Nrf2-dependent persistent oxidative stress results in stress-induced vulnerability to depression.

Stressful life events produce a state of vulnerability to depression in some individuals. The mechanisms that contribute to vulnerability to depression remain poorly understood. A rat model of intense stress (social defeat (SD), first hit) produced vulnerability to depression in 40% of animals. Only vulnerable animals developed a depression-like phenotype after a second stressful hit (chronic mild stress). We found that this vulnerability to depression resulted from a persistent state of oxidative stress, which was reversed by treatment with antioxidants. This persistent state of oxidative stress was due to low brain-derived neurotrophic factor (BDNF) levels, which characterized the vulnerable animals. We found that BDNF constitutively controlled the nuclear translocation of the master redox-sensitive transcription factor Nrf2, which activates antioxidant defenses. Low BDNF levels in vulnerable animals prevented Nrf2 translocation and consequently prevented the activation of detoxifying/antioxidant enzymes, ultimately resulting in the generation of sustained oxidative stress. Activating Nrf2 translocation restored redox homeostasis and reversed vulnerability to depression. This mechanism was confirmed in Nrf2-null mice. The mice displayed high levels of oxidative stress and were inherently vulnerable to depression, but this phenotype was reversed by treatment with antioxidants. Our data reveal a novel role for BDNF in controlling redox homeostasis and provide a mechanistic explanation for post-stress vulnerability to depression while suggesting ways to reverse it. Because numerous enzymatic reactions produce reactive oxygen species that must then be cleared, the finding that BDNF controls endogenous redox homeostasis opens new avenues for investigation.

Nrf2-dependent persistent oxidative stress results in stress-induced vulnerability to depression.

This corrects the article DOI: 10.1038/mp.2016.144.

Oxidative stress-driven parvalbumin interneuron impairment as a common mechanism in models of schizophrenia.

Parvalbumin inhibitory interneurons (PVIs) are crucial for maintaining proper excitatory/inhibitory balance and high-frequency neuronal synchronization. Their activity supports critical developmental trajectories, sensory and cognitive processing, and social behavior. Despite heterogeneity in the etiology across schizophrenia and autism spectrum disorder, PVI circuits are altered in these psychiatric disorders. Identifying mechanism(s) underlying PVI deficits is essential to establish treatments targeting in particular cognition. On the basis of published and new data, we propose oxidative stress as a common pathological mechanism leading to PVI impairment in schizophrenia and some forms of autism. A series of animal models carrying genetic and/or environmental risks relevant to diverse etiological aspects of these disorders show PVI deficits to be all accompanied by oxidative stress in the anterior cingulate cortex. Specifically, oxidative stress is negatively correlated with the integrity of PVIs and the extracellular perineuronal net enwrapping these interneurons. Oxidative stress may result from dysregulation of systems typically affected in schizophrenia, including glutamatergic, dopaminergic, immune and antioxidant signaling. As convergent end point, redox dysregulation has successfully been targeted to protect PVIs with antioxidants/redox regulators across several animal models. This opens up new perspectives for the use of antioxidant treatments to be applied to at-risk individuals, in close temporal proximity to environmental impacts known to induce oxidative stress.

Tollip, an early regulator of the acute inflammatory response in the substantia nigra.

BACKGROUND: Tollip is a ubiquitously expressed protein, originally described as a modulator of the IL-1R/TLR-NF-κB signaling pathways. Although this property has been well characterized in peripheral cells, and despite some evidence of its expression in the central nervous system, the role of Tollip in neuroinflammation remains poorly understood. The present study sought to explore the implication of Tollip in inflammation in the substantia nigra pars compacta, the structure affected in Parkinson's disease. METHODS: We first investigated Tollip distribution in the midbrain by immunohistochemistry. Then, we addressed TLR4-mediated response by intra-nigral injections of lipopolysaccharide (LPS), a TLR4 agonist, on inflammatory markers in Tollip knockout (KO) and wild-type (WT) mice. RESULTS: We report an unexpectedly high Tollip immunostaining in dopaminergic neurons of the mice brain. Second, intra-nigral injection of LPS led to increased susceptibility to neuroinflammation in Tollip KO compared to Tollip WT mice. This was demonstrated by a significant increase of tumor necrosis factor alpha (TNF-α), interleukin 1 beta (IL-1ß), interleukin 6 (IL-6), and interferon gamma (IFN-γ) messenger RNA (mRNA) in the midbrain of Tollip KO mice upon LPS injection. Consistently, brain rAAV viral vector transduction with a nuclear factor kappa B (NF-κB)-inducible reporter gene confirmed increased NF-κB activation in Tollip KO mice. Lastly, Tollip KO mice displayed higher inducible NO synthase (iNOS) production, both at the messenger and protein level when compared to LPS-injected WT mice. Tollip deletion also aggravated LPS-induced oxidative and nitrosative damages, as indicated by an increase of 8-oxo-2'-deoxyguanosine and nitrotyrosine immunostaining, respectively. CONCLUSIONS: Altogether, these findings highlight a critical role of Tollip in the early phase of TLR4-mediated neuroinflammation. As brain inflammation is known to contribute to Parkinson's disease, Tollip may be a potential target for neuroprotection.

Glutathione deficit impairs myelin maturation: relevance for white matter integrity in schizophrenia patients.

Schizophrenia pathophysiology implies both abnormal redox control and dysconnectivity of the prefrontal cortex, partly related to oligodendrocyte and myelin impairments. As oligodendrocytes are highly vulnerable to altered redox state, we investigated the interplay between glutathione and myelin. In control subjects, multimodal brain imaging revealed a positive association between medial prefrontal glutathione levels and both white matter integrity and resting-state functional connectivity along the cingulum bundle. In early psychosis patients, only white matter integrity was correlated with glutathione levels. On the other side, in the prefrontal cortex of peripubertal mice with genetically impaired glutathione synthesis, mature oligodendrocyte numbers, as well as myelin markers, were decreased. At the molecular levels, under glutathione-deficit conditions induced by short hairpin RNA targeting the key glutathione synthesis enzyme, oligodendrocyte progenitors showed a decreased proliferation mediated by an upregulation of Fyn kinase activity, reversed by either the antioxidant N-acetylcysteine or Fyn kinase inhibitors. In addition, oligodendrocyte maturation was impaired. Interestingly, the regulation of Fyn mRNA and protein expression was also impaired in fibroblasts of patients deficient in glutathione synthesis. Thus, glutathione and redox regulation have a critical role in myelination processes and white matter maturation in the prefrontal cortex of rodent and human, a mechanism potentially disrupted in schizophrenia.

Childhood sexual and physical abuse: age at exposure modulates impact on functional outcome in early psychosis patients.

BACKGROUND: Evidence suggests a relationship between exposure to trauma during childhood and functional impairments in psychotic patients. However, the impact of age at the time of exposure has been understudied in early psychosis (EP) patients. METHOD: Two hundred and twenty-five patients aged 18-35 years were assessed at baseline and after 2, 6, 18, 24, 30 and 36 months of treatment. Patients exposed to sexual and/or physical abuse (SPA) were classified according to age at the time of first exposure (Early SPA: before age 11 years; Late SPA: between ages 12 and 15 years) and then compared to patients who were not exposed to such trauma (Non-SPA). The functional level in the premorbid phase was measured with the Premorbid Adjustment Scale (PAS) and with the Global Assessment of Functioning (GAF) scale and the Social and Occupational Functioning Assessment Scale (SOFAS) during follow-up. RESULTS: There were 24.8% of patients with a documented history of SPA. Late SPA patients were more likely to be female (p = 0.010). Comparison with non-SPA patients revealed that: (1) both Early and Late SPA groups showed poorer premorbid social functioning during early adolescence, and (2) while patients with Early SPA had poorer functional level at follow-up with lower GAF (p = 0.025) and lower SOFAS (p = 0.048) scores, Late SPA patients did not. CONCLUSION: Our results suggest a link between exposure to SPA and the later impairment of social functioning before the onset of the disease. EP patients exposed to SPA before age 12 may present long-lasting functional impairment, while patients exposed at a later age may improve in this regard and have a better functional outcome.

[Schizophrenia: genes, environment and neurodevelopment]. / Gènes, environnement et neurodéveloppement: te cas de la schizophrénie.

Psychoses are complex diseases resulting from the interaction between genetic vulnerability factors and various environmental risk factors during the brain development and leading to the emergence of the clinical phenotype at the end of adolescence. Among the mechanisms involved, a redox imbalance plays an important role, inducing oxidative stress damaging to developing neurons. As a consequence, the excitatory/inhibitory balance in cortex and the pathways connecting brain areas are both impaired. Childhood and adolescence appear as critical periods of vulnerability for deleterious environmental insults. Antioxidants, applied during the environmental impacts, should allow preventing these impairments as well as their clinical consequences.

Alpha rhythm and hypofrontality in schizophrenia.

OBJECTIVE: To reveal the EEG correlates of resting hypofrontality in schizophrenia (SZ). METHOD: We analyzed the whole-head EEG topography in 14 patients compared to 14 matched controls by applying a new parameterization of the multichannel EEG. We used a combination of power measures tuned for regional surface mapping with power measures that allow evaluation of global effects. RESULTS: The SZ-related EEG abnormalities include i) a global decrease in absolute EEG power robustly manifested in the alpha and beta frequency bands, and ii) a relative increase in the alpha power over the prefrontal brain regions against its reduction over the posterior regions. In the alpha band both effects are linked to the SZ symptoms measured with Positive and Negative Symptom Scales and to chronicity. CONCLUSION: As alpha activity is related to regional deactivation, our findings support the concept of hypofrontality in SZ and expose the alpha rhythm as a sensitive indicator of it.

Impulsivity in early psychosis: A complex link with violent behaviour and a target for intervention.

BACKGROUND: Violent behaviour (VB) occurs in first episode of schizophrenia and can have devastating impact both on victims and patients themselves. A better knowledge of the underlying mechanisms of VB may pave the way to preventive treatments. OBJECTIVES: 1) To explore the nature of the link between impulsivity and VB in early psychosis (EP) patients; 2) To explore the interactions between impulsivity and substance abuse, insight, and positive symptoms, the main dynamic risk factors of VB described to date. DESIGN AND METHODS: Post hoc analysis of data acquired in the frame of a 36-months EP cohort study. A total of 265 EP patients, aged 18 to 35, treated at TIPP (Treatment and early Intervention in Psychosis Program), at the Department of Psychiatry in Lausanne, Switzerland, were included in the study. Logistic regression analyzes were performed as well as mediation analysis and interaction analysis RESULTS: Our data suggest that impulsivity is a predictor of VB when analyzed independently and as part of a multi-factorial model. Impulsivity continues to differentiate violent patients from non-violent ones at the end of the program. In addition, the relationship between impulsivity and VB is not mediated by substance abuse. Finally, the effect of impulsivity on the probability of VB is potentiated by the interaction of different levels of insight and positive symptoms. CONCLUSIONS: Early intervention strategies in psychotic disorders should include evaluation of impulsivity considering it is linked to increased risk of VB and may respond to treatment.

Potential mechanisms of development-dependent adverse effects of the herbicide paraquat in 3D rat brain cell cultures.

Exposure to environmental toxicants during vulnerable windows of brain development is suspected to raise the prevalence for neurological dysfunctions at later stages in life. Differentiation processes and changes in morphology, as well as a lack of physiological barriers, might be reasons that render a developing brain more susceptible to neurotoxicants than an adult. However, also the intrinsic capacity of cells to combat toxicant induced cellular stress might differ between the immature- and mature brain. In order to study whether this intrinsic protection capacity differs between immature and maturated brain cells we chose to study the maturation-dependent adverse effects of the known neurotoxicant Paraquat Dichloride (PQ) in 3D rat brain cell cultures. This in vitro system consists of the major brain cell types - neurons, astrocytes, oligodendrocytes and microglia - and over the time in vitro cultured cells undergo differentiation and maturation into a tissue-like organization. PQ was applied repeatedly over ten days in the sub-micromolar range, and effects were evaluated on neurons and glial cells. We observed that despite a higher PQ-uptake in mature cultures, PQ-induced adverse effects on glutamatergic-, GABAergic- and dopaminergic neurons, as assessed by gene expression and enzymatic activity, were more pronounced in immature cultures. This was associated with a stronger astrogliosis in immature- as compared to mature cultures, as well as perturbations of the glutathione-mediated defense against oxidative stress. Furthermore we observed evidence of microglial activation only in mature cultures, whereas immature cultures appeared to down-regulate markers for neuroprotective M2-microglial phenotype upon PQ-exposure. Taken together our results indicate that immature brain cell cultures have less intrinsic capacity to cope with cellular stress elicited by PQ as compared to mature cells. This may render immature brain cells more susceptible to the adverse effects of PQ.

Synaptic plasticity impairment and hypofunction of NMDA receptors induced by glutathione deficit: relevance to schizophrenia.

Increasing evidence suggests that the metabolism of glutathione, an endogenous redox regulator, is abnormal in schizophrenia. Patients show a deficit in glutathione levels in the cerebrospinal fluid and prefrontal cortex and a reduction in gene expression of the glutathione synthesizing enzymes. We investigated whether such glutathione deficit altered synaptic transmission and plasticity in slices of rat hippocampus, with particular emphasis on NMDA receptor function. An approximately 40% decrease in brain glutathione levels was induced by s.c. administration of L-buthionine-(S,R)-sulfoximine, an inhibitor of glutathione synthesis. Such glutathione deficit did not affect the basal synaptic transmission, but produced several NMDA receptor-dependent and -independent effects. Glutathione deficit caused an increase in excitability of CA1 pyramidal cells. The paired-pulse facilitation was diminished in glutathione-depleted slices, in a manner that was independent of NMDA receptor activity. This suggests that lowering glutathione levels altered presynaptic mechanisms involved in neurotransmitter release. NMDA receptor-dependent long-term potentiation induced by high-frequency stimulation was impaired in glutathione-depleted slices. Pharmacologically isolated NMDA receptor-mediated field excitatory postsynaptic potentials were significantly smaller in L-buthionine-(S,R)-sulfoximine-treated than in control slices. Hypofunction of NMDA receptors under glutathione deficit was explained at least in part by an excessive oxidation of the extracellular redox-sensitive sites of the NMDA receptors. These results indicate that a glutathione deficit, like that observed in schizophrenics, alters short- and long-term synaptic plasticity and affects NMDA receptor function. Thus, glutathione deficit could be one causal factor for the hypofunction of NMDA receptors in schizophrenia.

Redox dysregulation, neuroinflammation, and NMDA receptor hypofunction: A "central hub" in schizophrenia pathophysiology?

Accumulating evidence points to altered GABAergic parvalbumin-expressing interneurons and impaired myelin/axonal integrity in schizophrenia. Both findings could be due to abnormal neurodevelopmental trajectories, affecting local neuronal networks and long-range synchrony and leading to cognitive deficits. In this review, we present data from animal models demonstrating that redox dysregulation, neuroinflammation and/or NMDAR hypofunction (as observed in patients) impairs the normal development of both parvalbumin interneurons and oligodendrocytes. These observations suggest that a dysregulation of the redox, neuroimmune, and glutamatergic systems due to genetic and early-life environmental risk factors could contribute to the anomalies of parvalbumin interneurons and white matter in schizophrenia, ultimately impacting cognition, social competence, and affective behavior via abnormal function of micro- and macrocircuits. Moreover, we propose that the redox, neuroimmune, and glutamatergic systems form a "central hub" where an imbalance within any of these "hub" systems leads to similar anomalies of parvalbumin interneurons and oligodendrocytes due to the tight and reciprocal interactions that exist among these systems. A combination of vulnerabilities for a dysregulation within more than one of these systems may be particularly deleterious. For these reasons, molecules, such as N-acetylcysteine, that possess antioxidant and anti-inflammatory properties and can also regulate glutamatergic transmission are promising tools for prevention in ultra-high risk patients or for early intervention therapy during the first stages of the disease.

Impaired fornix-hippocampus integrity is linked to peripheral glutathione peroxidase in early psychosis.

Several lines of evidence implicate the fornix-hippocampus circuit in schizophrenia. In early-phase psychosis, this circuit has not been extensively investigated and the underlying mechanisms affecting the circuit are unknown. The hippocampus and fornix are vulnerable to oxidative stress at peripuberty in a glutathione (GSH)-deficient animal model. The purposes of the current study were to assess the integrity of the fornix-hippocampus circuit in early-psychosis patients (EP), and to study its relationship with peripheral redox markers. Diffusion spectrum imaging and T1-weighted magnetic resonance imaging (MRI) were used to assess the fornix and hippocampus in 42 EP patients compared with 42 gender- and age-matched healthy controls. Generalized fractional anisotropy (gFA) and volumetric properties were used to measure fornix and hippocampal integrity, respectively. Correlation analysis was used to quantify the relationship of gFA in the fornix and hippocampal volume, with blood GSH levels and glutathione peroxidase (GPx) activity. Patients compared with controls exhibited lower gFA in the fornix as well as smaller volume in the hippocampus. In EP, but not in controls, smaller hippocampal volume was associated with high GPx activity. Disruption of the fornix-hippocampus circuit is already present in the early stages of psychosis. Higher blood GPx activity is associated with smaller hippocampal volume, which may support a role of oxidative stress in disease mechanisms.

Glutamate-induced homocysteic acid release from astrocytes: possible implication in glia-neuron signaling.

Glial cells synthesise neuroactive substances and release them upon neurotransmitter receptor activation. Homocysteic acid (HCA), an endogenous agonist for glutamatergic N-methyl-D-aspartate (NMDA) receptors, is predominantly localised in glial cells. We have previously demonstrated the release of HCA from mouse astrocytes in culture following activation of beta-adrenergic receptors. Moreover, a release of HCA has also been observed in vivo upon physiological stimulation of sensory afferents in the thalamus. Here we report the glutamate-induced release of HCA from astrocytes. The effect of glutamate was mediated by the activation of ionotropic (NMDA and non-NMDA) as well as by metabotropic receptors. In addition, the release of HCA was Ca(2+)- and Na(+)-dependent, and its mechanism involved the activation of the Na+/Ca(2+)-exchanger. Furthermore, we provide evidence for the presence of functional NMDA receptors on astrocytes, which are coupled to an intracellular Ca2+ increase via stimulation of the Na+/Ca(2+)-exchanger. Our data thus favour a participation of glial cells in excitatory neurotransmission and corroborate the role of HCA as a "gliotransmitter."

An animal model with relevance to schizophrenia: sex-dependent cognitive deficits in osteogenic disorder-Shionogi rats induced by glutathione synthesis and dopamine uptake inhibition during development.

Low glutathione levels have been observed in the prefrontal cortex and the cerebrospinal fluid of schizophrenic patients, possibly enhancing the cerebral susceptibility to oxidative stress. We used osteogenic disorder Shionogi mutant rats, which constitute an adequate model of the human redox regulation because both are unable to synthesize ascorbic acid. To study the long-term consequences of a glutathione deficit, we treated developing rats with L-buthionine-(S,R)-sulfoximine (BSO), an inhibitor of glutathione synthesis, and later investigated their behavior until adulthood. Moreover, some rats were treated with the dopamine uptake inhibitor GBR 12909 in order to elevate dopamine extracellular levels, thereby mimicking the dopamine hyperactivity proposed to be involved in schizophrenia. BSO and GBR 12909 alone or in combination minimally affected the development of spontaneous alternation or basic sensory and motor skills. A major effect of BSO alone or in combination with GBR 12909 was the induction of cataracts in both sexes, whereas GBR 12909 induced an elevation of body weight in females only. Sex and age-dependent effects of the treatments were observed in a test of object recognition. At postnatal day 65, whereas male rats treated with both BSO and GBR 12909 failed to discriminate between familiar and novel objects, females were not affected. At postnatal day 94, male object recognition capacity was diminished by BSO and GBR 12909 alone or in combination, whereas females were only affected by the combination of both drugs. Inhibition of brain glutathione synthesis and dopamine uptake in developing rats induce long-term cognitive deficits occurring in adulthood. Males are affected earlier and more intensively than females, at least concerning object recognition. The present study suggests that the low glutathione levels observed in schizophrenic patients may participate in the development of some of their cognitive deficits.

Differential effects of (D)- and (L)-homocysteic acid on the membrane potential of cat caudate neurons in situ.

The enantiomers of homocysteic acid have been applied by microiontophoresis to neurons of the cat caudate nucleus in situ. The (L)-enantiomer elicited a bursty firing pattern similar to the one caused by N-methyl-D-aspartate, but differing from the N-methyl-D-aspartate pattern inasmuch as (L)-homocysteate induced depolarization shifts were shorter and had a smaller amplitude. (L)-Homocysteate induced excitations could be strongly inhibited by the selective N-methyl-D-aspartate antagonist 2-amino-7-phosphonoheptanoic acid but they were less sensitive to this antagonist than N-methyl-D-aspartate itself. (D)-Homocysteate elicited a more regular firing pattern similar to the one caused by non-N-methyl-D-aspartate excitatory amino acids such as quisqualate. These excitations were only rarely inhibited by 2-amino-7-phosphonoheptanoic acid. Our results suggest that (L)-homocysteate, a transmitter candidate at central mammalian synapses, is a mixed excitatory amino acid agonist with a strong preference for N-methyl-D-aspartate receptors in the cat caudate nucleus, while (D)-homocysteate has a predominant action at non-N-methyl-D-aspartate excitatory amino acid receptors.

Release of the nitric oxide precursor, arginine, from the thalamus upon sensory afferent stimulation, and its effect on thalamic neurons in vivo.

The neurophysiology and neuroanatomy of the thalamus have been extensively studied in a variety of species and sensory systems. The identity of the neurotransmitter(s) which mediate the excitation from ascending sensory afferents on to thalamic relay neurons is, however, still unclear, although it appears to be a substance which is a ligand for excitatory amino acid receptors, as the responses of ventrobasal thalamus neurons to natural stimulation of somatosensory afferents arising from the mustachial vibrissae of the rat are mediated by ionotropic excitatory amino acid receptors, when stimulation is performed using an air-jet directed at the vibrissa receptor field. In an effort to determine the transmitter of these sensory afferents, we have attempted to detect the release of amino acids in the ventrobasal thalamus in vivo upon such stimuli. We have thus used a similar natural stimulation protocol, together with push-pull perfusion and recording in the ventrobasal thalamus, and we describe the release of the amino acid, arginine, in this brain area following physiological stimulation of afferents. Furthermore, we show that application of L-arginine on to thalamic relay neurons can facilitate sensory synaptic transmission, possibly via the synthesis of the diffusable messenger, free radical gas, nitric oxide. This may represent a novel, local positive-feedback, modulatory system which could enhance the responsiveness of thalamic neurons to sensory input.

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.

Release of endogenous amino acids, including homocysteic acid and cysteine sulphinic acid, from rat hippocampal slices evoked by electrical stimulation of Schaffer collateral-commissural fibres.

This study examined the release of endogenous amino acids from acute hippocampal slices, upon stimulation of the Schaffer collateral-commissural fibres. One-minute samples of superfusate were collected via a cannula placed over the CA1 stratum radiatum, and were analysed by reversed-phase high performance liquid chromatography. Evoked potentials were recorded to ascertain stimulation efficacy. Four minutes of continuous 50 Hz stimulation produced a tetrodotoxin-sensitive release of aspartate and glycine in the second minute of stimulation, as well as a tetrodotoxin-sensitive release of cysteine sulphinic acid, during stimulation and of homocysteic acid, following stimulation. Such 50 Hz stimulation also produced a tetrodotoxin-insensitive decrease in methionine levels, but no significant changes in any of the other 15 amino acids measured. Four minutes of continuous 1 Hz stimulation produced no changes in the levels of any of the amino acids measured, but four 600-ms trains of 100 Hz stimulation, which, unlike the 1 Hz stimulation, produced long-term potentiation, resulted in significant increases in levels of cysteine sulphinic acid and homocysteic acid, but not of any of the other amino acids measured. These results suggest that aspartate, glycine, homocysteic acid, and cysteine sulphinic acid play a role in synaptic transmission in the Schaffer collateral-commissural fibres, and that cysteine sulphinic acid and homocysteic acid may be released specifically by high-frequency stimulation.

Localization and release of homocysteic acid, an excitatory sulfur-containing amino acid.

In addition to the excitatory role played by the amino acid transmitters glutamate and aspartate in the central nervous system, their sulfur-containing analogues homocysteic acid (HCA) and cysteine sulfinic acid (CSA) may also play a similar role. HCA is released and taken up by rat CNS tissue; it excites neurons predominantly via NMDA receptors whenever present, and is neurotoxic. The pattern of HCA-like immunoreactivity in the rat indicates a localization of HCA mostly in glial elements, although its presence in nerve terminals and neuronal perikarya cannot be excluded. In the cerebellum of newborn and adult animals, the Bergmann glial cells and the astrocyte endfeet are immunoreactive, either in the presence or in the absence of climbing fibers. In the cortex, hippocampus, and retina, labeling is seen in both glial and neuronal elements. Excitatory signaling involving glial elements is discussed.