| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
1Interdepartmental Ph.D. Program for Neuroscience and 2Department of Physiology, David Geffen School of Medicine, University of California, Los Angeles, California
Submitted 27 February 2007; accepted in final form 29 March 2007
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
|---|
|
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
|---|
; Traub et al. 2004) and in pathophysiological conditions such as epilepsy (Jin et al. 2006; Smith and Dudek 2002). Within the hippocampus, recurrent excitatory synapses are most prominent between CA3 pyramidal cells (Ishizuka et al. 1990; Miles and Wong 1986, 1987) and the high degree of local circuit connectivity provided by these synapses is thought to enable the hippocampal CA3 region to function as a center for autoassociative memory (see Rolls and Kesner 2006 for review). In contrast, relatively few recurrent excitatory synaptic connections are thought to exist between pyramidal cells in the hippocampal CA1 region (Deuchars and Thomson 1996; Knowles and Schwartzkroin 1981). Instead, the output of CA1 pyramidal cells primarily regulates the activity of local inhibitory interneurons and neurons in brain structures outside the hippocampus (Amaral 1993; Cenquizca and Swanson 2006; Freund and Buzsaki 1996).
Hippocampal CA1 pyramidal cells are not devoid of recurrent excitatory synaptic connections however. Although much less common than in the CA3 region, recurrent synapses between CA1 pyramidal cells have been demonstrated in a number of studies (Aniksztejn et al. 2001; Christian and Dudek 1988; Crépel et al. 1995; Deuchars and Thomson 1996; Klishin et al. 1995; Mlinar et al. 2001; Radpour and Thomson 1991; Shao and Dudek 2004). Importantly, the properties of these synapses appear to be very different from Schaffer collateral/commissural fiber synapses arising from CA3 pyramidal cells. For example, although CA3 pyramidal cell synapses onto CA1 pyramidal cells exhibit robust paired-pulse facilitation, paired recordings of CA1-CA1 synapses have shown that recurrent synapses exhibit paired-pulse depression (Deuchars and Thomson 1996). This suggests that the basal probability of transmitter release is strikingly different at these two synapses. Moreover, recurrent excitatory synapses between CA1 pyramidal cells are more sensitive to the inhibitory effects of adenosine (Klishin et al. 1995) and serotonin (Mlinar et al. 2001, 2003), suggesting that Schaffer collateral and recurrent synaptic inputs onto CA1 pyramidal cells are uniquely regulated by different neuromodulators.
Like Schaffer collateral fiber synapses onto CA1 pyramidal cells, recurrent synaptic connections between CA1 pyramidal cells are mediated by both AMPA- and NMDA-type glutamate receptors (Crépel et al. 1997; Deuchars and Thomson 1996). The presence of NMDA receptors at recurrent synaptic connections suggests that these synapses may exhibit NMDA receptor-dependent forms of synaptic plasticity. Consistent with this, activity-dependent changes in the strength of recurrent synaptic connections between CA1 pyramidal cells are thought to have a crucial role in the generation of 
-frequency oscillations of neuronal activity in the hippocampal CA1 region (Traub et al. 2004; Whittington et al. 1997). Moreover, recurrent synapses between CA1 pyramidal cells are enhanced in experimental models of epilepsy (Shao and Dudek 2004; Smith and Dudek 2001, 2002). Thus activity-dependent changes in synaptic strength at recurrent synapses in the CA1 region are likely to be involved in both normal hippocampal function and in pathophysiological states. It is not yet known, however, whether recurrent synapses in the CA1 region exhibit long-term potentiation and/or long-term depression. Moreover, the cellular mechanisms that might underlie either long or short-term forms of plasticity at these synapses are unclear. This is most likely due to the fact that the relatively sparse synaptic connections between CA1 pyramidal cells make experimental studies of these synapses exceedingly difficult.
Here we describe the results of studies examining plasticity at synaptic connections between CA1 pyramidal cells. We find that under certain experimental conditions Schaffer collateral/commissural fiber activation can elicit surprisingly robust polysynaptic excitatory postsynaptic potentials (EPSPs) due to activation of recurrent connections between CA1 pyramidal cells. To determine whether recurrent synapses onto CA1 pyramidal cells exhibit short-term forms of synaptic plasticity, we examined how activating networks of interconnected pyramidal cells in the hippocampal CA1 region with different patterns of Schaffer collateral/commissural fiber stimulation modulates polysynaptic EPSPs. In contrast to CA3 pyramidal cell inputs, recurrent synapses onto CA1 pyramidal cells exhibit strong, GABAB receptor-mediated paired-pulse depression. In addition, short trains of low-frequency stimulation (LFS) that have no effect on transmission at Schaffer collateral fiber synapses induced a profound, but rapidly reversible, depression of polysynaptic EPSPs that was mediated, in part, by activation of A1 type adenosine receptors. Our results thus indicate that CA1-CA1 recurrent synapses exhibit forms of short-term synaptic plasticity that are strikingly different from CA3 pyramidal cell synapses onto CA1 pyramidal cells.
|
METHODS |
|---|
|
TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
|---|
Standard techniques were used to prepare 400 µm thick hippocampal slices from hippocampi obtained from halothane anesthetized, 8 –16 wk old, male C57Bl/6 mice (Charles River Laboratories, Wilmington, MA). All techniques were approved by the UCLA Institutional Animal Care and Use Committee. Briefly, transverse slices were cut approximately parallel to the alvear fibers using a manual tissue chopper and the CA3 region was completely removed. Slices were then maintained at 30°C in an interface-slice type recording chamber perfused (
2–3 ml/min) with an oxygenated (95% O2-5% CO2) artificial cerebrospinal fluid (ACSF) containing 124.0 mM NaCl, 4.4 mM KCl, 25 mM NaHCO3, 1.0 mM NaH2PO4, 2.0 mM CaCl2, 1.2 mM MgSO4, and 10 mM glucose and allowed to recover for 
1 h prior to an experiment. All experiments were done at 30°C. For recordings, a slice was transferred into a submerged-slice recording chamber perfused with ACSF containing 100 µM picrotoxin to block GABAA receptor-mediated inhibitory synaptic transmission. A bipolar, nichrome wire stimulating electrode was placed in stratum radiatum of the hippocampal CA1 region to activate Schaffer collateral/commissural fiber synapses (hereafter referred to simply as Schaffer collateral fibers) and an extracellular glass microelectrode filled with ACSF (resistance = 5–10 M
) was used to record field excitatory postsynaptic potentials (fEPSPs). At the start of each experiment the strength of presynaptic fiber stimulation was adjusted to evoke fEPSPs with durations of 50 ms from onset (stimulation rate = 0.02 Hz).
Whole cell current clamp recordings and data analysis
Whole cell current clamp recordings were performed using low resistance (2 –6 M) patch electrodes filled with a solution containing 120.0 mM potassium gluconate, 20.0 mM KCl, 2.0 mM MgCl2, 4.0 mM Na2-ATP, 0.3 mM Tris-GTP, 14 mM phosphocreatine, and 10 mM HEPES (pH = 7.3). If needed, constant injections of hyperpolarizing current were used to maintain membrane potentials between –65 and –75 mV and 50 ms long pulses of hyperpolarizing current (0.1 nA) were injected every 20 s to monitor both series and input resistance. In most experiments we attempted to adjust the intensity of Schaffer collateral fiber stimulation to a level that evoked polysynaptic EPSPs that were below threshold for postsynaptic action potential generation. In many cells, however, robust polysynaptic EPSPs that evoked action potentials were elicited by even the lowest stimulation strengths that evoked polysynaptic responses. In all figures shown below action potentials are truncated for clarity. During baseline recordings presynaptic fiber stimulation pulses were delivered once every 20 s.
Data acquisition and analysis were performed using either the Experimenter's Workbench/Common Processing software package from DataWave Technologies (Longmont, CO) or PClamp (Molecular Devices, Sunnyvale, CA). In both field and whole cell recordings we used the area under the postsynaptic response (response onset to return to resting membrane potential) to measure the magnitude of polysynaptic EPSPs. In some experiments the initial rising slope of EPSPs was used to measure the magnitude of monosynaptic inputs. Paired and unpaired t-test or, where appropriate, one-way repeated measure ANOVAs followed by Bonferroni t-test for repeated comparisons to control were used to determine statistical significance.
8-cyclopentyl-1,3-dipropylxanthine (DPCPX), CGP55845, LY341495, SCH23390, isomoltane hemifumarate, CP93129 and D-APV were obtained from Tocris Bioscience (Ellisville, MO). All other compounds were obtained from Sigma-Aldrich (St. Louis, MO).
|
RESULTS |
|---|
|
TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
|---|
We first examined whether recurrent excitatory connections between hippocampal CA1 pyramidal cells could be reliably detected using extracellular recordings in slices where fast inhibitory synaptic transmission was blocked. Consistent with previous reports (Crépel et al. 1997), high-intensity Schaffer collateral fiber stimulation elicited prolonged fEPSPs in slices bathed in normal ACSF containing 100 µM picrotoxin and these responses were strongly depressed when polysynaptic transmission was suppressed by bath application of a high-Ca2+/Mg2+ ACSF containing 4.0 mM CaCl2 and 6.0 mM MgSO4 (Fig. 1A). To explore the potential contribution of polysynaptic EPSPs in these recordings in more detail we next examined whether two previously identified unique properties of CA1 recurrent synapses can be detected in extracellularly recorded responses elicited by Schaffer collateral fiber stimulation in dis-inhibited slices. First, paired recordings of CA1 to CA1 excitatory synapses have shown that recurrent synapses onto CA1 pyramidal cells exhibit pronounced paired-pulse depression (Deuchars and Thomnson 1996). Thus we measured fEPSPs elicited by pairs of presynaptic fiber stimulation pulses delivered with inter-pulse intervals of 200 –800 ms. As shown in Fig. 1B, the prolonged fEPSPs elicited in the absence of inhibitory synaptic transmission exhibited paired-pulse depression at all intervals tested. Second, synaptic transmission at recurrent synapses onto CA1 pyramidal cells is facilitated during low-frequency trains of presynaptic action potentials (Radpour and Thomson 1991). Thus as another test of the potential contribution of polysynaptic EPSPs to responses elicited by Schaffer collateral fiber stimulation in dis-inhibited slices we examined whether the area of fEPSPs were enhanced during short trains of LFS. Surprisingly, we found that synaptic responses underwent a rapid-onset but modest depression at the start of a 30 s long train of 1 Hz that persisted for approximately 
10 s (fEPSP area was reduced to 83 ± 2% of baseline, n = 13, P < 0.001 compared with baseline). A slower onset, but more profound suppression then developed throughout the rest of the train (after 30 s of 1 Hz stimulation fEPSP area was reduced to 51 ± 3% of baseline, P < 0.001 compared with baseline). LFS did not, however, induce a lasting depression of fEPSP area and responses rapidly returned to baseline levels (5 min post1Hz stimulation fEPSP area was 94 ± 3% of baseline). Thus while the inhibition of fEPSP area induced by high-Ca2+/Mg2+ACSF and the presence of paired-pulse depression are consistent with the idea that polysynaptic EPSPs contribute to postsynaptic responses generated by Schaffer collateral fiber stimulation when inhibitory synaptic transmission is blocked, the LFS-induced suppression of postsynaptic responses appears to be inconsistent with previously identified properties of recurrent excitatory synaptic connections between CA1 pyramidal cells.
|
Although extracellular recordings have been used by others to study polysynaptic EPSPs in the hippocampal CA1 region (Crépel et al. 1997), the postsynaptic responses recorded with extracellular electrodes under our experimental conditions are complex and not only reflect EPSPs but also postsynaptic cell firing. These responses are thus difficult to interpret and the potential contribution of polysynaptic EPSPs to the evoked responses is unclear. Because extracellular recordings seemed less then ideal for studying recurrent synaptic connections between CA1 pyramidal cells we turned to whole cell current-clamp recordings from individual CA1 pyramidal cells to examine whether Schaffer collateral fiber stimulation can elicit polysynaptic responses in CA1 pyramidal cells. As shown in Fig. 2 A, intensities of presynaptic fiber stimulation that elicited monosynaptic EPSPs under control conditions elicited numerous polysynaptic EPSPs following bath application of picrotoxin. In addition, the latency to peak depolarization of the polysynaptic responses was decreased by increases in presynaptic fiber stimulation intensity (Fig. 2B), a hallmark of polysynaptic EPSPs recorded in the hippocampal CA1 region (Crépel et al. 1997; Mlinar et al. 2001). In the presence of picrotoxin Schaffer collateral fiber stimulation-evoked polysynaptic EPSPs like those shown in Fig. 2, A and B were surprisingly common and observed in virtually every cell tested (126 of 135 cells).
|
; Commins et al. 2002; Harris et al. 2001; Knopp et al. 2005) but are instead due to local excitatory connections between CA1 pyramidal cells. Moreover, activation of 5-HT1B receptors with CP93129 (10 µM), which selectively inhibits local excitatory synapses between CA1 pyramidal cells (Mlinar et al. 2001, 2003), strongly suppressed polysynaptic EPSPs elicited by Schaffer collateral fiber stimulation (Fig. 2C). Finally, bath application of high-Ca2+/Mg2+ ACSF completely suppressed polysynaptic responses leaving only monosynaptic EPSPs (Fig. 2D). Thus in agreement with previous findings (Crépel et al. 1997; Mlinar et al. 2001, 2003), surprisingly robust network-driven polysynaptic EPSPs due to CA1-CA1 recurrent synaptic connections can be elicited in hippocampal CA1 pyramidal cells when GABAA receptor-mediated inhibitory synaptic transmission is blocked.
Paired recordings searching for CA1-CA1 pyramidal cell synaptic connections have revealed very low connection rates ranging from 0 to 1% (Deuchars and Thomson 1996Knowles and Schwartkroin 1981) and it was thus surprising that such robust polysynaptic EPSPs could be so readily elicited by Schaffer collateral fiber stimulation in our experiments. One feature of our experimental conditions that could facilitate CA1 pyramidal cell firing in response to Schaffer collateral fiber activation, and thus enhance detection of polysynaptic EPSPs due to recurrent synapses, is the somewhat high concentrations of KCl (4.4 mM) used in our ACSF. We thus performed a limited number of experiments comparing postsynaptic responses elicited by Schaffer collateral fiber stimulation in cells first bathed in a modified ACSF containing 2.4 mM KCl and then bathed in ACSF containing 4.4 mM KCl (100 µM picrotoxin present in both conditions). As shown in Fig. 2E, clear polysynaptic EPSPs were rarely observed in cells bathed in ACSF containing 2.4 mM KCl while robust polysynaptic EPSPs could be detected following application of ACSF containing 4.4 mM KCl. This suggests that slightly elevated concentrations of extracellular K+ facilitates detection of EPSPs due to recurrent CA1-CA1 pyramidal cell synapses; most likely because it increases CA1 pyramidal cell excitability and thus enhances CA1 pyramidal cell firing in response to activation of Schaffer collateral fiber inputs.
Short-term plasticity at recurrent synapses onto CA1 pyramidal cells
To determine whether recurrent excitatory synapses in the hippocampal CA1 region exhibit short-term forms of synaptic plasticity we examined the effects of pairs of presynaptic fiber stimulation pulses delivered with inter-pulse intervals of 200 –800 ms. Consistent with the results from our extracellular recording experiments, polysynaptic EPSPs exhibited robust paired-pulse depression at all inter-pulse intervals tested (Fig. 3A). Indeed, at the shortest inter-pulse interval tested polysynaptic EPSPs appeared to be completely suppressed and only monosynaptic EPSPs were evoked by the second stimulation pulse (Fig. 3A). In contrast, monosynaptic Schaffer collateral fiber inputs onto CA1 pyramidal cells recorded in slices where polysynaptic EPSPs were blocked by bath application of high-Ca2+/Mg2+ ACSF exhibited no paired-pulse depression over these same inter-pulse intervals (Fig. 3B).
|
|
) we next examined whether both paired-pulse and LFS-induced depression of polysynaptic EPSPs could be observed under ionic conditions that more closely match those found in vivo. As shown in Fig. 4, E and F, polysynaptic EPSPs were strongly depressed during a 30 s long train of 1 Hz stimulation in slices bathed in a "physiological" ACSF containing 1.2 mM Ca2+, 1.0 Mg2+, and 3.5 mM K+ (Sanchez-Vives and McCormick 2000). In contrast, the initial slope of the postsynaptic EPSPs was unchanged during 1 Hz stimulation (Fig. 4F). Thus as we observed in cells bathed in our standard ACSF, a short train of 1 Hz stimulation has no effect on the monosynaptic, Schaffer collateral fiber inputs onto CA1 pyramidal cells bathed in physiological ACSF. We also observed robust paired-pulse depression of polysynaptic EPSPs in slices bathed in physiological ACSF. In these experiments the area of postsynaptic responses elicited by the second of two Schaffer collateral fiber stimulation pulses delivered with an inter-pulse interval of 500 ms was reduced 20 ± 4% of the first response in cells bathed in physiological ACSF (n = 9) and was reduced to 22 ± 5% of the first response in cells bathed in our standard ACSF (n = 8). Together, these findings indicate that recurrent synapses between CA1 pyramidal cells exhibit robust activity-dependent depression under ionic conditions that mimic those found in vivo. Paired-pulse depression of recurrent synapses is mediated by GABAB receptor activation
Activation of inhibitory interneurons by Schaffer collateral fiber stimulation can elicit GABAB receptor-mediated slow inhibitory postsynaptic potentials (IPSPs) in CA1 pyramidal cells that can last for several hundred milliseconds and strongly inhibit pyramidal cell firing (Dutar and Nicoll 1988). Because the polysynaptic EPSPs evoked by Schaffer collateral fiber stimulation arise from a network of excitatory connections between CA1 pyramidal cells, it seemed likely that decreases in CA1 pyramidal cell firing due to slow IPSPs could importantly contribute to activity-dependent changes in recurrent synaptic connections between CA1 pyramidal cells. Indeed, in most cells a small, but clear, hyperpolarization lasting hundreds of milliseconds was present following polysynaptic EPSPs evoked by Schaffer collateral fiber stimulation (Fig. 5A). Consistent with the identification of these hyperpolarizations as slow IPSPs, they were completely abolished by the GABAB receptor antagonist CGP55845 (Fig. 5A). Thus to determine whether slow IPSPs contribute to paired-pulse depression of polysynaptic EPSPs we next examined the effects of bath applied CGP55845 (5 µM) on the paired-pulse depression of polysynaptic EPSPs induced by pairs of Schaffer collateral fiber stimulation pulses (inter-pulse interval = 500 ms) delivered once every 20 s. As shown in Fig. 5, B and C, CGP55845 (5 µM) completely abolished paired-pulse depression of polysynaptic EPSPs. Paired-pulse ratios (area of the 2nd response/area of the 1st response x 100) were 25 ± 5% during baseline and were 123 ± 11% 5 min post CGP55845 application (n = 8, P < 0.001). Paired-pulse depression of polysynaptic EPSPs induced by pairs of stimulating pulses delivered with inter-pulse intervals of 200 to 800 ms was also blocked in slices continuously bathed in ACSF containing 5 µM CGP55845 (Fig. 6, A and B).
|
|
; Lanthorn and Cotman 1981; Olpe et al. 1982), suggesting thatpresynaptic GABAB receptor activation might also contribute to paired-pulse depression of polysynaptic EPSPs. Indeed, the time course of paired-pulse depression of polysynaptic EPSPs was considerably longer than the time course of slow IPSPs (Fig. 7A), suggesting that slow IPSPs alone cannot account for paired-pulse depression of polysynaptic EPSPs. We thus next examined whether selectively blocking only the postsynaptic effects of GABAB receptor activation with the G-protein-activated, inwardly rectifying potassium (GIRK) channel blocker SCH23390 (Kuzhikandathil and Oxford 2002) mimicked the effects of CGP55845 on paired-pulse depression of polysynaptic EPSPs. Consistent with previous reports (Huang et al. 2005), SCH23390 (20 µM) strongly inhibited slow IPSPs recorded in cells where fast IPSPs were blocked with picrotoxin (100 µM) and a combination of CNQX (25 µM) and D-APV (50 µM) was used to suppress excitatory synaptic transmission (Fig. 7B). SCH23390 had no effect, however, on paired-pulse depression of polysynaptic EPSPs (Fig. 7C). Thus while blocking all GABAB receptors abolishes paired-pulse of depression of polysynaptic EPSPs, selectively blocking the effects of postsynaptic GABAB receptor activation has no effect. This indicates that much of the paired-pulse depression of polysynaptic EPSPs is due to activation of presynaptic GABAB receptors on recurrent synapses.
|
To investigate whether GABAB receptor activation also contributes to the suppression of recurrent synapses during LFS we next examined the effects of blocking GABAB receptors with CGP55845 on the depression of EPSP area induced by a 30 s long train of 1 Hz stimulation. As shown in Fig. 8, A and B, the initial depression of EPSP area that occurs during the first 10 s of 1 Hz stimulation was significantly attenuated in cells bathed in ACSF containing 5 µM CGP55845. However, the more profound suppression of EPSP area that occurs later in the train was completely unaffected by CGP55845 (at the end of 1 Hz stimulation EPSP area was reduced to 10 ± 2% of baseline in control cells, n = 6, compared with 8 ± 3% of baseline in cells bathed in ACSF containing 5 µM CGP55845, n = 10). Thus while GABAB receptor activation appears to be responsible for the initial component of the depression of recurrent synapses onto CA1 pyramidal cells induced by 1 Hz stimulation, it has no role in the delayed component of the suppression that occurs later during the 1 Hz train.
|
) we next examined whether mGluR activation might contribute to LFS-induced depression of polysynaptic EPSPs. Bath application of the broad spectrum mGluR antagonist LY341495 (20 µM) had no effect on LFS-induced depression (n = 8, data not shown), suggesting that mGluRs are not involved. Blocking mGluRs with LY341495 also had no effect on paired-pulse depression of polysynaptic EPSPs (n = 6, data not shown). Based on previous studies showing that activation of 5-HT1B (Mlinar et al. 2001, 2003) and A1 type adenosine receptors (Klishin et al. 1995) can potently suppress recurrent synapses onto CA1 pyramidal cells we also examined whether 5-HT1B or adenosine receptor activation might contribute to activity-dependent changes in synaptic transmission at these synapses. Bath application of the 5-HT1B receptor antagonist isomaltane (10 µM) had no effect on the depression of polysynaptic EPSPs during 1 Hz stimulation (n = 7, data not shown), suggesting that serotonin release does not contribute to the suppression of recurrent synapses during low-frequency stimulation. Blocking adenosine receptors with the A1 receptor selective antagonist DPCPX (200 nM) had no effect on paired-pulse depression of polysynaptic EPSPs (n = 7, data not shown) and also had no effect on the early component of the synaptic depression induced by 1 Hz stimulation (Fig. 8, C and D). The late component of the suppression induced by 1 Hz stimulation was, however, strongly attenuated by DPCPX. As shown in Fig. 8, C and D, EPSP area was reduced to 6 ± 2% of baseline at the end of 1 Hz stimulation in control cells (n = 5) while EPSP area was reduced to only 44 ± 7% of baseline in the presence of DPCPX (n = 8, P < 0.001 compared with control). This indicates that a significant component of the late phase of LFS-induced depression of recurrent synapses onto CA1 pyramidal cells is due to accumulation of extracellular adenosine and activation of A1 type adenosine receptors. Inhibiting GIRK channels with SCH23390 (20 µM) had no effect on 1 Hz stimulation-induced suppression of polysynaptic EPSPs (n = 9) (Fig. 8, E and F). This suggests that the effects of GABA and adenosine are not due to decreases in network excitability caused by activation of postsynaptic, GIRK channel-coupled receptors but are instead mediated by presynaptic receptors. Thus while activation of presynaptic GABAB and A1 receptors contributes to the depression of recurrent synapses during LFS, our results indicate that GABA and adenosine mediate temporally distinct phases of the suppression of recurrent synapses during 1-Hz stimulation. |
DISCUSSION |
|---|
|
TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
|---|
; Knowles and Schwartkroin 1981), rough estimates suggest that a given CA1 pyramidal cell could receive potentially dozens of synaptic inputs from other CA1 pyramidal cells in a transverse hippocampal slice (Traub et al. 2004). Consistent with this, we find that polysynaptic EPSPs due to recurrent synaptic connections can be readily evoked in CA1 pyramidal cells by Schaffer collateral fiber stimulation in slices where fast GABAA receptor-mediated inhibitory synaptic transmission is blocked, especially when neuronal excitability is enhanced by modestly elevated levels of extracellular K+. Activity-dependent modulation of recurrent synaptic connections between CA1 pyramidal cells
Using these experimental conditions, we find that polysynaptic EPSPs elicited by Schaffer collateral fiber stimulation exhibit robust paired-pulse depression. The fact that presynaptic fiber stimulation strengths that elicited polysynaptic EPSPs also evoked slow IPSPs initially suggested to us that paired-pulse depression could result from slow IPSPs elicited by the first stimulation pulse that suppress CA1 pyramidal cell firing in response to subsequent pulses of presynaptic fiber stimulation. Consistent with this, blocking slow IPSPs with the GABAB receptor blocker CGP55845 completely abolished paired-pulse depression of polysynaptic EPSPs. However, inhibiting sIPSPs with the GIRK channel blocker SCH23390 had no effect on paired-pulse depression. Thus rather than reflecting a network level phenomenon arising from slow IPSP-induced changes in CA1 pyramidal cell excitability, our results suggest that the paired-pulse depression is an intrinsic property of synaptic transmission at recurrent synapses mediated by presynaptic GABAB receptors. Our results do not, however, rule out the possibility that postsynaptic GABAB receptor-mediated slow IPSPs might also be involved, especially at shorter inter-pulse intervals where the slow IPSP is most robust.
Polysynaptic EPSPs due to recurrent synapses also underwent a profound short-term depression in response to short trains of 1-Hz Schaffer collateral fiber stimulation that exhibited at least two temporally and pharmacologically distinct phases. At the start of the LFS train there was a nearly immediate, but modest, suppression of polysynaptic EPSPs that persisted from 5–10 s. This initial depression was then followed by a slower onset but more profound suppression of polysynaptic EPSPs such that by the end of the 1-Hz train, only monosynaptic EPSPs remained. Blocking GABAB receptors completely abolished the initial phase of the LFS-induced depression suggesting that the cellular mechanisms responsible for this component of the depression are most likely the same as those that responsible for paired-pulse depression, i.e., decreases in glutamate release due to activation of presynaptic GABAB receptors. The slower, but more robust, phase of synaptic depression, however, was completely insensitive to CGP55845 and instead significantly inhibited by the A1 adenosine receptor antagonist DPCPX.
The effects of DPCPX on LFS-induced depression of polysynaptic EPSPs suggests that adenosine accumulation during 1-Hz stimulation is responsible for a large component of the late phase of LFS-induced depression. Previous studies have shown that activity-dependent increases in adenosine and activation of presynaptic A1 receptors can inhibit transmitter release at Schaffer collateral fiber inputs onto CA1 pyramidal cells, although only during much higher frequencies of synaptic stimulation than those used in our experiments (Brager and Thompson 2003; Manzoni et al. 1994; Mitchell et al. 1993). We find, however, that monosynaptic EPSPs elicited by Schaffer collateral fiber stimulation are not inhibited by short trains of 1-Hz stimulation. Our results thus suggest that recurrent synapses are much more sensitive to the inhibitory effects of adenosine than Schaffer collateral fiber synapses and can be strongly suppressed by activity-dependent increases in extracellular adenosine levels that are below that needed to significantly effect transmission at Schaffer collateral fiber synapses. Although the molecular mechanisms responsible for this differential sensitivity to adenosine are unclear, our results are consistent with the finding that under some experimental conditions ambient levels of extracellular adenosine are sufficient to completely suppress recurrent synapses in the hippocampal CA1 region (Klishin et al. 1995). In addition, although presynaptic GABAB receptor activation can inhibit transmission at Schaffer collateral fiber synapses (Isaacson et al. 1993; Lanthorn and Cotman 1981; Olpe et al. 1982), we find that pairs of presynaptic fiber stimulation pulses delivered with interpulse intervals of several 100 ms have no effect on monosynaptic EPSPs but induce a GABAB receptor-mediated paired-pulse depression of transmission at recurrent synapses. Recurrent synapses onto CA1 pyramidal cells thus also appear to be more sensitive to the inhibitory effects of GABA than Schaffer collateral fiber inputs.
Although our results clearly show that A1 adenosine receptor activation has an important role in LFS-induced depression of recurrent synapses a number of important questions remain unresolved. First, the source of adenosine responsible for modulating recurrent synapses during 1-Hz stimulation is unclear. One possibility is that Schaffer collateral fiber stimulation elicits adenosine release from glial cells (Pascual et al. 2005; Serrano et al. 2006). However, Schaffer collateral fiber stimulation-induced adenosine release from glial cells requires GABA release from inhibitory interneurons and subsequent activation of astrocyte GABAB receptors (Serrano et al. 2006). Thus our observation that blocking GABAB receptors with CGP55845 has little effect on LFS-induced depression of polysynaptic EPSPs suggests that adenosine release from glial cells is unlikely to have an important role. Other potential sources of adenosine, such as ATP released as a co-transmitter from excitatory synapses and its conversion to adenosine by extracellular 5'-ectonucleotidase (Dunwiddie et al. 1997; Masino et al. 2002) or the transport of adenosine out of pyramidal cells (Brundege and Dunwiddie 1996) appear to be more likely candidates. A second unresolved issue stems from the fact that blocking adenosine receptors only prevents a portion of LFS-induced suppression of polysynaptic EPSPs in CA1 pyramidal cells. This indicates that additional, and as yet unknown, mechanisms also importantly contribute to LFS-induced depression at recurrent synapses. Paired recordings of CA1–CA1 synapses have demonstrated that recurrent synapses exhibit paired-pulse depression (Deuchars and Thomson 1996), suggesting that the probability of transmitter release at these synapse is high. Thus the LFS-induced depression that remains in the presence of DPCPX may simply reflect depletion of the readily releasable pool of synaptic vesicles. Alternatively, other neuromodulators, such as endocannabinoids (Ohno-Shosaku et al. 2002; Staiker and Mackie 2005) might also be involved. Finally, it is surprising that we observed such robust LFS-induced depression of recurrent synapses during LFS. whereas a previous study of recurrent synapses onto CA1 pyramidal cells found that these synapses exhibit frequency-dependent facilitation during 1-Hz stimulation (Radpour and Thomson 1991). Importantly, recurrent synapses were activated via Schaffer collateral fiber stimulation in our experiments while minimal presynaptic fiber stimulation techniques were used in the study by Radpour and Thomason (1991). It thus seems likely that the much larger population of CA1 pyramidal cells activated by the stimulation techniques used in our experiments facilitates accumulation of extracellular adenosine and other neuromodulators involved in LFS-induced depression.
Functional implications of activity-dependent depression at recurrent synapses onto CA1 pyramidal cells
The ability of certain patterns of synaptic activity to strongly depress recurrent synapses without affecting Schaffer collateral fiber synapses has potentially important implications for both normal and pathophysiological processes in the hippocampus. For instance, short-term depression at recurrent synapses may uncouple CA1 pyramidal cells during certain patterns of neuronal activity and thus prevent synchronous bursting that could lead to epileptiform-like activity. Indeed, recurrent synapses between CA1 pyramidal cells are increased in experimental models of epilepsy (Shao and Dudek 2004; Smith and Dudek 2001, 2002), and the density of both A1 adenosine receptors and GABAB receptors in the hippocampus is reduced in some forms of epilepsy (Glass et al. 1996; Rebola et al. 2003; Straessle et al. 2003). In normal hippocampal function, the selective suppression of recurrent synapses during periods of low-frequency neuronal activity may transform the CA1 region from a sparsely connected recurrent network into a predominately feedforward circuit. LFS-induced and paired-pulse depression at recurrent synapses onto pyramidal cells may thus provide an activity-dependent mechanism for altering the configuration of microcircuits in the hippocampal CA1 region and thereby strongly influence information processing in the hippocampus. Our results indicate that two different neurotransmitters, GABA and adenosine, have a crucial role in this process yet operate over very different temporal domains, i.e., GABAB receptor activation suppresses recurrent synapses over hundreds of milliseconds, whereas the suppression of recurrent synapses during low-frequency trains of presynaptic action potentials lasting many seconds is largely mediated by adenosine receptor activation. Interestingly, downregulation of presynaptic voltage-gated calcium channels by 
subunits of heterotrimeric G proteins is thought to underlie the inhibition of synaptic transmission by a number of neurotransmitters (Tedford and Zamponi 2006), including GABA and adenosine (Wu and Saggau 1994, 1995). Because protein kinase C (PKC)-dependent phosphorylation of calcium channels can inhibit the ability of subunits to downregulate channel activity (see Tedford and Zamponi 2006 for review), neurotransmitters acting through receptors coupled to PKC activation could strongly inhibit the GABA and adenosine-mediated suppression of recurrent synapses during LFS. Indeed, PKC activation attenuates the ability of adenosine and GABA to inhibit voltage-gated calcium channels in hippocampal neurons (Swartz 1993) and blocks the ability of both adenosine (Thompson et al. 1992) and GABA (Thompson and Gäwiler 1992) to inhibit excitatory synaptic transmission in the hippocampus (Thompson et al. 1992). Thus activity-dependent depression at recurrent synapses between CA1 pyramidal cells may not only represent a mechanism that allows different patterns of neuronal activity to reconfigure the circuitry of the hippocampal CA1 region but also represent a target where modulatory neurotransmitters acting through receptors coupled to PKC activation can act to alter information processing in the hippocampus.
|
GRANTS |
|---|
|
TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
|---|
|
ACKNOWLEDGMENTS |
|---|
|
TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
|---|
|
FOOTNOTES |
|---|
Address for reprint requests and other correspondence: T. J. O'Dell, Dept. of Physiology, David Geffen School of Medicine at UCLA, 53-231 Center for the Health Sciences, Box 951751, Los Angeles, CA 90095-1751 (E-mail: todell{at}mednet.ucla.edu)
|
REFERENCES |
|---|
|
TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
|---|
Aniksztejn L, Demarque M, Morozov Y, Ben-Ari Y, Represa A. Recurrent CA1 collateral axons in developing rat hippocampus. Brain Res 913: 195–200, 2001.[CrossRef][Web of Science][Medline]
Anwyl R. Metabotropic glutamate receptors: electrophysiological properties and role in plasticity. Brain Res Rev 29: 83–120, 1999.[CrossRef][Medline]
Berger TW, Swanson GW, Milner TA, Lynch GS, Thompson RF. Reciprocal anatomical connections between hippocampus and subiculum in the rabbit: evidence for subicular innervation of region superior. Brain Res 183: 265–276, 1980.[CrossRef][Web of Science][Medline]
Brager DH, Thompson SM. Activity-dependent release of adenosine contributes to short-term depression at CA3-CA1 synapses in rat hippocampus. J Neurophysiol 89: 22–26, 2003.
Brundege JM, Dunwiddie TV. Modulation of excitatory synaptic transmission by adenosine release from single hippocampal pyramidal neurons. J Neurosci 16: 5603–5612, 1996.
Cenquizca LA, Swanson LW. Analysis of direct hippocampal cortical field CA1 projections to diencephalon in the rat. J Comp Neurol 497: 101–114, 2006.[CrossRef][Web of Science][Medline]
Christian EP, Dudek FE. Electrophysiological evidence from glutamate microapplications for local excitatory circuits in the CA1 area of rat hippocampal slices. J Neurophysiol 59: 110–123, 1998.[CrossRef]
Commins S, Aggleton JP, O'Mara SM. Physiological evidence for a possible projection from dorsal subiculum to hippocampal area CA1. Exp Brain Res 146: 155–160, 2002.[CrossRef][Web of Science][Medline]
Crépel V, Khazipov R, Ben-Ari Y. Blocking GABAA inhibition reveals AMPA- and NMDA-receptor-mediated polysynaptic responses in the CA1 region of the rat hippocampus. J Neurophysiol 77: 2971–2082, 1997.
Deuchars J, Thomson AM. CA1 pyramid-pyramid connections in rat hippocampus in vitro: dual intracellular recordings with biocytin filling. Neurosci 74: 1009–1018, 1996.[Web of Science][Medline]
Dunwiddie TV, Diao L, Proctor WR. Adenine nucleotides undergo rapid, quantitative conversion to adenosine in the extracellular space in rat hippocampus. J Neurosci 17: 7673–7682, 1997.
Dutar P, Nicoll RA. A physiological role for GABAB receptors in the central nervous system. Nature 332: 156–158, 1988.[CrossRef][Medline]
Freund TF, Buzsaki G. Interneurons of the hippocampus. Hippocampus 6: 347–470, 1996.[CrossRef][Web of Science][Medline]
Glass M, Faull RLM, Bullock JY, Jansen K, Mee EW, Walker EB, Synek BJL, Dragunow M. Loss of A1 adenosine receptors in human temporal lobe epilepsy. Brain Res 710: 56–68, 1996.[CrossRef][Web of Science][Medline]
Harris E, Witter MP, Weinstein G, Stewart M. Intrinsic connectivity of the rat subiculum. I. Dendritic morphology and patterns of axonal arborization by pyramidal neurons. J Comp Neurol 435: 490–505, 2001.[CrossRef][Web of Science][Medline]
Huang CS, Shi S-H, Ule J, Ruggiu M, Barker LA, Darnell RB, Jan YN, Jan LY. Common molecular pathways mediate long-term potentiation of synaptic excitation and slow synaptic inhibition. Cell 123: 105–118, 2005.[CrossRef][Web of Science][Medline]
Isaacson JS, Solis JM, Nicoll RA. Local and diffuse synaptic actions of GABA in the hippocampus. Neuron 10: 165–175, 1993.[CrossRef][Web of Science][Medline]
Ishizuka N, Weber J, Amaral DG. Organization of intrahippocampal projections originating from CA3 pyramidal cells in the rat. J Comp Neurol 295: 580–623, 1990.[CrossRef][Web of Science][Medline]
Jin X, Prince DA, Huguenard JR Enhanced excitatory synaptic connectivity in layer V pyramidal neurons of chronically injured epileptogenic neocortex in rats. J Neurosci 26: 4891–4900, 2006.
Klishin A, Tsintsadze T, Lozovaya N, Krishtal O. Latent N-methyl-D-aspartate receptors in the recurrent excitatory pathway between hippocampal CA1 pyramidal neurons: Ca2+-dependent activation by blocking A1 adenosine receptors. Proc Natl Acad Sci USA 92: 12431–12435, 1995.
Knowles WB, Schwartzkroin PA. Local circuit synaptic interactions in hippocampal brain slices. J Neurosci 1: 318–322, 1981.[Abstract]
Knopp A, Kivi A, Wozny C, Heinemann U, Behr J. Cellular and network properties of the subiculum in the pilocarpine model of temporal lobe epilepsy. J Comp Neurol 483: 476–488, 2005.[CrossRef][Web of Science][Medline]
Kuzhikandathil EV, Oxford GS. Classic D1 dopamine receptor antagonist R-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3benzazepine hydrochloride (SCH23390) directly inhibits G-protein-coupled inwardly rectifying potassium channels. Mol Pharmacol 62: 119–126, 2002.
Lanthorn TH, Cotman CW. Baclofen selectively inhibits excitatory synaptic transmission in the hippocampus. Brain Res 225: 171–178, 1981.[CrossRef][Web of Science][Medline]
Manzoni OJ, Manabe T, Nicoll RA. Release of adenosine by activation of NMDA receptors in the hippocampus. Science 265: 2098–2101, 1994.
Masino SA, Dio L, Illes P, Zahniser NR, Larson GA, Johansson B, Fredholm BB, Dunwiddie TV. Modulation of hippocampal glutamatergic transmission by ATP is dependent on adenosine A1 Receptors. J Pharmacol Exp Therapeutics 303: 356–363.
Mitchell JD, Lupica CR, Dunwiddie TV. Activity-dependent release of endogenous adenosine modulates synaptic responses in the rat hippocampus. J Neurosci 13: 3439–3447, 1993.[Abstract]
Miles R, Wong RK. Excitatory synaptic interactions between CA3 neurones in the guinea-pig hippocampus. J Physiol 373: 397–418, 1986.
Miles R, Wong RK. Inhibitory control of local excitatory circuits in the guinea-pig hippocampus. J Physiol 388: 611–629, 1987.
Mlinar B, Pugliese MA, Corradetti R. Selective inhibition of local excitatory synaptic transmission by serotonin through an unconventional receptor in the CA1 region of rat hippocampus. J Physiol 534 1: 141–158, 2001.
Mlinar B, Falsini C, Corradetti R. Pharmacological characterization of 5-HT1B receptor-mediated inhibition of local excitatory synaptic transmission in the CA1 region of rat hippocampus. Br J Pharmacol 138: 71–80, 2003.[CrossRef][Web of Science][Medline]
Ohno-Shosaku T, Tsubokawa H, Mizushima I, Yoneda N, Zimmer A, Kano M. Presynaptic cannabinoid sensitivity is a major determinant of depolarization-induced retrograde suppression at hippocampal synapses. J Neurosci 22: 3864–3872, 2002.
Olpe HR, Baudry M, Fagni L, Lynch G. The blocking action of baclofen on excitatory synaptic transmission in the rat hippocampal slices. J Neurosci 2: 698–703, 1982.[Abstract]
Pascual O, Casper KB, Kubera C, Zhang J, Revilla-Sanchez R, Sul JY, Takano H, Moss SJ, McCarthy K, Haydon PG. Astrocytic purinergic signaling coordinates synaptic networks. Science 310: 113–116, 2005.
Radpour S, Thomson AM. Coactivation of local circuit NMDA receptor meditaed epsps induces lasting enhancement of minimal schaffer collateral epsps in slices of rat hippocampus. Eur J Neurosci 3: 602–613, 1991.[CrossRef][Web of Science][Medline]
Rebola N, Coelho JE, Costenla AR, Lopes LV, Parada A, Oliveria CR, Soares-da-Silva P, de Menconça A, Cunha RA. Decrease of adenosine A1 receptor density and of adenosine neuromodulation in the hippocampus of kindled rats. Eur J Neurosci 18: 820–828, 2003.[CrossRef][Web of Science][Medline]
Rolls ET, Kesner RP. A computational theory of hippocampal function and empirical tests of the theory. Prog Neurobiol 79: 1–48, 2006.[CrossRef][Web of Science][Medline]
Sanchez-Vives MV, McCormick DA. Cellular and network mechanisms of rhythmic recurrent activity in neocortex. Nat Neurosci 3: 1027–1034, 2000.[CrossRef][Web of Science][Medline]
Serrano A, Haddjeri N, Lacaille J-C, Robitaille R. GABAergic network activation of glial cells underlies hippocampal heterosynaptic depression. J Neurosci 26: 5370–5382, 2006.
Shao L-R, Dudek FE. Increased Excitatory synaptic activity and local connectivity of hippocampal CA1 pyramidal cells in rats with kainate-induced epilepsy. J Neurophysiol 92: 1366–1373, 2004.
Silberberg G, Grillner S, LeBeau FEN, Maex R, Markram H. Synaptic pathways in neural microcircuits. Trends Neurosci 28: 541–551, 2005.[CrossRef][Web of Science][Medline]
Smith BN, Dudek FE. Short- and long-term changes in CA1 network excitability after kainate treatment in rats. J Neurophysiol 85: 1–9, 2001.
Smith BN, Dudek FE. Network interactions mediated by new excitatory connections between CA1 pyramidal cells in rats with kainate-induced epilepsy. J Neurophysiol 87: 1655–1658, 2002.
Straessle A, Loup F, Arabadzisz D, Ohning GV, Fritschy J-M. Rapid and long-term alterations of hippocampal GABAB receptors in a mouse model of temporal lobe epilepsy. Eur J Neurosci 18: 2213–2226, 2003.[CrossRef][Web of Science][Medline]
Straiker A, Mackie K. Depolarization-induced suppression of excitation in murine autaptic hippocampal neurons. J Physiol 569: 501–517, 2005.
Swartz KJ. Modulation of Ca2+ channels by protein kinase C in rat central and peripheral neurons: disruption of G protein-mediated inhibition. Neuron 11: 305–320, 1993.[CrossRef][Web of Science][Medline]
Tedford HW, Zamponi GW. Direct G protein modulation of Cav2 calcium channels. Pharmacol Rev 58: 837–862, 2006.
Thompson SM, Gähwiler BH. Comparison of the actions of baclofen at pre- and postsynaptic receptors in the rat hippocampus in vitro. J Physiol 451: 329–245, 1992.
Thompson SM, Hass HL, Gähwiler BH. Comparison of the actions of adenosine at pre- and postsynaptic receptors in the rat hippocampus in vitro. J Physiol 451: 347–363, 1992.
Traub RD, Bibbig A, LeBeau FEN, Buhl E, Whittington MA. Cellular mechanisms of neuronal population oscillations in the hippocampus in vitro. Annu Rev Neurosci 27: 247–278, 2004.[CrossRef][Web of Science][Medline]
Whittington MA, Traub RD, Faulkner HH, Standford IM, Jefferys JGR. Recurrent excitatory postsynaptic potentials induced by synchronized fast cortical oscillations. Proc Natl Acad Sci USA 94: 12198–12203, 1997.
Wu L-G, Saggau P. Adenosine inhibits evoked synaptic transmission primarily by reducing presynaptic calcium influx in area CA1 of hippocampus. Neuron 12: 1139–1148, 1994.[CrossRef][Web of Science][Medline]
Wu L-G, Saggau P. GABAB receptor-mediated presynaptic inhibition in guinea pig hippocampus is caused by a reduction of presynaptic calcium influx. J Physiol 495: 649–657, 1995.[Web of Science]
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| Visit Other APS Journals Online |
) nor the slope (
) of monosynaptic EPSPs elicited by Schaffer collateral fiber stimulation exhibit paired-pulse depression (n = 6). Slices were bathed in high Ca2+/Mg2+ ACSF to block polysynaptic EPSPs. Inset: EPSPs elicited by pairs of presynaptic fiber stimulation pulses delivered with interpulse intervals of 200–800 ms in a representative cell. Calibration bars are 5 mV and 50 ms.
) are significantly suppressed at all 3 time points (*P < 0.05 and #P < 0.001 compared with baseline) while the monosynaptic component of the postsynaptic responses (initial slope, 
) is not affected. Inset: superimposed EPSPs recorded during baseline and at the end of the 1-Hz train. Calibration bars are 10 mV and 15 ms.
