Lateral diffusion of glutamate receptors was proposed as a mechanism for regulating receptor numbers at synapses and affecting synaptic functions, especially the efficiency of synaptic transmission. However, a direct link between receptor lateral diffusion and change in synaptic function has not yet been established. In the present study, we demonstrated NMDA receptor (NMDAR) lateral diffusion in CA1 neurons in hippocampal slices by detecting considerable recovery of spontaneous or evoked EPSCs from the block of (+)-MK-801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d] cyclohepten-5,10-imine maleate], an irreversible NMDAR open-channel blocker. We observed changes on both the number and the composition of synaptic NMDAR on recovery. More importantly, after the recovery, long-term potentiation (LTP)-producing protocol induced only LTD (long-term depression) instead of LTP. In contrast, a complete recovery from competitive NMDAR blocker d,l-AP-5 was observed without subsequent changes on synaptic plasticity. Our data suggest a revised model of NMDAR trafficking wherein extrasynaptic NMDARs, mostly NR1/NR2B receptors, move laterally into synaptic sites, resulting in altered rule of synaptic modification. Thus, CA1 synapses exhibit a novel form of metaplasticity in which the direction of synaptic modification can be reverted through subtype-specific lateral diffusion of NMDA receptors.
Synaptic NMDA-type glutamate receptors play critical roles during brain development, plasticity, and pathology (Constantine-Paton and Cline, 1998; Zoghbi et al., 2000). In the hippocampus, NMDA receptors (NMDARs) are heteromultimeric complexes assembled from NR1 subunit and one or more of the two NR2 subunits, NR2A and NR2B (Sheng et al., 1994). The different NR2 subunits (NR2A–NR2D) display distinct kinetic and pharmacological properties on heteromeric receptors. The NR1/NR2A composition display faster decay constant than NR1/NR2B receptors (Monyer et al., 1994). Because both NR2A and NR2B have very long cytoplasmic tails that have been shown to play critical roles in directing the trafficking to and stabilization of synaptic sites (Osten et al., 2000), they are good candidates to participate in controlling the number and composition of NMDARs at synapse.
Metaplasticity was defined as the plasticity of synaptic plasticity. Metaplasticity could occur when priming synaptic or cellular activity (or inactivity) leads to persistent change in the direction or degree of synaptic plasticity induced by a given form of synaptic activation (Abraham and Bear, 1996). It is proposed that activity-dependent changes in the NR2A/NR2B ratio are involved in the metaplastic regulation of long-term depression (LTD)/long-term potentiation (LTP) thresholds (Philpot et al., 2001; Chen and Bear, 2007); therefore, NMDAR regulation provides a molecular basis for sliding synaptic modification (Bear, 2003).
NMDARs are often regarded as a relatively stable complex of postsynaptic membrane, anchored by proteins such as postsynaptic density-95 (PSD-95), gephyrin, and PSD proteins into molecule scaffold (Kneussel and Betz, 2000; Sheng, 2001). Using optical and electrophysiological approaches, recent findings suggest that ionotropic glutamatergic receptors, including AMPA receptors (AMPARs) and NMDARs, are more mobile than we initially thought. Besides trafficking directly between postsynaptic membrane and cytoplasm pools as previously studies suggest, the receptors can also move laterally and bidirectionally between synaptic and extrasynaptic sites along the postsynaptic membrane plane (Borgdorff and Choquet, 2002; Tovar and Westbrook, 2002; Choquet and Triller, 2003; Ehlers et al., 2007). This provides a new route for receptor trafficking and a potential new pathway for synaptic modification (Triller and Choquet, 2005; Cognet et al., 2006; Ehlers et al., 2007), including rapid bidirectional switching of synaptic NMDA receptors as most recently observed (Bellone and Nicoll, 2007). However, the consequence of such receptor trafficking on the properties of synaptic plasticity has not been investigated, partially attributable to relative difficulty in inducing the typical synaptic plasticity model: LTP on cultured neurons (Tovar and Westbrook, 2002; Adesnik et al., 2005) (but see Bekkers and Stevens 1990; Bi and Poo 1998; Gerkin et al., 2007). In the present study, we used electrophysiological recordings on hippocampal slices and demonstrated that extrasynaptic NMDARs (exclusively NR1/NR2B) could move into synaptic membrane via lateral diffusion during recovery from block of (+)MK-801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d] cyclohepten-5,10-imine maleate], a noncompetitive NMDA open-channel blocker, causing change of synaptic NR2 subunit composition (NR2A/NR2B ratio) and resulting in an altered rule of synaptic modification. Our data thus provide a direct conceivable link between receptor lateral diffusion and a novel form of synaptic metaplasticity.
Materials and Methods
Hippocampal slices preparation.
Male Sprague Dawley rats, 3 weeks old, were anesthetized with ethyl ether and decapitated, and whole hippocampus was removed from the brain. Coronal brain slices (350 μm thickness) were cut using a vibrantly blade microtome in ice-cold artificial CSF (ACSF) containing the following (in mm) 126 NaCl, 2.5 KCl, 1 MgCl2, 1 CaCl2, 1.25 KH2PO4, 26 NaHCO3, and 20 glucose. ACSF was bubbled continuously with carbogen (95%O2/5%CO2) to adjust the pH to 7.4. Fresh slices were incubated in chamber with carbogenated ACSF and recovered at 34°C for at least 1.5 h before they were transferred to recording chamber.
Conventional whole-cell recordings were made with patch pipettes containing the following (in mm): 132.5 Cs-gluconate, 17.5 CsCl, 2 MgCl2, 0.5 EGTA, 10 HEPES, 4 ATP, and 5 QX-314 [N-(2,6-dimethylphenylcarbamoylmethyl)triethylammonium bromide], pH adjusted to 7.2 by CsOH. Hippocampal slices were perfused with ice-cold ACSF that was bubbled continuously with carbogen (95%O2/5%CO2) to adjust the pH to 7.4. Evoked synaptic responses were evoked at 0.05 Hz except during the induction of LTP. LTP was induced with 200 synaptic stimuli at 2 Hz during a 2.5 min depolarization to 0 mV (Chen et al., 1999). Excitatory postsynaptic responses were evoked by stimulating the Schaffer fibers through a constant-current pulse delivered by a bipolar tungsten electrode. EPSCs were recorded with continuously low-Mg2+ (0.25 mm) ACSF perfusion containing bicuculline methiodide (BMI) (10 μm) to block GABAA receptor-mediated inhibitory synaptic currents. CA1 neurons were viewed under upright microscopy (BX51 WI, Nomarski; Olympus Optical, Tokyo, Japan) and recorded with Axopatch-200B amplifier (Molecular Devices, Palo Alto, CA). Data were low-pass filtered at 2 kHz and acquired at 5–10 kHz. The series resistance (R s) was always monitored during recording for fear that reseal of ruptured membrane, which will cause change of both EPSC kinetics and amplitude. Cells in which R s or capacitance deviated by >20% from initial values were excluded from analysis. Also, cells with R s >20 MΩ at any time during the recording were excluded from analysis. For experiments using paired-pulse stimulation, paired stimuli were delivered at a frequency of 0.125 Hz with an interstimulus interval of 100 ms. The paired-pulse protocol was used to accelerate the level of EPSCs of block and monitor large changes in release probability. For perforated-patch recording, amphetericin (600 μg/ml) was added in pipette solution (Rae et al., 1991). Recording was only made after access resistance decreased below 60 MΩ.
Normally spontaneous NMDAR-mediated currents were recorded with AMPAR blocker 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide (NBQX) (5 μm) as well as GABAA receptor blocker BMI (10 μm). If subsequent experiments on synaptic plasticity was conducted, only GABAA receptor blocker BMI (10 μm) was included in perfusion for two reasons. First, AMPAR blocker NBQX is not easy to be washed out; thus, we could only record subsequent LTP/LTD of NMDAR-mediated EPSCs (not LTP/LTD of AMPAR-mediated EPSCs) if we apply NBQX in the recovery period (see Fig. 5, data before vertical dashed-line). Conversely, it is not difficult to distinguish between AMPAR- and NMDAR-mediated EPSCs according to their rise and decay time. Indeed, there were some fused currents composed of both components. However, the percentage of these currents is quite low. Spontaneous events were averaged and analyzed using Minianalysis Software (Synaptosoft, Decatur, GA). The decay time was defined as the time from the current peak to 37% of the peak. Standard extracellular recordings were conducted to monitor evoked field potentials from using glass microelectrodes filled with the saline solution (5–10 MΩ resistance) and placed in CA1 stratum radiatum at least 60–80 μm away from the cell body layer. The current intensity of test stimuli (25–50 μA) was set to produce half-maximal EPSPs. The baseline was recorded at least 10 min to ensure the stability of the response. Field EPSP LTP was induced by two 100 Hz, 1 s trains (at a current set to produce 75% of the maximized EPSP response) with 120 s interval between the two stimuli. Data were collected with pClamp9.2 software and analyzed using Clampfit9.2 (Molecular Devices). The slope of the field potential was measured beginning at 10% and ending at 50% of the initial phase of EPSP response.
Block of NMDA-mediated currents with channel blockers.
Evoked or spontaneous synaptic NMDA receptor-mediated currents were blocked by coapplication of presynaptic electrical paired-pulse stimuli that facilitate activation of postsynaptic NMDA receptors and the whole-cell MK-801 perfusion that selectively block open NMDA channels activated by paired-pulse stimulation. Agonist-evoked block of whole-cell NMDA receptor was obtained by coapplication of whole-cell puffing of NMDA to activate all membrane NMDAR and MK-801 perfusion to simultaneously block all open receptor channels. All above recordings were conducted in low-Mg2+ (0.25 mm) ACSF containing the GABAA blocker BMI (10 μm) and the AMPAR blocker NBQX (5 μm). Spontaneous NMDAR-mediated currents were considered synaptic if the rise times were <8–9 ms and peak amplitude was at least two times baseline to peak noise (2 pA). Further addition of tetrodotoxin (TTX) (0.5 μm) to block action potentials reduced NMDAR-mediated currents frequency by <5%, indicating that most of these events were miniature NMDAR-mediated currents.
All blockers including BMI, NBQX, MK-801, AP-5, and NMDA were purchased from Sigma (St. Louis, MO). Dendritic spine motility inhibitor latrunculin B was also from Sigma.
The measurement of decay time (τ) of NMDAR-mediated currents is the time from current peak to 37% of peak. Data were analyzed by ANOVA with a post hoc Student's t test. All mean are presented with their SEs. For decay time of spontaneous EPSCs (sEPSCs), we used nonparametric and Wilcoxon's signed-ranks test because the distribution of spontaneous EPSC decay time does not fit the normal school.
NMDAR-mediated EPSC recovery from MK-801 block
Evoked NMDAR-mediated EPSCs were recorded in whole-cell patch mode at a holding potential of −70 mV with low-Mg2+ (0.25 mm) ACSF containing the GABAA antagonist BMI (10 μm) and the AMPA receptor antagonist NBQX (5 μm), which could be abolished by the NMDA receptor competitive blocker d,l-AP-5 or no-competitive blocker MK-801 (Fig. 1 A). Because the efficiency of perfusion is a very important factor that could affect the rate of receptor diffusion and, on slices, the perfusion efficiency is usually lower than that on cultured neurons, we highly increased ACSF perfusion rate up to 5 ml/min to guarantee the efficiency of wash-in and washout of the drugs. We used paired-pulse stimulation to facilitate the presynaptic release of glutamate, which in turn facilitates the activation of synaptic NMDAR currents (Fig. 1 B). Simultaneously applying an open NMDA receptor channel blocker MK-801 with paired-pulse stimulation given at 0.125 Hz could deactivate NMDA currents and keep them closed. This protocol selectively and irreverently blocks synaptic NMDA receptor, as was used previously by Tovar and Westbrook (2002) on cultured autaptic hippocampal neurons. After MK-801 block and washout, evoked NMDAR-mediated currents recorded at 0.05 Hz gradually recover and, at 30 min after MK-801 washout, increase up to 46% (43.11 ± 3.42%; n = 7) of initial evoked EPSCs amplitude (Fig. 1 C). The recovery of EPSCs might account for insertion of new NMDA receptors directly into postsynaptic membrane from the intracellular pool. We test this possibility by whole-cell coapplication of NMDA and MK-801. NMDA (1 mm) was applied through local puffing on recorded cells for 3 min to activate all NMDA receptors on whole-cell membrane, while MK-801 (10 μm) was simultaneously applied to irreversibly block all activated NMDA receptors on membrane, including synaptic and extrasynaptic NMDA receptors. Thus, we should block all activated NMDAR in the membrane. However, we did not see any significant NMDAR-mediated EPSC recovery at the timescale demonstrated previously on receptor trafficking (2.12 ± 1.15%; n = 3) (Fig. 1 D). Thus, these data preclude the contribution of direct insertion of receptors to observed EPSC recovery.
Recovery is consistent with lateral diffusion of extrasynaptic receptors into synapse
To clarify the reason for the above anomalous EPSC recovery, we used occluding methods used previously by Tovar and Westbrook (2002). The above anomalous EPSC recovery could account for some possibilities classified as presynaptic and postsynaptic possibilities. In the presynaptic point of view, increased neurotransmitter release probability could lead to more postsynaptic receptor activation. We test whether it could happen in our case by detecting paired-pulse ratio before MK-801 application and after recovery from MK-801 block. We found that the paired-pulse ratio displays no significant change (control, 1.46 ± 0.08; recovery, 1.38 ± 0.06; n = 6; p = 0.452, ANOVA and post hoc test) (supplemental Fig. 1, available at www.jneurosci.org as supplemental material), indicating that this presynaptic factor was not involve in the observed EPSC recovery. Another presynaptic possibility includes migration of presynaptic terminals or active zones that might cause equal receptor density between synaptic and extrasynaptic sites. Because new synapse formation is involved in dendrite filopodia motility (Lendvai et al., 2000), we used a filopodia motility inhibitor latrunculin B to test this possibility (Fischer et al., 1998). We did not find any influence of latrunculin B on recovery (control, 43.11 ± 3.42% vs latrunculin B, 42.85 ± 3.55%; p = 0.960, ANOVA and post hoc test). In addition, the course of EPSC recovery was faster than assembly of new synapses reported previously (Friedman et al., 2000).
Beside presynaptic possibilities, postsynaptic factors could also account for EPSC recovery. Theoretically, MK-801 has high affinity to NMDA receptors and can irreversibly blocked open-state channels. However, MK-801 might still unbind from a bound NMDA receptor if the closed channels reopen. Could the above EPSC recovery be attributable to the MK-801 unbinding? We tested this possibility by using NMDA receptor-competitive antagonist d,l-AP-5. MK-801 can block NMDAR and cannot unbind unless the channels reopen. AP-5 can prevent binding of presynaptically released glutamate with postsynaptic NMDARs and opening of these receptor channels. Thus, by preventing glutamate from rebinding to the channels already blocked by MK-801, we prevent reopening of blocked NMDAR and therefore prevent unbinding of NMDA receptor from MK-801. After partial recovery for 10 min after MK-801 block, we perfused slices with AP-5, which abolished evoked EPSCs immediately and completely. After removal of AP-5, however, the extent of recovery was even higher than that before AP-5 application and was not different from recovery in the absence of AP-5. These results suggest blocking of unbinding by d,l-AP-5 (100 μm) has no effect on the extent of recovery, as displayed after removal of the antagonist (Fig. 2 A,C). Some NMDA receptors might have faster unbinding rate attributable to relatively lower affinity to MK-801; this would also lead to partial EPSCs recovery. To investigate this possibility, we allowed EPSC recovery from selective synaptic block by MK-801 and AP-5. After a 10 min recovery period, coapplication of NMDA and MK-801 leads to a complete and immediate EPSC block (Fig. 2 B,C). If the recovery from sequential block of MK-801 and AP-5 was attributable to unbinding of lower-affinity receptors to MK-801, subsequent MK-801 application could not completely abolish the recovered EPSCs. Therefore, these data preclude the contribution of low-affinity receptor unbinding to EPSC recovery. In addition, our data on the change of NMDAR subunit composition can also argue strongly against the possible role of NMDARs unbinding from MK-801 (see data below). Finally, we did not observe a gradual increase of spontaneous NMDA EPSC amplitude and frequency under control condition, suggesting no net increase in either synaptic site containing NMDARs or gradual activation of previously “silent” synapse.
After ruling out all above possibilities, a potential scenario becomes clear, that is, addition of new receptors into synaptic sites. Because the above results argue against the direct insertion of new receptors into synaptic membrane from an intracellular pool, an alterative trafficking route is the adding of extrasynaptic NMDA receptors into synaptic sites via lateral diffusion from extrasynaptic sites. This deduction was supported by recent studies that demonstrated lateral diffusion as the trafficking pathway of glutamatergic receptors using electrophysiological and optical tools (Borgdorff and Choquet, 2002; Tovar and Westbrook, 2002; Choquet and Triller, 2003). Our present data in hippocampal neurons further determine that this receptor trafficking property could also occur on slices other than cultured neurons. By using the above occluding experiments, we proposed NMDA receptor lateral diffusion as a possible route to change the synaptic receptor number or component and, as a result, potential change of synaptic plasticity (see data below).
NR2 subunit composition switch after recovery from MK-801 block
Previous studies report that presynaptically released glutamate after electrical stimulation could spill over in synaptic cleft and activate perisynaptic or extrasynaptic NMDA receptors, the NR2B subunit of which is believed to have higher affinity to glutamate than NR2A (Clark and Cull-Candy, 2002; Scimemi et al., 2004). This property could blur to some extent the boundary between synaptic and perisynaptic or extrasynaptic and therefore jeopardize the effect we observe through evoked EPSC recording. To examine the above observed EPSC recovery after MK-801 block in more detail, we also detected spontaneous NMDA-mediated currents (sEPSCs) before and after recovery from MK-801 block. We observed a similar recovery trend after synaptic receptor activation with low-frequency paired-pulse stimulation given at 0.125 Hz and MK-801 coapplication. After 30 min MK-801 washout, the recovery of sEPSC amplitude was 65.12 ± 6.46% of initial value (n = 7) (Fig. 3 A, C, left, E), whereas whole-cell coapplication of NMDA with MK-801 could completely abolish sEPSCs without any obvious recovery after 30 min washout (1.90 ± 0.32%; n = 3) (Fig. 3 B) (for data recorded in perforated patch mode, see supplemental Fig. 2, available at www.jneurosci.org as supplemental material), suggesting no direct insertion of intracellular receptors into synaptic sites. The recovery of spontaneous NMDAR-mediated EPSCs is activity independent because similar recovery trend was observed under TTX (0.5 μm) application (supplemental Fig. 3, available at www.jneurosci.org as supplemental material).
One big advantage we can take through spontaneous EPSC recording is that, besides amplitude detecting, we can also simultaneously examine the possible alteration of event frequency across the whole process. The spontaneous NMDAR-mediated EPSC frequency also gradually recovered up to 38% of initial value (36.65 ± 5.63%; n = 7) (Fig. 3 A,D,E). This result suggests that the above EPSC amplitude recovery ascribe, at least in part, to recovery of numbers of active NMDA receptors at synapses.
All above observations were conducted in conventional whole-cell patch-clamp recording mode. The disadvantage of this protocol is possible dialysis of intracellular organic components, including receptors released from internal stores, which is demonstrated to be conserved and ready to traffic into postsynaptic membrane. Our above data argue against direct and “vertical” insertion (in contrast to lateral diffusion) of NMDA receptors from the intracellular pool to synaptic membrane, but we do not know whether intracellular components also take a role in this recovery. To examine this possibility, we repeated the above spontaneous EPSC recordings using perforated patch mode. We found in this recording configuration that the EPSC frequency recovery markedly increased from 36.65 ± 5.63 to 67.55 ± 6.22% (p = 0.011, ANOVA and post hoc test) (Fig. 3 D,E), whereas sEPSC amplitude could recover to 73.85 ± 7.13% of the initial value compared with 65.12 ± 6.46% recovery level in conventional recording mode (no significance, p = 0.485, ANOVA and post hoc test) (Fig. 3 E). Therefore, unless otherwise stated in text, most of following data were obtained in perforated patch configuration. Complete recovery on both amplitude and frequency was observed from AP-5 block (91.50 ± 13.55%; p > 0.05) (supplemental Fig. 2B, available at www.jneurosci.org as supplemental material). These results suggest that intracellular components could also be involved in the above recovery.
Because receptor density and NR2 subunit composition between synaptic and extrasynaptic sites are quite different, extrasynaptic NR2s are predominantly NR2B-containing NMDA receptors, but most NR2A-containing NMDA receptors are synaptic (Rumbaugh and Vicini 1999; Sinor et al., 2000) (but see Thomas et al., 2006), communication between those two sites through lateral diffusion under certain conditions might induce alteration of both synaptic numbers and subunit composition, and finally synaptic modification. To examine whether composition of the NMDA receptor subunit also changes after the recovery from MK-801 block, we checked the sEPSC current kinetics before and after recovery. We found that the averaged decay time of sEPSCs after recovery from MK-801 block was prolonged compared with control (control, 19.76 ± 1.35 ms vs recovery, 25.92 ± 1.17 ms; n = 6; p = 0.028, nonparametric and Wilcoxon's signed ranks test) (Fig. 4 B,F). A similar change was also observed on evoked EPSCs (control, 109.07 ± 6.57 ms vs recovery, 130.27 ± 4.67 ms; n = 7; p = 0.018, nonparametric and Wilcoxon's signed ranks test) (Fig. 4 E,F) recorded in perforated patch mode. In contrast, no change on decay was observed after recovery from AP-5 block (Fig. 4 A,E,F).
To further examine whether the prolongation of EPSC decay is attributable to NMDAR composition switch, that is, increase of NR2B component or decrease of NR2A component, we perfused the slice with the NR2B antagonist ifenprodil (3 μm) to check whether the ifenprodil-sensitive NMDAR component changed after recovery from MK-801 block. We observed a significant elevation of NR2B-mediated NMDA EPSC component by detecting the area under the current curve, which we called current density here. The ratio of NR2A/NR2B current density markedly decreased from 1.30 ± 0.08 to 0.79 ± 0.04 (p < 0.001, ANOVA and post hoc test) (Fig. 4 D), at least partially attributable to increase of NR2B-mediated EPSCs and/or decrease of NR2A-mediated EPSCs. We double checked this increase of NR2B component by using a potent, selective nontransportable inhibitor of excitatory amino acid transporters called dl-threo-beta-benzyloxyaspartic acid (TBOA). TBOA application could block recycling of presynaptically released glutamate and cause accumulation of glutamate in the synaptic cleft. As a result, both synaptic and perisynaptic NMDARs are activated, represented by prolongation of EPSC decay time (Diamond 2001; Massey et al., 2004; Scimemi et al., 2004; Bartlett et al., 2007). This effect was similar to the decay change after MK-801 washout. Furthermore, TBOA application after MK-801 washout did not produce additional effects on decay time (MK-801/recovery, 25.71 ± 1.67 ms vs MK-801/recovery + TBOA, 25.46 ± 0.68 ms; p > 0.05) (supplemental Fig. 4, available at www.jneurosci.org as supplemental material). Combined with the above findings, these results suggest that, after removal of MK-801 block, which inactivates synaptic NMDARs including NR2A-containing NMDARs, relatively more perisynaptic or extrasynaptic NR2B-containing NMDARs laterally diffuse into synaptic sites and cause the observed change on NMDAR-mediated EPSC kinetics, although this diffusion could be bidirectional.
Differential regulation of synaptic plasticity after recovery from MK-801 and AP-5 block
We further test whether there are any alterations on synaptic plasticity along with above NR2 composition switch caused by extrasynaptic NMDAR lateral diffusion. Among 21 neurons recorded, sEPSC frequency of seven neurons could almost completely recover to initial level (93.15 ± 5.97%; n = 7; p = 0.056, ANOVA and post hoc test) (Fig. 5 D). We conducted the following experiments on these neurons with full sEPSC frequency recovery. After precluding presynaptic factors affecting sEPSC recovery, full sEPSC frequency recovery usually indicates complete recovery of numbers of excitable postsynaptic NMDA receptors being activated. Then we used the standard LTP-producing stimulus protocol (2 Hz, 200 pulses during a 2.5 min depolarization to 0 mV), which was proven to be very effective in LTP induction to examine whether there were any changes in synaptic plasticity (Chen et al., 1999). Indeed, we can produce LTP of AMPAR-mediated EPSCs using this protocol (Fig. 5 E). However, we surprisingly observed that LTD of AMPAR-mediated EPSCs was induced after recovery from MK-801 block using the same stimulating protocol, which suggests a conversion of LTP into LTD (Fig. 5 B,D). In contrast, after complete sEPSC recovery from AP-5 block in terms of both NMDAR number and composition, standard LTP-producing stimulus can still induce LTP with a magnitude comparable with that observed in control (Fig. 5 A,C).
Conversion of synaptic plasticity direction is highly related to a change of NMDAR subunit composition after recovery from MK-801 block
Several possibilities could be responsible for the observed conversion after recovery from MK-801 block. One explanation is the incomplete recovery of total NMDA receptor function. Although the above conversion of synaptic plasticity direction was selectively conducted on neurons with full sEPSC frequency recovery, incomplete NMDARs function recovery cause by incomplete amplitude recovery could also be responsible for the above conversion. To examine this possibility, we adjust the concentration of the NMDA antagonist AP-5 to a level that could induce similar amplitude inhibition as the inhibition after recovery from MK-801 block (supplemental Fig. 5, available at www.jneurosci.org as supplemental material). d,l-AP-5 at a concentration of 5–8 μm could suppress NMDAR-mediated EPSCs by 18.40 ± 2.26%, similar to amplitude decrease after recovery from MK-801 block (20.76 ± 3.39%) (Fig. 6 A). Then we used the same LTP-producing stimulating protocol immediately after AP-5 application. In contrast to marked enhancement in control, neither potentiation nor depression of evoked EPSCs was observed in all our observations (Fig. 6 B). Our data suggest that alteration of synaptic plasticity direction does not ascribe to possible decrease of total NMDA receptor function. In addition, consistent with a previous report, partial blockade of NMDA currents by AP-5 to different levels governs polarity of synaptic plasticity (Nishiyama et al., 2000) (supplemental Fig. 5, available at www.jneurosci.org as supplemental material).
Another explanation is that the NMDAR subunit composition change after recovery is highly related to the observed conversion. NR2 subunit composition (NR2A/NR2B ratio) was proven as an important determinant for regulating the LTP/LTD crossover point (Philpot et al., 2001; Bear, 2003). Considering the difference of NR2 subunit composition after recovery from MK-801 block and from AP-5 block as well as the resulting differential regulation of synaptic plasticity, this conversion of synaptic plasticity direction could be highly related to a switch of NR2 subunit composition induced by extrasynaptic NR1/NR2B moving into synaptic sites through lateral diffusion, as the above data suggest. This assumption was further supported by experiments using the NR2B blocker ifenprodil. Perfusing slices with a low concentration of ifenprodil (0.6 μm) after recovery from MK-801 block could partially block NR2B-mediated EPSCs and increase the NR2A/NR2B ratio to a level closed to control. However, the conversion of synaptic plasticity direction from LTP to LTD was abolished and even reverted to short-term potentiation (STP) (Fig. 7). Moreover, neither LTP nor LTD was induced after full blockade of NR2B-mediated EPSCs with a saturated concentration of ifenprodil (3 μm) before LTP-producing stimulation (supplemental Fig. 6, available at www.jneurosci.org as supplemental material). In contrast, a low concentration of AP-5 (8–10 μm) with similar NMDAR-mediated EPSC inhibition did not block LTD induction (Fig. 7). These results further demonstrate that the synaptic NR2A/NR2B ratio is a key factor in determining the polarity of metaplasticity.
Differential regulation of synaptic plasticity was confirmed using field potential recording
All our above findings were obtained using whole-cell patch-clamp configuration. If NMDARs could move into synapses from extrasynaptic sites through lateral diffusion and cause changes in the rule of synaptic modification as suggested by above results, we should also make similar observations on extracellular field potential recording. Thus, we detect whether LTP of AMPAR-mediated EPSPs can be induced with a standard stimulating protocol (100 Hz, 1 s train) after recovery from AP-5 and MK-801 block, respectively. We got similar findings as we observed on whole-cell recording. LTP with a similar enhancement level to the control group was produced after 30 min AP-5 washout (Fig. 8 A,B). However, LTD instead of LTP was produced after recovery from MK-801 (Fig. 8 C). Moreover, full blockade of NR2B-mediated EPSCs with ifenprodil (3 μm) before tetanic stimulation abolished the observed conversion (supplemental Fig. 6, available at www.jneurosci.org as supplemental material). These data highly support our above findings on EPSCs, which strongly suggest that NMDAR lateral diffusion from extrasynaptic into synaptic sites was responsible for the observed change of synaptic plasticity direction.
Mechanism underlying NMDAR-mediated EPSC recovery
The recovery of NMDAR-mediated EPSCs from MK-801 block could be interpreted by several presynaptic and postsynaptic scenarios. In the present study, we preclude one by one those possibilities other than NMDAR lateral diffusion (see Results). Thus, the remaining likely scenario would be adding of new NMDA receptors into synaptic sites through receptor lateral diffusion along postsynaptic membrane plane from extrasynaptic sites in which NR2B outnumbers NR2A. Because NR2B-containing NMDAR-mediated EPSCs tends to have longer decay time, adding more NR2B into synaptic sites should cause prolongation of EPSCs. Indeed, our observation demonstrated this assumption. The recovery of spontaneous NMDAR-mediated EPSCs is activity independent because a similar recovery trend was observed under TTX (0.5 μm) application. Two recent reports from the same laboratory that show NMDAR surface diffusion by optical means highly support this finding that neuronal activity modified AMPAR but not NMDAR mobility (Groc et al., 2004, 2006).
It is noteworthy that our finding in NR2 subunit composition change is at odds with previous report on cultured autaptic hippocampal neurons (Tovar and Westbrook, 2002). In that study, Tovar and Westbrook found that the sensitivity of evoked EPSCs to ifenprodil before and after recovering from MK-801 block was similar. This inconsistency might ascribe to differences in the experiment protocol and the preparation used. First, the recording configuration we used here for EPSCs decay detection is perforated patching. In Figure 3, we show that frequency recovery of spontaneous EPSCs in whole-cell configuration is significantly lower than that in perforated patch. After occluding presynaptic contribution to observed EPSCs recovery, neurons with lower frequency recovery usually implicate lower recovery of synaptic NMDAR numbers. Actually, we also did not find any marked change on decay of NMDAR EPSCs recorded in the conventional whole-cell mode that was used by Tovar and Westbrook. According to our assumption, lower recovery means that fewer extrasynaptic NR2B “invade” into synaptic sites and lead to smaller change on EPSC kinetics that might not be easily detectable. Second, we should emphasize that the observation of the change of sEPSC decay only happened on the seven selected neurons with full recovery of spontaneous EPSC frequency on which LTP or LTD induction were conducted afterward (the neurons followed by LTP/LTD experiments in Fig. 5). When we pool all the data from 21 neurons, we also could not detect any alteration on sEPSC decay, which is consistent with results on evoked EPSCs by Tovar and Westbrook. Therefore, the level of NMDAR-mediated EPSC recovery primarily affects the change on current decay. Other factors such as the different preparations and stimulation protocols could also responsible for the inconsistency. Our findings on evoked EPSCs was also confirmed by spontaneous and miniature NMDAR-mediated EPSCs recordings that permit us to detect more subtle alteration on current kinetics. In addition, to reproduce the observation by Tovar and Westbrook on cultured autaptic hippocampal neurons, we made some adjustments on our experimental protocols conducted on slices (supplemental text, available at www.jneurosci.org as supplemental material). A recently investigation suggests that such trafficking does not occur at adult synapses (Harris and Pettit, 2007), similar to the conclusions of another recent paper (Bellone and Nicoll, 2007). Our adjustment on experimental protocols could address, at least in part, why no EPSC recovery was observed under their condition. Synapse maturation was referred to as a reason for occluding effects of synaptic NMDARs switching at adult synapses, i.e., all NR2 subunit changes have already occurred by maturation (Bellone and Nicoll, 2007). However, at 2 postnatal weeks, the alteration of NMDAR expression level has not yet arrived at its peak. The peak of NR2 switch usually occurs at approximately 3 postnatal weeks (Monyer et al., 1994). Therefore, the plasticity involving NR2 subunit switch could still occur at the age of animals we used here. Importantly, finding by both Nicoll's and our laboratories suggest that subunit-specific movement of NMDARs underlies switching of polarity of synaptic plasticity, which will broaden our knowledge on the role of NMDAR trafficking itself on synaptic plasticity.
NR2A/NR2B ratio is a determinative factor in NMDAR lateral diffusion-induced synaptic metaplasticity
The previous reports concerning the absolute role of NMDAR subtypes in synaptic plasticity are very controversial (Liu et al., 2004; Massey et al., 2004; Weitlauf et al., 2005; Bartlett et al., 2007; Morishita et al., 2007). We favor that the NR2A/NR2B ratio is a major factor in determining the polarity of metaplasticity caused by partial NMDAR inactivity. We artificially modulated the NR2A/NR2B ratio with the NR2B-specific blocker ifenprodil. After recovery from MK-801 block, a low concentration of ifenprodil (0.6 μm) partially blocked NR2B-mediated EPSCs and increased the NR2A/NR2B ratio to a level close to control. Interestingly, the conversion of synaptic plasticity direction from LTP to LTD was abolished and even reverted to STP. Moreover, neither LTP nor LTD was observed after full blockade of NR2B with a high concentration of ifenprodil (3 μm) (supplemental Fig. 6, available at www.jneurosci.org as supplemental material), probably attributable to high-level inhibition of total NMDAR function. In contrast, we observed a complete recovery on both EPSC amplitude and frequency after AP-5 washout without change on NR2 subunit composition. Subsequent LTP-producing stimulus protocol could still induce LTP. These results further demonstrate that synaptic NR2A/NR2B ratio is a key factor in determining the polarity of metaplasticity.
Then why should the continual normal movement of receptors between synaptic and extrasynaptic sites result in redistribution in favor of NR2B-containing NMDARs? The exchange between synaptic and extrasynaptic sites is bidirectional and keeps a dynamic equilibrium under the normal condition (Triller and Choquet, 2005). Surface mobility between NR2A and NR2B subunits are different and depend in part on the NMDAR subtypes with NR2A-containing NMDARs being more stable than NR2B-containing ones (Groc et al., 2006). We hypothesize that, after synaptic NR2A being blocked by MK-801, NR2B subunit tends to be more mobile and extrasynaptic NR2B could be more easily invade into synaptic sites, whereas inactivated synaptic NR2A is relatively more stable, resulting in more excitable NR2B versus NR2A in synaptic sites. This mechanism is different from the Bienenstock-Cooper-Munro theory reported previously, which assumes that a decrease in the NR2A/NR2B ratio is responsible for sliding the crossover point to the left and favors LTP over LTD (Philpot et al., 2001; Bear, 2003). This inconsistency might be attributable to different underlying mechanisms in different brain regions as a recent study suggested (Crozier et al., 2007). Many different mechanisms probably contribute to metaplasticity, as is also the case for synaptic plasticity. In the present study, the inactivity history by two different NMDAR antagonists displays a differential influence on subsequent EPSC recovery and synaptic plasticity direction, probably attributable to their different pharmacological properties in blocking NMDAR activity. AP-5 is a competitive antagonist and exerts its action via blocking NMDAR binding with agonist, whereas the noncompetitive antagonist MK-801 could irreversibly block open-state NMDAR channels.
A possible model of NMDA receptor trafficking
Increased sEPSC recovery level recorded in perforated patch mode compared with that recorded in conventional whole-cell mode suggests possible involvement of intracellular components in observed EPSC recovery. However, the results that no recovery after coapplication of NMDA and MK801 hints at no direct NMDAR insertion into synaptic sites. This inconsistency could be reconciled by a hypothesis that, during coapplication of NMDA with MK-801, NMDARs could insert in the extrasynaptic sites first. The number of inserted new NMDARs is very limited compared with numerous NMDARs already blocked in the whole-cell membrane. As a result, very few NMDARs could diffuse into synaptic sites, and most of them stay in extrasynaptic sites. Thus, we could not detect any obvious synaptic EPSC recovery. In contrast, when only synaptic NMDARs were blocked by MK-801, the source of unblocked NMDARs in extrasynaptic sites is abundant, and intracellular NMDARs insertion in extrasynaptic sites might contribute to this recovery. Therefore this “detour” through extrasynaptic sites could be a new model of NMDAR trafficking. The timescales of NMDAR lateral diffusion (usually occurring in seconds) and insertion from intracellular stores to the extrasynaptic cell surface (usually in minutes) match very well with timescale of the EPSC recovery observed in the present study. The recent findings of stable endocytic zones in regions lateral to the synapse/PSD of mature neurons suggest a requirement for lateral diffusion of receptors away from synapses. Although not yet identified, it is possible that specialized exocytic zones are also organized laterally to synapses (Lau and Zukin, 2007).
We should emphasize here that, to date, we have no direct evidence to demonstrate the insertion of NMDARs into extrasynaptic membrane. We also could not rule out other possibilities including that dialysis might affect the interaction of NMDAR with scaffold proteins, which might in turns greatly affect NMDAR surface trafficking and thus recovery after MK-801. Additional studies will be helpful to address the remaining questions.
This work was supported by National Natural Science Foundation of China Grant 30500145 and Young Investigator Natural Science Foundation of Jiangsu Province Grant BK2005424 (W.L.). We thank Dr. Guo-Qiang Bi at University of Pittsburgh, Dr. Li I. Zhang at University of Southern California, and Dr. Guo-song Liu at Tsing-hua University for their critical and beneficial comments on this manuscript. We also thank members of Dr. Shu-ming Duan's laboratory at Institute of Neuroscience at Chinese Academy of Sciences for their kind assistance on perforated-patch recording.
- Correspondence should be addressed to Dr. Wei Lu, Department of Neurobiology, Nanjing Medical University, Nanjing, Jiangsu Province 210029, China.
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