Upregulation of indoleamine 2,3-dioxygenase (IDO) by proinflammatory cytokines has been implicated as a biological mediator of inflammation-related mood disorders. Clinical reports on this neuro-immune interaction remain correlative, while mechanism-centered preclinical experiments have focused on a relatively narrow, and somewhat controversial, survey of depression-like behaviors that include the forced swim and tail suspension tests. Here, we sought to determine whether peripheral immune challenge with E. coli, lipopolysaccharides (LPS) precipitates the development of translationally relevant depression-like behaviors and to investigate the role of IDO in mediating these LPS-induced behaviors. Intraperitoneal injection of C57BL/6J mice with LPS resulted in a robust, but transient, reduction in exploratory locomotor activity (eLMA) that returned to near baseline levels by 24h. Sucrose preference, a preclinical correlate of anhedonia, was diminished by more than 20% in LPS-treated compared to saline-treated control mice, and LPS induced a significant increase in anxiety-like behavior at 24h that was independent eLMA. Pretreatment of mice with an IDO inhibitor, 1-methyltryptophan (1MT), ablated the anxiogenic effects of LPS, while having no impact on sickness associated changes in body weight or eLMA. Additionally, 1MT pretreatment attenuated the LPS-induced reduction in sucrose preference, which was also confirmed in IDO-1 null mice. Interestingly, acute systemic administration of L-kynurenine, the enzymatic product of IDO, precipitated an anhedonic and anxiogenic effect in naïve mice without effect on eLMA. In a preclinical model, these data implicate IDO as a pivotal mediator of LPS-induced depression- and anxiety-like behavior.

The kynurenine pathway of tryptophan metabolism has an important role in mediating the behavioral effects of inflammation, which has implications in understanding neuropsychiatric comorbidity and for the development of novel therapies. Inhibition of the rate-limiting enzyme, indoleamine 2,3-dioxygenase (IDO), prevents the development of many of these inflammation-induced preclinical behaviors. However, dysregulation in the balance of downstream metabolism, where neuroactive kynurenines are generated, is hypothesized to be a functionally important pathogenic feature of inflammation-induced depression. Here we utilized two novel transgenic mouse strains to directly test the hypothesis that neurotoxic kynurenine metabolism causes depressive-like behavior following peripheral immune activation. Wild-type (WT) or kynurenine 3-monooxygenase (KMO)-deficient (KMO−/−) mice were administered either lipopolysaccharide (LPS, 0.5 mg kg−1) or saline intraperitoneally. Depressive-like behavior was measured across multiple domains 24 h after immune challenge. LPS precipitated a robust depressive-like phenotype, but KMO−/− mice were specifically protected from LPS-induced immobility in the tail suspension test (TST) and reduced spontaneous alternations in the Y-maze. Direct administration of 3-hydroxykynurenine, the metabolic product of KMO, caused a dose-dependent increase in depressive-like behaviors. Mice with targeted deletion of 3-hydroxyanthranilic acid dioxygenase (HAAO), the enzyme that generates quinolinic acid, were similarly challenged with LPS. Similar to KMO−/− mice, LPS failed to increase immobility during the TST. Whereas kynurenine metabolism was generally increased in behaviorally salient brain regions, a distinct shift toward KMO-dependent kynurenine metabolism occurred in the dorsal hippocampus in response to LPS. Together, these results demonstrate that KMO is a pivotal mediator of hippocampal-dependent depressive-like behaviors induced by peripheral LPS challenge.

BackgroundActivation of the tryptophan degrading enzyme indoleamine-2,3-dioxygenase 1 (IDO1) is associated with the development of behavioral signs of depression. Systemic immune challenge induces IDO1 in both the periphery and the brain, leading to increased circulating and brain concentrations of kynurenines. However, whether IDO1 activity within the brain is necessary for the manifestation of depression-like behavior of mice following a central immune challenge remains to be elucidated.MethodsWe investigated the role of brain IDO1 in mediating depression-like behavior of mice in response to intracerebroventricular injection of saline or lipopolysaccharide (LPS, 10 ng).ResultsLPS increased the duration of immobility in the tail suspension test and decreased preference for a sucrose solution. These effects were associated with an activation of central but not peripheral IDO1, as LPS increased brain kynurenine but had no effect on plasma concentrations of kynurenine. Interestingly, genetic deletion or pharmacological inhibition of IDO1, using 1-methyl-tryptophan, abrogated the reduction in sucrose preference induced by intracerebroventricular LPS. 1-Methyl-tryptophan also blocked the LPS-induced increase in duration of immobility during the tail suspension test.ConclusionsThese data indicate that activation of brain IDO1 is sufficient to induce depression-like behaviors of mice in response to central LPS.

Mounting evidence demonstrates that kynurenine metabolism may play an important pathogenic role in the development of multiple neurological and neuropsychiatric disorders. The kynurenine pathway consists of two functionally distinct branches that generate both neuroactive and oxidatively reactive metabolites. In the brain, the rate-limiting enzyme for one of these branches, kynurenine 3-monooxygenase (KMO), is predominantly expressed in microglia and has emerged as a pivotal point of metabolic regulation. KMO substrate and expression levels are upregulated by pro-inflammatory cytokines and altered by functional genetic mutations. Increased KMO metabolism results in the formation of metabolites that activate glutamate receptors and elevate oxidative stress, while recent evidence has revealed neurodevelopmental consequences of reduced KMO activity. Together, the evidence suggests that KMO is positioned at a critical metabolic junction to influence the development or trajectory of a myriad of neurological diseases. Understanding the mechanism(s) by which alterations in KMO activity are able to impair neuronal function, and viability will enhance our knowledge of related disease pathology and provide insight into novel therapeutic opportunities. This review will discuss the influence of KMO on brain kynurenine metabolism and the current understanding of molecular mechanisms by which altered KMO activity may contribute to neurodevelopment, neurodegenerative, and neuropsychiatric diseases.

BackgroundInflammation increases the risk of developing depression-related symptoms, and tryptophan metabolism is an important mediator of these behavior changes. Peripheral immune activation results in central up-regulation of pro-inflammatory cytokine expression, microglia activation, and the production of neurotoxic kynurenine metabolites. The neuroinflammatory and kynurenine metabolic response to peripheral immune activation has been largely characterized at the whole brain level. It is unknown if this metabolic response exhibits regional specificity even though the unique indoleamine 2,3-dioxygenase (IDO)-dependent depressive-like behaviors are known to be controlled by discrete brain regions. Therefore, regional characterization of neuroinflammation and kynurenine metabolism might allow for better understanding of the potential mechanisms that mediate inflammation-associated behavior changes.MethodsFollowing peripheral immune challenge with lipopolysaccharide (LPS), brain tissue from behaviorally relevant regions was analyzed for changes in mRNA of neuroinflammatory targets and kynurenine pathway enzymes. The metabolic balance of the kynurenine pathway was also determined in the peripheral circulation and these brain regions.ResultsPeripheral LPS treatment resulted in region-independent up-regulation of brain expression of pro-inflammatory cytokines and glial cellular markers indicative of a neuroinflammatory response. The expression of kynurenine pathway enzymes was also largely region-independent. While the kynurenine/tryptophan ratio was elevated significantly in both the plasma and in each brain regions evaluated, the balance of kynurenine metabolism was skewed toward production of neurotoxic metabolites in the hippocampus.ConclusionsThe upstream neuroinflammatory processes, such as pro-inflammatory cytokine production, glial cell activation, and kynurenine production, may be similar throughout the brain. However, it appears that the balance of downstream kynurenine metabolism is a tightly regulated brain region-dependent process.

Chronic stress or inflammation increases tryptophan metabolism along the kynurenine pathway (KP), and the generation of neuroactive kynurenine metabolites contributes to subsequent depressive-like behaviors. Microglia regulate KP balance by preferentially producing oxidative metabolites, including quinolinic acid. Research has focused on the interplay between cytokines and HPA axis-derived corticosteroids in regulating microglial activity and effects of KP metabolites directly on neurons; however, the potential role that KP metabolites have directly on microglial activity is unknown. Here, murine microglia were stimulated with lipopolysaccharide(LPS). After 6 h, mRNA expression of interleukin(IL)-1β, IL-6, tumor necrosis factor(TNF)-α and inducible nitric oxide synthase(iNOS) was dose-dependently increased along with the rate-limiting enzymes for oxidative KP metabolism, indoleamine-2,3-dioxygenase(IDO)-1 and kynurenine 3-monooxygenase(KMO). By 24 h post-LPS, kynurenine and quinolinic acid in the media was elevated. Inhibiting KMO with Ro 61-8048 during LPS challenge attenuated extracellular nitrite accumulation and expression of KMO and TNF-α in response to LPS. Similarly, primary microglia isolated from KMO mice exhibited a significantly reduced pro-inflammatory response to LPS compared to WT controls. To determine whether the substrate (kynurenine) or end product (quinolinic acid) of KMO-dependent metabolism modulates the LPS response, microglia were treated with increasing concentrations of L-kynurenine or quinolinic acid in combination with LPS or saline. Interestingly, quinolinic acid did not impact the microglial LPS response. However, L-kynurenine had dose-dependent inhibitory effect on the LPS response. These data are the first to show an anti-inflammatory effect of KMO inhibition on microglia during immune challenge and suggest that KP metabolic balance may play a direct role in regulating microglia activity.

Autism spectrum disorder is a neurodevelopmental syndrome diagnosed primarily by persistent deficits in social interactions and communication, unusual sensory reactivity, motor stereotypies, repetitive behaviors, and restricted interests. No FDA‐approved medical treatments exist for the diagnostic symptoms of autism. Here we interrogate multiple pharmacological targets in two distinct mouse models that incorporate well‐replicated autism‐relevant behavioral phenotypes. Compounds that modify inhibitory or excitatory neurotransmission were selected to address hypotheses based on previously published biological abnormalities in each model. Shank3B is a genetic model of a mutation found in autism and Phelan‐McDermid syndrome, in which deficits in excitatory neurotransmission and synaptic plasticity have been reported. BTBR is an inbred strain model of forms of idiopathic autism in which reduced inhibitory neurotransmission and excessive mTOR signaling have been reported. The GABA‐A receptor agonist gaboxadol significantly reduced repetitive self‐grooming in three independent cohorts of BTBR. The TrkB receptor agonist 7,8‐DHF improved spatial learning in Shank3B mice, and reversed aspects of social deficits in BTBR. CX546, a positive allosteric modulator of the glutamatergic AMPA receptor, and d‐cycloserine, a partial agonist of the glycine site on the glutamatergic NMDA receptor, did not rescue aberrant behaviors in Shank3B mice. The mTOR inhibitor rapamycin did not ameliorate social deficits or repetitive behavior in BTBR mice. Comparison of positive and negative pharmacological outcomes, on multiple phenotypes, evaluated for replicability across independent cohorts, enhances the translational value of mouse models of autism for therapeutic discovery. GABA agonists present opportunities for personalized interventions to treat components of autism spectrum disorder. Autism Res 2019, 12: 401–421 © 2019 The Authors. Autism Research published by International Society for Autism Research published by Wiley Periodicals, Inc. Lay Summary Many of the risk genes for autism impair synapses, the connections between nerve cells in the brain. A drug that reverses the synaptic effects of a mutation could offer a precision therapy. Combining pharmacological and behavioral therapies could reduce symptoms and improve the quality of life for people with autism. Here we report reductions in repetitive behavior by a GABA‐A receptor agonist, gaboxadol, and improvements in social and cognitive behaviors by a TrkB receptor agonist, in mouse models of autism.

Background:Brain-derived neurotrophic factor (BDNF) deficiency confers vulnerability to stress, but the mechanisms are unclear. BDNF+/- mice exhibit behavioral, physiological, and neurochemical changes following low-level stress that are hallmarks of major depression. After immune challenge, neuroinflammation-induced changes in tryptophan metabolism along the kynurenine pathway mediate depressive-like behaviors.Methods:We hypothesized that BDNF+/- mice would be more susceptible to stress-induced neuroinflammation and kynurenine metabolism, so BDNF+/- or wild-type littermate mice were subject to repeated unpredictable mild stress. Proinflammatory cytokine expression and kynurenine metabolites were measured.Results:Unpredictable mild stress did not induce neuroinflammation. However, only wild-type mice produced the neuroprotective factors interleukin-10 and kynurenic acid in response to repeated unpredictable mild stress. In BDNF+/- mice, kynurenine was metabolized preferentially to the neurotoxic intermediate 3-hydroxykynurenine following repeated unpredictable mild stress.Conclusions:Our data suggest that BDNF may modulate kynurenine pathway metabolism during stress and provide a novel molecular mechanism of vulnerability and resilience to the development of stress-precipitated psychiatric disorders.