In vivo evidence for the link between L- and D-serine metabolism in rat cerebral cortex

Disturbance of neurotransmitter metabolites in peripheral blood of schizophrenia patients treated with olanzapine: a preliminary targeted metabolomic study

BACKGROUND

The aim of this research was to characterize changes in peripheral blood neurotransmitter metabolites in olanzapine-treated schizophrenia (SCZ) and to identify potential biomarkers for SCZ. Concurrently, the relationship between these differential neurotransmitters and cognitive function is explored.

METHODS

We recruited 40 SCZ treated with single-agent olanzapine and 40 healthy controls (HC). Cognitive function and psychopathology were assessed using the MCCB and PANSS, respectively. Neurotransmitter levels were determined by targeted metabolomics approach using liquid chromatography-mass spectrometry (LC/MS).

RESULTS

SCZ showed cognitive impairment in all domains of the MCCB compared to HC. Interestingly, a 4-neurotransmitter panel consisting of 3-Methoxytyramine hydrochloride (3-MT), 3,4-Dihydroxyphenylacetate (DOPAC), arginine, and r-aminobutyric acid (GABA) illustrated the highest determinative score between SCZ and HC. Arginine was positively correlated with PANSS general psychopathology scores. 3-MT independently predicted the verbal learning scores only in SCZ, whereas GABA independently predicted the social cognition scores only. Furthermore, GABA independently predicted the working memory scores only in HC.

CONCLUSIONS

The collective assessment of these four neurotransmitters (3-MT, DOPAC, arginine, and GABA) holds considerable promise as potential biomarkers for SCZ. Moreover, 3-MT and GABA may enhance our understanding of cognitive dysfunction in SCZ, particularly in areas of verbal learning and social cognitive dysfunction.

Blood D-serine levels correlate with aging and dopaminergic treatment in Parkinson's disease

We recently described increased D- and L-serine concentrations in the striatum of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated monkeys, the post-mortem caudate-putamen of human Parkinson's disease (PD) brains and the cerebrospinal fluid (CSF) of de novo living PD patients. However, data regarding blood D- and L-serine levels in PD are scarce. Here, we investigated whether the serum profile of D- and L-serine, as well as the other glutamate N-methyl-D-aspartate ionotropic receptor (NMDAR)-related amino acids, (i) differs between PD patients and healthy controls (HC) and (ii) correlates with clinical-demographic features and levodopa equivalent daily dose (LEDD) in PD. Eighty-three consecutive PD patients and forty-one HC were enrolled. PD cohort underwent an extensive clinical characterization. Serum levels of D- and L-serine, L-glutamate, L-glutamine, L-aspartate, L-asparagine and glycine were determined using High Performance Liquid Chromatography. In age- and sex-adjusted analyses, no differences emerged in the serum levels of D-serine, L-serine and other NMDAR-related amino acids between PD and HC. However, we found that D-serine and D-/Total serine ratio positively correlated with age in PD but not in HC, and also with PD age at onset. Moreover, we found that higher LEDD correlated with lower levels of D-serine and the other excitatory amino acids. Following these results, the addition of LEDD as covariate in the analyses disclosed a selective significant increase of D-serine in PD compared to HC (Δ ≈ 38%). Overall, these findings suggest that serum D-serine and D-/Total serine may represent a valuable biochemical signature of PD.

D-Serine: A Cross Species Review of Safety

Background:D-Serine, a direct, full agonist at the D-serine/glycine modulatory site of the N-methyl-D-aspartate-type glutamate receptors (NMDAR), has been assessed as a treatment for multiple psychiatric and neurological conditions. Based on studies in rats, concerns of nephrotoxicity have limited D-serine research in humans, particularly using high doses. A review of D-serine's safety is timely and pertinent, as D-serine remains under active study for schizophrenia, both directly (R61 MH116093) and indirectly through D-amino acid oxidase (DAAO) inhibitors. The principal focus is on nephrotoxicity, but safety in other physiologic and pathophysiologic systems are also reviewed. Methods: Using the search terms "D-serine," "D-serine and schizophrenia," "D-serine and safety," "D-serine and nephrotoxicity" in PubMed, we conducted a systematic review on D-serine safety. D-serine physiology, dose-response and efficacy in clinical studies and dAAO inhibitor safety is also discussed. Results: When D-serine doses >500 mg/kg are used in rats, nephrotoxicity, manifesting as an acute tubular necrosis syndrome, seen within hours of administration is highly common, if not universal. In other species, however, D-serine induced nephrotoxicity has not been reported, even in other rodent species such as mice and rabbits. Even in rats, D--serine related toxicity is dose dependent and reversible; and does not appear to be present in rats at doses producing an acute Cmax of <2,000 nmol/mL. For comparison, the Cmax of D-serine 120 mg/kg, the highest dose tested in humans, is ~500 nmol/mL in acute dosing. Across all published human studies, only one subject has been reported to have abnormal renal values related to D-serine treatment. This abnormality did not clearly map on to the acute tubular necrosis syndrome seen in rats, and fully resolved within a few days of stopping treatment. DAAO inhibitors may be nephroprotective. D-Serine may have a physiologic role in metabolic, extra-pyramidal, cardiac and other systems, but no other clinically significant safety concerns are revealed in the literature. Conclusions: Even before considering human to rat differences in renal physiology, using current FDA guided monitoring paradigms, D-serine appears safe at currently studied maximal doses, with potential safety in combination with DAAO inhibitors.

NMDARs, Coincidence Detectors of Astrocytic and Neuronal Activities

Synaptic plasticity is an extensively studied cellular correlate of learning and memory in which NMDARs play a starring role. One of the most interesting features of NMDARs is their ability to act as a co-incident detector. It is unique amongst neurotransmitter receptors in this respect. Co-incident detection is possible because the opening of NMDARs requires membrane depolarisation and the binding of glutamate. Opening of NMDARs also requires a co-agonist. Although the dynamic regulation of glutamate and membrane depolarization have been well studied in coincident detection, the role of the co-agonist site is unexplored. It turns out that non-neuronal glial cells, astrocytes, regulate co-agonist availability, giving them the ability to influence synaptic plasticity. The unique morphology and spatial arrangement of astrocytes at the synaptic level affords them the capacity to sample and integrate information originating from unrelated synapses, regardless of any pre-synaptic and post-synaptic commonality. As astrocytes are classically considered slow responders, their influence at the synapse is widely recognized as modulatory. The aim herein is to reconsider the potential of astrocytes to participate directly in ongoing synaptic NMDAR activity and co-incident detection.

Converging Evidence on D-Amino Acid Oxidase-Dependent Enhancement of Hippocampal Firing Activity and Passive Avoidance Learning in Rats

BACKGROUND

N-methyl-D-aspartate (NMDA) receptor activation requires the binding of a co-agonist on the glycine-binding site. D-serine is the main endogenous co-agonist of NMDA receptors, and its availability significantly depends on the activity of the metabolic enzyme D-amino acid oxidase (DAAO). Inhibition of DAAO increases the brain levels of D-serine and modulates a variety of physiological functions, including cognitive behavior.

METHODS

Here, we examined the effects of a novel 4-hydroxypyridazin-3(2H)-one derivative DAAO inhibitor, Compound 30 (CPD30), on passive avoidance learning and on neuronal firing activity in rats.

RESULTS

D-serine administration was applied as reference, which increased cognitive performance and enhanced hippocampal firing activity and responsiveness to NMDA after both local and systemic application. Similarly to D-serine, CPD30 (0.1 mg/kg) effectively reversed MK-801-induced memory impairment in the passive avoidance test. Furthermore, local iontophoretic application of CPD30 in the vicinity of hippocampal pyramidal neurons significantly increased firing rate and enhanced their responses to locally applied NMDA. CPD30 also enhanced hippocampal firing activity after systemic administration. In 0.1- to 1.0-mg/kg doses, CPD30 increased spontaneous and NMDA-evoked firing activity of the neurons. Effects of CPD30 on NMDA responsiveness emerged faster (at 10 minutes post-injection) when a 1.0-mg/kg dose was applied compared with the onset of the effects of 0.1 mg/kg CPD30 (at 30 minutes post-injection).

CONCLUSIONS

The present results confirm that the inhibition of DAAO enzyme is an effective strategy for cognitive enhancement. Our findings further facilitate the understanding of the cellular mechanisms underlying the behavioral effects of DAAO inhibition in the mammalian brain.

Improvement in regional CBF by L-serine contributes to its neuroprotective effect in rats after focal cerebral ischemia

To investigate the mechanisms underlying the neuroprotective effect of L-serine, permanent focal cerebral ischemia was induced by occlusion of the middle cerebral artery while monitoring cerebral blood flow (CBF). Rats were divided into control and L-serine-treated groups after middle cerebral artery occlusion. The neurological deficit score and brain infarct volume were assessed. Nissl staining was used to quantify the cortical injury. L-serine and D-serine levels in the ischemic cortex were analyzed with high performance liquid chromatography. We found that L-serine treatment: 1) reduced the neurological deficit score, infarct volume and cortical neuron loss in a dose-dependent manner; 2) improved CBF in the cortex, and this effect was inhibited in the presence of apamin plus charybdotoxin while the alleviation of both neurological deficit score and infarct volume was blocked; and 3) increased the amount of L-serine and D-serine in the cortex, and inhibition of the conversion of L-serine into D-serine by aminooxyacetic acid did not affect the reduction of neurological deficit score and infarct volume by L-serine. In conclusion, improvement in regional CBF by L-serine may contribute to its neuroprotective effect on the ischemic brain, potentially through vasodilation which is mediated by the small- and intermediate-conductance Ca²⁺-activated K⁺ channels on the cerebral blood vessel endothelium.

Serine racemase: an unconventional enzyme for an unconventional transmitter

The discovery of large amounts of D-serine in the brain challenged the dogma that only L-amino acids are relevant for eukaryotes. The levels of D-serine in the brain are higher than many L-amino acids and account for as much as one-third of L-serine levels. Several studies in the last decades have demonstrated a role of D-serine as an endogenous agonist of N-methyl-D-aspartate receptors (NMDARs). D-Serine is required for NMDAR activity during normal neurotransmission as well as NMDAR overactivation that takes place in neurodegenerative conditions. Still, there are many unanswered questions about D-serine neurobiology, including regulation of its synthesis, release and metabolism. Here, we review the mechanisms of D-serine synthesis by serine racemase and discuss the lessons we can learn from serine racemase knockout mice, focusing on the roles attributed to D-serine and its cellular origin.

Pharmacokinetics and cerebral distribution of glycine administered to rats

High doses of glycine have been reported to improve negative schizophrenic symptoms, suggesting that ingested glycine activates glutamatergic transmission via N-methyl-D-aspartate (NMDA) receptors. However, the pharmacokinetics of administered glycine in the brain has not been evaluated. In the present study, the time- and dose-dependent distributions of administered glycine were investigated from a pharmacokinetic viewpoint. Whole-body autoradiography of radiolabeled glycine was performed, and time-concentration curves for glycine and serine in plasma, cerebrospinal fluid (CSF), and brain tissues were obtained. Furthermore, pharmacokinetic parameters were calculated. For a more detailed analysis, the amount of glycine uptake in the brain was evaluated using the brain uptake index method. Radiolabeled glycine was distributed among periventricular organs in the brain. Oral administration of 2 g/kg of glycine significantly elevated the CSF glycine concentration above the ED50 value for NMDA receptors. The glycine levels in CSF were 100 times lower than those in plasma. Glycine levels were elevated in brain tissue, but with a slower time-course than in CSF. Serine, a major metabolite of glycine, was elevated in plasma, CSF, and brain tissue. Glycine uptake in brain tissue increased in a dose-dependent manner. Time-concentration curves revealed that glycine was most likely transported via the blood-CSF barrier and activated NMDA receptors adjacent to the ventricles. The pharmacokinetic analysis and the brain uptake index for glycine suggested that glycine was transported into brain tissue by passive diffusion. These results provide further insight into the potential therapeutic applications of glycine.

D-serine: the right or wrong isoform?

Only recently, d-amino acids have been identified in mammals. Of these, d-serine has been most extensively studied. d-Serine was found to play an important role as a neurotransmitter in the human central nervous system (CNS) by binding to the N-methyl-d-aspartate receptor (NMDAr), similar to glycine. Therefore, d-serine may well play a role in all physiological and pathological processes in which NMDArs have been implied. In this review, we discuss the findings implying an important role for d-serine in human physiology (CNS development and memory and learning) and pathology (excitotoxicity, perinatal asphyxia, amyotrophic lateral sclerosis (ALS), Alzheimer's disease, epilepsy, schizophrenia and bipolar disorder). We will debate on the relative contribution of d-serine versus glycine and conclude with clinical applications derived from these results and future directions to progress in this field. In general, adequate concentrations of d-serine are required for normal CNS development and function, while both decreased and increased concentrations can lead to CNS pathology. Therefore, d-serine appears to be the right isoform when present in the right concentrations.

Contributions of the D-serine pathway to schizophrenia

The glutamate neurotransmitter system is one of the major candidate pathways for the pathophysiology of schizophrenia, and increased understanding of the pharmacology, molecular biology and biochemistry of this system may lead to novel treatments. Glutamatergic hypofunction, particularly at the NMDA receptor, has been hypothesized to underlie many of the symptoms of schizophrenia, including psychosis, negative symptoms and cognitive impairment. This review will focus on D-serine, a co-agonist at the NMDA receptor that in combination with glutamate, is required for full activation of this ion channel receptor. Evidence implicating D-serine, NMDA receptors and related molecules, such as D-amino acid oxidase (DAO), G72 and serine racemase (SRR), in the etiology or pathophysiology of schizophrenia is discussed, including knowledge gained from mouse models with altered D-serine pathway genes and from preliminary clinical trials with D-serine itself or compounds modulating the D-serine pathway. Abnormalities in D-serine availability may underlie glutamatergic dysfunction in schizophrenia, and the development of new treatments acting through the D-serine pathway may significantly improve outcomes for many schizophrenia patients.

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.

Plasma alanine levels increase in patients with schizophrenia as their clinical symptoms improve-Results from the Juntendo University Schizophrenia Projects (JUSP)

Since oral administration of d-alanine, an agonist that binds to the glycine site of N-methyl-d-aspartate (NMDA) receptors, improves the positive and cognitive symptoms of patients with schizophrenia, measurement of endogenous plasma alanine levels could serve as a clinical marker for schizophrenia severity and improvement. Mean plasma alanine levels were compared in healthy controls and patients with schizophrenia during the clinical course of the disease.

METHODS

eighty-one Japanese patients with schizophrenia and 50 age- and gender-matched healthy controls were studied. Plasma alanine levels were measured twice, during the acute stage and during the remission stage, using high-performance liquid chromatography. On admission, lower plasma alanine levels in patients with schizophrenia were accompanied by more severe schizophrenic symptoms, especially positive symptoms. The plasma alanine levels in patients with schizophrenia increased significantly from the time of admission to discharge, when they were significantly higher than control levels. An increase in plasma alanine levels from the acute stage to the remission stage of schizophrenia was correlated with improvement in symptoms. Drug-naïve patients did not show a significant difference in plasma alanine levels when compared with healthy controls. The measurement of plasma alanine levels may be a therapeutic marker for schizophrenia.

Potential pathophysiological role of D-amino acid oxidase in schizophrenia: immunohistochemical and in situ hybridization study of the expression in human and rat brain

D-Amino acid oxidase (DAO) is a peroxisomal flavoenzyme that catalyzes oxidative deamination of a wide range of D-amino acids. Among the possible substrates of DAO in vivo, D-serine is proposed to be a neuromodulator of the N-methyl-D-aspartate (NMDA) type glutamate receptor. The gene for DAO was reported to be associated with schizophrenia. Since DAO is expected to be one of the key enzymes in the regulation of NMDA neurotransmission, the modulation of the enzyme activity is expected to be therapeutical for neuronal disorders. In search of the pathophysiological role of DAO, we analyzed the distribution of DAO mRNA and protein in the rat and human brain. In rat, the distribution of DAO mRNA was newly detected in choroid plexus (CP) epithelial cells in addition to glial cells of pons, medulla oblongata, and especially Bergmann glia of cerebellum. Moreover, to investigate how DAO expression level is altered in schizophrenia, we performed immunohistochemistry in the human brain. In agreement with the results in the rat brain, the immunoreactivity for DAO was detected in glial cells of rhombencephalon and in CP. Furthermore, higher level of DAO expression was observed in schizophrenic CP epithelial cells than that in non-schizophrenic cases. These results suggest that an increase in DAO expression in parts of the brain is involved in aberrant D-amino acid metabolism. In particular, gene expression of DAO in CP suggests that DAO may regulate D-amino acid concentration by modulating the cerebrospinal fluid and may be regarded as a potential therapeutic target for schizophrenia.

L-Serine lowers while glycine increases blood pressure in chronic L-NAME-treated and spontaneously hypertensive rats

OBJECTIVE

To determine the acute hemodynamic effects of the nonessential amino acid, glycine, and its precursor, L-serine, in normotensive and hypertensive rats.

METHODS

Changes in mean arterial pressure and heart rate evoked by comparable intravenously administered doses (0.3-3.0 mmol/kg) of L-serine, D-serine and glycine were examined in anaesthetized normotensive 14-week-old male Sprague-Dawley, Wistar-Kyoto (WKY) rats, spontaneously hypertensive rats and WKY rats subjected to chronic nitric oxide synthase inhibition by treatment with NG nitro L-arginine methyl ester (0.7 mg/ml in drinking water for 5 days).

RESULTS

L-Serine evoked a greater maximal fall in mean arterial pressure [L-serine vs. D-serine in Sprague-Dawley rats, mean +/- standard error of the mean values (mmHg): 30 +/- 3 vs. 20 +/- 5, P < 0.05; in control WKY rats: 46 +/- 3 vs. 30 +/- 4, P < 0.05; in NG nitro L-arginine methyl ester-treated WKY rats: 93 +/- 6 vs. 41 +/- 5, P < 0.01; in spontaneously hypertensive rats: 81 +/- 7 vs. 39 +/- 5 P < 0.01]. The effects of L-serine were significantly reduced in rats pretreated with a combination of apamin and charybdotoxin, inhibitors of the small conductance and intermediate conductance calcium-activated potassium (KCa) channels. Glycine elicited a dose-dependent fall in mean arterial pressure in normotensive WKY rats (25 +/- 4; P < 0.01) and evoked pressor responses in both spontaneously hypertensive rats (29 +/- 3; P < 0.01) and NG nitro L-arginine methyl ester-pretreated hypertensive WKY (39 +/- 5; P < 0.01) rats. Both the depressor and pressor responses to glycine were abolished by pretreatment with the N-methyl D-aspartate receptor antagonist, MK-801.

CONCLUSION

The profound stereo-selective antihypertensive effect of L-serine is neither mediated nor mimicked by glycine. It does not require N-methyl D-aspartate receptor activation by glycine but likely involves activation of endothelial KCa channels. L-Serine is a potential antihypertensive agent.

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.

Glycine and D: -serine, but not D: -cycloserine, attenuate prepulse inhibition deficits induced by NMDA receptor antagonist MK-801

RATIONALE

Several agents that stimulate the glycine site of N-methyl-D: -aspartate (NMDA) receptors have been reported to moderately improve both negative symptoms and cognitive dysfunctions in patients with schizophrenia. However, differences in efficacy have also been reported, and further comparative pharmacological studies are still needed.

OBJECTIVES

We aimed to explore the effects of two glycine site agonists of the NMDA receptor, glycine and D: -serine, and a partial agonist, D: -cycloserine, on prepulse inhibition (PPI) deficits induced by a NMDA receptor antagonist, MK-801, in mice. Furthermore, we performed in vivo microdialysis and additional PPI measurements using a selective glycine site antagonist to verify if the beneficial effects observed after the systemic administration of glycine were due to glycine itself via its activity at the glycine site.

RESULTS

High doses of glycine (1.6 g/kg) and D: -serine (1.8 and 2.7 g/kg) significantly attenuated MK-801-induced PPI deficits. In contrast, D: -cycloserine did not show any amelioration of MK-801-induced PPI deficits at doses ranging from 7.5 mg/kg to 60 mg/kg. The selective glycine site antagonist, L-701,324 (10 mg/kg), antagonized the effect of glycine on MK-801-induced PPI deficits. Furthermore, in vivo microdialysis demonstrated that intraperitoneal injection of glycine significantly increased glycine and L: -serine levels, but decreased D: -serine levels in the prefrontal cortex.

CONCLUSIONS

The findings of the present study suggest that glycine and D: -serine but not D: -cycloserine could attenuate PPI deficits associated with NMDA receptor hypofunction via NMDA glycine sites in the brain.

D-serine signalling as a prominent determinant of neuronal-glial dialogue in the healthy and diseased brain

Rather different from their initial image as passive supportive cells of the CNS, the astrocytes are now considered as active partners at synapses, able to release a set of gliotransmitter-like substances to modulate synaptic communication within neuronal networks. Whereas glutamate and ATP were first regarded as main determinants of gliotransmission, growing evidence indicates now that the amino acid D-serine is another important player in the neuronal-glial dialogue. Through the regulation of glutamatergic neurotransmission through both N-methyl-D-aspartate (NMDA-R) and non-NMDA-R, D-serine is helping in modelling the appropriate connections in the developing brain and influencing the functional plasticity within neuronal networks throughout lifespan. The understanding of D-serine signalling, which has increased linearly in the last few years, gives new insights into the critical role of impaired neuronal-glial communication in the diseased brain, and offers new opportunities for developing relevant strategies to treat cognitive deficits associated to brain disorders.

D-serine is a key determinant of glutamate toxicity in amyotrophic lateral sclerosis

Excitotoxicity has been implicated in the pathogenesis of amyotrophic lateral sclerosis (ALS). More recently, glial involvement has been shown to be essential for ALS-related motoneuronal death. Here, we identified an N-methyl-D-aspartate (NMDA) receptor co-agonist, D-serine (D-Ser), as a glia-derived enhancer of glutamate (Glu) toxicity to ALS motoneurons. Cell death assay indicated that primary spinal cord neurons from ALS mice were more vulnerable to NMDA toxicity than those from control mice, in a D-Ser-dependent manner. Levels of D-Ser and its producing enzyme, serine racemase, in spinal cords of ALS mice were progressively elevated, dominantly in glia, with disease progression. In vitro, expression of serine racemase was induced not only by an extracellular pro-inflammatory factor, but also by transiently expressed G93A-superoxide dismutase1 in microglial cells. Furthermore, increases of D-Ser levels were also observed in spinal cords of both familial and sporadic ALS patients. Collectively, Glu toxicity enhanced by D-Ser overproduced in glia is proposed as a novel mechanism underlying ALS motoneuronal death, and this mechanism may be regarded as a potential therapeutic target for ALS.

D-serine: a new word in the glutamatergic neuro-glial language

Gliotransmission is a process in which astrocytes are dynamic elements that influence synaptic transmission and synaptogenesis. The best-known gliotransmitters are glutamate and ATP. However, in the past decade, it has been demonstrated that D-serine, a D-amino acid, acts as a gliotransmitter in glutamatergic synapses. The physiological relevance of D-serine is sustained by the way in which it modulates the action of glutamatergic neurotransmission, neuronal migration and long-term potentiation (LTP). In addition, the synthesis and degradation mechanisms of D-serine have been proposed as potential therapeutic targets for the treatment of Alzheimer's disease, schizophrenia and related disorders. In the present review, detailed information is provided about the physiological and physiopathological relevance of D-serine, including metabolic and regulation aspects.

Correlation between plasma levels of glutamate, alanine and serine with severity of depression

The goal of this study was to evaluate the utility of using plasma levels of amino acids as an indicator of the severity of depression. The samples were collected from 23 depressed patients receiving antidepressant medication, and were compared to 31 healthy subjects. The plasma levels of amino acids were determined using HPLC with fluorometric detection. The severity of depression was evaluated using the Hamilton Depression Rating Scale (HAM-D) scores. Plasma levels of glutamate, glutamine, glycine and taurine were significantly increased in the depressed patients compared to the controls. Statistical analysis indicated a positive correlation between glutamate and alanine levels and HAM-D scores and a negative correlation of L-serine with HAM-D scores. The results indicate that plasma level of glutamate, alanine and L-serine could reflect the severity of depression rather than glutamine, glycine and taurine.

Alterations in D-amino acid levels in the brains of mice and rats after the administration of D-amino acids

To mutant ddY/DAO(-) mice lacking D-amino-acid oxidase activity and normal ddY/DAO(+) mice, five D-amino acids (D-Asp, D-Ser, D-Ala, D-Leu and D-Pro) were orally administered for two weeks, and the D-amino acid levels were examined in seven brain regions. The levels of D-Asp markedly increased in the pituitary and pineal glands in both strains. In the ddY/DAO(+) mice, the levels of the other D-amino acids did not significantly change in most of the brain regions. While in the ddY/DAO(-) mice the levels of D-Ser significantly increased in most of the brain regions except for the cerebrum and hippocampus. The levels of D-Ala and D-Leu increased in all regions but the levels of D-Pro did not significantly change. The same five D-amino acids were intravenously injected into Wistar rats and the D-amino acid levels in their brains were examined for 60 min after the administration. The levels of D-Asp markedly increased in the pineal gland 3 min after the administration, while the levels of D-Ser, D-Ala, and D-Pro increased both in the pineal and pituitary glands, the levels of D-Leu increased in all brain regions. These results are useful for the elucidation of the origins and regulation of D-amino acids in the mammalian body.

Evidence for involvement of glial cell activity in the control of extracellular D-serine contents in the rat brain

The continuous intra-cortical infusion of a glia toxin, fluorocitrate, at the concentration of 1 mM caused a decrease in the cortical extracellular contents of an intrinsic coagonist for the N-methyl-D-aspartate (NMDA) type glutamate receptor, D-serine, by peaking at 40 min by -25% but produced an increase in those of glycine and L-serine. The attenuated glial activity by fluorocitrate was verified by a marked reduction in the extracellular glutamine contents. The present findings suggest that a group of glial cells such as a population of the protoplasmic astrocytes could, at least in part, participate differently in the regulation of the extracellular release of D-serine and another NMDA coagonist glycine in the medial frontal cortex of the rat.

Gliotransmission at central glutamatergic synapses: D-serine on stage

Long ignored and only considered as housekeeping cells for neurons, astroglial cells in the last decade have gained increasing attention as key players of higher functions in healthy brain, but also in diseases. This revolution in our way to think the active brain culminates in the concept of a tripartite synapse, which considers glial cells and notably astrocytes as an integral dynamic partner of synapses. Glia not only listens but also talks to neurons through the release of neuroactive substances. Recently much attention has been paid to the role played by the atypical amino acid D-serine in this signalling pathway. This molecule synthesized through racemization of L-serine fulfils most criteria as a gliotransmitter and as the endogenous ligand for the strychnine-insensitive glycine binding site of the NMDA receptors. D-serine is considered to be a permissive factor for long-term changes in synaptic plasticity and neuronal migration through activation of NMDA receptors. It is also known that disturbance of NMDA receptors activity can cause cell death. Not surprisingly, then, D-serine has also been found to promote neurons death in experimental models of beta-amyloid peptide-induced neuroinflammation and of ischaemia by overactivating the NMDA receptors. Finally, in a more recent past, studies have pointed to the molecular mechanisms leading to D-serine release into and removal from the synaptic cleft.

Combining desorption electrospray ionization mass spectrometry and nuclear magnetic resonance for differential metabolomics without sample preparation

Desorption electrospray ionization mass spectrometry (DESI-MS) and nuclear magnetic resonance (NMR) spectroscopy are used to provide data on urine examined without sample preparation to allow differentiation between diseased (lung cancer) and healthy mice. Principal component analysis (PCA) is used to shortlist compounds with potential for biomarker screening which are responsible for significant differences between control urine samples and samples from diseased animals. Similar PCA score plots have been achieved by DESI-MS and NMR, using a subset of common detected metabolites. The common compounds detected by DESI and NMR have the same changes in sign of their concentrations thereby indicating the usefulness of corroborative analytical methods. The effects of different solvents and surfaces on the DESI mass spectra are also evaluated and optimized. Over 80 different metabolites were successfully identified by DESI-MS and tandem mass spectrometry experiments, with no prior sample preparation.

Effect of aminooxyacetic acid on extracellular level of D-serine in rat striatum: an in vivo microdialysis study

To elucidate the effect of an inhibitor of pyridoxal phosphate-dependent enzymes, aminooxyacetic acid, on the activity of serine racemase in vivo, we have investigated the effect of aminooxyacetic acid on the extracellular concentration of D-serine in the rat striatum using an in vivo microdialysis technique. The intrastriatal perfusion of aminooxyacetic acid caused a significant decline in the extracellular concentration of D-serine. These data, together with the fact that serine racemase is a pyridoxal phosphate-dependent enzyme, suggest that the aminooxyacetic acid-induced reduction of the extracellular D-serine may be at least in part due to the drug's ability to inhibit serine racemase.

Mutual regulation between serine and nitric oxide metabolism in human glioblastoma cells

D-Serine indirectly caused dose- and time-dependent inhibition of neuronal nitric oxide synthase (nNOS) without affecting endothelial nitric oxide synthase (eNOS) in human glioblastoma cell line U87. Activity of D-amino acid oxidase (DAAO), catalyzing the oxidative deamination of d-amino acid, was enhanced by NO in a dose-dependent manner. Recently, we have reported that serine racemase (SR) is inhibited by NO and activated by D-serine through nitrosylation and denitrosylation, respectively [K. Shoji, S. Mariotto, A.R. Ciampa, H. Suzuki, Regulation of serine racemase activity by D-serine and nitric oxide in human glioblastoma cells, Neurosci. Lett., in press]. Thus, the metabolism of both d-serine and NO in U87 cells is functionally correlated in a complex manner. Suppression of NO production by d-serine in U87 cells contrasts its known action in enhancing nNOS in neurons.

Cloning of a D-serine-regulated transcript dsr-2 from rat cerebral neocortex

D-serine is now considered to be an endogenous co-agonist of the NMDA receptor in mammalian brain. To obtain insight into the molecular mechanisms underlying D-serine metabolism and function, we explored transcripts that are responsive to D-serine in the neocortex of the 8-day-old infant rat by a differential cloning technique, RNA arbitrarily primed PCR. We isolated a novel D-serine inducible transcript, D-serine-responsive transcript-2 (dsr-2), that was exclusively expressed in the brain. Sequence analysis of the corresponding cDNAs to the transcript revealed that the dsr-2 mRNA consists of 7199 nucleotides with an open reading frame encoding 111 amino acids. The dsr-2 gene was located on the reverse strand within an intron of the neurexin-3alpha gene, mapped to rat chromosome 6q24-31. The regional distribution of the basal expression of dsr-2 and its ontogenic changes in the brain closely correlated with those of free D-serine and of NMDA receptor R2B subunit mRNA, but were somewhat different from those of the neurexin-3alpha transcript. These findings suggest that dsr-2 may be involved in D-serine metabolism and/or function, and in the interactions between D-serine, NMDA receptor and neurexin-3alpha, in mammalian brain.

Inhibition of D-serine accumulation in the Xenopus oocyte by expression of the rat ortholog of human 3'-phosphoadenosine 5'-phosphosulfate transporter gene isolated from the neocortex as D-serine modulator-1

D-serine in mammalian brains has been suggested to be an endogenous co-agonist of the NMDA-type glutamate receptor. We have explored the molecules regulating D-serine uptake and release from the rat neocortex cDNA library using a Xenopus oocyte expression system, and isolated a cDNA clone designated as dsm-1 (D-serine modulator-1) encoding a protein that reduces the accumulation of D-serine to the oocyte. dsm-1 is the rat orthologue of the human 3'-phosphoadenosine 5'-phosphosulfate transporter 1 (PAPST1) gene. The hydropathy analysis of the deduced amino acid sequence of the Dsm-1 protein predicts the 10 transmembrane domains with a long hydrophobic stretch in the C-terminal like some amino acid transporters. The dsm-1 mRNA is predominantly expressed in the forebrain areas that are enriched with D-serine and NMDA receptors, and in the liver. The transient expression of dsm-1 in COS-7 cells demonstrates a partially Golgi apparatus-related punctuate distribution throughout the cytoplasm with a concentration near the nucleus. dsm-1-expressing oocytes diminishes the sodium-dependent and -independent accumulation of D-serine and the basal levels of the intrinsic D-serine and increases the rate of release of the pre-loaded D-serine. These findings indicate that dsm-1 may, at least in part, be involved in the D-serine translocation across the vesicular or plasma membranes in the brain, and thereby control the extra- and intracellular contents of D-serine.

Preferred stereoselective brain uptake of d-serine — a modulator of glutamatergic neurotransmission

Although it has long been presumed that d-amino acids are uncommon in mammalians, substantial amounts of free d-serine have been detected in the mammalian brain. d-Serine has been demonstrated to be an important modulator of glutamatergic neurotransmission and acts as an agonist at the strychnine-insensitive glycine site of N-methyl-d-aspartate receptors. The blood-to-brain transfer of d-serine is thought to be extremely low, and it is assumed that d-serine is generated by isomerization of l-serine in the brain. Stimulated by the observation of a preferred transport of the d-isomer of proline at the blood-brain barrier, we investigated the differential uptake of [3H]-d-serine and [3H]-l-serine in the rat brain 1 h after intravenous injection using quantitative autoradiography. Surprisingly, brain uptake of [3H]-d-serine was significantly higher than that of [3H]-l-serine, indicating a preferred transport of the d-enantiomer of serine at the blood-brain barrier. This finding indicates that exogenous d-serine may have a direct influence on glutamatergic neurotransmission and associated diseases.

Identification of multiple serine racemase (SRR) mRNA isoforms and genetic analyses of SRR and DAO in schizophrenia and D-serine levels

BACKGROUND

We previously reported a reduction in serum levels of D-serine, an endogenous co-agonist of the N-methyl-D-aspartate (NMDA) receptor, in schizophrenia, supporting the hypofunction hypothesis of NMDA neurotransmission in schizophrenia. In this study, we examined the genetic roles of serine racemase (SRR), an enzyme catalyzing the formation of D-serine from L-serine, and D-amino-acid oxidase (DAO) in the susceptibility to schizophrenia and the regulation of serum D-serine levels.

METHODS

We determined the complete cDNA and genomic structures of SRR and performed mutation screening. Single nucleotide polymorphisms (SNPs) in SRR and DAO were tested for their association with schizophrenia in both case-control and family-based designs and for correlation with serum levels of D-serine.

RESULTS

Genomic analyses revealed that human brain SRR transcripts consist of four isoforms with one major species, which were derived from alternative use of various 5' end exons. Genetic association analyses showed no significant association between SRR/DAO and schizophrenia. We replicated the decreased serum D-serine levels in schizophrenia in the sample set, but D-serine levels did not correlate with SRR/DAO genotypes.

CONCLUSIONS

The SRR/DAO are not likely to be major genetic determinants in the development of schizophrenia or control of serum D-serine levels.

Regulation of serine racemase activity by amino acids

The effects of various amino acids on the activity of serine racemase, purified from mouse brain, were examined. Those acting as inhibitors included compounds with electron withdrawing groups on the beta-carbon of alanine (beta-halo-alanines and L-serine-O-sulfate), which can act as enzyme-activated inhibitors, and compounds containing beta-SH groups (cysteine and homocysteine) which react with enzyme-bound pyridoxal phosphate to form thiazolidine derivatives. Glycine and a series of metabolites related to L-aspartic acid (L-aspartic acid, L-asparagine, and oxaloacetic acid) were also found to be competitive inhibitors of the racemase. The Ki values for glycine and aspartic acid inhibition were 0.15 and 1.9 mM, respectively, indicating that alterations in the concentrations of these amino acids might play a role in the regulation of D-serine synthesis.

The pharmacological stimulation of NMDA receptors via co-agonist site: an fMRI study in the rat brain

d-Serine has been proposed as an endogenous modulator at the co-agonist glycine-binding site of N-methyl-d-aspartate (NMDA) receptors. There is still some debate as to whether this site is saturated in vivo, but it seems likely that this depends on regional differences in local glycine or d-serine concentrations. In order to identify areas where the co-agonist site was not fully activated in vivo, we studied the effect of intraperitoneal d-serine administration in the rat brain using functional magnetic resonance imaging (fMRI). Using contrast agent injection, the variations in the relative cerebral blood volume (CBVrel) in several regions of interest were evaluated. d-Serine (50 mg/kg) elicited a significant statistical increase in the CBVrel in the hippocampus. This effect was inhibited by the specific full antagonist of the co-agonist glycine site L-701,324 indicating that the hippocampal activation occurred through the binding of the agonist d-serine to the glycine-binding site of NMDA receptors. This result demonstrates that in the hippocampus, the co-agonist sites of NMDA receptors are not endogenously saturated under our experimental conditions, suggesting an important role of d-serine in the modulation of receptor function in the hippocampus.

Effect of systemic administration of D-serine on the levels of D- and L-serine in several brain areas and periphery of rat

To obtain further insight into the distribution and metabolism of exogenous D-serine, we have investigated the effect of the intraperitoneal administration of D-serine (10 mmol/kg) on the concentrations of D- and L-serine in several brain areas and periphery of infant and adult rats. The administration produced a significant augmentation of the D-serine levels not only in the cortex but also in the hippocampus, striatum, cerebellum and periphery. The rapid decline in the enhanced D-serine levels was observed in the periphery and cerebellum, whereas the injection caused a prolonged elevation of the D-serine levels in the cortex and hippocampus. The application caused a slight increase in the L-serine levels in several brain areas and periphery 3 or 6 h after the injection, whereas a significant decrease in the L-serine concentration was observed in the periphery, diencephalon and cerebellum 3 or 7 days after the injection. Because a structural abnormality and N-methyl-D-aspartate (NMDA) receptor hypofunction has been demonstrated in the cortex and hippocampus of schizophrenic subjects, D-serine treatment may offer a new therapeutic approach to diseases related to the hypofunction of NMDA receptors such as schizophrenia.

D-serine efficacy as add-on pharmacotherapy to risperidone and olanzapine for treatment-refractory schizophrenia

BACKGROUND

D-serine, a selective full agonist at the glycine site of N-methyl-D-aspartate glutamate receptor, might presently be the compound of choice for counteracting the hypothesized dysfunction of this receptor class in schizophrenia. Studies performed with Taiwanese patients indicate that D-serine significantly improves schizophrenia symptoms when used as adjuvant to conventional neuroleptics but not to clozapine. We assessed the efficacy and safety of D-serine adjuvant treatment for Occidental schizophrenia patients treated with newer atypical antipsychotics.

METHODS

Thirty-nine risperidone- or olanzapine-treated schizophrenia patients participated in a double-blind, placebo-controlled, 6-week crossover trial with 30 mg/kg/day D-serine added to their antipsychotic medication. Measures of clinical efficacy and side effects were determined biweekly throughout the study. Clinical laboratory parameters and amino acid serum levels were monitored.

RESULTS

D-serine administration induced increased serine serum levels (p < .001) and resulted in significant (p < .001) improvements in negative, positive, cognitive, and depression symptoms, as measured by the Positive and Negative Syndrome Scale. For approximately one third of the sample, D-serine treatment resulted in significant (>20%) reductions in Brief Psychiatric Rating Scale total scores. D-serine was well tolerated, and no detrimental changes in clinical laboratory parameters were noted.

CONCLUSIONS

These findings 1) indicate that risperidone and olanzapine efficacy might be augmented with D-serine adjuvant treatment; 2) confirm D-serine efficacy against main schizophrenia symptom domains; and 3) warrant the assessment of D-serine antipsychotic monotherapy for this illness.

Serine racemase modulates intracellular D-serine levels through an alpha,beta-elimination activity

Mammalian brain contains high levels of d-serine, an endogenous co-agonist of N-methyl D-aspartate type of glutamate receptors. D-Serine is synthesized by serine racemase, a brain enriched enzyme converting L- to D-serine. Degradation of D-serine is achieved by D-amino acid oxidase, but this enzyme is not present in forebrain areas that are highly enriched in D-serine. We now report that serine racemase catalyzes the degradation of cellular D-serine itself, through the alpha,beta-elimination of water. The enzyme also catalyzes water alpha,beta-elimination with L-serine and L-threonine. alpha,beta-Elimination with these substrates is observed both in vitro and in vivo. To investigate further the role of alpha,beta-elimination in regulating cellular D-serine, we generated a serine racemase mutant displaying selective impairment of alpha,beta-elimination activity (Q155D). Levels of D-serine synthesized by the Q155D mutant are several-fold higher than the wild-type both in vitro and in vivo. This suggests that the alpha,beta-elimination reaction limits the achievable D-serine concentration in vivo. Additional mutants in vicinal residues (H152S, P153S, and N154F) similarly altered the partition between the alpha,beta-elimination and racemization reactions. alpha,beta-Elimination also competes with the reverse serine racemase reaction in vivo. Although the formation of L- from D-serine is readily detected in Q155D mutant-expressing cells incubated with physiological D-serine concentrations, reversal with wild-type serine racemase-expressing cells required much higher D-serine concentration. We propose that alpha,beta-elimination provides a novel mechanism for regulating intracellular D-serine levels, especially in brain areas that do not possess D-amino acid oxidase activity. Extracellular D-serine is more stable toward alpha,beta-elimination, likely due to physical separation from serine racemase and its elimination activity.

The N-methyl D-aspartate receptor glycine site and D-serine metabolism: an evolutionary perspective

The N-methyl D-aspartate (NMDA) type of glutamate receptor requires two distinct agonists to operate. Glycine is assumed to be the endogenous ligand for the NMDA receptor glycine site, but this notion has been challenged by the discovery of high levels of endogenous d-serine in the mammalian forebrain. I have outlined an evolutionary framework for the appearance of a glycine site in animals and the metabolic events leading to high levels of D-serine in brain. Sequence alignments of the glycine-binding regions, along with the scant experimental data available, suggest that the properties of invertebrate NMDA receptor glycine sites are probably different from those in vertebrates. The synthesis of D-serine in brain is due to a pyridoxal-5'-phosphate (B(6))-requiring serine racemase in glia. Although it remains unknown when serine racemase first evolved, data concerning the evolution of B(6) enzymes, along with the known occurrences of serine racemases in animals, point to D-serine synthesis arising around the divergence time of arthropods. D-Serine catabolism occurs via the ancient peroxisomal enzyme d-amino acid oxidase (DAO), whose ontogenetic expression in the hindbrain of mammals is delayed until the postnatal period and absent from the forebrain. The phylogeny of D-serine metabolism has relevance to our understanding of brain ontogeny, schizophrenia and neurotransmitter dynamics.

Distribution and MK-801-induced expression of serine racemase mRNA in rat brain by real-time quantitative PCR

We have used real-time quantitative PCR methods to evaluate the effect of the systemic administration of (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine (MK-801; 0.4 mg/kg) on the gene expression of serine racemase in several brain areas of the rat. The levels of serine racemase mRNA in all the brain areas transiently increased after the administration. The present findings suggest that there is a link between the expression of serine racemase mRNA and the activity of the NMDA receptors.

Effect of the intracerebroventricular and systemic administration of L-serine on the concentrations of D- and L-serine in several brain areas and periphery of rat

To gain further insight into the metabolic mechanism of endogenous D-serine, the effect of the intracerebroventricular and intraperitoneal administration of L-serine on the concentrations of D- and L-serine in several brain areas and periphery was investigated. The intracerebroventricular injection of L-serine caused a rapid and marked increase in the L-serine levels in almost all brain regions of adult rats. This administration also produced a gradual increase in the D-serine levels in the forebrain, whereas a slight but significant elevation of D-serine was found in the cerebellum and pons-medulla. The intraperitoneal administration of L-serine caused a marked increase in the L-serine levels in all brain regions of both infant and adult rats. The treatment induced a significant augmentation of the D-serine levels in all brain regions of infant rats with higher concentrations in the cerebellum and cortex, whereas no significant change was observed in the cerebellum and pons-medulla of adult rats. These in vivo observation, together with the fact that immunohistochemical studies have indicated that both D-serine and serine racemase are highly concentrated in Bergmann glia of developing cerebellum, suggest that D-serine can be synthesized not only in the forebrain but also in the hindbrain by serine racemase. Furthermore, because the drastic decline in the cerebellar D-serine level coincides well with a dramatic increase in the cerebellar D-amino acid oxidase during early postnatal development, synthesized D-serine may be metabolized by D-amino acid oxidase in the hindbrain of adult rats.

Neurobiology through the looking-glass: D-serine as a new glial-derived transmitter

D-Amino acids have been known to be present in bacteria for more than 50 years, but only recently they were identified in mammals. The occurrence of D-amino acids in mammals challenge classic concepts in biology in which only L-amino acids would be present or thought to play important roles. Recent discoveries uncovered a role of endogenous D-serine as a putative glial-derived transmitter that regulates glutamatergic neurotransmission in mammalian brain. Free D-serine levels in the brain are about one third of L-serine values and its extracellular concentration is higher than many common L-amino acids. D-Serine occurs in protoplasmic astrocytes, a class of glial cells that ensheath the synapses and modulate neuronal activity. Biochemical and electrophysiological studies suggest that endogenous D-serine is a physiological modulator at the co-agonist site of NMDA-type of glutamate receptors. We previously showed that D-serine is synthesized by a glial serine racemase, a novel enzyme converting L- to D-serine in mammalian brain. The enzyme requires pyridoxal 5'-phosphate and it was the first racemase to be cloned from eucaryotes. Inhibitors of serine racemase have therapeutic implications for pathological processes in which over-stimulation of NMDA receptors takes place, such as stroke and neurodegenerative diseases. Here, we review the role of endogenous D-serine in modulating NMDA neurotransmission, its biosynthetic apparatus and the potential usefulness of serine racemase inhibitors as a novel neuroprotective strategy to decrease glutamate/NMDA excitotoxicity.

Cell density inversely regulates D- and L-aspartate levels in rat pheochromocytoma MPT1 cells

In a previous report (FEBS Lett. 434 (1998) 231), we demonstrated for the first time that D-aspartate (D-Asp) is synthesized in rat pheochromocytoma 12 (PC12) cells. This unique amino acid is believed to act as a novel messenger in mammalian cell regulation. However, the dynamics of D-Asp homeostasis in mammalian cells is yet to be elucidated. In this communication, we demonstrate that D-Asp is also synthesized in MPT1 cells (a subclone of PC12 cells) and that the D- and L-Asp levels in cells are regulated by cell density of the culture. Our data show that D-Asp levels increase, while in contrast, L-Asp levels decrease as a function of increased cell density. Conversely, in PC12 cells, which do not express the glutamate transporter involved in the incorporation of D- and L-Asp into cells, L-Asp levels decrease upon cell density increase while D-Asp concentrations remain almost unchanged. The results indicate that the biochemical behaviors of D- and L-Asp in mammalian cells are distinct and that the cellular levels of these stereoisomers appear to be under different control mechanisms.

Cloning of a D-serine-regulated transcript dsr-1 from the rat cerebral cortex

To obtain insight into the molecular mechanisms underlying the metabolism and functions of endogenous d-serine, we have explored d-serine-regulated transcripts in the neocortex of the infant rat treated with acute d-serine administration by using an RNA fingerprinting technique. Cloning and sequence analysis of the corresponding cDNAs to the identified transcripts have revealed that the dsr-1 (d-serine responsive transcript-1) mRNA is presumed to contain a novel sequence at the 5'-region, while the 631-base nucleotide sequence of its 3'-end is identical with that of rat M9.2 mRNA encoding a subunit of vacuolar type proton-ATPase. The predicted two open reading frames and their deduced amino acid sequences suggest that the dsr-1 product has a membrane spanning domain. The dsr-1 transcript was detected as a single band around 2.1 kb on the Northern blot. RT-PCR analyses have indicated that the dsr-1 transcript is expressed predominantly in the brain, lung, and testis, and that acute intraperitoneal injection of d-serine significantly upregulates dsr-1 expression in the neocortex 3 and 15 h later without affecting the levels of the M9.2 gene transcript. These results suggest that dsr-1 products may be involved in the d-serine-related metabolic or signaling pathways in mammalian brains.

Effects of N-methyl-D-aspartate, kainate or veratridine on extracellular concentrations of free D-serine and L-glutamate in rat striatum: an in vivo microdialysis study

Using an in vivo microdialysis technique, we have investigated the effect of N-methyl-D-aspartate (NMDA) or kainate on the extracellular concentrations of free D-serine and L-glutamate in the striatum. A intrastriatal perfusion of NMDA or kainate caused a significant increase in the extracellular release of L-glutamate, but a significant decrease in that of D-serine. Co-perfusion of an NMDA receptor antagonist, MK-801, or an alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid/kainate receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), with NMDA or kainate significantly reversed the NMDA- or kainate-induced decrease in the extracellular level of D-serine, respectively. The NMDA- or kainate-evoked increase in the extracellular L-glutamate level was also reversed by co-perfusion of MK-801 or CNQX, respectively. Because D-serine acts as a potent and selective agonist for the glycine site of the NMDA receptor and because intracerebroventricularly injected D-serine is accumulated in the astrocytes, D-serine could be taken up by the astrocytes following synaptic activation. Furthermore, because cortical ablation to remove corticostriatal glutamatergic inputs attenuates the excitotoxic effects of kainate in the striatum, L-glutamate may enhance its own release through a presynaptic NMDA and/or non-NMDA receptor-mediated mechanism.

IV glycine and oral D-cycloserine effects on plasma and CSF amino acids in healthy humans

BACKGROUND

The amino acid glycine, modulates neurotransmission via actions at GLY-A receptor and GLY-B receptor. The latter are coagonist sites associated with N-Methyl-D-Aspartate (NMDA) glutamate receptors. The central bioavailability of peripherally administered glycine has not been adequately characterized in humans.

METHODS

Healthy human subjects were administered either oral D-cycloserine (50 mg or placebo) and intravenous glycine (saline, 100 mg/kg or 200 mg/kg) in random order over 4 test days under double-blind conditions. Cerebrospinal fluid was collected by lumbar puncture performed on the first test day was analyzed to determine amino acid levels. The acoustic startle response was measured on the second test day.

RESULTS

Intravenous glycine dose-dependently increased both serum and CSF glycine and serine levels. Neither glycine nor DCS produced any significant effects on behavior, cognition or the acoustic startle response. Neither IV glycine nor DCS were associated with any toxicity.

CONCLUSIONS

Thus, peripheral glycine administration raised CSF glycine levels without producing any clear central nervous system effects. Glycine and D-cycloserine did not worsen cognitive test performance and did not induce behavioral symptoms on their own. The possibility that glycine and D-cycloserine enhanced cognitive test performance cannot be excluded given the psychometric limitations of the test battery.

Alteration in the D-amino acid content of the rat pineal gland under anesthesia

In a previous report (Hamase, K. et al., Biochim Biophys Acta 1134: 214-222 (1997)), we showed that the rat pineal gland contains D-leucine (D-Leu) as well as D-aspartic acid (D-Asp). In this communication we report alterations in the content of these D-amino acids during anesthesia. The D-Asp content was significantly increased from 2.8 to 5.0, 4.8 and 5.8 nmol/pineal gland by administration of ether, urethane and pentobarbital, respectively. In contrast, the D-Leu content was decreased by administration of urethane or pentobarbital. The D-Leu content decreased from 4.2 to 2.2 pmol/pineal gland 4 hours after administration of urethane, although the content remained unchanged until 1.5 hours after administration. The content of the L-enantiomers of these amino acids were not affected by anesthesia. The urethane-induced decrease in D-leucine content was almost completely suppressed by a beta-agonist, (-)-isoproterenol, whereas the agonist itself had no effect.

Biosynthesis of D-aspartate in mammalian cells

In this communication, we demonstrate that D-aspartate (D-Asp) is synthesized in pheochromocytoma cells (PC12). To our knowledge this is the first report of biosynthesis of D-Asp in mammalian cells. Synthesis of D-Asp was demonstrated by its time-dependent accumulation in the cell culture, and by the fact that this accumulation was proportional to the number of inoculated cells. D-Asp in PC12 cells was identified by (i) co-elution with authentic D-Asp on two different HPLC columns, an octadesyl silica column and a Pirkle-type chiral column, (ii) reversed elution order of D-Asp and L-Asp on another Pirkle-type chiral column with an opposite configuration, and (iii) sensitivity to D-Asp oxidase. In the cells the amount of D-Asp was approx. 12-14% of total Asp and no other investigated D-amino acid was detected. The amount of D-Asp did not increase during the culture of mouse 3T3 fibroblasts and human neuroblastoma NB-1 cells. Immunocytochemical staining with anti-D-Asp antiserum demonstrated that D-Asp synthesized is present in the cytoplasm of the cells.

D-aspartate uptake into cultured rat pinealocytes and the concomitant effect on L-aspartate levels and melatonin secretion

Significant amounts of D-aspartate (Asp) are found in mammalian tissues and D-Asp is presumed to play some significant, but as yet undefined physiological role. However, it is not known whether D-Asp is synthesized in mammals. In this study, we addressed this issue in cultured rat pinealocytes, parenchymal cells of the pineal gland, which contain significant amounts of D-Asp. Biosynthesis of D-Asp was found to be minimal to non-existent in cultured rat pinealocytes. We then investigated the mechanism of uptake of D-Asp into these cells and its consequent effect on cell function. D-Asp was efficiently taken up into cells, in a time- and dose-dependent manner. Interestingly, the L-Asp levels in the cells and media decreased concomitantly with the uptake of D-Asp. This decrease was not due to D-Asp cytotoxicity, since the cellular levels of othernted. D-Serine and D-alanine were not taken up efficiently into the cells and the cellular levels of L-serine and L-alanine were unchanged. Also, immunocytochemical staining with anti-D-Asp antibody showed that D-Asp, which had been taken up into the cells, was dispersed throughout the cytoplasm. In response to norepinephrine stimulation, pinealocytes, which had been pretreated with D-Asp released D-Asp as well as L-Asp. In these cells, norepinephrine-induced secretion of melatonin, a pineal hormone, was suppressed. The mechanism of this suppression is discussed here.