Glutamate is a mediator of neurotoxicity in secretions of activated HIV-1-infected macrophages

We sought to identify neurotoxin(s) secreted by HIV-1-infected mononuclear phagocytes that could contribute to the pathophysiology of HIV-1-associated dementia (HAD). Neurotoxic factors were characterized in batches of conditioned media (CM) from human monocyte-derived macrophages (MDM) infected with HIV-1(ADA) and/or activated with lipopolysaccharide (LPS). All of the neurotoxicity was: present in the <3000-Da fraction; blocked by 5 microM MK801; and not trypsin sensitive or extractable into polar organic solvents. Glutamate measured in CM accounted for all neurotoxic effects observed from HIV/LPS CM in astrocyte-poor neuronal cultures and may contribute to the pathophysiology of HIV-1-associated dementia.

Elevated cerebrospinal fluid quinolinic acid levels are associated with region-specific cerebral volume loss in HIV infection

Neuronal injury, dendritic loss and brain atrophy are frequent complications of infection with human immunodeficiency virus (HIV) type 1. Activated brain macrophages and microglia can release quinolinic acid, a neurotoxin and NMDA (N-methyl-D-aspartate) receptor agonist, which we hypothesize contributes to neuronal injury and cerebral volume loss. In the present cross-sectional study of 94 HIV-1-infected patients, elevated CSF quinolinic acid concentrations correlated with worsening brain atrophy, quantified by MRI, in regions vulnerable to excitotoxic injury (the striatum and limbic cortex) but not in regions relatively resistant to excitotoxicity (the non-limbic cortex, thalamus and white matter). Increased CSF quinolinic acid concentrations also correlated with higher CSF HIV-1 RNA levels. In support of the specificity of these associations, blood levels of quinolinic acid were unrelated to striatal and limbic volumes, and CSF levels of beta(2)-microglobulin, a non-specific and non-excitotoxic marker of immune activation, were unrelated to regional brain volume loss. These results are consistent with the hypothesis that quinolinic acid accumulation in brain tissue contributes to atrophy in vulnerable brain regions in HIV infection and that virus replication is a significant driver of local quinolinic acid biosynthesis.

Depletion of systemic macrophages by liposome-encapsulated clodronate attenuates striatal macrophage invasion and neurodegeneration following local endotoxin infusion in gerbils

CNS-localized inflammation with microglial activation and macrophage infiltration contributes to the pathogenesis of a broad spectrum of neurologic diseases. A direct injection of lipopolysaccharide (LPS) into the striatum of gerbils induced lectin-positive macrophage parenchymal invasion, minimal local microglial staining but extensive neurodegeneration (cresyl violet and silver staining) when evaluated 4 days later. In mice, LPS activated microglia (increased lectin staining of morphologically identified cells) with substantially less macrophage invasion but no neurodegeneration was seen at 4 days post LPS infusion. To evaluate the role of infiltrating macrophages in the neurodegenerative response in gerbils, peripheral macrophages were depleted by an intravenous injection of liposome-encapsulated clodronate. This preparation depleted spleen and liver macrophages (>95%), decreased blood monocytes by 55% and attenuated striatal macrophage infiltration (32 to 73% in five representative sections). Notably, the liposome-encapsulated clodronate reduced the severity of LPS-induced neurodegeneration, as visualized by cresyl violet staining and quantified in 20 serially stained silver sections (total volume, 1.32+/-0.41 mm(3) in liposome-encapsulated clodronate-treated versus 3.04+/-0.72 mm(3) in saline-treated controls). These results indicate that a local LPS infusion in gerbil brain may be a useful model in which to investigate the role of invading macrophages and other inflammatory responses in neurodegeneration in inflammatory neurological disease.

Mechanism of increases in L-kynurenine and quinolinic acid in renal insufficiency

Marked increases in metabolites of the L-tryptophan-kynurenine pathway, L-kynurenine and quinolinic acid (Quin), were observed in serum and cerebrospinal fluid (CSF) of both the rat and human with renal insufficiency. The mechanisms responsible for their accumulation after renal insufficiency were investigated. In patients with chronic renal insufficiency, elevated levels of serum L-kynurenine and Quin were reduced by hemodialysis. In renal-insufficient rats, Quin and L-kynurenine levels in serum, brain, and CSF were also increased parallel to the severity of renal insufficiency. Urinary excretion of Quin (3.5-fold) and L-kynurenine (2.8-fold) was also increased. Liver L-tryptophan 2,3-dioxygenase activity (TDO), a rate-limiting enzyme of the kynurenine pathway, was increased in proportion to blood urea nitrogen and creatinine levels. Kynurenine 3-hydroxylase and quinolinic acid phosphoribosyltransferase were unchanged, but the activities of kynureninase, 3-hydroxyanthranilate dioxygenase, and aminocarboxymuconate-semialdehyde decarboxylase (ACMSDase) were significantly decreased. Systemic administrations of pyrazinamide (ACMSDase inhibitor) increased serum Quin concentrations in control rats, demonstrating that changes in body ACMSDase activities in response to renal insufficiency are important factors for the determination of serum Quin concentrations. We hypothesize the following ideas: that increased serum L-kynurenine concentrations are mainly due to the increased TDO and decreased kynureninase activities in the liver and increased serum Quin concentrations are due to the decreased ACMSDase activities in the body after renal insufficiency. The accumulation of CSF L-kynurenine is caused by the entry of increased serum L-kynurenine, and the accumulation of CSF Quin is secondary to Quin from plasma and/or Quin precursor into the brain.

Labeled kynurenine pharmacokinetic modeling studies in gerbils. Nonequilibrium between infused and endogenous kynurenine

In order to complete pharmacokinetic studies on the central vs. peripheral origin of several tryptophan metabolites, we infused gerbils with labelled kynurenine (2H4 or 15N2). Osmotic minipumps charged with kynurenine solutions were surgically implanted subcutaneously in adult female gerbils (50-60 g). After a variable number of hours, the gerbils were sacrificed and organs taken for determination of labelled/unlabelled kynurenine ratios using mass spectrometric assay of a pentafluorobenzyl derivative as described previously. Surprisingly high ratios of 2H to 1H-kynurenine were measured in the kidney (0.25-0.40) and urine (4.0-8.0), although the ratio of deuterium labelled to endogenous kynurenine remained below detection limits (< 0.05) in serum and other tissues. Infusion of greater quantities of 2H4-kynurenine confirmed these observations in gerbils in which ratios of 2H4-to-1H kynurenine were measurable in serum and tissues. Synthesis and infusion of 15N2-kynurenine demonstrated that these effects were not due to deuterium isotope substitution. The data demonstrate a non-equilibrium between infused and endogenous kynurenine, which is related to differential rates of protein binding and the rapid clearance of free, infused kynurenine by kidney.

U50,488 protection against HIV-1-related neurotoxicity: involvement of quinolinic acid suppression

The pathogenesis of human immunodeficiency virus type 1 (HIV-1) encephalopathy has been associated with multiple factors including the neurotoxin quinolinate (an endogenous N-methyl-D-aspartate [NMDA] receptor ligand) and viral proteins. The kappa opioid receptor (KOR) agonist U50,488 recently has been shown to inhibit HIV-1 p24 antigen production in acutely infected microglial cell cultures. Using primary human brain cell cultures in the present study, we found that U50,488 also suppressed in a dose-dependent manner the neurotoxicity mediated by supernatants derived from HIV-1-infected microglia. This neuroprotective effect of U50,488 was blocked by the KOR selective antagonist nor-binaltorphimine. The neurotoxic activity of the supernatants from HIV-1-infected microglia was blocked by the NMDA receptor antagonists 2-amino-5-phosphonovalerate and MK-801. HIV-1 infection of microglial cell cultures induced the release of quinolinate, and U50,488 dose-dependently suppressed quinolinate release by infected microglial cell cultures with a corresponding inhibition of HIV-1 p24 antigen levels. These findings suggest that the kappa opioid ligand U50,488 may have therapeutic potential in HIV-1 encephalopathy by attenuating microglial cell production of the neurotoxin quinolinate and viral proteins.

Depletion of systemic macrophages by liposome-encapsulated clodronate attenuates increases in brain quinolinic acid during CNS-localized and systemic immune activation

Quinolinic acid is a neurotoxic tryptophan metabolite produced locally during immune activation. The present study tested the hypothesis that macrophages are an important source. In normal gerbils, the macrophage toxin liposome-encapsulated clodronate depleted blood monocytes and decreased quinolinic acid levels in liver (85%), duodenum (33%), and spleen (51%) but not serum or brain. In a model of CNS inflammation (an intrastriatal injection of 5 microg of lipopolysaccharide), striatal quinolinic acid levels were markedly elevated on day 4 after lipopolysaccharide in conjunction with infiltration with macrophages (lectin stain). Liposome-encapsulated clodronate given 1 day before intrastriatal lipopolysaccharide markedly reduced parenchymal macrophage invasion in response to lipopolysaccharide infusion and attenuated the increases in brain quinolinic acid (by 60%). A systemic injection of lipopolysaccharide (450 microg/kg) increased blood (by 38-fold), lung (34-fold), liver (23-fold), spleen (8-fold), and striatum (25-fold) quinolinic acid concentrations after 1 day. Liposome-encapsulated clodronate given 4 days before systemic lipopolysaccharide significantly attenuated the increases in quinolinic acid levels in blood (by 80%), liver (87%), spleen (80%), and striatum (68%) but had no effect on the increases in quinolinic acid levels in lung. These results are consistent with the hypothesis that macrophages are an important local source of quinolinic acid in brain and systemic tissues during immune activation.

Quinolinic acid is extruded from the brain by a probenecid-sensitive carrier system: a quantitative analysis

Although the neurotoxic tryptophan-kynurenine pathway metabolite quinolinic acid originates in brain by both local de novo synthesis and entry from blood, its concentrations in brain parenchyma, extracellular fluid, and CSF are normally below blood values. In the present study, an intraperitoneal injection of probenecid (400 mg/kg), an established inhibitor of acid metabolite transport in brain, into gerbils, increased quinolinic acid concentrations in striatal homogenates, CSF, serum, and homogenates of kidney and liver. Direct administration of probenecid (10 mM) into the brain compartment via an in vivo microdialysis probe implanted into the striatum also caused a progressive elevation in both quinolinic acid and homovanillic acid concentrations in the extracellular fluid compartment but was without effect on serum quinolinic acid levels. A model of microdialysis transport showed that the elevations in extracellular fluid quinolinic acid and homovanillic acid levels following intrastriatal application are consistent with probenecid block of a microvascular acid transport mechanism. We conclude that quinolinic acid in brain is maintained at concentrations below blood levels largely by active extrusion via a probenecid-sensitive carrier system.

Quinolinic acid in the cerebrospinal fluid of children after traumatic brain injury

OBJECTIVE

To measure quinolinic acid, a macrophage-derived neurotoxin, in the cerebrospinal fluid (CSF) of children after traumatic brain injury (TBI) and to correlate CSF quinolinic acid concentrations to clinically important variables.

DESIGN

A prospective, observational study.

SETTING

The pediatric intensive care unit in Children's Hospital of Pittsburgh, a tertiary care, university-based children's hospital.

PATIENTS

Seventeen critically ill children following severe TBI (Glasgow Coma Scale score <8) whose care required the placement of an intraventricular catheter for continuous drainage of CSF.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Patients ranged in age from 2 mos to 16 yrs (mean 6.0 yrs). CSF was collected immediately on placement of the ventricular catheter and daily thereafter. Quinolinic acid concentration was measured by gas chromatography/mass spectroscopy in 69 samples (4.0 +/- 0.4 [SEM] samples per patient). CSF quinolinic acid concentration progressively increased after injury (p = .034, multivariate analysis) and was increased in nonsurvivors vs. survivors (p = .002, multivariate analysis). CSF quinolinic acid concentration was not associated with age. Although overall CSF quinolinic acid concentration was not associated with shaken injury (p = .16, multivariate analysis), infants suffering with shaken infant syndrome had increased admission CSF quinolinic acid concentrations compared with children with accidental mechanisms of injury (p = .027, Mann-Whitney Rank Sum test).

CONCLUSIONS

A large and progressive increase in the macrophage-derived neurotoxin quinolinic acid is seen following severe TBI in children. The increase is strongly associated with increased mortality. Increased CSF quinolinic acid concentration on admission in children with shaken infant syndrome could reflect a delay in presentation to medical attention or age-related differences in quinolinic acid production. These findings raise the possibility that quinolinic acid may play a role in secondary injury after TBI in children and suggest an interaction between inflammatory and excitotoxic mechanisms of injury following TBI.

Conditioned immune response to interferon-gamma in humans

We determined whether a classical conditioning paradigm may be used to condition immunologic responses in normal human subjects receiving an optimal immunostimulating dose of recombinant human interferon-gamma (rhIFN-gamma). We conducted a placebo-controlled, double-blind study of 31 normal volunteers in order to determine whether an initially immune-neutral stimulus, oral propylene glycol (PG), could eventually elicit an immune response as a consequence of its being paired with a known immunostimulatory dose and schedule of rhIFN-gamma. Subjects were randomly assigned to one of three groups: (A) rhIFN-gamma injections paired with PG; (B) normal saline injections paired with PG; (C) rhIFN-gamma injections alone. During the 4-week study, subjects received progressively fewer injections so that, by the final week of the study, no injections were given and groups A and B received only PG. The principal outcome measures were serum concentrations of quinolinic acid (QUIN) and neopterin, two nonspecific but sensitive markers of immune activation, and expression of Fc receptors (CD64) on peripheral blood mononuclear cells. RhIFN-gamma injections produced significant and predictable alterations in each of the measured immune parameters. No group B subject made an immune response. Mean serum QUIN levels were significantly higher at the end of week three for subjects in the experimental condition (group A) than for subjects receiving rhIFN-gamma alone (group C) despite receiving identical doses of rhIFN-gamma. Similarly, the predicted decay in mean serum neopterin levels from the end of week 1 to the end of week 2 was seen in group C but not in group A. The exposure of group A to PG blunted the decline of CD64 expression in week four. The data suggest that the pairing of an unconditioned stimulus (rhIFN-gamma) and a conditioned stimulus (PG) permits the conditioned stimulus alone to prolong a cytokine-induced response in normal humans.

Effects of electromagnetic fields on the levels of biogenic amine metabolites, quinolinic acid, and beta-endorphin in the cerebrospinal fluid of dairy cows

Eight multiparous non-lactating pregnant Holstein cows at 198 +/- 35 d of gestation, weighing 608 +/- 24 kg, were confined to wooden metabolic cages in an electric and magnetic field chamber with a 12:12 h light:dark cycle. Subarachnoidal catheters were installed 5 d before the activation of the electric and magnetic fields. The cows were exposed to electric and magnetic fields (60 Hz, 10 kV/m and 30 microT) continuously except for the feeding and cleaning time for an average of 21.44 +/- 1.4 h per day for a period of 30 d. Cerebrospinal fluid samples were collected on three consecutive days before an exposure period of 30 d, on the last 3 d of the exposure period, and for 3 d starting 5 d after the exposure period. The concentrations of beta-endorphin, tryptophan, 5-hydroxyindoleacetic acid, homovanillic acid, 3-methoxy-4-hydroxyphenylethyleneglycol and quinolinic acid in cerebrospinal fluid were determined. There was a significant increase in quinolinic acid, and a trend towards an increase in tryptophan, findings consistent with a weakening of the blood-brain barrier due to exposure to the electric and magnetic fields.

Cerebrospinal fluid tau protein is not elevated in HIV-associated neurologic disease in humans. HIV Neurobehavioral Research Center Group (HNRC)

We measured the concentrations of the neuron-specific protein, tau, in the cerebrospinal fluid (CSF) of 32 neurologically characterized HIV-infected (HIVpos) subjects and nine matched seronegative (HIVneg) controls using a sensitive ELISA assay. Of 32 HIVpos subjects, nine had HIV-associated neurocognitive disorders, and nine had clinically diagnosed peripheral neuropathies. CSF tau levels in subjects with HIV-associated neurocognitive disorders were similar to those in HIVneg subjects (185 +/- 83 vs. 223 +/- 106 pg/ml; P = 57). CSF tau levels in HIVpos subjects with peripheral neuropathies did not differ from those without neuropathies (320 +/- 190 vs. 251 +/- 185; P = 23). In summary, CSF tau levels were not elevated in patients with HIV-associated neurologic disease.

Sources of the neurotoxin quinolinic acid in the brain of HIV-1-infected patients and retrovirus-infected macaques

This study investigated the sources of quinolinic acid, a neurotoxic tryptophan-kynurenine pathway metabolite, in the brain and blood of HIV-infected patients and retrovirus-infected macaques. In brain, quinolinic acid concentrations in HIV-infected patients were elevated by > 300-fold to concentrations that exceeded cerebrospinal fluid (CSF) by 8.9-fold. There were no significant correlations between elevated serum quinolinic acid levels with those in CSF and brain parenchyma. Because nonretrovirus-induced encephalitis confounds the interpretation of human postmortem data, rhesus macaques infected with retrovirus were used to examine the mechanisms of increased quinolinic acid accumulations and determine the relationships of quinolinic acid to encephalitits and systemic responses. The largest kynurenine pathway responses in brain were associated with encephalitis and were independent of systemic responses. CSF quinolinic acid levels were also elevated in all infected macaques, but particularly those with retrovirus-induced encephalitis. In contrast to the brain changes, there was no difference in any systemic measure between macaques with encephalitis vs. those without. Direct measures of the amount of quinolinic acid in brain derived from blood in a macaque with encephalitis showed that almost all quinolinic acid (>98%) was synthesized locally within the brain. These results demonstrate a role for induction of indoleamine-2,3dioxygenase in accelerating the local formation of quinolinic acid within the brain tissue, particularly in areas of encephalitis, rather than entry of quinolinic acid into the brain from the meninges or blood. Strategies to reduce QUIN production, targeted at intracerebral sites, are potential approaches to therapy.

Quinolinic acid is increased in CSF and associated with mortality after traumatic brain injury in humans

We tested the hypothesis that quinolinic acid, a tryptophan-derived N-methyl-D-aspartate agonist produced by macrophages and microglia, would be increased in CSF after severe traumatic brain injury (TBI) in humans, and that this increase would be associated with outcome. We also sought to determine whether therapeutic hypothermia reduced CSF quinolinic acid after injury. Samples of CSF (n = 230) were collected from ventricular catheters in 39 patients (16 to 73 years old) during the first week after TBI, (Glasgow Coma Scale [GCS] < 8). As part of an ongoing study, patients were randomized within 6 hours after injury to either hypothermia (32 degrees C) or normothermia (37 degrees C) treatments for 24 hours. Otherwise, patients received standard neurointensive care. Quinolinic acid was measured by mass spectrometry. Univariate and multivariate analyses were used to compare CSF quinolinic acid concentrations with age, gender, GCS, time after injury, mortality, and treatment (hypothermia versus normothermia). Quinolinic acid concentration in CSF increased maximally to 463 +/- 128 nmol/L (mean +/- SEM) at 72 to 83 hours after TBI. Normal values for quinolinic acid concentration in CSF are less than 50 nmol/L. Quinolinic acid concentration was increased 5- to 50-fold in many patients. There was a powerful association between time after TBI and increased quinolinic acid (P < 0.00001), and quinolinic acid was higher in patients who died than in survivors (P = 0.003). Age, gender, GCS, and treatment (32 degrees C versus 37 degrees C) did not correlate with CSF quinolinic acid. These data reveal a large increase in quinolinic acid concentration in CSF after TBI in humans and raise the possibility that this macrophage-derived excitotoxin may contribute to secondary damage.

Quinolinic acid in vivo synthesis rates, extracellular concentrations, and intercompartmental distributions in normal and immune-activated brain as determined by multiple-isotope microdialysis

Quinolinic acid (QUIN) kills neurons by activation of NMDA receptors that are accessed via the extracellular fluid (ECF). In vivo microdialysis was employed to quantify the dynamics of ECF QUIN levels. [(13)C7]QUIN was perfused through the probe for in vivo calibration to accurately quantify ECF QUIN concentrations. Osmotic pumps infused [(2H)3]QUIN subcutaneously to quantify blood contributions to ECF and tissue levels. Local QUIN production rates and influx and efflux rates across the blood-brain barrier were calculated from the extraction fraction of [(13)C7]QUIN, probe geometry, tissue diffusion coefficients, the extracellular volume fraction, and [(2)H3]QUIN/QUIN ratios in blood and dialysates. In normal brain, 85% of ECF QUIN levels (110 nM) originated from blood, whereas 59% of tissue homogenate QUIN (130 pmol/g) originated from local de novo synthesis. During systemic immune activation (intraperitoneal injection of endotoxin), blood QUIN levels increased (10.2-fold) and caused a rise in homogenate (10.8-fold) and ECF (18.5-fold) QUIN levels with an increase in the proportions of QUIN derived from blood. During CNS inflammation (local infusion of endotoxin), increases in brain homogenate (246-fold) and ECF (66-fold) QUIN levels occurred because of an increase in local synthesis rate (146-fold) and a reduction in efflux/influx ratio (by 53%). These results demonstrate that brain homogenate measures are a reflection of ECF concentrations, although there are quantitative differences in the values obtained. The mechanisms that maintain ECF QUIN levels at low values cannot do so when there are large increases in local brain synthesis or when there are large elevations in blood QUIN concentrations.

Species heterogeneity between gerbils and rats: quinolinate production by microglia and astrocytes and accumulations in response to ischemic brain injury and systemic immune activation

Quinolinic acid is an excitotoxic kynurenine pathway metabolite, the concentration of which increases in human brain during immune activation. The present study compared quinolinate responses to systemic and brain immune activation in gerbils and rats. Global cerebral ischemia in gerbils, but not rats, increased hippocampus indoleamine-2,3-dioxygenase activity and quinolinate levels 4 days postinjury. In a rat focal ischemia model, small increases in quinolinate concentrations occurred in infarcted regions on days 1, 3, and 7, although concentrations remained below serum values. In gerbils, systemic immune activation by an intraperitoneal injection of endotoxin (1 mg/kg of body weight) increased quinolinate levels in brain, blood, lung, liver, and spleen, with proportional increases in lung indoleamine-2,3-dioxygenase activity at 24 h postinjection. In rats, however, no significant quinolinate content changes occurred, whereas lung indoleamine-2,3-dioxygenase activity increased slightly. Gerbil, but not rat, brain microglia and peritoneal monocytes produced large quantities of [13C(6)]-quinolinate from L-[13C(6)]tryptophan. Gerbil astrocytes produced relatively small quantities of quinolinate, whereas rat astrocytes produced no detectable amounts. These results demonstrate that the limited capacity of rats to replicate elevations in brain and blood quinolinic acid levels in response to immune activation is attributable to blunted increases in local indoleamine-2,3-dioxygenase activity and a low capacity of microglia, astrocytes, and macrophages to convert L-tryptophan to quinolinate.

Prospective study of neurological responses to treatment with macrophage-targeted glucocerebrosidase in patients with type 3 Gaucher's disease

We prospectively evaluated the clinical and biochemical responses to enzyme-replacement therapy (ERT) with macrophage-targeted glucocerebrosidase (Ceredase) infusions in 5 patients (age, 3.5-8.5 years) with type 3 Gaucher's disease. The patients were followed for up to 5 years. Enzyme dosage ranged from 120 to 480 U/kg of body weight/month. Systemic manifestations of the disease regressed in all patients. Neurological deficits remained stable in 3 patients and slightly improved in 1. One patient developed myoclonic encephalopathy. Cognitive deterioration occurred in 1 patient and electroencephalographic deterioration in 2. Sequential cerebrospinal fluid (CSF) samples were obtained during the first 3 years of treatment in 3 patients and were analyzed for biochemical markers of disease burden. Glucocerebroside and psychosine levels were not elevated in these specimens, whereas chitotriosidase and quinolinic acid were elevated in 2 patients. Progressive decrease in the CSF levels of these latter macrophage markers during 3 years of treatment implies a decreased number of Gaucher cells in the cerebral perivascular space. Similar changes were not observed in the patient who had a poor neurological outcome. In conclusion, ERT reverses systemic manifestations of type 3 Gaucher's disease and appears to reduce the burden of Gaucher cells in the brain-CSF compartment in some patients.

Different kynurenine pathway enzymes limit quinolinic acid formation by various human cell types

Substantial increases in the tryptophan-kynurenine pathway metabolites, l-kynurenine and the neurotoxin quinolinic acid, occur in human brain, blood and systemic tissues during immune activation. Studies in vitro have shown that not all human cells are capable of synthesizing quinolinate. To investigate further the mechanisms that limit l-kynurenine and quinolinate production, the activities of kynurenine pathway enzymes and the ability of different human cells to convert pathway intermediates into quinolinate were compared. Stimulation with interferon gamma substantially increased indoleamine 2,3-dioxygenase activity and L-kynurenine production in primary peripheral blood macrophages and fetal brains (astrocytes and neurons), as well as cell lines derived from macrophage/monocytes (THP-1), U373MG astrocytoma, SKHEP1 liver and lung (MRC-9). High activities of kynurenine 3-hydroxylase, kynureninase or 3-hydroxyanthranilate 3,4-dioxygenase were found in interferon-gamma-stimulated macrophages, THP-1 cells and SKHEP1 cells, and these cells made large amounts of quinolinate when supplied with L-tryptophan, L-kynurenine, 3-hydroxykynurenine or 3-hydroxyanthranilate. Quinolinate production by human fetal brain cultures and U373MG cells was restricted by the low activities of kynurenine 3-hydroxylase, kynureninase and 3-hydroxyanthranilate 3,4-dioxygenase, and only small amounts of quinolinate were synthesized when cultures were supplied with L-tryptophan or 3-hydroxyanthranilate. In MRC-9 cells, quinolinate was produced only from 3-hydroxykynurenine and 3-hydroxyanthranilate, consistent with their low kynurenine 3-hydroxylase activity. The results are consistent with the notion that indoleamine 2,3-dioxygenase is an important regulatory enzyme in the production of L-kynurenine and quinolinate. Kynurenine 3-hydroxylase and, in some cells, kynureninase and 3-hydroxyanthranilate 3,4-dioxygenase are important determinants of whether a cell can make quinolinate.

Quinolinic acid accumulation in injured spinal cord: time course, distribution, and species differences between rat and guinea pig

Experimental compression injury of the spinal cord in guinea pigs results in delayed neurologic deficits that continue to increase in severity for several days following trauma, coincident with inflammatory responses, including invasion of the lesion by mononuclear phagocytes and increased levels of the neurotoxin quinolinic acid (QUIN). Inflammatory responses and QUIN elevation also occur following spinal cord contusion in rats, but maximal neurologic deficits develop immediately. In this study, somatosensory evoked potentials (SEP) and tissue, serum, and cerebrospinal fluid levels of QUIN were measured in guinea pigs and rats following similar compression injuries of the thoracic spinal cord. SEP changes differed between the species, consistent with other neurological changes. In guinea pigs, increases in QUIN levels at the lesion site began at 1 day postinjury, achieved maximal elevation (100-fold) by 12 days, then declined, but remained above serum levels at 25 days postinjury. A similar increase occurred in adjacent areas of the spinal cord, with lower peak levels. In rats, tissue QUIN at the center of the lesion remained below serum levels at all times, increasing moderately (<10-fold) up to 7 days, then decreasing between 7 and 25 days. These data demonstrate differences in the time course and magnitude of QUIN accumulation and neurological deficit between guinea pig and rat, which may relate to differences in secondary pathological mechanisms. Such profound differences may affect the use of these species for evaluation of experimental therapy in this and other inflammatory conditions of the central nervous system.

Marked increases in concentrations of apolipoprotein in the cerebrospinal fluid of poliovirus-infected macaques: relations between apolipoprotein concentrations and severity of brain injury

Apolipoproteins in cerebrospinal fluid (CSF) might have important functional roles in the pathophysiology of brain and lipid metabolism in the vascular component. The present study examined apolipoprotein A-I (apo-A-I) and apolipoprotein E (apo-E) levels in CSF and serum from poliovirus-infected macaques. Poliovirus-infected macaques developed motor deficits and were classified into three groups: (1) muscle weakness in one or both legs; (2) partial paralysis in one or both legs; (3) complete paralysis in one or both legs. No motor deficits were evident in the control or sham-treated macaques. Apo-A-I concentrations in CSF were markedly elevated in poliovirus-infected macaques with weakness, partial or complete paralysis, in comparison with either control or sham-treated animals, and were proportional to the severity of motor impairment. Apo-E concentrations in CSF were also significantly elevated in poliovirus-infected macaques with complete paralysis. The magnitude of increase in CSF apo-A-I or apo-E concentrations was also closely associated with the degree of histologic neurological damage and inflammation (lesion scores). However, no changes in serum apo-A-I and apo-E concentrations were observed in the poliovirus-infected macaques compared with control macaques. Furthermore there were no significant correlations apo-A-I or apo-E concentrations between serum and CSF. We hypothesize that the elevation of apo-A-I and apo-E concentrations after poliovirus infection is caused by immune stimulation within the central nervous system (CNS). Measures of CSF apo-A-I and apo-E levels might serve as a useful marker for the severity and/or the range of CNS injury.

Quantification of local de novo synthesis versus blood contributions to quinolinic acid concentrations in brain and systemic tissues

The source of the neurotoxin quinolinic acid (QUIN) in brain and systemic tissues under normal and pathologic circumstances reflects either de novo synthesis from L-tryptophan and other precursors, or entry of QUIN itself from the blood. To quantify the relative contributions of blood- versus tissue-derived QUIN, [13C7]-QUIN was infused subcutaneously via osmotic pumps (0.55 microliter/h, 30 mM) in gerbils, and the fraction of QUIN in tissue (Tl; measured in tissue homogenates) derived from blood (Bl; measured in serum) was calculated by the formula ([13C7]QUINTi/QUINTi)/([13C7]QUINBl/ QUINBl). In controls, blood QUIN contributed 38-49% of QUIN in brain, 70% in CSF, between 40 and 70% in kidney, heart, and skeletal muscle, but < 5% in spleen, lung, liver, and intestine. Systemic endotoxin (450 micrograms/kg) increased blood, brain, CSF, and systemic tissue QUIN levels. Notably, the relative proportion of QUIN derived from blood in brain, spleen, lung, and intestine was unchanged by endotoxin, but increased in kidney, heart, and skeletal muscle. In contrast, cerebral ischemic injury (10 min of bilateral carotid artery occlusion) increased regional brain QUIN concentrations at 4 days post ischemia, with a proportional increase in the amount of QUIN derived from de novo synthesis by brain tissue. In the blood and systemic tissues of postischemic gerbils, there were no changes in systemic tissue or blood QUIN levels, or changes in the relative proportions of blood- versus systemic tissue-derived QUIN. These results establish that the brain normally synthesizes QUIN, that the blood is a significant source of QUIN in controls and during acute systemic immune activation, and that the rate of QUIN formation by brain tissue increases in conditions of brain and systemic immune activation.

Neuroinvasion by simian immunodeficiency virus coincides with increased numbers of perivascular macrophages/microglia and intrathecal immune activation

During peak viremia and initial antibody response, rhesus macaques infected with pathogenic and nonpathogenic isolates of SIV show distinct differences in viral load and tissue distribution. Animals infected with pathogenic isolates of SIV invariably have virus in the CSF and brain parenchyma by two weeks postinoculation, whereas animals infected with nonpathogenic isolates do not. Mechanisms underlying neuroinvasion by SIV and HIV are unknown, but recruitment of latently infected mononuclear cells from the peripheral circulation (Trojan horse theory) is frequently proposed. Circulating monocytes, from which perivascular macrophage/microglia are derived, are a likely vehicle for cell-associated transport of virus across the blood-brain barrier. This transport and the kinetics of perivascular macrophage/microglial turnover in the CNS likely depend on endothelial and leukocyte adhesion molecules such as vascular cell adhesion molecule-1 (VCAM-1), which has previously been shown to be upregulated on cerebrovascular endothelium in SIV encephalitis. To investigate the role of peripheral monocyte recruitment into the perivascular macrophage/microglial cell pool at the time of initial viral neuroinvasion, we examined the temporal relationships among perivascular macrophage/microglia density, endothelial VCAM-1 expression and localization of viral nucleic acid in the CNS of macaques acutely infected with pathogenic and nonpathogenic molecular clones of SIV. The concentration of CSF quinolinic acid, a marker of intrathecal immune and macrophage activation, was examined concurrently. We found that significant increases in the density of perivascular macrophages/microglia coincided with viral neuroinvasion and marked elevations in CSF quinolinic acid. Furthermore, combined in situ hybridization and immunohistochemistry demonstrated that infected perivascular cells were macrophages/microglia. These findings provide evidence suggesting that neuroinvasion occurs through an influx of infected monocytes which take up residence in the CNS as perivascular macrophages/microglia. VCAM-1 expression, however, was not clearly correlated with these events, thus its contribution to initial viral neuroinvasion is unclear.

Human microglia convert l-tryptophan into the neurotoxin quinolinic acid

Immune activation leads to accumulations of the neurotoxin and kynurenine pathway metabolite quinolinic acid within the central nervous system of human patients. Whereas macrophages can convert L-tryptophan to quinolinic acid, it is not known whether human brain microglia can synthesize quinolinic acid. Human microglia, peripheral blood macrophages and cultures of human fetal brain cells (astrocytes and neurons) were incubated with [13C6]L-tryptophan in the absence or presence of interferon gamma. [13C6]Quinolinic acid was identified and quantified by gas chromatography and electron-capture negative-chemical ionization mass spectrometry. Both L-kynurenine and [13C6]quinolinic acid were produced by unstimulated cultures of microglia and macrophages. Interferon gamma, an inducer of indoleamine 2,3-dioxygenase, increased the accumulation of L-kynurenine by all three cell types (to more than 40 microM). Whereas large quantities of [13C6]quinolinic acid were produced by microglia and macrophages (to 438 and 1410 nM respectively), minute quantities of [13C6]quinolinic acid were produced in human fetal brain cultures (not more than 2 nM). Activated microglia and macrophage infiltrates into the brain might be an important source of accelerated conversion of L-tryptophan into quinolinic acid within the central nervous system in inflammatory diseases.

Neurobehavioral manifestations of symptomatic HIV-1 disease in children: can nutritional factors play a role?

Central nervous system (CNS) abnormalities are significant and frequent complications of human immunodeficiency virus (HIV-1) infection in infants and children. Although the predominant cause of neurological and neuropsychological abnormalities appears to be related to HIV infection of the CNS, other factors including malnutrition may also play a role. We retrospectively evaluated the association of change in body weight with changes in neurocognitive function, ventricular brain ratio, and cerebrospinal quinolinic acid levels in a small cohort of children (n=15; mean age 6.3 years) with symptomatic HIV-1 disease before and after 6 months of antiretroviral therapy with continuous intravenous infusion of zidovudine (ZVD). Significant increases in weight and neurocognitive function as well as decreases in ventricular brain ratio and cerebrospinal quinolinic acid levels were noted after therapy. Only the relation between increase in weight and decrease in ventricular brain ratio was statistically significant (P< .01); contrary to expectations, an increase in weight seemed to correlate with a decrease in neurocognitive function (NS). Another group of children treated at the same time with oral intermittent ZVD, but otherwise receiving the same care did not show the same magnitude of improvement in neurocognitive function. These results seem to suggest that general supportive and medical care as well as nutritional factors may only play a limited role in the neurocognitive improvements after antiretroviral therapy with continuous infusion ZVD. Our sample size was, however, small and the nutritional measure rather global; thus these findings have to be considered as very preliminary.

Brain extracellular quinolinic acid in chronic experimental hepatic encephalopathy as assessed by in vivo microdialysis: acute effects of L-tryptophan

Increased brain quinolinic acid (QUIN) levels have been suggested to play a role in hepatic encephalopathy (HE). Previous brain tissue studies have been unable to confirm this hypothesis. Because QUIn is a potent NMDA-receptor agonist, it also is relevant to determine brain extracellular QUIN levels in HE. For this purpose, we assessed frontal neocortical extracellular QUIN levels by in vivo microdialysis in rats subjected to a portacaval shunt (PCS). We also evaluated the acute effects of altered L-tryptophan (L-TRP) availability on brain extracellular QUIN levels. The basal extracellular L-TRP levels were significantly (p < .001) higher in the PCS rats than in the sham-operated controls. However, the QUIN level (p < .05) and the QUIN to L-TRP ratio (p < .01) were significantly lower in the PCS rats. Elevated L-TRP availability increased the QUIN levels to a similar degree in both sham and PCS rats. This study, in conjunction with our previous results, does thereby not support a major involvement of QUIN in the pathogenesis of HE.

Macrophage Activation Factors in the Brains of AIDS Patients

HIV, soluble HLA class I (sHLA-I), quinolinic acid (QUIN), and the monokines IL-1&beta;, IL-6, and TNF-&#945; were measured by ELISA and PCR in brain tissue of 60 AIDS autopsies without evidence of CNS opportunistic infections. Individual cases showed good interrogational correlations for the factors measured. There was a positive correlation between concentrations of IL-1&beta; and IL-6. Brain viral burden correlated with intraparenchymal levels of sHLA-I, IL-1&beta;, and IL-6. Comparison of neuritic damage and levels of immune mediators implicates macrophage activation factors in the etiology of neurologic damage in AIDS.

Quinolinic acid in patients with systemic lupus erythematosus and neuropsychiatric manifestations

OBJECTIVE

To evaluate the relationship between quinolinic acid, a neuroactive metabolite of L-tryptophan, and neuropsychiatric manifestations of systemic lupus erythematosus (SLE).

METHODS

Forty specimens of cerebrospinal fluid (CSF) were obtained from 39 patients with SLE who were evaluated for 40 episodes of neuropsychiatric dysfunction. The diagnosis of the neuropsychiatric dysfunction was determined clinically. CSF and serum specimens were analyzed for levels of quinolinic acid without knowledge of the clinical diagnosis.

RESULTS

Neuropsychiatric dysfunction attributed to SLE (NPSLE) was confirmed in 30 patient-episodes (Group 1), whereas in the other 10 (Group 2) other etiologies were felt to explain their CNS dysfunction. The median levels of CSF quinolinic acid for Group 1 (232.5 nmol/l) were significantly higher than those for Group 2 (median 38.2 nmol/l) (p < 0.014). CSF and serum quinolinic acid levels correlated significantly (p < 0.003) but there was not correlation between CSF quinolinic acid and CSF protein concentrations or white blood cell counts.

CONCLUSION

We conclude that elevated quinolinic acid levels in the CSF and serum may be associated with NPSLE and could possibly play a role in its pathogenesis.

The kynurenine pathway and neurologic disease. Therapeutic strategies

The neurotoxic effects of QUIN have been well established. Clinical conditions have been identified where substantial elevations in CNS QUIN levels occur. There is a relationship between the severity of neurologic impairments and macrophage activation, with the magnitude of the increases in QUIN. The magnitude of QUIN increases in experimental immune activation, and macrophages in vitro, are highest in non-human primates, intermediate in gerbils and guinea pigs, and lowest in mice and rats. Macrophages in vitro are a useful screening system to evaluate potential inhibitors of the kynurenine pathway. Several models of CNS inflammation are available, including brain injury in post-ischemic gerbils and spinal cord injury in guinea pigs. 4-Chloro-3-hydroxyanthranilate is a potent inhibitor of QUIN production by macrophages and reduces QUIN accumulations in spinal cord injury. Such reductions are associated with significant neurologic improvements in the early post-injury period. The results support further investigation of QUIN as a mediator of neurologic dysfunction and damage in neurologic diseases.

Quinolinic acid levels in a murine retrovirus-induced immunodeficiency syndrome

Mice infected with the retrovirus mixture designated LP-BM5 murine leukemia virus (MuLV) develop an immunosuppressive disease. Quinolinic acid (QUIN) is an endogenous neurotoxic N-methyl-D-aspartate agonist that may contribute to the pathogenesis of HIV-associated neurologic disease. In the present study, the levels of QUIN in brain and blood were measured in mice infected with LP-BM5 MuLV and compared with those in uninfected mice and mice infected with the nonpathogenic strain of ecotropic MuLV (helper component of LP-BM5 MuLV). Infection with LP-BM5 MuLV resulted in progressive increases in blood QUIN levels beginning 2 weeks after inoculation that peaked by 16 weeks postinfection. QUIN levels were also increased in cerebral cortex, hippocampus, and striatum. In systemic tissues, QUIN levels were increased in lung, liver, and spleen. In contrast, infection with the ecotropic viral component of the LP-BM5 MuLV mixture was not associated with any changes in brain, blood, or systemic tissue QUIN levels, even though helper virus burdens were comparable to those in mice infected with LP-BM5 MuLV. Treatment of LP-BM5 MuLV-infected mice with the antiretroviral agent zidovudine (azidothymidine) significantly reduced blood and brain QUIN levels in association with reductions in viral load in brain and spleen. These observations suggest that elevated QUIN production is not attributable to productive infection with retrovirus per se but occurs in response to an agent or agents, such as cytokines, that are produced by the host in response to virus infection.

Evidence of excitotoxicity in the brain of the ornithine carbamoyltransferase deficient sparse fur mouse

Ornithine carbamoyltransferase deficiency (OCTD) is the most common inborn error of urea synthesis. An X-linked disorder, OCTD males commonly present with hyperammonemic coma in the newborn period. There is a high rate of mortality and morbidity, with most survivors sustaining severe brain damage and resultant developmental disabilities. Although ammonia is presumed to be the principal neurotoxin, there is evidence that other neurochemical alterations may also be involved. The OCTD sparse fur (spf/Y) mouse has proven to be a useful model of this disease with similar metabolic and neurochemical alterations to those found in the human disease. In this study, the levels of the tryptophan derived excitotoxin quinolinic acid were examined in the brains of spf/Y mice. In addition, the neuropathology was examined using both light and electron microscopic approaches. Consistent with reports in children with urea cycle disorders, the levels of tryptophan and quinolinic acid were increased two-fold in various brain regions of the spf/Y mouse. Quinolinic acid, an agonist at the N-methyl-D-aspartate (NMDA) receptors, is known to produce selective cell loss in the striatum. We found a significant loss of medium spiny neurons and increased numbers of reactive oligodendroglia and microglia in the striatum of spf/Y mice. These neurochemical and neuropathological observations are consistent with an excitotoxic influence on brain injury in OCTD. It leads us to suggest that administration of NMDA receptor antagonists may ameliorate brain damage in children with inborn errors of urea synthesis.

Quinolinic acid production is related to macrophage tropic isolates of HIV-1

We sought to determine whether the neurotoxin quinolinic acid (QUIN) was produced by macrophages or lymphocytes infected with isolates of HIV-1 with varying degrees of macrophage tropism derived from patients with varying stages of AIDS dementia complex (ADC). Highly macrophage tropic isolates and minimally macrophage tropic isolates were used to inoculate macrophages and QUIN production was measured. Similarly, QUIN production from macrophages was monitored using a purified cell free highly macrophage tropic isolate and laboratory isolates SF33 and SF2. Each of these experiments was also performed with lymphocytes. We found that macrophages infected with macrophage tropic isolates of HIV-1 led to QUIN production while lymphocytes did not produce QUIN. The ability of the HIV-1 infected macrophages to produce QUIN was related to the viral inoculum and the degree of macrophage tropism of the isolate. The severity of ADC in the patient from whom a particular isolate was derived was not per se a determining factor for QUIN production. Purified cell free ADC isolates also led to QUIN production by macrophages thereby suggesting that HIV-1 infection alone is capable of inducing QUIN production.

Quinolinic acid in tumors, hemorrhage and bacterial infections of the central nervous system in children

A potential mechanism that may contribute to neurological deficits following central nervous system infection in children was investigated. Quinolinic acid (QUIN) is a neurotoxic metabolite of the kynurenine pathway that accumulates within the central nervous system following immune activation. The present study determined whether the levels of QUIN are increased in the cerebrospinal fluid of children with infections of the CNS, hydrocephalus, tumors or hemorrhage. Extremely high QUIN concentrations were found in patients with bacterial infections or the CNS, despite treatment with antimicrobial agents. CSF QUIN levels were also elevated to a lesser degree in patients with hydrocephalus or tumors. CSF L-kynurenine levels increased in parallel to the accumulations in QUIN, which is consistent with increased activity of the first enzyme of the kynurenine pathway, indoleamine-2,3-dioxygenase. The CSF levels of neopterin, a marker of immune and macrophage activation, were also increase in patients with infections. The cytokines tumor necrosis factor-alpha and interleukin-6 were also detected in some patients' samples, and were highest in patients with infection. These results suggest that QUIN is a sensitive marker of the presence of immune activation within the CNS. Further studies of QUIN as a potential contributor to neurologic dysfunction and neurodegeneration in children with CNS inflammation are warranted.

Metabolism of L-tryptophan to kynurenate and quinolinate in the central nervous system: effects of 6-chlorotryptophan and 4-chloro-3-hydroxyanthranilate

The metabolism of L-tryptophan to the neuroactive kynurenine pathway metabolites, L-kynurenine, kynurenate and quinolinate, and the effects of two inhibitors of quinolinate synthesis (6-chlorotryptophan and 4-chloro-3-hydroxyanthranilate) were investigated by mass spectrometric assays in cultured cells and in vivo. Cell lines obtained from astrocytoma, neuroblastoma, macrophage/monocytes, lung, and liver metabolized L-[13C6]-tryptophan to L-[13C6]kynurenine and [13C6]kynurenate, particularly after indoleamine-2,3-dioxygenase induction by interferon-gamma. Kynurenine aminotransferase activity was measurable in all cell types examined but was unaffected by interferon-gamma. These results suggest that many cell types can be sources of kynurenate following immune activation. In vivo synthesis of L-[13C6]kynurenine and [13C6]kynurenate from L-[13C6]tryptophan was studied in the CSF of macaques infected with poliovirus, as a model of inflammatory neurologic disease. The effects of 6-chlorotryptophan and 4-chloro-3-hydroxyanthranilate on the synthesis of kynurenate were different. 6-Chlorotryptophan attenuated formation of L-[13C6]kynurenine and [13C6]kynurenate and was converted to 4-chlorokynurenine and 7-chlorokynurenate. It may be an effective prodrug for the delivery of 7-chlorokynurenate, which is a potent antagonist of NMDA receptors. In contrast, 4-chloro-3-hydroxyanthranilate did not reduce accumulation of L-[13C6]kynurenine and [13C6]kynurenate. 6-Chlorotryptophan and 4-chloro-3-hydroxyanthranilate are useful tools to manipulate concentrations of quinolinate and kynurenate in the animal models of neurologic disease to evaluate physiological roles of these neuroactive metabolites.

Brain quinolinic acid in chronic experimental hepatic encephalopathy: effects of an exogenous ammonium acetate challenge

Elevated brain concentrations of the neurotoxin and NMDA receptor agonist quinolinic acid (QUIN) have been demonstrated in portacaval-shunted (PCS) rats, a chronic hepatic encephalopathy (HE) model. Increased brain QUIN levels have also been shown in acute hyperammonemic rats. In the present study, the plasma and brain (neocortical) QUIN levels in chronic PCS rats were investigated. The study also included a single exogenous ammonium acetate (NH4Ac; 5.2 mmol/kg, i.p.) challenge to precipitate a reversible hepatic coma. Compared with sham-operated controls, chronic PCS rats exhibited decreased rather than increased plasma and brain QUIN levels. The plasma-to-brain QUIN ratio was not found to be altered. The NH4Ac administration induced coma in all of the PCS rats 20-25 min after the challenge, and this coma was resolved within 60-75 min. No relevant temporal relationship between changes in brain QUIN levels and the neurological status in the PCS rats was observed. Therefore, our results do not support the contention that increased brain QUIN levels per se are involved in the pathogenesis of HE.

Increased human immunodeficiency virus (HIV) type 1 DNA content and quinolinic acid concentration in brain tissues from patients with HIV encephalopathy

Levels of human immunodeficiency virus type 1 (HIV-1) DNA and quinolinic acid were examined in areas of the central nervous system (CNS) and lymphoid organs (LN) from 5 AIDS patients with no clinically apparent CNS compromise (group I), 7 with CNS opportunistic diseases (group II), and 8 with HIV encephalopathy (group III). The brains from patients with HIV encephalopathy not only contained higher levels of HIV-1 DNA (cerebrum, P < .01; cerebellum, P < .05) as assessed by quantitative polymerase chain reaction but also showed a higher rate of viral pol region mutations suggestive of zidovudine or didanosine resistance than brains from patients in group I or II (P < .01). CNS quinolinic acid concentrations were significantly higher in group II and III patients than in group I (P = .03), even though quinolinic acid levels in LN were comparable among the 3 groups. These data suggest that CNS inflammatory changes associated with HIV encephalopathy may be triggered by a local productive HIV-1 infection within the CNS.

Zidovudine treatment prolongs survival and decreases virus load in the central nervous system of rhesus macaques infected perinatally with simian immunodeficiency virus

To assess the potential therapeutic effects of zidovudine, rhesus macaques were inoculated with simian immunodeficiency virus (SIV) strain SMM/B670 at birth and infused either continuously or intermittently with zidovudine for 6-7 months. Zidovudine did not prevent infection but did significantly increase survival time, which was associated with lower serum p26 viral core antigen levels, a lower virus burden in the cerebrospinal fluid (CSF), and lower CSF quinolinic acid levels than in untreated monkeys. Two of 5 infected, untreated monkeys developed motor impairment within 6 months following infection, whereas motor impairments did not occur in infected, zidovudine-treated monkeys until after the drug was discontinued. Zidovudine treatment was well tolerated by rhesus infants with minimal, transient side effects. These results demonstrate that zidovudine treatment significantly decreases virus load within the central nervous system (CNS) and delays the onset of CNS dysfunction and immune disease in rhesus monkeys perinatally infected with SIV.

Quinolinic acid accumulation and functional deficits following experimental spinal cord injury

Quinolinic acid (QUIN) is an excitotoxic tryptophan metabolite that is produced by activated macrophages. Accumulations of QUIN are implicated in the aetiology of a broad spectrum of human neurological diseases, particularly inflammatory conditions. To determine whether QUIN is an endogenous neurotoxin requires agents that reduce QUIN synthesis, and animal models where QUIN levels increase in association with neurological disease. Compression injury of the spinal cord of guinea pigs results in secondary neurological deficits, related to inflammation and macrophage activation. We evaluated whether 4-chloro-3-hydroxyanthranilate (4Cl-3HAA), an inhibitor of 3-hydroxyanthranilate-3,4-dioxygenase, reduces QUIN accumulations in this model and influences the progression of neurological deficits. Intraperitoneal injections of 4Cl-3HAA (100 mg/kg every 12 h) attenuated QUIN accumulations in spinal cord following injury, and reduced the severity of delayed functional deficits. Intraperitoneal injections of the macrophage toxin, silica, also reduced QUIN levels and attenuated neurological deficits. A direct subdural infusion of Cl-3HAA into the injured spinal cord (50 microM, 1 microliter/h) promptly exacerbated functional impairments, which suggests that the infusate had direct toxic effects. These studies demonstrate that guinea pigs with spinal cord injury constitute a useful model to study the mechanisms that increase central nervous system (CNS) QUIN levels in conditions of CNS inflammation, and to evaluate the neurochemical and neurological effects of agents designed to reduce the accumulations of QUIN and other potential pathogenic mediators within the CNS. The results are consistent with a contributory role for QUIN in the pathogenesis of secondary functional impairments following spinal cord injury, although the possibility that 4Cl-3HAA had additional effects independent of QUIN cannot be excluded. Further studies are required to determine whether the beneficial effects of 4Cl-3HAA are sustained. While it is unknown whether secondary inflammatory processes contribute significantly to neurological deficits in human spinal cord injury, strategies that reduce the accumulation of QUIN are worthy of consideration and evaluation as a therapeutic target.

The relationship between plasma and brain quinolinic acid levels and the severity of hepatic encephalopathy in animal models of fulminant hepatic failure

Quinolinic acid is an excitatory, neurotoxic tryptophan metabolite proposed to play a role in the pathogenesis of hepatic encephalopathy. This involvement was investigated in rat and rabbit models of fulminant hepatic failure at different stages of hepatic encephalopathy. Although plasma and brain tryptophan levels were significantly increased in all stages of hepatic encephalopathy, quinolinic acid levels increased three-to sevenfold only in the plasma, CSF, and brain regions of animals in stage IV hepatic encephalopathy. Plasma-CSF and plasma-brain quinolinic acid levels in rats and rabbits with fulminant hepatic failure were strongly correlated, with CSF and brain concentrations approximately 10% those of plasma levels. Moreover, there was no significant regional difference in brain quinolinic acid concentrations in either model. Extrahepatic indoleamine-2,3-dioxygenase activity was not altered in rats in stage IV hepatic encephalopathy, but hepatic L-tryptophan-2,3-dioxygenase activity was increased. These results suggest that quinolinic acid synthesized in the liver enters the plasma and then accumulates in the CNS after crossing a permeabilized blood-brain barrier in the end stages of liver failure. Furthermore, the observation of low brain concentrations of quinolinic acid only in stage IV encephalopathy suggests that the contribution of quinolinic acid to the pathogenesis of hepatic encephalopathy in these animal models is minor.

Early intrathecal events in rhesus macaques (Macaca mulatta) infected with pathogenic or nonpathogenic molecular clones of simian immunodeficiency virus

BACKGROUND

Encephalitis is a common and devastating sequela of HIV infection in humans and of simian immunodeficiency virus (SIV) infection in rhesus macaques. We used the SIV-infected rhesus macaque model to study early intrathecal events in the pathogenesis of lentiviral encephalitis.

EXPERIMENTAL DESIGN

To examine early events and to compare the neuroinvasiveness and neurovirulence of pathogenic (SIVmac239) and nonpathogenic (SIVmac1A11) molecular clones of SIV and the role of host immunity in the early postinfection period, we inoculated groups of rhesus macaques with each of these clones and compared them with a third group of animals inoculated with pathogenic uncloned SIV (SIVmac). We collected paired cerebrospinal fluid and sera before and at intervals after inoculation and determined albumin and IgG concentrations, SIV-specific humoral immune response, and concentrations of quinolinic acid. Two animals from each group were killed and necropsied at 2, 8, 13, and 23 weeks after inoculation. Routine histopathology and semi-quantitative in situ hybridization were performed on tissue from multiple levels of the central nervous system (CNS).

RESULTS

SIVmac and SIVmac-239 invaded both the meninges and the CNS parenchyma simultaneously within 2 weeks of inoculation, whereas nonpathogenic SIVmac-1A11 was not neuroinvasive. Gross disruption of the blood-brain barrier was not detected at any time. However, elevated IgG indices and high levels of cerebrospinal fluid quinolinic acid denoted intrathecal immune activation soon after viral neuroinvasion. Virus load in the CNS declined as the immune response peaked but subsequently increased with waning immunity. One macaque that never developed an SIV-specific immune response died with severe SIV encephalitis.

CONCLUSIONS

Our findings support the following hypotheses of early events in SIV neuropathogenesis: (a) Pathogenic virus invades the CNS within days of i.v. inoculation and elicits an intrathecal immune response, including intrathecal synthesis of IgG and macrophage activation; (b) the immune response initially is associated with a decreased virus load in the CNS; (c) as immunodeficiency develops, virus load in the CNS increases once again; and (d) both virus and host factors are important in determining the course of events.

Cerebrospinal fluid levels of kynurenine pathway metabolites in patients with eating disorders: relation to clinical and biochemical variable

In brain, most L-tryptophan is metabolized to indoleamines, whereas in systemic tissues L-tryptophan is catabolized to kynurenine pathway metabolites. Among these latter compounds are: quinolinic acid, an N-methyl-D-aspartate receptor agonist; kynurenic acid, an antagonist of excitatory amino acid receptors that also reduces quinolinic acid-mediated neurotoxicity; and L-kynurenine, a possible convulsant. Because the metabolism of L-tryptophan through the kynurenine pathway is dependent upon adequate nutrition, we sought to determine whether the impaired nutrition characteristic of eating-disordered patients might be associated with specific disturbances in this metabolic pathway. Cerebrospinal fluid levels of L-tryptophan, quinolinic acid, kynurenic acid, L-kynurenine, and 5-hydroxyindoleacetic acid were measured in medication-free female patients meeting DSM-III-R criteria for either anorexia nervosa (n = 10) or normal-weight bulimia nervosa (n = 22), studied at varying stages of nutritional recovery. Eight healthy, normal-weight females served as a comparison group. Cerebrospinal fluid levels of kynurenic acid were significantly reduced in underweight anorectics, compared to normal females, but returned to normal values with restoration of normal body weight. Although cerebrospinal fluid quinolinic acid levels were not different from controls, the ratio of quinolinic acid to kynurenic acid was significantly increased during the underweight phase of anorexia nervosa. Furthermore, in the eating-disordered patients, kynurenic acid levels in cerebrospinal fluid correlated positively with percent-of-population average body weight.(ABSTRACT TRUNCATED AT 250 WORDS)

The relationship between plasma and brain quinolinic acid levels and the severity of hepatic encephalopathy

BACKGROUND/AIMS

Quinolinic acid is an endogenous neuroexcitant derived from tryptophan. Brain quinolinic acid concentrations are reportedly elevated in chronic liver failure. The aim of this study was to determine if brain quinolinic acid levels correlate with the severity of hepatic encephalopathy.

METHODS

Postmortem samples of selected brain regions and plasma samples taken at several stages of encephalopathy were obtained from patients with acute and chronic liver failure. Quinolinic acid levels were measured by mass spectroscopy using [18O]quinolinic acid.

RESULTS

Plasma quinolinic acid levels were significantly increased by stage I encephalopathy in patients with acute liver failure and by stages II and III in patients with chronic liver failure. Brain quinolinic acid levels were elevated only in patients with acute liver failure and were uniformly distributed at concentrations below those observed in plasma.

CONCLUSIONS

The uniform distribution of quinolinic acid at subplasma concentrations in the brains of patients with acute liver failure suggests that it is synthesized peripherally and enters the brain across a permeabilized blood-brain barrier. Whereas the elevation of brain quinolinic acid levels in patients who died of acute but not chronic liver failure suggests that the involvement of quinolinic acid in the pathogenesis of hepatic encephalopathy is minimal, it could predispose these patients to seizures.

6-Chloro-D,L-tryptophan, 4-chloro-3-hydroxyanthranilate and dexamethasone attenuate quinolinic acid accumulation in brain and blood following systemic immune activation

Accumulations of the neurotoxin quinolinic acid (QUIN) occur in the brain and blood following immune activation and are attributed to increased metabolism of L-tryptophan through the kynurenine pathway. Systemic administration of 4-chloro-3-hydroxyanthranilate (an inhibitor of 3-hydroxyanthranilate-3,4-dioxygenase), 6-chloro-D,L-tryptophan (a substrate of the kynurenine pathway) and dexamethasone (an anti-inflammatory agent) attenuated the accumulation of QUIN in the brain and blood following systemic pokeweed mitogen administration to mice. 6-Chloro-D,L-tryptophan and dexamethasone also attenuated the increases in brain and lung indoleamine-2,3-dioxygenase activity and elevations in plasma L-kynurenine levels. We conclude that QUIN formation can be modified by drugs which act at different levels of the cascade of events that link immune stimulation to increased kynurenine pathway metabolism.

Cerebrospinal fluid nitrite/nitrate levels in neurologic diseases

Nitric oxide has been proposed to mediate cytotoxic effects in inflammatory diseases. To investigate the possibility that overproduction of nitric oxide might play a role in the neuropathology of inflammatory and noninflammatory neurological diseases, we compared levels of the markers of nitric oxide, nitrite plus nitrate, in the CSF of controls with those in patients with various neurologic diseases, including Huntington's and Alzheimer's disease, amyotrophic lateral sclerosis, and HIV infection. We found that there were no significant increases in the CSF levels of these nitric oxide metabolites, even in patients infected with HIV or in monkeys infected with poliovirus, both of which have significantly elevated levels of the neurotoxin quinolinic acid and the marker of macrophage activation, neopterin. However, CSF quinolinic acid, neopterin, and nitrite/nitrate levels were significantly increased in a small group of patients with bacterial and viral meningitis.

Viral load and its relationship to quinolinic acid, TNF alpha, and IL-6 levels in the CNS of retroviral infected mice

Mouse models of infection of the central nervous system (CNS) have been used to study retroviral-induced neurologic disease. Ecotropic-neurotropic murine leukemia virus (MuLV) infection of susceptible neonatal mice causes a neurologic disease characterized by progressive hindlimb paralysis. The lesions consist of chronic noninflammatory spongiform change predominantly involving brainstem and spinal cord. Two molecularly cloned strains of MuLV, ts-1, a temperature-sensitive mutant of Moloney MuLV, and pNE-8, derived from a feral mouse isolate Cas-Br-E, were used in this study. Infected mice were sacrificed at regular intervals postinoculation throughout the time-course of disease. The neuropathology was evaluated using standard histological and immunohistopathological techniques. Tissue concentrations of viral proteins and potentially cytotoxic factors were compared with the histopathology in select regions of the CNS. Areas of extensive vacuolation with neuronal and oligodendroglial infection were observed in spinal cord, brainstem, and cerebellum. High titers of infectious virus were observed within CNS lesions, whereas low titers were observed in morphologically uninvolved areas. Western blot analysis revealed abundant production of viral envelope proteins, which correlated well with infectious virus titers. Serum quinolinic acid (QUIN) concentrations in both groups of noninfected and infected mice were similar. However, CNS tissue concentrations of QUIN, TNF alpha, and IL-6 in ts-1 infected mice were significantly higher than in pNE-8 infected or noninfected mice. The difference in concentration of these factors may be the result of greater activation of macrophages/microglia in ts-1 infected mice. During murine retroviral encephalitis, CNS damage may be mediated by direct infection of CNS cells and may be enhanced by indirect effects of neurotoxic factors possibly secreted by infected/activated macrophages.