Switching off LTP: mGlu and NMDA receptor-dependent novelty exploration-induced depotentiation in the rat hippocampus

Administration of Atosiban, an oxytocin receptor antagonist, ameliorates autistic-like behaviors in a female rat model of valproic acid-induced autism

Autism spectrum disorder (ASD) is a pervasive developmental disorder with gender differences. Oxytocin (OXT) is currently an important candidate drug for autism, but the lack of data on female autism is a big issue. It has been reported that the effect of OXT is likely to be different between male and female ASD patients. In the study, we specifically explored the role of the OXT signaling pathway in a VPA-induced female rat's model of autism. The data showed that there was an increase of either oxytocin or its receptor expressions in both the hippocampus and the prefrontal cortex of VPA-induced female offspring. To determine if the excess of OXT signaling contributed to autism symptoms in female rats, exogenous oxytocin and oxytocin receptor antagonists Atosiban were used in the experiment. It was found that exogenous oxytocin triggered autism-like behaviors in wild-type female rats by intranasal administration. More interestingly, several autism-like deficits including social interaction, anxiety, and repeat stereotypical sexual behavior in the VPA female offspring were significantly attenuated by oxytocin receptor antagonists Atosiban. Moreover, Atosiban also effectively improved the synaptic plasticity impairment induced by VPA in female offspring. Our results suggest that oxytocin receptor antagonists significantly improve autistic-like behaviors in a female rat model of valproic acid-induced autism.

Mismatch novelty exploration training shifts VPAC1 receptor-mediated modulation of hippocampal synaptic plasticity by endogenous VIP in male rats

Novelty influences hippocampal-dependent memory through metaplasticity. Mismatch novelty detection activates the human hippocampal CA1 area and enhances rat hippocampal-dependent learning and exploration. Remarkably, mismatch novelty training (NT) also enhances rodent hippocampal synaptic plasticity while inhibition of VIP interneurons promotes rodent exploration. Since VIP, acting on VPAC1 receptors (Rs), restrains hippocampal LTP and depotentiation by modulating disinhibition, we now investigated the impact of NT on VPAC1 modulation of hippocampal synaptic plasticity in male Wistar rats. NT enhanced both CA1 hippocampal LTP and depotentiation unlike exploring an empty holeboard (HT) or a fixed configuration of objects (FT). Blocking VIP VPAC1Rs with PG 97269 (100 nM) enhanced both LTP and depotentiation in naïve animals, but this effect was less effective in NT rats. Altered endogenous VIP modulation of LTP was absent in animals exposed to the empty environment (HT). HT and FT animals showed mildly enhanced synaptic VPAC1R levels, but neither VIP nor VPAC1R levels were altered in NT animals. Conversely, NT enhanced the GluA1/GluA2 AMPAR ratio and gephyrin synaptic content but not PSD-95 excitatory synaptic marker. In conclusion, NT influences hippocampal synaptic plasticity by reshaping brain circuits modulating disinhibition and its control by VIP-expressing hippocampal interneurons while upregulation of VIP VPAC1Rs is associated with the maintenance of VIP control of LTP in FT and HT animals. This suggests VIP receptor ligands may be relevant to co-adjuvate cognitive recovery therapies in aging or epilepsy, where LTP/LTD imbalance occurs.

Brief environmental enrichment elicits metaplasticity on the insular cortex in vivo and reduces the strength of conditioned taste aversion

Environmental enrichment (EE) is known to improve memory and cognition and modulate the impact of aversive stimuli in animals, promoting the development of resilience to stressful situations. Likewise, it is known that EE can modulate synaptic plasticity as is the case of long-term potentiation (LTP). These findings have been described initially in ex vivo preparations, suggesting that the effects of EE are the result of an early modification of the synaptic excitability and transmission. In this regard, it is known that metaplasticity refers to the persistent modification, by previous activity, in the ability to induce synaptic plasticity. Our previous studies have shown that prior training in conditioned taste aversion (CTA) prevents the subsequent induction of LTP in the projection from the basolateral nucleus of the amygdala (Bla) to the insular cortex (IC) in vivo. In addition, we have shown that CTA extinction allows the induction but not the maintenance of IC-LTP of the Bla-IC pathway. Recently, we also showed that prior exposure to environmental enrichment for three weeks reduces the strength of CTA, restoring the brain-derived neurotrophic factor (BDNF) levels in the IC. The present study aimed to analyze the effects of brief exposure to an enriched environment on the strength of aversive memory, as well as on the in vivo IC-LTP. To do so, adult rats were exposed for seven days to an EE, either before CTA training or LTP induction in the Bla-IC pathway. Our results demonstrate that a seven-day exposure to an enriched environment attenuates the aversive response to a strong CTA and allows the induction but not the maintenance of LTP in the insular cortex. These findings provide evidence that metaplastic regulation in a neocortical region takes part in the mechanisms through which brief exposure to enriched environments attenuates an aversive response.

Ca2+-stimulated ADCY1 and ADCY8 regulate distinct aspects of synaptic and cognitive flexibility

The type 1 and 8 adenylyl cyclase (ADCY1 and ADCY8) exclusively account for Ca2+-stimulated cyclic AMP (cAMP) production and regulate activity-dependent synaptic modification. In this study, we examined distinct forms of synaptic plasticity in the hippocampus of Adcy1-/- and Adcy8-/- mice. We found that, at the Schaffer collateral-CA1 synapses, while the Adcy8-/- mice displayed normal long-term potentiation (LTP) following various induction protocols with high-frequency stimulation (HFS), the Adcy1-/- mice showed protocol-dependent deficits in LTP. We also found that long-term depression (LTD) requires ADCY1 but not ADCY8. Interestingly, both Adcy1-/- and Adcy8-/- mice showed defective synaptic depotentiation (i.e., activity-dependent reversal of LTP); the deficits in Adcy8-/- mice were dependent on the induction protocol. Examination of spatial memory found that ADCY1 is required for the formation of both initial and reversal memory. ADCY8 is only required for reversal memory formation. These data demonstrate that ADCY1 and ADCY8 play distinct roles in regulating synaptic and cognitive flexibility that involves bidirectional modification of synaptic function.

Protein kinase C mediates hypoxia-induced long-term potentiation of NMDA neurotransmission in the visual retinocollicular pathway

The identification of processes and mechanisms underlying the early stage of hypoxic injury of the retinocollicular pathway may be beneficial for the future prevention and treatment of navigation, orientation, and visual attention impairments. Previously, we have demonstrated that short-term hypoxia led to long-term potentiation (LTP) of NMDA neurotransmission in the background of long-term depression of GABA_A retinocollicular transmission. Here, we sought to obtain insight into the mechanisms of hypoxia-induced LTP of NMDA retinocollicular neurotransmission and the role of the protein kinase C (PKC) signaling pathway in it. To investigate these, we recorded pharmacologically isolated NMDA transmission in cocultivated pairs of rat retinal ganglion cells and superficial superior colliculus neurons under normoxic and hypoxic conditions, using the paired patch-clamp technique and method of fast local superfusion. We tested the involvement of the PKC by adding the potent and selective inhibitor chelerythrine chloride (ChC, 5 μM). We observed that hypoxia-induced LTP of NMDA neurotransmission is associated with the shortening of current kinetics. We also found that the PKC signaling pathway mediates hypoxia-induced LTP and associated shortening of NMDA currents. The ChC completely blocked the induction of LTP by hypoxia and associated kinetic changes. Contrary effects of ChC were observed with already induced LTP. ChC led to the reversal of LTP to the initial synaptic strength but the current kinetics remain irreversibly shortened. Our results show that ChC is a promising agent for the prevention and treatment of hypoxic injuries of NMDA retinocollicular neurotransmission and provide necessary electrophysiological basics for further research.

Object location learning in mice requires hippocampal somatostatin interneuron activity and is facilitated by mTORC1-mediated long-term potentiation of their excitatory synapses

Hippocampus-dependent learning and memory originate from long-term synaptic changes in hippocampal networks. The activity of CA1 somatostatin interneurons (SOM-INs) during aversive stimulation is necessary for contextual fear memory formation. In addition, mTORC1-dependent long-term potentiation (LTP) of SOM-IN excitatory input synapses from local pyramidal cells (PC-SOM synapses) contributes to the consolidation of fear motivated spatial and contextual memories. Although, it remains unknown if SOM-IN activity and LTP are necessary and sufficient for novelty motivated spatial episodic memory such as the object location memory, and if so when it is required. Here we use optogenetics to examine whether dorsal CA1 SOM-IN activity and LTP are sufficient to regulate object location memory. First, we found that silencing SOM-INs during object location learning impaired memory. Second, optogenetic induction of PC-SOM synapse LTP (TBS_opto) given 30 min before object location training, resulted in facilitation of memory. However, in mice with mTORC1 pathway genetically inactivated in SOM-INs, which blocks PC-SOM synapse LTP, TBS_opto failed to facilitate object location memory. Our results indicate that SOM-IN activity is necessary during object location learning and that optogenetic induction of PC-SOM synapse LTP is sufficient to facilitate consolidation of object location memory. Thus, hippocampal somatostatin interneuron activity is required for object location learning, a hippocampus-dependent form of novelty motivated spatial learning that is facilitated by plasticity at PC-SOM synapses.

MST1 mediates neuronal loss and cognitive deficits: A novel therapeutic target for Alzheimer's disease

Alzheimer's disease (AD) is the most prevalent form of dementia in the old adult and characterized by progressive cognitive decline and neuronal damage. The mammalian Ste20-like kinase1/2 (MST1/2) is a core component in Hippo signaling, which regulates neural stem cell proliferation, neuronal death and neuroinflammation. However, whether MST1/2 is involved in the occurrence and development of AD remains unknown. In this study we reported that the activity of MST1 was increased with Aβ accumulation in the hippocampus of 5xFAD mice. Overexpression of MST1 induced AD-like phenotype in normal mice and accelerated cognitive decline, synaptic plasticity damage and neuronal apoptosis in 2-month-old 5xFAD mice, but did not significantly affect Aβ levels. Mechanistically, MST1 associated with p53 and promoted neuronal apoptosis by phosphorylation and activation of p53, while p53 knockout largely reversed MST1-induced AD-like cognitive deficits. Importantly, either genetic knockdown or chemical inactivation of MST1 could significantly improve cognitive deficits and neuronal apoptosis in 7-month-old 5xFAD mice. Our results support the idea that MST1-mediated neuronal apoptosis is an essential mechanism of cognitive deficits and neuronal loss for AD, and manipulating the MST1 activity as a potential strategy will shed light on clinical treatment for AD or other diseases caused by neuronal injury.

NMDA Receptor-Dependent Synaptic Depression in Potentiated Synapses of the Anterior Cingulate Cortex of adult Mice

Long-term potentiation (LTP) is an important molecular mechanism for chronic pain in the anterior cingulate cortex (ACC), a key cortical region for pain perception and emotional regulation. Inhibiting ACC LTP via various manipulations or pharmacological treatments blocks chronic pain. Long-term depression (LTD) is another form of synaptic plasticity in the ACC, which is also proved to be involved in the mechanisms of chronic pain. However, less is known about the interactive relationship between LTP and LTD in the ACC. Whether the synaptic depression could be induced after synaptic LTP in the ACC is not clear. In the present study, we used multi-channel field potential recording systems to study synaptic depression after LTP in the ACC of adult mice. We found that low frequency stimulus (LFS: 1 Hz, 15 min) inhibited theta burst stimulation (TBS)-induced LTP at 30 min after the induction of LTP. However, LFS failed to induce depression at 90 min after the induction of LTP. Furthermore, NMDA receptor antagonist AP-5 blocked the induction of synaptic depression after potentiation. The GluN2B-selective antagonist Ro25-6981 also inhibited the phenomenon in the ACC, while the GluN2A-selective antagonist NVP-AAM077 and the GluN2C/D-selective antagonist PPDA and UBP145 had no any significant effect. These results suggest that synaptic LTP can be depressed by LTD in a time dependent manner, and GluN2B-containing NMDA receptors play important roles in this form of synaptic depression.

Legumain Knockout Protects Against Aβ1-42-Induced AD-like Cognitive Deficits and Synaptic Plasticity Dysfunction Via Inhibiting Neuroinflammation Without Cleaving APP

Neuroinflammation is the important pathological feature of Alzheimer's disease (AD). Legumain, a lysosomal cysteine protease, plays an important role in neuroinflammation during ischemic stroke and depressive disorder. Legumain is involved in AD process through cleaving APP; however, it is unclear if legumain can possibly modulate neuroinflammation without cleaving APP in AD. Thus, we established a mouse model of AD by single intracerebroventricular injections of Aβ1-42 in legumain knockout (KO) mice. The behavioral tests showed that legumain-KO effectively ameliorated cognitive impairment induced by Aβ1-42. Moreover, legumain deprivation significantly improves the synaptic plasticity damages in Aβ1-42-treated mice. Moreover, legumain-KO considerably inhibited the activation of microglia and reduced the expression of inflammatory cytokines in the hippocampus of Aβ1-42-treated mice. Interestingly, we found that legumain-KO inhibited TLR4/MyD88/NF-κB pathway, which was activated by Aβ1-42 in the hippocampus. In conclusion, our results suggested that legumain-KO reduced the level of neuroinflammation that was associated with inhibiting TLR4/MyD88/NF-κB pathways, thereby improving the hippocampal synaptic plasticity and reducing the cognitive impairments in Aβ1-42-treated mice. Legumain knockout blocked microglia activation by inhibiting TLR4/MyD88/NF-κB signaling pathways, and further reduced inflammatory cytokine expression. As a result, legumain knockout alleviated synaptic damage and cognitive impairment induced by Aβ1--42.

Graphene Oxide Ameliorates the Cognitive Impairment Through Inhibiting PI3K/Akt/mTOR Pathway to Induce Autophagy in AD Mouse Model

Alzheimer's disease (AD) is a neurodegenerative disease of the central nervous system characterised by cognitive impairment. Its major pathological feature is the deposition of β-amyloid (Aβ) peptide, which triggers a series of pathological cascades. Autophagy is a main pathway to eliminate abnormal aggregated proteins, and increasing autophagy represents a plausible treatment strategy against relative overproduction of neurotoxic Aβ. Graphene oxide (GO) is an emerging carbon-based nanomaterial. As a derivative of graphene with neuroprotective effects, it can effectively increase the clearance of abnormally aggregated protein. In this article, we investigated the protective function of GO in an AD mouse model. GO (30 mg/kg, intraperitoneal) was administered for 2 weeks. The results of the Morris water maze test and the novel object recognition test suggested that GO ameliorated learning and memory impairments in 5xFAD mice. The long-term potentiation and depotentiation from the perforant path to the dentate gyrus in the hippocampus were increased with GO treatment in 5xFAD mice. Furthermore, GO upregulated the expression of synapse-related proteins and increased the cell density in the hippocampus. Our results showed that GO up-regulated LC3II/LC3I and Beclin-1 and decreased p62 protein levels in 5xFAD mice. In addition, GO downregulated the PI3K/Akt/mTOR signalling pathway to induce autophagy. These results have revealed the protective potential of GO in AD.

Exposure to bacterial lipopolysaccharidein early life affects the expression of ionotropic glutamate receptor genes and is accompanied by disturbances in long-term potentiation and cognitive functions in young rats

Infections in childhood play an essential role in the pathogenesis of cognitive and psycho-emotional disorders. One of the possible mechanisms of these impairments is changes in the functional properties of NMDA and AMPA glutamate receptors in the brain. We suggest that bacterial infections during the early life period, which is critical for excitatory synapse maturation, can affect the subunit composition of NMDA and AMPA receptors. In the present study, we investigated the effect of repetitive lipopolysaccharide (LPS) intraperitoneal (i.p.) administration (25 μg/kg/day on P14, 16, and 18), mimicking an infectious disease, on the expression of subunits of NMDA and AMPA receptors in young rats. We revealed a substantial decrease of GluN2B subunit expression in the hippocampus at P23 using Western blot analysis and real-time polymerase chain reaction assay. Moderate changes were also found in GluN1, GluN2A, and GluA1 mRNA expression. The LPS-treated rats exhibited decreased exploratory and locomotor activity in the open field test and the impairment of spatial learning in the Morris water maze. Behavioral impairments were accompanied by a significant reduction in long-term hippocampal synaptic potentiation. Our data indicate that LPS-treatment in the critical period for excitatory synapse maturation alters ionotropic glutamate receptor gene expression, disturbs synaptic plasticity, and alters behavior.

Memory Susceptibility to Retroactive Interference Is Developmentally Regulated by NMDA Receptors

Retroactive interference (RI) occurs when new incoming information impairs an existing memory, which is one of the primary sources of forgetting. Although long-term potentiation (LTP) reversal shows promise as the underlying neural correlate, the key molecules that control the sensitivity of memory circuits to RI are unknown, and the developmental trajectory of RI effects is unclear. Here we found that depotentiation in the hippocampal dentate gyrus (DG) depends on GluN2A-containing NMDA receptors (NMDARs). The susceptibility of LTP to disruption progressively increases with the rise in the GluN2A/GluN2B ratio during development. The vulnerability of hippocampus-dependent memory to interference from post-learning novelty exploration is subject to similar developmental regulation by NMDARs. Both GluN2A overexpression and GluN2B downregulation in the DG promote RI-induced forgetting. Altogether, our results suggest that a switch in GluN2 subunit predominance may confer age-related differences to depotentiation and underlie the developmental decline in memory resistance to RI.

Development of Depotentiation in Adult-Born Dentate Granule Cells

Activity-dependent synaptic plasticity, i.e., long-term potentiation (LTP), long-term depression (LTD) and LTP reversal, is generally thought to make up the cellular mechanism underlying learning and memory in the mature brain, in which N-methyl-D-aspartate subtype of glutamate (NMDA) receptors and neurogenesis play important roles. LTP reversal may be the mechanism of forgetting and may mediate many psychiatric disorders, such as schizophrenia, but the specific mechanisms underlying these disorders remain unclear. In addition, LTP reversal during the development of adult-born dentate granule cells (DGCs) remains unknown. We found that the expression of the NMDA receptor subunits NR2A and NR2B displayed dynamic changes during the development of postnatal individuals and the maturation of adult-born neurons and was coupled with the change in LTP reversal. The susceptibility of LTP reversal progressively increases with the rise in the expression of NR2A during the development of postnatal individual and adult-born neurons. In addition, NMDA receptor subunits NR2A, but not NR2B, mediated LTP reversal in the DGCs of the mouse hippocampus.

Legumain acts on neuroinflammatory to affect CUS-induced cognitive impairment

Cognitive impairment has been widely recognized as a central feature of depression. Legumain, a lysosomal cysteine protease, plays an important role in cancer, atherosclerosis, inflammation and other pathological conditions. Meanwhile, it has been reported that the activation of legumain aggravates the cognitive impairment in neurodegenerative diseases. In this study, we explored the role of legumain in cognitive impairment of stressed mice. Legumain knockout (legumain KO) and wildtype (WT) mice were divided into four groups: control group, chronic mild unpredictable stressed (CUS) group, legumain KO group and legumain KO + CUS group. Our results demonstrated that CUS (4 weeks) induced cognitive impairment in mice effectively based on Morris water maze (MWM) test and novel object recognition (NOR) test and decreased the synaptic plasticity. Additionally, CUS exposure significantly decreased the expression of hippocampal synapse related proteins and the cell density in the DG region, accompanied by increasing the expression of hippocampal inflammatory cytokines and promoting the activation of microglia in the hippocampus. Legumain KO distinctly restored the CUS-induced negative effects on the indicators mentioned above. In conclusion, our results suggested that legumain may be an effective therapeutic target for cognitive impairment as was seen within the CUS model and legumain KO reduced the level of neuroinflammation, thereby improving the hippocampal synaptic plasticity and cognitive impairment of stressed mice.

Pre-plaque Aß-Mediated Impairment of Synaptic Depotentiation in a Transgenic Rat Model of Alzheimer's Disease Amyloidosis

How endogenously produced soluble amyloid ß-protein (Aß) affects synaptic plasticity in vulnerable circuits should provide insight into early Alzheimer's disease pathophysiology. McGill-R-Thy1-APP transgenic rats, modeling Alzheimer's disease amyloidosis, exhibit an age-dependent soluble Aß-mediated impairment of the induction of long-term potentiation (LTP) by 200 Hz conditioning stimulation at apical CA3-to-CA1 synapses. Here, we investigated if synaptic weakening at these synapses in the form of activity-dependent persistent reversal (depotentiation) of LTP is also altered in pre-plaque rats in vivo. In freely behaving transgenic rats strong, 400 Hz, conditioning stimulation induced stable LTP that was NMDA receptor- and voltage-gated Ca²⁺ channel-dependent. Surprisingly, the ability of novelty exploration to induce depotentiation of 400 Hz-induced LTP was impaired in an Aß-dependent manner in the freely behaving transgenic rats. Moreover, at apical synapses, low frequency conditioning stimulation (1 Hz) did not trigger depotentiation in anaesthetized transgenic rats, with an age-dependence similar to the LTP deficit. In contrast, at basal synapses neither LTP, induced by 100 or 200 Hz, nor novelty exploration-induced depotentiation was impaired in the freely behaving transgenic rats. These findings indicate that activity-dependent weakening, as well as strengthening, is impaired in a synapse- and age-dependent manner in this model of early Alzheimer's disease amyloidosis.

Etidronate-zinc Complex Ameliorated Cognitive and Synaptic Plasticity Impairments in 2-Vessel Occlusion Model Rats by Reducing Neuroinflammation

As one of the bisphosphonate derivatives, etidronate has proved to be beneficial to spatial learning and memory deficits caused by two-vessel occlusion (2-VO). In this study, the novel drug etidronate-zinc complex (Eti-Zn) was used to detect its role in synaptic plasticity and learning and memory functions in a rat model of 2-VO. Chronic cerebral hypoperfusion was induced by permanent occlusion of the common carotid artery bilaterally in adult Sprague-Dawley rats. Eti-Zn (20 mg/kg/day, tail vein injection) was administered for 7 days after a two-week operation. After treatment, a series of tests were carried out. Here, we found that Eti-Zn could reduce spatial learning and memory impairments in 2-VO model rats via the Morris water maze test. We also found that animals treated with Eti-Zn showed preference for the new-object in the novel object recognition test. In addition, the long-term potentiation and depotentiation from the Schaffer collaterals to the CA1 region in the hippocampus were enhanced by Eti-Zn treatment in 2-VO model rats. Furthermore, Eti-Zn significantly up-regulated NMDA receptor (NR) 2A, NR2B, postsynaptic density protein 95 and synaptophysin levels and prevented the destruction of dendritic spines. Moreover, Eti-Zn treatment reduced both the over-activation of microglia and the expressions of neuroinflammatory cytokines (TNF-α, IL-1β and IL-6) in the hippocampus. The increased NF-κB signaling pathway in the hippocampus of 2-VO rats was reversed after Eti-Zn treatment. In summary, these findings suggest that Eti-Zn could ameliorate the synaptic plasticity and cognitive impairments by reducing neuroinflammation in 2-VO model rats.

Peripheral Interventions Enhancing Brain Glutamate Homeostasis Relieve Amyloid β- and TNFα- Mediated Synaptic Plasticity Disruption in the Rat Hippocampus

Dysregulation of glutamate homeostasis in the interstitial fluid of the brain is strongly implicated in causing synaptic dysfunction in many neurological and psychiatric illnesses. In the case of Alzheimer's disease (AD), amyloid β (Aβ)-mediated disruption of synaptic plasticity and memory can be alleviated by interventions that directly remove glutamate or block certain glutamate receptors. An alternative strategy is to facilitate the removal of excess glutamate from the nervous system by activating peripheral glutamate clearance systems. One such blood-based system, glutamate oxaloacetate transaminase (GOT), is activated by oxaloacetate, which acts as a co-substrate. We report here that synthetic and AD brain-derived Aβ-mediated inhibition of synaptic long-term potentiation in the hippocampus is alleviated by oxaloacetate. Moreover the effect of oxaloacetate was GOT-dependent. The disruptive effects of a general inhibitor of excitatory amino acid transport or TNFα, a pro-inflammatory mediator of Aβ action, were also reversed by oxaloacetate. Furthermore, another intervention that increases peripheral glutamate clearance, peritoneal dialysis, mimicked the beneficial effect of oxaloacetate. These findings lend support to the promotion of the peripheral clearance of glutamate as a means to alleviate synaptic dysfunction that is caused by impaired glutamate homeostasis in the brain.

Cardiac Arrest Induces Ischemic Long-Term Potentiation of Hippocampal CA1 Neurons That Occludes Physiological Long-Term Potentiation

Ischemic long-term potentiation (iLTP) is a form of synaptic plasticity that occurs in acute brain slices following oxygen-glucose deprivation. In vitro, iLTP can occlude physiological LTP (pLTP) through saturation of plasticity mechanisms. We used our murine cardiac arrest and cardiopulmonary resuscitation (CA/CPR) model to produce global brain ischemia and assess whether iLTP is induced in vivo, contributing to the functionally relevant impairment of pLTP. Adult male mice were subjected to CA/CPR, and slice electrophysiology was performed in the hippocampal CA1 region 7 or 30 days later. We observed increased miniature excitatory postsynaptic current amplitudes, suggesting a potentiation of postsynaptic AMPA receptor function after CA/CPR. We also observed increased phosphorylated GluR1 in the postsynaptic density of hippocampi after CA/CPR. These data support the in vivo induction of ischemia-induced plasticity. Application of a low-frequency stimulus (LFS) to CA1 inputs reduced excitatory postsynaptic potentials in slices from mice subjected to CA/CPR, while having no effects in sham controls. These results are consistent with a reversal, or depotentiation, of iLTP. Further, depotentiation with LFS partially restored induction of pLTP with theta burst stimulation. These data provide evidence for iLTP following in vivo ischemia, which occludes pLTP and likely contributes to network disruptions that underlie memory impairments.

NLRP3-dependent synaptic plasticity deficit in an Alzheimer's disease amyloidosis model in vivo

Pro-inflammatory mechanisms have recently emerged as an important component of early Alzheimer's disease (AD) pathogenesis. A particularly attractive therapeutic strategy is to selectively prevent the disruptive effects of activation of the innate immune system in the brain at an early transitional stage by reducing the production or directly neutralizing pro-inflammatory cytokines, in particular IL-1β and TNF-α. Here we tested their in vivo effects on synaptic plasticity deficits, which provide sensitive and robust measures of synaptic failure, in a rat model of AD amyloidosis. Using electrophysiological techniques we longitudinally studied the effects of the NLRP3 inflammasome inhibitor Mcc950, the IL-1 receptor antagonist (anakinra) and an anti-TNF-α agent (etanercept) in awake freely moving transgenic rats overexpressing AD associated β-amyloid precursor protein at a pre-plaque stage of amyloidosis. Repeated treatment with Mcc950 reversibly abrogated the inhibition of long-term potentiation. The IL-1 receptor antagonist and etanercept also had a similar beneficial effect on the deficit in synaptic plasticity. Our findings support the clinical development of Mcc950 and clinically available IL-1- and TNF-α-neutralizing agents in early AD.

Long Term Depression in Rat Hippocampus and the Effect of Ethanol during Fetal Life

Alcohol (ethanol) disturbs cognitive functions including learning and memory in humans, non-human primates, and laboratory animals such as rodents. As studied in animals, cellular mechanisms for learning and memory include bidirectional synaptic plasticity, long-term potentiation (LTP), and long-term depression (LTD), primarily in the hippocampus. Most of the research in the field of alcohol has analyzed the effects of ethanol on LTP; however, with recent advances in the understanding of the physiological role of LTD in learning and memory, some authors have examined the effects of ethanol exposure on this particular signal. In the present review, I will focus on hippocampal LTD recorded in rodents and the effects of fetal alcohol exposure on this signal. A synthesis of the findings indicates that prenatal ethanol exposure disturbs LTD concurrently with LTP in offspring and that both glutamatergic and γ-aminobutyric acid (GABA) neurotransmissions are altered and contribute to LTD disturbances. Although the ultimate mode of action of ethanol on these two transmitter systems is not yet clear, novel suggestions have recently appeared in the literature.

Roles for Arc in metabotropic glutamate receptor-dependent LTD and synapse elimination: Implications in health and disease

The Arc gene is robustly transcribed in specific neural ensembles in response to experience-driven activity. Upon induction, Arc mRNA is transported to dendrites, where it can be rapidly and locally translated by activation of metabotropic glutamate receptors (mGluR1/5). mGluR-induced dendritic synthesis of Arc is implicated in weakening or elimination of excitatory synapses by triggering endocytosis of postsynaptic AMPARs in both hippocampal CA1 and cerebellar Purkinje neurons. Importantly, CA1 neurons with experience-induced Arc mRNA are susceptible, or primed for mGluR-induced long-term synaptic depression (mGluR-LTD). Here we review mechanisms and function of Arc in mGluR-LTD and synapse elimination and propose roles for these forms of plasticity in Arc-dependent formation of sparse neural representations of learned experience. We also discuss accumulating evidence linking dysregulation of Arc and mGluR-LTD in human cognitive disorders such as intellectual disability, autism and Alzheimer's disease.

VEGF-Mediated Cognitive and Synaptic Improvement in Chronic Cerebral Hypoperfusion Rats Involves Autophagy Process

Chronic cerebral hypoperfusion (CCH) is associated with various neurodegenerative diseases characterized by cognitive impairment. Dozens of studies including ours have indicated that exogenous administration of vascular endothelial growth factor (VEGF) could exert effective cognitive protection during ischemia. Nevertheless, the underlying mechanism has not been well clarified. To address this issue, we explored the synaptic mechanisms in vivo since hippocampal synaptic function is essential to the learning and memory process. Besides, the role of autophagy in cognitive dysfunction under conditions of CCH is still controversial. And abnormal autophagy could threaten normal neurotransmission at synapse where a large amount of protein synthesis and degradation take place. Hence, we further examined whether the altered synaptic function was associated with autophagy. The results showed that CCH impaired spatial cognition as evidenced in Morris water maze. We further found that VEGF mitigated impaired hippocampal synaptic function including basal synaptic transmission, paired-pulse facilitation, short-term, long-term plasticity, depotentiation, and the level of synaptic proteins as assessed by electrophysiological examination and western blot assay. Furthermore, our results demonstrated that CCH could induce excessive autophagy which could be inhibited by VEGF. Thus, we speculated that VEGF could ameliorate impaired synaptic function induced by CCH because of its ability to inhibit excessive autophagy, and eventually improve spatial learning and memory function. Importantly, our findings shed light on potential therapeutic strategies to be exploited in the usage of VEGF.

Targeting glutamatergic and cellular prion protein mechanisms of amyloid β-mediated persistent synaptic plasticity disruption: Longitudinal studies

Alzheimer's disease amyloid-β (Aβ) oligomers are synaptotoxic, inappropriately increasing extracellular glutamate concentration and glutamate receptor activation to thereby rapidly disrupt synaptic plasticity. Thus, acutely promoting brain glutamate homeostasis with a blood-based scavenging system, glutamate-oxaloacetate transaminase (GOT), and blocking metabotropic glutamate 5 (mGlu5) receptor or its co-receptor cellular prion protein (PrP), prevent the acute inhibition of long-term potentiation (LTP) by exogenous Aβ. Here, we evaluated the time course of the effects of such interventions in the persistent disruptive effects of Aβ oligomers, either exogenously injected in wild type rats or endogenously generated in transgenic rats that model Alzheimer's disease amyloidosis. We report that repeated, but not acute, systemic administration of recombinant GOT type 1, with or without the glutamate co-substrate oxaloacetate, reversed the persistent deleterious effect of exogenous Aβ on synaptic plasticity. Moreover, similar repetitive treatment reversibly abrogated the inhibition of LTP monitored longitudinally in freely behaving transgenic rats. Remarkably, brief repeated treatment with an mGlu5 receptor antagonist, basimglurant, or an antibody that prevents Aβ oligomer binding to PrP, ICSM35, also had similar reversible ameliorative effects in the transgenic rat model. Overall, the present findings support the ongoing development of therapeutics for early Alzheimer's disease based on these complementary approaches.

The atypical antipsychotic olanzapine disturbs depotentiation by modulating mAChRs and impairs reversal learning

Antipsychotic medication is an essential component for treating schizophrenia, which is a serious mental disorder that affects approximately 1% of the global population. Olanzapine (Olz), one of the most frequently prescribed atypical antipsychotics, is generally considered a first-line drug for treating schizophrenia. In contrast to psychotic symptoms, the effects of Olz on cognitive symptoms of schizophrenia are still unclear. In addition, the mechanisms by which Olz affects the neural circuits associated with cognitive function are unknown. Here we show that Olz interrupts depotentiation (reversal of long-term potentiation) without disturbing de novo LTP (long-term potentiation) and LTD (long-term depression). At hippocampal SC-CA1 synapses, inhibition of NMDARs (N-methyl-d-aspartate receptors), mGluRs (metabotropic glutamate receptors), or mAChRs (muscarinic acetylcholine receptors) disrupted depotentiation. In addition, co-activation of NMDARs, mGluRs, and mAChRs reversed stably expressed LTP. Olz inhibits the activation of mAChRs, which amplifies glutamate signaling through enhanced NMDAR opening and Gq (Gq class of G protein)-mediated signal transduction. Behaviorally, Olz impairs spatial reversal learning of mice in the Morris water maze test. Our results uncover a novel mechanism underpinning the cognitive modulation of Olz and show that the anticholinergic property of Olz affects glutamate signaling and synaptic plasticity.

G Protein-Gated K+ Channel Ablation in Forebrain Pyramidal Neurons Selectively Impairs Fear Learning

BACKGROUND

Cognitive dysfunction occurs in many debilitating conditions including Alzheimer's disease, Down syndrome, schizophrenia, and mood disorders. The dorsal hippocampus is a critical locus of cognitive processes linked to spatial and contextual learning. G protein-gated inwardly rectifying potassium ion (GIRK/Kir3) channels, which mediate the postsynaptic inhibitory effect of many neurotransmitters, have been implicated in hippocampal-dependent cognition. Available evidence, however, derives primarily from constitutive gain-of-function models that lack cellular specificity.

METHODS

We used constitutive and neuron-specific gene ablation models targeting an integral subunit of neuronal GIRK channels (GIRK2) to probe the impact of GIRK channels on associative learning and memory.

RESULTS

Constitutive Girk2^-/- mice exhibited a striking deficit in hippocampal-dependent (contextual) and hippocampal-independent (cue) fear conditioning. Mice lacking GIRK2 in gamma-aminobutyric acid neurons (GAD-Cre:Girk2^flox/flox mice) exhibited a clear deficit in GIRK-dependent signaling in dorsal hippocampal gamma-aminobutyric acid neurons but no evident behavioral phenotype. Mice lacking GIRK2 in forebrain pyramidal neurons (CaMKII-Cre(+):Girk2^flox/flox mice) exhibited diminished GIRK-dependent signaling in dorsal, but not ventral, hippocampal pyramidal neurons. CaMKII-Cre(+):Girk2^flox/flox mice also displayed a selective impairment in contextual fear conditioning, as both cue fear and spatial learning were intact in these mice. Finally, loss of GIRK2 in forebrain pyramidal neurons correlated with enhanced long-term depression and blunted depotentiation of long-term potentiation at the Schaffer collateral/cornu ammonis 1 synapse in the dorsal hippocampus.

CONCLUSIONS

Our data suggest that GIRK channels in dorsal hippocampal pyramidal neurons are necessary for normal learning involving aversive stimuli and support the contention that dysregulation of GIRK-dependent signaling may underlie cognitive dysfunction in some disorders.

Quinolinic acid injection in mouse medial prefrontal cortex affects reversal learning abilities, cortical connectivity and hippocampal synaptic plasticity

Intracerebral injection of the excitotoxic, endogenous tryptophan metabolite, quinolinic acid (QA), constitutes a chemical model of neurodegenerative brain disease. Complementary techniques were combined to examine the consequences of QA injection into medial prefrontal cortex (mPFC) of C57BL6 mice. In accordance with the NMDAR-mediated synapto- and neurotoxic action of QA, we found an initial increase in excitability and an augmentation of hippocampal long-term potentiation, converting within two weeks into a reduction and impairment, respectively, of these processes. QA-induced mPFC excitotoxicity impaired behavioral flexibility in a reversal variant of the hidden-platform Morris water maze (MWM), whereas regular, extended MWM training was unaffected. QA-induced mPFC damage specifically affected the spatial-cognitive strategies that mice use to locate the platform during reversal learning. These behavioral and cognitive defects coincided with changes in cortical functional connectivity (FC) and hippocampal neuroplasticity. FC between various cortical regions was assessed by resting-state fMRI (rsfMRI) methodology, and mice that had received QA injection into mPFC showed increased FC between various cortical regions. mPFC and hippocampus (HC) are anatomically as well as functionally linked as part of a cortical network that controls higher-order cognitive functions. Together, these observations demonstrate the central functional importance of rodent mPFC as well as the validity of QA-induced mPFC damage as a preclinical rodent model of the early stages of neurodegeneration.

Separate Ionotropic and Metabotropic Glutamate Receptor Functions in Depotentiation vs. LTP: A Distinct Role for Group1 mGluR Subtypes and NMDARs

Depotentiation (DP) is a mechanism by which synapses that have recently undergone long-term potentiation (LTP) can reverse their synaptic strengthening within a short time-window after LTP induction. Group 1 metabotropic glutamate receptors (mGluRs) were shown to be involved in different forms of LTP and long-term depression (LTD), but little is known about their roles in DP. Here, we generated DP by applying low-frequency stimulation (LFS) at 5 Hz after LTP had been induced by a single train of theta-burst-stimulation (TBS). While application of LFS for 2 min (DP2′) generated only a short-lasting DP that was independent of the activation of N-methyl-D-aspartate receptors (NMDARs) and group 1 mGluRs, LFS given for 8 min (DP8′) induced a robust DP that was maintained for at least 2 h. This strong form of DP was contingent on NMDAR activation. Interestingly, DP8′ appears to include a metabotropic NMDAR function because it was blocked by the competitive NMDAR antagonist D-AP5 but not by the use-dependent inhibitor MK-801 or high Mg²⁺. Furthermore, DP8′ was enhanced by application of the mGluR1 antagonist (YM 298198, 1 μM). The mGluR5 antagonist 2-Methyl-6(phenylethynyl) pyridine (MPEP, 40 μM), in contrast, failed to affect it. The induction of LTP, in turn, was NMDAR dependent (as tested with D-AP5), and blocked by MPEP but not by YM 298198. These results indicate a functional dissociation of mGluR1 and mGluR5 in two related and consecutively induced types of NMDAR-dependent synaptic plasticity (LTP → DP) with far-reaching consequences for their role in plasticity and learning under normal and pathological conditions.

Depotentiation from Potentiated Synaptic Strength in a Tristable System of Coupled Phosphatase and Kinase

Long-term potentiation (LTP) of synaptic strength is strongly implicated in learning and memory. On the other hand, depotentiation, the reversal of synaptic strength from potentiated LTP state to the pre-LTP level, is required in extinction of the obsolete memory. A generic tristable system, which couples the phosphatase and kinase switches, exclusively explains how moderate and high elevation of intracellular calcium concentration triggers long-term depression (LTD) and LTP, respectively. The present study, introducing calcium influx and calcium release from internal store into the tristable system, further show that significant elevation of cytoplasmic calcium concentration switches activation of both kinase and phosphatase to their basal states, thereby depotentiate the synaptic strength. A phase-plane analysis of the combined model was employed to explain the previously reported depotentiation in experiments and predict a threshold-like effect with calcium concentration. The results not only reveal a mechanism of NMDAR- and mGluR-dependent depotentiation, but also predict further experiments about the role of internal calcium store in induction of depotentiation and extinction of established memories.

Repetitive transcranial magnetic stimulation effectively facilitates spatial cognition and synaptic plasticity associated with increasing the levels of BDNF and synaptic proteins in Wistar rats

Repetitive transcranial magnetic stimulation (rTMS) is a non-invasive technique, by which cognitive deficits can be alleviated. Furthermore, rTMS may facilitate learning and memory. However, its underlying mechanism is still little known. The aim of this study was to investigate if the facilitation of spatial cognition and synaptic plasticity, induced by rTMS, is regulated by enhancing pre- and postsynaptic proteins in normal rats. Morris water maze (MWM) test was performed to examine the spatial cognition. The synaptic plasticity, including long-term potentiation (LTP) and depotentiation (DEP), presynaptic plasticity paired-pulse facilitation (PPF), from the hippocampal Schaffer collaterals to CA1 region was subsequently measured using in vivo electrophysiological techniques. The expressions of brain-derived neurotrophic factor (BDNF), presynaptic protein synaptophysin (SYP) and postsynaptic protein NR2B were measured by Western blot. Our data show that the spatial learning/memory and reversal learning/memory in rTMS rats were remarkably enhanced compared to that in the Sham group. Furthermore, LTP and DEP as well as PPF were effectively facilitated by 5Hz-rTMS. Additionally, the expressions of BDNF, SYP and NR2B were significantly increased via magnetic stimulation. The results suggest that rTMS considerably increases the expressions of BDNF, postsynaptic protein NR2B and presynaptic protein SYP, and thereby significantly enhances the synaptic plasticity and spatial cognition in normal animals.

Do group I metabotropic glutamate receptors mediate LTD?

Synapses undergo significant structural and functional reorganization in response to varying patterns of stimulation. These forms of plasticity are considered fundamental to cognition and neuronal homeostasis. An increasing number of reports highlight the importance of activity-dependent synaptic strengthening (long term potentiation: LTP) for learning. However, the functional significance of activity-dependent weakening of synapses (long term depression: LTD) remains relatively poorly understood. One form of synaptic weakening, induced by group I metabotropic glutamate receptors (mGluRs), has received significant attention from a mechanistic point of view and because of its augmentation in a murine model of Fragile X Syndrome. Yet, studies of this form of plasticity often yield confusing, contradictory results. These conflicting findings are likely attributable to the bulk stimulation and recording techniques often used to study synaptic plasticity (typically involving evoked extracellular recordings, which represent the summed activity of many synapses). Such studies inherently blur the identity of the synapses undergoing change, thus giving the illusion that synapses per se are being modified when in fact this may only be true of a specific subset of synapses. Indeed, studies employing minimal synaptic activation paint a fundamentally different picture of what is commonly called "mGluR-LTD". Here, I review the evidence in favour of group I mGluRs as mediators of various forms of synaptic downregulation and attempt to explain discrepancies in the literature. I argue that, while multiple forms of synaptic weakening may be triggered by these receptors, the canonical form of group I mGluR-mediated depression, mGluR-LTD, is in fact not a depression of basal synaptic responses. Rather, it is a reversal of established LTP and thus a form of depotentiation. Far from being arbitrary, this distinction has significant implications for the role of group I mGluRs in cognition, both in the healthy brain and in pathological conditions. Further, the differential actions of group I mGluRs at naïve and potentiated synapses suggest these receptors signal in a state-dependent manner to regulate various stages of the learning process.

The reemergence of long-term potentiation in aged Alzheimer's disease mouse model

Mouse models of Alzheimer’s disease (AD) have been developed to study the pathophysiology of amyloid β protein (Aβ) toxicity, which is thought to cause severe clinical symptoms such as memory impairment in AD patients. However, inconsistencies exist between studies using these animal models, specifically in terms of the effects on synaptic plasticity, a major cellular model of learning and memory. Whereas some studies find impairments in plasticity in these models, others do not. We show that long-term potentiation (LTP), in the CA1 region of hippocampal slices from this mouse, is impared at Tg2576 adult 6–7 months old. However, LTP is inducible again in slices taken from Tg2576 aged 14–19 months old. In the aged Tg2576, we found that the percentage of parvalbumin (PV)-expressing interneurons in hippocampal CA1-3 region is significantly decreased, and LTP inhibition or reversal mediated by NRG1/ErbB signaling, which requires ErbB4 receptors in PV interneurons, is impaired. Inhibition of ErbB receptor kinase in adult Tg2576 restores LTP but impairs depotentiation as shown in aged Tg2576. Our study suggests that hippocampal LTP reemerges in aged Tg2576. However, this reemerged LTP is an insuppressible form due to impaired NRG1/ErbB signaling, possibly through the loss of PV interneurons.

Orexin A induces bidirectional modulation of synaptic plasticity: Inhibiting long-term potentiation and preventing depotentiation

The orexin system consists of two peptides, orexin A and B and two receptors, OX1R and OX2R. It is implicated in learning and memory regulation while controversy remains on its role in modulating hippocampal synaptic plasticity in vivo and in vitro. Here, we investigated effects of orexin A on two forms of synaptic plasticity, long-term potentiation (LTP) and depotentiation of field excitatory postsynaptic potentials (fEPSPs), at the Schaffer Collateral-CA1 synapse of mouse hippocampal slices. Orexin A (≧30 nM) attenuated LTP induced by theta burst stimulation (TBS) in a manner antagonized by an OX1R (SB-334867), but not OX2R (EMPA), antagonist. Conversely, at 1 pM, co-application of orexin A prevented the induction of depotentiation induced by low frequency stimulation (LFS), i.e. restoring LTP. This re-potentiation effect of sub-nanomolar orexin A occurred at LFS of 1 Hz, but not 2 Hz, and with LTP induced by either TBS or tetanic stimulation. It was significantly antagonized by SB-334867, EMPA and TCS-1102, selective OX1R, OX2R and dual OXR antagonists, respectively, and prevented by D609, SQ22536 and H89, inhibitors of phospholipase C (PLC), adenylyl cyclase (AC) and protein kinase A (PKA), respectively. LFS-induced depotentiation was antagonized by blockers of NMDA, A1-adenosine and type 1/5 metabotropic glutamate (mGlu1/5) receptors, respectively. However, orexin A (1 pM) did not affect chemical-induced depotentiation by agonists of these receptors. These results suggest that orexin A bidirectionally modulates hippocampal CA1 synaptic plasticity, inhibiting LTP via OX1Rs at moderate concentrations while inducing re-potentiation via OX1Rs and OX2Rs, possibly through PLC and AC-PKA signaling at sub-nanomolar concentrations.

Antidepressant-like effects and possible mechanisms of amantadine on cognitive and synaptic deficits in a rat model of chronic stress

The aim of this study was to examine whether amantadine (AMA), as a low-affinity noncompetitive N-methyl-d-aspartate (NMDA) receptor antagonist, is able to improve cognitive deficits caused by chronic stress in rats. Male Wistar rats were divided into four groups: control, control + AMA, stress and stress + AMA groups. The chronic stress model combined chronic unpredictable stress (CUS) with isolated feeding. Animals were exposed to CUS continued for 21 days. AMA (25 mg/kg) was administrated p.o. for 20 days from the 4th day of CUS to the 23rd. Weight and sucrose consumption were measured during model establishing period. Spatial memory was evaluated using the Morris water maze (MWM) test. Following MWM testing, both long-term potentiation (LTP) and depotentiation were recorded in the hippocampal CA1 region. NR2B and postsynaptic density protein 95 (PSD-95) proteins were measured by Western-blot analysis. AMA increased weight and sucrose consumption of stressed rats. Spatial memory and reversal learning in stressed rats were impaired relative to controls, whereas AMA significantly attenuated cognitive impairment. AMA also mitigated the chronic stress-induced impairment of hippocampal synaptic plasticity, in which both the LTP and depotentiation were significantly inhibited in stressed rats. Moreover, AMA enhanced the expression of hippocampal NR2B and PSD-95 in stressed rats. The data suggest that AMA may be an effective therapeutic agent for depression-like symptoms and associated cognitive disturbances.

Longitudinal testing of hippocampal plasticity reveals the onset and maintenance of endogenous human Aß-induced synaptic dysfunction in individual freely behaving pre-plaque transgenic rats: rapid reversal by anti-Aß agents

Long before synaptic loss occurs in Alzheimer’s disease significant harbingers of disease may be detected at the functional level. Here we examined if synaptic long-term potentiation is selectively disrupted prior to extracellular deposition of Aß in a very complete model of Alzheimer’s disease amyloidosis, the McGill-R-Thy1-APP transgenic rat. Longitudinal studies in freely behaving animals revealed an age-dependent, relatively rapid-onset and persistent inhibition of long-term potentiation without a change in baseline synaptic transmission in the CA1 area of the hippocampus. Thus the ability of a standard 200 Hz conditioning protocol to induce significant NMDA receptor-dependent short- and long-term potentiation was lost at about 3.5 months of age and this deficit persisted for at least another 2–3 months, when plaques start to appear. Consistent with in vitro evidence for a causal role of a selective reduction in NMDA receptor-mediated synaptic currents, the deficit in synaptic plasticity in vivo was associated with a reduction in the synaptic burst response to the conditioning stimulation and was overcome using stronger 400 Hz stimulation. Moreover, intracerebroventricular treatment for 3 days with an N-terminally directed monoclonal anti- human Aß antibody, McSA1, transiently reversed the impairment of synaptic plasticity. Similar brief treatment with the BACE1 inhibitor LY2886721 or the γ-secretase inhibitor MRK-560 was found to have a comparable short-lived ameliorative effect when tracked in individual rats. These findings provide strong evidence that endogenously generated human Aß selectively disrupts the induction of long-term potentiation in a manner that enables potential therapeutic options to be assessed longitudinally at the pre-plaque stage of Alzheimer’s disease amyloidosis.

Elevated activation of CaMKIIα in the CPEB3-knockout hippocampus impairs a specific form of NMDAR-dependent synaptic depotentiation

Cytoplasmic polyadenylation element binding protein 3 (CPEB3) is a sequence-specific RNA-binding protein that confines the strength of glutamatergic synapses by translationally downregulating the expression of multiple plasticity-related proteins (PRPs), including the N-methyl-D-aspartate receptor (NMDAR) and the postsynaptic density protein 95 (PSD95). CPEB3 knockout (KO) mice exhibit hippocampus-dependent abnormalities related not only to long-term spatial memory but also to the short-term acquisition and extinction of contextual fear memory. In this study, we identified a specific form of NMDAR-dependent synaptic depotentiation (DPT) that is impaired in the adult CPEB3 KO hippocampus. In parallel, cultured KO neurons also exhibited delayed morphological and biochemical responses under NMDA-induced chemical long-term depression (c-LTD). The c-LTD defects in the KO neurons include elevated activation of calcium/calmodulin-dependent protein kinase II alpha subunit (CaMKIIα), increased Ser831 phosphorylation of GluA1 and slow degradation of PSD95 and GluA1. Because transient pharmacological suppression of CaMKIIα activity during the DPT-initiating phase successfully reversed the LTP in the KO hippocampus, DPT and c-LTD in the two different systems shared common molecular defects due to the absence of CPEB3. Together, our results suggest that CPEB3 deficiency imbalances NMDAR-activated CaMKIIα signaling, which consequently fails to depress synaptic strength under certain stimulation conditions.

Experience-induced Arc/Arg3.1 primes CA1 pyramidal neurons for metabotropic glutamate receptor-dependent long-term synaptic depression

A novel experience induces the Arc/Arg3.1 gene as well as plasticity of CA1 neural networks. To understand how these are linked, we briefly exposed GFP reporter mice of Arc transcription to a novel environment. Excitatory synaptic function of CA1 neurons with recent in vivo Arc induction (ArcGFP+) was similar to neighboring noninduced neurons. However, in response to group 1 metabotropic glutamate receptor (mGluR) activation, ArcGFP+ neurons preferentially displayed long-term synaptic depression (mGluR-LTD) and robust increases in dendritic Arc protein. mGluR-LTD in ArcGFP+ neurons required rapid protein synthesis and Arc, suggesting that dendritic translation of Arc underlies the priming of mGluR-LTD. In support of this idea, novelty exposure increased Arc messenger RNA in CA1 dendrites and promoted mGluR-induced translation of Arc in hippocampal synaptoneurosomes. Repeated experience suppressed synaptic transmission onto ArcGFP+ neurons and occluded mGluR-LTD ex vivo. mGluR-LTD priming in neurons with similar Arc activation history may contribute to encoding a novel environment.

Initiation, labile, and stabilization phases of experience-dependent plasticity at neocortical synapses

Alteration of sensory input can change the strength of neocortical synapses. Selective activation of a subset of whiskers is sufficient to potentiate layer 4-layer 2/3 excitatory synapses in the mouse somatosensory (barrel) cortex, a process that is NMDAR dependent. By analyzing the time course of sensory-induced synaptic change, we have identified three distinct phases for synaptic strengthening in vivo. After an early, NMDAR-dependent phase where selective whisker activation is rapidly translated into increased synaptic strength, we identify a second phase where this potentiation is profoundly reduced by an input-specific, NMDAR-dependent depression. This labile phase is transient, lasting only a few hours, and may require ongoing sensory input for synaptic weakening. Residual synaptic strength is maintained in a third phase, the stabilization phase, which requires mGluR5 signaling. Identification of these three phases will facilitate a molecular dissection of the pathways that regulate synaptic lability and stabilization, and suggest potential approaches to modulate learning.

Ex vivo depotentiation of conditioning-induced potentiation at thalamic input synapses onto the lateral amygdala requires GluN2B-containing NMDA receptors

We have previously characterized the ex vivo depotentiation (depotentiation(ex vivo)) of conditioning-induced synaptic potentiation at thalamic input synapses onto the lateral amygdala (T-LA synapses) as a potential cellular substrate for fear extinction: both depotentiation(ex vivo) and fear extinction require NMDA receptors, mitogen-activated protein kinases, metabotropic glutamate receptor 1, de novo protein synthesis and AMPA receptor internalization in the amygdala. Surprisingly, as shown in our and other previous studies, ifenprodil, an antagonist of GluN2B-containing NMDA receptors, fails to inhibit depotentiation(ex vivo) at a saturating concentration (10μM), although it has been suggested that GluN2B-containing NMDA receptors are required for fear extinction. Because ifenprodil is also known to act on other molecular targets in addition to GluN2B-containing NMDA receptors, especially at high concentrations (i.e., ≥10μM), the ineffectiveness of 10μM of ifenprodil may be due to its side effects. Therefore, in the present study, we tested Ro25-6981, a more specific antagonist of GluN2B-containing NMDA receptors, and a lower concentration (3μM) of ifenprodil, which may reduce any possible side effects. Ro25-6981 (3μM) blocked both depotentiation(ex vivo) and late-phase long-term potentiation at T-LA synapses. While 10μM ifenprodil failed to inhibit depotentiation(ex vivo), a lower concentration (3μM) of ifenprodil blocked depotentiation(ex vivo). Together, our findings suggest that depotentiation(ex vivo) requires GluN2B-containing NMDA receptors.