-term
depression is important for controlling the rate of persistent activity
in the model, it is not used to terminate this activity. The actual
biological mechanisms by which persistent activity might be terminated
are not understood and are not addressed in these simulations.
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DISCUSSION |
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The results presented here demonstrate that excitatory synapses in
rat medial prefrontal cortex exhibit short-term depression and
short-term facilitation that are similar to those observed in primary
visual cortex (Varela et al. 1997). These experiments also revealed, however, that short bursts of action potentials at
moderate frequency produce synaptic augmentation of up to 50%, lasting
seconds to minutes. This property was observed using field-potential recordings, intracellular recordings with extracellular stimulation, and intracellular recordings from synaptically connected pairs of layer
V pyramidal cells, the latter indicating that augmentation is a
property of intrinsic synapses. The capacity for augmentation is
expressed differentially in the cortex, because it was largely absent
in at least one class of synapse in the visual cortex, namely
excitatory synapses onto layer II/III pyramidal cells.
Rapid depression and facilitation
During a 50-Hz, 15-stimulus train, synaptic currents in layer V
pyramidal neurons in prefrontal cortex are reduced to between 20 and
40% of their initial amplitude. Both the magnitude of this depression
and its frequency dependence were similar to those observed at synapses
onto layer III pyramidal cells in primary visual cortex using the same
techniques (see Fig. 1B in Abbott et al.
1997; see also Varela et al. 1997
, 1999
). The
fact that short-term synaptic depression has been observed in all
cortical areas that have been examined (Buhl et al.
1997
; Galarreta and Hestrin 1998a
;
Markram and Tsodyks 1996
; Reyes et al.
1998
; Tamas et al. 1998
; Tarczy-Hornoch
et al. 1998
; Thomson 1997
; Thomson et al.
1993
, 1996
; Varela et al. 1997
) suggests that it
is a general property of cortical circuitry. This may reflect a general
need in cortical circuits for synapse-specific gain control, which short-term depression can provide (Abbott et al. 1997
).
The ubiquity of short-term depression may however, be a particular
feature of the developing cortex and may give way to more heterogeneous dynamic properties of synapses in the adult (Angulo et al.
1999
; Reyes et al. 1998
; Y. Zhuo and S. B. Nelson, unpublished observations).
Synaptic responses in prefrontal cortex typically exhibited less
paired-pulse depression (i.e., between the first and second stimulus in
the train) than that expected on the basis of steady state responses
(Fig. 2, B and C). This may reflect simultaneous facilitation which can be readily demonstrated at cortical synapses under conditions of reduced transmitter release (Varela et al. 1997). Short-term depression and short-term facilitation are
known to be coexpressed in the same synaptic pathways at some central synapses (Stevens and Wang 1995
), however their relative
proportions can vary between different output synapses of the same
neuron (Markram et al. 1998
; Reyes et al.
1998
). We have not attempted to separate the influence of
facilitation and depression at synapses in prefrontal cortex, but the
fact that the two forms of plasticity can overlap temporally means that
paired pulse protocols may be of limited value for predicting steady
state synaptic responses.
Synaptic augmentation
Some of the earliest studies concerning synaptic plasticity, such
as those carried out on the vertebrate neuromuscular junction (Feng 1941) and spinal cord (Kuno 1964
),
were concerned with synaptic enhancement lasting seconds to minutes.
These forms of plasticity are now classified under the rubric
"short-term enhancement," defined as an activity dependent increase
in synaptic efficacy produced by transient increases in residual
presynaptic Ca2+ concentration (Fisher et
al. 1997
). Short-term enhancement has been separated into four
distinct temporal phases (Zengel and Magleby 1982
), F1,
F2, augmentation (AUG), and posttetanic potentiation (PTP), lasting
tens of milliseconds, hundreds of milliseconds, seconds, and tens of
seconds, respectively. What we describe here in prefrontal cortex
operates on the timescale of the latter two, AUG and PTP. For
simplicity and because we have no mechanistic evidence demonstrating
two separate processes in the prefrontal cortex, we refer to both
phases collectively as "augmentation."
A large body of evidence links augmentation to increased residual
presynaptic Ca2+ in model systems including the
crustacean neuromuscular junction (Wojtowicz and Atwood
1988), vertebrate neuromuscular junction (Katz and
Miledi 1968
), and hippocampal CA3 (Regehr et al.
1994
). Given this degree of phylogenetic conservation, it seems
reasonable to hypothesize that the cortical augmentation described here
would operate by the same mechanism. None of the data presented here are inconsistent with this hypothesis and two observations provide support. First, induction of augmentation at individual synapses demonstrates that it is a homosynaptic process not requiring widespread network activation or release of neuromodulators. Second, augmentation was observed under voltage-clamp conditions at
70 mV, ruling out
mechanisms requiring strong postsynaptic depolarization.
Despite the similarities in augmentation between the prefrontal cortex
and the model systems mentioned above, a number of differences exist.
Most striking is the magnitude of the augmentation and the required
amount of conditioning stimulation. Cortical augmentation saturated at
~40-60% enhancement after a moderate 15-stimulus, 50-Hz train (Fig.
4). By contrast, at the frog neuromuscular junction augmentation does
not reach a limit even with 30-Hz trains of 30-s durations
(Magleby and Zengel 1975). With such long trains, synaptic transmission can be increased up to fivefold, although this is
under conditions of reduced Ca2+ and elevated
Mg2+. At mossy fiber synapses in the hippocampus,
in the presence of physiological Ca2+
concentrations, extended conditioning trains (e.g., 3 s of 120 Hz)
produce two- to threefold enhancement of synaptic transmission, whereas
shorter trains produce less (Griffith 1990
;
Regehr et al. 1994
). In both of these cases the dynamic
range of synaptic enhancement and of the required conditioning stimulus
were substantially larger than that observed in cortex, suggesting
mechanistic differences. One possibility is that the expression of
augmentation is limited by competing depression in the cortex. In
sensorimotor cortex, an NMDA receptor dependent form of short-term
enhancement has been observed (Castro-Alamancos and Connors
1996
; Thomson et al. 1993
). This raises the
possibility of multiple mechanisms of short-term synaptic enhancement
coexisting in the cortex.
The finding that augmentation was differentially expressed at
prefrontal and visual cortical synapses suggests the possibility that
the dynamic properties of synapses are specialized along different
cortical pathways to allow expression of different patterns of
activity. The differences we observed may reflect primarily an
interareal difference or may reflect a difference between particular interlaminar pathways. We cannot answer this question here because our
field-potential comparison was restricted to only one pathway in each
area: the previously studied layer IV to II/III pathway in visual
cortex (Varela et al. 1997) and the layer II/III to V
pathway in prefrontal cortex. To assess whether these plasticity differences arise primarily within or between cortical areas it will be
necessary to systematically and quantitatively examine plasticity
properties in several pathways in each of the two cortical areas.
Possible functions of synaptic augmentation in the cerebral cortex
Although the functional role of augmentation has been difficult to
assess at any of the wide variety of synaptic sites at which it is
expressed, attempts have been made to link augmentation to various
functions ranging from simple enhancement of neuromuscular transmission
and temporal integration of neuronal signals to roles in learning and
memory (reviewed by Fisher et al. 1997; Magleby 1987
).
Here we present a specific hypothesis that may have particular relevance for cortical processing. We suggest that augmentation could act to enhance the ability of a neuronal circuit to sustain persistent activity evoked by a transient stimulus. In this scenario, augmentation would temporarily boost the level of recurrent excitation throughout a cortical network until it overcomes inhibitory transmission. This would lead to runaway excitation if not checked by short-term depression, which acts to limit the maximal total excitation in the network. We tested these ideas in a simple neuronal network model. Under the conditions tested, if the recurrent connections were not sufficiently strong, the model exhibited persistent activity in response to a transient input only when augmentation was present.
It has been generally assumed that persistent activity in the PFC is
maintained by reverberation in a strongly recurrent network. Previous
modeling studies have demonstrated that sufficiently strong recurrent
connections can lead to network bistability between a resting state and
a persistently active state (Amari 1977; Amit 1995
; Camperi and Wang 1998
; Wang
1999a
). The results of this study suggest that synaptic
augmentation may be a dynamic mechanism for temporarily boosting the
efficacy of recurrent synapses. Such a mechanism offers several
advantages compared with permanent synaptic enhancement (such as that
induced by long-term potentiation). First, it may be more metabolically
efficient to strengthen synapses temporarily during periods of
persistent activity and to conserve synaptic resources during periods
of baseline activity. Second, temporary enhancement may provide greater
flexibility and control. In this regard, it may be important that
synaptic augmentation is activity dependent and can vary in a graded
fashion depending on the level of neuronal firing. Finally, because the
short-term plasticity of synapses can be strongly affected by
manipulations that alter presynaptic release probability
(Tsodyks and Markram 1997
; Varela et al.
1997
), an additional level of flexibility and regulation of
persistent activity may be conferred by the action of neuromodulators
such as dopamine. Further work will be required to determine whether
dopamine and other neuromodulators have presynaptic actions at these
synapses, and if so how these actions affect the dynamic properties
studied here.
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ACKNOWLEDGMENTS |
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This work was supported by National Science Foundation Grants IBN 9511094 to S. B. Nelson and IBN-973306 to X.-J. Wang, the Sloan Foundation, National Institutes of Health Grants EY-11116 to S. B. Nelson and NS-36853 and K02 NS-01893C to G. G. Turrigiano, and by a postdoctoral fellowship from the Medical Research Council of Canada to C. Hempel.
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FOOTNOTES |
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Address for reprint requests: C. Hempel, Dept. of Biology MS 008, Brandeis University, Waltham, MA 02454.
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 25 October 1999; accepted in final form 1 February 2000.
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REFERENCES |
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