Effects of acute tryptophan depletion on plasma and cerebrospinal fluid tryptophan and 5-hydroxyindoleacetic acid in normal volunteers

A central serotonin regulating gene polymorphism (TPH2) determines vulnerability to acute tryptophan depletion-induced anxiety and ventromedial prefrontal threat reactivity in healthy young men

Serotonin (5-HT) has long been implicated in adaptive emotion regulation as well as the development and treatment of emotional dysregulations in mental disorders. Accumulating evidence suggests a genetic vulnerability may render some individuals at a greater risk for the detrimental effects of transient variations in 5-HT signaling. The present study aimed to investigate whether individual variations in the Tryptophan hydroxylase 2 (TPH2) genetics influence susceptibility for behavioral and neural threat reactivity dysregulations during transiently decreased 5-HT signaling. To this end, interactive effects between TPH2 (rs4570625) genotype and acute tryptophan depletion (ATD) on threat reactivity were examined in a within-subject placebo-controlled pharmacological fMRI trial (n = 51). A priori genotype stratification of extreme groups (GG vs. TT) allowed balanced sampling. While no main effects of ATD on neural reactivity to threat-related stimuli and mood state were observed in the entire sample, accounting for TPH2 genotype revealed an ATD-induced increase in subjective anxious arousal in the GG but not the TT carriers. The effects were mirrored on the neural level, such that ATD specifically reduced ventromedial prefrontal cortex reactivity towards threat-related stimuli in the GG carriers. Furthermore, the ATD-induced increase in subjective anxiety positively associated with the extent of ATD-induced changes in ventromedial prefrontal cortex activity in response to threat-related stimuli in GG carriers. Together the present findings suggest for the first time that individual variations in TPH2 genetics render individuals susceptible to the anxiogenic and neural effects of a transient decrease in 5-HT signaling.

Facial expressions of pain: the role of the serotonergic system

RATIONALE

Although interest in the neurobiology of facial communication of pain has increased over the last decades, little is known about which neurotransmitter systems might be involved in regulating facial expressions of pain.

OBJECTIVES

We aim to investigate whether the serotonergic system (5-HT), which has been implicated in various aspects of pain processing as well as in behavioral response inhibition, might play a role in facial expressions of pain. Using acute tryptophan depletion (ATD) to manipulate 5-HT function, we examined its effects on facial and subjective pain responses.

METHODS

In a double-blind, placebo-controlled within-subject design, 27 participants received either an ATD or a control drink in two separate sessions. Approximately 5-h post-oral consumption, we assessed pain thresholds (heat, pressure) as well as facial and subjective responses to phasic heat pain. Moreover, situational pain catastrophizing and mood were assessed as affective state indicators.

RESULTS

ATD neither influenced pain thresholds nor self-report ratings, nor catastrophizing or mood. Only facial responses were significantly affected by ATD. ATD led to a decrease in pain-indicative as well as in pain-non-indicative facial responses to painful heat, compared to the control condition.

CONCLUSIONS

Decrease in brain 5-HT synthesis via ATD significantly reduced facial responses to phasic heat pain; possibly due to (i) diminished disposition to display social behavior or due to (ii) decreased facilitation of excitatory inputs to the facial motor neuron.

A Low-Protein, High-Carbohydrate Diet Exerts a Neuroprotective Effect on Mice with 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine-Induced Parkinson's Disease by Regulating the Microbiota-Metabolite-Brain Axis and Fibroblast Growth Factor 21

Parkinson's disease (PD) is closely linked to lifestyle factors, particularly dietary patterns, which have attracted interest as potential disease-modifying factors. Eating a low-protein, high-carbohydrate (LPHC) diet is a promising dietary intervention against brain aging; however, its protective effect on PD remains elusive. Here, we found that an LPHC diet ameliorated 1-methyl-4-phenyl-1,2,3,6-tetrathydropyridine (MPTP)-induced motor deficits, decreased dopaminergic neuronal death, and increased the levels of striatal dopamine, serotonin, and their metabolites in PD mice. Levels of fibroblast growth factor 21 (FGF-21), a member of the fibroblast growth factor family, were elevated in PD mice following LPHC treatment. Furthermore, the administration of FGF-21 exerted a protective effect on MPTP-induced PC12 cells, similar to the effect of an LPHC diet in MPTP-induced mice. Sequencing of the 16S rDNA from fecal microbiota revealed that an LPHC diet normalized the gut bacterial composition imbalance in PD mice, as evidenced by the increased abundance of the genera Bifidobacterium, Ileibacterium, Turicibacter, and Blautia and decreased abundance of Bilophila, Alistipes, and Bacteroides. PICRUSt-predicted fecal microbiome function revealed that an LPHC diet suppressed lipopolysaccharide biosynthesis and the citrate cycle (TCA cycle), biosynthesis of ubiquinone and other terpenoid-quinones, and oxidative phosphorylation pathways caused by MPTP, and enhanced the biosynthesis of amino acids, carbohydrate metabolism, and biosynthesis of other secondary metabolites. A nonmetabolomic analysis of the serum and feces showed that an LPHC diet significantly increased the levels of aromatic amino acids (AAAs), including tryptophan, tyrosine, and phenylalanine. In addition, an LPHC diet elevated the serum concentrations of bile acids (BAs), particularly tauroursodeoxycholic acid (TUDCA) and taurine. Collectively, our current findings point to the potential mechanism of administering an LPHC diet in attenuating movement impairments in MPTP-induced PD mice, with AAAs, microbial metabolites (TUDCA and taurine), and FGF-21 as key mediators along the gut-microbiota-brain axis.

The effects of α-lactalbumin supplementation and handgrip contraction on soleus motoneuron excitability

INTRODUCTION

We tested two strategies that hypothetically increase serotonin availability (α-lactalbumin consumption and a remote submaximal handgrip contraction) on estimates of persistent inward currents (PICs) amplitude of soleus muscle in healthy participants.

METHODS

With a randomised, double-blind, and cross-over design, 13 healthy participants performed triangular-shaped ramp contractions with their plantar flexors (20% of maximal torque), followed by a 30-s handgrip sustained contraction (40% of maximal force) and consecutive repeated triangular-shaped contractions. This was performed before and after the consumption of either 40 g of α-lactalbumin, an isonitrogenous beverage (Zein) or an isocaloric beverage (Corn-starch). Soleus motor units discharge rates were analysed from high-density surface electromyography signals. PICs were estimated by calculating the delta frequency (ΔF) of motor unit train spikes using the paired motor unit technique.

RESULTS

ΔF (0.19 pps; p = 0.001; d = 0.30) and peak discharge rate (0.20 pps; p < 0.001; d = 0.37) increased after the handgrip contraction, irrespective of the consumed supplement. No effects of α-lactalbumin were observed.

CONCLUSIONS

Our results indicate that 40 g of α-lactalbumin was unable to modify intrinsic motoneuron excitability. However, performing a submaximal handgrip contraction before the plantar flexion triangular contraction was capable of increasing ΔF and discharge rates on soleus motor units. These findings highlight the diffused effects of serotonergic input, its effects on motoneuron discharge behaviour, and suggest a cross-effector effect within human motoneurons.

Effects of α-lactalbumin on strength, fatigue and psychological parameters: a randomised double-blind cross-over study

PURPOSE

The neurotransmitter serotonin has a strong effect on behaviour and motor control. Regarding motor control, serotonin contributes to the development of fatigue and is also involved in the ability of motor neurones to operate across a large range of forces (gain control). The consumption of tryptophan-rich supplements (such as α-lactalbumin) is of interest because this amino acid is the only precursor for brain serotonin synthesis. Therefore, the purpose of this study was to determine the effects of α-lactalbumin supplementation on neuromuscular performance.

METHODS

Using a randomised double-blind cross-over design, 16 healthy participants performed plantar flexor and handgrip maximal voluntary contractions, a 30-s submaximal handgrip contraction, and a plantar flexor fatigue protocol before and 90 min after consuming either 40 g of α-lactalbumin, an isonitrogenous beverage (Zein) or an isocaloric beverage (corn-starch). Sleepiness, mood, and cognition were assessed to evaluate any psychological effects.

RESULTS

α-Lactalbumin decreased force steadiness by 25% during the sustained submaximal handgrip contraction (p < 0.01) and induced greater fatigue (15% reduction in total torque-time integral, p = 0.01) during the fatigue protocol. These effects were not observed for the other control beverages. No effects were found for maximal or explosive strength, or psychological measurements.

CONCLUSIONS

40 g of α-lactalbumin increased handgrip force variability and reduced performance during fatiguing muscle contractions but did not influence brief maximal contractions or psychological parameters in healthy individuals. These findings support the hypothesis that the consumption of α-lactalbumin can increase motor neurone input-output gain and exacerbate central fatigue during sustained maximal exercise.

Exploring the Tryptophan Metabolic Pathways in Migraine-Related Mechanisms

Migraine is a complex neurovascular disorder, which causes intense socioeconomic problems worldwide. The pathophysiology of disease is enigmatic; accordingly, therapy is not sufficient. In recent years, migraine research focused on tryptophan, which is metabolized via two main pathways, the serotonin and kynurenine pathways, both of which produce neuroactive molecules that influence pain processing and stress response by disturbing neural and brain hypersensitivity and by interacting with molecules that control vascular and inflammatory actions. Serotonin has a role in trigeminal pain processing, and melatonin, which is another product of this pathway, also has a role in these processes. One of the end products of the kynurenine pathway is kynurenic acid (KYNA), which can decrease the overexpression of migraine-related neuropeptides in experimental conditions. However, the ability of KYNA to cross the blood-brain barrier is minimal, necessitating the development of synthetic analogs with potentially better pharmacokinetic properties to exploit its therapeutic potential. This review summarizes the main translational and clinical findings on tryptophan metabolism and certain neuropeptides, as well as therapeutic options that may be useful in the prevention and treatment of migraine.

Effect of 5-Hydroxytryptophan on Fatigue in Quiescent Inflammatory Bowel Disease: A Randomized Controlled Trial

BACKGROUND & AIMS

Fatigue is highly prevalent among patients with inflammatory bowel disease (IBD), and only limited treatment options are available. Based on the hypothetical link between low serum tryptophan concentrations and fatigue, we determined the effect of 5-hydroxytryptophan supplementation on fatigue in patients with inactive IBD.

METHODS

A multicenter randomized controlled trial was performed at 13 Belgian hospitals, including 166 patients with IBD in remission but experiencing fatigue, defined by a fatigue visual analog scale (fVAS) score of ≥5. Patients were treated in a crossover manner with 100 mg oral 5-hydroxytryptophan or placebo twice daily for 2 consecutive periods of 8 weeks. The primary end point was the proportion of patients reaching a ≥20% reduction in fVAS after 8 weeks of intervention. Secondary outcomes included changes in serum tryptophan metabolites, Functional Assessment of Chronic Illness Therapy Fatigue scale, and scores for depression, anxiety, and stress. The effect of the intervention on the outcomes was evaluated by linear mixed modeling.

RESULTS

During 5-hydroxytryptophan treatment, a significant increase in serum 5-hydroxytryptophan (estimated mean difference, 52.66 ng/mL; 95% confidence interval [CI], 39.34-65.98 ng/mL; P < .001) and serotonin (3.0 ng/mL; 95 CI, 1.97-4.03 ng/mL; P < .001) levels was observed compared with placebo. The proportion of patients reaching ≥20% reduction in fVAS was similar in placebo- (37.6%) and 5-hydroxytryptophan (35.6%)-treated patients (P = .830). The fVAS reduction (-0.18; 95% CI, -0.81 to 0.46; P = .581) and Functional Assessment of Chronic Illness Therapy Fatigue scale increase (0.68; 95% CI, -2.37 to 3.73; P = .660) were both comparable between 5-hydroxytryptophan and placebo treatment as well as changes in depression, anxiety, and stress scores.

CONCLUSIONS

Despite a significant increase in serum 5-hydroxytryptophan and serotonin levels, oral 5-hydroxytryptophan did not modulate IBD-related fatigue better than placebo. (Trial Registration: Belgian Federal Agency for Medication and Health Products, EudraCT number: 2017-005059-10 and ClinicalTrials.gov: NCT03574948, https://clinicaltrials.gov/ct2/show/NCT03574948.).

Nutritional importance of tryptophan for improving treatment in depression and diabetes

The importance of nutrients in our diet is becoming increasingly recognized. From the viewpoint of protein synthesis and other physiologic and metabolic functions, all amino acids are important, but some of these amino acids are not synthesized endogenously. This subset, called essential amino acids, comprise dietarily indispensable nutrients. Tryptophan, an essential amino acid, is the sole precursor of neuronal as well as peripheral serotonin (5-hydroxytryptamine). Its systemic or oral administration increases serotonin synthesis because tryptophan hydroxylase, the rate-limiting enzyme of 5-hydroxytryptamine biosynthesis, is physiologically unsaturated with its substrate. Central serotonin is implicated in a number of psychiatric illnesses, including depression, and in responses to stress. Acting peripherally, serotonin affects vasoconstriction, intestinal motility, control of T cell–mediated immunity, and liver and pancreatic functions. Depression and diabetes are 2 highly prevalent diseases that often coexist. There is evidence that occurrence of depression is 2–3 times higher in people with diabetes mellitus. A comorbid condition of diabetes and depression worsens the treatment and increases risk for death. Stress, known for its causal role in depression, can also enhance risk for diabetes. Stress-induced decreases in the circulating levels of tryptophan can impair brain and pancreatic serotonin-dependent functions to precipitate these diseases. The importance of tryptophan supplementation for improving therapeutic intervention in depression and diabetes is the focus of this article. A deficiency of this essential amino acid may enhance risk for depression as well as diabetes, and can also weaken treatment efficacy of medicinal compounds for treating these diseases. Guidelines for optimal levels of circulating tryptophan can help if supplements of this amino acid can improve treatment efficacy.

Positive Effects of α-Lactalbumin in the Management of Symptoms of Polycystic Ovary Syndrome

To date, the involvement of α-Lactalbumin (α-LA) in the management of polycystic ovary syndrome (PCOS) refers to its ability to improve intestinal absorption of natural molecules like inositols, overcoming the inositol resistance. However, due to its own aminoacidic building blocks, α-LA is involved in various biological processes that can open new additional applications. A great portion of women with PCOS exhibit gastrointestinal dysbiosis, which is in turn one of the triggering mechanisms of the syndrome. Due to its prebiotic effect, α-LA can recover dysbiosis, also improving the insulin resistance, obesity and intestinal inflammation frequently associated with PCOS. Further observations suggest that altered gut microbiota negatively influence mental wellbeing. Depressive mood and low serotonin levels are indeed common features of women with PCOS. Thanks to its content of tryptophan, which is the precursor of serotonin, and considering the strict link between gut and brain, using α-LA contributes to preserving mental well-being by maintaining high levels of serotonin. In addition, considering women with PCOS seeking pregnancy, both altered microbiota and serotonin levels can induce later consequences in the offspring. Therefore, a deeper knowledge of potential applications of α-LA is required to transition to preclinical and clinical studies extending its therapeutic advantages in PCOS.

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.

Effects of tryptophan depletion on anxiety, a systematic review

Vulnerability markers for onset of anxiety disorders are scarce. In depression, patients at risk tend to respond with a negative mood to 'acute tryptophan depletion' (ATD), while healthy volunteers and current patients do not. The serotonergic system thus provides indications for vulnerability for depression. It is unknown whether ATD reveals vulnerability in anxiety too. This study systematically reviews the effects of ATD on anxiety and assesses whether challenging anxiety modifies the response. PubMed, Embase and PsychInfo were systematically searched up to April 2019 for studies in which (1) healthy volunteers or patients with a (remitted) anxiety disorder underwent ATD and (2) levels of anxiety were reported. In total, 21 studies were included. Studies conducted in healthy volunteers (n = 13), and patients with a remitted (n = 6) or current (panic, social or generalised) anxiety disorder (n = 4). Studies were mostly of poor quality and heterogeneous regarding population, challenge test used and outcome measures. ATD did not consistently affect anxiety in any of the groups. Moreover, a challenge test after ATD (n = 17 studies) did not consistently provoke anxiety in healthy volunteers or remitted patients. A 35% CO₂ challenge did consistently increase anxiety in patients with a current panic disorder (PD). To conclude, this systematic review found no clear indications that ATD provokes anxiety in those at risk for anxiety disorders. Hence, unlike in depression, ATD does not indicate vulnerability to develop an anxiety disorder. Because included studies were heterogeneous and mostly of poor quality, there is an urgent need for high quality research in homogeneous samples.

Nutritional Support of Neurodevelopment and Cognitive Function in Infants and Young Children-An Update and Novel Insights

Proper nutrition is crucial for normal brain and neurocognitive development. Failure to optimize neurodevelopment early in life can have profound long-term implications for both mental health and quality of life. Although the first 1000 days of life represent the most critical period of neurodevelopment, the central and peripheral nervous systems continue to develop and change throughout life. All this time, development and functioning depend on many factors, including adequate nutrition. In this review, we outline the role of nutrients in cognitive, emotional, and neural development in infants and young children with special attention to the emerging roles of polar lipids and high quality (available) protein. Furthermore, we discuss the dynamic nature of the gut-brain axis and the importance of microbial diversity in relation to a variety of outcomes, including brain maturation/function and behavior are discussed. Finally, the promising therapeutic potential of psychobiotics to modify gut microbial ecology in order to improve mental well-being is presented. Here, we show that the individual contribution of nutrients, their interaction with other micro- and macronutrients and the way in which they are organized in the food matrix are of crucial importance for normal neurocognitive development.

The regulation of liver cytochrome P450 expression and activity by the brain serotonergic system in different experimental models

Introduction: Cytochrome P450 (CYP) metabolizes vital endogenous (steroids, vitamins) and exogenous (drugs, toxins) substrates. Studies of the last decade have revealed that the brain dopaminergic and noradrenergic systems are involved in the regulation of CYP. Recent research indicates that the brain serotonergic system is also engaged in its regulation.Areas covered: This review focuses on the role of the brain serotonergic system in the regulation of liver CYP expression. It shows the effect of lesion and activation of the serotonergic system after peripheral or intracerebral injections of neurotoxins, serotonin precursor, or serotonin (5-HT) receptor agonists. An opposite role of the hypothalamic paraventricular and arcuate nuclei and 5-HT receptors present therein in the regulation of CYP is described. The engagement of those nuclei in the neuroendocrine regulation of CYP by hypothalamic releasing or inhibiting hormones, pituitary hormones, and peripheral gland hormones are shown.Expert opinion: In general, the brain serotonergic system negatively regulates liver cytochrome P450. However, the effects of serotonergic agents on the enzyme expression depend on their mechanism of action, the route of administration (intracerebral/peripheral), as well as on local intracerebral site of injection and 5-HT receptor-subtypes present therein.

The Role of Tryptophan and Tyrosine in Executive Function and Reward Processing

The serotonergic precursor tryptophan and the dopaminergic precursor tyrosine have been shown to be important modulators of mood, behaviour and cognition. Specifically, research on the function of tryptophan has characterised this molecule as particularly relevant in the context of pathological disorders such as depression. Moreover, a large body of evidence has now been accumulated to suggest that tryptophan may also be involved in executive function and reward processing. Despite some clear differentiation with tryptophan, the data reviewed in this paper illustrates that tyrosine shares similar functions with tryptophan in the regulation of executive function and reward, and that these processes in turn, rather than acting in isolation, causally influence each other.

Inverse changes in raphe and cortical 5-HT1B receptor availability after acute tryptophan depletion in healthy human subjects

Serotonergic neurotransmission plays a key role in the pathophysiology and treatment of various neuropsychiatric diseases. The purpose of this study was to investigate changes in serotonergic neurotransmission after acute tryptophan depletion (ATD) using positron emission tomography (PET) with [¹¹ C]P943, a 5-HT_1B receptor radioligand previously shown to be sensitive to changes in 5-HT. Five healthy subjects were scanned on a high resolution PET scanner twice on the same day, before and approximately 5 hours after ingesting capsules containing an amino acid mixture that lacks tryptophan. For each scan, emission data were acquired for 120 min after intravenous bolus injection of [¹¹ C]P943. Binding potential (BP_ND ) values were estimated from parametric images using the second version of the multilinear reference tissue model (MRTM2, t* = 20 min) with cerebellar grey matter used as a reference region. The change in [¹¹ C]P943 binding (ΔBP_ND , %) was calculated as (BP_ND,post  - BP_ND,pre )/(BP_ND,pre ) × 100, and correlation analysis was performed to measure linear associations of ΔBP_ND between raphe and other regions of interest (ROIs). ΔBP_ND ranged from -6% to 45% in the raphe, with positive values indicating reduced competition from 5-HT. In cortical regions, ΔBP_ND ranged from -28% to 7%. While these changes did not reach significance, there were significant negative correlations of ΔBP_ND of the raphe with those of cerebral cortical regions and the thalamus (e.g., r = -.96, p = .011 for average cortex). These findings support the hypothesis that raphe serotonin is a critical modulator of cortical serotonin release via projecting neurons in healthy human subjects.

Effects of serotonin depletion and dopamine depletion on bimodal divided attention

Objectives: This study aimed to explore the effects of acute phenylalanine tyrosine depletion (APTD) and acute tryptophan depletion (ATD) on bimodal divided attention. A balanced amino acid mixture (BAL) served as control condition.Methods: Fifty-three healthy adults (final analyzed sample was N = 49, age: M = 23.8 years) were randomly assigned to APTD, ATD or BAL in a double-blind, between-subject approach. Divided attention was assessed after 4 h. Blood samples were taken before and 6 h after challenge intake.Results: Amino acid concentrations following challenge intake significantly decreased (all P ≤ 0.01). There was a significant difference in the mean reaction time (RT) towards auditory stimuli, but not towards visual stimuli between the groups. Post-hoc comparison of mean RTs (auditory stimuli) showed a significant difference between ATD (RT = 604.0 ms, SD = 56.9 ms) and APTD (RT = 556.4 ms, SD = 54.2 ms; P = 0.037), but no RT difference between ATD and BAL or APTD and BAL (RT = 573.6 ms, SD = 45.7 ms).Conclusions: The results indicate a possible dissociation between the effects of a diminished brain 5-HT and DA synthesis on the performance in a bimodal divided attention task. The difference was exclusively observed within the RT towards auditory signals.

Functional consequences of acute tryptophan depletion on raphe nuclei connectivity and network organization in healthy women

Previous research on central nervous serotonin (5-HT) function provided evidence for a substantial involvement of 5-HT in the regulation of brain circuitries associated with cognitive and affective processing. The underlying neural networks comprise core subcortical/cortical regions such as amygdala and medial prefrontal cortex, which are assumed to be modulated amongst others by 5-HT. Beside the use of antidepressants, acute tryptophan depletion (ATD) is a widely accepted technique to manipulate of 5-HT synthesis and its respective metabolites in humans by means of a dietary and non-pharmacological tool. We used a double-blind, randomized, cross-over design with two experimental challenge conditions, i.e. ATD and tryptophan (TRP) supplementation (TRYP+) serving as a control. The aim was to perturb 5-HT synthesis and to detect its impact on brain functional connectivity (FC) of the upper serotonergic raphe nuclei, the amygdala and the ventromedial prefrontal cortex as well as on network organization using resting state fMRI. 30 healthy adult female participants (age: M ​= ​24.5 ​± ​4.4 ​yrs) were included in the final analysis. ATD resulted in a 90% decrease of TRP in the serum relative to baseline. Compared to TRYP ​+ ​for the ATD condition a significantly lower FC of the raphe nucleus to the frontopolar cortex was detected, as well as greater functional coupling between the right amygdala and the ventromedial prefrontal cortex. FC of the raphe nucleus correlated significantly with the magnitude of TRP changes for both challenge conditions (ATD & TRYP+). Network-based statistical analysis using time series from 260 independent anatomical ROIs revealed significantly greater FC after ATD compared to TRYP+ in several brain regions being part of the default-mode (DMN) and the executive-control networks (ECN), but also of salience or visual networks. Finally, we observed an impact of ATD on the rich-club organization in terms of decreased rich-club coefficients compared to TRYP+. In summary we could confirm previous findings that the putative decrease in brain 5-HT synthesis via ATD significantly alters FC of the raphe nuclei as well as of specific subcortical/cortical regions involved in affective, but also in cognitive processes. Moreover, an ATD-effect on the so-called rich-club organization of some nodes with the high degree was demonstrated. This may indicate effects of brain 5-HT on the integration of information flow from several brain networks.

Branched-Chain Amino Acids and Seizures: A Systematic Review of the Literature

BACKGROUND

Up to 40% of patients with epilepsy experience seizures despite treatment with antiepileptic drugs; however, branched-chain amino acid (BCAA) supplementation has shown promise in treating refractory epilepsy.

OBJECTIVES

The purpose of this systematic review was to evaluate all published studies that investigated the effects of BCAAs on seizures, emphasizing therapeutic efficacy and possible underlying mechanisms.

METHODS

On 31 January, 2017, the following databases were searched for relevant studies: MEDLINE (OvidSP), EMBASE (OvidSP), Scopus (Elsevier), the Cochrane Library, and the unindexed material in PubMed (National Library of Medicine/National Institutes of Health). The searches were repeated in all databases on 18 February, 2019. We only included full-length preclinical and clinical studies that were published in the English language that examined the effects of BCAA administration on seizures.

RESULTS

Eleven of 2045 studies met our inclusion criteria: ten studies were conducted in animal models and one study in human subjects. Seven seizure models were investigated: the strychnine (one study), pentylenetetrazole (two studies), flurothyl (one study), picrotoxin (two studies), genetic absence epilepsy in rats (one study), kainic acid (two studies), and methionine sulfoximine (one study) paradigms. Three studies investigated the effect of a BCAA mixture whereas the other studies explored the effects of individual BCAAs on seizures. In most animal models and in humans, BCAAs had potent anti-seizure effects. However, in the methionine sulfoximine model, long-term BCAA supplementation worsened seizure propagation and caused neuron loss, and in the genetic absence epilepsy in rats model, BCAAs exhibited pro-seizure effects.

CONCLUSIONS

The contradictory effects of BCAAs on seizure activity likely reflect differences in the complex mechanisms that underlie seizure disorders. Some of these mechanisms are likely mediated by BCAA's effects on glucose, glutamate, glutamine, and ammonia metabolism, activation of the mechanistic target of rapamycin signaling pathway, and their effects on aromatic amino acid transport and neurotransmitter synthesis. We propose that a better understanding of mechanisms by which BCAAs affect seizures and neuronal viability is needed to advance the field of BCAA supplementation in epilepsy.

Sugar rush or sugar crash? A meta-analysis of carbohydrate effects on mood

The effect of carbohydrate (CHO) consumption on mood is much debated, with researchers reporting both mood improvements and decrements following CHO ingestion. As global consumption of sugar-sweetened products has sharply increased in recent years, examining the validity of claims of an association between CHOs and mood is of high importance. We conducted a systematic review and meta-analysis to evaluate the relationship between acute CHO ingestion and mood. We examined the time-course of CHO-mood interactions and considered the role of moderator variables potentially affecting the CHO-mood relationship. Analysis of 176 effect sizes (31 studies, 1259 participants) revealed no positive effect of CHOs on any aspect of mood at any time-point following their consumption. However, CHO administration was associated with higher levels of fatigue and less alertness compared with placebo within the first hour post-ingestion. These findings challenge the idea that CHOs can improve mood, and might be used to increase the public's awareness that the 'sugar rush' is a myth, inform health policies to decrease sugar consumption, and promote healthier alternatives.

The Effects of Tryptophan Enhancement and Depletion on Plasma Catecholamine Levels in Healthy Individuals

OBJECTIVE

Central nervous system (CNS) serotonin (5-HT) exerts both excitatory and inhibitory effects on the sympathetic nervous system (SNS) in animals. In this study, we examine the effects of tryptophan enhancement and depletion on plasma catecholamine levels in humans.

METHODS

The total sample consisted of 164 healthy men and women who were tested for 2 days. Seventy-nine participants were randomized to a tryptophan enhancement condition and 85 to a tryptophan depletion condition. Both protocols consisted of a "sham day," followed by an "active day." Blood samples for assessment of plasma norepinephrine and epinephrine levels were collected before and after tryptophan enhancement/depletion. Data were analyzed using general linear models. Separate analyses were conducted for each study arm and for each measure.

RESULTS

In the depletion condition, both epinephrine (F(5,330) = 2.69, p = .021) and norepinephrine (F(5,335) = 2.79, p = .018) showed small increases on active versus "sham" depletion days. There were also significant day by time interactions for epinephrine (F(3,171) = 39.32, p < .0001) and norepinephrine (F(3,195) = 31.09, p < .0001) levels in the enhancement arm. Tryptophan infusion resulted in a marked increase in epinephrine (Premean = 23.92 (12.23) versus Postmean = 81.57 (62.36)) and decrease in norepinephrine (Premean = 257.2 (106.11) versus Postmean = 177.04 (87.15)), whereas levels of both catecholamines were stable on the "sham day."

CONCLUSIONS

CNS 5-HT exerts both inhibitory and excitatory effects on SNS activity in humans, potentially due to stimulation of CNS 5-HT receptors that have shown to have inhibitory (5-HT1A) and excitatory (5-HT1A and/or 5-HT2) SNS effects in animal models.

Depressed mood induction in early cigarette withdrawal is unaffected by acute monoamine precursor supplementation

BACKGROUND

Cigarette smoking is the leading preventable cause of death; however, quitting is difficult and early relapse is common. Dysphoric mood during early cigarette withdrawal is associated with relapse, and with the exception of bupropion and nortriptyline, few interventions have been developed to prevent this. During early cigarette withdrawal there is an elevation in the levels of monoamine oxidase-A (MAO-A), which removes monoamines excessively and induces oxidative stress and is implicated in creating sad mood. Hence, we conducted a randomized, placebo-controlled, double-blind crossover trial of a dietary supplement designed to counter the effects of elevated MAO-A levels on vulnerability to depressed mood.

METHODS

Twenty-one otherwise healthy cigarette smokers completed the protocol, receiving either active dietary supplement followed by washout and placebo or the same in reverse order. The dietary supplement was composed of monoamine precursors (2 g tryptophan, 10 g tyrosine) and blueberry antioxidants (blueberry juice with blueberry extract). Vulnerability to depressed mood was measured by the change in scores of depressed mood on the visual analog scale (VAS) following the sad mood induction paradigm (MIP).

RESULTS

There was a significant increase in VAS depressed mood scores after the sad MIP during supplement and placebo, but no difference between active and placebo conditions. There was also a significant increase in urge-to-smoke scores after sad MIP during supplement and placebo but no difference between active and placebo conditions. Reliability of the increase in VAS after MIP was very good.

CONCLUSION

The dietary supplement had negligible effect on depressed mood, but sad MIP is a very reliable method that can be applied in future studies to assess other interventions for preventing dysphoric mood during early cigarette withdrawal.

Acute tryptophan loading decreases functional connectivity between the default mode network and emotion-related brain regions

It has been shown that the functional architecture of the default mode network (DMN) can be affected by serotonergic challenges and these effects may provide insights on the neurobiological bases of depressive symptomatology. To deepen our understanding of this possible interplay, we used a double-blind, randomized, cross-over design, with a control condition and two interventions to decrease (tryptophan depletion) and increase (tryptophan loading) brain serotonin synthesis. Resting-state fMRI from 85 healthy subjects was acquired for all conditions 3 hr after the ingestion of an amino acid mixture containing different amounts of tryptophan, the dietary precursor of serotonin. The DMN was derived for each participant and session. Permutation testing was performed to detect connectivity changes within the DMN as well as between the DMN and other brain regions elicited by the interventions. We found that tryptophan loading increased tryptophan plasma levels and decreased DMN connectivity with visual cortices and several brain regions involved in emotion and affect regulation (i.e., putamen, subcallosal cortex, thalamus, and frontal cortex). Tryptophan depletion significantly reduced tryptophan levels but did not affect brain connectivity. Subjective ratings of mood, anxiety, sleepiness, and impulsive choice were not strongly affected by any intervention. Our data indicate that connectivity between the DMN and emotion-related brain regions might be modulated by changes in the serotonergic system. These results suggest that functional changes in the brain associated with different brain serotonin levels may be relevant to understand the neural bases of depressive symptoms.

Neuroticism predicts the impact of serotonin challenges on fear processing in subgenual anterior cingulate cortex

The personality trait neuroticism is associated with increased vulnerability to anxiety and mood disorders, conditions linked with abnormal serotonin neurotransmission and emotional processing. The interaction between neuroticism and serotonin during emotional processing is however not understood. Here we investigate how individual neuroticism scores influence the neural response to negative emotional faces and their sensitivity to serotonergic tone. Twenty healthy participants performed an emotional face task under functional MRI on three occasions: increased serotonin tone following infusion of a selective serotonin reuptake inhibitor (SSRI), decreased serotonin tone following acute tryptophan depletion (ATD) protocol, and no serotonin challenge (control). During the task, participants performed a gender-discrimination task of neutral, fearful or angry facial expressions. Individual variations in neuroticism scores were associated with neural response of subgenual anterior cingulate cortex to fearful facial expressions. The association was however opposite under the two serotoninergic challenges. The fear-related response in this region and individual neuroticism scores correlated negatively during citalopram challenge and positively during ATD. Thus, neuroticism scores were associated with the relative impact of serotonin challenges on fear processing in subgenual anterior cingulate cortex. This finding may link to a neural mechanism for the variable therapeutic effect of SSRI treatment observed in clinical populations.

Risk-seeking for losses is associated with 5-HTTLPR, but not with transient changes in 5-HT levels

RATIONALE

Serotonin (5-HT) plays a key role in different aspects of value-based decision-making. A recent framework proposed that tonic 5-HT (together with dopamine, DA) codes future average reward expectations, providing a baseline against which possible choice outcomes are compared to guide decision-making.

OBJECTIVES

To test whether high 5-HT levels decrease loss aversion, risk-seeking for gains, and risk-seeking for losses.

METHODS

In a first session, 611 participants were genotyped for 5-HTTLPR and performed a mixed gambles (MGA) task and two probability discounting tasks for gains and losses, respectively (PDG/PDL). Afterwards, a subsample of 105 participants (44 with S/S, 6 with S/L, 55 with L/L genotype) completed the pharmacological study using a crossover design with tryptophan depletion (ATD), loading (ATL), and balanced (BAL) conditions. The same decision constructs were assessed.

RESULTS

We found increased risk-seeking for losses in S/S compared to L/L individuals at the first visit (p = 0.002). Neither tryptophan depletion nor loading affected decision-making, nor did we observe an interaction between intervention and 5-HTTLPR genotype.

CONCLUSION

Our data do not support the idea that transient changes of tonic 5-HT affect value-based decision-making. We provide evidence for an association of 5-HTTLPR with risk-seeking for losses, independent of acute 5-HT levels. This indicates that the association of 5-HTTLPR and risk-seeking for losses is mediated via other mechanisms, possibly by differences in the structural development of neural circuits of the 5-HT system during early life phases.

Applications for α-lactalbumin in human nutrition

α-Lactalbumin is a whey protein that constitutes approximately 22% of the proteins in human milk and approximately 3.5% of those in bovine milk. Within the mammary gland, α-lactalbumin plays a central role in milk production as part of the lactose synthase complex required for lactose formation, which drives milk volume. It is an important source of bioactive peptides and essential amino acids, including tryptophan, lysine, branched-chain amino acids, and sulfur-containing amino acids, all of which are crucial for infant nutrition. α-Lactalbumin contributes to infant development, and the commercial availability of α-lactalbumin allows infant formulas to be reformulated to have a reduced protein content. Likewise, because of its physical characteristics, which include water solubility and heat stability, α-lactalbumin has the potential to be added to food products as a supplemental protein. It also has potential as a nutritional supplement to support neurological function and sleep in adults, owing to its unique tryptophan content. Other components of α-lactalbumin that may have usefulness in nutritional supplements include the branched-chain amino acid leucine, which promotes protein accretion in skeletal muscle, and bioactive peptides, which possess prebiotic and antibacterial properties. This review describes the characteristics of α-lactalbumin and examines the potential applications of α-lactalbumin for human health.

Tryptophan supplementation and serotonin function: genetic variations in behavioural effects

The neurotransmitter serotonin has a role in affective disorders such as depression and anxiety, as well as sleep, cognitive function and appetite. This review examines the evidence that serotonin-related genotypes may moderate the behavioural effects of supplementation with the serotonin precursor amino acidl-tryptophan (TRP), on which synthesis of serotonin (or 5-hydroxytryptamine; 5-HT) depends. However, 95 % of serotonin is synthesised and used in the periphery, and TRP is also metabolised via non-5-HT routes such as the kynurenine pathway. Moreover, understanding of genotypes involved in regulation of serotonin raises questions over the generalisability of TRP effects on behaviour across individuals with varied serotonergic genotypes. To date, only differences between variants of the 5-HT transporter-linked promoter region (5-HTTLPR) have been investigated in relation to behavioural effects of TRP supplementation. Effects of 5-HTTLPR genotypes are usually compared between the alleles that are either high (L/L′) or low (S/S′) expressing of mRNA for the 5-HT transporter receptor. Yet, another key genetic variable is sex: in women, the S/S′ genotype predicts sensitivity to improved mood and reduced cortisol by TRP supplementation, during stressful challenges, whereas the L/L′ genotype protects against stress-induced mood deterioration. In men, the L/L′ genotype may confer risk of stress-induced increases in negative affect; there are insufficient data to assess effects on male S/S′ genotypes. However, better-powered studies to detect sex by genotype by stress by TRP interactions, as well as consideration of more genotypes, are needed before strong conclusions and recommendations for behavioural effects of TRP treatment can be reached.

Effects of tryptophan depletion on selective serotonin reuptake inhibitor-remitted patients with obsessive compulsive disorder

BACKGROUND

Serotonergic antidepressants are first-line medication therapies for obsessive-compulsive disorder, however it is not known if synaptic serotonin availability is important for selective serotonin reuptake inhibitor efficacy. The present study tested the hypothesis that temporary reduction in central serotonin transmission, through acute tryptophan depletion, would result in an increase in anxiety in selective serotonin reuptake inhibitor-remitted obsessive-compulsive disorder patients.

METHODS

Eight patients (four males) with obsessive-compulsive disorder who showed sustained clinical improvement with selective serotonin reuptake inhibitor treatment underwent acute tryptophan depletion in a randomized, double-blind, placebo-controlled, within-subjects design, over two days one week apart. Five hours after consumption of the depleting/sham drink the participants performed a personalized obsessive-compulsive disorder symptom exposure task. Psychological responses were measured using the Spielberger State Anxiety Inventory, Yale-Brown Obsessive Compulsive Scale and Visual Analogue Scales.

RESULTS

Free plasma tryptophan to large neutral amino acid ratio decreased by 93% on the depletion day and decreased by 1% on the sham day, as anticipated. Psychological rating scores as measured by Visual Analogue Scale showed a significant decrease in perceived control and increase in interfering thoughts at the time of provocation on the depletion day but not on the sham day. A measure of convergent validity, namely Visual Analogue Scale Similar to past, was significantly higher at the time of provocation on both the depletion and sham days. Both the depletion and time of provocation scores for Visual Analogue Scale Anxiety, Spielberger State Anxiety Inventory, Yale-Brown Obsessive Compulsive Scale and blood pressure were not significant.

CONCLUSIONS

Acute tryptophan depletion caused a significant decrease in perceived control and increase in interfering thoughts at the time of provocation. Acute tryptophan depletion had no effect on the Spielberger State Anxiety Inventory or Visual Analogue Scale Anxiety measures, which suggests that the mechanism of action of selective serotonin reuptake inhibitors may be different to that seen in panic, social anxiety and post-traumatic stress disorder. Successful selective serotonin reuptake inhibitor treatment of obsessive-compulsive disorder may involve the ability of serotonin to switch habitual responding to goal-directed behaviour.

A Biomarker to Differentiate between Primary and Cocaine-Induced Major Depression in Cocaine Use Disorder: The Role of Platelet IRAS/Nischarin (I1-Imidazoline Receptor)

The association of cocaine use disorder (CUD) and comorbid major depressive disorder (MDD; CUD/MDD) is characterized by high prevalence and poor treatment outcomes. CUD/MDD may be primary (primary MDD) or cocaine-induced (CUD-induced MDD). Specific biomarkers are needed to improve diagnoses and therapeutic approaches in this dual pathology. Platelet biomarkers [5-HT2A receptor and imidazoline receptor antisera selected (IRAS)/nischarin] were assessed by Western blot in subjects with CUD and primary MDD (n = 16) or CUD-induced MDD (n = 9; antidepressant free, AD-; antidepressant treated, AD+) and controls (n = 10) at basal level and/or after acute tryptophan depletion (ATD). Basal platelet 5-HT2A receptor (monomer) was reduced in comorbid CUD/MDD subjects (all patients: 43%) compared to healthy controls, and this down-regulation was independent of AD medication (decreases in AD-: 47%, and in AD+: 40%). No basal differences were found for IRAS/nischarin contents in AD+ and AD- comorbid CUD/MDD subjects. The comparison of IRAS/nischarin in the different subject groups during/after ATD showed opposite modulations (i.e., increases and decreases) in response to low plasma tryptophan levels with significant differences discriminating between the subgroups of CUD with primary MDD and CUD-induced MDD. These specific alterations suggested that platelet IRAS/nischarin might be useful as a biomarker to discriminate between primary and CUD-induced MDD in this dual pathology.

Genetic and biochemical changes of the serotonergic system in migraine pathobiology

Migraine is a brain disorder characterized by a piercing headache which affects one side of the head, located mainly at the temples and in the area around the eye. Migraine imparts substantial suffering to the family in addition to the sufferer, particularly as it affects three times more women than men and is most prevalent between the ages of 25 and 45, the years of child rearing. Migraine typically occurs in individuals with a genetic predisposition and is aggravated by specific environmental triggers. Attempts to study the biochemistry of migraine began as early as the 1960s and were primarily directed at serotonin metabolism after an increase of 5-hydroxyindoleacetic acid (5-HIAA), the main metabolite of serotonin was observed in urine of migraineurs. Genetic and biochemical studies have primarily focused on the neurotransmitter serotonin, considering receptor binding, transport and synthesis of serotonin and have investigated serotonergic mediators including enzymes, receptors as well as intermediary metabolites. These studies have been mainly assayed in blood, CSF and urine as the most accessible fluids. More recently PET imaging technology integrated with a metabolomics and a systems biology platform are being applied to study serotonergic biology. The general trend observed is that migraine patients have alterations of neurotransmitter metabolism detected in biological fluids with different biochemistry from controls, however the interpretation of the biological significance of these peripheral changes is unresolved. In this review we present the biology of the serotonergic system and metabolic routes for serotonin and discuss results of biochemical studies with regard to alterations in serotonin in brain, cerebrospinal fluid, saliva, platelets, plasma and urine of migraine patients.

Tryptophan metabolism, its relation to inflammation and stress markers and association with psychological and cognitive functioning: Tasmanian Chronic Kidney Disease pilot study

Background

Adults with chronic kidney disease (CKD) exhibit alterations in tryptophan metabolism, mainly via the kynurenine pathway, due to higher enzymatic activity induced mainly by inflammation. Indoles produced by gut-microflora are another group of tryptophan metabolites related to inflammation and conditions accompanying CKD. Disruptions in tryptophan metabolism have been associated with various neurological and psychological disorders. A high proportion of CKD patients self-report symptoms of depression and/or anxiety and decline in cognitive functioning. This pilot study examines tryptophan metabolism in CKD and explores associations with psychological and cognitive functioning.

Methods

Twenty-seven adults with CKD were part of 49 patients recruited to participate in a prospective pilot study, initially with an eGFR of 15–29 mL/min/1.73 m². Only participants with viable blood samples and complete psychological/cognitive data at a 2-year follow-up were included in the reported cross-sectional study. Serum samples were analysed by Liquid Chromatography coupled to Mass Spectrometry, for tryptophan, ten of its metabolites, the inflammation marker neopterin and the hypothalamic–pituitary–adrenal (HPA) axis marker cortisol.

Results

The tryptophan breakdown index (kynurenine / tryptophan) correlated with neopterin (Pearson R = 0.51 P = 0.006) but not with cortisol. Neopterin levels also correlated with indoxyl sulfate (R = 0.68, P < 0.0001) and 5 metabolites of tryptophan (R range 0.5–0.7, all P ≤ 0.01), which were all negatively related to eGFR (P < 0.05). Higher levels of kynurenic acid were associated with lower cognitive functioning (Spearman R = −0.39, P < 0.05), while indole-3 acetic acid (IAA) was correlated with anxiety and depression (R = 0.52 and P = 0.005, R = 0.39 and P < 0.05, respectively).

Conclusions

The results of this preliminary study suggest the involvement of inflammation in tryptophan breakdown via the kynurenine pathway, yet without sparing tryptophan metabolism through the 5-HT (serotonin) pathway in CKD patients. The multiple moderate associations between indole-3 acetic acid and psychological measures were a novel finding. The presented pilot data necessitate further exploration of these associations within a large prospective cohort to assess the broader significance of these findings.

Electronic supplementary material

The online version of this article (doi:10.1186/s12882-016-0387-3) contains supplementary material, which is available to authorized users.

The Association between Baseline Subjective Anxiety Rating and Changes in Cardiac Autonomic Nervous Activity in Response to Tryptophan Depletion in Healthy Volunteers

The aim of this study was to investigate the influence of serotonin on anxiety and autonomic nervous system (ANS) function; the correlation between subjective anxiety rating and changes of ANS function following tryptophan depletion (TD) in healthy volunteers was examined. Twenty-eight healthy participants, consisting of 15 females and 13 males, with an average age of 33.3 years, were recruited.Baseline Chinese Symptom Checklist-90-Revised and ANS function measurements were taken. TD was carried out on the testing day, and participants provided blood samples right before and 5 hours after TD. ANS function, somatic symptoms, and Visual Analogue Scales (VASs) were determined after TD. Wilcoxon signed rank test and Spearman ρ correlation were adapted for analyses of the results.The TD procedure reduced total and free plasma tryptophan effectively. After TD, the sympathetic nervous activity increased and parasympathetic nervous activity decreased. Baseline anxiety ratings positively correlated with post-TD changes in sympathetic nervous activity, VAS ratings, and physical symptoms. However, a negative correlation with post-TD changes in parasympathetic nervous activity was found.The change in ANS function after TD was associated with the severity of anxiety in healthy volunteers. This supports the fact that the effect of anxiety on heart rate variability is related to serotonin vulnerability. Furthermore, it also shows that the subjective anxiety rating has a biological basis related to serotonin.

Influence of Tryptophan and Serotonin on Mood and Cognition with a Possible Role of the Gut-Brain Axis

The serotonergic system forms a diffuse network within the central nervous system and plays a significant role in the regulation of mood and cognition. Manipulation of tryptophan levels, acutely or chronically, by depletion or supplementation, is an experimental procedure for modifying peripheral and central serotonin levels. These studies have allowed us to establish the role of serotonin in higher order brain function in both preclinical and clinical situations and have precipitated the finding that low brain serotonin levels are associated with poor memory and depressed mood. The gut-brain axis is a bi-directional system between the brain and gastrointestinal tract, linking emotional and cognitive centres of the brain with peripheral functioning of the digestive tract. An influence of gut microbiota on behaviour is becoming increasingly evident, as is the extension to tryptophan and serotonin, producing a possibility that alterations in the gut may be important in the pathophysiology of human central nervous system disorders. In this review we will discuss the effect of manipulating tryptophan on mood and cognition, and discuss a possible influence of the gut-brain axis.

The neurobiology of aggression and violence

Aggression and violence represent a significant public health concern and a clinical challenge for the mental healthcare provider. A great deal has been revealed regarding the neurobiology of violence and aggression, and an integration of this body of knowledge will ultimately serve to advance clinical diagnostics and therapeutic interventions. We will review here the latest findings regarding the neurobiology of aggression and violence. First, we will introduce the construct of aggression, with a focus on issues related to its heterogeneity, as well as the importance of refining the aggression phenotype in order to reduce pathophysiologic variability. Next we will examine the neuroanatomy of aggression and violence, focusing on regional volumes, functional studies, and interregional connectivity. Significant emphasis will be on the amygdala, as well as amygdala-frontal circuitry. Then we will turn our attention to the neurochemistry and molecular genetics of aggression and violence, examining the extensive findings on the serotonergic system, as well as the growing literature on the dopaminergic and vasopressinergic systems. We will also address the contribution of steroid hormones, namely, cortisol and testosterone. Finally, we will summarize these findings with a focus on reconciling inconsistencies and potential clinical implications; and, then we will suggest areas of focus for future directions in the field.

Acute serotonin depletion releases motivated inhibition of response vigour

RATIONALE

The neurotransmitter serotonin has long been implicated in the motivational control of behaviour. Recent theories propose that the role of serotonin can be understood in terms of an interaction between a motivational and a behavioural activation axis. Experimental support for these ideas, however, has been mixed.

OBJECTIVES

In the current study, we aimed to investigate the role of serotonin (5HT) in behavioural vigour as a function of incentive motivation.

METHODS

We employed dietary acute tryptophan depletion (ATD) to lower the 5HT precursor tryptophan during the performance of a speeded visual discrimination task. Feedback valence and feedback probability were manipulated independently and cued prior to target onset. On feedback trials, fast correct responses led to either reward or avoidance of punishment, while slow or incorrect responses led to reward omission or punishment.

RESULTS

We show that behavioural responding is inhibited under high incentive motivation (i.e. high-feedback probability) at baseline 5HT levels and that lowering these leads to behavioural disinhibition, while leaving accuracy unaffected. Surprisingly, there were no differential effects of motivational valence, with 5HT depletion releasing behavioural inhibition under both appetitive and aversive motivation.

CONCLUSIONS

Our findings extend current theories on the role of 5HT in behavioural inhibition by showing that reductions in serotonin lead to increased behavioural vigour only if there is a motivational drive to inhibit behaviour at baseline.

Serotonin metabolites in the cerebrospinal fluid in sudden infant death syndrome

Forensic biomarkers are needed in sudden infant death syndrome (SIDS) to help identify this group among other sudden unexpected deaths in infancy. Previously, we reported multiple serotonergic (5-HT) abnormalities in nuclei of the medulla oblongata that help mediate protective responses to homeostatic stressors. As a first step toward their assessment as forensic biomarkers of medullary pathology, here we test the hypothesis that 5-HT-related measures are abnormal in the cerebrospinal fluid (CSF) of SIDS infants compared with those of autopsy controls. Levels of CSF 5-hydroxyindoleacetic acid (5-HIAA) and homovanillic acid (HVA), the degradative products of 5-HT and dopamine, respectively, were measured by high-performance liquid chromatography in 52 SIDS and 29 non-SIDS autopsy cases. Tryptophan (Trp) and tyrosine (Tyr), the substrates of 5-HT and dopamine, respectively, were also measured. There were no significant differences in 5-HIAA, Trp, HVA, or Tyr levels between the SIDS and non-SIDS groups. These data preclude the use of 5-HIAA, HVA, Trp, or Tyr measurements as CSF autopsy biomarkers of 5-HT medullary pathology in infants who have died suddenly and unexpectedly. They do, however, provide important information about monoaminergic measurements in human CSF at autopsy and their developmental profile in infancy that is applicable to multiple pediatric disorders beyond SIDS.

Bright ambient light conditions reduce the effect of tryptophan depletion in healthy females

INTRODUCTION

Tryptophan depletion (TD) is an established method to influence the serotonergic system and mood. The purpose of this study was to examine the effect of TD under different ambient light conditions, measured through serotonin-associated plasma levels and a visual analog scale (VAS), on healthy females.

METHODS

Thirty-eight healthy female s-allele carriers of the serotonin transporter promoter gene (5-HTTLPR) were administered a TD under dim light conditions (75 lx). A sub-group of 8 participants repeated the procedure randomized in two additional light conditions (585 lx and 1530 lx respectively). Prior to, and 5h following administration of TD, various variables (serotonin-associated plasma levels, VAS) were measured. Due to not normal distributed data, non-parametric statistical tests were used.

RESULTS

Overall analysis showed a significant mood lowering effect of TD. Moreover, TD decreased all measured serotonin-associated plasma levels significantly. Significant differences in varying light conditions were found for the VAS and plasma tryptophan, with the greatest effect of TD in the 75 lx condition.

CONCLUSION

Results of our study showed an influence of even slight differences in ambient light intensity on the effect of TD concerning mood as well as on the serotonergic system.

Acute tryptophan depletion reduces nitric oxide synthase in the rat hippocampus

Acute tryptophan depletion (ATD) is extensively used to investigate the role of central serotonin (5-HT). However, several studies reported that ATD had no significant effect on central 5-HT concentration and some ATD-induced changes was independent of 5-HT in the rodent brain. Therefore, the potential mechanism of ATD might not be ascribed solely to changes in the central 5-HT system. In recent studies, evidence suggests that nitric oxide synthase (NOS) is closely associated with ATD-induced changes in modulation of cerebral blood flow and metabolism, cognitive, and locomotor activity. Thus, NOS is implicated to be an underlying factor contributing to ATD-induced changes. In the present study, the effect of ATD upon central NOS levels in the rat was evaluated. Male Sprague-Dawley (SD) rats were orally administered a tryptophan-free protein-carbohydrate mixture. Then, ATD effects upon affective behavior and spatial memory were assessed by the forced swimming test (FST) and Morris water maze test, respectively. Further, NOS activity and neuronal NOS (nNOS) protein levels in the hippocampus were measured after ATD. Our experimental results showed that ATD had no influence on affective behavior in the FST or spatial memory in SD rats. Interestingly, a significant reduction of both constitutive NOS activity and nNOS protein levels after ATD was found in the hippocampus. These findings demonstrate ATD does not influence affective behavior and spatial memory despite a direct effect on hippocampal NOS. Our study might provide a valuable clue for exploring earlier reported ATD-induced behavioral and neurochemical changes in rodents.

Food-derived serotonergic modulators: effects on mood and cognition

The most frequently described drugs in the treatment of mood disorders are selective serotonin reuptake and monoamine oxidase (MAO) inhibitors, enhancing serotonin levels in the brain. However, side-effects have been reported for these drugs. Because serotonin levels in the brain are dependent on the availability of the food-derived precursor tryptophan, foods such as chicken, soyabeans, cereals, tuna, nuts and bananas may serve as an alternative to improve mood and cognition. Here we discuss the effects of high- or low-tryptophan-containing food, as well as plant extracts with a modest monoamine reuptake and MAO-A inhibition functional profile, on mood and cognition in healthy and vulnerable human subjects and rodents. Together the studies suggest that there is an inverted U-shaped curve for plasma tryptophan levels, with low and too high tryptophan levels impairing cognition, and moderate to high tryptophan levels improving cognition. This relationship is found for both healthy and vulnerable subjects. Whereas this relationship may also exist for mood, the inverted U-shaped curve for plasma tryptophan levels and mood may be based on different tryptophan concentrations in healthyv.vulnerable individuals. Animal studies are emerging and allow further understanding of effects and the mode of action of food-derived serotonergic components on mood, cognition and mechanisms. Ultimately, insight into the concentrations of tryptophan and other serotonergic components in food having beneficial effects on mood and cognition in healthy, but particularly vulnerable, subjects may support well-being in our highly demanding society.

Acute tryptophan depletion in humans: a review of theoretical, practical and ethical aspects

The acute tryptophan depletion (ATD) technique has been used extensively to study the effect of low serotonin in the human brain. This review assesses the validity of a number of published criticisms of the technique and a number of previously unpublished potential criticisms. The conclusion is that ATD can provide useful information when results are assessed in conjunction with results obtained using other techniques. The best-established conclusion is that low serotonin function after tryptophan depletion lowers mood in some people. However, this does not mean that other variables, altered after tryptophan depletion, are necessarily related to low serotonin. Each aspect of brain function has to be assessed separately. Furthermore, a negative tryptophan depletion study does not mean that low serotonin cannot influence the variable studied. This review suggests gaps in knowledge that need to be filled and guidelines for carrying out ATD studies.

Playing it safe but losing anyway--serotonergic signaling of negative outcomes in dorsomedial prefrontal cortex in the context of risk-aversion

Risk avoidance is an important determinant of human behavior. The neurotransmitter serotonin has been implicated in processing negative outcomes caused by risky decisions. However, it is unclear whether serotonin provides a neurobiological link between making a risk aversive decision and the response to a negative outcome. Using pharmacological fMRI, we manipulated the availability of serotonin in healthy volunteers while performing a gambling task. The same group of participants was studied in three fMRI sessions: (i) during intravenous administration of the SSRI citalopram to increase the serotonergic tone, (ii) after acute tryptophan depletion (ATD) to reduce central serotonin levels, or (iii) without interventions. ATD and citalopram had opposite effects on outcome related activity in dorsomedial prefrontal cortex (dmPFC) and amygdala. Relative to the control condition, ATD increased and citalopram decreased the neural response to negative outcomes in dmPFC. Conversely, ATD decreased and citalopram increased the neural response to negative outcomes in left amygdala. Critically, these pharmacological effects were restricted to negative outcomes that were caused by low-risk decisions and led to a high missed reward. ATD and citalopram did not alter the neural response to positive outcomes in dmPFC, but relative to ATD, citalopram produced a bilateral increase in the amygdala response to large wins caused by high-risk choices. The results show a selective involvement of the serotonergic system in neocortical processing of negative outcomes resulting from risk-averse decisions, thereby linking risk aversion and processing of negative outcomes in goal-directed behaviors.

Serotonin 2A receptors, citalopram and tryptophan-depletion: a multimodal imaging study of their interactions during response inhibition

Poor behavioral inhibition is a common feature of neurological and psychiatric disorders. Successful inhibition of a prepotent response in 'NoGo' paradigms requires the integrity of both the inferior frontal gyrus (IFG) and the serotonergic system. We investigated individual differences in serotonergic regulation of response inhibition. In 24 healthy adults, we used (18)F-altanserin positron emission tomography to assess cerebral 5-HT2A receptors, which have been related to impulsivity. We then investigated the impact of two acute manipulations of brain serotonin levels on behavioral and neural correlates of inhibition using intravenous citalopram and acute tryptophan depletion during functional magnetic resonance imaging. We adapted the NoGo paradigm to isolate effects on inhibition per se as opposed to other aspects of the NoGo paradigm. Successful NoGo inhibition was associated with greater activation of the right IFG compared to control trials with alternative responses, indicating that the IFG is activated with inhibition in NoGo trials rather than other aspects of invoked cognitive control. Activation of the left IFG during NoGo trials was greater with citalopram than acute tryptophan depletion. Moreover, with the NoGo-type of response inhibition, the right IFG displayed an interaction between the type of serotonergic challenge and neocortical 5-HT2A receptor binding. Specifically, acute tryptophan depletion (ATD) produced a relatively larger NoGo response in the right IFG in subjects with low 5-HT2A BPP but reduced the NoGo response in those with high 5-HT2A BPP. These links between serotonergic function and response inhibition in healthy subjects may help to interpret serotonergic abnormalities underlying impulsivity in neuropsychiatric disorders.

The effects of acute tryptophan depletion on impulsivity and mood in adolescents engaging in non-suicidal self-injury

OBJECTIVE

Non-suicidal self-injury (NSSI) is associated with impaired emotion regulation and impulsivity. Low serotonin (5-hydroxytryptamine) function is associated with NSSI, impaired emotion regulation and impulsivity. We investigated the effects of experimentally lowered 5-hydroxytryptamine activity, via acute tryptophan depletion (ATD), on impulsive action, reflection impulsivity and mood in female adolescents engaging in NSSI.

METHODS

Thirty-two female adolescents engaging in NSSI participated in a parallel group ATD study. Following ATD, impulsive action was assessed using the Continuous Performance Test, Identical Pairs Version. Reflection impulsivity was assessed using the Matching Familiar Figures Test. Mood-lowering was examined using the Profile of Mood States.

RESULTS

Following ATD, the participants showed an impulsive response style (as reflected in their low β) and increased attentional capacity (as reflected in their elevated d'). ATD did not affect reflection impulsivity or mood.

CONCLUSIONS

Acute tryptophan depletion caused an impulsive response style and increased attentional capacity. Importantly, the findings suggest that low serotonin function is a vulnerability among female adolescents for engaging in NSSI when in emotional distress.

Effects and side effects associated with the non-nutritional use of tryptophan by humans

The daily nutritional requirement for L-tryptophan (Trp) is modest (5 mg/kg). However, many adults choose to consume much more, up to 4-5 g/d (60-70 mg/kg), typically to improve mood or sleep. Ingesting L-Trp raises brain tryptophan levels and stimulates its conversion to serotonin in neurons, which is thought to mediate its actions. Are there side effects from Trp supplementation? Some consider drowsiness a side effect, but not those who use it to improve sleep. Though the literature is thin, occasional side effects, seen mainly at higher doses (70-200 mg/kg), include tremor, nausea, and dizziness, and may occur when Trp is taken alone or with a drug that enhances serotonin function (e.g., antidepressants). In rare cases, the "serotonin syndrome" occurs, the result of too much serotonin stimulation when Trp is combined with serotonin drugs. Symptoms include delirium, myoclonus, hyperthermia, and coma. In 1989 a new syndrome appeared, dubbed eosinophilia myalgia syndrome (EMS), and was quickly linked to supplemental Trp use. Key symptoms included debilitating myalgia (muscle pain) and a high peripheral eosinophil count. The cause was shown not to be Trp but a contaminant in certain production batches. This is not surprising, because side effects long associated with Trp use were not those associated with the EMS. Over 5 decades, Trp has been taken as a supplement and as an adjunct to medications with occasional modest, short-lived side effects. Still, the database is small and largely anecdotal. A thorough, dose-related assessment of side effects remains to be conducted.