Depressive relapse following acute tryptophan depletion in patients with major depressive disorder

Organic cation transporter 2 contributes to SSRI antidepressant efficacy by controlling tryptophan availability in the brain

Selective serotonin reuptake inhibitors (SSRI) are common first-line treatments for major depression. However, a significant number of depressed patients do not respond adequately to these pharmacological treatments. In the present preclinical study, we demonstrate that organic cation transporter 2 (OCT2), an atypical monoamine transporter, contributes to the effects of SSRI by regulating the routing of the essential amino acid tryptophan to the brain. Contrarily to wild-type mice, OCT2-invalidated mice failed to respond to prolonged fluoxetine treatment in a chronic depression model induced by corticosterone exposure recapitulating core symptoms of depression, i.e., anhedonia, social withdrawal, anxiety, and memory impairment. After corticosterone and fluoxetine treatment, the levels of tryptophan and its metabolites serotonin and kynurenine were decreased in the brain of OCT2 mutant mice compared to wild-type mice and reciprocally tryptophan and kynurenine levels were increased in mutants' plasma. OCT2 was detected by immunofluorescence in several structures at the blood-cerebrospinal fluid (CSF) or brain-CSF interface. Tryptophan supplementation during fluoxetine treatment increased brain concentrations of tryptophan and, more discreetly, of 5-HT in wild-type and OCT2 mutant mice. Importantly, tryptophan supplementation improved the sensitivity to fluoxetine treatment of OCT2 mutant mice, impacting chiefly anhedonia and short-term memory. Western blot analysis showed that glycogen synthase kinase-3β (GSK3β) and mammalian/mechanistic target of rapamycin (mTOR) intracellular signaling was impaired in OCT2 mutant mice brain after corticosterone and fluoxetine treatment and, conversely, tryptophan supplementation recruited selectively the mTOR protein complex 2. This study provides the first evidence of the physiological relevance of OCT2-mediated tryptophan transport, and its biological consequences on serotonin homeostasis in the brain and SSRI efficacy.

No effects of acute tryptophan depletion on anxiety or mood in weight-recovered female patients with anorexia nervosa

BACKGROUND

Previous studies have suggested that individuals recovered from anorexia nervosa (AN) are characterized by increased serotonergic (5-HT) activity that might be related to elevated levels of anxiety. Assuming these traits to be also present in individuals at risk for AN, it was further hypothesized that restricting food intake might be a means to temporarily alleviate dysphoric affective states by reducing central nervous availability of tryptophan (TRP), the sole precursor of 5-HT. One study that supported this hypothesis found anxiolytic effects in individuals with a history of AN during an experimentally induced short-term depletion of TRP supply to the brain.

METHODS

In this placebo-controlled, double-blind cross-over study, 22 patients weight-recovered from AN (recAN) and 25 healthy control participants (HC) completed questionnaires assessing anxiety and momentary mood during acute tryptophan depletion (ATD), a dietary intervention that lowers central 5-HT synthesis.

RESULTS

The ATD procedure effectively reduced the ratio of TRP to competing for large neutral amino acids in the peripheral blood, indicating decreased TRP supply to the brain. Effects of ATD on anxiety and mood did not differ between recAN and HC. Bayesian null hypothesis testing confirmed these initial results.

DISCUSSION

Our results do not support the hypothesis that short-term depletion of TRP and its impact on the brain 5-HT reduces anxiety or improves mood in AN. As the evidence for the role of 5-HT dysfunction on affective processes in patients with AN is limited, further studies are needed to assess its relevance in the pathophysiology of AN.

Alterations in Fecal Microbiomes and Serum Metabolomes of Fatigued Patients With Quiescent Inflammatory Bowel Diseases

BACKGROUND & AIMS

Fatigue is frequent and disabling in patients with inflammatory bowel diseases (IBD) but its mechanisms are poorly understood. We investigated alterations in fecal microbiomes and serum metabolomes and proteomes in patients with quiescent IBD, with vs without fatigue.

METHODS

We performed a prospective observational study of patients (44% women; mean age, 39.8 y) with clinically and endoscopically quiescent Crohn's disease (n = 106) or ulcerative colitis (n = 60) at a tertiary hospital, from March 2016 through December 2018. Fatigue was assessed using the functional assessment of chronic illness therapy-fatigue scoring system and defined as a score of 43 or less. We performed metabolomic analysis of serum samples using liquid chromatography-mass spectrometry methods and proteomic analysis using multiplex proximity extension assay (PEA) technology. Stool samples were obtained from 50 patients and analyzed by shotgun metagenomic sequencing on Illumina HiSeq platform.

RESULTS

Of the 166 study participants, 91 (55%) were fatigued. Serum samples from patients with fatigue (n = 59) did not have significant increases in levels of inflammatory cytokines compared with serum samples from nonfatigued patients (n = 72). We found a statistically significant difference in a cluster of 18 serum metabolites between patients with fatigue (n = 84) vs without fatigue (n = 72) (P = .033); serum samples from patients with fatigue had significant reductions in levels of methionine (P = .020), tryptophan (P = .042), proline (P = .017), and sarcosine (P = .047). Fecal samples from patients with fatigue had a less diverse gut microbiome, with significant reductions in butyrate-producing bacteria, including Faecalibacterium prausnitzii (P = .0002, q =.007) and Roseburia hominis (P = .0079, q = 0.105). This fatigue-like microbiome was associated with fatigue scales and correlated with progressive depletion of metabolites from serum samples.

CONCLUSIONS

In an analysis of fecal and serum samples from 166 patients with IBD, we found alterations in serum metabolites and fecal microbes that were associated with fatigue.

Impact of Supplementation and Nutritional Interventions on Pathogenic Processes of Mood Disorders: A Review of the Evidence

This narrative review was conducted using searches of the PubMed/Medline and Google Scholar databases from inception to November 2019. Clinical trials and relevant articles were identified by cross-referencing major depressive disorder (and/or variants) with the following terms: folate, homocysteine, S-adenosylmethionine (SAMe), L-acetylcarnitine, alpha-lipoic acid, N-acetylcysteine, L-tryptophan, zinc, magnesium, vitamin D, omega-3 fatty acids, coenzyme Q10, and inositol. Manual reviews of references were also performed using article reference lists. Abnormal levels of folate, homocysteine, and SAMe have been shown to be associated with a higher risk of depression. Numerous studies have demonstrated antidepressant activity with L-methylfolate and SAMe supplementation in individuals with depression. Additionally, the amino acids L-acetylcarnitine, alpha-lipoic acid, N-acetylcysteine, and L-tryptophan have been implicated in the development of depression and shown to exert antidepressant effects. Other agents with evidence for improving depressive symptoms include zinc, magnesium, omega-3 fatty acids, and coenzyme Q10. Potential biases and differences in study designs within and amongst the studies and reviews selected may confound results. Augmentation of antidepressant medications with various supplements targeting nutritional and physiological factors can potentiate antidepressant effects. Medical foods, particularly L-methylfolate, and other supplements may play a role in managing depression in patients with inadequate response to antidepressant therapies.

Potential Role of Neuroactive Tryptophan Metabolites in Central Fatigue: Establishment of the Fatigue Circuit

Central fatigue leads to reduced ability to perform mental tasks, disrupted social life, and impaired brain functions from childhood to old age. Regarding the neurochemical mechanism, neuroactive tryptophan metabolites are thought to play key roles in central fatigue. Previous studies have supported the “tryptophan-serotonin enhancement hypothesis” in which tryptophan uptake into extensive brain regions enhances serotonin production in the rat model of exercise-induced fatigue. However, serotonin was transiently released after 30 minutes of treadmill running to exhaustion, but this did not reflect the duration of fatigue. In addition, as the vast majority of tryptophan is metabolized along the kynurenine pathway, possible involvement of the tryptophan-kynurenine pathway in the mechanism of central fatigue induction has been pointed out. More recently, our study demonstrated that uptake of tryptophan and kynurenine derived from the peripheral circulation into the brain enhances kynurenic acid production in rat brain in sleep deprivation–induced central fatigue, but without change in serotonin activity. In particular, dynamic change in glial-neuronal interactive processes within the hypothalamus-hippocampal circuit causes central fatigue. Furthermore, increased tryptophan-kynurenine pathway activity in this circuit causes reduced memory function. This indicates a major potential role for the endogenous tryptophan-kynurenine pathway in central fatigue, which supports the “tryptophan-kynurenine enhancement hypothesis.” Here, we review research on the basic neuronal mechanism underlying central fatigue induced by neuroactive tryptophan metabolites. Notably, these basic findings could contribute to our understanding of latent mental problems associated with central fatigue.

Light therapy and serotonin transporter binding in the anterior cingulate and prefrontal cortex

OBJECTIVE

To investigate the effects of light therapy on serotonin transporter binding (5-HTT BPND ), an index of 5-HTT levels, in the anterior cingulate and prefrontal cortices (ACC and PFC) of healthy individuals during the fall and winter. Twenty-five per cent of healthy individuals experience seasonal mood changes that affect functioning. 5-HTT BPND has been found to be higher across multiple brain regions in the fall and winter relative to spring and summer, and elevated 5-HTT BPND may lead to extracellular serotonin loss and low mood. We hypothesized that, during the fall and winter, light therapy would reduce 5-HTT BPND in the ACC and PFC, which sample brain regions involved in mood regulation.

METHOD

In a single-blind, placebo-controlled, counterbalanced, crossover design, [(11) C]DASB positron emission tomography was used measure 5-HTT BPND following light therapy and placebo conditions during fall and winter.

RESULTS

In winter, light therapy significantly decreased 5-HTT BPND by 12% in the ACC relative to placebo (F1,9 = 18.04, P = 0.002). In the fall, no significant change in 5-HTT BPND was found in any region across conditions.

CONCLUSION

These results identify, for the first time, a central biomarker associated with the intervention of light therapy in humans which may be applied to further develop this treatment for prevention of seasonal depression.

Stress, serotonin, and hippocampal neurogenesis in relation to depression and antidepressant effects

Chronic stressful life events are risk factors for developing major depression, the pathophysiology of which is strongly linked to impairments in serotonin (5-HT) neurotransmission. Exposure to chronic unpredictable stress (CUS) has been found to induce depressive-like behaviours, including passive behavioural coping and anhedonia in animal models, along with many other affective, cognitive, and behavioural symptoms. The heterogeneity of these symptoms represents the plurality of corticolimbic structures involved in mood regulation that are adversely affected in the disorder. Chronic stress has also been shown to negatively regulate adult hippocampal neurogenesis, a phenomenon that is involved in antidepressant effects and regulates subsequent stress responses. Although there exists an enormous body of data on stress-induced alterations of 5-HT activity, there has not been extensive exploration of 5-HT adaptations occurring presynaptically or at the level of the raphe nuclei after exposure to CUS. Similarly, although hippocampal neurogenesis is known to be negatively regulated by stress and positively regulated by antidepressant treatment, the role of neurogenesis in mediating affective behaviour in the context of stress remains an active area of investigation. The goal of this review is to link the serotonergic and neurogenic hypotheses of depression and antidepressant effects in the context of stress. Specifically, chronic stress significantly attenuates 5-HT neurotransmission and 5-HT1A autoreceptor sensitivity, and this effect could represent an endophenotypic hallmark for mood disorders. In addition, by decreasing neurogenesis, CUS decreases hippocampal inhibition of the hypothalamic-pituitary-adrenal (HPA) axis, exacerbating stress axis overactivity. Similarly, we discuss the possibility that adult hippocampal neurogenesis mediates antidepressant effects via the ventral (in rodents; anterior in humans) hippocampus' influence on the HPA axis, and mechanisms by which antidepressants may reverse chronic stress-induced 5-HT and neurogenic changes. Although data are as yet equivocal, antidepressant modulation of 5-HT neurotransmission may well serve as one of the factors that could drive neurogenesis-dependent antidepressant effects through these stress regulation-related mechanisms.

Novel insights into depression and antidepressants: a synergy between synaptogenesis and neurogenesis?

Major depressive disorder has been associated with manifold pathophysiological changes. These include metabolic abnormalities in discreet brain areas; modifications in the level of stress hormones, neurotransmitters, and neurotrophic factors; impaired spinogenesis and synaptogenesis in crucial brain areas, such as the prefrontal cortex and the hippocampus; and impaired neurogenesis in the hippocampus. Antidepressant therapy facilitates remission by reversing most of these disturbances, indicating that these dysfunctions may participate causally in depressive symptomatology. However, few attempts have been made to integrate these different pathophysiologies into one model. The present chapter endeavors (1) to review the extant literature in the field, with particular focus on the role of neurogenesis and synaptogenesis in depression; (2) and to suggest a possible interplay between these two processes, as well as, describe the ways by which improving both neurogenesis and synaptogenesis may enable effective recovery by acting on a larger neuronal network.

Modulation of default-mode network activity by acute tryptophan depletion is associated with mood change: a resting state functional magnetic resonance imaging study

Recently, resting-state fMRI (R-fMRI) has attracted interest based on its ability to detect the default mode network. We examined the effect of acute tryptophan depletion (ATD) on the fractional amplitude of low-frequency fluctuation (fALFF) during the resting state, and the correlation between changes of mood and fALFF following ATD. We manipulated the central serotonergic levels of 21 right-handed healthy males (mean age=21.57±1.83 years) following ATD. A within-subjects, double-blind, placebo-controlled, and counter-balanced design was employed. Following ATD or sham depletion, subjects completed the Profile of Mood States (POMS) and underwent 5-min R-fMRI scans. Our findings show that the fALFF of the middle orbitofrontal cortex and precuneus was significantly decreased and the fALFF of the superior parietal lobule, paracentral lobule and precentral gyrus was significantly increased after ATD. The fALFF of the orbitofrontal cortex was negatively correlated with depressive mood. The fALFF of the superior parietal lobule was positively correlated with anger-hostility and the fALFF of the paracentral lobule was negatively correlated with vigor-activity. The middle orbitofrontal cortex plays a key role in serotonin depletion-induced brain changes and individual differences in depressive mood change. These results serve to further elucidate the mechanism of ATD-induced relapse in remitted MDD patients.

The diagnosis of a personality disorder increases the likelihood for seropositivity to Toxoplasma gondii in psychiatric patients

Individuals serologically positive for the chronic infection with the parasite Toxoplasma gondii (TG) display certain personality traits differently from uninfected individuals. Experimental data in mice demonstrate that TG infection modulates behaviour. However, psychiatric patients with a personality disorder have not yet been investigated systematically. In our sample containing 896 psychiatric inpatients with the primary diagnoses of schizophrenia, major depression, schizoaffective or bipolar disorder and 214 psychiatrically unaffected controls (same geographic region, sampled during same time period) we analysed for effects of the additional diagnosis of a personality disorder in the patients. Psychiatrically, a patient can meet the criteria of a personality disorder additionally to any of the mentioned primary diagnoses. We applied logistic regression and cross-table statistics, separated groups by the presence/absence of a personality disorder (ICD-10) and adjusted for age between groups. We found that among all patients the additional diagnosis of a personality disorder was significantly associated with TG infection. Furthermore, only in the patients with an additional personality disorder medium titre responses (1:16-1:64) were associated with chronic course and high C-reactive protein (CRP) levels whereas high titre response (>1:64) was associated with a more acute recurrent clinical course. In the older individuals only there was a preponderance of medium titre responses (1:16-1:64) among the patients with personality disorder compared to those without and controls. We conclude that TG infection and the host's response to it make a difference for the diagnosis of a personality disorder. Our data support that TG infection can modulate human behaviour and personality traits.

Electrocortical effects of acute tryptophan depletion on emotive facial processing in depression-prone individuals

This study assessed the effects of acute tryptophan depletion (ATD), which transiently lowers CNS 5-HT, on electrocortical responses to facial expression processing in individuals with a family history of depression (FH+). Electroencephalograph (EEG)-derived event-related potentials (ERPs) were acquired from 18 FH+ individuals during a facial expression recognition task (neutral and sad, joy and surprise at 50% and 100% intensities). Both early positive (P1 and P2) and the face-specific N170 ERP components were differentially altered by emotional intensity and valence. Increased depression, confusion and total mood disturbance scores, and decreased calmness, were observed with ATD (versus placebo). ATD was also associated with enhanced P1 and P2 amplitudes for sad versus joyful expressions. The N170 was not modulated by treatment, but was affected by emotive valence. Therefore, ATD enhanced ERP-indexed early processing of sad facial expressions, and altered the processing of positive ones, in FH+ individuals.

Chronic citalopram administration causes a sustained suppression of serotonin synthesis in the mouse forebrain

BACKGROUND

Serotonin (5-HT) is a neurotransmitter with important roles in the regulation of neurobehavioral processes, particularly those regulating affect in humans. Drugs that potentiate serotonergic neurotransmission by selectively inhibiting the reuptake of serotonin (SSRIs) are widely used for the treatment of psychiatric disorders. Although the regulation of serotonin synthesis may be an factor in SSRI efficacy, the effect of chronic SSRI administration on 5-HT synthesis is not well understood. Here, we describe effects of chronic administration of the SSRI citalopram (CIT) on 5-HT synthesis and content in the mouse forebrain.

METHODOLOGY/PRINCIPAL FINDINGS

Citalopram was administered continuously to adult male C57BL/6J mice via osmotic minipump for 2 days, 14 days or 28 days. Plasma citalopram levels were found to be within the clinical range. 5-HT synthesis was assessed using the decarboxylase inhibition method. Citalopram administration caused a suppression of 5-HT synthesis at all time points. CIT treatment also caused a reduction in forebrain 5-HIAA content. Following chronic CIT treatment, forebrain 5-HT stores were more sensitive to the depleting effects of acute decarboxylase inhibition.

CONCLUSIONS/SIGNIFICANCE

Taken together, these results demonstrate that chronic citalopram administration causes a sustained suppression of serotonin synthesis in the mouse forebrain. Furthermore, our results indicate that chronic 5-HT reuptake inhibition renders 5-HT brain stores more sensitive to alterations in serotonin synthesis. These results suggest that the regulation of 5-HT synthesis warrants consideration in efforts to develop novel antidepressant strategies.

Serotonin transporter genotype and depressive phenotype determination by discriminant analysis of glucose metabolism under acute tryptophan depletion

Acute tryptophan depletion (ATD) putatively results in a transient reduction in central serotonin transmission, and induces depressed mood in some un-medicated subjects with remitted major depressive disorder (MDD). The 5-HT transporter promoter region length polymorphism (5-HTTLPR) has been shown to influence behavioral and metabolic responses to ATD, as well as the risk for developing MDD within the context of stress. The current study investigates the relationships between 5-HTTLPR genotype, neurophysiologic response to ATD, and diagnostic phenotype (healthy control subjects versus MDD subjects differentiated by their depressive response to ATD) using (18)FDG-PET. Un-medicated subjects with remitted MDD and healthy controls were genotyped for the long (l) and short (s) alleles of the 5-HTTLPR polymorphism and categorized into one of three genotypes. On two separate occasions, subjects received either a placebo or an amino acid mixture designed to deplete plasma tryptophan, followed by (18)FDG-PET scanning. Depressive symptoms were rated to determine the diagnostic phenotype. Descriptive and predictive discriminant analyses were performed using brain regional metabolic data to classify according to phenotype and genotype. Overall, 79% of the cases were classified correctly by genotype, and 85% were classified correctly by phenotype. In a leave-one-out cross-validation, 72% of the subjects were classified correctly as carrying an s-allele, and 79% of the subjects were classified correctly by primary diagnosis. The robust nature of the classification results indicates that much of the variance in metabolic response to ATD is accounted for by genotypic and phenotypic category.

The chronobiology and neurobiology of winter seasonal affective disorder

This review summarizes research on the chronobiology and neurobiology of winter seasonal affective disorder (SAD), a recurrent subtype of depression characterized by a predictable onset in the fall/winter months and spontaneous remission in the spring/summer period. Chronobiological mechanisms related to circadian rhythms, melatonin, and photoperiodism play a significant role in many cases of SAD, and treatment of SAD can be optimized by considering individual differences in key chronobiological markers. Converging evidence also points to a role for the major monoamine neurotransmitters serotonin, norepinephrine, and dopamine in one or more aspects of SAD. Ultimately, as with other psychiatric illnesses, SAD is best considered as a complex disorder resulting from the interaction of several vulnerability factors acting at different levels, the various genetic mechanisms that underlie them, and the physical environment. Models of SAD that emphasize its potential role in human evolution will also be discussed.

Bright light exposure during acute tryptophan depletion prevents a lowering of mood in mildly seasonal women

We investigated the influence of bright light exposure on the mood-lowering effect of acute tryptophan depletion (ATD). Mildly seasonal healthy young women without a personal or family history of psychiatric disorders remained in either dim or bright light during two test days. Tryptophan-deficient and nutritionally balanced amino acid mixtures were administered in counterbalanced order. Mood state was assessed using the Profile of Mood States (POMS) and Visual Analogue Scales (VAS). In dim light, ATD decreased POMS scores across most subscales, indicating a worsening of mood. In bright light, mood was unaffected by ATD. Thus, bright light blocked the worsening of mood caused by ATD. This was also observed on the positive mood VAS. These results indicate a direct, immediate interaction between bright light and serotonin function. Bright light might help protect against ATD-induced mood change by increasing serotonin above the threshold level below which there is a lowering of mood.

An index of 5-HT synthesis changes during early antidepressant treatment: alpha-[11C]methyl-L-tryptophan PET study

The antidepressant selective serotonin transporter inhibitors (SSRIs) are clinically active after a delay of several weeks. Indeed, the rapid increase of serotonin (5-HT) caused by SSRIs, stimulates the 5-HT(1A) autoreceptors, which exert a negative feedback on the 5-HT neurotransmission. Only when autoreceptors are desensitized, can SSRIs exert their therapeutic activity. The 5-HT(1A) receptor antagonist pindolol has been used to accelerate the clinical effects of antidepressant by preventing the negative feedback. Using the alpha-[(11)C]methyl-L-tryptophan/positron emission tomography (PET), the goal of the present double-blind, randomized study was to compare the changes in alpha-[(11)C]methyl-L-tryptophan trapping, an index of serotonin synthesis, in patients suffering from unipolar depression treated with the SSRI citalopram (20 mg/day) plus placebo versus patients treated with citalopram plus pindol (7.5 mg/day). PET and Hamilton depression rating scale (HDRS-17) were performed at baseline, and after 10 and 24 days of antidepressant treatment. Results show that the combination citalopram plus pindol, compared to citalopram alone shows a more rapid and greater increase of an index of 5-HT synthesis in prefrontal cortex (BA 9). This research is the first human PET study demonstrating that, after 24 days, the combination SSRIs plus pindolol produces a greater increase of the metabolism of serotonin in the prefrontal cortex, an area associated to depressive symptoms.

A controlled prospective study of toxoplasma gondii infection in individuals with schizophrenia: beyond seroprevalence

Toxoplasma gondii (TG) infection has been reported to be more frequent in schizophrenia. The interaction of the lifelong persisting parasite with the host's immune system involves T-cell/interferon-gamma-induced degradation of tryptophan and provides a challenge to the host well beyond a possible role in the etiology of schizophrenia. The hypothesis we tested in this study was that TG infection may be more frequent (serofrequency) and/or more intense (serointensity) in patients with schizophrenia or major depression compared with psychiatrically healthy controls. In addition, these measures are associated with the clinical course. We did a cross-sectional, prospective investigation of individuals with schizophrenia (n = 277) and major depression (n = 465) admitted to our department (2002-2005) and of healthy controls (n = 214), with all groups adjusted for age and geographic home region. Serofrequency was comparable between the groups, but serointensity was significantly higher in the patients. In individuals with schizophrenia, serointensity was significantly positively associated with C-reactive protein levels and leukocyte counts, and first-episode patients yielded significantly higher serotiters. Immunomodulatory medication was associated with decreased serotiters. In addition, the route of infection appears to differ between patients and controls. Thus, our results support increased host responses to TG infection in the patients, as well as increased titers in first-episode patients with schizophrenia; this may relate to the shifted T-helper 1/2 status described in these patients. Therefore, we suggest that TG infection, particularly in individuals with schizophrenia, is an important environmental factor in the interaction between psychiatric vulnerability, genetic background, immunomodulation, and the neurotransmitter systems.

Serotonin transporter polymorphism mediates vulnerability to loss of incentive motivation following acute tryptophan depletion

The serotonin (5-HT) system is implicated in incentive motivational processes. The present study utilized the acute tryptophan depletion (ATD) procedure to investigate the effect of temporarily lowering 5-HT synthesis on motivation in healthy volunteers, stratifying the results by allelic variation at the serotonin transporter gene (5-HTTLPR). ATD resulted in a robust reduction in plasma tryptophan concentration. Consistent with a previous study, ATD attenuated motivationally speeded action on the Cued-Reinforcement Reaction Time task. The present investigation revealed that this effect was restricted to volunteers of the ss genotype, whereas ll volunteers exhibited intact motivationally speeded action following ATD (treatment x reinforcement probability x genotype interaction: F1,26=5.8, p=0.024). Furthermore, tryptophan availability to the brain was correlated positively with motivationally speeded action following ATD in the ss genotype group (rho13=0.71, p=0.006), whereas this correlation was negative in the ll genotype group (rho14=-0.60, p=0.023). This is the first study to suggest that allelic variation at the 5-HTTLPR mediates motivational responses to ATD in healthy volunteers. These data indicate that the s allele at the 5-HTTLPR may confer risk for depression via its effect on incentive motivational processing, and highlight the importance of genetic variation in determining individual responses to pharmacological treatments.

Effects of tryptophan depletion on the serotonin transporter in healthy humans

BACKGROUND

Lowering of brain serotonin by acute tryptophan depletion (TD) frequently leads to transient symptoms of depression in vulnerable individuals but not in euthymic healthy subjects with a negative family history of depression. The effects of TD on regional serotonin transporter binding potential (5-HTT BP), an index of 5-HTT density and affinity, were studied in healthy individuals using 3-(11)C-amino-4-(2-dimethylaminomethylphenylsulfanyl)benzonitrile ([11C]DASB) positron emission tomography (PET). Adaptive decreases in 5-HTT density and/or affinity during TD would be a possible compensatory mechanism to maintain sufficient extracellular serotonin levels during TD, thereby preventing a depressive relapse.

METHODS

Regional noninvasive 5-HTT BP was found in 25 healthy subjects using [11C]DASB PET. Fourteen subjects were scanned twice, once after TD and once after sham depletion, and 11 other healthy subjects were scanned twice to measure test-retest reliability of the method.

RESULTS

None of the healthy subjects experienced depressive symptoms during TD and there was no difference in regional 5-HTT BP during TD as compared with sham depletion.

CONCLUSIONS

Acute changes in 5-HTT density or affinity are unlikely to play a role in protecting healthy subjects against mood symptoms during TD. Other mechanisms that may be associated with greater resilience against acute lowering of extracellular serotonin should be explored to gain further insight into the neurochemical basis of different vulnerabilities to short-term depressive relapse.

Update on the biology of seasonal affective disorder

The etiology and pathophysiology of seasonal affective disorder (SAD) has been linked to the seasons and to light since its first conceptualization. Aspects of SAD that make it particularly amenable to biological investigation include the predictable recurrent episodes, the rapid response to a nonpharmacologic treatment, the specific neurovegetative features, and the availability of rich animal models of seasonality. This paper reviews new findings for the major biological hypotheses for SAD, focusing on circadian rhythms, neurotransmitters, and molecular genetics. Integrative issues and future directions for the study of SAD, including the heuristic value of a dual-vulnerability hypothesis that conceptualizes seasonality as a dimensional construct and the importance of studying endophenotypes, will be discussed.

Tryptophan depletion and serotonin loss in selective serotonin reuptake inhibitor-treated depression: an [(18)F] MPPF positron emission tomography study

BACKGROUND

Recurrence of depressive symptoms after tryptophan depletion (TD) in selective serotonin reuptake inhibitor (SSRI)-treated depression is an important, unexplained phenomenon. With [(18)F] MPPF positron emission tomography (PET), serotonin (5-hydroxytryptamine, 5-HT) 1A receptor binding potential (5-HT(1A)BP) was measured after TD in various brain regions in citalopram-treated depression. This 5-HT(1A)BP measurement is sensitive to changes in extracellular 5-HT in animal models.

METHODS

Eight remitted patients with major depressive disorder received [(18)F] MPPF PET scans twice: once after TD and once after sham depletion. Behavioral measures were evaluated with the Hamilton Depression Rating Scale and visual analog scales.

RESULTS

No effect on regional 5-HT(1A)BP was observed after TD, despite an 86% decrease in total plasma tryptophan and transient depressive relapse in six of eight patients.

CONCLUSIONS

Large-magnitude changes in extracellular 5-HT are not crucial for the mood effects observed in SSRI-treated subjects after TD. Therefore, greater consideration must be given to other mechanisms that involve vulnerability to small perturbations in extracellular 5-HT, such as impairment of signal transduction.

The effects of tyrosine depletion in normal healthy volunteers: implications for unipolar depression

RATIONALE

In recent years, there has been a growing interest in the role of dopamine (DA) both in the pathogenesis of unipolar depression and in motivated behaviour. The innovative technique of acute tyrosine depletion presents an opportunity to characterise further its function in these domains.

OBJECTIVE

The present study examined the physiological, subjective and cognitive effects of acute tyrosine depletion in healthy volunteers.

METHODS

A double-blind, placebo-controlled, parallel group design was employed. Half of the participants ingested a balanced amino-acid mixture (BAL) and the other half received an identical mixture except that tyrosine and phenylalanine were absent (TYR-free). Plasma amino acid concentrations and subjective ratings were monitored at both baseline (T(0)) and 5 h following consumption (T(5)) of the mixtures. A comprehensive neuropsychological test battery was also administered at T(5).

RESULTS

Relative to the BAL group, the reduction in TYR availability to the brain was more marked in the TYR-free group. Employment of psychological rating scales revealed that, compared with the BAL group, the TYR-free group became less content and more apathetic. For the affective go/no-go task, whilst the BAL group exhibited a happy latency bias, the TYR-free group demonstrated a sad latency bias. Furthermore, in the decision-making task, the rate at which the TYR-free group increased their bets in response to more likely outcomes was lower than that of the BAL group. Taken together, these neuropsychological findings strikingly paralleled those reported in previous investigations of unipolar depression. The experimental groups could not be differentiated on any of the other neuropsychological measures, including more classical assessments of fronto-executive function.

CONCLUSION

These findings are consistent with the hypothesis that dopaminergic factors are particularly involved in disrupted affect/reward-based processing characteristic of clinical depression.

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.

The effects of acute tryptophan depletion on neuropsychological function

Serotonin (5-HT, 5-hydroxytryptamine) may have an important role in the maintenance of normal neuropsychological functioning. The method of acute tryptophan depletion (ATD) provides a pharmacological challenge by which central 5-HT levels can be temporarily decreased and effects on learning, memory and mood examined. Twenty healthy male volunteers were recruited to take part in this within-subject, double-blind, crossover study. Neuropsychological function was evaluated 4-6 h after ingestion of a control or 52 g tryptophan (TRP) depleting amino-acid drink. ATD significantly lowered levels of plasma total and free TRP (p < 0.001), but this did not affect mood or performance on tests of verbal and visuo-spatial learning and memory, attention or executive function. These results contradict previous findings; however, the degree of disruption of central 5-HT levels resulting from the use of the 52 g amino-acid protocol may be an important factor in explaining the lack of effect. By utilizing more specific probes of individual 5-HT receptor subtypes, future studies can fully explore the role of 5-HT in neuropsychological functioning and may elucidate the factors determining vulnerability to the effects of serotonergic dysfunction.

Le test de déplétion en tryptophane : aspects méthodologiques et pratiques

INTRODUCTION

This paper focuses on the methodology and behavioural results of the tryptophan depletion challenge.

METHODS

A Medline search (1985-2002) using the keywords 'tryptophan depletion' and 'mood' has been performed.

RESULTS

Rapid depletion is obtained by morning intake under fasting condition of a tryptophan-free amino-acid mixture. Subjects with a family history of mood disorders and depressed patients receiving serotoninergic drugs demonstrate a mood-lowering effect. However, these effects are limited or absent in normal volunteers and naive depressed patients.

CONCLUSION

The tryptophan depletion challenge has largely contributed to the understanding of the physiopathology of depression. However, the mood response to acute tryptophan depletion challenge in healthy volunteers is not as sensitive as a 'depression model'.

The role of serotonin in human mood and social interaction. Insight from altered tryptophan levels

Alterations in brain tryptophan levels cause changes in brain serotonin synthesis, and this has been used to study the implication of altered serotonin levels in humans. In the acute tryptophan depletion (ATD) technique, subjects ingest a mixture of amino acids devoid of tryptophan. This results in a transient decline in tissue tryptophan and in brain serotonin. ATD can result in lower mood and increase in irritability or aggressive responding. The magnitude of the effect varies greatly depending on the susceptibility of the subject to lowered mood or aggressivity. Unlike ATD, tryptophan can be given chronically. Tryptophan is an antidepressant in mild to moderate depression and a small body of data suggests that it can also decrease aggression. Preliminary data indicate that tryptophan also increases dominant behavior during social interactions. Overall, studies manipulating tryptophan levels support the idea that low serotonin can predispose subjects to mood and impulse control disorders. Higher levels of serotonin may help to promote more constructive social interactions by decreasing aggression and increasing dominance.