Glutamate-induced release of the nitric oxide precursor, arginine, from glial cells

Monosodium glutamate altered renal architecture and modulated expression of NMDA-R, eNOS, and nNOS in normotensive and hypertensive rats

Monosodium glutamate (MSG) administration has been shown to pronounce hypertension and oxidative status with increased renal blood flow (RBF), however, the precise mechanisms of action have never been demonstrated. This study aimed to investigate the MSG action by studying the alteration in renal architecture and specific protein expression in 2-kidney-1-clip hypertensive comparing to sham operative normotensive rats. The administered doses of MSG were 80, 160, or 320 mg/kg BW daily for 8 weeks. Using routine chemical staining, the congestion of glomerular capillaries, a lesser renal corpuscles and glomeruli size, a widen Bowman capsule's space, an increase in mesangial cell proliferation and mesangial matrix, renal interstitial fibrosis, focal cloudy swelling of renal tubular epithelial cells were observed. Immunological study revealed an increase in the expression of N-methyl-D-aspartate receptor (NMDA-R) and endothelial nitric oxide synthase (eNOS) but a decrease in neuronal NOS (nNOS). It is suggested that MSG may upregulate the NMDA-R levels which responsible for the oxidative stress, glomerular injury, and renal interstitial fibrosis. The NMDA-R may also stimulate eNOS overexpression which resulted in renal microvascular dilatation, a raise in RBF and GFR, and natriuresis and diuresis promotion. Long-term exposure of MSG may trigger adaptation of tubuloglomerular feedback through nNOS downregulation.

Silva H, P. de Carvalho R, Ventura ALM, Calaza KC, Silveira MS, Kubrusly RCC, de Melo Reis RA. The Healthy and Diseased Retina Seen through Neuron-Glia Interactions

The retina is the sensory tissue responsible for the first stages of visual processing, with a conserved anatomy and functional architecture among vertebrates. To date, retinal eye diseases, such as diabetic retinopathy, age-related macular degeneration, retinitis pigmentosa, glaucoma, and others, affect nearly 170 million people worldwide, resulting in vision loss and blindness. To tackle retinal disorders, the developing retina has been explored as a versatile model to study intercellular signaling, as it presents a broad neurochemical repertoire that has been approached in the last decades in terms of signaling and diseases. Retina, dissociated and arranged as typical cultures, as mixed or neuron- and glia-enriched, and/or organized as neurospheres and/or as organoids, are valuable to understand both neuronal and glial compartments, which have contributed to revealing roles and mechanisms between transmitter systems as well as antioxidants, trophic factors, and extracellular matrix proteins. Overall, contributions in understanding neurogenesis, tissue development, differentiation, connectivity, plasticity, and cell death are widely described. A complete access to the genome of several vertebrates, as well as the recent transcriptome at the single cell level at different stages of development, also anticipates future advances in providing cues to target blinding diseases or retinal dysfunctions.

HSP70 protects rats and hippocampal neurons from central nervous system oxygen toxicity by suppression of NO production and NF-κB activation

During diving, central nervous system oxygen toxicity may cause drowning or barotrauma, which has dramatically limited the working benefits of hyperbaric oxygen in underwater operations and clinical applications. The aim of this study is to understand the effects and the underlying mechanism of heat shock protein 70 on central nervous system oxygen toxicity and its mechanisms in vivo and in vitro. Rats were given geranylgeranylacetone (800 mg/kg) orally to induce hippocampal expression of heat shock protein 70 and then treated with hyperbaric oxygen. The time course of hippocampal heat shock protein 70 expression after geranylgeranylacetone administration was measured. Seizure latency and first electrical discharge were recorded to evaluate the effects of HSP70 on central nervous system oxygen toxicity. Effects of inhibitors of nitric oxide synthase and nuclear factor-κB on the seizure latencies and changes in nitric oxide, nitric oxide synthase, and nuclear factor-κB levels in the hippocampus tissues were examined. In cell experiments, hippocampal neurons were transfected with a virus vector carrying the heat shock protein 70 gene (H3445) before hyperbaric oxygen treatment. Cell viability, heat shock protein 70 expression, nitric oxide, nitric oxide synthase, and NF-κB levels in neurons were measured. The results showed that heat shock protein 70 expression significantly increased and peaked at 48 h after geranylgeranylacetone was given. Geranylgeranylacetone prolonged the first electrical discharge and seizure latencies, which was reversed by neuronal nitric oxide synthase, inducible nitric oxide synthase and NF-κB inhibitors. Nitric oxide, nitric oxide synthase, and inducible nitric oxide synthase levels in the hippocampus were significantly increased after hyperbaric oxygen exposure, but reversed by geranylgeranylacetone, while heat shock protein 70 inhibitor quercetin could inhibit this effect of geranylgeranylacetone. In the in vitro study, heat shock protein 70-overexpression decreased the nitric oxide, nitric oxide synthase, and inducible nitric oxide synthase levels as well as the cytoplasm/nucleus ratio of nuclear factor-κB and protected neurons from hyperbaric oxygen-induced cell injury. In conclusion, overexpression of heat shock protein 70 in hippocampal neurons may protect rats from central nervous system oxygen toxicity by suppression of neuronal nitric oxide synthase and inducible nitric oxide synthase-mediated nitric oxide production and translocation of nuclear factor-κB to nucleus. Impact statement Because the pathogenesis of central nervous system oxygen toxicity (CNS-OT) remains unclear, there are few interventions available. To develop an efficient strategy against CNS-OT, it is necessary to understand its pathogenesis and in particular, the relevant key factors involved. This study examined the protective effects of heat shock protein 70 (HSP70) on CNS-OT via in vivo and in vitro experiments. Our results indicated that overexpression of HSP70 in hippocampal neurons may protect rats from CNS-OT by suppression of nNOS and iNOS-mediated NO production and the activation of NF-κB. These findings contribute to clarification of the role of HSP70 in CNS-OT and provide us a potential novel target to prevent CNS-OT. Clarification of the involvement of NO, NOS and NF-κB provides new insights into the mechanism of CNS-OT and may help us to develop new approach against it by interfering these molecules.

Insulin transport into the brain

While there is a growing consensus that insulin has diverse and important regulatory actions on the brain, seemingly important aspects of brain insulin physiology are poorly understood. Examples include: what is the insulin concentration within brain interstitial fluid under normal physiologic conditions; whether insulin is made in the brain and acts locally; does insulin from the circulation cross the blood-brain barrier or the blood-CSF barrier in a fashion that facilitates its signaling in brain; is insulin degraded within the brain; do privileged areas with a "leaky" blood-brain barrier serve as signaling nodes for transmitting peripheral insulin signaling; does insulin action in the brain include regulation of amyloid peptides; whether insulin resistance is a cause or consequence of processes involved in cognitive decline. Heretofore, nearly all of the studies examining brain insulin physiology have employed techniques and methodologies that do not appreciate the complex fluid compartmentation and flow throughout the brain. This review attempts to provide a status report on historical and recent work that begins to address some of these issues. It is undertaken in an effort to suggest a framework for studies going forward. Such studies are inevitably influenced by recent physiologic and genetic studies of insulin accessing and acting in brain, discoveries relating to brain fluid dynamics and the interplay of cerebrospinal fluid, brain interstitial fluid, and brain lymphatics, and advances in clinical neuroimaging that underscore the dynamic role of neurovascular coupling.

Role of nitric oxide in the regulation of motor function. An overview of behavioral, biochemical and histological studies in animal models

A compelling body of evidence suggests that nitric oxide (NO), a unique gaseous neurotransmitter and neuromodulator plays a key role in the regulation of motor function. Recently, the interest of researchers concentrates on the NO - soluble guanylyl cyclase (sGC) - cyclic GMP (cGMP) signaling pathway in the striatum as a new target for the treatment of Parkinson's disease (PD). The aim of the study is to review the available literature referring to the role of NO in the integration of basal ganglia functions. First, attention has been focused on behavioral effects of NO donors and neuronal nitric oxide synthase (nNOS) inhibitors in the modulation of motor behavior. Then, disturbances in the nitrergic neurotransmission in PD and its 6-OHDA animal model have been presented. Moreover, the most current data demonstrating the contribution of both dopamine and glutamate to the regulation of NO biosynthesis in the striatum have been analyzed. Finally, the role of NO in the tonic and phasic dopamine release as well as in the regulation of striatal output pathways also has been discussed.

Immunohistochemical evidence for impaired nitric oxide signaling of the locus coeruleus in bipolar disorder

Nitric oxide (NO) is an important messenger in brain signaling and influences the balance of monoaminergic and glutamatergic neurotransmission. Alterations of NO signaling are thought to play a crucial role in the pathophysiology of mood disorders. The locus coeruleus (LC) comprises the largest group of norepinephrine containing neurons in the mammalian brain. These norepinephrinergic LC neurons are able to generate NO. Immunohistochemical staining of neuronal nitric oxide synthase (nNOS)-immunoreactive (ir) neurons was performed in the LC of the brains of 10 patients with bipolar I disorder (BD), 8 patients with major depressive disorder (MDD) and 16 control cases (C). Analysis of variance (ANOVA) revealed significant differences between the groups, and post hoc tests indicated a lower nNOS-ir neuron number in bipolar patients than in controls (left -34%, right -17%). The total number of Nissl-stained LC neurons showed no changes between major depressive disorder patients, bipolar patients and controls. In the mood disorder patients, illness duration correlated negatively with nNOS-ir neuronal number (r=-0.74, p=0.002). A reduced relative amount of NO in the LC of bipolar patients is likely a result of a compensation for increased glutamatergic activity. The current data on nNOS suggest a dysregulation of the nitrergic system in bipolar disorder. Future studies may clarify the potential role of glial cells in the context of the described nNOS deficit.

The metabolic coupling of arginine metabolism to nitric oxide generation by astrocytes

Arginine, the only known precursor of nitric oxide, enters the brain parenchyma from the blood through the endothelial cells or from the cerebral spinal fluid through the ependymal cells. Astrocytes, whose processes abut the endothelium and ependymum, take up arginine through cationic amino acid transporters and release arginine through this transport system to the synapses that astrocytes shield. Some of these synapses are excitatory, and liberate glutamate into the synaptic cleft. Glutamate induces arginine release from astrocytes, making it available to the neuron. Neurons can take up arginine to be used in nitric oxide-mediated processes, such as neurotransmission. Thus, neural and nonneural cells act in concert to affect neuron physiology in an elegantly integrated system. This review focuses on the components of the interaction between astrocytes and neurons in nitric oxide biology.

Inhibition of protein synthesis by activation of NMDA receptors in cultured retinal cells: a new mechanism for the regulation of nitric oxide production

The synthesis of nitric oxide (NO) is limited by the intracellular availability of L-arginine. Here we show that stimulation of NMDA receptors promotes an increase of intracellular L-arginine which supports an increase in the production of NO. Although L-[3H]arginine uptake measured in cultured chick retina cells incubated in the presence of cycloheximide (CHX, a protein synthesis inhibitor) was inhibited approximately 75% at equilibrium, quantitative thin-layer chromatography analysis showed that free intracellular L-[3H]arginine was six times higher in CHX-treated than in control cultures. Extracellular L-[3H]citrulline levels increased threefold in CHX-treated groups, an effect blocked by NG-nitro-L-arginine, a NO synthase (NOS) inhibitor. NMDA promoted a 40% increase of free intracellular L-[3H]arginine in control cultures, an effect blocked by the NMDA antagonist 2-amino 5-phosphonovaleric acid. In parallel, NMDA promoted a reduction of 40-50% in the incorporation of 35[S]methionine or L-[3H]arginine into proteins. Western blot analysis revealed that NMDA stimulates the phosphorylation of eukaryotic elongation factor 2 (eEF2, a factor involved in protein translation), an effect inhibited by (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK801). In conclusion, we have shown that the stimulation of NMDA receptors promotes an inhibition of protein synthesis and a consequent increase of an intracellular L-arginine pool available for the synthesis of NO. This effect seems to be mediated by activation of eEF2 kinase, a calcium/calmodulin-dependent enzyme which specifically phosphorylates and blocks eEF2. The results raise the possibility that NMDA receptor activation stimulates two different calmodulin-dependent enzymes (eEF2 kinase and NOS) reinforcing local NO production by increasing precursor availability together with NOS catalytic activity.

Induction of NO synthase and glial acidic fibrillary protein in astrocytes in the temporal cortex of the rat with audiogenic epileptiform reactions

The localizations of NADPH-diaphorase (NADPH-d), inducible NO synthase (iNOS), and glial acid fibrillary protein (GFAP) in astrocytes of the temporal cortex were studied in Krushinskii-Molodkina rats, which are genetically predisposed to audiogenic convulsive seizures. Convulsive reactions were induced in rats by three exposures to acoustic stimuli. Controls consisted of Wistar rats and Krushinskii-Molodkina rats not subjected to acoustic stimulation, these not developing convulsive reactions. The neocortex of animals with audiogenic convulsions consistently showed foci of brain tissue damage. Foci, of diameter 300-400 microm, were located in layers III-V and were groupings of NADPH-d-positive astrocytes; these were seen in both hemispheres. Astrocytes in foci of damage expressed iNOS and had elevated GFAP levels. The numbers of GFAP-immunopositive cells were increased by 25-37% in damage foci as compared with levels in controls and undamaged areas of the cortex. The induction of NO synthase and GFAP in astrocytes seen here indicates the involvement of glia in compensatory NO-dependent mechanisms formed in damage foci in response to audiogenic convulsive seizures.

Neuronal and inducible nitric oxide synthase expression in the rat cerebellum following portacaval anastomosis

In order to determine the role of neuronal nitric oxide synthase (nNOS) and inducible nitric oxide synthase (iNOS) in the pathogenesis of experimental hepatic encephalopathy (HE), the expression of both was analyzed in the cerebellum of rats 1 month and 6 months after performing portacaval anastomosis (PCA). In control cerebella, nNOS immunoreactivity was mainly observed in the molecular layer (ML), whereas the Purkinje cells did not express nNOS. However, nNOS expression was detected in the Purkinje cells at 1 month after PCA, correlating with a decrease in nNOS expression in the ML--part of an overall reduction in cerebellar nNOS concentrations (as determined by Western blotting). At 6 months post-PCA, a significant increase in nNOS expression was observed in the ML, as well as increased nNOS immunoreactivity in the Purkinje cells. nNOS immunoreactivity was also observed in the Bergmann glial cells of PCA-treated rats. While no immunoreactivity for iNOS was seen in the cerebella of control rats, iNOS immunoreactivity was significantly induced in the cerebellum 1 month after PCA. In addition, the expression of iNOS was greater at 6 months than at 1 month post-PCA. Immunohistochemical analysis revealed this iNOS to be localized in the Purkinje cells and Bergmann glial cells. The induction of iNOS in astroglial cells has been associated with pathological conditions. Therefore, the iNOS expression observed in the Bergmann glial cells might play a role in the pathogenesis of HE, the harmful effects of PCA being caused by them via the production of excess nitric oxide. These results show that nNOS and iNOS are produced in the Purkinje cells and Bergmann glial cells following PCA, implicating nitric oxide in the pathology of HE.

Translational neurochemical research in acute human brain injury: the current status and potential future for cerebral microdialysis

Microdialysis (MD) was introduced as an intracerebral sampling method for clinical neurosurgery by Hillered et al. and Meyerson et al. in 1990. Since then MD has been embraced as a research tool to measure the neurochemistry of acute human brain injury and epilepsy. In general investigators have focused their attention to relative chemical changes during neurointensive care, operative procedures, and epileptic seizure activity. This initial excitement surrounding this technology has subsided over the years due to concerns about the amount of tissue sampled and the complicated issues related to quantification. The interpretation of mild to moderate MD fluctuations in general remains an issue relating to dynamic changes of the architecture and size of the interstitial space, blood-brain barrier (BBB) function, and analytical imprecision, calling for additional validation studies and new methods to control for in vivo recovery variations. Consequently, the use of this methodology to influence clinical decisions regarding the care of patients has been restricted to a few institutions. Clinical studies have provided ample evidence that intracerebral MD monitoring is useful for the detection of overt adverse neurochemical conditions involving hypoxia/ischemia and seizure activity in subarachnoid hemorrhage (SAH), traumatic brain injury (TBI), thromboembolic stroke, and epilepsy. There is some data strongly suggesting that MD changes precede the onset of secondary neurological deterioration following SAH, hemispheric stroke, and surges of increased ICP in fulminant hepatic failure. These promising investigations have relied on MD-markers for disturbed glucose metabolism (glucose, lactate, and pyruvate) and amino acids. Others have focused on trying to capture other important neurochemical events, such as excitotoxicity, cell membrane degradation, reactive oxygen species (ROS) and nitric oxide (NO) formation, cellular edema, and BBB dysfunction. However, these other applications need additional validation. Although these cerebral events and their corresponding changes in neurochemistry are important, other promising MD applications, as yet less explored, comprise local neurochemical provocations, drug penetration to the human brain, MD as a tool in clinical drug trials, and for studying the proteomics of acute human brain injury. Nevertheless, MD has provided new important insights into the neurochemistry of acute human brain injury. It remains one of very few methods for neurochemical measurements in the interstitial compartment of the human brain and will continue to be a valuable translational research tool for the future. Therefore, this technology has the potential of becoming an established part of multimodality neuro-ICU monitoring, contributing unique information about the acute brain injury process. However, in order to reach this stage, several issues related to quantification and bedside presentation of MD data, implantation strategies, and quality assurance need to be resolved. The future success of MD as a diagnostic tool in clinical neurosurgery depends heavily on the choice of biomarkers, their sensitivity, specificity, and predictive value for secondary neurochemical events, and the availability of practical bedside methods for chemical analysis of the individual markers. The purpose of this review was to summarize the results of clinical studies using cerebral MD in neurosurgical patients and to discuss the current status of MD as a potential method for use in clinical decision-making. The approach was to focus on adverse neurochemical conditions in the injured human brain and the MD biomarkers used to study those events. Methodological issues that appeared critical for the future success of MD as a routine intracerebral sampling method were addressed.

Release of arginine, glutamate and glutamine in the hippocampus of freely moving rats: Involvement of nitric oxide

Using in vivo microdialysis, we have monitored the release of three amino acids (arginine, glutamate and glutamine) in the hippocampus of freely moving rats in response to various drugs. In response to N-methyl-d-aspartate (NMDA) infusion, extracellular glutamate was increased, glutamine was decreased and arginine remained unchanged. By contrast, alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (AMPA) elicited an increase in arginine release but had no effect on either glutamate or glutamine. When S-nitroso-N-acetylpenicillamine (SNAP), a nitric oxide (NO) donor, was infused into the hippocampus, an increase in glutamate, a decrease in glutamine and no change in arginine were recorded. The effect of SNAP on extracellular glutamine levels was reversed by prior infusion of the guanylate cyclase inhibitor oxadiazolo[4,3-alpha]quinoxalin-1-one (ODQ), however its effect on glutamate release was unchanged. Interestingly, SNAP was found to promote the release of arginine in the presence of ODQ. We also assessed the effect of two nitric oxide synthase inhibitors, N-nitro-l-arginine methylester (l-NAME) and 7-nitroindazole (7-NI), on the release of these amino acids. l-NAME was found to increase arginine and glutamate levels but decrease those of glutamine. In contrast, 7-NI reduced the release of all three amino acids. The results presented here confirm some but not all of the findings previously obtained using in vitro preparations. In addition, they suggest that complex relationships exist between the release of these amino acids, and that endogenous NO plays an important role in regulating their release.

Low nNOS protein in the locus coeruleus in major depression

Disruptions of glutamatergic and noradrenergic signaling have been postulated to occur in depressive disorders. Glutamate provides excitatory input to the noradrenergic locus coeruleus (LC). In this study, the location of immunoreactivity against neuronal nitric oxide synthase (nNOS), an intracellular mediator of glutamate receptor activation, was examined in the normal human LC, and potential changes in nNOS immunoreactivity that might occur in major depression were evaluated. Tissue containing LC, and a non-limbic, LC projection area (cerebellum) was obtained from 11 to 12 matched pairs of subjects with major depression and control subjects lacking major psychiatric diagnoses. In the LC region, nNOS immunoreactivity was found in large neuromelanin-containing neurons, small neurons lacking neuromelanin, and glial cells. Levels of nNOS immunoreactivity were significantly lower in the LC (- 44%, p < 0.05), but not in the cerebellum, when comparing depressed with control subjects. nNOS levels were positively correlated with brain pH values in depressed, but not control, subjects in both brain regions. Low levels of nNOS in the LC may reflect altered excitatory input to this nucleus in major depression. However, pH appears to effect preservation of nNOS immunoreactivity in subjects with depression. This factor may contribute, in part, to low levels of nNOS in depression.

Arginine release from rat cerebellar astrocytes: autocrine roles for glutamate and nitric oxide?

In this study we have investigated the relationship between glutamate and arginine release from cultured cerebellar astrocytes. We found that the nitric oxide (NO) donor S-nitroso-N-acetylpenicillamine (SNAP) promoted the release of both amino acids in a concentration-dependent manner, and that these responses were partially reversed by a guanylate cyclase inhibitor. Application of the non-NMDA glutamate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) resulted in a 60% reduction in basal arginine release but no change in that of glutamate. This effect was not overcome by the subsequent addition of SNAP despite a two-fold increase in glutamate release. Incubation with the nitric oxide synthase (NOS) inhibitor N-nitro-L-arginine methyl ester (L-NAME) elicited 40 and 60% reductions in the basal release of glutamate and arginine, respectively. Basal release of both amino acids was restored by the addition of SNAP. We conclude that glutamate released from cerebellar astrocytes in response to increased levels of extracellular NO acts in an autocrine manner to promote arginine release via the activation of non-NMDA receptors. In addition, our data suggest that basal glutamate release is regulated to some extent by tonic NO synthesis in these cells.

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."

Selective amino acids changes in the medial and lateral preoptic area in the formalin test in rats

A combination of microdialysis in freely moving rats and capillary zone electrophoresis coupled to laser induced fluorescence detection was used to measure extracellular concentrations of amino acid neurotransmitters in different hypothalamic areas during noxious stimulation. Arginine, glutamate and aspartate were monitored every 30 s before and after a s.c. injection of formalin (5%, 50 microl) or saline (0.9%) in the right hind paw. In the medial and lateral preoptic area, calcium and nerve impulse dependent increases of arginine, glutamate and aspartate were observed during the first 2 min after formalin injection. However, amino acid changes were not detected in the lateral hypothalamus or in the ventromedial nucleus when compared with pre-injection levels or with the levels from animals injected with saline in the hind paw. Flinching behavior was also scored during the first 10 min following the formalin or saline injection. Flinching frequency was maximum at minute 2 after formalin injection, whereas saline injection did not elicited any flinching behavior. These results show that nociceptive stimulation induces rapid and differential amino acids changes in discrete areas of the hypothalamus that can be associated with pain-related behavior.

Comparison of microdialysate arginine and glutamate levels in severely head-injured patient

L-arginine concentrations in the brain are of interest following TBI because L-arginine is the immediate precursor of nitric oxide (NO). In addition, in vitro studies suggest that glutamate, which is a mediator of secondary injury after TBI, may stimulate release of arginine from glial cells. This study examines arginine concentrations in brain tissue using the microdialysis technique after human TBI. From 78 TBI patients, a total of 1739 microdialysate samples were collected using a CMA-70 probe perfused with normal saline at 2 microliters/min and concentrations of amino acids in microdialysate were determined. Amino acid concentrations for each patient were averaged for 8-hour periods during the first 3 days after injury, and daily for postinjury days 4 and 5. Following an initial rapid decrease in arginine, the dialysate arginine concentrations were low on days 1-3 and then increased over the days 4-5 after injury. In contrast, the microdialysate glutamate levels decreased slowly over the first 48 hours after TBI and thereafter remained low. Thirty-five episodes of jugular venous desaturation (SjvO2 < 50%) occurred during monitoring. Arginine and glutamate levels simultaneously doubled during desaturation and decreased as the clinical episode resolved. The low concentrations of arginine during the first 3 days after TBI may indicate that substrate unavailability could contribute to the decreased NO concentrations that have been observed after TBI. The simultaneous increase in glutamate and arginine during ischemic events is consistent with experimental data which has observed that glutamate induces release of arginine.

Peroxynitrite stimulates L-arginine transport system y(+) in glial cells. A potential mechanism for replenishing neuronal L-arginine

We have reported previously that peroxynitrite stimulates L-arginine release from astrocytes, but the mechanism responsible for such an effect remains elusive. To explore this issue, we studied the regulation of L-[(3)H]arginine transport by either exogenous or endogenous peroxynitrite in glial cells. A 2-fold peroxynitrite-mediated stimulation of l-arginine release in C6 cells was found to be Na(+)-independent, was prevented by 5 mm L-arginine and, although only in the presence of Na(+), was blocked by 5 mm L-alanine or L-leucine. Peroxynitrite-mediated stimulation of L-arginine uptake was trans-stimulated by 10 mm L-arginine and was inhibited in a dose-dependent fashion (k(i) of approximately 40 microm) by the system y(+) inhibitor N-ethylmaleimide in C6 cells. Endogenous production of peroxynitrite in lipopolysaccharide-treated astrocytes triggered an increased L-arginine transport activity without affecting Cat1 l-arginine transporter mRNA levels. However, Western blot analyses of peroxynitrite-treated astrocytes and C6 glial cells revealed a 3-nitrotyrosinated anti-Cat1-immunopositive band, strongly suggesting peroxynitrite-mediated Cat1 nitration. Furthermore, peroxynitrite stimulation of L-arginine release was abolished in fibroblast cells homozygous for a targeted inactivation of the Cat1 gene. Finally, peroxynitrite-triggered L-arginine released from astrocytes was efficiently taken up by neurons in an insert-based co-culture system. These results strongly suggest that peroxynitrite-mediated activation of the Cat1 transporter in glial cells may serve as a mechanism focused to replenish L-arginine in the neighboring neurons.

Glial-neuronal transfer of arginine and S-nitrosothiols in nitric oxide transmission

The arginine-nitric oxide (Arg-NO) and the S-nitrosothiols systems, two less well-studied aspects of NO transmission in the central nervous system, are reviewed. A growing body of evidence suggested that they play a crucial role in NO synthesis and activity. l-Arginine, the NO precursor, is predominantly localized in glia. Together with in vitro and in vivo results of arginine release, this suggests a transfer of arginine from glia to neurons in order to supply NO synthase with its substrate. NO biosynthesis may thus involve the co-occurrence of the glial-neuronal transfer of arginine and of NOS activation. The arginine availability may shed light on the dual, beneficial and toxic effects of NO. At low arginine concentrations, neuronal NO synthase generates NO and superoxide, favouring the production of the toxin peroxynitrite. NMDA-induced excitotoxicity in neuronal cells is dependent on arginine availability and glia may play a neuroprotective role by supplying arginine. The reversible S-nitros(yl)ation of thiol containing molecules may represent an important cellular signal transduction mechanism, probably comparable to phosphorylation. S-nitrosothiols, in particular through the presence and release of S-nitroso-cysteinylglycine in sensory thalamus, may act as a local buffering system in NO transmission. This may represent a novel specific facilitating mechanism in order to enhance transmission of persistent stimuli.

Neutrophil-derived glutamate regulates vascular endothelial barrier function

Endothelial barrier function is altered by the release of soluble polymorphonuclear leukocyte (PMN)-derived mediators during inflammatory states. However, endogenous pathways to describe such changes are only recently appreciated. Using an in vitro endothelial paracellular permeability model, cell-free supernatants from formylmethionylleucylphenylalanine-stimulated PMNs were observed to significantly alter endothelial permeability. Biophysical and biochemical analysis of PMN supernatants identified PMN-derived glutamate in modulating endothelial permeability. Furthermore, novel expression of metabotropic glutamate receptor 1 (mGluR1), mGluR4, and mGluR5 by human brain and dermal microvascular endothelial cells was demonstrated by reverse transcription-PCR, in situ hybridization, immunofluorescence, and Western blot analysis. Treatment of human brain endothelia with glutamate or selective, mGluR group I or III agonists resulted in a time-dependent loss of phosphorylated vasodilator-stimulated phosphoprotein (VASP) and significantly increased endothelial permeability. Glutamate-induced decreases in brain endothelial barrier function and phosphorylated VASP were significantly attenuated by pretreatment of human brain endothelia with selective mGluR antagonists. These observations were extended to an in vivo hypoxic mouse model in which pretreatment with mGluR antagonists significantly decreased fluorescein isothiocyanate-dextran flux across the blood-brain barrier. We conclude that activated human PMNs release glutamate and that endothelial expression of group I or III mGluRs function to decrease human brain endothelial VASP phosphorylation and barrier function. These results identify a novel pathway by which PMN-derived glutamate may regulate human endothelial barrier function.

Glial-derived arginine, the nitric oxide precursor, protects neurons from NMDA-induced excitotoxicity

Excitotoxic neuronal cell death is characterized by an overactivation of glutamate receptors, in particular of the NMDA subtype, and the stimulation of the neuronal nitric oxide synthase (nNOS), which catalyses the formation of nitric oxide (NO) from l-arginine (L-Arg). At low L-Arg concentrations, nNOS generates NO and superoxide (O2(.)(-)), favouring the production of the toxin peroxynitrite (ONOO-). Here we report that NMDA application for five minutes in the absence of added L-Arg induces neuronal cell death, and that the presence of L-Arg during NMDA application prevents cell loss by blocking O2(.)(-) and ONOO- formation and by inhibiting mitochondrial depolarization. Because L-Arg is transferred from glial cells to neurons upon activation of glial glutamate receptors, we hypothesized that glial cells play an important modulator role in excitotoxicity by releasing L-Arg. Indeed, as we further show, glial-derived L-Arg inhibits NMDA-induced toxic radical formation, mitochondrial dysfunction and cell death. Glial cells thus may protect neurons from excitotoxicity by supplying L-Arg. This potential neuroprotective mechanism may lead to an alternative approach for the treatment of neurodegenerative diseases involving excitotoxic processes, such as ischemia.

The hypothalamus and hypertension

Most forms of hypertension are associated with a wide variety of functional changes in the hypothalamus. Alterations in the following substances are discussed: catecholamines, acetylcholine, angiotensin II, natriuretic peptides, vasopressin, nitric oxide, serotonin, GABA, ouabain, neuropeptide Y, opioids, bradykinin, thyrotropin-releasing factor, vasoactive intestinal polypeptide, tachykinins, histamine, and corticotropin-releasing factor. Functional changes in these substances occur throughout the hypothalamus but are particularly prominent rostrally; most lead to an increase in sympathetic nervous activity which is responsible for the rise in arterial pressure. A few appear to be depressor compensatory changes. The majority of the hypothalamic changes begin as the pressure rises and are particularly prominent in the young rat; subsequently they tend to fluctuate and overall to diminish with age. It is proposed that, with the possible exception of the Dahl salt-sensitive rat, the hypothalamic changes associated with hypertension are caused by renal and intrathoracic cardiopulmonary afferent stimulation. Renal afferent stimulation occurs as a result of renal ischemia and trauma as in the reduced renal mass rat. It is suggested that afferents from the chest arise, at least in part, from the observed increase in left auricular pressure which, it is submitted, is due to the associated documented impaired ability to excrete sodium. It is proposed, therefore, that the hypothalamic changes in hypertension are a link in an integrated compensatory natriuretic response to the kidney's impaired ability to excrete sodium.

Neuronal and glial coexpression of argininosuccinate synthetase and inducible nitric oxide synthase in Alzheimer disease

The enzyme argininosuccinate synthetase (ASS) is the rate limiting enzyme in the metabolic pathway leading from L-citrulline to L-arginine, the physiological substrate of all isoforms of nitric oxide synthases (NOS). ASS and inducible NOS (iNOS) expression in neurons and glia was investigated by immunohistochemistry in brains of Alzheimer disease (AD) patients and nondemented, age-matched controls. In 3 areas examined (hippocampus, frontal, and entorhinal cortex), a marked increase in neuronal ASS and iNOS expression was observed in AD brains. GFAP-positive astrocytes expressing ASS were not increased in AD brains versus controls, whereas the number of iNOS expressing GFAP-positive astrocytes was significantly higher in AD brains. Density measurements revealed that ASS expression levels were significantly higher in glial cells of AD brains. Colocalization of ASS and iNOS immunoreactivity was detectable in neurons and glia. Occasionally, both ASS-and iNOS expression was detectable in CD 68-positive activated microglia cells in close proximity to senile plaques. These results suggest that neurons and astrocytes express ASS in human brain constitutively, whereas neuronal and glial ASS expression increases parallel to iNOS expression in AD. Because an adequate supply of L-arginine is indispensable for prolonged NO generation, coinduction of ASS enables cells to sustain NO generation during AD by replenishing necessary supply of L-arginine.

Arginine metabolism and the synthesis of nitric oxide in the nervous system

The biochemistry and physiology of L-arginine have to be reconsidered in the light of the recent discovery that the amino acid is the only substrate of all isoforms of nitric oxide synthase (NOS). Generation of nitric oxide, NO, a versatile molecule in signaling processes and unspecific immune defense, is intertwined with synthesis, catabolism and transport of arginine which thus ultimately participates in the regulation of a fine-tuned balance between normal and pathophysiological consequences of NO production. The complex composition of the brain at the cellular level is reflected in a complex differential distribution of the enzymes of arginine metabolism. Argininosuccinate synthetase (ASS) and argininosuccinate lyase which together can recycle the NOS coproduct L-citrulline to L-arginine are expressed constitutively in neurons, but hardly colocalize with each other or with NOS in the same neuron. Therefore, trafficking of citrulline and arginine between neurons necessitates transport capacities in these cells which are fulfilled by well-described carriers for cationic and neutral amino acids. The mechanism of intercellular exchange of argininosuccinate, a prerequisite also for its proposed function as a neuromodulator, remains to be elucidated. In cultured astrocytes transcription and protein expression of arginine transport system y(+) and of ASS are upregulated concomittantly with immunostimulant-mediated induction of NOS-2. In vivo ASS-immunoreactivity was found in microglial cells in a rat model of brain inflammation and in neurons and glial cells in the brains of Alzheimer patients. Any attempt to estimate the contributions of arginine transport and synthesis to substrate supply for NOS has to consider competition for arginine between NOS and arginase, the latter enzyme being expressed as mitochondrial isoform II in nervous tissue. Generation of NOS inhibitors agmatine and methylarginines is documented for the nervous system. Suboptimal supply of NOS with arginine leads to production of detrimental peroxynitrite which may result in neuronal cell death. Data have been gathered recently which point to a particular role of astrocytes in neural arginine metabolism. Arginine appears to be accumulated in astroglial cells and can be released after stimulation with a variety of signals. It is proposed that an intercellular citrulline-NO cycle is operating in brain with astrocytes storing arginine for the benefit of neighbouring cells in need of the amino acid for a proper synthesis of NO.

Glutamate neurotransmission and nitric oxide interaction within the ventrolateral medulla during cardiovascular responses to muscle contraction

We previously reported that nitric oxide, within the RVLM and CVLM, plays an opposing role in modulating cardiovascular responses during static muscle contraction [B.J. Freda, R.S. Gaitonde, R. Lillaney, A. Ally, Cardiovascular responses to muscle contraction following microdialysis of nitric oxide precursor into ventrolateral medulla, Brain Res. 828 (1999) 60-67]. In this study, we determined whether the effects of administering L-arginine, a precursor for the synthesis of nitric oxide, and N(G)-monomethyl-L-arginine (L-NMMA), a nitric oxide synthase inhibitor, into the rostral (RVLM) and caudal (CVLM) ventrolateral medulla on cardiovascular responses elicited during static muscle contraction were mediated via an alteration of localized glutamate concentrations using microdialysis techniques. In experiments within the RVLM (n=8), muscle contraction increased MAP and HR by 21+/-2 mmHg and 22+/-3 bpm, respectively. Glutamate increased from 1.1+/-0.4 to 4.4+/- 0.6 ng/5 microl measured from bilateral RVLM areas. Microdialysis of L-arginine (1.0 microM) for 30 min attenuated the contraction-evoked increases in MAP, HR, and glutamate levels. After subsequent microdialysis of L-NMMA (1.0 microM) into the RVLM, contraction augmented the pressor and tachycardic responses and glutamate release. In experiments within CVLM (n=8), muscle contraction increased MAP and HR by 22+/-3 mmHg and 20+/-2 bpm, respectively. Glutamate increased from 0.8+/-0. 4 to 3.6+/-0.6 ng/5 microl measured from the CVLM. L-Arginine augmented the cardiovascular responses and glutamate release and L-NMMA attenuated all the effects. Results suggest that nitric oxide within the RVLM and CVLM plays opposing roles in modulating cardiovascular responses during static exercise via decreasing and increasing, respectively, extracellular glutamate levels.

Noxious stimulation increases glutamate and arginine in the periaqueductal gray matter in rats: a microdialysis study

The periaqueductal gray matter (PAG) is an important center in the modulation of behavioral responses during nociception and stress. In the present experiment, extracellular excitatory amino acid overflow in the PAG was measured every 30 s during noxious stimulation. A combination of in vivo brain microdialysis in freely moving rats and capillary zone electrophoresis with laser induced-fluorescence detection allowed us to detect short lasting changes of excitatory amino acid in dialysates. A formalin injection in the hindpaw of the rat increased glutamate, arginine and aspartate concentration in PAG dialysates. This increase was calcium and nerve impulse-dependent, suggesting neuronal and glial origin of glutamate and arginine, respectively. Handling, pinching or saline injection in the hind paw did not increase glutamate showing that this neurochemical phenomenon is related to painful and persistent noxious stimulation. The results suggest that a rapid excitation of the PAG occurs during noxious stimulation. The role of glutamate and arginine in analgesia is discussed.

Changes in the concentrations of amino acids in the cerebrospinal fluid that correlate with pain in patients with fibromyalgia: implications for nitric oxide pathways

Substance P (SP), a putative nociceptive transmitter, is increased in the CSF of patients with fibromyalgia syndrome (FMS). Because excitatory amino acids (EAAs) also appear to transmit pain, we hypothesized that CSF EAAs may be similarly involved in this syndrome. We found that the mean concentrations of most amino acids in the CSF did not differ amongst groups of subjects with primary FMS (PFMS), fibromyalgia associated with other conditions (SFMS), other painful conditions not exhibiting fibromyalgia (OTHER) or age-matched, healthy normal controls (HNC). However, in SFMS patients, individual measures of pain intensity, determined using an examination-based measure of pain intensity, the tender point index (TPI), covaried with their respective concentrations of glutamine and asparagine, metabolites of glutamate and aspartate, respectively. This suggests that re-uptake and biotransformation mask pain-related increases in EAAs. Individual concentrations of glycine and taurine also correlated with their respective TPI values in patients with PFMS. While taurine is affected by a variety of excitatory manipulations, glycine is an inhibitory transmitter as well as a positive modulator of the N-methyl-D-asparate (NMDA) receptor. In both PFMS and SFMS patients, TPI covaried with arginine, the precursor to nitric oxide (NO), whose concentrations, in turn, correlated with those of citrulline, a byproduct of NO synthesis. These events predict involvement of NO, a potent signaling molecule thought to be involved in pain processing. Together these metabolic changes that covary with the intensity of pain in patients with FMS may reflect increased EAA release and a positive modulation of NMDA receptors by glycine, perhaps resulting in enhanced synthesis of NO.

L-arginine uptake and release by cultured avian retinal cells: differential cellular localization in relation to nitric oxide synthase

The availability of L-arginine is of pivotal importance for the synthesis of nitric oxide, a signaling molecule in the CNS. Here we show the presence of a high-affinity L-arginine uptake system (Km of 4.4 +/- 0.5 microM and a Vmax of 26.0 +/- 0.9 fmol/well/min) in cultured chick retinal cells. Different compounds, such as N(G)-mono-methyl-L-arginine and L-lysine, were able to inhibit the uptake that was also inhibited 60-70% in the absence of sodium and/or calcium ions. No trans stimulation was observed when cells were preloaded with L-lysine. The data indicate that the L-arginine uptake in cultured retinal cells is partially mediated by the y+ system, but has a great contribution of the B(0,+) system. Autoradiographic studies revealed that the uptake is predominant in glial cells and can also be detected in neurons, whereas immunocytochemistry of nitric oxide synthase and L-citrulline showed that the enzyme is present in neurons and photoreceptors, but not in glial cells. L-[3H]Arginine is released from purified glial cultures incubated with high concentrations of potassium in the extracellular medium. Moreover, the amino acid released from preloaded glial cells was taken up by purified neuronal cultures. These results indicate that L-arginine released from glial cells is taken up by neurons and used as substrate for the synthesis of nitric oxide.

Glutamate-stimulated activation of DNA synthesis via mitogen-activated protein kinase in primary astrocytes: involvement of protein kinase C and related adhesion focal tyrosine kinase

Glutamate is the major excitatory neurotransmitter in the CNS. Although its role in neurons has been studied extensively, little is known about its function in astrocytes. We studied the effects of glutamate on signaling pathways in primary astrocytes. We found that the tyrosine kinase related adhesion focal tyrosine kinase (RAFTK) is tyrosine phosphorylated in response to glutamate in a time- and dose-dependent manner. This phosphorylation was pertussis toxin (PTX) sensitive and could be attenuated by the depletion of Ca2+ from intracellular stores. RAFTK tyrosine phosphorylation was mediated primarily by class I/II metabotropic glutamate receptors and depends on protein kinase C (PKC) activation. Glutamate treatment of primary astrocytes also results in a significant increase in the activity of the mitogen-activated protein kinases [extracellular signal-related kinases 1/2 (ERK1/2)]. Like RAFTK phosphorylation, ERK1/2 activation is PTX sensitive and can be attenuated by the depletion of intracellular Ca2+ and by PKC inhibition, suggesting that RAFTK might mediate the glutamate-dependent activation of ERK1/2. Furthermore, we demonstrated that glutamate stimulation of primary astrocytes leads to a significant increase in DNA synthesis. Glutamate-stimulated DNA synthesis is PTX sensitive and can be inhibited by the MAP kinase kinase inhibitor PD98059, suggesting that in primary astrocytes, glutamate might signal via RAFTK and MAP kinase to promote DNA synthesis and cell proliferation.

The role of astrocytes and noradrenaline in neuronal glucose metabolism

In the classical model the energy requirements during neuronal activation are provided by the delivery of additional glucose directly into the extracellular compartment that results from the increase in local cerebral blood flow (rCBF). The present review proposes that astrocytes play a key role in the response to neuronal activation. Arginine for the synthesis of NO, which has a major role in the increase in rCBF, is released from astrocytes in response to stimulation of astrocytic glutamate receptors. The increased delivery of glucose by the blood stream enters astrocytes, where some of it is converted to glycogen. During neuronal activation there is a decrease in extracellular glucose owing to increased utilization followed by a delayed increase; this results from stimulation of astrocytic beta-adrenergic receptors, which leads to a breakdown of glycogen and the export of glucose.

Peroxynitrite anion stimulates arginine release from cultured rat astrocytes

The biosynthesis of the physiological messenger nitric oxide (*NO) in neuronal cells is thought to depend on a glial-derived supply of the *NO synthase substrate arginine. To expand our knowledge of the mechanism responsible for this glial-neuronal interaction, we studied the possible roles of peroxynitrite anion (ONOO-), superoxide anion (O2*-), *NO, and H2O2 in L-[3H]arginine release in cultured rat astrocytes. After 5 min of incubation at 37 degrees C, initial concentrations of 0.05-2 mM ONOO- stimulated the release of arginine from astrocytes in a concentration-dependent way; this effect was maximum from 1 mM ONOO- and proved to be approximately 400% as compared with control cells. ONOO(-)-mediated arginine release was prevented by arginine transport inhibitors, such as L-lysine and N(G)-monomethyl-L-arginine, suggesting an involvement of the arginine transporter in the effect of ONOO-. In situ xanthine/xanthine oxidase-generated O2*- (20 nmol/min) stimulated arginine release to a similar extent to that found with 0.1 mM ONOO-, but this effect was not prevented by arginine transport inhibitors. *NO donors, such as sodium nitroprusside, S-nitroso-N-acetylpenicillamine, or 1-[2-(2-aminoethyl)-N-(2-ammonioethyl)amino]diazen-1-ium+ ++-1,2-diolate, and H2O2 did not significantly modify arginine release. As limited arginine availability for neuronal *NO synthase activity may be neurotoxic due to ONOO- formation, our results suggest that ONOO(-)-mediated arginine release from astrocytes may contribute to replenishing neuronal arginine, hence avoiding further generation of ONOO- within these cells.

7-Nitroindazole, a selective inhibitor of nNOS, increases hippocampal extracellular glutamate concentration in status epilepticus induced by kainic acid in rats

The glutamate extracellular concentration is controlled by metabolic and neuronal pathways via release and uptake mechanisms. Stimulation of glutamate receptors induces neuronal nitric oxide (NO) release, which in turn modulates glutamate transmission. In this study, the influence of neuronally derived NO on hippocampal glutamate extracellular concentration was investigated in conditions of intense metabolic activation, i.e., during status epilepticus induced by systemic kainic acid (KA). Glutamate, arginine and citrulline concentrations were measured by microdialysis coupled to HPLC. Experiments were performed in conscious rats implanted with a microdialysis probe within the hippocampal CA3 area. Three groups were used: (1) rats treated with KA i.p. (12 mg/kg) and vehicle locally, via the microdialysis probe (n = 9); (2) rats given KA i.p. and a selective inhibitor of neuronal NO synthase, 7-nitroindazole (7-NI, 1.25 mM) locally (n = 13); (3) rats treated with saline i.p. and 7-NI locally (n = 7). Infusion of 7-NI or vehicle was performed throughout the second hour of status epilepticus. In groups 1 and 3, no significant modifications of extracellular glutamate, arginine and citrulline concentrations were measured. In group 2, the local application of 7-NI in the hippocampus during status epilepticus significantly increased extracellular glutamate and arginine concentrations, whereas citrulline concentration remained constant. The concomitant increases of extracellular glutamate and arginine concentrations under local 7-NI perfusion in seizure conditions, suggest that glutamate and arginine are linked in a common metabolic pathway and/or that glutamate is involved in the cross-talk between glia and neurons. A cerebrovascular effect of 7-NI which triggers glutamate release may also occur.

Induction of argininosuccinate synthetase in rat brain glial cells after striatal microinjection of immunostimulants

The enzyme argininosuccinate synthetase (ASS) initiates the metabolic pathway leading from L-citrulline to L-arginine, the only physiological substrate of all isoforms of nitric oxide synthases. The presence of ASS in glial cells in vivo was investigated by immunohistochemical methods in a model of rat brain inflammation. Phosphate-buffered saline or a mixture of bacterial lipopolysaccharide and interferon-gamma was injected into the left striatum, and animals were killed 24 hours later. Ipsilateral and contralateral sides of brain sections were incubated with an antiserum against ASS or antibodies against cell-specific markers. In the three areas examined, striatum, corpus callosum, and cortex, a strong induction of ASS immunoreactivity was observed in glial cells after injection of immunostimulants. A detailed quantitative analysis of double-stained sections revealed that ASS was almost exclusively expressed in reactive, ED1-positive microglial cells/brain macrophages in immunostimulant- or sham-injected ipsilateral sides of the sections. Furthermore, ASS/ED1 costaining was observed in perivascular cells. Colocalization of ASS with astroglial marker glial fibrillary acidic protein was given only occasionally after immunostimulation. ASS-positive neurons were detected in control and experimental animals; staining intensity was comparable in both cases. The results suggest that neurons express ASS constitutively, whereas the enzyme is induced in glial cells in response to proinflammatory stimuli. This finding is the first demonstration of an induction of a pathway auxiliary to generation of nitric oxide in brain in response to immunostimulants and provides new insight into neural arginine metabolism.

Cationic proteins inhibit L-arginine uptake in rat alveolar macrophages and tracheal epithelial cells. Implications for nitric oxide synthesis

Eosinophil-derived cationic proteins play an essential role in the pathogenesis of bronchial asthma. We tested whether cationic proteins interfere with the cationic amino-acid transport in alveolar macrophages (AMPhi) and tracheal epithelial cells, and whether L-arginine-dependent pathways were affected. The effect of cationic polypeptides on cellular uptake of [(3)H]-L-arginine, nitrite accumulation, and the turnover of [(3)H]-L-arginine by nitric oxide (NO) synthase and arginase (formation of [(3)H]-L-citrulline and [(3)H]-L-ornithine, respectively) were studied. Poly-L-arginine reduced [(3)H]-L-arginine uptake in rat AMPhi and tracheal epithelial cells in a concentration-dependent manner (at 300 microgram/ml by 70%). Poly-L-lysine, protamine, and major basic protein (each up to 300 microgram/ml) tested in rat AMPhi inhibited [(3)H]-L-arginine uptake by 35 to 50%. During 6 h incubation in amino acid-free Krebs solution, rat AMPhi, precultured in the absence or presence of LPS (1 microgram/ml), accumulated 1.4 and 3.5 nmol/10(6) cells nitrite, respectively. Addition of 100 microM L-arginine increased nitrite accumulation by 70 and 400% in control and lipopolysaccharide-treated AMPhi, respectively. Nitrite accumulation in the presence of L-arginine was reduced by poly-L-arginine and poly-L-lysine (100 and 300 microgram/ml) by 60 to 85% and 20 to 30%, respectively. Poly-L-arginine, but not poly-L-lysine, inhibited nitrite accumulation already in the absence of extracellular L-arginine. Poly-L-arginine (300 microgram/ml) inhibited [(3)H]-L-citrulline formation by AMPhi stronger than that of [(3)H]-L-ornithine. We conclude that cationic proteins can inhibit cellular transport of L-arginine and this can limit NO synthesis. Poly-L-arginine inhibits L-arginine uptake more effectively than other cationic proteins and exerts additional direct inhibitory effects on NO synthesis.

Interleukin-1beta activates forebrain glial cells and increases nitric oxide production and cortical glutamate and GABA release in vivo: implications for Alzheimer's disease

Interleukin-1beta (10 U) was injected into the nucleus basalis of adult male Wistar rats. The inflammation-induced changes in glial cell morphology and expression of inducible nitric oxide synthase in the injected area, the release of acetylcholine, GABA and glutamate from the ipsilateral cortex, the production of nitrite levels in the injected area and ipsilateral cortex, and changes in motor activity were investigated. Saline-injected rats were used as control. Interleukin-1beta induced an activation of both microglia and astrocytes which was already evident 24 h after injection. Seven days after injection, many reactive microglial cells and astrocytes were seen in the injected area and in other brain regions of the same hemisphere. Microglia reaction, but not astrocyte activation, disappeared 30 days post-injection. Seven days after interleukin-1beta injection, many cells immunopositive for inducible nitric oxide synthase were found surrounding the injection site. Inducible nitric oxide synthase-positive cells were identified, by double staining immunohistochemistry, in the reactive microglial cells and, by electron microscope examination, in the perineuronal subpopulation of resident activated microglia. Microdialysis investigations revealed a transient increase in reactive nitrogen intermediates (at seven days post-injection), a delayed (at 30 days post-injection) increase in GABA and glutamate release, and no changes in acetylcholine release in the ipsilateral cortex in interleukin-1beta, but not saline, injected rats. Inhibition of inducible nitric oxide synthase expression by N(G)-nitro-L-arginine methyl ester administration prevented the increase in nitrogen intermediates and GABA release, but not in glutamate release. Our findings suggest that an inflammatory reaction of the basal forebrain facilitates GABA release through the production of nitric oxide.