D-serine as a neuromodulator: regional and developmental localizations in rat brain glia resemble NMDA receptors

Metabolism of the neuromodulator D-serine

Over the past years, accumulating evidence has indicated that D-serine is the endogenous ligand for the glycine-modulatory binding site on the NR1 subunit of N-methyl-D-aspartate receptors in various brain areas. D-Serine is synthesized in glial cells and neurons by the pyridoxal-5' phosphate-dependent enzyme serine racemase, and it is released upon activation of glutamate receptors. The cellular concentration of this novel messenger is regulated by both serine racemase isomerization and elimination reactions, as well as by its selective degradation catalyzed by the flavin adenine dinucleotide-containing flavoenzyme D-amino acid oxidase. Here, we present an overview of the current knowledge of the metabolism of D-serine in human brain at the molecular and cellular levels, with a specific emphasis on the brain localization and regulatory pathways of D-serine, serine racemase, and D-amino acid oxidase. Furthermore, we discuss how D-serine is involved with specific pathological conditions related to N-methyl-D-aspartate receptors over- or down-regulation.

Neuron-astrocyte interactions in the medial nucleus of the trapezoid body

The calyx of Held (CoH) synapse serves as a model system to analyze basic mechanisms of synaptic transmission. Astrocyte processes are part of the synaptic structure and contact both pre- and postsynaptic membranes. In the medial nucleus of the trapezoid body (MNTB), midline stimulation evoked a current response that was not mediated by glutamate receptors or glutamate uptake, despite the fact that astrocytes express functional receptors and transporters. However, astrocytes showed spontaneous Ca²⁺ responses and neuronal slow inward currents (nSICs) were recorded in the postsynaptic principal neurons (PPNs) of the MNTB. These currents were correlated with astrocytic Ca²⁺ activity because dialysis of astrocytes with BAPTA abolished nSICs. Moreover, the frequency of these currents was increased when Ca²⁺ responses in astrocytes were elicited. NMDA antagonists selectively blocked nSICs while D-serine degradation significantly reduced NMDA-mediated currents. In contrast to previous studies in the hippocampus, these NMDA-mediated currents were rarely synchronized.

The structure of mammalian serine racemase: evidence for conformational changes upon inhibitor binding

Serine racemase is responsible for the synthesis of D-serine, an endogenous co-agonist for N-methyl-D-aspartate receptor-type glutamate receptors (NMDARs). This pyridoxal 5'-phosphate-dependent enzyme is involved both in the reversible conversion of L- to D-serine and serine catabolism by alpha,beta-elimination of water, thereby regulating D-serine levels. Because D-serine affects NMDAR signaling throughout the brain, serine racemase is a promising target for the treatment of disorders related to NMDAR dysfunction. To provide a molecular basis for rational drug design the x-ray crystal structures of human and rat serine racemase were determined at 1.5- and 2.1-A resolution, respectively, and in the presence and absence of the orthosteric inhibitor malonate. The structures revealed a fold typical of beta-family pyridoxal 5'-phosphate enzymes, with both a large domain and a flexible small domain associated into a symmetric dimer, and indicated a ligand-induced rearrangement of the small domain that organizes the active site for specific turnover of the substrate.

D-amino acid oxidase generates agonists of the aryl hydrocarbon receptor from D-tryptophan

The aryl hydrocarbon receptor (AHR) is well-known for its role in mediating the toxic and adaptive responses to xenobiotic compounds. Recent studies also indicate that AHR ligands are endogenously produced and may be essential for normal development. Previously, we showed that the endogenous enzyme, aspartate aminotransferase (AST), generates the AHR proagonist, indole-3-pyruvic acid (I3P), by deamination of its substrate L-tryptophan. We hypothesized that other enzymatic pathways capable of producing I3P may generate AHR agonists in vivo. We now demonstrate that the enzyme d-amino acid oxidase (DAAO) catalyzes the production of AHR agonists through the enzymatic conversion of D-tryptophan to I3P. Moreover, we provide evidence that the nonenzymatic oxidation and condensation of I3P is a critical step in the generation of receptor agonists by DAAO and AST. Products of this process include two novel agonists, 1,3-di(1H-indol-3-yl)propan-2-one and 1-(1H-indol-3-yl)-3-(3H-indol-3-ylidene) propan-2-one [characterized in the accompanying paper, Chowdhury et al. ( 2009 ) Chem. Res. Toxicol. , DOI: 10.1021/tx9000418 ], both of which can potently activate the AHR at concentrations in the nanomolar range. These results show that endogenous AHR activity can be modulated by I3P production from amino acid precursors through multiple enzymatic pathways, including those catalyzed by DAAO and AST.

L-serine synthesis in the central nervous system: a review on serine deficiency disorders

The de novo synthesis of the amino acid L-serine plays an essential role in the development and functioning of the central nervous system (CNS). L-serine displays many metabolic functions during different developmental stages; among its functions providing precursors for amino acids, protein synthesis, nucleotide synthesis, neurotransmitter synthesis and L-serine derived lipids. Patients with congenital defects in the L-serine synthesizing enzymes present with severe neurological abnormalities and underscore the importance of this synthetic pathway. In this review, we will discuss the cellular functions of the L-serine pathway, structure and enzymatic properties of the enzymes involved and genetic defects associated with this pathway.

High-dose glycine treatment of refractory obsessive-compulsive disorder and body dysmorphic disorder in a 5-year period

This paper describes an individual who was diagnosed with obsessive-compulsive disorder (OCD) and body dysmorphic disorder (BDD) at age 17 when education was discontinued. By age 19, he was housebound without social contacts except for parents. Adequate trials of three selective serotonin reuptake inhibitors, two with atypical neuroleptics, were ineffective. Major exacerbations following ear infections involving Group A β-hemolytic streptococcus at ages 19 and 20 led to intravenous immune globulin therapy, which was also ineffective. At age 22, another severe exacerbation followed antibiotic treatment for H. pylori. This led to a hypothesis that postulates deficient signal transduction by the N-methyl-D-aspartate receptor (NMDAR). Treatment with glycine, an NMDAR coagonist, over 5 years led to robust reduction of OCD/BDD signs and symptoms except for partial relapses during treatment cessation. Education and social life were resumed and evidence suggests improved cognition. Our findings motivate further study of glycine treatment of OCD and BDD.

The neurobiology of D-amino acid oxidase and its involvement in schizophrenia

D-amino acid oxidase (DAO) is a flavoenzyme that metabolizes certain D-amino acids, notably the endogenous N-methyl D-aspartate receptor (NMDAR) co-agonist, D-serine. As such, it has the potential to modulate the function of NMDAR and to contribute to the widely hypothesized involvement of NMDAR signalling in schizophrenia. Three lines of evidence now provide support for this possibility: DAO shows genetic associations with the disorder in several, although not all, studies; the expression and activity of DAO are increased in schizophrenia; and DAO inactivation in rodents produces behavioural and biochemical effects, suggestive of potential therapeutic benefits. However, several key issues remain unclear. These include the regional, cellular and subcellular localization of DAO, the physiological importance of DAO and its substrates other than D-serine, as well as the causes and consequences of elevated DAO in schizophrenia. Herein, we critically review the neurobiology of DAO, its involvement in schizophrenia, and the therapeutic value of DAO inhibition. This review also highlights issues that have a broader relevance beyond DAO itself: how should we weigh up convergent and cumulatively impressive, but individually inconclusive, pieces of evidence regarding the role that a given gene may have in the aetiology, pathophysiology and pharmacotherapy of schizophrenia?

d-Serine enhancement of NMDA receptor-mediated calcium increases in rat retinal ganglion cells

NMDA receptor (NMDAR) activation is enhanced by d-serine or glycine acting at a specific binding site. Previous work has shown d-serine enhancement of NMDAR currents in retinal ganglion cells. One of the major functions of most NMDA channels is to permit calcium influx into cells. We show that d-serine enhances glutamate-induced calcium responses in immunopanned retinal ganglion cells. This effect was specific to NMDA receptors as similar results were found with NMDA, but not kainate, and was reduced or blocked by modulators of the NMDAR coagonist binding site. d-Serine and glycine enhanced glutamate-induced calcium responses in a dose-dependent manner and at equimolar concentrations there was no difference in the efficacy of the coagonists. In isolated retinas NMDA-induced calcium responses were enhanced by d-serine coapplication in 46% of ganglion cells. Endogenous d-serine degradation by treatment with d-amino acid oxidase caused a approximately 45% decrease in the NMDA-induced response that could be reversed by coapplication with d-serine. d-Serine and glycine were equally effective in enhancing glutamatergic calcium responses. Endogenous d-serine contributes to NMDAR activation in retinal wholemounts and some but not all retinal ganglion cells may experience saturating levels of d-serine or glycine.

Reduced serine racemase expression contributes to age-related deficits in hippocampal cognitive function

To gain insight into the contribution of d-serine to impaired cognitive aging, we compared the metabolic pathway and content of the amino acid as well as d-serine-dependent synaptic transmission and plasticity in the hippocampus of young and old rats of the Wistar and Lou/C/Jall strains. Wistar rats display cognitive impairments with aging that are not found in the latter strain, which is therefore considered a model of healthy aging. Both mRNA and protein levels of serine racemase, the d-serine synthesizing enzyme, were decreased in the hippocampus but not in the cerebral cortex or cerebellum of aged Wistar rats, whereas the expression of d-amino acid oxidase, which degrades the amino acid, was not affected. Consequently, hippocampal levels of endogenous d-serine were significantly lower. In contrast, serine racemase expression and d-serine levels were not altered in the hippocampus of aged Lou/C/Jall rats. Ex vivo electrophysiological recordings in hippocampal slices showed a marked reduction in N-methyl-d-aspartate-receptor (NMDA-R)-mediated synaptic potentials and theta-burst-induced long-term potentiation (LTP) in the CA1 area of aged Wistar rats, which were restored by exogenous d-serine. In contrast, NMDA-R activation, LTP induction and responses to d-serine were not altered in aged Lou/C/Jall rats. These results further strengthen the notion that the serine racemase-dependent pathway is a prime target of hippocampus-dependent cognitive deficits with aging. Understanding the processes that specifically affect serine racemase during aging could thus provide key insights into the treatment of memory deficits in the elderly.

Intraperiaqueductal gray glycine and D-serine exert dual effects on rostral ventromedial medulla ON- and OFF-cell activity and thermoceptive threshold in the rat

We have studied the involvement of the N-methyl-D-aspartate receptor (NMDAR) glycine site and the strychnine-sensitive glycine receptor (GlyR) in the ventrolateral periaqueductal gray (VL-PAG) on nociceptive behavior (tail flick) and pain-related changes on neuronal activity in the rostral ventromedial medulla (RVM). Glycine or D-serine increased the tail-flick latency, reduced OFF-cell pause, and delayed its onset and increased the time between the onset of the OFF-cell pause and the tail withdrawal. Conversely, they decreased the ongoing activity of the ON cell, the tail-flick-induced ON-cell firing, whereas they delayed the onset of increased tail-flick-induced ON-cell firing. Also, glycine or D-serine reduced the interval between the onset of the increased ON-cell firing and tail withdrawal. Whereas 7-Cl-kynurenic acid (7-Cl-KYN) prevented such effects, strychnine did not do so. A higher dose of 7-Cl-KYN or strychnine was per se able to reduce or increase tail-flick latency and increase or reduce ON-cell activities, respectively. A higher dose of glycine was hyperalgesic in the presence of 7-Cl-KYN, whereas such an effect was prevented by strychnine. These data suggest 1) a dual role of glycine in producing hyperalgesia or analgesia by stimulating the GlyR or the NMDARs within the VL-PAG, respectively; 2) consistently that RVM ON and OFF cells display opposite firing patterns to the stimulation of the VL-PAG NMDAR glycine site and GlyR activation; and 3) a tonic role of these receptors within the VL-PAG-RVM antinociceptive descending pathway.

Functional and immunocytochemical characterization of D-serine transporters in cortical neuron and astrocyte cultures

D-serine is an endogenous coagonist of N-methyl-D-aspartate (NMDA) receptors that plays an important role in synaptic function, neuronal development, and excitotoxicity. Mechanisms of D-serine transport are important in regulation of extracellular D-serine concentration and therefore of these critical processes. D-serine can be transported with low affinity through the Na(+)-dependent amino acid transporter termed ASCT2, whereas high-affinity D-serine uptake has been reported through the Na(+)-independent transporter termed asc-1. We investigated immunoreactivity for ASCT2 and asc-1 and D-serine transport kinetics in cultured cortical neurons and astrocytes to gain insight into how D-serine transporters regulate CNS D-serine levels. Both neurons and astrocytes exhibited low-affinity Na(+)-dependent D-serine uptake (K(T) > 1 mM) with broad substrate selectivity that was consistent with uptake through ASCT2. Both neurons and astrocytes also stained positively for ASCT2 in immunocytochemistry studies. Neurons but not astrocytes stained positively for the high-affinity D-serine transporter asc-1, but no evidence of functional asc-1 could be detected in neurons with conditions that produced such activity in cortical synaptosomes. These data support ASCT2 function in both neuron and astrocyte cultures and identify a discrepancy between observed asc-1 immunoreactivity and lack of functional asc-1 activity in neuron cultures. Together these findings further our knowledge of the processes that govern D-serine regulation.

The involvement of the NMDA receptor D-serine/glycine site in the pathophysiology and treatment of schizophrenia

Hypofunction of the N-methyl-D-aspartate receptor (NMDAR) has been implicated in the pathophysiology of schizophrenia. The NMDAR contains a D-serine/glycine site on the NR1 subunit that may be a promising therapeutic target for psychiatric illness. This review outlines the complex regulation of endogenous NMDAR D-serine/glycine site agonists and explores their contribution to schizophrenia pathogenesis and their potential clinical utility. Genetic studies have associated genes influencing NMDAR D-serine/glycine site activation with an increased susceptibility to schizophrenia. Postmortem studies have identified abnormalities in several transcripts affecting D-serine/glycine site activity, consistent with in vivo reports of alterations in levels of endogenous D-serine/glycine site agonists and antagonists. Genetically modified mice with aberrant NMDAR D-serine/glycine site function model certain features of the negative and cognitive symptoms of schizophrenia, and similar behavioral abnormalities have been observed in other candidate genes models. Compounds that directly activate the NMDAR D-serine/glycine site or inhibit glycine transport have demonstrated beneficial effects in preclinical models and clinical trials. Future pharmacological approaches for schizophrenia treatment may involve targeting enzymes that affect D-serine synthesis and metabolism.

Targeted disruption of serine racemase affects glutamatergic neurotransmission and behavior

A subset of glutamate receptors that are specifically sensitive to the glutamate analog N-methyl-D-aspartate (NMDA) are molecular coincidence detectors, necessary for activity-dependent processes of neurodevelopment and in sensory and cognitive functions. The activity of these receptors is modulated by the endogenous amino acid D-serine, but the extent to which D-serine is necessary for the normal development and function of the mammalian nervous system was previously unknown. Decreased signaling at NMDA receptors has been implicated in the pathophysiology of schizophrenia based on pharmacological evidence, and several human genes related to D-serine metabolism and glutamatergic neurotransmission have been implicated in the etiology of schizophrenia. Here we show that genetically modified mice lacking the ability to produce D-serine endogenously have profoundly altered glutamatergic neurotransmission, and relatively subtle but significant behavioral abnormalities that reflect hyperactivity and impaired spatial memory, and that are consistent with elevated anxiety.

Serine racemase is associated with schizophrenia susceptibility in humans and in a mouse model

Abnormal N-methyl-d-aspartate receptor (NMDAR) function has been implicated in the pathophysiology of schizophrenia. d-serine is an important NMDAR modulator, and to elucidate the role of the d-serine synthesis enzyme serine racemase (Srr) in schizophrenia, we identified and characterized mice with an ENU-induced mutation that results in a complete loss of Srr activity and dramatically reduced d-serine levels. Mutant mice displayed behaviors relevant to schizophrenia, including impairments in prepulse inhibition, sociability and spatial discrimination. Behavioral deficits were exacerbated by an NMDAR antagonist and ameliorated by d-serine or the atypical antipsychotic clozapine. Expression profiling revealed that the Srr mutation influenced several genes that have been linked to schizophrenia and cognitive ability. Transcript levels altered by the Srr mutation were also normalized by d-serine or clozapine treatment. Furthermore, analysis of SRR genetic variants in humans identified a robust association with schizophrenia. This study demonstrates that aberrant Srr function and diminished d-serine may contribute to schizophrenia pathogenesis.

Functional characterization and in silico docking of full and partial GluK2 kainate receptor agonists

Two structural models have been developed to explain how agonist binding leads to ionotropic glutamate receptor (iGluR) activation. At alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) iGluRs, full and partial agonists close the agonist-binding domain (ABD) to different degrees whereas agonist-induced domain closure is apparently fixed at N-methyl-D-aspartate receptors. Although kainate (KA) iGluRs are thought to behave like AMPA receptors, the issue has not been formally tested because of the paucity of available receptor agonists. Here we identify a series of structurally related full and partial agonists at GluK2 (formerly GluR6) KARs and predict their docking mode using the in silico ligand-docking program FITTED. As expected, the neurotransmitter L-Glu behaved as a full agonist but modest reduction (e.g., L-serine or L-aspartate) or elongation (e.g., L-alpha-aminoadipate) in chain length generated weak partial agonists. It is noteworthy that in silico ligand-docking predicted that most partial agonists select for the closed and not, as expected, the open or intermediate conformations of the GluK2 ABD. Experiments using concanavalin-A to directly report conformations in the intact GluK2 receptor support this prediction with the full agonist, L-Glu, indistinguishable in this regard from weak partial agonists, D- and L-Asp. Exceptions to this were KA and domoate, which failed to elicit full closure as a result of steric hindrance by a key tyrosine residue. Our data suggest that alternative structural models need to be considered to describe agonist behavior at KARs. Finally, our study identifies the responsiveness to several neurotransmitter candidates establishing the possibility that endogenous amino acids other than L-Glu may regulate native KARs at central synapses.

Glial-neuronal interactions--implications for plasticity and drug addiction

Among neuroscientists, astrocytes have for long played Cinderella to their neuron stepsisters. While the importance of glia in regulating brain activity was predicted by Ramon y Cajal more than a century ago (Garcia-Marin et al., Trends. Neurosci. 30:479-787, 2007), these cells, until recently, have been thought to play mainly a passive part in synaptic signaling. Results obtained over the last decade have begun to suggest otherwise. Experiments carried out in a number of labs have shown that glial cells, especially astrocytes, directly participate in synaptic signaling and potentially regulate synaptic plasticity and network excitability. The presence of signaling pathways on astrocytes that are analogous to those at presynaptic terminals suggests a role for these cells in network plasticity. Findings that the same signaling pathways can be activated by receptors for drugs of abuse present on astrocytes suggest a role for these cells in the addictive process. In this review, we summarize current understanding of astrocytic role in synaptic signaling and suggest that a complete understanding of the process of addiction requires a better understanding of the functional role of these cells.

Genetic inactivation of D-amino acid oxidase enhances extinction and reversal learning in mice

Activation of the N-methyl-D-aspartate receptor (NMDAR) glycine site has been shown to accelerate adaptive forms of learning that may benefit psychopathologies involving cognitive and perseverative disturbances. In this study, the effects of increasing the brain levels of the endogenous NMDAR glycine site agonist D-serine, through the genetic inactivation of its catabolic enzyme D-amino acid oxidase (DAO), were examined in behavioral tests of learning and memory. In the Morris water maze task (MWM), mice carrying the hypofunctional Dao1(G181R) mutation demonstrated normal acquisition of a single platform location but had substantially improved memory for a new target location in the subsequent reversal phase. Furthermore, Dao1(G181R) mutant animals exhibited an increased rate of extinction in the MWM that was similarly observed following pharmacological administration of D-serine (600 mg/kg) in wild-type C57BL/6J mice. In contextual and cued fear conditioning, no alterations were found in initial associative memory recall; however, extinction of the contextual fear memory was facilitated in mutant animals. Thus, an augmented level of D-serine resulting from reduced DAO activity promotes adaptive learning in response to changing conditions. The NMDAR glycine site and DAO may be promising therapeutic targets to improve cognitive flexibility and inhibitory learning in psychiatric disorders such as schizophrenia and anxiety syndromes.

Cellular distribution of the neutral amino acid transporter subtype ASCT2 in mouse brain

ASCT2 is an ASC (alanine-, serine-, cysteine-preferring) neutral amino acid exchanger that may regulate CNS function by transporting amino acid substrates including L-serine, L-cysteine, L-glutamine, L-glutamate and D-serine. Despite the potentially important role of ASCT2 in influencing metabolic and signaling functions of these amino acids in brain, there has been little description of its distribution in brain tissue. We employed a commercially available human ASCT2 antibody in immunohistochemistry studies in adult mouse brain and found a wide regional distribution for ASCT2 that was limited to dendrites labeled by anti-microtubule-associated protein-2 in cortex, hippocampus and striatum. No ASCT2 immunoreactivity was observed in areas labeled by antibodies against a neuronal cell body marker (NeuN), or either of the astrocyte markers, glial fibrillary acidic protein or S100beta. In cerebellum both Purkinje cell bodies and dendrites were positive for ASCT2 immunoreactivity. In support of a dendritic localization for ASCT2 in cortex, low affinity (K(T) > 1 mM), Na(+)-dependent D-serine and L-glutamine uptake characteristic of ASCT2-mediated transport was observed in P2 synaptosomal preparations. These results suggest that ASCT2 may be an important neuronal neutral amino acid transporter and highlight a discrepancy between findings of astrocyte ASCT2 function in tissue culture and brain in situ.

D-cycloserine enhances short-delay, but not long-delay, conditioned taste aversion learning in rats

NMDA receptors have been implicated in conditioned taste aversion (CTA), a form of associative learning with the unique temporal characteristic of associating taste and toxic stimuli across very long delays. d-cycloserine (DCS), an NMDA receptor agonist, has been shown to enhance short-delay CTA learning. Here we examined the interaction of DCS with varying temporal parameters of CTA. DCS (15 mg/kg) administered prior to the pairing of 0.125% saccharin and LiCl (38 mM, 12 ml/kg) enhanced CTA when there was a short delay between the taste-toxin pairing (10 min), but not when there was a long delay (45 min). DCS activity remained at effective levels over the long delay, because DCS administered 60 min prior to a short-delay pairing enhanced CTA. The interaction of DCS with the delay between taste stimulus onset and LiCl injection was investigated by administering DCS and then 5 min access to saccharin 45 min prior to a short-delay pairing of saccharin and LiCl. DCS failed to enhance CTA in rats pre-exposed to saccharin, even with a short delay between the second saccharin exposure and LiCl injection. These results suggest that DCS enhancement of CTA is dependent on mechanisms underlying gustatory processing during long-delay taste-toxin associations.

Spinal N-methyl-D-aspartate receptors and nociception-evoked release of primary afferent substance P

Dorsal horn N-methyl-D-aspartate (NMDA) receptors contribute significantly to spinal nociceptive processing through an effect postsynaptic to non-primary glutamatergic axons, and perhaps presynaptic to the primary afferent terminals. The present study sought to examine the regulatory effects of NMDA receptors on primary afferent release of substance P (SP), as measured by neurokinin 1 receptor (NK1r) internalization in the spinal dorsal horn of rats. The effects of intrathecal NMDA alone or in combination with D-serine (a glycine site agonist) were initially examined on basal levels of NK1r internalization. NMDA alone or when co-administered with D-serine failed to induce NK1r internalization, whereas activation of spinal TRPV1 receptors by capsaicin resulted in a notable NK1r internalization. To determine whether NMDA receptor activation could potentiate NK1r internalization or pain behavior induced by a peripheral noxious stimulus, intrathecal NMDA was given prior to an intraplantar injection of formalin. NMDA did not alter the formalin-induced NK1r internalization nor did it enhance the formalin paw flinching behavior. To further characterize the effects of presynaptic NMDA receptors, the NMDA antagonists DL-2-amino-5-phosphonopentanoic acid (AP-5) and MK-801 were intrathecally administered to assess their regulatory effects on formalin-induced NK1r internalization and pain behavior. AP-5 had no effect on formalin-induced NK1r internalization, whereas MK-801 produced only a modest reduction. Both antagonists, however, reduced the formalin paw flinching behavior. In subsequent in vitro experiments, perfusion of NMDA in spinal cord slice preparations did not evoke basal release of SP or calcitonin gene-related peptide (CGRP). Likewise, perfusion of NMDA did not enhance capsaicin-evoked release of the two peptides. These results suggest that presynaptic NMDA receptors in the spinal cord play little if any role on the primary afferent release of SP.

To gate or not to gate: are the delta subunits in the glutamate receptor family functional ion channels?

The two delta receptor subunits remain the most puzzling enigma within the ionotropic glutamate receptor family. Despite the recent elucidation of the ligand-binding domain structure of delta2, many fundamental questions with regard to the subunits' mechanism of function still remain unanswered. Of necessity, the majority of studies on delta receptors focused on the metabotropic function of delta2, since electrophysiological approaches to date are limited to the characterization of spontaneous currents through the delta2-lurcher mutant. Indeed, accumulated evidence primarily from delta2-deficient transgenic mice suggest that major physiological roles of delta2 are mediated via metabotropic signaling by the subunit's C terminus. Why then would the subunits retain a conserved ion channel domain if they do not form functional ion channels? Any progress with regard to ionotropic function of the two delta subunits has been hampered by their largely unknown pharmacology. Even now that a pharmacological profile for delta2 is being established on the basis of the ligand-binding domain structure, wild-type delta2 channels in heterologous expression systems stay closed in the presence of molecules that have been demonstrated to bind to the receptor's ligand-binding domain. In this paper, we review the current knowledge of delta subunits focusing on the disputed ionotropic function.

Inhibition of D-amino-Acid oxidase activity induces pain relief in mice

(1). We investigated the effects of inhibiting D: -amino-acid oxidase (DAO) activity on nociceptive responses through the use of mutant ddY/DAO(-) mice, which lack DAO activity, and through the application of a selective inhibitor of DAO, sodium benzoate, in the tail flick test, hot-plate test, formalin test, and acetic acid-induced writhing test. (2). Compared with normal ddY/DAO+ mice, ddY/DAO(- )mice showed significantly prolonged tail withdrawal latency in the tail flick test and licking/jumping latency in the hot-plate test, as well as significantly reduced duration of licking/biting in the late phase of the formalin test and the number of abdominal writhing in the acetic acid-induced writhing test. (3). In addition, we investigated the effects of sodium benzoate in Kunming mice having normal DAO activity. (4). Intravenous administration of sodium benzoate (400 mg/kg) significantly inhibited pain responses of the late phase of the formalin test and abdominal writhing responses in the acetic acid-induced writhing test, with no effects on the early phase flinch responses in the formalin test, nociceptive responses in the tail flick test, or hot-plate test. (5). These results suggest that DAO acts as a pro-nociceptive factor in pain, particularly chronic pain, transmission and modulation, and may be a target for pain treatment.

Role for D-serine within the ventral tegmental area in the development of cocaine's sensitization

Repeated exposure to cocaine results in motor sensitization that, in the ventral tegmental area (VTA), is associated to enhanced glutamate release, which in turn leads to enhanced calcium levels in dopaminergic neurons. Calcium influx activates calcium-calmodulin-dependent protein kinases such as CaMKII. D-Serine could participate on these effects, and the objective was to discern the role of VTA D-serine after a sensitizing regimen of cocaine (10 mg/kg daily), and to discern consequent expression changes in CaMKII and its activated form. For this purpose, D-serine, sodium benzoate (inhibitor of D-amino acid oxidase, the degradating enzyme of D-serine), and 7-chlorokynurenate (inhibitor of the glycine site of NMDA receptors) were injected into the VTA (in either the induction or expression phase of sensitization), and activation state of CaMKII was assessed through blotting. The findings indicated that intra-VTA administration of D-serine (5 mM) and sodium benzoate (100 and 200 microg/microl) during the induction phase (not expression) reliably augmented the expression of behavioral sensitization to cocaine, providing evidence that D-serine in the VTA participates in the initiation of motor sensitization to this psychostimulant drug. Intra-VTA infusions of D-serine, sodium benzoate and 7-chlorokynurenate did not elicit a motor effect of their own. Confirming the important role of NMDA receptors and their activation at the glycine site, the employment of 7-chlorokynurenate (2 and 5 microg/microl) led to blocking of the development of sensitization to cocaine. CaMKII within the VTA was found to participate in D-serine's effects because this kinase, that is activated after repeated cocaine, was further activated after co-treatment with D-serine or sodium benzoate. Besides CaMKII activity was otherwise reduced by 7-chlorokynurenate.

D-serine augments NMDA-NR2B receptor-dependent hippocampal long-term depression and spatial reversal learning

The contributions of hippocampal long-term depression (LTD) to explicit learning and memory are poorly understood. Electrophysiological and behavioral studies examined the effects of modulating NMDA receptor-dependent LTD on spatial learning in the Morris water maze (MWM). The NMDA receptor co-agonist D-serine substantially enhanced NR2B-dependent LTD, but not long-term potentiation (LTP) or depotentiation, in hippocampal slices from adult wild type mice. Exogenous D-serine did not alter MWM acquisition, but substantially enhanced subsequent reversal learning of a novel target location and performance in a delayed-matching-to-place task. Conversely, an NR2B antagonist disrupted reversal learning and promoted perseveration. Endogenous synaptic D-serine likely saturates during LTP induction because exogenous D-serine rescued deficient LTP and MWM acquisition in Grin1(D481N) mutant mice having a lower D-serine affinity. Thus, D-serine may enhance a form of hippocampal NR2B-dependent LTD that contributes to spatial reversal learning. By enhancing this form of synaptic plasticity, D-serine could improve cognitive flexibility in psychiatric disorders characterized by perseveration of aberrant ideation or behaviors.

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.

In vitro and in vivo pharmacological profile of AS057278, a selective d-amino acid oxidase inhibitor with potential anti-psychotic properties

Non-competitive N-methyl-d-aspartate (NMDA) blockers induce schizophrenic-like behavior in healthy volunteers and exacerbate symptomatology in schizophrenic patients. Hence, a compound able to enhance NMDA neurotransmission by increasing levels of d-serine, an endogenous full agonist at the glycine site of the NMDA receptors, could have anti-psychotic activity. One way to increase d-serine levels is the inhibition of d-amino acid oxidase (DAAO), the enzyme responsible for d-serine oxidation. Indeed AS057278, a potent in vitro (IC(50)=0.91 microM) and ex vivo (ED(50)=2.2-3.95 microM) DAAO inhibitor, was able to increase d-serine fraction in rat cortex and midbrain (10 mg/kg i.v.). AS057278 was able to normalize phencyclidine (PCP)-induced prepulse inhibition after acute (80 mg/kg) and chronic (20 mg/kg b.i.d.) oral administration in mice. Finally, AS057278 after oral chronic treatment (10 mg/kg b.i.d.) was able to normalize PCP-induced hyperlocomotion. These results suggest that AS057278 has the potential to anti-psychotic action toward both cognitive and positive symptoms of schizophrenia.

Regulation of N-methyl-D-aspartate receptors by astrocytic D-serine

NMDA receptors (NMDARs) are key glutamatergic receptors in the CNS. Their permeability to Ca2+ and their voltage-dependent Mg2+ block make them essential for synaptic transmission, synaptic plasticity, rhythmogenesis, gene expression and excitotoxicity. One very peculiar property is that their activation requires the binding of both glutamate and a co-agonist like glycine or D-serine. There is a growing body of evidence indicating that D-serine, rather than glycine as originally thought, is the endogenous ligand for NMDARs in many brain structures. D-serine is synthesized mainly in glial cells and it is released upon activation of glutamate receptors. Its concentration in the synaptic cleft controls the number of NMDAR available for activation by glutamate. Consequently, the glial environment of neurons has a critical impact on the direction and magnitude of NMDAR-dependent synaptic plasticity.

Roles of l-serine and sphingolipid synthesis in brain development and neuronal survival

Sphingolipids represent a class of membrane lipids that contain a hydrophobic ceramide chain as its common backbone structure. Sphingolipid synthesis requires two simple components: l-serine and palmitoyl CoA. Although l-serine is classified as a non-essential amino acid, an external supply of l-serine is essential for the synthesis of sphingolipids and phosphatidylserine (PS) in particular types of central nervous system (CNS) neurons. l-Serine is also essential for these neurons to undergo neuritogenesis and to survive. Biochemical analysis has shown that l-serine is synthesized from glucose and released by astrocytes but not by neurons, which is the major reason why this amino acid is an essential amino acid for neurons. Biosynthesis of membrane lipids, such as sphingolipids, PS, and phosphatidylethanolamine (PE), in neurons is completely dependent on this astrocytic factor. Recent advances in lipid biology research using transgenic mice have demonstrated that synthesis of endogenous l-serine and neuronal sphingolipids is essential for brain development. In this review, we discuss the metabolic system that coordinates sphingolipid synthesis with the l-serine synthetic pathway between neurons and glia. We also discuss the crucial roles of the metabolic conversion of l-serine to sphingolipids in neuronal development and survival. Human diseases associated with serine and sphingolipid biosynthesis are also discussed.

The serine racemase mRNA is predominantly expressed in rat brain neurons

D-serine is an endogenous and obligatory coagonist for the glycine site of the N-methyl-D-aspartate receptor in the mammalian brain. D-serine is synthesized from L-serine by serine racemase; immunohistochemical studies have long been believed to indicate that serine racemase and D-serine occur predominantly in astrocytes. However, we have recently demonstrated in the primary cultures that both the mRNA and protein levels of serine racemase are higher in neurons than in astrocytes. Here we report the application of in situ hybridization based on tyramide signal amplification for the detection of serine racemase mRNA in sections of the adult rat brain. Serine racemase mRNA could be demonstrated in a large number of neurons throughout the brain, especially in the forebrain such as the cerebral cortex, striatum, and hippocampus. This is the first study to demonstrate the exact localization of serine racemase mRNA at the cellular or tissue level. These results suggest that neuron-derived D-serine could modulate neurotransmission via the glycine site of N-methyl-D-aspartate receptors.

d-Amino acid oxidase and serine racemase in human brain: normal distribution and altered expression in schizophrenia

The N-methyl-D-aspartate receptor co-agonist d-serine is synthesized by serine racemase and degraded by D-amino acid oxidase. Both D-serine and its metabolizing enzymes are implicated in N-methyl-D-aspartate receptor hypofunction thought to occur in schizophrenia. We studied D-amino acid oxidase and serine racemase immunohistochemically in several brain regions and compared their immunoreactivity and their mRNA levels in the cerebellum and dorsolateral prefrontal cortex in schizophrenia. D-Amino acid oxidase immunoreactivity was abundant in glia, especially Bergmann glia, of the cerebellum, whereas in prefrontal cortex, hippocampus and substantia nigra, it was predominantly neuronal. Serine racemase was principally glial in all regions examined and demonstrated prominent white matter staining. In schizophrenia, D-amino acid oxidase mRNA was increased in the cerebellum, and as a trend for protein. Serine racemase was increased in schizophrenia in the dorsolateral prefrontal cortex but not in cerebellum, while serine racemase mRNA was unchanged in both regions. Administration of haloperidol to rats did not significantly affect serine racemase or D-amino acid oxidase levels. These findings establish the major cell types wherein serine racemase and D-amino acid oxidase are expressed in human brain and provide some support for aberrant D-serine metabolism in schizophrenia. However, they raise further questions as to the roles of D-amino acid oxidase and serine racemase in both physiological and pathophysiological processes in the brain.

D-serine regulates CREB phosphorylation induced by NMDA receptor activation in Müller glia from the retina

D-serine is an N-methyl-D-aspartate (NMDA) receptor coagonist predominantly produced by glial cells in the brain and the retina. Whereas a role for D-serine as a modulator of NMDA receptors in neurons has been suggested, its function in glial cells has not been analyzed. We here show that D-serine modulates gene expression in Müller glial cells from the retina through the induction of transcription factor CREB phosphorylation and the expression of the immediate-early gene c-fos. Pharmacological analysis indicates that D-serine effect involves NMDA receptor activation. Comparison of the effect of D-serine in Müller cells, hippocampal astrocytes and hippocampal neurons suggests that D-serine could function as a retinal NMDA receptor coagonist activating functionally relevant transcription factor pathways in glial cells.

Reviewing the role of the genes G72 and DAAO in glutamate neurotransmission in schizophrenia

We review the role of two susceptibility genes; G72 and DAAO in glutamate neurotransmission and the aetiology of schizophrenia. The gene product of G72 is an activator of DAAO (D-amino acid oxidase), which is the only enzyme oxidising D-serine. D-serine is an important co-agonist for the NMDA glutamate receptor and plays a role in neuronal migration and cell death. Studies of D-serine revealed lower serum levels in schizophrenia patients as compared to healthy controls. Furthermore, administration of D-serine as add-on medication reduced the symptoms of schizophrenia. The underlying mechanism of the involvement of G72 and DAAO in schizophrenia is probably based on decreased levels of D-serine and decreased NMDA receptor functioning in patients. The involvement of this gene is therefore indirect support for the glutamate dysfunction hypothesis in schizophrenia.

Ionotropic glutamate-like receptor delta2 binds D-serine and glycine

The orphan glutamate-like receptor GluRdelta2 is predominantly expressed in Purkinje cells of the central nervous system. The classification of GluRdelta2 to the ionotropic glutamate receptor family is based on sequence similarities, because GluRdelta2 does not form functional homomeric glutamate-gated ion channels in transfected cells. Studies in GluRdelta2(-/-) knockout mice as well as in mice with naturally occurring mutations in the GluRdelta2 gene have demonstrated an essential role of GluRdelta2 in cerebellar long-term depression, motor learning, motor coordination, and synaptogenesis. However, the lack of a known agonist has hampered investigations on the function of GluRdelta2. In this study, the ligand-binding core of GluRdelta2 (GluRdelta2-S1S2) was found to bind neutral amino acids such as D-serine and glycine, as demonstrated by isothermal titration calorimetry. Direct evidence for binding of D-serine and structural rearrangements in the binding cleft of GluRdelta2-S1S2 is provided by x-ray structures of GluRdelta2-S1S2 in its apo form and in complex with D-serine. Functionally, D-serine and glycine were shown to inactivate spontaneous ion-channel conductance in GluRdelta2 containing the lurcher mutation (EC(50) values, 182 and 507 microM, respectively). These data demonstrate that the GluRdelta2 ligand-binding core is capable of binding ligands and that cleft closure of the ligand-binding core can induce conformational changes that alter ion permeation.

Glia: they make your memories stick!

Synaptic plasticity underlies higher brain functions such as learning and memory. At glutamatergic synapses in the vertebrate central nervous system, plasticity usually requires changes in the number of postsynaptic AMPA receptors. Recently, several studies have revealed that glial cells play an important role in regulating postsynaptic AMPA receptor density. This is accomplished through the release of gliotransmitters such as D-serine, ATP and TNF-alpha. More specifically, the availability of D-serine, the endogenous co-agonist of N-methyl-D-aspartate receptors in many brain areas, governs the induction of long-term potentiation and long-term depression. Meanwhile, ATP and TNF-alpha trigger long-lasting increases in synaptic strength at glutamatergic hypothalamic and hippocampal inputs, respectively, through mechanisms that promote AMPA receptor insertion in the absence of coincident presynaptic and postsynaptic activity. These data clearly demonstrate a vital role for glia in plasticity and argue that their contributions to brain function extend well beyond their outdated role as cellular 'glue'.

Contribution of endogenous glycine and d-serine to excitotoxic and ischemic cell death in rat cerebrocortical slice cultures

N-methyl-D-aspartate (NMDA) receptors, whose activation requires glycine site stimulation, play crucial roles in various physiological and pathological conditions in the brain. We investigated the regulatory roles of potential endogenous glycine site agonists, glycine and d-serine, in excitotoxic and ischemic cell death in the cerebral cortex. Cytotoxicity of NMDA on rat cerebrocortical slice cultures was potentiated by addition of glycine or d-serine. In contrast, cell death induced by oxygen/glucose deprivation (OGD) was not affected by exogenous glycine or d-serine, although blockade of NMDA receptors by MK-801 abolished cell death. In addition, higher concentrations of 2,7-dichlorokynurenic acid (DCKA), a competitive glycine site antagonist, were required to suppress OGD-induced cell death than those to suppress NMDA cytotoxicity. We also found that OGD triggered a robust increase in extracellular glycine. A glycine transporter blocker ALX 5407 increased the extracellular level of glycine, and the protective effect of DCKA against NMDA cytotoxicity was diminished in the presence of ALX 5407. Sensitivity of NMDA cytotoxicity to DCKA was also diminished by l-serine that increased the extracellular level of d-serine. These results indicate that both glycine and d-serine can act as endogenous ligands for NMDA receptor glycine site in the cerebral cortex, and that endogenous glycine may saturate the glycine site under ischemic conditions. The present findings are important for the interpretation of the mechanisms of NMDA and OGD cytotoxicity.

Why is d-serine nephrotoxic and α-aminoisobutyric acid protective?

d-Serine selectively causes necrosis of S3 segments of proximal tubules in rats. This leads to aminoaciduria and glucosuria. Coinjection of the nonmetabolizable amino acid α-aminoisobutyric acid (AIB) prevents the tubulopathy. d-serine is selectively reabsorbed in S3, thereby gaining access to peroxisomal d-amino acid oxidase (d-AAO). d-AAO-mediated metabolism produces reactive oxygen species. We determined the fractional excretion of amino acids and glucose in rats after intraperitoneal injection of d-serine alone or together with reduced glutathione (GSH) or AIB. Both compounds prevented the hyperaminoaciduria. We measured GSH concentrations in renal tissue before (control) and after d-serine injection and found that GSH levels decreased to ∼30% of control. This decrease was prevented when equimolar GSH was coinjected with d-serine. To find out why AIB protected the tubule from d-serine toxicity, we microinfused d-[14C]serine or [14C]AIB (0.36 mmol/l) together with [3H]inulin in late proximal tubules in vivo and measured the radioactivity in the final urine. Fractional reabsorption of d-[14C]serine and [14C]AIB amounted to 55 and 70%, respectively, and 80 mmol/l of AIB or d-serine mutually prevented reabsorption to a great extent. d-AAO activity measured in vitro (using d-serine as substrate) was not influenced by a 10-fold higher AIB concentration. We conclude from these results that 1) d-AAO-mediated d-serine metabolism lowers renal GSH concentrations and thereby provokes tubular damage because reduction of reactive oxygen species by GSH is diminished and 2) AIB prevents d-serine-induced tubulopathy by inhibition of d-serine uptake in S3 segments rather than by interfering with intracellular d-AAO-mediated d-serine metabolism.

Evidence from gene knockout studies implicates Asc-1 as the primary transporter mediating d-serine reuptake in the mouse CNS

In the mammalian central nervous system, transporter-mediated reuptake may be critical for terminating the neurotransmitter action of D-serine at the strychnine insensitive glycine site of the NMDA receptor. The Na(+) independent amino acid transporter alanine-serine-cysteine transporter 1 (Asc-1) has been proposed to account for synaptosomal d-serine uptake by virtue of its high affinity for D-serine and widespread neuronal expression throughout the brain. Here, we sought to validate the contribution of Asc-1 to D-serine uptake in mouse brain synaptosomes using Asc-1 gene knockout (KO) mice. Total [(3)H]D-serine uptake in forebrain and cerebellar synaptosomes from Asc-1 knockout mice was reduced to 34 +/- 5% and 22 +/- 3% of that observed in wildtype (WT) mice, respectively. When the Na(+) dependent transport components were removed by omission of Na(+) ions in the assay buffer, D-serine uptake in knockout mice was reduced to 8 +/- 1% and 3 +/- 1% of that measured in wildtype mice in forebrain and cerebellum, respectively, suggesting Asc-1 plays a major role in the Na(+) independent transport of D-serine. Potency determination of D-serine uptake showed that Asc-1 mediated rapid high affinity Na(+) independent uptake with an IC(50) of 19 +/- 1 microm. The remaining uptake was mediated predominantly via a low affinity Na(+) dependent transporter with an IC(50) of 670 +/- 300 microm that we propose is the glial alanine-serine-cysteine transporter 2 (ASCT2) transporter. The results presented reveal that Asc-1 is the only high affinity D-serine transporter in the mouse CNS and is the predominant mechanism for D-serine reuptake.

d-Serine inhibits AMPA receptor-mediated current in rat hippocampal neurons

d-Serine, a recently identified gliotransmitter, serves as an endogenous coagonist binding to the glycine site of N-methyl-d-aspartate (NMDA) receptors. However, it is not clear whether this native ligand is able to bind to and modulate α-amino-3-hydroxyl-5-methyl-4-isoxazolepropionate (AMPA) receptors. In the present study, we showed that d-serine was able to concentration-dependently inhibit kainate-induced AMPA receptor-mediated current in acutely isolated hippocampal neurons. The blocking action of d-serine on AMPA receptors was characterized by a shift in concentration–response curve of kainate-induced current to the right with no change in the maximal response and independent of holding potential in the range of –80 to +60 mV. This is consistent with a model that d-serine is a competitive antagonist on AMPA receptors. In contrast, l-serine did not exert such an inhibitory action. Consistent with this observation, we found that several d-isoforms, but not l-isoforms, of endogenous and exogenous amino acids were able to block AMPA receptors. These results indicate that there is a low affinity and stereo-selective site at the agonist binding pocket of AMPA receptors for these d-amino acids. More importantly, vesicular-released endogenous d-serine from astrocytes could potentially modulate AMPA receptors in synaptic transmission in hippocampus.

d-Aspartic acid: An endogenous amino acid with an important neuroendocrine role

D-Aspartic acid (d-Asp), an endogenous amino acid present in vertebrates and invertebrates, plays an important role in the neuroendocrine system, as well as in the development of the nervous system. During the embryonic stage of birds and the early postnatal life of mammals, a transient high concentration of d-Asp takes place in the brain and in the retina. d-Asp also acts as a neurotransmitter/neuromodulator. Indeed, this amino acid has been detected in synaptosomes and in synaptic vesicles, where it is released after chemical (K(+) ion, ionomycin) or electric stimuli. Furthermore, d-Asp increases cAMP in neuronal cells and is transported from the synaptic clefts to presynaptic nerve cells through a specific transporter. In the endocrine system, instead, d-Asp is involved in the regulation of hormone synthesis and release. For example, in the rat hypothalamus, it enhances gonadotropin-releasing hormone (GnRH) release and induces oxytocin and vasopressin mRNA synthesis. In the pituitary gland, it stimulates the secretion of the following hormones: prolactin (PRL), luteinizing hormone (LH), and growth hormone (GH) In the testes, it is present in Leydig cells and is involved in testosterone and progesterone release. Thus, a hypothalamus-pituitary-gonads pathway, in which d-Asp is involved, has been formulated. In conclusion, the present work is a summary of previous and current research done on the role of d-Asp in the nervous and endocrine systems of invertebrates and vertebrates, including mammals.