Acute tryptophan depletion in bulimia: effects on large neutral amino acids

Acute tryptophan depletion alters affective touch perception

RATIONALE

Affiliative tactile interactions help regulate physiological arousal and confer resilience to acute and chronic stress. C-tactile afferents (CTs) are a population of unmyelinated, low threshold mechanosensitive cutaneous nerve fibres which respond optimally to a low force stimulus, moving at between 1 and 10 cm/s. As CT firing frequencies correlate positively with subjective ratings of touch pleasantness, they are hypothesised to form the first stage of encoding affiliative tactile interactions. Serotonin is a key modulator of social responses with known effects on bonding.

OBJECTIVES

The aim of the present study was to determine the effect of acutely lowering central serotonin levels on perceptions of CT-targeted affective touch.

METHODS

In a double blind, placebo-controlled design, the effect of acute tryptophan depletion (ATD) on 25 female participants' ratings of directly and vicariously experienced touch was investigated. Psychophysical techniques were used to deliver dynamic tactile stimuli; some velocities were targeted to optimally activate CTs (1-10 cm/s), whereas other, faster and slower strokes fell outside the CT optimal range. Discriminative tactile function, cold pain threshold and tolerance were also measured.

RESULTS

ATD significantly increased pleasantness ratings of both directly and vicariously experienced affective touch, increasing discrimination of the specific hedonic value of CT targeted velocities. While ATD had no effect on either tactile or cold pain thresholds, there was a trend for reduced tolerance to cold pain.

CONCLUSIONS

These findings are consistent with previous reports that depletion of central serotonin levels modulates neural and behavioural responsiveness to appetitive sensory signals.

Effects of Acute Tryptophan Depletion on Human Taste Perception

Taste perception has been reported to vary with changes in affective state. Distortions of taste perception, including blunted recognition thresholds, intensity, and hedonic ratings have been identified in those suffering from depressive disorders. Serotonin is a key neurotransmitter implicated in the etiology of anxiety and depression; systemic and peripheral manipulations of serotonin signaling have previously been shown to modulate taste detection. However, the specific effects of central serotonin function on taste processing have not been widely investigated. Here, in a double-blind placebo-controlled study, acute tryptophan depletion was used to investigate the effect of reduced central serotonin function on taste perception. Twenty-five female participants aged 18-28 attended the laboratory on two occasions at least 1 week apart. On one visit, they received a tryptophan depleting drink and on the other, a control drink was administered. Approximately, 6 h after drink consumption, they completed a taste perception task which measured detection thresholds and supra-threshold perceptions of the intensity and pleasantness of four basic tastes (sweet, sour, bitter, and salt). While acutely reducing central levels of serotonin had no effect on the detection thresholds of sweet, bitter, or sour tastes, it significantly enhanced detection of salt. For supra-threshold stimuli, acutely reduced serotonin levels significantly enhanced the perceived intensity of both bitter and sour tastes and blunted pleasantness ratings of bitter quinine. These findings show manipulation of central serotonin levels can modulate taste perception and are consistent with previous reports that depletion of central serotonin levels enhances neural and behavioral responsiveness to aversive signals.

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.

Better to give than to take? Interactive social decision-making in severe major depressive disorder

BACKGROUND

Although recent studies focusing on major depressive disorder (MDD) suggest altered social decision-making, studies using the Ultimatum Game (UG) in patients with severe, clinical MDD do not exist. Moreover, all aforementioned studies so far focused on responder behavior and thus fairness considerations; to this date, no one investigated social interactive behavior which involves proposer behavior possibly requiring second-order mentalizing as well.

METHODS

Thirty-nine MDD patients and 22 healthy controls played a modified UG, both in the roles of responder and proposer against the same partner.

RESULTS

MDD patients accepted both fair and unfair offers as many times as the healthy controls in their role as responder. Importantly, however, in the role of proposer MDD patients offered significantly more than the control group did.

LIMITATIONS

Most patients were treated with psychotropic medication.

CONCLUSIONS

Responder behavior demonstrates that MDD patients are capable of making social decisions on fairness considerations in the same way as healthy controls do. The observed proposer behavior, however, could indicate that MDD patients are more focused on avoiding rejection. These findings provide unique evidence that social decision-making--as studied in a realistic context--is disturbed in MDD.

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.

Equivalent effects of acute tryptophan depletion on REM sleep in ecstasy users and controls

INTRODUCTION

This study sought to test the association between 3,4-methylenedioxymethamphetamine use, serotonergic function and sleep.

MATERIALS AND METHODS

Ambulatory polysomnography was used to measure three nights sleep in 12 ecstasy users and 12 controls after screening (no intervention), a tryptophan-free amino acid mixture (acute tryptophan depletion (ATD)) and a tryptophan-supplemented control mixture.

RESULTS

ATD significantly decreased rapid eye movement (REM) sleep onset latency, increased the amount of REM sleep and increased the amount of stage 2 sleep in the first 3 h of sleep. There was no difference between ecstasy users' and controls' sleep on the screening night or after ATD.

DISCUSSION

These findings imply that the ecstasy users had not suffered significant serotonergic damage as indexed by sleep.

L-Tryptophan: Basic Metabolic Functions, Behavioral Research and Therapeutic Indications

An essential component of the human diet, L-tryptophan is critical in a number of metabolic functions and has been widely used in numerous research and clinical trials. This review provides a brief overview of the role of L-tryptophan in protein synthesis and a number of other metabolic functions. With emphasis on L-tryptophan's role in synthesis of brain serotonin, details are provided on the research uses of L-tryptophan, particularly L-tryptophan depletion, and on clinical trials that have been conducted using L-tryptophan supplementation. The ability to change the rates of serotonin synthesis in the brain by manipulating concentrations of serum tryptophan is the foundation of much research. As the sole precursor of serotonin, experimental research has shown that L-tryptophan's role in brain serotonin synthesis is an important factor involved in mood, behavior, and cognition. Furthermore, clinical trials have provided some initial evidence of L-tryptophan's efficacy for treatment of psychiatric disorders, particularly when used in combination with other therapeutic agents.

Diminished central nervous 5-HT neurotransmission and mood self-ratings in children and adolescents with ADHD: no clear effect of rapid tryptophan depletion

INTRODUCTION

Research on 5-HT-functioning in adult patients and healthy subjects using rapid tryptophan depletion (RTD) has indicated weak but stable effects on mood ratings. Altered mood in children and adolescents with attention-deficit/hyperactivity disorder (ADHD) can confound the differential diagnosis between severe ADHD and mood disorders such as pediatric bipolar disorder. The present study investigated the effects of RTD induced lowered central nervous 5-HT-levels on mood self-ratings in children with ADHD.

METHODS

Seventeen boys with ADHD participated in the study in a double-blind within-subject crossover-design. They were administered RTD within an amino acid drink lacking tryptophan, thus lowering central nervous 5-HT-synthesis. On another day they received a placebo. Self-rated mood was assessed on both days at baseline conditions and at three different post-drink time-points.

RESULTS

RTD had no clear effect on mood within the whole sample. Low scorers on venturesomeness were more strongly affected by RTD in terms of feelings of inactivity and negative feelings compared to high venture patients.

CONCLUSIONS

Our data did not show a significant effect of RTD on mood self-ratings. However, the findings must be considered as preliminary and require further replication, in particular as they could be due to sampling bias.

Serotonin-induced decrease of intracellular Ca(2+) release in platelets of bulimic patients normalizes during treatment

Numerous symptoms related to eating disorders have been shown to be influenced by serotonergic (5-HT) functioning, with the 5-HT(2A) receptor subtype being one of the most relevant involved in the pathophysiology of bulimia nervosa (BN). In line with this, Ca(2+) mobilization as mediated by 5-HT(2) receptors in platelets was shown to serve as a peripheral model for central nervous 5-HT functioning. Here, the 5-HT-induced intracellular Ca(2+) mobilization in platelets was measured in 13 female normal weight bulimic patients (14-18 years) upon admission and at the end of inpatient treatment. Findings were compared to 21 age-matched healthy female adolescents. 5-HT-induced Ca(2+) release was significantly decreased in bulimic patients upon admission and normalized during inpatient treatment. Antidepressive medication caused a significant improvement. The data provide further evidence that altered 5-HT(2) receptor functioning is involved in the pathophysiological underpinnings in BN.

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.

Mood is indirectly related to serotonin, norepinephrine and dopamine levels in humans: a meta-analysis of monoamine depletion studies

Dysfunction in the monoamine systems of serotonin (5-HT), norepinephrine (NE) and dopamine (DA) may causally be related to major depressive disorder (MDD). Monoamine depletion studies investigate the direct effects of monoamines on mood. Acute tryptophan depletion (ATD) or para-chlorophenylalanine (PCPA) deplete 5-HT, acute phenylalanine/tyrosine depletion (APTD) or alpha-methyl-para-tyrosine (AMPT) deplete NE/DA. Available depletion studies found conflicting results in heterogeneous populations: healthy controls, patients with previous MDD in remission and patients suffering from MDD. The decrease in mood after 5-HT and NE/DA depletion in humans is reviewed and quantified. Systematic search of MEDLINE and EMBASE (1966-October 2006) and cross-references was carried out. Randomized studies applying ATD, PCPA, APTD or AMPT vs control depletion were included. Pooling of results by meta-analyses was stratified for studied population and design of the study (within or between subjects). Seventy-three ATD, 2 PCPA, 10 APTD and 8 AMPT studies were identified of which 45 ATD and 8 APTD studies could be meta-analyzed. 5-HT or NE/DA depletion did not decrease mood in healthy controls. 5-HT or NE/DA depletion slightly lowered mood in healthy controls with a family history of MDD. In drug-free patients with MDD in remission, a moderate mood decrease was found for ATD, without an effect of APTD. ATD induced relapse in patients with MDD in remission who used serotonergic antidepressants. In conclusion, monoamine depletion studies demonstrate decreased mood in subjects with a family history of MDD and in drug-free patients with MDD in remission, but do not decrease mood in healthy humans. Although depletion studies usefully investigate the etiological link of 5-HT and NE with MDD, they fail to demonstrate a causal relation. They presumably clarify a vulnerability trait to become depressed. Directions for further investigation of this vulnerability trait are proposed.

Relação da síndrome do excesso de treinamento com estresse, fadiga e serotonina

A grande exigência do esporte competitivo tem provocado sérias conseqüências em atletas envolvidos em treinamento de alto nível. Por sua vez, a mudança dos padrões estéticos tem levado indivíduos a buscarem, por meio do exercício físico, a redução da massa corporal, o aumento da massa muscular e do condicionamento aeróbio. É comum atletas e não atletas excederem os limites de suas capacidades físicas e psicológicas ocasionando o desenvolvimento da síndrome do excesso de treinamento (overtraining), a qual é definida como um distúrbio neuroendócrino (hipotálamo-hipofisário) que resulta do desequilíbrio entre a demanda do exercício e a possibilidade de assimilação de treinamento, acarretando alterações metabólicas, com conseqüências que abrangem não apenas o desempenho, mas também outros aspectos fisiológicos e emocionais. Altos índices de estresse físico, sócio-cultural e psíquico são fatores que colaboram com o seu aparecimento, bem como alterações neuroendócrinas provocadas por aspectos nutricionais, que levam a flutuações serotoninérgicas. Alterações nos níveis de serotonina cerebral podem ser associadas ao aparecimento do estado de fadiga física, que pode se estabelecer de forma crônica, constituindo-se um dos sintomas do quadro. Deficiências ou desequilíbrios em neurotransmissores e neuromoduladores também podem ser causados por severo ou longo estresse. O objetivo do presente artigo de revisão é fazer uma análise dos fatores que contribuem de forma sinérgica para o aparecimento da síndrome do excesso de treinamento.

Acute tryptophan depletion. Part I: rationale and methodology

OBJECTIVE

Acute tryptophan depletion (ATD) is an experimental technique that has been widely used over the last decade to investigate the role of serotonin (5-HT) in a variety of disorders. This review, the first of two articles, describes the rationale behind this technique and provides detail on how it is applied in research settings.

METHOD

The authors outline the development of this technique with reference to the seminal literature and more recent findings from neuroimaging and neuroendocrine studies. This is supplemented by the authors' clinical experience of over 5 years of continuous experimental work with this paradigm in over 50 subjects.

RESULTS

Acute tryptophan depletion is a method that significantly reduces central 5-HT in human subjects. Non-serotonergic explanations of the effects of ATD have not been confirmed, supporting the specificity of this method.

CONCLUSIONS

The ATD technique is a valid method of manipulating central 5-HT levels. The second article in this series will review the application of ATD in depression, anxiety and other psychiatric conditions.

Acute tryptophan depletion. Part II: clinical effects and implications

OBJECTIVE

This is the second of two articles reviewing the modern dietary technique of acute tryptophan depletion (ATD), a method of transiently reducing central serotonin levels in both healthy volunteers and clinical populations. This article details the clinical studies to date and discusses the implications of this research methodology.

METHOD

The authors present a review of clinical ATD studies collated from a MEDLINE search, unpublished communications and the authors' considerable experience with this paradigm.

RESULTS

Following from the initial use of ATD in subjects with depressive illness, studies of anxiety disorders and other psychiatric illnesses have been reported. Sleep, aggressive and cognitive effects are also active areas of research and are reviewed here.

CONCLUSIONS

Acute tryptophan depletion remains a useful psychiatric research tool. The findings from the clinical studies reviewed here are summarized and implications for future research detailed.

The effects of high-dose and low-dose tryptophan depletion on mood and cognitive functions of remitted depressed patients

It has frequently been demonstrated that acute tryptophan depletion (ATD) induces a transient depressed mood in some patients who are in remission from depression. However, the effects of ATD on cognitive processes in remitted depressed patients have not been investigated. The aim of the present study was to investigate the effects of different extents of depletion on mood and cognitive tasks involving neutral and emotional stimuli. Twenty patients in remission or in partial remission from depression received ATD in a double-blind, crossover design. Mood was assessed at both sessions before, at +6.5 h and +24 h after depletion. Cognitive assessment in both sessions started at +4.75 h, and also before and after the whole procedure. The ATD mixtures induced the expected reductions of plasma tryptophan levels. High-dose ATD induced a depressive response in a subsample of patients and impaired the processing of positive information independent of mood change. Attention for neutral stimuli (Stroop interference) improved in a dose-dependent manner. ATD may affect mood and cognition via different pathways: one implicated in mood regulation and the processing of emotional information, and one for the processing of neutral information. The first pathway may be more important for discriminating vulnerability to impaired serotonin function. The comparison of the effects of high-dose and low-dose ATD is useful for those studies aiming to investigate the relationships among 5-HT, mood and cognition.

Eating disorders: clinical features and pathophysiology

Anorexia nervosa (AN) and bulimia nervosa (BN) are disorders of eating and weight-related behavior that together afflict some 1-3% of women in the United States. One of the remarkable features about each of the eating disorders is how persistent the disordered eating behavior becomes once it has begun. Substantial psychological, social, and physiological disturbances are associated with eating disorders, and it has been very difficult to disentangle those factors that may result from the disturbed behavior from the factors that may have predisposed individuals to, or precipitated the development of, the disorder. This article will briefly review the definitions, phenomenology, and identified risk factors for development of each of the major eating disorders. Pathophysiology will be discussed, with a particular focus on candidate factors that might sustain disordered eating behavior, as informed by clinical and basic science research. Future research directions will be suggested.

The role of serotonin in panic: evidence from tryptophan depletion studies

There has been growing interest in the role of serotonin (5-hydroxytryptamine, 5-HT) in anxiety, including pathological states such as panic disorder. The technique of tryptophan depletion (TD), which causes an acute, temporary and reversible reduction in brain 5-HT levels, is a useful minimally invasive paradigm to aid the research of the role of 5-HT in various disorders. This review discusses the evidence supporting the hypothesis that 5-HT function is of importance in the neurobiology of panic disorder and considers in more detail how our understanding has been influenced by work using the technique of TD. Possible avenues for future research are also discussed.

Coming to Terms With Risk Factors for Eating Disorders: Application of Risk Terminology and Suggestions for a General Taxonomy

The aims of the present review are to apply a recent risk factor approach (H. C. Kraemer et al., 1997) to putative risk factors for eating disorders, to order these along a timeline, and to deduce general taxonomic questions. Putative risk factors were classified according to risk factor type, outcome (anorexia nervosa, bulimia nervosa, binge-eating disorder, full vs. partial syndromes), and additional factor characteristics (specificity, potency, need for replication). Few of the putative risk factors were reported to precede the onset of the disorder. Many factors were general risk factors; only few differentiated between the 3 eating disorder syndromes. Common risk factors from longitudinal and cross-sectional studies were gender, ethnicity, early childhood eating and gastrointestinal problems, elevated weight and shape concerns, negative self-evaluation, sexual abuse and other adverse experiences, and general psychiatric morbidity. Suggestions are made for the conceptualization of future risk factor studies.

Monoamine depletion in psychiatric and healthy populations: review

A number of techniques temporarily lower the functioning of monoamines: acute tryptophan depletion (ATD), alpha-methyl-para-tyrosine (AMPT) and acute phenylalanine/tyrosine depletion (APTD). This paper reviews the results of monoamine depletion studies in humans for the period 1966 until December 2002. The evidence suggests that all three interventions are specific, in terms of their short-term effects on one or two neurotransmitter systems, rather than on brain protein metabolism in general. The AMPT procedure is somewhat less specific, affecting both the dopamine and norepinephrine systems. The behavioral effects of ATD and AMPT are remarkably similar. Neither procedure has an immediate effect on the symptoms of depressed patients; however, both induce transient depressive symptoms in some remitted depressed patients. The magnitude of the effects, response rate and quality of response are also comparable. APTD has not been studied in recovered major depressive patients. Despite the similarities, the effects are distinctive in that ATD affects a subgroup of recently remitted patients treated with serotonergic medications, whereas AMPT affects recently remitted patients treated with noradrenergic medications. The evidence also suggests that ATD and APTD affect different cognitive functions, in particular different memory systems. Few studies investigated cognitive effects of the procedures in patients. Patients who are in remission for longer may also be vulnerable to ATD and AMPT, but the relationship with prior treatment is much weaker. For these patients, individual vulnerability markers are the more important determinants of depressive response, making these techniques potentially useful models of vulnerability to depression.

Anxiolytic effects of acute tryptophan depletion in anorexia nervosa

OBJECTIVE

Recent studies have raised the question as to whether a dysregulation of the neurotransmitter serotonin may contribute to the alterations in mood seen in anorexia nervosa (AN). People with AN tend to be anxious, obsessional, perfectionistic, and harm avoidant. These traits are premorbid and persist after recovery. It has been suggested that increased activity of brain serotonin systems could contribute to this pathologic condition. Dieting in AN, which serves to reduce plasma levels of tryptophan (TRP), may serve to reduce symptoms of dysphoric mood.

METHOD

Fourteen women currently symptomatic with AN (ILL AN), 14 women recovered from AN (REC AN), and 15 healthy control women (CW) underwent acute tryptophan depletion (ATD). Measures of psychological state were self-assessed at baseline and hourly after ATD to determine whether ATD would reduce negative mood.

RESULTS

ILL AN and REC AN had significantly higher mean baseline TRP/LNAA (tryptophan/large neutral amino acids) ratios compared with CW. In contrast to placebo, the ATD challenge demonstrated a significantly greater reduction in the TRP/LNAA ratio for ILL AN (-95%) and REC AN (-84%) compared with CW (-70 %). Both the ILL AN and REC AN had a significant reduction in anxiety on the ATD day compared with the placebo day.

DISCUSSION

These data demonstrate that a dietary-induced reduction of TRP, the precursor of serotonin, is associated with decreased anxiety in people with AN. Restricting dietary intake may represent a mechanism through which individuals with AN modulate a dysphoric mood.

Milnacipran in the treatment of bulimia nervosa: a report of 16 cases

Controlled trials in patients with bulimia nervosa have demonstrated efficacy of antidepressant medications with serotonergic function (e.g. fluoxetine) as well as noradrenergic function (e.g. desipramine). Sixteen out-patients with bulimia nervosa according to DSM-IV criteria were treated in a drug surveillance with 100 mg of milnacipran, a specific serotonin and noradrenaline reuptake inhibitor (SNRI). Ten patients completed the 8-week observation period. The reasons for premature attrition were improvement in one patient (no. 12), a generalized exanthema in one patient (no. 7), severe nausea in one patient (no. 8) and non-compliance due to non-drug-related reasons in three patients (no. 1, 2, and 16). An intent-to-treat analysis exhibited a significant reduction in weekly binge eating and vomiting frequency from baseline to the end of treatment. Three patients stopped binge eating and purging completely during the last week of treatment. Furthermore, there was a concomitant decrease of depression ratings (HAMD, BDI). Our preliminary data give rise to the notion that milnacipran may be promising in the treatment of bulimia nervosa.

Effects of tryptophan depletion on central and peripheral chemoreflexes in man

Klein (Arch. Gen. Psychiatry 50, 306-317, 1993) suggests that panic attacks are the result of a defective 'suffocation alarm' threshold that presents with carbon dioxide (CO(2)) hypersensitivity, exaggerated ventilatory response and panic in panic disorder (PD) patients. Serotonergic deficiencies enhance this ventilatory response in PD patients, as per 'suffocation alarm' theory predictions, suggesting that serotonin (5-HT) normalizes the ventilatory response. Other research supports a serotonin system-mediated stimulation of ventilation. Knowledge of 5-HT's role on ventilatory output and its neurophysiological sources impacts on the 'suffocation alarm' theory validity and predictive value. We used tryptophan depletion (TRP-) in concert with a modified Read rebreathing test to determine the effect of deficient serotonergic modulation on the central and peripheral chemoreflex threshold and sensitivity of response to CO(2) in 11 healthy men. TRP- did not affect central or peripheral chemoreflex threshold or sensitivity of response to CO(2). However, basal ventilation was significantly elevated during TRP-. In contrast to 'suffocation alarm' theory predictions, decreased 5-HT neurotransmission does not significantly affect the respiratory chemoreflex response to CO(2), impacting on non-chemoreflex drives to breathe. Panic associated respiratory abnormalities may be related to defective 5-HT modulation of non-chemoreflex drives to breathe, unrelated to any respiratory chemoreflex abnormality.

The effects of tryptophan depletion on mood and psychiatric symptoms

BACKGROUND

The number of studies using tryptophan depletion (TD) challenge has increased markedly in the past few years. Recently, a number of negative results have been published, implicating that the effect of TD on mood may be less consistent than previously thought.

METHODS

The literature on the mood effects of TD in psychiatric patients and healthy volunteers was reviewed.

RESULTS

TD has a mood-lowering effect in subgroups of recovered depressed patients, patients with seasonal affective disorder and vulnerable healthy subjects. The mood effect in former patients is of a different quality, however, than the effect in healthy subjects. Some recent negative studies in depression might be explained by insufficient lowering of plasma tryptophan levels. Preliminary evidence exists for an effect of TD on bulimia nervosa, autism, aggression and substance dependence.

CONCLUSIONS

The effects of TD on mood may be more consistent than suggested by a number of recent negative studies. Response to TD in recovered depressed patients is associated with prior treatment. However, even in SSRI-treated patients the relapse rates are not higher than 50-60%, which needs to be explained. The clinical usefulness of the response to TD in recovered patients (prediction of relapse after treatment discontinuation) and in symptomatic patients (prediction of treatment refractoriness) deserves more research attention. Further suggestions for future research include the cognitive effects of TD in recovered depressed patients and the effect of dietary habits on response to TD.

Can nutrient supplements modify brain function?

Over the past 40 y, several lines of investigation have shown that the chemistry and function of both the developing and the mature brain are influenced by diet. Examples are the effect of folate deficiency on neural tube development during early gestation, the influence of essential fatty acid deficiency during gestation and postnatal life on the development of visual function in infants, and the effects of tryptophan or tyrosine intake (alone or as a constituent of dietary protein) on the production of the brain neurotransmitters derived from them (serotonin and the catecholamines, respectively). Sometimes the functional effects are clear and the underlying biochemical mechanisms are not (as with folate and essential fatty acids); in other cases (such as the amino acids tyrosine and tryptophan), the biochemical effects are well understood, whereas the effect on brain function is not. Despite the incomplete knowledge base on the effects of such nutrients, investigators, physicians, and regulatory bodies have promoted the use of these nutrients in the treatment of disease. Typically, these nutrients have been given in doses above those believed to be required for normal health; after they have been given in pure form, unanticipated adverse effects have occasionally occurred. If this pharmacologic practice is to continue, it is important from a public safety standpoint that each nutrient be examined for potential toxicities so that appropriate purity standards can be developed and the risks weighed against the benefits when considering their use.

5-HT and human anxiety. Evidence from studies using acute tryptophan depletion

There is abundant evidence that serotonin (5-hydroxytryptamine, 5-HT) is involved in anxiety in both animals and humans but there is conflicting evidence for the precise role it plays. Acute tryptophan depletion provides a technique for investigating a global reduction in brain 5-HT function and we have investigated its effect on anxiety in drug-free panic disorder patients and normal volunteers. We found little effect on general levels of anxiety but it enhanced the effect of a panic challenge using 5% carbon dioxide (5%CO2) in panic disorder patients. The effect in normal volunteers was less clear with no overall effect following 5%CO2 challenge or the psychological challenge of a simulated public speaking task. These results are discussed in relation to the literature and are broadly supportive of the hypothesis that 5-HT acts to inhibit panic anxiety at the level of the periaqueductal grey but facilitates general and conditioned anxiety at the level of medial temporal lobe structures.

Effects of tryptophan depletion on carbon dioxide provoked panic in panic disorder patients

Results of an earlier study in healthy volunteers suggest that the serotonergic system is involved in anxiety-related mechanisms. We studied the influence of tryptophan depletion on the response to a 35% carbon dioxide challenge. Twenty-four panic disorder patients received a mixture of amino acids, either with or without tryptophan, under double-blind conditions. There was a significant increase in anxiety as well as in neurovegetative symptoms in the depletion group, compared to the placebo condition. Furthermore, when we compare the results of the placebo group with earlier panic provocation studies, it also seems that a balanced amino acid mixture might have a protective effect against a panic provocation. We conclude that the panic-enhancing effect of tryptophan depletion as well as the potential protective effect of tryptophan administration in panic disorder patients can be explained by the Deakin-Graeff theory of anxiety.

Tryptophan depletion impairs memory consolidation but improves focussed attention in healthy young volunteers

Animal and human studies have provided evidence for serotonergic modulation of cognitive processes. However, the exact nature of this relationship is not clear. We used the acute tryptophan depletion (ATD) method to investigate the effects of lowered serotonin synthesis on cognitive functions in 17 healthy young volunteers. The study was conducted according to a placebo-controlled, double-blind, crossover design. Cognitive performance and mood were assessed at baseline and 5 and 9 h after administration of ATD. A specific impairment of word recognition, without effects on short-term memory, occurred during ATD. No memory deficits were seen if ATD was induced after acquisition of new words. The Stroop Test and dichotic listening task demonstrated a modality independent improvement of focussed attention after ATD. Fluency was also improved after ATD. ATD did not alter speed of information processing, divided attention or planning functions. These results indicate that serotonin is essential in the process of long-term memory consolidation, primarily in the first 30 min after acquisition. Improvement of specific cognitive processes by lowered 5-HT function may be linked to the removal of inhibitory actions of 5-HT in the cortex.

Effect of acute tryptophan depletion on CO2-induced anxiety in patients with panic disorder and normal volunteers

BACKGROUND

Uncertainties remain about the role of serotonin in the aetiology and treatment of panic disorder.

AIMS

To investigate the effect of reducing brain serotonin function on anxiety at rest, and following 5% CO2 provocation in normal controls and patients with panic disorder.

METHOD

Twenty drug-free patients with DSM-III-R panic disorder and 19 controls received a tryptophan-free amino acid drink on one occasion and a control drink on the other in a double-blind, balanced protocol. 5% CO2 was given as a panic challenge after 270 minutes.

RESULTS

Plasma tryptophan fell by more than 80% both patients and controls after the tryptophan-free drink. Tryptophan depletion did not alter resting anxiety. In patients alone, tryptophan depletion caused a greater anxiogenic response and an increased rate of panic attacks (9 v. 2, P < 0.05) after 5% CO2 challenge. No normal volunteers panicked.

CONCLUSIONS

Serotonin may directly modulate panic anxiety in patients with panic disorder. This may underlie the efficacy of serotonergic antidepressants in treating panic disorder.

Effects of acute tryptophan depletion on mood in bulimia nervosa

BACKGROUND

The present study investigated the role of serotonin in the pathophysiology of bulimia nervosa (BN) by studying the affective and appetitive responses of women ill with BN to an acute tryptophan depletion (ATD) paradigm.

METHODS

Twenty-two women with BN and 16 healthy control women (CW) were studied on 2 separate days during the follicular stage of the menstrual cycle. Participants drank a control mix of essential amino acids (100 g + 4.6 g tryptophan) on one day and a tryptophan deficient (100 g - 4.6 g tryptophan) mixture (ATD) on the other in a double-blind fashion. Mood/appetite ratings and blood samples were taken at baseline and at intervals up to 420 minutes. Participants were then presented with an array of foods and were allowed to binge and vomit if they desired.

RESULTS

CW and BN women had a similar and significant reduction in plasma tryptophan levels and the tryptophan: LNAA ratio after ATD. After ATD, the BN women had a significantly greater increase in peak (minus baseline) depression, mood lability, sadness and desire to binge compared to the CW. BN subjects and CW had similar peak changes in mood after the control amino acid mixture. BN subjects and CW consumed similar amounts of food after the two amino acid treatments.

CONCLUSIONS

Women with BN seem more vulnerable to the mood lowering effects of ATD, suggesting they have altered modulation of central 5-HT neuronal systems.

Mood effects of 24-hour tryptophan depletion in healthy first-degree relatives of patients with affective disorders

BACKGROUND

Acute tryptophan (TRP) depletion was evaluated in healthy volunteers with or without a family history of major affective disorder (FH+ versus FH-).

METHODS

Twenty-seven subjects (16 FH+, 11 FH-) received 100 g of an amino acid mixture with and without TRP according to a placebo-controlled, double-blind cross-over design and a diet devoid of TRP for the next 24 hours.

RESULTS

The ratio TRP/large neutral amino acids declined to 22% of baseline values after 6 hours, and increased during the night reaching 85% of baseline after 24 hours. Overall, after 6 hours, TRP depletion lead to a lowering of mood, but after 24 hours, these changes were no longer detected. Mood changes and gastrointestinal side effects were significantly more evident in FH+ subjects than in FH- subjects.

CONCLUSIONS

Our data support the hypothesis that subjects with a positive family history for depression are predisposed to increased vulnerability to the adverse consequences of serotonergic imbalance.

Serotonin neuronal function and selective serotonin reuptake inhibitor treatment in anorexia and bulimia nervosa

Anorexia nervosa (AN) and bulimia nervosa (BN) are disorders characterized by aberrant patterns of feeding behavior and weight regulation, and disturbances in attitudes toward weight and shape and the perception of body shape. Emerging data support the possibility that substantial biologic and genetic vulnerabilities contribute to the pathogenesis of AN and BN. Multiple neuroendocrine and neurotransmitter abnormalities have been documented in AN and BN, but for the most part, these disturbances are state-related and tend to normalize after symptom remission and weight restoration; however, elevated concentrations of 5-hydroxyindoleacetic acid in the cerebrospinal fluid after recovery suggest that altered serotonin activity in AN and BN is a trait-related characteristic. Elevated serotonin activity is consistent with behaviors found after recovery from AN and BN, such as obsessionality with symmetry and exactness, harm avoidance, perfectionism, and behavioral over control. In BN, serotonergic modulating antidepressant medications suppress symptoms independently of their antidepressant effects. Selective serotonin reuptake inhibitors (SSRIs) are not useful when AN subjects are malnourished and under-weight; however, when given after weight restoration, fluoxetine may significantly reduce the extremely high rate of relapse normally seen in AN. Nonresponse to SSRI medication in ill AN subjects could be a consequence of an inadequate supply of nutrients, which are essential to normal serotonin synthesis and function. These data raise the possibility that a disturbance of serotonin activity may create a vulnerability for the expression of a cluster of symptoms that are common to both AN and BN and that nutritional factors may affect SSRI response in depression, obsessive-compulsive disorder, or other conditions characterized by disturbances in serotonergic pathways.

Serotonergic 'vulnerability' in affective disorder: a study of the tryptophan depletion test and relationships between peripheral and central serotonin indexes in citalopram-responders

A double-blind study of the tryptophan depletion (TD) challenge was performed on a sample consisting of 20 patients with a major depressive disorder in clinical remission after citalopram treatment. TD was induced by the intake of 43 g of an amino acid mixture containing the five large neutral amino acids. The control group received the same mixture, to which 2.3 g tryptophan had been added. Five of the 12 challenged patients showed a worsening of depressive symptoms during the day of the test. In contrast, there was no mood alteration in the eight control patients. Baseline cortisol levels were significantly higher in responders to TD compared to those in non-responders and controls. Platelet serotonin-receptor function and plasma prolactin levels were correlated. There was a significant positive correlation in the baseline data between rated mood state and plasma cortisol and a significant inverse correlation between related mood state and plasma tryptophan concentration. Thus low mood appeared to be associated with low serotonin precursor availability as well as with high cortisol levels.

Rapid depletion of plasma tryptophan: a review of studies and experimental methodology

Evidence that the neurotransmitter serotonin (5-hydroxytryptamine, 5-HT) plays a role in the pathophysiology of mood disorders has been accumulating over the past three decades. Recent studies on this neurotransmitter have extended across the spectrum of psychiatric disorder, suggesting a role for 5-HT in psychosis, aggression, eating disorders and addiction. However, much of the evidence has come from post-mortem examination of the brain or measures of peripheral rather than central 5-HT function. The technique of tryptophan depletion allows investigation of brain 5-HT function in living subjects by examining the behavioural responses to this pharmacological challenge. This review considers the current status of tryptophan depletion as an experimental technique and discusses the implications of findings both in affective disorders and in a range of other psychiatric syndromes. MEDLINE and PSYCHLIT searches were completed for the years 1966 to November 1996 using the key words 'serotonin', '5-hydroxytryptamine', 'tryptophan' and 'depletion'. In addition relevant journals were hand-searched for the period from 1980 to December 1996. Forty-four double-blind studies in humans and three clinical case reports were identified; these cover a range of psychiatric disorders including mood disorders and psychoses, anxiety and eating disorders and specific behaviours such as appetite, aggression and craving. The studies reviewed utilized a variety of differing methodologies reducing the extent to which results can be generalized. A series of studies in depressed patients (before and after treatment with antidepressants) and their first-degree relatives have shown the importance of an intact 5-HT system in the action of antidepressants and offer new insights into the biology of affective disorder. The mood change induced by tryptophan depletion may predict those patients likely to respond to 5-HT-specific drugs. Rapid tryptophan depletion has also been reported to exacerbate both panic and aggression in vulnerable individuals. Effects in other disorders are conflicting and further research is needed to clarify these findings.

Effects of fluoxetine administration on mood response to tryptophan depletion in healthy subjects

Short-term reduction in plasma tryptophan (tryptophan depletion) produces a relapse of depressive symptoms in 60% of previously depressed patients recently recovered with serotonin reuptake inhibitor treatment. Tryptophan depletion does not consistently increase depressive symptoms in unmedicated depressed patients or in depressed patients whose symptoms are remitted with a norepinephrine reuptake inhibitor. These data suggest that serotonin reuptake inhibitor treatment itself may confer vulnerability to the development of depressive symptoms during tryptophan depletion. In order to further investigate this possibility, six healthy individuals underwent double-blind placebo-controlled tryptophan depletion before and following six weeks of treatment with fluoxetine 20 mg/day. No increased vulnerability to the mood-lowering effects of tryptophan depletion occurred as a result of fluoxetine treatment. Additionally, fluoxetine treatment itself was not associated with changes in mood or quality of life in these healthy volunteers. These data indicate that serotonin reuptake inhibitor treatment alone does not produce the depressive effects of tryptophan depletion that are observed in serotonin reuptake inhibitor-treated depressed and obsessive compulsive disorder patients.

Relapse of depression after rapid depletion of tryptophan

BACKGROUND

Major depression is a common disorder but the pathophysiology is poorly understood. Current hypotheses implicate deficient function of brain serotonin pathways because drugs that selectively increase brain serotonin activity are effective antidepressants. However, there is no direct evidence that lowered serotonin function causes major depression. We aimed to assess whether lowering of brain serotonin activity by depletion of its amino acid precursor, tryptophan, could provoke a short-term relapse of clinically significant symptoms in women vulnerable to major depressive disorder.

METHODS

We studied 15 women who had suffered recurrent episodes of major depression but had recovered and were no longer on drug treatment. Patients received two amino acid mixtures in a double-blind crossover design. One of the mixtures was nutritionally balanced and contained tryptophan and the other was identical except it contained no tryptophan. Participants were scored on the Hamilton rating scale for depression (HAMD) before and 7 h after drinking each mixture. They also completed hourly self-rated measures of mood during this period. Blood samples were also taken at baseline and 7 h for measurement of plasma tryptophan.

FINDINGS

The tryptophan-free mixture produced a 75% reduction in plasma tryptophan concentration. After drinking the tryptophan-free mixture, ten of the 15 women experienced temporary but clinically significant depressive symptoms. The mean difference in total HAMD scores (7 h minus baseline) were significantly higher after the tryptophan-free mixture than after the nutritionally balanced mixture (7.3 vs 0.15 [95% CI 4.5-9.9]; p < 0.001). No changes in mood were seen after taking the nutritionally balanced mixture.

INTERPRETATION

We conclude that rapid lowering of brain serotonin function can precipitate clinical depressive symptoms in well, untreated individuals who are vulnerable to major depressive disorder. The findings support a key role for deficient serotonin function in the aetiology of depression.

Acute tyrosine depletion reduces tyrosine hydroxylation rate in rat central nervous system

An amino acid cocktail was devised that would rapidly reduce central nervous system (CNS) tyrosine levels in rats following gastric intubation. The effect of this treatment on in vivo tyrosine hydroxylation rate was examined. Serum tyrosine (TYR) levels, the serum ratio of TYR to the sum of its transport competitors, and CNS TYR concentrations fell substantially within 60 minutes of intubation and remained low for at least 3 hr. In vivo tyrosine hydroxylation rate, evaluated in hypothalamus and retina 2 hr after amino acid intubation, also declined significantly. The results suggest that an amino acid mixture can be devised that will cause an acute reduction in TYR levels and hydroxylation rate in rat CNS. This procedure may ultimately prove applicable to humans to examine functional consequences in particular CNS regions of reducing neuronal catecholamine synthesis.

Serotonergic mechanisms of cocaine effects in humans

The objective of this study was to investigate the role of serotonin (5-HT) in mediating the effects of cocaine in humans. To accomplish this, 12 subjects each participated in two randomized, double-blind test sessions separated by 1 week. In one session, subjects underwent acute depletion of the 5-HT amino acid precursor tryptophan (TRP), followed by a test dose of intranasal cocaine. In the other session, the cocaine test dose was preceded by sham depletion. Subject ratings of cocaine "high" were significantly lower following active TRP depletion than after the sham procedure. Subjects also showed an earlier but less sustained rise in self-rated nervousness during active TRP depletion. These findings are consistent with the hypothesis that 5-HT may be involved in mediating the euphorigenic and modulating the anxiogenic effects of cocaine in humans, either directly or through actions on other (e.g., dopaminergic) systems.

Comparison of the effects of amino acid mixture and placebo on plasma tryptophan to large neutral amino acid ratio

To assess the possible role of altered central serotonin function in psychiatric disorders, investigators have utilized pharmacological challenge testing with an amino acid mixture to decrease blood tryptophan concentration and, indirectly, brain serotonin levels. The aim of this pilot study was to assess the effectiveness of a modified amino mixture, administered in capsule form, in decreasing plasma tryptophan levels. Studies were conducted in six healthy, medication-free female volunteers. Following double-blind, randomized, cross-over design, subjects received on separate days capsules containing a tryptophan-free amino acid mixture (31.5 grams) or lactose placebo. Over the six hours following amino acid administration, plasma tryptophan concentrations decreased to 21% of baseline values, while the tryptophan/large neutral amino acid ratio decreased to 6% of baseline. Subjects reported minimal symptoms of nausea or other side effects following amino acid administration. The results suggest that the modified amino acid mixture may be useful in assessing behavioral responses to acute tryptophan depletion challenge testing.