 |
 Previous Article | Next Article 
The Journal of Neuroscience, January 1, 2001, 21(1):27-34
Involvement of the Secretory Pathway for AMPA Receptors in
NMDA-Induced Potentiation in Hippocampus
Greg
Broutman and
Michel
Baudry
Neuroscience Program, University of Southern California, Los
Angeles, California 90089-2520
 |
ABSTRACT |
A chemical form of synaptic potentiation was produced with a brief
bath application of NMDA to rat hippocampal slices. Two methods were
used to assess changes in membrane-bound AMPA receptors. Traditional
subcellular fractionation was used to isolate synaptic membranes;
alternatively, membrane receptors were cross-linked with the
membrane-impermeable reagent bis(sulfosuccinimidyl) suberate, and levels of nonmembrane receptors were determined. In both cases, Western blots were used to determine the content of receptor subunits in various subcellular fractions. NMDA-induced potentiation was associated with increased levels of glutamate receptor 1 (GluR1) and GluR2/3 subunits of AMPA receptors in synaptic membrane
preparations, whereas no change was observed in whole homogenates. Both
KN-62, an inhibitor of calcium/calmodulin kinase, and calpain inhibitor III, a calpain inhibitor, inhibited NMDA-induced potentiation and
changes in GluR1 and GluR2/3 subunits of AMPA receptors. Brefeldin A
(BFA) inhibits protein trafficking between the Golgi apparatus and cell
membranes. Pretreatment of hippocampal slices with BFA significantly
decreased NMDA-induced potentiation and completely prevented an
NMDA-induced increase in GluR1 levels in membrane fractions. Thus, the
levels of GluR1 and GluR2/3 subunits of AMPA receptors are rapidly
upregulated in synaptic membranes under conditions associated with
potentiation of synaptic responses, and this upregulation requires a
functional secretory pathway.
Key words:
AMPA receptors; brefeldin A; exocytosis; long-term
potentiation; Golgi; hippocampal slice
| |
INTRODUCTION |
Since its discovery nearly 30 years
ago (Bliss and Lomo, 1973 ), long-term potentiation (LTP) has been
heralded as a cellular mechanism for memory formation. The detailed
cellular and molecular mechanisms that underlie this phenomenon remain
a hotly debated issue. Our laboratory has proposed that LTP could occur
because of modifications of AMPA receptors (Bi et al., 1998).
This hypothesis was first based on the existence of a calcium-dependent
upregulation of glutamate binding in synaptic membranes (Lynch and
Baudry, 1984). It was later found that the binding and
electrophysiological properties of one subtype of glutamate receptors,
the AMPA receptors, were indeed modified as a result of LTP (Staubli et
al., 1992; Maren et al., 1993; Kolta et al., 1998). Lynch and
colleagues argued for an increase in the kinetics of the
receptor-associated ion channel (Ambros-Ingerson and Lynch, 1993;
Ambros-Ingerson et al., 1993), whereas Berger and colleagues proposed a
redistribution of existing receptors within the postsynaptic density
(Xie et al., 1997). Alternatively, translocation of receptors from an intracellular pool to synaptic membranes could result in increased functional AMPA receptors (Standley et al., 1996).
Other lines of investigation have led to the notion of silent synapses,
synapses with functional NMDA receptors but lacking functional
AMPA receptors (Isaac et al., 1995, 1996, 1999; Liao et al., 1995).
After LTP induction, such synapses would become activated because of
the "unmasking" of functional AMPA receptors. Both the distribution
of AMPA receptors and the morphology of synaptic contacts are regulated
by synaptic activity (Engert and Bonhoeffer, 1999; Maletic-Savatic et
al., 1999; Toni et al., 1999), and a rapid cycling of AMPA receptors in
and out of synaptic membranes has been documented (Song et al., 1998;
Carroll et al., 1999; Luscher et al., 1999; Noel et al., 1999; Shi et
al., 1999).
Several methods have been used to produce widespread LTP or long-term
depression (LTD) by chemical instead of electrical stimulation. A brief application of a high glycine concentration produced a long-lasting potentiation that shared mechanisms similar to those observed for tetanus-induced potentiation (Shahi et al., 1993; Musleh et al., 1997). A mixture of NMDA, glycine, and spermine elicited
an LTP-like increase in synaptic transmission (Thibault et al., 1989),
whereas a brief NMDA application induced LTD in CA1 (Lee et al., 1998).
In the present study, we used a brief application of NMDA in
hippocampal slices to produce a long-lasting increase in synaptic
transmission and to determine changes in AMPA receptor subunits in
various subcellular fractions and possible mechanisms underlying such
changes. In particular, we tested the effects of the drug brefeldin A
(BFA) on NMDA-induced potentiation and changes in synaptic AMPA
receptors. BFA is a fungal metabolite that causes the Golgi membrane to
fuse with the endoplasmic reticulum (ER), thereby inhibiting exocytosis
of newly synthesized proteins (Misumi et al., 1986; Oda et al., 1987;
Klausner et al., 1992). Our results indicate that AMPA receptors are
inserted from an intracellular pool into synaptic membranes under
conditions associated with LTP-like increased synaptic transmission and
that a functional secretory pathway is required for this process.
| |
MATERIALS AND METHODS |
Electrophysiology in hippocampal slices. Transverse
hippocampal slices (400 µm thick) were prepared from adult (3- to
4-month-old) Sprague Dawley rats using a McIlwain tissue chopper.
Slices were immediately placed in ice-cold cutting buffer containing
(in mM): 124 NaCl, 3 KCl, 1.25 KH2PO4, 1 CaCl2, 3 MgCl2, 26 NaHCO3, 10 glucose, and 2 L-ascorbate, and saturated with
95% O2-5% CO2. Slices
then had the CA3 region removed with a razor blade before being
transferred to an interface chamber. Slices were maintained at 32°C
and constantly perfused with an artificial CSF (ACSF) containing (in
mM): 124 NaCl, 3 KCl, 1.25 KH2PO4, 3 CaCl2, 1 MgCl2, 26 NaHCO3, 10 glucose, and 2 L-ascorbate; in addition, they
were constantly oxygenated with a 95% O2-5%
CO2 mixture at a rate of 1 ml/min. Extracellular
field recordings were obtained in CA1 using a bipolar stimulating
electrode and a glass recording electrode (containing 2 M NaCl) placed in stratum radiatum. Stimulus
intensity was set to approximately one-third of the intensity required
to evoke a population spike, and responses were evoked every 30 sec (0.033 Hz, pulse duration of 0.1 msec). All drugs were perfused in ACSF.
NMDA treatment of hippocampal slices. Hippocampal slices
were prepared and placed in an interface chamber as described above (electrophysiology). After a 1 hr equilibration period, slices were
treated with 50 µM NMDA for 5 min. Generally,
three slices were treated per group so as to obtain enough tissue for
Western blots. Slices were collected 15 min and, where indicated, 1 hr after the end of the NMDA treatment and placed in ice-cold sucrose solution containing 0.32 M sucrose, 10 mM EDTA, and 10 µM
leupeptin (homogenization solution). Slices were then stored at
 70°C for later use (i.e., preparation of membranes and Western blots).
Membrane preparation. Crude synaptic membranes were prepared
from hippocampal slices by homogenizing them by sonication in the
homogenization solution. Aliquots of the homogenates were processed for
Western blots. The rest of the homogenate was then centrifuged
at 24,000 × g at 4°C for 20 min. The supernatant was discarded, and the pellet was resuspended in distilled water containing 100 µM EGTA. Samples were then centrifuged
again as described above, and the supernatant was again discarded. The
pellet was resuspended in Tris-acetate buffer (100 mM, pH 7.4) containing 100 µM EGTA and centrifuged as described above.
This last centrifugation step was repeated, and the final pellet was
resuspended in ice-cold Tris-acetate buffer and immediately used for
Western blots.
Cross-linking of membrane proteins. Control or NMDA-treated
slices were collected and treated with the cross-linker agent bis(sulfosuccinimidyl) suberate (BS3) as
described previously by Hall et al. (1997). Briefly, slices were first
washed twice in saline solution (SS) containing (in mM): 137 NaCl, 5.3 KCl, 170 Na2HPO4, 220 KH2PO4, 10 HEPES, 33 glucose, and 44 sucrose, pH 7.3. One-half of the slices were kept in
SS, and the other half were placed in SS containing 1 mg/ml BS3 [a concentration sufficient to
saturate all of the cross-linking sites and to leave intracellular
proteins intact (Hall et al., 1997)] and incubated for 30 min at
37°C with agitation. After the incubation, slices were washed three
times in harvest buffer [a modified SS additionally containing (in
mM): 1 EDTA, 1 PMSF, and 50 ethanolamine].
Slices were then saved in harvest buffer and frozen at 70°C until
they were used for Western blots.
Western blots. Samples were thawed on ice, and slices used
in cross-linking studies were homogenized by sonication. Protein assays
were performed using the Bio-Rad protein assay (Bio-Rad, Hercules,
CA) to determine protein concentration. Equal volumes of 2×
sample buffer (2% SDS, 50 mM Tris-HCl, pH 6.8, 10% 2-mercaptoethanol, 10% glycerol, and 0.1% bromophenol blue) were
added to samples of either whole homogenates or synaptic membranes, and
samples were boiled for 10 min. Aliquots containing equal amounts of
proteins were run on SDS-polyacrylamide gels containing 8%
polyacrylamide, and proteins were transferred onto nitrocellulose
membranes. The membranes were incubated in Tris-buffered saline (TBS)
containing 3% gelatin for 1 hr at room temperature before an overnight
incubation with primary antibodies in TBS containing 1% gelatin and
0.05% Tween 20. Glutamate receptor 1 (GluR1) and GluR2/3 antibodies were obtained from Chemicon (Temecula, CA) (1:2000 dilution). Immunostaining was detected by incubating with an alkaline
phosphatase-conjugated secondary antibody (Bio-Rad) also in TBS
containing 1% gelatin and 0.05% Tween 20 for 2 hr. Quantification of
blots was done using ImageQuant software (Molecular Dynamics,
Sunnyvale, CA).
| |
RESULTS |
NMDA application induces a long-lasting increase in synaptic
transmission in CA1
Perfusion of hippocampal slices with 50 µM NMDA for
5 min induced repeated bursts of high-frequency activity throughout the slices within 2-3 min after the end of perfusion. The CA3 region was
therefore removed to minimize this NMDA-induced epileptiform activity.
NMDA treatment also caused a rapid loss of EPSPs evoked in CA1 stratum
radiatum by stimulation of the Schaffer collateral pathway. This loss
of EPSP is attributed to NMDA-induced depolarization of CA1 pyramidal
neurons and thus to a loss of driving force, an effect also seen with a
similar protocol used to induce LTD in slices prepared from juvenile
animals (Lee et al., 1998). After NMDA washout, EPSPs not only
recovered but exhibited a large increase in amplitude compared with
baseline values (Fig. 1). By 1 hr after NMDA treatment, EPSP amplitudes had increased by ~86 ± 12%
(n = 8) above baseline values. This level of
potentiation remained constant for at least 3 hr after NMDA treatment,
and this phenomenon is hereafter referred to as NMDA-LTP.
View larger version (18K):
[in this window]
[in a new window]
|
Figure 1.
NMDA-induced potentiation in rat hippocampal
slices. Hippocampal slices were prepared as described in Materials and
Methods. Stimulating and recording electrodes were positioned in CA1.
After a 10 min baseline recording, either 50 µM NMDA was
perfused for 5 min (bar; A) or three
high-frequency stimulation bursts (arrows) were
given (B), separated by 5 min. Twenty minutes
after the last high-frequency stimulation burst, 50 µM
NMDA was perfused for 5 min. Results represent the amplitudes of evoked
responses and are expressed as percentage of responses averaged over
the 10 min baseline period. They are means ± SEM of eight and
seven experiments, respectively. Insets are traces
(averages of 5 successive responses) from representative experiments at
the indicated times. Calibration: 1 mV, 10 msec.
|
|
Several experiments were performed to compare NMDA-LTP with traditional
tetanus-induced LTP. We first determined whether calpain was activated
in NMDA-LTP as in the case of tetanus-induced LTP (del Cerro et al.,
1990; Denny et al., 1990; Vanderklish et al., 1995). Slices treated
with NMDA were processed for immunoblotting with spectrin antibodies,
and the levels of the 150 kDa breakdown product generated by
calpain-mediated degradation of spectrin were quantified. NMDA
treatment resulted in a significant (41 ± 6%; n = 8; p < 0.001; Student's t test) increase
in this breakdown product, an effect that was blocked by preincubating
slices with calpain inhibitor III (CalI III), a membrane-permeable
inhibitor of calpain (5 ± 8%) (data not shown). We also tested
the effects of CalI III on NMDA-LTP. Slices were preincubated with CalI
III before NMDA application. Treatment with CalI III caused a slight but significant increase in synaptic responses (Fig.
2A). Moreover, CalI III
completely blocked NMDA-LTP (5 ± 15 vs 146 ± 15%;
n = 6; p < 0.001; Student's
t test) (Fig. 2A).
View larger version (23K):
[in this window]
[in a new window]
|
Figure 2.
Effects of inhibitors of both calpain and CamKII
on NMDA-LTP. Hippocampal slices were preincubated with either 10 µM CalI III (A) or 2 µM KN-62 (B; dotted line)
before application of 50 µM NMDA (solid
bar). Results represent the amplitudes of evoked responses and
are expressed as percentage of the average baseline responses recorded
over 10 min before NMDA application. They are means ± SEM of six
and seven experiments, respectively.
|
|
Several studies have indicated that calcium/calmodulin kinase type II
(CamKII) plays a critical role in LTP (Malinow et al., 1989; Pettit et
al., 1994; Lledo et al., 1995; Blitzer et al., 1998). Therefore, we
determined the effects of KN-62, a CamKII inhibitor, on NMDA-LTP (Fig.
2B). KN-62 treatment had little effect on basal
synaptic responses; however, after 1 hr, NMDA-induced potentiation was
completely inhibited when compared with control slices (5 ± 14 vs 192 ± 35%, respectively; n = 7;
p < 0.001; Student's t test).
Finally, we also determined whether NMDA-LTP could be occluded by
tetanus-induced LTP, a test that has been widely used to assess whether
chemically induced LTP (or LTD) shares mechanisms similar to those
observed for tetanus-induced LTP (or LTD). Tetanus-induced LTP
was elicited by three episodes of high-frequency stimulation resulting
in saturation of LTP (140 ± 20%; n = 7). Under
these conditions, NMDA application did not produce any further increase in synaptic responses (Fig. 1B).
Changes in AMPA receptor distribution after
NMDA-induced potentiation
Potentiation was induced by NMDA treatment of hippocampal slices,
and slices were collected 15 min after the end of NMDA treatment. Slices were then used to prepare crude synaptic membranes or were treated with BS3 to obtain an estimate of
the levels of nonplasma membrane AMPA receptors.
Synaptic membranes
Slices were homogenized, and aliquots of the homogenates were used
to prepare a crude synaptic membrane fraction. Western blots from
homogenates and crude synaptic fractions were processed with antibodies
against GluR1 or GluR2/3 subunits of AMPA receptors. NMDA treatment
resulted in a significant increase in the abundance of both GluR1
(+39 ± 7%; n = 15; p < 0.01;
Student's t test) and GluR2/3 (+24 ± 6%;
n = 6; p < 0.01; Student's
t test) subunits in synaptic membrane fractions (Fig.
3A,B). A similar increase in
GluR1 subunits was also present in slices collected 1 hr after NMDA
treatment (data not shown). However, there was no significant increase
in GluR1 levels in homogenates of NMDA-treated slices at either time
point (+6 ± 11%; n = 5).
View larger version (69K):
[in this window]
[in a new window]
|
Figure 3.
Changes in synaptic AMPA but not NMDA receptor
subunits with NMDA-LTP. Hippocampal slices were treated with 50 µM NMDA for 5 min and were collected 15 min after the end
of NMDA treatment. Synaptic membranes were prepared, and aliquots of
whole homogenates and membranes were processed for immunoblots with
antibodies recognizing either the GluR1 and GluR2/3 subunits of AMPA
receptors (A, B, respectively) or the NR1
subunits of the NMDA receptor (C). Top
panels, Representative Western blots. Bottom
panels, Quantification of Western blots similar to those shown
in top panels. Results represent optical density, are
expressed as percentage of values measured in samples from control
slices, and are means ± SEM of 15 (GluR1), 7 (GluR2/3), and 4 (NR1) experiments. *p < 0.01; Student's
t test.
|
|
As a control, we tested whether other synaptic proteins were
upregulated by NMDA treatment. We determined the levels of NR1 subunits
of NMDA receptors in the same fraction used to measure the levels of
GluR1 subunits of AMPA receptors. NMDA treatment did not modify the
levels of NR1 subunits in synaptic membranes when compared with levels
found in membranes prepared from control slices (+2 ± 5%;
n = 4) (Fig. 3C).
Cross-linking
To confirm the results obtained with crude synaptic
membrane fractions, we evaluated the levels of intracellular AMPA
receptor subunits after NMDA treatment. The membrane-impermeable
cross-linking reagent BS3 was used to
effectively remove all surface receptors, as described previously by
Hall et al. (1997). After BS3 treatment,
slices were homogenized, and the number of non-cross-linked receptors
(presumably representing intracellular receptors) was assessed with
Western blots. As a control, whole homogenates of slices were incubated
with BS3, and these samples showed little
immunoreactivity when probed with GluR1 antibodies in Western blots
(data not shown). This is presumably because most proteins in the
homogenate are exposed to BS3 and thus
cross-linked, thereby forming large aggregates of proteins that do not
penetrate polyacrylamide gels. Treatment of control slices with
BS3 resulted in a 47 ± 13% decrease
in the levels of GluR1 compared with levels found in slices not treated
with BS3 (Fig. 4A). This
suggests that ~53% of the total number of GluR1 subunits are located
intracellularly under control conditions. After NMDA treatment,
incubation with BS3 resulted in a 44 ± 11% decrease in the levels of GluR1 subunits when compared with
values found in control slices (Fig. 4B), indicating that
there were significantly less intracellular receptors after NMDA
treatment. When compared with whole homogenates from control slices,
levels of GluR1 in NMDA-treated slices incubated with BS3 were decreased by ~66%, indicating
that ~66% of GluR1 subunits were membrane bound. Because levels of
membrane-bound GluR1 subunits in control slices were ~47% of total,
this suggests that the number of membrane-bound receptor subunits
increased by ~40%, a value in good agreement with that obtained with
the subcellular fractionation approach.
View larger version (38K):
[in this window]
[in a new window]
|
Figure 4.
Changes in nonplasma membrane AMPA receptors with
NMDA-LTP. Hippocampal slices were prepared as described in Materials
and Methods. They were then treated with or without
BS3 to determine the levels of intracellular AMPA
receptor subunits (GluR1) with Western blots of aliquots from whole
homogenates (WH). Top panels,
Representative blots. Bottom panels, Quantitative
analysis of blots similar to those shown in top panels.
A, Effects of BS3 in control slices.
Homogenates were prepared from slices incubated in the absence
(WH) or presence
(BS3) of BS3 and
processed for immunoblots with GluR1 antibodies. B,
Effects of BS3 in slices treated with
(NMDA) or without (Control) 50 µM NMDA for 5 min. Results represent percentage of
values measured in whole homogenate (A) or
control (B) and are means ± SEM of four
experiments. *p < 0.05; Student's
t test.
|
|
NMDA-induced changes in AMPA receptor distribution are calpain-
and CamKII-dependent
Because we observed that NMDA-LTP was blocked by inhibitors of
calpain and CamKII, we determined whether these inhibitors could
prevent NMDA-induced changes in AMPA receptor subunit distribution. Preincubation of slices with the calpain inhibitor CalI III completely blocked an NMDA-induced increase in the levels of GluR1 and GluR2/3 subunits (Fig. 5A). Preincubation of slices with the CamKII
inhibitor KN-62 also completely inhibited an NMDA-induced increase in
the levels of both GluR1 and GluR2/3 subunits of AMPA receptors in synaptic membranes (Fig. 5B). In Western blots using the
GluR1 antibody, KN-62 alone caused a small yet significant decrease in
immunoreactivity. However, this effect of KN-62 was not significant in
blots using the GluR2/3 antibody, although the same trend was seen.
View larger version (69K):
[in this window]
[in a new window]
|
Figure 5.
Effects of calpain and CamKII inhibitors on
NMDA-induced changes in AMPA receptor subunits. Hippocampal slices were
preincubated with either 10 µM CalI III for 25 min
(A) or 2 µM KN-62 for 30 min
(B) before application of 50 µM
NMDA for 5 min and were collected 15 min after the end of NMDA
treatment. Synaptic membranes were prepared, and aliquots of whole
homogenates and membranes were processed for immunoblots with
antibodies recognizing either the GluR1 or GluR2/3 subunits of AMPA
receptors. Top panels, Representative Western blots.
Bottom panels, Quantification of Western blots similar
to those shown in top panels. Results represent optical
density, are expressed as percentage of values measured in samples from
control slices, and are means ± SEM of four (GluR1) and four
(GluR2/3) experiments for CalI III and six (GluR1) and four (GluR2/3)
experiments for KN-62. *p < 0.03; Student's
t test.
|
|
Brefeldin A inhibits NMDA-induced potentiation and changes in AMPA
receptor subunits in membrane fractions
Hippocampal slices were pretreated for 25 min with BFA before
being subjected to NMDA treatment. BFA treatment had no effect on basal
synaptic responses, but after 1 hr, NMDA-induced potentiation was
almost completely inhibited when compared with control slices [+18 ± 12 (n = 8) vs +86 ± 12%
(n = 7), respectively; p < 0.02; Student's t test] (Fig. 6A). BFA
treatment also completely inhibited an NMDA-induced increase in the
levels of GluR1 subunits of AMPA receptors in synaptic membranes
(2 ± 3 vs +37 ± 9%, respectively; n = 4;
p < 0.02; Student's t test) (Fig.
6B).
View larger version (38K):
[in this window]
[in a new window]
|
Figure 6.
Effects of brefeldin A on NMDA-induced LTP and
changes in GluR1 subunits. A, Hippocampal slices were
prepared, and stimulating and recording electrodes were positioned in
CA1. After a 10 min baseline period, BFA (10 µg/ml) was added to the
perfusion medium (dotted line). Twenty-five minutes
later, NMDA (50 µM) was added for 5 min (solid
bar). Amplitudes of EPSPs were measured, and values are
expressed as percentage of the average values measured during the 10 min preceding NMDA application. Data are means ± SEM of seven
experiments. B, Hippocampal slices were incubated in the
absence or presence of BFA. They were then treated with or without NMDA
(50 µM, 5 min). Fifteen minutes later, slices were
collected, and synaptic membranes were prepared as described in
Materials and Methods. Aliquots of membrane samples were processed for
immunoblots with antibodies against GluR1 subunits of AMPA receptors.
Top panel, Representative Western blots. Bottom
panel, Quantification of Western blots similar to those shown
in top panel. Results represent optical density of the
GluR1-immunoreactive band and are expressed as percentage of values
measured in control conditions (no BFA, no NMDA). Data are means ± SEM of four experiments. *p < 0.02; Student's
t test.
|
|
| |
DISCUSSION |
Because only a small fraction of synapses are affected by the
electrical stimulation protocols used to induce plasticity, chemical-pharmacological stimulation protocols have been developed to
study biochemical changes associated with synaptic plasticity. Thus,
several laboratories have used brief activation of NMDA receptors to
produce widespread changes in synaptic efficacy in CA1 from hippocampal
slices (Kauer et al., 1988; Thibault et al., 1989; Shahi et al., 1993;
Lee et al., 1998). In our study, the application of 50 µM NMDA for 5 min to hippocampal slices produced a
long-lasting increase (>3 hr) in synaptic efficacy. In addition, NMDA-induced increased synaptic transmission was associated with calpain activation as evidenced by increased formation of a
calpain-mediated spectrin breakdown product and inhibition by a calpain
inhibitor. It was also associated with the requirement for CamKII
because it was blocked with an inhibitor of CamKII. Finally, NMDA-LTP was occluded by previous LTP saturation by tetanus-induced LTP. Thus,
NMDA-LTP shares many features of tetanus-induced LTP, and it is
reasonable to conclude that these two forms of potentiation share
similar biochemical mechanisms. However, we cannot completely exclude
the possibility that several consequences of NMDA application are not
related to what happens in tetanus-induced LTP.
NMDA-LTP was associated with a rapid (<15 min) and long-lasting (>1
hr) increase in the levels of synaptic AMPA receptor subunits. The
decrease in synaptic responses observed at 15 min after NMDA treatment
is likely because of the prolonged depolarization elicited by NMDA
application that would mask any potentiation that could be present at
this early time point. Because there was no change in receptor subunits
in the homogenate, it is clear that the increase in synaptic AMPA
receptors is independent of protein synthesis. Two potential mechanisms
could account for such an upregulation of synaptic receptors: (1) a
redistribution of existing membrane receptors to synaptic sites or (2)
a translocation of intracellular receptors into synaptic membranes. Our
results support the latter rather than the former mechanism. First, the
observation of a decrease in the levels of GluR1 subunits in
NMDA-treated slices versus control slices incubated with the
cross-linking reagent BS3 is best
explained by a decrease in the number of intracellular subunits.
Second, BFA, which inhibits anterograde protein secretion from the
Golgi, inhibited both NMDA-LTP and an NMDA-induced increase in synaptic
AMPA receptor subunits. Therefore, NMDA-LTP is associated with
upregulation of synaptic AMPA receptors, and this upregulation is
likely because of the translocation of receptors from intracellular, Golgi-associated sites to synaptic sites.
View larger version (25K):
[in this window]
[in a new window]
|
Figure 7.
Postulated mechanisms for the involvement of the
secretory pathway for AMPA receptors in LTP. A, A pool
of subsynaptic receptors cycles in and out of the postsynaptic
densities. After an LTP-inducing stimulus, a regulated exocytic pathway
transports newly synthesized receptors from the Golgi apparatus to this
pool. The resulting increase in the number of subsynaptic receptors
would lead to an increase in the number of receptors to be inserted
into the membrane. B, AMPA receptors cycle into and out
of the synaptic membrane. This cycling of receptors involves a
constitutively active as well as a regulated exocytic pathway, both of
which involve Golgi-associated receptors. Under these conditions, the
constitutive pathway would be involved in normal homeostatic regulation
of the synaptic AMPA receptor population, whereas the regulated pathway
would be involved in adding to the existing synaptic population only
after LTP-inducing stimuli.
|
|
Several data have indicated the existence of an intracellular pool of
AMPA receptors (Baude et al., 1994, 1995; Henley, 1995; Standley et
al., 1998; Rubio and Wenthold, 1999), although relatively little is
known about these receptors, especially with regard to their function
and location in dendritic and synaptic compartments. Based on
biochemical studies, we proposed previously that these intracellular
receptors represent unglycosylated, immature states of the receptors
(Standley et al., 1998). The data obtained with BS3 also clearly support the existence of
a relatively large pool of intracellular receptors, as shown previously
in dissociated neurons (Hall et al., 1997). Recent evidence has
suggested that AMPA receptors can rapidly cycle in and out of synaptic
membranes and that this cycling is regulated by synaptic activity and
could possibly be involved in synaptic plasticity (Luscher et al.,
2000; Malinow et al., 2000; Turrigiano, 2000). In particular, infusion of botulinum toxin to prevent exocytosis resulted in a 30% decrease in
AMPA receptor-mediated synaptic responses (Luscher et al., 1999).
Because the same authors had previously observed a different result
with intracellular infusion of botulinum toxin (Lledo et al., 1998),
they proposed that this drug might have a different effect on regulated
and constitutive exocytosis. Two recent studies have provided strong
evidence for the involvement of a clathrin-dependent endocytotic
process in LTD in hippocampus and cerebellum (Man et al., 2000; Wang
and Linden, 2000). Our results provide some clues regarding the links
between the secretory pathway and exocytosis and the involvement of
these processes in LTP. The results obtained with BFA indicate that
receptors originating in the ER-Golgi apparatus are critically
involved in LTP but not in baseline responses. Two possible scenarios
could account for these results (Fig. 7). In the first one, a
subsynaptic pool of receptors could be located beneath synaptic
membranes (Fig. 7A). Receptors from this pool could be
rapidly inserted into the membrane, whereas membrane receptors could be
endocytosed as a result of normal synaptic activity. The size of the
membrane pool of receptors would be dependent on the size of the
subsynaptic pool, as well as on the rates of receptor insertion and
internalization. LTP would activate a regulated exocytic pathway, and
receptors from the ER-Golgi complex would shuttle into the pool of
cycling receptors. This would effectively increase the number of
receptors inserted into the synapse. This hypothesis implies the
existence of two distinct intracellular receptor populations, with one
population associated with the Golgi and one (a subsynaptic population)
that is independent of the Golgi. Alternatively, it is possible that
all the intracellular receptors are associated with the ER-Golgi
complex (Fig. 7B). In this case, there could be both a
constitutive and a regulated exocytic pathway contributing to receptor
insertion in synaptic membranes. The constitutive pathway could account
for the normal maintenance of the synaptic receptor population, whereas
the regulated pathway would be activated during LTP, thus adding to the
constitutive pathway the number of receptors to be inserted into the
synapse. Either scenario requires the existence of a regulated exocytic pathway and, as mentioned above, there is some evidence for such a
pathway (Luscher et al., 1999). A recent study describes the existence
of a calcium-dependent dendritic exocytosis that is dependent on CamKII
activity (Maletic-Savatic et al., 1998; Maletic-Savatic and Malinow,
1998). This could well be identical to the regulated exocytic pathway
that is disrupted by BFA in our experiments. Interestingly, the
delivery of AMPA receptors to the synapse also requires CamKII (Hayashi
et al., 2000), a result in good agreement with our data with KN-62. In
addition, both scenarios require that the ER-Golgi complex be located
fairly close to synaptic membranes. The notion that the spine apparatus
represents an extension of the ER has been around for a long time
(Harris and Stevens, 1988; Martone et al., 1993), and recent evidence
further supports this concept (Spacek and Harris, 1997). Because BFA
did not affect baseline responses or the number of synaptic AMPA
receptors while completely blocking NMDA-induced modifications, the
scenario presented in Figure 7A would appear to be more
likely than the one in Figure 7B. However, our results
differ from those obtained by Matthies et al. (1999). Although these
authors found that BFA inhibited tetanus-induced LTP, they did observe
a decrease in baseline responses after BFA perfusion in hippocampal
slices. Therefore, more work remains to be done to determine the exact
relationship between the secretory pathway and the exocytic processes
in glutamatergic terminals. In any event, it does appear more and more
likely that the keys to understanding molecular mechanisms of synaptic
plasticity at excitatory synapses lie in the understanding of the
exocytosis and endocytosis pathways for the AMPA receptors.
| |
FOOTNOTES |
Received Aug. 18, 2000; revised Oct. 9, 2000; accepted Oct. 12, 2000.
The research was supported by Grant AG14751 from the National Institute
on Aging (principal investigator C. E. Finch). We thank Dr. S. Standley (currently at the National Institutes of Health)
for helpful discussions at the origin of this work.
Correspondence should be addressed to Michel Baudry, HNB 124, University of Southern California, Los Angeles, CA 90089-2520. E-mail:
baudry{at}neuro.usc.edu.
| |
REFERENCES |
-
Ambros-Ingerson J,
Lynch G
(1993)
Channel gating kinetics and synaptic efficacy: a hypothesis for expression of long-term potentiation.
Proc Natl Acad Sci USA
90:7903-7907[Abstract/Free Full Text].
-
Ambros-Ingerson J,
Xiao P,
Larson J,
Lynch G
(1993)
Waveform analysis suggests that LTP alters the kinetics of synaptic receptor channels.
Brain Res
620:237-244[Web of Science][Medline].
-
Baude A,
Molnar E,
Latawiec D,
McIlhinney RA,
Somogyi P
(1994)
Synaptic and nonsynaptic localization of the GluR1 subunit of the AMPA-type excitatory amino acid receptor in the rat cerebellum.
J Neurosci
14:2830-2843[Abstract].
-
Baude A,
Nusser Z,
Molnar E,
McIlhinney RA,
Somogyi P
(1995)
High-resolution immunogold localization of AMPA type glutamate receptor subunits at synaptic and non-synaptic sites in rat hippocampus.
Neuroscience
69:1031-1055[Web of Science][Medline].
-
Bi X,
Standley S,
Baudry M
(1998)
Posttranslational regulation of ionotropic glutamate receptors and synaptic plasticity.
Int Rev Neurobiol
42:227-284[Web of Science][Medline].
-
Bliss TV,
Lomo T
(1973)
Long-lasting potentiation of synaptic transmission in the dentate area of the anaesthetized rabbit following stimulation of the perforant path.
J Physiol (Lond)
232:331-356[Abstract/Free Full Text].
-
Blitzer RD,
Connor JH,
Brown GP,
Wong T,
Shenolikar S,
Iyengar R,
Landau EM
(1998)
Gating of CaMKII by cAMP-regulated protein phosphatase activity during LTP.
Science
280:1940-1942[Abstract/Free Full Text].
-
Carroll RC,
Beattie EC,
Xia H,
Luscher C,
Altschuler Y,
Nicoll RA,
Malenka RC,
von Zastrow M
(1999)
Dynamin-dependent endocytosis of ionotropic glutamate receptors.
Proc Natl Acad Sci USA
96:14112-14117[Abstract/Free Full Text].
-
del Cerro S,
Larson J,
Oliver MW,
Lynch G
(1990)
Development of hippocampal long-term potentiation is reduced by recently introduced calpain inhibitors.
Brain Res
530:91-95[Web of Science][Medline].
-
Denny JB,
Polan-Curtain J,
Ghuman A,
Wayner MJ,
Armstrong DL
(1990)
Calpain inhibitors block long-term potentiation.
Brain Res
534:317-320[Web of Science][Medline].
-
Engert F,
Bonhoeffer T
(1999)
Dendritic spine changes associated with hippocampal long-term synaptic plasticity.
Nature
399:66-70[Medline].
-
Hall RA,
Hansen A,
Andersen PH,
Soderling TR
(1997)
Surface expression of the AMPA receptor subunits GluR1, GluR2, and GluR4 in stably transfected baby hamster kidney cells.
J Neurochem
68:625-630[Web of Science][Medline].
-
Harris KM,
Stevens JK
(1988)
Dendritic spines of rat cerebellar Purkinje cells: serial electron microscopy with reference to their biophysical characteristics.
J Neurosci
8:4455-4469[Abstract].
-
Hayashi Y,
Shi SH,
Esteban JA,
Piccini A,
Poncer JC,
Malinow R
(2000)
Driving AMPA receptors into synapses by LTP and CaMKII: requirement for GluR1 and PDZ domain interaction.
Science
287:2262-2267[Abstract/Free Full Text].
-
Henley JM
(1995)
Subcellular localization and molecular pharmacology of distinct populations of [3H]-AMPA binding sites in rat hippocampus.
Br J Pharmacol
115:295-301[Web of Science][Medline].
-
Isaac JT,
Nicoll RA,
Malenka RC
(1995)
Evidence for silent synapses: implications for the expression of LTP.
Neuron
15:427-434[Web of Science][Medline].
-
Isaac JT,
Nicoll RA,
Malenka RC
(1999)
Silent glutamatergic synapses in the mammalian brain.
Can J Physiol Pharmacol
77:735-737[Web of Science][Medline].
-
Isaac JT,
Oliet SH,
Hjelmstad GO,
Nicoll RA,
Malenka RC
(1996)
Expression mechanisms of long-term potentiation in the hippocampus.
J Physiol (Paris)
90:299-303[Web of Science][Medline].
-
Kauer JA,
Malenka RC,
Nicoll RA
(1988)
NMDA application potentiates synaptic transmission in the hippocampus.
Nature
334:250-252[Medline].
-
Klausner RD,
Donaldson JG,
Lippincott-Schwartz J
(1992)
Brefeldin A: insights into the control of membrane traffic and organelle structure.
J Cell Biol
116:1071-1080[Free Full Text].
-
Kolta A,
Lynch G,
Ambros-Ingerson J
(1998)
Effects of aniracetam after LTP induction are suggestive of interactions on the kinetics of the AMPA receptor channel.
Brain Res
788:269-286[Web of Science][Medline].
-
Lee HK,
Kameyama K,
Huganir RL,
Bear MF
(1998)
NMDA induces long-term synaptic depression and dephosphorylation of the GluR1 subunit of AMPA receptors in hippocampus.
Neuron
21:1151-1162[Web of Science][Medline].
-
Liao D,
Hessler NA,
Malinow R
(1995)
Activation of postsynaptically silent synapses during pairing-induced LTP in CA1 region of hippocampal slice.
Nature
375:400-404[Medline].
-
Lledo PM,
Hjelmstad GO,
Mukherji S,
Soderling TR,
Malenka RC,
Nicoll RA
(1995)
Calcium/calmodulin-dependent kinase II and long-term potentiation enhance synaptic transmission by the same mechanism.
Proc Natl Acad Sci USA
92:11175-11179[Abstract/Free Full Text].
-
Lledo PM,
Zhang X,
Sudhof TC,
Malenka RC,
Nicoll RA
(1998)
Postsynaptic membrane fusion and long-term potentiation.
Science
279:399-403[Abstract/Free Full Text].
-
Luscher C,
Xia H,
Beattie EC,
Carroll RC,
von Zastrow M,
Malenka RC,
Nicoll RA
(1999)
Role of AMPA receptor cycling in synaptic transmission and plasticity.
Neuron
24:649-658[Web of Science][Medline].
-
Luscher C,
Nicoll RA,
Malenka RC,
Muller D
(2000)
Synaptic plasticity and dynamic modulation of the postsynaptic membrane.
Nat Neurosci
3:545-550[Web of Science][Medline].
-
Lynch G,
Baudry M
(1984)
The biochemistry of memory: a new and specific hypothesis.
Science
224:1057-1063[Abstract/Free Full Text].
-
Maletic-Savatic M,
Malinow R
(1998)
Calcium-evoked dendritic exocytosis in cultured hippocampal neurons. I. trans-Golgi network-derived organelles undergo regulated exocytosis.
J Neurosci
18:6803-6813[Abstract/Free Full Text].
-
Maletic-Savatic M,
Koothan T,
Malinow R
(1998)
Calcium-evoked dendritic exocytosis in cultured hippocampal neurons. II. Mediation by calcium/calmodulin-dependent protein kinase II.
J Neurosci
18:6814-6821[Abstract/Free Full Text].
-
Maletic-Savatic M,
Malinow R,
Svoboda K
(1999)
Rapid dendritic morphogenesis in CA1 hippocampal dendrites induced by synaptic activity.
Science
283:1923-1927[Abstract/Free Full Text].
-
Malinow R,
Schulman H,
Tsien RW
(1989)
Inhibition of postsynaptic PKC or CaMKII blocks induction but not expression of LTP.
Science
245:862-866[Abstract/Free Full Text].
-
Malinow R,
Mainen ZF,
Hayashi Y
(2000)
LTP mechanisms: from silence to four-lane traffic.
Curr Opin Neurobiol
10:352-357[Web of Science][Medline].
-
Man YH,
Lin JW,
Ju WH,
Ahmadian G,
Liu L,
Becker LE,
Sheng M,
Wang YT
(2000)
Regulation of AMPA receptor-mediated synaptic transmission by clathrin-dependent receptor internalization.
Neuron
25:649-662[Web of Science][Medline].
-
Maren S,
Tocco G,
Standley S,
Baudry M,
Thompson RF
(1993)
Postsynaptic factors in the expression of long-term potentiation (LTP): increased glutamate receptor binding following LTP induction in vivo.
Proc Natl Acad Sci USA
90:9654-9658[Abstract/Free Full Text].
-
Martone ME,
Zhang Y,
Simpliciano VM,
Carragher BO,
Ellisman MH
(1993)
Three-dimensional visualization of the smooth endoplasmic reticulum in Purkinje cell dendrites.
J Neurosci
13:4636-4646[Abstract].
-
Matthies Jr H,
Kretlow J,
Matthies H,
Smalla KH,
Staak S,
Krug M
(1999)
Glycosylation of proteins during a critical time window is necessary for the maintenance of long-term potentiation in the hippocampal CA1 region.
Neuroscience
91:175-183[Web of Science][Medline].
-
Misumi Y,
Miki K,
Takatsuki A,
Tamura G,
Ikehara Y
(1986)
Novel blockade by brefeldin A of intracellular transport of secretory proteins in cultured rat hepatocytes.
J Biol Chem
261:11398-11403[Abstract/Free Full Text].
-
Musleh W,
Bi X,
Tocco G,
Yaghoubi S,
Baudry M
(1997)
Glycine-induced long-term potentiation is associated with structural and functional modifications of
 -amino-3-hydroxyl-5-methyl-4-isoxazolepropionic acid receptors.
Proc Natl Acad Sci USA
94:9451-9456[Abstract/Free Full Text]. -
Noel J,
Ralph GS,
Pickard L,
Williams J,
Molnar E,
Uney JB,
Collingridge GL,
Henley JM
(1999)
Surface expression of AMPA receptors in hippocampal neurons is regulated by an NSF-dependent mechanism.
Neuron
23:365-376[Web of Science][Medline].
-
Oda K,
Hirose S,
Takami N,
Misumi Y,
Takatsuki A,
Ikehara Y
(1987)
Brefeldin A arrests the intracellular transport of a precursor of complement C3 before its conversion site in rat hepatocytes.
FEBS Lett
214:135-138[Web of Science][Medline].
-
Pettit DL,
Perlman S,
Malinow R
(1994)
Potentiated transmission and prevention of further LTP by increased CaMKII activity in postsynaptic hippocampal slice neurons.
Science
266:1881-1885[Abstract/Free Full Text].
-
Rubio ME,
Wenthold RJ
(1999)
Differential distribution of intracellular glutamate receptors in dendrites.
J Neurosci
19:5549-5562[Abstract/Free Full Text].
-
Shahi K,
Marvizon JC,
Baudry M
(1993)
High concentrations of glycine induce long-lasting changes in synaptic efficacy in rat hippocampal slices.
Neurosci Lett
149:185-188[Web of Science][Medline].
-
Shi SH,
Hayashi Y,
Petralia RS,
Zaman SH,
Wenthold RJ,
Svoboda K,
Malinow R
(1999)
Rapid spine delivery and redistribution of AMPA receptors after synaptic NMDA receptor activation.
Science
284:1811-1816[Abstract/Free Full Text].
-
Song I,
Kamboj S,
Xia J,
Dong H,
Liao D,
Huganir RL
(1998)
Interaction of the N-ethylmaleimide-sensitive factor with AMPA receptors.
Neuron
21:393-400[Web of Science][Medline].
-
Spacek J,
Harris KM
(1997)
Three-dimensional organization of smooth endoplasmic reticulum in hippocampal CA1 dendrites and dendritic spines of the immature and mature rat.
J Neurosci
17:190-203[Abstract/Free Full Text].
-
Standley S,
Bi X,
Baudry M
(1996)
Glutamate receptor regulation and synaptic plasticity.
In: Long-term potentiation, Vol III (Baudry M,
Davis JL,
eds), pp 17-40. Cambridge, MA: MIT.
-
Standley S,
Tocco G,
Wagle N,
Baudry M
(1998)
High- and low-affinity -[3H]amino-3-hydroxy-5-methylisoxazole-4-propionic acid ([3H]AMPA) binding sites represent immature and mature forms of AMPA receptors and are composed of differentially glycosylated subunits.
J Neurochem
70:2434-2445[Web of Science][Medline].
-
Staubli U,
Ambros-Ingerson J,
Lynch G
(1992)
Receptor changes and LTP: an analysis using aniracetam, a drug that reversibly modifies glutamate (AMPA) receptors.
Hippocampus
2:49-57[Web of Science][Medline].
-
Thibault O,
Joly M,
Muller D,
Schottler F,
Dudek S,
Lynch G
(1989)
Long-lasting physiological effects of bath applied N-methyl-D-aspartate.
Brain Res
476:170-173[Web of Science][Medline].
-
Toni N,
Buchs PA,
Nikonenko I,
Bron CR,
Muller D
(1999)
LTP promotes formation of multiple spine synapses between a single axon terminal and a dendrite.
Nature
402:421-425[Medline].
-
Turrigiano GG
(2000)
AMPA receptors unbound: membrane cycling and synaptic plasticity.
Neuron
26:5-8[Web of Science][Medline].
-
Vanderklish P,
Saido TC,
Gall C,
Arai A,
Lynch G
(1995)
Proteolysis of spectrin by calpain accompanies theta-burst stimulation in cultured hippocampal slices.
Brain Res Mol Brain Res
32:25-35[Medline].
-
Wang YT,
Linden DJ
(2000)
Expression of cerebellar long-term depression requires postsynaptic clathrin-mediated endocytosis.
Neuron
25:635-647[Web of Science][Medline].
-
Xie X,
Liaw JS,
Baudry M,
Berger TW
(1997)
Novel expression mechanism for synaptic potentiation: alignment of presynaptic release site and postsynaptic receptor.
Proc Natl Acad Sci USA
94:6983-6988[Abstract/Free Full Text].
Copyright © 2001 Society for Neuroscience 0270-6474/01/21127-08$05.00/0
This article has been cited by other articles:
|
|
|
|
 
S. Qiu, A. K. Jebelli, J. H. Ashe, and M. C. Curras-Collazo
Domoic Acid Induces a Long-Lasting Enhancement of CA1 Field Responses and Impairs Tetanus-Induced Long-term Potentiation in Rat Hippocampal Slices
Toxicol. Sci.,
September 1, 2009;
111(1):
140 - 150.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
H. Jourdi, Y.-T. Hsu, M. Zhou, Q. Qin, X. Bi, and M. Baudry
Positive AMPA Receptor Modulation Rapidly Stimulates BDNF Release and Increases Dendritic mRNA Translation
J. Neurosci.,
July 8, 2009;
29(27):
8688 - 8697.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
S. Qiu and E. J. Weeber
Reelin Signaling Facilitates Maturation of CA1 Glutamatergic Synapses
J Neurophysiol,
March 1, 2007;
97(3):
2312 - 2321.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
S. Qiu, L. F. Zhao, K. M. Korwek, and E. J. Weeber
Differential Reelin-Induced Enhancement of NMDA and AMPA Receptor Activity in the Adult Hippocampus
J. Neurosci.,
December 13, 2006;
26(50):
12943 - 12955.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
A. C. Boudreau and M. E. Wolf
Behavioral Sensitization to Cocaine Is Associated with Increased AMPA Receptor Surface Expression in the Nucleus Accumbens
J. Neurosci.,
October 5, 2005;
25(40):
9144 - 9151.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
A. A. Bagal, J. P. Y. Kao, C.-M. Tang, and S. M. Thompson
Long-term potentiation of exogenous glutamate responses at single dendritic spines
PNAS,
October 4, 2005;
102(40):
14434 - 14439.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
H. Jourdi, X. Lu, T. Yanagihara, J. C. Lauterborn, X. Bi, C. M. Gall, and M. Baudry
Prolonged Positive Modulation of {alpha}-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid (AMPA) Receptors Induces Calpain-Mediated PSD-95/Dlg/ZO-1 Protein Degradation and AMPA Receptor Down-Regulation in Cultured Hippocampal Slices
J. Pharmacol. Exp. Ther.,
July 1, 2005;
314(1):
16 - 26.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
S. M. Rodrigues, C. R. Farb, E. P. Bauer, J. E. LeDoux, and G. E. Schafe
Pavlovian Fear Conditioning Regulates Thr286 Autophosphorylation of Ca2+/Calmodulin-Dependent Protein Kinase II at Lateral Amygdala Synapses
J. Neurosci.,
March 31, 2004;
24(13):
3281 - 3288.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
B. BARTHOLOME, C. M. SPIES, T. GABER, S. SCHUCHMANN, T. BERKI, D. KUNKEL, M. BIENERT, A. RADBRUCH, G.-R. BURMESTER, R. LAUSTER, et al.
Membrane glucocorticoid receptors (mGCR) are expressed in normal human peripheral blood mononuclear cells and up-regulated after in vitro stimulation and in patients with rheumatoid arthritis
FASEB J,
January 1, 2004;
18(1):
70 - 80.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
P. V. Perestenko and J. M. Henley
Characterization of the Intracellular Transport of GluR1 and GluR2 {alpha}-Amino-3-hydroxy-5-methyl-4-isoxazole Propionic Acid Receptor Subunits in Hippocampal Neurons
J. Biol. Chem.,
October 31, 2003;
278(44):
43525 - 43532.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
J. A. Esteban
AMPA Receptor Trafficking: A Road Map for Synaptic Plasticity
Mol. Interv.,
October 1, 2003;
3(7):
375 - 385.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
M. Hirasawa and Q. J. Pittman
From the Cover: Nifedipine facilitates neurotransmitter release independently of calcium channels
PNAS,
May 13, 2003;
100(10):
6139 - 6144.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
B. Lin, A. C. Arai, G. Lynch, and C. M. Gall
Integrins Regulate NMDA Receptor-Mediated Synaptic Currents
J Neurophysiol,
May 1, 2003;
89(5):
2874 - 2878.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
E. A. Kramar, J. A. Bernard, C. M. Gall, and G. Lynch
Integrins Modulate Fast Excitatory Transmission at Hippocampal Synapses
J. Biol. Chem.,
March 14, 2003;
278(12):
10722 - 10730.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
J. Ko, S. Kim, J. G. Valtschanoff, H. Shin, J.-R. Lee, M. Sheng, R. T. Premont, R. J. Weinberg, and E. Kim
Interaction between Liprin-alpha and GIT1 Is Required for AMPA Receptor Targeting
J. Neurosci.,
March 1, 2003;
23(5):
1667 - 1677.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
L. Fang, J. Wu, Q. Lin, and W. D. Willis
Calcium-Calmodulin-Dependent Protein Kinase II Contributes to Spinal Cord Central Sensitization
J. Neurosci.,
May 15, 2002;
22(10):
4196 - 4204.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
TOP
ABSTRACT
INTRODUCTION
Compound via MeSH
