The neurotransmitter candidature of sulphur-containing excitatory amino acids in the mammalian central nervous system

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.

A Novel Stereospecific Bioluminescent Assay for Detection of Endogenous d-Cysteine

The presence of endogenous d-stereoisomers of amino acids in mammals dispels a long-standing dogma about their existence. d-Serine and d-aspartate function as novel neurotransmitters in mammals. However, the stereoisomer with the fastest, spontaneous in vitro racemization rate, d-cysteine, has not been reported. We utilized a novel, stereospecific, bioluminescent assay to identify endogenous d-cysteine in substantial amounts in the eye, brain, and pancreas of mice. d-Cysteine is enriched in mice embryonic brains at day E9.5 (4.5 mM) and decreases progressively with development (μM levels). d-Cysteine is also present in significantly higher amounts in the human brain white matter compared with gray matter. In the luciferase assay, d-cysteine conjugates with cyano hydroxy benzothiazole in the presence of a base and reducing agent to form d-luciferin. d-Luciferin, subsequently, in the presence of firefly luciferase and ATP, emits bioluminescence proportional to the concentration of d-cysteine. The assay is stereospecific and allows the quantitative estimation of endogenous d-cysteine in tissues in addition to its specificity for d-cysteine. Future efforts aimed at bioluminescent in vivo imaging of d-cysteine may allow a more noninvasive means of its detection, thereby elucidating its function.

Reducing the Kidney Uptake of High Contrast CXCR4 PET Imaging Agents via Linker Modifications

Purpose: The C-X-C chemokine receptor 4 (CXCR4) is highly expressed in many subtypes of cancers, notably in several kidney-based malignancies. We synthesized, labeled, and assessed a series of radiotracers based on a previous high contrast PET imaging radiopharmaceutical [68Ga]Ga-BL02, with modifications to its linker and metal chelator, in order to improve its tumor-to-kidney contrast ratio. Methods: Based on the design of BL02, a piperidine-based cationic linker (BL06) and several anionic linkers (tri-Aad (BL17); tri-D-Glu (BL20); tri-Asp (BL25); and tri-cysteic acid (BL31)) were substituted for the triglutamate linker. Additionally, the DOTA chelator was swapped for a DOTAGA chelator (BL30). Each radiotracer was labeled with 68Ga and evaluated in CXCR4-expressing Daudi xenograft mice with biodistribution and/or PET imaging studies. Results: Of all the evaluated radiotracers, [68Ga]Ga-BL31 showed the most promising biodistribution profile, with a lower kidney uptake compared to [68Ga]Ga-BL02, while retaining the high imaging contrast capabilities of [68Ga]Ga-BL02. [68Ga]Ga-BL31 also compared favorably to [68Ga]Ga-Pentixafor, with superior imaging contrast in all non-target organs. The other anionic linker-based radiotracers showed either equivocal or worse contrast ratios compared to [68Ga]Ga-BL02; however, [68Ga]Ga-BL25 also showed lower kidney uptake, as compared to that of [68Ga]Ga-BL02. Meanwhile, [68Ga]Ga-BL06 had high non-target organ uptake and relatively lower tumor uptake, while [68Ga]Ga-BL30 showed significantly increased kidney uptake and similar tumor uptake values. Conclusions: [68Ga]Ga-BL31 is an optimized CXCR4-targeting radiopharmaceutical with lower kidney retention that has clinical potential for PET imaging and radioligand therapy.

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.

Solid-State 17O NMR Studies of Sulfonate Jump Dynamics in Crystalline Sulfonic Acids: Insights into the Hydrogen Bonding Effect

We report variable-temperature (VT) ¹⁷O solid-state nuclear magnetic resonance (NMR) spectra for three crystalline sulfonic acids: l-cysteic acid monohydrate (CA), 3-pyridinesulfonic acid (PSA), and p-toluenesulfonic acid monohydrate (TSA). We were able to analyze the experimental VT ¹⁷O NMR spectra to obtain the activation barriers for SO₃^- jumps in these systems. Using the density functional-based tight-binding (DFTB) method, we performed potential energy surface scans for SO₃^- jumps in the crystal lattice of CA, PSA, and TSA, as well as for three related crystalline sulfonic acids (taurine, homotaurine, and 4-aminobutane-1-sulfonic acid) for which relevant ¹⁷O solid-state NMR data are available in the literature. The calculated activation barriers are in reasonable agreement with the experimental values. On the basis of the DFTB results, we hypothesized that activation barriers for SO₃^- jumps in the crystal lattice depend largely on the hydrogen bonding energy difference between the ground state and the transition state.

Caffeic acid improves glucose utilization and maintains tissue ultrastructural morphology while modulating metabolic activities implicated in neurodegenerative disorders in isolated rat brains

Impaired glucose utilization has been implicated in the pathophysiology of neurodegenerative diseases. The neuroprotective effect of caffeic acid (CA) was investigated in the isolated rat brain by determining its ability to promote glucose uptake, mitigate redox imbalance, modulate purinergic and cholinergic activities, elemental distribution, and maintain tissue morphology. Isolated rat brains were incubated for 2 hours with glucose, CA and glucose, and metformin and glucose. There was an increased glucose uptake, glutathione level, superoxide dismutase, and catalase activities in brain tissues incubated with CA compared to the controls. Incubation with CA also led to significantly decreased levels of malondialdehyde, nitric oxide, acetylcholinesterase, butyrylcholinesterase, and ATPase activities. Electron microscopy (scanning electron microscopy and transmission electron microscopy) analysis portrayed a maintenance of tissue ultrastructural morphology in 2CA-incubated tissues as indicated by the intact synaptic vesicles, blood vessels, dendritic and neuronal network, mitochondria, and presynaptic membrane. CA also restored altered elemental levels in brain tissues. These results indicate the stimulatory potential of CA on brain glucose utilization with simultaneous neuroprotective activities.

Kolaviron stimulates glucose uptake with concomitant modulation of metabolic activities implicated in neurodegeneration in isolated rat brain, without perturbation of tissue ultrastructural morphology

Reduced glucose uptake usually occurs in type 2 diabetes due to down-regulation of brain glucose transporters. The potential of kolaviron, a biflavonoid from Garcinia kola to stimulate glucose uptake and suppress glucose-induced oxidative toxicity were investigated in rat brain. Its molecular interactions with the target proteins were investigated in silico. Kolaviron was incubated with excised rat brain in the presence of glucose for 2 h, with metformin serving as a positive control. Kolaviron caused a significant (p < 0.05) increase in glucose uptake, glutathione level, superoxide dismutase, catalase, ATPase, ENTPDase and 5'-nucleotidase activities, while concomitantly depleting malondialdehyde level, acetylcholinesterase and butyrylcholinesterase activities compared to brains incubated with glucose only. Electron microscopy (SEM and TEM) analysis revealed kolaviron had little or no effect on the ultrastructural morphology of brain tissues as evidenced by the intact dendritic and neuronal network, blood vessels, mitochondria, synaptic vesicles, and pre-synaptic membrane. SEM-EDX analysis revealed a restorative effect of glucose-induced alteration in brain elemental concentrations, with total depletion of aluminum and zinc. MTT analysis revealed kolaviron had no cytotoxic effect on HT-22 cells. Molecular docking revealed a potent interaction between kolaviron and catalase at the SER114 and MET350 residues, with a binding energy of 12 kcal/mol. Taken together, these results portray the potential of kolaviron to stimulate glucose uptake while concomitantly coffering a neuroprotective effect.

BJ-1103, 6-aminopyridin-3-ol skeletal compound, modulates neuroprotective and anti-neuroinflammatory effects in murine hippocampal and microglial cells via Nrf2-mediated heme oxygenase-1 expression

BJ-1103, as a 6-aminopyridin-3-ol skeletal compound, was originally developed as an antioxidant against free radicals and oxidative stress was prepared from pyridoxine·HCl by the reported procedure. In the present study, we examined the effect of BJ-1103 on neuroprotection and neuroinflammation. Our data showed that BJ-1103 can protect HT22 cells against glutamate-induced cell cytotoxicity. And, BJ-1103 also inhibited LPS-induced inflammatory action. In addition, BJ-1103-induced heme oxygenase-1 (HO-1) expression and elevated HO-1 activities in the two cell lines studied. Additionally, BJ-1103 treatment induced nuclear transcription factor erythroid-2 related factor 2 (Nrf2) and increased the promoter activity of antioxidant response elements (AREs). We have demonstrated using the Nrf2 siRNA, HO inhibitor or HO-1 siRNA that BJ-1103 suppressed neurotoxicity and neuroinflammation through the Nrf2-mediated HO-1 expression. These results demonstrated that BJ-1103 may have good therapeutic agent against neurodegenerative diseases that are induced by oxidative stress and neuroinflammation.

Behavioral and biochemical evidence of the role of acetaldehyde in the motivational effects of ethanol

Since Chevens' report, in the early 50's that his patients under treatment with the aldehyde dehydrogenase inhibitor, antabuse, could experience beneficial effects when drinking small volumes of alcoholic beverages, the role of acetaldehyde (ACD) in the effects of ethanol has been thoroughly investigated on pre-clinical grounds. Thus, after more than 25 years of intense research, a large number of studies have been published on the motivational properties of ACD itself as well as on the role that ethanol-derived ACD plays in the effects of ethanol. Accordingly, in particular with respect to the motivational properties of ethanol, these studies were developed following two main strategies: on one hand, were aimed to challenge the suggestion that also ACD may exert motivational properties on its own, while, on the other, with the aid of enzymatic manipulations or ACD inactivation, were aimed to test the hypothesis that ethanol-derived ACD might have a role in ethanol motivational effects. Furthermore, recent evidence significantly contributed to highlight, as possible mechanisms of action of ACD, its ability to commit either dopaminergic and opioidergic transmission as well as to activate the Extracellular signal Regulated Kinase cascade transduction pathway in reward-related brain structures. In conclusion, and despite the observation that ACD seems also to have inherited the elusive nature of its parent compound, the behavioral and biochemical evidence reviewed points to ACD as a neuroactive molecule able, on its own and as ethanol metabolite, to exert motivational effects.

Pressor response to L-cysteine injected into the cisterna magna of conscious rats involves recruitment of hypothalamic vasopressinergic neurons

The sulfur-containing non-essential amino acid L-cysteine injected into the cisterna magna of adult conscious rats produces an increase in blood pressure. The present study examined if the pressor response to L-cysteine is stereospecific and involves recruitment of hypothalamic vasopressinergic neurons and medullary noradrenergic A1 neurons. Intracisternally injected D-cysteine produced no cardiovascular changes, while L-cysteine produced hypertension and tachycardia in freely moving rats, indicating the stereospecific hemodynamic actions of L-cysteine via the brain. The double labeling immunohistochemistry combined with c-Fos detection as a marker of neuronal activation revealed significantly higher numbers of c-Fos-positive vasopressinergic neurons both in the supraoptic and paraventricular nuclei and tyrosine hydroxylase containing medullary A1 neurons, of L-cysteine-injected rats than those injected with D-cysteine as iso-osmotic control. The results indicate that the cardiovascular responses to intracisternal injection of L-cysteine in the conscious rat are stereospecific and include recruitment of hypothalamic vasopressinergic neurons both in the supraoptic and paraventricular nuclei, as well as of medullary A1 neurons. The findings may suggest a potential function of L-cysteine as an extracellular signal such as neuromodulators in central regulation of blood pressure.

Effect of (L)-cysteine on acetaldehyde self-administration

Acetaldehyde (ACD), the first metabolite of ethanol, has been implicated in several behavioural actions of alcohol, including its reinforcing effects. Recently, we reported that l-cysteine, a sequestrating agent of ACD, reduced oral ethanol self-administration and that ACD was orally self-administered. This study examined the effects of l-cysteine pre-treatment during the acquisition and maintenance phases of ACD (0.2%) self-administration as well as on the deprivation effect after ACD extinction and on a progressive ratio (PR) schedule of reinforcement. In a separate PR schedule of reinforcement, the effect of l-cysteine was assessed on the break-point produced by ethanol (10%). Furthermore, we tested the effect of l-cysteine on saccharin (0.2%) reinforcement. Wistar rats were trained to self-administer ACD by nose poking on a fixed ratio (FR1) schedule in 30-min daily sessions. Responses on an active nose-poke caused delivery of ACD solution, whereas responses on an inactive nose-poke had no consequences. l-cysteine reduced the acquisition (40 mg/kg), the maintenance and the deprivation effect (100 mg/kg) of ACD self-administration. Furthermore, at the same dose, l-cysteine (120 mg/kg) decreased both ACD and ethanol break point. In addition, l-cysteine was unable to suppress the different responses for saccharin, suggesting that its effect did not relate to an unspecific decrease in a general motivational state. Compared to saline, l-cysteine did not modify responses on inactive nose-pokes, suggesting an absence of a non-specific behavioural activation. Taken together, these results could support the hypotheses that ACD possesses reinforcing properties and l-cysteine reduces motivation to self-administer ACD.

Effects of L-cysteine on reinstatement of ethanol-seeking behavior and on reinstatement-elicited extracellular signal-regulated kinase phosphorylation in the rat nucleus accumbens shell

BACKGROUND

Alcoholism is a neuroadaptive disorder, and the understanding of the mechanisms of the high rates of relapse, which characterize it, represents one of the most demanding challenges in alcoholism and addiction research. The extracellular signal-regulated kinase (ERK) is an intracellular kinase, critical for neuroplasticity in the adult brain that is suggested to play a fundamental role in the molecular mechanisms underlying drug addiction and relapse. We previously observed that a nonessential amino acid, L-cysteine, significantly decreases oral ethanol (EtOH) self-administration, reinstatement of EtOH-drinking behavior, and EtOH self-administration break point.

METHODS

Here, we tested whether L-cysteine can affect the ability of EtOH priming to induce reinstatement of EtOH-seeking behavior. In addition, we determined the ability of EtOH priming to induce ERK phosphorylation as well as the ability of L-cysteine to affect reinstatement-elicited ERK activation. To these purposes, Wistar rats were trained to nose-poke for a 10% v/v EtOH solution. After stable drug-taking behavior was obtained, nose-poking for EtOH was extinguished, and reinstatement of drug seeking, as well as reinstatement-elicited pERK, was determined after an oral, noncontingent, priming of EtOH (0.08 g/kg). Rats were pretreated with either saline or L-cysteine (80 to 120 mg/kg) 30 minutes before testing for reinstatement.

RESULTS

The findings of this study confirm that the noncontingent delivery of a nonpharmacologically active dose of EtOH to rats, whose previous self-administration behavior had been extinguished, results in significant reinstatement into EtOH-seeking behavior. In addition, the results indicate that reinstatement selectively activates ERK phosphorylation in the shell of the nucleus accumbens (Acb) and that pretreatment with L-cysteine reduces either reinstatement of EtOH seeking and reinstatement-elicited pERK in the AcbSh.

CONCLUSIONS

Altogether, these results indicate that L-cysteine could be an effective pharmacological agent for the prevention of behavioral and molecular correlates of EtOH-primed reinstatement of EtOH seeking and that the shell of the Acb represents a critical neural substrate for priming-elicited reinstatement mechanisms involving ERK phosphorylation.

Amino acids that centrally influence blood pressure and regional blood flow in conscious rats

Functional roles of amino acids have increasingly become the focus of research. This paper summarizes amino acids that influence cardiovascular system via the brain of conscious rats. This paper firstly describes why amino acids are selected and outlines how the brain regulates blood pressure and regional blood flow. This section includes a concise history of amino acid neurotransmitters in cardiovascular research and summarizes brain areas where chemical stimulations produce blood pressure changes mainly in anesthetized animals. This is followed by comments about findings regarding several newly examined amino acids with intracisternal stimulation in conscious rats that produce changes in blood pressure. The same pressor or depressor response to central amino acid stimulations can be produced by distinct mechanisms at central and peripheral levels, which will be briefly explained. Thereafter, cardiovascular actions of some of amino acids at the mechanism level will be discussed based upon findings of pharmacological and regional blood flow measurements. Several examined amino acids in addition to the established neurotransmitter amino acids appear to differentially activate brain structures to produce changes in blood pressure and regional blood flows. They may have physiological roles in the healthy brain, but pathological roles in the brain with cerebral vascular diseases such as stroke where the blood-brain barrier is broken.

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.

Conversion of methionine into homocysteic acid in heavily oxidized proteomics samples

Analysis of protein oxidation is necessary in numerous areas of biochemistry, including hydroxyl radical surface mapping, oxidative stress assays, and pharmaceutical stability testing. Mass spectrometry is one of the tools most often used to identify protein oxidation products, and previous studies have attempted to identify and characterize all of the major oxidation products detected by mass spectrometry for each amino acid residue. In this note, we present evidence that in heavily oxidized protein samples, such as those produced by hydroxyl radical surface mapping, a major oxidation product of methionine is homocysteic acid. The formation of homocysteic acid from methionine was previously unrecognized in other mass spectrometric analyses, and has important implications for the analysis of oxidized samples, as well as potential implications as to the functional consequences of methionine oxidation.

l-Cysteine reduces oral ethanol self-administration and reinstatement of ethanol-drinking behavior in rats

Our previous findings have shown that l-cysteine, a non essential amino acid, prevented ethanol (EtOH) induced conditioned place preference. The aim of the present study was to examine the effect of l-cysteine on the acquisition and maintenance of oral EtOH self-administration and on the reinstatement of EtOH-drinking behavior in Wistar rats. Rats were pretreated intraperitoneally with saline or l-cysteine (20 and 40 mg/kg) 30 min before each acquisition trial, in an operant nose-poking paradigm where they were given the opportunity to orally self-administer tap water or EtOH (5-10% v/v). Further, to evaluate if l-cysteine reduces the acquired oral EtOH self-administration, we carried out an independent experiment in which rats were trained to self-administer EtOH (10%); after all groups of rats developed similarly stable oral EtOH self-administration, the effect of l-cysteine (0, 40, 60, 80 and 100mg/kg) was tested. An additional group of rats was pretreated with saline or l-cysteine (80 mg/kg) and tested on reinstatement after EtOH extinction and, at the end of last reinstatement session, were utilized to measure blood and brain EtOH levels. The animals that had access to EtOH solution discriminated between the active and inactive nose-pokes and showed rates of active nose-pokes significantly higher than the tap water group. Furthermore, rats self-administering EtOH (10%) also demonstrated extinction behavior and gradually reinstated active nose-poke responding when EtOH was reintroduced. l-cysteine reduced both the acquisition and maintenance of oral EtOH self-administration. The reduced reinstatement of EtOH-drinking behavior was paralleled by a significant reduction of EtOH intake and correlated with blood and brain EtOH levels. The efficacy of l-cysteine on the various phases of alcohol drinking in rats, could represent an interesting pharmacological approach and could open a new line of research for the development of therapies to reduce EtOH intake in alcoholic patients.

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

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

Synaptic Vesicle Protein NTT4/XT1 (SLC6A17) Catalyzes Na+-coupled Neutral Amino Acid Transport

The SLC6 family of structurally related, Na(+)-dependent transporter proteins is responsible for presynaptic reuptake of the majority of neurotransmitters. Within this family are a number of orphan transporters, including NTT4/XT1 (SLC6A17), a protein first identified over 15 years ago. NTT4/XT1 is expressed exclusively in the nervous system and specifically on synaptic vesicles in glutamatergic and some GABAergic neurons. Despite extensive efforts by a number of groups, no substrate has been reported for NTT4/XT1. Here we use a combination of molecular manipulations to increase expression of the NTT4/XT1 protein at the plasma membrane and to directly demonstrate that it catalyzes neutral amino acid transport. The substrate profile of the NTT4/XT1-dependent activity is similar to that of the closely related B(0)AT2/SBAT1 (SLC6A15), including a submillimolar apparent affinity for proline and leucine and a low millimolar apparent affinity for glutamine. The transport activity is Na(+)-dependent and Cl(-)-independent and is inhibited by low pH as is SLC6A15, suggesting redundant roles for these proteins. This characterization of NTT4/XT1 offers important insights into neurotransmitter metabolism as well as the mechanistic differences among the structurally related, but functionally divergent, SLC6 proteins.

Reduction of ethanol-derived acetaldehyde induced motivational properties by L-cysteine

BACKGROUND

Experimental evidences suggest that acetaldehyde (ACD) contributes to the positive motivational properties of ethanol (EtOH) as assessed by the place conditioning paradigm; indeed, we found that by reducing ACD production and/or by using ACD-sequestrating agents, EtOH is deprived from its motivational properties. Thiol products, such as the amino acid cysteine, are known to be effective ACD-sequestering agents. Cysteine is able to covalently bind ACD thereby forming a stable, nontoxic 2-methyl-thiazolidine-4-carboxylic acid compound. Thus, we treated rats with l-cysteine before intragastric administration of EtOH or ACD.

METHODS

Male Wistar rats were pretreated intraperitoneally with saline or l-cysteine (10, 20, or 30 mg/kg), before intragastric administration of saline, EtOH (1 g/kg), or ACD (20 mg/kg). The specificity of l-cysteine effect was addressed using morphine-induced conditioned place preference (cpp) (2.5 mg/kg, i.p.).

RESULTS

l-cysteine dose-dependently prevented both EtOH and ACD-induced cpp but did not interfere with morphine-induced cpp, suggesting that l-cysteine specifically modulates the motivational properties of EtOH.

CONCLUSION

The present results further underscore the role of EtOH-derived ACD in EtOH-induced motivational properties. l-cysteine, by binding EtOH-derived ACD, would deprive it of its rewarding properties and reduce its abuse liability.

Homocysteine and Parkinson's disease: a dangerous liaison?

Homocysteine, a sulphur-containing amino acid formed by demethylation of methionine, is involved in numerous processes of methyl group transfer, all playing pivotal roles in the biochemistry of the human body. Increased levels of plasma homocysteine (hyperhomocysteinemia) - which may result from a deficiency of folate, vitamin B6 or B12 or mutations in enzymes regulating the catabolism of homocysteine - are associated with a wide range of clinical manifestations, mostly affecting the central nervous system (e.g., mental retardation, cerebral atrophy and epileptic seizures). Recent evidence suggests that changes in the metabolic fate of homocysteine, leading to hyperhomocysteinemia, may also play a role in the pathophysiology of neurodegenerative disorders, particularly Parkinson's disease (PD). The nervous system might be particularly sensitive to homocysteine, due to the excitotoxic-like properties of the amino acid. However, experimental findings have shown that homocysteine does not seem to posses direct, cytotoxic activity, while the amino acid has proven able to synergize with more specific neurotoxic insults. Hyperhomocysteinemia has been repeatedly reported in PD patients; the increase, however, seems mostly related to the methylated catabolism of l-Dopa, the main pharmacological treatment of PD. Therefore, hyperhomocysteinemia may not be specific to movement disorders or other neurological diseases, the condition being, in fact, rather the result of the combinations of different factors, mainly metabolic, but also genetic and pharmacological, intervening in the neurodegenerative process.

The SLC6 orphans are forming a family of amino acid transporters

Transporters in the human genome are grouped in solute carrier families (SLC). The SLC6 family is one of the biggest transporter families in the human genome comprising 20 members. It is usually referred to as the neurotransmitter transporter family because its founding members encode transporters for the neurotransmitters GABA, noradrenaline, serotonin and dopamine. The family also includes a number of 'orphan' transporters, the function of which has remained elusive until recently. Identification of the broadly specific neutral amino acid transporter SLC6A19 (also called B(0)AT1) suggested that all orphan transporters may in fact be amino acid transporters. This was subsequently confirmed by the identification of SLC6A20 as the long-sought IMINO system, a proline transporter found in kidney, intestine and brain. Very recently, SLC6A15 was identified as the neutral amino acid transporter B(0)AT2. All amino acid transporters appear to cotransport only 1Na(+) together with the amino acid substrate. Both, B(0)AT1 and B(0)AT2 are chloride independent, whereas IMINO is chloride dependent. The amino acid transporters of the SLC6 family are functionally and sequence related to the recently crystallized leucine transporter from Aquifex aeolicus. The structure elegantly explains many of the mechanistic features of the SLC6 amino acid transporters.

The orphan transporter v7-3 (slc6a15) is a Na+-dependent neutral amino acid transporter (B0AT2)

Transporters of the SLC6 (solute carrier 6) family play an important role in the removal of neurotransmitters in brain tissue and in amino acid transport in epithelial cells. In the present study, we demonstrate that mouse v7-3 (slc6a15) encodes a transporter for neutral amino acids. The transporter is functionally and sequence related to B(0)AT1 (slc6a19) and was hence named B(0)AT2. Leucine, isoleucine, valine, proline and methionine were recognized by the transporter, with values of K(0.5) (half-saturation constant) ranging from 40 to 200 microM. Alanine, glutamine and phenylalanine were low-affinity substrates of the transporter, with K(0.5) values in the millimolar range. Transport of neutral amino acids via B(0)AT2 was Na+-dependent, Cl--independent and electrogenic. Superfusion of mouse B(0)AT2-expressing oocytes with amino acid substrates generated robust inward currents. Na+-activation kinetics of proline transport and uptake under voltage clamp suggested a 1:1 Na+/amino acid co-transport stoichiometry. Susbtrate and co-substrate influenced each other's K(0.5) values, suggesting that they share the same binding site. A mouse B(0)AT2-like transport activity was detected in synaptosomes and cultured neurons. A potential role of B(0)AT2 in transporting neurotransmitter precursors and neuromodulators is proposed.

The central nervous system in animal models of hyperhomocysteinemia

Growing epidemiological evidence of associations between mildly elevated plasma homocysteine with age-related cognitive impairment, neurodegenerative and cerebrovascular disease has stimulated interest in the role of homocysteine in neurological and neuropsychiatric disorders. Homocysteine is an intermediate in the folate, vitamin B12 and B6 dependent pathways of one-carbon and sulfur amino acid metabolism. Impairments of these pathways may cause CNS dysfunction by promoting the intracellular generation of homocysteine, which is postulated to have vasotoxic and neurotoxic properties. It might also inhibit the methylation of myelin basic protein and membrane phospholipids, or disrupt biogenic amine metabolism and many other vital CNS reactions. However, it is unclear which, if any, of these putative mechanisms underlies the epidemiological associations. Genetic mouse models of hyperhomocysteinemia suggest that the primary metabolic disturbances rather than homocysteine per se may be important in determining neurological outcomes. However, severe and early developmental abnormalities in these mice limit their usefulness for understanding the relation of hyperhomocysteinemia to adult CNS disorders. Pharmacologic and dietary studies on homocysteine in rodents have reported heightened neuronal sensitivity to neurotoxic insults, neurochemical abnormalities and cerebrovascular dysfunction. Such studies are consistent with a causal relationship, but they fail to distinguish between effects that might result from a dietary imbalance and those that might be caused by homocysteine per se. Future work should be directed towards refining these models in order to distinguish between the effects of homocysteine and its determinants on neurological and behavioral outcomes that represent different CNS disorders.

Implications for hyperhomocysteinemia: not homocysteine but its oxidized forms strongly inhibit neuronal network activity

Severe hyperhomocysteinemia (50-200 microM) often presents itself with acute neuronal dysfunction including seizures and psychosis. Its moderate form (15-50 microM) is associated with cognitive impairment and dementia. We investigated the neuropharmacological effects of homocysteine and its oxidized forms, homocysteinesulfinic acid (HCSA) and homocysteic acid (HCA), on neuronal network function utilizing dissociated cortical neurons from embryonic Wistar rats on microelectrode arrays. All substances inhibited dose-dependently and reversibly spontaneous neuronal network activity within seconds: L-HCSA and L-HCA blocked spontaneous spike rate (SSR) significantly at very low concentrations, with an IC50 of 1.9 and 1.3 microM, respectively; whereas the dose-response curve of D,L-homocysteine revealed an IC50 of 401 microM. These effects were antagonized by 2-amino-5-phosphonovaleric acid (APV) pointing to the NMDA receptor as mediator of this fast and reversible inhibition of network activity. We conclude that a neuronal dysfunction observed in hyperhomocysteinemia is likely due to HCSA and HCA since effective concentrations of homocysteine are not reached in patients.

L-homocysteine sulfinic acid and other acidic homocysteine derivatives are potent and selective metabotropic glutamate receptor agonists

Moderate hyperhomocysteinemia is associated with several diseases, including coronary artery disease, stroke, Alzheimer's disease, schizophrenia, and spina bifida. However, the mechanisms for their pathogenesis are unknown but could involve the interaction of homocysteine or its metabolites with molecular targets such as neurotransmitter receptors, channels, or transporters. We discovered that L-homocysteine sulfinic acid (L-HCSA), L-homocysteic acid, L-cysteine sulfinic acid, and L-cysteic acid are potent and effective agonists at several rat metabotropic glutamate receptors (mGluRs). These acidic homocysteine derivatives 1) stimulated phosphoinositide hydrolysis in the cells stably expressing the mGluR1, mGluR5, or mGluR8 (plus Galpha(qi9)) and 2) inhibited the forskolin-induced cAMP accumulation in the cells stably expressing mGluR2, mGluR4, or mGluR6, with different potencies and efficacies depending on receptor subtypes. Of the four compounds, L-HCSA is the most potent agonist at mGluR1, mGluR2, mGluR4, mGluR5, mGluR6, and mGluR8. The effects of the four agonists were selective for mGluRs because activity was not discovered when L-HCSA and several other homocysteine derivatives were screened against a large panel of cloned neurotransmitter receptors, channels, and transporters. These findings imply that mGluRs are candidate G-protein-coupled receptors for mediating the intracellular signaling events induced by acidic homocysteine derivatives. The relevance of these findings for the role of mGluRs in the pathogenesis of homocysteine-mediated phenomena is discussed.

Endogenous sulphur-containing amino acids: potent agonists at presynaptic metabotropic glutamate autoreceptors in the rat central nervous system

We have recently demonstrated that presynaptically located metabotropic glutamate (mGlu) autoreceptors regulate synaptic glutamate release both in vitro and in vivo. We now report a positive modulatory action of the sulphur-containing amino acids (SCAAs), L-cysteic acid (CA) and L-cysteine sulphinic acid (CSA), at presynaptic group I mGlu receptors, specifically of the mGlu5 subtype, acting to enhance synaptic glutamate release from the rat forebrain in vitro. Neuronal glutamate release was monitored using electrically-evoked efflux of preloaded [(3)H]-D-aspartate from rat forebrain hemisections. Both CA (3 - 100 muM) and CSA (1 - 100 microM), in addition to the selective group I mGlu receptor agonist, (S)-3,5-dihydroxyphenylglycine ((S)-DHPG), concentration-dependently enhanced electrically-stimulated efflux of [(3)H]-D-aspartate from the rat forebrain slices. Basal efflux of label remained unchanged. The inhibitory activity of the broad spectrum mGlu receptor antagonist, (+/-)-alpha-methyl-4-carboxyphenylglycine ((+/-)-MCPG; 200 microM), coupled with the inactivity of the selective mGlu1 receptor antagonists, (R,S)-1-aminoindan-1,5-dicarboxylic acid ((R,S)-AIDA; 100 - 500 microM) and the more potent (+)-2-methyl-4-carboxyphenylglycine (LY367385; 10 microM) against these responses, indicates an action of the SCAAs at the mGlu5 receptor subtype. This proposal is supported by the potent inhibition of these responses by the selective, non-competitive mGlu5 receptor antagonist, 2-methyl-6-(phenylethynyl)pyridine (MPEP; 10 microM). The observed enhancement of the responses to high concentrations of CA by the selective mGlu5 receptor desensitization inhibitor, cyclothiazide (CYZ; 10 microM), is also consistent with this concept. Administration of the agonists in the presence of bovine serum albumin (BSA; 5 - 15 mg ml(-1)) markedly attenuated the positive modulatory responses observed, strongly supporting a role for arachidonic acid in the expression of these mGlu5 receptor-mediated responses. The regulatory actions of SCAAs on synaptic glutamate release demonstrated in the present study may provide a physiological function for these putative neurotransmitter amino acids in the mammalian brain. These central actions of the SCAAs may have wide-ranging implications for a range of neurological and neuropsychiatric disease states and their treatment.

Risk assessment of alcohol withdrawal seizures with a Kohonen feature map

Recently, it has been suggested that alcohol-induced hyperhomocysteinaemia in patients suffering from chronic alcoholism might be a risk factor for alcohol withdrawal seizures. In the present follow-up study 12 patients with chronic alcoholism who suffered from withdrawal seizures had significantly higher levels of homocysteine (Hcy) on admission (71.43 +/- 25.84 mol/l) than patients (n = 37) who did not develop seizures (32.60 +/- 24.87 mol/l; U = 37.50, p = 0.0003). Using a logistic regression analysis, withdrawal seizures were best predicted by a high Hcy level on admission (p < 0.01; odds ratio 2.07). Based on these findings we developed an artificial neural network system (Kohonen feature map, KFM) for an improved prediction of the risk of alcohol withdrawal seizures. Forty-nine patients with chronic alcoholism (12 with alcohol withdrawal seizures and 37 without seizures) were randomized into a training set and a test set. Best results for sensitivity of the KFM was 83.3% (five of six seizure patients were predicted correctly) with a specificity of 94.4% (one false positive prediction of 19 patients). We conclude that in patients with alcohol-induced hyperhomocysteinaemia the KFM is a useful tool to predict alcohol withdrawal seizures.

Homocysteine and alcoholism

Chronic alcohol consumption can induce alterations in the function and morphology of most if not all brain systems and structures. However, the exact mechanism of brain damage in alcoholics remains unknown. Partial recovery of brain function with abstinence suggests that a proportion of the deficits must be functional in origin (i.e. plastic changes of nerve cells) while neuronal loss from selected brain regions indicates permanent and irreversible damage. There is growing evidence that chronic alcoholism is associated with a derangement in the sulfur amino acid metabolism. Recently, it has been shown that excitatory amino acid (EAA) neurotransmitters and homocysteine levels are elevated in patients who underwent withdrawal from alcohol. Furthermore, it has been found that homocysteine induces neuronal cell damage by stimulating NMDA receptors as well as by producing free radicals. Homocysteine neurotoxicity via overstimulation of N-methyl-D-aspartate receptors may contribute to the pathogenesis of both brain shrinkage and withdrawal seizures linked to alcoholism.

Plasma homocysteine is a predictor of alcohol withdrawal seizures

An adaptive consequence of prolonged ethanol consumption is a compensatory up-regulation of NMDA receptors in certain brain areas. Taking into account that homocysteine and its breakdown products (i.e. homocysteic acid) are putative neurotransmitters and agonists at the NMDA receptor, the aim of this study was to assess the influence of levels of homocysteine on alcohol withdrawal seizures. Six patients with chronic alcoholism who suffered from withdrawal seizures had significantly higher levels of homocysteine on admission (84.7 +/- 29.8 micromol/l) than patients (n = 26) who did not develop seizures (30.2 +/- 23.2 micromol/l; U = 8.0, p = 0.0007). Furthermore, seizure patients had significantly lower levels of folate and significantly higher blood alcohol concentrations. Using a logistic regression analysis, withdrawal seizures were best predicted by a high homocysteine level on admission (p < 0.01; odds ratio = 1.05). Homocysteine levels on admission may be a useful screening method to identify patients at risk for withdrawal seizures.

Conservation of the ligand recognition site of metabotropic glutamate receptors during evolution

Mammalian metabotropic glutamate receptors (mGluRs) are classified into 3 groups based on their sequence similarity and ligand recognition selectivity. Recently, we identified a Drosophila mGluR (DmGlu(A)R) which is about equidistant, phylogenetically, from the 3 mGluR groups. However, both the G-protein coupling selectivity and the pharmacological profile of DmGlu(A)R, as analysed with mutated G-proteins and a few compounds, look similar to those of mammalian group-II mGluRs. In the present study we carefully examined the pharmacological profile of DmGlu(A)R, and compared it to those of the rat mGlu(1a), mGlu(2) and mGlu(4a) receptors, representative of group-I, II and III respectively. The pharmacological profile of DmGlu(A)R was found to be similar to that of mGlu(2)R, and only very small differences could be identified at the level of their pharmacophore models. These data strongly suggest that the binding sites of these two receptors are similar. To further document this idea, a 3D model of the mGlu(2) binding domain was constructed based on the low sequence similarity with periplasmic amino acid binding proteins, and was used to identify the residues that possibly constitute the ligand recognition pocket. Interestingly, this putative binding pocket was found to be very well conserved between DmGlu(A)R and the mammalian group-II receptors. These data indicate that there has been a strong selective pressure during evolution to maintain the ligand recognition selectivity of mGluRs.

Glutamate as a neurotransmitter in the brain: review of physiology and pathology

Glutamate is the principal excitatory neurotransmitter in brain. Our knowledge of the glutamatergic synapse has advanced enormously in the last 10 years, primarily through application of molecular biological techniques to the study of glutamate receptors and transporters. There are three families of ionotropic receptors with intrinsic cation permeable channels [N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and kainate]. There are three groups of metabotropic, G protein-coupled glutamate receptors (mGluR) that modify neuronal and glial excitability through G protein subunits acting on membrane ion channels and second messengers such as diacylglycerol and cAMP. There are also two glial glutamate transporters and three neuronal transporters in the brain. Glutamate is the most abundant amino acid in the diet. There is no evidence for brain damage in humans resulting from dietary glutamate. A kainate analog, domoate, is sometimes ingested accidentally in blue mussels; this potent toxin causes limbic seizures, which can lead to hippocampal and related pathology and amnesia. Endogenous glutamate, by activating NMDA, AMPA or mGluR1 receptors, may contribute to the brain damage occurring acutely after status epilepticus, cerebral ischemia or traumatic brain injury. It may also contribute to chronic neurodegeneration in such disorders as amyotrophic lateral sclerosis and Huntington's chorea. In animal models of cerebral ischemia and traumatic brain injury, NMDA and AMPA receptor antagonists protect against acute brain damage and delayed behavioral deficits. Such compounds are undergoing testing in humans, but therapeutic efficacy has yet to be established. Other clinical conditions that may respond to drugs acting on glutamatergic transmission include epilepsy, amnesia, anxiety, hyperalgesia and psychosis.

Glutamate in life and death of retinal amacrine cells

1. Glutamate is the neurotransmitter released by bipolar cells at their synapses with amacrine cells. The amacrine cells express ionotropic (NMDA, AMPA and kainate) and metabotropic (mGluR1, mGluR2, mGluR4 and mGluR7) glutamate receptors and may take up glutamate from the synaptic cleft. 2. Activation of the ionotropic glutamate receptors increases the intracellular free calcium concentration ([Ca2+]i), owing to Ca2+ entry through the receptor-associated channels as well as through voltage-gated Ca2+ channels. The [Ca2+]i response to glutamate may be amplified by Ca2+-induced Ca2+ release from intracellular sources. 3. Activation of NMDA and non-NMDA glutamate receptors stimulates the release of GABA and acetylcholine from amacrine cells. GABA is released by a Ca2+-dependent mechanism and by reversal of the neurotransmitter transporter. 4. Excessive activation of glutamate receptors during ischemia leads to amacrine cell death. An increase in [Ca2+]i due to Ca2+ influx through NMDA and AMPA/kainate receptor channels is related to cell death in studies in vitro. In other studies, it was shown that nitric oxide may also take part in the process of cell damage during ischemia.