Release of neuroactive substances: homocysteic acid as an endogenous agonist of the NMDA receptor

Gut Symptoms, Gut Dysbiosis and Gut-Derived Toxins in ALS

Many pathogenetic mechanisms have been proposed for amyotrophic lateral sclerosis (ALS). Recently, there have been emerging suggestions of a possible role for the gut microbiota. Gut microbiota have a range of functions and could influence ALS by several mechanisms. Here, we review the possible role of gut-derived neurotoxins/excitotoxins. We review the evidence of gut symptoms and gut dysbiosis in ALS. We then examine a possible role for gut-derived toxins by reviewing the evidence that these molecules are toxic to the central nervous system, evidence of their association with ALS, the existence of biochemical pathways by which these molecules could be produced by the gut microbiota and existence of mechanisms of transport from the gut to the blood and brain. We then present evidence that there are increased levels of these toxins in the blood of some ALS patients. We review the effects of therapies that attempt to alter the gut microbiota or ameliorate the biochemical effects of gut toxins. It is possible that gut dysbiosis contributes to elevated levels of toxins and that these could potentially contribute to ALS pathogenesis, but more work is required.

The role of hydrogen sulfide in the retina

The discovery of the hydrogen sulfide (H2S) and the transsulfuration pathway (TSP) responsible for its synthesis in the mammalian retina has highlighted this molecule's wide range of physiological processes that influence cellular signaling, redox homeostasis, and cellular metabolism. The multi-level regulatory program that influences H2S levels in the retina depends on the relative expression and activity of TSP enzymes, which regulate the abundance of competitive substrates that support or abrogate H2S synthesis. In addition, and apart from TSP, intracellular H2S levels are regulated by mitochondrial sulfide oxidizing pathways. Retinal layers natively express differing levels of TSP enzymes, which highlight the differences in the metabolite and substrate requirement. Recent studies indicate that these systems are susceptible to pathophysiologies affecting the retina. Dysregulation at any level can upset the balance of redox and signaling processes and possibly upset oxidative stress, apoptotic signaling, ion channels, and immune response within this sensitive tissue. H2S donors are a potential therapeutic in such cases and have been demonstrated to bridge the gap, positively impacting the damaged retina. Here, we review the recent findings of H2S, how its multi-level regulation impacts the retina, and how its dysregulation is implicated in retinal pathologies.

Elevated Levels of Homocysteinesulfinic Acid in the Plasma of Patients with Amyotrophic Lateral Sclerosis: A Potential Source of Excitotoxicity?

OBJECTIVES

Excitotoxicity is thought to be involved in the pathogenesis of amyotrophic lateral sclerosis (ALS). One possible source of excitotoxicity is the presence of sulphur amino acids (SAAs). In the brain of subjects with ALS, there are increased levels of taurine. In the metabolism of methionine to taurine, excitatory sulphur amino acids (SAAs) are formed. These could potentially contribute to excitotoxicity in ALS. The present study has examined whether plasma levels of SAAs in 38 ALS patients differ from those of 30 healthy controls.

METHODS

Plasma levels of SAAs were measured by liquid chromatography mass spectrometry.

RESULTS

There were no significant changes in plasma cysteic acid, cysteine sulfinic acid, and homocysteic acid in ALS patients compared to healthy subjects. Significant elevations in plasma homocysteinesulfinic acid (HCSA) levels (p < 0.0001) were observed in the ALS patients (75.91 ± 15.38 nM) compared to healthy controls (54.06 ± 8.503 nM); 50% of the ALS patients had HCSA levels that were 1.5-2-folds higher than those of controls. Plasma levels of HCSA differed significantly (p = 0.0440) between patients with bulbar onset and spinal onset (68.57 ± 14.20 vs. 79.30 ± 14.95 nM, respectively).

CONCLUSION

HCSA is elevated in the blood of subjects with ALS. Since HCSA can be transported from the blood to the CNS by active transport, has neurotransmitter properties, and can activate synaptic receptors including NMDAR and metabotropic glutamate receptor, it is possible that increases in HCSA could influence glutamatergic neurotransmission and potentially contribute to excitotoxicity in some ALS patients.

Metazoan evolution and diversity of glutamate receptors and their auxiliary subunits

Glutamate is the major excitatory neurotransmitter in vertebrate and invertebrate nervous systems. Proteins involved in glutamatergic neurotransmission, and chiefly glutamate receptors and their auxiliary subunits, play key roles in nervous system function. Thus, understanding their evolution and uncovering their diversity is essential to comprehend how nervous systems evolved, shaping cognitive function. Comprehensive phylogenetic analysis of these proteins across metazoans have revealed that their evolution is much more complex than what can be anticipated from vertebrate genomes. This is particularly true for ionotropic glutamate receptors (iGluRs), as their current classification into 6 classes (AMPA, Kainate, Delta, NMDA1, NMDA2 and NMDA3) would be largely incomplete. New work proposes a classification of iGluRs into 4 subfamilies that encompass 10 classes. Vertebrate AMPA, Kainate and Delta receptors would belong to one of these subfamilies, named AKDF, the NMDA subunits would constitute another subfamily and non-vertebrate iGluRs would be organised into the previously unreported Epsilon and Lambda subfamilies. Similarly, the animal evolution of metabotropic glutamate receptors has resulted in the formation of four classes of these receptors, instead of the three currently recognised. Here we review our current knowledge on the animal evolution of glutamate receptors and their auxiliary subunits. This article is part of the special issue on 'Glutamate Receptors - Orphan iGluRs'.

Hyperreflexia and enhanced ripple oscillations in the taurine-deficient mice

In this study, we examined neuronal excitability and skeletal muscle physiology and histology in homozygous knockout mice lacking cysteine sulfonic acid decarboxylase (CSAD-KO). Neuronal excitability was measured by intracerebral recording from the prefrontal cortex. Skeletal muscle response was measured through stretch reflex in the ankle muscles. Specifically, we measured the muscle tension, amplitude of electromyogram and velocity of muscle response. Stretch reflex responses were evoked using a specialized stretching device designed for mice. The triceps surae muscle was stretched at various speeds ranging from 18 to 18,000° s^-1. A transducer recorded the muscle resistance at each velocity and the corresponding EMG. We also measured the same parameter in anesthetized mice. We found that at each velocity, the CSAD-KO mice generated more tension and exhibited higher EMG responses. To evaluate if the enhanced response was due to neuronal excitability or changes in the passive properties of muscles, we anesthetize mice to eliminate the central component of the reflex. Under these conditions, CSAD-KO mice still exhibited an enhanced stretch reflex response, indicating ultrastructural alterations in muscle histology. Consistent with this, we found that sarcomeres from CSAD-KO muscles were shorter and thinner when compared to control sarcomeres. Neuronal excitability was further investigated using intracerebral recordings of brain waves from the prefrontal cortex. We found that extracellular field potentials in CSAD-KO mice were characterized by reduced amplitude of low-frequency brain waves (delta, theta, alpha, beta and gamma) and increased in the high low-frequency brain waves (slow and fast ripples). Increased slow and fast ripple rates serve as a biomarker of epileptogenic brain. We have previously shown that taurine interacts with GABA_A receptors and induces biochemical changes in the GABAergic system. We suggest that taurine deficiency leads to alterations in the GABAergic system that contribute to the enhanced stretch reflex in CSAD-KO mice through biochemical mechanisms that involve alterations not only at the spinal level but also at the cortical level.

Targeting therapy for homocysteic acid in the blood represents a potential recovery treatment for cognition in Alzheimer's disease patients

At present, we have no reliable means of recovering cognitive impairment in Alzheimer's disease (AD) patients. We hypothesized that homocysteic acid (HA) in the blood might represent one such pathogen that could be excreted into the urine. Since DHA is known to reduce circulating levels of homocysteine, and since exercise attenuates this effect, it follows that supplementation of the diet with DHA, along with increased levels of physical activity, may help to reduce cognitive impairment in AD patients. Our hypothesis was proven to be correct because memory problems in 3xTg- AD mice (a model for AD in which animals develop amyloid pathology), and in a mouse model of familial AD, were recovered following treatment with an anti-HA antibody and not by amyloid treatment. Interestingly, 3xTg-AD mice with amyloid pathology showed increased levels of HA level. This could perhaps be explained by the fact that amyloid precursor protein and/or presenilin increases calcium influx, which could then increase levels of superoxide and consequently increase levels of HA from homocysteine or methionine. Our hypothesis is also partially supported by an open clinical trial of certain dietary supplements that has shown impressive results. Also there are other treatments hypothesis which would be possible for the effective therapies, such as ribonucleoprotein therapy, a β-secretase inhibitor treatment and the metabolic enhancement treatment.

Methotrexate-induced leukoencephalopathy presenting as stroke in the emergency department

Methotrexate-induced leukoencephalopathy is to be considered as a potential etiology in any patient presenting with stroke-like symptoms after receiving methotrexate. One of our cases suggests that the method of administration of the methotrexate can be IV or intrathecal and still results in leukoencephalopathy.

Main path and byways: non-vesicular glutamate release by system xc(-) as an important modifier of glutamatergic neurotransmission

System xc(-) is a cystine/glutamate antiporter that exchanges extracellular cystine for intracellular glutamate. Cystine is intracellularly reduced to cysteine, a building block of GSH. As such, system xc(-) can regulate the antioxidant capacity of cells. Moreover, in several brain regions, system xc(-) is the major source of extracellular glutamate. As such this antiporter is able to fulfill key physiological functions in the CNS, while evidence indicates it also plays a role in certain brain pathologies. Since the transcription of xCT, the specific subunit of system xc(-), is enhanced by the presence of reactive oxygen species and inflammatory cytokines, system xc(-) could be involved in toxic extracellular glutamate release in neurological disorders that are associated with increased oxidative stress and neuroinflammation. System xc(-) has also been reported to contribute to the invasiveness of brain tumors and, as a source of extracellular glutamate, could participate in the induction of peritumoral seizures. Two independent reviews (Pharmacol. Rev. 64, 2012, 780; Antioxid. Redox Signal. 18, 2013, 522), approached from a different perspective, have recently been published on the functions of system xc(-) in the CNS. In this review, we highlight novel achievements and insights covering the regulation of system xc(-) as well as its involvement in emotional behavior, cognition, addiction, neurological disorders and glioblastomas, acquired in the past few years. System xc(-) constitutes an important source of extrasynaptic glutamate in the brain. By modulating the tone of extrasynaptic metabotropic or ionotropic glutamate receptors, it affects excitatory neurotransmission, the threshold for overexcitation and excitotoxicity and, as a consequence, behavior. This review describes the current knowledge of how system xc(-) is regulated and involved in physiological as well as pathophysiological brain functioning.

Retinal Ganglion Cell Loss and Mild Vasculopathy in Methylene Tetrahydrofolate Reductase (Mthfr)-Deficient Mice: A Model of Mild Hyperhomocysteinemia

PURPOSE

Methylenetetrahydrofolate reductase (Mthfr) is a key enzyme in homocysteine-methionine metabolism. We investigated Mthfr expression in retina and asked whether mild hyperhomocysteinemia, due to Mthfr deficiency, alters retinal neurovascular structure and function.

METHODS

Expression of Mthfr was investigated at the gene and protein level using quantitative (q) RT-PCR, in situ hybridization, immunoblotting, and immunohistochemistry (IHC). The Mthfr+/+ and Mthfr+/- mice were subjected to comprehensive evaluation using ERG, funduscopy, fluorescein angiography (FA), spectral-domain optical coherence tomography (SD-OCT), HPLC, and morphometric and IHC analysis of glial fibrillary acidic protein (GFAP) at 8 to 24 weeks.

RESULTS

Gene and protein analyses disclosed widespread retinal expression of Mthfr. Electroretinography (ERG) revealed a significant decrease in positive scotopic threshold response in retinas of Mthfr+/- mice at 24 weeks. Fundus examination in mice from both groups was normal; FA revealed areas of focal vascular leakage in 20% of Mthfr+/- mice at 12 to 16 weeks and 60% by 24 weeks. The SD-OCT revealed a significant decrease in nerve fiber layer (NFL) thickness at 24 weeks in Mthfr+/- compared to Mthfr+/+ mice. There was a 2-fold elevation in retinal hcy at 24 weeks in Mthfr+/- mice by HPLC and IHC. Morphometric analysis revealed an approximately 20% reduction in cells in the ganglion cell layer of Mthfr+/- mice at 24 weeks. The IHC indicated significantly increased GFAP labeling suggestive of Müller cell activation.

CONCLUSIONS

Mildly hyperhomocysteinemic Mthfr+/- mice demonstrate reduced ganglion cell function, thinner NFL, and mild vasculopathy by 24 weeks. The retinal phenotype is similar to that of hyperhomocysteinemic mice with deficiency of cystathionine-β-synthase (Cbs) reported earlier. The data support the hypothesis that hyperhomocysteinemia may be causative in certain retinal neurovasculopathies.

The cystine/glutamate antiporter system x(c)(-) in health and disease: from molecular mechanisms to novel therapeutic opportunities

The antiporter system x(c)(-) imports the amino acid cystine, the oxidized form of cysteine, into cells with a 1:1 counter-transport of glutamate. It is composed of a light chain, xCT, and a heavy chain, 4F2 heavy chain (4F2hc), and, thus, belongs to the family of heterodimeric amino acid transporters. Cysteine is the rate-limiting substrate for the important antioxidant glutathione (GSH) and, along with cystine, it also forms a key redox couple on its own. Glutamate is a major neurotransmitter in the central nervous system (CNS). By phylogenetic analysis, we show that system x(c)(-) is a rather evolutionarily new amino acid transport system. In addition, we summarize the current knowledge regarding the molecular mechanisms that regulate system x(c)(-), including the transcriptional regulation of the xCT light chain, posttranscriptional mechanisms, and pharmacological inhibitors of system x(c)(-). Moreover, the roles of system x(c)(-) in regulating GSH levels, the redox state of the extracellular cystine/cysteine redox couple, and extracellular glutamate levels are discussed. In vitro, glutamate-mediated system x(c)(-) inhibition leads to neuronal cell death, a paradigm called oxidative glutamate toxicity, which has successfully been used to identify neuroprotective compounds. In vivo, xCT has a rather restricted expression pattern with the highest levels in the CNS and parts of the immune system. System x(c)(-) is also present in the eye. Moreover, an elevated expression of xCT has been reported in cancer. We highlight the diverse roles of system x(c)(-) in the regulation of the immune response, in various aspects of cancer and in the eye and the CNS.

Ligand-specific deactivation time course of GluN1/GluN2D NMDA receptors

N-methyl-D-aspartate (NMDA) receptors belong to the family of ionotropic glutamate receptors that mediate a majority of excitatory synaptic transmission. One unique property of GluN1/GluN2D NMDA receptors is an unusually prolonged deactivation time course following the removal of L-glutamate. Here we show, using x-ray crystallography and electrophysiology, that the deactivation time course of GluN1/GluN2D receptors is influenced by the conformational variability of the ligand-binding domain (LBD) as well as the structure of the activating ligand. L-glutamate and L-CCG-IV induce significantly slower deactivation time courses compared with other agonists. Crystal structures of the isolated GluN2D LBD in complex with various ligands reveal that the binding of L-glutamate induces a unique conformation at the backside of the ligand-binding site in proximity to the region at which the transmembrane domain would be located in the intact receptors. These data suggest that the activity of the GluN1/GluN2D NMDA receptor is controlled distinctively by the endogenous neurotransmitter L-glutamate.

The glutamate agonist homocysteine sulfinic acid stimulates glucose uptake through the calcium-dependent AMPK-p38 MAPK-protein kinase C zeta pathway in skeletal muscle cells

Homocysteine sulfinic acid (HCSA) is a homologue of the amino acid cysteine and a selective metabotropic glutamate receptor (mGluR) agonist. However, the metabolic role of HCSA is poorly understood. In this study, we showed that HCSA and glutamate stimulated glucose uptake in C2C12 mouse myoblast cells and increased AMP-activated protein kinase (AMPK) phosphorylation. RT-PCR and Western blot analysis revealed that C2C12 expresses mGluR5. HCSA transiently increased the intracellular calcium concentration. Although α-methyl-4-carboxyphenylglycine, a metabotropic glutamate receptor antagonist, blocked the action of HCSA in intracellular calcium response and AMPK phosphorylation, 6-cyano-7-nitroquinoxaline-2,3-dione, an AMPA antagonist, did not exhibit such effects. Knockdown of mGluR5 with siRNA blocked HCSA-induced AMPK phosphorylation. Pretreatment of cells with STO-609, a calmodulin-dependent protein kinase kinase (CaMKK) inhibitor, blocked HCSA-induced AMPK phosphorylation, and knockdown of CaMKK blocked HCSA-induced AMPK phosphorylation. In addition, HCSA activated p38 mitogen-activated protein kinase (MAPK). Expression of dominant-negative AMPK suppressed HCSA-mediated phosphorylation of p38 MAPK, and inhibition of AMPK and p38 MAPK blocked HCSA-induced glucose uptake. Phosphorylation of protein kinase C ζ (PKCζ) was also increased by HCSA. Pharmacologic inhibition or knockdown of p38 MAPK blocked HCSA-induced PKCζ phosphorylation, and knockdown of PKCζ suppressed the HCSA-induced increase of cell surface GLUT4. The stimulatory effect of HCSA on cell surface GLUT4 was impaired in FITC-conjugated PKCζ siRNA-transfected cells. Together, the above results suggest that HCSA may have a beneficial role in glucose metabolism in skeletal muscle cells via stimulation of AMPK.

Glutamate receptor ion channels: structure, regulation, and function

The mammalian ionotropic glutamate receptor family encodes 18 gene products that coassemble to form ligand-gated ion channels containing an agonist recognition site, a transmembrane ion permeation pathway, and gating elements that couple agonist-induced conformational changes to the opening or closing of the permeation pore. Glutamate receptors mediate fast excitatory synaptic transmission in the central nervous system and are localized on neuronal and non-neuronal cells. These receptors regulate a broad spectrum of processes in the brain, spinal cord, retina, and peripheral nervous system. Glutamate receptors are postulated to play important roles in numerous neurological diseases and have attracted intense scrutiny. The description of glutamate receptor structure, including its transmembrane elements, reveals a complex assembly of multiple semiautonomous extracellular domains linked to a pore-forming element with striking resemblance to an inverted potassium channel. In this review we discuss International Union of Basic and Clinical Pharmacology glutamate receptor nomenclature, structure, assembly, accessory subunits, interacting proteins, gene expression and translation, post-translational modifications, agonist and antagonist pharmacology, allosteric modulation, mechanisms of gating and permeation, roles in normal physiological function, as well as the potential therapeutic use of pharmacological agents acting at glutamate receptors.

Subacute methotrexate neurotoxicity and cerebral venous sinus thrombosis in a 12-year-old with acute lymphoblastic leukemia and methylenetetrahydrofolate reductase (MTHFR) C677T polymorphism: homocysteine-mediated methotrexate neurotoxicity via direct endothelial injury

From as early as the 1970s methotrexate has been associated with disseminated necrotizing leukoencephalopathy and other neurotoxic sequelae. Yet, a clear mechanism for methotrexate-induced neurotoxicity has not been established. The authors describe the case of a 12-year-old male with acute lymphoblastic leukemia and a homozygous methylenetetrahydrofolate reductase C677T mutation, who developed subacute methotrexate-induced toxicity and cerebral venous thrombosis after receiving intrathecal methotrexate. The role of homocysteine as a possible mediator in methotrexate-induced neurotoxicity via direct endothelial injury is discussed.

Treatment of Alzheimer's disease with anti-homocysteic acid antibody in 3xTg-AD male mice

Alzheimer's disease (AD) is an age-associated progressive neurodegenerative disorder with dementia, the exact pathogenic mechanisms of which remain unknown. We previously reported that homocysteic acid (HA) may be one of the pathological biomarkers in the brain with AD and that the increased levels of HA may induce the accumulation of intraneuronal amyloid-beta (Aβ) peptides. In this study, we further investigated the pathological role of HA in a mouse model of AD. Four-month-old prepathological 3xTg-AD mice exhibited higher levels of HA in the hippocampus than did age-matched nontransgenic mice, suggesting that HA accumulation may precede both Aβ and tau pathologies. We then fed 3-month-old 3xTg-AD mice with vitamin B6-deficient food for 3 weeks to increase the HA levels in the brain. Concomitantly, mice received either saline or anti-HA antibody intraventricularly via a guide cannula every 3 days during the course of the B6-deficient diet. We found that mice that received anti-HA antibody significantly resisted cognitive impairment induced by vitamin B6 deficiency and that AD-related pathological changes in their brains was attenuated compared with the saline-injected control group. A similar neuroprotective effect was observed in 12-month-old 3xTg-AD mice that received anti-HA antibody injections while receiving the regular diet. We conclude that increased brain HA triggers memory impairment and that this condition deteriorates with amyloid and leads to subsequent neurodegeneration in mouse models of AD.

L-methylfolate, methylcobalamin, and N-acetylcysteine in the treatment of Alzheimer's disease-related cognitive decline

Neuroinflammatory oxidative stress occurs early in AD pathology. Elevated blood Hcy is a useful marker for such neuroinflammation. Hcy contributes to pathological cascades involving AP and NFTs. In AD, Hcy should be lowered by B-vitamin supplements and NAC.

Endogenous elevation of homocysteine induces retinal neuron death in the cystathionine-beta-synthase mutant mouse

PURPOSE

To determine the effects of endogenous elevation of homocysteine on the retina using the cystathionine beta-synthase (cbs) mutant mouse.

METHODS

Retinal homocysteine in cbs mutant mice was measured by high-performance liquid chromatography (HPLC). Retinal cryosections from cbs(-/-) mice and cbs(+/-) mice were examined for histologic changes by light and electron microscopy. Morphometric analysis was performed on retinas of cbs(+/-) mice maintained on a high-methionine diet (cbs(+/-) HM). Changes in retinal gene expression were screened by microarray.

RESULTS

HPLC analysis revealed an approximate twofold elevation in retinal homocysteine in cbs(+/-) mice and an approximate sevenfold elevation in cbs(-/-) mice. Distinct alterations in the ganglion, inner plexiform, inner nuclear, and epithelial layers were observed in retinas of cbs(-/-) and 1-year-old cbs(+/-) mice. Retinas of cbs(+/-) HM mice demonstrated an approximate 20% decrease in cells of the ganglion cell layer (GCL), which occurred as early as 5-weeks after onset of the HM diet. Microarray analysis revealed alterations in expression of several genes, including increased expression of Aven, Egr1, and Bat3 in retinas of cbs(+/-) HM mice.

CONCLUSIONS

This study provides the first analysis of morphologic and molecular effects of endogenous elevations of retinal homocysteine in an in vivo model. Increased retinal homocysteine alters inner and outer retinal layers in cbs homozygous mice and older cbs heterozygous mice, and it primarily affects the cells of the GCL in younger heterozygous mice. Elevated retinal homocysteine alters expression of genes involved in endoplasmic reticular stress, N-methyl-d-aspartate (NMDA) receptor activation, cell cycle, and apoptosis.

Increased homocysteine-induced release of excitatory amino acids in the striatum of spontaneously hypertensive stroke-prone rats

BACKGROUND AND PURPOSE

Increased plasma [homocysteine] is associated with stroke but its direct effects on the brain during a stroke are unknown. Since excitatory amino acids are important in inducing brain damage, we examined the effect of homocysteine on the release of various amino acids in the striatum of spontaneously hypertensive stroke-prone (SHSP) rats before and after a stroke.

METHODS

In vivo microdialysis was carried out in the striatum of anesthetized SHSP rats before and after signs of stroke. Animals were exposed to 20 and 200 muM homocysteine in the microdialysis solution and then the microdialysates were analyzed 30 min later for amino acid content. Brain cryosections were silver-stained to quantify infarcts in the non-ischemic and the damaged tissues in pre-stroke and post-stroke rats.

RESULTS

Both pre-stroke and post-stroke animals had similar levels of all amino acids in the striatum. Homocysteine did not alter amino acid release in rats prior to stroke but induced a significant increase in the release of all amino acids tested in the post-stroke rats. However, the increase was significantly greater with the excitatory amino acids glutamate and aspartate, and with tyrosine in post-stroke animals as compared to those in pre-stroke, normal animals. The mean pixel density of the gray matter of post-stroke animals was significantly decreased following homocysteine treatment indicating the presence of neurological damage.

CONCLUSIONS

Homocysteine-induced neurological damage in post-infarct SHSP rats was associated with a hypersecretion of excitatory amino acids. Patients with hyperhomocysteinemia may be at risk for augmented brain damage from an ischemic infarct due to a selective activation of neuronal excitatory amino acids.

Subunit-Specific Agonist Activity at NR2A-, NR2B-, NR2C-, and NR2D-ContainingN-Methyl-d-aspartate Glutamate Receptors

The four N-methyl-d-aspartate (NMDA) receptor NR2 subunits (NR2A-D) have different developmental, anatomical, and functional profiles that allow them to serve different roles in normal and neuropathological situations. Identification of subunit-selective NMDA receptor agonists, antagonists, or modulators could prove to be both valuable pharmacological tools as well as potential new therapeutic agents. We evaluated the potency and efficacy of a wide range of glutamate-like compounds at NR1/NR2A, NR1/NR2B, NR1/NR2C, and NR1/NR2D receptors. Twenty-five of 53 compounds examined exhibited agonist activity at the glutamate binding site of NMDA receptors. Concentration-response relationships were determined for these agonists at each NR2 subunit. We find consistently higher potency at the NR2D subunit for a wide range of dissimilar structures, with (2S,4R)-4-methylglutamate (SYM2081) showing the greatest differential potency between NR2A- and NR2D-containing receptors (46-fold). Analysis of chimeric NR2A/D receptors suggests that enhanced agonist potency for NR2D is controlled by residues in both of the domains (Domain1 and Domain2) that compose the bilobed agonist binding domain. Molecular dynamics (MD) simulations comparing a crystallography-based hydrated NR1/NR2A model with a homology-based NR1/NR2D hydrated model of the agonist binding domains suggest that glutamate exhibits a different binding mode in NR2D compared with NR2A that accommodates a 4-methyl substitution in SYM2081. Mutagenesis of functionally divergent residues supports the conclusions drawn based on the modeling studies. Despite high homology and conserved atomic contact residues within the agonist binding pocket of NR2A and NR2D, glutamate adopts a different binding orientation that could be exploited for the development of subunit selective agonists and competitive antagonists.

Delayed neurotoxicity associated with therapy for children with acute lymphoblastic leukemia

Most children diagnosed today with acute lymphoblastic leukemia (ALL) will be cured. However, treatment entails risk of neurotoxicity, causing deficits in neurocognitive function that can persist in the years after treatment is completed. Many of the components of leukemia therapy can contribute to adverse neurologic sequelae, including craniospinal irradiation, nucleoside analogs, corticosteroids, and antifolates. In this review, we describe the characteristic radiographic findings and neurocognitive deficits seen among survivors of childhood ALL. We summarize what is known about the pathophysiology of delayed treatment-related neurotoxicity, with a focus on the toxicity resulting from pharmacologic disruption of folate physiology within the central nervous system. Finally, we suggest testable strategies to ameliorate the symptoms of treatment-related neurotoxicity or decrease its incidence.

Steroid-induced alterations of mood and behavior in children during treatment for acute lymphoblastic leukemia

Despite their therapeutic utility, children may experience emotional and behavioral side effects from steroids during treatment for leukemia. However, clinical manifestations and treatment options have rarely been described in the pediatric literature. The aim of this current paper is to address this knowledge gap via four brief case examples and a review of the literature. In addition, we review recent data to indicate how concurrent use of antifolates may place these children at particular risk for adverse psychological reactions.

Role of Magnesium, Coenzyme Q10, Riboflavin, and Vitamin B12 in Migraine Prophylaxis

Migraine is a neurovascular syndrome characterized by recurrent headache associated with other symptoms, eventually preceded by aura. This chapter reviews the involvement of some mineral, coenzyme, and vitamin defects in the pathogenesis of migraine headaches and focuses on their potential therapeutic use in the preventive treatment for migraine. The therapeutic potential of magnesium, coenzyme Q(10), riboflavin, and vitamin B(12) can be cautiously inferred from some published open clinical trials; it should, however, be considered that double-blind randomized larger studies are needed to correctly estimate the impact of the placebo effect in these promising therapies.

Release of homocysteic acid from rat thalamus following stimulation of somatosensory afferents in vivo: feasibility of glial participation in synaptic transmission

The sulphur-containing amino acid homocysteic acid (HCA) is present in and released in vitro from nervous tissue and is a potent neuronal excitant, predominantly activating N-methyl-d-aspartate (NMDA) receptors. However, HCA is localised not in neurones but in glial cells [Eur J Neurosci 3 (1991) 1370], and we have shown that it is released from astrocytes in culture upon glutamate receptor activation [Neuroscience 124 (2004) 377]. We now report the in vivo release of HCA from ventrobasal (VB) thalamus following natural stimulation of somatosensory afferents arising from the facial vibrissae of the rat. Simultaneously with multi-unit recording, [35S]-methionine, a HCA precursor, was perfused through a push-pull cannula in VB thalamus of anaesthetized rats. Perfusates were collected before, during and after 4 min stimulation of the vibrissal afferents with an air jet. A marked release of radiolabeled HCA was observed during and after the stimulation. Furthermore, the beta-adrenoreceptor agonist isoproterenol, which is known to evoke HCA release from glia in vitro, was found to increase the efflux of HCA in the perfusate in vivo. In separate experiments, the excitatory actions of iontophoretically applied HCA on VB neurones were inhibited by the NMDA receptor antagonist CPP, but not by the non-NMDA antagonist CNQX. These results suggest a possible "gliotransmitter" role for HCA in VB thalamus. The release of HCA from glia might exert a direct response or modulate responses to other neurotransmitters in postsynaptic neurons, thus enhancing excitatory processes.

Homocysteine in breast cyst fluid

BACKGROUND

Elevated plasma homocysteine concentrations have been reported in a variety of carcinoma, including those of the breast. The risk of breast cancer is higher in patients suffering from gross cystic disease. The breast cyst fluid contains unusual amounts of low- and high- molecular substances, including steroid hormones and their conjugates. The present study was undertaken to find out the presence of homocysteine in the fluid filling the cysts and have its concentration compared with other thiols, levels of Na+/K+ ratio and steroid hormones. Materials and methods Fourteen women suffering from gross cystic disease were enrolled in this study. Cystic concentrations of homocysteine (Hcy), cysteine (Cys), cysteinylglycine (Cys-Gly) and glutathione (GSH) were determined by high performance liquid chromatography, with fluorescence detection; estradiol (E2), progesterone, allopregnanolone and pregnenolone sulfate (PregS) by RIA methods.

RESULTS

Mean levels of Hcy, Cys, Cys-Gly, Na+/K+, E2 and PregS in the fluid filling the breast cysts were significantly higher than the corresponding plasma concentrations. In addition, a negative correlation was found between cystic Hcy and the Na+/K+ ratio (Rs = -0.72, P = 0.003) and positive correlations between cyst Hcy and estradiol (Rs = 0.64, P = 0.018) and Hcy and PregS (Rs = 0.60, P = 0.025). Conclusion The study provides the first evidence of thiol concentrations in the breast cyst fluid. The finding of a negative correlation between homocysteine and the Na+/K+ ratio support the idea that the homocysteine concentration in breast cysts might be used clinically as a marker for the development of breast cancer disease.

Changes in uptake of vitamin B(12) and trace metals in brains of mice treated with clioquinol

Clioquinol is a hydroxyquinoline antibiotic that has been associated with severe side-effects in the CNS. The syndrome caused by clioquinol treatment, subacute myelo-optic neuropathy (SMON), is considered as one of the worst drug disasters of this century. The precise biochemical mechanism behind SMON is not fully understood. Clioquinol can form strong lipophilic chelates with divalent cations and therefore it has been speculated that the drug may disturb the retention of vitamin B(12) through chelation of Co(2+). In the present study, the tissue distribution and uptake capacity of [57Co]cyanocobalamin were estimated in mice treated with clioquinol or saline. The concentrations of some trace metals were also determined in brain tissue. Accumulation of vitamin B(12) in the brain and its concentration in blood were decreased by clioquinol treatment. The mean concentrations of several trace metals were also lowered in the brain while the concentration of cobalt in the brain was not affected, suggesting that clioquinol does not bind to the cobalt in vitamin B(12). Moreover, a significant decrease in the levels of S-adenosylmethionine (SAM) was observed in the brain after clioquinol treatment. This may be a consequence of decreased vitamin B(12) levels. From these results, it can be concluded that chronic treatment with clioquinol may alter the tissue homeostasis of vitamin B(12) in the brain.

Plasma levels of neuroexcitatory amino acids in patients with migraine or tension headache

Plasma amino acids were analysed in patients with migraine with (9) and without (80) aura, in patients with tension headache (14) and in controls (62). The neuroexcitatory amino acids glutamic acid, glutamine, glycine, cysteic acid and homocysteic acid were elevated in migraine patients while total thiols (cysteine/cystine) were reduced. Patients with tension headache had values which were similar to those of controls. Tryptophan was elevated in migraine patients without aura only. Studies on two patients showed that the raised resting excitatory amino acid levels became still further elevated during a migraine attack. These results show that high concentrations of neurotransmitter amino acids occur normally in migraine patients and suggest that this profile may be a contributory factor in migraine attacks. Tension headache, however, has different biochemical parameters.

Folate, vitamin B12, and neuropsychiatric disorders

Folate and vitamin B12 are required both in the methylation of homocysteine to methionine and in the synthesis of S-adenosylmethionine. S-adenosylmethionine is involved in numerous methylation reactions involving proteins, phospholipids, DNA, and neurotransmitter metabolism. Both folate and vitamin B12 deficiency may cause similar neurologic and psychiatric disturbances including depression, dementia, and a demyelinating myelopathy. A current theory proposes that a defect in methylation processes is central to the biochemical basis of the neuropsychiatry of these vitamin deficiencies. Folate deficiency may specifically affect central monoamine metabolism and aggravate depressive disorders. In addition, the neurotoxic effects of homocysteine may also play a role in the neurologic and psychiatric disturbances that are associated with folate and vitamin B12 deficiency.

Hyperhomocysteinaemia--a common finding in a psychogeriatric population

Plasma homocysteine concentration is a sensitive marker for cobalamin and folate deficiency. The previously reported high incidence of increased plasma homocysteine in psychogeriatric patients and the association between reduced concentrations of cobalamin, folate and neuropsychiatric symptoms led to the present study on 741 consecutive psychogeriatric patients. The concentrations of plasma homocysteine correlated significantly with blood folate, serum cobalamin and serum creatinine both in demented (n = 295) and in non-demented patients with other psychiatric disorders (n = 215). Plasma homocysteine concentrations were significantly increased in both the demented and the non-demented patients, whereas only the demented patients had lower blood folate and serum creatinine concentrations than 163 control subjects. Almost all of the different diagnostic groups of demented and non-demented patients exhibited significantly increased plasma homocysteine concentrations compared with control subjects. Significantly decreased blood folate concentrations were mainly found in the different diagnosis groups of demented patients. Plasma homocysteine concentrations in both demented and non-demented patients with serum cobalamin and blood folate above the lower 20th percentile of these vitamins in the control subjects were also studied. Despite these vitamin concentrations, both groups of patients still exhibited significantly higher plasma homocysteine concentrations than the control subjects, which may indicate an increased frequency of impaired genetic capacity to metabolize homocysteine in these patients. Patients with either dementia of vascular cause or a history of other occlusive arterial disease had a significantly higher plasma homocysteine concentration than those without a history of vascular disease.

Specificity of cysteine sulfinate decarboxylase (CSD) for sulfur-containing amino-acids

Cysteine sulfinate decarboxylase (CSD) which decarboxylates cysteine sulfinic acid (CSA) to form hypotaurine is thought to be involved in the biosynthesis of taurine. It was recently localized in astrocytes in the cerebellum and hippocampus by immunocytochemistry. Another sulfur-containing amino-acid (SCAA), homocysteic acid (HCA), was also found in astrocytes in these regions. We therefore investigated the specificity of CSD vs CSA and HCA as well as the related analogs homocysteine sulfinic acid (HCSA) and cysteic acid (CA). CSD was immunotrapped from brain and liver tissue supernatant using a specific CSD antiserum and Protein-A Sepharose. It was then incubated with the L-form of the various SCAA. Reaction products were identified and quantified by pre-column o-phthalaldehyde derivatization HPLC. CA and HCA from 2.5 to 25 mM inhibited the formation of hypotaurine from CSA (0.25 mM). Moreover, the inhibition curves were parallel for liver and brain CSD. CA or HCA (25 mM) elicited a near-total inhibition. HCSA did not produce a significant inhibition up to 25 mM. Incubation with 25 mM CSA or CA led to the formation of hypotaurine and taurine, respectively. The ratio of formation of taurine to that of hypotaurine was similar for CSD from liver and brain. In contrast no homotaurine, the decarboxylated reaction product of HCA, could be detected following incubation with 25 mM HCA. According to the sensitivity of the HPLC analysis this indicates that the decarboxylation of HCA, if any, was 130-fold and 50-fold less than that of CSA by CSD from liver and brain, respectively, in our experimental conditions. Similarly, following incubation with HCSA, no new peak appeared on the chromatogram when compared to a blank sample. These results show that CSD from either brain or liver has a high specificity for CSA and CA, which are the SCAA involved in the biosynthesis of taurine. HCA is an inhibitor of CSD but does not appear to be a substrate for CSD in vitro. HCSA is neither a substrate nor an inhibitor of CSD in vitro. Accordingly, CSD is unlikely to play a role in the metabolism of HCA or HCSA in vivo.

Decreased methionine adenosyltransferase activity in erythrocytes of patients with dementia disorders

ATP:1-methionine S-adenosyltransferase (EC 2.5.1.6, MAT) activity was analyzed in erythrocytes from nine patients with a clinical diagnosis of probable Alzheimer's disease (Pro.AD), four with possible Alzheimer's disease (Pos.AD), three with mild cognitive dysfunction (MCD) and two with dementia of vascular origin (VD), and 10 age-matched control subjects. Significantly lower kinetic parameters (Vmax and Km towards methionine) for MAT were observed in all the dementia cases. In the subgroup of Pro.AD patients who also had low plasma levels of vitamin B12 (B12), the reduction in MAT Km was significantly correlated with an increase in the serum levels of homocysteine, while no such correlation was observed in all the other dementia groups. Treatment for 6 months of this subgroup of Pro.AD patients with B12 (1 mg x 7 days + 1 mg/week, i.m.), S-adenosylmethionine (SAM, 200 mg twice daily, p.o.) and folate (2.5 mg every 2 days, p.o.) caused a significant decrease in homocysteine in parallel with a significant increase in Km for MAT. These findings support the hypothesis that aberrations in the B12 dependent transmethylation reactions might be involved in the pathogenesis of dementia, and suggest that the evaluation of erythrocyte MAT activity may be a useful marker for the detection of such an aberration.

Are neuropsychiatric manifestations of folate, cobalamin and pyridoxine deficiency mediated through imbalances in excitatory sulfur amino acids?

Folate, cobalamin and pyridoxine deficiency are associated with psychiatric or neurological symptomatology. Disturbances in sulfur amino acid metabolism leading to accumulation of homocysteine occurs in all three conditions as the metabolism of homocysteine depends on enzymes requiring these vitamins as cofactors. Oxidation products of homocysteine (homocysteine sulfinic acid and homocysteic acid) and cysteine (cysteine sulfinic acid and cysteic acid) are excitatory sulfur amino acids and may act as excitatory neurotransmitters, whereas taurine and hypotaurine (decarboxylation products of cysteic acid and cysteine sulfinic acid) may act as inhibitory transmitters. Homocysteic acid and cysteine sulfinic acid have been considered as endogenous ligands for the N-methyl-D-aspartate (NMDA) type of glutamate receptors. The profile of these sulfur amino acid neurotransmitters could be altered in a similar fashion in states of decreased availability of folate, cobalamin or pyridoxine. It is proposed that the mechanism of neuropsychiatric manifestations in all three conditions result from a combination of two insults to homocysteine catabolism in the brain.

The chemical nature of the main central excitatory transmitter: a critical appraisal based upon release studies and synaptic vesicle localization

The chemical nature of the central transmitter responsible for fast excitatory events and other related phenomena is analysed against the historical background that has progressively clarified the structure and function of central synapses. One of the problems posed by research in this field has been whether one or more of the numerous excitatory substances endogenous to the brain is responsible for fast excitatory synaptic transmission, or if such a substance is, or was, a previously unknown one. The second question is related to the presence in the CNS of three main receptor types related to fast excitatory transmission, the so-called alpha-amino-3-hydroxy-5-methylisoxazole propionic acid, kainate and N-methyl-D-aspartate receptors. This implies the possibility that each receptor type might have its own endogenous agonist, as has sometimes been suggested. To answer such questions, an analysis was done of how different endogenous substances, including L-glutamate, L-aspartate, L-cysteate, L-homocysteate, L-cysteine sulfinate, L-homocysteine sulfinate, N-acetyl-L-aspartyl glutamate, quinolinate, L-sulfoserine, S-sulfo-L-cysteine, as well as possible unknown compounds, were able to fulfil the more important criteria for transmitter identification, namely identity of action, induced release, and presence in synaptic vesicles. The conclusion of this analysis is that glutamate is clearly the main central excitatory transmitter, because it acts on all three of the excitatory receptors, it is released by exocytosis and, above all, it is present in synaptic vesicles in a very high concentration, comparable to the estimated number of acetylcholine molecules in a quantum, i.e. 6000 molecules. Regarding a possible transmitter role for aspartate, for which a large body of evidence has been presented, it seems, when this evidence is carefully scrutinized, that it is either inconclusive, or else negative. This suggests that aspartate is not a classical central excitatory transmitter. From this analysis, it is suggested that the terms alpha-amino-3-hydroxy-5-methylisoxazole propionic acid, kainate and N-methyl-D-aspartate receptors, should be changed to that of glutamate receptors, and, more specifically, to GLUA, GLUK and GLUN receptors, respectively. When subtypes are described, a Roman numeral may be added, as in GLUNI, GLUNII, and so on.

Plasma homocysteine in vascular disease and in nonvascular dementia of depressed elderly people

Depression among elderly people with reversible cognitive loss often manifests with concomitant vascular disease and can also precede the development of nonvascular degenerative dementia. Little is known about etiological factors for reversible or irreversible dementias in older depressed people. The amino acid homocysteine (HC), which is both a vascular disease risk factor and a precursor of the excitotoxic amino acids cysteine and homocysteic acid, could play a role in the pathophysiology of such individuals. Twenty-seven depressed elderly acute inpatients by DSM-III-R criteria had significantly higher plasma homocysteine levels and lower cognitive screening test scores than did 15 depressed young adult inpatients. HC was highest in the older patients who had concomitant vascular diseases (n = 14). HC was lowest in the older depressives who had neither vascular illnesses nor dementia (n = 8), comparable to the young adult depressives. Higher HC correlated significantly with poorer cognition only in the nonvascular geriatric patients (rs = -0.53). The findings extend earlier work showing higher HC in vascular patients from general medical populations, and also suggest a possible metabolic factor in certain dementias associated with late-life depression.

Membrane currents induced byl-homocysteic acid in mouse cultured hippocampal neurons

The concentration-response relationship of membrane currents induced by L-homocysteic acid was studied on mouse embryonic hippocampal neurons in culture (n = 56). In the majority of neurons two phases in the dose-response relationship could be distinguished. The first was characterized by responses to 3-100 microM L-homocysteic acid which desensitized with a time-constant greater than 1 s in a concentration-dependent manner and were antagonized by 30 microM D-L-2-amino-5-phosphonovaleric acid indicating activation of the N-methyl-D-aspartate receptors. At higher concentrations of L-homocysteic acid this component was strongly depressed. The second phase was characterized by sustained responses that were concentration-dependent (1 mM L-homocysteic acid maximum concentration tested) and were not blocked by D-L-2-amino-5-phosphonovaleric acid indicating activation of non-N-methyl-D-aspartate receptors. Eight neurons did not exhibit these two-phase characteristics in the concentration-response relationship at the beginning of the recording. The magnitude of responses to L-homocysteic acid was positively related to concentration and the responses were partially blocked by D-L-2-amino-5-phosphonovaleric acid. In these neurons, however, repeated applications of L-homocysteic acid at concentrations 30 microM up to 300 microM resulted in a long-lasting, three- to four-fold increase of the membrane current. This increase was completely blocked by D-L-2-amino-5-phosphonovaleric acid (50-100 microM) suggesting that it was produced by activation of receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Alzheimer's Disease: A ‘cobalaminergic’ hypothesis

An association between Alzheimer's Disease (AD) and low CSF and serum vitamin B12 (B12) has recently been described (1, 2, 3). This is apparently independent of nutritional intake (4). It has been suggested that such patients may exhibit an atypical form of cobalamin deficiency (3, 4). It is therefore proposed that these deficiencies may be aetiologically important, at least in sub-groups of AD, and a mechanism is described whereby B12 deficiency may result in the characteristic neurotransmitter changes of the disease. The hypothesis generates predictions regarding biochemical evaluation of such patients and suggests associations between the neurochemical disturbances and structural abnormalities of AD.

Effects of ageing on the content in sulfur-containing amino acids in rat brain

Concentrations of the sulfur-containing amino acids methionine, homocysteic acid, cysteic acid and taurine were measured in brain structures of young and old Wistar rats in an attempt to establish a possible link between the increase in oxidative stress with ageing and changes in tissue levels of these amino acids. Contrary to data reported by others, in all brain structures of young and old rats homocysteic acid levels could not be quantified. Compared with young rats, in old animals taurine and methionine concentrations significantly decreased in striatum and cortex; decreased taurine levels were also found in nucleus accumbens and cerebellum and lower concentrations of methionine were found in midbrain, hippocampus and pons-medulla. Cysteic acid levels either did not change or significantly increased in cortex and hippocampus. These results are discussed taking into account the biosynthesis of sulfur-containing amino acids in rat brain and the decrease in glutathione in relation to oxidative stress with ageing. Changes in aspartic acid, glutamic acid, serine, glutamine, glycine and GABA concentrations with ageing were also determined in the same brain structures and were in good agreement with those previously reported (Strolin Benedetti et al., 1990 a, b).

Evidence that activation of N-methyl-D-aspartate (NMDA) and non-NMDA receptors within the nucleus tractus solitarii triggers swallowing

Swallowing is a patterned motor activity generated by neurons located within the nucleus tractus solitarii (NTS). Previous experiments have shown that administration of excitatory amino acids within the NTS induces swallowing. The present study was undertaken to identify the receptor subtypes involved in this effect. Pressure microinjections of L-glutamate (10-100 pmol), quisqualate (0.1-10 pmol) and N-methyl-D-aspartate (NMDA, 0.1-10 pmol) were performed into the NTS of decerebrate rats. Glutamate and quisqualate microinjections elicited short series of swallows while NMDA microinjections induced long-lasting, rhythmic swallowing. Pretreatment with the selective NMDA antagonist, DL-2-amino-5-phosphonovalerate (50 pmol), almost completely suppressed the response elicited by NMDA (10 pmol) but did not induce a significant modification of swallowing triggered by either glutamate (25 pmol) or quisqualate (10 pmol). Pretreatment with 6-cyano-7-nitroquinoxaline-2,3-dione (50 pmol), a selective blocker of non-NMDA receptors, suppressed the swallows elicited by glutamate and strongly inhibited the response elicited by quisqualate microinjections. The same pretreatment induced only a slight modification of the swallowing elicited by NMDA. These data demonstrate that deglutition can be triggered by activating either NMDA or non-NMDA receptors localized within the NTS, and therefore suggest that both receptor subtypes may be involved in swallowing elicited under physiological conditions.

Cysteine sulfinic acid can enhance the central depressant effect of ethanol in mice

The interaction between ethanol and cysteine sulfinic acid was examined in male Swiss-Webster mice. The loss of the righting reflex (LORR) was used as a measurement of central nervous system depression. In addition, the interaction between ethanol and cysteic acid, a metabolite of cysteine sulfinic acid, was studied. Immediately after the animals regained the righting reflex following ethanol injection (IP), mice were given an ICV injection of saline, cysteine sulfinic acid (1, 15 or 25 mumol/kg) or cysteic acid (1, 15, or 25 mumol/kg). There occurred a return to the LORR within 30 s after the ICV injection of drugs. The return to the LORR by the administration of the amino acids in the presence of ethanol occurred in a dose-dependent fashion. When cysteine sulfinic acid or cysteic acid (25 mumol/kg, ICV) was injected in the absence of ethanol, no loss of the righting reflex occurred. In other experiments, bicuculline methiodide was given ICV with cysteine sulfinic acid (25 mumol/kg), cysteic acid (25 mumol/kg), or GABA (25 mumol/kg) in the presence of ethanol. Bicuculline methiodide, a GABA antagonist, reduced the effects of the three amino acids to produce a return to the LORR in the presence of ethanol. These results indicate that cysteine sulfinic acid, an excitatory amino acid, and cysteic acid can enhance the central depressant properties of ethanol. Since bicuculline antagonized the effects of these two amino acids, a GABAergic mechanism may be involved in the interaction between ethanol and cysteine sulfinic acid or cysteic acid.

Antibodies to glutamate and aspartate recognize non-endogenous ligands for excitatory amino acid receptors

Antisera raised against glutaraldehyde conjugates of glutamate (Glu) and aspartate (Asp) with hemocyanin proved highly specific for their respective unconjugated amino acid haptens when tested in immunocytochemical blocking experiments on sections of the rat spinal cord. In addition, immunocytochemical staining by the Glu antiserum was effectively blocked by quisqualate but not by kainate or N-methyl-D-aspartate (NMDA); staining with the Asp antiserum was effectively blocked by kainate, to a lesser extent by quisqualate, and was not affected by NMDA. These results may be explained by assuming that the specific binding regions of the antibodies tested share certain recognition characteristics with endogenous binding sites or receptors for excitatory amino acids and their agonists.

Stimulation of dopamine release from cultured rat mesencephalic cells by naturally occurring excitatory amino acids: involvement of both N-methyl-D-aspartate (NMDA) and non-NMDA receptor subtypes

In rat mesencephalic cell cultures, L-glutamate at concentrations ranging from 100 microM to 1 mM stimulated release of [3H]dopamine that was attenuated by the non-N-methyl-D-aspartate (non-NMDA) receptor antagonist 6,7-dinitroquinoxalinedione, but not by the selective NMDA receptor antagonists (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate (MK-801; 10 microM) and 3-(2-carboxypiperazine-4-yl)propyl-1-phosphonate (300 microM). Even at 1 mM glutamate, this release was Ca2+ dependent. These observations suggest that the release was mediated by a non-NMDA receptor. Only release stimulated by a lower concentration (10 microM) of glutamate was inhibited by MK-801 (10 microM), indicating that glutamate at this concentration activates the NMDA receptor. By contrast, L-aspartate at concentrations of 10 microM to 1 mM evoked [3H]dopamine release that was completely inhibited by MK-801 (10 microM) and was also Ca2+ dependent (tested at 1 and 10 mM aspartate). Thus, effects of aspartate involved activation of the NMDA receptor. Sulfur-containing amino acids (L-homocysteate, L-homocysteine sulfinate, L-cysteate, L-cysteine sulfinate) also evoked [3H]dopamine release. Release evoked by submillimolar concentrations of these amino acids was attenuated by MK-801 (10 microM), indicating involvement of the NMDA receptor.(ABSTRACT TRUNCATED AT 250 WORDS)

Localization of N-methyl-D-aspartate receptors in the rat striatum: effects of specific lesions on the [3H]3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid binding

The binding of [3H]3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid ([3H]CPP), a rigid analogue of 2-amino-7-phosphonoheptanoic acid (AP7) and reported to be a selective N-methyl-D-aspartate (NMDA) antagonist, was studied in rat striatal membranes using a centrifugation procedure to separate bound and free radioligand. [3H]CPP bound with high affinity (KD = 272 nM) in a saturable, reversible, and protein concentration-dependent manner. Specific binding was suggested to involve a single class of noninteracting binding sites. The most potent [3H]CPP binding inhibitors tested were CPP, L-glutamate, 2-amino-5-phosphonovalerate, and AP7. NMDA, L-aspartate, and alpha-aminoadipate were also shown to be efficient in inhibiting the binding, whereas quisqualate, D,L-2-amino-4-phosphonobutyrate, kainate, L-glutamate diethylester, and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid were found to be essentially inactive. These data are therefore consistent with the view that [3H]CPP selectively binds to NMDA receptors in the rat striatum. Lesions of intrastriatal neurons using local injections of kainic acid revealed a marked decrease in [3H]CPP binding, suggesting an almost exclusively postsynaptic location of binding sites in the striatum. Conversely, bilateral lesion of corticostriatal glutamatergic fibers resulted in an increased number of [3H]CPP striatal binding sites, providing evidence for a putative supersensitivity response to this striatal deafferentation. Interestingly, lesion of the nigrostriatal dopaminergic neurons using intranigral 6-hydroxydopamine injections resulted, 2-3 weeks later, in a similar increase in the number of [3H]CPP striatal binding sites.(ABSTRACT TRUNCATED AT 250 WORDS)

A study in rat brain cortex synaptic vesicles of endogenous ligands for N-methyl-D-aspartate receptors

The presence of endogenous ligands for the N-methyl-D-aspartate receptor was looked for in highly purified rat brain cortex synaptic vesicles, the contents of which were extracted and fractionated by gel filtration on Sephadex G-10, or by three different high-voltage electrophoresis procedures. The presence of endogenous ligands was detected by their ability to compete with 50 nM L-[3H]glutamate for binding to whole rat brain N-methyl-D-aspartate receptors. The receptor preparations used were those present in purified postsynaptic densities, in which the quisqualate receptors were blocked by 10 microM quisqualate. Synaptic vesicles had a high content of N-methyl-D-aspartate receptor ligands, which on fractionation always coincided with glutamate or aspartate. A variable and very small amount of a highly acidic endogenous ligand was also found. The latter substance did not coincide in the electrophoresis with homocysteic, cysteic, quinolinic, cysteine sulphinic or homocysteine sulphinic acids, or with N-acetyl-aspartyl-glutamic acid, S-sulphocysteine or sulphoserine. We also found that a single centrifugation, in 0.25 M sucrose, 25 mM Tris-citrate, pH 7.1, of purified synaptic vesicles, at 135,000 gmax for 45 min, led to a 51% loss of endogenous glutamate, but did not change their aspartate content. Thus, in uncentrifuged vesicles the glutamate/aspartate ratio was 9.4, while in centrifuged ones the ratio was 3.9 ATP markedly enhanced L-[3H]glutamate uptake into synaptic vesicles, but did not change the binding of L-[3H]aspartate. Differences in labelled aspartate and glutamate efflux from the vesicles were also found.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of excitatory amino acid receptors in K+- and glutamate-evoked release of endogenous adenosine from rat cortical slices

K+ and glutamate released endogenous adenosine from superfused slices of rat parietal cortex. The absence of Ca2+ markedly diminished K+- but not glutamate-evoked adenosine release. Tetrodotoxin decreased K+- and glutamate-evoked adenosine release by 40 and 20%, respectively, indicating that release was mediated in part by propagated action potentials in the slices. Inhibition of ecto-5'-nucleotidase by alpha,beta-methylene ADP and GMP decreased basal release of adenosine by 40%, indicating that part of the adenosine was derived from the extracellular metabolism of released nucleotide. In contrast, inhibition of ecto-5'-nucleotidase did not affect release evoked by K+ or glutamate, suggesting that adenosine was released as such. Inhibition of glutamate uptake by dihydrokainate potentiated glutamate-evoked release of adenosine. Glutamate-evoked adenosine release was diminished 50 and 55% by the N-methyl-D-aspartate (NMDA) receptor antagonists, DL-2-amino-5-phosphonovaleric acid and (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate (MK-801), respectively. The remaining release in the presence of MK-801 was diminished a further 66% by the non-NMDA receptor antagonist, 6,7-dinitroquinoxaline-2,3-dione, suggesting that both NMDA and non-NMDA receptors were involved in glutamate-evoked adenosine release. Surprisingly, K+-evoked adenosine release was also diminished about 30% by NMDA antagonists, suggesting that K+-evoked adenosine release may be partly mediated indirectly through the release of an excitatory amino acid acting at NMDA receptors.

Excitatory sulphur amino acid-evoked neurotransmitter release from rat brain synaptosome fractions

The neuroactive sulphur-containing amino acids L-cysteate (CA), L-cysteine sulphinate (CSA), L-homocysteine sulphinate (HSA), S-sulpho-L-cysteine (SC) and L-homocysteate (HCA) evoked the release of previously accumulated D-[3H]aspartate from rat brain cerebrocortical and cerebellar synaptosome fractions in a manner that was wholly Ca2+-independent. However, analysis of endogenous release by hplc revealed the presence of both Ca2+-dependent and -independent component of L-glutamate release but only a Ca2+-independent component of L-aspartate release. CA, CSA, HSA and SC but not HCA evoked the release of previously accumulated [3H]GABA from synaptosome fractions by a mechanism shown to comprise both a Ca2+-dependent and -independent component. The specific antagonists of the N-methyl-D-aspartate (NMDA) receptor, 3-[(+/-)-2-carboxypiperazin-4-yl]propyl-l-phosphonic acid (CPP) and the relatively selective competitive quisqualate (QUIS)/kainate (KA) receptor antagonist, 6-cyano-7-dinitroquinoxalinedione (CNQX), were ineffective in blocking the excitatory sulphur amino acid-evoked release of either D-[3H]aspartate, [3H]GABA or of endogenous established transmitter amino acids.