Tryptophan in alcoholism treatment I: kynurenine metabolites inhibit the rat liver mitochondrial low Km aldehyde dehydrogenase activity, elevate blood acetaldehyde concentration and induce aversion to alcohol

Alcohol Use Disorder and the Gut-Brain Axis: A Narrative Review of the Role of Gut Microbiota and Implications for Treatment

Alcohol use disorder (AUD) affects millions of people worldwide and can lead to deleterious physical and social consequences. Recent research has highlighted not only the effect of alcohol on the gut microbiome, but also the role of the gut microbiome and the gut-brain axis in the development and maintenance of alcohol use disorder. This review provides an overview of the reciprocal relationship between alcohol consumption and the gut microbiome, including the effects of alcohol on gut microbial composition, changes in gut microbial metabolites in response to alcohol consumption, and how gut microbial metabolites may modulate alcohol use behavior. We also discuss the gut-mediated mechanisms of neuroinflammation that contribute to and result from AUD, including disruption of the intestinal barrier, toll-like receptor signaling, and the activation of glial cells and immune cells. Finally, we review the current evidence on gut microbial-directed therapies for AUD and discuss the implications of this research for our understanding of the pathophysiology of AUD and future research directions.

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.

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.

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.

Gut microbiota, kynurenine pathway and mental disorders - Review

The intestine and the gut-associated limphoid tissue constitute the largest immunity organ of the human body. Among several possible tryptophan metabolism routes, the kynurenine pathway can be influenced by the gut microbiota. Disturbances of gut biodiversity may cause increased gut permeability and cause systemic inflammation, also related to central nervous system. Proinflammatory cytokines induce kynurenine pathway enzymes resulting in formation of neuroactive metabolites, which are being associated with several psychiatric disorders. The kynurenine pathway may also be influenced by certain bacteria species directly. The aim of this review is to highlight the current knowledge on the interaction of gut microbiota and the central nervous system with the kynurenine pathway taken into special account. Up to date study results on specific psychiatric disorders such as schizophrenia, bipolar disorder, Alzheimer's disease, autism spectrum disorders, depression and alcoholism are presented. Available evidence suggests that toxicity of kynurenine metabolites may be reduced by adjunction of probiotics which can affect proinflammatory cytokines. Due to their potential for modulation of the kynurenine pathway, gut microbiota pose an interesting target for future therapies.

Addiction and the kynurenine pathway: A new dancing couple?

Drug use poses a serious threat to health systems throughout the world and the number of consumers rises relentlessly every year. The kynurenine pathway, main pathway of tryptophan degradation, has drawn interest in this field due to its relationship with addictive behaviour. Recently it has been confirmed that modulation of kynurenine metabolism at certain stages of the pathway can reduce, prevent or abolish drug seeking-like behaviours in studies with several different drugs. In this review, we present an up-to-date summary of the evidences of a relationship between drug use and the kynurenine pathway, both the alterations of the pathway due to drug use as well as modulation of the pathway as a potential approach to treat drug addiction. The review discusses ethanol, nicotine, cannabis, amphetamines, cocaine and opioids and new prospects in the drug research field are proposed.

The effect of a selective estrogen receptor modulator – raloxifene – on the levels of tryptophan and kynurenic acid in the livers of rats as studied via RP-HPLC-FL

The levels of tryptophan (TRP) and its main metabolite kynurenic acid (KYNA) were measured in rat livers treated with raloxifene – a selective estrogen receptor modulator. The research was conducted by applying high-performance liquid chromatography on a 5 μm Zorbax Eclipse XDB-C18 column. Selective fluorescence detection (FL) was performed at an excitation of 219 nm and emission of 360 nm for TRP and KYNA. The assays showed good linearity (R2 >0.95) within the tested ranges of 0.045-0.20 µg mL−1, 0.025-0.32 µg mL−1, respectively, for KYNA and TRP. The limits of the detection were found to be 0.1480 ng mL−1 for KYNA and 0.0332 ng mL-1 for TRP. The deproteinization of the liver homogenate samples was accomplished by 80% methanol addition combined with boiling precipitation. The average recovery values were between 94.84% and 99.54% with RSDs no more than 5.5%. The work revealed that raloxifene decreased the mean value of tryptophan, as compared with the control group, while simultaneously leaving kynurenic acid at the same level. For the first time the research suggests that, in the case of raloxifene therapy, tryptophan is not metabolized via the kynurenine pathway.

Organ-differential responses to ethanol and kynurenic acid, a component of alcoholic beverages in gene transcription

Excessive alcohol (ethanol) use has long been known to affect human health negatively. However, the underlying molecular basis is incompletely understood. Moreover, consumption of alcohol is often mixed with kynurenic acid (KYNA), an abundant tryptophan metabolite produced during fermentation. The combined effect of ethanol and KYNA on host gene expression has not been investigated. The current study used mice as the model to interrogate the impact of ethanol and/or KYNA on global gene transcription. Adult male mice were administered with 2 g/kg ethanol and/or 0.1 mg/kg KYNA by gavage once a day for a week. Three organs: brain, kidney, and liver were collected and their total RNAs extracted for transcriptome sequencing and quantitative real-time PCR. Gene ontology, Kyoto encyclopedia of genes, and genomes pathway analyses revealed that alcohol affects the three organs differentially. Furthermore, the gene expression profile from alcohol and KYNA co-administration was significantly different from that of alcohol or KYNA administration alone. Strikingly, Indolamine 2,3-dioxygenase 1, a rate-limiting enzyme in tryptophan metabolism, was significantly increased in the brain after a combined exposure of alcohol and KYNA, suggesting that Trp metabolism was skewed towards the kynurenine pathway in the brain. Our systemic analysis provides new insights into the mechanism whereby alcohol and KYNA affects organ functions.

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.

Genome-Wide Association Studies of Metabolite Concentrations (mGWAS): Relevance for Nephrology

Metabolites are small molecules that are intermediates or products of metabolism, many of which are freely filtered by the kidneys. In addition, the kidneys have a central role in metabolite anabolism and catabolism, as well as in active metabolite reabsorption and/or secretion during tubular passage. This review article illustrates how the coupling of genomics and metabolomics in genome-wide association analyses of metabolites can be used to illuminate mechanisms underlying human metabolism, with a special focus on insights relevant to nephrology. First, genetic susceptibility loci for reduced kidney function and chronic kidney disease (CKD) were reviewed systematically for their associations with metabolite concentrations in metabolomics studies of blood and urine. Second, kidney function and CKD-associated metabolites reported from observational studies were interrogated for metabolite-associated genetic variants to generate and discuss complementary insights. Finally, insights originating from the simultaneous study of both blood and urine or by modeling intermetabolite relationships are summarized. We also discuss methodologic questions related to the study of metabolite concentrations in urine as well as among CKD patients. In summary, genome-wide association analyses of metabolites using metabolite concentrations quantified from blood and/or urine are a promising avenue of research to illuminate physiological and pathophysiological functions of the kidney.

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.

Increasing kynurenine brain levels reduces ethanol consumption in mice by inhibiting dopamine release in nucleus accumbens

Recent research suggests that ethanol (EtOH) consumption behaviour can be regulated by modifying the kynurenine (KYN) pathway, although the mechanisms involved have not yet been well elucidated. To further explore the implication of the kynurenine pathway in EtOH consumption we inhibited kynurenine 3-monooxygenase (KMO) activity with Ro 61-8048 (100 mg/kg, i.p.), which shifts the KYN metabolic pathway towards kynurenic acid (KYNA) production. KMO inhibition decreases voluntary binge EtOH consumption and EtOH preference in mice subjected to "drinking in the dark" (DID) and "two-bottle choice" paradigms, respectively. This effect seems to be a consequence of increased KYN concentration, since systemic KYN administration (100 mg/kg, i.p.) similarly deters binge EtOH consumption in the DID model. Despite KYN and KYNA being well-established ligands of the aryl hydrocarbon receptor (AhR), administration of AhR antagonists (TMF 5 mg/kg and CH-223191 20 mg/kg, i.p.) and of an agonist (TCDD 50 μg/kg, intragastric) demonstrates that signalling through this receptor is not involved in EtOH consumption behaviour. Ro 61-8048 did not alter plasma acetaldehyde concentration, but prevented EtOH-induced dopamine release in the nucleus accumbens shell. These results point to a critical involvement of the reward circuitry in the reduction of EtOH consumption induced by KYN and KYNA increments. PNU-120596 (3 mg/kg, i.p.), a positive allosteric modulator of α7-nicotinic acetylcholine receptors, partially prevented the Ro 61-8048-induced decrease in EtOH consumption. Overall, our results highlight the usefulness of manipulating the KYN pathway as a pharmacological tool for modifying EtOH consumption and point to a possible modulator of alcohol drinking behaviour.

Tryptophan Metabolism in Rat Liver After Administration of Tryptophan, Kynurenine Metabolites, and Kynureninase Inhibitors

Rat liver tryptophan (Trp), kynurenine pathway metabolites, and enzymes deduced from product/substrate ratios were assessed following acute and/or chronic administration of kynurenic acid (KA), 3-hydroxykynurenine (3-HK), 3-hydroxyanthranilic acid (3-HAA), Trp, and the kynureni-nase inhibitors benserazide (BSZ) and carbidopa (CBD). KA activated Trp 2,3-dioxygenase (TDO), possibly by increasing liver 3-HAA, but inhibited kynurenine aminotransferase (KAT) and kynureninase activities with 3-HK as substrate. 3-HK inhibited kynureninase activity from 3-HK. 3-HAA stimulated TDO, but inhibited kynureninase activity from K and 3-HK. Trp (50 mg/kg) increased kynurenine metabolite concentrations and KAT from K, and exerted a temporary stimulation of TDO. The kynureninase inhibitors BSZ and CBD also inhibited KAT, but stimulated TDO. BSZ abolished or strongly inhibited the Trp-induced increases in liver Trp and kynurenine metabolites. The potential effects of these changes in conditions of immune activation, schizophrenia, and other disease states are discussed.

Tryptophan metabolism, disposition and utilization in pregnancy

Tryptophan (Trp) requirements in pregnancy are several-fold: (1) the need for increased protein synthesis by mother and for fetal growth and development; (2) serotonin (5-HT) for signalling pathways; (3) kynurenic acid (KA) for neuronal protection; (4) quinolinic acid (QA) for NAD(+) synthesis (5) other kynurenines (Ks) for suppressing fetal rejection. These goals could not be achieved if maternal plasma [Trp] is depleted. Although plasma total (free + albumin-bound) Trp is decreased in pregnancy, free Trp is elevated. The above requirements are best expressed in terms of a Trp utilization concept. Briefly, Trp is utilized as follows: (1) In early and mid-pregnancy, emphasis is on increased maternal Trp availability to meet the demand for protein synthesis and fetal development, most probably mediated by maternal liver Trp 2,3-dioxygenase (TDO) inhibition by progesterone and oestrogens. (2) In mid- and late pregnancy, Trp availability is maintained and enhanced by the release of albumin-bound Trp by albumin depletion and non-esterified fatty acid (NEFA) elevation, leading to increased flux of Trp down the K pathway to elevate immunosuppressive Ks. An excessive release of free Trp could undermine pregnancy by abolishing T-cell suppression by Ks. Detailed assessment of parameters of Trp metabolism and disposition and related measures (free and total Trp, albumin, NEFA, K and its metabolites and pro- and anti-inflammatory cytokines in maternal blood and, where appropriate, placental and fetal material) in normal and abnormal pregnancies may establish missing gaps in our knowledge of the Trp status in pregnancy and help identify appropriate intervention strategies.

Elevation of Kynurenine Metabolites in Rat Liver and Serum: A Potential Additional Mechanism of the Alcohol Aversive and Anti-cancer Effects of Disulfiram?

Aims

The tryptophan metabolites 3-hydroxykynurenine (3-HK) and 3-hydroxyanthranilic acid (3-HAA) inhibit the liver mitochondrial low K_m aldehyde dehydrogenase and possess alcohol-aversive and immunosuppressant properties. As the disulfiram (DS) metabolite carbon disulphide activates enzymes forming 3-HK and 3-HAA, we investigated if repeated disulfiram treatment increases the hepatic and serum levels of these 2 metabolites.

Methods

Livers and sera of male Wistar rats were analysed for tryptophan and kynurenine metabolites after repeated DS treatment for 7 days.

Results

DS increased liver and serum [3-HK] and [3-HAA] possibly by increasing the flux of tryptophan down the hepatic kynurenine pathway and activation of kynurenine hydroxylase and kynureninase.

Conclusions

We provisionally suggest that elevation of some kynurenine metabolites may be an additional mechanism of the alcohol-aversive and anticancer effects of disulfiram.

Multiparameter rodent chronic model for complex evaluation of alcoholism-mediated metabolic violations

BACKGROUND

Despite of the wide spectrum of alcoholism experimental models, the majority of them are very specialized on the short list of investigated parameters and could not provide reproduction of complex metabolic changes in the rats. The aim of the present study was to estimate whether rats selected by high alcohol preference, allowed free access to 15% alcohol for 150 days, develop simultaneous multilevel disturbances of cell macromolecules structure, metabolism and oxidative/nitrosative stress.

METHODS

Wistar albino male rats were divided into groups: I - rats selected by preferences to alcohol were used for chronic alcoholism modeling by replacing water with 15% ethanol (150 days), II - control. Contents of amino acids in serum, liver mRNA CYP2E1 and CYP3A2 expression, DNA fragmentation and lipid peroxidation levels, the reduced glutathione content, superoxide dismutase, catalase, iNOS and cNOS activities were evaluated.

RESULTS

In serum of ethanol-treated rats contents of aspartic acid, serine, glycine, alanine and valine were decreased whereas contents of histidine, methionine and phenylalanine were increased. Liver CYP2E1, CYP3A2 mRNA expression, DNA fragmentation levels significantly elevated. Level of cNOS in ethanol-treated rat's hepatocytes was within the normal limits, whereas iNOS activity was raised 1.6 times. Liver pro- and anti-oxidant system alterations were shown.

CONCLUSIONS

Rats' chronic 15% alcohol consumption (150 days) led solely to complex metabolomic changes at different levels, which simultaneously characterized cell macromolecules structure, metabolism, and oxidative/nitrosative stress. Rodent model of chronic alcoholism in the proposed modification could be an adequate and reasonably priced tool for further preclinical development and testing of pharmacotherapeutic agents.

A mathematical model of tryptophan metabolism via the kynurenine pathway provides insights into the effects of vitamin B-6 deficiency, tryptophan loading, and induction of tryptophan 2,3-dioxygenase on tryptophan metabolites

Vitamin B-6 deficiency is associated with impaired tryptophan metabolism because of the coenzyme role of pyridoxal 5'-phosphate (PLP) for kynureninase and kynurenine aminotransferase. To investigate the underlying mechanism, we developed a mathematical model of tryptophan metabolism via the kynurenine pathway. The model includes mammalian data on enzyme kinetics and tryptophan transport from the intestinal lumen to liver, muscle, and brain. Regulatory mechanisms and inhibition of relevant enzymes were included. We simulated the effects of graded reduction in cellular PLP concentration, tryptophan loads and induction of tryptophan 2,3-dioxygenase (TDO) on metabolite profiles and urinary excretion. The model predictions matched experimental data and provided clarification of the response of metabolites in various extents of vitamin B-6 deficiency. We found that moderate deficiency yielded increased 3-hydroxykynurenine and a decrease in kynurenic acid and anthranilic acid. More severe deficiency also yielded an increase in kynurenine and xanthurenic acid and more pronounced effects on the other metabolites. Tryptophan load simulations with and without vitamin B-6 deficiency showed altered metabolite concentrations consistent with published data. Induction of TDO caused an increase in all metabolites, and TDO induction together with a simulated vitamin B-6 deficiency, as has been reported in oral contraceptive users, yielded increases in kynurenine, 3-hydroxykynurenine, and xanthurenic acid and decreases in kynurenic acid and anthranilic acid. These results show that the model successfully simulated tryptophan metabolism via the kynurenine pathway and can be used to complement experimental investigations.

Aldehyde dehydrogenase inhibitors: a comprehensive review of the pharmacology, mechanism of action, substrate specificity, and clinical application

Aldehyde dehydrogenases (ALDHs) belong to a superfamily of enzymes that play a key role in the metabolism of aldehydes of both endogenous and exogenous derivation. The human ALDH superfamily comprises 19 isozymes that possess important physiological and toxicological functions. The ALDH1A subfamily plays a pivotal role in embryogenesis and development by mediating retinoic acid signaling. ALDH2, as a key enzyme that oxidizes acetaldehyde, is crucial for alcohol metabolism. ALDH1A1 and ALDH3A1 are lens and corneal crystallins, which are essential elements of the cellular defense mechanism against ultraviolet radiation-induced damage in ocular tissues. Many ALDH isozymes are important in oxidizing reactive aldehydes derived from lipid peroxidation and thereby help maintain cellular homeostasis. Increased expression and activity of ALDH isozymes have been reported in various human cancers and are associated with cancer relapse. As a direct consequence of their significant physiological and toxicological roles, inhibitors of the ALDH enzymes have been developed to treat human diseases. This review summarizes known ALDH inhibitors, their mechanisms of action, isozyme selectivity, potency, and clinical uses. The purpose of this review is to 1) establish the current status of pharmacological inhibition of the ALDHs, 2) provide a rationale for the continued development of ALDH isozyme-selective inhibitors, and 3) identify the challenges and potential therapeutic rewards associated with the creation of such agents.

Tryptophan in alcoholism treatment II: inhibition of the rat liver mitochondrial low Km aldehyde dehydrogenase activity, elevation of blood acetaldehyde concentration and induction of aversion to alcohol by combined administration of tryptophan and benserazide

AIMS

The aims were to provide proofs of mechanism and principle by establishing the ability of the amino acid L-tryptophan (Trp) combined with the kynureninase inhibitor benserazide (BSZ) to inhibit the liver mitochondrial low K(m) aldehyde dehydrogenase (ALDH) activity after administration and in vivo and to induce aversion to alcohol.

METHODS

Trp, BSZ or both were administered to male Wistar rats and ALDH activity was determined both in vitro in liver homogenates and in vivo (by measuring acetaldehyde accumulation in blood after ethanol administration). Alcohol consumption was studied in an aversion model in rats and in alcohol-preferring C57 mice.

RESULTS

Combined administration of Trp + BSZ, but neither compound alone, produced a strong inhibition of ALDH activity and an increase in blood acetaldehyde concentration after ethanol, and induced aversion to alcohol in rats and decreased preference in mice. Another kynureninase inhibitor, carbidopa, induced aversion to alcohol by itself, which was reversed by Trp co-administration.

CONCLUSIONS

The present results establish a prior art for the use of a combination of Trp plus BSZ in the treatment of alcoholism by aversion, which merits rapid clinical development.