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Long-term depression in the CNS

Graham L. Collingridge1,2,3, Stephane Peineau1,4,5,6, John G. Howland7 & Yu Tian Wang8,9.

1 MRC Centre for Synaptic Plasticity, Department of Anatomy, School of Medical Sciences, University Walk, Bristol, BS8 1TD, UK.

2 Brain Research Centre, Department of Psychiatry, University of British Columbia, Vancouver, British Columbia, V6T 1Z3, Canada.

3 Department of Brain and Cognitive Sciences, Seoul National University, Seoul, 151747, Korea.

4 Inserm, U676, H?pital Robert Debré, B?timent Ecran – Etage +3, 48 Boulevard Sérurier, 75019 Paris, France.

5 Université Paris 7, Faculté de Médecine Denis Diderot, Paris, France.

6 PremUP, Paris, France.

7 Neural Systems and Plasticity Research Group, Depts. of Psychology and Physiology, University of Saskatchewan, 9 Campus Drive, Saskatoon, Saskatchewan, S7N 5A5, Canada.

8 Brain Research Centre, Department of Medicine, Coastal Health Research Institute, University of British Columbia, Vancouver, British Columbia V6T 2B5, Canada.

9 Center for Neuropsychiatry and Graduate Institute for Immunology, China Medical University Hospital, Taichung, Taiwan.

Long-term depression (LTD) in the CNS has been the subject of intense investigation as a process that may be involved in learning and memory and in various pathological conditions. Several mechanistically distinct forms of this type of synaptic plasticity have been identified and their molecular mechanisms are starting to be unravelled. Most studies have focused on forms of LTD that are triggered by synaptic activation of either NMDARs (N-methyl-D-aspartate receptors) or metabotropic glutamate receptors (mGluRs). Converging evidence supports a crucial role of LTD in some types of learning and memory and in situations in which cognitive demands require a flexible response. In addition, LTD may underlie the cognitive effects of acute stress, the addictive potential of some drugs of abuse and the elimination of synapses in neurodegenerative diseases.

The two major forms of long-lasting synaptic plasticity in the mammalian brain — long-term potentiation (LTP) and long-term depression (LTD) — are characterized by a long-lasting increase or decrease in synaptic strength, respectively. Both processes are thought to be involved in information storage and therefore in learning and memory and other physiological processes. The last few years have seen rapid advances in our understanding of the molecular mechanisms that are involved in the generation of LTD and in the functions of LTD in health and disease.

There are several types of LTD and these can be defined in various ways (Box 1). LTD can be homosynaptic (induced in the conditioned input) or heterosynaptic (induced in a non-conditioned input) and can be induced de novo or following LTP (in which case it is called depotentiation). Although these different forms of LTD may seem similar, they use distinct molecular mechanisms and probably have different functions. A clear definition of, and distinction between, the various forms of LTD is therefore crucial for the understanding of this family of synaptic plasticity mechanisms.

  

LTD can be induced by prolonged periods of low-frequency stimulation (LFS), by pairing baseline synaptic stimulation with depolarization (known as pairing), by appropriately timed back-propagating action potentials (a form of spike-timing dependent plasticity (STDP)), or by application of an appropriate receptor agonist (known as chemical LTD (chem-LTD)) (Box 1). For a more detailed description of the discovery and characterization of these forms of LTD see Ref. 1.

In this Review, we first discuss the mechanisms that are involved in the generation of LTD, classified according to their induction, expression and signalling mechanisms. We then focus on the role of LTD in physiological and pathological processes in different regions of the CNS.

Mechanisms of induction

Most synapses that undergo LTD use L-glutamate as their neurotransmitter. L-glutamate acts on NMDA (N-methyl-D-aspartate) receptors (NMDARs), AMPA (α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid) receptors (AMPARs), kainate receptors (KARs) and metabotropic glutamate receptors (mGluRs)2.

NMDAR-dependent LTD. Depotentiation3 and de novo LTD4 require, like LTP5, NMDAR activation at many different synapses in the brain (Supplementary information S1 (table)). Thus, NMDARs are widespread triggers for synaptic plasticity, but do not determine the direction of change in synaptic efficiency.

NMDAR-dependent LTD (NMDAR-LTD) is usually induced by LFS but can also be activated as a form of STDP. In addition, a brief application of NMDA can lead to a long-lasting depression of synaptic transmission (a form of chem-LTD)6. Mutual occlusion between LFS-induced LTD and NMDA-induced synaptic depression7 suggests that they use common mechanisms. However, there are also important differences between LFS-induced and chemically induced LTD8, 9. The following paragraphs refer to LFS-induced NMDAR-LTD except where chem-LTD is specifically mentioned.

NMDARs are tetramers of various subunits (GluN subunits)2. They are usually composed of two GluN1 subunits and two GluN2 subunits, although GluN3 subunits sometimes replace GluN2. The two GluN2 subunits can be identical (GluN2A, GluN2B, GluN2C or GluN2D), forming a diheteromer, or they can be different from each other, forming a triheteromer together with two identical GluN1 subunits10.

Although genetic and pharmacological studies have suggested that NMDAR-LTD of AMPAR-mediated synaptic transmission (LTD(A)) involves the activation of specific NMDAR subtypes11, there is considerable flexibility — for example, GluN2B-containing NMDARs are required for LTD, but only under certain circumstances12, 13, 14, 15, 16. It is therefore likely that various NMDAR subtypes can trigger LTD and that the actual subtype(s) that are involved depend on various factors, such as the induction protocol that is employed, expression levels (which vary according to brain region and developmental stage17) and environmental conditions (for example, access to a running wheel)18. In addition, in young adult animals proBDNF (the precursor of brain-derived neurotrophic factor (BDNF)), acting via neurotrophin receptor P75 (p75NTR), increases GluN2B expression and so enables a GluN2B-sensitive form of NMDAR-LTD to occur19. Furthermore, hippocampal NMDAR-LTD that is induced in adult animals by blocking L-glutamate uptake may also be dependent on GluN2B activation12, 13.

De novo NMDAR-LTD in CA1 is pronounced early in development, but is more difficult to induce in brain slices from adult animals20, 21 or in intact rodent hippocampi in vivo12, 22, 23. However, hippocampal LTD can be induced using certain protocols23 and is facilitated by exposing an animal to novelty24 or mild stress12, 20, 21, 22. The effects of stress can be mimicked by blocking L-glutamate uptake12, 21. As the CNS develops, glutamate transporter mechanisms may limit activation of the NMDARs that would otherwise trigger LTD. Some studies have also shown strain differences, with LTD being more easily induced in Wistar and Sprague–Dawley strains than in hooded rat strains25, 26. However, there is also evidence that NMDAR-LTD is important for certain forms of learning and memory in the adult animal in the absence of stress (see below).

mGluR-dependent LTD. A second major form of LTD requires the activation of mGluRs. The patterns of activation that are required to induce mGluR-LTD are generally similar to those required to induce NMDAR-LTD, although there are differences depending on the synapse type. For example, NMDAR-LTD is usually induced at CA1 synapses by single-shock LFS, whereas mGluR-LTD is usually induced by paired-pulse LFS27. A form of mGluR-LTD can also be induced by the application of the group I mGluR agonist 3,5-dihydroxyphenylglycine (DHPG)28.

The finding that the weak group I mGluR antagonist L(+)-2-amino-3-phosphonopropionic acid (LAP3) blocks de novo LTD at CA1 synapses provided the first suggestion that mGluRs may be involved in LTD29. The first selective mGluR antagonist that was discovered, α-methyl-4-carboxyphenylglycine (MCPG)30, blocks both depotentiation11 and de novo LTD31 in CA1. Subsequent studies revealed that several mGluR subtypes can mediate LTD and that the subtype involved depends on the brain region — for example, mGluR1 and mGluR2 receptors mediate de novo LTD at cerebellar parallel fibre–Purkinje cell synapses32 and hippocampal–mossy fibre synapses33, respectively.

Although NMDAR-LTD and mGluR-LTD involve distinct receptors and signalling mechanisms, some types of LTD require synergistic interactions between the two receptor subtypes28. In the perirhinal cortex, LTD involves a tripartite interaction between NMDARs, group I mGluRs and group II mGluRs34.

Less common inducers of LTD. Ca2+-permeable AMPARs can trigger LTD in a subset of inhibitory neurons in the CA3 area of the hippocampus35 and a form of LTD that was described in the perirhinal cortex is induced by the activation of kainate receptors36. There are also examples of LTD that do not require activation of any class of glutamate receptor37, such as LTD that is induced by activation of muscarinic receptors38. Various forms of chem-LTD do not involve the activation of glutamate receptors, but in most cases it is unknown whether they can also be induced by the action of a synaptically-released neurotransmitter or modulator.

In most studies, LTD involves alterations in AMPAR-mediated synaptic transmission, but can also involve alterations in NMDAR39-, KAR36-, and mGluR40-mediated synaptic transmission (known as LTD(N), LTD(K) and LTD(mGluR), respectively). These other forms of LTD have begun to be investigated — for example, it has been shown that LTD(N) is typically triggered by the activation of NMDARs and in some cases39, but not always41, co-activation of mGluRs is required.

Mechanisms of expression

LTD is mediated by persistent pre- and postsynaptic changes, and the proportion of presynaptic compared with postsynaptic alterations probably depends on various factors, including the type of synapse and the developmental stage of the animal. It is possible that LTD expression involves shrinkage and elimination of both pre- and postsynaptic elements42, 43, 44, 45, but the mechanisms behind such LTD-associated structural changes are not well understood. Below, we discuss mechanisms of LTD expression about which more data exist.

Alterations in glutamate release. There is evidence that LTD46 — including NMDAR-LTD47 and mGluR-LTD48— can involve a reduction in the probability of glutamate release. This could be triggered by changes in the presynaptic terminal or by postsynaptic changes that are communicated across the synapse via a retrograde messenger. Several retrograde messengers have been proposed to be involved in LTD, including nitric oxide in NMDAR-LTD47 and lipoxygenase metabolites in hippocampal mGluR-LTD49. Endocannabinoids can also function as retrograde messengers in the striatum50, neocortex51 and cerebellum52. In the hippocampus the endocanabinoids act as mediators of a form of heterosynaptic mGluR-induced LTD that is prominent early in development53. Endocannabinoid release during LTP can also lead to LTD of GABA (γ-aminobutyric acid)-mediated synaptic transmission and this affects the subsequent plasticity of the network54.

Alterations in receptors. NMDAR-LTD(A) is mainly a postsynaptic phenomenon and this has been most directly demonstrated in hippocampal slices using L-glutamate uncaging in close proximity to synapses at which LTD was induced chemically or with LFS55, 56. Although various mechanisms may underlie the decrease in sensitivity to L-glutamate, most of the evidence points to the removal of AMPARs from the synapse11. However, LTD can also be expressed by an alteration of the conductance properties of the receptors. Specifically, at synapses at which LTP is associated with an increase in single-channel conductance (γ), subsequent depotentiation involves a decrease in this parameter57. In the same study, γ was not altered during de novo LTD or during depotentiation when γ had not been increased by LTP. This implies that multiple mechanisms of LTD can coexist at the same population of synapses.

mGluR-LTD can also involve postsynaptic receptor changes. In the cerebellum, mGluR-LTD involves reduced postsynaptic sensitivity to L-glutamate58. However, the mechanisms of AMPAR trafficking in NMDAR-LTD and mGluR-LTD may differ — for example, in the CA1, NMDAR-LTD but not DHPG-LTD (a form of mGluR-LTD that is induced by application of DHPG) was associated with a decrease in sensitivity to L-glutamate56. It is worth noting that AMPARs are probably not the only type of receptor that are modified in LTD as there is evidence that DHPG-LTD(N) involves the internalisation of NMDARs59.

Induction to expression

NMDAR-LTD and mGluR-LTD use different signal transduction mechanisms and these have been most extensively investigated in the context of AMPAR trafficking (Figs 1, 2).

Figure 1 | Signalling mechanisms involved in NMDAR-dependent LTD. a | Calmodulin (CaM) detects Ca2+ (shown by graded purple clouds) that enters via NMDARs and this leads, through a Ser/Thr protein phosphatase cascade, to activation of protein phosphatase 1 (PP1) a key enzyme in synaptically-induced LTD. PP1 can dephosphorylate various targets, including ser845 on the AMPAR subunit GluA1 and ser295 of postsynaptic density protein 95 (PSD95). b | GluA2-containing AMPARs are stabilised at synapses by an interaction with N-ethylmaleimide-sensitive factor (NSF). The neuronal calcium sensor protein hippocalcin (HPC) is a high-affinity Ca2+ sensor that can target adaptor protein 2 (AP2) to GluA2 and therefore displace NSF and initiate clathrin-mediated endocytosis of AMPARs. Ras-related protein (RalA)-binding protein 1 (RalBP1) may also be involved in the NMDAR-dependent targeting of AP2, where it associates with RalA. c | In some circumstances, protein interacting with C kinase 1 (PICK1) may aid the NMDAR-dependent disassociation of AMPARs from AMPAR-binding protein–glutamate receptor interacting protein (ABP–GRIP), potentially via the targeted phosphorylation of ser880 of GluA2 by protein kinase Cα (PKCα). PICK1, by binding actin-related protein 2/3 (Arp2/3) and F-actin, also acts as a negative regulator of Arp2/3-mediated actin polymerization. d | NMDAR-LTD is associated with phosphorylation (by protein tyrosine kinases (PTKs)) of tyr876 of GluA2 and this may also aid the exchange of PICK1 for ABP–GRIP. e | Glycogen synthase kinase-3β (GSK3β) is required for NMDAR-LTD during which it can be activated by PP1. The upstream regulators of GSK3β (the phosphoinositide 3-kinase (PI3K)–Akt pathway) enable the direct regulation of LTD by LTP. The release of cytochrome c from the mitochondria may activate caspase-9 and caspase-3, which can cleave Akt, possibly resulting in GSK3β activation. AKAP, A-kinase anchor protein; ARAP, AMPAR-associated protein; I-1, inhibitor 1; PP2B, protein phosphatase 2B; RyR, ryanodine receptor.

Figure 2 | Signalling mechanisms involved in mGluR-LTD(A). A | Signalling mechanisms that are involved in metabotropic glutamate receptor (mGluR)-dependent, AMPAR-mediated long-term depression (LTD(A)) in the hippocampus. Aa | Stimulation of group I mGluRs (here, mGlu5) leads to activation of phosphoinositide-specific phospholipase C (PLC). This can trigger the release of Ca2+ from intracellular stores and the activation of protein kinase C (PKC). In some forms of mGluR-LTD, PICK1 may target PKCα to phosphorylate ser880 of GluA2 to displace AMPAR-binding protein and glutamate receptor interacting protein (ABP–GRIP) and permit the removal of AMPARs from synapses. GRIP may then be sequestered by microtubule associated protein 1B (MAP1B). NCS1 has been implicated as a Ca2+ sensor and as a targeting molecule that is involved in this cascade. Ab | Several lines of evidence implicate p38 mitogen-activated protein kinase (p38 MAPK) and, to a lesser extent, extracellular signal-regulated kinases (ERKs) in mGluR-LTD, however, the downstream effectors are largely unknown. Ac | Activation of protein tyrosine phosphatases (PTPs) is also required for mGluR-LTD. Although the identity of the PTPs that are involved has not been firmly established, one candidate is striatal-enriched protein phosphatase (STEP). Ad | Arc is also involved in mGluR-LTD and it may help to initiate dynamin-dependent endocytosis of AMPARs. Ae | In some studies mGluR-LTD has been shown to require rapid (in a few minutes) de novo protein synthesis, with Arc, MAP1B and STEP as candidate protein molecules and eukaryotic elongation factor-2 kinase (eEF2K/eEF2) as one of the putative regulators of translation. Af | There is also evidence that the phosphoinositide 3-kinase (PI3K)-Akt-mammalian target of rapamicin (mTOR) pathway may control translation during mGluR-LTD. B | Signalling mechanisms that are involved in mGluR-LTD(A) in the cerebellum. At parallel fibre synapses onto Purkinje cells in the cerebellum, a form of mGluR-LTD(A) that requires activation of mGlu1 receptors has been extensively characterized. There are several similarities between cerebellar mGluR-LTD(A) and hippocampal mGluR-LTD(A) but some striking differences have also been found. Most notably, this form of LTD involves Ca2+ entry through voltage-gated Ca2+ channels (VGCC) and there is a requirement for 'orphan' glutamate receptor delta 2 (GluD2) subunits. A role for the nitric oxide–cyclic guanosine monophosphate (NO–cGMP) cascade has also been identified. Ser/Thr phosphatases negatively regulate this form of LTD(A) (not shown). CaM, calmodulin; CaMKII, Ca2+/calmodulin-dependent protein kinase II; DAG, dyacylglycerol; IP3, inositol trisphosphate; IP3R, inositol trisphosphate receptor; PIKE, PI 3-kinase enhancer; PP1, protein phosphatase 1; PP2A, protein phosphatase 2A; PP2B, protein phosphatase 2B.

NMDAR-LTD(A). In NMDAR-LTD(A), Ca2+ that enters through NMDARs binds to calmodulin to activate protein phosphatase 2B (PP2B; also known as calcineurin), which dephosphorylates inhibitor-1 and this leads to the activation of protein phosphatase 1 (PP1)60 (Fig. 1). PP1 then dephosphorylates its substrate(s), including ser845 on the AMPAR subunit GluA1 (Ref. 11) and this leads to LTD. In addition, Ca2+ entry triggers Ca2+ release from intracellular stores61 and this may serve to activate Ca2+ sensitive enzymes that are located further away from the postsynaptic density (PSD), where endocytosis may occur.

The first clue to the molecular mechanism that drives the endocytosis of AMPARs during NMDAR-LTD(A) was the observation that disruption of an interaction between GluA2 and N-ethylmaleimide-sensitive factor (NSF; an ATPase involved in membrane fusion events) causes AMPAR internalisation, mimicking NMDAR-LTD(A)11. Later, it was shown that clathrin-mediated endocytosis was involved in this process and that the clathrin adaptor protein AP2 also binds to the NSF site on the GluA2 subunit. This suggests that AMPARs are stabilised on the membrane by NSF and that AP2 replaces NSF to initiate AMPAR endocytosis during NMDAR-LTD(A)11.

A potential mechanism for the triggering of this exchange involves hippocalcin, a member of the neuronal calcium sensor (NCS) family. On sensing small rises in Ca2+ (10?7 to 10?5 M) hippocalcin translocates to the plasma membrane, where it forms a complex with AP2 and GluA2 that may initiate clathrin-mediated AMPAR endocytosis62. Consistent with this model, inhibition of hippocalcin function blocks NMDAR-LTD(A)62. Another AP2 targeting molecule may be Ras-related protein (RalA)-binding protein 1 (RalBP1), which binds to postsynaptic density protein 95 (PSD95) during NMDAR-LTD(A), following activation of the small GTPase RalA63. In addition, the small GTPase Rab5 has been implicated in LTD64. Other steps in clathrin-mediated endocytosis that underlies NMDAR-LTD(A) are also beginning to be determined — for example, calcyon, a protein that regulates clathrin assembly is involved in NMDAR-LTD(A) in the hippocampus65.

Protein interacting with C kinase 1 (PICK1) is another protein that binds directly to GluA2 and has been implicated in NMDAR-LTD(A). PICK1 is a low-affinity Ca2+ sensor66 that can also bind protein kinase Cα (PKCα) and that can sense, and perhaps induce, membrane curvature. PICK1 competes with the scaffolding proteins AMPAR-binding protein (ABP) and glutamate receptor interacting protein (GRIP) for binding to the carboxy-terminal (C-terminal) region of GluA2 and it also promotes internalization of GluA2-containing AMPARs11. It was originally proposed that during NMDAR-LTD(A) PICK1 may promote the synaptic removal of AMPARs by inducing the PKCα-mediated phosphorylation of ser880 of GluA2 to dissociate AMPARs from ABP–GRIP. However, most of the experimental data do not support this hypothesis. NMDA-induced internalisation of AMPARs is not dependent on the phosphorylation status of ser880 or on PICK1 (Ref. 67) and PKC inhibitors do not affect NMDAR-LTD(A)68, 69. Experiments that use inhibitors of the GluA2–PICK1 interaction have yielded conflicting results. One study acutely applied an interfering peptide and reported no effect on NMDAR-LTD(A), whereas a subsequent report described a partial inhibition11. A small-molecule inhibitor of the GluA2–PICK1 interaction was also found to partially inhibit NMDAR-LTD(A)70. These partial effects are in contrast with the abolition of NMDAR-LTD(A) that results from chronic manipulation of PICK1 (Ref. 71). Thus, the presence of PICK1 may be necessary for NMDAR-LTD(A) in the long-term but it probably plays only a minor part, if any, in the release of AMPARs from their synaptic tethers.

A more important role of ABP–GRIP may be to anchor AMPARs at non-synaptic sites (intracellular or extrasynaptic sites on the plasma membrane)11, as recently confirmed by paired-cell recording experiments72. NMDAR-LTD(A) becomes unstable if the ability of AMPARs to bind ABP–GRIP is impaired. This implies that by retaining AMPARs at non-synaptic sites this scaffolding molecule is crucial for the expression of this form of LTD. PICK1 may enable the disassociation of AMPARs from ABP–GRIP at these non-synaptic sites, thereby enabling de-depression (re-potentiation) of synaptic transmission11. Another key function of PICK1 in NMDAR-LTD(A) may be to enable actin depolymerization through an interaction with F-actin and the actin-related protein 2/3 (Arp2/3 complex), and through this process to modify neuronal architecture73.

NMDAR-LTD(A) is classically assumed to require the activation of phosphatases. However, studies using kinase inhibitors have implicated various serine/threonine (Ser/Thr) protein kinases in this process as well. These include protein kinase A (PKA)74, cyclin-dependent kinase 5 (Ref. 75), P38 mitogen-activated protein kinase (p38MAPK)76 and glycogen synthase kinase-3 (GSK3)68, 77. As with all inhibitor studies, potential off-target effects should be taken into account and this is particularly important for protein kinases because the mammalian genome encodes over 500 protein kinases. A role for GSK3 in NMDAR-LTD(A) is supported by the effects of six different GSK3 inhibitors68 including lithium, which may exert some of its therapeutic actions via this mechanism. A direct link between the protein phosphatase cascade and GSK3 was observed during NMDAR-LTD(A); PP1 dephosphorylates GSK3β and its upstream inhibitor Akt and these actions result in activation of GSK3β77 (Fig. 3). An additional mechanism of GSK3 activation may occur via caspase-3 (Ref. 78). This protease is activated during NMDAR-LTD(A) through a cascade involving cytochrome c and caspase-9 and is able to cleave Akt, thus removing its tonic inhibition of GSK3 (Ref. 78). The finding that both caspases and GSK3β, an enzyme that is deregulated in patients with Alzheimer's disease, are involved in NMDAR-LTD raises the possibility that the neurodegeneration that underlies Alzheimer's disease and related dementias may be caused at least in part by pathological activation of this form of LTD (Box 2).

  

Figure 3 | Molecular interactions between long-term potentiation and long-term depression. a | A schematic of electrode placements for induction of input-specific long-term potentiation (LTP) and long-term depression (LTD) in CA1. The hippocampal slices were obtained from 2-week-old rats. The box shows the dendritic region where enzyme activity was assessed. b | LTP can be induced after 60 shocks at 0 mV as shown by the persistent increase in the amplitude of excitatory postsynaptic currents (EPSCs) after stimulus (stim) application (top left). There is no change in response following an LTP stimulus due to washout of unknown constituents that are required for LTP when baseline recording is extended beyond about 10 minutes (top right). De novo LTD can be induced by 300 shocks at -40 mV and becomes evident by a persistent decrease in EPSC amplitude (bottom left). De novo LTD is inhibited — leaving just a transient depression — if an LTP stimulus (delivered after washout of LTP) is applied first (bottom right). c | Glycogen synthase kinase-3 β (GSK3β) is inhibited after an LTP-inducing stimulus (by phosphorylation at ser9), whereas LTD-inducing stimuli have the opposite effect. d | A diagram showing the cellular mechanism for LTD inhibition by LTP. An LTD-inducing stimulus activates protein phosphatase 1 (PP1), which dephosphorylates GSK3β to activate it and permit the induction of LTD. An LTP stimulus activates the phosphoinositide 3-kinase (PI3K)-Akt pathway, which phosphorylates GSK3β to inhibit it, thus preventing LTD. (Note that the LTP stimulus can inhibit LTD without inducing LTP, as the regulation can occur after washout of LTP.) *, significant difference from control; GluN1, NMDAR (N-methyl-D-aspartate receptor) subunit 1; GluN2, NMDAR subunit 2; GluA1, AMPAR subunit 1; GluA2, AMPAR subunit 2. Parts b and c are modified, with permission, from Ref. 77 ? (2007) Cell Press.

  

NMDAR-LTD(A) is associated with tyrosine phosphorylation of GluA2 and this suggests that protein tyrosine kinases (PTKs) are also involved. PTKs of the sarcoma (Src) family phosphorylate GluA2 at a tyrosine residue in a tyrosine-rich region of the C-terminal tail of GluA2 and this is thought to be required for AMPAR endocytosis11. Consistent with this idea, a peptide that mimics this tyrosine-rich region has been found to block NMDAR-LTD(A)79 (Box 3).

  

What are the targets of enzymes that are activated during NMDAR-LTD(A) and underlie an alteration in the synaptic expression of AMPARs? A major target seems to be PSD95, which positions calcineurin near the mouth of the NMDAR channel through an interaction with A-kinase anchor protein (AKAP)-150 (Ref. 80) and which is dephosphorylated on ser295 during LTD to enable the removal of PSD95 from the synapse and thereby permit AMPAR endocytosis81.

These mechanisms occur rapidly after NMDAR-LTD(A) is triggered. However, protein synthesis is required for LTD to be sustained as inhibitors of translation cause a recovery of synaptic transmission in a few hours25. How these newly synthesized proteins sustain LTD for longer periods of time is not known but regulators of gene transcription that may be involved in NMDAR-LTD(A) are starting to be investigated82.

mGluR-LTD(A). mGluR-LTD(A) involves signal transduction mechanisms that are different from those that underlie NMDAR-LTD(A)83. Any of the seven mGluR subtypes that are expressed in the brain could conceivably trigger LTD, potentially through different cellular mechanisms. mGluR-LTD(A) in CA1 is triggered predominantly through activation of mGlu5 receptors (Fig. 2), although mGlu1 receptors may initiate certain forms of LTD(A) in this region too84. A clearer picture of the role of mGlu1 receptors in mGluR-LTD(A) has emerged from studies of the parallel fibre–Purkinje cell synapse in the cerebellum, where their expression levels are high32 (Fig. 2).

The canonical signalling pathway of group I mGluRs involves the hydrolysis of phosphatidyl inositol to generate inositol trisphosphate (IP3) and diacylglycerol (DAG), which in turn can activate PKC. This pathway is involved in mGluR-LTD(A) triggered by both mGlu1 and mGlu5 receptors in the hippocampus69, cerebellum85 and perirhinal cortex86. A common way to induce mGluR-LTD(A) is to apply the group I selective agonist DHPG28. Most strikingly, DHPG-LTD(A) can be induced in the absence of Ca2+ (Ref. 87) and is unaffected by inhibition of PKC88. This could be because group I mGluRs can signal through both Ca2+ dependent and independent pathways, depending on how they are activated.

PICK1 is also required for mGluR-LTD(A) at different synapses11, 86, 89. In the perirhinal cortex PICK1 forms a complex with the prototypic member of the NCS family NCS-1, which could act as a high-affinity Ca2+ sensor for mGluR-LTD(A)86. As PICK1 can also bind PKCα, it is possible that NCS-1 attracts the PICK1–PKC complex to the GluA2 subunit of AMPARs in response to Ca2+ signals, resulting in phosphorylation of the subunit and disassociation from ABP–GRIP.

Several other protein kinases have been implicated in various forms of mGluR-LTD(A), including p38MAPK90, 91, 92 and possibly extracellular signal-regulated kinase (ERK)93. Phosphoinositide 3-kinase (PI3K) has been associated with DHPG-LTD(A)94. In terms of dephosphorylation, mGluR-LTD(A) involves protein tyrosine phosphatases (PTPs)90 rather than Ser/Thr protein phosphatases. DHPG-LTD involves the tyrosine dephosphorylation of GluA2 (Ref. 95) and this is associated with the endocytosis of AMPARs96. that can be blocked by the GluA23Y peptide191 (Box 3).

In summary, the two most extensively studied forms of LTD — hippocampal NMDAR-LTD(A) and mGluR-LTD(A) — involve different protein kinases and phosphatases as well as different reciprocal changes in tyrosine phosphorylation. In addition, there may be two pools of synaptic AMPARs, one linked to ABP–GRIP and the other bound indirectly to PSD95 via a transmembrane AMPAR regulatory protein (TARP). As both forms of LTD can probably occur at the same synapse, this use of different anchoring proteins and the regulated disassociation via different phosphorylation cascades may preserve the independence of the two processes during the initial phase of the process (Figs 1, 2).

New protein synthesis is required for the expression of mGluR-LTD(A) under certain circumstances97. However, mGluR-LTD(A) can also be induced in the presence of protein synthesis inhibitors90 with no alterations in its magnitude. This suggests that, at least during its initial phase, mGluR-LTD can be independent of protein synthesis. A number of factors may determine whether protein synthesis is required and these may include the developmental stage of the animal98.

In cases of LTD that involve protein synthesis, the protein synthesis-dependent component of LTD is observed within minutes of mGluR-LTD(A) induction, which clearly requires rapid de novo synthesis of one or more proteins84. Candidates for these proteins are the immediate early gene Arc/Arg3.1 (Arc)192, striatal-enriched protein phosphatase (STEP)99 and microtubule associated protein 1B (MAP1B)100. All three proteins seem to be involved in the internalisation of AMPARs following mGluRs stimulation99, 100, 101. How can mGluR-LTD(A) be both protein synthesis dependent and independent? One possibility is that crucial proteins, such as Arc, STEP and MAP1B may be in relatively short supply and are only present in sufficient amounts under certain circumstances to enable mGluR-LTD without new protein synthesis. Interestingly, mGluR-LTD(A) is facilitated in mice that lack the gene encoding fragile X mental retardation protein (FMRP)102 and it is also resistant to protein synthesis inhibitors103. Therefore, by suppressing transcription, FMRP may (under normal circumstances) limit the synthesis of proteins and thereby confer susceptibility to protein synthesis inhibition. Collectively, these results suggest that two pathways — potentially one involving ERK and protein synthesis and the other involving p38MAPK — may converge at the level of PTPs and endocytic machinery (Fig. 2).

In addition to group I mGluRs, activation of other Gq-coupled receptors can also induce LTD(A) (known as Gq-LTD(A)) and there are indications that the signalling mechanisms that underlie these other forms of LTD are not necessarily the same. In the hippocampus, pharmacological activation of muscarinic (M) 1 receptors induced LTD(A), with the involvement of ABP–GRIP, liprin-α and the tyrosine phosphatase leukocyte common antigen-related (LAR) family receptor protein tyrosine phosphatase (LAR-RPTP) within the postsynaptic neuron104 However, DHPG-LTD(A) occurred independently of this cascade, raising the possibility that Gq-LTD(A) may be highly compartmentalised in neurons. M1 receptors and mGluRs may also act synergistically in LTD, with M1 receptor providing a basal activation of PKC, which is further stimulated by mGluR activation resulting in LTD(A) via presynaptic mechanisms105. Clearly the inter-relationships between different forms of LTD are complex but provide considerable flexibility and scope for modulation. In fact, mGluR-LTD(A) may be the result of interactions with several other neuromodulators, ranging from classical neurotransmitters to ephrins106 as exemplified by the interactions between mGluRs and endocannabinoids or dopamine in the striatum84.

Compared to LTD(A), far less is known about the signalling mechanisms involved in the other forms of LTD. A clearer picture of how different receptors trigger and express LTD and how these processes are modulated and affected is likely to emerge over the next few years.