Department of Neurobiology, Duke University Medical Center, Durham, North Carolina 27710
![]() |
ABSTRACT |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Kauer, Julie A.
Blockade of hippocampal long-term potentiation by sustained tetanic
stimulation near the recording site. Specific patterns of
electrical stimulation trigger several forms of synaptic plasticity in
hippocampal pyramidal cells, including a long-term potentiation (LTP)
of excitatory synaptic transmission. I investigated the effect of
commonly used stimulation protocols at different distances from the
recording site. Sustained electrical stimulation (100 Hz, 1 s)
delivered close to the recording site prevented LTP induction; the same
stimulation from a second electrode placed farther away subsequently
produced LTP at the same recording site. Strong stimulation near the
recording site could also interfere with LTP triggered from a distal
site. In contrast to sustained high-frequency stimulation, intermittent
stimulation ( burst pattern) delivered close to the recording site
produced normal LTP. These data support the hypothesis that strong
stimulation releases a factor that acts locally to prevent
LTP.
![]() |
INTRODUCTION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Patterned stimulation of excitatory afferent
pathways in the hippocampus produces long-term synaptic plasticity
widely thought to underlie information storage. In general,
low-frequency stimulation (0.5-5 Hz) leads to long-term depression
(LTD), whereas high-frequency stimulation (20-200 Hz) elicits
long-term potentiation (LTP). However, several investigators reported
that high-frequency stimulation, when delivered repetitively or at high
stimulus strengths, prevents normal LTP induction (Abraham and
Huggett 1997; Bashir and Collingridge 1992
).
Such stimulation can also trigger heterosynaptic LTD at neighboring
excitatory synapses (Abraham and Goddard 1983
;
Christie et al. 1994
; Dunwiddie and Lynch
1978
; Scanziani et al. 1996
). Here I report that
one important factor governing the outcome of high-frequency electrical
stimulation is the proximity between the recording and stimulating
electrodes. Brief stimulation at 100 Hz produced little or no LTP near
the stimulation site but robust LTP at a distance from the stimulation
site. A less intense stimulus protocol subsequently triggered LTP at
synapses close to the stimulation site. These results suggest that
sustained tetanic stimulation renders nearby synapses incapable of
undergoing LTP, possibly via release of a local extracellular signal.
![]() |
METHODS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Hippocampal slice preparation
Slices were prepared from male Sprague-Dawley rats aged 17-28
days as previously described (McMahon and Kauer 1997).
With the use of a vibratome, coronal slices (400 µm) were cut from the middle third of the hippocampus into ice-cold artificial
cerebrospinal fluid (ACSF) (in mM: 119 NaCl, 26 NaHCO3, 2.5 KCl, 1.0 NaH2PO4, 2.5 CaCl2, 1.3 MgCl2, 11 D-glucose) saturated with 95%
O2-5% CO2. Slices were held after cutting for
1-6 h in an interface chamber at room temperature and then transferred
to a recording chamber where the slice was held submerged. The
recording solution included the
-aminobutyric acid-A
(GABAA) receptor antagonist picrotoxin (100 µM), and
Ca2+ and Mg2+ were each increased to 4 mM to
prevent epileptiform bursting. The bath temperature was maintained
between 28 and 30°C.
Extracellular stimulation and recording
Field potentials were recorded from area CA1 with glass
microelectrodes filled with 2 M NaCl. Bipolar stainless steel
electrodes were generally used to stimulate (Frederick Haer), but in
some experiments glass pipettes filled with ACSF and connected via silver wire were used (see RESULTS). Test stimuli were 100 µs in duration, delivered every 10 s. In most experiments,
alternating stimulation was delivered to each of two stimulating
electrodes, placed in stratum (s.) radiatum so as to activate
nonoverlapping groups of afferent fibers. The recording electrode was
placed in s. radiatum either "near" (200 µm) or "far"
(>500 µm) from the stimulating electrode.
Field potentials were recorded with an Axopatch amplifier and filtered at 2 kHz for storage on a PC. The initial slope of the field potential was measured with custom software written in the Axobasic programming environment and kindly donated by Dr. Daniel Madison. Results are reported as the means ± SE.
LTP inducing protocol
Sustained tetanic stimulation was delivered at 100 Hz for 1 s at 1.5 times the test intensity; this train was repeated twice. In
some experiments, a burst protocol was also used, consisting of a
burst of 4 pulses at 100 Hz, delivered 10 times 200 ms apart; 4 such
trains were delivered at 20-s intervals. The stimulus intensity was not
increased during
burst stimulation except where noted. In all
experiments, the presynaptic fiber volley was carefully monitored to
ensure no change after tetanus; in rare instances in which the fiber
volley changed, data were discarded.
![]() |
RESULTS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Stimulating electrodes were placed on either side of an
extracellular recording electrode to activate independent groups of Schaffer collaterals. One stimulating electrode was placed 200 µm
of the recording electrode (near), and a second stimulating electrode
was placed
500 µm away (far) (Fig.
1A). When a sustained tetanus
(100 Hz for 1 s, repeated twice) was delivered through the
stimulating electrode near the recording site, LTP was often attenuated
or entirely blocked after tetanus (Fig. 1, B and
C). However, an identical tetanus delivered 15 min later
through the far stimulating electrode produced robust LTP recorded at
the same recording site. On average, much less LTP could be generated at the near than at the far stimulation site (LTP at 10 min
posttetanus: after far tetanus, 166 ± 11%, n = 9; after near tetanus, 128 ± 5%, n = 27). The
posttetanic potentiation (PTP) was also smaller after stimulation
nearby than after stimulation farther away (PTP
1 min after tetanus:
after far tetanus, 212 ± 11%, n = 9; after near
tetanus, 152 ± 10%, n = 29). To control for
possible nonspecific effects of stainless steel electrodes, additional
experiments utilized glass stimulating electrodes. The type of
stimulating electrode made no difference to the result; stimulation
near the recording site always produced less LTP than stimulation at a distance (LTP 10 min after near tetanus, with a glass electrode, 126 ± 12%, n = 6).
|
To investigate further the relationship between LTP and distance, a
single stimulating electrode was placed near one recording electrode
and far from a second recording electrode (Fig.
2A). A sustained tetanus
caused very little LTP at the nearby recording site, whereas robust LTP
was simultaneously triggered at the distant recording site (Fig. 2,
B and C). Moreover, after the sustained tetanus
failed to elicit much potentiation at the near recording site,
subsequent less intense stimulation ( burst) effectively triggered
LTP. These results emphasize that neither the presynaptic afferents nor
the associated synapses are damaged by the strong stimulation, as the
relevant synapses can support LTP after weaker stimuli.
|
Stimulus intensity was routinely increased during the sustained tetani
but was not increased during that burst stimulation. To test directly
whether the increased current near the recording site alone can account
for the lack of LTP, the effects of each stimulus pattern were compared
when current intensities were matched (1.5 times test intensity).
Sustained tetanus produced 110 +4.3% potentiation 10 min after tetanus
(n = 5), whereas 10-15 min later burst stimulation
at 1.5 times test intensity elicited significantly more LTP (141 +4.9%
potentiation, 10 min after
burst). These results show that
increased current intensity alone does not prevent LTP at a nearby
recording site.
Sustained tetanic stimulation might release a diffusible factor that blocks LTP in the area local to the stimulating electrode (Fig. 3A). This idea was tested by delivering tetanic stimulation simultaneously via two stimulating electrodes, one near the recording site (to release the putative factor) and one farther away (to initiate LTP). LTP, usually triggered by the farther stimulating electrode, was blocked or attenuated by simultaneous sustained tetanus through the near stimulating electrode (Fig. 3, B and C). Fifteen minutes later, when a sustained tetanus was again delivered only through the far electrode, LTP was triggered. Thus sustained tetanic stimulation interferes with local LTP generation even at synapses not directly activated by the local stimulus.
|
A previous study showed that multiple burst stimulations in area
CA1 failed to elicit LTP because of activation of adenosine A1 receptors (Abraham and Huggett 1997
). An
A1 purinergic receptor antagonist, CPT (2-10 µM
8-cyclopentyltheophylline; RBI), was added in some experiments in which
tetanic stimulation was delivered
200 µm of the recording site. The
excitatory postsynaptic potential slope at 5 min posttetanus was
92 ± 7% of control values, indicating that CPT cannot reinstate
LTP in these experiments.
![]() |
DISCUSSION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
These experiments support the idea that sustained tetanic
stimulation in the hippocampal slice blocks LTP near the stimulating electrode. Tetanic stimulation near the recording site routinely produces less LTP than stimulation far from the recording site. LTP can
subsequently be induced with intermittent stimulation of the same
synaptic circuit. In addition, the results indicate that a signal may
spread heterosynaptically from the locally stimulated site to attenuate
LTP at neighboring synapses. Previous work suggested that strong
stimulation can block LTP in hippocampal slices (Abraham and
Huggett 1997; Bashir and Collingridge 1992
).
This study demonstrates that one important variable in these
experiments is the distance between the stimulating and recording
electrodes.
What causes this attenuation of LTP? One possibility is that LTP is
induced but masked by a simultaneous synaptic depression (Bashir
and Collingridge 1992). This seems unlikely because robust LTP
can subsequently be induced by
burst stimulation, as though the
relevant synapses have not already undergone significant LTP. Experiments in which both a near and a far stimulating electrode were
simultaneously tetanically stimulated suggest that LTP induction is
heterosynaptically blocked by local stimulation. Very strong local
depolarization, perhaps because of a large postsynaptic influx of
extracellular Ca2+ or to excessive direct depolarization by
the stimulating electrode, could be involved in blocking LTP. In a
possibly related finding, heterosynaptic LTD after tetanic stimulation
is increased by blocking GABAergic inhibition, which would increase
intracellular depolarization and enhance entry of postsynaptic
Ca2+ (Christie et al. 1994
). However, the
depression of PTP I observed after nearby strong stimulation suggests a
possible involvement of presynaptic terminals.
Anomalies in synaptic transmission were previously noted during
recordings close to the stimulus site in hippocampal slices (Dingledine et al. 1987); a stimulating electrode 200 µm from an intracellularly recorded pyramidal cell was less effective in driving the cell to threshold than a more distant stimulating electrode. This result is consistent with the idea that local stimulation effectively limits the excitability of local pyramidal cells.
I hypothesize that strong stimulation causes the local release of a neuromodulator that prevents LTP. Because the blockade of LTP is not maintained over hundreds of microns, the spread of the purported neuromodulator must be limited. It could be released nonspecifically to diffuse a short distance through the local extracellular space or alternatively could be released synaptically from nerve terminals that only travel short distances in the slice preparation. The hippocampus contains afferent fibers that could release myriad neuromodulators during the stimulation used in the current experiments. For example, the axons of local circuit interneurons are known to travel a few hundred microns through s. radiatum and can release a variety of peptides as well as GABA (although GABAA receptors are not likely to mediate the lack of LTP because they were blocked in these experiments).
In summary, a mechanism was identified that can locally suppress hippocampal LTP. Although it is not yet known under what physiological conditions this suppressive mechanism is activated, it is clearly seen after high-frequency firing of afferents such as that seen during epileptiform bursting. The results can be explained by the local release of a neuromodulator by strong stimulation. If such a neuromodulator were released in situ, it would be expected to attenuate LTP heterosynaptically near sites of its release.
![]() |
ACKNOWLEDGMENTS |
---|
I thank Drs. Lori McMahon and Susan Jones for helpful comments on the manuscript.
This project was supported by National Institute of Neurological Disorders and Stroke Grant NS-30500.
![]() |
FOOTNOTES |
---|
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 6 July 1998; accepted in final form 5 October 1998.
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|