Postsynaptic induction and presynaptic expression of group 1 mGluR-dependent LTD in the hippocampal CA1 region

A novel mechanism of hippocampal LTD involving muscarinic receptor-triggered interactions between AMPARs, GRIP and liprin-alpha

BACKGROUND

Long-term depression (LTD) in the hippocampus can be induced by activation of different types of G-protein coupled receptors, in particular metabotropic glutamate receptors (mGluRs) and muscarinic acetylcholine receptors (mAChRs). Since mGluRs and mAChRs activate the same G-proteins and isoforms of phospholipase C (PLC), it would be expected that these two forms of LTD utilise the same molecular mechanisms. However, we find a distinct mechanism of LTD involving GRIP and liprin-alpha.

RESULTS

Whilst both forms of LTD require activation of tyrosine phosphatases and involve internalisation of AMPARs, they use different molecular interactions. Specifically, mAChR-LTD, but not mGluR-LTD, is blocked by peptides that inhibit the binding of GRIP to the AMPA receptor subunit GluA2 and the binding of GRIP to liprin-alpha. Thus, different receptors that utilise the same G-proteins can regulate AMPAR trafficking and synaptic efficacy via distinct molecular mechanisms.

CONCLUSION

Our results suggest that mAChR-LTD selectively involves interactions between GRIP and liprin-alpha. These data indicate a novel mechanism of synaptic plasticity in which activation of M1 receptors results in AMPAR endocytosis, via a mechanism involving interactions between GluA2, GRIP and liprin-alpha.

Mechanisms of Group I mGluR-Dependent Long-Term Depression of NMDA Receptor–Mediated Transmission at Schaffer Collateral–CA1 Synapses

The mechanisms underlying group I metabotropic glutamate receptor (mGluR)-dependent long-term depression (LTD) of N-methyl-d-aspartate receptor (NMDAR)-mediated synaptic currents (EPSCsNMDAR) are poorly understood. Here we investigated the effects of ( R,S)-3,5-dihydroxyphenylglycine (DHPG), a selective agonist of group I mGluRs, on the EPSCsNMDAR in area CA1 of acute hippocampal slices from 6- to 8-wk Sprague-Dawley rats. DHPG acutely and persistently depressed the isolated EPSCNMDAR and transiently slowed its decay rate. Combined antagonism of mGluR1 and mGluR5 blocked the effects of DHPG. Strong calcium buffering with intracellular BAPTA did not reduce the acute depression or LTD, making the involvement of elevated postsynaptic calcium unlikely. The acute depression and LTD were not mediated by activation of tyrosine kinases or phosphatases, nor were they dependent on protein synthesis. However, the LTD was prevented by the intracellular actin-stabilizer jasplakinolide, raising the possibility that it was associated with a lateral movement of NMDARs. Supporting this hypothesis, when the effective spatial spread of synaptically released glutamate was increased using the glutamate transporter inhibitor TBOA, the resultant EPSCNMDAR did not undergo LTD in response to DHPG. Importantly, isolation of the extrasynaptic EPSCNMDAR by blockade of synaptic NMDARs with MK-801 showed that this was not due to a potentiation of the preexisting extrasynaptic component. These findings indicate that LTD of NMDAR-mediated synaptic transmission occurs via lateral movement of receptors away from the synapse.

Infrared-guided laser stimulation as a tool for elucidating the synaptic site of expression of long-term synaptic plasticity

Long-term potentiation is a synaptic mechanism thought to be involved in learning and memory. Long-term depression (LTD), an activity-dependent decrease in synaptic efficacy, may be an equally important mechanism that permits neural networks to store information more effectively. Two forms of LTD have been identified in the mammalian central nervous system, which are induced by the synaptic activation of N-methyl-D: -aspartate (NMDA) and metabotropic glutamate (mGlu) receptors, respectively. Whereas the expression mechanisms of NMDA receptor-dependent LTD have been demonstrated to be postsynaptic, those of mGlu receptor-dependent LTD have not been clearly identified. In order to address this issue, a variety of different electrophysiological methods have been used. A very elegant way to realize this experimental approach is provided by the development of photolytic application of glutamate, which allows the temporally and spatially highly specific activation of any neuron or any part of the neuron. By means of simultaneous application of electrical and photolytic stimulation techniques, it has been demonstrated that mGlu receptor-induced LTD is compatible with a presynaptic mechanism of expression.

Tyrosine dephosphorylation regulates AMPAR internalisation in mGluR-LTD

Long-term depression (LTD) can be induced at hippocampal CA1 synapses by activation of either NMDA receptors (NMDARs) or group I metabotropic glutamate receptors (mGluRs), using their selective agonists NMDA and (RS)-3,5-dihydroxyphenylglycine (DHPG), respectively. Recent studies revealed that DHPG-LTD is dependent on activation of postsynaptic protein tyrosine phosphatases (PTPs), which transiently dephosphorylate tyrosine residues in AMPA receptors (AMPARs). Here we show that while both endogenous GluR2 and GluR3 AMPAR subunits are tyrosine phosphorylated at basal activity, only GluR2 is dephosphorylated in DHPG-LTD. The tyrosine dephosphorylation of GluR2 does not occur in NMDA-LTD. Conversely, while NMDA-LTD is associated with the dephosphorylation of GluR1-serine-845, DHPG-LTD does not alter the phosphorylation of this site. The increased AMPAR endocytosis in DHPG-LTD is PTP-dependent and involves tyrosine dephosphorylation of cell surface AMPARs. Together, these results indicate that the subunit selective tyrosine dephosphorylation of surface GluR2 regulates AMPAR internalisation in DHPG-LTD but not in NMDA-LTD in the hippocampus.

In vivo metabotropic glutamate receptor 5 (mGluR5) antagonism prevents cocaine-induced disruption of postsynaptically maintained mGluR5-dependent long-term depression

Metabotropic glutamate receptor 5 (mGluR5) plays a critical role in psychostimulant-induced behavior, yet it is unclear whether mGluR5 is activated by psychostimulant administration, or whether its role is constitutive. We previously reported that activation of mGluR5 with the group I mGluR agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) can induce a long-term depression (DHPG-LTD) of glutamatergic transmission in the bed nucleus of the stria terminalis (BNST), and that ex vivo induction of this LTD is disrupted by repeated in vivo administration of cocaine. Here we demonstrate that DHPG-LTD is not maintained by alterations in glutamate release, and that postsynaptic endocytosis is necessary. Furthermore, we find that a single administration of cocaine produces a transient disruption of DHPG-LTD, and the duration of this disruption was increased by repeated days of cocaine administration. The disruption produced by cocaine was not permanent, because DHPG-LTD could be induced 10 d after cocaine administration. To test the role of mGluR5 in vivo in the cocaine-induced disruption of DHPG-LTD, we injected mice with the mGluR5 antagonist 2-methyl-6-(phenylethynyl)-pyridine before cocaine. mGluR5 antagonism during in vivo cocaine administration rescued subsequent ex vivo induction of DHPG-LTD. The effects of in vivo cocaine could be mimicked by application of cocaine to BNST-containing slices, suggesting that the actions of cocaine are local. Thus, using a novel strategy of in vivo antagonist-induced rescue of ex vivo agonist effects for the same receptor, we provide evidence suggesting that mGluR5 activation is actively recruited by in vivo cocaine.

Postsynaptic signals mediating induction of long-term synaptic depression in the entorhinal cortex

The entorhinal cortex receives a large projection from the piriform cortex, and synaptic plasticity in this pathway may affect olfactory processing. In vitro whole cell recordings have been used here to investigate postsynaptic signalling mechanisms that mediate the induction of long-term synaptic depression (LTD) in layer II entorhinal cortex cells. To induce LTD, pairs of pulses, using a 30-millisecond interval, were delivered at 1 Hz for 15 minutes. Induction of LTD was blocked by the NMDA receptor antagonist APV and by the calcium chelator BAPTA, consistent with a requirement for calcium influx via NMDA receptors. Induction of LTD was blocked when the FK506 was included in the intracellular solution to block the phosphatase calcineurin. Okadaic acid, which blocks activation of protein phosphatases 1 and 2a, also prevented LTD. Activation of protein phosphatases following calcium influx therefore contributes to induction of LTD in layer II of the entorhinal cortex.

Group S8A serine proteases, including a novel enzyme cadeprin, induce long-lasting, metabotropic glutamate receptor-dependent synaptic depression in rat hippocampal slices

Long-term potentiation and long-term depression (LTD) are forms of synaptic plasticity in the central nervous system. We now report that a group of chymotrypsin-like serine proteases, especially members of the S8A subfamily, induce LTD of evoked potentials in rat hippocampal slices. The proteolytic activity of these enzymes is required for the induction of LTD, as serine protease inhibitors prevent the effect. The depression is partly mediated by the suppression of transmitter release from glutamatergic terminals but also involves an elevation of action potential threshold with no change of post-synaptic membrane potential or input resistance. We have also isolated a novel and more potent related enzyme, cadeprin, from Aspergillus. The LTD produced by all of these proteases is not dependent on receptors for several transmitter systems, including N-methyl-d-aspartate or adenosine receptors, but is prevented by blocking group I metabotropic glutamate receptors. The activity of cadeprin, subtilisin and other S8A serine proteases may shed light on the mechanisms of LTD and a related endogenous molecule could have a physiological or pathological role as a modulator of synaptic plasticity in the mammalian hippocampus.

Loss of metabotropic glutamate receptor-dependent long-term depression via downregulation of mGluR5 after status epilepticus

Synaptic plasticity is thought to be a key mechanism of information storage in the CNS. Different forms of synaptic long-term potentiation have been shown to be impaired in neurological disorders. Here, we show that metabotropic glutamate receptor (mGluR)-dependent long-term depression (LTD), but not NMDA receptor-dependent LTD at Schaffer collateral-CA1 synapses, is profoundly impaired after status epilepticus. Brief application of the group I mGluR agonist (R,S)-3,5-dihydroxyphenylglycine (100 microM; 5 min) induced mGluR LTD in control, but not in pilocarpine-treated rats. Experiments in the presence of selective inhibitors of either mGluR5 [2-methyl-6-(phenylethynyl)-pyridine] or mGluR1 [7-(hydroxyimino)cyclopropachromen-carboxylate ethyl ester and (S)-(+)-alpha-amino-4-carboxy-2-methylbenzeneacetic acid] demonstrate that loss of mGluR LTD is most likely attributable to a loss of mGluR5 function. Quantitative real-time reverse transcription PCR revealed a specific downregulation of mGluR5 mRNA, but not of mGluR1 mRNA in the CA1 region. Furthermore, we detected a strong reduction in mGluR5 protein expression by immunofluorescence and quantitative immunoblotting. Additionally, the scaffolding protein Homer that mediates coupling of mGluR5 to downstream signaling cascades was downregulated. Thus, we conclude that the reduction of mGluR LTD after pilocarpine-induced status epilepticus is the result of the subtype-specific downregulation of mGluR5 and associated downstream signaling components.

Group I mGluRs and long-term depression: potential roles in addiction?

Addiction is an enormous societal problem. A number of recent studies have focused on adaptations at glutamatergic synapses that may play a role in the behavioral responses to drugs of abuse. These studies have largely focused on NMDA receptor-dependent forms of synaptic plasticity such as NMDA receptor-dependent long-term potentiation (LTP) and long-term depression (LTD). A growing body of evidence, however, suggests that metabotropic glutamate receptors (mGluRs) also play important roles in the behavioral responses to drugs of abuse and participate in producing synaptic plasticity at glutamate synapses. In this review, we focus first on the evidence supporting a role for mGluRs in addiction and then on the properties of mGluR-dependent forms of synaptic plasticity, focusing in particular on Gq-linked receptor-induced LTD.

N-methyl-D-aspartate receptor-dependent regulation of the glutamate transporter excitatory amino acid carrier 1

The neuronal transporter excitatory amino acid carrier 1 (EAAC1) is enriched in perisynaptic regions, where it may regulate synaptic spillover of glutamate. In this study we examined potential interactions between EAAC1 and ionotropic glutamate receptors. N-Methyl-D-aspartate (NMDA) receptor subunits NR1, NR2A, and NR2B, but not the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor subunit GluR2, were co-immunoprecipitated with EAAC1 from neuron-enriched hippocampal cultures. A similar interaction was observed in C6 glioma and human embryonic kidney cells after co-transfection with Myc epitope-tagged EAAC1 and NMDA receptor subunits. Co-transfection of C6 glioma with the combination of NR1 and NR2 subunits dramatically increased (approximately 3-fold) the amount of Myc-EAAC1 that can be labeled with a membrane-impermeable biotinylating reagent. In hippocampal cultures, brief (5 min), robust (100 microM NMDA, 10 microM glycine) activation of the NMDA receptor decreased biotinylated EAAC1 to approximately 50% of control levels. This effect was inhibited by an NMDA receptor antagonist, intracellular or extracellular calcium chelators, or hypertonic sucrose. Glutamate, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid with cyclothiazide, and thapsigargin mimicked the effects of NMDA. These studies suggest that NMDA receptors interact with EAAC1, facilitate cell surface expression of EAAC1 under basal conditions, and control internalization of EAAC1 upon activation. This NMDA receptor-dependent regulation of EAAC1 provides a novel mechanism that may shape excitatory signaling during synaptic plasticity and/or excitotoxicity.

Susceptibility to induction of long-term depression is associated with impaired memory in aged Fischer 344 rats

The current study employed aged and young male Fischer 344 rats to examine the relationship between long-term depression (LTD), age, and memory. Memory performance was measured on two tasks that are sensitive to hippocampal function; inhibitory avoidance and spatial discrimination on the Morris water maze. The slope of the extracellular excitatory postsynaptic field potential was recorded from CA3-CA1 synapses in hippocampal slices. Low frequency stimulation (LFS) induced a modest LTD only in aged animals under standard recording conditions. The decrease in synaptic transmission examined only in aged animals correlated with memory scores on the spatial task and LTD was not observed in aged animals with the highest memory scores. LTD induction was facilitated by increasing the Ca(2+)/Mg(2+) ratio of the recording medium or employing a paired-pulse stimulation paradigm. Age differences disappeared when LFS was delivered under conditions of elevated Ca(2+)/Mg(2+) in the recording medium. Using multiple induction episodes under conditions which facilitate LTD-induction, no age-related difference was observed in the maximum level of LTD. The results indicate that the increased susceptibility to LTD induction is associated with impaired memory and results from a shift in the induction process. The possible relationship between LTD and memory function is discussed.

Comparison of cellular mechanisms of long-term depression of synaptic strength at perforant path-granule cell and Schaffer collateral-CA1 synapses

This chapter compares the cellular mechanisms that have been implicated in the induction and expression of long-term depression (LTD) at Schaffer collateral-CA1 synapses to perforant path-dentate gyrus (DG) synapses. In general, Schaffer collateral LTD and long-term potentiation (LTP) both appear to be a complex combination of many alterations in synaptic transmission that occur at both presynaptic and postsynaptic sites, while at perforant path synapses, most evidence has focused on postsynaptic long-term alterations. Within the DG, the medial perforant path is far more studied than lateral perforant path synapses, where most evidence relates to the induction of heterosynaptic LTD at lateral perforant path synapses when LTP is induced in the medial perforant path. Of course, there remain many other classes of synapses in the DG where synaptic plasticity, including LTD, have been largely neglected. It is clear that a better understanding of the range of DG loci where long-lasting activity-dependent plasticity, both LTD and LTP, are expressed will be essential to improve our understanding of the cognitive roles of such DG plasticity.

NMDA-dependent, but not group I metabotropic glutamate receptor-dependent, long-term depression at Schaffer collateral-CA1 synapses is associated with long-term reduction of release from the rapidly recycling presynaptic vesicle pool

Postsynaptic alterations have been suggested to account for NMDA receptor (NMDAR)-dependent long-term depression (LTD) and long-term potentiation of synaptic strength, although there is substantial evidence supporting changes in presynaptic release. Direct chemical activation of either NMDA or group I metabotropic glutamate receptor (mGluR1) elicits LTD of similar magnitudes, but it is unknown whether they share common expression mechanisms. Using dual-photon laser-scanning microscopy of FM1-43 [N-(3-triethylammoniumpropyl)-4-(4-(dibutylamino)styryl)pyridinium dibromide] to directly visualize presynaptic vesicular release from the rapidly recycling vesicle pool (RRP) at Schaffer collateral terminals in field CA1 of rat hippocampal slices, we found that a persistent reduction in vesicular release from the RRP is induced by NMDA-LTD but not by mGluR1-LTD. Variance-mean analyses of Schaffer collateral release probability (P(r)) at varying extracellular calcium concentrations confirmed that NMDA-LTD was associated with reduced P(r), whereas mGluR1-LTD was not. Pharmacological isolation of NMDAR-dependent and mGluR-dependent forms of stimulus-evoked LTD revealed that both are composed of a combination of presynaptic and postsynaptic alterations. However, when group I mGluR-dependent LTD was isolated by combining an NMDAR blocker with a group II mGluR antagonist, this form of LTD was purely postsynaptic. The nitric oxide synthase inhibitor N omega-nitro-L-arginine blocked the induction of NMDA-LTD but did not alter mGluR-LTD, consistent with a selective role for nitric oxide as a retrograde messenger mediating NMDA-LTD. These data demonstrate that single synapses can express multiple forms of LTD with different sites of expression, that NMDA-LTD is a combination of presynaptic and postsynaptic alterations, but that group I mGluR-LTD appears to be expressed entirely postsynaptically.

Metabotropic glutamate receptors and epilepsy

Metabotropic glutamate receptors (mGluRs) play an important role in the initiation of ictal discharges by participating in the interictal-ictal transition, and may play a crucial role in recruiting normal brain tissue into synchronized discharges, thereby facilitating propagation of seizure activity. In this article we present a review of mGluRs and epilepsy studies. Structural features of mGluRs offer multiple possibilities for synthetic compounds to modulate their activity, and for many reasons these compounds are good candidates for therapeutic applications. Group I mGluRs enhance excitatory transmission as much as groups II and III mGluRs can modulate those effects. Finally, main avenues to induce epileptogenesis are considered: activation of Ca2+ channels and Ca2+/CaMKII cascade, overexpression of AMPA and/or KA receptors, enhanced NMDARs function, activation of protooncogenes leading to a steady epileptogenic state, enhancement of INaP currents, blockade of A and/or M K(+) currents, calcium channelopathies, diminished number of GABARs or functions, and down-regulation of glutamate transporters. Deregulation of mGluR signaling functions including deficits in groups II and III mGluRs or hyperactivation of group I mGluRs may occur in some forms of epilepsy, therefore targeting these mechanisms with specific pharmacological tools could provide new developments for original therapeutic approaches.

Exocytosis of vesicular zinc reveals persistent depression of neurotransmitter release during metabotropic glutamate receptor long-term depression at the hippocampal CA3-CA1 synapse

Exocytosis can be directly measured in mammalian brain slices by fluorescence detection of vesicular zinc release. Detection of the low-level evoked zinc signal [Zn]t was first demonstrated at the zinc-rich hippocampal mossy fiber pathway and required the use of high-frequency presynaptic stimulation. Here, we show that release after individual action potentials can be reliably detected even at non-mossy fiber, zinc-poor synapses in the hippocampus, a major enhancement in the temporal resolution of the technique. Short-term facilitation of release properties of zinc-positive CA3-CA1 Schaffer collateral/commissural synapses in the stratum radiatum differ from those at mossy fibers but are similar to those measured for the EPSP [field EPSP (fEPSP)]. The N-type Ca2+ channel toxin omega-conotoxin GVIA inhibited both the [Zn]t and fEPSP equally, and the modulation of neurotransmitter release by neuropeptide Y, baclofen, and adenosine as revealed by [Zn]t closely resembles that measured for the fEPSP. A long-standing controversy in hippocampal synaptic plasticity involves the site of long-term depression (LTD) at these synapses. Using zinc release as a direct marker for exocytotic events and a surrogate marker for glutamate release, we demonstrate that persistent depression of presynaptic release occurs in the late expression of DHPG [(S)-3,5-dihydroxyphenylglycine]-induced LTD at this synapse. The ability to examine release dynamics with zinc fluorescence detection will facilitate exploration of the molecular pharmacology and plasticity of exocytosis at many CNS synapses.

[Endogenous cannabinoid receptor ligands--anandamide and 2-arachidonoylglycerol]

Marijuana has been used as a traditional medicine and a pleasure-inducing drug for thousands of years around the world, especially in Asia. Delta(9)-tetrahydrocannabinol, major psychoactive component of marijuana, has been shown to interact with specific cannabinoid receptors, thereby eliciting a variety of pharmacological responses in experimental animals and human. In 1990, the gene encoding a cannabinoid receptor (CB1) was cloned. This prompted the search for endogenous ligands. In 1992, N-arachidonoylethanolamine (anandamide) was isolated from pig brain as an endogenous ligand, and in 1995, 2-arachidonoylglycerol was isolated from rat brain and canine gut as another endogenous ligand. Both anandamide and 2-arachidonoylglycerol exhibit various cannabimimetic activities. The results of structure-activity relationship experiments, however, revealed that 2-arachidonoylglycerol, but not anandamide, is the intrinsic natural ligand for the cannabinoid receptor. 2-arachidonoylglycerol is a degradation product of inositol phospholipids that links the function of the cannabinoid receptors with the enhanced inositol phospholipid turnover in stimulated tissues and cells. The possible physiological roles of cannabinoid receptors and 2-arachidonoylglycerol in various mammalian tissues such as those of the nervous and inflammatory cells are demonstrated. Furthermore, the future development of therapeutic drugs coming from this endocannabinoid system are discussed.

Extracellular-signal regulated kinase 1-dependent metabotropic glutamate receptor 5-induced long-term depression in the bed nucleus of the stria terminalis is disrupted by cocaine administration

The bed nucleus of the stria terminalis (BNST) is a key component of the CNS stress and reward circuit. Synaptic plasticity in this region could in part underlie the persistent behavioral alterations in generalized anxiety and addiction. Group I metabotropic glutamate receptors (mGluRs) have been implicated in stress, addiction, and synaptic plasticity, but their roles in the BNST are unknown. We find that activation of group I mGluRs in the dorsal BNST induces depression of excitatory synaptic transmission through two distinct mechanisms. First, a combined activation of group I mGluRs (mGluR1 and mGluR5) induces a transient depression that is cannabinoid 1 receptor dependent. Second, as with endocannabinoid-independent group I mGluR long-term depression (LTD) in the adult hippocampus, we find that activation of mGluR5 induces an extracellular signal-regulated kinase (ERK)-dependent LTD. Surprisingly, our data demonstrate that this LTD requires the ERK1 rather than ERK2 isoform, establishing a key role for this isoform in the CNS. Finally, we find that this LTD is dramatically reduced after multiple exposures but not a single exposure to cocaine, suggesting a role for this form of plasticity in the actions of psychostimulants on anxiety and reward circuitries and their emergent control of animal behavior.

Tyrosine phosphatases regulate AMPA receptor trafficking during metabotropic glutamate receptor-mediated long-term depression

Two forms of long-term depression (LTD), triggered by activation of NMDA receptors (NMDARs) and metabotropic glutamate receptors (mGluRs), respectively, can be induced at CA1 synapses in the hippocampus. Compared with NMDAR-LTD, relatively little is known about mGluR-LTD. Here, we show that protein tyrosine phosphatase (PTP) inhibitors, orthovanadate and phenylarsine oxide, selectively block mGluR-LTD induced by application of the group I mGluR agonist (RS)-3,5-dihydroxyphenylglycine (DHPG-LTD), because NMDAR-LTD is unaffected by these inhibitors. Furthermore, DHPG-LTD measured using whole-cell recording is similarly blocked by either bath-applied or patch-loaded PTP inhibitors. These inhibitors also block the changes in paired-pulse facilitation and coefficient of variation that are associated with the expression of DHPG-LTD. DHPG treatment of hippocampal slices was associated with a decrease in the level of tyrosine phosphorylation of GluR2 AMPA receptor (AMPAR) subunits, an effect blocked by orthovanadate. Finally, in dissociated hippocampal neurons, orthovanadate blocked the ability of DHPG to reduce the number of AMPA receptor clusters on the surface of dendrites. Again, the effects of PTP blockade were selective, because NMDA-induced decreases in surface AMPAR clusters was unaffected by orthovanadate. Together, these data suggest that activation of postsynaptic PTP results in tyrosine dephosphorylation of AMPARs and their removal from the synapse.

Developmental switch in synaptic mechanisms of hippocampal metabotropic glutamate receptor-dependent long-term depression

The presynaptic and postsynaptic properties of synapses change over the course of postnatal development. Therefore, synaptic plasticity mechanisms would be expected to adapt to these changes to facilitate alterations of synaptic strength throughout ontogeny. Here, we identified developmental changes in long-term depression (LTD) mediated by group 1 metabotropic glutamate receptors (mGluRs) and dendritic protein synthesis in hippocampal CA1 slices (mGluR-LTD). In slices prepared from adolescent rats [postnatal day 21 (P21) to P35], mGluR activation induces LTD and a long-term decrease in AMPA receptor (AMPAR) surface expression, both of which require protein synthesis. In neonatal animals (P8-P15), mGluR-LTD is independent of protein synthesis and is not associated with changes in the surface expression of AMPARs. Instead, mGluR-LTD at neonatal synapses results in large decreases in presynaptic function, measured by changes in paired-pulse facilitation and the rate of blockade by the use-dependent NMDA receptor blocker (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate. Conversely, mGluR-LTD at mature synapses results in little or no change in presynaptic function, suggesting a postsynaptic mechanism of expression. The developmental switch in the synaptic mechanisms of LTD would differentially affect synapse dynamics and perhaps information processing over the course of postnatal development.

Long-term depression at olfactory nerve synapses

The synapses formed by the olfactory nerve (ON) convey sensory information to olfactory glomeruli, the first stage of central odor processing. Morphological and behavioral studies suggest that glomerular odor processing is plastic in neonate rodents. However, long-term synaptic plasticity, a cellular correlate of functional and structural plasticity, has not yet been demonstrated in this system. Here, we report that ON-->mitral cell (MC) synapses of 5- to 8-d-old mice express long-term depression (LTD) after brief low-frequency ON stimulation. Pharmacological techniques and imaging of presynaptic calcium signals demonstrate that ON-MC LTD is expressed presynaptically and requires the activation of metabotropic glutamate receptors but does not require fast synaptic transmission. LTD at the ON--> MC synapse is potentially relevant for the establishment, maintenance, and experience-dependent refinement of odor maps in the olfactory bulb.

Differential roles for group 1 mGluR subtypes in induction and expression of chemically induced hippocampal long-term depression

Although metabotropic glutamate receptors (mGluRs) mGluR1 and mGluR5 are often found to have similar functions, there is considerable evidence that the two receptors also serve distinct functions in neurons. In hippocampal area CA1, mGluR5 has been most strongly implicated in long-term synaptic depression (LTD), whereas mGluR1 has been thought to have little or no role. Here we show that simultaneous pharmacological blockade of mGluR1 and mGluR5 is required to block induction of LTD by the group 1 mGluR agonist, (RS)-3,5-dihydroxyphenylglycine (DHPG). Blockade of mGluR1 or mGluR5 alone has no effect on LTD induction, suggesting that activation of either receptor can fully induce LTD. Consistent with this conclusion, mGluR1 and mGluR5 both contribute to activation of extracellular signal-regulated kinase (ERK), which has previously been shown to be required for LTD induction. In contrast, selective blockade of mGluR1, but not mGluR5, reduces the expression of LTD and the associated decreases in AMPA surface expression. LTD is also reduced in mGluR1 knockout mice confirming the involvement of mGluR1. This shows a novel role for mGluR1 in long-term synaptic plasticity in CA1 pyramidal neurons. In contrast to DHPG-induced LTD, synaptically induced LTD with paired-pulse low-frequency stimulation persists in the pharmacological blockade of group 1 mGluRs and in mGluR1 or mGluR5 knockout mice. This suggests different receptors and/or upstream mechanisms for chemically and synaptically induced LTD.

Induction and expression mechanisms of postsynaptic NMDA receptor-independent homosynaptic long-term depression

The induction of long-term depression (LTD) can be divided into two main forms, one dependent upon activation of postsynaptic NMDAR, and another independent of postsynaptic NMDAR. Non-postsynaptic NMDAR-LTD (non-NMDAR-LTD) occurs in many regions of the brain, and encompasses a wide variety of induction and expression mechanisms. In this article, the induction and expression mechanisms of such LTD in over 10 brain regions are described, with a number of common mechanisms compared across a large range of types of LTD. The article describes the involvement of different presynaptic or postsynaptic receptors in the induction of non-NMDAR-LTD, especially metabotropic glutamate receptors, cannabinoid receptors and dopamine receptors. An increase in presynaptic or postsynaptic intracellular Ca concentration is a key event in induction, commonly followed by activation of certain kinases, especially PKC, p38 MAPK and ERK. Expression mechanisms are either presynaptic via a reduction in release probability, or postsynaptic involving a decrease in AMPAR via phosphorylation of a glutamate receptor subunit, especially GluR2, followed by clathrin-mediated endocytosis. Retrograde signalling from postsynaptic to presynaptic occurs when induction is postsynaptic and expression is presynaptic.

Investigations of the protein synthesis dependency of mGluR-induced long-term depression in the dentate gyrus of freely moving rats

Hippocampal long-term depression (LTD) comprises an activity-dependent weakening of synaptic strength. In this study we compared persistent LTD induced by the group I mGluR agonist, DHPG, or the group III mGluR agonist, AP4, in the dentate gyrus of freely moving rats. The role of protein translation, using the translation inhibitors, anisomycin and emetine, was also investigated. Potentials were evoked from medial perforant path-dentate gyrus granule cell synapses of male Wistar rats by means of chronically implanted electrodes. Immediately after intracerebral (ventricular) application of DHPG or AP4 robust LTD (>24 h) occurred. Paired-pulse analysis during LTD, and application of mGluR antagonists after stabilisation of depression, supported that LTD genuinely occurred and that the depression was not a consequence of persistence of the agonists at the synapse. Application of a protein synthesis inhibitor 2 h prior to DHPG injection inhibited the expression of LTD (from ca. 6 h post-injection) but did not affect LTD induced by AP4. These data highlight differences in chemical LTD elicited by group I and group III mGluRs. Whereas AP4-induced LTD may arise as a result of modulation of presynaptic glutamate release mechanisms, the protein synthesis dependency of DHPG-induced LTD suggests an additional postsynaptic expression mechanism for this phenomenon.

Driving cellular plasticity and survival through the signal transduction pathways of metabotropic glutamate receptors

Metabotropic glutamate receptors (mGluRs) share a common molecular morphology with other G protein-linked receptors, but there expression throughout the mammalian nervous system places these receptors as essential mediators not only for the initial development of an organism, but also for the vital determination of a cell's fate during many disorders in the nervous system that include amyotrophic lateral sclerosis, Parkinson's disease, Alzheimer's disease, Huntington's disease, Multiple Sclerosis, epilepsy, trauma, and stroke. Given the ubiquitous distribution of these receptors, the mGluR system impacts upon neuronal, vascular, and glial cell function and is activated by a wide variety of stimuli that includes neurotransmitters, peptides, hormones, growth factors, ions, lipids, and light. Employing signal transduction pathways that can modulate both excitatory and inhibitory responses, the mGluR system drives a spectrum of cellular pathways that involve protein kinases, endonucleases, cellular acidity, energy metabolism, mitochondrial membrane potential, caspases, and specific mitogen-activated protein kinases. Ultimately these pathways can converge to regulate genomic DNA degradation, membrane phosphatidylserine (PS) residue exposure, and inflammatory microglial activation. As we continue to push the envelope for our understanding of this complex and critical family of metabotropic receptors, we should be able to reap enormous benefits for both clinical disease as well as our understanding of basic biology in the nervous system.

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

Previous studies have shown that brief application of group I metabotropic glutamate receptor (mGluR) agonist (S)-3, 5-dihydroxyphenylglycine (DHPG) to hippocampal slices can induce a chemical form of long-term depression (DHPG-LTD) in the hippocampal CA1 region; however, the expression mechanisms of this LTD remain unclear. We show here that the expression of DHPG-LTD can be specifically reversed by application of the broad-spectrum mGluR antagonists, (S)-alpha-methyl-4-carboxyphenylglycine (MCPG) and LY341495, and mGluR5 antagonist, 2-methyl-6-(phenylethyl)pyridine, but not by NMDA receptor antagonist, D-2-amino-5-phosphonopentanoic acid, mGluR1 antagonist, LY367385, group II mGluR antagonist, (2S)-alpha-ethylglutamic acid, or group III mGluR antagonist, (S)-2-amino-2-methyl-4-phosphonobutanic acid (MAP4). In addition, the ability of MCPG to reverse DHPG-LTD was mimicked by the protein tyrosine phosphatase inhibitors, phenylarsine oxide and orthovanadate, but not phospholipase C inhibitor, U73122, protein kinase C inhibitor, bisindolylmaleimide 1, p38 mitogen-activated protein kinase inhibitor, SB203580, or protein phosphatases 1/2 A inhibitor, okadaic acid. Moreover, MCPG reversed the DHPG-LTD without affecting the paired-pulse facilitation. The expression of DHPG-LTD was associated with the reduction of both tyrosine phosphorylation and surface expression of AMPA receptor GluR2 subunits. Together, these results suggest that sustained activation of mGluR5 and in turn triggering a protein tyrosine phosphatase-dependent regulation of postsynaptic expression of AMPA receptors may contribute to the expression of DHPG-LTD.

Long-term potentiation persists in an occult state following mGluR-dependent depotentiation

Depotentiation, the reversal of long-term potentiation (LTP), can be induced by activation of metabotropic glutamate receptors (mGluRs) or NMDA receptors (NMDARs). Although NMDAR-dependent depotentiation is due to a protein phosphatase-dependent erasure of LTP, the notion that mGluR-dependent depotentiation also involves LTP erasure is controversial. To address this issue we used electrophysiological and biochemical approaches to investigate mGluR-dependent depotentiation in hippocampal slices. Activating group I mGluRs with (R,S)-3,5-dihydroxyphenylglycine (DHPG) induced robust depotentiation in both the CA1 and CA3 regions of hippocampal slices. Western immunoblotting of samples prepared from DHPG-treated slices revealed, however, that activation of group I mGluRs causes a transient increase in phosphorylation of AMPA receptor GluR1 subunits at sites crucial for LTP and under some conditions causes persistent activation of alphaCamKII. The paradoxical ability of DHPG to induce depotentiation while at the same time activating signaling pathways involved in LTP suggests that LTP might not be erased by mGluR-dependent depotentiation. Consistent with this, DHPG-induced depotentiation did not restore the ability of high-frequency stimulation to induce LTP at synapses that had previously undergone saturating levels of LTP. In addition, blocking the expression of DHPG-induced LTD revealed hidden LTP at depotentiated synapses. Our results indicate that LTP and mGluR-dependent LTD can co-exist at excitatory synapses.

Glutamate transporter studies reveal the pruning of metabotropic glutamate receptors and absence of AMPA receptor desensitization at mature calyx of Held synapses

We examined the effect of glutamate transporter blockade at the calyx of Held synapse. In immature synapses [defined as postnatal day 8 (P8) to P10 rats], transporter blockade causes tonic activation of NMDA receptors and strong inhibition of the AMPA receptor-mediated EPSC amplitude. EPSC inhibition was blocked with a metabotropic glutamate receptor (mGluR) antagonist [1 microm LY341495 (2S-2-amino-2-(1S,2S-2-carboxycycloprop-1-yl)-3-(xanth-9-yl)propanoic acid)], suggesting that elevated resting glutamate concentration specifically activates group II and group III mGluRs. Using mGluR subtype-specific agonists and antagonists, we determined that increased glutamate activates presynaptic mGluR2/3 and mGluR8 receptors but not mGluR4, although this receptor is present. Surprisingly, in older animals (P16-P18), transporter blockade had no effect on EPSC amplitude because of a developmental downregulation of group II/III mGluR activation in rats and mice. In contrast to other CNS synapses, we observed no effect of transporter blockade on EPSC decay kinetics, although expression of glutamate transporters was strong in nearby glial processes at both P9 and P17. Finally, using a low-affinity AMPA receptor antagonist (gamma-D-glutamylglycine), we show that desensitization occurs at P8-P10 but is absent at P16-P18, even during trains of high-frequency (100-300 Hz) stimulation. We suggest that diffusion and transporter activation are insufficient to clear synaptically released glutamate at immature calyces, resulting in significant desensitization. Thus, mGluRs may be expressed in the immature calyx to help limit glutamate release. In the more mature calyx, there is a far smaller diffusional barrier attributable to the highly fenestrated synaptic terminal morphology, so AMPA receptor desensitization is avoided and mGluR-mediated inhibition is not necessary.

LTP and LTD: an embarrassment of riches

LTP and LTD, the long-term potentiation and depression of excitatory synaptic transmission, are widespread phenomena expressed at possibly every excitatory synapse in the mammalian brain. It is now clear that "LTP" and "LTD" are not unitary phenomena. Their mechanisms vary depending on the synapses and circuits in which they operate. Here we review those forms of LTP and LTD for which mechanisms have been most firmly established. Examples are provided that show how these mechanisms can contribute to experience-dependent modifications of brain function.

Glutamate transporters and metabotropic receptors regulate excitatory neurotransmission in the medial entorhinal cortex of the rat

In layer III of the medial entorhinal cortex (mEC), a region that is especially prone to cell damage in Alzheimer's disease, schizophrenia and epilepsy, effects of blocking glutamate uptake on excitatory synaptic transmission were studied. Two competitive glutamate transporter antagonists, TBOA and tPDC, reduced the amplitude of pharmacologically isolated AMPAR and NMDAR mediated EPSPs/EPSCs without changing the time course of the events. This effect was mimicked by tACPD, an agonist of groups I and II metabotropic glutamate receptors (mGluRs). The competitive groups I and II mGluR antagonist MCPG blocked the depression of the EPSC amplitude induced by tACPD and also prevented the effect of either TBOA or tPDC. Furthermore, EGLU, which selectively antagonizes group II mGluRs, blocked the effect of tPDC and LY3414965, a specific group I mGluR antagonist, abolished the reduction of amplitude caused by TBOA. Additionally, application of TBOA increased the paired-pulse index, suggesting a presynaptic mechanism for the depression of EPSP/EPSC amplitude. The present data suggest that glutamate transporters and group I/II mGluRs regulate excitatory synaptic transmission in the mEC. Presynaptic mGluRs may limit excessive glutamate accumulation if uptake becomes compromised.

Extracellular signal-regulated protein kinase activation is required for metabotropic glutamate receptor-dependent long-term depression in hippocampal area CA1

Activation of group 1 metabotropic glutamate receptors (mGluRs) induces long-term depression (LTD) of synaptic transmission that relies on dendritic protein synthesis. We investigated the signal transduction pathways required for mGluR-LTD to identify candidate mechanisms for mGluR regulation of synaptic protein synthesis. Our results demonstrate a role for extracellular signal-regulated protein kinase (ERK), a subclass of the mitogen-activated protein kinases (MAPKs), in mGluR-LTD in area CA1 of the rat hippocampus. Inhibitors of the upstream kinase of ERK, MAP/ERK kinase significantly reduce mGluR-LTD induced by the group 1 agonist dihydroxyphenylglycine (DHPG) and synaptic stimulation but do not affect NMDA receptor-dependent LTD. In contrast, inhibitors of p38 MAPK were ineffective against DHPG-induced LTD. Consistent with the role of ERK in mGluR-LTD, we observed that DHPG treatment of hippocampal slices (isolated CA1), at concentrations that induce LTD, results in a robust phosphorylation of ERK but not of p38 MAPK. These results point to ERK as an important regulator of mGluR-LTD and a potential mechanism for mGluR regulation of synaptic protein synthesis.

Frequency-dependent requirement for calcium store-operated mechanisms in induction of homosynaptic long-term depression at hippocampus CA1 synapses

For induction of long-term depression (LTD), mechanisms dependent on N-methyl-D-aspartate receptors (NMDARs) and on intracellular calcium stores have been separately known. How these two mechanisms coexist at the same synapses is not clear. Here, induction of LTD at hippocampal Schaffer collateral-to-CA1 pyramidal cell synapses was shown to depend on NMDARs throughout the theoretically predicted activation range for LTD induction. With stimulation at 1 Hz, the largest LTD was induced in a store-independent manner. With stimulation at 0.5 and 2.0 Hz the induced LTD was much smaller, and dependence on calcium stores appeared. Under caffeine application, an enlarged LTD was induced with 0.5 Hz stimulation. Postsynaptic blockade of ryanodine receptors prevented this caffeine-induced enhancement of LTD. It is therefore suggested that calcium release from calcium stores facilitated by caffeine contributed to the LTD enhancement, and that the caffeine effect was exerted on the postsynaptic side. Induction of this enhanced LTD was resistant to NMDAR blockade. We thus propose that the store-dependent mechanism for LTD induction is dormant at the centre of the theoretically predicted activation range for LTD induction, but operates at the fringes of this activation range, with its contribution more emphasized when ample calcium release occurs.

Rap1-induced p38 Mitogen-activated Protein Kinase Activation Facilitates AMPA Receptor Trafficking via the GDI·Rab5 Complex

Recent evidence has emphasized the importance of p38 mitogen-activated protein kinase (MAPK) in the induction of metabotropic glutamate receptor (mGluR)-dependent long term depression (LTD) at hippocampal CA3-CA1 synapses. However, the cascade responsible of mGluR to activate p38 MAPK and the signaling pathway immediately downstream from it to induce synaptic depression is poorly understood. Here, we show that transient activation of group I mGluR with the selective agonist (S)-3,5-dihydroxyphenylglycine (DHPG) activates p38 MAPK through G protein betagamma-subunit, small GTPase Rap1, and MAPK kinase 3/6 (MKK3/6), thus resulting in mGluR5-dependent LTD. Furthermore, our data clearly show that an accelerating AMPA receptor endocytosis by stimulating the formation of guanyl nucleotide dissociation inhibitor-Rab5 complex is a potential downstream processing of p38 MAPK activation to mediate DHPG-LTD. These results suggest an important role for Rap1-MKK3/6-p38 MAPK pathway in the induction of mGluR-dependent LTD by directly coupling to receptor trafficking machineries to facilitate the loss of synaptic AMPA receptors.

A chemical LTP induced by co-activation of metabotropic and N-methyl-d-aspartate glutamate receptors in hippocampal CA1 neurons

In CA1 neurons of guinea pig hippocampal slices, long-term depression (LTD) was induced in the field EPSP response in the absence of test synaptic inputs (one stimulus every 20 s) by application of the metabotropic glutamate receptor (mGluR) agonist, aminocyclopentane-1S, 3R-dicarboxylic acid (ACPD). This effect was blocked and long-term potentiation (LTP) was induced by co-application of N-methyl-D-aspartate (NMDA) during ACPD perfusion (ACPD/NMDA-induced LTD). These results indicate that the state of NMDA receptor activation during ACPD perfusion determines whether LTP or LTD is induced in hippocampal CA1 neurons. Co-application of an inositol 1, 4, 5-trisphosphate (IP3) receptor inhibitor, 2-aminotheoxydiphenyl borate, during ACPD application had no effect on the ACPD/NMDA-induced LTP, but increased the magnitude of the ACPD-induced LTD, suggesting that the ACPD/NMDA-induced LTP involves NMDA receptors, but not IP3 receptors, whereas the converse applies to the ACPD-induced LTD.

Extracellular signal-regulated kinase 1/2 is required for the induction of group I metabotropic glutamate receptor-mediated epileptiform discharges

Transient stimulation of group I metabotropic glutamate receptors (mGluRs) induces persistent prolonged epileptiform discharges in hippocampal slices via a protein synthesis-dependent process. At present, the signaling process underlying the induction of these epileptiform discharges remains unknown. We examined the possible role of extracellular signal-regulated kinases (ERK1 and ERK2) because these kinases can be activated by group I mGluRs, and their activation may regulate gene expression and alter protein synthesis. The group I mGluR agonist (S)-3,5-dihydroxyphenylglycine (DHPG; 50 microm) induced activation of ERK1/2 in hippocampal slices. 2-(2-Diamino-3-methoxyphenyl-4H-1-benzopyran-4-one (PD98059) (50 microm) a specific inhibitor of mitogen-activated protein kinase kinase (MEK), suppressed ERK1/2 activation by DHPG. PD98059 or another MEK inhibitor, 1,4-diamino-2,3-dicyano-1,4-bis[2-aminophenylthio]butadiene (10 microm), also prevented the induction of the prolonged epileptiform discharges by DHPG. In the presence of ionotropic glutamate receptor inhibitors and tetrodotoxin (blockers), DHPG-induced epileptiform discharges were suppressed, whereas ERK1/2 activation persisted. Protein kinase C inhibitors (2-[1-(3-dimethylaminopropyl)-5-methoxyindol-3-yl]-3-(1H-indol-3-yl) maleimide, 1 microm; or chelerythrine, 10 microm) did not prevent the generation of DHPG-induced epileptiform discharges, nor did they suppress the activation of ERK1/2 by DHPG in slices pretreated with the blockers. Genistein (30 microm), a broad-spectrum tyrosine kinase inhibitor, suppressed the DHPG-induced epileptiform discharges and the ERK1/2 activation in the presence of blockers. Induction of DHPG-mediated epileptiform discharges was also suppressed by 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-D]pyrimidine (10 microm), an Src-family tyrosine kinase inhibitor. The study shows that group I mGluRs activate ERK1/2 through a tyrosine kinase-dependent process and that this activation of ERK1/2 is necessary for the induction of prolonged epileptiform discharges in the hippocampus.

12-lipoxygenase metabolites of arachidonic acid mediate metabotropic glutamate receptor-dependent long-term depression at hippocampal CA3-CA1 synapses

Arachidonic acid metabolites have been proposed as signaling molecules in hippocampal long-term potentiation (LTP) and long-term depression (LTD) for >15 years. However, the functional role of these molecules remains controversial. Here we used a multidisciplinary biochemical, electrophysiological, and genetic approach to examine the function of the 12-lipoxygenase metabolites of arachidonic acid in long-term synaptic plasticity at CA3-CA1 synapses. We found that the 12-lipoxygenase pathway is required for the induction of metabotropic glutamate receptor-dependent LTD (mGluR-LTD), but is not required for LTP: (1) Hippocampal homogenates were capable of synthesizing the 12-lipoxygenase metabolite of arachidonic acid, 12(S)-hydroxyeicosa-5Z,8Z,10E,14Z-tetraenoic acid (HETE). (2) Stimulation protocols that induce mGluR-LTD lead to a release of 12-(S)-HETE from acute hippocampal slices. (3) A mouse in which the leukocyte-type 12-lipoxygenase (the neuronal isoform) was deleted through homologous recombination was deficient in mGluR-LTD, but showed normal LTP. (4) Pharmacological inhibition of 12-lipoxygenase also blocked induction of mGluR-LTD. (5) Finally, direct application of 12(S)-HPETE, but not 15(S)-HPETE, to hippocampal slices induced a long-term depression of synaptic transmission that mimicked and occluded mGluR-LTD induced by synaptic stimulation. Thus, 12(S)-hydroperoxyeicosa-5Z, 8Z, 10E, 14Z-tetraenoic acid (12(S)-HPETE), a 12-lipoxygenase metabolite of arachidonic acid, satisfies all of the criteria of a messenger molecule that is actively recruited for the induction of mGluR-LTD.

Synaptically stimulated induction of group i metabotropic glutamate receptor-dependent long-term depression and depotentiation is inhibited by prior activation of metabotropic glutamate receptors and PROTEIN KINASE C

We have investigated metaplasticity of the group I metabotropic glutamate receptor (mGluR)-dependent long-term depression (LTD) and depotentiation (DP) induced by physiological synaptic stimulation in the medial perforant path of the dentate gyrus in vitro. Group I mGluR-LTD/DP was inhibited by prior preconditioning brief high frequency stimulation (HFS) if the preconditioning HFS induced long-term potentiation (LTP) or if the induction of LTP was inhibited by an NMDA receptor antagonist. The inhibitory effect of the preconditioning HFS on LTD/DP was dependent upon activation of mGluRs, as it was blocked by the presence of the mGluR antagonist (S)-alpha-methyl-4-carboxyphenylglycine during the preconditioning stimulation. The inhibitory effect of the preconditioning HFS involved stimulation of PKC, as the presence of the PKC inhibitor bisindolylmaleimide (BIS) during the preconditioning stimulation prevented the inhibitory effect of such preconditioning stimulation. Activation of PKC was also necessary for the induction of mGluR-LTD itself, as the PKC inhibitor BIS prevented the induction of the mGluR-LTD. We suggest that the physiological stimulation of mGluRs by the preconditioning stimulation produces a PKC-dependent inactivation of subsequent group I mGluR functioning and thereby an inhibition of induction of group I mGluR-dependent LTD/DP induction.

Cooperativity between activation of metabotropic glutamate receptors and NMDA receptors in the induction of LTP in hippocampal CA1 neurons

In CA1 neurons of guinea pig hippocampal slices, long-term potentiation (LTP) was induced by 10 min application of 10 microM aminocyclopentane-1S, 3R-dicarboxylic acid (ACPD), the metabotropic glutamate receptor (mGluR) agonist, in the presence of test synaptic inputs (once every 20 s). In contrast, long-term depression (LTD) was induced by application of 10 microM ACPD in the absence of test inputs. When 10 microM ACPD was applied in the presence of test inputs, co-application of the N-methyl-D-aspartate (NMDA) receptor antagonist, D,L-2-amino-5-phosphonovalerate resulted in LTD induction when used at 50 microM. In ACPD-induced LTP, the delivery of test synaptic inputs to CA1 neurons could be replaced by co-application of NMDA (100 nM) during ACPD perfusion. These results suggest that, in CA1 neurons, a co-operative effect involving the activation of both mGluRs and NMDA receptors is required to trigger the process involved in ACPD-induced LTP. In addition, ACPD-induced LTD was blocked by co-application of an inositol 1,4,5-trisphosphate (IP3) receptor inhibitor, 2-aminotheoxydiphenyl borate (10 microM), which had no effect on ACPD-induced LTP. The results of the present study, therefore, indicate that ACPD-induced LTP involves NMDA receptors, but not IP3 receptors, whereas the converse applies to ACPD-induced LTD.

Effects of stimulus frequency and age on bidirectional synaptic plasticity in the dentate gyrus of freely moving rats

We investigated the frequency-dependent transition from homosynaptic long-term depression (LTD) to long-term potentiation (LTP) at the lateral perforant pathway/dentate gyrus synapse in adult (90 days of age) and immature (15 days of age) awake, freely moving rats. Dentate-evoked field potentials were recorded and analyzed using the population spike amplitude and the field EPSP slope measures following sustained stimulation (900 pulses) of the lateral perforant pathway at various frequencies (1, 3, 7, 30, 50, or 200 Hz). Our results indicate that both the strength and the direction (LTP or LTD) of synaptic plasticity vary as a function of activation frequency: sustained low-frequency stimulation ranging from 1 to 7 Hz results in depression of activated synapses, whereas high-frequency stimulation (30-200 Hz) produces potentiation. In addition, a significant (P < 0.01) ontogenetic shift in the frequency of transition from LTD to LTP was observed; the transition frequency in immature animals was significantly lower than that obtained in adult animals. These observations agree strongly with the prediction of the Bienenstock-Cooper-Munro theory of synapse modification, indicating perhaps a neurophysiological basis for this theoretical model of learning in the dentate gyrus of awake behaving rats.

Endocannabinoids contribute to short-term but not long-term mGluR-induced depression in the hippocampus

Activation of postsynaptic group 1 metabotropic glutamate receptors (mGluRs) by the agonist DHPG causes a long-term depression (DHPG-LTD) of excitatory transmission in the CA1 region of the hippocampus, as well as causing the release of endocannabinoids from pyramidal cells. As cannabinoid agonists cause a presynaptic inhibition at these synapses and DHPG-LTD is thought to be expressed, at least in part, by a presynaptic mechanism, we examined the possibility that endocannabinoids mediated DHPG-LTD. We find that antagonists of cannabinoid receptors reduce the acute depression induced by DHPG, but have no effect on the lasting depression. Furthermore, both the acute and the lasting effects of DHPG were unaffected in the CB1 knockout mouse. These findings suggest that endocannabinoids, acting on a non-CB1 cannabinoid receptor, contribute to the acute depression but not to DHPG-LTD. Presumably some other retrograde signalling mechanism is responsible for DHPG-LTD.

Role of endogenous cannabinoids in synaptic signaling

Research of cannabinoid actions was boosted in the 1990s by remarkable discoveries including identification of endogenous compounds with cannabimimetic activity (endocannabinoids) and the cloning of their molecular targets, the CB1 and CB2 receptors. Although the existence of an endogenous cannabinoid signaling system has been established for a decade, its physiological roles have just begun to unfold. In addition, the behavioral effects of exogenous cannabinoids such as delta-9-tetrahydrocannabinol, the major active compound of hashish and marijuana, await explanation at the cellular and network levels. Recent physiological, pharmacological, and high-resolution anatomical studies provided evidence that the major physiological effect of cannabinoids is the regulation of neurotransmitter release via activation of presynaptic CB1 receptors located on distinct types of axon terminals throughout the brain. Subsequent discoveries shed light on the functional consequences of this localization by demonstrating the involvement of endocannabinoids in retrograde signaling at GABAergic and glutamatergic synapses. In this review, we aim to synthesize recent progress in our understanding of the physiological roles of endocannabinoids in the brain. First, the synthetic pathways of endocannabinoids are discussed, along with the putative mechanisms of their release, uptake, and degradation. The fine-grain anatomical distribution of the neuronal cannabinoid receptor CB1 is described in most brain areas, emphasizing its general presynaptic localization and role in controlling neurotransmitter release. Finally, the possible functions of endocannabinoids as retrograde synaptic signal molecules are discussed in relation to synaptic plasticity and network activity patterns.

Age-dependent alterations of long-term synaptic plasticity in thyroid-deficient rats

Thyroid hormone deficiency during a critical period of development profoundly affects cognitive functions such as attention, learning, and memory, but the synaptic alterations underlying these deficits remain unexplored. The present study examines the effect of congenital hypothyroidism on long-term synaptic plasticity. This plasticity is believed to be essential for learning and memory and for activity-dependent regulation of synapse formation in the developing brain. We found that the neonatal expression of long-term potentiation (LTP), long-term depression (LTD), depotentiation, and de-depression in hippocampal slices from hypothyroid animals was similar to that of controls. To examine the postnatal development of these plasticities, we used slices from neonatal (2-3 weeks) and adult (7-8 weeks) rats. This work demonstrates that the ability to express all these forms of synaptic plasticity is reduced in an age-dependent manner in control rats. LTP and depotentiation are also downregulated in adult hypothyroid rats, but we have found that de-depression is not affected during maturation. In addition, these animals express LTD at ages at which controls fail to induce it. In contrast, input/output experiments have shown greater levels of basal synaptic efficacy in hypothyroid adults, and this effect is probably related to the higher probability of release observed by paired-pulse experiments. Nevertheless, these effects appear to be unrelated to the differences observed in long-term synaptic plasticity, as no correlation was found between basal synaptic efficacy and the degree of LTD and de-depression. Furthermore, the NMDA-receptor antagonist amino-phosphonopentanoic acid (APV) completely blocked LTD, which suggests a postsynaptic locus of this alteration. Because LTD has been associated with novelty acquisition, we suggest that the greater LTD observed in adult hypothyroid rats might be related to the hyperactivity of these animals. However, other possibilities such as a retarded maturation of synaptic plasticity must be taken into account.

NMDA receptor-dependent long-term synaptic depression in the entorhinal cortex in vitro

The entorhinal cortex receives a large projection from the piriform (primary olfactory) cortex and, in turn, provides the hippocampal formation with most of its cortical sensory input. Synaptic plasticity in this pathway may therefore affect the processing of olfactory information and memory encoding. We have recently found that long-term synaptic depression (LTD) can be induced in this pathway in vivo by repetitive paired-pulse stimulation but not by low-frequency (1 Hz) stimulation with single pulses. Here, we have used field potential recordings to investigate the stimulation parameters and transmitter receptors required for the induction of LTD in the rat entorhinal cortex in vitro. The effectiveness of low-frequency stimulation (900 pulses at 1 or 5 Hz) and repeated delivery of pairs of stimulation pulses (30-ms interpulse interval) was assessed. Only repeated paired-pulse stimulation resulted in lasting LTD, and a low-intensity paired-pulse stimulation protocol that induces LTD in vivo was only effective in the presence of the GABA(A) receptor antagonist bicuculline (50 microM). LTD could also be induced in normal ACSF, however, by increasing the number of pulse-pairs delivered and by increasing the stimulation intensity during LTD induction. The induction of LTD was blocked by constant bath application of the N-methyl-d-aspartate (NMDA) glutamate receptor antagonist d-2-amino-5-phosphonovalerate (50 microM), indicating that LTD is dependent on NMDA receptor activation. However, LTD was not blocked by the group I/II mGluR antagonist (RS)-alpha-ethyl-4-carboxyphenylglycine (500 microM) or by bicuculline (50 microM). The induction of LTD in the entorhinal cortex in vitro is therefore dependent on intense stimulation that recruits activation of NMDA receptors, but does not require concurrent activation of mGluRs or inhibitory synaptic inputs.

On long-term depression induced by activation of G-protein coupled receptors

In this mini-review I consider the mechanisms by which activation of glutamate and acetylcholine metabotropic receptors can result in the induction of long-term depression. Two regions of the CNS will receive particular attention; the CA1 region of hippocampus and the perirhinal cortex.

Interactions between adenosine and metabotropic glutamate receptors in the rat hippocampal slice

1. We have examined excitatory postsynaptic potentials and paired-pulse interactions in rat hippocampal slices to obtain more information about the site and mechanism of interactions between metabotropic glutamate receptors and adenosine receptors. 2. The results show that the suppression of adenosine sensitivity is explained by a selectively reduced responsiveness to A(1) receptor stimulation, and does not involve any facilitation of A(2A) adenosine receptors, since it can be obtained in the absence of endogenous adenosine and is not prevented by the A(2A) receptor blocker ZM241385. 3. The glutamate receptors involved are of the group I class since the suppression of adenosine sensitivity is produced by ACPD and the group I selective compound DHPG. Furthermore, the effects of DHPG could be prevented by LY367385, a selective antagonist at the mGlu(1a) subtype of group I receptors. The selective antagonist at mGlu(5) receptors, SIB1893, did not prevent the suppression of adenosine sensitivity by DHPG. Blockade of the DHPG/adenosine interaction was also obtained by superfusion with the protein kinasae C inhibitor chelerythrine. 4. Since the suppression of adenosine responses by metabotropic receptor agonists was seen in the paired-pulse paradigm, we conclude that the observed interactions occur at the level of the presynaptic terminals. 5. The interaction with adenosine receptors is not specific, but applies also to a suppression of responses mediated by the GABA(B) receptor agonist baclofen. 6. We conclude that activation of the mGlu1a subtype of receptor can suppress responses mediated via adenosine A1 receptors, probably by activating protein kinase C. Since the changes induced by metabotropic glutamate receptor agonists last for at least 60 min, the data also imply that these interactions could play an important role in changes of synaptic function long after even transient increases of glutamate release in the CNS.

Cellular and molecular connections between sleep and synaptic plasticity

The hypothesis that sleep promotes learning and memory has long been a subject of active investigation. This hypothesis implies that sleep must facilitate synaptic plasticity in some way, and recent studies have provided evidence for such a function. Our knowledge of both the cellular neurophysiology of sleep states and of the cellular and molecular mechanisms underlying synaptic plasticity has expanded considerably in recent years. In this article, we review findings in these areas and discuss possible mechanisms whereby the neurophysiological processes characteristic of sleep states may serve to facilitate synaptic plasticity. We address this issue first on the cellular level, considering how activation of T-type Ca(2+) channels in nonREM sleep may promote either long-term depression or long-term potentiation, as well as how cellular events of REM sleep may influence these processes. We then consider how synchronization of neuronal activity in thalamocortical and hippocampal-neocortical networks in nonREM sleep and REM sleep could promote differential strengthening of synapses according to the degree to which activity in one neuron is synchronized with activity in other neurons in the network. Rather than advocating one specific cellular hypothesis, we have intentionally taken a broad approach, describing a range of possible mechanisms whereby sleep may facilitate synaptic plasticity on the cellular and/or network levels. We have also provided a general review of evidence for and against the hypothesis that sleep does indeed facilitate learning, memory, and synaptic plasticity.

Activation of mGlu receptors induces LTD without affecting postsynaptic sensitivity of CA1 neurons in rat hippocampal slices

Two forms of long-term depression (LTD) of excitatory synaptic transmission have been identified in the mammalian CNS, which are induced by the synaptic activation of N-methyl-D-aspartate (NMDA) and metabotropic glutamate (mGlu) receptors, respectively. The mGlu receptor-dependent form of LTD can be activated by application of 3,5-dihydroxyphenylglycine (DHPG), a group I selective mGlu receptor agonist. DHPG-induced LTD is increasingly being used to investigate the mechanisms of mGlu receptor-dependent LTD. However, recent experiments have argued for both a pre- and postsynaptic locus of expression of DHPG-induced LTD. In the present study we report that DHPG-induced LTD is not associated with changes in the sensitivity of CA1 neurons to bath applied AMPA. Furthermore, in contrast to homosynaptic LTD, DHPG-induced LTD is also not associated with changes in sensitivity to focally uncaged L-glutamate. These data do not support the notion that DHPG-induced LTD requires a modification of AMPA receptors, such as their internalisation, but are compatible with a presynaptic mechanism of expression.

Corticostriatal LTP requires combined mGluR1 and mGluR5 activation

Metabotropic glutamate receptors (mGluRs) have been demonstrated to play a role in synaptic plasticity. It has been recently shown that mGluR1 is involved in corticostriatal long-term depression, by means of pharmacological approach and by using mGluR1-knockout mice. Here, we report that both mGluR1 and mGluR5 are involved in corticostriatal long-term potentiation (LTP). In particular, the mGluR1 antagonist LY 367385, as well as the mGluR5 antagonist MPEP, reduce LTP amplitude. Moreover, blockade of both mGluR1 and mGluR5 by LY 367385 and MPEP co-administration fully suppresses LTP. Accordingly, group II and group III mGluRs antagonists fail to affect LTP induction. Interestingly, LTP amplitude is also significantly reduced in both mGluR1- and mGluR5-knockout mice. The differential function of mGluR1 and mGluR5 in corticostriatal synaptic plasticity may play a role in the modulation of the motor activity mediated by the basal ganglia, thus providing a substrate for the pharmacological treatment of motor disorders involving the striatum.

The group I metabotropic glutamate receptor agonist (S)-3,5-dihydroxyphenylglycine induces a novel form of depotentiation in the CA1 region of the hippocampus

The ability of activation of group I metabotropic glutamate receptor (mGluR) to induce depotentiation was investigated at Schaffer collateral-CA1 synapses of rat hippocampal slices. Brief bath application (5 min) of group I mGluR agonist (S)-3,5-dihydroxyphenylglycine (DHPG) (10 microm) induced a long-term depression of synaptic transmission or depotentiation (DEP) of previously established long-term potentiation (LTP), which was independent of NMDA or A(1) adenosine receptor activation. This DHPG-DEP was observed when DHPG was delivered 3 min after LTP induction. However, when DHPG was applied at 10 or 30 min after LTP induction, significantly less depotentiation was found. DHPG-DEP (1) is reversible and has the ability to unsaturate LTP, (2) is synapse specific, (3) does not require concurrent synaptic stimulation, (4) is mechanistically distinct from NMDA receptor-dependent depotentiation, (5) requires mGluR5 activation, (6) requires rapamycin-sensitive mRNA translation signaling, (7) does not require phospholipase C or protein phosphatase activation, and (8) is not associated with a change in paired-pulse (PP) facilitation. In addition, the ability of DHPG to reverse LTP was mimicked by a long train of low-frequency (1 Hz/15 min) PP stimulation. Moreover, the expression of DHPG-DEP is associated with a reduction in the increase of the surface expression of AMPA receptors seen with LTP. These results suggest that the activation of mGluR5 and in turn the triggering of a protein synthesis-dependent internalization of synaptic AMPA receptors may contribute to the DHPG-DEP in the CA1 region of the hippocampus.

Two distinct signaling pathways upregulate NMDA receptor responses via two distinct metabotropic glutamate receptor subtypes

Molecular processes regulating the gain of NMDA receptors modulate diverse physiological and pathological responses in the CNS. Group I metabotropic glutamate receptors (mGluRs), which neighbor NMDA receptors and which can be coactivated by synaptically released glutamate, couple to several different second messenger pathways, each of which could target NMDA receptors. In CA3 pyramidal cells we show that the activation of mGluR1 potentiates NMDA current via a G-protein-independent mechanism involving Src kinase activation. In contrast, mGluR5-mediated enhancement of NMDA current requires G-protein activation, triggering a signaling cascade including protein kinase C and Src. These results indicate that one neurotransmitter, glutamate, can activate two distinct and independent signaling systems to target the same effector. These two pathways are likely to contribute significantly to the highly differentiated control of NMDA receptor function.