Abstract: It is increasingly accepted that early cognitive impairment in Alzheimer's disease results in considerable part from synaptic dysfunction caused by the accumulation of a range of oligomeric assemblies of amyloid β-protein (Aβ). Most studies have used synthetic Aβ peptides to explore the mechanisms of memory deficits in rodent models, but recent work suggests that Aβ assemblies isolated from human (AD) brain tissue are far more potent and disease-relevant. Although reductionist experiments show Aβ oligomers to impair synaptic plasticity and neuronal viability, the responsible mechanisms are only partly understood. Glutamatergic receptors, GABAergic receptors, nicotinic receptors, insulin receptors, the cellular prion protein, inflammatory mediators, and diverse signaling pathways have all been suggested. Studies using AD brain-derived soluble Aβ oligomers suggest that only certain bioactive forms (principally small, diffusible oligomers) can disrupt synaptic plasticity, including by binding to plasma membranes and changing excitatory-inhibitory balance, perturbing mGluR, PrP, and other neuronal surface proteins, down-regulating glutamate transporters, causing glutamate spillover, and activating extrasynaptic GluN2B-containing NMDA receptors. We synthesize these emerging data into a mechanistic hypothesis for synaptic failure in Alzheimer's disease that can be modified as new knowledge is added and specific therapeutics are developed.

Abstract: Excitotoxicity is a phenomenon that describes the toxic actions of excitatory neurotransmitters, primarily glutamate, where the exacerbated or prolonged activation of glutamate receptors starts a cascade of neurotoxicity that ultimately leads to the loss of neuronal function and cell death. In this process, the shift between normal physiological function and excitotoxicity is largely controlled by astrocytes since they can control the levels of glutamate on the synaptic cleft. This control is achieved through glutamate clearance from the synaptic cleft and its underlying recycling through the glutamate-glutamine cycle. The molecular mechanism that triggers excitotoxicity involves alterations in glutamate and calcium metabolism, dysfunction of glutamate transporters, and malfunction of glutamate receptors, particularly N-methyl-D-aspartic acid receptors (NMDAR). On the other hand, excitotoxicity can be regarded as a consequence of other cellular phenomena, such as mitochondrial dysfunction, physical neuronal damage, and oxidative stress. Regardless, it is known that the excessive activation of NMDAR results in the sustained influx of calcium into neurons and leads to several deleterious consequences, including mitochondrial dysfunction, reactive oxygen species (ROS) overproduction, impairment of calcium buffering, the release of pro-apoptotic factors, among others, that inevitably contribute to neuronal loss. A large body of evidence implicates NMDAR-mediated excitotoxicity as a central mechanism in the pathogenesis of many neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS), Alzheimer's disease (AD), and epilepsy. In this review article, we explore different causes and consequences of excitotoxicity, discuss the involvement of NMDAR-mediated excitotoxicity and its downstream effects on several neurodegenerative disorders, and identify possible strategies to study new aspects of these diseases that may lead to the discovery of new therapeutic approaches. With the understanding that excitotoxicity is a common denominator in neurodegenerative diseases and other disorders, a new perspective on therapy can be considered, where the targets are not specific symptoms, but the underlying cellular phenomena of the disease.

Abstract: It is widely accepted that glutamate-mediated neuronal hyperexcitation plays a causative role in eliciting seizures. Among glutamate receptors, the roles of N-methyl-D-aspartate (NMDA) and α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors in physiological and pathological conditions represent major clinical research targets. It is well known that agonists of NMDA or AMPA receptors can elicit seizures in animal or human subjects, while antagonists have been shown to inhibit seizures in animal models, suggesting a potential role for NMDA and AMPA receptor antagonists in anti-seizure drug development. Several such drugs have been evaluated in clinical studies; however, the majority, mainly NMDA-receptor antagonists, failed to demonstrate adequate efficacy and safety for therapeutic use, and only an AMPA-receptor antagonist, perampanel, has been approved for the treatment of some forms of epilepsy. These results suggest that a misunderstanding of the role of each glutamate receptor in the ictogenic process may underlie the failure of these drugs to demonstrate clinical efficacy and safety. Accumulating knowledge of both NMDA and AMPA receptors, including pathological gene mutations, roles in autoimmune epilepsy, and evidence from drug-discovery research and pharmacological studies, may provide valuable information enabling the roles of both receptors in ictogenesis to be reconsidered. This review aimed to integrate information from several studies in order to further elucidate the specific roles of NMDA and AMPA receptors in epilepsy.

Abstract: A subanesthetic dose of ketamine causes acute psychotomimetic symptoms and sustained antidepressant effects. In prefrontal cortex, the prevailing disinhibition hypothesis posits that N-methyl-d-aspartate receptor (NMDAR) antagonists such as ketamine act preferentially on GABAergic neurons. However, cortical interneurons are heterogeneous. In particular, somatostatin-expressing (SST) interneurons selectively inhibit dendrites and regulate synaptic inputs, yet their response to systemic NMDAR antagonism is unknown. Here, we report that ketamine acutely suppresses the activity of SST interneurons in the medial prefrontal cortex of the awake mouse. The deficient dendritic inhibition leads to greater synaptically evoked calcium transients in the apical dendritic spines of pyramidal neurons. By manipulating NMDAR signaling via GluN2B knockdown, we show that ketamine's actions on the dendritic inhibitory mechanism has ramifications for frontal cortex-dependent behaviors and cortico-cortical connectivity. Collectively, these results demonstrate dendritic disinhibition and elevated calcium levels in dendritic spines as important local-circuit alterations driven by the administration of subanesthetic ketamine.

Abstract: N-methyl-D-aspartate (NMDA) receptor antagonists such as phencyclidine (PCP), dizocilpine (MK-801) and ketamine have long been considered a model of schizophrenia, both in animals and humans However, ketamine has been recently approved for treatment-resistant depression, although with severe restrictions Interestingly, the dosage in both conditions is similar, and positive symptoms of schizophrenia appear before antidepressant effects emerge Here, we describe the temporal mechanisms implicated in schizophrenia-like and antidepressant-like effects of NMDA blockade in rats, and postulate that such effects may indicate that NMDA receptor antagonists induce similar mechanistic effects, and only the basal pre-drug state of the organism delimitates the overall outcome Hence, blockade of NMDA receptors in depressive-like status can lead to amelioration or remission of symptoms, whereas healthy individuals develop psychotic symptoms and schizophrenia patients show an exacerbation of these symptoms after the administration of NMDA receptor antagonists

Abstract: Background: An increasing number of studies have shown that the brain–gut–microbiota axis may significantly contribute to Alzheimer’s disease (AD) pathogenesis. Moreover, impaired memory and learning involve the dysfunction neurotransmission of glutamate, the agonist of the N-methyl-d-aspartate receptor and a major excitatory neurotransmitter in the brain. This systematic review aimed to summarize the current cutting-edge research on the gut microbiota and glutamate alterations associated with dementia. Methods: PubMed, the Cochrane Collaboration Central Register of Controlled Clinical Trials, and Cochrane Systematic Reviews were reviewed for all studies on glutamate and gut microbiota in dementia published up until Feb 2020. Results: Several pilot studies have reported alterations of gut microbiota and metabolites in AD patients and other forms of dementia. Gut microbiota including Bacteroides vulgatus and Campylobacter jejuni affect glutamate metabolism and decrease the glutamate metabolite 2-keto-glutaramic acid. Meanwhile, gut bacteria with glutamate racemase including Corynebacterium glutamicum, Brevibacterium lactofermentum, and Brevibacterium avium can convert l-glutamate to d-glutamate. N-methyl-d-aspartate glutamate receptor (NMDAR)-enhancing agents have been found to potentially improve cognition in AD or Parkinson’s disease patients. These findings suggest that d-glutamate (d-form glutamate) metabolized by the gut bacteria may influence the glutamate NMDAR and cognitive function in dementia patients. Conclusions: Gut microbiota and glutamate are potential novel interventions to be developed for dementia. Exploring comprehensive cognitive functions in animal and human trials with glutamate-related NMDAR enhancers are warranted to examine d-glutamate signaling efficacy in gut microbiota in patients with AD and other neurodegenerative dementias.

Abstract: Parkinson's disease (PD) is a common neurodegenerative disease, the pathological features of which include the presence of Lewy bodies and the neurodegeneration of dopaminergic neurons in the substantia nigra pars compacta. However, until recently, research on the pathogenesis and treatment of PD have progressed slowly. Glutamate and dopamine are both important central neurotransmitters in mammals. A lack of enzymatic decomposition of extracellular glutamate results in glutamate accumulating at synapses, which is mainly absorbed by excitatory amino acid transporters (EAATs). Glutamate exerts its physiological effects by binding to and activating ligand-gated ion channels [ionotropic glutamate receptors (iGluRs)] and a class of G-protein-coupled receptors [metabotropic glutamate receptors (mGluRs)]. Timely clearance of glutamate from the synaptic cleft is necessary because high levels of extracellular glutamate overactivate glutamate receptors, resulting in excitotoxic effects in the central nervous system. Additionally, increased concentrations of extracellular glutamate inhibit cystine uptake, leading to glutathione depletion and oxidative glutamate toxicity. Studies have shown that oxidative glutamate toxicity in neurons lacking functional N-methyl-D-aspartate (NMDA) receptors may represent a component of the cellular death pathway induced by excitotoxicity. The association between inflammation and excitotoxicity (i.e., immunoexcitotoxicity) has received increased attention in recent years. Glial activation induces neuroinflammation and can stimulate excessive release of glutamate, which can induce excitotoxicity and, additionally, further exacerbate neuroinflammation. Glutamate, as an important central neurotransmitter, is closely related to the occurrence and development of PD. In this review, we discuss recent progress on elucidating glutamate as a relevant neurotransmitter in PD. Additionally, we summarize the relationship and commonality among glutamate excitotoxicity, oxidative toxicity, and immunoexcitotoxicity in order to posit a holistic view and molecular mechanism of glutamate toxicity in PD.

Abstract: Alzheimer's disease (AD) is an age-related dementia and neurodegenerative disorder, characterized by Aβ and tau protein deposition impairing learning, memory and suppressing synaptic plasticity of neurons. Increasing evidence suggests that there is a link between the glucose and glutamate alterations with age that down-regulates glucose utilization reducing glutamate levels in AD patients. Deviations in brain energy metabolism reinforce the development of AD by hampering glutamate levels in the brain. Glutamate is a nonessential amino acid and the major excitatory neurotransmitter synthesized from glucose. Alterations in cerebral glucose and glutamate levels precede the deposition of Aβ plaques. In the brain, over 40% of neuronal synapses are glutamatergic and disturbances in glutamatergic function have been implicated in pathophysiology of AD. Nevertheless, targeting the glutamatergic system seems to be a promising strategy to develop novel, improved therapeutics for AD. Here, we review data supporting the involvement of the glutamatergic system in AD pathophysiology as well as the efficacy of glutamatergic agents in this neurodegenerative disorder. We also discuss exciting new prospects for the development of improved therapeutics for this devastating disorder.

Abstract: Experience-dependent learning and memory require multiple forms of plasticity at hippocampal and cortical synapses that are regulated by N-methyl-D-aspartate receptors (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type ionotropic glutamate receptors (NMDAR, AMPAR). These plasticity mechanisms include long-term potentiation (LTP) and depression (LTD), which are Hebbian input-specific mechanisms that rapidly increase or decrease AMPAR synaptic strength at specific inputs, and homeostatic plasticity that globally scales-up or -down AMPAR synaptic strength across many or even all inputs. Frequently, these changes in synaptic strength are also accompanied by a change in the subunit composition of AMPARs at the synapse due to the trafficking to and from the synapse of receptors lacking GluA2 subunits. These GluA2-lacking receptors are most often GluA1 homomeric receptors that exhibit higher single-channel conductance and are Ca2+-permeable (CP-AMPAR). This review article will focus on the role of protein phosphorylation in regulation of GluA1 CP-AMPAR recruitment and removal from hippocampal synapses during synaptic plasticity with an emphasis on the crucial role of local signaling by the cAMP-dependent protein kinase (PKA) and the Ca2+calmodulin-dependent protein phosphatase 2B/calcineurin (CaN) that is coordinated by the postsynaptic scaffold protein A-kinase anchoring protein 79/150 (AKAP79/150).

Abstract: N-Methyl-d-Aspartate Receptors (NMDARs) are ionotropic glutamate-gated receptors. NMDARs are tetramers composed by several homologous subunits of GluN1-, GluN2-, or GluN3-type, leading to the existence in the central nervous system of a high variety of receptor subtypes with different pharmacological and signaling properties. NMDAR subunit composition is strictly regulated during development and by activity-dependent synaptic plasticity. Given the differences between GluN2 regulatory subunits of NMDAR in several functions, here we will focus on the synaptic pool of NMDARs containing the GluN2A subunit, addressing its role in both physiology and pathological synaptic plasticity as well as the contribution in these events of different types of GluN2A-interacting proteins.

Abstract: As a major metabolite of kynurenine in the oxidative metabolism of tryptophan, kynurenic acid is of considerable biological and clinical importance as an endogenous antagonist of glutamate in the central nervous system. It is most active as an antagonist at receptors sensitive to N-methyl-D-aspartate (NMDA) which regulate neuronal excitability and plasticity, brain development and behaviour. It is also thought to play a causative role in hypo-glutamatergic conditions such as schizophrenia, and a protective role in several neurodegenerative disorders, notably Huntington's disease. An additional hypothesis, that kynurenic acid could block nicotinic receptors for acetylcholine in the central nervous system has been proposed as an alternative mechanism of action of kynurenate. However, the evidence for this alternative mechanism is highly controversial, partly because at least eight earlier studies concluded that kynurenic acid blocked NMDA receptors but not nicotinic receptors and five subsequent, independent studies designed to repeat the results have failed to do so. Many studies considered to support the alternative 'nicotinic' hypothesis have been based on the use of analogs of kynurenate such as 7-chloro-kynurenic acid, or putatively nicotinic modulators such as galantamine, but a detailed analysis of the pharmacology of these compounds suggests that the results have often been misinterpreted, especially since the pharmacology of galantamine itself has been disputed. This review examines the evidence in detail, with the conclusion that there is no confirmed, reliable evidence for an antagonist activity of kynurenic acid at nicotinic receptors. Therefore, since there is overwhelming evidence for kynurenate acting at ionotropic glutamate receptors, especially NMDAR glutamate and glycine sites, with some activity at GPR35 sites and Aryl Hydrocarbon Receptors, results with kynurenic acid should be interpreted only in terms of these confirmed sites of action.

Abstract: Synaptic plasticity is a fundamental property of neurons referring to the activity-dependent changes in the strength and efficacy of synaptic transmission at preexisting synapses. Such changes can last from milliseconds to hours, days, or even longer and are involved in learning and memory as well as in development and response of the brain to injuries. Several types of synaptic plasticity have been described across neuronal types, brain regions, and species, but all of them share in one way or another capital importance of Ca2+-mediated processes. In this chapter, we will focus on the Ca2+-dependent events necessary for the induction and expression of multiple forms of synaptic plasticity.

Abstract: NMDA receptors play crucial roles in excitatory synaptic transmission. Rare variants in GRIN2A encoding the GluN2A subunit are associated with a spectrum of disorders, ranging from mild speech and language delay to intractable neurodevelopmental disorders, including but not limited to developmental and epileptic encephalopathy. A de novo missense variant, p.Ser644Gly, was identified in a child with this disorder, and Grin2a knock-in mice were generated to model and extend understanding of this intractable childhood disease. Homozygous and heterozygous mutant mice exhibited altered hippocampal morphology at 2 weeks of age, and all homozygotes exhibited lethal tonic-clonic seizures by mid-third week. Heterozygous adults displayed susceptibility to induced generalized seizures, hyperactivity, repetitive and reduced anxiety behaviours, plus several unexpected features, including significant resistance to electrically-induced limbic seizures and to pentylenetetrazole induced tonic-clonic seizures. Multielectrode recordings of neuronal networks revealed hyperexcitability and altered bursting and synchronicity. In heterologous cells, mutant receptors had enhanced NMDA receptor agonist potency and slow deactivation following rapid removal of glutamate, as occurs at synapses. NMDA receptor-mediated synaptic currents in heterozygous hippocampal slices also showed a prolonged deactivation time course. Standard anti-epileptic drug monotherapy was ineffective in the patient. Introduction of NMDA receptor antagonists was correlated with a decrease in seizure burden. Chronic treatment of homozygous mouse pups with NMDA receptor antagonists significantly delayed the onset of lethal seizures but did not prevent them. These studies illustrate the power of using multiple experimental modalities to model and test therapies for severe neurodevelopmental disorders, while revealing significant biological complexities associated with GRIN2A developmental and epileptic encephalopathy.

Abstract: The levels of the astrocyte markers (GFAP, S100B) were increased unevenly in patients with schizophrenia. Reactive astrogliosis was found in approximately 70% of patients with schizophrenia. The astrocytes play a major role in etiology and pathogenesis of schizophrenia. Astrocytes produce the components that altered in schizophrenia extracellular matrix system which are involved in inflammation, functioning of interneurons, glio-, and neurotransmitter system, especially glutamate system. Astrocytes activate the interneurons through glutamate release and ATP. Decreased expression of astrocyte glutamate transporters was observed in patients with schizophrenia. Astrocytes influence on N-methyl-d-aspartate (NMDA) receptors via D-serine, an agonist of the glycine-binding site of NMDA receptors, and kynurenic acid, an endogenous antagonist. NMDA receptors, on its turn, control the impulses of dopamine neurons. Therefore following theories of schizophrenia are proposed. They are a) activation of astrocytes for neuroinflammation, b) glutamate and dopamine theory, as astrocyte products control the activity of NMDA receptors, which influence on the dopamine neurons.

Abstract: Excitatory signaling mediated by NMDARs has been shown to regulate mood disorders. However, current treatments targeting NMDAR subtypes have shown limited success in treating patients, highlighting a need for alternative therapeutic targets. Here, we identify a role for GluN2D-containing NMDARs in modulating emotional behaviors and neural activity in the bed nucleus of the stria terminalis (BNST). Using a GluN2D KO mouse line (GluN2D-/-), we assessed behavioral phenotypes across tasks modeling emotional behavior. We then used a combination of ex vivo electrophysiology and in vivo fiber photometry to assess changes in BNST plasticity, cell-specific physiology, and cellular activity profiles. GluN2D-/- male mice exhibit evidence of exacerbated negative emotional behavior, and a deficit in BNST synaptic potentiation. We also found that GluN2D is functionally expressed on corticotropin-releasing factor (CRF)-positive BNST cells implicated in driving negative emotional states, and recordings in mice of both sexes revealed increased excitatory and reduced inhibitory drive onto GluN2D-/- BNST-CRF cells ex vivo and increased activity in vivo Using a GluN2D conditional KO line (GluN2Dflx/flx) to selectively delete the subunit from the BNST, we find that BNST-GluN2Dflx/flx male mice exhibit increased depressive-like behaviors, as well as altered NMDAR function and increased excitatory drive onto BNST-CRF neurons. Together, this study supports a role for GluN2D-NMDARs in regulating emotional behavior through their influence on excitatory signaling in a region-specific manner, and suggests that these NMDARs may serve as a novel target for selectively modulating glutamate signaling in stress-responsive structures and cell populations.SIGNIFICANCE STATEMENT Excitatory signaling mediated through NMDARs plays an important role in shaping emotional behavior; however, the receptor subtypes/brain regions through which this occurs are poorly understood. Here, we demonstrate that loss of GluN2D-containing NMDARs produces an increase in anxiety- and depressive-like behaviors in mice, deficits in BNST synaptic potentiation, and increased activity in BNST-CRF neurons known to drive negative emotional behavior. Further, we determine that deleting GluN2D in the BNST leads to increased depressive-like behaviors and increased excitatory drive onto BNST-CRF cells. Collectively, these results demonstrate a role for GluN2D-NMDARs in regulating the activity of stress-responsive structures and neuronal populations in the adult brain, suggesting them as a potential target for treating negative emotional states in mood-related disorders.

Abstract: LAR-type receptor phosphotyrosine-phosphatases (LAR-RPTPs) are presynaptic adhesion molecules that interact trans-synaptically with multitudinous postsynaptic adhesion molecules, including SliTrks, SALMs, and TrkC. Via these interactions, LAR-RPTPs are thought to function as synaptogenic wiring molecules that promote neural circuit formation by mediating the establishment of synapses. To test the synaptogenic functions of LAR-RPTPs, we conditionally deleted the genes encoding all three LAR-RPTPs, singly or in combination, in mice before synapse formation. Strikingly, deletion of LAR-RPTPs had no effect on synaptic connectivity in cultured neurons or in vivo, but impaired NMDA-receptor-mediated responses. Deletion of LAR-RPTPs decreased NMDA-receptor-mediated responses by a trans-synaptic mechanism. In cultured neurons, deletion of all LAR-RPTPs led to a reduction in synaptic NMDA-receptor EPSCs, without changing the subunit composition or the protein levels of NMDA-receptors. In vivo, deletion of all LAR-RPTPs in the hippocampus at birth also did not alter synaptic connectivity as measured via AMPA-receptor-mediated synaptic responses at Schaffer-collateral synapses monitored in juvenile mice, but again decreased NMDA-receptor mediated synaptic transmission. Thus, LAR-RPTPs are not essential for synapse formation, but control synapse properties by regulating postsynaptic NMDA-receptors via a trans-synaptic mechanism that likely involves binding to one or multiple postsynaptic ligands.

Abstract: Mobile genetic elements, such as human endogenous retroviruses (HERVs), produce proteins that regulate brain cell functions and synaptic transmission and have been implicated in the etiology of neurological and neurodevelopmental psychiatric disorders. However, the mechanisms by which these proteins of retroviral origin alter brain cell communication remain poorly understood. Here, we combined single-molecule tracking, calcium imaging, and behavioral approaches to demonstrate that the envelope protein (Env) of HERV type W, which is normally silenced but expressed in patients with neuropsychiatric conditions, alters the N-methyl-d-aspartate receptor (NMDAR)-mediated synaptic organization and plasticity through glia- and cytokine-dependent changes. Env expression in the developing hippocampus was sufficient to induce behavioral impairments at the adult stage that were prevented by Env neutralization or tuning of NMDAR trafficking. Thus, we show that a HERV gene product alters glutamate synapse maturation and generates behavioral deficits, further supporting the possible etiological interplay between genetic, immune, and synaptic factors in psychosis.

Abstract: Molecular imaging with positron emission tomography (PET) and single photon emission computed tomography (SPECT) is a well-established and important in vivo technique to evaluate fundamental biological processes and unravel the role of neurotransmitter receptors in various neuropsychiatric disorders. Specific ligands are available for PET/SPECT studies of dopamine, serotonin, and opiate receptors, but corresponding development of radiotracers for receptors of glutamate, the main excitatory neurotransmitter in mammalian brain, has lagged behind. This state of affairs has persisted despite the central importance of glutamate neurotransmission in brain physiology and in disorders such as stroke, epilepsy, schizophrenia, and neurodegenerative diseases. Recent years have seen extensive efforts to develop useful ligands for molecular imaging of subtypes of the ionotropic (N-methyl-D-aspartate (NMDA), kainate, and AMPA/quisqualate receptors) and metabotropic glutamate receptors (types I, II, and III mGluRs). We now review the state of development of radioligands for glutamate receptor imaging, placing main emphasis on the suitability of available ligands for reliable in vivo applications. We give a brief account of the radiosynthetic approach for selected molecules. In general, with the exception of ligands for the GluN2B subunit of NMDA receptors, there has been little success in developing radiotracers for imaging ionotropic glutamate receptors; failure of ligands for the PCP/MK801 binding site in vivo doubtless relates their dependence on the open, unblocked state of the ion channel. Many AMPA and kainite receptor ligands with good binding properties in vitro have failed to give measurable specific binding in the living brain. This may reflect the challenge of developing brain-penetrating ligands for amino acid receptors, compounded by conformational differences in vivo. The situation is better with respect to mGluR imaging, particularly for the mGluR5 subtype. Several successful PET ligands serve for investigations of mGluRs in conditions such as schizophrenia, depression, substance abuse and aging. Considering the centrality and diversity of glutamatergic signaling in brain function, we have relatively few selective and sensitive tools for molecular imaging of ionotropic and metabotropic glutamate receptors. Further radiopharmaceutical research targeting specific subtypes and subunits of the glutamate receptors may yet open up new investigational vistas with broad applications in basic and clinical research.

Abstract: OBJECTIVE The aim was to demonstrate that antibodies from patients with anti-N-methyl-d-aspartate receptor (NMDAR) encephalitis alter the levels of dopamine 1 receptor (D1R) and dopamine 2 receptor (D2R) and cause psychotic-like features in mice METHODS Cultured rat hippocampal neurons were treated with cerebrospinal fluid (CSF) from patients with anti-NMDAR encephalitis or controls, and the effects on clusters of D1R and D2R were quantified In vivo studies included 71 C57BL/6J mice that were chronically infused with CSF from patients or controls through ventricular catheters connected to subcutaneous osmotic pumps Prepulse inhibition of the acoustic startling reflex (PPI; a marker of psychotic-like behavior), memory, locomotor activity, and the density of cell-surface and synaptic D1R, D2R, and NMDAR clusters were examined at different time points using reported techniques RESULTS In cultured neurons, CSF from patients, but not from controls, caused a significant decrease of cell-surface D1R and an increase of D2R clusters In mice, CSF from patients caused a significant decrease of synaptic and total cell-surface D1R clusters and an increase of D2R clusters associated with a decrease of PPI These effects were accompanied by memory impairment and a reduction of surface NMDARs, as reported in this model The psychotic-like features, memory impairment, and changes in levels of D1R, D2R, and NMDAR progressively improved several days after the infusion of CSF from patients stopped INTERPRETATION In addition to memory deficits and reduction of NMDARs, CSF antibodies from patients with anti-NMDAR encephalitis cause reversible psychotic-like features accompanied by changes (D1R decrease, D2R increase) in cell-surface dopamine receptor clusters ANN NEUROL 2020 ANN NEUROL 2020;88:603-613

Abstract: Although numerous studies have indicated that chronic stress causes cognitive dysfunction with the impairment of synaptic structures and functions, the relationship between cognitive deficits induced by repeated restraint stress and the level of NMDA receptors in the subregion of the hippocampus has been relatively unknown until now. In this study, 3-week-old male Sprague-Dawley rats were exposed to repeated restraint stress for seven consecutive days, their cognitive functions were evaluated through behavioral tests, and then they were sacrificed for electrophysiological, morphological, and biochemical assays. Chronic repeated restraint stress led to cognitive and electrophysiological impairments, with a reduced density of dendritic spines. We also found that the protein level of NMDA receptors only increased in the hippocampal CA3 region. Nevertheless, repeated restraint stress-induced cognitive and synaptic dysfunction were effectively reversed by Ro25-6981, an inhibitor of the GluN2B receptor. These findings suggest that repeated restraint stress-induced synaptic and cognitive deficits are probably mediated through NMDA receptors.

Abstract: Alzheimer’s disease (AD) is the leading cause of dementia. Non-competitive N-Methyl-D-aspartate (NMDA) receptor antagonist memantine improved cognition and molecular alterations after preclinical treatment. Nevertheless, clinical results are discouraging. In vivo efficacy of the RL-208, a new NMDA receptor blocker described recently, with favourable pharmacokinetic properties was evaluated in Senescence accelerated mice prone 8 (SAMP8), a mice model of late-onset AD (LOAD). Oral administration of RL-208 improved cognitive performance assessed by using the three chamber test (TCT), novel object recognition test (NORT), and object location test (OLT). Consistent with behavioural results, RL-208 treated-mice groups significantly changed NMDAR2B phosphorylation state levels but not NMDAR2A. Calpain-1 and Caspase-3 activity was reduced, whereas B-cell lymphoma-2 (BCL-2) levels increased, indicating reduced apoptosis in RL-208 treated SAMP8. Superoxide Dismutase 1 (SOD1) and Glutathione Peroxidase 1 (GPX1), as well as a reduction of hydrogen peroxide (H2O2), was also determined in RL-208 mice. RL-208 treatment induced an increase in mature brain-derived neurotrophic factor (mBDNF), prevented Tropomyosin-related kinase B full-length (TrkB-FL) cleavage, increased protein levels of Synaptophysin (SYN) and Postsynaptic density protein 95 (PSD95). In whole, these results point out to an improvement in synaptic plasticity. Remarkably, RL-208 also decreased the protein levels of Cyclin-Dependent Kinase 5 (CDK5), as well as p25/p35 ratio, indicating a reduction in kinase activity of CDK5/p25 complex. Consequently, lower levels of hyperphosphorylated Tau (p-Tau) were found. In sum, these results demonstrate the neuroprotectant role of RL-208 through NMDAR blockade.

Abstract: Sleep apnea causes cognitive deficits and is associated with several neurologic diseases. Intermittent hypoxia (IH) is recognized as a principal mediator of pathophysiology associated with sleep apnea, yet the basis by which IH contributes to impaired cognition remains poorly defined. Using a mouse model exposed to IH, this study examines how the transcription factor, hypoxia inducible factor 1a (HIF1a), contributes to disrupted synaptic physiology and spatial memory. In wild-type mice, impaired performance in the Barnes maze caused by IH coincided with a loss of NMDA receptor (NMDAr)-dependent long-term potentiation (LTP) in area CA1 and increased nuclear HIF1a within the hippocampus. IH-dependent HIF1a signaling caused a two-fold increase in expression of the reactive oxygen species (ROS) generating enzyme NADPH oxidase 4 (NOX4). These changes promoted a pro-oxidant state and the downregulation of GluN1 within the hippocampus. The IH-dependent effects were not present in either mice heterozygous for Hif1a (HIF1a+/-) or wild-type mice treated with the antioxidant manganese (III) tetrakis(1-methyl-4-pyridyl) porphyrin (MnTMPyP). Our findings indicate that HIF1a-dependent changes in redox state are central to the mechanism by which IH disrupts hippocampal synaptic plasticity and impairs spatial memory. This mechanism may enhance the vulnerability for cognitive deficit and lower the threshold for neurologic diseases associated untreated sleep apnea.

Abstract: The impact of pannexin-1 (Panx1) channels on synaptic transmission is poorly understood. Here, we show that selective block of Panx1 in single postsynaptic hippocampal CA1 neurons from male rat or mouse brain slices causes intermittent, seconds long increases in the frequency of sEPSC following Schaffer collateral stimulation. The increase in sEPSC frequency occurred without an effect on evoked neurotransmission. Consistent with a presynaptic origin of the augmented glutamate release, the increased sEPSC frequency was prevented by bath-applied EGTA-AM or TTX. Manipulation of a previously described metabotropic NMDAR pathway (i.e., by preventing ligand binding to NMDARs with competitive antagonists or blocking downstream Src kinase) also increased sEPSC frequency similar to that seen when Panx1 was blocked. This facilitated glutamate release was absent in transient receptor potential vanilloid 1 (TRPV1) KO mice and prevented by the TRPV1 antagonist, capsazepine, suggesting it required presynaptic TRPV1. We show presynaptic expression of TRPV1 by immunoelectron microscopy and link TRPV1 to Panx1 because Panx1 block increases tissue levels of the endovanilloid, anandamide. Together, these findings demonstrate an unexpected role for metabotropic NMDARs and postsynaptic Panx1 in suppression of facilitated glutamate neurotransmission.SIGNIFICANCE STATEMENT The postsynaptic ion and metabolite channel, pannexin-1, is regulated by metabotropic NMDAR signaling through Src kinase. This pathway suppresses facilitated release of presynaptic glutamate during synaptic activity by regulating tissue levels of the transient receptor potential vanilloid 1 agonist anandamide.

Abstract: The NMDA receptor (NMDAR) is inhibited by synaptically released zinc This inhibition is thought to be the result of zinc diffusion across the synaptic cleft and subsequent binding to the extracellular domain of the NMDAR However, this model fails to incorporate the observed association of the highly zinc-sensitive NMDAR subunit GluN2A with the postsynaptic zinc transporter ZnT1, which moves intracellular zinc to the extracellular space Here, we report that disruption of ZnT1-GluN2A association by a cell-permeant peptide strongly reduced NMDAR inhibition by synaptic zinc in mouse dorsal cochlear nucleus synapses Moreover, synaptic zinc inhibition of NMDARs required postsynaptic intracellular zinc, suggesting that cytoplasmic zinc is transported by ZnT1 to the extracellular space in close proximity to the NMDAR These results challenge a decades-old dogma on how zinc inhibits synaptic NMDARs and demonstrate that presynaptic release and a postsynaptic transporter organize zinc into distinct microdomains to modulate NMDAR neurotransmission