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Background: Continuous subcutaneous apomorphine infusion (CSAI) has been used globally since the 1980s for Parkinson disease (PD) motor fluctuations but has not been available in the United States (US)., Objective: Evaluate CSAI for motor fluctuations in the US setting., Methods: This open-label study (NCT02339064) enrolled patients with PD experiencing ≥3 hours (h) daily OFF time despite optimized levodopa and current/prior use of at least one other adjunctive therapy. CSAI was initiated with a 1-2 mg bolus followed by 1 mg/h infusion titrated to optimal efficacy and tolerability. Following titration, patients entered a 52-week maintenance period., Results: Of 99 patients treated, 85 completed the titration period, 69 completed maintenance week 12 and 48 completed maintenance week 52. Common treatment-related adverse events included infusion site nodules and erythema, dyskinesia, nausea, and somnolence, each of which occurred more frequently during the titration period. Reduction in OFF time began at CSAI initiation and reached a mean of 3.0 ± 3.18 h/day by maintenance week 12 (primary efficacy endpoint), with a corresponding increase in Good ON time of 3.1 ± 3.35 h/day. By maintenance week 12, 68% of patients rated themselves as much or very much improved, 62% had at least a 2-h reduction in daily OFF time, and mean concomitant oral levodopa and levodopa equivalent doses (excluding CSAI) had been reduced by 198 mg/day and 283 mg/day, respectively. Improvements were maintained through week 52., Conclusions: This study supports the clinical utility of CSAI to reduce OFF time and increase Good ON time in patients with motor fluctuations inadequately controlled with oral therapy., Competing Interests: Declaration of conflicting interestsThe authors declared the following potential conflicts of interest with respect to the research, authorship, and/or publication of this article: Stuart H. Isaacson, Peter LeWitt, Alberto J. Espay, Rajesh Pahwa, Pinky Agarwal, Holly A. Shill, Jennifer Hui, and Khashayar Dashtipour were investigators in the InfusON study and report fees for consultancy from Supernus Pharmaceuticals Inc and US WorldMeds, LLC. Gianpiera Ceresoli-Borroni, Andrea Formella, and Peibing Qin, are employed by Supernus Pharmaceuticals Inc. In addition, Dr Isaacson reports honoraria for CME, consultant, research grants, and/or promotional speaker on behalf of: Abbvie, Acadia, Acorda, Addex, Affiris, Alexza, Allevion, Amneal, Annovis, Aptinyx, Athira, Bial, Biogen, BlueRock, Britannia, Bukwang, Cala, Cerecor, Cerevel, CND, Eli Lilly, Enterin, Esteve, Fasikl, GE Healthcare, Global Kinetics, Inhibikase, Intra-Cellular Therapies, Ipsen, Jazz, Kyowa Kirin, Lundbeck, Medscape, Merz, Michael J. Fox Foundation, Mitsubishi Tanabe, Neuralys, Neurocrine, NeuroDerm, Neurolive, Exeltis, Novartis, ONO Pharmaceutical, Parkinson Study Group, Pharma2B, Praxis, Revance, Roche, Sage, Sanofi, Scion, Stoparkinson, Sunovion, Sun Pharma, Supernus, Teva, Theravance, Transposon, and UCB. Dr Espay has received grant support from the NIH and the Michael J Fox Foundation; personal compensation as a consultant/scientific advisory board member for NeuroDerm, Amneal, Acadia, Avion Pharmaceuticals, Acorda, Kyowa Kirin, Supernus (formerly, US WorldMeds), NeuroDiagnostics, Inc (SYNAPS Dx), and Herantis Pharma; and publishing royalties from Lippincott Williams & Wilkins, Cambridge University Press, and Springer. He cofounded REGAIN Therapeutics and is co-inventor of the patent “Compositions and methods for treatment and/or prophylaxis of proteinopathies.” He serves on the editorial boards of the Journal of Parkinson's Disease, Journal of Alzheimer's Disease, European Journal of Neurology, Movement Disorders Clinical Practice, and JAMA Neurology. Dr Pahwa serves as a consultant for Abbott, AbbVie, ACADIA, Acorda, Allevion, Amneal, Appello, Biogen, BioVie, CalaHealth, Convatec, Fasikl, Genetech, Insightec, Jazz, Keiferx, Kyowa, Lundbeck, Merz, Mitsubishi Tanabe, Neurocrine, Ono, PhotoPharmics, PSG, Reagenxbio, Sage, Sun Biopharma, Supernus, UCB. He receives research support from Abbott, AbbVie, Alexza, Amylyx, Annovis, ASK Bio, Attune, Biogen, Bluerock, Bukwang, Cerevance, Cerevel, EIP, Fasikl, GemVax, Global Kinetics, Inhibikase, Intra-cellular Therapies (ITI), Jazz, Michael J Fox Foundation, Neurocrine, NeuroDerm, ONO, Parkinson's Foundation, Praxis, Roche, Sage, Scion, Sun Pharma, Teva, Theravance, UCB. Dr Agarwal has received compensation to serve as consultant and speaker for Acadia, Acorda, Adamas, Amneal, US WorldMeds and reports research grants from the Michael J Fox Foundation, Merck, Astellas, Parkinson Study Group, CHDI Foundation, US WorldMeds, Lilly USA, Biogen Idec, Acadia Pharmaceuticals, Genentech (Roche), Prilenia Therapeutics, Sun Pharma Advanced Research Company Limited (SPARC) and Centogene AG. Dr Shill reports consultancy for Biogen, Acadia, Mitsubishi Tanabe Pharma America, Inc; honoraria for advisory boards from Kyowa Kirin Inc, Jazz Pharmaceuticals and research funding from Barrow Neurological Foundation, Impax Laboratories LLC, US World Meds, NIH, Parkinson Study Group/Biogen, Sunovion, Intec Pharma, Cala Health. Dr Hui reports consultancy for Sunovion. She has research grants from US WorldMeds, Sunovion, and Roche. Dr Dashtipour has received compensation to serve as an advisor and speaker from Allergan, Acadia, Abbvie, Acorda, Amneal, Ipsen, Lundbeck, Neurocrine, Teva and US WorldMeds.Dr Lew reports consultancy for Acorda, Supernus, Neurocrine, Kyowa, Amneal, UCB, and Global Kinetics. He has research grants from the Parkinson's Study Group, Michael J. Fox Foundation, Anenberg foundation, UCB, Jazz Pharmaceuticals, Neuraly, NIAA, Jazz Pharmaceuticals, Inhibikase Therapeutics, Biogen, Ono Pharma. Dr LeWitt has received compensation from consulting or lectures from Abide, Amneal, Biogen, Bukwang, Cavion, Denali, Hoffmann-La Roche, Jazz Pharmaceuticals, Kyowa, Mitsubishi NeuroDerm Ltd, Neurocrine, ONO Pharma USA, Revance, US WorldMeds, and Voyager Therapeutics; research grant support from Acorda Therapeutics, Biotie Therapies, Lundbeck, Merz, Michael J Fox Foundation for Parkinson's Research, Parkinson Study Group, Pharma 2B, Revance, Roche, Sunovion, Sun Pharma, and Supernus. Peter LeWitt and Alberto Espay are Editorial Board Members of this journal but were not involved in the peer-review process of this article nor had access to any information regarding its peer-review.

The objective of this study was to compare the effectiveness of subcutaneous (SC) and sublingual (SL) formulations of apomorphine for the treatment of motor fluctuations in Parkinson's disease using a pharmacokinetics (PK)/pharmacodynamics (PD) modeling approach. The PK of SC and SL apomorphine are best described by a one-compartment model with first-order absorption and a two-compartment model with delayed absorption, respectively. The PK/PD model relating apomorphine plasma concentrations to the Unified Parkinson's Disease Rating Scale (UPDRS) motor scores was described by a sigmoidal E max model assuming effective concentration = drug concentration in an effect compartment. Apomorphine concentrations and UPDRS motor scores were simulated from the PK/PD models using 500 hypothetical subjects. UPDRS motor score change from baseline was evaluated using time to clinically relevant response, response duration, area under the curve, maximal response, and time to maximal response. Higher doses of each apomorphine formulation were associated with shorter time to response, longer response duration, and greater maximal response. Although the mean maximal responses to SC and SL apomorphine were comparable, the time to response was four times shorter (7 vs. 31 min) and time to maximal response was two times shorter (27 vs. 61 min) for 4 mg SC vs. 50 mg SL. Thus, faster onset of action was observed for the SC formulation compared to SL. These data may be useful for physicians when selecting "on demand" therapy for patients with Parkinson's disease experiencing motor fluctuations., (© 2024. The Author(s).)

Impulsive aggressive (IA, or impulsive aggression) behavior describes an aggregate set of maladaptive, aggressive behaviors occurring across multiple neuropsychiatric disorders. IA is reactive, eruptive, sudden, and unplanned; it provides information about the severity, but not the nature, of its associated primary disorder. IA in children and adolescents is of serious clinical concern for patients, families, and physicians, given the detrimental impact pediatric IA can have on development. Currently, the ability to properly identify, monitor, and treat IA behavior across clinical populations is hindered by two major roadblocks: (1) the lack of an assessment tool designed for and sensitive to the set of behaviors comprising IA, and (2) the absence of a treatment indicated for IA symptomatology. In this review, we discuss the clinical gaps in the approach to monitoring and treating IA behavior, and highlight emerging solutions that may improve clinical outcomes in patients with IA.

Objective: To evaluate efficacy and safety of SPN-810 (extended-release molindone) in a Phase-2b, randomized, double-blind, placebo-controlled, dose-ranging study of children (6-12 years) with ADHD and persistent impulsive aggression (IA). Method: After lead-in, children were randomized to (a) placebo ( N = 31); (b) low-dose ( N = 29, 12/18 mg/day); (c) medium-dose ( N = 30, 24/36 mg/day); and (4) high-dose ( N = 31, 36/54 mg/day) groups. Treatment included ~2.5-week titration, 3-week maintenance, and 1-week tapering/conversion, alongside existing monotherapy (stimulants/nonstimulants) and behavioral therapy. The primary endpoint was change in Retrospective-Modified Overt Aggression Scale (R-MOAS) score at end of study, with safety monitored. Results: A total of 95 (78.5%) children completed the study. Aggression (R-MOAS) improved with low and medium doses (low dose: p = .031; medium dose: p = .024; high dose: p = .740). The most common adverse events were headache (10.0%), sedation (8.9%), and increased appetite (7.8%). Conclusion: These results suggest SPN-810 may be effective in reducing residual IA behaviors in children with ADHD. Research is still needed to support the benefit-risk profile of SPN-810 in pediatric populations.

Objective: Impulsive aggression (IA) is a maladaptive form of aggressive behavior that is an associated feature of neuropsychiatric disorders, including attention-deficit/hyperactivity disorder (ADHD). As one of the most common forms of aggressive behavior, IA is a serious clinical concern. Recognition, monitoring, and management of IA symptoms are complicated by the lack of IA-specific psychometric instruments and evidence-based treatments. A recently developed electronic observer-reported outcome instrument has been validated in children for monitoring the frequency of 15 IA-related behaviors in the context of ADHD. This study seeks to first determine if the behaviors included in the pediatric IA diary are applicable to adolescents with ADHD, and second, compare the reliability of adolescent versus parent reporters. Methods: We evaluated the utility of the pediatric IA diary through concept elicitation and cognitive interviews with 17 pairs of parents and adolescents (aged 13-17 years) with IA and ADHD, supplemented with 15 new behaviors potentially applicable to adolescents. Results: The behaviors most frequently reported by adolescents included arguing (93.8%), raising their voice/shouting/yelling (93.8%), hitting others (87.5%), slamming (87.5%), pushing/shoving (81.3%), breaking (75.0%), fighting (75.0%), throwing (75.0%), and cursing (68.8%). The behaviors most commonly reported by parents included raising their voice/shouting/yelling (94.1%), arguing (88.2%), being disrespectful/mean/rude (88.2%), slamming (88.2%), throwing (88.2%), cursing (82.4%), hitting others (82.4%), pushing/shoving (82.4%), breaking (76.5%), name-calling (76.5%), and threatening (70.6%). Of all commonly reported behaviors, only being "disrespectful/mean/rude" and "breaking" are not part of the pediatric IA diary, likely due to the imprecision of these terms. No significant usability issues were found for the IA diary device. Conclusions: These findings suggest that the 15-item pediatric IA diary should be applicable to adolescent populations to appropriately characterize IA behaviors in individuals with ADHD. Furthermore, this study indicated that parents may be more reliable reporters of IA behavior than adolescents.

Objective: To establish the validity and reliability of a provisional 30-item impulsive aggression (IA) diary in children (ages 6-12 years, inclusive) with attention-deficit/hyperactivity disorder (ADHD). Methods: The provisional 30-item IA diary was administered for 14 days to parents of children with ADHD and IA symptoms ( n  = 103). Key inclusion criteria: confirmed ADHD diagnosis; signs of IA as measured by a Retrospective-Modified Overt Aggression Scale (R-MOAS) score ≥20 and an Aggression Questionnaire score of -2 to -5. Analyses included inter-item correlations, exploratory factor analysis (EFA), item response theory (IRT) modeling, internal consistency, test-retest reliability (TRT), concurrent validity (estimated by correlation between the IA diary and the R-MOAS/Nisonger Child Behavior Rating Form), and known-groups methods. Results: The prevalence rates of 15 (50.0%) items were found to be too low (<1%) for analysis; three items with prevalence rates ≤1% were retained, as content validity was deemed high by clinical experts. The remaining 12 behavior items had prevalence rates of 2.7%-73.6%. EFA and IRT models confirmed two subdomains in the IA diary included within a general domain of IA behavior frequency, yielding a single total behavioral frequency score (TBFS). Internal consistency was high for this TBFS (marginal reliability = 0.86 and α = 0.73). TRT for the TBFS, based on the intraclass correlation coefficient, was 0.8. Concurrent validity of TBFS with R-MOAS ranged from r  = 0.49 to r  = 0.62. Conclusion: The final 15-item IA diary is a reliable, psychometrically validated IA measurement tool that will allow clinicians and researchers to assess the frequency of IA behavior.

Proper drug categorization enables clinicians to readily identify the agents most appropriate for patients in need. Currently, patients with maladaptive aggression do not all always fall into a single existing diagnostic or treatment category. Such is the case for those with impulsive aggression (IA). IA is an associated feature of numerous neuropsychiatric disorders, and can be described as eruptive, aggressive behavior or a 'short fuse'. Although agents from a broad spectrum of drug classes have been used to treat maladaptive aggression, few have been tested distinctly in patients with IA, and there is no drug specifically indicated by the US Food and Drug Administration (US FDA) for IA. Further, current treatments often fail to sufficiently treat IA symptomatology. These issues create an unclear and inadequate treatment path for patients. Here we will propose the establishment of a class of anti-maladaptive aggression agents to begin addressing this clinical issue. The development of such a class would unify the various drugs currently used to treat maladaptive aggression and streamline the treatment approach towards IA. As an important case example of the range of candidate drugs that could fit into a new anti-maladaptive aggression agent category, we will review an investigational IA pharmacotherapy. SPN-810 (extended-release molindone) is currently being investigated as a novel treatment for children with IA and ADHD. Based on these studies we will review how SPN-810 may be well suited for a new, anti-maladaptive aggression drug class and more precisely, a proposed subgroup of IA modulators. The goal of this review is to begin improving the identification of and therapeutic approach for maladaptive aggression as well as IA through more precise anti-maladaptive aggression agent categorization.

The kynurenine pathway (KP) of tryptophan degradation contains three neuroactive metabolites: the neuroinhibitory agent kynurenic acid (KYNA) and, in a competing branch, the free radical generator 3-hydroxykynurenine (3-HK) and the excitotoxin quinolinic acid (QUIN). These three "kynurenines" derive from a common precursor, L-kynurenine, and are recognized for their role in brain physiology and pathophysiology. Inhibition of kynurenine 3-hydroxylase, the enzyme responsible for 3-HK formation, shifts KP metabolism in the mature brain toward enhanced KYNA formation. We now tested the cerebral effects of kynurenine 3-hydroxylase inhibition in immature rodents. Rat pups treated with the kynurenine 3-hydroxylase inhibitor UPF 648 (30 mg/kg, i.p.) 10 min after birth showed substantial increases in cerebral and liver kynurenine and KYNA levels up to 24 hr later, whereas 3-HK and QUIN levels were simultaneously decreased. Administered to pregnant rats or mice on the last day of gestation, UPF 648 (50 mg/kg, i.p.) produced qualitatively similar changes (i.e., large increases in kynurenine and KYNA and reductions in 3-HK and QUIN) in the brain and liver of the offspring. Rat pups delivered by UPF 648-treated mothers and immediately exposed to neonatal asphyxia showed further enhanced brain KYNA levels. These studies demonstrate that acute kynurenine 3-hydroxylase inhibition effectively shifts cerebral KP metabolism in neonatal rodents toward increased KYNA formation. Selective inhibitors of this enzyme may therefore provide neuroprotection in newborns and will also be useful for the experimental evaluation of the long-term effects of perinatal KP impairment.

The brain and cerebrospinal fluid levels of kynurenic acid (KYNA), a metabolite of the kynurenine pathway of tryptophan degradation and antagonist of the glycine(B) receptor and the alpha7 nicotinic acetylcholine receptor, are elevated in persons with schizophrenia. To evaluate whether this increase is related to antipsychotic medication, we examined the effects of haloperidol (HAL), clozapine (CLOZ) or raclopride (RAC) on brain KYNA levels in rats. Animals received either acute drug injections or ingested the drugs chronically with the drinking water. Acute application or one-week drug exposure had no effect on brain KYNA levels. After one month, HAL, CLOZ and RAC all caused significant reductions in KYNA levels in striatum, hippocampus and frontal cortex. Quantitatively similar reductions in the brain tissue content of KYNA were observed after one year of HAL administration. All these effects were accompanied by equivalent decreases in the extracellular concentration of KYNA, measured by striatal microdialysis. Separate animals received an intrastriatal infusion of (3)H-kynurenine to probe the entire kynurenine pathway acutely in rats treated with HAL for one year. These animals showed reduced (3)H-KYNA production, but no changes in the formation of other kynurenine pathway metabolites. By enhancing glutamatergic and cholinergic neurotransmission, reduced brain KYNA levels may play a role in the clinical effects of prolonged antipsychotic medication.

In the rodent brain, astrocytes are known to be the primary source of kynurenate (KYNA), an endogenous antagonist of both the glycine(B) and the alpha7 nicotinic acetylcholine receptor. In the present study, primary human astrocytes were used to examine the characteristics and regulation of de novo KYNA synthesis in vitro. To this end, cells were exposed to KYNA's bioprecursor L-kynurenine, and newly formed KYNA was recovered from the extracellular milieu. The production of KYNA was stereospecific and rose with increasing L-kynurenine concentrations, reaching a plateau in the high microM range. In an analogous experiment, astrocytes also readily produced and liberated the potent, specific glycine(B) receptor antagonist 7-chlorokynurenate from L-4-chlorokynurenine. KYNA synthesis was dose-dependently reduced by L-leucine or L-phenylalanine, two amino acids that compete with L-kynurenine for cellular uptake, and by aminooxyacetate, a non-specific aminotransferase inhibitor. In contrast, KYNA formation was stimulated by 5 mM pyruvate or oxaloacetate, which act as co-substrates of the transamination reaction. Aglycemic or depolarizing (50 mM KCl or 100 microM veratridine) conditions had no effect on KYNA synthesis. Subsequent studies using tissue homogenate showed that both known cerebral kynurenine aminotransferases (KAT I and KAT II) are present in astrocytes, but that KAT II appears to be singularly responsible for KYNA formation under physiological conditions. Taken together with previous results, these data suggest that very similar mechanisms control KYNA synthesis in the rodent and in the human brain. These regulatory events are likely to influence the neuromodulatory effects of astrocyte-derived KYNA in the normal and diseased human brain.

Two tryptophan metabolites, the anti-excitotoxic N-methyl-D-aspartate (NMDA) receptor antagonist kynurenic acid (KYNA) and the free radical generator 3-hydroxykynurenine (3-HK), have been proposed to influence neuronal viability in the mammalian brain. In rats, the brain content of both KYNA and 3-HK decreases immediately after birth, possibly to ensure normal postnatal functioning of NMDA receptors. Because complications of birth asphyxia have been suggested to be associated with anomalous NMDA receptor function, we examined the acute effects of an asphyctic insult on the brain levels of KYNA and 3-HK in neonatal rats. Asphyxia was induced in animals delivered by cesarean section on the last day of gestation, using the procedure introduced by Bjelke et al. (Brain Res 543: 1-9, 1991). KYNA and 3-HK levels were determined in the brain at seven time points between 10 min and 24 h after asphyxia. Up to 6 h, asphyxia caused 160-267% increases in KYNA levels. In the same tissues, 3-HK levels decreased (significantly at five of the seven time points), demonstrating an asphyxia-induced shift in kynurenine pathway metabolism toward the neuroprotectant KYNA. This shift might constitute the brain's attempt to counter the ill effects of birth asphyxia. Furthermore, the transient increase in the brain KYNA/3-HK ratio in these animals might be causally related to the well-documented detrimental long-term effects of asphyxia.

Competing enzymatic mechanisms degrade the tryptophan metabolite L-kynurenine to kynurenate, an inhibitory and neuroprotective compound, and to the neurotoxins 3-hydroxykynurenine and quinolinate. Kynurenine 3-hydroxylase inhibitors such as PNU 156561 shift metabolism towards enhanced kynurenate production, and this effect may underlie the recently discovered anticonvulsant and neuroprotective efficacy of these drugs. Using electrophysiological and neurotoxicological endpoints, we now used PNU 156561 as a tool to examine the functional interplay of kynurenate, 3-hydroxykynurenine and quinolinate in the rat hippocampus in vivo. First, population spike amplitude in area CA1 and the extent of quinolinate-induced excitotoxic neurodegeneration were studied in animals receiving acute or prolonged intravenous infusions of L-kynurenine, PNU 156561, (L-kynurenine+PNU 156561) or kynurenate. Only the latter two treatments, but not L-kynurenine or PNU 156561 alone, caused substantial inhibition of evoked responses in area CA1, and only prolonged (3h) infusion of (L-kynurenine+PNU 156561) or kynurenate was neuroprotective. Biochemical analyses in separate animals revealed that the levels of kynurenate attained in both blood and brain (hippocampus) were essentially identical in rats receiving extended infusions of L-kynurenine alone or (L-kynurenine+PNU 156561) (4 and 7microM, respectively, after an infusion of 90 or 180min). However, addition of the kynurenine 3-hydroxylase inhibitor resulted in a significant decrement in the formation of 3-hydroxykynurenine and quinolinate in both blood and brain. These data suggest that the ratio between kynurenate and 3-hydroxykynurenine and/or quinolinate in the brain is a critical determinant of neuronal excitability and viability. The anticonvulsant and neuroprotective potency of kynurenine 3-hydroxylase inhibitors may therefore be due to the drugs' dual action on both branches of the kynurenine pathway of tryptophan degradation.

The kynurenine pathway of tryptophan degradation contains several metabolites which may influence brain physiology and pathophysiology. The brain content of one of these compounds, kynurenic acid (KYNA), decreases precipitously around the time of birth, possibly to avoid deleterious N-methyl-D-aspartate (NMDA) receptor blockade during the perinatal period. The present study was designed to determine the levels of KYNA, the free radical generator 3-hydroxykynurenine (3-HK), and their common precursor L-kynurenine (L-KYN) between gestational day 16 and adulthood in rat brain and liver. The cerebral activities of the biosynthetic enzymes of KYNA and 3-HK, kynurenine aminotransferases (KATs) I and II and kynurenine 3-hydroxylase, respectively, were measured at the same ages. Additional studies were performed to assess whether and to what extent kynurenines in the immature brain derive from the mother, and to examine the short-term effects of birth asphyxia on brain KYNA and 3-HK levels. The results revealed that 1) the brain and liver content of L-KYN, KYNA and 3-HK is far higher pre-term than postnatally; 2) KAT I and kynurenine 3-hydroxylase activities are quite uniform between E-16 and adulthood, whereas KAT II activity rises sharply after postnatal day 14; 3) during the perinatal period, KYNA, but not L-KYN, may originate in part from the maternal circulation; and 4) oxygen deprivation at birth affects the brain content of both KYNA and 3-HK 1 h but not 24h later.

Intrastriatal injection of the endogenous excitotoxin quinolinate in experimental animals causes a lesion which duplicates many features of Huntington's disease (HD). This lesion can be prevented by a related metabolite, kynurenate. Since kynurenate levels are reduced in the HD neostriatum, a deficiency in brain kynurenate may be the cause of neuron loss in HD. In order to investigate the relationship between excitotoxic neurodegeneration and kynurenate formation, effects of a unilateral quinolinate injection on several measures of kynurenate metabolism were studied in the rat striatum and substantia nigra. Within 2 hours, quinolinate caused an approximately 100% increase in striatal kynurenate levels in the absence of changes in its bioprecursor L-kynurenine or its biosynthetic enzymes kynurenine aminotransferases (KATs) I and II. This increase was more dramatic after 2 days (+735%) and was accompanied by an increase in L-kynurenine (+182%). No change or a slight decrease in enzyme activities were detected at this time-point. More chronic excitotoxic lesions produced a substantial increase in kynurenate levels (by approximately 2-, 4- and 4-fold, respectively, after 7 days, 1 and 5 months). Lesion-induced changes in KAT II activity essentially paralleled those seen with kynurenate, whereas KAT I remained slightly decreased at all timepoints. Nigral KAT II activity was increased ipsilaterally 2 days, 1 and 5 months after the striatal quinolinate injection. Kinetic analyses, performed in the striatum 5 months after the quinolinate injection, showed an almost 3-fold decrease in Km values for KAT II in the absence of v(max) changes. These findings indicate that 1) different mechanisms regulate kynurenate production at different stages after an intrastriatal quinolinate injection; 2) an increased substrate affinity to KAT II is responsible for the elevation of kynurenate in the chronically lesioned rat striatum; and 3) qualitative differences in kynurenate metabolism exist between the HD neostriatum and the excitotoxin-lesioned rat striatum, supporting the idea that (a decrease in) kynurenate tone may play a primary role in the pathophysiology of HD.

Using in vivo and in vitro paradigms, the regulation and function of the brain metabolite kynurenic acid (KYNA) was examined in rats on postnatal days (PND) 7 and 14. As shown previously in adult rats, glucose removal and d-amphetamine (d-Amph) administration caused decreases in KYNA formation, while exposure to pyruvate up-regulated KYNA synthesis. The effect of glucose deprivation was substantially blunted in immature animals. In PND 14 rats, d-Amph pre-treatment exacerbated the excitotoxic effects of an intrastriatal N-methyl-D-aspartate (NMDA) injection. This potentiation was prevented by m-nitrobenzoylalanine, a kynurenine 3-hydroxylase inhibitor that also antagonized the KYNA reduction caused by d-Amph. These and additional experiments with the competitive NMDA receptor antagonist CGP 40116 indicate the existence of a functionally significant, novel high-affinity receptor for KYNA in the brain.

Several brain-specific mechanisms control the formation of the endogenous excitatory amino acid receptor antagonist kynurenic acid (KYNA) in the adult rat brain. Two of these, dopaminergic neurotransmission and cellular energy metabolism, were examined in the brain of immature (postnatal day 7) rats. The results indicate that during the early postnatal period cerebral KYNA synthesis is exceptionally amenable to modulation by dopaminergic mechanisms but rather insensitive to fluctuations in cellular energy status. These findings may be of relevance for the role of KYNA in the function and dysfunction of the developing brain.