Quinolinic acid accumulation and functional deficits following experimental spinal cord injury

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.

Effects of the 3-hydroxyanthranilic acid analogue NCR-631 on anoxia-, IL-1 beta- and LPS-induced hippocampal pyramidal cell loss in vitro

The kynurenine pathway intermediate 3-hydroxyanthranilic acid (3-HANA) is converted by 3-HANA 3,4-dioxygenase (3-HAO) to the putative neuropathogen quinolinic acid (QUIN). In the present study, the neuroprotective effects of the 3-HANA analogue and 3-HAO inhibitor NCR-631 was investigated using organotypic cultures of rat hippocampus. An anoxic lesion was induced by exposing the cultures to 100% N2 for 150 min, resulting in a pronounced loss of pyramidal neurons, as identified using NMDA-R1 receptor subunit immunohistochemistry. NCR-631 provided a concentration-dependent protective effect against the anoxia. NCR-631 was also found to counteract the loss of pyramidal neurons in two models of neuroinflammatory-related damage; incubation with either LPS (10 ng/ml) or IL-1 beta (10 IU/ml). The findings suggest that NCR-631 has neuroprotective properties and that it may be a useful tool to study the role of kynurenines in neurodegeneration.

Expression of the kynurenine enzymes in macrophages and microglial cells: regulation by immune modulators

The regulation of the expression of indoleamine 2,3-dioxygenase (IDO) was studied in cloned murine macrophages (MT2) and microglial (N11) cells. Both cell lines express IDO and inducible nitric oxide synthase activity after interferon-gamma (IFN-gamma) stimulation. The regulation of IDO expression appears to differ in the two cell lines. Nitric oxide (NO) production negatively modulates the expression of IDO activity in IFN-gamma-primed macrophages, thereby indicating a cross-talk between the kynurenine and nitridergic pathways in these cells. Conversely, this down-regulation of IDO activity by NO does not occour in microglial cells. A differential regulation of IDO expression in the two cell lines was also observed with LPS and picolinic acid. Together with previous findings, these results indicate the existence of marked differences in the regulation of the expression of the kynurenine pathway enzymes between macrophages and microglial cells.

Species differences in L-tryptophan-kynurenine pathway metabolism: quantification of anthranilic acid and its related enzymes

Anthranilic acid (AA) has attracted considerable attention as one of the L-tryptophan-kynurenine pathway metabolites in the central nervous system. In this study, a highly sensitive and accurate method for the quantification of AA has been developed using reversed-phase high-performance liquid chromatography with electrochemical detection. Serum and cerebrospinal fluid (CSF) AA concentrations in different animal species were measured. CSF AA concentrations in rabbits were 1.1 +/- 0.1 nmol/liter, which were 5. 7-33.0 times lower than those in other species studied. Serum AA concentrations, however, were slightly higher in rabbits than in other species. In contrast, the concentrations of L-kynurenine (L-KYN) in both serum and CSF were substantially higher in rabbits than in other species. Tissue kynurenine pathway enzymes, indoleamine 2,3-dioxygenase (IDO), tryptophan 2,3-dioxygenase, kynurenine 3-hydroxylase, and kynureninase were determined in rabbits, rats, gerbils, and mice. These enzymes varied among species, especially lung IDO activities in rabbits were 146-516 times higher than those found in other species, but rabbit liver kynurenine 3-hydroxylase activities were lower by one order of magnitude than those of the other species tested. Furthermore, brain kynurenine 3-hydroxylasae activities were 12.3-23.2 times higher in gerbils than those in the other species tested. In addition, AA concentrations in serum following intravenous administration of L-KYN (5 mg/kg) were also measured in rabbits. AA levels peaked sharply within 5 min after administration and decreased in a time-dependent manner. At 5 min after administration, CSF L-KYN and AA concentrations were also increased by 1.76- and 2.56-fold, respectively, compared with basal levels. Increased AA concentrations in CSF following L-KYN administration may reflect the entry of AA into the CSF after conversion to AA in systemic tissue and/or the local synthesis of AA from L-KYN in the CNS.

Methylprednisolone inhibition of TNF-alpha expression and NF-kB activation after spinal cord injury in rats

Post-traumatic inflammatory reaction has been implicated in the secondary injury after SCI. TNF-alpha is a key inflammatory mediator, which plays a pathogenetic role in cell death in inflammatory disorders and traumatic brain injury. TNF-alpha exerts its effector actions, at least partially, through the activation of a pro-inflammatory transcription factor, NF-kB, which in turn upregulates such genes as iNOS, cytokines, adhesive molecules, and others. Consistent with a post-traumatic inflammatory reaction after SCI, we noted an increase in TNF-alpha expression by Western blotting (4.5-fold increase at 1 day after SCI, P<0.01) and immunohistochemistry in a rat SCI model. Post-traumatic TNF-alpha expression was accompanied by an increase in NF-kB binding activity in nuclear proteins isolated from the injured cord (3.9-fold increase, P<0.01). MP is the only drug proven effective in improving neurological function in patients with acute SCI. The mechanism of action of MP is not fully understood, but is thought to be related to its antioxidant effects. MP is also a potent anti-inflammatory agent, which has been recently shown to inhibit NF-kB binding activity. MP (30 mg/kg, i.v.) given immediately after SCI reduced TNF-alpha expression by 55% (P<0.01) and NF-kB binding activity. These findings suggest that post-traumatic inflammatory activity that is mediated by the TNF-alpha-NF-kB cascade can be suppressed by MP.

Temporal and spatial changes of quinolinic acid immunoreactivity in the gerbil hippocampus following transient cerebral ischemia

Quinolinic acid (QUIN) is an endogenous neurotoxin which originates from the kynurenine pathway of tryptophan metabolism. An increase of brain QUIN level occurs in several degenerative and inflammatory disorders, but the cellular source of QUIN is still a matter of controversy. In the present study, the gerbil model of transient global ischemia was used to investigate the time course and the cellular localization of QUIN immunoreactivity. Neurodegeneration was evident in the subiculum and in the CA1 area of the hippocampus 4, 7 and 14 days after ischemia. QUIN positive cells, with microglia-like morphology, appeared in the subiculum and in the CA1, 4 days after ischemia. At 7 days post-ischemia they extended to the whole CA1, disappearing at 14 days. Neither neurodegeneration nor QUIN positive cells could be detected in ischemic gerbils sacrificed at 1 and 2 days after ischemia and in sham-operated animals. These findings suggest that microglia-like cells infiltrating the degenerating areas of the hippocampus represent the major source of QUIN following transient ischemia in the gerbil. Thus, in situ production of QUIN in vulnerable brain regions may contribute to the pathophysiological mechanisms of delayed brain injury.

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.

Traumatic spinal cord injury induces nuclear factor-kappaB activation

Inflammatory responses are a major component of secondary injury and play a central role in mediating the pathogenesis of acute and chronic spinal cord injury (SCI). The nuclear factor-kappaB (NF-kappaB) family of transcription factors is required for the transcriptional activation of a variety of genes regulating inflammatory, proliferative, and cell death responses of cells. In this study we examined the temporal and cellular expression of activated NF-kappaB after traumatic SCI. We used a contusion model (N.Y.U. Impactor) to initiate the early biochemical and molecular changes that occur after traumatic injury to reproduce the pathological events associated with acute inflammation after SCI. The activation and cellular distribution of activated NF-kappaB was evaluated by using a monoclonal antibody that selectively recognizes activated p65 in a NF-kappaB dimer. Immunohistochemical and Western blot analyses demonstrated that NF-kappaB activation occurred as early as 0.5 hr postinjury and persisted for at least 72 hr. Using electrophoretic mobility shift assays (EMSA), we demonstrate that NF-kappaB is activated after SCI. In our immunohistochemical, Western, and EMSA experiments there are detectable levels of activated NF-kappaB in our control animals. Using double-staining protocols, we detected activated NF-kappaB in macrophages/microglia, endothelial cells, and neurons within the injured spinal cord. Colocalization of activated NF-kappaB with the NF-kappaB-dependent gene product, inducible nitric oxide synthase (iNOS), suggests functional implications for this transcription factor in the pathogenesis of acute spinal cord injury. Although there is considerable evidence for the involvement of an inflammatory reaction after traumatic SCI, this is the first evidence for the activation of NF-kappaB after trauma. Strategies directed at blocking the initiation of this cascade may prove beneficial as a therapeutic approach for the treatment of acute SCI.

Altered tryptophan metabolism in mice with herpes simplex virus encephalitis: increases in spinal cord quinolinic acid

Mice infected with the herpes simplex virus, type-1, developed a paralysis which was associated with increased levels of the neurotoxin quinolinic acid (QUIN). The largest increases in QUIN were observed in the spinal cord with much smaller changes in the rostral forebrain or serum. The time course for the paralysis coincided with the increase in spinal cord QUIN, a maximal 40-fold elevation, at 7-10 days post infection. The time course suggested that the increases in QUIN were due to its local synthesis. Consistent with this possibility, herpes virus-infected mice had increased activities of indoleamine 2,3-dioxygenase and kynurenine hydroxylase (two key enzymes in QUIN formation), when compared to non-infected controls. Since QUIN is formed by activated macrophages, these new data are consistent with QUIN formation as part of the host response to a pathogen whose importance is discussed.

S-aryl cysteine S,S-dioxides as inhibitors of mammalian kynureninase

A series of 2-amino-S-aryl cysteine S,S-dioxides have been synthesised and shown to inhibit kynureninase an important enzyme in the biosynthesis of the known excitotoxic moiety quinolinic acid. The most potent of these, 2-amino-5-methyl-S-phenyl cysteine S,S-dioxide 6d, inhibits interferon-gamma induced synthesis of quinolinic acid in human macrophages.

Synthesis and biochemical evaluation of N-(4-phenylthiazol-2-yl)benzenesulfonamides as high-affinity inhibitors of kynurenine 3-hydroxylase

In this paper we describe the synthesis, structure-activity relationship (SAR), and biochemical characterization of N-(4-phenylthiazol-2-yl)benzenesulfonamides as inhibitors of kynurenine 3-hydroxylase. The compounds 3,4-dimethoxy-N-[4-(3-nitrophenyl)thiazol-2-yl]benzenesulfonamide 16 (IC50 = 37 nM, Ro-61-8048) and 4-amino-N-[4-[2-fluoro-5-(trifluoromethyl)phenyl]-thiazol-2-yl] benzenesulfonamide 20 (IC50 = 19 nM) were found to be high-affinity inhibitors of this enzyme in vitro. In addition, both compounds blocked rat and gerbil kynurenine 3-hydroxylase after oral administration, with ED50's in the 3-5 mumol/kg range in gerbil brain. In a microdialysis experiment in rats, 16 dose dependently increased kynurenic acid concentration in the extracellular hippocampal fluid. A dose of 100 mumol/kg po led to a 7.5-fold increase in kynurenic acid outflow. These new compounds should allow detailed investigation of the pathophysiological role of the kynurenine pathway after neuronal injury.

Cloning and functional expression of human kynurenine 3-monooxygenase

Kynurenine 3-monooxygenase, an NADPH-dependent flavin monooxygenase, catalyses the hydroxylation of L-kynurenine to L-3-hydroxykynurenine. By hybridization screening using a cDNA probe encoding the entire exon 2 of Drosophila melanogaster kynurenine 3-monooxygenase, we isolated a 2.0 kb cDNA clone coding for the corresponding human liver enzyme. The deduced amino acid sequence of the human protein consists of 486 amino acids with a predicted molecular mass of 55,762 Da. Transfection of the human cDNA in HEK-293 cells resulted in the functional expression of the enzyme with kinetic properties similar to those found for the native human protein. RNA blot analysis of human tissues revealed the presence of a major mRNA species of approximately 2.0 kb in liver, placenta and kidney.

Spinal cord compression injury in guinea pigs: structural changes of endothelium and its perivascular cell associations after blood-brain barrier breakdown and repair

This study examines morphological changes of the blood-brain barrier (BBB) after spinal cord compression. The lowest thoracic segment (T13) of female guinea pigs was injured and the BBB was tested from 7 days to 5.5 months postinjury using intravenously injected horseradish peroxidase (HRP) as a tracer. Tracer leakage in the injured segment was verified with the light microscope and the fine structure of capillaries was examined. Diffuse tissue staining was observed at T13 up to 2 weeks following injury. A leaky BBB correlated with expected changes in the fine structure of endothelial cell junctions. These were predominantly nonoverlapping cell junctions which, in many instances, were separated by clefts between adjacent cells. At early survival times, numerous capillary profiles with juxtaposed astrocyte foot processes were noted in addition to altered cell associations. Complete sealing of the BBB against interstitial HRP leakage was not observed until 17 days postinjury. After the first week, some of the endothelial cells were contacted by macrophages, processes of perivascular microglia, and processes of swollen and degenerating astrocytes. Perivascular spaces varied in extent and contained amorphous deposits of extracellular materials in addition to supernumerary layers of basal lamina. The early changes were followed by profound tissue restructuring due to loss of both neurons and glia. At longer survival times the BBB to HRP repaired. Endothelial cells formed complex overlapping junctions with zonulae occludentes. Most of the capillaries in the injured segment were no longer in direct contact with astrocyte foot processes, although reactive astrocytes constituted the predominant cell type in the remaining gray matter. Substantial expansion of perivascular spaces was evident. The cytoplasm of endothelial cells had numerous pinocytotic vesicles. Perivascular spaces contained layers of assembled collagen arranged perpendicularly to each other in addition to amorphous matrix materials. The findings suggest that decoupling of astrocyte foot processes from endothelial cell surfaces does not prevent reformation of tight junctions. It remains to be examined what effects the larger perivascular spaces, extracellular matrix deposits, and changes of cell associations may have on transport systems and ionic buffering. The data are relevant for estimating an opportune time for application of barrier-impermeable drugs to the lesion area.

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.

Enzyme-catalyzed production of the neuroprotective NMDA receptor antagonist 7-chlorokynurenic acid in the rat brain in vivo

NMDA receptors play a critical role in neurotransmission and are also involved in the occurrence of excitotoxic nerve cell death. Synthetic halogenated analogs of the endogenous broad spectrum excitatory amino acid receptor blocker kynurenic acid are among the most potent and selective antagonists of the glycine co-agonist site of the NMDA receptor complex. Pharmacological blockade of this site provides neuroprotection in animal models of cerebral ischemia, epilepsy and neurodegenerative disorders, and does not appear to be associated with some of the undesirable side effects linked to classic competitive and non-competitive NMDA receptor antagonists. Here we demonstrate the neuroprotective quantities of 7-chloro-kynurenic acid (7-Cl-KYNA), one of the most selective and well-studied glycine site antagonists, can be synthesized in the brain from its bioprecursor L-4-chlorokynurenine (4-Cl-KYN). Intracerebral infusion of 4-Cl-KYN dose-dependently reduced quinolinate neurotoxicity in the rat hippocampus after enzymatic conversion to 7-Cl-KYNA by kynurenine aminotransferase. In accordance with previous studies demonstrating that kynurenine aminotransferase is preferentially localized in astrocytes, both the enzymatic formation of 7-Cl-KYNA and the neuroprotective potency of 4-Cl-KYN were substantially reduced following an intrahippocampal injection of the gliotoxin fluorocitrate. In situ produced 7-Cl-KYNA offers a novel neuroprotective strategy for targeting the glycine/NMDA site while avoiding excessive receptor blockade and reducing the clinical risks associated with conventional NMDA receptor antagonism.

Primary demyelination and regeneration of ascending axons in the dorsal funiculus of the rat spinal cord following photochemically induced injury

The extent of primary demyelination and regeneration of ascending axons in the dorsal funiculus of the rat spinal cord was investigated following photochemically-induced ischaemic injury. Groups of rats were killed at intervals from 48h to 1 month after injury and a combination of light and electron microscopy and counting of axons in specific sites was used to study the axonal changes. Unmyelinated axons were noted in the dorsal rim of the lesion at its centre and at the centre of the gracile fasciculus at the caudal end of the lesion 7 days after injury. By 1 month, axons in these sites were thinly myelinated by Schwann cells or oligodendrocytes. In order to differentiate between remyelination of demyelinated axons and myelination of regenerated axons, axon counts were performed. The number of sub-pial axons present at the lesion centre did not change significantly from 48h to 1 month after injury, whereas the number of axons at the caudal end of the lesion increased significantly from 4 to 10 days after injury. We therefore conclude that sub-pial axons at the lesion centre are demyelinated between 4 and 7 days after injury and subsequently remyelinated by Schwann cells. At the caudal end of the lesion, a specific population of small diameter axons located at the centre of the gracile fasciculus regenerates for a distance of approximately 1 mm between 4 and 10 days after injury; these axons are then myelinated by oligodendrocytes or Schwann cells. In contrast, larger diameter axons of the gracile fasciculus do not show a regenerative response, demonstrating the variability of axonal responses to injury.

Reconstruction of the glial environment of a photochemically induced lesion in the rat spinal cord by transplantation of mixed glial cells

It is becoming increasingly apparent that the astrocytic environment is critical to the normal development and functioning of the CNS, and that acute injury to the spinal cord causes destruction of glial cells in addition to neurones and axons. The aims of this study were to assess the viability of reconstructing the astrocytic environment of a cystic spinal cord lesion by transplantation of glial cells and to examine the effect of the transplanted cells on meningeal cell invasion and revascularisation of the lesion and on axonal regeneration. Neonatal rat and kitten mixed glial cells and the CG-4 rat O-2A progenitor cell line were transplanted into a lesion produced in the dorsal funiculus of the rat spinal cord by photochemical infarction. The animals were killed 4 weeks after injury, their cords examined with light and electron microscopy and compared with control animals that were injected with medium alone. Transplantation of all three preparations resulted in increased numbers of astrocytes in the area of Wallerian degeneration cranial to the lesion and within the cyst. Mixed glial cell cultures prepared from neonatal rat forebrain contained cells with in vitro characteristics of type-1 astrocytes, and produced dense clusters of astrocytes that were surrounded by meningeal cells, resulting in a fragmented environment in the cyst. In contrast, glial cell cultures prepared from kitten forebrain and the CG-4 cell line produced cells that filled the cyst with a loose network of fine processes and reduced meningeal cell infiltration of the lesion. The CG-4 cell line significantly increased the density of blood vessels in the centre of the lesion and the number of spared axons present dorsal to the lesion, but none of the preparations significantly increased the number of axons regenerating at the caudal end of the lesion. We conclude that O-2A progenitor-derived astrocytes are more suitable for reconstruction of the glial environment of a cystic lesion in the rat spinal cord than 'type-1 like' astrocytes and would therefore be the cell of choice to engineer to produce factors that promote axonal regeneration.

Intrathecal infusion of the nitric oxide synthase inhibitor N-methyl L-arginine after experimental spinal cord injury in guinea pigs

The potential role of nitric oxide (NO) production in secondary pathologic processes that follow spinal cord injury was examined in a guinea pig model that shows secondary loss of function for at least 3 days after trauma. Lateral compression injury of the lower thoracic cord was performed under ketamine/xylazine/acepromazine anesthesia. A fine polyethylene cannula was inserted through an incision in the dura rostral to the injury and run along the dorsal subdural space to the lesion level. The tube was connected to an osmotic pump delivering 1 microL/h of a 10 mM solution of either N-methyl-L-arginine or N-methyl-D-arginine in normal saline (pH 7.2). N-Methyl-L-arginine blocks both constitutive and inducible forms of NO synthase (NOS), present in neurons and inflammatory cells, respectively: N-methyl-D-arginine is the inactive stereoisomer. Two groups of 10 animals were used. Behavioral analysis and somatosensory evoked potential measurements were performed daily for 3 days, then the animals were fixed and survival of white matter at the center of the injury was evaluated, using toluidine-blue stained, 1 microns plastic sections. No significant difference was found between treated and control groups in degree or rate of secondary loss of spinal cord function or in the cross-sectional area of surviving white matter. These data do not support the hypothesis that local NO production by phagocytes, neurons, or other cells plays a significant role in secondary pathology of injury in this model.

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.