Metabolic shift of the kynurenine pathway impairs alcohol and cocaine seeking and relapse

Impact of Substance Use Disorder on Tryptophan Metabolism Through the Kynurenine Pathway: A Narrative Review

Background/Objectives: Substance use disorder is a crisis impacting many people in the United States. This review aimed to identify the effect addictive substances have on the kynurenine pathway. Tryptophan is an essential amino acid metabolized by the serotonin and kynurenine pathways. The metabolites of these pathways play a role in the biological reward system. Addictive substances have been shown to cause imbalances in the ratios of these metabolites. With current treatment and therapeutic options being suboptimal, identifying biochemical mechanisms that are impacted during the use of addictive substances can provide alternative options for treatment or drug discovery. Methods: A systematic literature search was conducted to identify studies evaluating the relationship between substance use disorder and tryptophan metabolism through the kynurenine pathway. A total of 32 articles meeting eligibility criteria were used to review the relationship between the kynurenine pathway, tryptophan breakdown, and addictive substances. Results: The use of addictive substances dysregulates tryptophan metabolism and kynurenine metabolite concentrations. This imbalance directly affects the dopamine reward system and is thought to promote continued substance use. Conclusions: Further studies are needed to fully evaluate the metabolites of the kynurenine pathway, along with other options for treatment to repair the metabolite imbalance. Several possible therapeutics have been identified; drugs that restore homeostasis, such as Ro 61-8048 and natural products like Tinospora cordifolia or Decaisnea insignis, are promising options for the treatment of substance use disorder.

Integrative Pharmacology in the Treatment of Substance Use Disorders

The detrimental physical, mental, and socioeconomic effects of substance use disorders (SUDs) have been apparent to the medical community for decades. However, it has become increasingly urgent in recent years to develop novel pharmacotherapies to treat SUDs. Currently, practitioners typically rely on monotherapy. Monotherapy has been shown to be superior to no treatment at all for most substance classes. However, many randomized controlled trials (RCTs) have revealed that monotherapy leads to poorer outcomes when compared with combination treatment in all specialties of medicine. The results of RCTs suggest that monotherapy frequently fails since multiple dysregulated pathways, enzymes, neurotransmitters, and receptors are involved in the pathophysiology of SUDs. As such, research is urgently needed to determine how various neurobiological mechanisms can be targeted by novel combination treatments to create increasingly specific yet exceedingly comprehensive approaches to SUD treatment. This article aims to review the neurobiology that integrates many pathophysiologic mechanisms and discuss integrative pharmacology developments that may ultimately improve clinical outcomes for patients with SUDs. Many neurobiological mechanisms are known to be involved in SUDs including dopaminergic, nicotinic, N-methyl-D-aspartate (NMDA), and kynurenic acid (KYNA) mechanisms. Emerging evidence indicates that KYNA, a tryptophan metabolite, modulates all these major pathophysiologic mechanisms. Therefore, achieving KYNA homeostasis by harmonizing integrative pathophysiology and pharmacology could prove to be a better therapeutic approach for SUDs. We propose KYNA-NMDA-α7nAChRcentric pathophysiology, the "conductor of the orchestra," as a novel approach to treat many SUDs concurrently. KYNA-NMDA-α7nAChR pathophysiology may be the "command center" of neuropsychiatry. To date, extant RCTs have shown equivocal findings across comparison conditions, possibly because investigators targeted single pathophysiologic mechanisms, hit wrong targets in underlying pathophysiologic mechanisms, and tested inadequate monotherapy treatment. We provide examples of potential combination treatments that simultaneously target multiple pathophysiologic mechanisms in addition to KYNA. Kynurenine pathway metabolism demonstrates the greatest potential as a target for neuropsychiatric diseases. The investigational medications with the most evidence include memantine, galantamine, and N-acetylcysteine. Future RCTs are warranted with novel combination treatments for SUDs. Multicenter RCTs with integrative pharmacology offer a promising, potentially fruitful avenue to develop novel therapeutics for the treatment of SUDs.

Role of Neuronal Nicotinic Acetylcholine Receptors in Cannabinoid Dependence

Cannabis is among the most widely consumed psychoactive drugs around the world and cannabis use disorder (CUD) has no current approved pharmacological treatment. Nicotine and cannabis are commonly co-used which suggests there to be overlapping neurobiological actions supported primarily by the co-distribution of both receptor systems in the brain. There appears to be strong rationale to explore the role that nicotinic receptors play in cannabinoid dependence. Preclinical studies suggest that the ɑ7 nAChR subtype may play a role in modulating the reinforcing and discriminative stimulus effects of cannabinoids, while the ɑ4β2 * nAChR subtype may be involved in modulating the motor and sedative effects of cannabinoids. Preclinical and human genetic studies point towards a potential role of the ɑ5, ɑ3, and β4 nAChR subunits in CUD, while human GWAS studies strongly implicate the ɑ2 subunit as playing a role in CUD susceptibility. Clinical studies suggest that current smoking cessation agents, such as varenicline and bupropion, may also be beneficial in treating CUD, although more controlled studies are necessary. Additional behavioral, molecular, and mechanistic studies investigating the role of nAChR in the modulation of the pharmacological effects of cannabinoids are needed.

Dysregulation of tryptophan metabolism and distortion of cell signaling after oral exposure to ethanol and Kynurenic acid

Kynurenic acid (KYNA), an unavoidable tryptophan metabolite during fermentation is naturally blended with alcohol in all alcoholic beverages. Thus, alcohol drinking inevitably results in co-intake of KYNA. Effects of alcohol or KYNA on human health have been widely studied. However, the combined effects of both remain unknown. Here we report that alcohol and KYNA have a synergistic impact of on global gene expression, especially the gene sets related to tryptophan metabolism and cell signaling. Adult mice were exposed to alcohol (ethanol) and/or KYNA daily for a week. Transcriptomes of the brain, kidney and liver were profiled via bulk RNA sequencing. Results indicate that while KYNA alone largely promotes, and alcohol alone mostly inhibits gene expression, alcohol and KYNA co-administration has a stronger inhibition of global gene expression. Tryptophan metabolism is severely skewed towards kynurenine pathway by decreasing tryptophan hydroxylase 2 and increasing tryptophan dioxygenase. Quantification of tryptophan metabolic enzymes corroborates the transcriptional changes of these enzymes. Furthermore, the co-administration greatly enhances the GnRH signaling pathway. This research provides critical data to better understand the effects of alcohol and KYNA in mix on human health.

Tryptophan and Substance Abuse: Mechanisms and Impact

Addiction, the continuous misuse of addictive material, causes long-term dysfunction in the neurological system. It substantially affects the control strength of reward, memory, and motivation. Addictive substances (alcohol, marijuana, caffeine, heroin, methamphetamine (METH), and nicotine) are highly active central nervous stimulants. Addiction leads to severe health issues, including cardiovascular diseases, serious infections, and pulmonary/dental diseases. Drug dependence may result in unfavorable cognitive impairments that can continue during abstinence and negatively influence recovery performance. Although addiction is a critical global health challenge with numerous consequences and complications, currently, there are no efficient options for treating drug addiction, particularly METH. Currently, novel treatment approaches such as psychological contingency management, cognitive behavioral therapy, and motivational enhancement strategies are of great interest. Herein, we evaluate the devastating impacts of different addictive substances/drugs on users' mental health and the role of tryptophan in alleviating unfavorable side effects. The tryptophan metabolites in the mammalian brain and their potential to treat compulsive abuse of addictive substances are investigated by assessing the functional effects of addictive substances on tryptophan. Future perspectives on developing promising modalities to treat addiction and the role of tryptophan and its metabolites to alleviate drug dependency are discussed.

Proteomics and weighted gene correlated network analysis reveal glutamatergic synapse signaling in diazepam treatment of alcohol withdrawal

Background: Alcohol use disorder (AUD) is characterized by chronic excessive alcohol consumption, often alternating with periods of abstinence known as alcohol withdrawal syndrome (AWS). Diazepam is the preferred benzodiazepine for treatment of alcohol withdrawal syndrome under most circumstances, but the specific mechanism underlying the treatment needs further research. Methods: We constructed an animal model of two-bottle choices and chronic intermittent ethanol exposure. LC-MS/MS proteomic analysis based on the label-free and intensity-based quantification approach was used to detect the protein profile of the whole brain. Weighted gene correlated network analysis was applied for scale-free network topology analysis. We established a protein-protein interaction network based on the Search Tool for the Retrieval of Interacting Genes (STRING) database and Cytoscape software and identified hub proteins by CytoHubba and MCODE plugins of Cytoscape. The online tool Targetscan identified miRNA-mRNA pair interactions. Results: Seven hub proteins (Dlg3, Dlg4, Shank3, Grin2b, Camk2b, Camk2a and Syngap1) were implicated in alcohol withdrawal syndrome or diazepam treatment. In enrichment analysis, glutamatergic synapses were considered the most important pathway related to alcohol use disorder. Decreased glutamatergic synapses were observed in the late stage of withdrawal, as a protective mechanism that attenuated withdrawal-induced excitotoxicity. Diazepam treatment during withdrawal increased glutamatergic synapses, alleviating withdrawal-induced synapse inhibition. Conclusion: Glutamatergic synapses are considered the most important pathway related to alcohol use disorder that may be a potential molecular target for new interventional strategies.

Influx of kynurenine into the brain is involved in the reduction of ethanol consumption induced by Ro 61-8048 after chronic intermittent ethanol in mice

BACKGROUND AND PURPOSE

The kynurenine pathway has been proposed as a new target for modulating drug abuse. We previously demonstrated that inhibition of kynurenine 3-monooxygenase (KMO) using Ro 61-8048 reduces ethanol consumption in a binge drinking model. Here we investigate the effect of the kynurenine pathway modulation in ethanol -dependent mice.

EXPERIMENTAL APPROACH

Adult male and female mice were subjected to the Chronic Intermittent Ethanol (CIE) paradigm. On the last day of CIE, mice were treated with Ro 61-8048, Ro 61-8048 + PNU-120596, a positive allosteric modulator of α7nAChR, and Ro 61-8048 + L-leucine or probenecid, which block the influx or efflux of kynurenine from the brain, respectively. Ethanol, water consumption and preference were measured and kynurenine levels in plasma and limbic forebrain were determined.

KEY RESULTS

Ro 61-8048 decreases consumption and preference for ethanol in both sexes exposed to the CIE model, an effect that is prevented by PNU-120596. The Ro 61-8048-induced decrease in ethanol consumption depends on the influx of kynurenine into the brain.

CONCLUSION AND IMPLICATIONS

Inhibition of KMO reduces ethanol consumption and preference in both male and female mice subjected to CIE model by a mechanism involving α7nAChR. Moreover, the effect which is mediated centrally depends on the influx of peripheral kynurenine to the brain and can be prolonged by blocking the efflux of kynurenine from the brain. Here, for the first time we demonstrate that the modulation of the kynurenine pathway is a valid strategy for the treatment of ethanol dependence in both sexes.

FACTORS CONTRIBUTING TO THE ESCALATION OF ALCOHOL CONSUMPTION

Understanding factors that contribute to the escalation of alcohol consumption is key to understanding how an individual transitions from non/social drinking to AUD and to providing better treatment. In this review, we discuss how the way ethanol is consumed as well as individual and environmental factors contribute to the escalation of ethanol consumption from intermittent low levels to consistently high levels. Moreover, we discuss how these factors are modelled in animals. It is clear a vast array of complex, interacting factors influence changes in alcohol consumption. Some of these factors act early in the acquisition of ethanol consumption and initial escalation, while others contribute to escalation of ethanol consumption at a later stage and are involved in the development of alcohol dependence. There is considerable need for more studies examining escalation associated with the formation of dependence and other hallmark features of AUD, especially studies examining mechanisms, as it is of considerable relevance to understanding and treating AUD.

Alterations of kynurenine pathway in alcohol use disorder and abstinence: a link with gut microbiota, peripheral inflammation and psychological symptoms

The gut-brain communication is mostly driven by the immune, metabolic and neural pathways which remained poorly explored in patients with alcohol use disorder (AUD). The metabolites arising from the tryptophan-kynurenine pathway have gained considerable attention since they are at the interface between intestinal bacteria, host immune response and brain functions. This study described the circulating levels of kynurenine metabolites in AUD patients, at the onset (T1) and end (T2) of a 3-week detoxification program, and tested correlations between those metabolites and inflammatory markers, the gut microbiota and the psychological symptoms. Increased concentration of the neurotoxic metabolite quinolinic acid (QUIN) and decreased levels of the neuroprotector metabolite kynurenic acid (KYNA) which both modulate glutamatergic neurotransmission were observed in AUD patients, particularly at T2. The inflammatory marker hsCRP was associated with several metabolic ratios of the kynurenine pathway. Tryptophan, KYNA and QUIN were correlated with depression, alcohol craving and reaction time, respectively. Analysis of gut microbiota revealed that bacteria known as short-chain fatty acid producers, as well as bacterial metabolites including butyrate and medium-chain fatty acids were associated with some metabolites of the tryptophan-kynurenine pathway. Targeting the glutamatergic neurotransmission through the modulation of the kynurenine pathway, by manipulating the gut microbiota, might represent an interesting alternative for modulating alcohol-related behavior.

Evidence for Modulation of Substance Use Disorders by the Gut Microbiome: Hidden in Plain Sight

The gut microbiome modulates neurochemical function and behavior and has been implicated in numerous central nervous system (CNS) diseases, including developmental, neurodegenerative, and psychiatric disorders. Substance use disorders (SUDs) remain a serious threat to the public well-being, yet gut microbiome involvement in drug abuse has received very little attention. Studies of the mechanisms underlying SUDs have naturally focused on CNS reward circuits. However, a significant body of research has accumulated over the past decade that has unwittingly provided strong support for gut microbiome participation in drug reward. β-Lactam antibiotics have been employed to increase glutamate transporter expression to reverse relapse-induced release of glutamate. Sodium butyrate has been used as a histone deacetylase inhibitor to prevent drug-induced epigenetic alterations. High-fat diets have been used to alter drug reward because of the extensive overlap of the circuitry mediating them. This review article casts these approaches in a different light and makes a compelling case for gut microbiome modulation of SUDs. Few factors alter the structure and composition of the gut microbiome more than antibiotics and a high-fat diet, and butyrate is an endogenous product of bacterial fermentation. Drugs such as cocaine, alcohol, opiates, and psychostimulants also modify the gut microbiome. Therefore, their effects must be viewed on a complex background of cotreatment-induced dysbiosis. Consideration of the gut microbiome in SUDs should have the beneficial effects of expanding the understanding of SUDs and aiding in the design of new therapies based on opposing the effects of abused drugs on the host's commensal bacterial community. SIGNIFICANCE STATEMENT: Proposed mechanisms underlying substance use disorders fail to acknowledge the impact of drugs of abuse on the gut microbiome. β-Lactam antibiotics, sodium butyrate, and high-fat diets are used to modify drug seeking and reward, overlooking the notable capacity of these treatments to alter the gut microbiome. This review aims to stimulate research on substance abuse-gut microbiome interactions by illustrating how drugs of abuse share with antibiotics, sodium butyrate, and fat-laden diets the ability to modify the host microbial community.

The Tryptophan System in Cocaine-Induced Depression

Major depression disorder (MDD) is the most prevalent psychiatric comorbid condition in cocaine use disorder (CUD). The comorbid MDD might be primary-MDD (CUD-primary-MDD) or cocaine-induced MDD (CUD-induced-MDD), and their accurate diagnoses and treatment is a challenge for improving prognoses. This study aimed to assess the tryptophan/serotonin (Trp/5-HT) system with the acute tryptophan depletion test (ATD), and the kynurenine pathway in subjects with CUD-primary-MDD, CUD-induced-MDD, MDD and healthy controls. The ATD was performed with a randomized, double-blind, crossover, and placebo-controlled design. Markers of enzymatic activity of indoleamine 2,3-dioxygenase/tryptophan 2,3-dioxygenase, kynurenine aminotransferase (KAT) and kynureninase were also established. Following ATD, we observed a decrease in Trp levels in all groups. Comparison between CUD-induced-MDD and MDD revealed significant differences in 5-HT plasma concentrations (512 + 332 ng/mL vs. 107 + 127 ng/mL, p = 0.039) and the Kyn/5-HT ratio (11 + 15 vs. 112 + 136; p = 0.012), whereas there were no differences between CUD-primary-MDD and MDD. Effect size coefficients show a gradient for all targeted markers (d range 0.72-1.67). Results suggest different pathogenesis for CUD-induced-MDD, with lower participation of the tryptophan system, probably more related to other neurotransmitter pathways and accordingly suggesting the need for a different pharmacological treatment approach.

Plasma tryptophan and kynurenine pathway metabolites in abstinent patients with alcohol use disorder and high prevalence of psychiatric comorbidity

BACKGROUND

Alterations in tryptophan (TRP) metabolism has been linked to drug exposure and mental disorders. However, most of studies have been performed without considering the co-occurrence of both disorders in the context of addiction. This cross-sectional study examines TRP metabolism through the serotonin (5-HT) and kynurenine (KYN) pathways in subjects with alcohol use disorders (AUD) and high prevalence of psychiatric comorbidity.

METHODS

For this purpose, male and female abstinent AUD patients (N = 130) and healthy controls (N = 80) were clinically evaluated for substance use and mental disorders, and blood samples were collected to determine plasma concentrations of TRP, 5-HT, KYN and kynurenic acid (KA) using high performance liquid chromatography. Clinical and biochemical variables were analyzed for potential associations considering AUD, psychiatric comorbidity and sex.

RESULTS

TRP concentrations were significantly associated with an interaction effect between AUD diagnosis and sex (p < .01): TRP concentrations were lower in male AUD patients but higher in female AUD patients compared with their controls. KYN and KA concentrations were significantly associated with AUD diagnosis (p < .01 and p < .05, respectively). Thus, AUD patients showed significantly higher KYN concentrations and lower KA concentrations than controls. Regarding 5-HT concentrations, there were sex differences in the alcohol group (p < .05) and female AUD patients showed lower 5-HT concentrations than male AUD patients. Moreover, there was a significant interaction effect between psychiatric comorbidity and sex on TRP concentrations in the alcohol group (p < .01). Whereas male patients with both comorbid substance use and mental disorders showed lower TRP concentrations than male non-comorbid patients, female patients with comorbid mental disorders showed higher TRP concentrations than female non-comorbid patients.

CONCLUSION

While alterations in the KYN pathway appear to be directly associated with a history of AUD, altered TRP concentrations are associated with the presence of comorbid psychiatric disorders. Finally, sex differences in TRP metabolism must be considered in future studies.

Preliminary comparative analysis of kynurenine pathway metabolites in chronic ketamine users, schizophrenic patients, and healthy controls

OBJECTIVE

The serum kynurenine pathway metabolites kynurenic acid (KYNA), kynurenine (KYN), and tryptophan (TRP) were examined in chronic ketamine users and in schizophrenic patients. The correlations of the metabolites with sociodemographic data, clinical characteristics, and drug use status were analyzed.

METHODS

Seventy-nine healthy controls, 78 ketamine users, and 80 schizophrenic patients were recruited. Serum TRP, KYN, and KYNA levels were measured by high-performance liquid chromatography following tandem mass spectrometry (MS/MS). Psychotic symptoms were evaluated using the positive and negative syndrome scale (PANSS), the Beck Depression Inventory (BDI), and the Beck Anxiety Inventory (BAI).

RESULTS

Serum levels of TRP, KYNA, and KYN (in ketamine users only) were lower in ketamine users and schizophrenic patients than in controls (p < .05). TRP and KYN were lower in ketamine users than in schizophrenic patients (p < .01). KYNA levels were positively correlated with the current frequency of ketamine use in ketamine users (p = .031), and serum KYNA levels were negatively correlated with the duration of schizophrenia (p = .015).

CONCLUSION

TRP, KYNA, and KYN were lower in chronic ketamine users than in controls, and the alterations were in the same direction as those observed in schizophrenic patients.

Sobriety and Satiety: Is NAD+ the Answer?

Nicotinamide adenine dinucleotide (NAD+) is an essential pyridine nucleotide that has garnered considerable interest in the last century due to its critical role in cellular processes associated with energy production, cellular protection against stress and longevity. Research in NAD+ has been reinvigorated by recent findings that components of NAD+ metabolism and NAD-dependent enzymes can influence major signalling processes associated with the neurobiology of addiction. These studies implicate raising intracellular NAD+ levels as a potential target for managing and treating addictive behaviour and reducing cravings and withdrawal symptoms in patients with food addiction and/or substance abuse. Since clinical studies showing the use of NAD+ for the treatment of addiction are limited, this review provides literature evidence that NAD+ can influence the neurobiology of addiction and may have benefits as an anti-addiction intervention.

Serotonin is the main tryptophan metabolite associated with psychiatric comorbidity in abstinent cocaine-addicted patients

The lack of effective treatments and a high rate of relapse in cocaine addiction constitute a major health problem. The present study was conducted to examine the expression of tryptophan-derived metabolites in the context of cocaine addiction and psychiatric comorbidity, which is common in addicted subjects. Abstinent patients with cocaine use disorder (CUD) and control subjects were recruited for a cross-sectional study. Participants were assessed with a semi-structured diagnostic interview (PRISM) based on DSM-IV-TR for substance and mental disorders. Plasma concentrations of tryptophan metabolites and their association with relevant CUD-related variables and psychiatric comorbidity were explored. We observed decreased plasma kynurenic acid concentrations in the cocaine group, however no associations between CUD-related variables and tryptophan-derived metabolites were found. In contrast, 5-HT concentrations were increased in CUD-patients and the diagnosis of different psychiatric disorders in the cocaine group was related to higher plasma 5-HT concentrations compared with non-comorbid patients. Therefore, while changes in plasma kynurenic acid concentrations appear to be directly associated with lifetime CUD, changes in 5-HT concentrations are associated with psychiatric comorbidity. These results emphasize the need to find potential biomarkers for a better stratification of cocaine-addicted patients in order to develop therapeutic approaches to prevent cocaine relapse.

NAD+ cellular redox and SIRT1 regulate the diurnal rhythms of tyrosine hydroxylase and conditioned cocaine reward

The diurnal regulation of dopamine is important for normal physiology and diseases such as addiction. Here we find a novel role for the CLOCK protein to antagonize CREB-mediated transcriptional activity at the tyrosine hydroxylase (TH) promoter, which is mediated by the interaction with the metabolic sensing protein, Sirtuin 1 (SIRT1). Additionally, we demonstrate that the transcriptional activity of TH is modulated by the cellular redox state, and daily rhythms of redox balance in the ventral tegmental area (VTA), along with TH transcription, are highly disrupted following chronic cocaine administration. Furthermore, CLOCK and SIRT1 are important for regulating cocaine reward and dopaminergic (DAergic) activity, with interesting differences depending on whether DAergic activity is in a heightened state and if there is a functional CLOCK protein. Taken together, we find that rhythms in cellular metabolism and circadian proteins work together to regulate dopamine synthesis and the reward value for drugs of abuse.

Changes in brain kynurenine levels via gut microbiota and gut-barrier disruption induced by chronic ethanol exposure in mice

Inflammatory processes have been shown to modify tryptophan (Trp) metabolism. Gut microbiota appears to play a significant role in the induction of peripheral and central inflammation. Ethanol (EtOH) exposure alters gut permeability, but its effects on Trp metabolism and the involvement of gut microbiota have not been studied. We analyzed several parameters of gut-barrier and of peripheral and central Trp metabolism following 2 different EtOH consumption patterns in mice, the binge model, drinking in the dark (DID), and the chronic intermittent (CI) consumption paradigm. Antibiotic treatment was used to evaluate gut microbiota involvement in the CI model. Mice exposed to CI EtOH intake, but not DID, show bacterial translocation and increased plasma LPS immediately after EtOH removal. Gut-barrier permeability to FITC-dextran is increased by CI, and, furthermore, intestinal epithelial tight-junction (TJ) disruption is observed (decreased expression of zonula occludens 1 and occludin) associated with increased matrix metalloproteinase (MMP)-9 activity and iNOS expression. CI EtOH, but not DID, increases kynurenine (Kyn) levels in plasma and limbic forebrain. Intestinal bacterial decontamination prevents the LPS increase but not the permeability to FITC-dextran, TJ disruption, or the increase in MMP-9 activity and iNOS expression. Although plasma Kyn levels are not affected by antibiotic treatment, the elevation of Kyn in brain is prevented, pointing to an involvement of microbiota in CI EtOH-induced changes in brain Trp metabolism. Additionally, CI EtOH produces depressive-like symptoms of anhedonia, which are prevented by the antibiotic treatment thus pointing to an association between anhedonia and the increase in brain Kyn and to the involvement of gut microbiota.-Giménez-Gómez, P., Pérez-Hernández, M., O'Shea, E., Caso, J. R., Martín-Hernández, D., Cervera, L. A., Centelles. M. L. G.-L., Gutiérrez-Lopez, M. D., Colado, M. I. Changes in brain kynurenine levels via gut microbiota and gut-barrier disruption induced by chronic ethanol exposure in mice.

Hypothesis kynurenic and quinolinic acids: The main players of the kynurenine pathway and opponents in inflammatory disease

I hypothesize that the intermediates of the kynurenine (Kyn) pathway (KP) of tryptophan (Trp) degradation kynurenic acid (KA) and quinolinic acid (QA) play opposite roles in inflammatory diseases, with KA being antiinflammatory and QA being immunosuppressant. Darlington et al. have demonstrated a decrease in the ratio of plasma 3-hydroxyanthranilic acid to anthranilic acid ([3-HAA]/[AA]) in many inflammatory conditions and proposed that this decrease either reflects inflammatory disease or is an antiinflammatory response. I argue in favour of the latter possibility and provide evidence that KA is responsible for the decrease in this ratio by increasing AA formation from Kyn through activation of the kynureninase reaction. Immunosuppression has been attributed to some Kyn metabolites tested at concentrations far greater than could occur in microenvironments. So far, only QA has been shown using immunohistochemistry to reach immunosuppressive levels. Future immune studies of the KP should focus on QA as the potentially main microenvironmentally measurable immunosuppressant and should include KA as an antiinflammatory metabolite.

Assessing and Modulating Kynurenine Pathway Dynamics in Huntington's Disease: Focus on Kynurenine 3-Monooxygenase

The link between disturbances in kynurenine pathway (KP) metabolism and Huntington's disease (HD) pathogenesis has been explored for a number of years. Several novel genetic and pharmacological tools have recently been developed to modulate key regulatory steps in the KP such as the reaction catalyzed by the enzyme kynurenine 3-monooxygenase (KMO). This insight has offered new options for exploring the mechanistic link between this metabolic pathway and HD, and provided novel opportunities for the development of candidate drug-like compounds. Here, we present an overview of the field, focusing on some novel approaches for interrogating the pathway experimentally.

Preclinical Studies of Cannabinoid Reward, Treatments for Cannabis Use Disorder, and Addiction-Related Effects of Cannabinoid Exposure

Cannabis use has become increasingly accepted socially and legally, for both recreational and medicinal purposes. Without reliable information about the effects of cannabis, people cannot make informed decisions regarding its use. Like alcohol and tobacco, cannabis can have serious adverse effects on health, and some people have difficulty discontinuing their use of the drug. Many cannabis users progress to using and becoming addicted to other drugs, but the reasons for this progression are unclear. The natural cannabinoid system of the brain is complex and involved in many functions, including brain development, reward, emotion, and cognition. Animal research provides an objective and controlled means of obtaining information about: (1) how cannabis affects the brain and behavior, (2) whether medications can be developed to treat cannabis use disorder, and (3) whether cannabis might produce lasting changes in the brain that increase the likelihood of becoming addicted to other drugs. This review explains the tactics used to address these issues, evaluates the progress that has been made, and offers some directions for future research.

Rat animal models for screening medications to treat alcohol use disorders

The purpose of this review is to present animal research models that can be used to screen and/or repurpose medications for the treatment of alcohol abuse and dependence. The focus will be on rats and in particular selectively bred rats. Brief introductions discuss various aspects of the clinical picture, which provide characteristics of individuals with alcohol use disorders (AUDs) to model in animals. Following this, multiple selectively bred rat lines will be described and evaluated in the context of animal models used to screen medications to treat AUDs. Next, common behavioral tests for drug efficacy will be discussed particularly as they relate to stages in the addiction cycle. Tables highlighting studies that have tested the effects of compounds using the respective techniques are included. Wherever possible the Tables are organized chronologically in ascending order to describe changes in the focus of research on AUDs over time. In general, high ethanol-consuming selectively bred rats have been used to test a wide range of compounds. Older studies usually followed neurobiological findings in the selected lines that supported an association with a propensity for high ethanol intake. Most of these tests evaluated the compound's effects on the maintenance of ethanol drinking. Very few compounds have been tested during ethanol-seeking and/or relapse and fewer still have assessed their effects during the acquisition of AUDs. Overall, while a substantial number of neurotransmitter and neuromodulatory system targets have been assessed; the roles of sex- and age-of-animal, as well as the acquisition of AUDs, ethanol-seeking and relapse continue to be factors and behaviors needing further study. This article is part of the Special Issue entitled "Alcoholism".

Kynurenine Pathway of Tryptophan Metabolism: Regulatory and Functional Aspects

Regulatory and functional aspects of the kynurenine (K) pathway (KP) of tryptophan (Trp) degradation are reviewed. The KP accounts for ~95% of dietary Trp degradation, of which 90% is attributed to the hepatic KP. During immune activation, the minor extrahepatic KP plays a more active role. The KP is rate-limited by its first enzyme, Trp 2,3-dioxygenase (TDO), in liver and indoleamine 2,3-dioxygenase (IDO) elsewhere. TDO is regulated by glucocorticoid induction, substrate activation and stabilization by Trp, cofactor activation by heme, and end-product inhibition by reduced nicotinamide adenine dinucleotide (phosphate). IDO is regulated by IFN-γ and other cytokines and by nitric oxide. The KP disposes of excess Trp, controls hepatic heme synthesis and Trp availability for cerebral serotonin synthesis, and produces immunoregulatory and neuroactive metabolites, the B₃ “vitamin” nicotinic acid, and oxidized nicotinamide adenine dinucleotide. Various KP enzymes are undermined in disease and are targeted for therapy of conditions ranging from immunological, neurological, and neurodegenerative conditions to cancer.