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

Serine-deficiency syndromes

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

SUMMARY

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

Bergmann glial GlyT1 mediates glycine uptake and release in mouse cerebellar slices

Glycine is an inhibitory neurotransmitter and is critical for NMDA receptor activation. These roles are dependent on extracellular glycine levels, which are regulated by Na(+)/Cl(-)-dependent glycine transporters (GlyTs) in neurones and glia. The glial GlyT subtype GlyT1 is well located to activate NMDA receptors. However, glial GlyTs have not been studied in an intact system thus far. Whole-cell patch-clamp recordings were obtained from Bergmann glia in mice cerebellar slices to determine whether these glia express functional GlyT1 that can mediate both glycine uptake and efflux. In the presence of a glycine receptor blocker, glycine and a substrate agonist for GlyT1, sarcosine, induced voltage-dependent inward currents that were abolished by removing external Na(+), identifying them as transport currents. Inhibitors of glycine transport through GlyT1 (sarcosine and (N-[3-(4'-fluorophenyl)-3-(4'-phenylphenoxy)propyl]sarcosine (NFPS)) reduced glycine currents by approximately 85%, consistent with positive immunostaining for GlyT1 in Bergmann glia while inhibitors of glycine transport through GlyT2 (4-benzyloxy-3,5-dimethoxy-N-[1-(dimethylaminocyclopently)methyl]benzamide (ORG 25543) and amoxapine) or through systems A and ASC did not affect glycine transport currents. Following internal glycine perfusion during the recording, outward currents progressively developed at -50 mV and external glycine-induced uptake currents were reduced. Using paired recordings of a Bergmann glial cell and a granule cell in the whole cell and outside-out modes, respectively, depolarizations of Bergmann glia to +20 mV induced a 73% increase in the open probability of glycine receptor channels in membrane patches of granule cells. This increase was prevented when NFPS was included in the bath solution. Overall, these results demonstrate for the first time that Bergmann glia express functional GlyT1 that can work in reverse at near-physiological ionic and internal glycine conditions in brain slices. These glial GlyTs can probably mediate glycine efflux under conditions of metabolic impairments like ischaemia.

Endogenous d-Serine Is Involved in Induction of Neuronal Death by N-Methyl-d-aspartate and Simulated Ischemia in Rat Cerebrocortical Slices

Emerging evidence indicates that D-serine rather than glycine serves as an endogenous agonist at glycine site of the N-methyl-D-aspartate (NMDA) subtype of glutamate receptors, in several nervous tissues, including the developing cerebellum and the retina. Here, we examined whether endogenous D-serine plays a significant role in neuronal damage resulting from excitotoxic insults in the cerebral cortex, using rat brain slices maintained in a defined salt solution. Neuronal cell death induced by application of NMDA or by oxygen-glucose deprivation (simulated ischemia) was markedly suppressed by a competitive glycine site antagonist 2,7-dichlorokynurenic acid. Addition of glycine or D-serine did not augment neuronal damage by NMDA or simulated ischemia, indicating that sufficient amount of glycine site agonist(s) is supplied endogenously within the slices. Application of D-amino acid oxidase, an enzyme that degrades D-serine, markedly inhibited neuronal damage by NMDA and simulated ischemia, which was reversed by addition of excess D-serine or glycine. Sensitivity to the glycine site antagonist of NMDA- or ischemia-induced damage was not affected by the presence of a non-NMDA receptor antagonist, suggesting that kainate receptor-stimulated D-serine release as demonstrated in primary cultured astrocytes does not contribute significantly to the extent of neuronal injury in these settings. The present results suggest that endogenous supply of D-serine as a glycine site agonist is important for neuronal injury involving NMDA receptor overactivation in the cerebral cortex.

Tonic and phasic alteration in amygdala 5-HT, glutamate and GABA transmission after prefrontal cortex damage in rats

The relationship between the ventromedial prefrontal cortex and the amygdala during the presentation of an unconditioned fear stimulus was assessed. Rats underwent bilateral ibotenic acid or vehicle administration into the ventromedial prefrontal cortex. Five weeks later, the behavior as well as the neurochemical changes in the amygdala was evaluated before and after a brief cat presentation. Lesioned animal freezing behavior increased 10 times when compared to controls. In the right basolateral amygdala, basal concentrations of 5-HT, 5-HIAA, glutamate and serine were elevated but basal level of GABA was diminished in lesioned animals relative to controls. Sham but not lesioned animals increased 5-HT and decreased GABA and serine levels after cat presentation. Phasic changes in glutamate were not detected either in lesioned or shams but the difference in amygdala glutamate between lesioned and shams persisted after cat presentation. These data show that increased serotonin and glutamate tone and decreased gabaergic tone in the amygdala correlate to elevated fear and anxiety after prefrontal cortex ibotenic acid lesion. The lesion also seems to produce a failure of adaptive changes in neurotransmitter systems revealing lost of control of the ventromedial prefrontal cortex over the amygdala in frightening situations.

Active release of glycine or D-serine saturates the glycine site of NMDA receptors at the cerebellar mossy fibre to granule cell synapse

The current and calcium influx generated by NMDA receptors depend on the concentration of the coagonist glycine, or its analogue d-serine, in the synaptic cleft. If there is no release of glycine, the ionic stoichiometry of the glial GlyT1 glycine transporters expressed near NMDA receptors in the brain should be able to lower the extracellular glycine concentration to below the EC50 for coactivation of NMDA receptors. We examined whether changing the glycine or d-serine concentration in the superfusion solution altered the NMDA receptor mediated component of the synaptic current at the rat cerebellar mossy fibre to granule cell synapse. Adding up to 100 microM glycine or d-serine had no effect, implying that the glycine site is saturated. Using the competitive glycine site antagonist 7-chlorokynurenate, and plausible values for the kinetic parameters of NMDA receptors, we estimate that during activation of the mossy fibres the concentration of glycine or d-serine in the synaptic cleft is at least 4.6 microM or 1.5 microM, respectively, requiring active release of glycine or d-serine.

WITHDRAWN: Inhibitory effects of D-Serine on hippocampal synapse transmission

Ahead of Print article withdrawn by publisher.

Opiate receptors and beyond: 30 years of neural signaling research

Identification of opiate receptors some 30 years ago provided tools that brought major new insights into how these drugs act and led to the discovery of a novel group of atypical neurotransmitters, the peptide enkephalins being the first. The ligand binding techniques that were used to identify opiate receptors were employed to characterize receptors for all of the major neurotransmitters in the brain leading to additional insights into the actions of many drugs, such as neuroleptics. These techniques also permitted characterization of intracellular signaling systems such as the IP3 receptor and immunophilins. Even more novel than the enkephalins have been the gaseous neurotransmitters NO and CO and D-serine.

D-Serine as a glial modulator of nerve cells

Until the last decade, it was widely accepted that D-amino acids had no functional role in higher organisms, but that they were restricted to lower organisms, such as bacteria, where they are integrated into the proteoglycans of the cell wall. However, D-serine proved to be an effective coagonist at the "glycine-binding" site of the N-methyl-D-aspartate (NMDA) glutamate receptors, and this observation led to chemical analyses that have now revealed the presence of high levels of D-serine in the central nervous system, including many regions of the brain and retina. D-Serine has been localized to astrocytes and can be released by glutamate through stimulation of AMPA receptors. A new enzyme, serine racemase has been localized to glial cells and converts L-serine to D-serine. Degradation of D-serine takes place through D-amino acid oxidase, an enzyme once thought to metabolize D-amino acids from external sources. Although the "glycine-binding" site of NMDA receptors was initially regarded as a saturated site, evidence in many brain regions has established that this site is not saturated and is therefore modulated by interactions between glial cells and neurons. In some, but not all, studies, D-serine enhances NMDA-mediated currents; a light-evoked enhancement to NMDA currents has been reported in the retina. D-serine also plays a role in synaptic and cellular development, particularly in the cerebellum, where the normal developmental sequences underlying synapse formation onto Purkinje cells and the migration of granule cells are dependent on NMDA receptors during a time when high levels of D-serine are expressed in the Bergmann glia and other cerebellar astrocytes. D-serine must be added to the list of agents through which glial cells participate in controlling the excitability of neurons.

Distribution and pharmacology of alanine-serine-cysteine transporter 1 (asc-1) in rodent brain

A polyclonal antibody against the Na+-independent alanine-serine-cysteine transporter 1 (asc-1) was raised and the specificity of the antibody verified by Western blots performed on membranes prepared from HEK293 cells transiently transfected with the cloned murine asc-1. The antibody was then used to localize the transporter in the brain of two rodent species by using immunohistochemistry at the light and electron microscopical level. asc-1-immunoreactivity (asc-1-ir) was widely distributed throughout the mouse and rat brain. Areas with high levels of asc-1-ir included hypothalamus, the medial septal area, globus pallidus, entopeduncular nucleus, cingulate and retrosplenial cortices. Moderate asc-1-ir was observed in several areas including layers III and V of the neocortex, thalamus, nucleus accumbens, caudate putamen, bed nucleus of stria terminalis, all amygdaloid nuclei, hippocampus (CA1-CA3 and hilus of the dentate gyrus), as well as several brainstem nuclei. asc-1-ir was observed as punctuate staining consistent with varicosities matching neuronal cell bodies and dendritic fields. At the ultrastructural level, asc-1-ir was mainly confined to presynaptic terminals. Immunostaining in either glial cell bodies or perivascular sites was not observed and white matter was completely devoid of asc-1-ir. Furthermore, the pharmacology of the Na+-independent uptake site for [3H]d-serine in rat brain synaptosomal P2 fractions was compared with the substrate specificity of the cloned human asc-1 transporter and a high degree of correlation was demonstrated. We conclude that asc-1-ir is widespread in the brain and limited to neuronal structures and that asc-1 may contribute to synaptic clearance of d-serine in brain.

Contribution of astrocytes to hippocampal long-term potentiation through release of D-serine

Repetitive correlated activation of pre- and postsynaptic neurons induced long-term potentiation (LTP) of synaptic transmission among hippocampal neurons grown on a layer of astrocytes (mixed cultures) but not among neurons cultured in glial conditioned medium. Supplement of D-serine, an agonist for the glycine-binding site of N-methyl-D-aspartate (NMDA) receptors, enhanced NMDA receptor activation and enabled LTP induction in glial conditioned medium cultures. The induction of LTP in both mixed cultures and hippocampal slices was suppressed by NMDA receptor antagonists, glycine-binding-site blockers of NMDA receptors, or an enzyme that degrades endogenous D-serine. By providing extracellular D-serine that facilitates activation of NMDA receptors, astrocytes thus play a key role in long-term synaptic plasticity.

Effects of endogenous agonists, glycine and D-serine, on in vivo specific binding of [11C]L-703,717, a PET radioligand for the glycine-binding site of NMDA receptors

A positron-emitter (carbon-11) labeled antagonist for the glycine-binding site of NMDA receptors, [(11)C]L-703,717, has a unique in vivo binding characteristic, in which it preferentially binds to cerebellar-specific NMDA receptors consisting of a GluRepsilon3 subunit and eventually accumulates in rodent cerebellum under in vivo conditions, but not under in vitro conditions. In order to understand the in vivo-specific site and subunit localization of this radioligand, we examined the effect of the endogenous glycine site agonists, glycine and D-serine, on in vivo [(11)C]L-703,717 binding. An increase in extracellular glycine concentration by treatment with a glycine transporter 1 (GlyT1)-selective inhibitor, NFPS ethyl ester, significantly decreased the cerebellar localization of [(11)C]L-703,717 in rats. D-serine is known to be concentrated in mammalian forebrain regions. The lack of D-serine detection in the cerebellum may be due to the fact that it has the highest enzymatic activity of D-amino acid oxidase (DAO). It was found that the cerebellar localization of [(11)C]L-703,717 is greatly diminished in mutant mice lacking DAO, in which D-serine content in the cerebellum is drastically increased from a nondetectable level in normal mice. These studies indicate that [(11)C]L-703,717 is susceptible to inhibition by glycine site agonists in its in vivo binding, and suggest that regional differences in inhibitions by endogenous agonists may be a crucial factor in the site- and subunit-specific binding of this glycine-site antagonist.

CE/Electrospray Ionization-MS Analysis of Underivatized d/l-Amino Acids and Several Small Neurotransmitters at Attomole Levels through the Use of 18-Crown-6-tetracarboxylic Acid as a Complexation Reagent/Background Electrolyte

A new capillary electrophoresis/mass spectrometry technique is introduced for attomole detection of primary amines (including several neurotransmitters), amino acids, and their d/l enantiomers in one run through the use of a complexation reagent while using only approximately 1 nL of sample. The technique uses underivatized amino acids in conjunction with an underivatized capillary, which significantly reduces cost and analysis time. It was found that when (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid (18-C-6-TCA, MW 440) was used as the background electrolyte/complexation reagent during the capillary electrophoresis/electrospray ionization-mass spectrometry (CE/ESI-MS) analysis of underivatized amino acids, stable complexes were formed between the amino acids and the 18-C-6-TCA molecules. These complexes, which exhibited high ionization efficiencies, were detectable at attomole levels for most amino acids. The detection limits of the AA/18-C-6-TCA complexes were on the average more than 2 orders of magnitude lower than that of the free amino acids in solution. In addition to lower detection limits under CE/ESI-MS, a solution of 18-C-6-TCA in the concentration range of 5-30 mM provided high separation efficiency for mixtures of l-amino acids as well as mixtures of d/l-amino acids. By using a solution of 18-C-6-TCA as the background electrolyte in conjunction with an underivatized, 130-cm-long, 20-microm-i.d., 150-microm-o.d. fused-silica capillary and by monitoring the m/z range of the amino acid/18-C-6-TCA complexes (m/z 515-700), most of the standard amino acids and many of their enantiomers were separated and detected with high separation efficiency and high sensitivity (nanomolar concentration detection limits) in one run. The solutions of 18-C-6-TCA also worked well as the CE/ESI-MS BGE for low-level detection of several neurotransmitters and some of their d/l enantiomers as well as for the analysis of amino acids at endogenous levels in lysed red blood cells.

D-Serine alleviates retrograde amnesia of a visual discrimination task in rats with a lesion of the perirhinal cortex

D-Serine has been suggested to be a potent endogenous glycine-site agonist on the N-methyl-D-aspartate receptor, thereby having a potential role in the process of learning and memory. In rats, perirhinal cortex (PC) constitutes a particularly important structure for mnemonic processing, and damage to this area induces both anterograde and retrograde amnesia. In the present work, we show that intraperitoneal administration of 1000 mg/kg D-serine immediately after bilateral lesion of PC produced complete restoration of retrograde memory in rats, measured by a visual brightness discrimination task, while a higher dose (3000 mg/kg) did not show any reliable effect. Uptake of the drug into the brain was confirmed using high performance liquid chromatography (HPLC).

D-serine and serine racemase are present in the vertebrate retina and contribute to the physiological activation of NMDA receptors

d-serine has been proposed as an endogenous modulator of N-methyl-d-aspartate (NMDA) receptors in many brain regions, but its presence and function in the vertebrate retina have not been characterized. We have detected d-serine and its synthesizing enzyme, serine racemase, in the retinas of several vertebrate species, including salamanders, rats, and mice and have localized both constituents to Müller cells and astrocytes, the two major glial cell types in the retina. Physiological studies in rats and salamanders demonstrated that, in retinal ganglion cells, d-serine can enhance excitatory currents elicited by the application of NMDA, as well as the NMDA receptor component of light-evoked synaptic responses. Application of d-amino acid oxidase, which degrades d-serine, reduced the magnitude of NMDA receptor-mediated currents, raising the possibility that endogenous d-serine serves as a ligand for setting the sensitivity of NMDA receptors under physiological conditions. These observations raise exciting new questions about the role of glial cells in regulating the excitability of neurons through release of d-serine.

Analysis of underivatized amino acids and their D/L-enantiomers by sheathless capillary electrophoresis/electrospray ionization mass spectrometry

Capillary electrophoresis/electrospray ionization-mass spectrometry (CE/ESI-MS) was applied to the analysis of underivatized amino acids and the separation of their D/L-enantiomers. Under full-scan mode, all standard protein amino acids were separated and detected at low-femtomole levels using a 130-cm-long, 20-microm-i.d., 150-microm-o.d. underivatized fused-silica capillary with 1 M formic acid as the background electrolyte. The CE/ESI-MS technique was also applied to the separation of L-arginine from L-canavanine (a close analogue of arginine where the terminal methylene linked to the guanidine group of arginine is replaced by an oxygen atom) in a complex mixture containing all standard protein amino acids. The utility of CE/ESI-MS in the analysis of real-world samples was demonstrated by the identification of two metabolic diseases (PKU and tyrosinemia) through blood analysis with minimal sample preparation. In addition, the on-line separation of 11 underivatized L-amino acids from their D-enantiomers was achieved by using a 30 mM solution of (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid as the background electrolyte.

NOVELNEURALMODULATORS

The discovery that nitric oxide (NO) is produced by neurons and regulates synaptic activity has challenged the definition of a neurotransmitter. NO is not stored in synaptic vesicles and does not act at conventional receptors on the surface of adjacent neurons. The toxic gases carbon monoxide (CO) and hydrogen sulfide (H2S) are also produced by neurons and modulate synaptic activity. D-serine synthesis and release by astrocytes as an endogenous ligand for the "glycine" site of N-methyl D-aspartate (NMDA) receptors defy the concept that a neurotransmitter must be synthesized by neurons. We review the properties of these "atypical" neural modulators.

Metabolic disorders and mental retardation

The metabolic and anatomical substrate of most forms of mental retardation is not known. Because the basis of normal brain function is not sufficiently understood, the basis of abnormal function is understood poorly. Even in disorders where the fundamental biochemical defect is known, such as phenylketonuria (PKU) and other enzyme defects, the exact basis for brain dysfunction is uncertain. The outcome for treated PKU, galactosemia, homocystinuria, and lysosomal disorders is not yet optimal. The various forms of nonketotic hyperglycinemia often respond poorly to current therapy. Less familiar disorders, with or without seizures, such as deficient synthesis of serine or creatine and impaired glucose transport into the brain, and disorders with variable malformations, such as Smith-Lemli-Opitz (SLO) syndrome and the congenital disorders of glycosylation (CDGs), may initially be thought to be a nonspecific form of developmental delay. Less familiar disorders, with or without seizures and disorders with variable malformations may initially be thought to be a nonspecific form of developmental delay. Simple tests of urine, blood, and cerebrospinal fluid may lead to a diagnosis, accurate genetic counseling, and better treatment. Metabolic brain imaging (magnetic resonance spectroscopy (MRS)) has also helped to reveal biochemical abnormalities within the brain.

Glycine tranporter-1 blockade potentiates NMDA-mediated responses in rat prefrontal cortical neurons in vitro and in vivo

The N-methyl-D-aspartate (NMDA) receptor (NMDA-R) has pivotal roles in neural development, learning, memory, and synaptic plasticity. Functional impairment of NMDA-R has been implicated in schizophrenia. NMDA-R activation requires glycine to act on the glycine-B (GlyB) site of the NMDA-R as an obligatory co-agonist with glutamate. Extracellular glycine near NMDA-R is regulated effectively by a glial glycine transporter (GlyT1). Using whole-cell voltage-clamp recordings in prefrontal cortex (PFC) slices, we have shown that exogenous GlyB site agonists glycine and D-serine, or a specific GlyT1 inhibitor N[3-(4'-fluorophenyl)-3-(4'-phenylphenoxy)propyl]sarcosine (NFPS) in the presence of exogenous glycine (10 microM), potentiated synaptically evoked NMDA excitatory postsynaptic currents (EPSCs) in vitro. Furthermore, in urethan-anesthetized rats, microiontophoretic NMDA pulses excite single PFC neurons. When these responses were blocked by approximately 50% to approximately 90% on continuous iontophoretic application of the GlyB site, antagonist (+)HA-966, intravenous NFPS (5 mg/kg), or a GlyB site agonist D-serine (50 mg/kg iv) reversed this (+)HA-966 block. NFPS may elevate endogenous glycine levels sufficiently to displace (+)HA-966 from the GlyB sites of the NMDA-R, thus enabling reactivation of the NMDA-Rs by iontophoretic NMDA applications. D-Serine (50-100 mg/kg iv) or NFPS (1-2 mg/kg iv) alone also augmented NMDA-evoked excitatory responses. These data suggest that direct GlyB site stimulation by D-serine, or blockade of GLYT1 to elevate endogenous glycine to act on unsaturated GlyB sites on NMDA-Rs, potentiated NMDA-R-mediated firing responses in rat PFC. Hence, blockade of GlyT1 to elevate glycine near the NMDA-R may activate hypofunctional NMDA-R, which has been implicated to play a critical role in the pathophysiology of schizophrenia.

Mouse brain serine racemase catalyzes specific elimination of L-serine to pyruvate

D-Serine was previously identified in mammalian brain and was shown to be a co-agonist at the 'glycine' site of the N-methyl-D-aspartate (NMDA)-type receptors. Racemization of serine is catalyzed by serine racemase, a pyridoxal 5'-phosphate-dependent enzyme expressed mainly in brain and liver. NMDA receptor overactivation has been implicated in a number of pathological conditions and inhibitors of serine racemase are thus potentially interesting targets for therapy. We expressed recombinant mouse serine racemase in insect cells and purified it to near homogeneity. The enzyme is a non-covalent homodimer in solution and requires divalent cations Mg(2+), Ca(2+) or Mn(2+) for activity but not for dimerization. In addition to the racemization it also catalyzes specific elimination of L-Ser to pyruvate. D-Serine is eliminated much less efficiently. Both L-serine racemization and elimination activities of serine racemase are of comparable magnitude, display alkaline pH optimum and are negligible below pH 6.5.

D-Amino acids in mammals and their diagnostic value

Substantial amounts of D-amino acids are present in mammalian tissues; their function, origin and relationship between pathophysiological processes have been of great interest over the last two decades. In the present article, analytical methods including chromatographic, electrophoretic and enzymatic methods to determine D-amino acids in mammalian tissues are reviewed, and the distribution of these D-amino acids in mammals is discussed. An overview of the function, origin and relationship between the amino acids and pathophysiological processes is also given.

Cloning and characterization of Disc1, the mouse ortholog of DISC1 (Disrupted-in-Schizophrenia 1)

We cloned the mouse ortholog of DISC1 (Disrupted-in-Schizophrenia 1), a candidate gene for schizophrenia. Disc1 is 3163 nucleotides long and has 60% identity with the human DISC1. Disc1 encodes 851 amino acids and has 56% identity with the human protein. Disc1 maps to the DISC1 syntenic region in the mouse, and genomic structure is conserved. A Disc1 splice variant deletes a portion of Disc1 beginning at amino acids orthologous to the human truncation. Bioinformatic analysis and cross-species comparisons revealed sequence conservation distributed across the genes and conservation of leucine zipper and coiled-coil domains in both orthologs. In situ hybridization in adult mouse brain revealed a restricted expression pattern, with highest levels in the dentate gyrus of the hippocampus and lower expression in CA1-CA3 of the hippocampus, cerebellum, cerebral cortex, and olfactory bulbs. Identification of Disc1 will facilitate the study of DISC1's function and creation of mouse models of DISC1 disruption.

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.

Vesicular release mechanisms in astrocytic signalling

Astrocytes play an important role in chemical signalling, acting as receptive as well as secretory elements. They can express receptors for essentially all classical neurotransmitter substances and for a large variety of peptides. Recent evidence indicates that astrocytes are involved in the information processing within the nervous system. Astrocytes respond to various neurotransmitters with elevations in intracellular calcium which can either be long-duration Ca(2+) spikes or oscillations in Ca(2+) levels. Astrocytic excitation can be propagated to adjacent astrocytes in the form of Ca(2+) waves. Due to their intimate spatial relationship with synaptic contacts, astrocytes can directly respond to synaptically released messengers and communicate, via signalling substances, with neurons in a reciprocal manner. Cultured astrocytes and astroglioma cells express synaptic vesicle proteins and members of the synaptic SNARE complex. Astrocytes can release a variety of messenger substances via receptor-mediated mechanisms implicating their potential for regulated exocytosis and the participation of proteins of the SNARE complex.

D-Aspartic acid in bovine dentine non-collagenous phosphoprotein

In tooth dentine, owing to its slow metabolism after its formation, racemized and transformed D-aspartic acid remains in the tissue and accumulates with age. However, no dentinal proteins which contain D-aspartic acid have been identified. In this study, a non-collagenous phosphoprotein was purified from bovine dentine. Its molecular mass was about 130 kDa and its amino acid composition was very similar to that of bovine dentine phosphophoryn. The purified protein contained a large proportion of aspartic acid residues and some of them were stereoinverted from the L-isomer to the D-isomer. The D-/L-aspartic acid ratio of dentine non-collagenous phosphoproteins purified from 8-month-old fetal, postnatal and 1-year-old bovine first incisors showed that the stereoinversion tended to increase with age. These results suggest that the purified non-collagenous phosphoprotein is a candidate for the protein in dentine containing D-aspartic acid.

Glutamate and GABA systems as targets for novel antidepressant and mood-stabilizing treatments

Glutamate and gamma-amino butyric acid (GABA) systems are emerging as targets for development of medications for mood disorders. There is increasing preclinical and clinical evidence that antidepressant drugs directly or indirectly reduce N-methyl-D-aspartate glutamate receptor function. Drugs that reduce glutamatergic activity or glutamate receptor-related signal transduction may also have antimanic effects. Recent studies employing magnetic resonance spectroscopy also suggest that unipolar, but not bipolar, depression is associated with reductions in cortical GABA levels. Antidepressant and mood-stabilizing treatments also appear to raise cortical GABA levels and to ameliorate GABA deficits in patients with mood disorders. The preponderance of available evidence suggests that glutamatergic and GABAergic modulation may be an important property of available antidepressant and mood-stabilizing agents. Future research will be needed to develop and evaluate new agents with specific glutamate and GABA receptor targets in the treatment of mood disorders.

Cellular and subcellular distribution of D-aspartate oxidase in human and rat brain

The unusual amino acid D-aspartate is present in significant amounts in brain and endocrine glands and is supposed to be involved in neurotransmission and neurosecretion (Wolosker et al. [2000] Neuroscience 100:183-189). D-aspartate oxidase is the only enzyme known to metabolize D-aspartate and could regulate its level in different regions of the brain. We examined the cellular and subcellular distribution of this enzyme and its mRNA in human and rat brain by immunohistochemistry, in situ hybridization, and immunoelectron microscopy. D-aspartate oxidase protein and mRNA are ubiquitous. The protein shows a granular pattern, particularly within neurons and to a significantly lesser extent in astrocytes and oligodendrocytes. No evidence for a synaptic association was observed. Whereas between most positive neurons only gradual differences were observed, in the hypothalamic paraventricular nucleus, neurons with high enzyme content were found next to others with no labeling. cDNA cloning of D-aspartate oxidase corroborates an inherent targeting signal sequence for protein import into peroxisomes. Immunoelectron microscopy showed that the protein is localized in single membrane-bound organelles, apparently peroxisomes.

Direct calcium binding results in activation of brain serine racemase

Serine racemase (SR) is a brain enzyme present in glial cells, where it isomerizes L-serine into D-serine that, in turn, diffuses and coactivates the N-methyl-D-aspartate receptor through the binding to the so-called "glycine site." We have developed a method for the slow expression of SR in a eukaryotic vector that permits the correct insertion of the prosthetic group into the active site, rendering functional SR with a K(m) toward L-serine of 4.8 mm. Divalent cations such as calcium or manganese were necessary for complete enzyme activity, whereas the presence of chelators such as EDTA completely inhibited the enzyme. Moreover, direct binding of calcium to SR was evidenced using (45)Ca(2+). Gel filtration of the recombinant SR revealed the protein to be in a dimer-tetramer equilibrium. The addition of EDTA to a calcium-saturated serine racemase evokes a profound conformational change, as monitored by both fluorescence and circular dichroism techniques. Fluorescence titration allowed us to calculate a binding constant for calcium of 6.2 microm. Reagents that react with sulfhydryl groups, such as cystamine, were potent inhibitors of SR, in a clear reflection that one or more cysteine residues are important for enzyme activity. Additionally, 16 serine analogues were tested as a putative SR substrate or inhibitors. Significant inhibition was only observed for L-Ser-O-sulfate, L-cycloserine, and L-cysteine. Finally, activation of brain SR as a result of the changes in calcium concentration was studied in primary astrocytes. Treatment of astrocytes with the calcium ionophore, as well as with compounds that augment the intracellular calcium levels such as glutamate or kainate led to an increase in the amount of d-serine present in the extracellular medium. These results suggest that there might be a glutamatergic-mediated regulation of SR activity by intracellular calcium concentration.

Glial transport of the neuromodulator D-serine

D-Serine is an endogenous agonist of NMDA receptors that occurs in astrocytes in gray matter areas of the brain. D-Serine is synthesized from L-serine by the activity of a glial enriched serine racemase, but little is known on the properties of D-serine transport and factors regulating its synaptic concentration. In the present report we characterize the transport of D-serine in astrocytes. In primary astrocyte cultures, D-serine uptake is dependent on sodium ions and exhibits both low affinity and low specificity for D-serine. The kinetics of D-serine transport resembles that of ASCT type transporters as several small neutral amino acids strongly inhibit the uptake of D-serine. D-Serine fluxes are coupled to counter-movement of L-serine and to a less extent to other small neutral amino acids. Thus, addition of D-serine to cell cultures elicits robust efflux of intracellular L-serine. Conversely, physiological concentrations of L-serine induce efflux of preloaded D-serine from astrocytes. L-Serine was more effective than kainate, which have been previously shown to induce D-serine release from astrocytes upon stimulation of non-NMDA type of glutamate receptors. The features of D-serine transport we describe reveal possible new mechanisms controlling the synaptic concentration of D-serine.

Modulation of defensive behavior by periaqueductal gray NMDA/glycine-B receptor

Glutamate (GLU) associated with glycine, act as co-transmitter at the N-methyl-D-aspartate/glycine-B (NMDA/GLY(B)) receptor. Dorsal periaqueductal gray (dPAG) neurons express NMDA/GLY(B) receptors suggesting a GLU physiological role in mediating the responses elicited by stimulation of this area. Immunohistochemical data provided evidence of a possible correlation among elevated plus-maze (EPM), fear-like defensive behavior, and dPAG activity. The present data show that whereas the NMDA/GLY(B) receptor agonists increased the open-arm avoidance responses in the EPM, the antagonists had the opposite effects. Microinjection of NMDA/GLY(B) receptor agonists within the dPAG during test sessions in the EPM resulted in an enduring learned fear response detected in the retest. Therefore, in addition to the proposed role for the dPAG in panic attacks (escape), these findings suggest that the dPAG can also participate in more subtle anxiety-like behaviors.

3-Phosphoglycerate dehydrogenase, a key enzyme for l-serine biosynthesis, is preferentially expressed in the radial glia/astrocyte lineage and olfactory ensheathing glia in the mouse brain

l-Serine is synthesized from glycolytic intermediate 3-phosphoglycerate and is an indispensable precursor for the synthesis of proteins, membrane lipids, nucleotides, and neuroactive amino acids d-serine and glycine. We have recently shown that l-serine and its interconvertible glycine act as Bergmann glia-derived trophic factors for cerebellar Purkinje cells. To investigate whether such a metabolic neuron-glial relationship is fundamental to the developing and adult brain, we examined by in situ hybridization and immunohistochemistry the cellular expression of 3-phosphoglycerate dehydrogenase (3PGDH), the initial step enzyme for de novo l-serine biosynthesis in animal cells. At early stages when the neural wall consists exclusively of the ventricular zone, neuroepithelial stem cells expressed 3PGDH strongly and homogeneously. Thereafter, 3PGDH expression was downregulated and eventually disappeared in neuronal populations, whereas its high expression was transmitted to the radial glia and later to astrocytes in the gray and white matters. In addition, 3PGDH was highly expressed throughout development in the olfactory ensheathing glia, a specialized supporting cell that thoroughly ensheathes olfactory nerves. These results establish a fundamental link of the radial glia/astrocyte lineage and olfactory ensheathing glia to l-serine biosynthesis in the brain. We discuss this finding in the context of the hypothesis that 3PGDH expression in these glia cells contributes to energy metabolism in differentiating and differentiated neurons and other glia cells, which are known to be vulnerable to energy loss.

D-Aspartate is stored in secretory granules and released through a Ca(2+)-dependent pathway in a subset of rat pheochromocytoma PC12 cells

D-Aspartate in mammalian neuronal and neuroendocrine cells is suggested to play a regulatory role(s) in the neuroendocrine function. Although D-aspartate is known to be released from neuroendocrine cells, the mechanism underlying the release is less understood. Rat pheochromocytoma PC12 cells contain an appreciable amount of D-aspartate (257 +/- 31 pmol/10(7) cells). Indirect immunofluorescence microscopy with specific antibodies against d-aspartate indicated that the amino acid is present within a particulate structure, which is co-localized with dopamine and chromogranin A, markers for secretory granules, but not with synaptophysin, a marker for synaptic-like microvesicles. After sucrose density gradient centrifugation of the postnuclear particulate fraction, about 80% of the d-aspartate was recovered in the secretory granule fraction. Upon the addition of KCl, an appreciable amount of D-aspartate (about 40 pmol/10(7) cells at 10 min) was released from cultured cells on incubation in the presence of Ca(2+) in the medium. The addition of also triggered d-aspartate release. Botulinum neurotoxin type E inhibited about 40% of KCl- and Ca(2+)-dependent d-aspartate release followed by specific cleavage of 25-kDa synaptosomal-associated protein. alpha-Latrotoxin increased the intracellular [Ca(2+)] and caused the Ca(2+)-dependent d-aspartate release. Bafilomycin A1 dissipated the intracellular acidic regions and inhibited 40% of the Ca(2+)-dependent D-aspartate release. These properties are similar to those of the exocytosis of dopamine. Furthermore, digitonin-permeabilized cells took up radiolabeled d-aspartate depending on MgATP, which is sensitive to bafilomycin A1 or 3,5-di-tert-butyl-4-hydroxybenzylidene-malononitrile. Taken together, these results strongly suggest that d-aspartate is stored in secretory granules and then secreted through a Ca(2+)-dependent exocytotic mechanism. Exocytosis of D-aspartate further supports the role(s) of D-aspartate as a chemical transmitter in neuroendocrine cells.

Effect of the glycine modulatory site of the N-methyl-D-aspartate receptor on synaptic responses in kitten visual cortex

In visual cortical slices taken from kittens, the administration of D-serine, an agonist of the N-methyl-D-aspartate receptor complex, significantly enhanced synaptically evoked responses using field potential recordings in lower layer II/III. Expression of normally appearing long-term potentiation (LTP) took place in the presence of the agonist during high-frequency stimulation (HFS). The administration of an antagonist of this receptor, 7-chloro-kynurenic acid (7-Cl KY) alone had no appreciable effect on low-frequency synaptic transmission while HFS failed to induce LTP. Combined administration of D-serine and 7-Cl KY resulted in no alteration of low-frequency synaptic transmission, although expression of LTP was normally obtained. These results suggest that in visual cortex of kittens, the availability of this glycine binding site of the NMDA receptor is necessary for the expression of LTP.

A new strategy to decrease N-methyl-D-aspartate (NMDA) receptor coactivation: inhibition of D-serine synthesis by converting serine racemase into an eliminase

Serine racemase is a brain-enriched enzyme that synthesizes d-serine, an endogenous modulator of the glycine site of N-methyl-d-aspartate (NMDA) receptors. We now report that serine racemase catalyzes an elimination reaction toward a nonphysiological substrate that provides a powerful tool to study its neurobiological role and will be useful to develop selective enzyme inhibitors. Serine racemase catalyzes robust elimination of l-serine O-sulfate that is 500 times faster than the physiological racemization reaction, generating sulfate, ammonia, and pyruvate. This reaction provides the most simple and sensitive assay to detect the enzyme activity so far. We establish stable cell lines expressing serine racemase and show that serine racemase can also be converted into a powerful eliminase in cultured cells, while the racemization of l-serine is inhibited. Likewise, l-serine O-sulfate inhibits the synthesis of d-serine in primary astrocyte cultures. We conclude that the synthetic compound l-serine O-sulfate is a better substrate than l-serine as well as an inhibitor of d-serine synthesis. Inhibition of serine racemase provides a new strategy to selectively decrease NMDA receptor coactivation and may be useful in conditions in which overstimulation of NMDA receptors plays a pathological role.

Immunohistochemical demonstration of L-serine distribution in the rat brain

L-Serine has been suggested by in vitro studies to be an important neurotrophic factor which supports survival and neurite outgrowth of neurons. It is also a precursor of D-serine, a putative neurotransmitter. In the present study, we raised antibodies against L-serine in a rabbit and examined immunohistochemical distribution of the amino acid in the rat brain. In the hippocampus and the cerebellar cortex, where neurotrophic effects of L-serine have been indicated, L-serine immunoreactivity was found primarily in astrocytes. In the brain stem, where neuronal distribution of D-serine was reported, positive staining for L-serine was located primarily in neurons. Regional differences of cellular distribution of L-serine were indicated.

D-aspartate disposition in neuronal and endocrine tissues: ontogeny, biosynthesis and release

High levels of D-aspartate occur in the brain and endocrine glands, such as pineal, adrenal and pituitary. In the brain, D-aspartate levels are highest in embryonic and early postnatal stages. Notably high levels occur in the early postnatal cortical plate and subventricular zone of the cerebral cortical cultures, implying a role in development. In embryonic neuronal primary culture cells, we detected high levels of endogenous D-aspartate and demonstrated biosynthesis of [14C]D-aspartate using [14C]L-aspartate as precursor. Synthesis of D-aspartate in cell cultures is inhibited by amino-oxyacetic acid, an inhibitor of pyridoxal phosphate-dependent enzymes. In the rat adrenal medulla, D-aspartate is depleted by treatment of the animals with intraperitoneal nicotine injections. In adrenal slices, D-aspartate is released by depolarization with KCl or acetylcholine, implying physiological release by activation of the cholinergic innervation of the adrenal. Our characterization of D-aspartate ontogeny, biosynthesis and depolarization-induced release implies specific physiological roles for this amino acid.

Immunohistochemical evidences for localization and production of D-serine in some neurons in the rat brain

D-Amino acids are thought not to occur in mammalian tissues. However, previous studies reported D-serine was present only in astrocytes in the rat brain. In the present study, it was indicated by a highly sensitive immunocytochemical method with a D-serine specific antibody that D-serine was contained not only in astrocytes but also in some neurons, such as pyramidal neurons in the cerebral cortex, and neurons in the nucleus of the trapezoid body. Some amacrine cells also showed strong immunoreactivity for D-serine in the eyes which were injected with colchicine into the corpus vitreum.

Atypical neural messengers

Notions of what constitutes a neurotransmitter have changed markedly with the advent in the past decade of synaptic molecules, which satisfy key neurotransmitter criteria but differ radically from classical transmitters. Thus, NO and carbon monoxide are neither stored in synaptic vesicles nor released by exocytosis. These gases do not act via traditional receptors on postsynaptic membranes. In addition, zinc, stored together with glutamate in synaptic vesicles, appears to act as an 'antagonist' co-transmitter at the NMDA receptor, and although localized exclusively to glia, D-serine fulfills most neurotransmitter criteria as an endogenous ligand for the 'glycine' site of NMDA receptors.

Reciprocal communication systems between astrocytes and neurones

Over the past decade, a growing body of evidence has emerged on the existence in the brain of a close bidirectional communication system between neurones and astrocytes. This article reviews recent advances in understanding the rules governing these interactions and describes putative, novel functions attributable to astrocytes in neuronal transmission. Astrocytes can respond to the neurotransmitter released from active synaptic terminals, with cytosolic Ca(2+) oscillations whose frequency is under the dynamic control of neuronal activity. In response to these neuronal signals, astrocytes can signal back to neurones by releasing various neurone active compounds, such as the excitatory neurotransmitter glutamate. Interestingly, there is accumulating evidence that glutamate is released via a Ca(2+)-dependent mechanism which may share common properties with neurotransmitter exocytosis in neurones. This bidirectional communication system between neurones and astrocytes may lead to profound changes in neuronal excitability and synaptic transmission. While there clearly is an enormous amount of experimental and theoretical work yet to figure out, a coherent view is now emerging which incorporates the astrocyte, with the presynaptic terminal and the postsynaptic target neurone, as a possible third functional element of the synapse.

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.

Determination of minute amounts of D-leucine in various brain regions of rat and mouse using column-switching high-performance liquid chromatography

A highly sensitive method for the determination of minute amounts of D-Leu in biological samples was developed. For accurate and sensitive determination, a column-switching system using a micro ODS column and a chiral column was adopted. After pre-column derivatization of D- and L-Leu with NBD-F, the derivatives of the enantiomers were purified on a micro ODS column as a DL mixture. The eluted DL-Leu was then introduced to the chiral column, and each enantiomer was determined. The calibration curve for D-Leu, which was constructed by adding known amounts of D-Leu to a rat hippocampus, was linear from 1 to 1000 fmol (r>0.999), and the detection limit of added D-Leu was 1 fmol (S/N=5). Within-day and day-to-day precisions of D-Leu determination using the same homogenate of rat hippocampus were 5.11 and 5.25% (RSD), respectively. The content of D-Leu in rat hippocampus was 0.69 nmol/g wet tissue (the percentage of D-enantiomer for total Leu was 0.97%), which was consistent with the reported value. The distribution of D-Leu in mouse brain was also investigated, and the presence of D-Leu in various regions of the mammalian brain was first observed.

Cloning and characterization of a human brain Na(+)-independent transporter for small neutral amino acids that transports D-serine with high affinity

We isolated a cDNA for the human homologue of system asc transporter Asc-1 from human brain. The encoded protein designated as hAsc-1 (human Asc-1) exhibited 91 % sequence identity to mouse Asc-1. Consistent with mouse Asc-1, hAsc-1 required 4F2 heavy chain for its functional expression in Xenopus oocytes. hAsc-1 exhibited the properties of amino acid transport system asc which transports small neutral amino acids in a Na(+)-independent manner. hAsc-1 transported D-serine at high affinity with a K(m) value of 22.8 microM. In brain, 2.0 kb mRNA was highly expressed. hAsc-1 gene was mapped to human chromosome 19, region q12-q13.1. Because of the high-affinity transport with the K(m) value close to the physiological concentration of D-serine, together with the high levels of expression in brain, hAsc-1 is proposed to play significant roles in the D-serine mobilization in brain.

D-serine is an endogenous ligand for the glycine site of the N-methyl-D-aspartate receptor

Functional activity of N-methyl-D-aspartate (NMDA) receptors requires both glutamate binding and the binding of an endogenous coagonist that has been presumed to be glycine, although D-serine is a more potent agonist. Localizations of D-serine and it biosynthetic enzyme serine racemase approximate the distribution of NMDA receptors more closely than glycine. We now show that selective degradation of d-serine with D-amino acid oxidase greatly attenuates NMDA receptor-mediated neurotransmission as assessed by using whole-cell patch-clamp recordings or indirectly by using biochemical assays of the sequelae of NMDA receptor-mediated calcium flux. The inhibitory effects of the enzyme are fully reversed by exogenously applied D-serine, which by itself did not potentiate NMDA receptor-mediated synaptic responses. Thus, D-serine is an endogenous modulator of the glycine site of NMDA receptors and fully occupies this site at some functional synapses.

Interactions between GABA and glycine at inhibitory amino acid receptors on rat olfactory bulb neurons

Whole cell voltage-clamp electrophysiology was used to examine interactions between GABA and glycine at inhibitory amino acid receptors on rat olfactory bulb neurons in primary culture. Membrane currents evoked by GABA and glycine were selectively inhibited by low concentrations of bicuculline and strychnine, respectively, suggesting that they activate pharmacologically distinct receptors. However, GABA- and glycine-mediated currents showed cross-inhibition when the two amino acids were applied sequentially. Application of one amino acid inhibited the response to immediate subsequent application of the other. In the majority of neurons, GABA inhibited subsequent glycine-evoked currents and glycine inhibited subsequent GABA-evoked currents. In a small proportion of neurons, however, GABA inhibited glycine-evoked currents but glycine had little effect on GABA-evoked currents. The reverse was true in other neurons, suggesting that alterations in chloride gradients alone did not account for the cross-inhibition. Furthermore, no cross-inhibition was observed between GABA- and glycine-evoked currents in some neurons. The amplitude of the current evoked by the coapplication of saturating concentrations of GABA and glycine in these neurons was nearly the sum of the currents evoked by GABA and glycine alone. In contrast, the currents were not additive in neurons demonstrating cross-inhibition. These results suggest that olfactory bulb neurons heterogeneously express a population of inhibitory amino acid receptors that can bind either GABA or glycine. Interactions between GABA and glycine at inhibitory amino acid receptors may provide a mechanism to modulate inhibitory synaptic transmission.

Serine racemase: a glial enzyme synthesizing D-serine to regulate glutamate-N-methyl-D-aspartate neurotransmission

Although D amino acids are prominent in bacteria, they generally are thought not to occur in mammals. Recently, high levels of D-serine have been found in mammalian brain where it activates glutamate/N-methyl-D-aspartate receptors by interacting with the "glycine site" of the receptor. Because amino acid racemases are thought to be restricted to bacteria and insects, the origin of D-serine in mammals has been puzzling. We now report cloning and expression of serine racemase, an enzyme catalyzing the formation of D-serine from L-serine. Serine racemase is a protein representing an additional family of pyridoxal-5' phosphate-dependent enzymes in eukaryotes. The enzyme is enriched in rat brain where it occurs in glial cells that possess high levels of D-serine in vivo. Occurrence of serine racemase in the brain demonstrates the conservation of D-amino acid metabolism in mammals with implications for the regulation of N-methyl-D-aspartate neurotransmission through glia-neuronal interactions.

Effects of D-cycloserine, a positive modulator of N-methyl-D-aspartate receptors, and ST 587, a putative alpha-1 adrenergic agonist, individually and in combination, on the non-delayed and delayed foraging behaviour of rats assessed in the radial arm maze

The present study investigated whether alpha-1 adrenergic and glutamatergic N-methyl-D-aspartate (NMDA) receptor-mediated mechanisms interact in memory processes, by examining the effects of individual and combined systemic administration of ST 587, a putative alpha-1 agonist, and D-cycloserine (DCS), a partial agonist at the glycine-B binding site of the NMDA receptor, on the performance of rats in non-delayed and delayed (4-6 h) foraging behaviour in the radial arm maze task, using the delayed non-matching to sample (DNMTS) version. The results indicated that DCS (5.0 mg/kg) decreased working memory errors, i.e. the number of re-entries into the previously visited arms during the sampling phase. In addition, both ST 587 (100 microg/kg) and DCS (10 mg/kg), when administered alone 30 min before a sampling phase, improved retention of this task as reflected by the increased number of correct choices before the first error during the retention phase. The combined administration of ST 587 and DCS, however, did not lead to better retention in the DNMTS task compared with the administration of each of the drugs alone. Combinations of sub-threshold doses of ST 587 (50 or 75 microg/kg) and DCS (5.0 or 7.5 mg/kg) also did not improve retention in this task. DCS (5.0 or 7.5 mg/kg) increased activity as indicated by the increased number of arm entries in a given time during the sampling phase. These findings suggest that the systemic administration of a positive modulator of the NMDA receptor facilitates hippocampal-dependent memory functions, but that these effects are not enhanced by combined administration with an alpha-1 agonist, even though the alpha-1 agonist is effective when given alone. The results support the idea that NMDA receptors modulate both mnemonic and non-mnemonic functions in the brain.

D-aspartate localization in the rat pituitary gland and retina

Rat pituitary gland and retina were probed with anti-D-aspartate (D-Asp) antibody previously prepared in this laboratory [Lee et al., Biochem. Biophys. Res. Commun., 231 (1997) 505-508]. D-Asp immunoreactivity (IR) was observed only in the posterior lobe of the pituitary gland of 3-day-old rats, whereas the anterior and posterior lobes were also positive in 3-week and 6-week-old rats, respectively. In the anterior lobe, intense IR was scattered throughout the lobe and the D-Asp-positive cells appeared to be prolactin-containing cells or some other very closely related type of cell. In the retina, D-Asp IR was observed only in the ganglion cell and nerve fiber region of 3-day-old rats. In contrast, during the transient increase in D-Asp levels in 7-day-old rats, D-Asp IR was additionally evident in regions where differentiating bipolar cells had begun to make contact with other types of cells. The functional relevance of D-Asp localization in these tissues is discussed.

Rapid determination of aspartate enantiomers in tissue samples by microdialysis coupled on-line with capillary electrophoresis

Microdialysis was coupled on-line with derivatization by o-phthalaldehyde and beta-mercaptoethanol and optically gated capillary electrophoresis to determine D- and L-aspartate in tissue samples obtained from rats. The microdialysis probe was inserted into a homogenized tissue sample which allowed generation of a continuous sample stream that was filtered and deproteinated. With 7.5 mM beta-cyclodextrin (CD) in the electrophoresis buffer, the enantiomers of interest could be resolved in 3 s with an electric field of 2500 V/cm over a separation length of 15 mm. Values of D- and L-aspartate in different tissues agreed well with those obtained by an HPLC procedure that required protein precipitation, centrifugation, and extraction. The speed and compatibility with automation of the microdialysis/CE method may make it a general approach for a variety of applications involving high-throughput analysis or sensorlike operation.