Paired-pulse Low-frequency Stimulation Research Articles

MGluR5 Facilitates Long-Term Synaptic Depression in a Stress-Induced Depressive Mouse Model.

Glutamatergic system has been known to play a role in the pathogenesis of major depression disorder by inducing N-methyl-d-aspartate receptor-dependent long-term depression (LTD) or metabotropic glutamate receptors (mGluR)-dependent LTD. Here, we characterized the LTD in a chronic social defeat stress (CSDS)-induced depressive mouse model. CSDS was used to induce the depressive-like behaviors in C57BL/6 male mice, which were assessed using sucrose preference test and social interaction test. The synaptic strength including LTD and long-term potentiation (LTP) induced by paired-pulse low frequency stimulation (PP-LFS) was measured using whole-cell recording technique. CSDS induced depressive-like behaviors and facilitated PP-LFS-induced LTD in hippocampal CA3-CA1 pathway in the susceptible mice. Interestingly, mGluR5 but not N-methyl-d-aspartate receptor mediated the PP-LFS-induced LTD. In addition, mGluR5 agonist dihydroxyphenylglycine promoted PP-LFS-induced LTD specifically in susceptible mice, which was diminished by activating the BDNF/TrkB signaling pathway. Our results suggest that mGluR5-dependent LTD might be responsible for the development of depressive-like behaviors in CSDS-induced depression mice model.

Increased Homer1-mGluR5 mediates chronic stress-induced depressive-like behaviors and glutamatergic dysregulation via activation of PERK-eIF2α

Glutamatergic dysregulation has served as one common pathophysiology of major depressive disorder (MDD) and a promising target for treatment intervention. Previous studies implicate neurotransmission via metabotropic glutamate receptors (mGluRs) and Homer1 in stress-induced anhedonia, but the mechanisms have not been well elucidated. In the present study, we used two different animal models of depression, chronic social defeat stress (CSDS) and chronic restraint stress (CRS), to investigate the expression of Homer1 isoforms and functional interaction with mGluRs. We found that chronic stress selectively upregulated the expression of Homer1b/c in the hippocampus, whereas the level of Homer1a was unchanged. Additionally, there was a significant negative correlation between the levels of Homer1-mGluR5 signaling and depressive-like behaviors. Both application of paired-pulse low-frequency stimulation (PP-LFS) and the selective group 1 mGluRs agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) significantly enhanced mGluR-dependent long-term depression (LTD) at CA3-CA1 pyramidal cell synapses in slices from susceptible mice, whereas there was no change in NMDAR-dependent LTD induced by LFS. Furthermore, these effects were associated with the internalization of surface AMPARs in hippocampal pyramidal neurons, including reduced the expression of AMPARs and amplitude of AMPARs-mediated mEPSC. Finally, we found that chronic stress activated the KR-like ER kinase-eukaryotic initiation factor 2α (PERK-eIF2α) signaling pathway, subsequently phosphorylated cAMP response element binding protein (CREB) at the S129 and reduced the BDNF level, eventually leading to the impairment of synaptic transmission and depressive-like behaviors. Therefore, our study suggests that PERK-eIF2α acts as a critical target downstream of Homer1-mGluR5 complex to mediate chronic stress-induced depressive-like behaviors, and highlights them as a potential target for the treatment of mood disorder.

Acid-sensing ion channel 1a is required for mGlu receptor dependent long-term depression in the hippocampus

Acid-sensing ion channels (ASICs), members of the degenerin/epithelial Na+ channel superfamily, are widely distributed in the mammalian nervous system. ASIC1a is highly permeable to Ca2+ and are thought to be important in a variety of physiological processes, including synaptic plasticity, learning and memory. To further understand the role of ASIC1a in synaptic transmission and plasticity, we investigated metabotropic glutamate (mGlu) receptor-dependent long-term depression (LTD) in the hippocampus. We found that ASIC1a channels mediate a component of LTD in P30-40 animals, since the ASIC1a selective blocker psalmotoxin-1 (PcTx1) reduced the magnitude of LTD induced by application of the group I mGlu receptor agonist (S)-3,5-Dihydroxyphenylglycine (DHPG) or induced by paired-pulse low frequency stimulation (PP-LFS). Conversely, PcTx1 did not affect LTD in P13-18 animals. We also provide evidence that ASIC1a is involved in group I mGlu receptor-induced increase in action potential firing. However, blockade of ASIC1a did not affect DHPG-induced polyphosphoinositide hydrolysis, suggesting the involvement of some other molecular partners in the functional crosstalk between ASIC1a and group I mGlu receptors. Notably, PcTx1 was able to prevent the increase in GluA1 S845 phosphorylation at the post-synaptic membrane induced by group I mGlu receptor activation. These findings suggest a novel function of ASIC1a channels in the regulation of group I mGlu receptor synaptic plasticity and intrinsic excitability.

Acute food deprivation enhances fear extinction but inhibits long-term depression in the lateral amygdala via ghrelin signaling

Fear memory-encoding thalamic input synapses to the lateral amygdala (T-LA) exhibit dynamic efficacy changes that are tightly correlated with fear memory strength. Previous studies have shown that auditory fear conditioning involves strengthening of synaptic strength, and conversely, fear extinction training leads to T-LA synaptic weakening and occlusion of long-term depression (LTD) induction. These findings suggest that the mechanisms governing LTD at T-LA synapses may determine the behavioral outcomes of extinction training. Here, we explored this hypothesis by implementing food deprivation (FD) stress in mice to determine its effects on fear extinction and LTD induction at T-LA synapses. We found that FD increased plasma acylated ghrelin levels and enhanced fear extinction and its retention. Augmentation of fear extinction by FD was blocked by pretreatment with growth hormone secretagogue receptor type-1a antagonist D-Lys3-GHRP-6, suggesting an involvement of ghrelin signaling. Confirming previous findings, two distinct forms of LTD coexist at thalamic inputs to LA pyramidal neurons that can be induced by low-frequency stimulation (LFS) or paired-pulse LFS (PP-LFS) paired with postsynaptic depolarization, respectively. Unexpectedly, we found that FD impaired the induction of PP-LFS- and group I metabotropic glutamate receptor agonist (S)-3,5-dihydroxyphenylglycine (DHPG)-induced LTD, but not LFS-induced LTD. Ghrelin mimicked the effects of FD to impair the induction of PP-LFS- and DHPG-induced LTD at T-LA synapses, which were blocked by co-application of D-Lys3-GHRP-6. The sensitivity of synaptic transmission to 1-naphthyl acetyl spermine was not altered by either FD or ghrelin treatment. These results highlight distinct features of fear extinction and LTD at T-LA synapses.

Roles of protein synthesis inhibitors in long-term potentiation and depotentiation in hippocampal CA1 region of adult rats

To study the roles of protein synthesis inhibitors in long-term potentiation(LTP) and depotentiation(DP) in hippocampal CA1 region of adult rats. Standard extracellular recording technique was used to record field EPSP(fEPSP) evoked by Schaffer collateral stimulation from the CA1 subfield of adult rat hippocampal slices. Paired-pulse low-frequency stimulation(PP-LFS) or high-intensity paired-pulse low-frequency stimulation(HI-PP-LFS) was delivered to induce depotentiation 2 h after LTP induction induced by six theta-burst stimulations. Protein synthesis inhibitors were applied before and after LTP induction to study their roles in LTP and DP in hippocampal CA1 region of adult rats. When HI-PP-LFS was applied at 2 h after LTP induction, the depotentiation was induced. The mean fEPSP slopes reduced from 346.2%±26.3% to 207.1%±21.6%. This depotentiation was named as partial LTP depotentiation and maintained at least for 30 min. The percentage of depotentiation was 59.81%. Application of protein synthesis inhibitors, anisomycin and cycloheximide prior to tetanus resulted in smaller LTP compared to control group, and almost complete depotentiation was induced by HI-PP-LFS. With application of protein synthesis inhibitors anisomycin and cycloheximide 90 min after LTP induction, HI-PP-LFS still induced partial LTP depotentiation. However, there was no significant difference in the percentage of depotentiation between this group and control group. HI-PP-LFS partially reverses late phase LTP. When protein synthesis inhibitors are applied prior to tetanus, LTP amplitude is markedly reduced, and HI-PP-LFS completely reverses late-phase LTP. Application of protein synthesis inhibitors after LTP induction does not significantly affect either the amplitude or depotentiation of LTP.

Costimulation of AMPA and metabotropic glutamate receptors underlies phospholipase C activation by glutamate in hippocampus.

Glutamate, a major neurotransmitter in the brain, activates ionotropic and metabotropic glutamate receptors (iGluRs and mGluRs, respectively). The two types of glutamate receptors interact with each other, as exemplified by the modulation of iGluRs by mGluRs. However, the other way of interaction (i.e., modulation of mGluRs by iGluRs) has not received much attention. In this study, we found that group I mGluR-specific agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) alone is not sufficient to activate phospholipase C (PLC) in rat hippocampus, while glutamate robustly activates PLC. These results suggested that additional mechanisms provided by iGluRs are involved in group I mGluR-mediated PLC activation. A series of experiments demonstrated that glutamate-induced PLC activation is mediated by mGluR5 and is facilitated by local Ca(2+) signals that are induced by AMPA-mediated depolarization and L-type Ca(2+) channel activation. Finally, we found that PLC and L-type Ca(2+) channels are involved in hippocampal mGluR-dependent long-term depression (mGluR-LTD) induced by paired-pulse low-frequency stimulation, but not in DHPG-induced chemical LTD. Together, we propose that AMPA receptors initiate Ca(2+) influx via the L-type Ca(2+) channels that facilitate mGluR5-PLC signaling cascades, which underlie mGluR-LTD in rat hippocampus.

Aberrant NMDA-dependent LTD after perinatal ethanol exposure in young adult rat hippocampus.

Irreversible cognitive deficits induced by ethanol exposure during fetal life have been ascribed to a lower NMDA-dependent synaptic long-term potentiation (LTP) in the hippocampus. Whether NMDA-dependent long-term depression (LTD) may also play a critical role in those deficits remains unknown. Here, we show that in vitro LTD induced with paired-pulse low frequency stimulation is enhanced in CA1 hippocampus field of young adult rats exposed to ethanol during brain development. Furthermore, single pulse low frequency stimulation, ineffective at this age (LFS600), induced LTD after ethanol exposure accompanied with a stronger response than controls during LFS600, thus revealing an aberrant form of activity-dependent plasticity at this age. Blocking NMDA receptor or GluN2B containing NMDA receptor prevented both the stronger response during LFS600 and LTD whereas Zinc, an antagonist of GluN2A containing NMDA receptor, was ineffective on both responses. In addition, LFS600-induced LTD was revealed in controls only with a reduced-Mg(2+) medium. In whole dissected hippocampus CA1 field, perinatal ethanol exposure increased GluN2B subunit expression in the synaptic compartment whereas GluN2A was unaltered. Using pharmacological tools, we suggest that LFS600 LTD was of synaptic origin. Altogether, we describe a new mechanism by which ethanol exposure during fetal life induces a long-term alteration of synaptic plasticity involving NMDA receptors, leading to an aberrant LTD. We suggest this effect of ethanol may reflect a delayed maturation of the synapse and that aberrant LTD may also participates to long-lasting cognitive deficits in fetal alcohol spectrum disorder.

Distinct molecular components for thalamic- and cortical-dependent plasticity in the lateral amygdala.

N-methyl-D-aspartate receptor (NMDAR)-dependent long-term depression (LTD) in the lateral nucleus of the amygdala (LA) is a form of synaptic plasticity thought to be a cellular substrate for the extinction of fear memory. The LA receives converging inputs from the sensory thalamus and neocortex that are weakened following fear extinction. Combining field and patch-clamp electrophysiological recordings in mice, we show that paired-pulse low-frequency stimulation can induce a robust LTD at thalamic and cortical inputs to LA, and we identify different underlying molecular components at these pathways. We show that while LTD depends on NMDARs and activation of the protein phosphatases PP2B and PP1 at both pathways, it requires NR2B-containing NMDARs at the thalamic pathway, but NR2C/D-containing NMDARs at the cortical pathway. LTD appears to be induced post-synaptically at the thalamic input but presynaptically at the cortical input, since post-synaptic calcium chelation and NMDAR blockade prevent thalamic but not cortical LTD. These results highlight distinct molecular features of LTD in LA that may be relevant for traumatic memory and its erasure, and for pathologies such as post-traumatic stress disorder (PTSD).

Deletion of CPEB3 Enhances Hippocampus-Dependent Memory via Increasing Expressions of PSD95 and NMDA Receptors

Long-term memory requires activity-dependent synthesis of plasticity-related proteins (PRPs) to strengthen synaptic efficacy and consequently consolidate memory. Cytoplasmic polyadenylation element binding protein (CPEB)3 is a sequence-specific RNA-binding protein that regulates translation of several PRP RNAs in neurons. To understand whether CPEB3 plays a part in learning and memory, we generated CPEB3 knock-out (KO) mice and found that the null mice exhibited enhanced hippocampus-dependent, short-term fear memory in the contextual fear conditioning test and long-term spatial memory in the Morris water maze. The basal synaptic transmission of Schaffer collateral-CA1 neurons was normal but long-term depression evoked by paired-pulse low-frequency stimulation was modestly facilitated in the juvenile KO mice. Molecular and cellular characterizations revealed several molecules in regulating plasticity of glutamatergic synapses are translationally elevated in the CPEB3 KO neurons, including the scaffolding protein PSD95 and the NMDA receptors along with the known CPEB3 target, GluA1. Together, CPEB3 functions as a negative regulator to confine the strength of glutamatergic synapses by downregulating the expression of multiple PRPs and plays a role underlying certain forms of hippocampus-dependent memories.

Haploinsufficiency of Cyfip1 Produces Fragile X-Like Phenotypes in Mice

BackgroundCopy number variation (CNV) at the 15q11.2 region, which includes a gene that codes for CYFIP1 (cytoplasmic FMR1 interacting protein 1), has been implicated in autism, intellectual disability and additional neuropsychiatric phenotypes. In the current study we studied the function of Cyfip1 in synaptic physiology and behavior, using mice with a disruption of the Cyfip1 gene.Methodology/Principal FindingsWe observed that in Cyfip1 heterozygous mice metabotropic glutamate receptor (mGluR)-dependent long-term depression (LTD) induced by paired-pulse low frequency stimulation (PP-LFS) was significantly increased in comparison to wildtype mice. In addition, mGluR-LTD was not affected in the presence of protein synthesis inhibitor in the Cyfip1 heterozygous mice, while the same treatment inhibited LTD in wildtype littermate controls. mGluR-agonist (RS)-3,5-dihydroxyphenylglycine (DHPG)-induced LTD was also significantly increased in hippocampal slices from Cyfip1 heterozygous mice and again showed independence from protein synthesis only in the heterozygous animals. Furthermore, we observed that the mammalian Target of Rapamycin (mTOR) inhibitor rapamycin was only effective at reducing mGluR-LTD in wildtype animals. Behaviorally, Cyfip1 heterozygous mice showed enhanced extinction of inhibitory avoidance. Application of both mGluR5 and mGluR1 antagonist to slices from Cyfip1 heterozygous mice reversed the increase in DHPG-induced LTD in these mice.Conclusions/SignificanceThese results demonstrate that haploinsufficiency of Cyfip1 mimics key aspects of the phenotype of Fmr1 knockout mice and are consistent with the hypothesis that these effects are mediated by interaction of Cyfip1 and Fmrp in regulating activity-dependent translation. The data provide support for a model where CYFIP1 haploinsufficiency in patients results in intermediate phenotypes increasing risk for neuropsychiatric disorders.

Distinct mechanisms contribute to agonist and synaptically induced metabotropic glutamate receptor long-term depression

Metabotropic glutamate receptor mediated long-term depression (mGlu receptor LTD) is evoked in hippocampal area CA1 chemically by the agonist 3,5-Dihydroxyphenylglycine (DHPG, agonist LTD) and synaptically by paired-pulse low frequency stimulation (PP-LFS, synaptic LTD). We tested the hypothesis that different expression mechanisms regulate mGlu receptor LTD in 15–21 day old rats through neurophysiologic recordings in CA1. Several findings, in fact, suggest that agonist and synaptic mGlu receptor LTD are expressed through different mechanisms. First, neither LTD occluded the other. Second, a low calcium solution enhanced agonist LTD but did not alter synaptic LTD. Third, application of the mGlu receptor antagonist LY341495 (2S-2-amino-2-(1S,2S-2-carboxycyclopropyl-1-yl)-3-(xanth-9-yl)propanoic acid) reversed agonist LTD expression, but did not alter synaptic LTD. Finally, an N-type, voltage-dependent calcium channel antagonist, ω-conotoxin GVIA (CTX), reduced agonist LTD expression by 35%, but did not alter synaptic LTD. CTX also blocked the increase in the paired-pulse ratio associated with agonist LTD. This study cautions against assuming that agonist and synaptic LTD are equivalent as several components underlying their expression appear to differ. Moreover, the data suggest that agonist LTD, but not synaptic LTD, has a presynaptic, N-channel mediated component.

Increased Expression of Phospho-Cofilin in CA1 and Subiculum Areas after Theta-Burst Stimulation of Schaffer Collateral-Commissural Fibers in Rat Hippocampal Slices

Activity-dependent structural plasticity of dendritic spines of pyramidal neurons in the central neuron system has been proposed to be a cellular basis of learning and memory. Long-term potentiation (LTP) is accompanied by changes in synaptic morphology and structural remodeling of dendritic spines. However, there is considerable uncertainty as to the nature of the adjustment. The present study tested whether immunoreactive phospho-cofilin, an index of altered actin filament assembly, could be increased by theta-burst stimulations (TBS), which is an effective stimulation pattern for inducing LTP in the hippocampus. The slope of fEPSPs evoked by TBS to Schaffer collateral-commissural fibers in hippocampal slices was measured, and p-cofilin expression was examined using immunofluorescence techniques. Results indicated that saturated L-LTP was produced by multiple TBS episodes to Schaffer collateral-commissural fibers in the hippocampal CA1 area, and TBSs also increased immunoreactive p-cofilin expression in the stratum radiatum of the hippocampal CA1 area and pyramidal layer of the subiculum. D-2-amino-5-phosphonovalerate (D-APV) prevented LTP and expression of p-cofilin immunoreactive induced by multiple TBS episodes in the stratum radiatum of the hippocampal CA1 area. Two paired-pulse low-frequency stimulation (PP-LFS) episodes to Schaffer collateral-commissural fibers induced long-term depression (LTD), and did not affect p-cofilin expression in the stratum radiatum of the hippocampal CA1 area. These results suggest that LTP induction is associated with altered actin filament assembly. Moreover, the CA1 and subiculum areas of the hippocampal formation possibly cooperate with each other in important physiological functions, such as learning and memory, or in pathological diseases, such as epilepsy.

Extinction of cued fear memory involves a distinct form of depotentiation at cortical input synapses onto the lateral amygdala

The amygdala is known to be a critical storage site of conditioned fear memory. Among the two major pathways to the lateral amygdala (LA), the cortical pathway is known to display a presynaptic long-term potentiation which is occluded with fear conditioning. Here we show that fear extinction results in a net depression of conditioning-induced potentiation at cortical input synapses onto the LA (C-LA synapses). Fear conditioning induced a significant potentiation of excitatory postsynaptic currents at C-LA synapses compared with naïve and unpaired controls, whereas extinction apparently reversed this potentiation. Paired-pulse low-frequency stimulation (pp-LFS) induced synaptic depression in the C-LA pathway of fear-conditioned rats, but not in naïve or unpaired controls, indicating that the pp-LFS-induced depression is specific to associative learning-induced changes (pp-LFS-induced depotentiation(ex vivo)). Importantly, extinction occluded pp-LFS-induced depotentiation(ex vivo), suggesting that extinction shares some mechanisms with the depotentiation. pp-LFS-induced depotentiation(ex vivo) required NMDA receptor (NMDAR) activity, consistent with a previous finding that blockade of amygdala NMDARs impaired fear extinction. In addition, pp-LFS-induced depotentiation(ex vivo) required activity of group II metabotropic glutamate receptors (mGluRs), known to be present at presynaptic terminals, but not AMPAR internalization, consistent with a presynaptic mechanism for pp-LFS-induced depotentiation(ex vivo). This result is in contrast with another form of ex vivo depotentiation in the thalamic pathway that requires both group I mGluR activity and AMPAR internalization. We thus suggest that extinction of conditioned fear involves a distinct form of depotentiation at C-LA synapses, which depends upon both NMDARs and group II mGluRs.

GABA A receptor inhibition does not affect mGluR-dependent LTD at hippocampal Schaffer collateral-CA1 synapses

Hippocampal synaptic plasticity between Schaffer collaterals and CA1 pyramidal neurons can be induced by activation of N-methyl- d-aspartate receptors (NMDARs) or of metabotropic glutamate receptors (mGluRs). Inhibitory GABAergic interneurons in this region abundantly terminate on pyramidal neurons and may thus influence synaptic plasticity. Although NMDAR-dependent synaptic plasticity is known to be influenced by inhibitory interneurons, little is known about the role of GABA on mGluR-dependent plasticity. Here, we used field potential recordings of the Schaffer collateral-CA1 synapses in rat hippocampal slices in order to study the effect of GABA A receptor (GABA AR) inhibition on mGluR-dependent long-term depression (LTD). Without GABA AR blockade, mGluR-dependent LTD was induced pharmacologically by the group I mGluR agonist (RS)-3,5-dihydroxyphenylglycine (DHPG, 100 μM, 10 min) as well as electrically by paired-pulse low-frequency stimulation (PP-LFS, 900 paired pulses at 1 Hz) resulting in a stable depression of the field response lasting at least 80 min after LTD induction. The GABA AR antagonist gabazine (5 μM) itself caused an increase of field responses suggesting an endogenous GABA release inhibiting CA1 field potentials. However, when either DHPG or PP-LFS was applied during GABA AR inhibition, the field responses were significantly reduced. Moreover, normalizing these responses to experiments without GABA AR blockade, there was no significant effect of gabazine on both DHPG- and PP-LFS-induced LTD. Thus, our results show that mGluR-dependent LTD at Schaffer collateral-CA1 synapses is unaffected by GABA AR mediated synaptic transmission.

Activation of Muscarinic Acetylcholine Receptors Induces a Nitric Oxide–Dependent Long-Term Depression in Rat Medial Prefrontal Cortex

Cholinergic neurotransmission in the medial prefrontal cortex (mPFC) is critical for normal processing of cue detection and cognitive performance. However, the mechanism by which cholinergic system modifies mPFC synaptic function remains unclear. Here we show that activation of muscarinic acetylcholine receptors (mAChRs) by carbamoylcholine (CCh) induces long-term depression (CCh-LTD) of excitatory synaptic transmission on mPFC layer V pyramidal neurons. The induction of CCh-LTD is dependent on M(1) mAChR activation but does not require N-methyl-D-aspartate receptor activation or coincident synaptic stimulation. Activation of phospholipase C (PLC), protein kinase C (PKC), and postsynaptic Ca(2+) release from inositol 1,4,5-triphosphate (IP(3)) receptor-sensitive internal stores are required for CCh-LTD induction. The expression of CCh-LTD is likely to be presynaptic because it is accompanied by a decrease in 1/(coefficient of variance)(2) and an increase in synaptic failure and paired-pulse ratio of synaptic responses. CCh-LTD is blocked by nitric oxide (NO) synthase inhibitors, soluble guanylyl cyclase (sGC) inhibitor, and protein kinase G (PKG) inhibitor. Synaptic stimulation of M(1) mAChRs with prolonged paired-pulse low-frequency stimulation also triggers LTD. These results suggest that activation of M(1) mAChRs can induce LTD on mPFC layer V pyramidal neurons through the activation of postsynaptic PLC/PKC/IP(3) receptor- and subsequently presynaptic NO/sGC/PKG-dependent signaling processes.

Fragile X mental retardation protein replacement restores hippocampal synaptic function in a mouse model of fragile X syndrome

SUMMARYFragile X Syndrome (FXS) is caused by a mutation that silences the Fragile X Mental Retardation gene (FMR1) which encodes the Fragile X Mental Retardation Protein (FMRP). To determine if FMRP replacement can rescue phenotypic deficits in an fmr1 knockout (KO) mouse model of FXS, we constructed an Adeno-Associated Virus-based viral vector that expresses the major CNS isoform of FMRP. Using this vector we tested whether FMRP replacement could rescue the fmr1 KO phenotype of enhanced long-term-depression (LTD), a form of synaptic plasticity that may be linked to cognitive impairments associated with FXS. Extracellular excitatory postsynaptic field potentials were recorded from CA3-CA1 synaptic contacts in hippocampal slices from wild-type and fmr1 KO mice in the presence of AP-5 and anisomycin. Paired-pulse low frequency stimulation (PP-LFS) induced LTD is enhanced in slices obtained from fmr1 KO compared with wild-type mice. Analyses of hippocampal synaptic function in fmr1 KO mice that received hippocampal injections of vector showed that the PP-LFS induced LTD was restored to wild-type levels. These results indicate that expression of the major CNS isoform of FMRP alone is sufficient to rescue this phenotype and suggest that post-developmental protein replacement may have the potential to improve cognitive function in FXS.

NR2A‐containing NMDA receptors are required for L‐LTP induction and depotentiation in CA1 region of hippocampal slices

Long-term potentiation (LTP) is a well-characterized form of synaptic plasticity that fulfills many of the criteria for the neural correlate of memory. LTP reversal (or depotentiation, DP) is thought to correlate with prevention or elimination of memory storage. LTP during and immediately after induction can be easily reversed by afferent stimulation, when applied within the optimal time window. The aim of the present study was to determine whether later-phase LTP (L-LTP) could be reversed by special patterned stimulation applied at 2 h after LTP induction, as well as to characterize the receptor mechanisms underlying this reversal. Field excitatory postsynaptic potentials evoked by Schaffer collateral stimulation were recorded from the CA1 subfield of adult rat hippocampal slices. Results demonstrated that stable LTP, which was induced by six theta-burst stimulations, was mediated by NR2A-containing N-methyl-d-aspartate receptors (NMDARs). This L-LTP was partially reversed by high-intensity paired-pulse low-frequency stimulation (HI-PP-LFS) and was inhibited by Zn(2+) (30 nm), a voltage-independent NR2A-NMDAR antagonist. However, NR2B-NMDAR antagonists (Ro 25-6981, 1 mum) displayed no effect on L-LTP reversal. L-LTP partial reversal was also induced by HI-PP-LFS, when the protein synthesis inhibitors anisomycin (25 microm) and cycloheximide (60 microm) were applied following LTP induction. These results suggested that NR2A-containing NMDARs are required for L-LTP induction and DP in the hippocampal CA1 area of adult rats. Moreover, HI-PP-LFS was an effective stimulation pattern to induce DP.

MGluR5 Positive Allosteric Modulators Facilitate both Hippocampal LTP and LTD and Enhance Spatial Learning

Highly selective positive allosteric modulators (PAMs) of metabotropic glutamate receptor subtype 5 (mGluR5) have emerged as a potential approach to treat positive symptoms associated with schizophrenia. mGluR5 plays an important role in both long term potentiation (LTP) and long term depression (LTD), suggesting that mGluR5 PAMs may also have utility in improving impaired cognitive function. However, if mGluR5 PAMs shift the balance of LTP and LTD or induce a state in which afferent activity induces lasting changes in synaptic function that are not appropriate for a given pattern of activity, this could disrupt rather than enhance cognitive function. We determined the effect of selective mGluR5 PAMs on induction of LTP and LTD at the Schaffer collateral – CA1 synapse in the hippocampus. mGluR5-selective PAMs significantly enhanced threshold theta burst stimulation (TBS)-induced LTP. In addition, mGluR5 PAMs enhanced both DHPG-induced LTD and LTD induced by delivery of paired-pulse low frequency stimulation. Selective potentiation of mGluR5 had no effect on LTP induced by suprathreshold TBS or saturated LTP. The finding that potentiation of mGluR5-mediated responses to stimulation of glutamatergic afferents enhances both LTP and LTD supports the hypothesis that activation of mGluR5 by endogenous glutamate contributes to both forms of plasticity. Furthermore, two systemically active mGluR5 PAMs enhanced performance in the Morris water maze, a measure of hippocampus-dependent spatial learning. Discovery of small molecules that enhance both LTP and LTD in an activity-appropriate manner demonstrates a unique action on synaptic plasticity that may provide a novel approach for treatment of impaired cognitive function.

Elongation Factor 2 and Fragile X Mental Retardation Protein Control the Dynamic Translation of Arc/Arg3.1 Essential for mGluR-LTD

Group I metabotropic glutamate receptors (mGluR) induce long-term depression (LTD) that requires protein synthesis. Here, we demonstrate that Arc/Arg3.1 is translationally induced within 5 min of mGluR activation, and this response is essential for mGluR-dependent LTD. The increase in Arc/Arg3.1 translation requires eEF2K, a Ca(2+)/calmodulin-dependent kinase that binds mGluR and dissociates upon mGluR activation, whereupon it phosphorylates eEF2. Phospho-eEF2 acts to slow the elongation step of translation and inhibits general protein synthesis but simultaneously increases Arc/Arg3.1 translation. Genetic deletion of eEF2K results in a selective deficit of rapid mGluR-dependent Arc/Arg3.1 translation and mGluR-LTD. This rapid translational mechanism is disrupted in the fragile X disease mouse (Fmr1 KO) in which mGluR-LTD does not require de novo protein synthesis but does require Arc/Arg3.1. We propose a model in which eEF2K-eEF2 and FMRP coordinately control the dynamic translation of Arc/Arg3.1 mRNA in dendrites that is critical for synapse-specific LTD.

Prolonged Reversal of Long-term Potentiation that is N-methyl-D-aspartate Receptor Independent: Implications for Memory Formation

Reversal of long-term potentiation (LTP) by low-frequency stimulation (LFS) is often referred to as depotentiation (DP), a phenomenon that is time-dependent. The present study aimed to determine whether LTP could still be reversed when the stimulation was applied beyond the optimal time window in hippocampal slices from adult rats. Field excitatory postsynaptic potentials (fEPSPs) were recorded from the strata radiatum in CA1, following stimulation of Schaffer collaterals. Theta-burst stimulation (TBS) induced LTP that could be reversed by repeated paired-pulse LFS (PP-LFS) after almost 3 h post-TBS. Only when synapse strength reached a plateau did application of PP-LFS trigger DP. In addition, it was surprising to observe that PP-LFS, which generally induces LTD in adult rats, evoked an LTP-like further strengthening in previously potentiationed synapses, even in the presence of APV, a competitive antagonist of N-methyl-D-aspartate receptors (NMDA-Rs). Our results suggest that LTP can be reversed NMDAR-independently more than 2 h after TBS by PP-LFS in adult hippocampus and that saturation of LTP is effective to promote this process.