Metabolism of [5-3H]kynurenine in the rat brain in vivo: evidence for the existence of a functional kynurenine pathway

Comparative effects of oxygen on indoleamine 2,3-dioxygenase and tryptophan 2,3-dioxygenase of the kynurenine pathway

Indoleamine 2,3-dioxygenase (IDO) reacts with either oxygen or superoxide and tryptophan (trp) or other indoleamines while tryptophan 2,3-dioxygenase (TDO) reacts with oxygen and is specific for trp. These enzymes catalyze the rate-limiting step in the kynurenine (KYN) pathway from trp to quinolinic acid (QA) with TDO in kidney and liver and IDO in many tissues, including brain where it is low but inducible. QA, which does not cross the blood-brain barrier, is an excitotoxin found in the CNS during various pathologies and is associated with convulsions. We proposed that HBO-induced convulsions result from increased flux through the KYN pathway via oxygen stimulation of IDO. To test this, TDO and IDO of liver and brain, respectively, of Sprague Dawley rats were assayed with oxygen from 0 to 6.2 atm HBO. TDO activity was appreciable at even 30 microM oxygen and rose steeply to a maximum at 40 microM. Conversely, IDO had almost no detectable activity at or below 100 microM oxygen and maximum activity was not reached until about 1150 microM. (Plasma contains about 215 microM oxygen and capillaries about 20 microM oxygen when rats breathe air.) KYN was 60% higher in brains of HBO-convulsed rats compared to rats breathing air. While the oxygen concentration inside cells of rats breathing air or HBO is not known precisely, it is clear that the rate-limiting, IDO-catalyzed step in the brain KYN pathway (but not liver TDO) can be greatly accelerated in rats breathing HBO.

Xanthurenic acid provokes formation of unfolded proteins in endoplasmic reticulum of the lens epithelial cells

The role of xanthurenic acid in a cell is unknown, but it is suspected to provoke several diseases. This study shows that accumulation of xanthurenic acid in the lens epithelial cells leads to an overexpression of endoplasmic reticulum (ER) resident stress chaperones proteins, glucose-regulated protein (Grp94), and calreticulin. Both chaperones proteins are overexpressed in the presence of unfolded proteins. A formation of the unfolded protein in the presence of xanthurenic acid may take place due to covalent binding of xanthurenic acid to protein. Grp94 is responsible for scavenging of the unfolded proteins. The results suggest that Grp94 scavenged xanthurenic acid-modified proteins, and for this reason become preferentially yellow-stained in the presence of yellow xanthurenic acid. Such a modified Grp94 is weakly recognized by anti-Grp94 antibody. An end point of the xanthurenic acid accumulation in the cell is the cell death. In conclusion xanthurenic acid can lead to cell pathology.

3-Hydroxykynurenine potentiates quinolinate but not NMDA toxicity in the rat striatum

L-3-Hydroxykynurenine (L-3-HK) and quinolinate (QUIN) are two metabolites of the kynurenine pathway, the major route of tryptophan degradation in mammals. L-3-HK is a known generator of highly reactive free radicals, whereas QUIN is an endogenous excitotoxin acting specifically at N-methyl-D-aspartate (NMDA) receptors. This study was designed to examine possible synergistic interactions between L-3-HK and QUIN in the rat brain in vivo. Intrastriatal coinjection of 5 nmol L-3-HK and 15 nmol QUIN, i.e. doses which caused no or minimal neurodegeneration on their own, resulted in substantial neuronal loss, determined both behaviourally (apomorphine-induced rotations) and histologically (quantitative assessment of lesion size). The excitotoxic nature of the lesion was verified by tyrosine hydroxylase immunohistochemistry, showing the survival of dopaminergic striatal afferents. There was also a relative sparing of large striatal neurons, and neurodegeneration was prevented both by NMDA receptor blockade (using CGP 40116) and free radical scavenging [using N-tert-butyl-alpha-(2-sulphophenyl)-nitrone, S-PBN]. The pro-excitotoxic features of L-3-HK were especially pronounced at low QUIN doses and were not observed when QUIN was substituted by NMDA. Notably, the effect of L-3-HK was not due to its intracerebral conversion to QUIN and was duplicated by equimolar D,L-3-HK. These data indicate that an elevation of L-3-HK levels constitutes a significant hazard in situations of excitotoxic injury. Pharmacological interventions aimed at decreasing L-3-HK formation may therefore be particularly useful for the treatment of neurological diseases which are associated with an abnormally enhanced flux through the kynurenine pathway.

Quantitative differences in the effects of de novo produced and exogenous kynurenic acid in rat brain slices

Kynurenic acid (KYNA) is an antagonist of (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptors and it blocks the glycine site of the NMDA receptor preferentially (IC50 = 7.9 microM). KYNA is produced endogenously by transamination of its precursor L-kynurenine (L-KYN). We tested the hypothesis that effects of endogenous, de novo produced KYNA, following bath-application of L-KYN to slices, would be different than effects of commercially-synthesized (exogenous) KYNA. The ability to block spontaneous epileptiform activity, induced by lowering extracellular magnesium, was examined in area CA3 of hippocampus and the entorhinal cortex. At a concentration of 200 microM L-KYN, which produced 0.89 +/- 0.20 microM KYNA, there were fewer slices with spontaneous epileptiform activity than slices exposed to 2 microM exogenous KYNA. The results indicate a more potent neuromodulatory action of endogenous KYNA than has been previously realized.

Kynurenic acid in brain and cerebrospinal fluid of fetal, newborn, and adult sheep and effects of placental embolization

Concentrations of the endogenous glutamate receptor antagonist kynurenic acid (KA) were measured in various brain regions and in cisternal cerebrospinal fluid of fetal, newborn, and adult sheep. KA concentrations were significantly higher in the fetal brain and cerebrospinal fluid at 90 and 140 d gestation compared with postnatal ages. In fetuses of 132-139 d gestation, KA concentrations in cerebrospinal fluid collected by drainage from an indwelling cisternal catheter increased significantly after infusion of the organic acid transport inhibitor probenecid (100 or 200 mg/kg, i.v.) indicating active transport of KA out of the fetal brain. In fetuses in which the umbilical circulation had been chronically restricted from 120 to 140 d gestation by partial embolization of the placenta, plasma concentrations of the KA precursor kynurenine were significantly lower than in control fetuses, and KA concentrations in the hypothalamus and hippocampus were significantly reduced; other brain regions were not affected. These results indicate that the production of KA is higher in the fetal brain compared with the newborn and adult brain. Because KA diminishes the risk of excitotoxic neuronal damage under hypoxic-ischemic conditions, the high levels of KA in the brain before birth may have a neuroprotective function. The decrease of KA concentrations in the hypothalamus and hippocampus after umbilical embolization suggests that, after chronic hypoxia in utero, these regions of the brain may become more vulnerable to subsequent episodes of acute hypoxia or ischemia encountered in late gestation or during parturition.

2-Oxoacids regulate kynurenic acid production in the rat brain: studies in vitro and in vivo

This study was designed to examine the role of 2-oxoacids in the enzymatic transamination of L-kynurenine to the excitatory amino acid receptor antagonist, kynurenate, in the rat brain. In brain tissue slices incubated in Krebs-Ringer buffer with a physiological concentration of L-kynurenine, pyruvate, and several other straight- and branched-chain 2-oxoacids, substantially restored basal kynurenate production in a dose-dependent manner without increasing the intracellular concentration of L-kynurenine. All 2-oxoacids tested also reversed or attenuated the hypoglycemia-induced decrease in kynurenate synthesis, but only pyruvate and oxaloacetate also substantially restored intracellular L-kynurenine accumulation. Thus, 2-oxoacids increase kynurenate formation in the brain primarily by functioning as co-substrates of the transamination reaction. This was supported further by the fact that the nonspecific kynurenine aminotransferase inhibitors (aminooxy)acetic acid and dichlorovinylcysteine prevented the effect of pyruvate on kynurenate production in a dose-dependent manner. Moreover, all 2-oxoacids tested attenuated or prevented the effects of veratridine, quisqualate, or L-alpha-aminoadipate, which reduce the transamination of L-kynurenine to kynurenate. Finally, dose-dependent increases in extracellular kynurenate levels in response to an intracerebral perfusion with pyruvate or alpha-ketoisocaproate were demonstrated by in vivo microdialysis. Taken together, these data show that 2-oxoacids can directly augment the de novo production of kynurenate in several areas of the rat brain. 2-Oxoacids may therefore provide a novel pharmacological approach for the manipulation of excitatory amino acid receptor function and dysfunction.

Effects of oxygen on 3-hydroxyanthranilate oxidase of the kynurenine pathway

Iron containing 3-Hydroxyanthranilate oxidase (3HAO) converts 3-hydroxyanthranilate (3HAA) and dioxygen into a precursor which spontaneously converts to quinolinic acid (QA). 3HAO participates in de novo biosynthesis of NAD in mammalian kidney and liver, and it is present in low concentrations in brain where its function is controversial. However, QA increases in spinal fluid and is associated with convulsions in AIDS dementia, Huntington's disease, and CNS inflammation. QA is a known N-methyl, D-aspartate receptor agonist and excitotoxin that causes convulsions when injected into the brain. Hyperbaric oxygen (HBO) also causes convulsions and we investigated the interrelationships among the stimulating and toxic effects of oxygen and the role of iron in vitro using rat liver enzyme which is reported to be identical to brain enzyme and is more abundant. 3HAO requires dioxygen as a substrate but it was inactivated approximately 40% by 5.2 atm HBO in vitro in 15 min. The apparent Km was 2.6 x 10(-4) M for oxygen and 5 x 10(-5) M for 3HAA, and these values did not change for enzyme that was half-inactivated by HBO oxygen. Thus, oxygen-inactivation appears to be all-or-none for individual enzyme molecules. Freshly prepared enzyme was activated about 3-fold by incubation with acidic iron. Iron-staining of 3HAO, separated by gel electrophoresis after partial purification by FPLC, showed that loss of iron and loss of enzyme activity during HBO exposure were correlated. The apparent oxygen Km of 3HAO is far higher than the oxygen concentration in brain cells. Thus, 3HAO is capable of being stimulated initially in animals breathing HBO, and subsequently of being inactivated with potential significance for brain QA and convulsions.

Effects of central and peripheral administration of kynurenic acid on hippocampal evoked responses in vivo and in vitro

Kynurenic acid is an excitatory amino acid antagonist with preferential activity at the N-methyl-D-aspartate subtype of glutamate receptors. It is produced endogenously in the brain, but is synthesized more effectively in the periphery. The influence of peripheral kynurenic acid on brain function is unclear because kynurenic acid is likely to penetrate the blood-brain barrier poorly. To determine the potential central effects of peripheral kynurenic acid, we compared its effects in the hippocampus after peripheral or direct administration. The hippocampus of the rat was chosen as a test system because this region receives glutamatergic inputs, and because responses to stimulation of these inputs can be compared after peripheral drug administration in vivo, and after direct administration of drugs in vitro. Peripherally-administered kynurenic acid was injected via a catheter in the jugular vein. Bath-application to hippocampal slices was used to test effects of direct administration. Area CA1 pyramidal cells and dentate gyrus granule cells were examined by extracellular recording and stimulation of area CA3 or the perforant path, respectively. Pairs of identical stimuli were used to assess paired-pulse inhibition and paired-pulse facilitation. Kynurenic acid decreased evoked responses in area CA1 and the dentate gyrus, both in vivo and in vitro. Effective concentrations were in the low micromolar range, and therefore were likely to be mediated by antagonism of N-methyl-D-aspartate receptors. In both preparations, area CA1 was more sensitive than the dentate gyrus, and paired-pulse facilitation was affected, but not paired-pulse inhibition. Control solutions had no effect. We conclude that kynurenic acid can enter the brain after peripheral administration, and that peripheral and direct effects in the hippocampus are qualitatively similar. Therefore, we predict that effects of endogenous kynurenic acid that was synthesized peripherally or centrally would be similar. Furthermore, the results suggest that modulation of the glycine site of the N-methyl-D-aspartate receptor, for example by kynurenic acid, may vary considerably among different brain areas.

Maternal, umbilical, and amniotic fluid concentrations of tryptophan and kynurenine after labor or cesarean section

A major route of tryptophan metabolism is via the hepatic and cerebral synthesis of kynurenine, a substance subsequently used by astrocytes in the brain for the production of the neuroactive substances kynurenic acid and quinolinic acid. Both kynurenic and quinolinic acids have been implicated in modulating the activity of excitatory amino acid pathways in the brain, the former as a neuroprotectant because of its antagonist properties, and the latter as an excitotoxin because of its agonist actions, at NMDA receptors. We therefore determined the concentrations of tryptophan and kynurenine in maternal venous and umbilical cord blood, and in amniotic fluid, of infants after labor and vaginal delivery, and after delivery by cesarean section. Concentrations of tryptophan and kynurenine were significantly higher in umbilical vein plasma compared with maternal venous plasma. Tryptophan and kynurenine concentrations in umbilical vein plasma and amniotic fluid were significantly higher after labor, compared with samples obtained from infants of the same gestational age delivered by cesarean section. There was no umbilical vein-to-artery concentration difference for kynurenine in samples obtained after either labor or cesarean section, but there was a significant gradient for tryptophan in samples obtained after vaginal delivery, indicating increased transfer of this amino acid during labor. There was a significant correlation between umbilical vein tryptophan and kynurenine concentrations for both the labor and cesarean section groups, and plasma kynurenine concentrations were also significantly correlated with both umbilical vein cortisol concentrations and the duration of the second stage of labor in the vaginally delivered infants. These results suggest that the placental transfer of tryptophan and the fetal synthesis of kynurenine are increased during labor. These findings have implications for understanding the vulnerability of the infant brain to ischemic/hypoxic damage in the perinatal period. By analogy with the adult brain, the molar ratio of these substances is likely to determine the susceptibility of the brain to seizure and excitotoxic damage.

Systemic d-amphetamine administration causes a reduction of kynurenic acid levels in rat brain

Tissue levels of the endogenous excitatory amino acid receptor antagonist kynurenic acid (KYNA) and of its bioprecursor L-kynurenine were measured in rats of different ages after d-amphetamine administration. In adult animals, extracellular KYNA concentrations were also determined in vivo by hippocampal microdialysis. In the adult brain, d-amphetamine caused a transient, dose-dependent decrease in tissue content and extracellular levels of KYNA, reaching a nadir of approximately 70% of control values after 1 h at 5 mg/kg. Quantitatively similar decrements were observed in four different brain regions. Seven, 14 and 28-day-old pups were particularly sensitive to the drug, showing a reduction in forebrain KYNA levels to 25%, 40% and 35% of control values, respectively, 1 h after the administration of 5 mg/kg d-amphetamine. Notably, no changes in brain L-kynurenine levels and in liver L-kynurenine and KYNA concentrations were found after d-amphetamine administration. Thus, endogenous monoamines released by d-amphetamine may interfere with the transamination of L-kynurenine to KYNA specifically in the brain. These results suggest that d-amphetamine increases excitatory amino acid receptor function temporarily by reducing the levels of endogenous KYNA.

Characterization of rat brain kynurenine aminotransferases I and II

The endogenous neuroprotectant kynurenic acid (KYNA) is produced by irreversible transamination of L-kynurenine (KYN). In the brain, two distinct kynurenine aminotransferases (KAT I and KAT II) are responsible for the formation of KYNA. The present experiments were designed to examine the respective roles of the two KATs in the normal rat brain. To this end, the two enzymes were partially purified, and their characteristics were examined. KAT I (identical with glutamine transaminase K) had an optimal pH of 9.5, preferred pyruvate as a cosubstrate and was potently inhibited by glutamine. KAT II (identical with L-alpha-aminoadipate transaminase) had a neutral optimal pH, showed no preference for pyruvate, and was essentially insensitive to inhibition by glutamine. KAT II was selectively inhibited by quisqualic acid (IC50: 520 microM). The endogenous substrate 3-hydroxykynurenine had an approximately 10-fold preference for KAT II. The distinct properties of the two enzymes made it possible to measure brain KAT I and KAT II in parallel by using dialyzed tissue homogenate (to remove interfering endogenous amino acids). Under these conditions, both enzymes presented essentially the same apparent Km values as the partially purified enzymes. In lesioned, neurondepleted brain tissue and in brain regions other than the cerebellum, KYNA derived primarily from KAT II at physiologic pH. In summary, the present study describes a simple methodology for the simultaneous determination of the two KYNA-producing enzymes in small rat brain tissue samples and provides baseline values for future work in experimentally challenged animals.

Protection against quinolinic acid-mediated excitotoxicity in nigrostriatal dopaminergic neurons by endogenous kynurenic acid

Endogenous excitotoxins have been implicated in the degeneration of dopaminergic neurons in the substantia nigra compacta of patients with Parkinson's disease. One such agent quinolinic acid is an endogenous excitatory amino acid receptor agonist. This study examined whether an increased level of endogenous kynurenic acid, an excitatory amino acid receptor antagonist, can protect nigrostriatal dopamine neurons against quinolinic acid-induced excitotoxic damage. Nigral infusion of quinolinic acid (60 nmoles) or N-methyl-D- aspartate (15 nmoles) produced a significant depletion in striatal tyrosine hydroxylase activity, a biochemical marker for dopaminergic neurons. Three hours following the intraventricular infusion of nicotinylalanine (5.6 nmoles), an agent that inhibits kynureninase and kynurenine hydroxylase activity, when combined with kynurenine (450 mg/kg i.p.), the precursor of kynurenic acid, and probenecid (200 mg/kg i.p.), an inhibitor of organic acid transport, the kynurenic acid in the whole brain and substantia nigra was increased 3.3-fold and 1.5-fold respectively when compared to rats that received saline, probenecid and kynurenine. This elevation in endogenous kynurenic acid prevented the quinolinic acid-induced reduction in striatal tyrosine hydroxylase. However, 9 h following the administration of nicotinylalanine with kynurenine and probenecid, a time when whole brain kynurenic acid levels had decreased 12-fold, quinolinic acid injections produced a significant depletion in striatal tyrosine hydroxylase. Intranigral infusion of quinolinic acid in rats that received saline with kynurenine and probenecid resulted in a significant depletion of ipsilateral striatal tyrosine hydroxylase. Administration of nicotinylalanine in combination with kynurenine and probenecid also blocked N-methyl-D-aspartate-induced depletion of tyrosine hydroxylase. Tyrosine hydroxylase immunohistochemical assessment of the substantia nigra confirmed quinolinic acid-induced neuronal cell loss and the ability of nicotinylalanine in combination with kynurenine and probenecid to protect neurons from quinolinic acid-induced toxicity. The present study demonstrates that increases in endogenous kynurenic acid can prevent the loss of nigrostriatal dopaminergic neurons resulting from a focal infusion of quinolinic acid or N-methyl-D-aspartate. The strategy of neuronal protection by increasing the brain kynurenic acid may be useful in retarding cell loss in Parkinson's disease and other neurodegenerative diseases where excitotoxic mechanisms have been implicated.

Metabolism of [5-(3)H]kynurenine in the developing rat brain in vivo: effect of intrastriatal ibotenate injections

Two metabolites of the kynurenine pathway of tryptophan degradation, the neurotoxin quinolinic acid (QUIN) and the neuroprotectant kynurenic acid (KYNA), may play a role in the initiation or propagation of brain diseases. In order to study their disposition during the acute and chronic stages of neurodegeneration, effects of an excitotoxic insult on the de novo synthesis of several kynurenine pathway metabolites were examined in vivo. Neuronal injury and lesions were produced in 7-day (PND 7), 14-day (PND 14) and young adult rats by an intrastriatal injection of the excitotoxin ibotenic acid. At 2 h, 2, 7 and 28 days later, the formation of tritiated KYNA, 3-hydroxykynurenine (3HK), xanthurenic acid and QUIN was assessed after an acute intrastriatal injection of their common bioprecursor, [5-(3)H]kynurenine. In all three age groups, the acute insult resulted in a shift towards enhanced KYNA formation, as indicated by 2-4 fold decreases in the 3HK/KYNA and QUIN/KYNA ratios in ibotenate-treated striata. At later post-lesion intervals, age-specific several-fold changes were observed in the flux through both the KYNA and QUIN branches of the kynurenine pathway. With aging, kynurenine conversion to QUIN and especially to 3HK, became increasingly more prominent, though KYNA synthesis was substantially activated as well. The acute toxin-induced changes in kynurenine metabolism, the propensity of the lesioned immature striatum to increase KYNA production preferentially, and the pronounced lesion-induced long-term increases in cerebral KYNA, 3HK and QUIN formation may participate in the modulation of NMDA receptor function following injury. In particular, changes in the production of these kynurenine pathway metabolites may play a role in mechanisms involved in endogenous neuroprotection, delayed neurodegeneration and regenerative processes.

Pretreatment with L-kynurenine, the precursor to the excitatory amino acid antagonist kynurenic acid, suppresses epileptiform activity in combined entorhinal/hippocampal slices

The kynurenine pathway converts tryptophan into various compounds, including L-kynurenine, which in turn can be converted to the excitatory amino acid receptor antagonist kynurenic acid. The hypothesis that endogenously-produced kynurenic acid could have physiological effects was tested in combined entorhinal/hippocampal slices from adult rats. Specifically, perfusion with L-kynurenine (1 mM) was examined for its ability to suppress epileptiform activity produced by subsequent perfusion with buffer lacking added magnesium (nominal 0 mM magnesium buffer). Importantly, treatment with L-kynurenine did not appear to have depressant effects in itself, but it prevented spontaneous epileptiform activity in all 64 slices subsequently perfused with 0 mM magnesium buffer. In contrast, 45 slices that were not pretreated with L-kynurenine exhibited spontaneous epileptiform activity. These data support the hypothesis that endogenously-produced kynurenic acid can be produced and released in brain slices, where it can suppress excitatory activity in an "anticonvulsant' manner. Therefore, manipulation of the kynurenine pathway might constitute a useful new direction for anticonvulsant drug development.

(R,S)-3,4-dichlorobenzoylalanine (FCE 28833A) causes a large and persistent increase in brain kynurenic acid levels in rats

Kynurenic acid is an endogenous excitatory amino-acid receptor antagonist with neuroprotective and anticonvulsant properties. We demonstrate here that systemic administration of the new and potent kynurenine 3-hydroxylase inhibitor (R,S)-3,4-dichlorobenzoylalanine (FCE 28833A) causes a dose-dependent elevation in endogenous kynurenine and kynurenic acid levels in rat brain tissue. In hippocampal microdialysates, peak increases of 10- and 80-fold above basal kynurenic acid concentrations, respectively, were obtained after a single oral or intraperitoneal administration of 400 mg/kg FCE 28833A. After intraperitoneal treatment with FCE 28833A, extracellular brain kynurenic acid levels remained significantly elevated for at least 22 h, rendering this compound a far more effective enhancer of kynurenic acid levels than the previously described kynurenine 3-hydroxylase blocker m-nitrobenzoylalanine. FCE 28833A and similar molecules may have therapeutic value in diseases which are linked to a hyperfunction of excitatory amino-acid receptors.

Hydrogen peroxide-mediated neuronal cell death induced by an endogenous neurotoxin, 3-hydroxykynurenine

3-Hydroxykynurenine (3-HK) is a tryptophan metabolite whose level in the brain is markedly elevated under several pathological conditions, including Huntington disease and human immunodeficiency virus infection. Here we demonstrate that micromolar concentrations (1-100 microM) of 3-HK cause cell death in primary neuronal cultures prepared from rat striatum. The neurotoxicity of 3-HK was blocked by catalase and desferrioxamine but not by superoxide dismutase, indicating that the generation of hydrogen peroxide and hydroxyl radical is involved in the toxicity. Measurement of peroxide levels revealed that 3-HK caused intracellular accumulation of peroxide, which was largely attenuated by application of catalase. The peroxide accumulation and cell death caused by 1-10 microM 3-HK were also blocked by pretreatment with allopurinol or oxypurinol, suggesting that endogenous xanthine oxidase activity is involved in exacerbation of 3-HK neurotoxicity. Furthermore, NADPH diaphorase-containing neurons were spared from toxicity of these concentrations of 3-HK, a finding reminiscent of the pathological characteristics of several neurodegenerative disorders such as Huntington disease. These results suggest that 3-HK at pathologically relevant concentrations renders neuronal cells subject to oxidative stress leading to cell death, and therefore that this endogenous compound should be regarded as an important factor in pathogenesis of neurodegenerative disorders.

Excitotoxic lesions of the rat striatum: different responses of kynurenine pathway enzymes during ontogeny

Excitotoxic lesions of the adult rat striatum result in reactive gliosis and an associated increase in the activities of the astrocytic enzymes 3-hydroxyanthranilic acid oxygenase (3HAO) and kynurenine aminotransferase (KAT), which are responsible for the biosynthesis of the neurotoxin quinolinic acid and the neuroprotectant kynurenic acid, respectively. Unilateral ibotenate injections were made in the striatum of 7-, 14-, 21- and 28-day- and 2.5-month-old rats to study the reaction of 3HAO and KAT when injury is inflicted during ontogeny. By one week, all lesioned striata showed a > 50 percent decrease in the activity of the neuronal marker enzyme glutamic acid decarboxylase. At this timepoint, lesion-induced elevations in 3HA0 activity increased progressively from 130 to 206, 280, 385 and 456 percent of the contralateral striatum in the five age groups studied. In contrast, in the same animals the respective increases in striatal KAT activity were 601, 350, 312, 259 and 159 percent (n = 6-13 per group). In all age groups, statistically significant lesion-induced increases in 3HA0 and KAT were seen up to 4 weeks after the ibotenate injection. Rats receiving an intrastriatal injection of ibotenate on postnatal day 7 also showed an increase in the striatal tissue level of kynurenic acid 1 week after the lesion. These data demonstrate that substantial qualitative differences exist between the immature and adult rat in the reaction of two glial enzymes to striatal injury. Moreover, the ability of the immature brain to mobilize kynurenic acid production preferentially may play a role in the brain's response to perinatal injury.