Differential roles of NR2A and NR2B-containing NMDA receptors in cortical long-term potentiation and long-term depression

Inhibiting proBDNF to mature BDNF conversion leads to ASD-like phenotypes in vivo

Autism Spectrum Disorders (ASD) comprise a range of early age-onset neurodevelopment disorders with genetic heterogeneity. Most ASD related genes are involved in synaptic function, which is regulated by mature brain-derived neurotrophic factor (mBDNF) and its precursor proBDNF in a diametrically opposite manner: proBDNF inhibits while mBDNF potentiates synapses. Here we generated a knock-in mouse line (BDNFmet/leu) in which the conversion of proBDNF to mBDNF is attenuated. Biochemical experiments revealed residual mBDNF but excessive proBDNF in the brain. Similar to other ASD mouse models, the BDNFmet/leu mice showed reduced dendritic arborization, altered spines, and impaired synaptic transmission and plasticity in the hippocampus. They also exhibited ASD-like phenotypes, including stereotypical behaviors and deficits in social interaction. Moreover, the plasma proBDNF/mBDNF ratio was significantly increased in ASD patients compared to normal children in a case-control study. Thus, deficits in proBDNF to mBDNF conversion in the brain may contribute to ASD-like behaviors, and plasma proBDNF/mBDNF ratio may be a potential biomarker for ASD.

Pyramidal and parvalbumin neurons modulate the process of electroacupuncture stimulation for stroke rehabilitation

Electroacupuncture (EA) stimulation has been shown to be beneficial in stroke rehabilitation; however, little is known about the neurological mechanism by which this peripheral stimulation approach treats for stroke. This study showed that both pyramidal and parvalbumin (PV) neuronal activity increased in the contralesional primary motor cortex forelimb motor area (M1FL) after ischemic stroke induced by focal unilateral occlusion in the M1FL. EA stimulation reduced pyramidal neuronal activity and increased PV neuronal activity. These results were obtained by a combination of fiber photometry recordings, in vivo and in vitro electrophysiological recordings, and immunofluorescence. Moreover, EA was found to regulate the expression/function of N-methyl-D-aspartate receptors (NMDARs) altered by stroke pathology. In summary, our findings suggest that EA could restore disturbed neuronal activity through the regulation of the activity of pyramidal and PV neurons. Furthermore, NMDARs we shown to play an important role in EA-mediated improvements in sensorimotor ability during stroke rehabilitation.

Imbalance of synaptic and extrasynaptic NMDA receptors induced by the deletion of CRMP1 accelerates age-related cognitive decline in mice

Collapsin response mediator protein 1 (CRMP1) is involved in semaphorin 3A signaling pathway, promoting neurite extension and growth cone collapse. It is highly expressed in the nervous system, especially the hippocampus. The crmp1 knockout (KO) mice display impaired spatial learning and memory, and this phenomenon seemingly tends to deteriorate with age. Here we investigated whether CRMP1 is involved in age-related cognitive decline in WT and crmp1 KO mice at adult, middle-aged and older stages. The results revealed that cognitive dysfunction in the Morris water maze task became more severe and decreased glutamate and glutamine level in middle-aged crmp1 KO mice. Additionally, increasing levels of extrasynaptic NMDA receptors and phosphorylation of Tau were observed in middle-aged crmp1 KO mice, leading to synaptic and neuronal loss in the CA3 regions of hippocampus. These findings suggest that deletion of CRMP1 accelerates age-related cognitive decline by disrupting the balance between synaptic and extrasynaptic NMDA receptors, resulting in the loss of synapses and neurons.

Long-term spaceflight composite stress induces depressive behaviors in model rats through disrupting hippocampus synaptic plasticity

INTRODUCTION

Long-term spaceflight composite stress (LSCS) can cause adverse effects on human systems, including the central nervous system, which could trigger anxiety and depression.

AIMS

This study aimed to identify changes in hippocampus synaptic plasticity under LSCS.

METHODS

The present study simulated the real long-term space station environment by conducting a 42-day experiment that involved simulating microgravity, isolation, noise, circadian rhythm disruptions, and low pressure. The mood and behavior of the rats were assessed by behavior test. Transmission electron microscopy and patch-clamp were used to detect the changes in synapse morphology and electrophysiology, and finally, the expression of NMDA receptor channel proteins was detected by western blotting.

RESULTS

The results showed that significant weight loss, anxiety, and depressive behaviors in rats were observed after being exposed to LSCS environment for 42 days. The synaptic structure was severely damaged, manifested as an obvious decrease in postsynaptic density thickness and synaptic interface curvature (p < 0.05; p < 0.05, respectively). Meanwhile, LTP was significantly impaired (p < 0.0001), and currents in the NMDAR channel were also significantly reduced (p < 0.0001). Further analysis found that LSCS decreased the expression of two key subtype proteins on this channel.

CONCLUSION

These results suggested that LSCS-induced depressive behaviors by impairing synaptic plasticity in rat hippocampus.

d-Serine Increases Release of Acetylcholine in Rat Submandibular Glands

d-serine has been observed in submandibular gland tissue in rats, but its functions remain to be clarified. Oral administration of d-serine, but not l-serine, increased its concentrations in the submandibular gland and pilocarpine-induced salivary secretion. In vivo microdialysis was used to collect the d- and l-enantiomers of amino acids from local interstitial fluid in the rat submandibular gland. The proportion of the d-form of serine in interstitial fluid was higher than that in plasma or saliva. Perfusion of the rat submandibular gland with d-serine and l-glutamic acid via the submandibular gland artery resulted in a significant increase in salivary secretion after stimulation of muscarinic receptors with carbachol. In vivo microdialysis applied to the submandibular glands of rats showed that infusion of d-serine along with l-glutamate through the microdialysis probe significantly elevated acetylcholine levels in local interstitial fluids in the submandibular glands of anesthetized rats as compared to that with l-glutamate alone in an N-methyl-d-aspartate receptor glycine site antagonist-sensitive manner. These results indicate that d-serine augments salivary secretion by increasing acetylcholine release in the salivary glands.

Induction of Activity-Regulated Cytoskeleton-Associated Protein and c-Fos Expression in an Animal Model of Anorexia Nervosa

Anorexia nervosa (AN) is a complex eating disorder characterized by reduced caloric intake to achieve body-weight loss. Furthermore, over-exercise is commonly reported. In recent years, animal models of AN have provided evidence for neuroplasticity changes in specific brain areas of the mesocorticolimbic circuit, which controls a multitude of functions including reward, emotion, motivation, and cognition. The activity-regulated cytoskeleton-associated protein (Arc) is an immediate early gene that modulates several forms of synaptic plasticity and has been linked to neuropsychiatric illness. Since the role of Arc in AN has never been investigated, in this study we evaluated whether the anorexic-like phenotype reproduced by the activity-based anorexia (ABA) model may impact its expression in selected brain regions that belong to the mesocorticolimbic circuit (i.e., prefrontal cortex, nucleus accumbens, and hippocampus). The marker of neuronal activation c-Fos was also assessed. We found that the expression of both markers increased in all the analyzed brain areas of ABA rats in comparison to the control groups. Moreover, a negative correlation between the density of Arc-positive cells and body-weight loss was found. Together, our findings suggest the importance of Arc and neuroplasticity changes within the brain circuits involved in dysfunctional behaviors associated with AN.

Extrasynaptic NMDA receptors in acute and chronic excitotoxicity: implications for preventive treatments of ischemic stroke and late-onset Alzheimer's disease

Stroke and late-onset Alzheimer's disease (AD) are risk factors for each other; the comorbidity of these brain disorders in aging individuals represents a significant challenge in basic research and clinical practice. The similarities and differences between stroke and AD in terms of pathogenesis and pathophysiology, however, have rarely been comparably reviewed. Here, we discuss the research background and recent progresses that are important and informative for the comorbidity of stroke and late-onset AD and related dementia (ADRD). Glutamatergic NMDA receptor (NMDAR) activity and NMDAR-mediated Ca²⁺ influx are essential for neuronal function and cell survival. An ischemic insult, however, can cause rapid increases in glutamate concentration and excessive activation of NMDARs, leading to swift Ca²⁺ overload in neuronal cells and acute excitotoxicity within hours and days. On the other hand, mild upregulation of NMDAR activity, commonly seen in AD animal models and patients, is not immediately cytotoxic. Sustained NMDAR hyperactivity and Ca²⁺ dysregulation lasting from months to years, nevertheless, can be pathogenic for slowly evolving events, i.e. degenerative excitotoxicity, in the development of AD/ADRD. Specifically, Ca²⁺ influx mediated by extrasynaptic NMDARs (eNMDARs) and a downstream pathway mediated by transient receptor potential cation channel subfamily M member (TRPM) are primarily responsible for excitotoxicity. On the other hand, the NMDAR subunit GluN3A plays a "gatekeeper" role in NMDAR activity and a neuroprotective role against both acute and chronic excitotoxicity. Thus, ischemic stroke and AD share an NMDAR- and Ca²⁺-mediated pathogenic mechanism that provides a common receptor target for preventive and possibly disease-modifying therapies. Memantine (MEM) preferentially blocks eNMDARs and was approved by the Federal Drug Administration (FDA) for symptomatic treatment of moderate-to-severe AD with variable efficacy. According to the pathogenic role of eNMDARs, it is conceivable that MEM and other eNMDAR antagonists should be administered much earlier, preferably during the presymptomatic phases of AD/ADRD. This anti-AD treatment could simultaneously serve as a preconditioning strategy against stroke that attacks ≥ 50% of AD patients. Future research on the regulation of NMDARs, enduring control of eNMDARs, Ca²⁺ homeostasis, and downstream events will provide a promising opportunity to understand and treat the comorbidity of AD/ADRD and stroke.

Neuron-astrocyte omnidirectional signaling in neurological health and disease

Astrocytes are an abundantly distributed population of glial cells in the central nervous system (CNS) that perform myriad functions in the normal and injured/diseased brain. Astrocytes exhibit heterogeneous phenotypes in response to various insults, a process known as astrocyte reactivity. The accuracy and precision of brain signaling are primarily based on interactions involving neurons, astrocytes, oligodendrocytes, microglia, pericytes, and dendritic cells within the CNS. Astrocytes have emerged as a critical entity within the brain because of their unique role in recycling neurotransmitters, actively modulating the ionic environment, regulating cholesterol and sphingolipid metabolism, and influencing cellular crosstalk in diverse neural injury conditions and neurodegenerative disorders. However, little is known about how an astrocyte functions in synapse formation, axon specification, neuroplasticity, neural homeostasis, neural network activity following dynamic surveillance, and CNS structure in neurological diseases. Interestingly, the tripartite synapse hypothesis came to light to fill some knowledge gaps that constitute an interaction of a subpopulation of astrocytes, neurons, and synapses. This review highlights astrocytes' role in health and neurological/neurodegenerative diseases arising from the omnidirectional signaling between astrocytes and neurons at the tripartite synapse. The review also recapitulates the disruption of the tripartite synapse with a focus on perturbations of the homeostatic astrocytic function as a key driver to modulate the molecular and physiological processes toward neurodegenerative diseases.

Stimulating β-adrenergic receptors promotes synaptic potentiation by switching CaMKII movement from LTD to LTP mode

Learning, memory and cognition are thought to require synaptic plasticity, specifically including hippocampal long-term potentiation and depression (LTP and LTD). LTP versus LTD is induced by high- versus low-frequency stimulation (HFS versus LFS) but, stimulating β-adrenergic receptors (βARs) enables LTP induction also by LFS (1 Hz) or theta frequencies (∼5 Hz) that don't cause plasticity by themselves. In contrast to HFS-LTP, such βAR-LTP requires Ca²⁺-flux through L-type voltage-gated Ca²⁺-channels, not NMDA-type glutamate receptors (NMDARs). Surprisingly, we found that βAR-LTP still required a non-ionotropic scaffolding function of the NMDAR: the stimulus-induced binding of the Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) to its GluN2B subunit that mediates CaMKII movement to excitatory synapses. In hippocampal neurons, β-adrenergic stimulation with isoproterenol transformed LTD-type CaMKII movement to LTP-type movement, resulting in CaMKII movement to excitatory instead of inhibitory synapses. Additionally, isoproterenol enabled induction of a major cell-biological feature of LTP in response to LTD stimuli: increased surface expression of GluA1 fused with super-ecliptic pHluorein (SEP-GluA1). Like for βAR-LTP in hippocampal slices, the isoproterenol effects on CaMKII movement and SEP-GluA1 surface expression involved L-type Ca²⁺-channels and specifically required β2-ARs. Taken together, these results indicate that isoproterenol transforms LTD stimuli to LTP signals by switching CaMKII movement and GluN2B binding to LTP mode.

Pseudo-transistors for emerging neuromorphic electronics

Artificial synaptic devices are the cornerstone of neuromorphic electronics. The development of new artificial synaptic devices and the simulation of biological synaptic computational functions are important tasks in the field of neuromorphic electronics. Although two-terminal memristors and three-terminal synaptic transistors have exhibited significant capabilities in the artificial synapse, more stable devices and simpler integration are needed in practical applications. Combining the configuration advantages of memristors and transistors, a novel pseudo-transistor is proposed. Here, recent advances in the development of pseudo-transistor-based neuromorphic electronics in recent years are reviewed. The working mechanisms, device structures and materials of three typical pseudo-transistors, including tunneling random access memory (TRAM), memflash and memtransistor, are comprehensively discussed. Finally, the future development and challenges in this field are emphasized.

The Role of Glutamate Receptors in Epilepsy

Glutamate is an essential excitatory neurotransmitter in the central nervous system, playing an indispensable role in neuronal development and memory formation. The dysregulation of glutamate receptors and the glutamatergic system is involved in numerous neurological and psychiatric disorders, especially epilepsy. There are two main classes of glutamate receptor, namely ionotropic and metabotropic (mGluRs) receptors. The former stimulate fast excitatory neurotransmission, are N-methyl-d-aspartate (NMDA), α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA), and kainate; while the latter are G-protein-coupled receptors that mediate glutamatergic activity via intracellular messenger systems. Glutamate, glutamate receptors, and regulation of astrocytes are significantly involved in the pathogenesis of acute seizure and chronic epilepsy. Some glutamate receptor antagonists have been shown to be effective for the treatment of epilepsy, and research and clinical trials are ongoing.

In the fast lane: Receptor trafficking during status epilepticus

Status epilepticus (SE) remains a significant cause of morbidity and mortality and often is refractory to standard first-line treatments. A rapid loss of synaptic inhibition and development of pharmacoresistance to benzodiazepines (BZDs) occurs early during SE, while NMDA and AMPA receptor antagonists remain effective treatments after BZDs have failed. Multimodal and subunit-selective receptor trafficking within minutes to an hour of SE involves GABA-A, NMDA, and AMPA receptors and contributes to shifts in the number and subunit composition of surface receptors with differential impacts on the physiology, pharmacology, and strength of GABAergic and glutamatergic currents at synaptic and extrasynaptic sites. During the first hour of SE, synaptic GABA-A receptors containing γ2 subunits move to the cell interior while extrasynaptic GABA-A receptors with δ subunits are preserved. Conversely, NMDA receptors containing N2B subunits are increased at synaptic and extrasynaptic sites, and homomeric GluA1 ("GluA2-lacking") calcium permeant AMPA receptor surface expression also is increased. Molecular mechanisms, largely driven by NMDA receptor or calcium permeant AMPA receptor activation early during circuit hyperactivity, regulate subunit-specific interactions with proteins involved with synaptic scaffolding, adaptin-AP2/clathrin-dependent endocytosis, endoplasmic reticulum (ER) retention, and endosomal recycling. Reviewed here is how SE-induced shifts in receptor subunit composition and surface representation increase the excitatory to inhibitory imbalance that sustains seizures and fuels excitotoxicity contributing to chronic sequela such as "spontaneous recurrent seizures" (SRS). A role for early multimodal therapy is suggested both for treatment of SE and for prevention of long-term comorbidities.

G-protein coupled estrogen receptor (GPER1) activation promotes synaptic insertion of AMPA receptors and induction of chemical LTP at hippocampal temporoammonic-CA1 synapses

It is well documented that 17β estradiol (E2) regulates excitatory synaptic transmission at hippocampal Shaffer-collateral (SC)-CA1 synapses, via activation of the classical estrogen receptors (ERα and ERβ). Hippocampal CA1 pyramidal neurons are also innervated by the temporoammonic (TA) pathway, and excitatory TA-CA1 synapses are reported to be regulated by E2. Recent studies suggest a role for the novel G-protein coupled estrogen receptor (GPER1) at SC-CA1 synapses, however, the role of GPER1 in mediating the effects of E2 at juvenile TA-CA1 synapses is unclear. Here we demonstrate that the GPER1 agonist, G1 induces a persistent, concentration-dependent (1-10 nM) increase in excitatory synaptic transmission at TA-CA1 synapses and this effect is blocked by selective GPER1 antagonists. The ability of GPER1 to induce this novel form of chemical long-term potentiation (cLTP) was prevented following blockade of N-methyl-D-aspartate (NMDA) receptors, and it was not accompanied by any change in paired pulse facilitation ratio (PPR). GPER1-induced cLTP involved activation of ERK but was independent of phosphoinositide 3-kinase (PI3K) signalling. Prior treatment with philanthotoxin prevented the effects of G1, indicating that synaptic insertion of GluA2-lacking α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors underlies GPER1-induced cLTP. Furthermore, activity-dependent LTP occluded G1-induced cLTP and vice versa, indicating that these processes have overlapping expression mechanisms. Activity-dependent LTP was blocked by the GPER1 antagonist, G15, suggesting that GPER1 plays a role in NMDA-dependent LTP at juvenile TA-CA1 synapses. These findings add a new dimension to our understanding of GPER1 in modulating neuronal plasticity with relevance to age-related neurodegenerative conditions.

Genetic removal of synaptic Zn2+ impairs cognition, alters neurotrophic signaling and induces neuronal hyperactivity

Vesicular Zn2+ (zinc) is released at synapses and has been demonstrated to modulate neuronal responses. However, mechanisms through which dysregulation of zinc homeostasis may potentiate neuronal dysfunction and neurodegeneration are not well-understood. We previously reported that accumulation of soluble amyloid beta oligomers (AβO) at synapses correlates with synaptic loss and that AβO localization at synapses is regulated by synaptic activity and enhanced by the release of vesicular Zn2+ in the hippocampus, a brain region that deteriorates early in Alzheimer's disease (AD). Significantly, drugs regulating zinc homeostasis inhibit AβO accumulation and improve cognition in mouse models of AD. We used both sexes of a transgenic mouse model lacking synaptic Zn2+ (ZnT3KO) that develops AD-like cognitive impairment and neurodegeneration to study the effects of disruption of Zn2+ modulation of neurotransmission in cognition, protein expression and activation, and neuronal excitability. Here we report that the genetic removal of synaptic Zn2+ results in progressive impairment of hippocampal-dependent memory, reduces activity-dependent increase in Erk phosphorylation and BDNF mRNA, alters regulation of Erk activation by NMDAR subunits, increases neuronal spiking, and induces biochemical and morphological alterations consistent with increasing epileptiform activity and neurodegeneration as ZnT3KO mice age. Our study shows that disruption of synaptic Zn2+ triggers neurodegenerative processes and is a potential pathway through which AβO trigger altered expression of neurotrophic proteins, along with reduced hippocampal synaptic density and degenerating neurons, neuronal spiking activity, and cognitive impairment and supports efforts to develop therapeutics to preserve synaptic zinc homeostasis in the brain as potential treatments for AD.

Adaptive control of synaptic plasticity integrates micro- and macroscopic network function

Synaptic plasticity configures interactions between neurons and is therefore likely to be a primary driver of behavioral learning and development. How this microscopic-macroscopic interaction occurs is poorly understood, as researchers frequently examine models within particular ranges of abstraction and scale. Computational neuroscience and machine learning models offer theoretically powerful analyses of plasticity in neural networks, but results are often siloed and only coarsely linked to biology. In this review, we examine connections between these areas, asking how network computations change as a function of diverse features of plasticity and vice versa. We review how plasticity can be controlled at synapses by calcium dynamics and neuromodulatory signals, the manifestation of these changes in networks, and their impacts in specialized circuits. We conclude that metaplasticity-defined broadly as the adaptive control of plasticity-forges connections across scales by governing what groups of synapses can and can't learn about, when, and to what ends. The metaplasticity we discuss acts by co-opting Hebbian mechanisms, shifting network properties, and routing activity within and across brain systems. Asking how these operations can go awry should also be useful for understanding pathology, which we address in the context of autism, schizophrenia and Parkinson's disease.

A critical role for long-term potentiation mechanisms in the maintenance of object recognition memory in perirhinal cortex revealed by the infusion of zeta inhibitory pseudosubstrate

Object recognition, the ability to discriminate between a novel and a familiar stimulus, is critically dependent upon the perirhinal cortex. Neural response reductions upon repetition of a stimulus, have been hypothesized to be the mechanism within perirhinal cortex that supports recognition memory function. Thus, investigations into the mechanisms of long-term depression (LTD) in perirhinal cortex has provided insight into the mechanism of object recognition memory formation, but the contribution of long-term potentiation (LTP) to object recognition memory formation has been less studied. Inhibition of atypical PKC activity by Zeta Inhibitory Pseudosubstrate (ZIP) impairs the maintenance of LTP but not LTD, thus here infusion of ZIP into the perirhinal cortex allowed us to investigate the contribution of LTP-like mechanisms to object recognition memory maintenance. Infusion of ZIP into the perirhinal cortex of rats 24 h after the sample phase impaired performance in an object recognition but not an object location task, in contrast infusion of ZIP into the hippocampus impaired performance in an object location but not an object recognition task. The impairment in object recognition by ZIP was prevented by administration of the peptide GluA2_3y, which blocks the endocytosis of GluA2 containing AMPA receptors. Finally, performance in a perceptual oddity task, which requires perirhinal cortex function, was not disrupted by ZIP. Together these results demonstrate the importance of LTP-like mechanisms to the maintenance of object recognition memory in the perirhinal cortex.

Restored Fyn Levels in Huntington’s Disease Contributes to Enhanced Synaptic GluN2B-Composed NMDA Receptors and CREB Activity

N-methyl-D-aspartate receptors (NMDARs) are important postsynaptic receptors that contribute to normal synaptic function and cell survival; however, when overactivated, as in Huntington’s disease (HD), NMDARs cause excitotoxicity. HD-affected striatal neurons show altered NMDAR currents and augmented ratio of surface to internal GluN2B-containing NMDARs, with augmented accumulation at extrasynaptic sites. Fyn protein is a member of the Src kinase family (SKF) with an important role in NMDARs phosphorylation and synaptic localization and function; recently, we demonstrated that Fyn is reduced in several HD models. Thus, in this study, we aimed to explore the impact of HD-mediated altered Fyn levels at post-synaptic density (PSD), and their role in distorted NMDARs function and localization, and intracellular neuroprotective pathways in YAC128 mouse primary striatal neurons. We show that reduced synaptic Fyn levels and activity in HD mouse striatal neurons is related to decreased phosphorylation of synaptic GluN2B-composed NMDARs; this occurs concomitantly with augmented extrasynaptic NMDARs activity and currents and reduced cAMP response element-binding protein (CREB) activation, along with induction of cell death pathways. Importantly, expression of a constitutive active form of SKF reestablishes NMDARs localization, phosphorylation, and function at PSD in YAC128 mouse neurons. Enhanced SKF levels and activity also promotes CREB activation and reduces caspase-3 activation in YAC128 mouse striatal neurons. This work supports, for the first time, a relevant role for Fyn protein in PSD modulation, controlling NMDARs synaptic function in HD, and favoring neuroprotective pathways and cell survival. In this respect, Fyn Tyr kinase constitutes an important potential HD therapeutic target directly acting at PSD.

Enhancing GluN2A-type NMDA receptors impairs long-term synaptic plasticity and learning and memory

N-methyl-D-aspartic acid type glutamate receptors (NMDARs) play critical roles in synaptic transmission and plasticity, the dysregulation of which leads to cognitive defects. Here, we identified a rare variant in the NMDAR subunit GluN2A (K879R) in a patient with intellectual disability. The K879R mutation enhanced receptor expression on the cell surface by disrupting a KKK motif that we demonstrated to be an endoplasmic reticulum retention signal. Expression of GluN2A_K879R in mouse hippocampal CA1 neurons enhanced the excitatory postsynaptic currents mediated by GluN2A-NMDAR but suppressed those mediated by GluN2B-NMDAR and the AMPA receptor. GluN2A_K879R knock-in mice showed similar defects in synaptic transmission and exhibited impaired learning and memory. Furthermore, both LTP and LTD were severely impaired in the KI mice, likely explaining their learning and memory defects. Therefore, our study reveals a new mechanism by which elevated synaptic GluN2A-NMDAR impairs long-term synaptic plasticity as well as learning and memory.

NMDA receptors mediate synaptic plasticity impairment of hippocampal neurons due to arsenic exposure

Endemic arsenism is a worldwide health problem. Chronic arsenic exposure results in cognitive dysfunction due to arsenic and its metabolites accumulating in hippocampus. As the cellular basis of cognition, synaptic plasticity is pivotal in arsenic-induced cognitive dysfunction. N-methyl-D-aspartate receptors (NMDARs) serve physiological functions in synaptic transmission. However, excessive NMDARs activity contributes to exitotoxicity and synaptic plasticity impairment. Here, we provide an overview of the mechanisms that NMDARs and their downstream signaling pathways mediate synaptic plasticity impairment due to arsenic exposure in hippocampal neurons, ways of arsenic exerting on NMDARs, as well as the potential therapeutic targets except for water improvement.

NMDA and AMPA receptor physiology and role in visceral hypersensitivity: a review

N-methyl-d-aspartate receptors (NMDARs) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors (AMPARs) are excitatory neurotransmission receptors of the central nervous system and play vital roles in synaptic plasticity. Although not fully elucidated, visceral hypersensitivity is one of the most well-characterized pathophysiologic abnormalities of functional gastrointestinal diseases and appears to be associated with increased synaptic plasticity. In this study, we review the updated findings on the physiology of NMDARs and AMPARs and their relation to visceral hypersensitivity, which propose directions for future research in this field with evolving importance.

Inhibition of CK2 mitigates Alzheimer's tau pathology by preventing NR2B synaptic mislocalization

Alzheimer’s disease (AD) is a neurodegenerative disorder that exhibits pathological changes in both tau and synaptic function. AD patients display increases in hyperphosphorylated tau and synaptic activity. Previous studies have individually identified the role of NR2B subunit-containing NMDA receptors in AD related synaptic dysfunction and aggregated tau without reconciling the conflicting differences and implications of NR2B expression. Inhibition of extrasynaptically located NR2B mitigates tau pathology in AD models, whereas the inhibition of synaptic NR2B replicates tau-associated hyperactivity. This suggests that a simultaneous increase in extrasynaptic NR2B and decrease in synaptic NR2B may be responsible for tau pathology and synaptic dysfunction, respectively. The synaptic location of NR2B is regulated by casein kinase 2 (CK2), which is highly expressed in AD patients. Here, we used patient brains diagnosed with AD, corticobasal degeneration, progressive supranuclear palsy or Pick’s disease to characterize CK2 expression across these diverse tauopathies. Human derived material was also utilized in conjunction with cultured hippocampal neurons in order to investigate AD-induced changes in NR2B location. We further assessed the therapeutic effect of CK2 inhibition on NR2B synaptic distribution and tau pathology. We found that aberrant expression of CK2, and synaptically translocated NR2B, is unique to AD patients compared to other tauopathies. Increased CK2 was also observed in AD-tau treated neurons in addition to the mislocalization of NR2B receptors. Tau burden was alleviated in vitro by correcting synaptic:extrasynaptic NR2B function. Restoring NR2B physiological expression patterns with CK2 inhibition and inhibiting the function of excessive extrasynaptic NR2B with Memantine both mitigated tau accumulation in vitro. However, the combined pharmacological treatment promoted the aggregation of tau. Our data suggests that the synaptic:extrasynaptic balance of NR2B function regulates AD-tau pathogenesis, and that the inhibition of CK2, and concomitant prevention of NR2B mislocalization, may be a useful therapeutic tool for AD patients.

Free d-Amino Acids in Salivary Gland in Rat

Free d-amino acids, which are enantiomers of l-amino acids, are found in mammals, including humans, and play an important role in a range of physiological functions in the central nervous system and peripheral tissues. Several d-amino acids have been observed in saliva, but their origin and the enzymes involved in their metabolism and catabolism remain to be clarified. In the present study, large amounts of d-aspartic acid and small amounts of d-serine and d-alanine were detected in all three major salivary glands in rat. No other d-enantiomers were detected. Protein expression of d-amino acid oxidase and d-aspartate oxidase, the enzymes responsible for the oxidative deamination of neutral and dicarboxylic d-amino acids, respectively, were detected in all three types of salivary gland. Furthermore, protein expression of the d-serine metabolic enzyme, serine racemase, in parotid glands amounted to approximately 40% of that observed in the cerebral cortex. The N-methyl-d-aspartic acid subunit proteins NR1 and NR2D were detected in all three major salivary glands. The results of the present study suggest that d-amino acids play a physiological role in a range of endocrine and exocrine function in salivary glands.

Nanoscale rules governing the organization of glutamate receptors in spine synapses are subunit specific

Heterotetrameric glutamate receptors are essential for the development, function, and plasticity of spine synapses but how they are organized to achieve this is not known. Here we show that the nanoscale organization of glutamate receptors containing specific subunits define distinct subsynaptic features. Glutamate receptors containing GluA2 or GluN1 subunits establish nanomodular elements precisely positioned relative to Synaptotagmin-1 positive presynaptic release sites that scale with spine size. Glutamate receptors containing GluA1 or GluN2B specify features that exhibit flexibility: GluA1-subunit containing AMPARs are found in larger spines, while GluN2B-subunit containing NMDARs are enriched in the smallest spines with neither following a strict modular organization. Given that the precise positioning of distinct classes of glutamate receptors is linked to diverse events including cell death and synaptic plasticity, this unexpectedly robust synaptic nanoarchitecture provides a resilient system, where nanopositioned glutamate receptor heterotetramers define specific subsynaptic regions of individual spine synapses.

Glutamate receptors comprise two obligate subunits and two subunits that confer distinct properties and functions to the specific tetramers, which also localize to distinct synaptic spines. Here, the authors use STimulated Emission Depletion nanoscopy (STED) to provide detailed insights into the spatial organization of glutamate receptor types.

Behind the scenes: Are latent memories supported by calcium independent plasticity?

N-methyl-D-aspartate receptors (NMDARs) can be considered to be the de facto "plasticity" receptors in the brain due to their central role in the activity-dependent modification of neuronal morphology and synaptic transmission. Since the 1980s, research on NMDARs has focused on the second messenger properties of calcium and the downstream signaling pathways that mediate alterations in neural form and function. Recently, NMDARs were shown to drive activity-dependent synaptic plasticity without calcium influx. How this "nonionotropic" plasticity occurs in vitro is becoming clearer, but research on its involvement in behavior and cognition is in its infancy. There is a partial overlap in the downstream signaling molecules that are involved in ionotropic and nonionotropic NMDAR-dependent plasticity. Given this, and prior studies of the cognitive impacts of ionotropic NMDAR plasticity, a preliminary model explaining how NMDAR nonionotropic plasticity affects learning and memory can be established. We hypothesize that nonionotropic NMDAR plasticity takes part in latent memory encoding in immature rodents through nonassociative depression of synaptic efficacy, and possibly shrinking of dendritic spines. Further, the late postnatal alteration in NMDAR composition in the hippocampus appears to reduce nonionotropic signaling and remove a restriction on memory retrieval. This framework substantially alters the canonical model of NMDAR involvement in spatial cognition and hippocampal maturation and provides novel and exciting inroads for future studies.

Differential regulation of STP, LTP and LTD by structurally diverse NMDA receptor subunit-specific positive allosteric modulators

Different types of memory are thought to rely on different types of synaptic plasticity, many of which depend on the activation of the N-Methyl-D Aspartate (NMDA) subtype of glutamate receptors. Accordingly, there is considerable interest in the possibility of using positive allosteric modulators (PAMs) of NMDA receptors (NMDARs) as cognitive enhancers. Here we firstly review the evidence that NMDA receptor-dependent forms of synaptic plasticity: short-term potentiation (STP), long-term potentiation (LTP) and long-term depression (LTD) can be pharmacologically differentiated by using NMDAR ligands. These observations suggest that PAMs of NMDAR function, depending on their subtype selectivity, might differentially regulate STP, LTP and LTD. To test this hypothesis, we secondly performed experiments in rodent hippocampal slices with UBP714 (a GluN2A/2B preferring PAM), CIQ (a GluN2C/D selective PAM) and UBP709 (a pan-PAM that potentiates all GluN2 subunits). We report here, for the first time, that: (i) UBP714 potentiates sub-maximal LTP and reduces LTD; (ii) CIQ potentiates STP without affecting LTP; (iii) UBP709 enhances LTD and decreases LTP. We conclude that PAMs can differentially regulate distinct forms of NMDAR-dependent synaptic plasticity due to their subtype selectivity.

This article is part of the Neuropharmacology Special Issue on ‘Glutamate Receptors – NMDA receptors’.

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NMDAR-dependent STP, LTP and LTD can be dissociated pharmacologically

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GluN2A/2B PAM UBP714 potentiates LTP and reduces LTD

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GluN2C/D PAM CIQ potentiates STP without affecting LTP

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NMDAR pan-PAM UBP709 potentiates LTD and reduces LTP

Impairments of Long-Term Synaptic Plasticity in the Hippocampus of Young Rats during the Latent Phase of the Lithium-Pilocarpine Model of Temporal Lobe Epilepsy

Status epilepticus (SE) causes persistent abnormalities in the functioning of neuronal networks, often resulting in worsening epileptic seizures. Many details of cellular and molecular mechanisms of seizure-induced changes are still unknown. The lithium–pilocarpine model of epilepsy in rats reproduces many features of human temporal lobe epilepsy. In this work, using the lithium–pilocarpine model in three-week-old rats, we examined the morphological and electrophysiological changes in the hippocampus within a week following pilocarpine-induced seizures. We found that almost a third of the neurons in the hippocampus and dentate gyrus died on the first day, but this was not accompanied by impaired synaptic plasticity at that time. A diminished long-term potentiation (LTP) was observed following three days, and the negative effect of SE on plasticity increased one week later, being accompanied by astrogliosis. The attenuation of LTP was caused by the weakening of N-methyl-D-aspartate receptor (NMDAR)-dependent signaling. NMDAR-current was more than two-fold weaker during high-frequency stimulation in the post-SE rats than in the control group. Application of glial transmitter D-serine, a coagonist of NMDARs, allows the enhancement of the NMDAR-dependent current and the restoration of LTP. These results suggest that the disorder of neuron–astrocyte interactions plays a critical role in the impairment of synaptic plasticity.

Neonatal NMDA blockade alters the LTP, LTD and cognitive functions in male and female Wistar rats

There is compelling evidence that neonatal blockade of NMDA receptors by phencyclidine (PCP) is associated with cognitive impairment in adulthood but little is known about the effects of early life PCP treatment on synaptic function later in life. Here, we sought to determine whether early life exposure to PCP alters the electrophysiologic function of hippocampal CA1 neurons in adult rats. To this end, male and female Wistar rats received either saline or PCP (10 mg/kg) on postnatal days (PND) 7, 9, and 11, and then underwent separate behavioral and electrophysiology tests in adulthood. Neonatal PCP treatment did not alter basic synaptic transmission and had only a modest effect on frequency following (FF) capacity but significantly decreased the paired-pulse facilitation (PPF) in the Schaffer collateral (SC)-CA1 pathway. We found that PCP treatment significantly attenuated the long-term potentiation (LTP) and long-term depression (LTD) in CA1 neurons accompanied by pronounced alteration in complex response profile in adult rats. The electrophysiology data were comparable in male and female rats and reliably associated with impaired spatial reference and working memories in these animals. Overall, this study suggests that blockade of NMDA receptors during early life deteriorates the short-term and long-term synaptic plasticity and complex response profile of CA1 neurons in adulthood.

Neuroprotective Effect of Moxibustion on Cerebral Ischemia/Reperfusion Injury in Rats by Downregulating NR2B Expression

Objective

Stroke is a common and frequently occurring disease of the central nervous system, which is characterized by high mortality and a high disability rate. Moxibustion is a common method for treating stroke in traditional Chinese medicine, but its neuroprotective mechanism is unknown. N-Methyl-D-Aspartate Receptor Subunit 2B (NR2B) plays an important role in neuronal apoptosis. The objective of this study was to explore the mechanisms underlying the neuroprotective effect of moxibustion on cerebral ischemia/reperfusion (I/R) injury based on NR2B.

Methods

Sprague-Dawley rats were randomly divided into 5 groups: the control group, I/R group, I/R + moxibustion group, I/R + Ro25-6981 (NR2B antagonist) group, and I/R + Ro25-6981 + moxibustion group. The cerebral ischemia/reperfusion model was induced by middle cerebral artery occlusion. Before the establishment of the model, the Ro25-6981 group received intraperitoneal injections of Ro25-6981, the moxibustion group received moxibustion, and the Ro25-6981 + moxibustion group received both interventions. The neurological dysfunction was evaluated by a neurological deficiency score (NDS). The infarct volume was examined by TTC (2,3,5-triphenyltetrazolium chloride) staining. The apoptosis rate of cerebral cells in the ischemic area was examined by TUNEL (terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling) staining, and the expression of Bcl-2, Bax, and caspase-3 was observed by western blot. NR2B and JNK were also observed by western blot.

Results

Compared with the I/R group, moxibustion significantly decreased the neurological deficiency score (P < 0.05) and the infarct rate (P < 0.01) in I/R rats which were similar to those in the Ro25-6981 group. After moxibustion treatment, there was a significant decrease in the apoptosis rate (P < 0.001) and the protein expression levels of Bax, caspase-3, and JNK (P < 0.001) and an increase in the expression of Bcl-2 (P < 0.01). Compared with the I/R group, moxibustion downregulated the expression of NR2B and decreased the activity of NR2B in the cerebral ischemia area (P < 0.001).

Conclusions

Moxibustion can improve neurological dysfunction and decrease infarction area and neuronal apoptosis caused by cerebral ischemia/reperfusion in rats. Its neuroprotective mechanism may be related to downregulating the expression of NR2B.

Understanding ethanol's acute effects on medial prefrontal cortex neural activity using state-space approaches

Acute ethanol (EtOH) intoxication results in several maladaptive behaviors that may be attributable, in part, to the effects of EtOH on neural activity in medial prefrontal cortex (mPFC). The acute effects of EtOH on mPFC function have been largely described as inhibitory. However, translating these observations on function into a mechanism capable of delineating acute EtOH's effects on behavior has proven difficult. This review highlights the role of acute EtOH on electrophysiological measurements of mPFC function and proposes that interpreting these changes through the lens of dynamical systems theory is critical to understand the mechanisms that mediate the effects of EtOH intoxication on behavior. Specifically, the present review posits that the effects of EtOH on mPFC N-methyl-d-aspartate (NMDA) receptors are critical for the expression of impaired behavior following EtOH consumption. This hypothesis is based on the observation that recurrent activity in cortical networks is supported by NMDA receptors, and, when disrupted, may lead to impairments in cognitive function. To evaluate this hypothesis, we discuss the representation of mPFC neural activity in low-dimensional, dynamic state spaces. This approach has proven useful for identifying the underlying computations necessary for the production of behavior. Ultimately, we hypothesize that EtOH-related alterations to NMDA receptor function produces alterations that can be effectively conceptualized as impairments in attractor dynamics and provides insight into how acute EtOH disrupts forms of cognition that rely on mPFC function. This article is part of the special Issue on 'Neurocircuitry Modulating Drug and Alcohol Abuse'.

Role of hippocampal NF-κB and GluN2B in the memory acquisition impairment of experiences gathered prior to cocaine administration in rats

Cocaine can induce severe neurobehavioral changes, among others, the ones involved in learning and memory processes. It is known that during drug consumption, cocaine-associated memory and learning processes take place. However, much less is known about the effects of this drug upon the mechanisms involved in forgetting.The present report focuses on the mechanisms by which cocaine affects memory consolidation of experiences acquired prior to drug administration. We also study the involvement of hippocampus in these processes, with special interest on the role of Nuclear factor kappa B (NF-κB), N-methyl-D-aspartate glutamate receptor 2B (GluN2B), and their relationship with other proteins, such as cyclic AMP response element binding protein (CREB). For this purpose, we developed a rat experimental model of chronic cocaine administration in which spatial memory and the expression or activity of several proteins in the hippocampus were assessed after 36 days of drug administration. We report an impairment in memory acquisition of experiences gathered prior to cocaine administration, associated to an increase in GluN2B expression in the hippocampus. We also demonstrate a decrease in NF-κB activity, as well as in the expression of the active form of CREB, confirming the role of these transcription factors in the cocaine-induced memory impairment.

Structure, Function, and Pharmacology of Glutamate Receptor Ion Channels

Many physiologic effects of l-glutamate, the major excitatory neurotransmitter in the mammalian central nervous system, are mediated via signaling by ionotropic glutamate receptors (iGluRs). These ligand-gated ion channels are critical to brain function and are centrally implicated in numerous psychiatric and neurologic disorders. There are different classes of iGluRs with a variety of receptor subtypes in each class that play distinct roles in neuronal functions. The diversity in iGluR subtypes, with their unique functional properties and physiologic roles, has motivated a large number of studies. Our understanding of receptor subtypes has advanced considerably since the first iGluR subunit gene was cloned in 1989, and the research focus has expanded to encompass facets of biology that have been recently discovered and to exploit experimental paradigms made possible by technological advances. Here, we review insights from more than 3 decades of iGluR studies with an emphasis on the progress that has occurred in the past decade. We cover structure, function, pharmacology, roles in neurophysiology, and therapeutic implications for all classes of receptors assembled from the subunits encoded by the 18 ionotropic glutamate receptor genes. SIGNIFICANCE STATEMENT: Glutamate receptors play important roles in virtually all aspects of brain function and are either involved in mediating some clinical features of neurological disease or represent a therapeutic target for treatment. Therefore, understanding the structure, function, and pharmacology of this class of receptors will advance our understanding of many aspects of brain function at molecular, cellular, and system levels and provide new opportunities to treat patients.

The Increased Channel Activity of N-Methyl-D-Aspartate Receptors at Extrasynaptic Sites in the Anterior Cingulate Cortex of Neonatal Rats Following Prolonged Ketamine Exposure

Background

Ketamine is a dissociative anesthetic, commonly used for analgesia and anesthesia in a variety of pediatric procedures. It acts as a non-competitive antagonist to block ion channels of the N-methyl-D-aspartate receptors (NMDARs). Our previous study showed that repeated ketamine exposure developed a compensatory increase in NMDAR-mediated currents in neurons of the anterior cingulate cortex (ACC) of neonatal rats, and this increase was largely mediated by the GluN2B subunit-containing receptors, a predominant type of NMDARs during embryonic and early development of the brain. These data provide the molecular evidence to support that immature neurons are highly vulnerable to the development of apoptotic cell death after prolonged ketamine exposure.

Methods

Using whole-cell patch-clamp electrophysiology in an in vitro preparation of rat forebrain slices containing the ACC, the present study aimed at further determining whether GluN2B-containing NMDARs at extrasynaptic sites of immature neurons were the major target of ketamine for developing a compensatory increase in NMDAR-mediated synaptic transmission.

Results

Our major findings were that GluN2B subunits played a significant role in mediating ketamine-induced blockade of NMDAR-mediated currents in neonatal neurons and GluN2B-containing NMDARs expressed at extrasynaptic sites in neonatal neurons were the major player in compensatory enhancement of NMDAR-mediated currents after repeated ketamine exposure.

Conclusion

These results provide new evidence to strongly indicate that GluN2B-containing NMDARs at extrasynaptic sites are the key molecule contributing to the high vulnerability of the neonatal brain to ketamine-induced neurotoxic effects.

Requirements of Postnatal proBDNF in the Hippocampus for Spatial Memory Consolidation and Neural Function

Mature brain-derived neurotrophic factor (BDNF) and its downstream signaling pathways have been implicated in regulating postnatal development and functioning of rodent brain. However, the biological role of its precursor pro-brain-derived neurotrophic factor (proBDNF) in the postnatal brain remains unknown. The expression of hippocampal proBDNF was blocked in postnatal weeks, and multiple behavioral tests, Western blot and morphological techniques, and neural recordings were employed to investigate how proBDNF played a role in spatial cognition in adults. The peak expression and its crucial effects were found in the fourth but not in the second or eighth postnatal week. Blocking proBDNF expression disrupted spatial memory consolidation rather than learning or memory retrieval. Structurally, blocking proBDNF led to the reduction in spine density and proportion of mature spines. Although blocking proBDNF did not affect N-methyl-D-aspartate (NMDA) receptor (NMDAR) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) subunits, the learning-induced phosphorylation of the GluN2B subunit level declined significantly. Functionally, paired-pulse facilitation, post-low-frequency stimulation (LFS) transiently enhanced depression, and GluN2B-dependent short-lasting long-term depression in the Schaffer collateral-CA1 pathway were weakened. The firing rate of pyramidal neurons was significantly suppressed around the target region during the memory test. Furthermore, the activation of GluN2B-mediated signaling could effectively facilitate neural function and mitigate memory impairment. The findings were consistent with the hypothesis that postnatal proBDNF played an essential role in synaptic and cognitive functions.

Role of N-methyl-d-aspartate receptor subunits GluN2A and GluN2B in auditory thalamocortical long-term potentiation in adult mice

Cortical neurons undergo continuous remodelling throughout development and into adulthood, associated with long-term changes in the synaptic transmission of thalamocortical pathways, i.e., long-term potentiation (LTP); such plasticity is input-specific, reflected in the frequency-specificity of the auditory system. It is well established that thalamocortical LTP is dependent on the activation of N-methyl-d-aspartate (NMDA) receptors. In this study, the roles of NMDA receptor subunits GluN2A and GluN2B in LTP induction were examined in thalamocortical pathways of the auditory system using subunit-selective pharmacological inhibition and in vivo tetanic stimulation of the auditory thalamus, while recording neural response in the primary auditory cortex. Long-term enhancement of thalamocortical field excitatory postsynaptic potentials (i.e., thalamocortical LTP) were induced by high frequency tetanic stimulation of the ventral division of the medial geniculate body. Such enhancement in thalamocortical fEPSPs was decreased when a GluN2A blocker (NVP-M077) was applied to the recording site in the primary auditory cortex and was increased when a GluN2B blocker (Ro25-6981) was applied. Our data suggest that the induction of thalamocortical LTP is dependent on the differential expression of the GluN2A and GluN2B subunits of NMDA receptors in thalamocortical circuits.

Chronic AdipoRon Treatment Mimics the Effects of Physical Exercise on Restoring Hippocampal Neuroplasticity in Diabetic Mice

Administration of exercise mimetic drugs could be a novel therapeutic approach to combat comorbid neurodegeneration and metabolic syndromes. Adiponectin is an adipocyte-secreted hormone. In addition to its antidiabetic effect, adiponectin mediates the antidepressant effect of physical exercise associated with adult hippocampal neurogenesis. The antidiabetic effect of the adiponectin receptor agonist AdipoRon has been demonstrated, but its potential pro-cognitive and neurotrophic effects in the hippocampus under diabetic condition are still unclear. This study reported that chronic AdipoRon treatment for 2 weeks improved hippocampal-dependent spatial recognition memory in streptozotocin-induced diabetic mice. Besides, AdipoRon treatment increased progenitor cell proliferation and neuronal differentiation in the hippocampal dentate gyrus (DG) of diabetic mice. Furthermore, AdipoRon treatment significantly increased dendritic complexity, spine density, and N-methyl-D-aspartate receptor-dependent long-term potentiation (LTP) in the dentate region, and increased BDNF levels in the DG of diabetic mice. AdipoRon treatment activated AMPK/PGC-1α signalling in the DG, whereas increases in cell proliferation and LTP were not observed when PGC-1α signalling was pharmacologically inhibited. In sum, chronic AdipoRon treatment partially mimics the benefits of physical exercise for learning and memory and hippocampal neuroplasticity in the diabetic brain. The results suggested that AdipoRon could be a potential physical exercise mimetic to improve hippocampal plasticity and hence rescue learning and memory impairment typically associated with diabetes.

Differential inhibitory effects of resveratrol on excitotoxicity and synaptic plasticity: involvement of NMDA receptor subtypes

Objectives: The neuroprotective effects of resveratrol against excitatory neurotoxicity have been associated with N-methyl-D-aspartate receptor (NMDAR) inhibition. This study examined the differential inhibitory effects of resveratrol on NMDAR-mediated responses in neuronal cells with different NMDAR subtype composition.Methods: The effects of resveratrol on NMDA-induced cell death and calcium influx in immature and mature rat primary cortical neurons were determined and compared. Moreover, the potencies and efficacies of resveratrol to inhibit NR1/NR2A, NR1/NR2B, NR1/NR2C, and NR1/NR2D NMDAR expressed in HEK 293 cells were evaluated.Results: Resveratrol significantly attenuated NMDA-induced cell death in mature neurons, but not in immature neurons. Resveratrol also concentration-dependently reduced NMDA-induced calcium influx among all NMDAR subtypes, but displayed NR2 subunit selectivity, with a potency rank order of NR2B = NR2D > NR2A = NR2C and an efficacy rank order of NR2B = NR2C > NR2A = NR2D. Data show the stronger inhibitory effects of resveratrol on NR1/NR2B than other subtypes. Moreover, resveratrol did not affect hippocampal long-term potentiation (LTP), but impaired long-term depression (LTD).Discussion: These findings reveal the specific NMDAR modulating profile of resveratrol, providing further insight into potential mechanisms underlying the protective effects of resveratrol on neurological disorders.

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.

Net decrease in spine-surface GluA1-containing AMPA receptors after post-learning sleep in the adult mouse cortex

The mechanisms by which sleep benefits learning and memory remain unclear. Sleep may further strengthen the synapses potentiated by learning or promote broad synaptic weakening while protecting the newly potentiated synapses. We tested these ideas by combining a motor task whose consolidation is sleep-dependent, a marker of synaptic AMPA receptor plasticity, and repeated two-photon imaging to track hundreds of spines in vivo with single spine resolution. In mouse motor cortex, sleep leads to an overall net decrease in spine-surface GluA1-containing AMPA receptors, both before and after learning. Molecular changes in single spines during post-learning sleep are correlated with changes in performance after sleep. The spines in which learning leads to the largest increase in GluA1 expression have a relative advantage after post-learning sleep compared to sleep deprivation, because sleep weakens all remaining spines. These results are obtained in adult mice, showing that sleep-dependent synaptic down-selection also benefits the mature brain.

The metaplastic effects of cordycepin in hippocampal CA1 area of rats

Metaplasticity is referred to adjustment in the requirements for induction of synaptic plasticity based on the prior history of activity. Synaptic plasticity, including long-term potentiation (LTP) and long-term depression (LTD), has been considered to be the neural processes underlying learning and memory. Previous observations that cordycepin (an adenosine derivative) improved learning and memory seemed to be contradictory to the findings that cordycepin inhibited LTP. Therefore, we speculated that the conflicting reports of cordycepin might be related to metaplasticity. In the current study, population spike (PS) in hippocampal CA1 area of rats was recorded by using electrophysiological method in vivo. The results showed that cordycepin reduced PS amplitude in hippocampal CA1 with a concentration-dependent relationship, and high frequency stimulation (HFS) failed to induce LTP when cordycepin was intrahippocampally administrated but improved LTP magnitude when cordycepin was pre-treated. Cordycepin increased LTD induced by activating N-Methyl-D-aspartate (NMDA) receptors and subsequently facilitated LTP induced by HFS. Furthermore, we found that 1,3-dipropyl-8-cyclopentylxanthine (DPCPX), an adenosine A1 receptors antagonist, could block the roles of cordycepin on LTD and LTP. Collectively, cordycepin was able to modulate metaplasticity in hippocampal CA1 area of rats through adenosine A1 receptors. These findings would be helpful to reconcile the conflicting reports in the literatures and provided new insights into the mechanisms underlying cognitive function promotions with cordycepin treatment.

Plasticity in Prefrontal Cortex Induced by Coordinated Synaptic Transmission Arising from Reuniens/Rhomboid Nuclei and Hippocampus

The nucleus reuniens and rhomboid nuclei of the thalamus (ReRh) are reciprocally connected to a range of higher order cortices including hippocampus (HPC) and medial prefrontal cortex (mPFC). The physiological function of ReRh is well predicted by requirement for interactions between mPFC and HPC, including associative recognition memory, spatial navigation, and working memory. Although anatomical and electrophysiological evidence suggests ReRh makes excitatory synapses in mPFC there is little data on the physiological properties of these projections, or whether ReRh and HPC target overlapping cell populations and, if so, how they interact. We demonstrate in ex vivo mPFC slices that ReRh and HPC afferent inputs converge onto more than two-thirds of layer 5 pyramidal neurons, show that ReRh, but not HPC, undergoes marked short-term plasticity during theta frequency transmission, and that HPC, but not ReRh, afferents are subject to neuromodulation by acetylcholine acting via muscarinic receptor M2. Finally, we demonstrate that pairing HPC followed by ReRh (but not pairing ReRh followed by HPC) at theta frequency induces associative, NMDA receptor dependent synaptic plasticity in both inputs to mPFC. These data provide vital physiological phenotypes of the synapses of this circuit and provide a novel mechanism for HPC-ReRh-mPFC encoding.

N-methyl-D-aspartate receptor blockade reduces plasticity-related tau expression and phosphorylation of tau at Ser416 residue but not Thr231 residue

The molecular mechanisms regulating N-methyl-D-aspartate (NMDA) receptor-dependent synaptic plasticity are complex, and the contribution of Tau protein in the physiological process is not fully understood. Herein, we investigated whether the blockade of NMDA receptor activation might change Tau phosphorylation during long-term potentiation (LTP) and long-term depression (LTD) via contribution of GSK3β as a major Tau kinase. For this, we recorded two components (synaptic and population spike components) of hippocampal field potential, which is evoked by the stimulation of the perforant pathway with high- and low-frequency stimulation (HFS and LFS). We found under a 20-µl volume of D-AP5 infusion lasting 1 h that,HFS caused significant synaptic depression, whereas LFS induced a synaptic potentiation. Both the HFS and LFS protocols resulted in a significant increase in population spike component but were characterized by a slow increase in amplitude that occurred with the LFS. D-AP5 attenuated HFS-induced population spike potentiation, but augmented LFS-induced population spike potentiation. The enzymatic activity of GSK-3β was decreased by D-AP5 infusion in the hippocampus, indicating that NMDA receptor activity modulates the enzymatic activity of GSK-3β. In addition, NMDA receptor blockade reduced tau expression and phosphorylation of tau at Ser416 residue, but not Thr231 residue. These findings confirm previous studies that D-AP5 applied to the DG in vivo blocks HFS-induced LTP, but we further also showed that the same dose of D-AP5 resulted in a slowly rising LFS-induced LTP and HFS-induced LTD. The formation of such an LTP, together with reduced enzymatic activity of GSK-3β and tau phosphorylation at Ser⁴¹⁶ epitope, can make it a candidate mechanism for prevention of taupathies.

GSK-3 and Tau: A Key Duet in Alzheimer's Disease

Glycogen synthase kinase-3 (GSK-3) is a ubiquitously expressed serine/threonine kinase with a plethora of substrates. As a modulator of several cellular processes, GSK-3 has a central position in cell metabolism and signaling, with important roles both in physiological and pathological conditions. GSK-3 has been associated with a number of human disorders, such as neurodegenerative diseases including Alzheimer’s disease (AD). GSK-3 contributes to the hyperphosphorylation of tau protein, the main component of neurofibrillary tangles (NFTs), one of the hallmarks of AD. GSK-3 is further involved in the regulation of different neuronal processes that are dysregulated during AD pathogenesis, such as the generation of amyloid-β (Aβ) peptide or Aβ-induced cell death, axonal transport, cholinergic function, and adult neurogenesis or synaptic function. In this review, we will summarize recent data about GSK-3 involvement in these processes contributing to AD pathology, mostly focusing on the crucial interplay between GSK-3 and tau protein. We further discuss the current development of potential AD therapies targeting GSK-3 or GSK-3-phosphorylated tau.

Leptin Contributes to Neuropathic Pain via Extrasynaptic NMDAR-nNOS Activation

Leptin is an adipocytokine that is primarily secreted by white adipose tissue, and it contributes to the pathogenesis of neuropathic pain in collaboration with N-methyl-D-aspartate receptors (NMDARs). Functional NMDARs are a heteromeric complex that primarily comprise two NR1 subunits and two NR2 subunits. NR2A is preferentially located at synaptic sites, and NR2B is enriched at extrasynaptic sites. The roles of synaptic and extrasynaptic NMDARs in the contribution of leptin to neuropathic pain are not clear. The present study examined whether the important role of leptin in neuropathic pain was related to synaptic or extrasynaptic NMDARs. We used a rat model of spared nerve injury (SNI) and demonstrated that the intrathecal administration of the NR2A-selective antagonist NVP-AAM077 and the NR2B-selective antagonist Ro25-6981 prevented and reversed mechanical allodynia following SNI. Administration of exogenous leptin mimicked SNI-induced behavioral allodynia, which was also prevented by NVP-AAM077 and Ro25-6981. Mechanistic studies showed that leptin enhanced NR2B- but not NR2A-mediated currents in spinal lamina II neurons of naïve rats. Leptin also upregulated the expression of NR2B, which was blocked by the NR2B-selective antagonist Ro25-6981, in cultured dorsal root ganglion (DRG) neurons. Leptin enhanced neuronal nitric oxide synthase (nNOS) expression, which was also blocked by Ro25-6981, in cultured DRG cells. However, leptin did not change NR2A expression, and the NR2A-selective antagonist NVP-AAM077 had no effect on leptin-enhanced nNOS expression. Our data suggest an important cellular link between the spinal effects of leptin and the extrasynaptic NMDAR-nNOS-mediated cellular mechanism of neuropathic pain.

Hyperhomocysteinemia is an emerging comorbidity in ischemic stroke

Hyperhomocysteinemia or systemic elevation of the amino acid homocysteine is a common metabolic disorder that is considered to be a risk factor for ischemic stroke. However, it is still unclear whether predisposition to hyperhomocysteinemia could contribute to the severity of stroke outcome. This review highlights the advantages and limitations of the current rodent models of hyperhomocysteinemia, describes the consequence of mild hyperhomocysteinemia on the severity of ischemic brain damage in preclinical studies and summarizes the mechanisms involved in homocysteine induced neurotoxicity. The findings provide the premise for establishing hyperhomocysteinemia as a comorbidity for ischemic stroke and should be taken into consideration while developing potential therapeutic agents for stroke treatment.

Leptin regulation of hippocampal synaptic function in health and disease

It is widely accepted that the metabolic hormone leptin regulates food intake and body weight via activation of hypothalamic leptin receptors. However, as leptin receptors are also highly expressed in other brain regions, such as the hippocampus, alterations in leptin responsiveness also impacts on key functions of the hippocampus, like learning and memory. Within the hippocampus, high levels of leptin receptors are expressed at excitatory synapses, and in accordance with a synaptic localization, leptin potently regulates synaptic transmission at both Schaffer collateral (SC) and temporoammonic (TA) inputs to CA1 pyramidal neurons. Increasing evidence from cellular and behavioral studies examining leptin action at CA1 synapses support the notion that leptin is a potential cognitive enhancer. However, the capacity of leptin to regulate synaptic efficacy at SC-CA1 and TA-CA1 synapses declines in an age-dependent manner. Moreover, clinical evidence that supports a link between circulating leptin levels and the risk of the age-related neurodegenerative disorder, Alzheimer's disease (AD) is accumulating. Consequently, it has been proposed that the leptin system is a potential therapeutic target in AD, and that boosting the hippocampal actions of leptin may be beneficial in the treatment of AD. Here we review recent progress in our understanding of the neuronal and hippocampal synaptic functions that are regulated by leptin and how alterations in the leptin system influence age-related CNS-related disorders like AD.

Melatonin reverts methamphetamine-induced learning and memory impairments and hippocampal alterations in mice

AIMS

Methamphetamine (METH) has become a major public health problem because of its abuse and profound neurotoxic effects, causing alterations in brain structure and function, and impairing cognitive functions, including attention, decision making, emotional memory, and working memory. This study aimed to determine whether melatonin (MEL), the circadian-control hormone, which has roles beyond circadian rhythm regulation, could restore METH-induced cognitive and neuronal impairment.

MAIN METHODS

Mice were treated with either METH (1 mg/kg) or saline for 7 days, followed by MEL (10 mg/kg) or saline for another 14 days. The Morris water maze (MWM) test was performed one day after the last saline or MEL injection. The hippocampal neuronal density, synaptic density, and receptors involved in learning and memory, along with downstream signaling molecules (NMDA receptor subunits GluN2A, GluN2B, and CaMKII) were investigated by immunoblotting.

KEY FINDINGS

METH administration significantly extended escape latency in learning phase and reduced the number of target crossings in memory test-phase as well as decreased the expression of BDNF, NMDA receptors, TrkB receptors, CaMKII, βIII tubulin, and synaptophysin. MEL treatment significantly ameliorated METH-induced increased escape latency, decreased the number of target crossings and decreased expression of BDNF, NMDA receptors, TrkB receptors, CaMKII, βIII tubulin and synaptophysin.

SIGNIFICANCE

METH administration impairs learning and memory in mice, and MEL administration restores METH-induced neuronal impairments which is probably through the changes in BDNF, NMDA receptors, TrkB receptors, CaMKII, βIII tubulin and synaptophysin. Therefore, MEL is potentially an innovative and promising treatment for learning and memory impairment of humans.