The neurotoxicity of sulfur-containing amino acids in energy-deprived rat hippocampal slices

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.

Inbred mice strain shows neurobehavioral changes when exposed to tannery effluent

The bovine leather processing (tanning industries) stands as a generating activity of potentially toxic waste. The emission of untreated effluents into the environment may cause serious harm to human and environmental health. Nevertheless, few studies have investigated the possible effects of intake of these effluents in experimental mammalian models. Thus, this study aimed to evaluate the neurobehavioral effects of chronic intake of different tannery effluent concentrations diluted with water (0.1, 1, and 5%) in male C57BL/6J mice. After 120 days of exposure, the animals were subjected to different behavioral tests, predictive of anxiety (elevated plus maze (EPM), open-field (OF), and neophobia test), depression (forced swim), and memory deficits (object recognition test). From the EPM test, it was observed that the mice exposed to 0.1, 1, and 5% of tannery effluents showed higher anxiety scores compared to the animals in the control group. However, the results of this study revealed no differences among the experimental groups in the proportion (percentage) of locomotion in the central quarters/total locomotion calculated (by OF), considered an indirect measure for anxiety. At neophobia test, all the animals exposed to chronic intake of tannery effluents showed higher latency time to start eating, which corresponds to an anxiogenic behavior. Regarding the forced swim test, it was observed that the animals exposed to tannery effluents had longer time in immobility behavior, suggesting a predictive behavior to depression. Finally, the object recognition test showed that the treatments did not cause damage to the animals' memory. The recognition rate of the new object did not differ among the experimental groups. Thus, it is concluded that male C57BL/6J mice (inbred strain) exposed to tannery effluents have predictive neurobehavioral changes of anxiety and depression, without memory deficit.

Stereoisomers of cysteine and its analogs Potential effects on chemo- and radioprotection strategies

The thiol-containing amino acid, cysteine, and its analogs are useful for a variety of protective applications, including protecting normal tissues against the unwanted side effects of cancer chemotherapeutic agents and radiation treatment. The protection can result from both direct action of the amino acid and/or after its conversion to glutathione (GSH), sulfate, or other sulfur-based protective substances. Unfortunately, high GSH levels have been implicated in the problematic development of tumor cells' resistance to therapy. Due to numerous differences in the metabolic processing of the cysteine stereoisomers, chemo- and radioprotective strategies might be developed using the D-form of the amino acid, which can participate in protection directly, but which cannot be used to support GSH biosynthesis. In this way, protection of normal tissue may be achieved, while the potential development of resistance in tumor cells is minimized. Greatly enhanced therapeutic efficacy of cancer treatment regimens may be the result.

The neuropsychiatry of inborn errors of metabolism

A number of metabolic disorders that affect the central nervous system can present in childhood, adolescence or adulthood as a phenocopy of a major psychiatric syndrome such as psychosis, depression, anxiety or mania. An understanding and awareness of secondary syndromes in metabolic disorders is of great importance as it can lead to the early diagnosis of such disorders. Many of these metabolic disorders are progressive and may have illness-modifying treatments available. Earlier diagnosis may prevent or delay damage to the central nervous system and allow for the institution of appropriate treatment and family and genetic counselling. Metabolic disorders appear to result in neuropsychiatric illness either through disruption of late neurodevelopmental processes (metachromatic leukodystrophy, adrenoleukodystrophy, GM2 gangliosidosis, Niemann-Pick type C, cerebrotendinous xanthomatosis, neuronal ceroid lipofuscinosis, and alpha mannosidosis) or via chronic or acute disruption of excitatory/inhibitory or monoaminergic neurotransmitter systems (acute intermittent porphyria, maple syrup urine disease, urea cycle disorders, phenylketonuria and disorders of homocysteine metabolism). In this manuscript we review the evidence for neuropsychiatric illness in major metabolic disorders and discuss the possible models for how these disorders result in psychiatric symptoms. Treatment considerations are discussed, including treatment resistance, the increased propensity for side-effects and the possibility of some treatments worsening the underlying disorder.

CaV3.2 is the major molecular substrate for redox regulation of T-type Ca2+ channels in the rat and mouse thalamus

Although T-type Ca(2+) channels in the thalamus play a crucial role in determining neuronal excitability and are involved in sensory processing and pathophysiology of epilepsy, little is known about the molecular mechanisms involved in their regulation. Here, we report that reducing agents, including endogenous sulfur-containing amino acid l-cysteine, selectively enhance native T-type currents in reticular thalamic (nRT) neurons and recombinant Ca(V)3.2 (alpha1H) currents, but not native and recombinant Ca(V)3.1 (alpha1G)- and Ca(V)3.3 (alpha1I)-based currents. Consistent with this data, T-type currents of nRT neurons from transgenic mice lacking Ca(V)3.2 channel expression were not modulated by reducing agents. In contrast, oxidizing agents inhibited all native and recombinant T-type currents non-selectively. Thus, our findings directly demonstrate that Ca(V)3.2 channels are the main molecular substrate for redox regulation of neuronal T-type channels. In addition, because thalamic T-type channels generate low-threshold Ca(2+) spikes that directly correlate with burst firing in these neurons, differential redox regulation of these channels may have an important function in controlling cellular excitability in physiological and pathological conditions and fine-tuning of the flow of sensory information into the central nervous system.

Hydrogen Sulfide Is a Mediator of Cerebral Ischemic Damage

BACKGROUND AND PURPOSE

We observed recently that elevated plasma cysteine levels are associated with poor clinical outcome in acute stroke patients. In a rat stroke model, cysteine administration increased the infarct volume apparently via its conversion to hydrogen sulfide (H2S). We therefore investigated the effects of H2S and the inhibition of its formation on stroke.

METHODS

Cerebral ischemia was studied in a rat stroke model created by permanent occlusion of the middle cerebral artery (MCAO). The resultant infarct volume was measured 24 hours after occlusion.

RESULTS

Administration of sodium hydrosulfide (NaHS, an H2S donor) significantly increased the infarct volume after MCAO. The NaHS-induced increase in infarct volume was abolished by the administration of dizolcilpine maleate (an N-methyl-d-aspartate receptor channel blocker). MCAO caused an increase in H2S level in the lesioned cortex as well as an increase in the H2S synthesizing activity. Administration of 4 different inhibitors of H2S synthesis reduced MCAO-induced infarct volume dose dependently. The potency of these inhibitors in effecting neuroprotection in vivo appeared to parallel their potency as inhibitors of H2S synthesis in vitro. It also appeared that most of the H2S synthesizing activity in the cortex results from the action of cystathionine beta-synthase.

CONCLUSIONS

The present results strongly suggest that H2S plays a part in cerebral ischemic damage after stroke. Inhibition of H2S synthesis should be investigated for its potential as a novel neuroprotective stroke therapy.

High Plasma Cyst(e)ine Level May Indicate Poor Clinical Outcome in Patients With Acute Stroke: Possible Involvement of Hydrogen Sulfide

Cysteine is known to cause neuronal cell death and has been reported to be elevated in brain ischemia, but it has not been studied in clinical stroke. In this study, we correlated plasma levels of cyst(e)ine with long-term clinical outcome at 3 months in acute stroke. Patients were classified into 3 groups at 3 months as follows: good outcome (Rankin 0-1, n = 11), poor outcome (Rankin 2-5, n = 20), and dead (n = 5). Their plasma cyst(e)ine levels within 24 hours of stroke onset were 61 +/- 12, 67 +/- 9, and 82 +/- 14 micromol/L (standard deviation), respectively. The correlation between early plasma cyst(e)ine levels and long-term clinical outcome assessed at 3 months is significant with p < 0.001. None of the other 4 amino acids studied showed any significant correlation. Cyst(e)ine was also significantly elevated in patients who had early stroke deterioration (p < 0.02). Dose-dependent administration of cysteine increased the infarct volume by approximately 30% in a rat stroke model. This effect of cysteine was abolished by aminooxyacetic acid, an inhibitor of the enzyme cystathionine beta-synthase that converts cysteine to hydrogen sulfide (H2S), indicating that this novel neuromodulator may be acting as a mediator of ischemic brain damage. Raised plasma cyst(e)ine in patients with stroke may reflect increased production of H2S in the brain and thus predispose to poor outcome in clinical stroke. Inhibition of H2S formation may therefore be a novel approach in acute stroke therapy.

Neurotoxicity induced by glutamate in glucose-deprived rat hippocampal slices is prevented by GMP

Guanosine-5'-monophosphate (GMP) was evaluated as a neuroprotective agent against the damage induced by glutamate in rat hippocampal slices submitted to glucose deprivation. In slices maintained under physiological conditions, glutamate (0.01 to 10 mM), Kainate, alpha-amino-3-hydroxi-5-methylisoxazole-propionic acid (AMPA), N-methyl-D-aspartate (NMDA), 1S,3R-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD), or L-2-amino-4-phosphonobutanoic acid (L-AP4) (100 microM) did not alter cell membrane permeability, as evaluated by lactate dehydrogenase (LDH) release assay. In slices submitted to glucose deprivation, GMP (from 0.5 mM) prevented LDH leakage and the loss of cell viability induced by 10 mM glutamate. LDH leakage induced by Kainate, AMPA, NMDA or 1S,3R-ACPD was fully prevented by 1 mM GMP. However, glutamate uptake was not altered in slices submitted to glucose deprivation and glutamate analogues. Glucose deprivation induced a significant decrease in ATP levels which was unchanged by addition of glutamate or GMP. Our results show that glucose deprivation decreases the energetic charge of cells, making hippocampal slices more susceptible to excitotoxicity and point to GMP as a neuroprotective agent acting as a glutamatergic antagonist.

l-Cysteine-induced brain damage in adult rats

The time-dependent brain damage induced in adult rats by a single dose of L-cysteine was examined morphologically. Five-week-old male Sprague-Dawley rats that received 1500 mg/kg of L-cysteine by intraperitoneal injection were examined at 12 and 24 h and 3, 7, and 14 days after administration. Pathological changes were seen in the cerebral and cerebellar cortex. Neuronal karyopyknosis was observed in the granular and molecular layers of the superficial cerebellar cortex at 12 h, and well-demarcated infarct-like lesions were seen with a widespread distribution in the cerebral cortex at 24 h. A large number of lipid phagocytes and glial cell proliferation were noted in the affected regions on days 3 to 14. The neuronal cell death observed in the cerebellar granular layer cells was demonstrated to be due to apoptosis by histopathological and ultrastructural examinations as well as by the terminal deoxyribonucleotide transferase-mediated dUTP nick-end labeling (TUNEL) method and agarose gel electrophoresis for DNA laddering. It was found that L-cysteine induced brain lesions mainly in the cerebral and cerebellar cortex in adult rats, in contrast to lesions in various regions as observed in neonatal rats. The histopathological findings reported here suggest that the pathogenesis of the brain damage induced by L-cysteine in adult rats differs from that in neonatal rats. It appears likely that L-cysteine-induced brain damage is secondary to impairment of blood flow or other unknown factors that are responsible for the subsequent development of brain lesions.

Mechanisms of L-cysteine neurotoxicity

We review here the possible mechanisms of neuronal degeneration caused by L-cysteine, an odd excitotoxin. L-Cysteine lacks the omega carboxyl group required for excitotoxic actions via excitatory amino acid receptors, yet it evokes N-methyl-D-aspartate (NMDA) -like excitotoxic neuronal death and potentiates the Ca2+ influx evoked by NMDA. Both actions are prevented by NMDA antagonists. One target for cysteine effects is thus the NMDA receptor. The following mechanisms are discussed now: (1) possible increase in extracellular glutamate via release or inhibition of uptake/degradation, (2) generation of cysteine alpha-carbamate, a toxic analog of NMDA, (3) generation of toxic oxidized cysteine derivatives, (4) chelation of Zn2+ which blocks the NMDA receptor-ionophore, (5) direct interaction with the NMDA receptor redox site(s), (6) generation of free radicals, and (7) formation of S-nitrosocysteine. In addition to these, we describe another new alternative for cytotoxicity: (8) generation of the neurotoxic catecholamine derivative, 5-S-cysteinyl-3,4-dihydroxyphenylacetate (cysdopac).

Net efflux of cysteine, glutathione and related metabolites from rat hippocampal slices during oxygen/glucose deprivation: dependence on gamma-glutamyl transpeptidase

Extracellular metabolism of the protective substance glutathione (gamma-glutamyl-cysteinyl-glycine) may generate cysteine, glycine, several gamma-glutamyl-containing dipeptides and possibly free glutamate, all of which could participate in neurotoxicity. In the present study, we have examined how blockage of gamma-glutamyl transpeptidase, the key enzyme in glutathione degradation, influences the extracellular concentrations of glutathione, cysteine and related metabolites during anoxia/aglycemia of rat hippocampal slices. The net efflux, i.e., the increase in extracellular concentration due to changes in release and/or uptake, of cysteine, cysteine sulfinate, gamma-glutamyl-glutamate, gamma-glutamyl-glutamine, glutathione, gamma-glutamyl-cysteine and glutamate increased as a result of anoxia/aglycemia. These increases in net efflux of cysteine, cysteine sulfinate, gamma-glutamyl-glutamate and gamma-glutamyl-glutamine were reduced or blocked by acivicin, an inhibitor of gamma-glutamyl transpeptidase. In contrast, acivicin caused an increase in both basal and anoxia/aglycemia-induced net efflux of glutathione whereas the basal and anoxia/aglycemia-induced efflux of glutamate was unchanged by acivicin treatment. The effect of acivicin on the efflux of gamma-glutamyl-cysteine was similar to that of glutathione although less pronounced. Addition of beta-mercaptoethanol to the incubation medium during and after 30 min of anoxia/aglycemia decreased the net efflux of cysteine sulfinate specifically, indicating that the increase in cysteine sulfinate during anoxia/aglycemia may be partly derived from the spontaneous oxidation of cysteine. The results suggest that gamma-glutamyl transpeptidase may be involved in the regulation of the extracellular concentrations of cysteine, several gamma-glutamyl-containing dipeptides and glutathione but not glutamate during ischemia.

Melatonin attenuates L-cysteine-induced seizures and lipid peroxidation in the brain of mice

The effect of melatonin, a potent free radical scavenger, on L-cysteine-induced seizures and lipid peroxidation was investigated in mice. When L-cysteine (1.25, or 5.0 mumol/animal) was injected intracerebroventricularly (i.c.v.) into mice, severe tonic seizures were observed for over 20 sec in 75% and 100% of the treated mice, respectively. However, when melatonin (20 or 100 mg/kg) was injected subcutaneously (sc) into mice 15 min before L-cysteine injection (1.25 mumol/animal, i.c.v.), the incidence of seizures was observed in only 35% and 20% of the treated mice, respectively. Furthermore, when L-cysteine (1.25 or 5.0 mumol/animal, i.c.v.) was injected into mice, lipid peroxidation in whole brain 20 min after injection was significantly increased by 56% or 67% as compared to that of the control. However, when the seizures induced by L-cysteine (1.25 mumol/animal) were abolished by preadministration of melatonin, the increased lipid peroxidation induced by L-cysteine was prevented. These results suggest that there may be a positive correlation between free radical formation and seizures induced by L-cysteine and that melatonin affords protection against the seizures as well as against the associated lipid peroxidation.

Preventive effect of N(G)-nitro-L-arginine against L-cysteine-induced seizures in mice

The effect of N(G)-nitro-L-arginine (NNA), an inhibitor of nitric oxide (NO) synthase on L-cysteine- induced neurotoxicity was investigated in mice. When L-cysteine (1, 2.5, 5 or 10 micromol/brain) was injected intracerebroventricular (i.c.v.) in mice, severe tonic seizures were observed for over 20 s in the treated mice in a dose-dependent manner. However, the tonic seizures induced by L-Cysteine were prevented by pretreatment with N(G)-nitro-L-arginine. Although L-cysteine (0.5, 1, 2.5, 5, 10 micromol/brain, i.c.v.) also caused a wild running (WR), NNA did not affect behavior. These results suggest that an overproduction of NO may be involve in the development of tonic seizures but not WR induced by L-cysteine.

Biological chemistry of thiols in the vasculature and in vascular-related disease

For all their similarities in structure and common chemistry, the functions of the amino thiols in vascular biology are remarkably different. This review details the basic chemistry of sulfhydryls that dictates their functions in health and disease. In addition, the biochemistry and metabolism of each thiol are outlined, in an effort to highlight its specific contributions to the normal biology and physiology of blood vessels and to the pathogenesis of vascular-related disease.

Hypoxia, excitotoxicity, and neuroprotection in the hippocampal slice preparation

The excitotoxic hypothesis postulates a central role for the excitatory amino acids (EAAs) and their receptors in the neuronal damage that ensues cerebral ischemia-hypoxia and numerous other brain disorders. A major premise of the excitotoxic hypothesis is that neuronal protection can be achieved via blockade of EAA receptors with specific antagonists. This paper describes the use of the rat hippocampal slice preparation in the evaluation of various EAAs and their analogues for their potency as excitotoxins (agonists) and antagonists of the NMDA and the kainate/AMPA glutamate receptor subtypes. The hypersensitivity of hypoxic hippocampal slices to the presence of excitotoxins provided us with an inexpensive, sensitive tool to distinguish between structurally similar compounds. Moreover, these studies indicate that hypoxic neuronal damage cannot solely result from an excitotoxic mechanism; the involvement of voltage-dependent calcium channels in such damage is likely, as is evident from experiments performed in calcium-depleted medium and with the non-competitive NMDA antagonist MK-801. At sub-toxic doses, quinolinate, a tryptophan metabolite implicated in Huntington's disease, appears to be a strong potentiator of the toxicity of all excitotoxins tested.

Protection by MK-801 against hypoxia-, excitotoxin-, and depolarization-induced neuronal damage in vitro

Exposure of rat hippocampal slices to 12-min hypoxia produced only mild neuronal damage, as 72% of all slices recovered their CA1-evoked population spike following a 30-min recovery period. However, when this hypoxic insult was administered in the presence of 2.5 microM kainate or AMPA, only 6 and 15% of the slices, respectively, recovered their neuronal function. This enhancement of hypoxic damage by kainate could be attenuated in a dose-dependent fashion by the kainate/AMPA antagonist GYKI 52466 but not by the competitive NMDA antagonist APV. Unexpectedly, the noncompetitive NMDA antagonist MK-801 also attenuated the kainate- and AMPA-enhanced hypoxic neuronal damage and was more efficacious than GYKI 52466. Considering (1) the ability of MK-801 to antagonize hypoxic neuronal damage in the absence or the presence of NMDA, kainate or AMPA; (2) the antihypoxic effect of MK-801 in the presence of APV + 7-chlorokynurenate, a pairing that supposedly blocks MK-801 binding to the NMDA receptor; (3) the ability of MK-801 to protect hippocampal slices against brain damage induced by depolarization + excitotoxin (50 mM KCl + mM glutamate for 60 min); and (4) the ability of diltiazem, an L-type calcium channel blocker, to protect hippocampal slices against hypoxic neuronal damage, we conclude that the mode of action of MK-801 cannot be explained by its NMDA receptor antagonistic properties alone. A possible blockade of Ca2+ channels, most likely of the L-type, by MK-801 should be considered along with other mechanisms.

Kainate toxicity in energy-compromised rat hippocampal slices: differences between oxygen and glucose deprivation

The effects of kainate (KA) on the recovery of neuronal function in rat hippocampal slices after hypoxia or glucose deprivation (GD) were investigated and compared to those of (R,S)-alpha-amino-3-hydroxy-5-methyl-4- isoxazoleproprionate (AMPA). KA and AMPA were found to be more toxic than either N-methyl-D-aspartate (NMDA), quinolinate, or glutamate, both under normal conditions and under states of energy deprivation. Doses as low as 1 microM KA or AMPA were sufficient to significantly reduce the recovery rate of neuronal function in slices after a standardized period of hypoxia or GD. The enhancement of hypoxic neuronal damage by both agonists could be partially blocked by the antagonist kynurenate, by the NMDA competitive antagonist AP5, and by elevating [Mg2+] in or by omitting Ca2+ from the perfusion medium. The AMPA antagonist glutamic acid diethyl ester was ineffective in preventing the enhanced hypoxic neuronal damage by either KA or AMPA. The antagonist of the glycine modulatory site on the NMDA receptor, 7-chlorokynurenate, did not block the KA toxicity but was able to block the toxicity of AMPA. 2,3-Dihydroxyquinoxaline completely blocked the KA- and AMPA-enhanced hypoxic neuronal damage. The KA-enhanced, GD-induced neuronal damage was prevented by Ca2+ depletion and partially antagonized by kynurenate but not by AP5 or elevated [Mg2+]. The results of the present study indicate that the KA receptor is involved in the mechanism of neuronal damage induced by hypoxia and GD, probably allowing Ca2+ influx and subsequent intracellular Ca2+ overload.(ABSTRACT TRUNCATED AT 250 WORDS)