Developmental Changes of Synaptic and Extrasynaptic NMDA Receptor Expression in Rat Cerebellar Neurons In Vitro

Therapeutic potential of N-methyl-D-aspartate receptor modulators in psychiatry

N-methyl-D-aspartate (NMDA) receptors mediate a slow component of excitatory synaptic transmission, are widely distributed throughout the central nervous system, and regulate synaptic plasticity. NMDA receptor modulators have long been considered as potential treatments for psychiatric disorders including depression and schizophrenia, neurodevelopmental disorders such as Rett Syndrome, and neurodegenerative conditions such as Alzheimer's disease. New interest in NMDA receptors as therapeutic targets has been spurred by the findings that certain inhibitors of NMDA receptors produce surprisingly rapid and robust antidepressant activity by a novel mechanism, the induction of changes in the brain that well outlast the presence of drug in the body. These findings are driving research into an entirely new paradigm for using NMDA receptor antagonists in a host of related conditions. At the same time positive allosteric modulators of NMDA receptors are being pursued for enhancing synaptic function in diseases that feature NMDA receptor hypofunction. While there is great promise, developing the therapeutic potential of NMDA receptor modulators must also navigate the potential significant risks posed by the use of such agents. We review here the emerging pharmacology of agents that target different NMDA receptor subtypes, offering new avenues for capturing the therapeutic potential of targeting this important receptor class.

GluN2B inhibition rescues impaired potentiation and epileptogenicity at associational-commissural CA3 synapses in a model of anti-NMDAR encephalitis

Anti-N-methyl-d-aspartate receptor (anti-NMDAR) encephalitis is an autoimmune epilepsy associated with memory deficits. Research has demonstrated that anti-NMDAR inhibit long-term potentiation, and, at the same time, lead to disinhibition in the form of epileptiform afterpotentials in the potentiated state. While both effects may give rise to the key symptoms of the disease, the molecular basis of being simultaneously inhibitory and disinhibitory is difficult to explain. Here, we explored a possible involvement of the GluN2B subunit. To this aim, we injected cerebrospinal fluid from anti-NMDAR encephalitis patients into the rat hippocampus and prepared brain slices for in vitro field potential recordings. Associational-commissural-fiber-CA3 synapses from anti-NMDAR-treated animals showed increased field potential amplitudes with concomitantly enhanced paired-pulse ratios as compared to control tissue. GluN2B inhibition by Ro25-6981 mimicked these effects in controls but had no effect in anti-NMDAR tissues indicating a presynaptic and occluding effect of anti-NMDAR. We then induced potentiation of associational-commissural-fiber-CA3 synapses, and confirmed that slices from anti-NMDAR-treated animals showed reduced potentiation and pronounced epileptiform afterpotentials. Intriguingly, both effects were absent when Ro25-6981 was added in vitro before inducing potentiation. These results indicate that GluN2B-containing NMDARs, partially expressed presynaptically, show differential sensitivity to anti-NMDAR, and that altered GluN2B function is particularly apparent in the potentiated state rather than under baseline conditions. Since GluN2B inhibition rescued the effects of anti-NMDAR in the potentiated state, this opens the possibility that at least a subgroup of patients could benefit from a GluN2B antagonist.

Spatial Learning and Memory Impairment in Growing Mice Induced by Major Oxidized Tyrosine Product Dityrosine

This study focused on the effects of oxidized tyrosine products (OTPs) and major component dityrosine (DT) on the brain and behavior of growing mice. Male and female mice were treated with daily intragastric administration of either tyrosine (Tyr; 420 μg/kg body weight), DT (420 μg/kg body weight), or OTPs (1909 μg/kg body weight) for 35 days. We found that pure DT and OTPs caused redox state imbalance, elevated levels of inflammatory factors, hippocampal oxidative damage, and neurotransmitter disorders while activating the mitochondrial apoptosis pathway in the hippocampus and downregulating the genes associated with learning and memory. These events eventually led to growing mice learning and memory impairment, lagging responses, and anxiety-like behaviors. Furthermore, the male mice exhibited slightly more oxidative damage than the females. These findings imply that contemporary diets and food-processing strategies of the modern world should be modified to reduce oxidized protein intake.

Immuno-Pharmacological Characterization of Presynaptic GluN3A-Containing NMDA Autoreceptors: Relevance to Anti-NMDA Receptor Autoimmune Diseases

Mouse hippocampal glutamatergic nerve endings express presynaptic release-regulating NMDA autoreceptors (NMDARs). The presence of GluN1, GluN2A, GluN2B, and GluN3A subunits in hippocampal vesicular glutamate transporter type 1-positive synaptosomes was confirmed with confocal microscopy. GluN2C, GluN2D, and GluN3B immunopositivity was scarcely present. Incubation of synaptosomes with the anti-GluN1, the anti-GluN2A, the anti-GluN2B, or the anti-GluN3A antibody prevented the 30 μM NMDA/1 μM glycine-evoked [³H]D-aspartate ([³H]D-ASP) release. The NMDA/glycine-evoked [³H]D-ASP release was reduced by increasing the external protons, consistent with the participation of GluN1 subunits lacking the N1 cassette to the receptor assembly. The result also excludes the involvement of GluN1/GluN3A dimers into the NMDA-evoked overflow. Complement (1:300) released [³H]D-ASP in a dizocilpine-sensitive manner, suggesting the participation of a NMDAR-mediated component in the releasing activity. Accordingly, the complement-evoked glutamate overflow was reduced in anti-GluN-treated synaptosomes when compared to the control. We speculated that incubation with antibodies had favored the internalization of NMDA receptors. Indeed, a significant reduction of the GluN1 and GluN2B proteins in the plasma membranes of anti-GluN1 or anti-GluN2B antibody-treated synaptosomes emerged in biotinylation studies. Altogether, our findings confirm the existence of presynaptic GluN3A-containing release-regulating NMDARs in mouse hippocampal glutamatergic nerve endings. Furthermore, they unveil presynaptic alteration of the GluN subunit insertion in synaptosomal plasma membranes elicited by anti-GluN antibodies that might be relevant to the central alterations occurring in patients suffering from autoimmune anti-NMDA diseases.

Calcium-Dependent Desensitization of NMDA Receptors

Glutamate receptors play the key role in excitatory synaptic transmission in the central nervous system (CNS). N-methyl-D-aspartate-activated glutamate receptors (NMDARs) are ion channels permeable to sodium, potassium, and calcium ions that localize to the pre- and postsynaptic membranes, as well as extrasynaptic neuronal membrane. Calcium entry into dendritic spines is essential for long-term potentiation (LTP) and long-term depression (LTD) of synaptic transmission. Both LTP and LTD represent morphological and functional changes occurring in the process of memory formation. NMDAR dysfunction is associated with epilepsy, schizophrenia, migraine, dementia, and neurodegenerative diseases. Prolonged activation of extrasynaptic NMDARs causes calcium overload and apoptosis of neurons. Here, we review recent findings on the molecular mechanisms of calcium-dependent NMDAR desensitization that ensures fast modulation of NMDAR conductance in the CNS and limits calcium entry into the cells under pathological conditions. We present the data on molecular determinants related to calcium-dependent NMDAR desensitization and functional interaction of NMDARs with other ion channels and transporters. We also describe association of NMDARs with lipid membrane microdomains.

High sensitivity of cerebellar neurons to homocysteine is determined by expression of GluN2C and GluN2D subunits of NMDA receptors

Homocysteine (HCY) induced neurotoxicity largely depends on interaction of this endogenous amino acid with glutamate NMDA receptors (NMDARs). This receptor type is composed by GluN1 and different GluN2 (A, B, C or D) subunits. However, the receptor activity of HCY in brain regions which differ in relative contribution of GluN2 subunits was not tested so far. In the current study, we explored the action of HCY on cerebellar neurons which natively express GluN2C and GluN2D subunits of NMDARs and compared this with the action of HCY on cortical neurons which are mainly composed by GluN2A and GluN2B subunits. To validate obtained results, we also studied the responses to HCY in recombinant GluN1/2C and GluN1/2D NMDARs expressed in HEK293T cells. Responses to HCY were compared to membrane currents evoked by glutamate or by the specific agonist NMDA. First, we found that on HEK cells expressing GluN1/2C or GluN1/2D NMDARs, HCY was full agonist producing membrane currents similar in amplitude to currents induced by glutamate. The EC₅₀ values for these particular receptor subtype activation were 80 μM and 31 μM, respectively. Then, we found that HCY similarly to NMDA, evoked large slightly desensitizing membrane currents in native NMDARs of cerebellar and cortical neurons. In cortical neurons, the ratio of the respective currents (I_HCY/I_NMDA) was 0.16 and did not significantly change during in vitro maturation. In sharp contrast, in cerebellar neurons, the ratio of currents evoked by HCY and NMDA was dramatically increased from 0.31 to 0.72 from 7 to 21 day in culture. We show that least 75% of HCY-induced currents in cerebellum were mediated by GluN2C- or GluN2D-containing NMDARs. Thus, our data revealed a large population of cerebellar NMDA receptors highly sensitive to HCY which suggest potential vulnerability of this brain region to pathological conditions associated with enhanced levels of this neurotoxic amino acid.