Free and conjugated amino acids in human CSF: influence of age and sex

Relation of the kynurenine pathway with normal age: A systematic review

BACKGROUND

The kynurenine pathway (KP) is gaining more attention as a common pathway involved in age-related conditions. However, which changes in the KP occur due to normal ageing is still largely unclear. The aim of this systematic review was to summarize the available evidence for associations of KP metabolites with age.

METHODS

We used an broad search strategy and included studies up to October 2023.

RESULTS

Out of 8795 hits, 55 studies were eligible for the systematic review. These studies suggest that blood levels of tryptophan decrease with age, while blood and cerebrospinal fluid levels of kynurenine and its ratio with tryptophan increase. Studies investigating associations between cerebrospinal fluid and blood levels of kynurenic acid and quinolinic acid with age reported either positive or non-significant findings. However, there is a large heterogeneity across studies. Additionally, most studies were cross-sectional, and only few studies investigated associations with other downstream kynurenines.

CONCLUSIONS

This systematic review suggests that levels of kynurenines are positively associated with age. Larger and prospective studies are needed that also investigate a more comprehensive panel of KP metabolites and changes during the life-course.

Elevated Levels of Arachidonic Acid-Derived Lipids Including Prostaglandins and Endocannabinoids Are Present Throughout ABHD12 Knockout Brains: Novel Insights Into the Neurodegenerative Phenotype

Derived from arachidonic acid (AA), the endogenous cannabinoid (eCB) 2-arachidonoyl glycerol (2-AG) is a substrate for α/β hydrolase domain-12 (ABHD12). Loss-of-function mutations of ABHD12 are associated with the neurodegenerative disorder polyneuropathy, hearing loss, ataxia, retinitis pigmentosa, and cataract (PHARC). ABHD12 knockout (KO) mice show PHARC-like behaviors in older adulthood. Here, we test the hypothesis that ABHD12 deletion age-dependently regulates bioactive lipids in the CNS. Lipidomics analysis of the brainstem, cerebellum, cortex, hippocampus, hypothalamus, midbrain, striatum and thalamus from male young (3–4 months) and older (7 months) adult ABHD12 KO and age-matched wild-type (WT) mice was performed on over 80 lipids via HPLC/MS/MS, including eCBs, lipoamines, 2-acyl glycerols, free fatty acids, and prostaglandins (PGs). Aging and ABHD12 deletion drove widespread changes in the CNS lipidome; however, the effects of ABHD12 deletion were similar between old and young mice, meaning that many alterations in the lipidome precede PHARC-like symptoms. AA-derived lipids were particularly sensitive to ABHD12 deletion. 2-AG increased in the striatum, hippocampus, cerebellum, thalamus, midbrain, and brainstem, whereas the eCB N-arachidonoyl ethanolamine (AEA) increased in all 8 brain regions, along with at least 2-PGs. Aging also had a widespread effect on the lipidome and more age-related changes in bioactive lipids were found in ABHD12 KO mice than WT suggesting that ABHD12 deletion exacerbates the effects of age. The most robust effects of aging (independent of genotype) across the CNS were decreases in N-acyl GABAs and N-acyl glycines. In conclusion, levels of bioactive lipids are dynamic throughout adulthood and deleting ABHD12 disrupts the wider lipidome, modulating multiple AA-derived lipids with potential consequences for neuropathology.

Identification of altered brain metabolites associated with TNAP activity in a mouse model of hypophosphatasia using untargeted NMR-based metabolomics analysis

Tissue non-specific alkaline phosphatase (TNAP) is a key player of bone mineralization and TNAP gene (ALPL) mutations in human are responsible for hypophosphatasia (HPP), a rare heritable disease affecting the mineralization of bones and teeth. Moreover, TNAP is also expressed by brain cells and the severe forms of HPP are associated with neurological disorders, including epilepsy and brain morphological anomalies. However, TNAP's role in the nervous system remains poorly understood. To investigate its neuronal functions, we aimed to identify without any a priori the metabolites regulated by TNAP in the nervous tissue. For this purpose we used ¹ H- and ³¹ P NMR to analyze the brain metabolome of Alpl (Akp2) mice null for TNAP function, a well-described model of infantile HPP. Among 39 metabolites identified in brain extracts of 1-week-old animals, eight displayed significantly different concentration in Akp2^-/- compared to Akp2^+/+ and Akp2^+/- mice: cystathionine, adenosine, GABA, methionine, histidine, 3-methylhistidine, N-acetylaspartate (NAA), and N-acetyl-aspartyl-glutamate, with cystathionine and adenosine levels displaying the strongest alteration. These metabolites identify several biochemical processes that directly or indirectly involve TNAP function, in particular through the regulation of ecto-nucleotide levels and of pyridoxal phosphate-dependent enzymes. Some of these metabolites are involved in neurotransmission (GABA, adenosine), in myelin synthesis (NAA, NAAG), and in the methionine cycle and transsulfuration pathway (cystathionine, methionine). Their disturbances may contribute to the neurodevelopmental and neurological phenotype of HPP.

Modulation of glycine sites enhances social memory in rats using PQQ combined with d-serine

The aim of study was to investigate the effects of pyrroloquinoline quinone (PQQ) combined with d-serine on the modulation of glycine sites in the brain of rats using social recognition test. Rats were divided into seven groups (n=10) and given repeated intraperitoneal (ip) injections of saline, MK-801 (0.5mg/kg), clozapine (1mg/kg), haloperidol (0.1mg/kg), d-serine (0.8g/kg), PQQ (2.0μg/kg), or d-serine (0.4g/kg) combined with PQQ (1.0μg/kg) for seven days. A social recognition test, including assessment of time-dependent memory impairment, was performed. A non-competitive NMDA receptor antagonist, MK-801, significantly impaired social memory, and this impairment was significantly repaired with an atypical antipsychotic (clozapine) but not with a typical antipsychotic (haloperidol). Likewise, d-serine combined with PQQ significantly improved MK-801-disrupted cognition in naïve rats, whereas haloperidol was ineffective. The present results show that the co-agonist NMDA receptor treated with PQQ and d-serine enhances social memory and may be an effective approach for treating the cognitive dysfunction observed in schizophrenic patients. PQQ stimulates glycine modulatory sites by which it may antagonize indirectly by removing glycine from the synaptic cleft or by binding the unsaturated site with d-serine in the brain, providing the insights into future research of central nervous system and drug discovery.

Glycine blocks long-term potentiation of GABAergic synapses in the ventral tegmental area

The mesocorticolimbic dopamine system, originating in the ventral tegmental area (VTA) is normally constrained by GABA-mediated synaptic inhibition. Accumulating evidence indicates that long-term potentiation of GABAergic synapses (LTPGABA) in VTA dopamine neurons plays an important role in the actions of drugs of abuse, including ethanol. We previously showed that a single infusion of glycine into the VTA of rats strongly reduces ethanol intake for 24h. In the current study, we examined the effect of glycine on the electrophysiological activities of putative dopamine VTA neurons in midbrain slices from ethanol-naïve rats. We report here that a 15-min exposure to 10 μM glycine prevented trains of high-frequency stimulation (HFS) from producing LTPGABA, which was rescued by the glycine receptor (GlyR) antagonist strychnine. Glycine also concentration-dependently decreased the frequency of spontaneous excitatory postsynaptic currents (sEPSCs). By contrast, glycine pretreatment did not prevent potentiation of inhibitory postsynaptic currents (IPSCs) during a continuous exposure to the nitric oxide (NO) donor, SNAP (S-nitroso-N-acetylpenicillamine), or a brief exposure to 10 μM glycine and 10 μM NMDA (N-methyl-D-aspartate), an agonist of NMDA-type glutamate receptors. Thus, the blockade of LTPGABA by glycine is probably resulted from suppressing glutamate release by activating the GlyRs on the glutamatergic terminals. This effect of glycine may contribute to the reduction in ethanol intake induced by intra-VTA glycine observed in vivo.

NMDA receptor and schizophrenia: a brief history

Although glutamate was first hypothesized to be involved in the pathophysiology of schizophrenia in the 1980s, it was the demonstration that N-methyl-D-aspartate (NMDA) receptor antagonists, the dissociative anesthetics, could replicate the full range of psychotic, negative, cognitive, and physiologic features of schizophrenia in normal subjects that placed the "NMDA receptor hypofunction hypothesis" on firm footing. Additional support came from the demonstration that a variety of agents that enhanced NMDA receptor function at the glycine modulatory site significantly reduced negative symptoms and variably improved cognition in patients with schizophrenia receiving antipsychotic drugs. Finally, persistent blockade of NMDA receptors recreates in experimental animals the critical pathologic features of schizophrenia including downregulation of parvalbumin-positive cortical GABAergic neurons, pyramidal neuron dendritic dysgenesis, and reduced spine density.

Changes in kinetics of amino acid uptake at the ageing ovine blood-cerebrospinal fluid barrier

Amino acids (AA) in brain are precisely controlled by blood-brain barriers, which undergo a host of changes in both morphology and function during ageing. The effect of these age-related changes on AA homeostasis in brain is not well described. This study investigated the kinetics of four AA (Leu, Phe, Ala and Lys) uptakes at young and old ovine choroid plexus (CP), the blood-cerebrospinal fluid (CSF) barrier (BCB), and measured AA concentrations in CSF and plasma samples. In old sheep, the weight of lateral CP increased, so did the ratio of CP/brain. The expansion of the CP is consistent with clinical observation of thicker leptomeninges in old age. AA concentrations in old CSF, plasma and their ratio were different from the young. Both V(max) and K(m) of Phe and Lys were significant higher compared to the young, indicating higher trans-stimulation in old BCB. Cross-competition and kinetic inhibition studies found the sensitivity and specificity of these transporters were impaired in old BCB. These changes may be the first signs of a compromised barrier system in ageing brain leading increased AA influx into the brain causing neurotoxicity.

Coagonist release modulates NMDA receptor subtype contributions at synaptic inputs to retinal ganglion cells

NMDA receptors (NMDARs) are tetrameric protein complexes usually comprising two NR1 and two NR2 subunits. Different combinations of four potential NR2 subunits (NR2A-D) confer diversity in developmental expression, subsynaptic localization, and functional characteristics, including affinity for neurotransmitter. NR2B-containing NMDARs, for example, exhibit relatively high affinity both for glutamate and the coagonist glycine. Although multiple NMDAR subtypes can colocalize at individual synapses, particular subtypes often mediate inputs from distinct functional pathways. In retinal ganglion cells (RGCs), NMDARs contribute to synaptic responses elicited by light stimulus onset ("ON") and offset ("OFF"), but roles for particular NMDAR subtypes, and potential segregation between the ON and OFF pathways, have not been explored. Moreover, elements in the retinal circuitry release two different NMDAR coagonists, glycine and d-serine, but the effects of endogenous coagonist release on the relative contribution of different NMDAR subtypes are unclear. Here, we show that coagonist release within the retina modulates the relative contribution of different NMDARs in the ON pathway of the rat retina. By pharmacologically stimulating functional pathways independently in acute slices and recording synaptic responses in RGCs, we show that ON inputs, but not OFF inputs, are mediated in part by NMDARs exhibiting NR2B-like pharmacology. Furthermore, suppressing release of NMDAR coagonist reduces NMDAR activation at ON synapses and increases the relative contribution of these putative NR2B-containing receptors. These results demonstrate direct evidence for evoked coagonist release onto NMDARs and indicate that modulating coagonist release may regulate the relative activation of different NMDAR subtypes in the ON pathway.

Glycine modulates synaptic NR2A- and NR2B-containing NMDA receptor-mediated responses in the rat visual cortex

In the central nervous system, activation of N-methyl-d-aspartate receptor (NMDA-R) glycine binding sites is a prerequisite for activation of synaptic NMDA-Rs by the excitatory neurotransmitter glutamate. Here we used patch-clamp recordings in transverse slice preparations to study whether the glycine binding site of the NMDA-R saturates and to determine their subunit composition in layer II/III pyramidal neurons of the rat visual cortex. We found that the NMDA-R-mediated component of miniature excitatory postsynaptic currents (mEPSCs) could be potentiated by exogenously applied glycine. Similar results were obtained by exogenously applied d-serine. A specific antagonist for NR2B-NMDA-Rs, Ro 25-6981, reduced NMDA-R-mediated mEPSCs, and glycine with Ro 25-6981 enhanced NMDA-R-mediated mEPSCs. Moreover, Zn2+, an NR2A-NMDA-R antagonist, also reduced NMDA-mediated mEPSCs and glycine with Zn2+ enhanced the NMDA-mediated mEPSCs. Our data indicate that the glycine binding site of synaptic NR2A-containing and NR2B-containing NMDA-Rs does not saturate and that glycine may act as a modulator of NMDA-R-mediated transmission in layer II/III pyramidal neurons of the rat visual cortex.

Simultaneous Analysis of d- and l-Serine in Cerebrospinal Fluid by Use of HPLC

Background: d-Serine is a coagonist for the glycine-binding site of the N-methyl-d-aspartate receptors and has been implicated in various neuropsychiatric functions such as learning, memory, and nociception, as well as schizophrenia and Alzheimer disease. We developed an HPLC method for d- and l-serine in cerebrospinal fluid (CSF).

Methods: The dabsylated racemic serine peak, automatically collected using a previously reported HPLC separation process for CSF amino acids, was desalted and subjected to a chiral resolution HPLC step with a Sumichiral column using an ultraviolet-visible detector.

Results: The limits of quantification (signal-to-noise ratio = 10) for d- and l-serine were 0.8 and 1.3 μmol/L, respectively. The mean imprecision values (CVs) for within-day measurements of d- and l-serine were 2.1% and 1.8%, respectively, and for between-day were 6.2% and 6.6%. Mean recovery of CSF serine (sum of d-serine + l-serine) applied to the Sumichiral column was 87%. The mean (SD) d-serine concentrations in 45 CSF samples obtained from 16 patients with chronic pain due to degenerative osteoarthritis of the knees, 16 with postherpetic neuralgia, and 13 with no pain were, respectively, 3.97 (0.44), 1.85 (0.21), and 2.72 (0.32) μmol/L.

Conclusion: d- and l-serine can be quantified with ultraviolet-visible detection of dabsyl derivatives. The dabsyl derivatives are stable and allow duplicate analysis of CSF samples in multisample runs.

Glycine Binding Sites of Presynaptic NMDA Receptors May Tonically Regulate Glutamate Release in the Rat Visual Cortex

In the CNS, activation of N-methyl-d-aspartate receptor (NMDA-R) glycine binding sites is a prerequisite for activation of postsynaptic NMDA-Rs by the excitatory neurotransmitter glutamate. Here we provide electrophysiological evidence that the glycine binding sites of presynaptic NMDA-Rs regulate glutamate release in layer II/III pyramidal neurons of the rat visual cortex. Specifically, our results reveal that the frequency of miniature excitatory postsynaptic currents is significantly reduced by 7-chloro-kynurenic acid (7-Cl KYNA), a NMDA-R glycine binding site antagonist, and glycine or d-serine reverses this effect. Similar results are obtained when the open-channel NMDA receptor blocker, MK-801, is included in the recording pipette. Our data indicate that the glycine binding site of postsynaptic NMDA-Rs is not saturated. Moreover, they suggest that presynaptic NMDA-Rs are located in layer II/III pyramidal neurons of the rat visual cortex and that the glycine binding site of presynaptic NMDA-Rs tonically regulates glutamate release.

Glutamatergic dysfunction in OCD

The role of glutamatergic dysfunction in the pathophysiology of OCD has hardly been explored despite recent reports implicating glutamatergic dysfunction in OCD. We decided to investigate CSF glutamate levels in adult OCD probands compared to psychiatrically normal controls. In total, 21 consenting psychotropic drug-naïve adult OCD patients, diagnosed using SCID-IV-CV, and 18 consenting psychiatrically normal controls with age within 10 years of age of the patients, who did not have any history of head injury or neurological illness, were included into the study. Aseptically collected and stored CSF samples obtained from the patients and control subjects were used for glutamate estimation, which was carried out by a modification of the procedure described by Lund (1986). CSF glutamate (micromol/l) level was found to be significantly higher [F(1,31)=6.846, p=0.014] in OCD patients (47.12+/-4.25) compared to control subjects (41.36+/-3.63) on analysis of covariance. There was no effect of gender, age, duration of illness, Y-BOCS score, or CGI-S score on CSF glutamate levels. Our study provides preliminary evidence implicating glutamatergic excess in the pathophysiology of OCD, which needs to be further explored by studies from other centers involving larger sample sets from different age groups.

Age-Related Increase of Kynurenic Acid in Human Cerebrospinal Fluid – IgG and β2-Microglobulin Changes

Kynurenic acid (KYNA) is an endogenous metabolite in the kynurenine pathway of tryptophan degradation and is an antagonist at the glycine site of the N-methyl-D-aspartate as well as at the alpha 7 nicotinic cholinergic receptors. In the brain tissue KYNA is synthesised from L-kynurenine by kynurenine aminotransferases (KAT) I and II. A host of immune mediators influence tryptophan degradation. In the present study, the levels of KYNA in cerebrospinal fluid (CSF) and serum in a group of human subjects aged between 25 and 74 years were determined by using a high performance liquid chromatography method. In CSF and serum KAT I and II activities were investigated by radioenzymatic assay, and the levels of beta(2)-microglobulin, a marker for cellular immune activation, were determined by ELISA. The correlations between neurochemical and biological parameters were evaluated. Two subject groups with significantly different ages, i.e. <50 years and >50 years, p < 0.001, showed statistically significantly different CSF KYNA levels, i.e. 2.84 +/- 0.16 fmol/microl vs. 4.09 +/- 0.14 fmol/microl, p < 0.001, respectively; but this difference was not seen in serum samples. Interestingly, KYNA is synthesised in CSF principally by KAT I and not KAT II, however no relationship was found between enzyme activity and ageing. A positive relationship between CSF KYNA levels and age of subjects indicates a 95% probability of elevated CSF KYNA with ageing (R = 0.6639, p = 0.0001). KYNA levels significantly correlated with IgG and beta(2)-microglobulin levels (R = 0.5244, p = 0.0049; R = 0.4253, p = 0.043, respectively). No correlation was found between other biological parameters in CSF or serum. In summary, a positive relationship between the CSF KYNA level and ageing was found, and the data would suggest age-dependent increase of kynurenine metabolism in the CNS. An enhancement of CSF IgG and beta(2)-microglobulin levels would suggest an activation of the immune system during ageing. Increased KYNA metabolism may be involved in the hypofunction of the glutamatergic and/or nicotinic cholinergic neurotransmission in the ageing CNS.

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

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

The glycine transporter type 1 inhibitor N-[3-(4'-fluorophenyl)-3-(4'-phenylphenoxy)propyl]sarcosine potentiates NMDA receptor-mediated responses in vivo and produces an antipsychotic profile in rodent behavior

Glycine acts as a necessary coagonist for glutamate at the NMDA receptor (NMDAR) complex by binding to the strychnine-insensitive glycine-B binding site on the NR1 subunit. The fact that glycine is normally found in the brain and spinal cord at concentrations that exceed those required to saturate this site has led to the speculation that glycine normally saturates NMDAR-containing synapses in vivo. However, additional lines of evidence suggest that synaptic glycine may be efficiently regulated in synaptic areas by the glycine transporter type 1 (GlyT1). The recent description of a potent and selective GlyT1 inhibitor (N-[3-(4'-fluorophenyl)-3-(4'-phenylphenoxy)propyl]sarcosine [NFPS]) provides a tool for evaluation of the hypothesis that inhibition of GlyT1 may increase synaptic glycine and thereby potentiate NMDAR function in vivo. In the present study, we found that (+)-NFPS demonstrated >10-fold greater activity in an in vitro functional glycine reuptake assay relative to the racemic compound. In vivo, (+/-)-NFPS significantly enhanced long-term potentiation in the hippocampal dentate gyrus induced by high-frequency electrical stimulation of the afferent perforant pathway. Furthermore, (+)-NFPS induced a pattern of c-Fos immunoreactivity comparable with the atypical antipsychotic clozapine and enhanced prepulse inhibition of the acoustic startle response in DBA/2J mice, a strain with low basal levels of prepulse inhibition. Collectively, these data suggest that selective inhibition of GlyT1 can enhance NMDAR-sensitive activity in vivo and also support the idea that GlyT1 may represent a novel target for developing therapeutics to treat disorders associated with NMDAR hypofunction.

Sequence identification and characterization of human carnosinase and a closely related non-specific dipeptidase

Carnosine (beta-alanyl-L-histidine) and homocarnosine (gamma-aminobutyric acid-L-histidine) are two naturally occurring dipeptides with potential neuroprotective and neurotransmitter functions in the brain. Peptidase activities degrading both carnosine and homocarnosine have been described previously, but the genes linked to these activities were unknown. Here we present the identification of two novel cDNAs named CN1 and CN2 coding for two proteins of 56.8 and 52.7 kDa and their classification as members of the M20 metalloprotease family. Whereas human CN1 mRNA and protein are brain-specific, CN2 codes for a ubiquitous protein. In contrast, expression of the mouse and rat CN1 orthologues was detectable only in kidney. The recombinant CN1 and CN2 proteins were expressed in Chinese hamster ovary cells and purified to homogeneity. CN1 was identified as a homodimeric dipeptidase with a narrow substrate specificity for Xaa-His dipeptides including those with Xaa = beta Ala (carnosine, K(m) 1.2 mM), N-methyl beta Ala, Ala, Gly, and gamma-aminobutyric acid (homocarnosine, K(m) 200 microM), an isoelectric point of pH 4.5, and maximal activity at pH 8.5. CN2 protein is a dipeptidase not limited to Xaa-His dipeptides, requires Mn(2+) for full activity, and is sensitive to inhibition by bestatin (IC(50) 7 nM). This enzyme does not degrade homocarnosine and hydrolyzes carnosine only at alkaline pH with an optimum at pH 9.5. Based on their substrate specificity and biophysical and biochemical properties CN1 was identified as human carnosinase (EC ), whereas CN2 corresponds to the cytosolic nonspecific dipeptidase (EC ).

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.

Non-competitive antagonism of N-methyl-d-aspartate by displacement of an endogenous glycine-like substance

N-methyl-D-aspartate (NMDA; 40 microM) induced depolarizations of cortical wedges that were reduced by 30 - 60% in the presence of D-2-amino-5-phosphonovalerate (D-AP5; 5 microM), ketamine (5 microM), dextrorphan (5 microM), magnesium (500 microM), kynurenate (200 microM), and 1-hydroxy-3-aminopyrrolidone-2 (HA-966; 200 microM). Superfusion with glycine (1 microM - 1 mM) did not enhance the action of NMDA in control medium and in media containing D-AP5, ketamine, dextrorphan, or magnesium. In the presence of kynurenate and HA-966, however, NMDA-induced depolarizations were enhanced in a dose-dependent manner by glycine (10 microM - 3.16 mM). NMDA antagonism produced by HA-966 appeared to be more completely reversed than that produced by kynurenate. This action of glycine was mimicked by D-serine but not by GABA or L-serine, and was resistant to strychnine (10 - 50 microM). Reduction of responses to quisqualate by kynurenate was not reversed by glycine. In these cortical wedges, spontaneous synaptic activity was observed in nominally magnesium-free medium and this epileptiform activity could be blocked by the above NMDA antagonists. Glycine and D-serine reversed only the effect of kynurenate and HA-966 on such synaptic activity. These results suggest there is an endogenous glycine-like compound acting on NMDA receptor-ionophore complexes and that displacement of this compound by HA-966 or kynurenate produces antagonism of NMDA.

Excitatory Amino Acids in Cerebrospinal Fluid of Patients with Acute Head Injuries

Background: The excitatory amino acids (EAAs) glutamate (Glu) and aspartate (Asp) play a role in the pathogenesis of postischemic and posttraumatic brain insult. The changes of EAAs in cerebrospinal fluid (CSF) of patients with traumatic brain injury are incompletely understood.

Methods: We used reversed-phase HPLC with precolumn derivatization with o-phthalaldehyde and fluorescence detection to measure Glu and Asp in CSF of 42 patients with acute head injury and 9 control adults without neurologic diseases. We assessed the Glasgow Coma Scale (GCS) on admission, the main lesion patterns on computed tomography (CT) scan within 24 h post trauma, and the Glasgow Outcome Scale (GOS) 3 months post injury.

Results: The mean concentrations of Glu and Asp in CSF in the brain-injured group were significantly higher than those of the control group (Glu, P &lt;0.001; Asp, P &lt;0.05). In patients admitted within 24 h after severe injury (n = 13), peak Glu values appeared within 48 h in 11 patients (85%), and the mean value remained higher than control values at day 7 (P &lt;0.02). The concentrations of EAAs were higher in patients with severe injuries (GCS ≤8) than in those with milder injuries (Glu, P &lt;0.001; Asp, P &lt;0.05). GCS and peak EAAs correlated negatively (Glu, rs = −0.5706, P &lt;0.001; Asp, rs = −0.5503, P &lt;0.001). The patients with focal brain contusion on initial CT scan (n = 8) had a significantly lower peak Glu value than the patients with other patterns (n = 8–15; P &lt;0.02 to 0.001). The peak value of EAAs in the poor-outcome group (including severe disability, vegetative state, and death) was significantly higher than in the good-outcome group (good recovery and moderate disability; Glu, P &lt;0.001; Asp, P &lt;0.01); GOS was closely correlated to the EAA values (Glu, rs = 0.5737, P &lt;0.001; Asp, rs = 0.5470, P &lt;0.001).

Conclusions: The EAA concentrations in CSF increase after acute head injury and remain higher for at least 1 week post injury in severely injured patients. The more severe the trauma, the more obvious the excitotoxicity induced by EAAs and the worse the outcome.

Effects of the seafood toxin domoic acid on glutamate uptake by rat astrocytes

Pronounced glutamic acid uptake was observed after only 15 min with glutamate concentrations of 60 nmol/mg protein when astrocytes were incubated with 1 mM glutamic acid. The uptake increased with time to a steady-state glutamate level of above 160 nmol/mg protein by 45 min. The uptake was energy dependent. Reduced temperature (0 degrees C) and ouabain (100 microM) inhibited uptake by 86.7% (P<0.001; n=18) and 84.4% (P<0.001; n=18), respectively, when compared with controls. After exposure of astrocytes to glutamate (1 mM) in the incubation medium, in the presence of domoic acid (10 and 100 microM) at 5 and 60 min, domoic acid (10 microM) elevated glutamate uptake by 64.0% (P<0.05; n=34) at 5 min but decreased glutamate uptake by 47.8% (P<0.01; n=19) at 60 min compared with controls. A higher dose of domoic acid (100 microM) decreased glutamate uptake by 49.6% (P<0.01; n=20) and 61.3% (P<0.001; n=20) at 5 and 60 min, respectively, compared with controls. This study suggests that domoic acid may induce neurotoxicity because of the failure of astrocytes to remove extracellular glutamate. This may contribute to excitotoxic injury.

Neuronal-glial interactions and behaviour

Both neurons and glia interact dynamically to enable information processing and behaviour. They have had increasingly intimate, numerous and differentiated associations during brain evolution. Radial glia form a scaffold for neuronal developmental migration and astrocytes enable later synapse elimination. Functionally syncytial glial cells are depolarised by elevated potassium to generate slow potential shifts that are quantitatively related to arousal, levels of motivation and accompany learning. Potassium stimulates astrocytic glycogenolysis and neuronal oxidative metabolism, the former of which is necessary for passive avoidance learning in chicks. Neurons oxidatively metabolise lactate/pyruvate derived from astrocytic glycolysis as their major energy source, stimulated by elevated glutamate. In astrocytes, noradrenaline activates both glycogenolysis and oxidative metabolism. Neuronal glutamate depends crucially on the supply of astrocytically derived glutamine. Released glutamate depolarises astrocytes and their handling of potassium and induces waves of elevated intracellular calcium. Serotonin causes astrocytic hyperpolarisation. Astrocytes alter their physical relationships with neurons to regulate neuronal communication in the hypothalamus during lactation, parturition and dehydration and in response to steroid hormones. There is also structural plasticity of astrocytes during learning in cortex and cerebellum.

Interactions of cholinergic and glutamatergic neuronal systems in the functional activation of cerebral blood flow response: a PET study in unanesthetized monkeys

The effects of somatosensory stimulation on the regional cerebral blood flow (rCBF) response were studied in unanesthetized monkeys under modulations of the glutamatergic and cholinergic systems using [15O]H2O and positron emission tomography (PET). Under a saline condition, vibrotactile stimulation elicited a significant increase in the rCBF response in the contralateral somatosensory cortex. The systemic administration of scopolamine, a muscarinic cholinergic receptor antagonist, resulted in the dose-dependent reduction of the rCBF response to the stimulation. The rCBF response abolished by scopolamine was recovered by the administration of physostigmine, a cholinesterase inhibitor in a dose-dependent manner. In addition, D-cycloserine, a partial agonist at the glycine site coupled to N-methyl-D-aspartate (NMDA) receptors, also restored the scopolamine-abolished rCBF response. The regional cerebral metabolic rate of glucose (rCMRglc) response, measured with [18F]-2-fluoro-2-deoxy-D-glucose, was not affected by the administration of scopolamine, physostigmine and/or D-cycloserine. The systemic administration of (+)-3-amino-1-hydroxy-2-pyrrolidone (HA-966), an antagonist of the glycine modulatory site on the NMDA receptors, induced the dose-dependent suppression of the rCBF response to the stimulation. The rCBF response abolished by HA-966 was restored by D-cycloserine, but not by physostigmine. The rCMRglc response was partially but significantly reduced by the administration of HA-966, and its reduction was restored by D-cycloserine, but not by physostigmine. These findings provided pharmacological evidence for an interaction between cholinergic and glutamatergic neuronal systems, the latter of which mediates the former by downstream regulation, in the functional rCBF response to somatosensory stimulation.

Decreased cerebrospinal fluid levels of neutral and basic amino acids in patients with Parkinson's disease

We measured the CSF levels of 21, and the plasma levels of 26, amino acids in 31 patients with Parkinson's disease (PD) and in 45 matched controls. We used an ion-exchange chromatography method. When compared to controls, PD patients had lower CSF levels of taurine, alanine, valine, leucine, isoleucine, ethanolamine, citrulline, ornithine, lysine, histidine, arginine, and alpha-aminobutyric acid. PD patients not treated with levodopa or with dopamine agonists had higher CSF tyrosine and phenylalanine levels than those not treated with these drugs and also than controls. PD patients had higher plasma levels of phosphoserine, threonine, methionine, tyrosine, sarcosine and alpha-aminoadipic acid, and lower plasma levels of valine, leucine, and tryptophan, than controls. The CSF/plasma ratio of many of these amino acids was significantly lower in PD patients than those of controls, suggesting that PD patients might have a dysfunction in the transport of neutral and basic amino acids across the blood-brain barrier.

Free D-aspartate and D-serine in the mammalian brain and periphery

It has long been assumed that L-forms of amino acids exclusively constitute free amino acid pools in mammals. However, a variety of studies in the last decade has demonstrated that free D-aspartate and D-serine occur in mammals and may have important physiological function in mammals. Free D-serine is confined predominantly to the forebrain structure, and the distribution and development of D-serine correspond well with those of the N-methyl-D-aspartate (NMDA)-type excitatory amino acid receptor. As D-serine acts as a potent and selective agonist for the strychnine-insensitive glycine site of the NMDA receptor, it is proposed that D-serine is a potential candidate for an NMDA receptor-related glycine site agonist in mammalian brain. In contrast, widespread and transient emergence of a high concentration of free D-aspartate is observed in the brain and periphery. Since the periods of maximal emergence of D-aspartate in the brain and periphery occur during critical periods of morphological and functional maturation of the organs, D-aspartate could participate in the regulation of these regulation of these developmental processes of the organs. This review deals with the recent advances in the studies of presence of free D-aspartate and D-serine and their metabolic systems in mammals. Since D-aspartate and D-serine have been shown to potentiate NMDA receptor-mediated transmission through the glutamate binding site and the strychnine-insensitive glycine binding site, respectively, and have been utilized extensively as potent and selective tools to study the excitatory amino acid system in the brain, we shall discuss also the NMDA receptor and uptake system of D-amino acids.

The pharmacology of amino-acid responses in septal neurons

Septal cholinergic neurons are known to play an important role in cognitive processes including learning and memory through afferent innervation of the hippocampal formation and cerebral cortex. The septum contains not only cholinergic neurons but also various types of neurons including GABA (gamma-aminobutyric acid)-ergic neurons. Although synaptic transmission in the septum is mediated primarily by the activation of excitatory and inhibitory amino-acid receptors, it is possible that a distinct phenotype of neuron is endowed with a different type for each of the amino-acid receptors and thus they play different roles from each other, since it has been demonstrated within the septum that there is a regional distribution of various types of amino-acid receptor subunits, their expression as different combinations within a specific cell may produce receptor channels with disparate functional properties. As a first step towards knowing the various functions of septal cholinergic neurons, we characterized the functional properties of glutamate, GABA (type A; GABAA) and glycine receptor channels on cultured rat septal neurons which were histologically identified to be cholinergic. These were similar to those of receptor channels on other types of neurons, except for the actions of some neuromodulators. The septal N-methyl-D-aspartate receptor channel was distinct in being less sensitive to Mg2+ and in a voltage-dependent action of Zn2+. The septal GABAA receptor channel exhibited a lanthanide site whose activation resulted in a positive allosteric interaction with a binding site of pentobarbital. The septal glycine receptor channel was only positively modulated by Zn2+; this action of Zn2+ was not accompanied by an inhibitory effect. Our data suggest that the amino-acid receptors on septal cholinergic neurons may play a distinct role compared to other types of neurons; this difference depends on the actions of neuromodulators and metal cations. It would be interesting to compare these effects recorded in tissue culture to those observed with septal cholinergic neurons in slice preparations.

High extracellular glycine does not potentiate N-methyl-D-aspartate-evoked depolarization in vivo

As N-methyl-D-aspartate receptor (NMDA) ionophore complexes have a distinct positive, allosteric regulatory site for glycine, it has been proposed that elevated extracellular glycine during or after cerebral ischaemia may induce excessive NMDA/glutamate receptor activation and, thereby, excitotoxicity. To test this hypothesis, we have perfused increasing concentrations of glycine, either alone or with co-application of NMDA, through a microdialysis probe implanted in the striatum of halothane anaesthetized rats. Changes in the extracellular field (DC) potential indicative of depolarization were recorded precisely at the site of drug application by an electrode incorporated within dialysis fibre. Microdialysis application of up to 1 mM of glycine had no effect on the basal DC potential. Above 10 mM, glycine produced concentration-dependent depolarizations, but the amplitude of these responses remained very small (e.g. 0.52 +/- 0.05 mV for 100 mM glycine, n = 10, i.e. around 30-fold smaller than that of a wave of spreading depression). Application of 200 microM NMDA via the microdialysis probe produced consistent short-lasting depolarizations (around 2.5 mV amplitude), but these were not potentiated by co-application of up to 100 mM glycine. These data do not support the view that increased extracellular concentrations of glycine, such as those observed in ischaemia, may be potentially excitotoxic. Nevertheless, as occupation of the glycine site coupled to the NMDA-receptor is required for NMDA/glutamate receptor activation, this site remains an attractive target for potential neuroprotective agents.

Neurotransmitter amino acids in cerebrospinal fluid of patients with Parkinson's disease

We measured the CSF and plasma levels of glutamate, glutamine, aspartate (only in plasma), asparagine, glutamine, glycine and GABA in 31 patients with Parkinson's disease and in 45 matched controls. We used an ion-exchange chromatography method. When compared to controls, PD patients had similar CSF levels of glutamate, glutamine, asparagine, and glycine higher CSF GABA levels higher plasma levels of glutamine, asparagine, and glycine, and lower plasma levels of aspartate. The CSF levels of the amino acids measured were not correlated with the clinical features of PD. Our results that CSF GABA levels are not decreased in PD as previously suggested.

The glycine/NMDA receptor antagonist HA-966 impairs visual recognition memory in rhesus monkeys

Recent studies have shown that strychnine-insensitive glycine binding sites positively modulate the N-methyl-D-asparate (NMDA) subclass of glutamate receptors, which are important in neural pathways involved in cognitive function. We examined the effect of (+/-)-3-amino-1-hydroxy-2-pyrrolidone (HA-966), a highly specific antagonist of this glycine modulatory site on the NMDA receptor, on visual recognition memory in four rhesus monkeys performing a computer-automated version of delayed nonmatching-to-sample (DNMS) with a list length of 20 trial-unique graphic symbols. In addition, the effect of HA-966 was compared with that of (+)-5-methyl-10, 11-dihydro-5H-dibenzo[a,d]cyclohepten-5, 10-imine (dizocilpine; MK-801), a noncompetitive NMDA channel blocker. Administration of HA-966 (0.1-10 mg/kg, i.m.) 30 min before testing impaired DNMS performance dose-dependently, starting at doses of 3.2 mg/kg; the memory deficity following the highest dose (10 mg/kg) was associated with prolonged response latencies. Similar impairments in recognition memory were observed following treatment with MK-801, though at much lower doses (3.2-32 micrograms/kg) than those at which HA-966 was effective. Administration of low doses of HA-966 (1 mg/kg) and MK-801 (10 micrograms/kg), each of which had no significant effect on performance when given alone, also failed to impair performance when given concurrently. Combined administration of both drugs, each at amnesia-producing doses (3.2 mg/kg of HA-966 plus 32 micrograms/kg of MK-801), markedly impaired performance in an additive, not a synergistic, manner. From these results, we propose that the recognition memory impairment observed in our monkeys following HA-966 administration is via an action on the glycine modulatory site of the NMDA receptor complex.

Reference values of neuroactive amino acids in the cerebrospinal fluid by high-performance liquid chromatography with electrochemical and fluorescence detection

Sampling and HPLC analysis procedures for CSF amino acid determinations were evaluated. In order to increase sensitivity, a precolumn derivatization of amino acids by o-phthalaldehyde-mercaptoethanol reagent was used. By using fluorimetric and electrochemical detection in series, positive peak identification can be obtained in a single chromatographic run. It is recommended to analyze freshly collected CSF. Amino acids are stable for short periods over a wide range of temperature, but storage at -80 degrees C is recommended. The CSF samples for the calculation of the reference values were taken from 40 healthy subjects, hospitalized for lumbar disk herniation, placed on the same diet and kept drug-free for at least 1 week. The mean values (mumol/l) and the ranges (in parentheses) were: 0.27 (0.09-0.63), 0.62 (0.18-1.15), 5.32 (3.05-11.50), 6.16 (2.90-13.30), 0.16 (0.03-0.22) for aspartic acid, glutamic acid, glycine, taurine and gamma-aminobutyric acid respectively.

Kinetic analysis of glutamate transport by the miniswine choroid plexus in vitro

Transport of glutamic acid by the choroid plexus, the blood-cerebrospinal fluid (CSF) barrier, was investigated by using the isolated choroid plexi from the fourth (FVCP) and lateral ventricles (LVCP) of the young adult miniswine in vitro. Glutamic acid uptake was very pronounced, with concentrations 7-fold (LVCP) and 2.4-fold (FVCP) higher in tissue than in medium after only 5 min of incubation with 1 microM glutamic acid. Tissue/medium ratios reached steady state by 15 min at 30-fold (LVCP) and 11-fold (FVCP). Uptake was energy-dependent and inhibited by ouabain and hypothermia. L-Aspartic acid was shown to be inhibitory in a concentration-dependent manner, suggesting that it shares a common transport system, whereas neither octanoic acid nor okadaic acid (transported by a separate fatty acid system) inhibited glutamic acid transport. At the same temperature, the labeled metabolite of glutamate (glutamine) in the tissue was 64.7%, 73.2%, and 72.5% of total radioactivity at 5, 30, and 60 min, respectively. The estimated Km values for glutamate uptake by the choroid plexus are 264 microM (FVCP) and 196 microM (LVCP); Vmax values are 87 (FVCP) and 147 (LVCP) nmol/g/min, respectively. These results indicate that, in addition to the metabolism of glutamate to glutamine, an active uptake mechanism is present in the choroid plexus of miniswine which may serve to regulate glutamic acid concentration in the CSF.

Cerebrospinal fluid amino acid levels in newborn infants with intracranial hemorrhage

Cerebrospinal fluid (CSF) amino acid levels including excitatory amino acids (i.e. glutamate and aspartate) in 25 preterm and 18 full-term newborn infants with no serious disease except intracranial hemorrhage (ICH) were measured. ICH was detected in 13 preterm and six full-term infants on the basis of the clinical, lumbar puncture (LP) and cranial ultrasonography (CraUSG) findings. Twelve preterm and 12 full-term infants who were neurologically healthy comprised the control group. The mean concentration of CSF amino acids did not differ between preterm and full-term infants. The CSF concentrations of taurine, threonine, glycine, alanine, valine, isoleucine, leucine, tyrosine and phenylalanine in preterm infants, and threonine, aspartic acid and alanine in full-term infants were significantly elevated in infants with ICH. These abnormalities, especially in preterm infants, are probably related to cerebral hypoxia in CSF amino acid concentrations in newborn infants with ICH.

N-Acetyl-aspartylglutamate (NAAG) in human cerebrospinal fluid: Determination by high performance liquid chromatography, and influence of biological variables

NAAG is one of the neuropeptides found in highest concentrations in the CNS. The presence of micromolar concentrations of NAAG in human CSF was demonstrated by using two different and complementary analytical approaches: 1) isocratic separation of endogenous NAAG by reverse-phase high performance liquid chromatography (HPLC) with dual wavelength detection and 2) derivatization of endogenous NAAG with acidic methanol and subsequent HPLC analysis of the derivative NAAG-trimethyl ester. The NAAG concentration was between 0.44µmol/l and 7.16µmol/l (mean of 2.19 ± 1.53µmol/l) in CSF samples from forty neuropsychiatric patients. Endogenous NAAG or [(3)H]NAAG added to CSF samples were not significantly degraded when the CSF was incubated at 37°C during one hour, suggesting that the peptide is a highly stable metabolite in the subarachnoid space. In addition, evidence is provided that NAAG does not present a concentration gradient along the lower subarachnoid space.

Potentiation of N-methyl-D-aspartate-evoked elevation of intracellular Ca2+ concentrations by exogenous glycine in cerebellar granule cells

The effect of glycine on the intracellular free Ca2+ concentration ([Ca2+])i response to N-methyl-D-aspartate (NMDA) was examined in small groups of cerebellar granule cells loaded with fura 2. NMDA alone evoked a long-lasting monophasic [Ca2+]i plateau, which was abolished by removal of extracellular Ca2+, or addition of the NMDA channel antagonist dizocilpine or the glycine site antagonist 5,7-dichlorokynurenic acid, virtually unaffected by the L-type Ca2+ channel antagonist (-)-PN 202 791, and greatly, though variably, potentiated by addition of glycine. In the presence of glycine the response to NMDA was clearly biphasic. However, there was no consistent relationship between the magnitudes of the peak and plateau phases of the response, and their temporal relationship was also highly variable. The potentiation seen with exogenous glycine was highly dependent on plating density, which may be the result of higher levels of endogenous glycine in more dense cultures. Our results provide an explanation of the inconsistent findings previously reported by different groups on the potentiation of the [Ca2+]i response to NMDA by exogenous glycine.

Cerebrospinal fluid as a tool in the diagnosis of neurometabolic diseases: amino acid analysis before and after acid hydrolysis

We perform systematically amino acid analysis of the CSF before and after strong acid hydrolysis in children with unexplained neurological disease. By comparing the amino acid pattern before and after hydrolysis, defects can be traced in the metabolism not only of amino acids but also of purines, peptides, N-acetylated amino acids and peptides, and probably other compounds. This method has yielded important information such as the identification of two "new" diseases, GABA transaminase deficiency and adenylosuccinase deficiency, and the discovery of a peculiar, acid-labile double peak in the CSF of patients with the transient neonatal hyperammonaemia syndrome and with urea cycle defects. This substance was subsequently identified by others as gamma-glutamylglutamine. As a consequence, we strongly recommend incorporating of this approach in the investigation of all children with unclear neurological disease.

Antiepileptogenic action of 7-chlorokynurenic acid on amygdala kindling of rats

To investigate the role of strychnine-insensitive glycine receptors in epilepsy, we studied the effects of 7-chlorokynurenic acid (7-CK), a selective strychnine-insensitive glycine receptor antagonist, on amygdala kindling development and previously amygdala-kindled seizures in rats. ICV administration of 7-CK (10 or 20 micrograms) suppressed amygdala kindling development, according to the motor seizure stage and afterdischarge development, in a dose-dependent manner. However, 7-CK had no significant effect on previously kindled seizures at either of these doses nor did 20 micrograms at any time (15 min, 30 min, 2 h, and 24 h) after injection studied. These results demonstrate that this selective strychnine-insensitive glycine receptor antagonist has antiepileptogenic activity and suggest a role for the glycine receptors in the contribution of the NMDA receptor complex to epileptogenic events.

The effect of age on concentrations of monoamines, amino acids, and their related substances in the cerebrospinal fluid

We studied age-related changes in the concentrations of monoamines, amino acids, and their related substances in the cerebrospinal fluid on 144 neurologically normal subjects. The concentrations of tyrosine, 3-O-methyldopa, dopamine (total), norepinephrine (total), homovanillic acid, p-hydroxyphenylacetic acid, and 5-hydroxytryptophan increased significantly with age (p < 0.05), and the concentration of 3.4-dihydroxyphenylacetic acid displayed a non-significant trend to decrease, whereas concentrations of other monoamine precursors and metabolites were unchanged. We found the significant positive correlations between the concentrations of HVA and 5-HIAA (p < 0.001), between tyrosine and tryptophan (p < 0.001), and between tyrosine and 3-O-methyldopa (p < 0.001). The concentrations of asparagine, glycine, taurine, and alanine increased significantly with age (p < 0.05), while glutamine, arginine, and threonine concentrations did not change with age. The aspartate, glutamate, and GABA concentrations displayed the non-significant trends to decrease in the elderly subjects. The concentrations of aspartate, glutamate, and GABA had mutually significant positive correlations (p < 0.05), but had significant negative correlations with the concentrations of some neutral amino acids. The urate and xanthine concentrations increased significantly with age (p < 0.01). These findings suggest that the concentrations of monoamine and amino acid transmitters and their related compounds in the cerebrospinal fluid reflect age-related changes in the synthesis, release, and reuptake mechanisms of the transmitters and their transport mechanisms across the blood-brain barrier.

Fasting plasma and CSF amino acid levels in amyotrophic lateral sclerosis: a subtype analysis

Data from the literature about plasma and CSF amino acid (AA) levels in amyotrophic lateral sclerosis (ALS) remain controversial. To refine such analyses we used HPLC, and report a study of plasma and CSF AA concentrations in patients with ALS, the type of the disease (spinal and bulbar onset) being precisely determined. In ALS, there is a decrease in the plasma levels of the large neutral amino acids (LNAA) alanine, isoleucine, leucine, methionine and tyrosine which was particularly striking in the bulbar type (p < 0.05). Plasma glutamate levels do not differ between ALS and controls, but are significantly increased in ALS with spinal onset and decreased in the bulbar type (p < 0.05 vs controls, p < 0.001 bulbar vs spinal). In CSF, the analysis of the whole ALS group shows no difference from controls. However, there is an increase of CSF serine, glutamine and alanine in ALS with spinal onset (p < 0.05). Our results do not support an abnormal profile of excitatory AA concentrations in ALS. The heterogeneous changes we observed, mainly concerning LNAAs, may be explained by a blood-CSF barrier disturbance in the disease. As AA levels clearly differ between ALS types, with low concentrations in bulbar ALS, this dual profile probably explains some of the discrepancies between previous studies.

Support for radiolabeling of a glycine recognition domain on the N-methyl-D-aspartate receptor ionophore complex by 5,7-[3H]dichlorokynurenate in rat brain

Pretreatment with Triton X-100 more than doubled the binding of radiolabeled 5,7-dichlorokynurenic acid (DCKA), a proposed antagonist at a glycine (Gly) recognition domain on the N-methyl-D-aspartate (NMDA) receptor ionophore complex, in rat brain synaptic membranes. The binding exhibited an inverse temperature dependency, reversibility, and saturability, the binding sites consisting of a single component with a high affinity (27.5 nM) and a relatively low density (2.87 pmol/mg of protein). The binding of both [3H]DCKA and [3H]Gly was similarly displaced by numerous putative agonists and antagonists at the Gly domain in a concentration-dependent manner at a concentration range of 100 nM to 0.1 mM. Among the 24 putative ligands tested, DCKA was the second most potent displacer of the binding of both radioligands with no intrinsic affinity for the binding of [3H]kainic acid and alpha-amino-3-hydroxy-5- [3H]methylisoxazole-4-propionic acid (AMPA) to the non-NMDA receptors. In contrast, the other proposed potent Gly antagonist, 5,7-dinitroquinoxaline-2,3-dione, was active in displacing the binding of [3H]glutamic ([3H]Glu) and D,L-(E)-2-amino-4-[3H]propyl-5-phosphono-3-pentenoic acids to the NMDA recognition domain with a relatively high affinity for the non-NMDA receptors. In addition, the proposed antagonist at the AMPA-sensitive receptor, 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline, not only displaced weakly the binding of both [3H]-Gly and [3H]DCKA, but also inhibited the binding of (+)-5-[3H]methyl-10,11- dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine ([3H]MK-801) to an ion channel associated with the NMDA-sensitive receptor in the presence of added Glu alone in a manner sensitive to antagonism by further added Gly. Clear correlations were seen between potencies of the displacers to displace [3H]DCKA binding and [3H]Gly binding, in addition to between the potencies to displace [3H]-DCKA or [3H]Gly binding and to potentiate or inhibit [3H]MK-801 binding. All quinoxalines tested were invariably more potent displacers of [3H]DCKA binding than [3H]Gly binding, whereas kynurenines were similarly effective in displacing the binding of both [3H]Gly and [3H]DCKA. These results undoubtedly give support to the proposal that [3H]DCKA is one useful radioligand available in terms of its high selectivity and affinity for the Gly domain in the brain. Possible multiplicity of the Gly domain is suggested by the differential pharmacological profiles between the binding of [3H]Gly and [3H]DCKA.

Cerebrospinal fluid amino acids, purines and pyrimidines as a tool in the study of metabolic brain diseases

After establishing more extended reference values for amino acids, purines and pyrimidines in cerebrospinal fluid (CSF) in infancy and childhood, we studied 1250 CSF-aliquots from patients who were undergoing a diagnostic lumbar puncture for diverse clinical indications. Our primary aim was to answer the question whether determination of the concentration of amino acids, purines and pyrimidines in CSF is a useful tool in screening for metabolic disorders in children with unexplained mental retardation. In unexplained mental retardation (95 patients) we observed varying abnormalities of CSF. These were reproducible in only 2 patients (a decrease of homocarnosine in combination with two unidentified compounds). Striking abnormalities in pyrimidine content which are limited to CSF are found in argininosuccinic aciduria and uraemia. In uraemia a general decrease in amino acids in CSF and increase of gamma-aminobutyric acid (GABA) was observed. The results obtained indicate that determination of amino acids, purines and pyrimidines in CSF is only of limited value in the diagnosis of unexplained mental retardation.

Excitatory Transmitter Amino Acid Release from the Ischemic Rat Cerebral Cortex: Effects of Adenosine Receptor Agonists and Antagonists

The effects of selective adenosine receptor agonists [N6-cyclopentyladenosine (CPA) and N-ethylcarboxamidoadenosine (NECA)] and antagonists [8-cyclopentyl-1,3-dipropylxanthine (DPCPX) and 9-chloro-2-(2-furanyl)-5,6-dihydro-1,2,4-triazolo[1,5-c]quinazoline-5-im ine (CGS-15943A)] on aspartate and glutamate release from the ischemic rat cerebral cortex were studied with the cortical cup technique. Cerebral ischemia (for 20 min) was elicited by four-vessel occlusion. Excitatory amino acid releases were compared from control ischemic rats and drug-treated rats. Basal levels of aspartate and glutamate release were not greatly affected by pretreatment with the adenosine receptor agonists or antagonists. However, CPA (10(-10) M) and NECA (10(-9) M) significantly inhibited the ischemia-evoked release of aspartate and glutamate into cortical superfusates. The ability to block ischemia-evoked release of excitatory amino acids was not evident at higher concentrations of CPA (10(-6) M) or NECA (10(-5) M). The selective A1 receptor antagonist DPCPX also had no effect on release when administered at a low dosage (0.01 mg/kg, i.p.) but blocked the ischemia-evoked release of aspartate and glutamate at a higher dosage (0.1 mg/kg). Evoked release was inhibited by the selective A2 receptor antagonist CGS-15943A (0.1 mg/kg, i.p.). Thus, adenosine and its analogs may suppress ischemia-evoked release of excitatory neurotransmitter amino acids via high-affinity A1 receptors, whereas coactivation of lower-affinity A2 receptors may block (or reverse) the A1-mediated response.

In vivo modulation of N-methyl-D-aspartate receptor-dependent long-term potentiation by the glycine modulatory site

The role of the glycine modulatory site in N-methyl-D-aspartate receptor function was examined by determining the effect of the glycine site antagonist, 7-chlorokynurenic acid, on the induction of long-term potentiation at the commissural-CA1 synapse in anesthetized rats. Robust long-term potentiation of population excitatory postsynaptic potentials and population spike responses recorded extracellularly in the stratum pyramidale and in stratum radiatum of CA1 developed after high frequency stimulation (100 Hz for 1 s) of commissural fibers during continuous intrahippocampal administration of vehicle solution (0.15 M NaCl). In contrast, infusion of either 7-chlorokynurenic acid (400 microM) or of the N-methyl-D-aspartate receptor antagonist, D-2-amino-5-phosphonovaleric acid (100 microM), significantly attenuated or completely blocked the development of long-term potentiation. When 7-chlorokynurenic acid was infused together with the glycine analog, D-serine (1 mM), long-term potentiation developed that was comparable to that observed in control animals. Intrahippocampal administration of D-serine alone was associated with slightly greater magnitude of long-term potentiation than observed in control animals. Collectively, these findings establish that in intact hippocampus, activity at the glycine modulatory site is necessary for activation of the N-methyl-D-aspartate receptor complex. Furthermore, these results suggest that the glycine modulatory site may not be fully saturated in vivo, and thus can serve to regulate N-methyl-D-aspartate receptor function.

Bilateral striopallidodentate calcinosis: cerebrospinal fluid, imaging, and electrophysiological studies

We report the genetic, clinical, electrophysiological, and imaging studies in a family with bilateral striopallidodentate calcinosis (Fahr's disease). The intracerebral calcium deposits occurred before onset of the symptoms in the third decade of life. Progressive neurological deterioration occurred in the fifth decade of life in the proband. Cerebrospinal fluid homocarnosine, a central nervous system-specific peptide, was increased twofold in patients with autosomal dominant bilateral stripallidodentate calcinosis; in sporadic cases, there was no detectable homocarnosine and a decreased level of histidine. With advancing age, the amount of calcification increases, but it has not been determined if a critical amount must be reached before symptoms occur. Computerized tomography is superior to magnetic resonance imaging for radiological diagnosis. Despite diffuse striatal calcification, striatal 6-[18F]fluoro-L-dopa uptake did not reveal any difference between patients and control subjects, from which we infer persisting integrity of the nigrostriatal dopaminergic pathway.

Excitatory amino acid receptors appear to mediate paroxysmal depolarizing shifts in rat neocortical neurons in vitro

This study was designed to assess some of the contributions of excitatory amino acids to locally evoked responses in neurons in slices from frontal motor cortex in Sprague-Dawley rats. Intracellular recordings were obtained from 54 cortical neurons. Paroxysmal depolarization shifts (PDS) were evoked by local single pulse stimulation in cortex or in a small number of cases (n = 2) occurred spontaneously. These potentials could be abolished by application of kynurenic acid, a broad spectrum excitatory amino acid receptor antagonist. They were enhanced in Mg(2+)-free medium and could then be antagonized by application of D,L-2-amino-5-phosphonovalerate (AP5), a selective blocker of the N-methyl-D-aspartate (NMDA) receptors.

Amino acids and glucose in human cerebrospinal fluid after acute ischaemic brain damage

In the search for potential biochemical markers of value for prognosis after acute hypoxic brain damage, amino acids and glucose were assessed in the cerebrospinal fluid (CSF) and glucose in blood. Samples were taken by lumbar puncture 4, 28, 76 and 172 h after resuscitation from 20 patients and once from 10 control patients. Eight of the resuscitated patients recovered neurologically but 12 remained comatose. The concentrations of alanine (P less than 0.001) and phenylalanine (P less than 0.035) differed most in 4-h samples between the groups. The concentration of alanine was higher in all patient groups with hypoxic brain damage as compared to the controls, the concentrations in patients dying within 76 h (disabled-s group) being higher than in the recovered patients. Phenylalanine in the disabled-s group was significantly higher than the control value. Furthermore, there were significant differences between various patient groups in the concentrations of glutamine, isoleucine, leucine, lysine, serine, tyrosine and valine. When taking into account the permeability of the BBB to these amino acids, alanine, valine and isoleucine most clearly represent brain amino acid metabolism. CSF glucose in the control group and in the recovered patients was lower than in patients dying within 76 h.