Abnormal serine-glycine metabolism in the brains of schizophrenics

Variations of blood D-serine and D-aspartate homeostasis track psychosis stages

Schizophrenia (SCZ) is a severe psychotic disorder characterized by a disruption in glutamatergic NMDA receptor (NMDAR)-mediated neurotransmission. Compelling evidence has revealed that NMDAR activation is not limited to L-glutamate, L-aspartate, and glycine since other free amino acids (AAs) in the atypical D-configuration, such as D-aspartate and D-serine, also modulate this class of glutamatergic receptors. Although dysregulation of AAs modulating NMDARs has been previously reported in SCZ, it remains unclear whether distinct variations of these biomolecules occur during illness progression from at-risk premorbid to clinically manifest stage. To probe this issue, we used High-Performance Liquid Chromatography (HPLC) to measure serum levels of D- and L-AAs that stimulate NMDARs across four groups of individuals diagnosed with (a) At-Risk Mental State (ARMS) for psychosis, (b) First Episode of Psychosis (FEP), (c) full-blown SCZ and (d) Healthy Donors (HD). We examined how diagnosis, demographic features, and antipsychotic treatment influence the variation of AA levels throughout psychosis progression. Finally, we explored the potential association between AA blood concentrations and clinical and cognitive measures related to psychosis. Our findings identified inter-group differences in serum AA composition, highlighting that the upregulation of D-serine/total serine and D-aspartate/total aspartate ratios represent a peculiar blood biochemical signature of early stages of psychosis progression, while increased L-glutamate, L-aspartate and glycine associate with chronic SCZ diagnosis. The present findings provide direct evidence for early dysregulation of D-AA metabolism and have potential implications for the identification of biomarkers for the early detection and staging of psychosis.

Association study of the excitatory amino acid transporter 2 (EAAT2) and glycine transporter 1 (GlyT1) gene polymorphism with schizophrenia in a Polish population

Background: Excitatory amino acid transporter 2 encoded by SLC1A2 is responsible for approximately 90% of glutamate uptake. Glycine transporter 1, encoded by SLC6A9, is responsible for maintaining a low concentration of the N-methyl-D-aspartate receptor (NMDAR) co-agonist - glycine in the synaptic cleft, suggesting its participation in the development of the NMDARs hypofunction described in schizophrenia. Aim: The aim of this study was to evaluate whether the functional polymorphism-181 A/C (rs4354668) of the SLC1A2 and the rs2486001 (IVS3+411 G/A) in the SLC6A9 are involved in schizophrenia development and its clinical picture in the Polish population. Methods: The study group consisted of 393 unrelated Caucasian patients (157 [39.9%] females and 236 [60.1%] males; mean age 41±12) diagnosed with schizophrenia according to the DSM-5, and 462 healthy controls. The results of the Positive and Negative Syndrome Scale (PANSS) were presented in the five-dimensional model. Polymorphisms of SLC1A2 and SLC6A9 were genotyped with the use of PCR-RFLP assay. Results: There were no statistically significant differences in the frequency of genotypes and alleles between the patients and controls for SLC1A2 and SLC6A9 polymorphisms in either the entire sample or after stratification according to gender. In the haplotype analysis, men with CA haplotype had more than 1.5 higher risk to develop schizophrenia than women (OR=1.63 [95% CI=1.17-2.27, p<0.05]). The influence of gender, genotypes of both analyzed polymorphisms and gender x genotype interactions on individual dimensions of the PANSS scale has not been observed. Also, there was no association of either polymorphism with suicide attempts. Conclusion: The results of the present study did not indicate an association of polymorphism-181 A/C (rs4354668) in SLC1A2 and rs2486001 in SLC6A9 with onset of schizophrenia and its psychopathology in a Polish population.

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.

Allosteric modulators for the treatment of schizophrenia: targeting glutamatergic networks

Schizophrenia is a highly debilitating mental disorder which afflicts approximately 1% of the global population. Cognitive and negative deficits account for the lifelong disability associated with schizophrenia, whose symptoms are not effectively addressed by current treatments. New medicines are needed to treat these aspects of the disease. Neurodevelopmental, neuropathological, genetic, and behavioral pharmacological data indicate that schizophrenia stems from a dysfunction of glutamate synaptic transmission, particularly in frontal cortical networks. A number of novel pre- and postsynaptic mechanisms affecting glutamatergic synaptic transmission have emerged as viable targets for schizophrenia. While developing orthosteric glutamatergic agents for these targets has proven extremely difficult, targeting allosteric sites of these targets has emerged as a promising alternative. From a medicinal chemistry perspective, allosteric sites provide an opportunity of finding agents with better drug-like properties and greater target specificity. Furthermore, allosteric modulators are better suited to maintaining the highly precise temporal and spatial aspects of glutamatergic synaptic transmission. Herein, we review neuropathological and genomic/genetic evidence underscoring the importance of glutamate synaptic dysfunction in the etiology of schizophrenia and make a case for allosteric targets for therapeutic intervention. We review progress in identifying allosteric modulators of AMPA receptors, NMDA receptors, and metabotropic glutamate receptors, all with the aim of restoring physiological glutamatergic synaptic transmission. Challenges remain given the complexity of schizophrenia and the difficulty in studying cognition in animals and humans. Nonetheless, important compounds have emerged from these efforts and promising preclinical and variable clinical validation has been achieved.

One-carbon metabolism in psychiatric illness

The cost of psychiatric illness to the UK economy was recently estimated at pound77 billion annually. Despite years of research no firm aetiological explanation exists, and with no physiological or biochemical markers diagnosis is made entirely on a behavioural basis. All current pharmacological therapies are associated with serious long-term side effects. Substantial evidence supports the involvement of one-carbon cycle dysregulation in psychiatric illness, but this is not currently used as a basis for diagnosis or treatment. The present paper reviews the evidence for one-carbon cycle dysregulation in schizophrenic, bipolar, depressed and autistic patients. Also presented are novel findings from the field of epigenetics, which demonstrate how the one-carbon cycle-derived methyl donor S-adenosylmethionine influences the expression of key genes in the brain affecting memory, learning, cognition and behaviour, genes whose expression is reduced to varying degrees in these patient groups. Clinical evidence that nutritional supplements can rectify one-carbon cycle activity, and restore normal gene expression, suggests a novel approach to the development of biochemical tests and simple, non-harmful treatments for some psychiatric patients. Conversely, evidence from animal studies highlights the dangers of exposing the unborn fetus to very high dietary levels of folic acid, a one-carbon cycle cofactor. Fetal adaptations to a high-folate environment may interfere with folate metabolism postnatally, with serious consequences for the epigenetic regulation of gene expression. The public health implications of these diverse scenarios indicate an urgent need for further research in this field.

No association between glutathione-synthesis-related genes and Japanese schizophrenia

AIMS

Schizophrenia is a major psychiatric disorder with complex genetic, environmental, and psychological causes, and oxidative stress may be involved in the pathogenesis of the disease. Glutathione (GSH), one of the main cellular non-protein antioxidants and redox regulators, and altered GSH levels have been reported in various regions in patients with schizophrenia. Three enzymes are responsible for GSH synthesis: glutamate cysteine ligase modifier (GCLM), glutamate cysteine ligase catalytic subunit (GCLC), and glutathione synthetase (GSS). Previously, positive associations between GCLM and schizophrenia were reported in Europeans, but not in the Japanese population. Thus, in this study, we investigated the association between the GSH synthesis genes (GCLM, GCLC, and GSS) and schizophrenia in Japanese individuals.

METHODS

Seventeen single-nucleotide polymorphisms (SNP) in GCLM, GCLC, and GSS were genotyped in 358 patients with schizophrenia and in 359 controls.

RESULTS

No SNP showed a significant association between their allelic or genotypic frequencies and schizophrenia. Case-control haplotype association analysis using windows of two or three SNP showed no significant associations with schizophrenia. The case-control haplotype analyses based on the ascertained linkage disequilibrium blocks also showed no significant associations in any genes with schizophrenia.

CONCLUSIONS

The three primary GSH synthesis genes do not have an apparent degree of association with schizophrenia in the Japanese population.

Significance of NMDA receptor-related glutamatergic amino acid levels in peripheral blood of patients with schizophrenia

Hypo-function of N-methyl d-aspartate (NMDA) receptors is strongly involved in the brain pathophysiology of schizophrenia. Several excitatory amino acids, such as endogenous glutamate, glycine, serine and alanine, which are involved in glutamate neurotransmission via NMDA receptors, were studied to further understand the pathophysiology of schizophrenia and to find a biological marker for this disease, particularly in peripheral blood. In this literature review, we connect several earlier clinical studies and several studies of excitatory amino acid levels in peripheral blood in a historical context. Finally, we join these results and our previous studies, the Juntendo University Schizophrenia Projects (JUSP), which investigated plasma glutamatergic amino acid levels in detail, and considered whether these amino acid levels may be diagnostic, therapeutic, or symptomatic biological markers. This review concludes that peripheral blood levels of endogenous glycine and alanine could be a symptomatic marker in schizophrenia, while peripheral blood levels of exogenous glycine and alanine in augmentation therapies could be therapeutic markers. Noteworthy peripheral blood levels of endogenous d-serine could reflect its brain levels, and may prove to be a useful diagnostic and therapeutic marker in schizophrenia. In addition, measurements of new endogenous molecules, such as glutathione, are promising. Finally, for future therapies with glutamatergic agents still being examined in animal studies, the results of these biological marker studies may lay the foundation for the development of next-generation antipsychotics.

Glycine transport inhibitors for the treatment of schizophrenia

Multiple lines of evidence indicate that hypofunction of glutamatergic neurotransmission via N-methyl-D-aspartate (NMDA) receptors might be implicated in the pathophysiology of schizophrenia, suggesting that increasing NMDA receptor function via pharmacological manipulation could provide a new strategy for the management of schizophrenia. Currently, the glycine modulatory sites on NMDA receptors present the most attractive therapeutic targets for the treatment of schizophrenia. One means of enhancing NMDA receptor neurotransmission is to increase the availability of the obligatory co-agonist glycine at modulatory sites on the NMDA receptors through the inhibition of glycine transporter-1 (GlyT-1) on glial cells. Clinical studies have demonstrated that the GlyT-1 inhibitor sarcosine (N-methyl glycine) shows antipsychotic activity in patients with schizophrenia. Accordingly, a number of pharmaceutical companies have developed novel and selective GlyT-1 inhibitors for the treatment of schizophrenia. This paper provides an overview of the various GlyT-1 inhibitors and their therapeutic potential.

One-carbon metabolism and schizophrenia: current challenges and future directions

Schizophrenia is a heterogeneous disease generally considered to result from a combination of heritable and environmental factors. Although its pathophysiology has not been fully determined, biological studies support the involvement of several possible components including altered DNA methylation, abnormal glutamatergic transmission, altered mitochondrial function, folate deficiency and high maternal homocysteine levels. Although these factors have been explored separately, they all involve one-carbon (C1) metabolism. Furthermore, C1 metabolism is well positioned to integrate gene-environment interactions by influencing epigenetic regulation. Here, we discuss the potential roles of C1 metabolism in the pathophysiology of schizophrenia. Understanding the contribution of these mechanisms could yield new therapeutic approaches aiming to counteract disease onset or progression.

Association analysis of glycine- and serine-related genes in a Japanese population of patients with schizophrenia

Differences in the levels of the glutamate-related amino acids glycine and serine in brain/plasma between schizophrenic patients and normal subjects and changes in the plasma concentrations of these amino acids according to the clinical course have been reported. It has been hypothesized that glycine and serine metabolism may be altered in schizophrenia. In fact, some genes related to the metabolism of these amino acids have been suggested to be candidate genes for schizophrenia. Thus, we performed a genomic case-control analysis of amino acid metabolism-related genes in Japanese patients with schizophrenia. Case-control genetic association analysis of PHGDH, SHMT1, SRR, and DAO was performed. In addition, the effect of the various genotypes resulting from these four genes on changes in plasma amino acid levels in schizophrenia was assessed. The genetic case-control analysis showed that no individual single-nucleotide polymorphism (SNP) in any of the four genes was associated with schizophrenia; only the two (rs3918347-rs4964770, P=0.0009) and three (rs3825251-rs3918347-rs4964770, P=0.002) SNP-based haplotype analysis of the DAO gene showed an association with schizophrenia even after correction for multiple testing. None of the genotypes studied was associated with changes in the plasma glycine and l- and d-serine levels during the schizophrenic clinical course. The DAO gene may be a susceptibility locus for schizophrenia.

Changes in plasma glycine, L-serine, and D-serine levels in patients with schizophrenia as their clinical symptoms improve: results from the Juntendo University Schizophrenia Projects (JUSP)

OBJECTIVES

Based on the hypothesis of NMDA receptor hypofunction in schizophrenia, plasma glycine, L-serine, and D-serine levels have been studied, since they could serve as biological markers. However, changes over time in the levels of these amino acids in schizophrenic patients have not been investigated. To clarify the mean plasma glycine, L-serine, and D-serine levels in patients with schizophrenia, levels of these amino acids were compared between healthy controls and patients with schizophrenia. The plasma levels of these amino acids during the clinical course of schizophrenia were also compared.

METHODS

Eighty-nine Japanese patients with schizophrenia and 50 age- and gender-matched healthy controls were studied. Plasma glycine, L-serine, and D-serine levels and their ratios were measured twice, during the acute stage and during the remission stage, using high-performance liquid chromatography.

RESULTS

The admission plasma glycine, L-serine, and D-serine levels of schizophrenic patients were higher than those of healthy controls. There were no significant differences between drug-naïve patients and healthy controls in the admission levels of the plasma amino acids, but chronically medicated patients had higher admission plasma glycine and D-serine levels. Only the D-serine level and the D-/L-serine ratio were markedly significantly increased in schizophrenic patients from the time of admission to the time of discharge as their clinical symptoms improved. In addition, the increase in the plasma D-serine levels of drug-naïve patients was correlated with improvements in positive symptoms.

CONCLUSIONS

Plasma amino acid levels, especially D-serine levels, could be useful as a "therapeutic" or "clinical state" marker in patients with acute schizophrenia.

Reduced D-serine to total serine ratio in the cerebrospinal fluid of drug naive schizophrenic patients

Several lines of evidence suggest that D-serine, an endogenous agonist of the glycine site on the NMDA receptors, might play a role in the pathophysiology of schizophrenia. The purpose of this study was to determine whether levels of D- and L-serine or D-serine ratio (D-serine/total serine) in cerebrospinal fluid (CSF) were altered in first episode and drug-naive schizophrenic patients. The CSF levels of D- and L-serine in 25 male first episode and drug-naive schizophrenic patients and 17 age-matched male healthy subjects were measured using a column-switching high performance liquid chromatography system. The percentage of D-serine in the total serine of patients was significantly (z = -2.01, p = 0.044) lower than that of controls. This study suggests that synthetic or metabolic pathways of D-serine may be abnormal in the brain of drug-naive schizophrenic patients, supporting the NMDA receptor dysfunction hypothesis of schizophrenia.

Serine-deficiency syndromes

PURPOSE OF REVIEW

Serine-deficiency disorders comprise a new group of neurometabolic diseases and are caused by defects in the biosynthesis of the amino acid L-serine. In contrast to most neurometabolic disorders, serine-deficiency disorders are potentially treatable. Furthermore, the severe neurological symptoms observed in patients underscore the important roles of the serine biosynthetic pathway in brain tissue. An overview of the patients with serine-deficiency disorders reported to date, the biochemical findings and the results of treatment with amino acids is presented.

RECENT FINDINGS

L-Serine biosynthesis plays an important role in multiple cellular reactions, particularly in the brain, as L-serine is a precursor of important metabolites such as nucleotides, phospholipids and the neurotransmitters glycine and D-serine. Disturbances of serine-glycine metabolism in relation to N-methyl-D-aspartate-receptor activation are supposed to play a role in psychiatric disease as well. Recent findings concerning these roles of L-serine-derived phospholipids and neurotransmitters are presented.

SUMMARY

Congenital microcephaly, seizures and severe psychomotor retardation are symptoms of serine deficiency and can be treated with supplementation of L-serine, sometimes combined with glycine. The symptoms observed in serine deficiency confirm that L-serine and L-serine-derived metabolites play important roles in the central nervous system.

L-serine in disease and development

The amino acid L-serine, one of the so-called non-essential amino acids, plays a central role in cellular proliferation. L-Serine is the predominant source of one-carbon groups for the de novo synthesis of purine nucleotides and deoxythymidine monophosphate. It has long been recognized that, in cell cultures, L-serine is a conditional essential amino acid, because it cannot be synthesized in sufficient quantities to meet the cellular demands for its utilization. In recent years, L-serine and the products of its metabolism have been recognized not only to be essential for cell proliferation, but also to be necessary for specific functions in the central nervous system. The findings of altered levels of serine and glycine in patients with psychiatric disorders and the severe neurological abnormalities in patients with defects of L-serine synthesis underscore the importance of L-serine in brain development and function. This paper reviews these recent insights into the role of L-serine and the pathways of L-serine utilization in disease and during development, in particular of the central nervous system.

Schizophrenia: a purinergic hypothesis

Knowledge of the physiological roles of the purinergic system and its influence on other neurotransmitter systems has greatly advanced. In this article, a purinergic model is proposed as an attempt to integrate several findings in schizophrenia. According to this hypothesis, a purinergic system dysfunction would mainly result in reduced adenosinergic activity. This model also addresses the systemic aspects of schizophrenia, based on peripheral roles of purines, such as modulation of the immune system.

A hyperglycinergic rat model for the pathogenesis of schizophrenia: preliminary findings

There is evidence of high glycine concentrations in the brains and periphery of schizophrenics. In the forebrain, glycine plays a major role as a co-agonist with glutamate at the excitatory N-methyl-D-aspartate (NMDA) receptors. This activity of glycine is involved in the normal functioning of the brain in adulthood and during neurodevelopment, and it may also cause neurotoxicity and brain abnormalities when its concentrations are high. To test the hypothesis that the high glycine concentrations observed in schizophrenics play an etiologic role in schizophrenia, an animal model was tested where rats were made hyperglycinic from life in utero to adulthood. The hyperglycinic rats showed abnormalities in sensory gating mechanisms, enlarged cerebral ventricles and diminished hippocampal dimensions. All of these abnormalities closely parallel observations reported in patients with schizophrenic psychoses. These results from a rat model suggest an etiologic role for high glycine concentration in the behavior and brain abnormalities of schizophrenic patients.

Delay to first antipsychotic medication in schizophrenia: impact on symptomatology and clinical course of illness

The possible adverse clinical effects of untreated psychosis in schizophrenic patients, particularly early in the course of illness, have been a topic of considerable interest in recent years. We examined the timing of first administration of antipsychotic medication after the first onset of psychotic symptoms in 103 patients with DSM-III-R diagnoses of schizophrenia, schizophreniform or schizo-affective disorder. Patients with a delay of one or more years between the onset of the first psychotic symptom and the initiation of antipsychotic treatment demonstrated more severe negative symptomatology on admission to hospital and more severe positive symptoms and negative symptoms at discharge. These effects were present in both first-admission patients, in whom the delay to treatment immediately preceded hospitalization and chronic patients with a history of multiple hospitalizations. Patients with one or more years of untreated psychosis prior to their first antipsychotic treatment displayed a more severe poverty syndrome at the time of admission and discharge and a more severe reality distortion syndrome at discharge from the index hospitalization. These findings were not related to age, premorbid functioning, duration of illness, first- vs multiple-episodes status, or dosage of antipsychotic medication at time of admission or discharge assessment. Findings from the present study suggest that failure to initiate antipsychotic treatment early in the course of the illness may be associated with a recurrent pattern of poorer treatment response and more severe and persistent positive and negative symptomatology. These findings indicate the importance of early detection of illness and early initiation of antipsychotic treatment for the first psychotic symptoms of schizophrenia.

A postmortem study of glycine and its potential precursors in chronic schizophrenics

We have measured the concentrations of glycine and its potential precursors, serine and threonine, in 20 areas of the postmortem brains of chronic schizophrenics and controls using high-performance liquid chromatography by pre-column derivatization with dimethyl-amino-azobenzene sulphonyl chloride. The regional distribution pattern of glycine in the postmortem brains with and without the disease was more similar to that of serine (r = 0.874, P < 0.0001) than to that of threonine (r = 0.476, P < 0.01). A multiple regression analysis with regressor variables including diagnosis, age at death and interval between death and freezing revealed that there is a significant difference between schizophrenics and controls in the contents of these amino acids in a number of brain areas. The level of glycine in the orbitofrontal cortex of schizophrenics was found to be significantly increased in schizophrenics, with a tendency to an increase in that of serine. The increase in glycine was also significantly high in the off-drug group of schizophrenics who had not taken antipsychotics more than 40 days before death. Prominent decreases in both glycine and serine were observed in the somesthetic cortex of the on-drug schizophrenics. Serine was found to be significantly decreased in the putamen of the off-drug schizophrenics. A marked decrease in threonine was also observed in the supramarginal cortex and posterior portion of the lateral occipitotemporal cortex of the off-drug group of schizophrenics and in the putamen of all schizophrenics. The highly similar distribution pattern of glycine and serine in the postmortem brains supports the close coupling of synthesis and metabolism between these chemicals in human brains. The increased content of glycine in the orbitofrontal cortex, the reduced level of serine in the putamen and the decrease in threonine in the cerebral cortices, which were prominent in the off-drug schizophrenics, may be involved in the pathophysiology of schizophrenia.

Psychosis and vulnerability to ECT-induced seizures

Medical records of patients with major depressive disorders who had received electroconvulsive therapy (ECT) for the first time were studied to test the hypothesis that psychotic patients are more vulnerable to seizures than nonpsychotic patients. This hypothesis was based on studies suggesting a putative purinergic deficiency in psychosis. Results showed that the duration of ECT-induced seizures as a measure of seizure vulnerability was significantly longer in psychotic than in nonpsychotic depressive patients. The association applied for the first ECT as well as for the course of eight ECTs. These findings were still present when covariates such as age, electrical energy applied, dosage of methohexital and succinylcholine, and psychotropic medications such as neuroleptics, benzodiazepines, and tricyclics were included in the statistical analysis. The results are discussed in the context of the role of neurotransmitters such as glutamate, gamma-aminobutyric acid, adenosine, and dopamine on seizure vulnerability and psychosis.

Neuroleptic effects on serine and glycine metabolism

The concentrations of serine and glycine and the activity of serine hydroxymethyltransferase (SHMT) are abnormal in plasma and brains of schizophrenics. To further elucidate the possible role of neuroleptics on the metabolism of serine and glycine and the activity of SHMT, we studied the plasma of controls and schizophrenics on and off medications, the brains of rats treated with haloperidol, and the activity of purified SHMT in the presence or absence of haloperidol and fluphenazine. Plasmas of neuroleptic-treated schizophrenics had nonsignificantly lower concentrations of serine and glycine. Brains of haloperidol-treated rats had significantly lower concentrations of serine and glycine. At therapeutic levels haloperidol and fluphenazine did not inhibit the activity of purified SHMT. The serine-glycine lowering effects of haloperidol and neuroleptics are discussed in the context of a possible neuroprotective potential of neuroleptics in schizophrenia.

Presence of the N-methyl-D-aspartate-associated glycine receptor agonist, D-serine, in human temporal cortex: comparison of normal, Parkinson, and Alzheimer tissues

The amino acid D-serine (D-Ser), previously recognized as a pharmacological tool for potentiating neuronal activity mediated by the N-methyl-D-aspartate (NMDA) receptor complex, in vitro and in vivo, has been observed in several brain regions of the rat and mouse, most prominently in cortex. In addition to reconfirming the presence and distribution of D-Ser in rat brain, we have observed, for the first time, endogenous, free D-Ser in temporal cortex of normal human brains at a level of 2.18 +/- 0.12 nmol/mg of protein, representing 15 +/- 2% of the free L-Ser pool. The D- and L-Ser specific content and the D/L-Ser ratio obtained from temporal cortex of Parkinson and Alzheimer brains did not differ significantly from those of controls. However, at the levels observed here, and considering its specificity and affinity for the NMDA-associated glycine receptor, endogenous D-Ser is a plausible NMDA receptor glycine site agonist.