Group 1 mGluR-dependent synaptic long-term depression (mGluR-LTD): mechanisms and implications for circuitry & disease

Many excitatory synapses express Group 1, or Gq coupled, metabotropic glutamate receptors (Gp1 mGluRs) at the periphery of their postsynaptic density. Activation of Gp1 mGluRs typically occurs in response to strong activity and triggers long-term plasticity of synaptic transmission in many brain regions including the neocortex, hippocampus, midbrain, striatum and cerebellum. Here we focus on mGluR-induced long-term synaptic depression (LTD) and review the literature that implicates Gp1 mGluRs in the plasticity of behavior, learning and memory. Moreover, recent studies investigating the molecular mechanisms of mGluR-LTD have discovered links to mental retardation, autism, Alzheimer’s disease, Parkinson’s disease and drug addiction. We discuss how mGluRs lead to plasticity of neural circuits and how the understanding of the molecular mechanisms of mGluR plasticity provides insight into brain disease. Cellular mechanisms of Group 1 mGluR-dependent synaptic plasticity Group 1 mGluRs are comprised of mGluR1 and mGluR5 and constitute a subclass of metabotropic glutamate receptors that are canonically linked to the Gαq/11 heterotrimeric Gproteins. Immunoreactivity for mGluR1 and mGluR5 are largely complementary in the CNS (figure 1, for a review see (Ferraguti and Shigemoto, 2006)). mGluR1 staining is most intense in Purkinje cells of the cerebellar cortex and mitral/tufted cells of the olfactory bulb. Strong expression is also observed in neurons of the lateral septum, the pallidum and in the thalamus. mGluR5 on the other hand is observed in the cerebral cortex, hippocampus, subiculum, olfactory bulb, striatum, nucleus accumbens and lateral septal nucleus. In the hippocampus, mGlu5 is mainly expressed in dendritic fields of the stratum radiatum, whereas mGluR1 is mostly found on cell bodies. Subcellularly, group 1 mGluRs are localized postsynaptically in a perisynaptic zone surrounding the ionotropic receptors (Lujan et al., 1996). At excitatory synapses mGluRs are thus well positioned for rapid and selective regulation of excitatory © 2009 Elsevier Inc. All rights reserved. Correspondence should be addressed to: Dr. Kimberly M. Huber, Department of Neuroscience, UT Southwestern Medical Center, 5323 Harry Hines Blvd., NA4.118, Dallas, Texas 75390-9011. [email protected]. Dr. Christian Lüscher, Dept. NeuFo, Centre médical universitaire, 1 Michel Servet, 1211 Geneva, Switzerland. [email protected]. Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. NIH Public Access Author Manuscript Neuron. Author manuscript; available in PMC 2011 February 25. Published in final edited form as: Neuron. 2010 February 25; 65(4): 445–459. doi:10.1016/j.neuron.2010.01.016. N IH PA Athor M anscript N IH PA Athor M anscript N IH PA Athor M anscript synaptic strength for example by redistribution of AMPA and NMDA receptors. Hence, Gp1 mGluRs are known to facilitate or induce both long-term depression (LTD) and potentiation (LTP) of synaptic strength (Anwyl, 1999;Bellone et al., 2008). Gp1 mGluRs also trigger plasticity of non-synaptic conductances that lead to enhanced neuronal excitability (Wong et al., 2004). The best-characterized synaptic plasticity induced by Gp1 mGluRs is a long-term depression (LTD) of excitatory synaptic strength. MGluR-LTD was first described at the granule cell parallel fiber (PF) synapses onto Purkinje cells (PC) in the cerebellum and subsequently has been demonstrated in diverse brain regions such as the hippocampus, neocortex, dorsal and ventral striatum and spinal cord (reviewed in (Bellone et al., 2008;Gladding et al., 2009;Jorntell and Hansel, 2006)). Consequently, much of the detailed molecular mechanisms of cerebellar mGluR-LTD are known and there is strong evidence for its role in cerebellar-dependent learning (Jorntell and Hansel, 2006). The study of hippocampal mGluR-LTD has lead to the discovery of novel cellular mechanisms with implications for disease, but its contribution to normal hippocampal function remains elusive. mGluR-LTD has also been demonstrated in medium spiny neurons (MSNs) of the striatum and dopamine neurons of the midbrain where there is some overlap among the underlying molecular mechanisms. At the systems level, there is strong evidence for a role of mGluR-LTD in goal directed learning, Parkinson’s disease and drug addiction (Fig. 1). For the purpose of this review, we will focus on the conserved mGluR-LTD mechanisms, their role in normal brain function and their implication for neurological diseases and drug addiction. Cerebellar and hippocampal mGluR-LTD: Conservation of a postsynaptic LTD expression mechanism At both hippocampal CA1 synapses and cerebellar parallel fiber to Purkinje cell (PF-PC) synapses brief activation of Gp1 mGluRs either by pharmacological or synaptic stimulation induces LTD. mGluR1 is the primary Gp1 mGluR expressed in cerebellar PCs and consequently is solely responsible for PF-PC LTD (Romano et al., 1995; Shigemoto et al., 1992). Coincident synaptic activation of mGluR1 at PF inputs onto PCs together with PC depolarization provided by climbing fibers from the inferior olive, is required to induce LTD. Climbing fiber-mediated depolarization activates voltage-dependent Ca2+ channels and increases intracellular Ca2+ concentration in PCs, which together with PF mediated mGluR1 activation, induces LTD specifically of the active PF inputs. The requirement for coincident parallel and climbing fiber activation in the induction of long-term synaptic depression of PFPC synapses was predicted by learning theorists Marr and Albus as well as the neuroscientist Ito and is a critical component of the learning mechanism ((Ito, 1982); reviewed in (Jorntell and Hansel, 2006) (Kano et al., 2008). In hippocampal CA1 pyramidal neurons, mGluR-LTD is typically induced with either prolonged low frequency synaptic stimulation (1–3 Hz, 5–15 min) of the CA3 Schaffer collateral axons or brief application of the Gp1 mGluR agonist, R,SDihydroxyphenylglycine (5–10 min; DHPG) and is observed in slice preparations as well as in vivo in awake, behaving rodents (Bolshakov and Siegelbaum, 1994; Huber et al., 2000; Kemp and Bashir, 1999; Manahan-Vaughan, 1997; Naie and Manahan-Vaughan, 2005; Palmer et al., 1997; Volk et al., 2007). Because the induction and expression mechanisms differ across development (Nosyreva and Huber, 2005) reviewed in (Bellone et al., 2008), here we will focus on mGluR-LTD mechanisms in mature CA1 neurons (>2nd postnatal week in rodents) where most recent work has focused. Although mGluR5 is highly expressed in area CA1, activation of either mGluR1 or mGluR5 appears to be sufficient for agonist-induced LTD (Fitzjohn et al., 1999; Hou and Klann, 2004; Palmer et al., 1997; Volk et al., 2006). Interestingly, mGluR1 activity is also required for LTD expression in CA1, but the cellular mechanism by which mGluR1 mediates LTD expression is unknown (Volk et al., 2006). Paired pulses of low frequency synaptic stimulation (1 Hz; 50 msec interstimulus interval; PP-LFS), induce LTD through activation of Gp1 mGluRs (mGluR1 and 5) in conjunction with the Gq-coupled M1 muscarinic acetylcholine receptors (mAChRs). In other words, mGluR1/5 antagonists alone Lüscher and Huber Page 2 Neuron. Author manuscript; available in PMC 2011 February 25. N IH PA Athor M anscript N IH PA Athor M anscript N IH PA Athor M anscript do not block LTD induced by PP-LFS, but only when combined with an M1 mAChR antagonist (Kemp and Bashir, 1999; Volk et al., 2006; Volk et al., 2007). Deafferented cholinergic fibers from the septal nucleus maintain the capacity to release acetylcholine when stimulated extracellularly in the stratum radiatum of acute hippocampal slices (Cole and Nicoll, 1983; Shinoe et al., 2005), thus explaining the specific contribution of M1 mAChRs to LTD induced with extracellular stimulation, in contrast to chemically induced mGluR-LTD. Gp1 mGluRs are canonically linked to activation of phospholipase C (PLCβ), inositol trisphosphate (IP3) generation, release of Ca2+ from intracellular stores and Protein Kinase C (PKC) activation, of which all are required for cerebellar mGluR-LTD (reviewed in (Kano et al., 2008). In contrast, hippocampal mGluR-LTD requires Gαq,, but occurs independently of postsynaptic Ca2+ increases, IP3 sensitive Ca2+ stores, PLC or PKC activity (Fitzjohn et al., 2001; Kleppisch et al., 2001; Moult et al., 2006) (Huber and Bear, unpublished} implicating distinct Gαq dependent signaling pathways. However it is important to emphasize that most of these findings were obtained using bath application of the Gp1 mGluR agonist DHPG to induce LTD. Recent work using glutamate uncaging onto individual spines of CA1 neurons suggests a role for mGluR-induced Ca2+ increases in the spine in mGluR-LTD (see (Holbro et al., 2009). Therefore, it will be important to test the requirement for postsynaptic Ca2+ and PLC using more physiological, mGluR-LTD paradigms in response to synaptically released glutamate. Although cerebellar and hippocampal mGluR-LTD rely on two distinct mGluR signaling pathways, both ultimately trigger endocytosis of ionotropic AMPA receptor subunits (GluR1 and 2 in CA1 and GluR2 only in PCs) and a long-term reduction in the number of postsynaptic surface AMPAR (Moult et al., 2006; Snyder et al., 2001; Steinberg et al., 2004; Wang and Linden, 2000) (Fig. 4). As discussed below, mGluR-LTD in other brain regions such as the midbrain and the striatum can be expressed by other preor postsynaptic mechanisms including insertion of lower conductance GluR2-containing AMPARs and the retrograde signaling of endocannabinoids. Therefore, mGluR-LTD can come about through a number of expression mechanisms (Bellone et al., 2008; Gladding et al., 2009). The mechanisms of mGluR-triggered AMPAR endocytosis are best understood in cerebellar PCs. At PCs, mGluR-LTD and AMPAR endocytosis are induced by Ca2+-dependent activation of Protein kinase C (PKC) and phosphorylation of GluR2 at Ser880 (Chung et al., 2003; Steinberg et al., 2006). This GluR2 phosphorylation reduces its affinity for the AMPAR scaffold GRIP, which leads to increased AMPAR endocytosis and reduced surface AMPAR expression (Chung et al., 2003). In contrast, mGluR-induced LTD and AMPAR endocytosis in CA1 do not require PKC, but instead rely on tyrosine dephosphorylation and the tyrosine phosphatase STEP (Striatal-enriched tyrosine phosphatase). mGluR activation results in dephosphorylation of GluR2 on Tyr residues; a process which requires STEP (Moult et al., 2006; Moult et al., 2002; Schnabel et al., 1999; Zhang et al., 2008) (Fig. 4). Recent exciting work implicates the matrix metalloproteinase (MMP) TACE (tumor necrosis factor-α-converting enzyme) in mGluR-triggered AMPAR endocytosis underlying both hippocampal and cerebellar mGluR-LTD. Gp1 mGluRs stimulate the enzymatic activity of TACE, which in turn cleaves the intramembrane protein neuronal pentraxin receptor (NPR) to release its extracellular pentraxin domain. The NPR ectodomain cleavage product clusters AMPARs through extracellular interactions and stimulates their endocytosis (Cho et al., 2008) (Fig. 4). The discovery of mGluR induced regulation of TACE and NPR cleavage revealed a novel mechanism by which mGluRs invoke synaptic plasticity. Therefore, many questions remain such as: How mGluRs regulate or couple to TACE activation? How NPR-induced clustering of AMPARs interacts with Tyr dephosphorylated GluR2 or other endocytic proteins implicated in mGluR-LTD such as Arc (see below)? Furthermore, determining if mGluRs activate other MMPs or intramembrane cleavage of other known TACE substrates such as TNFα or amyloid precursor protein (APP) will be important Lüscher and Huber Page 3 Neuron. Author manuscript; available in PMC 2011 February 25. N IH PA Athor M anscript N IH PA Athor M anscript N IH PA Athor M anscript to understand the complexities of mGluR regulation of synaptic function and its involvement in disease (Black, 2002; Blobel, 2000). It is important to point out that there are distinct forms of LTD, independent of mGluRs that coexist at CA1 synapses and the nucleus accumbens. These forms typically rely on activation of NMDA receptors (NMDARs). Like mGluR-LTD, NMDAR-dependent LTD is mediated by decreases in postsynaptic AMPAR number, but has distinct molecular mechanisms. mGluRLTD and NMDAR-LTD may also be induced at distinct synapses or affect distinct populations of surface AMPARs. In support of the former, 2-photon uncaging of glutamate onto individual spines of CA1 neurons expressing fluorescent endoplasmic reticulum (ER) proteins revealed that spines with an ER were susceptible to mGluR-LTD; whereas spines without an ER were refractory to LTD induction (Holbro et al., 2009). ER-containing spines were larger in volume and responded to glutamate with larger synaptic currents as well as mGluR-mediated Ca2+ transients, suggesting a role for Ca2+ released from ER-stores in hippocampal mGluR-LTD. Importantly, this data provides evidence that mGluR-LTD may function to selectively weaken strong synaptic inputs and/or destabilize stable, mature spines. A corollary of this hypothesis is that NMDAR-dependent LTD is more prevalent at smaller spines and/or weaker synapses without an ER. Recent evidence supports this view. A structural correlate of NMDAR-LTD is the separation of spines from their associated presynaptic boutons, the latter of which selectively occurs at smaller spines (Bastrikova et al., 2008; Becker et al., 2008). Interestingly, although mGluR-mediated Ca2+ transients spread to neighboring spines, these synapses were not depressed functionally, suggesting an additional role for the ER in maintaining spinespecific mGluR-LTD. The fact that the ER is also a site of ribosome localization supports data implicating local, synaptic translation in mGluR-LTD (see below). Striatal and mesolimbic mGluR-LTD: Cell type specific plasticity and circuit function In the striatum, excitatory synaptic inputs from cortical neurons can undergo mGluR-LTD. Such cortico-striatal afferents impinge on both the neurons of the direct and the indirect striatal pathways. In both cases excitatory afferents synapse on the so-called medium spiny neurons (MSN) that send out GABAergic projections. While direct pathway MSNs monosynaptically project to the output neurons of the basal ganglia, indirect pathway neurons first connect to the medial globus pallidus and the subthalamic nucleus, before reaching the output nuclei (Fig. 2). The two types of neurons also express distinct sets of receptors. D1 and M4 muscarinic receptors mark direct pathway MSNs, while D2 receptors are exclusively expressed on indirect pathway MSNs. Recently, expressing GFP under the control of M4 receptor (M4-GFP) or dopamine D2 receptor (D2-GFP) promoter allowed the visualization of direct and indirect pathway neurons respectively in living tissues (Gerfen, 2006). Meanwhile several similar mouse lines that express large constructs have been generated, thanks to bacterial artificial chromosome (BAC) technology, and improved our understanding of the role in plasticity for shaping the striatal networks (for a recent review see (Kreitzer and Malenka, 2008)). Previous work has shown that MSNs in general are capable of expressing forms of mGluRLTD independent of NMDA receptor activation, but suggested a dependence of D2 receptors (Calabresi et al., 1997). Thanks to BAC-transgenic mice, which express GFP selectively in D1/M4or D2-expressing neurons, two distinct forms of LTD have recently been extensively characterized. They both are induce by the activation of postsynaptic mGluRs, require endocannabinoids as retrograde messenger, and are expressed by presynaptic mechanisms. A striking difference is that mGluR-LTD in MSNs of the indirect pathway requires D2 receptor activation, while LTD in direct neurons is blocked when D1 receptors are activated (Kreitzer and Malenka, 2007; Shen et al., 2009). In all MSNs, LTD is typically initiated by high frequency stimulation (10–15 Hz; HFS; or strong pharmacological activation of the mGluRs), which triggers an induction mechanism in the postsynaptic neuron, followed by the presynaptic Lüscher and Huber Page 4 Neuron. Author manuscript; available in PMC 2011 February 25. N IH PA Athor M anscript N IH PA Athor M anscript N IH PA Athor M anscript expression of the plasticity (Choi and Lovinger, 1997) (Fig. 2). An endocannabinoid, most likely anandamide, serves as the retrograde signal originating in the postsynaptic cell and affecting transmitter release in the presynaptic partner. This mechanism has been formally demonstrated only for indirect pathway MSNs (Gerdeman et al., 2002). Other authors have therefore classified striatal LTD of glutamatergic transmission among the eCB-LTDs (Heifets and Castillo, 2009). Here, we will keep with a nomenclature that makes reference to the induction mechanism; mGluR-LTD (Bellone et al., 2008). When LTD is elicited in slice preparations of the dorsal striatum, extracellular HFS concomitantly activates glutamatergic axons from the cortex and dopaminergic fibers arising from the midbrain. This activates Gp I mGluRs along with D2 receptors in indirect pathway MSNs. Several experiments led to the conclusion that D2 receptor activation gates the induction of mGluR-LTD in these neurons. In fact, mGluR agonists alone will only transiently depress synaptic transmission, while in the presence of a D2 agonist LTD is observed (Kreitzer and Malenka, 2007). D2R are Gio coupled, hence liberating Gβγ dimers, which can recruit and activate PLCb (Hernandez-Lopez et al., 2000), eventually triggering the synthesis and release of endocannabinoids. In addition, L-type voltage-gated calcium channels may also positively modulate the mobilization of the endocannabinoids. Clearly pharmacological stimulation of CB1 receptors alone is not enough to cause mGluR-LTD in the dorsal striatum, and there is a requirement for low-frequency presynaptic activity during CB1R activation (Singla et al., 2007). The concomitant activation of CB1 receptors and presynaptic activity has been proposed to confer synapse specificity to mGluR-LTD thus limiting volume transmission of endocannabinoids. Although MSNs express higher levels of mGluR5 relative to mGluR1, pharmacological evidence suggests that the mGluR1 is the primary receptor to drive mGluR-LTD in the striatum (Gubellini et al., 2001), but these findings await confirmation in genetic mouse models. For direct pathway MSNs, in contrast, a precisely timed activation of the preand the postsynaptic neurons (i.e. a spike-time dependent plasticity (STDP) protocol) is very efficient to induce mGluR-LTD, provided D1 receptors are pharmacologically blocked (Shen et al., 2008). As mentioned above, in both types of MSNs, endocannabinoids diffuse retrogradely to the presynaptic terminal where they bind to CB1 receptors and reduce the release probability, presumably through inhibition of calcium channels and inhibition of adenylyl cyclase and PKA. It is believed that mGluR-LTD is maintained by a steady production of endocannabinoids, although the experimental evidence remains controversial. Alternatively, CB1 signaling may permanently reduce the release probability. Although not directly tested in the striatum, in other parts of the brain once LTD is established, washing in a CB1 antagonist typically does not reverse the depression (Heifets and Castillo, 2009). For example, mGluR and eCB dependent LTD of GABAergic synaptic transmission onto hippocampal CA1 neurons is expressed by an alteration of the release machinery through an effect on the active-zone protein RIM1a (Chevaleyre et al., 2007; Schoch et al., 2002). In the ventral striatum, a similar form of mGluR-LTD can be observed. In MSNs of the NAc Gp1 mGluRs are also expressed postsynaptically, and their activation eventually leads to the release of endocannabinoids and a long-lasting decrease of the presynaptic release probability (Robbe et al., 2002). Efficient protocols to induce LTD require sustained high frequency stimulation (e.g. 10–15 Hz for 1 minute), presumably so that enough glutamate is released to reach the perisynaptically located mGluRs by diffusion. Pharmacological stimulation mGluRs is sufficient to induce LTD. However the in vivo pre-exposure of the animal to a single injection of tetrahydrocannabinol (THC) or cocaine will block the expression of LTD through an unknown mechanism (Fourgeaud et al., 2004). Future studies will have to address the question whether heterogeneity in the neuronal population (and therefore plasticity) of the NAc exists, similar to the one described for the dorsal striatum. Lüscher and Huber Page 5 Neuron. Author manuscript; available in PMC 2011 February 25. N IH PA Athor M anscript N IH PA Athor M anscript N IH PA Athor M anscript Taken together, mGluR.LTD is expressed throughout the striatum. In the dorsal part, dopamine gates mGluR-LTD, albeit with opposing polarity. In the indirect pathway the presence of dopamine promotes LTD, while in the direct pathway dopamine prevents LTD. Whether a similar segregation also exists in the ventral striatum remains to be shown. mGluR-LTD contributes to balance direct and indirect pathways, dominated by a long-lasting inhibitory effect on the indirect pathway (Surmeier et al., 2007). The crucial role of dopamine in controlling plasticity has received much attention, because studies in awake non-human primates suggest that dopamine release from midbrain neurons represent a learning signal shaping goal directed actions (see below, (Schultz, 2006). In dopamine neurons of the VTA, normally no mGluR-LTD is observed, even when Gp 1 mGluRs are strongly stimulated. The plasticity can be unmasked, after an exposure to an addictive drug, such as cocaine. In fact mGluR-LTD has been identified as the mechanism by which cocaine-evoked potentiation of excitatory afferents onto dopamine neurons is reversed (Fig. 3) (Bellone and Lüscher, 2006) and could therefore also be referred to as a mGluRdepotentiation. In drug-naïve mice, pharmacological activation of GpI mGluRs only transiently affects synaptic transmission. This short-term depression involves retrograde release of endocannabinoids, which in the VTA unlike the NAc does not seem to permanently affect excitatory transmission, but may be involved in LTD of GABAergic transmission (Pan et al., 2008). Pharmacological as well as experiments in KO mice suggest that the short-term depression is mediated by mGluR5, while mGluR-LTD is induced by mGluR1. At the same synapse another form of LTD coexists that can be observed acute midbrain slices. When excitatory afferents onto DA neurons are stimulated with a low frequency (e.g. 1 Hz for 15 minutes) a form of LTD is triggered that depends neither on mGluRs nor NMDAR but is regulated by PKA (Gutlerner et al., 2002). The expression mechanism of mGluR-LTD in the VTA is unique. Rather than reducing the number of AMPA receptors (see above), mGluR-LTD in the VTA relies on an exchange of receptors with a distinct subunit composition (Fig. 3). Typically, in naïve rodents, AMPARs are GluR2-containing heteromers, as in many other parts of the brain. Within hours of a single exposure to cocaine a substantial fraction of AMPAR become GluR2-lacking, and excitatory transmission therefore calcium permeable. Activation of mGluR1 causes the selective removal of the GluR2-lacking AMPARs that are then replaced with GluR2 containing ones. mGluR-dependent LTD: A model of rapid translational control of synaptic function A common cellular mechanism for mGluR-LTD in many brain regions including cerebellar PCs, hippocampus, VTA and probably also striatum is the reliance on rapid (in minutes) protein synthesis (Huber et al., 2000; Karachot et al., 2001; Mameli et al., 2007; Waung and Huber, 2009). The rapid requirement for translation in mGluR-LTD predicts a role for locally synthesized proteins (Fig. 4). This was first demonstrated in the CA1 pyramidal neurons, where the protein synthesis required for LTD occurs in dendrites (Huber et al., 2000). As mentioned above, activation of Gq coupled, M1 muscarinic acetylcholine receptors (mAChRs) either with synaptic stimulation (PP-LFS) or an M1 mAChR agonist, induces LTD onto CA1 neurons that shares similar mechanisms as mGluR-LTD (Volk et al., 2007). Therefore, Gq coupled receptors activate a common postsynaptic, protein synthesis-dependent LTD mechanism that is mediated by a persistent decrease in AMPAR number. It has been hypothesized that Gq coupled receptors stimulate the rapid local synthesis of new proteins, which participate in the regulation of AMPAR endocytosis and/or trafficking after endocytosis (Fig.1). For the purpose of this review, these newly synthesized proteins will be referred to as “LTD proteins”. Although mGluR-LTD in the striatum and the VTA is mediated by distinct preor postsynaptic mechanisms from hippocampal and cerebellar mGluR-LTD, these forms of synaptic plasticity also rely on rapid protein synthesis (Mameli et al., 2007; Yin et al., 2006). From the study of Lüscher and Huber Page 6 Neuron. Author manuscript; available in PMC 2011 February 25. N IH PA Athor M anscript N IH PA Athor M anscript N IH PA Athor M anscript translation-dependent mGluR-LTD, researchers have gained knowledge of how synaptic activity regulates dendritic translation, what and how newly synthesized proteins alter synaptic function and discovered how disease linked proteins with translational control functions impact synaptic plasticity (Costa-Mattioli et al., 2009; Waung and Huber, 2009). It is important to note that mGluR-LTD occur independently of protein synthesis under some circumstances, for example, in hippocampal area CA1 in the Fragile X Syndrome disease model (Hou et al., 2006; Nosyreva and Huber, 2006) (discussed below) and in older rodents (3–4 months) (Moult et al., 2008), but see (Kumar and Foster, 2007). Interestingly, mGluR-LTD under these conditions is mediated by a similar postsynaptic mechanism as translation dependent LTD (i.e. tyrosine dephosphorylation and/or AMPAR endocytosis) (Kumar and Foster, 2007; Moult et al., 2008; Nosyreva and Huber, 2006). This result suggests that protein synthesis is dispensable for mGluR-LTD if existing levels of “LTD proteins” are sufficient to maintain decreases in surface AMPARs and LTD. Work in the last year has made great progress in determining the identity of the “LTD proteins” with regard to hippocampal mGluR-LTD. From these studies, a picture is emerging in which mGluRs synthesize a number of functionally related proteins that impact AMPAR trafficking. Recent work has implicated Activity-regulated cytoskeletal associated protein (Arc) in mGluRLTD (Park et al., 2008; Waung et al., 2008). Arc associates directly with dynamin 2 and endophilin, components of AMPAR endocytosis machinery, and functions to increase AMPAR endocytosis and decrease surface AMPARs (Chowdhury et al., 2006; Rial Verde et al., 2006; Shepherd et al., 2006). Consistent with this role, existing Arc protein is necessary for mGluRs to trigger AMPAR endocytosis and LTD (Fig. 4). The Arc gene is well known as an activity-dependent immediate early gene that upon induction the Arc mRNA rapidly localizes to dendrites (Link et al., 1995; Steward et al., 1998; Steward and Worley, 2001). In dendrites, Arc is rapidly (~5 min) translated in response to group 1 mGluRs and this rapid synthesis is required to maintain decreases in surface AMPARs and LTD. Interestingly, Arc levels remain elevated for the duration of LTD (at least one hour) and evidence suggests that Arc actively maintains LTD through a persistent increase in the endocytosis rate of AMPARs (Park et al., 2008; Waung et al., 2008). Because Arc transcription is induced by neuronal activity associated with salient experiences and learning (Guzowski et al., 1999; Guzowski et al., 2006; Link et al., 1995; Lyford et al., 1995). This suggests that mGluR-LTD may participate in the encoding of Arc-inducing experience. MGluRs induce rapid translation other proteins that regulate AMPAR trafficking, such as microtubule associated protein 1b (MAP1b) and STEP. STEP and MAP1b negatively regulate AMPAR surface expression and are required for mGluRs to reduce AMPAR surface expression, suggesting that they contribute to mGluR-LTD (Davidkova and Carroll, 2007; Zhang et al., 2008). STEP synthesis may function in LTD to dephosphorylate GluR2 and to maintain an increased endocytosis rate (Zhang et al., 2008) (Moult et al., 2006). MAP1b interacts with a GluR scaffold, GRIP, and may function to sequester GRIP and associated AMPARs from the synaptic surface (Davidkova and Carroll, 2007; Seog, 2004). In summary, mGluRs stimulate a coordinated translation of proteins that together reduce surface AMPAR expression. The “LTD proteins” that underlie cerebellar mGluR-LTD are unknown. Because the late phase of cerebellar LTD relies on newly translated proteins to maintain surface AMPAR decreases, Arc and MAP1b may play a similar role in the cerebellum. Although mGluR-LTD in the VTA and striatum is expressed via distinct postsynaptic or presynaptic mechanisms, it also requires rapid protein synthesis suggesting a common mechanism by which mGluR invoke plasticity. As mentioned above, in dopamine neurons of the VTA, mGluR-LTD is expressed by insertion of lower conductance, GluR2 containing AMPARs (Bellone and Lüscher, 2005). Interfering selectively with the synthesis of GluR2 through the diffusion of siRNA into the postsynaptic dopaminergic neuron blocked mGluRLüscher and Huber Page 7 Neuron. Author manuscript; available in PMC 2011 February 25. N IH PA Athor M anscript N IH PA Athor M anscript N IH PA Athor M anscript LTD (Mameli et al., 2007). In other words one of the proteins that needs to be synthesized in order to express mGluR-LTD in these neurons is GluR2, which is then reinserted into AMPARs that are built from scratch within minutes (Fig. 3). Only one report has examined the proteinsynthesis dependence of striatal mGluR-LTD, albeit without distinguishing direct and indirect pathway neurons (Yin et al., 2006). They find that in a slice preparation, where presynaptic cell bodies (i.e. the soma of the cortical neurons) had been surgically removed, the bath application but not postsynaptic filling of the MSN with the protein synthesis inhibitor anisomycin blocked mGluR-LTD. These results would suggest a presynaptic or astroglial locus of the protein synthesis. mGluR regulated translational activation in neurons Translational regulation is a major mechanism by which mGluRs induce plasticity, and therefore defining the signaling pathways by which mGluRs control translation provides insight into the plasticity mechanisms. Current evidence indicates that Gp1 mGluRs regulate translation at both the level of translation initiation and elongation as reviewed (Costa-Mattioli et al., 2009; Waung and Huber, 2009). Briefly, mGluRs appear to stimulate translation initiation through 2 major signaling pathways, the ERK-MAPK and PI3K–mTOR pathways. To initiate translation, mGluRs trigger phosphorylation of eukaryotic initiation factor 4E (eIF4E), and eIF4E binding protein (4EBP) as well as stimulate formation of the translation initiation (eIF4F) complex via the ERK and PI3K–mTOR signaling pathways (Banko et al., 2006; Ronesi and Huber, 2008a; Waung and Huber, 2009). MGluR activation of PI3K, mTOR and ERK also stimulates phosphorylation of p70 ribosomal S6 kinase (RSK) and ribosomal S6 in hippocampal slices which functions to increase translation of a subset of mRNAs (those with a 5’ terminal oligopyrimidine tract; 5’TOP) that encode ribosomes, translation factors, thus increasing the overall translational capacity of the neuron (Antion et al., 2008; Ronesi and Huber, 2008a). mGluR-LTD in CA1 relies on activation of both ERK and the PI3K–mTOR pathway (Gallagher et al., 2004; Hou and Klann, 2004). Similarly, mGluR-LTD in other brain regions such as the cerebellum and bed nucleus of the stria terminalis (BNST) also rely on ERK (Grueter et al., 2006; Ito-Ishida et al., 2006); whereas mTOR is required for LTD in the VTA (Mameli et al., 2007). Interaction of the mGluR5 C-terminal tail with the scaffold and signaling molecule Homer forms a critical link between mGluRs and activation of the translational apparatus as well as mGluR-LTD induction. Multimerization of Homer molecules scaffolds mGluR5 to PI3K enhancer (PIKE), a small GTPase that binds PI3K and stimulates its lipid kinase activity (Rong et al., 2003). Acute disruption of mGluR5-Homer interactions in hippocampal slices blocks mGluR-LTD, as well as mGluR stimulation of PI3K–mTOR, translation initiation and synthesis of Elongation factor 1a (EF1a), a 5’TOP mRNA (Ronesi and Huber, 2008a). As discussed below in the context of mGluR-LTD and addiction, mGluR1-Homer interactions in the VTA in vivo are required for mGluR1LTD and reversal of cocaine induced plasticity in this brain region (Mameli et al., 2009). MGluRs control translation elongation which also occurs through Homer interactions (Davidkova and Carroll, 2007; Park et al., 2008). MGluR5 forms direct and indirect (via Homer) association with Ca2+/calmodulin-dependent eukaryotic elongation factor 2 kinase (EF2K) (Park et al., 2008). Although somewhat counterintuitive, evidence indicates that mGluR5 inhibits translation elongation through stimulation of EF2K which, as the name implies, phosphorylates eukaryotic elongation factor 2 (EF2). Although phosphorylation of EF2 generally inhibits elongation, this is thought to make available more initiation factors for poorly initiated mRNAs such as Arc and MAP1b. In support of this hypothesis, mGluR stimulated Arc and MAP1b synthesis, as well as mGluR-LTD, are abolished with EF2K knockdown or KO; effects that are rescued by low concentrations of cycloheximide, a translation elongation inhibitor (Davidkova and Carroll, 2007; Park et al., 2008). Therefore, mGluR5 activation, through a Homer scaffold, concurrently stimulates Lüscher and Huber Page 8 Neuron. Author manuscript; available in PMC 2011 February 25. N IH PA Athor M anscript N IH PA Athor M anscript N IH PA Athor M anscript translation initiation while slightly inhibiting elongation, which coordinates translational activation of specific mRNAs required for LTD. The RNA binding protein Fragile X Mental Retardation Protein (FMRP) also contributes to the specificity of Gp1 mGluR translational activation of specific mRNAs as discussed below (Ronesi and Huber, 2008b; Waung and Huber, 2009). MGluR-dependent LTD, goal-directed learning and neurological disease of