Department of Anatomy and Neurobiology, University of Tennessee, Memphis, Tennessee 38163
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ABSTRACT |
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Galarreta, Mario and Shaul Hestrin. Burst Firing Induces a Rebound of Synaptic Strength at Unitary Neocortical Synapses. J. Neurophysiol. 83: 621-624, 2000. High-frequency activity produces transient depression at many synapses but also, as recently demonstrated, may accelerate the recovery from use-dependent depression. We have examined the possible consequences of this synaptic mechanism in neocortical excitatory synapses by recording simultaneously from presynaptic pyramidal neurons and their postsynaptic targets. Brief bursts of high-frequency spikes produced a strong depression of the amplitude of unitary excitatory postsynaptic currents (uEPSCs). However, when burst firing was combined with low-frequency ongoing activity, we found that the strong synaptic depression was followed by a transient rebound of synaptic strength. This rebound overshot the low-frequency baseline values and lasted 1-2 s. These results suggest that in the presence of ongoing activity, neocortical synapses may functionally facilitate following burst firing.
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INTRODUCTION |
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At many central synapses, synaptic strength is
transiently reduced during high-frequency stimulation. This form of
short-term synaptic plasticity is generally attributed to presynaptic
mechanisms (Zucker 1989) and is thought to have
significant computational and functional implications (Abbott et
al. 1997
; Galarreta and Hestrin 1998
;
Tsodyks and Markram 1997
). Recently, it has been shown
that high-frequency firing can additionally accelerate the rate at
which synapses recover from depression (Dittman and Regehr 1998
; Stevens and Wesseling 1998
; Wang
and Kaczmarek 1998
). This process may operate as a
frequency-dependent "boosting" of the synaptic strength
(Dittman and Regehr 1998
) and thus may play a role in
how central synapses respond to complex patterns of stimulation.
We have studied the effects of high-frequency firing on the recovery
from synaptic depression at neocortical excitatory synapses. These
synapses exhibit a prominent frequency-dependent depression (Abbott et al. 1997; Castro-Alamancos and Connors
1997
; Galarreta and Hestrin 1998
;
Stratford et al. 1996
; Thomson et al.
1993
; Tsodyks and Markram 1997
) and are thought
to sustain in vivo an ongoing activity. We used dual recordings to show
that in the presence of ongoing activity bursts of high-frequency
spikes are followed by a transient increase of synaptic strength.
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METHODS |
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Cortical slice preparation and cell identification
Experimental procedures were similar to those previously
described (Galarreta and Hestrin 1998). Sagittal
cortical slices (30°, 300 µm thick) were obtained from 14- to
18-day-old Wistar rats. After dissection, slices were incubated at
32-34°C for 30 min and then at room temperature (20-22°C) until
transferred to a submersion-type recording chamber. The extracellular
solution bathing the slices contained (in mM) 125 NaCl, 2.5 KCl, 1.25 NaH2PO4, 1 MgSO4, 2 CaCl2, 26 NaHCO3, 20 glucose, and 0.4 ascorbic acid (pH
7.4, 315 mOsm) and was continuously bubbled with a gas mixture of 95%
O2-5% CO2.
Neurons in layer five of the somatosensory cortex were visualized using
infrared differential interference contrast video microscopy with an
upright microscope (Axioskop, Zeiss, equipped with a ×40 water
immersion lens). Pyramidal and fast-spiking cells, a subtype of
GABAergic interneuron, were identified according to their morphological
appearance and their pattern of firing in response to depolarizing
pulses of current (Kawaguchi 1993). Biocytin was
included in the recording pipettes and cell classification was
confirmed morphologically. Similar results were obtained when the
postsynaptic neuron was a pyramidal or a fast-spiking cell and data
have been pooled together.
Recording and data analysis
We recorded simultaneously from pairs of synaptically connected
neurons. Patch electrodes (3-4 M) were filled with a solution containing (in mM) 95 K-methylsulfate, 40 KCl, 10 HEPES, 4 MgATP, 20 phosphocreatine(Na), 0.3 NaGTP, 0.2 EGTA, and 0.3% biocytin (pH 7.3, 295 mOsm). Experiments were performed at 32-33°C. We recorded from
the presynaptic neuron under current-clamp mode while the postsynaptic
cell was kept under voltage-clamp mode. Presynaptic action potentials
were generated by injecting brief pulses (3-5 ms) of depolarizing
current at the appropriate frequency. Experiments were interrupted
whenever the access resistance to the postsynaptic neuron increased
significantly. Signals were recorded using an Axopatch 200A and an
Axopatch 200B amplifiers (Axon Instruments). The voltage and current
output were filtered at 1-10 kHz and digitized at 16-bit resolution
(National Instruments). The sampling frequency was 5 or 10 kHz.
The amplitude of individual unitary excitatory postsynaptic currents (uEPSCs) was measured as the difference between the peak of the uEPSC (minimum value in a 1-ms window at the peak) and the mean current at a 2-ms window before the onset of the synaptic current. In Fig. 2, baseline was obtained by averaging the amplitude of the last uEPSCs before the burst and the first response in the burst. Data are described as mean ± SE. Statistical analysis testing two-sample hypothesis was performed using unpaired, two-tailed Student's t test. Differences were considered significant when P < 0.05.
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RESULTS |
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Dynamics of local excitatory synapses in neocortical slices (layer
5) were studied by recording simultaneously from presynaptic pyramidal
neurons and their postsynaptic targets; either other pyramidal neurons
(n = 6) or fast-spiking cells (n = 10).
With this method we could precisely control the presynaptic spiking activity and avoid possible modulatory effects caused by the
stimulation of other axons. Individual presynaptic action potentials
were produced by injecting brief current pulses (3-5 ms, 0.8-1 nA), and the uEPSCs were recorded from the postsynaptic cell under voltage-clamp mode (Vh = 70 mV).
It has been reported that in vivo cortical neurons exhibit ongoing
firing rates at the range of 1-50 Hz (Hubel 1959;
Mountcastle et al. 1969
). We tested the effect on
synaptic strength of steady activation at a frequency of 1 Hz (Fig.
1). Baseline response was obtained as the
average amplitude when stimulating at 0.25 Hz. Increasing the
stimulation rate to 1 Hz produced a depression of the uEPSC amplitude
to 77.9 ± 2.7% of baseline value (Fig. 1, n = 4). This result together with previous observations (Galarreta and Hestrin 1998
; Thomson and West 1993
;
Tsodyks and Markram 1997
; Varela et al.
1997
) suggests that the ongoing activity in vivo produces some
degree of depression.
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Given that in vivo neocortical neurons fire bursts of high-frequency
spikes superimposed on an ongoing single-spike activity (Bair et
al. 1994; Cattaneo et al. 1981
; Hubel
1959
), we examined the response to burst firing when these two
patterns are combined. Ongoing low-frequency spiking activity was
generated by producing presynaptic spikes at a frequency of 1 Hz. In
addition, a burst of 10 spikes at 100 Hz replaced an individual action
potential every 15 s (Fig.
2A). High-frequency bursts
produced a strong depression of the uEPSCs (Fig. 2A) and the
amplitude of the last uEPSC in the burst was 19.3 ± 3.5% of the
amplitude of the first response (n = 12 pairs). In
spite of this strong depression, we found that uEPSCs recorded 1 s
after the burst were larger than those obtained at baseline stimulation
rate (Fig. 2, A and B; P < 0.01, t-test). The increase of synaptic strength was transient and
the uEPSCs returned to baseline values 3 s after the burst. Similar results were obtained in 10 of 12 pairs. The average rebound of
synaptic strength, measured 1 s after the burst, was 31.4 ± 7.8% over baseline (Fig. 2B, n = 12).
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It was important to test whether the rebound of synaptic strength observed after burst firing required an ongoing activity. To study this issue we compared, in the same pairs, the postsynaptic responses produced by identical bursts of high-frequency spikes with and without sustained spiking at 1 Hz (Fig. 3). In the absence of 1 Hz stimulation, bursts were not followed by a transient increase of synaptic strength (Fig. 3A). In five pairs, the average amplitude of an uEPSC obtained 1 s after the burst was 99.0 ± 7.8% of baseline value (Fig. 3A3). In contrast, brief bursts superimposed on an ongoing low-frequency activity (1 Hz) were followed by a transient increase of uEPSC amplitude (Fig. 3B). In response to this protocol, the amplitude of the test response obtained 1 s after the burst was 140.6 ± 24.2% of baseline (Fig. 3B3; n = 5 pairs, P < 0.01).
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DISCUSSION |
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We have shown that at local excitatory neocortical synapses, the strong synaptic depression produced by bursts of high-frequency spikes is followed by a transient enhancement of synaptic strength. This rebound of synaptic efficacy occurred when synapses sustained an ongoing low level of activity, suggesting that synapses displaying activity-dependent depression may functionally facilitate in response to burst activity.
Previous work has shown that bursts of action potentials can accelerate
the recovery from activity-dependent depression (Dittman and
Regehr 1998; Stevens and Wesseling 1998
;
Wang and Kaczmarek 1998
). In agreement with these data,
we found that the uEPSC obtained 1 s after a single action
potential was significantly depressed (Fig. 1B), whereas the
response obtained 1 s after a burst of high-frequency spikes was
not (Fig. 3A). It is possible that an accelerated recovery
from depression may underlie the rebound of synaptic strength following
burst firing. Thus, when synapses are activated every several seconds
and use-dependent depression is minimal, the response after a burst of
high-frequency spikes will not overshoot baseline values (Fig.
3A). In contrast, when background activity is higher and
synapses operate under a partial degree of depression, burst-induced
acceleration of recovery will transiently reduce the level of
baseline synaptic depression (Fig. 3B).
In vivo experiments have revealed that burst activity in
neocortical neurons is associated with sensory stimuli (Bair et
al. 1994; Cattaneo et al. 1981
;
Livingstone et al. 1996
), suggesting that brief periods
of very rapid firing might play a role in encoding information
(Lisman 1997
). Because the patterns of stimulation we
used are within the range of frequencies observed in vivo, we suggest
that this rebound of synaptic efficacy may contribute to the readout of
the information contained in complex patterns of spikes.
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ACKNOWLEDGMENTS |
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We thank M. Chang for excellent technical assistance.
This work was supported by National Eye Institute Grant EY-09120 to S. Hestrin.
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FOOTNOTES |
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Address for reprint requests: M. Galarreta, Dept. of Anatomy and Neurobiology, University of Tennessee, Memphis, 855 Monroe Ave., Memphis, TN 38163.
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.
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REFERENCES |
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