Liver tryptophan 2,3-dioxygenase: a determinant of anxiety-like behaviour - studies with chronic nicotine administration in rats

Deletion of the tryptophan 2,3-dioxygenase ( TDO2 ) gene induces an anxiolytic-like behaviour in mice and TDO inhibition by allopurinol elicits an antidepressant-like effect in rats exposed to restraint stress. Chronic nicotine administration inhibits TDO activity, enhances brain serotonin synthesis and exerts anxiolytic- and antidepressant-like effects in rodent models. There is a strong association between anxiety, depression and tobacco use, which is stronger in women than in men. The present study aimed to examine the relationship between behavioural measures of anxiety and depression with liver TDO activity, brain tryptophan concentration and serotonin synthesis in rats treated chronically with nicotine. Behavioural measures included the elevated plus maze (EPM), open field (OFT) and forced swim (FST) tests. Biochemical measures included TDO activity, serum corticosterone and brain Trp, 5-HT and 5-HIAA concentrations. Anxiolytic-like and antidepressant-like effects of chronic nicotine were confirmed in association with TDO inhibition and elevation of brain Trp and 5-HT. Sex differences in behaviour were independent of the biochemical changes. At baseline, female rats performed better than males in OFT and FST. Nicotine was less anxiolytic in females in the open arm test. Nicotine treatment did not elicit different responses between sexes in the FST. Our findings support the notion that liver TDO activity exhibits a strong association with behavioural measures of anxiety and depression in experimental models, but provide little evidence for sex differences in behavioural response to nicotine. The TDO-anxiety link may be underpinned by kynurenine metabolites as well as serotonin.

Kynurenine pathway of tryptophan metabolism in pathophysiology and therapy of major depressive disorder

Serotonin deficiency in major depressive disorder (MDD) has formed the basis of antidepressant drug development and was originally attributed to induction of the major tryptophan (Trp)-degrading enzyme, liver Trp 2,3-dioxygenase (TDO), by cortisol, leading to decreased Trp availability to the brain for serotonin synthesis. Subsequently, the serotonin deficiency was proposed to involve induction of the extrahepatic Trp-degrading enzyme indoleamine 2,3-dioxygenase (IDO) by proinflammatory cytokines, with inflammation being the underlying cause. Recent evidence, however, challenges this latter concept, as not all MDD patients are immune-activated and, when present, inflammation is mild and/or transient. A wide range of antidepressant drugs inhibit the activity of liver TDO and bind specifically to the enzyme, but not to IDO. IDO induction is not a major event in MDD, but, when it occurs, its metabolic consequences may be masked and overridden by upregulation of kynurenine monooxygenase (KMO), the gateway to production of modulators of immune and neuronal functions. KMO appears to be activated in MDD by certain proinflammatory cytokines and antidepressants with anti-inflammatory properties may block this activation. We demonstrate the ability of the antidepressant ketamine to dock (bind) to KMO. The pathophysiology of MDD may be underpinned by both the serotonin deficiency and glutamatergic activation mediated respectively by TDO induction and N-methyl-D-aspartate receptor activation. Inhibition of TDO and KMO should be the focus of MDD pharmacotherapy.

Species Differences in Tryptophan Metabolism and Disposition

Major species differences in tryptophan (Trp) metabolism and disposition exist with important physiological, functional and toxicity implications. Unlike mammalian and other species in which plasma Trp exists largely bound to albumin, teleosts and other aquatic species possess little or no albumin, such that Trp entry into their tissues is not hampered, neither is that of environmental chemicals and toxins, hence the need for strict measures to safeguard their aquatic environments. In species sensitive to toxicity of excess Trp, hepatic Trp 2,3-dioxygenase (TDO) lacks the free apoenzyme and its glucocorticoid induction mechanism. These species, which are largely herbivorous, however, dispose of Trp more rapidly and their TDO is activated by smaller doses of Trp than Trp-tolerant species. In general, sensitive species may possess a higher indoleamine 2,3-dioxygenase (IDO) activity which equips them to resist immune insults up to a point. Of the enzymes of the kynurenine pathway beyond TDO and IDO, 2-amino-3-carboxymuconic acid-6-semialdehyde decarboxylase (ACMSD) determines the extent of progress of the pathway towards NAD⁺ synthesis and its activity varies across species, with the domestic cat (Felis catus) being the leading species possessing the highest activity, hence its inability to utilise Trp for NAD⁺ synthesis. The paucity of current knowledge of Trp metabolism and disposition in wild carnivores, invertebrates and many other animal species described here underscores the need for further studies of the physiology of these species and its interaction with Trp metabolism.

Effects of oral administration of an aqueous ginger extract on anxiety behavior and tryptophan and serotonin metabolism in the rat

Background: Zingiber officinale (ginger) is used widely as a herb and medicine. It contains among its constituents 6-Gingerol (a phenol) and quercetin (a flavonoid) that possess anxiolytic and antidepressant properties, but the potential biochemical mechanism(s) of these effects has not been assessed, particularly in relation to serotonin synthesis and neurotransmission. Aims and Objectives: We investigated the anxiolytic-like activity of an aqueous ginger extract by evaluating its influence on behavior, and its effects on serotonin metabolism and on metabolism and disposition of the serotonin precursor tryptophan (Trp) in rats. Materials and Methods: An aqueous ginger extract was given orally in a single daily dose equivalent to 500 mg of ginger material per kg body weight for 3 weeks. The elevated plus maze test of anxiety and Trp metabolism and disposition and brain serotonin synthesis and turnover were assessed in Ginger-treated and control rats. Results: When compared with controls, ginger-treated rats showed a significant increase in the time spent in the open arm, indicative of decreased anxiety. However there was no effect on locomotor activity in open field test. The extract caused significant decreases in activities of liver Trp 2,3-dioxygenase and significant increases in concentrations of serum Trp and corticosterone and brain Trp, serotonin and the major serotonin metabolite 5-hydroxyindoleacetic acid. Conclusion: An aqueous ginger extract exerts an anxiolytic effect in a behavioral model of anxiety, which may be caused by increased serotonin synthesis, and influences tryptophan metabolism and disposition in a manner analogous to antidepressant drugs.

Effect of Disaccharide Inclusion in Vitrification and Warming Solutions on Developmental Competence of Vitrified/Warmed Germinal Vesicle Stage Buffalo Oocytes

BACKGROUND

Cryopreservation of immature oocyte is a potential strategy for preserving the female germline, providing a non-seasonal, easily accessible source for reproduction and science. Exposure of oocytes to high concentrations of cryoprotectants during vitrification is toxic and can negatively impact the fertilization ability and development of vitrified/warmed oocytes.

OBJECTIVE

1) to evaluate the effects of exposure of buffalo germinal vesicle (GV) oocytes to different vitrification solutions (VS), either supplemented with or without sucrose, on cumulus expansion and nuclear maturation following IVM; and 2) to compare the effects of sucrose and trehalose in the warming solution on developmental competence of buffalo oocytes vitrified at the GV-stage.

MATERIALS AND METHODS

Cumulus oocyte complexes (COCs) obtained at slaughter from mature buffalo ovaries were randomly assigned into five groups: control - directly subjected to IVM); VS1 group - exposed to 20% ethylene glycol (EG) + 20% glycerol (GLY) + 0.5 M sucrose; VS2 group - exposed to 20% EG + 20% GLY; VS3 group - subjected to 20% EG+20% dimethyl sulfoxide (DMSO) + 0.5 M sucrose; and VS4 group - subjected to 20% EG+20% DMSO. Following cryoprotectant dilution, viable oocytes were matured in vitro for 22 h; cumulus expansion and nuclear maturation were then evaluated (Experiment 1). COCs were vitrified by solid surface vitrification (SSV) in a solution composed of 20% EG + 20% DMSO (VS4). Following vitrification, COCs were warmed in a solution composed of either sucrose or trehalose in decreasing concentrations (1 M, 0.5 M and 0.25 M). Morphologically viable oocytes were matured, fertilized and cultured in vitro. Cleavage and blastocyst rates were evaluated at 30 h and day 7 post-insemination (p.i.), respectively (Experiment 2).

RESULTS

Exposure of GV-buffalo oocytes to different cryoprotectant combinations did not significantly affect cumulus expansion following IVM. However, nuclear maturation rate (oocytes at MII) was significantly higher (P<0.05) in the groups exposed to sucrose-free vitrification solutions (VS2 and VS4) and not significantly different from the control. Compared with the control group, the cleavage and blastocyst rates were significantly (P<0.05) lower in oocytes vitrified and then warmed in a solution containing trehalose; whilst this was not the case when sucrose was present in the solution.

CONCLUSION

Our results suggest that exposure of buffalo GV-oocytes to sucrose-free vitrification solutions improved nuclear maturation after IVM. Moreover, warming of vitrified buffalo oocytes in sucrose-based solution improved preimplantation development following IVM and IVF compared to trehalose based media.

Quinolinate as a Marker for Kynurenine Metabolite Formation and the Unresolved Question of NAD+ Synthesis During Inflammation and Infection

Quinolinate (Quin) is a classic example of a biochemical double-edged sword, acting as both essential metabolite and potent neurotoxin. Quin is an important metabolite in the kynurenine pathway of tryptophan catabolism leading to the de novo synthesis of nicotinamide adenine dinucleotide (NAD⁺). As a precursor for NAD⁺, Quin can direct a portion of tryptophan catabolism toward replenishing cellular NAD⁺ levels in response to inflammation and infection. Intracellular Quin levels increase dramatically in response to immune stimulation [e.g., lipopolysaccharide (LPS) or pokeweed mitogen (PWM)] in macrophages, microglia, dendritic cells, and other cells of the immune system. NAD⁺ serves numerous functions including energy production, the poly ADP ribose polymerization (PARP) reaction involved in DNA repair, and the activity of various enzymes such as the NAD⁺-dependent deacetylases known as sirtuins. We used highly specific antibodies to protein-coupled Quin to delineate cells that accumulate Quin as a key aspect of the response to immune stimulation and infection. Here, we describe Quin staining in the brain, spleen, and liver after LPS administration to the brain or systemic PWM administration. Quin expression was strong in immune cells in the periphery after both treatments, whereas very limited Quin expression was observed in the brain even after direct LPS injection. Immunoreactive cells exhibited diverse morphology ranging from foam cells to cells with membrane extensions related to cell motility. We also examined protein expression changes in the spleen after kynurenine administration. Acute (8 h) and prolonged (48 h) kynurenine administration led to significant changes in protein expression in the spleen, including multiple changes involved with cytoskeletal rearrangements associated with cell motility. Kynurenine administration resulted in several expression level changes in proteins associated with heat shock protein 90 (HSP90), a chaperone for the aryl-hydrocarbon receptor (AHR), which is the primary kynurenine metabolite receptor. We propose that cells with high levels of Quin are those that are currently releasing kynurenine pathway metabolites as well as accumulating Quin for sustained NAD⁺ synthesis from tryptophan. Further, we propose that the kynurenine pathway may be linked to the regulation of cell motility in immune and cancer cells.

Kynurenine pathway and human systems

The essential amino acid L-tryptophan (Trp) appears to play an important role in aging by acting as a general regulator of protein homeostasis. The major route of Trp degradation, the kynurenine pathway (KP), produces a range of biologically active metabolites that can impact or be impacted by a variety of body systems, including the endocrine, haemopoietic, immune, intermediary metabolism and neuronal systems, with the end product of the KP, NAD⁺, being essential for vital cellular processes. An account of the pathway, its regulation and functions is presented in relation to body systems with a summary of previous studies of the impact of aging on the pathway enzymes and metabolites. A low-grade inflammatory environment characterized by elevation of cytokines and other immune modulators and consequent disturbances in KP activity develops with aging. The multifactorial nature of the aging process necessitates assessment of factors determining the progression of this mild dysfunction to age-related diseases and developing strategies aimed at arresting and reversing this progression.

The Plasma [Kynurenine]/[Tryptophan] Ratio and Indoleamine 2,3-Dioxygenase: Time for Appraisal

The plasma kynurenine to tryptophan ([Kyn]/[Trp]) ratio is frequently used to express or reflect the activity of the extrahepatic Trp-degrading enzyme indoleamine 2,3-dioxygenase (IDO). This ratio is increasingly used instead of measurement of IDO activity, which is often low or undetectable in immune and other cells under basal conditions, but is greatly enhanced after immune activation. The use of this ratio is valid in in vitro studies, eg, in cell cultures or isolated organs, but its ‘blanket’ use in in vivo situations is not, because of modulating factors, such as supply of nutrients; the presence of multiple cell types; complex structural and functional tissue arrangements; the extracellular matrix; and hormonal, cytokine, and paracrine interactions. Determinants other than IDO may therefore be involved in vivo. These are hepatic tryptophan 2,3-dioxygenase (TDO) activity and the flux of plasma-free Trp down the Kyn pathway. In addition, conditions leading to accumulation of Kyn, eg, inhibition of activities of Kyn monooxygenase and kynureninase, could lead to elevation of the aforementioned ratio. In this review, the origin of use of this ratio will be discussed, variations in extent of its elevation will be described, evidence against its indiscriminate use will be presented, and examining determinants other than IDO activity and their correlates will be proposed for future studies.

Hypothesis: Metabolic targeting of 5-aminolevulinate synthase by tryptophan and inhibitors of heme utilisation by tryptophan 2,3-dioxygenase as potential therapies of acute hepatic porphyrias

Metabolic targeting of liver 5-aminolevulinate synthase (5-ALAS) by inhibition of heme utilisation by tryptophan (Trp) 2,3-dioxygenase (TDO) or the use of tryptophan is proposed as a therapy of acute hepatic porphyrias. 5-ALAS, the rate-limiting enzyme of heme biosynthesis, is under negative feedback control by a small regulatory heme pool in the hepatic cytosol. Acute porphyric attacks, precipitated by fasting, certain hormones and some drugs, involve induction of 5-ALAS secondarily to depletion of the above pool, and the resultant elevation of 5-ALA levels initiates the abdominal and neurological symptoms of attacks. By utilising the regulatory heme, cytosolic TDO undermines the feedback control, thus allowing 5-ALAS induction to occur, e.g. upon glucocorticoid induction of TDO during fasting (starvation) and exogenous glucocorticoid administration. Currently, glucose therapy is the preferred strategy for reversing moderate attacks induced by fasting (calorie restriction), with more severe attacks being treated by intravenous heme preparations. Reversal of fasting-induced attacks by glucose is explained by the previously demonstrated reversal of increased heme utilisation by TDO. Inhibitors of this utilisation are therefore potential therapeutic targets in acute attacks and also for maintenance of a symptomless state. Existing TDO inhibitors other than glucose include allopurinol, nicotinamide and recently developed potent inhibitors such as LM10 used in cancer therapy. Based on studies in rats, the hypothesis predicts that the safety or otherwise of drugs in the hepatic porphyrias is determined by their ability to inhibit TDO utilisation of heme under basal conditions or after glucocorticoid induction or heme activation of TDO, in parallel with reciprocal changes in 5-ALAS induction. Tryptophan is also proposed as a potential therapy of acute attacks either alone or as an adjunct to the recently proposed 5-ALAS1 gene silencing. Trp increases heme biosynthesis by enhancing 5-ALA dehydratase activity and, based on a Trp-5-ALA model presented herein, Trp offers several advantages over heme therapy, namely rapid conversion of 5-ALA into heme, a greatly enhanced heme availability, a near complete inhibition of 5-ALAS induction, assumed rapid clearance of 5-ALA and hence accelerated resolution of symptoms of attacks, and finally provision of the neuroprotective metabolite kynurenic acid to neutralise the neurological symptoms. The hypothesis also addresses heme regulation in species lacking the TDO free apoenzyme and its glucocorticoid induction mechanism and proposes detailed assessment of heme biosynthesis in these species. Detailed proposals for testing the hypothesis are presented.

The EZ:Faast Family of Amino Acid Analysis Kits: Application of the GC-FID Kit for Rapid Determination of Plasma Tryptophan and Other Amino Acids

Plasma tryptophan (Trp) and other amino acids (AA) can be determined rapidly by gas (GC) or liquid (LC) chromatography using the Phenomenex EZ:Faast™ family of kits. Three kits are available: (1) GC-FID or -NPD, (2) GC-MS, and (3) LC-MS. The two GC kits can determine 32 AA, whereas the LC-MS can determine 5 additional AA. All three kits, however, share the same experimental procedure up to and including the preparation of derivatized AA. The method is based on solid-phase extraction (SPE), thus saving time on prior removal of plasma or other proteins and interfering substances and can be applied to other body fluids and experimental media and to supernatants of extracts of solid material. Briefly, SPE is performed using a proprietary cation-exchange mechanism. The acid solution of the internal standard ensures that the free amino acids are in an anionic form suitable for cationic binding. The alkaline nature of the elution medium ensures that the AA cations are released prior to derivatization. The latter involves production of chloroformate derivatives of both the amino and carboxylic acid groups. With experience, six plasma samples can be so processed within 12 min. The shortest analytical run is <7 min per sample using the GC-FID/NPD kit. Despite its many steps, the procedure becomes second nature and an enjoyable task. I have now used the GC-FID kit with manual injection to process >1600 plasma and other samples. Limit of detection of AA is 1 μM or less. The procedure has been validated and optimized for Trp and its main five brain uptake competitors.

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.

Targeting tryptophan availability to tumors: the answer to immune escape?

Tumoral immune escape is an obstacle to successful cancer therapy. Tryptophan (Trp) metabolites along the kynurenine pathway induce immunosuppression involving apoptosis of effector immune cells, which tumors use to escape an immune response. Production of these metabolites is initiated by indoleamine 2,3-dioxygenase (IDO1). IDO1 inhibitors, however, do not always overcome the immune escape and another enzyme expressed in tumors, Trp 2,3-dioxygenase (TDO2), has been suggested as the reason. However, without Trp, tumors cannot achieve an immune escape through either enzyme. Trp is therefore key to immune escape. In this perspective paper, Trp availability to tumors will be considered and strategies limiting it proposed. One major determinant of Trp availability is the large increase in plasma free (non-albumin-bound) Trp in cancer patients, caused by the low albumin and the high non-esterified fatty acid (NEFA) concentrations in plasma. Albumin infusions, antilipolytic therapy or both could be used, if indicated, as adjuncts to immunotherapy and other therapies. Inhibition of amino acid uptake by tumors is another strategy and α-methyl-DL-tryptophan or other potential inhibitors could fulfill this role. Glucocorticoid receptor antagonists may have a role in preventing glucocorticoid induction of TDO in host liver and tumors expressing it and in undermining the permissive effect of glucocorticoids on IDO1 induction by cytokines. Nicotinamide may be a promising TDO2 inhibitor lacking disadvantages of current inhibitors. Establishing the Trp disposition status of cancer patients and in various tumor types may provide the information necessary to formulate tailored therapeutic approaches to cancer immunotherapy that can also undermine tumoral immune escape.

Modulation of Tryptophan and Serotonin Metabolism as a Biochemical Basis of the Behavioral Effects of Use and Withdrawal of Androgenic-Anabolic Steroids and Other Image- and Performance-Enhancing Agents

Modulation of tryptophan (Trp) metabolism may underpin the behavioral effects of androgenic-anabolic steroids (AAS) and associated image and performance enhancers. Euphoria, arousal, and decreased anxiety observed with moderate use and exercise may involve enhanced cerebral serotonin synthesis and function by increased release of albumin-bound Trp and estrogen-mediated liver Trp 2,3-dioxygenase (TDO) inhibition and enhancement of serotonin function. Aggression, anxiety, depression, personality disorders, and psychosis, observed on withdrawal of AAS or with use of large doses, can be caused by decreased serotonin synthesis due to TDO induction on withdrawal, excess Trp inhibiting the 2 enzymes of serotonin synthesis, and increased cerebral levels of neuroactive kynurenines. Exercise and excessive protein and branched-chain amino acid intakes may aggravate the effects of large AAS dosage. The hypothesis is testable in humans and experimental animals by measuring parameters of Trp metabolism and disposition and related metabolic processes.

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.

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.

Assessment of the Human Kynurenine Pathway: Comparisons and Clinical Implications of Ethnic and Gender Differences in Plasma Tryptophan, Kynurenine Metabolites, and Enzyme Expressions at Baseline and After Acute Tryptophan Loading and Depletion

Tryptophan (Trp) metabolism via the kynurenine pathway (KP) was assessed in normal healthy US volunteers at baseline and after acute Trp depletion (ATD) and acute Trp loading (ATL) using amino acid formulations. The hepatic KP accounts for ~90% of overall Trp degradation. Liver Trp 2,3-dioxygenase (TDO) contributes ~70% toward Trp oxidation, with the remainder achieved by subsequent rate-limiting enzymes in the KP. TDO is not influenced by a 1.15 g Trp load, but is maximally activated by a 5.15 g dose. We recommend a 30 mg/kg dose for future ATL studies. ATD activates TDO and enhances the Trp flux down the KP via its leucine component. Higher plasma free [Trp] and lower total [Trp] are observed in women, with no gender differences in kynurenines. Kynurenic acid is lower in female Caucasians, which may explain their lower incidence of schizophrenia. African-American and Hispanic women have a lower TDO and Trp oxidation relative to free Trp than the corresponding men. African-American women have a potentially higher 3-hydroxyanthranilic acid/anthranilic acid ratio, which may protect them against osteoporosis. Future studies of the KP in relation to health and disease should focus on gender and ethnic differences.

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.

Standardization of formulations for the acute amino acid depletion and loading tests

The acute tryptophan depletion and loading and the acute tyrosine plus phenylalanine depletion tests are powerful tools for studying the roles of cerebral monoamines in behaviour and symptoms related to various disorders. The tests use either amino acid mixtures or proteins. Current amino acid mixtures lack specificity in humans, but not in rodents, because of the faster disposal of branched-chain amino acids (BCAAs) by the latter. The high content of BCAA (30-60%) is responsible for the poor specificity in humans and we recommend, in a 50g dose, a control formulation with a lowered BCAA content (18%) as a common control for the above tests. With protein-based formulations, α-lactalbumin is specific for acute tryptophan loading, whereas gelatine is only partially effective for acute tryptophan depletion. We recommend the use of the whey protein fraction glycomacropeptide as an alternative protein. Its BCAA content is ideal for specificity and the absence of tryptophan, tyrosine and phenylalanine render it suitable as a template for seven formulations (separate and combined depletion or loading and a truly balanced control). We invite the research community to participate in standardization of the depletion and loading methodologies by using our recommended amino acid formulation and developing those based on glycomacropeptide.

Mechanisms of the pellagragenic effect of leucine: stimulation of hepatic tryptophan oxidation by administration of branched-chain amino acids to healthy human volunteers and the role of plasma free tryptophan and total kynurenines

The pellagragenic effect of leucine (Leu) has been proposed to involve modulation of L-tryptophan (Trp) metabolism along the hepatic kynurenine pathway. Here, we discuss some of the mechanisms suggested and report the effects in healthy volunteers of single doses of Leu (4.05-6.75 g) administered in a 16-amino acid mixture on concentrations of plasma Trp and its kynurenine metabolites. Flux of Trp through Trp 2,3-dioxygenase (TDO) is dose-dependently enhanced most probably by Leu and can be attributed to TDO activation. Trp oxidation is better expressed using plasma total kynure-nines, rather than kynurenine, and free, rather than total, Trp. Increased hepatic Trp oxidation may be an additional mechanism of action of branched-chain amino acids in the acute Trp depletion test. Inhibition of intestinal absorption or hepatic uptake of Trp by Leu can be excluded. Potential mechanisms of the aggravation of pellagra symptoms by Leu are discussed.

Pellagra and alcoholism: a biochemical perspective

Historical and clinical aspects of pellagra and its relationship to alcoholism are reviewed from a biochemical perspective. Pellagra is caused by deficiency of niacin (nicotinic acid) and/or its tryptophan (Trp) precursor and is compounded by B vitamin deficiencies. Existence on maize or sorghum diets and loss of or failure to isolate niacin from them led to pellagra incidence in India, South Africa, Southern Europe in the 18th century and the USA following the civil war. Pellagra is also induced by drugs inhibiting the conversion of Trp to niacin and by conditions of gastrointestinal dysfunction. Skin photosensitivity in pellagra may be due to decreased synthesis of the Trp metabolite picolinic acid → zinc deficiency → decreased skin levels of the histidine metabolite urocanic acid and possibly also increased levels of the haem precursor 5-aminolaevulinic acid (5-ALA) and photo-reactive porphyrins. Depression in pellagra may be due to a serotonin deficiency caused by decreased Trp availability to the brain. Anxiety and other neurological disturbances may be caused by 5-ALA and the Trp metabolite kynurenic acid. Pellagra symptoms are resolved by niacin, but aggravated mainly by vitamin B6. Alcohol dependence can induce or aggravate pellagra by inducing malnutrition, gastrointestinal disturbances and B vitamin deficiencies, inhibiting the conversion of Trp to niacin and promoting the accumulation of 5-ALA and porphyrins. Alcoholic pellagra encephalopathy should be managed with niacin, other B vitamins and adequate protein nutrition. Future studies should explore the potential role of 5-ALA and also KA in the skin and neurological disturbances in pellagra.

Tryptophan: the key to boosting brain serotonin synthesis in depressive illness

It has been proposed that focusing on brain serotonin synthesis can advance antidepressant drug development. Biochemical aspects of the serotonin deficiency in major depressive disorder (MDD) are discussed here in detail. The deficiency is caused by a decreased availability of the serotonin precursor tryptophan (Trp) to the brain. This decrease is caused by accelerated Trp degradation, most likely induced by enhancement of the hepatic enzyme tryptophan 2,3-dioxygenase (TDO) by glucocorticoids and/or catecholamines. Induction of the extrahepatic Trp-degrading enzyme indolylamine 2,3-dioxygenase (IDO) by the modest immune activation in MDD has not been demonstrated and, if it occurs, is unlikely to make a significant contribution. Liver TDO appears to be a target of many antidepressants, the mood stabilisers Li(+) and carbamazepine and possibly other adjuncts to antidepressant therapy. The poor, variable and modest antidepressant efficacy of Trp is due to accelerated hepatic Trp degradation, and efficacy can be restored or enhanced by combination with antidepressants or other existing or new TDO inhibitors. Enhancing Trp availability to the brain is thus the key to normalisation of serotonin synthesis and could form the basis for future antidepressant drug development.

Specificity of the acute tryptophan and tyrosine plus phenylalanine depletion and loading tests I. Review of biochemical aspects and poor specificity of current amino Acid formulations

The acute tryptophan or tyrosine plus phenylalanine depletion and loading tests are powerful tools for studying the roles of serotonin, dopamine and noradrenaline in normal subjects and those with behavioural disorders. The current amino acid formulations for these tests, however, are associated with undesirable decreases in ratios of tryptophan or tyrosine plus phenylalanine to competing amino acids resulting in loss of specificity. This could confound biochemical and behavioural findings. Compositions of current formulations are reviewed, the biochemical principles underpinning the tests are revisited and examples of unintended changes in the above ratios and their impact on monoamine function and behaviour will be demonstrated from data in the literature. The presence of excessive amounts of the 3 branched-chain amino acids Leu, Ile and Val is responsible for these unintended decreases and the consequent loss of specificity. Strategies for enhancing the specificity of the different formulations are proposed.

Specificity of the Acute Tryptophan and Tyrosine Plus Phenylalanine Depletion and Loading Tests Part II: Normalisation of the Tryptophan and the Tyrosine Plus Phenylalanine to Competing Amino Acid Ratios in a New Control Formulation

Current formulations for acute tryptophan (Trp) or tyrosine (Tyr) plus phenylalanine (Phe) depletion and loading cause undesirable decreases in ratios of Trp or Tyr + Phe to competing amino acids (CAA), thus undermining the specificities of these tests. Branched-chain amino acids (BCAA) cause these unintended decreases, and lowering their content in a new balanced control formulation in the present study led to normalization of all ratios. Four groups (n = 12 each) of adults each received one of four 50 g control formulations, with 0% (traditional), 20%, 30%, or 40% less of the BCAA. The free and total [Trp]/[CAA] and [Phe + Tyr]/[BCAA + Trp] ratios all decreased significantly during the first 5 h following the traditional formulation, but were fully normalized by the formulation containing 40% less of the BCAA. We recommend the latter as a balanced control formulation and propose adjustments in the depletion and loading formulations to enhance their specificities for 5-HT and the catecholamines.

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

AIMS

The aims were to provide proofs of mechanism and principle by establishing the ability of kynurenine metabolites to inhibit the liver mitochondrial low K(m) aldehyde dehydrogenase (ALDH) activity after administration and in vivo, and to induce aversion to alcohol.

METHODS

Kynurenic acid (KA), 3-hydroxykynurenine (3-HK) and 3-hydroxyanthranilic acid (3-HAA) were administered to normal male Wistar rats and ALDH activity was determined both in vitro in liver homogenates and in vivo (by measuring blood acetaldehyde following ethanol administration). Alcohol consumption was studied in an aversion model in rats and in alcohol-preferring C57 mice.

RESULTS

ALDH activity was significantly inhibited by all three metabolites by doses as small as 1 mg/kg body wt. Blood acetaldehyde accumulation after ethanol administration was strongly elevated by KA and 3-HK and to a lesser extent by 3-HAA. All three metabolites induced aversion to alcohol in rats and decreased alcohol preference in mice.

CONCLUSIONS

The above kynurenine metabolites of tryptophan induce aversion to alcohol by inhibiting ALDH activity. An intellectual property covering the use of 3-HK and 3-HAA and derivatives thereof in the treatment of alcoholism by aversion awaits further development.

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.

Rapid Isocratic Liquid Chromatographic Separation and Quantification of Tryptophan and Six kynurenine Metabolites in Biological Samples with Ultraviolet and Fluorimetric Detection

A simple, rapid isocratic liquid chromatographic procedure with ultraviolet and fluorimetric detection is described for the separation and quantification of L-tryptophan (Trp) and six of its kynurenine metabolites (kynurenine, 3-hydroxykynurenine, and 3-hydroxyanthranilic, kynurenic, xanthurenic and anthranilic acids). Using the Perkin Elmer LC 200 system, a reverse phase Synergi 4 μ fusion-RP80 A column (250 × 4.6 mm) (Phenomenex), and a mobile phase of 10 mM sodium dihydrogen phosphate: methanol (73:27, by vol) at pH 2.8 and a flow rate of 1.0-1.2 ml/min at 37 °C, a run took ∼13 min. The run took <7 min at 40 °C and a 1.4 ml/min flow rate. Limits of detection of all 7 analytes were 5-72 nM and their recoveries from human plasma and rat serum and liver varied between 62% and 111%. This simple method is suitable for high throughput work and can be further developed to include quinolinic acid and other Trp metabolites.

Tryptophan metabolism in alcoholism

Acute and chronic alcohol (ethanol) intake and subsequent withdrawal exert major effects on tryptophan (Trp) metabolism and disposition in human subjects and experimental animals. In rats, activity of the rate-limiting enzyme of Trp degradation, liver Trp pyrrolase (TP), is enhanced by acute, but inhibited after chronic, ethanol administration, then enhanced during withdrawal. These changes lead to alterations in brain serotonin synthesis and turnover mediated by corresponding changes in circulating Trp availability to the brain. A low brain-serotonin concentration characterizes the alcohol-preferring C57BL/6J mouse strain and many alcohol-preferring rat lines. In this mouse strain, liver TP enhancement causes the serotonin decrease. In man, acute ethanol intake inhibits brain serotonin synthesis by activating liver TP. This may explain alcohol-induced depression, aggression and loss of control in susceptible individuals. Chronic alcohol intake in dependent subjects may be associated with liver TP inhibition and a consequent enhancement of brain serotonin synthesis, whereas subsequent withdrawal may induce the opposite effects. The excitotoxic Trp metabolite quinolinate may play a role in the behavioural disturbances of the alcohol-withdrawal syndrome. Some abstinent alcoholics may have a central serotonin deficiency, which they correct by liver TP inhibition through drinking. Further studies of the Trp and serotonin metabolic status in long-term abstinence in general and in relation to personality characteristics, alcoholism typology and genetic factors in particular may yield important information which should facilitate the development of more effective screening, and preventative and therapeutic strategies in this area of mental health.

Low-alcohol beers: contribution to blood-ethanol concentration and its elevation above the UK legal limit after 'topping-up'

AIMS

The aim of this study was to establish the contribution of low-alcohol beers to blood-ethanol concentration (BEC) and to test if 'topping-up' with these beverages can increase BEC above the 80 mg/dl UK legal limit.

METHODS

Healthy male and female volunteers received a dose of ethanol designed to give a BEC of just below 80 mg/dl, and then received one pint (600 ml) of a 1% v/v alcohol beer in the fasting state or after lunch, or of a zero-alcohol or a 0.5% v/v alcohol beer after fasting. BEC was determined enzymatically and data were subjected to ANOVA.

RESULTS

Topping-up with a pint of a 1% v/v alcohol beer increased BEC >80 mg/dl in fasting subjects, contributing an extra 12-17 mg/dl, which lasted longer in males (80 min) than in females (20 min). A 0.5% v/v alcohol beer increased BEC above 80 mg/dl only in males, which lasted for 60 min. After food intake, the 1% v/v alcohol beer increased BEC above 80 mg/dl transiently only in males.

CONCLUSIONS

Low-alcohol beers make a significant contribution to blood-ethanol concentration and can increase it above the UK legal limit. Their use as a 'top-up' should be discouraged. Low-alcohol beers have a place as a substitute for normal-strength beverages as a strategy for decreasing alcohol consumption in general and in countries where low legal alcohol limits are in force or being contemplated.

Activation of liver tryptophan pyrrolase mediates the decrease in tryptophan availability to the brain after acute alcohol consumption by normal subjects

AIMS

We have previously suggested that acute ethanol consumption by normal subjects decreases the availability of circulating tryptophan (Trp) to the brain by activating liver Trp pyrrolase, the first and rate-limiting enzyme of the (major) kynurenine pathway of Trp degradation. The aim of the present study was to examine this hypothesis further by measuring plasma levels of kynurenine metabolites following alcohol consumption.

METHODS

After an overnight fast and a light breakfast, each of 10 healthy subjects received one of five drinks (placebo and doses of ethanol of 0.2, 0.4, 0.6 and 0.8 g/kg body weight in tonic water) on five different occasions. Blood samples were withdrawn 2 h later and plasma was analysed for concentrations Trp, competing amino acids (CAA) and kynurenine metabolites.

RESULTS

Along with the depletion of plasma Trp and the decrease in its availability to the brain, as expressed by the ratio of [Trp]/[CAA], plasma kynurenine was elevated by doses of ethanol of 0.2-0.8 g/kg body weight. The ratio% of [kynurenine]/[Trp], an index of the expression of Trp pyrrolase activity, was also increased by all doses of ethanol.

CONCLUSIONS

We conclude that activation of liver Trp pyrrolase mediates the depletion of plasma Trp and the decrease in its availability to the brain induced by acute ethanol consumption.

Comparison of 50- and 100-g L -tryptophan depletion and loading formulations for altering 5-HT synthesis: pharmacokinetics, side effects, and mood states

RATIONALE

Differences in 5-hydroxytryptamine (5-HT) function have been the subject of extensive research in psychiatric studies. Many studies have manipulated L -tryptophan (Trp) levels to temporarily decrease (depletion) or increase (loading) 5-HT synthesis. While most researchers have used a 100-g formulation, there has been ongoing interest in using smaller-sized formulations.

OBJECTIVES

This study examined the time course of multiple plasma indicators of brain 5-HT synthesis after a 50-g depletion and loading as a comparison to the corresponding 100-g formulations that are typically used.

MATERIALS AND METHODS

Plasma was collected from 112 healthy adults at seven hourly intervals after consumption of either a 50- or 100-g depletion or loading. Self-ratings of mood and somatic symptoms were completed before and after Trp manipulations.

RESULTS

The primary findings were that (1) the 50- and 100-g formulations produced the expected changes in plasma indicators after both depletion (-89% and -96%, respectively) and loading (+570% and +372%, respectively); (2) the 100-g depletion showed more robust effects at the 4, 5, and 6 h measurements than the 50-g depletion; (3) there was significant attrition after both the 100-g depletion and loading, but not after either of the 50-g formulations; and (4) both the 50- and 100-g depletions produced increases in negative self-ratings of mood and somatic symptoms, while loading significantly increased negative ratings after the 100 g only.

CONCLUSIONS

There are important considerations when choosing among formulation sizes for use in Trp manipulation studies, and the complete 7-h time-course data set of the typical plasma Trp measures presented here may help researchers decide which methodology best suits their needs.

The effects of alcohol on laboratory-measured impulsivity after L: -Tryptophan depletion or loading

RATIONALE

Indirect evidence supports a link between serotonergic activity and individual differences in the behavioral response to alcohol, but few studies have experimentally demonstrated that an individual's biological state can influence the sensitivity to alcohol-induced behaviors.

OBJECTIVE

Our purpose was to temporarily modify serotonin synthesis in healthy individuals to determine how altered biological states may interact with alcohol administration to affect impulsive behavior.

MATERIALS AND METHODS

In a repeated-measures design, 18 normal controls consumed a 50-g L: -tryptophan (Trp) depleting (ATD) or loading (ATL) amino-acid beverage that temporarily decreased or increased (respectively) serotonin synthesis before receiving either a moderate dose of alcohol (0.65 g/kg) or placebo. All participants completed three impulsivity testing sessions on each of the five experimental days. Session one was a baseline session. Session two included testing after ATD-only or ATL-only. Session three included: (1) placebo after ATL (ATL+PBO); (2) placebo after ATD (ATD+PBO); (3) alcohol after ATL (ATL+ALC); (4) alcohol after ATD (ATD+ALC); and (5) Alcohol-only conditions. Impulsivity was assessed using the Immediate Memory Task (Dougherty et al., Behav Res Methods Instrum Comput 34:391-398, 2002), a continuous performance test yielding commission errors that have been previously validated as a component of impulsive behavior.

RESULTS

Primary findings were that ATD-only increased impulsive responding compared to ATL-only, and ATD+ALC increased commission errors to levels higher than either the ATL+ALC or Alcohol-only conditions.

CONCLUSIONS

These findings demonstrate that reduced serotonin synthesis can produce increased impulsivity even among non-impulsive normal controls, and that the behavioral effects of alcohol are, in part, dependent on this biological state.

Nicotine stimulation on extracellular glutamate levels in the nucleus accumbens of ethanol-withdrawn rats in vivo

BACKGROUND

Nicotine can release glutamate in the limbic system. Presynaptic activation of glutamate receptors might be relevant for the subsequent firing of excitatory postsynaptic potentials. This might be relevant in early ethanol withdrawal. The effects and differences of nicotine stimulation on glutamate response measured by microdialysis in the nucleus accumbens (NAC) between ethanol-withdrawn rats (EW group) and ethanol-naïve rats (control group) were investigated.

METHODS

Rats were ethanol-intoxicated according to a binge-drinking model: recurrent cycle of 4 days of intoxication (EW group) or 5% sucrose (control group), followed by a 3-day recovery. This was followed by a 2-day intoxication period and subsequent abstinence. After the last oral intake, microdialysis was performed in the left NAC for a 16-hour withdrawal period. At the end of the withdrawal period, a rated withdrawal score (RWS) was documented. Then, nicotine was given subcutaneously at a dose of 0.5 mg/kg and amino acid levels determined by microdialysis were followed for an additional 3 hours.

RESULTS

The RWS was not correlated to the last amount of ethanol received, but was correlated to the total amount of ethanol administered during the pretreatment period: the basal values of extracellular glutamate were found to be decreased in the EW group before withdrawal. Cessation of ethanol significantly increased glutamate levels with a peak between 4 and 10 hours after the last oral intake. Sixteen hours after ethanol withdrawal, the same level as in the control group was achieved. Nicotine significantly increased glutamate levels in the NAC of the EW group but not in ethanol-naïve rats.

CONCLUSIONS

This study showed that withdrawal of ethanol was associated with an increase in extracellular glutamate levels. Systemic administration of nicotine in vivo produced an increase in extracellular levels of glutamate in the core region of the NAC during ethanol withdrawal. This might be a relevant pathomechanism for increased craving either for alcohol or for nicotine after ethanol withdrawal.

Heterogeneity of serum tryptophan concentration and availability to the brain in patients with the chronic fatigue syndrome

We assessed the serotonin status of patients with the chronic fatigue syndrome (CFS). Tryptophan (Trp) availability to the brain, expressed as the ratio of concentration of serum Trp to the sum of those of its five competitors (CAA), and other parameters of Trp disposition were compared in 23 patients with the CFS and 42 healthy controls. The serum [free Trp]/[CAA] ratio was 43% higher in CFS patients, due to a 48% higher [free Trp]. [Total Trp] was also significantly higher (by 19%) in CFS patients, and, although the [total Trp]/[CAA] ratio did not differ significantly between the control and patient groups, the difference became significant when the results were co-varied with age and gender. [CAA] was not significantly different between groups, but was significantly lower in females, compared to males, of the CFS patient group. We have established normal ranges for Trp disposition parameters and propose criteria for defining the serotonin-biosynthetic status in humans. We have provisionally identified two subgroups of CFS patients, one with normal serotonin and the other with a high serotonin status. The relevance of our findings to, and their implications for, the pharmacological and other therapies of the chronic fatigue syndrome are discussed.

Alcohol and violence and the possible role of serotonin

BACKGROUND

There is undisputed evidence linking alcohol consumption and violence and other forms of aggressive behaviour, and also linking aggression with dysfunction of the brain indolylamine serotonin (5-hydroxytryptamine or 5-HT). Alcohol consumption also causes major disturbances in the metabolism of brain serotonin. In particular, acute alcohol intake depletes brain serotonin levels in normal (non-alcohol-dependent) subjects. On the basis of the above statements, it is suggested that, at the biological level, alcohol may induce aggressive behaviour in susceptible individuals, at least in part, by inducing a strong depletion of brain serotonin levels.

AIMS

In this article, evidence supporting these interrelationships and interactions will be summarized and discussed, the alcohol serotonin aggression hypothesis will be reiterated, and potential intervention strategies will be proposed.