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NMDA Receptors: Finally Fingered as Faulty in Schizophrenia?

7 November 2012. Despite abundant indirect evidence implicating N-methyl-D-aspartate receptors (NMDARs) in positive and cognitive symptoms of schizophrenia, it has proved surprisingly difficult to identify structural or functional differences in these receptors in the disorder, leading many researchers to try to account for these findings by studying signaling mechanisms downstream of NMDARs. A new study, led by Tom Weickert of Australia’s University of New South Wales, examined NMDAR subunits in the largest postmortem cohort to date, finding reduced expression of the NR1 and NR2C subunits. The study, published online on October 16 in Molecular Psychiatry, also linked a single-nucleotide polymorphism (SNP) in the gene coding for the NR2B subunit to both poor cognitive performance and reduced expression of the NR1 subunit in the dorsolateral prefrontal cortex of schizophrenia subjects.

NMDARs are activated by glutamate, the brain’s primary excitatory neurotransmitter, but also require co-activation by either D-serine or glycine. They play a myriad of roles throughout the nervous system, from neurodevelopment to learning and memory (see SRF related news story). Their versatility is made possible by a complex mix-and-match configuration: two obligatory glycine-binding NR1 subunits, which occur in eight variants via alternative splicing of the gene GRIN1, are paired with two glutamate-binding NR2 subunits, which have four isoforms (NR2A-D), and/or with two glycine/D-serine-binding NR3 subunits, which have two isoforms (NR3A and NR3B).

It has been known for decades that NMDAR antagonists such as phencyclidine (PCP) and ketamine have potent psychotomimetic effects in healthy subjects and worsen symptoms in those with schizophrenia, providing the first clues that NMDAR hypofunction might contribute to the psychosis seen in the illness (see SRF Current Hypotheses by Bita Moghaddam and Daniel Javitt). Reduced serum levels of the endogenous NMDAR agonist D-serine (Hashimoto et al., 2003), as well as increased activity of D-amino acid oxidase (Madeira et al., 1997), which degrades D-serine, have been reported in schizophrenia, but reports of NMDAR expression in postmortem tissue have yielded mixed results—with increased, decreased, and unchanged levels reported (Kristiansen et al., 2007).

In the current study, first author Cynthia Weickert and colleagues measured mRNA transcripts for the NR1, NR2A, NR2B, NR2C, and NR3A NMDAR subunits, as well as NR1 protein levels, in postmortem samples of dorsolateral prefrontal cortex (DLPFC) from schizophrenia subjects and matched controls. Levels of NR1 and NR2C mRNA were significantly lower—22 percent and 28 percent, respectively—in schizophrenia subjects’ DLPFC, and NR1 protein levels were significantly lower by 36 percent.

In a second component of the new study, the research team found that subjects with schizophrenia carrying the minor allele (C) of the NR2B gene SNP rs1805502 had significantly lower reasoning performance, as measured by the Arithmetic Subtest of the Weschler Adult Intelligence Scale (WAIS-III), than both schizophrenia subjects who did not carry the C allele and controls who did. In further postmortem analyses, the group found that NR1 mRNA and protein levels were significantly lower in C carrier schizophrenia subjects than C carrier controls, but no change was observed between control and schizophrenia subjects who did not carry the C allele. NR2C mRNA levels did not differ as a function of rs1805502 genotype in either controls or subjects with schizophrenia.

The significant reduction of both mRNA and protein of the obligatory NR1 subunit, “supports the theory,” the authors write, “that hypofunction of NMDARs exists endogenously in prefrontal cortex in schizophrenia and strongly supports that the hypofunction can reside within the NMDAR.” Weickert and colleagues argue that their NR2C findings may also speak to the cognitive deficits of schizophrenia, citing research showing deficits in fear conditioning and working memory in NR2C-knockout mice (Hillman et al., 2011).

The exact mechanism by which the NR2B SNP produces lower NR1 levels and cognitive deficits in schizophrenia remains unknown. Interestingly, although a SNP in NR1 that has previously been associated with schizophrenia (Zhao et al., 2006) was not associated with lower NR1 levels in the current study, one schizophrenia study has demonstrated a genetic interaction between the NR1 and NR2B SNPs (Qin et al., 2005).—Pete Farley.

Reference:
Weickert CS, Fung SJ, Catts VS, Schofield PR, Allen KM, Moore LT, Newell KA, Pellen D, Huang XF, Catts SV, Weickert TW. Molecular evidence of N-methyl-D-aspartate receptor hypofunction in schizophrenia. Mol Psychiatry. 2012 Oct 16. Abstract

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Related News: Probing the Role of NMDA Receptor NR2B in Development and Schizophrenia

Comment by:  Stefan KolataKazu Nakazawa
Submitted 21 February 2012
Posted 21 February 2012

The original NMDA receptor (NMDAR) hypofunction theory of schizophrenia was predicated on the discovery that, in adulthood, NMDAR antagonists mimicked disease symptomatology and exacerbated symptoms in schizophrenic patients (Javitt and Zukin, 1991). Recent advances have since shown that, in addition to this effect in adulthood, there may be a postnatal developmental sensitive period necessary for NMDAR hypofunction to later manifest as schizophrenia phenotypes. For instance, in mice, schizophrenia-like phenotypes were observed when NR1 (GluN1) was ablated selectively in corticolimbic interneurons after postnatal day 7, but not when the knockout occurred after adolescence (Belforte et al., 2010). Similarly, transient antagonism of NMDA during development later resulted in schizophrenia-like phenotypes in adult rats (Stefani and Moghaddam, 2005; Baier et al., 2009). In the present work by Wang et al. (2011), using an elegant molecular genetic technique, Benjamin Hall and his colleagues were able to show that it is perhaps the NR2B (GluN2B) subunit during this developmental period that is most critical for the later development of the symptomatology. NR2B is highly expressed during this postnatal sensitive period, and is only later replaced by NR2A (GluN2A) in most NMDA receptors. The present paper showed that an early replacement of NR2B with NR2A recapitulated some of the NMDA hypomorph phenotypes. While these results are very intriguing and dovetail nicely with the emerging thinking about the neurodevelopmental role of NMDARs, the possible involvement of NR2A itself in schizophrenia should not be lost. Impairment of NR2A results in several schizophrenia-like phenotypes, including a reduction in parvalbumin immunoreactivity, impaired fast-spiking interneuron maturation, altered dopamine metabolism, and a hyperlocomotion response in the open field that is rescued by antipsychotic treatment (Zhang and Sun, 2011; Miyamoto et al., 2001). Further studies of synapses, neurons, and neuronal networks regulated by NR2A and NR2B may lead to a better understanding of the mechanisms underlying the NMDAR hypofunction theory of schizophrenia.

References:

Baier PC, Blume A, Koch J, Marx A, Fritzer G, Aldenhoff JB, Schiffelholz T. Early postnatal depletion of NMDA receptor development affects behaviour and NMDA receptor expression until later adulthood in rats--a possible model for schizophrenia. Behav Brain Res. 2009 Dec 14;205(1):96-101. Abstract

Belforte JE, Zsiros V, Sklar ER, Jiang Z, Yu G, Li Y, Quinlan EM, Nakazawa K. Postnatal NMDA receptor ablation in corticolimbic interneurons confers schizophrenia-like phenotypes. Nat Neurosci. 2010 Jan;13(1):76-83. Abstract

Javitt DC, Zukin SR. Recent advances in the phencyclidine model of schizophrenia. Am J Psychiatry. 1991 Oct;148(10):1301-8. Abstract

Miyamoto Y, Yamada K, Noda Y, Mori H, Mishina M, Nabeshima T. Hyperfunction of dopaminergic and serotonergic neuronal systems in mice lacking the NMDA receptor epsilon1 subunit. J Neurosci. 2001 Jan 15;21(2):750-7. Abstract

Stefani MR, Moghaddam B. Transient N-methyl-D-aspartate receptor blockade in early development causes lasting cognitive deficits relevant to schizophrenia. Biol Psychiatry. 2005 Feb 15;57(4):433-6. Abstract

Zhang Z, Sun QQ. Development of NMDA NR2 subunits and their roles in critical period maturation of neocortical GABAergic interneurons. Dev Neurobiol. 2011 Mar;71(3):221-45. Abstract

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Related News: Boosting NMDA Receptors Improves Symptoms, Cognition in Schizophrenia

Comment by:  Hugo Geerts
Submitted 20 October 2013
Posted 20 October 2013
  I recommend the Primary Papers

The group around Dr. Hsien-Yuan Lane has published a number of papers on clinical trials in schizophrenia patients with agents that act on co-agonist sites of the NMDA-receptor. This time they report on the beneficial effects of augmentation therapy with high-dose benzoate, a D-amino acid oxidase inhibitor, on a number of clinical scales (about 25 subjects/treatment arm). The effect is substantial (effect sizes between 1.16 on the PANSS negative and 1.69 on the PANSS positive subscale). For instance, this effect size is about twice the value seen in clinical trials with bitopertin, a glycine transporter-1 inhibitor in a larger Phase II study (Umbricht et al., 2010). Only one dose of benzoate has been tested, so the issue of a possible inverse U-shape response that has been observed earlier for a similar target and supported by theoretical-mechanistic insights has not been addressed in this study. They took great care in balancing the treatment arms with regard to the type of basal antipsychotic medication and found that haldol and risperidone were particularly receptive for benzoate augmentation therapy.

Of interest is the observation that benzoate is a food additive (E210-E213) with an impressive record of safety, opening up the possibility of an easier treatment approach of lower levels of the drug be achieved using food strategies. It might therefore be of interest to test lower levels of benzoate as well.

Antipsychotics are often considered deleterious or neutral at best for cognitive improvement, so this augmentation study suggests that benzoate is able to reverse this trend of worsening. In addition, there were no correlations between changes in PANSS positive or EPS changes and changes in both PANSS negative or cognitive outcome. This suggests that the observed effect of the compound is unlikely to be indirectly due to an improvement in PANSS positive symptoms or motor side effects, suggesting a genuine impact on the negative or cognitive subscales.

With regard to cognition, from the MATRICS subscale, the authors only found speed of processing and visual learning and memory to be significantly improved with the active treatment. However, this is one of the few trials in which the global composite score increased more with treatment than the placebo, despite the possible practice effect. Nevertheless, it underscores the difficulty of improving all seven domains of the cognitive MATRICS scale.

With the caveat of low numbers in the treatment arm, this study has to be recommended because it once again suggests a path forward for glutamatergic strategies. The glutamatergic system is currently the focus of much research in psychiatric indications (such as ketamine in depression). However the major problem, unlike older dopaminergic and neuromodulatory strategies, is finding a balance between excitation and inhibition in the human brain, and the feedback mechanism that operates, that makes it sometimes difficult to find the best dose-range for any treatment paradigm. The authors of this paper, however, show that this is possible.

References:

Umbricht D, Yoo K, Youssef E, Dorflinger E, Martin-Facklam M, Bausch A, Arrowsmith R, Alberati D, Marder S, Santarelli L. Glycine Transporter Type 1 (GLYT1) Inhibitor RG1678: Positive Results of the Proof-of-Concept Study for the Treatment of Negative Symptoms in Schizophrenia. Neuropsychopharmacology. 2010; 35:S320-321.

View all comments by Hugo Geerts

Related News: Boosting NMDA Receptors Improves Symptoms, Cognition in Schizophrenia

Comment by:  Michael McFarland
Submitted 5 November 2013
Posted 11 November 2013

Sodium benzoate combined with ascorbic acid produces benzene, a known carcinogen. I hope that another D-amino acid oxidase inhibitor can be found easily.

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Related News: New Compounds May Help Probe the Glutamate Hypothesis of Schizophrenia

Comment by:  John Krystal, SRF Advisor
Submitted 23 February 2016
Posted 23 February 2016

Positive allosteric modulators of NR2A-containing NMDA receptors (NMDARs) represents an important new pharmacologic advance for probing the role of NMDARs in cognition and behavior and the pathophysiology and treatment of psychiatric disorders. There is a long-standing interest in finding ways to enhance NMDAR function using allosteric modulators of NMDARs that might avoid the potential of direct agonists to cause neurotoxicity. There are a number of psychiatric disorders and addictions where deficits in NMDAR function are implicated in disturbances in network function or plasticity that compromise recovery. This has been a particular focus in the area of schizophrenia research, where compromised glutamate synaptic signaling has been mimicked by the administration of NMDA glutamate receptor antagonists. Since at least the late 1980s, investigators have been interested in the possibility that drugs that enhance NMDAR function might have value in the treatment of symptoms and functional impairment associated with schizophrenia.

The first area of focus was amino acids that enhanced NMDAR function by stimulating the glycine/D-serine co-agonist site of the NMDAR. The initial approach was to orally administer naturally occurring glycine or D-amino acids, D-serine, or D-alanine. Studies with these agents produced small and somewhat inconsistent benefits across studies, raising concerns that this approach could not be successful. However, these amino acids were not really developed as drugs. The optimal dose level and pattern of administration was never established with these agents. From this perspective, any positive data provided tantalizing support for the hypothesis even though there were ongoing efforts to find alternatives with superior pharmacologic properties to enhance NMDAR function.

The first of these alternative agents was D-cycloserine, a partial agonist of the glycine site with a complex pharmacology. D-cycloserine has 30 to 50 percent of the activity of glycine at NR2A- and NR2B-containing NMDARs, meaning that in many behavioral assays, high-dose D-cycloserine produces effects (weak sedation, weak euphoria, memory impairment, etc.) that resemble low doses of NMDAR antagonists. Importantly, at high doses, D-cycloserine seemed to worsen symptoms in some patients diagnosed with schizophrenia. However, D-cycloserine is a nearly full or even super (greater activity than glycine) agonist at NR2C- and NR2D-containing NMDARs, and it is able to stimulate NR3-containing NMDARs that are not directly stimulated by glutamate. It is likely that by stimulating NR2C-, NR2D-, or NR3-containing receptors, beneficial NMDAR agonist-like effects attributed to D-cycloserine emerge. But the complex pharmacology of D-cycloserine limits its therapeutic potential.

The next class of drugs developed was glycine transporter-1 antagonists (GlyT1 inhibitors). These drugs are based on the premise that GlyT1 is a high-activity transporter that controls synaptic glycine levels below the saturation level, allowing receptors to be stimulated by exogenous glycine or GlyT1 inhibition. Some early agents had limited pharmacologic development but showed promising clinical results. Later agents received systematic study, and Roche initially developed exciting initial data, but positive results could not be replicated. Many questions remain, including the following:

1. Is glycine the right amino acid to target relative to D-serine? D-serine appears to have greater synaptic function relative to glycine, which may be more important for extrasynaptic signaling.

2. Is tonic inhibition of GlyT1 detrimental due to its potential to stimulate downregulation of NMDAR function? Instead, should these agents be administered intermittently to enhance network function and plasticity? (

3. Do GlyT1 inhibitors have an inverted U dose response to avoid stimulating NMDAR downregulation and to optimally modulate network functions?

There are a number of reasons that NR2A-containing NMDARs are an interesting target. NR2A-containing NMDARs are predominately synaptic receptors, while NR2B-containing NMDARs are widely distributed in synaptic and extrasynaptic spaces. When "overstimulated," extrasynaptic NR2B-containing NMDARs suppress BDNF levels and cause the pruning of dendritic spines and even dendrites themselves. They also have the potential to produce even more severe forms of toxicity. NR2A-containing NMDARs also emerge early in life and gradually displace NR2B-containing receptors during childhood and adolescence in many circuit elements. Since the risk for many psychiatric disorders also emerges during this period, the NR2A subunit is an intriguing target. In addition, while overstimulation of NR2B-containing receptors has negative effects on synaptic connectivity, NR2A-containing NMDARs are implicated in some neurotrophic processes. Further, there is interest in drugs that facilitate NR2A-mediated signaling (positive allosteric modulation) rather than stimulating NMDARs in order to avoid neurotoxicity that might be associated with directly stimulating these receptors. In this way, one might think of NR2A-positive allosteric modulators (PAMs) as analogous to the way that benzodiazepines enhance the activation of GABAA receptors.

The exciting paper by Hackos et al. in Neuron represents an important advance in the effort to develop NR2A PAMs. The researchers present an elegant story about the identification and validation of these drugs. Schizophrenia is an obvious disorder where these medications might play a role. But there are many other conditions where modulating NMDAR function via NR2A could be predicted to produce benefits. I think that this paper will stimulate interest and discussion in the field.

View all comments by John Krystal