Skip to main content
PMC home page
The Journal of Physiology logoLink to The Journal of Physiology
. 2008 Mar 20;586(Pt 10):2499–2510. doi: 10.1113/jphysiol.2008.153122

Co-activation of p38 mitogen-activated protein kinase and protein tyrosine phosphatase underlies metabotropic glutamate receptor-dependent long-term depression

Peter R Moult 1, Sônia A L Corrêa 1, Graham L Collingridge 1, Stephen M Fitzjohn 1, Zafar I Bashir 1
PMCID: PMC2464349  PMID: 18356198

Abstract

Long-term potentiation (LTP) and long-term depression (LTD) are forms of synaptic plasticity thought to contribute to learning and memory. Much is known about the mechanisms of NMDA receptor-dependent LTD in the CA1 region of rat hippocampus but there is still considerable uncertainty about the mechanisms of LTD induced by mGluR activation (mGluR-LTD). Furthermore, data on mGluR-LTD derives largely from studies using pharmacologically induced LTD. To investigate mGluR-LTD that is more physiologically relevant we have examined, in CA1 of adult rat hippocampus, mechanisms of synaptically induced mGluR-LTD. We provide the first demonstration that activation of protein tyrosine phosphatase (PTP) is essential for the induction of synaptically induced mGluR-LTD. In addition, we show that activation of p38 MAPK is also required for this form of LTD. Furthermore, LTD can be mimicked and occluded by activation of p38 MAPK, provided that protein tyrosine kinases (PTKs) are inhibited. These data therefore demonstrate that a novel combination of signalling cascades, requiring both activation of p38 MAPK and tyrosine de-phosphorylation, underlies the induction of synaptically induced mGluR-LTD.

The ability of synapses to undergo long-lasting alterations in efficiency, via the process of synaptic plasticity, is thought to be necessary for learning and memory (Bliss & Collingridge, 1993). Considerable knowledge has been gained concerning possible cellular mechanisms involved in learning and memory by the study of mechanisms of long-term potentiation (LTP). For example, the observation that inhibition of N-methyl-d-aspartate receptors (NMDARs) prevented the induction of LTP in the CA1 region of hippocampal slices (Collingridge et al. 1983) led to studies showing that inhibition of NMDARs impaired spatial learning in rodents (Morris et al. 1986). Similarly, the discovery of signalling mechanisms that contribute to the induction of LTP, such as CaMKII (Malinow et al. 1989), has also led to behavioural experiments implicating these signalling molecules (Silva et al. 1992) in learning and memory.

Different patterns of synaptic activation to those that induce LTP can result in long-term depression of baseline transmission (LTD) and depotentiation (DP) of pre-established LTP (Kemp & Bashir, 2001). There is a growing realization that LTD and DP may also be effective at processing and storing information that is essential for learning and memory (Braunewell & Manahan-Vaughan, 2001; Kemp & Manahan-Vaughan, 2007; Massey & Bashir, 2007). For example, spatial exploration is associated with the reversal of hippocampal LTP (Xu et al. 1998; Abraham et al. 2002) and hippocampal novelty acquisition can result in LTD (Manahan-Vaughan & Braunewell, 1999). However, whilst most studies of learning are carried out in adult animals, most information concerning signalling mechanisms of LTD has derived from studies in juvenile animals; thus there is a distinct lack of knowledge of the signalling mechanisms underlying LTD in adult tissue.

Like LTP, the induction of LTD (Dudek & Bear, 1992; Mulkey & Malenka, 1992) and DP (Fujii et al. 1991) can require the activation of NMDARs (NMDAR-LTD). However, protocols that readily induce NMDAR-LTD (such as 1 Hz stimulation) early in development are less effective at inducing LTD in adult tissue, unless animals are stressed (Xu et al. 1997; Yang et al. 2005) or l-glutamate uptake is compromised (Massey et al. 2004; Yang et al. 2005). In some circumstances LTD and DP require the activation of metabotropic glutamate receptors (mGluRs) rather than NMDARs (Bashir et al. 1993; Bashir & Collingridge, 1994; Bolshakov & Siegelbaum, 1994; Oliet et al. 1997). In contrast to NMDAR-LTD, mGluR-dependent LTD (mGluR-LTD) can be readily induced synaptically in adult tissue by delivering paired-pulse, low-frequency stimulation (PP-LFS; Kemp & Bashir, 1999; Huber et al. 2000; Massey & Bashir, 2007). Thus, mGluR-LTD may be the predominant form of LTD in adult tissue and it is therefore imperative to obtain a greater understanding of the signalling mechanisms underlying this form of LTD.

The purpose of the present study therefore was to establish the signalling cascades that are involved in synaptically induced mGluR-LTD (hereafter simply referred to as LTD) in the CA1 region of adult hippocampus. The results presented in this study show for the first time that LTD can be completely blocked by inhibition of either protein tyrosine phosphatases (PTPs) or p38 MAPK. Furthermore, LTD can be mimicked and occluded by activation of p38 MAPK provided that protein tyrosine kinases (PTKs) are inhibited. Thus, LTD relies on p38 MAPK activation and tyrosine dephosphorylation. These data therefore uncover a novel combination of signalling cascades underlying the induction of synaptically induced mGluR-LTD in the adult rat hippocampus.

Methods

Hippocampal slice preparation

Hippocampal slices (400 μm thick) were obtained from adult Wistar rats (10–15 weeks of age). Animals were killed by cervical dislocation in accordance with the UK Animal (Scientific Procedures) Act 1986. The brains were removed rapidly and placed in ice-cold artificial CSF (aCSF) consisting of the following (mm): 124 NaCl, 3 KCl, 26 NaHCO3, 1.25 NaH2PO4, 2 CaCl2, 1 MgSO4 and 10 d-glucose (bubbled with 95% O2–5% CO2). Parasaggital brain slices were then prepared and the hippocampus isolated from surrounding tissue. The CA3 region was removed in all cases. Slices were allowed to recover at room temperature in oxygenated aCSF for between 1 and 7 h before use; the duration of this period was noted for each experiment.

Extracellular recordings

Grease-gap recordings were obtained from the CA1 region as previously described (Blake et al. 1988; Moult et al. 2002). Briefly, slices were placed on a glass coverslip on the surface of an inclined temperature-controlled unit (maintained at 30°C). The slice was partially covered with absorbent paper and superfused at a rate of 2 ml min−1 with aCSF (composition as above) bubbled continuously with 95% O2–5% CO2. For all experiments, the GABAA receptor antagonist picrotoxin (50 μm) and the NMDAR antagonist L-689,560 (trans-2-carboxy-5,7-dichloro-4-phenylaminocarbonylamino-1,2,3,4-tetrahydroquinoline; 5 μm) were present throughout (Moult et al. 2002). The Schaffer collateral–commissural pathway was stimulated at 0.033 Hz at an intensity that evoked a field EPSP (fEPSP) slope of 50% of the maximum. LTD was induced with a paired-pulse low-frequency stimulation protocol (PP-LFS; 900 paired stimuli at 1 Hz with a 50 ms interpulse interval; Kemp & Bashir, 1999). Responses were digitized at 5 kHz, recorded on-line and analysed off-line using the LTP program (Anderson & Collingridge, 2001). The maximum slope was obtained from 20 to 80% of the incline of the response.

LTD was calculated 25–30 min after termination of PP-LFS and expressed as a percentage of the baseline. Statistical analysis was performed using Student's t tests. n signifies the number of times a given experiment was performed, with each experiment using a slice from a different rat. Experimental treatments and corresponding control experiments were interleaved throughout. Where multiple comparisons were made against one of the control groups, an ANOVA with the post hoc Bonferroni test was applied.

The following compounds were applied by addition to the perfusate: anisomycin, cycloheximide, genistein, L-689,560, LY294002, LY367385, lavendustin A, MPEP, PP2, Ro 31-8220, SB 203580, SP 600125, U0126 (all obtained from Tocris, UK), sodium orthovanadate and phenylarsineoxide (PAO) (obtained from Sigma-Aldrich, UK). Concentrations of each compound were selected based on the known effects of these agents in related in vitro studies. None of the agents tested affected baseline synaptic transmission when applied alone.

Analysis of p38 MAPK activation

Hippocampal slices for each experimental group were incubated in aCSF containing picrotoxin (50 μm) and L-689,560 (5 μm) for 30 min and then transferred for either 30 or 60 min into control aCSF (control group), anisomycin (20 μm) or cyclohexamide (60 μm). The slices from the different groups were then homogenized in Eppendorf Scientific tubes (Westbury, NY, USA) with a pellet pestle on ice in 11% (w/v) sucrose, 10 mm Hepes, pH 7.2, 100 μm genistein, 1 mm orthovanadate and a mixture of ‘complete’ protease inhibitors (Roche Products, Welwyn Garden City, UK) and phosphatase inhibitor mixture 1 (Sigma) to prevent degradation and dephosphorylation of proteins. Homogenized samples were then sonicated, rotated for 1–2 h at 4°C, centrifuged at 15000 g for 15 min at 4°C and then assayed for total protein concentration using Bio-Rad Bradford protein assay Kit (Hercules, CA, USA). Each sample was separated in 12% SDS-PAGE, transferred onto polyvinylidene difluoride membranes using an Atto HorizBlot electrophoretic transfer unit (Tokyo, Japan) with a discontinuous buffer system for 1.5 h at room temperature, as recommended by the manufacturer. Following the transfer, blots were blocked in solution containing 5% (w/v) milk and 0.1% (v/v) Tween 20 in TBS for 1 h at room temperature, and incubated with antibody for phosphorylated p38 MAPK (1: 1000 dilution, New England BioLabs, UK) overnight at 4°C. Blots were then washed several times with 0.1% TBS with Tween (TBST) and probed with horseradish peroxidase-conjugated secondary antibody for 1 h before being developed using ECL immunoblotting detection system. The immublots were then stripped and re-probed with an antibody recognizing p38 MAPK (1: 1000 dilution, New England BioLabs, UK). Immublots were analysed by densitometry using NIH ImageJ (http://rsb.info.nih.gov/ij/index.html). All of the values reported are mean ± s.e.m. and statistical analysis was performed using ANOVA.

Results

Synaptically induced mGluR-LTD is mGlu5R dependent

In agreement with previous studies, delivery of PP-LFS (Kemp & Bashir, 1999; Huber et al. 2000; Gallagher et al. 2004) resulted in LTD (depression to 67 ± 7% of baseline, n = 6 and P < 0.05; Fig. 1). In interleaved experiments the induction of LTD was not affected by the mGlu1R antagonist (S)-(+)-α-amino-4-carboxy-2-ethylbenzeneacetic acid (LY367385, 100 μm; 61 ± 7% of baseline; n = 6 and P > 0.05 compared to control LTD; Fig. 1A and B). In contrast, LTD was prevented by the specific mGlu5R antagonist 2-methyl-6-(phenylethynyl) pyridine hydrochloride (MPEP, 1 μm; 98 ± 9% of baseline; n = 6 and P < 0.05 compared to control LTD; Fig. 1C and D). These results show that LTD in adult hippocampus induced by PP-LFS results from activation of mGlu5R but not mGlu1R.

Figure 1. Synaptically induced mGluR-LTD requires activation of mGlu5R but not mGlu1R.

Figure 1

Open in a new tab

In this and subsequent figures each point is the average of four successive responses (points during PP-LFS are averages of 20 successive responses), and example traces are from an individual representative experiment taken 10 min into the baseline period (black) and 30 min after LTD-inducing protocol (grey). In all figures, filled circles show experiments performed in the presence of the relevant drug, whereas interleaved controls are represented by open circles. Drugs were applied for the time indicated by the bar. PP-LFS consists of 900 paired-pulses delivered at 1 Hz, with an inter-pair interval of 50 ms. In this and all subsequent figures, pooled data are represented as mean ± s.e.m.A, single example showing lack of block of LTD by the mGlu1R antagonist LY367385 (100 μm). B, pooled data showing interleaved controls and experiments performed in the presence of LY367385. C, single example showing block of LTD by the mGlu5R antagonists MPEP (1 μm). D, pooled data for experiments performed in the presence of MPEP and controls. Calibration in this and subsequent figures: 0.1 mV, 10 ms.

LTD is dependent on activation of protein tyrosine phosphatase

mGluR-LTD induced by application of the agonist DHPG (DHPG-LTD) is blocked by protein tyrosine phosphatase (PTP) inhibitors (Moult et al. 2002, 2006; Huang & Hsu, 2006). We have now examined the effects of PTP inhibition on synaptically induced mGluR-LTD. In the presence of the PTP inhibitor orthovanadate (1 mm), LTD was blocked (99 ± 10% of baseline; n = 6 and P < 0.05 compared to control LTD; Fig. 2A). Similarly, another PTP inhibitor, phenylarsineoxide (PAO; 15 μm), also prevented the induction of LTD (100 ± 10% of baseline; n = 6; P < 0.05 compared to control LTD; Fig. 2B). The above data suggest that the synaptic activation of mGlu5R and subsequent activation of PTP results in LTD.

Figure 2. LTD is blocked by PTP inhibitors, an effect prevented by inhibition of Src-family PTKs.

Figure 2

Open in a new tab

A and B, application of either of the PTP inhibitors orthovanadate (1 mm) or PAO (15 μm) prevents LTD. C and D, neither the broad spectrum PTK inhibitor lavendustin A (20 μm) nor the Src-family PTK inhibitor PP2 (10 μm) have an effect upon LTD. E, the Src-family PTK inhibitor PP2 (10 μm) prevents the blockade of LTD by orthovanadate.

LTD is dependent on an interaction between PTP and PTK

We have previously shown that PTK inhibitors do not directly tjp0586-2499-LTD but completely prevent the block of DHPG-LTD by PTP inhibitors (Moult et al. 2006). We therefore determined whether synaptically induced mGluR-LTD was similarly affected by PTK inhibitors. We found that LTD was not blocked by either the broad spectrum protein tyrosine kinase (PTK) inhibitor, lavendustin A (20 μm), or the Src-family PTK inhibitor, 3-(4-chlorophenyl)1-(1,1-dimethylethyl)-1 H-pyrazolo[3,4-d]pyrimidin-4-amine (PP2; 10 μm; 57 ± 9% and 58 ± 8%, respectively; for each n = 6 and P > 0.05 compared to control LTD; Fig. 2C and D). However, PP2 did prevent the block of LTD by orthovanadate (65 ± 8% of baseline, n = 6 and P > 0.05 compared to control LTD; Fig. 2E). This finding was not specific to the PP2–orthovanadate combination as a range of other PTK–PTP inhibitor combinations yielded the same result: PP2 and PAO, lavendustin A and orthovanadate, lavendustin A and PAO, genistein and orthovanadate, genistien and PAO (LTD was 60 ± 11%, 63 ± 7%, 69 ± 8%, 62 ± 9% and 63 ± 9% of baseline, respectively; n = 6 for each and P > 0.05 compared to control for each; data not shown).

The interaction between PTP and PTK uncovers the requirement for an additional signalling pathway in LTD

The finding that several different PTK inhibitors prevent the effects of the two PTP inhibitors verifies that these compounds are working via affecting tyrosine phosphorylation. This observation, however, raises two intriguing questions. Firstly, how does inhibition of PTKs prevent the block of LTD by PTP inhibitors? Secondly, if LTD simply requires tyrosine dephosphorylation then why does inhibition of PTKs not induce LTD? The simplest explanation for the first question is as follows: the PTK inhibitors prevent constitutive phosphorylation of the substrate(s) that are normally dephosphorylated by PTPs, such that the need for activation of PTPs is negated (and hence the PTP inhibitors become ineffective at blocking LTD). As for the second question, it follows therefore that LTD requires more than just dephosphorylation of tyrosine residues (otherwise PTK inhibitors would induce LTD). The simplest explanation for the observation that PTK inhibitors do not induce LTD is that there must be, in addition to tyrosine dephosphorylation, an additional signalling pathway(s) that is essential for the induction of LTD. Thus, we set out to determine the identity of this additional pathway.

LTD is independent of protein synthesis

It has been reported that both DHPG-LTD and synaptically induced mGluR-LTD can be either dependent or independent of protein synthesis (Huber et al. 2000; Nosyreva & Huber, 2005; Volk et al. 2006). We therefore used the protein synthesis inhibitors anisomycin or cycloheximide, at the same concentrations that have previously been shown to be effective in our (Massey et al. 2001) and other studies (Huber et al. 2000; Nosyreva & Huber, 2005; Volk et al. 2006). However, we found that LTD was unaffected by either anisomycin (20 μm; 62 ± 8% of baseline; n = 6 and P > 0.05 compared to control LTD; Fig. 3A) or cycloheximide (60 μm; 62 ± 7% of baseline; n = 5 and P > 0.05 compared to control LTD; Fig. 3B). Furthermore, baseline transmission was also unaffected by either anisomycin or cycloheximide. Thus, synaptically induced mGluR-LTD in adult CA1 is independent of protein synthesis.

Figure 3. LTD is not blocked by inhibitors of protein synthesis.

Figure 3

Open in a new tab

A and B, LTD was unaffected by the protein synthesis inhibitors anisomycin (20 μm) or cycloheximide (60 μm). C, there is no effect of protein synthesis inhibitors irrespective of whether experiments were performed 1–4 h after slice preparation or performed 4–7 h after slice preparation.

During the course of the experiments, slices were used betweetjp0586-2499-preparation time might have any effect on recruitment of a protein synthesis component of LTD, as previously reported (Bear, 2003). However, there was no evidence for the effect of post-preparation time on either the magnitude of LTD or recruitment of a protein synthesis component in LTD (Fig. 3C).

The additional pathway in LTD does not require activation of PKC, PI3K or ERK

The role of PKC in mGluR-LTD in the CA1 region of the hippocampus is unclear. PKC has been shown to be involved in a form of synaptically induced mGluR-LTD (Oliet et al. 1997) but not to be involved in DHPG-LTD (Schnabel et al. 1999). We therefore examined the effects on LTD of a PKC inhibitor, bisindolymaleimide IX/3-3[2,5dihydro-4-(1-methyl-1H-indol-3-yl)-2,5-dioxo-1H-pyrrol-1H-3-yl]-1H-indol-1-yl]propyl carbamimidothioic acid ester mesylate (Ro 31-8220). We used a supramaximal concentration (10 μm;Davies et al. 2000) for inhibition of PKC. This concentration, or lower, has previously been shown to be effective in hippocampal slices (Bortolotto & Collingridge, 2000; Wikstrom et al. 2003). Inhibition of PKC had no effect on LTD (65 ± 9%; n = 6 and P > 0.05 compared to control LTD; Fig. 4A). There was, however, a potentiation of the initial depression following PP-LFS (from 47 ± 4% to 18 ± 5% of baseline; P < 0.05).

Figure 4. LTD does not involve PKC, PI3K or ERK.

Figure 4

Open in a new tab

A, the PKC inhibitor Ro 31-8220 (10 μm) has no effect upon LTD. B, the PI3K inhibitor LY294002 (50 μm) has no effect on LTD. C, inhibition of MEK (a precursor of ERK activation) by U0126 (20 μm) does not block LTD.

Previous studies have suggested roles for PI3K (Hou & Klann, 2004) and Ras-activated ERK (Gallagher et al. 2004) in mGluR-LTD. Therefore we investigated the role of PI3K using the selective inhibitor LY294002. At a concentration of 50 μm, LY294002 has been shown to effectively inhibit PI3K-mediated effects in hippocampal slices (Daw et al. 2002; Peineau et al. 2007). However, LY294002 had no effect on LTD (69 ± 5% of baseline; n = 6 and P > 0.05 compared to control LTD; Fig. 4B). In addition, inhibition of MEK (a precursor to ERK activation), using U0126 (20 μm; a concentration previously shown to be effective in hippocampal slices; Gallagher et al. 2004) had no effect on LTD (67 ± 10% of baseline; n = 6 and P > 0.05 compared to control LTD; Fig. 4C).

LTD is dependent upon p38 MAPK but not JNK

Given previous evidence that mGluR-LTD may involve the Rap-activated p38 MAPK pathway (Bolshakov et al. 2000; Rush et al. 2002; Huang et al. 2004), we tested the requirement in LTD for the stress-activated kinase p38 MAPK. LTD was completely blocked by the p38 MAPK inhibitor SB 203580 (5 μm; 98 ± 6% of baseline; n = 6 and P < 0.05 compared to control LTD; Fig. 5A). These results show that p38 MAPK is essential for LTD. JNK1 is another stress-activated protein kinase which has been implicated in mGluR-LTD (Li et al. 2007). However, we found that the specific JNK inhibitor SP 600125, used at a concentration (20 μm) previously shown to be effective in hippocampal slices (Curran et al. 2003; Li et al. 2007), had no effect on LTD (69 ± 4% of baseline; n = 5 and P > 0.05 compared to control, Fig. 5B).

Figure 5. LTD is dependent upon p38 MAPK activation and tyrosine dephosphorylation.

Figure 5

Open in a new tab

A, the p38 MAPK inhibitor SB 203580 (5 μm) prevents the induction of LTD. B, the JNK inhibitor SP 600125 (20 μm) does not block LTD. Ca, pooled data showing that LTD seen in the presence of inhibitors of PTP (orhthovanadate; 1 mm) and Src-family PTK (PP2; 10 μm) is blocked by the p38 MAPK inhibitor SB 203580 (5 μm). Cb, in the same slices, PP-LFS induces LTD following washout of SB 203580.

LTD is dependent upon p38 MAPK activation and tyrosine dephosphorylation

Our results suggest that the PTP and the p38 MAPK pathways untjp0586-2499-LTD. Possibilities that explain the requirement of both p38 MAPK and PTP is that either these form part of a serial cascade or alternatively they are part of two separate but essential parallel pathways required for LTD induction. If the latter is correct then LTD induced by PP-LFS in the presence of PTK and PTP inhibitors (see Fig. 2E) should be blocked by p38 MAPK inhibition. Indeed, this was found to be the case; LTD was not induced by PP-LFS in the combined presence of the PTK inhibitor PP2, the PTP inhibitor orthovanadate and the p38 MAPK inhibitor SB 203580 (97 ± 2% of baseline; n = 5 and P < 0.05 compare to control; Fig. 5Ca). However, following washout of the p38 MAPK inhibitor, LTD was subsequently induced in the presence of PP2 and orthovanadate (66 ± 3% baseline; n = 5 and P > 0.05 compared to control; Fig. 5Cb). This result provides evidence that following mGlu5R stimulation the p38 MAPK and the PTP pathways operate in parallel and that both are required for the induction of synaptically induced mGluR-LTD.

Activation of p38 MAPK combined with tyrosine dephosphorylation is sufficient for LTD

Our results suggest that two cascades, activation of PTP and activation of p38 MAPK, are essential for the induction of LTD. The next question we wanted to address was whether there are additional necessary pathways or whether activation of these two pathways is sufficient for LTD. In the latter case, blockade of PTK (to decrease tyrosine phosphorylation) plus activation of p38 MAPK should result in the induction of LTD in the absence of PP-LFS. In order to perform these experiments we took advantage of the fact that anisomycin (Mahadevan et al. 1991; Cano et al. 1994; Cano et al. 1996) but not cycloheximide (Iordanov et al. 1997) has been reported to be a potent activator of p38 MAPK.

Firstly, we examined in hippocampal slices whether anisomycin activates p38 MAPK. Following application of 20 μm anisomycin a significant increase in the ratio of phosphorylated p38 MAPK/total p38 MAPK was observed. In contrast, however, cycloheximide (60 μm) had no effect (control: 0.35 ± 0.05; anisomycin: 0.81 ± 0.08, P < 0.05; cycloheximide: 0.38 ± 0.05, P > 0.05; n = 6; Fig. 6).

Figure 6. Anisomycin, but not cyclohexamide, increases phosphorylation of p38 MAPK.

Figure 6

Open in a new tab

A, left immunoblot shows an increase in phosphorylation of p38 MAPK after 1 h pre-incubation with anisomycin (An; 20 μm) compared with the untreated group (C). Cyclohexamide (Cy, 60 μm) did not increase p38 MAPK phosphorylation. Right immunoblot shows no change in total p38MAPK following anisomycin or cycloheximide treatment. B, pooled data from 6 experiments showing that anisomycin increases the ratio of phosphorylated p38 MAPK/total p38 MAPK.

Having established that anisomycin phosphorylates p38 MAPK wetjp0586-2499-LFS (70 ± 5% of baseline; n = 5; Fig. 7Aa). Furthermore, the lasting depression induced under these conditions occluded the subsequent induction of LTD by PP-LFS (98 ± 7% of baseline; n = 5; Fig. 7Ab), suggesting common mechanisms of induction. It is important to note that application of neither PP2 nor anisomycin alone affected baseline transmission (Figs 2 and 3).

Figure 7. Activation of p38 MAPK combined with tyrosine dephosphorylation is sufficient for LTD.

Figure 7

Open in a new tab

Aa, pooled data showing that application of the Src-family PTK inhibitor PP2 (10 μm) together with the p38 MAPK activator anisomycin (20 μm) produces synaptic depression. Ab, in the same slices, subsequent PP-LFS fails to induce LTD (data re-normalized to 20 min prior to PP-LFS). Ba, co-application of PP2 with the protein synthesis inhibitor cycloheximide (60 μm) does not induce synaptic depression. Bb, following application of PP2 and cycloheximide, subsequent PP-LFS induces LTD. Ca, pooled data showing that the p38 MAPK inhibitor SB 203580 (5 μm) blocks the depression induced by PP2 and anisomycin. Cb, after washout of SB 203580, but in the continuing presence of PP2 and anisomycin, synaptic depression is induced.

Co-tjp0586-2499- (98 ± 5% of baseline; n = 5; Fig. 7Bb) and LTD was not occluded by prior application of PP2 and cycloheximide (70 ± 8% of baseline, n = 6 and P > 0.05 compared to control LTD; Fig. 7Bb). This suggests that in the presence of a PTK inhibitor, the LTD induced by anisomycin was not due to inhibition of protein synthesis.

Finally, we confirmed that LTD induced in the presence of PP2 and anisomycin was indeed due to activation of p38 MAPK. Application of PP2 and anisomycin failed to produce lasting depression when delivered in the presence of the p38 MAPK inhibitor SB 203580 (110 ± 2%n = 5, P > 0.05, measured 50 min after application of anisomycin; Fig. 7Ca). However, following washout of SB 203580, lasting depression of transmission developed in the continued presence of PP2 and anisomycin (72 ± 5%, n = 5, P < 0.05; Fig. 7Cb). Therefore, LTD induced by anisomycin in the presence of PP2 was due to p38 MAPK activation.

Discussion

The results of the present study show, for the first time, that synaptically induced mGluR-LTD can be induced by activation of mGlu5R and the subsequent and parallel stimulation of both PTP and p38 MAPK. Given that LTD is readily induced in adult hippocampus and may be important in learning and memory (Braunewell & Manahan-Vaughan, 2001; Kemp & Manahan-Vaughan, 2007; Massey & Bashir, 2007) these findings raise the possibility that a signalling cascade involving mGlu5R, PTP and p38 MAPK may be engaged by learning and memory processes. mGluR-LTD exists in many brain regions and therefore this signalling mechanism is potentially of widespread physiological significance.

LTD requires activation of mGlu5R but not mGlu1R

LTD was prevented by the specific mGlu5R antagonist, MPEP, but not by the mGlu1R antagonist, LY367385. Using animals ranging from 12 days to 10 weeks of age and studying LTD induced by either DHPG or synaptic stimulation, most previous studies at CA1 synapses have also shown a role for mGlu5R in mGluR-LTD (Palmer et al. 1997; Fitzjohn et al. 1999; Huber et al. 2001; Faas et al. 2002; Hou & Klann, 2004; Huang et al. 2004). One recent report has suggested that DHPG-LTD requires inhibition of both mGlu1R and mGlu5R and that synaptically induced mGluR-LTD involves activation of receptors other than the eight known mGluR subtypes (Volk et al. 2006). The difference in these results cannot be attributed to age differences since 19- to 55-day-old animals were used by Volk et al. (2006). In the dentate gyrus, DHPG-LTD is also dependent upon activation of mGlu5R (Camodeca et al. 1999) and a form of LTD evoked by high-frequency stimulation is sensitive to inhibition of mGlu5R and is also partially sensitive to inhibition of mGlu1R (Wang et al. 2007). Our results are therefore consistent with most previous studies in identifying a critical role for mGlu5R in the induction of mGluR-LTD in hippocampus.

Signalling mechanisms not involved in LTD

In contrast to some previous studies of mGluR-LTD in tissue from younger animals (Oliet et al. 1997; Huber et al. 2000; Gallagher et al. 2004; Hou & Klann, 2004; Li et al. 2007), we found no role for PKC, PI3K, ERK, JNK or protein synthesis in LTD in adult hippocampus. Furthermore, we also demonstrate that the length of post-preparation incubation time has no effect on either the level of LTD or the recruitment of protein synthesis (Bear, 2003). In our study, we used the inhibitors of the signalling molecules at concentrations that have previously been shown to be effective in vitro. Therefore, it is highly unlikely that the negative results in this study are due to a lack of effect of the inhibitors. Instead, these negative results strongly mitigate against a role for these signalling cascades in LTD in adult hippocampus. Of course, we cannot exclude a role for one or more of these enzymes in LTD under other experimental conditions, such as at an earlier stage in development. A direct comparison across the life-span of the role of each of these signalling molecules in LTD is beyond the scope of the present article. Our intention has been to focus on adult tissue, which is directly relevant to studies of learning and memory, and to characterize synaptically induced mGluR-LTD.

Tyrosine dephosphorylation is essential for LTD

PTP activation plays an essential role in mGluR-LTD induced by application of DHPG (Moult et al. 2002, 2006; Huang & Hsu, 2006). We now provide the first demonstration that synaptically induced mGluR-LTD is also dependent on PTP activation. Furthermore, like DHPG-LTD (Moult et al. 2002), synaptically induced mGluR-LTD is not directly affected by inhibition of PTKs. However, inhibition of PTKs prevents PTP inhibitors from blocking synaptically induced mGluR-LTD, similar to our previous observations on DHPG-LTD (Moult et al. 2006). A regulation of PTPs by PTKs has also been reported for the modulation of NMDAR-mediated synaptic transmission (Coussens et al. 2000). That PTK inhibitors alone did not induce LTD suggests that other signalling cascades, beyond the regulation of tyrosine phosphorylation, are critically involved in the induction of LTD.

Co-activation of p38 MAPK and PTP is required for LTD

In CA1 and dentate gyrus there is evidence for the involvement of p38 MAPK in mGluR-LTD (Bolshakov et al. 2000; Rush et al. 2002; Huang et al. 2004). The data in the present study also support a role for the p38 MAPK pathway in LTD. However, the present data, demonstrating the essential requirement for both tyrosine dephosphorylation and p38 MAPK activation in synaptically induced mGluR-LTD, suggest a new level of complexity in LTD signalling mechanisms. This is shown, firstly, by the finding that either a PTP inhibitor or a p38 MAPK inhibitor fully blocks LTD and, secondly, by the finding that a PTK inhibitor co-applied with a p38MAPK activator mimicked and occluded LTD. This latter observation, that stimulation of p38 MAPK together with inhibition of PTKs mimicked and occluded LTD, demonstrates for the first time that activation of these two parallel pathways is sufficient to induce synaptically induced mGluR-LTD.

DHPG-LTD is associated with tyrosine dephosphorylation and internalization of the GluR2 subunit of AMPARs (Huang & Hsu, 2006; Moult et al. 2006). This suggests a simple scenario for LTD in which the critical tyrosine residue on GluR2 is normally basally phosphorylated by PTK. Synaptic activation of mGlu5R results in a PTP-dependent dephosphorylation of this residue and, in parallel, the activation of p38 MAPK. Activation of both of these processes results in removal of AMPA receptors from the synapse and thus the expression of LTD. The site(s) phosphorylated by p38 MAPK are not known, although one potential target is the Rab5–GDI (guanyl nucleotide dissociation inhibitor) complex that is involved in receptor removal into clathrin-coated pits (Huang et al. 2004). Further studies will be required to establish the actual tyrosine residues that are dephosphorylated by PTP activity and to identify both the PTPs and the Src-family PTKs involved in LTD.

In conclusion, the results of this study demonstrate, for the first time, that synaptically induced mGluR-LTD in adult CA1 relies on the parallel activation of p38 MAPK and PTP. Given that LTD is readily induced in adult hippocampus and that LTD may be important in learning and memory, these findings raise the possibility that a signalling cascade involving mGlu5R, PTP and p38 MAPK may be engaged by learning and memory processes.

References

  1. Abraham WC, Logan B, Greenwood JM, Dragunow M. Induction and experience-dependent consolidation of stable long-term potentiation lasting months in the hippocampus. J Neurosci. 2002;22:9626–9634. doi: 10.1523/JNEUROSCI.22-21-09626.2002. [DOI] [PMC free article] [PubMed] [Google Scholar]
  2. Anderson WW, Collingridge GL. The LTP Program: a data acquisition program for on-line analysis of long-term potentiation and other synaptic events. J Neurosci Methods. 2001;108:71–83. doi: 10.1016/s0165-0270(01)00374-0. [DOI] [PubMed] [Google Scholar]
  3. Bashir ZI, Collingridge GL. An investigation of depotentiation of long-term potentiation in the CA1 region of the hippocampus. Exp Brain Res. 1994;100:437–443. doi: 10.1007/BF02738403. [DOI] [PubMed] [Google Scholar]
  4. Bashir ZI, Jane DE, Sunter DC, Watkins JC, Collingridge GL. Metabotropic glutamate receptors contribute to the induction of long-term depression in the CA1 region of the hippocampus. Eur J Pharmacol. 1993;239:265–266. doi: 10.1016/0014-2999(93)91009-c. [DOI] [PubMed] [Google Scholar]
  5. Bear MF. Bidirectional synaptic plasticity: from theory to reality. Philos Trans R Soc Lond B Biol Sci. 2003;358:649–655. doi: 10.1098/rstb.2002.1255. [DOI] [PMC free article] [PubMed] [Google Scholar]
  6. Blake JF, Brown MW, Collingridge GL. CNQX blocks acidic amino acid induced depolarizations and synaptic components mediated by non-NMDA receptors in rat hippocampal slices. Neurosci Lett. 1988;89:182–186. doi: 10.1016/0304-3940(88)90378-3. [DOI] [PubMed] [Google Scholar]
  7. Bliss TV, Collingridge GL. A synaptic model of memory: long-term potentiation in the hippocampus. Nature. 1993;361:31–39. doi: 10.1038/361031a0. [DOI] [PubMed] [Google Scholar]
  8. Bolshakov VY, Carboni L, Cobb MH, Siegelbaum SA, Belardetti F. Dual MAP kinase pathways mediate opposing forms of long-term plasticity at CA3-CA1 synapses. Nat Neurosci. 2000;3:1107–1112. doi: 10.1038/80624. [DOI] [PubMed] [Google Scholar]
  9. Bolshakov VY, Siegelbaum SA. Postsynaptic induction and presynaptic expression of hippocampal long-term depression. Science. 1994;264:1148–1152. doi: 10.1126/science.7909958. [DOI] [PubMed] [Google Scholar]
  10. Bortolotto ZA, Collingridge GL. A role for protein kinase C in a form of metaplasticity that regulates the induction of long-term potentiation at CA1 synapses of the adult rat hippocampus. Eur J Neurosci. 2000;12:4055–4062. doi: 10.1046/j.1460-9568.2000.00291.x. [DOI] [PubMed] [Google Scholar]
  11. Braunewell KH, Manahan-Vaughan D. Long-term depression: a cellular basis for learning? Rev Neurosci. 2001;12:121–140. doi: 10.1515/revneuro.2001.12.2.121. [DOI] [PubMed] [Google Scholar]
  12. Camodeca N, Breakwell NA, Rowan MJ, Anwyl R. Induction of LTD by activation of group I mGluR in the dentate gyrus in vitro. Neuropharmacology. 1999;38:1597–1606. doi: 10.1016/s0028-3908(99)00093-3. [DOI] [PubMed] [Google Scholar]
  13. Cano E, Doza YN, Ben-Levy R, Cohen P, Mahadevan LC. Identification of anisomycin-activated kinases p45 and p55 in murine cells as MAPKAP kinase-2. Oncogene. 1996;12:805–812. [PubMed] [Google Scholar]
  14. Cano E, Hazzalin CA, Mahadevan LC. Anisomycin-activated protein kinases p45 and p55 but not mitogen-activated protein kinases ERK-1 and -2 are implicated in the induction of c-fos and c-jun. Mol Cell Biol. 1994;14:7352–7362. doi: 10.1128/mcb.14.11.7352. [DOI] [PMC free article] [PubMed] [Google Scholar]
  15. Collingridge GL, Kehl SJ, McLennan H. The antagonism of amino acid-induced excitations of rat hippocampal CA1 neurones in vitro. J Physiol. 1983;334:19–31. doi: 10.1113/jphysiol.1983.sp014477. [DOI] [PMC free article] [PubMed] [Google Scholar]
  16. Coussens CM, Williams JM, Ireland DR, Abraham WC. Tyrosine phosphorylation-dependent inhibition of hippocampal synaptic plasticity. Neuropharmacology. 2000;39:2267–2277. doi: 10.1016/s0028-3908(00)00087-3. [DOI] [PubMed] [Google Scholar]
  17. Curran BP, Murray HJ, O'Connor JJ. A role for c-Jun N-terminal kinase in the inhibition of long-term potentiation by interleukin-1b and long-term depression in the rat dentate gyrus in vitro. Neuroscience. 2003;118:347–357. doi: 10.1016/s0306-4522(02)00941-7. [DOI] [PubMed] [Google Scholar]
  18. Davies SP, Reddy H, Caivano M, Cohen P. Specificity and mechanism of action of some commonly used protein kinase inhibitors. Biochem J. 2000;351:95–105. doi: 10.1042/0264-6021:3510095. [DOI] [PMC free article] [PubMed] [Google Scholar]
  19. Daw MI, Bortolotto ZA, Saulle E, Zaman S, Collingridge GL, Isaac JT. Phosphatidylinositol 3 kinase regulates synapse specificity of hippocampal long-term depression. Nat Neurosci. 2002;5:835–836. doi: 10.1038/nn903. [DOI] [PubMed] [Google Scholar]
  20. Dudek SM, Bear MF. Homosynaptic long-term depression in area CA1 of hippocampus and effects of N-methyl-D-aspartate receptor blockade. Proc Natl Acad Sci U S A. 1992;89:4363–4367. doi: 10.1073/pnas.89.10.4363. [DOI] [PMC free article] [PubMed] [Google Scholar]
  21. Faas GC, Adwanikar H, Gereau RW, Saggau P. Modulation of presynaptic calcium transients by metabotropic glutamate receptor activation: a differential role in acute depression of synaptic transmission and long-term depression. J Neurosci. 2002;22:6885–6890. doi: 10.1523/JNEUROSCI.22-16-06885.2002. [DOI] [PMC free article] [PubMed] [Google Scholar]
  22. Fitzjohn SM, Kingston AE, Lodge D, Collingridge GL. DHPG-induced LTD in area CA1 of juvenile rat hippocampus; characterisation and sensitivity to novel mGlu receptor antagonists. Neuropharmacology. 1999;38:1577–1583. doi: 10.1016/s0028-3908(99)00123-9. [DOI] [PubMed] [Google Scholar]
  23. Fujii S, Saito K, Miyakawa H, Ito K, Kato H. Reversal of long-term potentiation (depotentiation) induced by tetanus stimulation of the input to CA1 neurons of guinea pig hippocampal slices. Brain Res. 1991;555:112–122. doi: 10.1016/0006-8993(91)90867-u. [DOI] [PubMed] [Google Scholar]
  24. Gallagher SM, Daly CA, Bear MF, Huber KM. Extracellular signal-regulated protein kinase activation is required for metabotropic glutamate receptor-dependent long-term depression in hippocampal area CA1. J Neurosci. 2004;24:4859–4864. doi: 10.1523/JNEUROSCI.5407-03.2004. [DOI] [PMC free article] [PubMed] [Google Scholar]
  25. Hou L, Klann E. Activation of the phosphoinositide 3-kinase-Akt-mammalian target of rapamycin signaling pathway is required for metabotropic glutamate receptor-dependent long-term depression. J Neurosci. 2004;24:6352–6361. doi: 10.1523/JNEUROSCI.0995-04.2004. [DOI] [PMC free article] [PubMed] [Google Scholar]
  26. Huang CC, Hsu KS. Sustained activation of metabotropic glutamate receptor 5 and protein tyrosine phosphatases mediate the expression of (S)-3,5-dihydroxyphenylglycine-induced long-term depression in the hippocampal CA1 region. J Neurochem. 2006;96:179–194. doi: 10.1111/j.1471-4159.2005.03527.x. [DOI] [PubMed] [Google Scholar]
  27. Huang CC, You JL, Wu MY, Hsu KS. Rap1-induced p38 mitogen-activated protein kinase activation facilitates AMPA receptor trafficking via the GDI.Rab5 complex. Potential role in (S)-3,5-dihydroxyphenylglycene-induced long term depression. J Biol Chem. 2004;279:12286–12292. doi: 10.1074/jbc.M312868200. [DOI] [PubMed] [Google Scholar]
  28. Huber KM, Kayser MS, Bear MF. Role for rapid dendritic protein synthesis in hippocampal mGluR-dependent long-term depression. Science. 2000;288:1254–1257. doi: 10.1126/science.288.5469.1254. [DOI] [PubMed] [Google Scholar]
  29. Huber KM, Roder JC, Bear MF. Chemical induction of mGluR5- and protein synthesis-dependent long-term depression in hippocampal area CA1. J Neurophysiol. 2001;86:321–325. doi: 10.1152/jn.2001.86.1.321. [DOI] [PubMed] [Google Scholar]
  30. Iordanov MS, Pribnow D, Magun JL, Dinh TH, Pearson JA, Chen SL, Magun BE. Ribotoxic stress response: activation of the stress-activated protein kinase JNK1 by inhibitors of the peptidyl transferase reaction and by sequence-specific RNA damage to the alpha-sarcin/ricin loop in the 28S rRNA. Mol Cell Biol. 1997;17:3373–3381. doi: 10.1128/mcb.17.6.3373. [DOI] [PMC free article] [PubMed] [Google Scholar]
  31. Kemp N, Bashir ZI. Induction of LTD in the adult hippocampus by the synaptic activation of AMPA/kainate and metabotropic glutamate receptors. Neuropharmacology. 1999;38:495–504. doi: 10.1016/s0028-3908(98)00222-6. [DOI] [PubMed] [Google Scholar]
  32. Kemp N, Bashir ZI. Long-term depression: a cascade of induction and expression mechanisms. Prog Neurobiol. 2001;65:339–365. doi: 10.1016/s0301-0082(01)00013-2. [DOI] [PubMed] [Google Scholar]
  33. Kemp A, Manahan-Vaughan D. Hippocampal long-term depression: master or minion in declarative memory processes? Trends Neurosci. 2007;30:111–118. doi: 10.1016/j.tins.2007.01.002. [DOI] [PubMed] [Google Scholar]
  34. Li XM, Li CC, Yu SS, Chen JT, Sabapathy K, Ruan DY. JNK1 contributes to metabotropic glutamate receptor-dependent long-term depression and short-term synaptic plasticity in the mice area hippocampal CA1. Eur J Neurosci. 2007;25:391–396. doi: 10.1111/j.1460-9568.2006.05300.x. [DOI] [PubMed] [Google Scholar]
  35. Mahadevan LC, Willis AC, Barratt MJ. Rapid histone H3 phosphorylation in response to growth factors, phorbol esters, okadaic acid, and protein synthesis inhibitors. Cell. 1991;65:775–783. doi: 10.1016/0092-8674(91)90385-c. [DOI] [PubMed] [Google Scholar]
  36. Malinow R, Schulman H, Tsien RW. Inhibition of postsynaptic PKC or CaMKII blocks induction but not expression of LTP. Science. 1989;245:862–866. doi: 10.1126/science.2549638. [DOI] [PubMed] [Google Scholar]
  37. Manahan-Vaughan D, Braunewell KH. Novelty acquisition is associated with induction of hippocampal long-term depression. Proc Natl Acad Sci U S A. 1999;96:8739–8744. doi: 10.1073/pnas.96.15.8739. [DOI] [PMC free article] [PubMed] [Google Scholar]
  38. Massey PV, Bashir ZI. Long-term depression: multiple forms and implications for brain function. Trends Neurosci. 2007;30:176–184. doi: 10.1016/j.tins.2007.02.005. [DOI] [PubMed] [Google Scholar]
  39. Massey PV, Bhabra G, Cho K, Brown MW, Bashir ZI. Activation of muscarinic receptors induces protein synthesis-dependent long-lasting depression in the perirhinal cortex. Eur J Neurosci. 2001;14:145–152. doi: 10.1046/j.0953-816x.2001.01631.x. [DOI] [PubMed] [Google Scholar]
  40. Massey PV, Johnson BE, Moult PR, Auberson YP, Brown MW, Molnar E, Collingridge GL, Bashir ZI. Differential roles of NR2A and NR2B-containing NMDA receptors in cortical long-term potentiation and long-term depression. J Neurosci. 2004;24:7821–7828. doi: 10.1523/JNEUROSCI.1697-04.2004. [DOI] [PMC free article] [PubMed] [Google Scholar]
  41. Morris RG, Anderson E, Lynch GS, Baudry M. Selective impairment of learning and blockade of long-term potentiation by an N-methyl-D-aspartate receptor antagonist, AP5. Nature. 1986;319:774–776. doi: 10.1038/319774a0. [DOI] [PubMed] [Google Scholar]
  42. Moult PR, Gladding CM, Sanderson TM, Fitzjohn SM, Bashir ZI, Molnar E, Collingridge GL. Tyrosine phosphatases regulate AMPA receptor trafficking during metabotropic glutamate receptor-mediated long-term depression. J Neurosci. 2006;26:2544–2554. doi: 10.1523/JNEUROSCI.4322-05.2006. [DOI] [PMC free article] [PubMed] [Google Scholar]
  43. Moult PR, Schnabel R, Kilpatrick IC, Bashir ZI, Collingridge GL. Tyrosine dephosphorylation underlies DHPG-induced LTD. Neuropharmacology. 2002;43:175–180. doi: 10.1016/s0028-3908(02)00110-7. [DOI] [PubMed] [Google Scholar]
  44. Mulkey RM, Malenka RC. Mechanisms underlying induction of homosynaptic long-term depression in area CA1 of the hippocampus. Neuron. 1992;9:967–975. doi: 10.1016/0896-6273(92)90248-c. [DOI] [PubMed] [Google Scholar]
  45. Nosyreva ED, Huber KM. Developmental switch in synaptic mechanisms of hippocampal metabotropic glutamate receptor-dependent long-term depression. J Neurosci. 2005;25:2992–3001. doi: 10.1523/JNEUROSCI.3652-04.2005. [DOI] [PMC free article] [PubMed] [Google Scholar]
  46. Oliet SH, Malenka RC, Nicoll RA. Two distinct forms of long-term depression coexist in CA1 hippocampal pyramidal cells. Neuron. 1997;18:969–982. doi: 10.1016/s0896-6273(00)80336-0. [DOI] [PubMed] [Google Scholar]
  47. Palmer MJ, Irving AJ, Seabrook GR, Jane DE, Collingridge GL. The group I mGlu receptor agonist DHPG induces a novel form of LTD in the CA1 region of the hippocampus. Neuropharmacology. 1997;36:1517–1532. doi: 10.1016/s0028-3908(97)00181-0. [DOI] [PubMed] [Google Scholar]
  48. Peineau S, Taghibiglou C, Bradley C, Wong TP, Liu L, Lu J, Lo E, Wu D, Saule E, Bouschet T, Matthews P, Isaac JT, Bortolotto ZA, Wang YT, Collingridge GL. LTP inhibits LTD in the hippocampus via regulation of GSK3b. Neuron. 2007;53:703–717. doi: 10.1016/j.neuron.2007.01.029. [DOI] [PubMed] [Google Scholar]
  49. Rush AM, Wu J, Rowan MJ, Anwyl R. Group I metabotropic glutamate receptor (mGluR)-dependent long-term depression mediated via p38 mitogen-activated protein kinase is inhibited by previous high-frequency stimulation and activation of mGluRs and protein kinase C in the rat dentate gyrus in vitro. J Neurosci. 2002;22:6121–6128. doi: 10.1523/JNEUROSCI.22-14-06121.2002. [DOI] [PMC free article] [PubMed] [Google Scholar]
  50. Schnabel R, Kilpatrick IC, Collingridge GL. An investigation into signal transduction mechanisms involved in DHPG-induced LTD in the CA1 region of the hippocampus. Neuropharmacology. 1999;38:1585–1596. doi: 10.1016/s0028-3908(99)00062-3. [DOI] [PubMed] [Google Scholar]
  51. Silva AJ, Stevens CF, Tonegawa S, Wang Y. Deficient hippocampal long-term potentiation in alpha-calcium-calmodulin kinase II mutant mice. Science. 1992;257:201–206. doi: 10.1126/science.1378648. [DOI] [PubMed] [Google Scholar]
  52. Volk LJ, Daly CA, Huber KM. Differential roles for group 1 mGluR subtypes in induction and expression of chemically induced hippocampal long-term depression. J Neurophysiol. 2006;95:2427–2438. doi: 10.1152/jn.00383.2005. [DOI] [PubMed] [Google Scholar]
  53. Wang Q, Chang L, Rowan MJ, Anwyl R. Developmental dependence, the role of the kinases p38 MAPK and PKC, and the involvement of tumor necrosis factor-R1 in the induction of mGlu-5 LTD in the dentate gyrus. Neuroscience. 2007;144:110–118. doi: 10.1016/j.neuroscience.2006.09.011. [DOI] [PubMed] [Google Scholar]
  54. Wikstrom MA, Matthews P, Roberts D, Collingridge GL, Bortolotto ZA. Parallel kinase cascades are involved in the induction of LTP at hippocampal CA1 synapses. Neuropharmacology. 2003;45:828–836. doi: 10.1016/s0028-3908(03)00336-8. [DOI] [PubMed] [Google Scholar]
  55. Xu L, Anwyl R, Rowan MJ. Behavioural stress facilitates the induction of long-term depression in the hippocampus. Nature. 1997;387:497–500. doi: 10.1038/387497a0. [DOI] [PubMed] [Google Scholar]
  56. Xu L, Anwyl R, Rowan MJ. Spatial exploration induces a persistent reversal of long-term potentiation in rat hippocampus. Nature. 1998;394:891–894. doi: 10.1038/29783. [DOI] [PubMed] [Google Scholar]
  57. Yang CH, Huang CC, Hsu KS. Behavioral stress enhances hippocampal CA1 long-term depression through the blockade of the glutamate uptake. J Neurosci. 2005;25:4288–4293. doi: 10.1523/JNEUROSCI.0406-05.2005. [DOI] [PMC free article] [PubMed] [Google Scholar]

Articles from The Journal of Physiology are provided here courtesy of The Physiological Society

RESOURCES