The NMDA receptor glycine modulatory site: a therapeutic target for improving cognition and reducing negative symptoms in schizophrenia

What Remains to Be Discovered in Schizophrenia Therapeutics: Contributions by Advancing the Molecular Mechanisms of Drugs for Psychosis and Schizophrenia

Schizophrenia is a frequently debilitating and complex mental disorder affecting approximately 1% of the global population, characterized by symptoms such as hallucinations, delusions, disorganized thoughts and behaviors, cognitive dysfunction, and negative symptoms. Traditional treatment has centered on postsynaptic dopamine antagonists, commonly known as antipsychotic drugs, which aim to alleviate symptoms and improve functioning and the quality of life. Despite the availability of these medications, significant challenges remain in schizophrenia therapeutics, including incomplete symptom relief, treatment resistance, and medication side effects. This opinion article explores advancements in schizophrenia treatment, emphasizing molecular mechanisms, novel drug targets, and innovative delivery methods. One promising approach is novel strategies that target neural networks and circuits rather than single neurotransmitters, acknowledging the complexity of brain region interconnections involved in schizophrenia. Another promising approach is the development of biased agonists, which selectively activate specific signaling pathways downstream of receptors, offering potential for more precise pharmacological interventions with fewer side effects. The concept of molecular polypharmacy, where a single drug targets multiple molecular pathways, is exemplified by KarXT, a novel drug combining xanomeline and trospium to address both psychosis and cognitive dysfunction. This approach represents a comprehensive strategy for schizophrenia treatment, potentially improving outcomes for patients. In conclusion, advancing the molecular understanding of schizophrenia and exploring innovative therapeutic strategies hold promise for addressing the unmet needs in schizophrenia treatment, aiming for more effective and tailored interventions. Future research should focus on these novel approaches to achieve better clinical outcomes and improve the functional level and quality of life for individuals with schizophrenia.

MIR137 polygenic risk for schizophrenia and ephrin-regulated pathway: Role in lateral ventricles and corpus callosum volume

Background/Objective. Enlarged lateral ventricle (LV) volume and decreased volume in the corpus callosum (CC) are hallmarks of schizophrenia (SZ). We previously showed an inverse correlation between LV and CC volumes in SZ, with global functioning decreasing with increased LV volume. This study investigates the relationship between LV volume, CC abnormalities, and the microRNA MIR137 and its regulated genes in SZ, because of MIR137's essential role in neurodevelopment. Methods. Participants were 1224 SZ probands and 1466 unaffected controls from the GENUS Consortium. Brain MRI scans, genotype, and clinical data were harmonized across cohorts and employed in the analyses. Results. Increased LV volumes and decreased CC central, mid-anterior, and mid-posterior volumes were observed in SZ probands. The MIR137-regulated ephrin pathway was significantly associated with CC:LV ratio, explaining a significant proportion (3.42 %) of CC:LV variance, and more than for LV and CC separately. Other pathways explained variance in either CC or LV, but not both. CC:LV ratio was also positively correlated with Global Assessment of Functioning, supporting previous subsample findings. SNP-based heritability estimates were higher for CC central:LV ratio (0.79) compared to CC or LV separately. Discussion. Our results indicate that the CC:LV ratio is highly heritable, influenced in part by variation in the MIR137-regulated ephrin pathway. Findings suggest that the CC:LV ratio may be a risk indicator in SZ that correlates with global functioning.

Title: A Boron‐Chelating Piperazine‐Tethered Schiff Base Can Modulate Excitability in Brain Slices in a Specific Frequency Range

The ability of boron‐containing compounds to make various bonds with biological targets draws attention in their use as new therapeutic agents. In this study, the effect of a newly synthesized molecule with the short name fmpemboron, “Difluoro [2‐[([2‐[4‐(2,3,4‐trimethoxybenzyl)piperazin‐1‐yl]ethyl] imino‐κN)methyl]phenolato‐κO]boron (5)”on the excitability of neurons in the brain was investigated for the first time. First of all, fmpemboron was synthesized and characterized. Secondly, virtual scanning of the molecule was performed using ADME and molecular docking methods. Then, epileptiform activities were induced in mouse brain slices using Mg‐free or 4AP methods, and the effect of fmpemboron (5) at different concentrations was examined. The absorbance peak wavelength of fmpemboron (5) is between 330–340 nm and the fluorescence emission peak is 435 nm. According to ADME and molecular docking results, fmpemboron (5) can cross the blood‐brain barrier (BBB) and has affinity for the NMDA receptor. It also significantly reduces the power of oscillations between 8–12 Hz and 13–29 Hz in epileptiform activities generated by the Mg‐free method. Administration of 10 μM fmpemboron (5) had a modulatory effect on epileptiform activities, indicating that fmpemboron (5) can affect various neurological functions through NMDA channels. However, in‐vivo dose‐dependent studies are required to further investigate the effects of this novel molecule.

A narrative review of treatment interventions to improve cognitive performance in schizophrenia, with an emphasis on at-risk and early course stages

Cognitive dysfunction is a core feature of schizophrenia (SCZ), which unfavorably affects SCZ patients' daily functioning and overall clinical outcome. An increasing body of evidence has shown that cognitive deficits are present not only at the beginning of the illness but also several years before the onset of psychosis. Nonetheless, the majority of treatment interventions targeting cognitive dysfunction in SCZ, using both pharmacological and nonpharmacological approaches, have focused on chronic patients rather than individuals at high risk or in the early stages of the disease. In this article, we provide a narrative review of cognitive interventions in SCZ patients, with a particular focus on pre-emptive interventions in at-risk/early course individuals when available. Furthermore, we discuss current challenges for these pre-emptive treatment interventions and provide some suggestions on how future work may ameliorate cognitive dysfunction in these individuals.

GlyCEST: Magnetic Resonance Imaging of Glycine—Distribution in the Normal Murine Brain and Alterations in 5xFAD Mice

The glycine level in the brain is known to be altered in neuropsychiatric disorders, such as schizophrenia and Alzheimer's disease (AD). Several studies have reported the in vivo measurement of glycine concentrations in the brain using proton magnetic resonance spectroscopy (¹H-MRS), but ¹H-MRS is not capable of imaging the distribution of glycine concentration with high spatial resolution. Chemical exchange saturation transfer magnetic resonance imaging (CEST-MRI) is a new technology that can detect specific molecules, including amino acids, in tissues. To validate the measurements of glycine concentrations in living tissues using CEST from glycine to water (GlyCEST), we extracted the brain tissues from mice and performed biochemical tests. In wild-type C57BL/6 mice, GlyCEST effects were found to be higher in the thalamus than in the cerebral cortex (P < 0.0001, paired t-test), and this result was in good agreement with the biochemical results. In 5xFAD mice, an animal model of AD, GlyCEST measurements demonstrated that glycine concentrations in the cerebral cortex (P < 0.05, unpaired t-test) and thalamus (P < 0.0001, unpaired t-test), but not in the hippocampus, were decreased compared to those in wild-type mice. These findings suggest that we have successfully applied the CEST-MRI technique to map the distribution of glycine concentrations in the murine brain. The present method also captured the changes in cerebral glycine concentrations in mice with AD. Imaging the distribution of glycine concentrations in the brain can be useful in investigating and elucidating the pathological mechanisms of neuropsychiatric disorders.

Low-Level Brain Somatic Mutations Are Implicated in Schizophrenia

BACKGROUND

Somatic mutations arising from the brain have recently emerged as significant contributors to neurodevelopmental disorders, including childhood intractable epilepsy and cortical malformations. However, whether brain somatic mutations are implicated in schizophrenia (SCZ) is not well established.

METHODS

We performed deep whole exome sequencing (average read depth > 550×) of matched dorsolateral prefrontal cortex and peripheral tissues from 27 patients with SCZ and 31 age-matched control individuals, followed by comprehensive and strict analysis of somatic mutations, including mutagenesis signature, substitution patterns, and involved pathways. In particular, we explored the impact of deleterious mutations in GRIN2B through primary neural culture.

RESULTS

We identified an average of 4.9 and 5.6 somatic mutations per exome per brain in patients with SCZ and control individuals, respectively. These mutations presented with average variant allele frequencies of 8.0% in patients with SCZ and 7.6% in control individuals. Although mutational profiles, such as the number and type of mutations, showed no significant difference between patients with SCZ and control individuals, somatic mutations in SCZ brains were significantly enriched for SCZ-related pathways, including dopamine receptor, glutamate receptor, and long-term potentiation pathways. Furthermore, we showed that brain somatic mutations in GRIN2B (encoding glutamate ionotropic NMDA receptor subunit 2B), which were found in two patients with SCZ, disrupted the location of GRIN2B across the surface of dendrites among primary cultured neurons.

CONCLUSIONS

Taken together, this study shows that brain somatic mutations are associated with the pathogenesis of SCZ.

Resveratrol Adjunct Therapy for Negative Symptoms in Patients With Stable Schizophrenia: A Double-Blind, Randomized Placebo-Controlled Trial

BACKGROUND

Patients with schizophrenia can generally manifest a broad variety of primary negative symptoms. The current study aimed to assess the efficacy and tolerability of resveratrol add-on therapy in the treatment of negative symptoms in patients with stable schizophrenia.

METHODS

In a randomized, double-blind, and placebo-controlled setting, schizophrenia patients were assigned to receive either 200 mg/d resveratrol or matched placebo in addition to a stable dose of risperidone for 8 weeks. Patients were assessed using the positive and negative syndrome scale, the extrapyramidal symptom rating scale, and Hamilton Depression Rating Scale over the trial period. The primary outcome was considered as the change in positive and negative subscale score from baseline to week 8 between the treatment arms.

RESULTS

A total 52 patients completed the trial (26 in each arm). Baseline characteristics of both groups were statistically similar (P  > .05). Despite the statistically similar behavior of positive symptoms between the groups across time (Greenhouse-Geisser corrected: F = 1.76, df = 1.88, P = .180), the resveratrol group demonstrated greater improvement in negative, general psychopathology, and total scores (Greenhouse-Geisser corrected: F = 12.25, df = 2.04, P < .001; F = 5.42, df = 1.56, P = .011; F = 7.64, df = 1.48, P = .003). HDRS scores and its changes, ESRS score, and frequency of other complications were not significantly different between resveratrol and placebo groups.

CONCLUSION

Adding resveratrol to risperidone can exhibit remarkable efficacy and safety in terms of management of schizophrenia-related negative symptoms.

Prenatal and Early Postnatal Cerebral d-Aspartate Depletion Influences l-Amino Acid Pathways, Bioenergetic processes, and Developmental Brain Metabolism

d-Amino acids were believed to occur only in bacteria and invertebrates. Today, it is well known that d-amino acids are also present in mammalian tissues in a considerable amount. In particular, high levels of free d-serine (d-Ser) and d-aspartate (d-Asp) are found in the brain. While the functions of d-Ser are well known, many questions remain unanswered regarding the role of d-Asp in the central nervous system. d-Asp is very abundant at the embryonic stage, while it strongly decreases after birth because of the expression of d-aspartate oxidase (Ddo) enzyme, which catalyzes the oxidation of this d-amino acid into oxaloacetate, ammonium, and hydrogen peroxide. Pharmacologically, d-Asp acts as an endogenous agonist of N-methyl d-aspartate and mGlu5 receptors, which are known to control fundamental brain processes, including brain development, synaptic plasticity, and cognition. In this work, we studied a recently generated knockin mouse model (R26^ddo/ddo), which was designed to express DDO beginning at the zygotic stage. This strategy enables d-Asp to be almost eliminated in both prenatal and postnatal lives. To understand which biochemical pathways are affected by depletion of d-Asp, in this study, we carried out a metabolomic and lipidomic study of ddo knockin brains at different stages of embryonic and postnatal development, combining nuclear magnetic resonance (NMR) and high-resolution mass spectrometry (HRMS) techniques. Our study shows that d-Asp deficiency in the brain influences amino acid pathways such as threonine, glycine, alanine, valine, and glutamate. Interestingly, d-Asp is also correlated with metabolites involved in brain development and functions such as choline, creatine, phosphocholine (PCho), glycerophosphocholine (GPCho), sphingolipids, and glycerophospholipids, as well as metabolites involved in brain energy metabolism, such as GPCho, glucose, and lactate.

Regulation of Glutamatergic Activity via Bidirectional Activation of Two Select Receptors as a Novel Approach in Antipsychotic Drug Discovery

Schizophrenia is a mental disorder that affects approximately 1-2% of the population and develops in early adulthood. The disease is characterized by positive, negative, and cognitive symptoms. A large percentage of patients with schizophrenia have a treatment-resistant disease, and the risk of developing adverse effects is high. Many researchers have attempted to introduce new antipsychotic drugs to the clinic, but most of these treatments failed, and the diversity of schizophrenic symptoms is one of the causes of disappointing results. The present review summarizes the results of our latest papers, showing that the simultaneous activation of two receptors with sub-effective doses of their ligands induces similar effects as the highest dose of each compound alone. The treatments were focused on inhibiting the increased glutamate release responsible for schizophrenia arousal, without interacting with dopamine (D₂) receptors. Ligands activating metabotropic receptors for glutamate, GABA_B or muscarinic receptors were used, and the compounds were administered in several different combinations. Some combinations reversed all schizophrenia-related deficits in animal models, but others were active only in select models of schizophrenia symptoms (i.e., cognitive or negative symptoms).

Evaluation of the Efficacy of BI 425809 Pharmacotherapy in Patients with Schizophrenia Receiving Computerized Cognitive Training: Methodology for a Double-blind, Randomized, Parallel-group Trial

BACKGROUND AND OBJECTIVE: Cognitive impairments associated with schizophrenia (CIAS) predict poor functional outcomes, but there are currently no approved pharmacological treatments for patients with CIAS. Additional cognitive stimulation may be required for pro-cognitive medications to improve efficacy, and computerized cognitive training (CCT) can be used to increase cognitive activity. A trial evaluating the effects of the novel glycine transporter inhibitor BI 425809 compared with placebo, on a background of regularly self-administered CCT in clinically stable patients with schizophrenia has commenced and its methodology is described here.

METHODS

This Phase II, multinational, randomized, double-blind, placebo-controlled, parallel-group trial will randomize 200 clinically stable outpatients, aged 18-50 years with established schizophrenia and no other major psychiatric disorder, 1:1 to BI 425809 or placebo once daily for 12 weeks. Following screening, which included a 2-week CCT run-in period, patients sufficiently compliant with CCT (target: ≥ 2 h of CCT per week during CCT run-in) will be randomized. During the 12-week treatment period, all patients should complete a total of approximately 30 h of CCT. The primary endpoint is change from baseline in neurocognitive function as measured by the neurocognitive composite score of the Measurement and Treatment Research to Improve Cognition in Schizophrenia Consensus Cognitive Battery (MCCB), after 12 weeks of treatment. Secondary endpoints include change from baseline in overall MCCB score, Schizophrenia Cognition Rating Scale, Positive and Negative Syndrome Scale, and safety (adverse events [AEs]) and serious AEs. Primary and secondary endpoints will be analyzed using the Restricted Maximum Likelihood-based mixed model for repeated measures. Novel endpoints include the Balloon Effort Task to evaluate patients' motivation and the Virtual Reality Functional Capacity Assessment Tool to assess skills for daily functioning.

DISCUSSION

This is one of the largest and longest trials to date to combine pharmacological therapy with CCT in patients with schizophrenia and will determine the benefit of combining BI 425809 pharmacotherapy with cognitive stimulation through self-administered CCT. This trial will further evaluate whether improvements in neurocognition translate into improved everyday functioning, whether self-administered CCT can be effectively implemented in a large multinational trial, and the role of motivation in neurocognitive and functional improvements.

TRIAL REGISTRATION

Registered on Clinicaltrials.gov on March 1, 2019 (NCT03859973).

Therapeutic potential and underlying mechanism of sarcosine (N-methylglycine) in N-methyl-D-aspartate (NMDA) receptor hypofunction models of schizophrenia

BACKGROUND

Compelling animal and clinical studies support the N-methyl-D-aspartate receptor (NMDAR) hypofunction hypothesis of schizophrenia and suggest promising pharmacological agents to ameliorate negative and cognitive symptoms of schizophrenia, including sarcosine, a glycine transporter-1 inhibitor.

AIMS AND METHODS

It is imperative to evaluate the therapeutic potential of sarcosine in animal models, which provide indispensable tools for testing drug effects in detail and elucidating the underlying mechanisms. In this study, a series of seven experiments was conducted to investigate the effect of sarcosine in ameliorating behavioral deficits and the underlying mechanism in pharmacological (i.e., MK-801-induced) and genetic (i.e., serine racemase-null mutant (SR^-/-) mice) NMDAR hypofunction models.

RESULTS

In Experiment 1, the acute administration of 500/1000 mg/kg sarcosine (i.p.) had no adverse effects on motor function and serum biochemical responses. In Experiments 2-4, sarcosine significantly alleviated MK-801-induced (0.2 mg/kg) brain abnormalities and behavioral deficits in MK-801-induced and SR^-/- mouse models. In Experiment 5, the injection of sarcosine enhanced CSF levels of glycine and serine in rat brain. In Experiments 6-7, we show for the first time that sarcosine facilitated NMDAR-mediated hippocampal field excitatory postsynaptic potentials and influenced the movement of surface NMDARs at extrasynaptic sites.

CONCLUSIONS

Sarcosine effectively regulated the surface trafficking of NMDARs, NMDAR-evoked electrophysiological activity, brain glycine levels and MK-801-induced abnormalities in the brain, which contributed to the amelioration of behavioral deficits in mouse models of NMDAR hypofunction.

N-Methyl-d-aspartate receptor co-agonist availability affects behavioral and neurochemical responses to cocaine: insights into comorbid schizophrenia and substance abuse

Both schizophrenia (SZ) and substance abuse (SA) exhibit significant heritability. Moreover, N-methyl-d-aspartate receptors (NMDARs) have been implicated in the pathophysiology of both SZ and SA. We hypothesize that the high prevalence of comorbid SA in SZ is due to dysfunction of NMDARs caused by shared risk genes. We used transgenic mice with a null mutation of the gene encoding serine racemase (SR), the enzyme that synthesizes the NMDAR co-agonist d-serine and an established risk gene for SZ, to recreate the pathology of SZ. We determined the effect of NMDAR hypofunction resulting from the absence of d-serine on motivated behavior by using intracranial self-stimulation and neurotransmitter release in the nucleus accumbens by using in vivo microdialysis. Compared with wild-type mice, SR-/- mice exhibited similar baseline intracranial self-stimulation thresholds but were less sensitive to the threshold-lowering (rewarding) and the performance-elevating (stimulant) effects of cocaine. While basal dopamine (DA) and glutamate release were elevated in the nucleus accumbens of SR-/- mice, cocaine-induced increases in DA and glutamate release were blunted. γ-Amino-butyric acid efflux was unaffected in the SR-/- mice. Together, these findings suggest that the impaired NMDAR function and a consequent decrease in sensitivity to cocaine effects on behavior are mediated by blunted DA and glutamate responses normally triggered by the drug. Projected to humans, NMDAR hypofunction due to mutations in SR or other genes impacting glutamatergic function in SZ may render abused substances less potent and effective, thus requiring higher doses to achieve a hedonic response, resulting in elevated drug exposure and increased dependence/addiction.

Schizophrenia, Not a Psychotic Disorder: Bleuler Revisited

Current diagnostic criteria delineate schizophrenia as a discrete entity essentially defined by positive symptoms. However, the role of positive symptoms in psychiatry is being questioned. There is compelling evidence that psychotic manifestations are expressed in the population in a continuum of varying degrees of severity, ranging from normality to full-blown psychosis. In most cases, these phenomena do not persist, but they constitute risk factors for psychiatric disorders in general. Psychotic symptoms are also present in most non-psychotic psychiatric diagnoses, being a marker of severity. Research revealed that hallucinations and delusions appear to have distinct, independent biological underpinnings-in the general population, in psychotic, and in non-psychotic disorders as well. On the other hand, negative symptoms were seen to be far more restricted to schizophrenia, have other underlying pathophysiology than positive symptoms, predict outcome and treatment response in schizophrenia, and start before the first psychotic outbreak. The current work discusses the concept of schizophrenia, suggesting that a greater emphasis should be put on cases where psychotic symptoms emerge in a premorbid subtly increasing negative/cognitive symptoms background. In those cases, psychosis would have a different course and outcome while psychosis occurring in the absence of such background deterioration would be more benign-probably having no, or a milder, underlying degenerative process. This reformulation should better drive psychopathological classification, face positive symptoms as epiphenomenon of the schizophrenia process, and dishevel stigma from schizophrenia and from delusions and hallucinations.

Glycine Transporters and Its Coupling with NMDA Receptors

Glycine plays two roles in neurotransmission. In caudal areas like the spinal cord and the brainstem, it acts as an inhibitory neurotransmitter, but in all regions of the CNS, it also works as a co-agonist with L-glutamate at N-methyl-D-aspartate receptors (NMDARs). The glycine fluxes in the CNS are regulated by two specific transporters for glycine, GlyT1 and GlyT2, perhaps with the cooperation of diverse neutral amino acid transporters like Asc-1 or SNAT5/SN2. While GlyT2 and Asc-1 are neuronal proteins, GlyT1 and SNAT5 are mainly astrocytic, although neuronal forms of GlyT1 also exist. GlyT1 has attracted considerable interest from the medical community and the pharmaceutical industry since compelling evidence indicates a clear association with the functioning of NMDARs, whose activity is decreased in various psychiatric illnesses. By controlling extracellular glycine, transporter inhibitors might potentiate the activity of NMDARs without activating excitotoxic processes. Physiologically, GlyT1 is a central actor in the cross talk between glutamatergic, glycinergic, dopaminergic, and probably other neurotransmitter systems. Many of these relationships begin to be unraveled by studies performed in recent years using genetic and pharmacological models. These studies are also clarifying the interactions between glycine, glycine transporters, and other co-agonists of the glycine site of NMDARs like D-serine. These findings are also relevant to understand the pathophysiology of devastating diseases like schizophrenia, depression, anxiety, epilepsy, stroke, and chronic pain.

Schizophrenia: synthetic strategies and recent advances in drug design

Schizophrenia is a complex and unpredictable mental disorder which affects several domains of cognition and behaviour. It is a heterogeneous illness characterised by positive, negative, and cognitive symptoms, often accompanied by signs of depression. In this tutorial review, we discuss recent progress in understanding the target sites and mechanisms of action of second-generation antipsychotic drugs. Progress in identifying and defining target sites has been accelerated recently by advances in neuroscience, and newly developed agents that regulate signalling by the main excitatory neurotransmitters in the brain are surveyed. Examples of novel molecules for the treatment of schizophrenia in preclinical and clinical development and their industrial sponsors are highlighted.

L-carnosine as an add-on to risperidone for treatment of negative symptoms in patients with stable schizophrenia: A double-blind, randomized placebo-controlled trial

Since l-carnosine has shown effectiveness in improvement of cognition in patients with schizophrenia, this 8-week, randomized, double-blind, placebo-controlled pilot study was conducted. Sixty-three patients with chronic schizophrenia, who were clinically stable on a stable dose of risperidone, entered the study. The patients were randomly assigned to l-carnosine (2 gr/day in two divided doses) or placebo for eight weeks. The patients were assessed using the positive and negative syndrome scale (PANSS), extrapyramidal symptom rating scale (ESRS), and Hamilton depression rating scale (HDRS) during the study course. Sixty patients completed the trial. L-carnosine resulted in greater improvement of negative scores as well as total PANSS scores but not positive subscale scores compared to placebo. HDRS scores and its changes did not differ between the two groups. Both groups demonstrated a constant ESRS score during the trial course. Frequency of other side effects was not significantly different between the two groups. In a multiple regression analysis model (controlled for positive, general psychopathology, depressive and extrapyramidal symptoms, as well as other variables), the treatment group significantly predicted changes in primary negative symptoms. In conclusion, l-carnosine add-on therapy can safely and effectively reduce the primary negative symptoms of patients with schizophrenia.

In Vivo Brain Glycine and Glutamate Concentrations in Patients With First-Episode Psychosis Measured by Echo Time-Averaged Proton Magnetic Resonance Spectroscopy at 4T

BACKGROUND

Accumulating evidence suggests the involvement of abnormal glutamateric neurotransmission and N-methyl-D-aspartate receptor hypofunction in the pathophysiology of psychotic disorders. The purpose of this study was to quantify in vivo glutamate (Glu) and glycine (Gly) levels in patients with first-episode psychosis as well as age-matched healthy control subjects with magnetic resonance spectroscopy (MRS).

METHODS

The subjects were 46 patients with first-episode psychosis (20 with a schizophrenia spectrum disorder, 26 with bipolar disorder) and 50 age-matched healthy control subjects. Glu and Gly levels were measured in vivo in the anterior cingulate cortex and posterior cingulate cortex of the subjects by using the echo time-averaged proton MRS technique at 4T (i.e., modified point resolved spectroscopy sequence: 24 echo time steps with 20-ms increments). Metabolite levels were quantified using LCModel with simulated basis sets.

RESULTS

Significantly higher Glu and Gly levels were found in both the anterior cingulate cortex and posterior cingulate cortex of patients with first-episode psychosis as compared with healthy control subjects. Glu and Gly levels were positively correlated in patients. Patients with a schizophrenia spectrum disorder and bipolar disorder showed similar abnormalities.

CONCLUSIONS

Our findings demonstrate abnormally elevated brain Glu and Gly levels in patients with first-episode psychosis by means of echo time-averaged proton MRS at 4T. The findings implicate dysfunction of N-methyl-D-aspartate receptor and glutamatergic neurotransmission in the pathophysiology of the acute early phase of psychotic illnesses.

In vivo gamma-aminobutyric acid and glutamate levels in people with first-episode schizophrenia: A proton magnetic resonance spectroscopy study

BACKGROUND

Gamma-aminobutyric acid (GABA) dysfunction and its consequent imbalance are implicated in the pathophysiology of schizophrenia. Reduced GABA production would lead to a disinhibition of glutamatergic neurons and subsequently cause a disruption of the modulation between GABAergic interneurons and glutamatergic neurons. In this study, levels of GABA, Glx (summation of glutamate and glutamine), and other metabolites in the anterior cingulate cortex were measured and compared between first-episode schizophrenia subjects and healthy controls (HC). Diagnostic potential of GABA and Glx as upstream biomarkers for schizophrenia was explored.

METHODS

Nineteen first-episode schizophrenia subjects and fourteen HC participated in this study. Severity of clinical symptoms of patients was measured with Positive and Negative Syndrome Scale (PANSS). Metabolites were measured using proton magnetic resonance spectroscopy, and quantified using internal water as reference.

RESULTS

First-episode schizophrenia subjects revealed reduced GABA and myo-inositol (mI), and increased Glx and choline (Cho), compared to HC. No significant correlation was found between metabolite levels and PANSS scores. Receiver operator characteristics analyses showed Glx had higher sensitivity and specificity (84.2%, 92.9%) compared to GABA (73.7%, 64.3%) for differentiating schizophrenia patients from HC. Combined model of both GABA and Glx revealed the best sensitivity and specificity (89.5%, 100%).

CONCLUSION

This study simultaneously showed reduction in GABA and elevation in Glx in first-episode schizophrenia subjects, and this might provide insights on explaining the disruption of modulation between GABAergic interneurons and glutamatergic neurons. Elevated Cho might indicate increased membrane turnover; whereas reduced mI might reflect dysfunction of the signal transduction pathway. In vivo Glx and GABA revealed their diagnostic potential for schizophrenia.

The effects of glycine on auditory mismatch negativity in schizophrenia

Glycine increases N-methyl-d-aspartate receptor (NMDAR) mediated glutamatergic function. Mismatch negativity (MMN) is a proposed biomarker of glutamate-induced improvements in clinical symptoms, however, the effect of glycine-mediated NMDAR activation on MMN in schizophrenia is not well understood. This study aimed to determine the effects of acute and 6-week chronic glycine administration on MMN in schizophrenia patients. MMN amplitude was compared at baseline between 22 patients (schizophrenia or schizoaffective disorder; receiving stable antipsychotic medication; multi-centre recruitment) and 21 age- and gender-matched controls. Patients underwent a randomised, double-blind, placebo-controlled clinical trial with glycine added to their regular antipsychotic medication (placebo, n=10; glycine, n=12). MMN was reassessed post-45-minutes of first dose (0.2g/kg) and post-6-weeks treatment (incremented to 0.6g/kg/day). Clinical symptoms were assessed at baseline and post-6-weeks treatment. At baseline, duration MMN was smaller in schizophrenia compared to controls. Acute glycine increased duration MMN (compared to placebo), whilst this difference was absent post-6-weeks treatment. Six weeks of chronic glycine administration improved PANSS-Total, PANSS-Negative and PANSS-General symptoms compared to placebo. Smaller baseline duration MMN was associated with greater PANSS-Negative symptoms and predicted (at trend level) PANSS-Negative symptom improvement post-6-weeks glycine treatment (not placebo). These findings support the benefits of chronic glycine administration and demonstrate, for the first time, that acute glycine improves duration MMN in schizophrenia. This result, together with smaller baseline duration MMN predicting greater clinical treatment response, suggests the potential for duration MMN as a biomarker of glycine-induced improvements in negative symptoms in schizophrenia.

The Emerging Role of Altered d-Aspartate Metabolism in Schizophrenia: New Insights From Preclinical Models and Human Studies

Besides d-serine, another d-amino acid with endogenous occurrence in the mammalian brain, d-aspartate, has been recently shown to influence NMDA receptor (NMDAR)-mediated transmission. d-aspartate is present in the brain at extracellular level in nanomolar concentrations, binds to the agonist site of NMDARs and activates this subclass of glutamate receptors. Along with its direct effect on NMDARs, d-aspartate can also evoke considerable l-glutamate release in specific brain areas through the presynaptic activation of NMDA, AMPA/kainate and mGlu5 receptors. d-aspartate is enriched in the embryonic brain of rodents and humans and its concentration strongly decreases after birth, due to the post-natal expression of the catabolising enzyme d-aspartate oxidase (DDO). Based on the hypothesis of NMDAR hypofunction in schizophrenia pathogenesis, recent preclinical and clinical studies suggested a relationship between perturbation of d-aspartate metabolism and this psychiatric disorder. Consistently, neurophysiological and behavioral characterization of Ddo knockout (Ddo ^-/-) and d-aspartate-treated mice highlighted that abnormally higher endogenous d-aspartate levels significantly increase NMDAR-mediated synaptic plasticity, neuronal spine density and memory. Remarkably, increased d-aspartate levels influence schizophrenia-like phenotypes in rodents, as indicated by improved fronto-hippocampal connectivity, attenuated prepulse inhibition deficits and reduced activation of neuronal circuitry induced by phencyclidine exposure. In healthy humans, a genetic polymorphism associated with reduced prefrontal DDO gene expression predicts changes in prefrontal phenotypes including greater gray matter volume and enhanced functional activity during working memory. Moreover, neurochemical detections in post-mortem brain of schizophrenia-affected patients have shown significantly reduced d-aspartate content in prefrontal regions, associated with increased DDO mRNA expression or DDO enzymatic activity. Overall, these findings suggest a possible involvement of dysregulated embryonic d-aspartate metabolism in schizophrenia pathophysiology and, in turn, highlight the potential use of free d-aspartate supplementation as a new add-on therapy for treating the cognitive symptoms of this mental illness.

Possible role of rare variants in Trace amine associated receptor 1 in schizophrenia

Schizophrenia (SZ) is a chronic mental illness with behavioral abnormalities. Recent common variant based genome wide association studies and rare variant detection using next generation sequencing approaches have identified numerous variants that confer risk for SZ, but etiology remains unclear propelling continuing investigations. Using whole exome sequencing, we identified a rare heterozygous variant (c.545G>T; p.Cys182Phe) in Trace amine associated receptor 1 gene (TAAR1 6q23.2) in three affected members in a small SZ family. The variant predicted to be damaging by 15 prediction tools, causes breakage of a conserved disulfide bond in this G-protein-coupled receptor. On screening this intronless gene for additional variant(s) in ~800 sporadic SZ patients, we identified six rare protein altering variants (MAF<0.001) namely p.Ser47Cys, p.Phe51Leu, p.Tyr294Ter, p.Leu295Ser in four unrelated north Indian cases (n=475); p.Ala109Thr and p.Val250Ala in two independent Caucasian/African-American patients (n=310). Five of these variants were also predicted to be damaging. Besides, a rare synonymous variant was observed in SZ patients. These rare variants were absent in north Indian healthy controls (n=410) but significantly enriched in patients (p=0.036). Conversely, three common coding SNPs (rs8192621, rs8192620 and rs8192619) and a promoter SNP (rs60266355) tested for association with SZ in the north Indian cohort were not significant (P>0.05). TAAR1 is a modulator of monoaminergic pathways and interacts with AKT signaling pathways. Substantial animal model based pharmacological and functional data implying its relevance in SZ are also available. However, this is the first report suggestive of the likely contribution of rare variants in this gene to SZ.

Understanding taurine CNS activity using alternative zebrafish models

Taurine is a highly abundant "amino acid" in the brain. Despite the potential neuroactive role of taurine in vertebrates has long been recognized, the underlying molecular mechanisms related to its pleiotropic effects in the brain remain poorly understood. Due to the genetic tractability, rich behavioral repertoire, neurochemical conservation, and small size, the zebrafish (Danio rerio) has emerged as a powerful candidate for neuropsychopharmacology investigation and in vivo drug screening. Here, we summarize the main physiological roles of taurine in mammals, including neuromodulation, osmoregulation, membrane stabilization, and antioxidant action. In this context, we also highlight how zebrafish models of brain disorders may present interesting approaches to assess molecular mechanisms underlying positive effects of taurine in the brain. Finally, we outline recent advances in zebrafish drug screening that significantly improve neuropsychiatric translational researches and small molecule screens.

An overview on D-amino acids

More than half a century ago researchers thought that D-amino acids had a minor function compared to L-enantiomers in biological processes. Many evidences have shown that D-amino acids are present in high concentration in microorganisms, plants, mammals and humans and fulfil specific biological functions. In the brain of mammals, D-serine (D-Ser) acts as a co-agonist of the N-methyl-D-aspartate (NMDA)-type glutamate receptors, responsible for learning, memory and behaviour. D-Ser metabolism is relevant for disorders associated with an altered function of the NMDA receptor, such as schizophrenia, ischemia, epilepsy and neurodegenerative disorders. On the other hand, D-aspartate (D-Asp) is one of the major regulators of adult neurogenesis and plays an important role in the development of endocrine function. D-Asp is present in the neuroendocrine and endocrine tissues and testes, and regulates the synthesis and secretion of hormones and spermatogenesis. Also food proteins contain D-amino acids that are naturally originated or processing-induced under conditions such as high temperatures, acid and alkali treatments and fermentation processes. The presence of D-amino acids in dairy products denotes thermal and alkaline treatments and microbial contamination. Two enzymes are involved in the metabolism of D-amino acids: amino acid racemase in the synthesis and D-amino acid oxidase in the degradation.

Metabolomics and lipidomics analyses by 1H nuclear magnetic resonance of schizophrenia patient serum reveal potential peripheral biomarkers for diagnosis

Using ¹H NMR-based metabolomics in association to chemometrics analysis, we analyzed here the metabolic differences between schizophrenia patients (SCZ) compared to healthy controls (HCs). HCs and SCZ patients underwent clinical interview using the Structured Clinical Interview for DSM Disorders (SCID). SCZ patients were further assessed by Positive and Negative Syndrome Scale (PANSS), Calgary Depression Scale, Global Assessment of Functioning Scale (GAF), and Clinical Global Impressions Scale (CGI). Using the principal component analysis (PCA) and supervised partial least-squares discriminate analysis (PLS-DA) in obtained NMR data, a clear group separation between HCs and SCZ patients was achieved. Interestingly, all metabolite compounds identified as exclusively present in the SCZ group, except for the gamma-aminobutyric acid (GABA), were never previously associated with mental disorders. Although the initial perception of an absence of obvious biological link among the different key molecules exclusively observed in each group, and no identification of any specific pathway yet, the present work represents an important contribution for the identification of potential biomarkers to inform diagnosis, as it was possible to completely separate the affected SCZ patients from HCs, with no outliers or exceptions. In addition, the data presented here reinforced the role of the modulation of glycolysis pathway and the loss of GABA interneuron/hyperglutamate hypothesis in SCZ.

Synaptic NMDA Receptor Activation Induces Ubiquitination and Degradation of STEP61

NMDA receptor signaling is critical for the development of synaptic plasticity, learning, and memory, and dysregulation of NMDAR signaling is implicated in a number of neurological disorders including schizophrenia (SZ). Previous work has demonstrated that the STriatal-Enriched protein tyrosine Phosphatase 61 kDa (STEP₆₁) is elevated in human SZ postmortem cortical samples and after administration of psychotomimetics to cultures or mice. Here, we report that activation of synaptic NMDAR by bicuculline or D-serine results in the ubiquitination and proteasomal degradation of STEP₆₁, and increased surface localization of GluN1/GluN2B receptors. Moreover, bicuculline or D-serine treatments rescue the motor and cognitive deficits in MK-801-treated mice and reduce STEP₆₁ in mouse frontal cortex. These results suggest that STEP₆₁ may contribute to the therapeutic effects of D-serine.

Neurometabolic abnormalities in schizophrenia and depression observed with magnetic resonance spectroscopy at 7 T

BACKGROUND

Examining neurometabolic abnormalities in critical brain areas in schizophrenia and major depressive disorder (MDD) may help guide future pharmacological interventions including glutamate-modulating treatments.

AIMS

To measure metabolite concentrations within the anterior cingulate cortex (ACC) and thalamus of people with schizophrenia and people with MDD.

METHODS

Spectra were acquired from 16 volunteers with schizophrenia, 17 with MDD and 18 healthy controls using magnetic resonance spectroscopy on a 7 Tesla scanner.

RESULTS

In the thalamus, there were lower glycine concentrations in the schizophrenia group relative to control (P=0.017) and MDD groups (P=0.012), and higher glutamine concentrations relative to healthy controls (P=0.009). In the thalamus and the ACC, the MDD group had lower myo-inositol concentrations than the control (P=0.014, P=0.009, respectively) and schizophrenia (P=0.004, P=0.002, respectively) groups.

CONCLUSION

These results support the glutamatergic theory of schizophrenia and indicate a potential glycine deficiency in the thalamus. In addition, reduced myo-inositol concentrations in MDD suggest its involvement in the disorder.

DECLARATION OF INTEREST

None.

COPYRIGHT AND USAGE

© The Royal College of Psychiatrists 2017. This is an open access article distributed under the terms of the Creative Commons Non-Commercial, No Derivatives (CC BY-NC-ND) license.

Supplementation with D-serine prevents the onset of cognitive deficits in adult offspring after maternal immune activation

Prenatal maternal infection contributes to the etiology of schizophrenia, with D-serine, an endogenous co-agonist of the N-methyl-D-aspartate (NMDA) receptor, playing a role in the pathophysiology of this disease. We examined whether supplementation with D-serine during juvenile and adolescent stages could prevent the onset of cognitive deficits, prodromal and the core symptoms of schizophrenia in adult offspring after maternal immune activation (MIA). Juvenile offspring exposed prenatally to poly(I:C) showed reduced expression of NMDA receptor subunits in the hippocampus. Supplementing drinking water with D-serine (600 mg/L from P28 to P56) prevented the onset of cognitive deficits in adult offspring after MIA, in a significant manner. This study shows that supplementing offspring with D-serine during juvenile and adolescent stages could prevent the onset of psychosis in adulthood, after MIA. Therefore, early intervention with D-serine may prevent the occurrence of psychosis in high-risk subjects.

New Pharmacotherapy Targeting Cognitive Dysfunction of Schizophrenia via Modulation of GABA Neuronal Function

Schizophrenia is considered a neurodevelopmental and neurodegenerative disorder. Cognitive impairment is a core symptom in patients with the illness, and has been suggested a major predictor of functional outcomes. Reduction of parvalbumin (PV)-positive γ-aminobutyric acid (GABA) interneurons has been associated with the pathophysiology of schizophrenia, in view of the link between the abnormality of GABA neurons and cognitive impairments of the disease. It is assumed that an imbalance of excitatory and inhibitory (E-I) activity induced by low activity of glutamatergic projections and PV-positive GABA interneurons in the prefrontal cortex resulted in sustained neural firing and gamma oscillation, leading to impaired cognitive function. Therefore, it is important to develop novel pharmacotherapy targeting GABA neurons and their activities. Clinical evidence suggests serotonin (5-HT) 1A receptor agonist improves cognitive disturbances of schizophrenia, consistent with results from preclinical studies, through mechanism that corrects E-I imbalance via the suppression of GABA neural function. On the other hand, T-817MA, a novel neurotrophic agent, ameliorated loss of PV-positive GABA neurons in the medial prefrontal cortex and reduction of gamma-band activity, as well as cognitive dysfunction in animal model of schizophrenia. In conclusion, a pharmacotherapy to alleviate abnormalities in GABA neurons through 5-HT1A agonists and T-817MA is expected to prevent the onset and/or progression of schizophrenia.

Protons Potentiate GluN1/GluN3A Currents by Attenuating Their Desensitisation

N-methyl-D-aspartate (NMDA) receptors are glutamate- and glycine-gated channels composed of two GluN1 and two GluN2 or/and GluN3 subunits. GluN3A expression is developmentally regulated, and changes in this normal pattern of expression, which occur in several brain disorders, alter synaptic maturation and function by unknown mechanisms. Uniquely within the NMDA receptor family, GluN1/GluN3 receptors produce glycine-gated deeply desensitising currents that are insensitive to glutamate and NMDA; these currents remain poorly characterised and their cellular functions are unknown. Here, we show that extracellular acidification strongly potentiated glycine-gated currents from recombinant GluN1/GluN3A receptors, with half-maximal effect in the physiologic pH range. This was largely due to slower current desensitisation and faster current recovery from desensitisation, and was mediated by residues facing the heterodimer interface of the ligand-binding domain. Consistent with the observed changes in desensitisation kinetics, acidic shifts increased the GluN1/GluN3A equilibrium current and depolarized the membrane in a glycine concentration-dependent manner. These results reveal novel modulatory mechanisms for GluN1/GluN3A receptors that further differentiate them from the canonical glutamatergic GluN1/GluN2 receptors and provide a new and potent pharmacologic tool to assist the detection, identification, and the further study of GluN1/GluN3A currents in native preparations.

Are there glutamate abnormalities in subjects at high risk mental state for psychosis? A review of the evidence

New approaches to underlying alterations in psychosis suggest increasing evidence of glutamatergic abnormalities in schizophrenia and an association between these abnormalities and certain core psychopathological alterations such as cognitive impairment and negative symptoms. Proton magnetic resonance spectroscopy ((1)H MRS) is an MR-based technique that enables investigators to study glutamate function by measuring in vivo glutamatergic indices in the brain. In this article we review the published studies of (1)H MRS in subjects with an at-risk mental state (ARMS) for psychosis. The primary aim was to investigate whether alterations in glutamate function are present before the illness develops in order to expand our understanding of glutamatergic abnormalities in prodromal phases. Three databases were consulted for this review. Titles and abstracts were examined to determine if they fulfilled the inclusion criteria. The reference lists of the included studies were also examined to identify additional trials. Eleven final studies were included in this review. Significant alterations in glutamate metabolites across different cerebral areas (frontal lobe, thalamus, and the associative striatum) in subjects with an ARMS for psychosis are reported in six of the trials. A longitudinal analysis in two of these trials confirmed an association between these abnormalities and worsening of symptoms and final transition to psychosis. Considering that five other studies found no significant differences across these same areas, we can conclude that more research is needed to confirm glutamatergic abnormalities in subjects with an ARMS for psychosis. However, future research must overcome the methodological limitations of existing studies to obtain reliable results.

Brexpiprazole: so far so good

This article describes the role of a newly approved antipsychotic agent brexpiprazole in the treatment of schizophrenia and major depressive disorder. This drug has high affinity for 5-HT1A, 5-HT2A, D2 and α1B,2C receptors. It displays partial agonism at 5-HT1A and D2 receptors and potent antagonism at 5-HT2A and α1B,2C adrenergic receptors. It also has some affinity (antagonism) for D3, 5-HT2B, 5-HT7 and α1A,1D receptors, and moderate affinity for H1 and low affinity for M1 receptors. These all lead to a favorable antipsychotic profile in terms of improvement of cognitive performance and sleep patterns, as well as effects on affective states and potential to treat core symptoms in schizophrenia and major depressive disorder, including cognitive deficits with a low risk of adverse effects (extrapyramidal symptoms, metabolic complications, weight gain, akathisia potential) that are commonly encountered with other typical and second-generation antipsychotic drugs. In our review, we have made an attempt to decipher the pharmacological profile of brexpiprazole from two major trials (VECTOR and BEACON). We have also tried to give a concise but detailed overview of brexpiprazole by head to head comparison of the pharmacological profile of brexpiprazole and its earlier congeners aripiprazole and prototype antipsychotic drug chlorpromazine by accessing individual summaries of product characteristics from the US Food and Drug Administration database, 2015. Relevant preclinical and clinical studies associated with this drug have been discussed with emphasis on efficacy and safety concerns. From the studies done so far, it can be concluded that brexpiprazole can be an effective monotherapy for schizophrenia and as an adjunct to other antidepressant medications in major depressive disorder.

Astroglia, Glutamatergic Transmission and Psychiatric Diseases

Astrocytes are primary homeostatic cells of the central nervous system. They regulate glutamatergic transmission through the removal of glutamate from the extracellular space and by supplying neurons with glutamine. Glutamatergic transmission is generally believed to be significantly impaired in the contexts of all major neuropsychiatric diseases. In most of these neuropsychiatric diseases, astrocytes show signs of degeneration and atrophy, which is likely to be translated into reduced homeostatic capabilities. Astroglial glutamate uptake/release and glutamate homeostasis are affected in all forms of major psychiatric disorders and represent a common mechanism underlying neurotransmission disbalance, aberrant connectome and overall failure on information processing by neuronal networks, which underlie pathogenesis of neuropsychiatric diseases.

Glycine transporter 1 is a target for the treatment of epilepsy

Glycine is the major inhibitory neurotransmitter in brainstem and spinal cord, whereas in hippocampus glycine exerts dual modulatory roles on strychnine-sensitive glycine receptors and on the strychnine-insensitive glycineB site of the N-methyl-D-aspartate receptor (NMDAR). In hippocampus, the synaptic availability of glycine is largely under control of glycine transporter 1 (GlyT1). Since epilepsy is a disorder of disrupted network homeostasis affecting the equilibrium of various neurotransmitters and neuromodulators, we hypothesized that changes in hippocampal GlyT1 expression and resulting disruption of glycine homeostasis might be implicated in the pathophysiology of epilepsy. Using two different rodent models of temporal lobe epilepsy (TLE)--the intrahippocampal kainic acid model of TLE in mice, and the rat model of tetanic stimulation-induced TLE--we first demonstrated robust overexpression of GlyT1 in the hippocampal formation, suggesting dysfunctional glycine signaling in epilepsy. Overexpression of GlyT1 in the hippocampal formation was corroborated in human TLE samples by quantitative real time PCR. In support of a role of dysfunctional glycine signaling in the pathophysiology of epilepsy, both the genetic deletion of GlyT1 in hippocampus and the GlyT1 inhibitor LY2365109 increased seizure thresholds in mice. Importantly, chronic seizures in the mouse model of TLE were robustly suppressed by systemic administration of the GlyT1 inhibitor LY2365109. We conclude that GlyT1 overexpression in the epileptic brain constitutes a new target for therapeutic intervention, and that GlyT1 inhibitors constitute a new class of antiictogenic drugs. These findings are of translational value since GlyT1 inhibitors are already in clinical development to treat cognitive symptoms in schizophrenia.

Metabotropic glutamate receptors as targets for new antipsychotic drugs: Historical perspective and critical comparative assessment

In this review, we aim to present, discuss and clarify our current understanding regarding the prediction of possible antipsychotic effects of metabotropic glutamate (mGlu) receptor ligands. The number of preclinical trials clearly indicates, that this group of compounds constitutes an excellent alternative to presently used antipsychotic therapy, being effective not only to positive, but also negative and cognitive symptoms of schizophrenia. Although the results of clinical trials that were performed for the group of mGlu2/3 agonists were not so enthusiastic as in animal studies, they still showed that mGlu ligands do not induced variety of side effects typical for presently used antipsychotics, and were generally well tolerated. The lack of satisfactory effectiveness towards schizophrenia symptoms of mGlu2/3 activators in humans could be a result of variety of uncontrolled factors and unidentified biomarkers different for each schizophrenia patient, that should be taken into consideration in the future set of clinical trials. The subject is still open for further research, and the novel classes of mGlu5 or mGlu2/3 agonists/PAMs were recently introduced, including the large group of compounds from the third group of mGlu receptors, especially of mGlu4 subtype. Finally, more precise treatment based on simultaneous administration of minimal doses of the ligands for two or more receptors, seems to be promising in the context of symptoms-specific schizophrenia treatment.

Pharmacological treatment of negative symptoms in schizophrenia

Effective treatment of negative symptoms is one of the most important unmet needs in schizophrenic disorders. Because the evidence on current psychopharmacological treatments is unclear, the authors reviewed the findings published to date by searching PubMed with the keywords negative symptoms, antipsychotics, antidepressants, glutamatergic compounds, monotherapy and add-on therapy and identifying additional articles in the reference lists of the resulting publications. The findings presented here predominantly focus on results of meta-analyses. Evidence for efficacy of current psychopharmacological medications is difficult to assess because of methodological problems and inconsistent results. In general, the second-generation antipsychotics (SGAs) do not appear to have good efficacy in negative symptoms, although some show better efficacy than first-generation antipsychotics, some of which also demonstrated efficacy in negative symptoms. Specific trials on predominant persistent negative symptoms are rare and have been performed with only a few SGAs. More often, trials on somewhat persistent negative symptoms evaluate add-on strategies to ongoing antipsychotic treatment. Such trials, mostly on modern antidepressants, have demonstrated some efficacy. Several trials with small samples have evaluated add-on treatment with glutamatergic compounds, such as the naturally occurring amino acids glycine and D-serine and new pharmacological compounds. The results are highly inconsistent, although overall efficacy results appear to be positive. The unsatisfactory and inconsistent results can be partially explained by methodological problems. These problems need to be solved in the future, and the authors propose some possible solutions. Further research is required to identify effective treatment for the negative symptoms of schizophrenia.

Functional magnetic resonance spectroscopy of glutamate in schizophrenia and major depressive disorder: anterior cingulate activity during a color-word Stroop task

Background:

Glutamate abnormalities have been suggested to be associated with symptoms of schizophrenia. Using functional magnetic resonance spectroscopy (¹H-fMRS), it is possible to monitor glutamate dynamically in the activated brain areas, which has yet to be reported in schizophrenia. It was hypothesized that subjects with schizophrenia would have weaker glutamatergic responses in the anterior cingulate to a color-word Stroop Task.

AIMS:

The aim of this study was to gain insight into the health of GLU neurotransmission and the GLU-GLN cycle in SZ using a ¹H-fMRS protocol.

Methods:

Spectra were acquired from the anterior cingulate of 16 participants with schizophrenia, 16 healthy controls and 16 participants with major depressive disorder (MDD) while performing the Stroop task in a 7T magnetic resonance imaging scanner. ¹H-fMRS spectra were acquired for 20 min in which there were three 4-min blocks of cross fixation interleaved with two 4-min blocks of the Stroop paradigm.

Results:

A repeated-measures analysis of variance revealed a main effect of time for glutamate concentrations of all groups (P<0.001). The healthy control group increased glutamate concentrations in the first run of the Stroop task (P=0.006) followed by a decrease in the recovery period (P=0.007). Neither the schizophrenia (P=0.107) nor MDD (P=0.081) groups had significant glutamate changes in the first run of the task, while the schizophrenia group had a significant increase in glutamine (P=0.005). The MDD group decreased glutamate concentrations in the second run of the task (P=0.003), as did all the groups combined (P=0.003).

Conclusions:

¹H-fMRS data were successfully acquired from psychiatric subjects with schizophrenia and mood disorder using a cognitive paradigm for the first time. Future study designs should further elucidate the glutamatergic response to functional activation in schizophrenia.

Efficacy of a glycine transporter 1 inhibitor TASP0315003 in animal models of cognitive dysfunction and negative symptoms of schizophrenia

RATIONALE

Since the hypofunction of the N-methyl-D-aspartate (NMDA) receptor is known to be involved in the pathophysiology of schizophrenia, the enhancement of NMDA receptor function through glycine modulatory sites is expected to be a useful approach for the treatment of schizophrenia.

OBJECTIVES

We investigated the efficacy of a glycine transporter 1 (GlyT1) inhibitor that potentiates NMDA receptor function by increasing synaptic glycine levels in animal models for cognitive dysfunction and negative symptoms, both of which are poorly managed by current antipsychotics.

RESULTS

A newly synthesized GlyT1 inhibitor, 3-chloro-N-{(S)-[3-(1-ethyl-1H-pyrazol-4-yl)phenyl][(2S)-piperidin-2-yl]methyl}-4-(trifluoromethyl)pyridine-2-carboxamide (TASP0315003) significantly improved cognitive deficit induced by MK-801 in the object recognition test in rats. Likewise, TASP0315003 significantly improved MK-801 impaired cognition in the social recognition test in rats and also enhanced social memory in treatment-naïve rats. In addition, repeated phencyclidine (PCP) treatment reduced the social interaction of paired mice, which may reflect negative symptoms such as social withdrawal, and both acute and sub-chronic treatment with TASP0315003 reversed the reduction in social interaction induced by PCP. Moreover, TASP0315003 additionally exhibited an antidepressant effect in the forced swimming test in rats. In contrast, TASP0315003 did not affect spontaneous locomotor activity or rotarod performance and did not induce catalepsy, indicating that TASP0315003 does not cause sedation or motor dysfunction, which is sometimes observed with the use of current antipsychotics.

CONCLUSIONS

These results suggest that GlyT1 inhibitors including TASP0315003 may be useful for the treatment of cognitive dysfunction and the negative symptoms of schizophrenia without having undesirable central nervous system side effects.

Glycine-dependent activation of NMDA receptors

Results from single-molecule and macroscopic electrophysiology and kinetic analysis provide a model for activation of the glutamate-bound NMDA receptor by glycine.

N-methyl-d-aspartate (NMDA) receptors are the only neurotransmitter receptors whose activation requires two distinct agonists. Heterotetramers of two GluN1 and two GluN2 subunits, NMDA receptors are broadly distributed in the central nervous system, where they mediate excitatory currents in response to synaptic glutamate release. Pore opening depends on the concurrent presence of glycine, which modulates the amplitude and time course of the glutamate-elicited response. Gating schemes for fully glutamate- and glycine-bound NMDA receptors have been described in sufficient detail to bridge the gap between microscopic and macroscopic receptor behaviors; for several receptor isoforms, these schemes include glutamate-binding steps. We examined currents recorded from cell-attached patches containing one GluN1/GluN2A receptor in the presence of several glycine-site agonists and used kinetic modeling of these data to develop reaction schemes that include explicit glycine-binding steps. Based on the ability to match a series of experimentally observed macroscopic behaviors, we propose a model for activation of the glutamate-bound NMDA receptor by glycine that predicts apparent negative agonist cooperativity and glycine-dependent desensitization in the absence of changes in microscopic binding or desensitization rate constants. These results complete the basic steps of an NMDA receptor reaction scheme for the GluN1/GluN2A isoform and prompt a reevaluation of how glycine controls NMDA receptor activation. We anticipate that our model will provide a useful quantitative instrument to further probe mechanisms and structure–function relationships of NMDA receptors and to better understand the physiological and pathological implications of endogenous fluctuations in extracellular glycine concentrations.

The sleep-promoting and hypothermic effects of glycine are mediated by NMDA receptors in the suprachiasmatic nucleus

The use of glycine as a therapeutic option for improving sleep quality is a novel and safe approach. However, despite clinical evidence of its efficacy, the details of its mechanism remain poorly understood. In this study, we investigated the site of action and sleep-promoting mechanisms of glycine in rats. In acute sleep disturbance, oral administration of glycine-induced non-rapid eye movement (REM) sleep and shortened NREM sleep latency with a simultaneous decrease in core temperature. Oral and intracerebroventricular injection of glycine elevated cutaneous blood flow (CBF) at the plantar surface in a dose-dependent manner, resulting in heat loss. Pretreatment with N-methyl-D-aspartate (NMDA) receptor antagonists AP5 and CGP78608 but not the glycine receptor antagonist strychnine inhibited the CBF increase caused by glycine injection into the brain. Induction of c-Fos expression was observed in the hypothalamic nuclei, including the medial preoptic area (MPO) and the suprachiasmatic nucleus (SCN) shell after glycine administration. Bilateral microinjection of glycine into the SCN elevated CBF in a dose-dependent manner, whereas no effect was observed when glycine was injected into the MPO and dorsal subparaventricular zone. In addition, microinjection of D-serine into the SCN also increased CBF, whereas these effects were blocked in the presence of L-701324. SCN ablation completely abolished the sleep-promoting and hypothermic effects of glycine. These data suggest that exogenous glycine promotes sleep via peripheral vasodilatation through the activation of NMDA receptors in the SCN shell.

D-serine uptake and release in PC-12 cells measured by chiral microchip electrophoresis-mass spectrometry

Previous work has established that D-serine (D-Ser) plays important roles in certain neurological processes. Study on its uptake/storage and release by neuronal cells is highly significant for elucidating relevant mechanisms. In this work, PC-12 cells were incubated with racemic Ser (100 μM each enantiomer). After incubation, both intra- and extracellular levels of D-Ser and L-Ser were quantified by chiral microchip electrophoresis with mass spectrometric detection. It was found the cells preferably took up D-Ser over L-Ser. After 120 min incubation, D-Ser percentage ([D-Ser]/([D-Ser] + [L-Ser]) in the culture media changed from 50% to 9% while inside the cells it increased from 13% to 67%. Small neutral amino acids such as threonine impaired D-Ser uptake. Ser release was studied by using PC-12 cells preloaded with D-Ser. KCl, Glu, and Gly evoked Ser release. Interestingly, while depolarization by KCl evoked release of Ser as a D-Ser/L-Ser mixture of 1:1 ratio, the stereoisomeric composition of Ser released due to Glu exposure varied with the exposure time, ranging from 73% D-Ser (i.e., [D-Ser] > [L-Ser]) at 2 min to 44% (i.e., [D-Ser] < [L-Ser]) at 14 min, clearly indicating a stereochemical preference for D-Ser in Ser release from neuronal cells evoked by Glu-receptor activation.

In vivo assessment of neurotransmitters and modulators with magnetic resonance spectroscopy: application to schizophrenia

In vivo measurement of neurotransmitters and modulators is now feasible with advanced proton magnetic resonance spectroscopy ((1)H MRS) techniques. This review provides a basic tutorial of MRS, describes the methods available to measure brain glutamate, glutamine, γ-aminobutyric acid, glutathione, N-acetylaspartylglutamate, glycine, and serine at magnetic field strengths of 3T or higher, and summarizes the neurochemical findings in schizophrenia. Overall, (1)H MRS holds great promise for producing biomarkers that can serve as treatment targets, prediction of disease onset, or illness exacerbation in schizophrenia and other brain diseases.