Influence of GluN2 subunit identity on NMDA receptor function

GluN2A- and GluN2B-containing pre-synaptic N-methyl-d-aspartate receptors differentially regulate action potential-evoked Ca2+ influx via modulation of SK channels

N-methyl-d-aspartate receptors (NMDARs) play a pivotal role in synaptic plasticity. While the functional role of post-synaptic NMDARs is well established, pre-synaptic NMDAR (pre-NMDAR) function is largely unexplored. Different pre-NMDAR subunit populations are documented at synapses, suggesting that subunit composition influences neuronal transmission. Here, we used electrophysiological recordings at Schaffer collateral-CA1 synapses partnered with Ca2+ imaging and glutamate uncaging at boutons of CA3 pyramidal neurones to reveal two populations of pre-NMDARs that contain either the GluN2A or GluN2B subunit. Activation of the GluN2B population decreases action potential-evoked Ca2+ influx via modulation of small-conductance Ca2+-activated K+ channels, while activation of the GluN2A population does the opposite. Critically, the level of functional expression of the subunits is subject to homeostatic regulation, bidirectionally affecting short-term facilitation, thus providing a capacity for a fine adjustment of information transfer. This article is part of a discussion meeting issue 'Long-term potentiation: 50 years on'.

Quinolone bioisosteres of phenolic GluN2B-selective NMDA receptor antagonists

Cyclopenta[g]quinolones of type 4 were designed with the aim to bioisosterically replace the phenol of potent GluN2B ligands such as ifenprodil and Ro 25-6981 by the quinolone system and to restrict the conformational flexibility of the aminopropanol substructure in a cyclopentane system. The designed ligands were synthesized in an eight-step sequence starting with terephthalaldehyde (5). Key steps pf the synthesis were the intramolecular Friedel-Crafts acylation of propionic acids 10 to yield the cyclopenta[g]quinolinediones 11 and the Mannich reaction of diketone 11a followed by conjugate addition at the α,β-unsaturated ketone 12a. Although the quinolones 13a, 15a, and 16a contain an H-bond donor group (secondary lactam) as ifenprodil and Ro 25-6981, they show only moderate GluN2B affinity (Ki > 410 nM). However, the introduction of lipophilic substituents at the quinolone N-atom resulted in more than 10-fold increased GluN2B affinity of the benzyl and benzyloxymethyl derivatives cis-13c (Ko = 36 nM) and 13e (Ko = 27 nM). All compounds are selective over the phencyclidine (PCP) binding site of the N-methyl-D-aspartate (NMDA) receptor. The benzyl derivative 13c showed six- and threefold selectivity over σ1 and σ2 receptors, respectively.

NMDA Receptor Antagonists: Emerging Insights into Molecular Mechanisms and Clinical Applications in Neurological Disorders

Neurodegenerative disorders (NDs) include a range of chronic conditions characterized by progressive neuronal loss, leading to cognitive, motor, and behavioral impairments. Common examples include Alzheimer's disease (AD) and Parkinson's disease (PD). The global prevalence of NDs is on the rise, imposing significant economic and social burdens. Despite extensive research, the mechanisms underlying NDs remain incompletely understood, hampering the development of effective treatments. Excitotoxicity, particularly glutamate-mediated excitotoxicity, is a key pathological process implicated in NDs. Targeting the N-methyl-D-aspartate (NMDA) receptor, which plays a central role in excitotoxicity, holds therapeutic promise. However, challenges, such as blood-brain barrier penetration and adverse effects, such as extrapyramidal effects, have hindered the success of many NMDA receptor antagonists in clinical trials. This review explores the molecular mechanisms of NMDA receptor antagonists, emphasizing their structure, function, types, challenges, and future prospects in treating NDs. Despite extensive research on competitive and noncompetitive NMDA receptor antagonists, the quest for effective treatments still faces significant hurdles. This is partly because the same NMDA receptor that necessitates blockage under pathological conditions is also responsible for the normal physiological function of NMDA receptors. Allosteric modulation of NMDA receptors presents a potential alternative, with the GluN2B subunit emerging as a particularly attractive target due to its enrichment in presynaptic and extrasynaptic NMDA receptors, which are major contributors to excitotoxic-induced neuronal cell death. Despite their low side-effect profiles, selective GluN2B antagonists like ifenprodil and radiprodil have encountered obstacles such as poor bioavailability in clinical trials. Moreover, the selectivity of these antagonists is often relative, as they have been shown to bind to other GluN2 subunits, albeit minimally. Recent advancements in developing phenanthroic and naphthoic acid derivatives offer promise for enhanced GluN2B, GluN2A or GluN2C/GluN2D selectivity and improved pharmacodynamic properties. Additional challenges in NMDA receptor antagonist development include conflicting preclinical and clinical results, as well as the complexity of neurodegenerative disorders and poorly defined NMDA receptor subtypes. Although multifunctional agents targeting multiple degenerative processes are also being explored, clinical data are limited. Designing and developing selective GluN2B antagonists/modulators with polycyclic moieties and multitarget properties would be significant in addressing neurodegenerative disorders. However, advancements in understanding NMDA receptor structure and function, coupled with collaborative efforts in drug design, are imperative for realizing the therapeutic potential of these NMDA receptor antagonists/modulators.

Early expression of GluN2A-containing NMDA receptors in a model of fragile X syndrome

NMDA-type glutamate receptors (NMDARs) play a crucial role in synaptogenesis, circuit development, and synaptic plasticity, serving as fundamental components in cellular models of learning and memory. Their dysregulation has been implicated in several neurological disorders and synaptopathies. NMDARs are heterotetrameric complexes composed of two GluN1 and two GluN2 subunits. The composition of GluN2 subunits determines the main biophysical properties of the channel, such as calcium permeability and gating kinetics, and influences the ability of the receptor to interact with postsynaptic proteins involved in normal synaptic physiology and plasticity, including scaffolding proteins and signaling molecules. During early development, NMDARs in the forebrain contain solely the GluN2B subunit, a necessary subunit for proper synaptogenesis and synaptic plasticity. As the animal matures, the expression of the GluN2A subunit increases, leading to a partial replacement of GluN2B-containing synaptic NMDARs with GluN2A-containing receptors. The switch in the synaptic GluN2A-to-GluN2B ratio has a significant impact on the kinetics of excitatory postsynaptic currents and diminishes the synaptic plasticity capacity. In this study, we present findings indicating that GluN2A expression occurs earlier in a mouse model of fragile X syndrome (FXS). This altered timing of GluN2A expression affects various important parameters of NMDAR-mediated excitatory postsynaptic currents, including maximal current amplitude, decay time, and response to consecutive stimuli delivered in close temporal proximity. These observations suggest that the early expression of GluN2A during a critical period when synapses and circuits are developing could be an underlying factor contributing to the formation of pathological circuits in the FXS mouse model.NEW & NOTEWORTHY NMDA receptors (NMDARs) play important roles in synaptic transmission and are involved in multiple neurological disorders. During development, GluN2A in the forebrain becomes incorporated into previously GluN2B-dominated NMDARs, leading to the "GluN2A/GluN2B ratio switch." This is a crucial step for normal brain development. Here we present findings indicating that GluN2A expression occurs earlier in the fragile X mouse and this could be an underlying factor contributing to the pathology found in the fragile X model.

Distinct GABAergic modulation of timing-dependent LTP in CA1 pyramidal neurons along the longitudinal axis of the mouse hippocampus

Synaptic plasticity in the hippocampus underlies episodic memory formation, with dorsal hippocampus being instrumental for spatial memory whereas ventral hippocampus is crucial for emotional learning. Here, we studied how GABAergic inhibition regulates physiologically relevant low repeat spike timing-dependent LTP (t-LTP) at Schaffer collateral-CA1 synapses along the dorsoventral hippocampal axis. We used two t-LTP protocols relying on only 6 repeats of paired spike-firing in pre- and postsynaptic cells within 10 s that differ in postsynaptic firing patterns. GABAA receptor mechanisms played a greater role in blocking 6× 1:1 t-LTP that recruits single postsynaptic action potentials. 6× 1:4 t-LTP that depends on postsynaptic burst-firing unexpectedly required intact GABAB receptor signaling. The magnitude of both t-LTP-forms decreased along the dorsoventral axis, despite increasing excitability and basal synaptic strength in this direction. This suggests that GABAergic inhibition contributes to the distinct roles of dorsal and ventral hippocampus in memory formation.

Loss of the sustained antidepressant-like effect of (2R,6R)-hydroxynorketamine in NMDA receptor GluN2D subunit knockout mice

Ketamine, an N-methyl-d-aspartate (NMDA) receptor antagonist, has attracted attention for its acute and sustained antidepressant effects in patients with depression. Hydroxynorketamine (HNK), a metabolite of ketamine, exerts antidepressant effects without exerting ketamine's side effects and has attracted much attention in recent years. However, the detailed pharmacological mechanism of action of HNK remains unclear. We previously showed that the GluN2D NMDA receptor subunit is important for sustained antidepressant-like effects of (R)-ketamine. Therefore, we investigated whether the GluN2D subunit is involved in antidepressant-like effects of (2R,6R)-HNK and (2S,6S)-HNK. Treatment with (2R,6R)-HNK but not (2S,6S)-HNK exerted acute and sustained antidepressant-like effects in the tail-suspension test in wildtype mice. Interestingly, sustained antidepressant-like effects of (2R,6R)-HNK were abolished in GluN2D-knockout mice, whereas acute antidepressant-like effects were maintained in GluN2D-knockout mice. When expression levels of GluN2A and GluN2B subunits were evaluated, a decrease in GluN2B protein expression in the nucleus accumbens was found in stressed wildtype mice but not in stressed GluN2D-knockout mice. These results suggest that the GluN2D subunit and possibly the GluN2B subunit are involved in the sustained antidepressant-like effect of (2R,6R)-HNK.

Pathological Interplay between Inflammation and Mitochondria Aggravates Glutamate Toxicity

Mitochondrial dysfunction and glutamate toxicity are associated with neural disorders, including brain trauma. A review of the literature suggests that toxic and transmission actions of neuronal glutamate are spatially and functionally separated. The transmission pathway utilizes synaptic GluN2A receptors, rapidly released pool of glutamate, evoked release of glutamate mediated by Synaptotagmin 1 and the amount of extracellular glutamate regulated by astrocytes. The toxic pathway utilizes extrasynaptic GluN2B receptors and a cytoplasmic pool of glutamate, which results from the spontaneous release of glutamate mediated by Synaptotagmin 7 and the neuronal 2-oxoglutarate dehydrogenase complex (OGDHC), a tricarboxylic acid (TCA) cycle enzyme. Additionally, the inhibition of OGDHC observed upon neuro-inflammation is due to an excessive release of reactive oxygen/nitrogen species by immune cells. The loss of OGDHC inhibits uptake of glutamate by mitochondria, thus facilitating its extracellular accumulation and stimulating toxic glutamate pathway without affecting transmission. High levels of extracellular glutamate lead to dysregulation of intracellular redox homeostasis and cause ferroptosis, excitotoxicity, and mitochondrial dysfunction. The latter affects the transmission pathway demanding high-energy supply and leading to cell death. Mitochondria aggravate glutamate toxicity due to impairments in the TCA cycle and become a victim of glutamate toxicity, which disrupts oxidative phosphorylation. Thus, therapies targeting the TCA cycle in neurological disorders may be more efficient than attempting to preserve mitochondrial oxidative phosphorylation.

Single-locus and Haplotype Associations of GRIN2B Gene with Autism Spectrum Disorders and the Demographic and Clinical Characteristics of Patients in Guilan, Iran

Autism spectrum disorders (ASDs) are described as generalized developmental disorders, with an average age of onset of 36 months. Genetic and environmental factors may contribute to this multifactorial disorder. The present study aimed to investigate the association of three GRIN2B polymorphisms, including rs1019385, rs1024893, and rs3764028, with ASDs. Based on the results, there was a significant difference regarding the genotype frequency of rs3764028 polymorphism between the control and case (ASD) groups (P = 0.027). According to the recessive model, this variant was associated with ASDs (P = 0.23). None of the eight haplotype models with frequencies above 0.5 showed significant differences between the case and control groups in terms of allelic frequency. The present results showed that the rs376028 variant was directly related to the phenotypic symptoms of ASDs.

Esketamine induces apoptosis of nasopharyngeal carcinoma cells through the PERK/CHOP pathway

Nasopharyngeal carcinoma, a malignant tumor prevalent in southeast Asia and north Africa, still lacks effective treatment. Esketamine, an N-methyl-D-aspartatic acid (NMDA) receptor (NMDAR) antagonist, is widely used in clinical anesthesia. Emerging evidence suggests that esketamine plays an important role in inhibiting tumor cell activity. However, the underlying mechanisms of esketamine on nasopharyngeal carcinoma remain unknown. In this study, we found that esketamine inhibited the proliferation and migration of nasopharyngeal carcinoma cells. Mechanically, transcriptome sequencing and subsequent verification experiments revealed that esketamine promoted the apoptosis of nasopharyngeal carcinoma cells through endoplasmic reticulum stress PERK/ATF4/CHOP signaling pathway mediated by NMDAR. Additionally, when combined with esketamine, the inhibitory effect of cisplatin on the proliferation of nasopharyngeal carcinoma cells was significantly enhanced. These findings provide new insights into future anti-nasopharyngeal carcinoma clinical strategies via targeting the NMDAR/PERK/CHOP axis alone or in combination with cisplatin.

The neuroprotective potential of phytochemicals in traumatic brain injury: mechanistic insights and pharmacological implications

Traumatic brain injury (TBI) leads to brain damage, comprising both immediate primary damage and a subsequent cascade of secondary injury mechanisms. The primary injury results in localized brain damage, while the secondary damage initiates inflammatory responses, followed by the disruption of the blood-brain barrier, infiltration of peripheral blood cells, brain edema, and the release of various immune mediators, including chemotactic factors and interleukins. TBI disrupts molecular signaling, cell structures, and functions. In addition to physical tissue damage, such as axonal injuries, contusions, and haemorrhages, TBI interferes with brain functioning, impacting cognition, decision-making, memory, attention, and speech capabilities. Despite a deep understanding of the pathophysiology of TBI, an intensive effort to evaluate the underlying mechanisms with effective therapeutic interventions is imperative to manage the repercussions of TBI. Studies have commenced to explore the potential of employing natural compounds as therapeutic interventions for TBI. These compounds are characterized by their low toxicity and limited interactions with conventional drugs. Moreover, many natural compounds demonstrate the capacity to target various aspects of the secondary injury process. While our understanding of the pathophysiology of TBI, there is an urgent need for effective therapeutic interventions to mitigate its consequences. Here, we aimed to summarize the mechanism of action and the role of phytochemicals against TBI progression. This review discusses the therapeutic implications of various phytonutrients and addresses primary and secondary consequences of TBI. In addition, we highlighted the roles of emerging phytochemicals as promising candidates for therapeutic intervention of TBI. The review highlights the neuroprotective roles of phytochemicals against TBI and the mechanistic approach. Furthermore, our efforts focused on the underlying mechanisms, providing a better understanding of the therapeutic potential of phytochemicals in TBI therapeutics.

Boltzmann's Theorem Revisited: Inaccurate Time-to-Action Clocks in Affective Disorders

Timely goal-oriented behavior is essential for survival and is shaped by experience. In this paper, a multileveled approach was employed, ranging from the polymorphic level through thermodynamic molecular, cellular, intracellular, extracellular, non-neuronal organelles and electrophysiological waves, attesting for signal variability. By adopting Boltzmann's theorem as a thermodynamic conceptualization of brain work, we found deviations from excitation-inhibition balance and wave decoupling, leading to wider signal variability in affective disorders compared to healthy individuals. Recent evidence shows that the overriding on-off design of clock genes paces the accuracy of the multilevel parallel sequencing clocks and that the accuracy of the time-to-action is more crucial for healthy behavioral reactions than their rapidity or delays. In affective disorders, the multilevel clocks run free and lack accuracy of responsivity to environmentally triggered time-to-action as the clock genes are not able to rescue mitochondria organelles from oxidative stress to produce environmentally-triggered energy that is required for the accurate time-to-action and maintenance of the thermodynamic equilibrium. This maintenance, in turn, is dependent on clock gene transcription of electron transporters, leading to higher signal variability and less signal accuracy in affective disorders. From a Boltzmannian thermodynamic and energy-production perspective, the option of reversibility to a healthier time-toaction, reducing entropy is implied. We employed logic gates to show deviations from healthy levelwise communication and the reversed conditions through compensations implying the role of nonneural cells and the extracellular matrix in return to excitation-inhibition balance and accuracy in the time-to-action signaling.

Characterizing Functional Contributions of Specific GluN2 Subunits to Individual Postsynaptic NMDAR Responses Using Biophysical Parameters

The NMDAR is a heterotetramer composed of two GluN1 subunits and two GluN2 and/or GluN3 subunits, with the GluN2 subunits exhibiting significant diversity in their structure and function. Recent studies have highlighted the importance of characterizing the specific roles of each GluN2 subunit across central nervous system regions and developmental stages, as well as their unique contributions to NMDAR-mediated signaling and plasticity. Understanding the distinct functions of GluN2 subunits is critical for the development of targeted therapeutic strategies for NMDAR-related disorders. However, measuring the functional contribution of individual GluN2 subtypes in ex vivo slices is challenging. Conventionally, pharmacological or genetic approaches are used, but, in many cases, this is not possible or is restricted to population-level NMDAR responses. Here, we describe a technique for using biophysical properties of miniature synaptic NMDAR responses as a proxy to measure the functional contribution of specific GluN2-NMDAR subunits to individual synapses within a neuron.

Understanding the role of the NMDA receptor subunit, GluN2D, in mediating NMDA receptor antagonist-induced behavioral disruptions in male and female mice

Noncompetitive NMDA receptor (NMDAR) antagonists like phencyclidine (PCP) and ketamine cause psychosis-like symptoms in healthy humans, exacerbate schizophrenia symptoms in people with the disorder, and disrupt a range of schizophrenia-relevant behaviors in rodents, including hyperlocomotion. This is negated in mice lacking the GluN2D subunit of the NMDAR, suggesting the GluN2D subunit mediates the hyperlocomotor effects of these drugs. However, the role of GluN2D in mediating other schizophrenia-relevant NMDAR antagonist-induced behavioral disturbances, and in both sexes, is unclear. This study aimed to investigate the role of the GluN2D subunit in mediating schizophrenia-relevant behaviors induced by a range of NMDA receptor antagonists. Using both male and female GluN2D knockout (KO) mice, we examined the effects of the NMDAR antagonist's PCP, the S-ketamine enantiomer (S-ket), and the ketamine metabolite R-norketamine (R-norket) on locomotor activity, anxiety-related behavior, and recognition and short-term spatial memory. GluN2D-KO mice showed a blunted locomotor response to R-norket, S-ket, and PCP, a phenotype present in both sexes. GluN2D-KO mice of both sexes showed an anxious phenotype and S-ket, R-norket, and PCP showed anxiolytic effects that were dependent on sex and genotype. S-ket disrupted spatial recognition memory in females and novel object recognition memory in both sexes, independent of genotype. This datum identifies a role for the GluN2D subunit in sex-specific effects of NMDAR antagonists and on the differential effects of the R- and S-ket enantiomers.

Salicylate- and Noise-induced Tinnitus. Different Mechanisms Producing the same Result? An Experimental Model

PURPOSE

Tinnitus, the generation of phantom sounds, can be the result of noise exposure, however, understanding of its underlying mechanisms is limited. Purpose of the study was is to determine whether different concentrations of salicylate can cause tinnitus of different intensity.

METHODS

For the purposes of this study 50 male Wistar rats were used. The animals were divided into 5 groups (10 rats in each group). The animals that did not receive any substance were allocated to the control group (Group A). The second group (Group B) of rats received salicylate (Sigma Aldrich) intraperitoneally for 7 days (300 mg/Kg/day). The 3rd group (Group C) received salicylate intraperitoneally for 7 days, but at twice the concentration of the animals in the second group (600 mg/kg/d). The 4th group (Group D) simultaneously received salicylate (300 mg/Kg/day) and pure Memantine (Sigma Aldrich, 10 mg/kg/d) intraperitoneally for 7 days. The 5th group (Group E) did not receive any substance but was exposed for 168 consecutive hours (7 days) to sound to induce tinnitus. Cochlear activity was evaluated with the use of Distortion Product Otoacoustic Emissions (DPOAEs). At the end of the experimental period, the animals were sacrificed, and the right cochlea was removed and prepared for further histological and immunohistochemical studies.

RESULTS

The DPOAEs of animals treated either with salicylate as monotherapy or salicylate combined with memantine were indistinguishable from the noise floor, did not differ significantly compared to the animals of the control group or those expose to constant noise. The cochlear structures of Group E remained anatomically and functionally unaffected from the exposure to constant noise. Memantine does not seem to offer substantial protection to the cochlear structures, according to histological examination and hearing tests, however, the rats receiving it exhibited better results in behavioral tests.

CONCLUSIONS

The administration of memantine does not contribute significantly to the reduction of tinnitus.

GluN2A and GluN2B N-Methyl-D-Aspartate Receptor (NMDARs) Subunits: Their Roles and Therapeutic Antagonists in Neurological Diseases

N-methyl-D-aspartate receptors (NMDARs) are ion channels that respond to the neurotransmitter glutamate, playing a crucial role in the permeability of calcium ions and excitatory neurotransmission in the central nervous system (CNS). Composed of various subunits, NMDARs are predominantly formed by two obligatory GluN1 subunits (with eight splice variants) along with regulatory subunits GluN2 (GluN2A-2D) and GluN3 (GluN3A-B). They are widely distributed throughout the CNS and are involved in essential functions such as synaptic transmission, learning, memory, plasticity, and excitotoxicity. The presence of GluN2A and GluN2B subunits is particularly important for cognitive processes and has been strongly implicated in neurodegenerative diseases like Parkinson's disease and Alzheimer's disease. Understanding the roles of GluN2A and GluN2B NMDARs in neuropathologies provides valuable insights into the underlying causes and complexities of major nervous system disorders. This knowledge is vital for the development of selective antagonists targeting GluN2A and GluN2B subunits using pharmacological and molecular methods. Such antagonists represent a promising class of NMDA receptor inhibitors that have the potential to be developed into neuroprotective drugs with optimal therapeutic profiles.

Negative allosteric modulators of NMDA receptors with GluN2B subunit: synthesis of β-aminoalcohols by epoxide opening and subsequent rearrangement

In order to obtain novel antagonists of GluN2B subunit containing NMDA receptors, aryloxiranes were opened with benzylpiperidines. Phenyloxiranes 6 and (indazolyl)oxirane 15 were opened regioselectively at the position bearing the aryl moiety. Reaction of the resulting β-aminoalcohols 7 and 16 with carboxylic acids under Mitsunobu conditions (DIAD, PPh3) led to rearrangement and after ester hydrolysis to the regioisomeric β-aminoalcohols 9 and 18. This strategy allows the synthesis of amino-ifenprodil 12 as well using phthalimide in the Mitsunobu reaction. Unexpectedly, the isomeric (indazolyl)oxirane 21 reacted with benzylpiperidines to afford both regioisomeric β-aminoalcohols 22 and 23. In radioligand receptor binding studies, the indazolyl derivative 18a, which can be regarded as indazole bioisostere of ifenprodil, showed high GluN2B affinity (Ki = 31 nM). Replacement of the benzylic OH moiety of ifenprodil by the NH2 moiety in amino-ifenprodil 12 also resulted in low nanomolar GluN2B affinity (Ki = 72 nM). In TEVC experiments, 18a inhibited the ion flux to the same extent as ifenprodil proving that the phenol of ifenprodil can be replaced bioisosterically by an indazole ring maintaining affinity and inhibitory activity. Whereas 10-fold selectivity was found for the ifenprodil binding site over σ1 receptors, only low preference for the GluN2B receptor over σ2 receptors was detected. The log D7.4 value of 18a (log D7.4 = 2.08) indicates promising bioavailability.

GRIN2A (NR2A): a gene contributing to glutamatergic involvement in schizophrenia

Involvement of the glutamate system, particularly N-methyl-D-aspartate (NMDA) receptor hypofunction, has long been postulated to be part of the pathophysiology of schizophrenia. An important development is provided by recent data that strongly implicate GRIN2A, the gene encoding the NR2A (GluN2A) NMDA receptor subunit, in the aetiology of the disorder. Rare variants and common variants are both robustly associated with genetic risk for schizophrenia. Some of the rare variants are point mutations likely affecting channel function, but most are predicted to cause protein truncation and thence result, like the common variants, in reduced gene expression. We review the genomic evidence, and the findings from Grin2a mutant mice and other models which give clues as to the likely phenotypic impacts of GRIN2A genetic variation. We suggest that one consequence of NR2A dysfunction is impairment in a form of hippocampal synaptic plasticity, producing deficits in short-term habituation and thence elevated and dysregulated levels of attention, a phenotype of relevance to schizophrenia and its cognitive aspects.

Differential responses of disease-related GRIN variants located in pore-forming M2 domain of N-methyl-D-aspartate receptor to FDA-approved inhibitors

N-methyl-D-aspartate receptors (NMDAR), ionotropic glutamate receptors, mediate a slow component of excitatory synaptic transmission in the central nervous system and play a key role in normal brain function and development. Genetic variations in GRIN genes encoding NMDAR subunits that alter the receptor's functional characteristics are associated with a wide range of neurological and neuropsychiatric conditions. Pathological GRIN variants located in the M2 re-entrant loop lining the channel pore cause significant functional changes, the most consequential alteration being a reduction in voltage-dependent Mg2+ inhibition. Voltage-dependent Mg2+ block is a unique feature of NMDAR biology whereby channel activation requires both ligand binding and postsynaptic membrane depolarization. Thus, loss of NMDAR Mg2+ block will have a profound impact on synaptic function and plasticity. Here, we choose 11 missense variants within the GRIN1, GRIN2A, and GRIN2B genes that alter residues located in the M2 loop and significantly reduce Mg2+ inhibition. Each variant was evaluated for tolerance to genetic variation using the 3-dimensional structure and assessed for functional rescue pharmacology via electrophysiological recordings. Three FDA-approved NMDAR drugs-memantine, dextromethorphan, and ketamine-were chosen based on their ability to bind near the M2 re-entrant loop, potentially rectifying dysregulated NMDAR function by supplementing the reduced voltage-dependent Mg2+ block. These results provide insight of structural determinants of FDA-approved NMDAR drugs at their binding sites in the channel pore and may further define conditions necessary for the use of such agents as potential rescue pharmacology.

Indazole as a Phenol Bioisostere: Structure-Affinity Relationships of GluN2B-Selective NMDA Receptor Antagonists

Negative allosteric modulation of GluN2B subunit-containing NMDA receptors prevents overstimulation, resulting in neuroprotective effects. Since the phenol of prominent negative allosteric modulators is prone to rapid glucuronidation, its bioisosteric replacement by an indazole was envisaged. The key step in the synthesis was a Sonogashira reaction of non-protected iodoindazoles with propargylpiperidine derivatives. Modification of the alkynyl moiety allowed the introduction of several functional groups. The synthesized indazoles showed very high GluN2B affinity but limited selectivity over σ receptors. Molecular dynamics simulations revealed the same molecular interactions with the ifenprodil binding site as the analogous phenols. In two-electrode voltage-clamp experiments, enantiomeric 3-(4-benzylpiperidin-1-yl)-1-(1H-indazol-5-yl)propan-1-ols (S)-10a and (R)-10a displayed higher inhibitory activity than ifenprodil. In contrast to phenolic GluN2B antagonists, the indazoles were not conjugated with glucuronic acid. It can be concluded that the phenol of potent GluN2B antagonists can be replaced bioisosterically by an indazole, retaining the high GluN2B affinity and activity but inhibiting glucuronidation.

Microglial Priming in Bilirubin-Induced Neurotoxicity

Neuroinflammation is a major contributor to bilirubin-induced neurotoxicity, which results in severe neurological deficits. Microglia are the primary immune cells in the brain, with M1 microglia promoting inflammatory injury and M2 microglia inhibiting neuroinflammation. Controlling microglial inflammation could be a promising therapeutic strategy for reducing bilirubin-induced neurotoxicity. Primary microglial cultures were prepared from 1-3-day-old rats. In the early stages of bilirubin treatment, pro-/anti-inflammatory (M1/M2) microglia mixed polarization was observed. In the late stages, bilirubin persistence induced dominant proinflammatory microglia, forming an inflammatory microenvironment and inducing iNOS expression as well as the release of tumor necrosis factor (TNF)-α, interleukin (IL)-6, and IL-1β. Simultaneously, nuclear factor-kappa B (NF-κB) was activated and translocated into the nucleus, upregulating inflammatory target genes. As well known, neuroinflammation can have an effect on N-methyl-D-aspartate receptor (NMDAR) expression or function, which is linked to cognition. Treatment with bilirubin-treated microglia-conditioned medium did affect the expression of IL-1β, NMDA receptor subunit 2A (NR2A), and NMDA receptor subunit 2B (NR2B) in neurons. However, VX-765 effectively reduces the levels of proinflammatory cytokines TNF-α, IL-6, and IL-1β, as well as the expressions of CD86, and increases the expressions of anti-inflammatory related Arg-1. A timely reduction in proinflammatory microglia could protect against bilirubin-induced neurotoxicity.

Pharmacological Potential of 3-Benzazepines in NMDAR-Linked Pathophysiological Processes

The number of N-Methyl-D-aspartate receptor (NMDAR) linked neurodegenerative diseases such as Alzheimer's disease and dementia is constantly increasing. This is partly due to demographic change and presents new challenges to societies. To date, there are no effective treatment options. Current medications are nonselective and can lead to unwanted side effects in patients. A promising therapeutic approach is the targeted inhibition of NMDARs in the brain. NMDARs containing different subunits and splice variants display different physiological properties and play a crucial role in learning and memory, as well as in inflammatory or injury processes. They become overactivated during the course of the disease, leading to nerve cell death. Until now, there has been a lack of understanding of the general functions of the receptor and the mechanism of inhibition, which need to be understood in order to develop inhibitors. Ideal compounds should be highly targeted and even splice-variant-selective. However, a potent and splice-variant-selective NMDAR-targeting drug has yet to be developed. Recently developed 3-benzazepines are promising inhibitors for further drug development. The NMDAR splice variants GluN1-1b-4b carry a 21-amino-acid-long, flexible exon 5. Exon 5 lowers the NMDAR's sensitivity to allosteric modulators by probably acting as an NMDAR modulator itself. The role of exon 5 in NMDAR modulation is still poorly understood. In this review, we summarize the structure and pharmacological relevance of tetrahydro-3-benzazepines.

Opioids and autism spectrum disorder: liaisons dangereuses?

A recent laboratory study in the Journal examined the effects of repeated exposures of neonatal mice to fentanyl on autism-like behaviour via opioid receptor-mediated DNA hypermethylation of the Grin2B gene, which encodes the GluN2B subunit of the NMDA receptor. These experiments provide mechanisms and biological plausibility but do not directly demonstrate that opioid exposure in early life induces autism spectrum disorder in humans. Experimental modelling of human neuropsychiatric disorders is extremely challenging since most subjective psychiatric symptoms used to establish diagnosis in humans cannot be convincingly ascertained in laboratory rodents. While some human epidemiological data show associations between repeated exposures to opioids during early life, it remains undetermined whether opioid exposure is an independent risk factor for developing autism spectrum disorder in the young.

A novel NMDA receptor test model based on hiPSC-derived neural cells

N-Methyl-D-aspartate receptors (NMDARs) are central for learning and information processing in the brain. Dysfunction of NMDARs can play a key role in the pathogenesis of neurodegeneration and drug addiction. The development of selective NMDAR modulators represents a promising strategy to target these diseases. Among such modulating compounds are ifenprodil and its 3-benzazepine derivatives. Classically, the effects of these NMDAR modulators have been tested by techniques like two-electrode voltage clamp (TEVC), patch clamp, or fluorescence-based assays. However, testing their functional effects in complex human systems requires more advanced approaches. Here, we established a human induced pluripotent stem cell-derived (hiPSC-derived) neural cell system and proved its eligibility as a test system for investigating NMDAR modulators and pharmaceutical effects on human neurons.

In vitro ADME characterization of a very potent 3-acylamino-2-aminopropionic acid-derived GluN2C-NMDA receptor agonist and its ester prodrugs

The GluN2C subunit exists predominantly, but not exclusively in NMDA receptors within the cerebellum. Antagonists such as UBP1700 and positive allosteric modulators including PYD-106 and 3-acylamino-2-aminopropionic acid derivatives such as UA3-10 ((R)-2-amino-3-{[5-(2-bromophenyl)thiophen-2-yl]carboxamido}propionic acid) represent promising tool compounds to investigate the role of GluN2C-containing NMDA receptors in the signal transduction in the brain. However, due to its high polarity the bioavailability and CNS penetration of the amino acid UA3-10 are expected to be rather low. Herein, three ester prodrugs 12a-c of the NMDA receptor glycine site agonist UA3-10 were prepared and pharmacokinetically characterized. The esters 12a-c showed higher lipophilicity (higher logD _7.4 values) than the acid UA3-10 but almost the same binding at human serum albumin. The acid UA3-10 was rather stable upon incubation with mouse liver microsomes and NADPH, but the esters 12a-c were fast hydrolyzed to afford the acid UA3-10. Incubation with pig liver esterase and mouse serum led to rapid hydrolysis of the esters 12a-c. The isopropyl ester 12c showed a promising logD _7.4 value of 3.57 and the highest stability in the presence of pig liver esterase and mouse serum. These results demonstrate that ester prodrugs of UA3-10 can potentially afford improved bioavailability and CNS penetration.

Chemical, pharmacodynamic and pharmacokinetic characterization of the GluN2B receptor antagonist 3-(4-phenylbutyl)-2,3,4,5-tetrahydro-1H-3-benzazepine-1,7-diol - starting point for PET tracer development

GluN2B-NMDA receptors play a key role in several neurological and neurodegenerative disorders. In order to develop novel negative allosteric GluN2B-NMDA receptor modulators, the concept of conformational restriction was pursued, i.e. the flexible aminoethanol substructure of ifenprodil was embedded into a more rigid tetrahydro-3-benzazepine system. The resulting tetrahydro-3-benzazepine-1,7-diol (±)-2 (WMS-1410) showed promising receptor affinity in receptor binding studies (K _i = 84 nM) as well as pharmacological activity in two-electrode-voltage-clamp experiments (IC ₅₀ = 116 nM) and in cytoprotective assays (IC ₅₀ = 18.5 nM). The interactions of (R)-2 with the ifenprodil binding site of GluN2B-NMDA receptors were analyzed on the molecular level and the "foot-in-the-door" mechanism was developed. Due to promising pharmacokinetic parameters (logD_7.4 = 1.68, plasma protein binding of 76-77%, sufficient metabolic stability) F-substituted analogs were prepared and evaluated as tracers for positron emission tomography (PET). Both fluorine-18-labeled PET tracers [¹⁸F]11 and [¹⁸F]15 showed high brain uptake, specific accumulation in regions known for high GluN2B-NMDA receptor expression, but no interactions with σ ₁ receptors. Radiometabolites were not observed in the brain. Both PET tracers might be suitable for application in humans.

GRIN2B-related neurodevelopmental disorder: current understanding of pathophysiological mechanisms

The GRIN2B-related neurodevelopmental disorder is a rare disease caused by mutations in the GRIN2B gene, which encodes the GluN2B subunit of NMDA receptors. Most individuals with GRIN2B-related neurodevelopmental disorder present with intellectual disability and developmental delay. Motor impairments, autism spectrum disorder, and epilepsy are also common. A large number of pathogenic de novo mutations have been identified in GRIN2B. However, it is not yet known how these variants lead to the clinical symptoms of the disease. Recent research has begun to address this issue. Here, we describe key experimental approaches that have been used to better understand the pathophysiology of this disease. We discuss the impact of several distinct pathogenic GRIN2B variants on NMDA receptor properties. We then critically review pivotal studies examining the synaptic and neurodevelopmental phenotypes observed when disease-associated GluN2B variants are expressed in neurons. These data provide compelling evidence that various GluN2B mutants interfere with neuronal differentiation, dendrite morphogenesis, synaptogenesis, and synaptic plasticity. Finally, we identify important open questions and considerations for future studies aimed at understanding this complex disease. Together, the existing data provide insight into the pathophysiological mechanisms that underlie GRIN2B-related neurodevelopmental disorder and emphasize the importance of comparing the effects of individual, disease-associated variants. Understanding the molecular, cellular and circuit phenotypes produced by a wide range of GRIN2B variants should lead to the identification of core neurodevelopmental phenotypes that characterize the disease and lead to its symptoms. This information could help guide the development and application of effective therapeutic strategies for treating individuals with GRIN2B-related neurodevelopmental disorder.

Progresses in GluN2A-containing NMDA Receptors and their Selective Regulators

NMDA receptors play an important physiological role in regulating synaptic plasticity, learning and memory. GluN2A subunits are the most abundant functional subunits of NMDA receptors expressed in mature brain, and their dysfunction is related to various neurological diseases. According to subunit composition, GluN2A-containing NMDA receptors can be divided into two types: diheteromeric and triheteromeric receptors. In this review, the expression, functional and pharmacological properties of different kinds of GluN2A-containing NMDA receptors as well as selective GluN2A regulators were described to further understand this type of NMDA receptors.

From Mouse to Man: N-Methyl-d-Aspartic Acid Receptor Activation as a Promising Pharmacotherapeutic Strategy for Autism Spectrum Disorders

The BALB/c mouse displays hypersensitivity to behavioral effects of MK-801 (dizocilpine), a noncompetitive N-methyl-d-aspartic acid (NMDA) receptor "open-channel" blocker, and shows both no preference for an enclosed stimulus mouse over an inanimate object and reduced social interaction with a freely behaving stimulus mouse. NMDA receptor agonist interventions improved measures of social preference and social interaction of the BALB/c mouse model of autism spectrum disorder (ASD). A "proof of principle/proof of concept" translational 10-week clinical trial with 8-week of active medication administration was conducted comparing 20 DSM-IV-TR-diagnosed older adolescent/young adult patients with ASD randomized to once-weekly pulsed administration (50 mg/d) versus daily administration of d-cycloserine (50 mg/d). The results showed that d-cycloserine, a partial glycine agonist, was well tolerated, the 2 dosing strategies did not differ, and improvement was noted on the "lethargy/social withdrawal" and "stereotypic behavior" subscales of the Aberrant Behavior Checklist. NMDA receptor activation contributes to the regulation of mTOR signaling, a pathologic point of convergence in several monogenic syndromic forms of ASD. Furthermore, both NMDA receptor hypofunction and imbalance between NMDA receptor activation mediated by GluN2B and GluN2A-containing NMDA receptors occur as "downstream" consequences of several genetically unrelated abnormalities associated with ASD. NMDA receptor-subtype selective "positive allosteric modulators (PAMs)" are particularly appealing medication candidates for future translational trials.

Different Forms of Disorder in NMDA-Sensitive Glutamate Receptor Cytoplasmic Domains Are Associated with Differences in Condensate Formation

The N-methyl-D-aspartate (NMDA)-sensitive glutamate receptor (NMDAR) helps assemble downstream signaling pathways through protein interactions within the postsynaptic density (PSD), which are mediated by its intracellular C-terminal domain (CTD). The most abundant NMDAR subunits in the brain are GluN2A and GluN2B, which are associated with a developmental switch in NMDAR composition. Previously, we used single molecule fluorescence resonance energy transfer (smFRET) to show that the GluN2B CTD contained an intrinsically disordered region with slow, hop-like conformational dynamics. The CTD from GluN2B also undergoes liquid-liquid phase separation (LLPS) with synaptic proteins. Here, we extend these observations to the GluN2A CTD. Sequence analysis showed that both subunits contain a form of intrinsic disorder classified as weak polyampholytes. However, only GluN2B contained matched patterning of arginine and aromatic residues, which are linked to LLPS. To examine the conformational distribution, we used discrete molecular dynamics (DMD), which revealed that GluN2A favors extended disordered states containing secondary structures while GluN2B favors disordered globular states. In contrast to GluN2B, smFRET measurements found that GluN2A lacked slow conformational dynamics. Thus, simulation and experiments found differences in the form of disorder. To understand how this affects protein interactions, we compared the ability of these two NMDAR isoforms to undergo LLPS. We found that GluN2B readily formed condensates with PSD-95 and SynGAP, while GluN2A failed to support LLPS and instead showed a propensity for colloidal aggregation. That GluN2A fails to support this same condensate formation suggests a developmental switch in LLPS propensity.

GluN2A-Selective NMDA Receptor Antagonists: Mimicking the U-Shaped Bioactive Conformation of TCN-201 by a [2.2]Paracyclophane System

Under physiological conditions, N-Methyl-D-Aspartate (NMDA) receptors play a crucial role for synaptic plasticity, long-term potentiation and long-term depression. However, overactivation of NMDA receptors can result in excitotoxicity, which is associated with various neurological and neurodegenerative diseases. The physiological properties of NMDA receptors are strongly dependent on the GluN2 subunit incorporated into the heterotetrameric NMDA receptor. Therefore, subtype selective NMDA receptor modulators are of high interest. Since prototypical GluN2A-NMDA receptor antagonists TCN-201 and its MPX-analogs adopt a U-shaped conformation within the binding pocket, paracyclophanes were designed containing the phenyl rings in an already parallel orientation. Docking studies of the designed paracyclophanes show a similar binding pose as TCN-201. [2.2]Paracyclophanes with a benzoate or benzamide side chain were prepared in four-step synthesis, respectively, starting with a radical bromination in benzylic 1-position of [2.2]paracyclophane. In two-electrode voltage clamp experiments using Xenopus laevis oocytes transfected with cRNAs for the GluN1-4a and GluN2A subunits, the esters and amides (conc. 10 μM) did not show considerable inhibition of ion flux. It can be concluded that the GluN2A-NMDA receptor does not accept ligands with a paracyclophane scaffold functionalized in benzylic 1-position, although docking studies had revealed promising binding poses for benzoic acid esters and benzamides.

Dose-dependent suppression of hippocampal contextual memory formation, place cells, and spatial engrams by the NMDAR antagonist (R)-CPP

We recently reported that the competitive NMDAR antagonist (R,S)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) does not suppress NMDAR-mediated field EPSPs (fEPSPNMDA) or long-term potentiation (LTP) in vitro at concentrations that block contextual conditioning in vivo. Here we tested one possible explanation for the mismatch - that the hippocampus is relatively resistant to CPP compared to other brain structures engaged in contextual fear conditioning. Using the context pre-exposure facilitation effect (CPFE) paradigm to separate the hippocampal and extra-hippocampal components of contextual learning, we found that the active enantiomer (R)-CPP suppressed the hippocampal component with an IC50 of 3.1 mg/kg, a dose that produces brain concentrations below those required to block fEPSPNMDA or LTP. Moreover, using in-vivo calcium imaging of place cells and spatial engrams to directly assess hippocampal spatial coding, we found that (R)-CPP dose-dependently reduced the development of place cells and interfered with the formation of stable spatial engrams when it was administered prior to exposing mice to a novel context. Both effects occurred at doses that interfered with freezing to context in CPFE experiments. We conclude that (R)-CPP blocks memory formation by interfering with hippocampal function, but that it does so by modulating NMDARs at sites that are not engaged in vitro in the same manner that they are in vivo - perhaps through interneuron circuits that do not contribute to fEPSPs and are not required to elicit LTP using standard induction protocols in vitro, but are essential for successful mnemonic function in vivo.

Postsynaptic Proteins at Excitatory Synapses in the Brain—Relationship with Depressive Disorders

Depressive disorders (DDs) are an increasingly common health problem that affects all age groups. DDs pathogenesis is multifactorial. However, it was proven that stress is one of the most important environmental factors contributing to the development of these conditions. In recent years, there has been growing interest in the role of the glutamatergic system in the context of pharmacotherapy of DDs. Thus, it has become increasingly important to explore the functioning of excitatory synapses in pathogenesis and pharmacological treatment of psychiatric disorders (including DDs). This knowledge may lead to the description of new mechanisms of depression and indicate new potential targets for the pharmacotherapy of illness. An excitatory synapse is a highly complex and very dynamic structure, containing a vast number of proteins. This review aimed to discuss in detail the role of the key postsynaptic proteins (e.g., NMDAR, AMPAR, mGluR5, PSD-95, Homer, NOS etc.) in the excitatory synapse and to systematize the knowledge about changes that occur in the clinical course of depression and after antidepressant treatment. In addition, a discussion on the potential use of ligands and/or modulators of postsynaptic proteins at the excitatory synapse has been presented.

NMDA Receptor C-Terminal Domain Signalling in Development, Maturity, and Disease

The NMDA receptor is a Ca²⁺-permeant glutamate receptor which plays key roles in health and disease. Canonical NMDARs contain two GluN2 subunits, of which 2A and 2B are predominant in the forebrain. Moreover, the relative contribution of 2A vs. 2B is controlled both developmentally and in an activity-dependent manner. The GluN2 subtype influences the biophysical properties of the receptor through difference in their N-terminal extracellular domain and transmembrane regions, but they also have large cytoplasmic Carboxyl (C)-terminal domains (CTDs) which have diverged substantially during evolution. While the CTD identity does not influence NMDAR subunit specific channel properties, it determines the nature of CTD-associated signalling molecules and has been implicated in mediating the control of subunit composition (2A vs. 2B) at the synapse. Historically, much of the research into the differential function of GluN2 CTDs has been conducted in vitro by over-expressing mutant subunits, but more recently, the generation of knock-in (KI) mouse models have allowed CTD function to be probed in vivo and in ex vivo systems without heterologous expression of GluN2 mutants. In some instances, findings involving KI mice have been in disagreement with models that were proposed based on earlier approaches. This review will examine the current research with the aim of addressing these controversies and how methodology may contribute to differences between studies. We will also discuss the outstanding questions regarding the role of GluN2 CTD sequences in regulating NMDAR subunit composition, as well as their relevance to neurodegenerative disease and neurodevelopmental disorders.

Calcium and activity-dependent signaling in the developing cerebral cortex

Calcium influx can be stimulated by various intra- and extracellular signals to set coordinated gene expression programs into motion. As such, the precise regulation of intracellular calcium represents a nexus between environmental cues and intrinsic genetic programs. Mounting genetic evidence points to a role for the deregulation of intracellular calcium signaling in neuropsychiatric disorders of developmental origin. These findings have prompted renewed enthusiasm for understanding the roles of calcium during normal and dysfunctional prenatal development. In this Review, we describe the fundamental mechanisms through which calcium is spatiotemporally regulated and directs early neurodevelopmental events. We also discuss unanswered questions about intracellular calcium regulation during the emergence of neurodevelopmental disease, and provide evidence that disruption of cell-specific calcium homeostasis and/or redeployment of developmental calcium signaling mechanisms may contribute to adult neurological disorders. We propose that understanding the normal developmental events that build the nervous system will rely on gaining insights into cell type-specific calcium signaling mechanisms. Such an understanding will enable therapeutic strategies targeting calcium-dependent mechanisms to mitigate disease.

Understanding global changes of the mouse brain proteome after vaginal infection with HSV-2 using a label-free shotgun approach

Herpes simplex virus type 2 (HSV-2) is a common human pathogen that establishes lifelong latency in neurons of the nervous system. The number of severe central nervous system infections caused by the virus has increased recently. However, the pathogenesis of HSV-2 infection in the nervous system is not fully understood. Here, we demonstrated global proteomic changes in the brain tissue in BALB/c mice vaginally infected with HSV-2. Data are available via ProteomeXchange with identifier PXD034186. A total of 249 differentially expressed proteins were identified in infected brain tissue. The GO and KEGG enrichment analysis of these proteins indicated that they were mainly involved in the regulation of synapse formation and synaptic excitability. In addition, genes affecting autophagy, the development of other neurodegenerative diseases, and signaling pathways relevant to other neurologic diseases were identified. Additional experiments, comparing the brain tissue of asymptomatic and symptomatic mice showed a differential expression of proteins involved in synapse formation and synaptic transmission. Others were involved in autophagy, addiction, and signaling pathways of other neurologic diseases. These results suggest that changes in synaptic structure and function, as well as autophagy, may be related to the development of neurologic abnormalities that follow HSV-2 infection. We also identified a protein GluN2A encoded by Grin2a was continuously expressed at high levels after infection. We propose that GluN2A may be a key molecule in the pathogenesis of HSV-2-induced neurologic diseases.

Phenol-benzoxazolone bioisosteres of GluN2B-NMDA receptor antagonists: Unexpected rearrangement during reductive alkylation with phenylcyclohexanone

Negative allosteric modulators of N-methyl- d-aspartate receptors containing the GluN2B subunit represent promising drug candidates for the treatment of various neurological disorders including stroke, epilepsy, and Parkinson's disease. To increase the bioavailability and GluN2B affinity, the phenol of the potent benzazepine-based inhibitor, WMS-1410 (3), was replaced bioisosterically by a benzoxazolone moiety and the phenylbutyl side chain was conformationally restricted in a phenylcyclohexyl substituent. A four-step, one-pot procedure transformed the oxazolo-benzazepine 7 into the phenylcyclohexyl derivative 11. The same protocol was applied to the methylated analog 12, which unexpectedly led to ring-contracted oxazolo-isoquinolines 18. This rearrangement was explained by the additional methyl moiety in the 8-position inhibiting the formation of the planar intermediate iminium ion with phenylcyclohexanone. The allyl protective group of 11 and 18 was removed with RhCl₃ and HCl to obtain the tricyclic compounds 5 and 19 without substituent at the oxazolone ring. The structures of the rearranged products 18 and 19 were elucidated by X-ray crystal structure analysis. The oxazolo-isoquinoline trans-18 with allyl moiety (K_i  = 89 nM) and the oxazolo-benzazepine 5 without substituent at the oxazolone ring (K_i  = 114 nM) showed GluN2B affinity in the same range as the lead compound 3. In two-electrode voltage clamp measurements, 5 displayed only weak inhibitory activity.

Role of N-Methyl-D-Aspartate Receptor NR2B Subunit in Inflammatory Arthritis-Induced Chronic Pain and Peripheral Sensitized Neuropathic Pain: A Systematic Review

Arthritis is a common clinical disease that affects millions of people in the world. The most common types of arthritis are osteoarthritis and rheumatoid arthritis. Inflammatory arthritis (IA), a chronic painful disease, is characterized by synovitis and cartilage destruction in the early stages. Pathologically, IA causes inflammatory changes in the joints and eventually leads to joint destruction. Pain is associated with inflammation and abnormal regulation of the nervous system pathways involved in pain promotion and inhibition. In addition, the occurrence of pain is associated with depression and anxiety. We found that there are many factors affecting pain, in addition to inflammatory factors, glutamate receptor may be the possible cause of long-term chronic pain caused by IA. N-methyl-d-aspartate receptor subunit 2B (NR2B) has been reported to involved in IA and nervous system diseases, especially peripheral neuropathic pain. In this review, we summarized the mechanisms of the NR2B subunit of the N-methyl-D-aspartate (NMDA) receptor in peripheral nerve sensitization during IA and chronic pain.

Protein quality control of N-methyl-D-aspartate receptors

N-methyl-D-aspartate receptors (NMDARs) are glutamate-gated cation channels that mediate excitatory neurotransmission and are critical for synaptic development and plasticity in the mammalian central nervous system (CNS). Functional NMDARs typically form via the heterotetrameric assembly of GluN1 and GluN2 subunits. Variants within GRIN genes are implicated in various neurodevelopmental and neuropsychiatric disorders. Due to the significance of NMDAR subunit composition for regional and developmental signaling at synapses, properly folded receptors must reach the plasma membrane for their function. This review focuses on the protein quality control of NMDARs. Specifically, we review the quality control mechanisms that ensure receptors are correctly folded and assembled within the endoplasmic reticulum (ER) and trafficked to the plasma membrane. Further, we discuss disease-associated variants that have shown disrupted NMDAR surface expression and function. Finally, we discuss potential targeted pharmacological and therapeutic approaches to ameliorate disease phenotypes by enhancing the expression and surface trafficking of subunits harboring disease-associated variants, thereby increasing their incorporation into functional receptors.

Negative allosteric modulators of NMDA receptors with GluN2B subunit: Alanine-derived benzoxazolone bioisosteres of 2-methyl-3-benzazepine-1,7-diols

Inspired by besonprodil, the phenol of potent negative allosteric modulators of GluN2B-N-methyl-d-aspartate (NMDA) receptors was replaced by a benzoxazolone system. To increase the similarity to the lead compounds, an additional methyl moiety was installed in the 8-position of tricyclic oxazolobenzazepines, resulting in compounds 6. The additional methyl moiety originates from alanine, which was introduced by a Mitsunobu reaction of benzoxazolylethanol 7 with N-triflyl-protected alanine methyl ester. A crucial feature of the synthesis was the protection of the oxazolone ring by an allyl moiety, which was cleaved off at the end of the synthesis by RhCl₃ -catalyzed isomerization. Due to the additional methyl moiety, the intramolecular Friedel-Crafts acylation of acid 10 to afford ketone 11 required careful optimization to minimize the formation of the side product tetrahydroisoquinoline 16. Alkylation or reductive alkylation of secondary amine 13 led to diastereomeric oxazolobenzazepines cis-14 and trans-14, which were separated by flash chromatography. Phenylbutyl derivatives cis-6a and trans-6a revealed twofold higher GluN2B affinity than analog 5a without 8-CH₃ group. The methylated oxazolobenzazepines 6 and 14 did not interact with the phencyclidine binding site of NMDA receptors and σ₂ receptors. However, the σ₁ receptor preferred cis-configured oxazolobenzazepines. The highest σ₁ receptor affinities were obtained for cis-14a (K_i  = 26 nM) and cis-6b (K_i  = 30 nM).

Phenol-Benzoxazolone bioisosteres: Synthesis and biological evaluation of tricyclic GluN2B-selective N-methyl- d-aspartate receptor antagonists

Tricyclic tetrahydrooxazolo[4,5-h]-[3]benzazepin-9-ols 22 were designed as phenol bioisosteres of tetrahydro-3-benzazepine-1,7-diols. Key features of the synthesis are the introduction of the trifluoromethylsulfonyl and allyl protective groups at the heterocyclic N-atoms. Two methods were developed to convert the triflyl-protected ketone 16 into tricyclic alcohols 21 bearing various N-substituents. According to the first method, trifluoromethanesulfinate was removed by K₂ CO₃ . Following the selective reduction of the imino moiety of 17 with NaBH(OAc)₃ afforded the aminoketone 18, which was reductively alkylated and reduced. According to the second method, both the imine and the ketone of the iminoketone 17 were reduced with NaBH₄ to yield the aminoalcohol 20, which was alkylated or reductively alkylated to form tertiary amines 21f-21r. In the last step, the allyl protective group of 21 was removed with RhCl₃ and HCl to obtain oxazolones 22. In receptor binding studies using [³ H]ifenprodil as radioligand ketone, 22m showed the highest GluN2B affinity (K_i  = 88 nM). However, a reduced affinity toward GluN2B subunit-containing N-methyl- d-aspartate (NMDA) receptors was observed for oxazolones 22 compared to bioisosteric 3-benzazepine-1,7-diols. High selectivity of 22m for the ifenprodil binding site of GluN2B-NMDA receptors over the 1-(1-phenylcyclohexyl)piperidine binding site and σ₂ receptors was observed, but only negligible selectivity over σ₁ receptors. In two-electrode voltage clamp experiments, the 4-phenylbutyl derivative 22d (K_i  = 422 nM) demonstrated 80% inhibition of ion flux at a concentration of 1 µM. The differences in GluN2B affinity and inhibitory activity are explained by docking studies. In conclusion, 22d is regarded as a novel scaffold of highly potent GluN1/GluN2B antagonists.

Behavioral Training Related Neurotransmitter Receptor Expression Dynamics in the Nidopallium Caudolaterale and the Hippocampal Formation of Pigeons

Learning and memory are linked to dynamic changes at the level of synapses in brain areas that are involved in cognitive tasks. For example, changes in neurotransmitter receptors are prerequisite for tuning signals along local circuits and long-range networks. However, it is still unclear how a series of learning events promotes plasticity within the system of neurotransmitter receptors and their subunits to shape information processing at the neuronal level. Therefore, we investigated the expression of different glutamatergic NMDA (GRIN) and AMPA (GRIA) receptor subunits, the GABAergic GABARG2 subunit, dopaminergic DRD1, serotonergic 5HTR1A and noradrenergic ADRA1A receptors in the pigeon’s brain. We studied the nidopallium caudolaterale, the avian analogue of the prefrontal cortex, and the hippocampal formation, after training the birds in a rewarded stimulus-response association (SR) task and in a simultaneous-matching-to-sample (SMTS) task. The results show that receptor expression changed differentially after behavioral training compared to an untrained control group. In the nidopallium caudolaterale, GRIN2B, GRIA3, GRIA4, DRD1D, and ADRA1A receptor expression was altered after SR training and remained constantly decreased after the SMTS training protocol, while GRIA2 and DRD1A decreased only under the SR condition. In the hippocampal formation, GRIN2B decreased and GABARG2 receptor expression increased after SR training. After SMTS sessions, GRIN2B remained decreased, GABARG2 remained increased if compared to the control group. None of the investigated receptors differed directly between both conditions, although differentially altered. The changes in both regions mostly occur in favor of the stimulus response task. Thus, the present data provide evidence that neurotransmitter receptor expression dynamics play a role in the avian prefrontal cortex and the hippocampal formation for behavioral training and is uniquely, regionally and functionally associated to cognitive processes including learning and memory.

Effects of NMDA receptor antagonists on behavioral economic indices of cocaine self-administration

BACKGROUND

Currently, there are no FDA-approved medications for the treatment of psychostimulant (e.g., cocaine) use disorders. Because the GluN2B subunit of the glutamate N-methyl-D-aspartate (NMDA) receptor is an important mediator of addiction-like behaviors, the goal of the current study was to determine if the GluN2B-selective antagonist Ro 63-1908 is efficacious in attenuating cocaine self-administration.

METHODS

Adult Sprague Dawley rats (24 males and 11 females) were implanted with indwelling catheters and were trained to self-administer cocaine (0.75 mg/kg/inf). Rats were then trained in a threshold procedure, in which the dose of cocaine decreased across six 6-min blocks (0.75, 0.27, 0.08, 0.03, 0.01, 0.003 mg/kg/inf). This procedure allowed for the quantification of behavioral economic indices of drug self-administration. Following training in the threshold procedure, rats were treated with the GluN2B-selective antagonist Ro 63-1908 (0, 0.1, 0.3, 1.0 mg/kg; s.c.). Rats also received treatments of the NMDA receptor channel blocker MK-801 (0, 0.01, 0.03, 0.06 mg/kg; s.c.).

RESULTS

Blocking NMDA receptors decreased initial intake (i.e., consumption during the first block), although Ro 63-1908 and MK-801 increased area under the curve (global measure of demand) and decreased demand elasticity, an effect observed primarily in males. Neither drug affected demand intensity (i.e., consumption of cocaine at a minimally constrained price).

CONCLUSIONS

While blocking the NMDA receptor decreases initial intake of cocaine, NMDA receptor antagonists make cocaine more inelastic with increasing price. These results suggest that NMDA receptor antagonists can exacerbate addiction-like behaviors during self-administration during extinction-like conditions that are observed in later blocks of the threshold procedure.

Laparotomy-Induced Peripheral Inflammation Activates NR2B Receptors on the Brain Mast Cells and Results in Neuroinflammation in a Vagus Nerve-Dependent Manner

Background: The pathophysiological mechanisms underlying postoperative cognitive dysfunction (POCD) remain unclear over the years. Neuroinflammation caused by surgery has been recognized as an important element in the development of POCD. Many studies also suggest that the vagus nerve plays an important role in transmitting peripheral injury signals to the central nervous system (CNS) and the resultant neuroinflammation. Previously, we have demonstrated that brain mast cells (BMCs), as the “first responders”, play a vital role in neuroinflammation and POCD. However, how the vagus nerve communicates with BMCs in POCD has not yet been clarified.

Methods: In the current study, we highlighted the role of the vagus nerve as a conduction highway in surgery-induced neuroinflammation for the first time. In our model, we tested if mice underwent unilateral cervical vagotomy (VGX) had less neuroinflammation compared to the shams after laparotomy (LP) at an early stage. To further investigate the roles of mast cells and glutamate in the process, we employed Kit^W-sh mice and primary bone marrow-derived MCs to verify the glutamate-NR2B axis on MCs once again.

Results: Our results demonstrated that there were higher levels of glutamate and BMCs activation as early as 4 h after LP. Meanwhile, vagotomy could partially block the increases and reduce neuroinflammation caused by peripheral inflammation during the acute phase. Excitingly, inhibition of NR2B receptor and knockout of mast cells can attenuateneuroinflammation induced by glutamate.

Conclusion: Taken together, our findings indicate that the vagus is a high-speed pathway in the transmission of peripheral inflammation to the CNS. Activation of BMCs triggered a neuroinflammatory cascade. Inhibition of NR2B receptor on BMCs can reduce glutamate-induced BMCs activation, neuroinflammation, and memory impairment, suggesting a novel treatment strategy for POCD.

Common synaptic phenotypes arising from diverse mutations in the human NMDA receptor subunit GluN2A

Dominant mutations in the human gene GRIN2A, encoding NMDA receptor (NMDAR) subunit GluN2A, make a significant and growing contribution to the catalogue of published single-gene epilepsies. Understanding the disease mechanism in these epilepsy patients is complicated by the surprising diversity of effects that the mutations have on NMDARs. Here we have examined the cell-autonomous effect of five GluN2A mutations, 3 loss-of-function and 2 gain-of-function, on evoked NMDAR-mediated synaptic currents (NMDA-EPSCs) in CA1 pyramidal neurons in cultured hippocampal slices. Despite the mutants differing in their functional incorporation at synapses, prolonged NMDA-EPSC current decays (with only marginal changes in charge transfer) were a common effect for both gain- and loss-of-function mutants. Modelling NMDA-EPSCs with mutant properties in a CA1 neuron revealed that the effect of GRIN2A mutations can lead to abnormal temporal integration and spine calcium dynamics during trains of concerted synaptic activity. Investigations beyond establishing the molecular defects of GluN2A mutants are much needed to understand their impact on synaptic transmission.

The cell-autonomous effect of five severe loss- or gain-of-function GluN2A (NMDA receptor) mutations is assessed on evoked NMDAR-mediated synaptic currents in CA1 pyramidal neurons in cultured mouse hippocampal slices. Data and modelling suggest that mutant-like NMDA-EPSCs can lead to abnormal temporal summation and spine calcium dynamics.

Synthesis of oxazolo-annulated 3-benzazepines designed by merging two negative allosteric NMDA receptor modulators

To improve the metabolic stability and receptor selectivity of ifenprodil (1), the benzoxazolone moiety of besonprodil (2) and the 3-benzazepone moiety of WMS-1410 (3) were merged to obtain oxazolobenzazepines of type 4. The 5-(hydroxyethyl)benzoxazolone 7 representing the first key intermediate was prepared in four steps starting with the 4-(2-hydroxyethyl)phenol (8). Mitsunobu reaction of primary alcohol 7 with N-sulfonylated glycine esters established the necessary side chain. The intramolecular Friedel-Crafts acylation of acid 12a containing the N-tosyl protective group led upon decarbonylation exclusively to the tricyclic tetrahydroisoquinoline 14. Protection of the amino moiety by the stronger electron-withdrawing triflyl group resulted in the desired 3-benzazepine 15 without the formation of analogous isoquinoline. The triflyl protective group was cleaved off by K₂ CO₃ -induced elimination of trifluoromethanesulfinate. In a one-pot three-step procedure, various oxazolobenzazepinediones 15 were obtained, which were reduced to afford the desired secondary alcohols 18.

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.

Targeted NMDA Receptor Interventions for Autism: Developmentally Determined Expression of GluN2B and GluN2A-Containing Receptors and Balanced Allosteric Modulatory Approaches

Various ASD risk alleles have been associated with impairment of NMDA receptor activation (i.e., NMDA Receptor Hypofunction) and/or disturbance of the careful balance between activation mediated by GluN2B-subtype and GluN2A-subtype-containing NMDA receptors. Importantly, although these various risk alleles affect NMDA receptor activation through different mechanisms, they share the pathogenic consequences of causing disturbance of highly regulated NMDA receptor activation. Disturbances of NMDA receptor activation due to sequence variants, protein termination variants and copy number variants are often cell-specific and regionally selective. Thus, translational therapeutic NMDA receptor agonist interventions, which may require chronic administration, must have specificity, selectivity and facilitate NMDA receptor activation in a manner that is physiologic (i.e., mimicking that of endogenously released glutamate and glycine/D-serine released in response to salient and relevant socio-cognitive provocations within discrete neural circuits). Importantly, knockout mice with absent expression and mice with haploinsufficient expression of the deleterious genes often serve as good models to test the potential efficacy of promising pharmacotherapeutic strategies. The Review considers diverse examples of “illness” genes, their pathogenic effects on NMDA receptor activation and, when available, results of studies of impaired sociability in mouse models, including “proof of principle/proof of concept” experiments exploring NMDA receptor agonist interventions and the development of promising positive allosteric modulators (PAMs), which serve as support and models for developing an inventory of PAMs and negative allosteric modulators (NAMs) for translational therapeutic intervention. Conceivably, selective PAMs and NAMs either alone or in combination will be administered to patients guided by their genotype in order to potentiate and/or restore disrupted balance between activation mediated by GluN2B-subtype and GluN2A-subtype containing NMDA receptors.

Borna disease virus docks on neuronal DNA double-strand breaks to replicate and dampens neuronal activity

Borna disease viruses (BoDV) have recently emerged as zoonotic neurotropic pathogens. These persistent RNA viruses assemble nuclear replication centers (vSPOT) in close interaction with the host chromatin. However, the topology of this interaction and its consequences on neuronal function remain unexplored. In neurons, DNA double-strand breaks (DSB) have been identified as novel epigenetic mechanisms regulating neurotransmission and cognition. Activity-dependent DSB contribute critically to neuronal plasticity processes, which could be impaired upon infection. Here, we show that BoDV-1 infection, or the singled-out expression of viral Nucleoprotein and Phosphoprotein, increases neuronal DSB levels. Of interest, inducing DSB promoted the recruitment anew of vSPOT colocalized with DSB and increased viral RNA replication. BoDV-1 persistence decreased neuronal activity and response to stimulation by dampening the surface expression of glutamate receptors. Taken together, our results propose an original mechanistic cross talk between persistence of an RNA virus and neuronal function, through the control of DSB levels.

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BoDV-1, its Nucleoprotein or Phosphoprotein cause neuronal DNA double-strand breaks (DSB)

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DNA double-strand breaks co-localize with BoDV-1 replication factories

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DNA DSB recruits BoDV-1 replication factories and promotes viral replication

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BoDV-1 inhibits neuronal activity by impeding surface expression of GluN2A receptors