Sex differences in the effect of acute tryptophan depletion on declarative episodic memory: A pooled analysis of nine studies

Association Study of Serotonin 1A Receptor Gene, Personality, and Anxiety in Women with Alcohol Use Disorder

Alcohol use disorder is considered a chronic and relapsing disorder affecting the central nervous system. The serotonergic system, mainly through its influence on the mesolimbic dopaminergic reward system, has been postulated to play a pivotal role in the underlying mechanism of alcohol dependence. The study aims to analyse the association of the rs6295 polymorphism of the 5HTR1A gene in women with alcohol use disorder and the association of personality traits with the development of alcohol dependence, as well as the interaction of the rs6295, personality traits, and anxiety with alcohol dependence in women. The study group consisted of 213 female volunteers: 101 with alcohol use disorder and 112 controls. NEO Five-Factor and State-Trait Anxiety Inventories were applied for psychometric testing. Genotyping of rs6295 was performed by real-time PCR. We did not observe significant differences in 5HTR1A rs6295 genotypes (p = 0.2709) or allele distribution (p = 0.4513). The AUD subjects scored higher on the anxiety trait (p < 0.0001) and anxiety state (p < 0.0001) scales, as well as on the neuroticism (p < 0.0001) and openness (p = 0134) scales. Significantly lower scores were obtained by the AUD subjects on the extraversion (p < 0.0001), agreeability (p < 0.0001), and conscientiousness (p < 0.0001) scales. Additionally, we observed a significant effect of 5HTR1A rs6295 genotype interaction and alcohol dependency, or lack thereof, on the openness scale (p = 0.0016). In summary, this study offers a comprehensive overview of alcohol dependence among women. It offers valuable insights into this complex topic, contributing to a more nuanced understanding of substance use among this specific demographic. Additionally, these findings may have implications for developing prevention and intervention strategies tailored to individual genetic and, most importantly, personality and anxiety differences.

Arbitration between model-free and model-based control is not affected by transient changes in tonic serotonin levels

BACKGROUND

Serotonin has been suggested to modulate decision-making by influencing the arbitration between model-based and model-free control. Disruptions in these control mechanisms are involved in mental disorders such as drug dependence or obsessive-compulsive disorder. While previous reports indicate that lower brain serotonin levels reduce model-based control, it remains unknown whether increases in serotonergic availability might thus increase model-based control. Moreover, the mediating neural mechanisms have not been studied yet.

AIM

The first aim of this study was to investigate whether increased/decreased tonic serotonin levels affect the arbitration between model-free and model-based control. Second, we aimed to identify the underlying neural processes.

METHODS

We employed a sequential two-stage Markov decision-task and measured brain responses during functional magnetic resonance imaging in 98 participants in a randomized, double-blind cross-over within-subject design. To investigate the influence of serotonin on the balance between model-free and model-based control, we used a tryptophan intervention with three intervention levels (loading, balanced, depletion). We hypothesized that model-based behaviour would increase with higher serotonin levels.

RESULTS

We found evidence that neither model-free nor model-based control were affected by changes in tonic serotonin levels. Furthermore, our tryptophan intervention did not elicit relevant changes in Blood-Oxygenation-Level Dependent activity.

Sex Differences in Tryptophan Metabolism: A Systematic Review Focused on Neuropsychiatric Disorders

Tryptophan (Tryp) is an essential amino acid and the precursor of several neuroactive compounds within the central nervous system (CNS). Tryp metabolism, the common denominator linking serotonin (5-HT) dysfunctions and neuroinflammation, is involved in several neuropsychiatric conditions, including neurological, neurodevelopmental, neurodegenerative, and psychiatric diseases. Interestingly, most of those conditions occur and progress in a sex-specific manner. Here, we explore the most relevant observations about the influence of biological sex on Tryp metabolism and its possible relation to neuropsychiatric diseases. Consistent evidence suggests that women have a higher susceptibility than men to suffer serotoninergic alterations due to changes in the levels of its precursor Tryp. Indeed, female sex bias in neuropsychiatric diseases is involved in a reduced availability of this amino acid pool and 5-HT synthesis. These changes in Tryp metabolism could lead to sexual dimorphism on the prevalence and severity of some neuropsychiatric disorders. This review identifies gaps in the current state of the art, thus suggesting future research directions. Specifically, there is a need for further research on the impact of diet and sex steroids, both involved in this molecular mechanism as they have been poorly addressed for this topic.

The role of serotonin in declarative memory: A systematic review of animal and human research

The serotonergic system is involved in diverse cognitive functions including memory. Of particular importance to daily life are declarative memories that contain information about personal experiences, general facts, and events. Several psychiatric or neurological diseases, such as depression, attention-deficit-hyperactivity disorder (ADHD), and dementia, show alterations in serotonergic signalling and attendant memory disorders. Nevertheless, understanding serotonergic neurotransmission and its influence on memory remained a challenge until today. In this systematic review, we summarize recent psychopharmacological studies in animals and humans from a psychological memory perspective, in consideration of task-specific requirements. This approach has the advantage that comparisons between serotonin (5-HT)-related neurochemical mechanisms and manipulations are each addressing specific mnemonic circuits. We conclude that applications of the same 5-HT-related treatments can differentially affect unrelated tasks of declarative memories. Moreover, the analysis of specific mnemonic phases (e.g., encoding vs. consolidation) reveals opposing impacts of increased or decreased 5-HT tones, with low 5-HT supporting spatial encoding but impairing the consolidation of objects and verbal memories. Promising targets for protein synthesis-dependent consolidation enhancements include 5-HT₄ receptor agonists and 5-HT₆ receptor antagonists, with the latter being of special interest for the treatment of age-related decline. Further implications are pointed out as base for the development of novel therapeutic targets for memory impairment of neuropsychiatric disorders.

Edible Mushrooms as a Potential Component of Dietary Interventions for Major Depressive Disorder

Dietary interventions for people suffering from major depressive disorder (MDD) are an ongoing field of research. In this article, we present a comprehensive background for understanding the possibility of using edible medicinal mushrooms as an adjunctive treatment for MDD. We start with a brief history of MDD, its diagnosis, epidemiology and treatment, and the effects of diet on depression symptoms, followed by a review of neurobiological, behavioral, and clinical studies of medicinal mushrooms. We specifically highlight the results of preclinical and clinical studies on dietary supplementation with three selected mushroom species: Lion’s mane (Hericium erinaceus), Caterpillar mushroom (Cordyceps militaris), and Lingzhi/Reishi (Ganoderma lucidum). Preliminary small-sample clinical studies suggest that Lion’s mane can influence well-being of humans. In the case of Reishi, the results of clinical studies are equivocal, while in the case of Caterpillar Mushroom, such studies are underway. Edible mushrooms contain 5-hydroxy-L-tryptophan (5-HTP), which is a direct precursor of serotonin—a neurotransmitter targeted in pharmacotherapy of MDD. Therefore, in light of the well-recognized role of stress as a pathogenic factor of MDD, we also describe the neurobiological mechanisms of the interaction between stress and serotonergic neurotransmission; and summarize the current state of knowledge on dietary supplementation with 5-HTP in MDD.

Serotonin transporter gene promoter hypomethylation in obsessive-compulsive disorder - Predictor of impaired response to exposure treatment?

Treatment resistance is common in obsessive-compulsive disorder (OCD) and associated with a significant burden for the individual patient. Accordingly, the identification of biomarkers as early predictors of the clinical response has become a central goal in the search for more efficacious and personalized treatments. Epigenetic mechanisms such as DNA methylation of the serotonin transporter gene (SLC6A4) have been suggested to predict therapy outcome in mental disorders closely related to OCD, but have not yet been investigated as such in OCD. The present therapy-epigenetic study therefore sought to address the potential role of SLC6A4 promoter methylation in the prediction of treatment response for the first time in OCD. Overall, 112 patients with primary OCD were investigated over the course of 8-10-week OCD-specific, cognitive behavioral therapy (CBT) comprising exposure and response prevention/management (phase I) and in vivo exposure exercises ('flooding', phase II). OCD symptoms were measured using the Yale-Brown Obsessive Compulsive Scale (Y-BOCS) at baseline as well as before and after the in vivo exposure phase. SLC6A4 promoter methylation at baseline was analyzed via pyrosequencing of sodium bisulfite-treated DNA extracted from blood cells. Lower baseline SLC6A4 promoter methylation predicted impaired treatment response (defined as reduction in Y-BOCS scores) in phase II (but not phase I) of CBT (β = -0.359, p = .002). SLC6A4 methylation may thus constitute a potential early biomarker predicting biologically mediated clinical changes elicited specifically by exposure treatment. These results carry promise for clinical application and in the future could aid in early treatment modification and personalized treatment efforts.

Untargeted metabolomic analysis and pathway discovery in perinatal asphyxia and hypoxic-ischaemic encephalopathy

Elucidating metabolic effects of hypoxic-ischaemic encephalopathy (HIE) may reveal early biomarkers of injury and new treatment targets. This study uses untargeted metabolomics to examine early metabolic alterations in a carefully defined neonatal population. Infants with perinatal asphyxia who were resuscitated at birth and recovered (PA group), those who developed HIE (HIE group) and healthy controls were all recruited at birth. Metabolomic analysis of cord blood was performed using direct infusion FT-ICR mass spectrometry. For each reproducibly detected metabolic feature, mean fold differences were calculated HIE vs. controls (ΔHIE) and PA vs. controls (ΔPA). Putative metabolite annotations were assigned and pathway analysis was performed. Twenty-nine putatively annotated metabolic features were significantly different in ΔPA after false discovery correction ( q < 0.05), with eight of these also significantly altered in ΔHIE. Altered putative metabolites included; melatonin, leucine, kynurenine and 3-hydroxydodecanoic acid which differentiated between infant groups (ΔPA and ΔHIE); and D-erythrose-phosphate, acetone, 3-oxotetradecanoic acid and methylglutarylcarnitine which differentiated across severity grades of HIE. Pathway analysis revealed ΔHIE was associated with a 50% and 75% perturbation of tryptophan and pyrimidine metabolism, respectively. We have identified perturbed metabolic pathways and potential biomarkers specific to PA and HIE, which measured at birth, may help direct treatment.

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Association between the serotonin transporter gene polymorphism and verbal learning in older adults is moderated by gender

The S allele of the functional 5-HTTLPR polymorphism has previously been associated with reductions in memory function. Given the change in function of the serotonergic system in older adults, and the functional consequences of memory decline in this age group, further investigation into the impact of 5-HTTLPR in healthy older adults is required. This investigation examined the effect of 5-HTTLPR variants (S carriers versus L/L homozygotes) on verbal and visual episodic memory in 438 healthy older adults participating in the Tasmanian Healthy Brain Project (age range 50-79 years, M=60.35, s.d.=6.75). Direct effects of 5-HTTLPR on memory processes, in addition to indirect effects through interaction with age and gender, were assessed. Although no direct effects of 5-HTTLPR on memory processes were identified, our results indicated that gender significantly moderated the impact that 5-HTTLPR variants exerted on the relationship between age and verbal episodic memory function as assessed by the Rey Auditory Verbal Learning Test. No significant direct or indirect effects were identified in relation to visual memory performance. Overall, this investigation found evidence to suggest that 5-HTTLPR genotype affects the association of age and verbal episodic memory for males and females differently, with the predicted negative effect of S carriage present in males but not females. Such findings indicate a gender-dependent role for 5-HTTLPR in the verbal episodic memory system of healthy older adults.

Effects of biperiden and acute tryptophan depletion and their combination on verbal word memory and EEG

BACKGROUND

Research on the neurobiological foundations of memory has shown that multiple neurotransmitters play an important role in memory processing. To study the interaction between neurotransmitters such as acetylcholine and serotonin, pharmacological models can be used. In this study, we tested the effects of the muscarinic M1 antagonist biperiden, acute tryptophan depletion (ATD), and the interaction between the two on episodic memory using the verbal learning task.

METHODS

The study was conducted according to a double-blind, placebo-controlled, four-way crossover design. Seventeen participants received biperiden (2.0 mg), ATD (SolugelP), a combination of both, or a placebo in counterbalanced order with a wash out of at least 7 days. A verbal learning task was performed while recording electroencephalography. The task consisted of an immediate and delayed recall as well as a recognition part.

RESULTS

Results revealed decreased scores on the delayed recall after biperiden and ATD separately but no significant interaction between the two. However, the event-related potential components P3b, N400, and P600 did show an interaction during encoding.

CONCLUSION

These results indicate that both BIP and ATD impair episodic memory. However, an interaction between the serotonergic and cholinergic system on memory performance is not supported.

Intake of tryptophan-enriched whey protein acutely enhances recall of positive loaded words in patients with multiple sclerosis

BACKGROUND & AIMS

Multiple sclerosis (MS) has physiological and/or immunological characteristics that diminish serotonin metabolism, a neurotransmitter associated with affective and cognitive functions. The aim was examine the acute and dose-dependent effects of a dietary tryptophan (TRP) enrichment on affective and cognitive functions in MS patients. We hypothesized that increased dietary availability of the amino acid TRP enhances serotonin concentrations and improves neuropsychological functions.

METHODS

In a double-blind, placebo-controlled, crossover study, MS patients with (n = 15) and without (n = 17) depressed mood ingested a whey protein mixture with 4 different amounts of TRP. Mood states, total plasma TRP and plasma TRP/ΣLNAA ratio were measured during each test session and cognitive tasks were conducted three hours after dietary intake.

RESULTS

A fast, transient and dose-dependent increase of total plasma TRP and TRP/ΣLNAA ratio was found. Ratings of negative mood decreased over time, independent of the TRP dose. Relative to whey-only, immediate word recall and delayed recognition improved after ingestion of the lowest added TRP dose and was mainly due to better recollection for positive loaded words. Executive functions were not affected by a difference in TRP availability.

CONCLUSIONS

A moderate addition of TRP to whey protein enhances memory processes without improving the mood state in MS. ccmo-registration number is NL32316.096.10.

Cognitive disruptions in stress-related psychiatric disorders: A role for corticotropin releasing factor (CRF)

This article is part of a Special Issue "SBN 2014". Stress is a potential etiology contributor to both post-traumatic stress disorders (PTSD) and major depression. One stress-related neuropeptide that is hypersecreted in these disorders is corticotropin releasing factor (CRF). Dysregulation of CRF has long been linked to the emotion and mood symptoms that characterize PTSD and depression. However, the idea that CRF also mediates the cognitive disruptions observed in patients with these disorders has received less attention. Here we review literature indicating that CRF can alter cognitive functions. Detailed are anatomical studies revealing that CRF is poised to modulate regions required for learning and memory. We also describe preclinical behavioral studies that demonstrate CRF's ability to alter fear conditioning, impair memory consolidation, and alter a number of executive functions, including attention and cognitive flexibility. The implications of these findings for the etiology and treatment of the cognitive impairments observed in stress-related psychiatric disorders are described.

Studying the effects of dietary body weight-adjusted acute tryptophan depletion on punishment-related behavioral inhibition

Background

Alterations in serotonergic (5-HT) neurotransmission are thought to play a decisive role in affective disorders and impulse control.

Objective

This study aims to reproduce and extend previous findings on the effects of acute tryptophan depletion (ATD) and subsequently diminished central 5-HT synthesis in a reinforced categorization task using a refined body weight–adjusted depletion protocol.

Design

Twenty-four young healthy adults (12 females, mean age [SD]=25.3 [2.1] years) were subjected to a double-blind within-subject crossover design. Each subject was administered both an ATD challenge and a balanced amino acid load (BAL) in two separate sessions in randomized order. Punishment-related behavioral inhibition was assessed using a forced choice go/no-go task that incorporated a variable payoff schedule.

Results

Administration of ATD resulted in significant reductions in TRP measured in peripheral blood samples, indicating reductions of TRP influx across the blood–brain barrier and related brain 5-HT synthesis. Overall accuracy and response time performance were improved after ATD administration. The ability to adjust behavioral responses to aversive outcome magnitudes and behavioral adjustments following error contingent punishment remained intact after decreased brain 5-HT synthesis. A previously observed dissociation effect of ATD on punishment-induced inhibition was not observed.

Conclusions

Our results suggest that neurodietary challenges with ATD Moja–De have no detrimental effects on task performance and punishment-related inhibition in healthy adults.

Biperiden selectively induces memory impairment in healthy volunteers: no interaction with citalopram

RATIONALE

Traditionally, the non-selective muscarinic antagonist scopolamine has been used to induce episodic memory impairments as found in Alzheimer's disease (AD). However, it also impairs attention and induces drowsiness. Muscarinic antagonists more selective for the M1 receptor might, therefore, be preferred.

OBJECTIVES

We examined the effects of the M1 antagonist biperiden on cognitive functions in order to test the specificity of this drug on memory performance. Additionally, we assessed whether the selective serotonin re-uptake inhibitor citalopram can reverse a possible biperiden-induced impairment.

METHODS

The study was conducted according to a double-blind, placebo-controlled, four-way cross-over design. Sixteen volunteers received biperiden (2 mg), citalopram (20 mg), a combination of the two, or a placebo in counterbalanced order with a washout of at least 4 days. Cognitive tests (verbal memory, continuous recognition memory, spatial memory, choice reaction) were performed 4 and 1 h after treatment with citalopram and biperiden, respectively.

RESULTS

Biperiden impaired memory performance in the verbal learning task, the continuous recognition memory test, and the spatial memory task. Effects on attention and side effects, as measured using the choice reaction time test and questionnaires respectively, could be neglected. Citalopram did not affect any of the memory or attention measures taken. Most importantly, citalopram was also unable to reverse the biperiden-induced memory impairments.

CONCLUSIONS

Our results, thus, show that the M1 antagonist biperiden may serve as a translational model to induce episodic memory deficits as seen in AD. However, the interactive influence of acetylcholine and serotonin on memory could not be confirmed.

Response inhibition and serotonin in autism: a functional MRI study using acute tryptophan depletion

Stereotyped, repetitive behaviours in autism may reflect deficits in serotonin-modulated inhibitory control. Daly et al. use fMRI to compare the effects of acute tryptophan depletion in adult males with autism and controls performing the Go/No-Go task. Opposite effects are seen in the two groups, consistent with altered inhibition in autism.

It has been suggested that the restricted, stereotyped and repetitive behaviours typically found in autism are underpinned by deficits of inhibitory control. The biological basis of this is unknown but may include differences in the modulatory role of neurotransmitters, such as serotonin, which are implicated in the condition. However, this has never been tested directly. We therefore assessed the modifying role of serotonin on inhibitory brain function during a Go/No-Go task in 14 adults with autism and normal intelligence and 14 control subjects that did not differ in gender, age and intelligence. We undertook a double-blind, placebo-controlled, crossover trial of acute tryptophan depletion using functional magnetic resonance imaging. Following sham, adults with autism relative to controls had reduced activation in key inhibitory regions of inferior frontal cortex and thalamus, but increased activation of caudate and cerebellum. However, brain activation was modulated in opposite ways by depletion in each group. Within autistic individuals depletion upregulated fronto-thalamic activations and downregulated striato-cerebellar activations toward control sham levels, completely ‘normalizing’ the fronto-cerebellar dysfunctions. The opposite pattern occurred in controls. Moreover, the severity of autism was related to the degree of differential modulation by depletion within frontal, striatal and thalamic regions. Our findings demonstrate that individuals with autism have abnormal inhibitory networks, and that serotonin has a differential, opposite, effect on them in adults with and without autism. Together these factors may partially explain the severity of autistic behaviours and/or provide a novel (tractable) treatment target.

Can formulation affect tryptophan depletion results? Hints from studies in experimental panic

RATIONALE

Acute Tryptophan Depletion (ATD) is a specific serotonergic challenge tool. Central serotonergic effects of different ATD procedures are possibly not those that are usually assumed.

OBJECTIVES

In this paper we review data of ATD in an experimental fear model to investigate whether and how methodological differences may affect fear outcomes. Next we point to discrepancies of studies in our laboratory in order to test the hypotheses formulated in the review.

METHODS

Literature was searched in PubMed and MEDLINE and studies of our laboratory were compared.

RESULTS

Eight studies were included in the review: five in patients with panic disorder, three in healthy individuals. Methodologically the studies in our laboratory were quite similar except for the applied ATD mixtures.

CONCLUSIONS

ATD exerts fear-enhancing effects in patients with panic disorders, more than in healthy individuals. However, our findings are inconclusive. The discrepant findings of studies in our laboratory can possibly be explained by differences in the ATD mixtures used. We suggest mechanisms as to how these might have affected the central availability of tryptophan and hence serotonin.

Cognitive effects of methylphenidate and levodopa in healthy volunteers

Our previous study showed enhanced declarative memory consolidation after acute methylphenidate (MPH) administration. The primary aim of the current study was to investigate the duration of this effect. Secondary, the dopaminergic contribution of MPH effects, the electrophysiological correlates of declarative memory, and the specificity of memory enhancing effects of MPH to declarative memory were assessed. Effects of 40 mg of MPH on memory performance were compared to 100mg of levodopa (LEV) in a placebo-controlled crossover study with 30 healthy volunteers. Memory performance testing included a word learning test, the Sternberg memory scanning task, a paired associates learning task, and a spatial working memory task. During the word learning test, event-related brain potentials (ERPs) were measured. MPH failed to enhance retention of words at a 30 min delay, but it improved 24 h delayed memory recall relative to PLA and LEV. Furthermore, during encoding, the P3b and P600 ERP latencies were prolonged and the P600 amplitude was larger after LEV compared to PLA and MPH. MPH speeded response times on the Sternberg Memory Scanning task and improved performance on the Paired Associates Learning task, relative to LEV, but not PLA. Performance on the Spatial working memory task was not affected by the treatments. These findings suggest that MPH and LEV might have opposite effects on memory.

Effects of the serotonin transporter polymorphism and history of major depression on overgeneral autobiographical memory

Overgeneral autobiographical memory (OGM) is a key memory deficit in major depressive disorder (MDD). Much research has examined cognitive mechanisms underlying OGM, but little work has investigated potential neurobiological influences. There is preliminary evidence that a genetic serotonergic vulnerability coupled with depressive symptoms may be associated with other memory impairments, and experimental research suggests a role for serotonin in OGM. We investigated whether a polymorphism in the promoter region of the serotonin transporter gene (5-HTTLPR) was associated with OGM in interaction with a lifetime history of MDD in 370 young adults in a longitudinal study of risk for emotional disorders. There was a significant interaction between 5-HTTLPR genotype and lifetime history of MDD in predicting OGM. Among S allele homozygotes, MDD history was associated with greater OGM, whereas no significant relationship between MDD history and OGM emerged among L carriers. Furthermore, there was evidence that a greater number of S alleles were associated with greater memory specificity in individuals without a history of MDD. Implications for understanding cognitive and biological risk for depression are discussed.

Short-term quetiapine treatment alters the use of reinforcement signals during risky decision-making and promotes the choice of negative expected values in healthy adult males

Effective decision-making can involve using environmental signals about the possible good and bad outcomes, and their probabilities, to select optimal actions. Problematic decision-making in psychiatric disorders, and particularly bipolar illness, may result from disrupted use of these reinforcement cues, leading to actions that reflect or precipitate pathological changes in mood. Previous experiments indicate that the processing of reinforcement cues while selecting between risky actions can be influenced by dopamine and serotonin activity. Quetiapine is an atypical antipsychotic agent with a complex pharmacology, including antagonist actions at 5-HT2A and, to a lesser extent, D2 receptors. Here, we investigated the effects of (short-term) treatment with quetiapine on the risky decision-making of healthy human adults. Twenty participants received 150 mg of quetiapine XL for 7 d, whereas 20 age- and IQ-matched participants received a placebo. On the eighth day, all participants completed a risky decision-making task that involved making a series of choices between two simultaneously presented gambles that differed in the magnitudes of their possible gains and losses, and the probabilities with which these outcomes were delivered. Quetiapine treatment was associated with a marked tendency to choose options with negative expected values compared with placebo treatment in male but not female participants. Our results demonstrate that antagonism of serotonin and dopamine receptor activity can alter the way individuals use information about gains and losses when selecting between risky actions, possibly reflecting gender-specific differences in risk attitudes. These effects may be beneficial by correcting decision-making biases that feature in mood disorders.

Food-derived serotonergic modulators: effects on mood and cognition

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

Bidirectional regulation of emotional memory by 5-HT1B receptors involves hippocampal p11

Cognitive impairments are common in depression and involve dysfunctional serotonin neurotransmission. The 5-HT1B receptor (5-HT(1B)R) regulates serotonin transmission, via presynaptic receptors, but can also affect transmitter release at heterosynaptic sites. This study aimed at investigating the roles of the 5-HT(1B)R, and its adapter protein p11, in emotional memory and object recognition memory processes by the use of p11 knockout (p11KO) mice, a genetic model for aspects of depression-related states. 5-HT(1B)R agonist treatment induced an impairing effect on emotional memory in wild type (WT) mice. In comparison, p11KO mice displayed reduced long-term emotional memory performance. Unexpectedly, 5-HT(1B)R agonist stimulation enhanced memory in p11KO mice, and this atypical switch was reversed after hippocampal adeno-associated virus mediated gene transfer of p11. Notably, 5-HT(1B)R stimulation increased glutamatergic neurotransmission in the hippocampus in p11KO mice, but not in WT mice, as measured by both pre- and postsynaptic criteria. Magnetic resonance spectroscopy demonstrated global hippocampal reductions of inhibitory GABA, which may contribute to the memory enhancement and potentiation of pre- and post-synaptic measures of glutamate transmission by a 5-HT(1B)R agonist in p11KO mice. It is concluded that the level of hippocampal p11 determines the directionality of 5-HT(1B)R action on emotional memory processing and modulates hippocampal functionality. These results emphasize the importance of using relevant disease models when evaluating the role of serotonin neurotransmission in cognitive deficits related to psychiatric disorders.

Revisiting the serotonin-aggression relation in humans: a meta-analysis

The inverse relation between serotonin and human aggression is often portrayed as "reliable," "strong," and "well established" despite decades of conflicting reports and widely recognized methodological limitations. In this systematic review and meta-analysis, we evaluate the evidence for and against the serotonin deficiency hypothesis of human aggression across 4 methods of assessing serotonin: (a) cerebrospinal fluid levels of 5-hydroxyindoleacetic acid (CSF 5-HIAA), (b) acute tryptophan depletion, (c) pharmacological challenge, and (d) endocrine challenge. Results across 175 independent samples and over 6,500 total participants were heterogeneous, but, in aggregate, revealed a small, inverse correlation between serotonin functioning and aggression, anger, and hostility (r = -.12). Pharmacological challenge studies had the largest mean weighted effect size (r = -.21), and CSF 5-HIAA studies had the smallest (r = -.06). Potential methodological and demographic moderators largely failed to account for variability in study outcomes. Notable exceptions included year of publication (effect sizes tended to diminish with time) and self- versus other-reported aggression (other-reported aggression was positively correlated to serotonin functioning). We discuss 4 possible explanations for the pattern of findings: unreliable measures, ambient correlational noise, an unidentified higher order interaction, and a selective serotonergic effect. Finally, we provide 4 recommendations for bringing much needed clarity to this important area of research: acknowledge contradictory findings and avoid selective reporting practices; focus on improving the reliability and validity of serotonin and aggression measures; test for interactions involving personality and/or environmental moderators; and revise the serotonin deficiency hypothesis to account for serotonin's functional complexity.

Study protocol for the randomised controlled trial: antiglucocorticoid augmentation of anti-Depressants in Depression (The ADD Study)

BACKGROUND

Some patients with depression do not respond to first and second line conventional antidepressants and are therefore characterised as suffering from treatment refractory depression (TRD). On-going psychosocial stress and dysfunction of the hypothalamic-pituitary-adrenal axis are both associated with an attenuated clinical response to antidepressants. Preclinical data shows that co-administration of corticosteroids leads to a reduction in the ability of selective serotonin reuptake inhibitors to increase forebrain 5-hydroxytryptamine, while co-administration of antiglucocorticoids has the opposite effect. A Cochrane review suggests that antiglucocorticoid augmentation of antidepressants may be effective in treating TRD and includes a pilot study of the cortisol synthesis inhibitor, metyrapone. The Antiglucocorticoid augmentation of anti-Depressants in Depression (The ADD Study) is a multicentre randomised placebo controlled trial of metyrapone augmentation of serotonergic antidepressants in a large population of patients with TRD in the UK National Health Service.

METHODS/DESIGN

Patients with moderate to severe treatment refractory Major Depression aged 18 to 65 will be randomised to metyrapone 500 mg twice daily or placebo for three weeks, in addition to on-going conventional serotonergic antidepressants. The primary outcome will be improvement in Montgomery-Åsberg Depression Rating Scale score five weeks after randomisation (i.e. two weeks after trial medication discontinuation). Secondary outcomes will include the degree of persistence of treatment effect for up to 6 months, improvements in quality of life and also safety and tolerability of metyrapone. The ADD Study will also include a range of sub-studies investigating the potential mechanism of action of metyrapone.

DISCUSSION

Strengths of the ADD study include broad inclusion criteria meaning that the sample will be representative of patients with TRD treated within the UK National Health Service, longer follow up, which to our knowledge is longer than any previous study of antiglucocorticoid treatments in depression, and the range of mechanistic investigations being carried out. The data set acquired will be a rich resource for a range of research questions relating to both refractory depression and the use of antiglucocorticoid treatments.

TRIAL REGISTRATION

Current Controlled Trials: ISRCTN45338259; EudraCT Number: 2009-015165-31.

Vortioxetine (Lu AA21004), a novel multimodal antidepressant, enhances memory in rats

The serotonergic system plays an important role in cognitive functions via various 5-HT receptors. Vortioxetine (Lu AA21004) in development as a novel multimodal antidepressant is a 5-HT3, 5-HT7 and 5-HT1D receptor antagonist, a 5-HT1B receptor partial agonist, a 5-HT1A receptor agonist and a 5-HT transporter (5-HTT) inhibitor in vitro. Preclinical studies suggest that 5-HT3 and 5-HT7 receptor antagonism as well as 5-HT1A receptor agonism may have a positive impact on cognitive functions including memory. Thus vortioxetine may potentially enhance memory. We investigated preclinical effects of vortioxetine (1-10mg/kg administered subcutaneously [s.c.]) on memory in behavioral tests, and on cortical neurotransmitter levels considered important in rat memory function. Contextual fear conditioning and novel object recognition tests were applied to assess memory in rats. Microdialysis studies were conducted to measure extracellular neurotransmitter levels in the rat medial prefrontal cortex. Vortioxetine administered 1h before or immediately after acquisition of contextual fear conditioning led to an increase in freezing time during the retention test. This mnemonic effect was not related to changes in pain sensitivity as measured in the hotplate test. Rats treated with vortioxetine 1h before training spent more time exploring the novel object in the novel object recognition test. In microdialysis studies of the rat medial prefrontal cortex, vortioxetine increased extracellular levels of acetylcholine and histamine. In conclusion, vortioxetine enhanced contextual and episodic memory in rat behavioral models. Further demonstration of its potential effect on memory functions in clinical settings is warranted.

The 5-HT4 receptor levels in hippocampus correlates inversely with memory test performance in humans

The cerebral serotonin (5-HT) system is involved in cognitive functions such as memory and learning and animal studies have repeatedly shown that stimulation of the 5-HT type 4 receptor (5-HT4 R) facilitates memory and learning and further that the 5-HT4 R modulates cellular memory processes in hippocampus. However, any associations between memory functions and the expression of the 5-HT4 R in the human hippocampus have not been investigated. Using positron emission tomography with the tracer [(11) C]SB207145 and Reys Auditory Verbal Learning Test we aimed to examine the individual variation of the 5-HT4R binding in hippocampus in relation to memory acquisition and consolidation in healthy young volunteers. We found significant, negative associations between the immediate recall scores and left and right hippocampal BPND , (p = 0.009 and p = 0.010 respectively) and between the right hippocampal BPND and delayed recall (p = 0.014). These findings provide evidence that the 5-HT4 R is associated with memory functions in the human hippocampus and potentially pharmacological stimulation of the receptor may improve episodic memory.

The effects of acute tryptophan depletion on neuropsychological function, mood and movement in the healthy elderly

Few studies have investigated the function of the serotonin (5-HT) system in the elderly. Previous studies have shown effects of reducing serotonin function, by acute tryptophan depletion (ATD), on neuropsychological function in healthy subjects but this technique has not previously been employed over a wide age range in the elderly. This study compared the effects of ATD on mood, cognitive function and motor function in two groups of healthy volunteers, one group aged 50-69 and the other aged 70-89. The effects of ATD were investigated in a double-blind, placebo-controlled, counterbalanced, crossover, randomized design. The effects of ATD were not significantly different between age groups, suggesting that there is relatively little functional change across these age ranges. Compared with studies in much younger age groups there was, however, more evidence of an adverse effect of ATD on psychomotor function and working memory. There was no effect of ATD on mood despite inclusion of subjects with a family history of depression.

Mechanism of acute tryptophan depletion: is it only serotonin?

The method of acute tryptophan depletion (ATD), which reduces the availability of the essential amino acid tryptophan (TRP), the dietary serotonin (5-hydroxytryptamine (5-HT)) precursor, has been applied in many experimental studies. ATD application leads to decreased availability of TRP in the brain and its synthesis into 5-HT. It is therefore assumed that a decrease in 5-HT release and subsequent blunted neurotransmission is the underlying mechanism for the behavioural effects of ATD. However, direct evidence that ATD decreases extracellular 5-HT concentrations is lacking. Furthermore, several studies provide support for alternative underlying mechanisms of ATD. This may question the utility of the method as a selective serotonergic challenge tool. As ATD is extensively used for investigating the role of 5-HT in cognitive functions and psychiatric disorders, the potential of alternative mechanisms and possible confounding factors should be taken into account. It is suggested that caution is required when interpreting ATD effects in terms of a selective serotonergic effect.

Age and sex effects on 5-HT(4) receptors in the human brain: a [(11)C]SB207145 PET study

Experimental studies indicate that the 5-HT(4) receptor activation influence cognitive function, affective symptoms, and the development of Alzheimer's disease (AD). The prevalence of AD increases with aging, and women have a higher predisposition to both AD and affective disorders than men. This study aimed to investigate sex and age effects on 5-HT(4) receptor-binding potentials in striatum, the limbic system, and neocortex. Positron-emission tomographic scans were conducted using the radioligand [(11)C]SB207145 in a cohort of 30 healthy subjects (mean age 44 years; range 20 to 86 years; 14 men and 16 women). The output parameter, BP(ND), was modeled using the simplified reference tissue model, and partial volume correction was performed with the Muller-Gartner method. A decline with age of 1% per decade was found only in striatum. Women had a 13% lower 5-HT(4) receptor binding in the limbic system. The lower limbic 5-HT(4) receptor binding in women supports a role for 5-HT(4) receptors in the sex-specific differences in emotional control and might contribute to the higher prevalence of affective diseases and AD in women. The relatively stable 5-HT(4) receptor binding with aging contrasts others in subtypes of receptors, which generally decrease with aging.

Evaluating the role of serotonin on neuropsychological function after breast cancer using acute tryptophan depletion

Although cognitive dysfunction is a prevalent and disruptive problem for many breast cancer survivors (BCSs), little research has examined its etiology. One potential mechanism that remains to be explored is serotonin. Serotonin has been implicated in normal and dysfunctional cognitive processes, and serotonin levels are significantly affected by estrogen withdrawal, a common side effect of breast cancer treatment. However, no study has evaluated serotonin's role on cognitive dysfunction in BCSs. The purpose of this study was to examine the role of serotonin in cognitive dysfunction in survivors by lowering central serotonin concentrations via acute tryptophan depletion (ATD). Based on previous research in noncancer populations, we hypothesized that alterations in central serotonin levels would induce cognitive dysfunction in these women controlling for confounding characteristics such as fluctuating mood and glucose levels. Secondarily, we explored whether genetic variations in serotonin genes would partly explain ATD. Participants included 20 female BCSs, posttreatment for nonmetastatic breast cancer, who received ATD or control in a double-blind, crossover design. Cognitive performance was measured at the 5-hr tryptophan/serotonin nadir on each test day using standardized neuropsychological tests. Specific impairment was noted in episodic memory (delayed recall) and motor speed during ATD versus control. ATD did not alter new learning (immediate recall), working memory, verbal fluency, or information processing speed. Findings suggest that serotonin may play a critical role in memory consolidation and motor functioning in BCSs.

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.

Effects of acute tryptophan depletion on neuropsychological and motor function in Parkinson's disease

Interactions between the 5-HT system and the dopaminergic system and cholinergic system may be important in determining cognitive function and motor function in Parkinson's disease (PD). Previous studies have shown effects of reducing serotonin function, by acute tryptophan depletion (ATD), on neuropsychological function. In particular, an adverse effect on verbal memory has been demonstrated. This study compared with the effects of ATD on cognitive and motor function in PD and healthy control subjects. The effects of ATD were investigated in a double-blind, placebo-controlled, counterbalanced, cross-over, randomised design in 20 patients with PD and 35 healthy controls matched for age, gender and premorbid IQ. There was a differential group effect of ATD on global cognitive function whereby the mean score on the modified mini mental state examination during ATD was lower than placebo in PD but higher in controls. There was a similar pattern of effects on verbal recognition. In a visual recognition task, ATD improved performance in the PD but not in the control group. In terms of psychomotor speed, there was also a group-specific effect with reduced latency of response during ATD in the PD group but increased latency in the control group. ATD has subtle neuropsychological effects, which differ significantly between PD and healthy control subjects. This suggests that the dopaminergic and cholinergic deficit of PD significantly modulates the effects of serotonin depletion, resulting in positive effects in some domains. Further investigation on the effects of specific serotonin antagonists may be merited in PD.

Tryptophan metabolism, from nutrition to potential therapeutic applications

Tryptophan is an indispensable amino acid that should to be supplied by dietary protein. Apart from its incorporation into body proteins, tryptophan is the precursor for serotonin, an important neuromediator, and for kynurenine, an intermediary metabolite of a complex metabolic pathway ending with niacin, CO(2), and kynurenic and xanthurenic acids. Tryptophan metabolism within different tissues is associated with numerous physiological functions. The liver regulates tryptophan homeostasis through degrading tryptophan in excess. Tryptophan degradation into kynurenine by immune cells plays a crucial role in the regulation of immune response during infections, inflammations and pregnancy. Serotonin is synthesized from tryptophan in the gut and also in the brain, where tryptophan availability is known to influence the sensitivity to mood disorders. In the present review, we discuss the major functions of tryptophan and its role in the regulation of growth, mood, behavior and immune responses with regard to the low availability of this amino acid and the competition between tissues and metabolic pathways for tryptophan utilization.

F15599, a preferential post-synaptic 5-HT1A receptor agonist: activity in models of cognition in comparison with reference 5-HT1A receptor agonists

We assessed the activity of F15599, a selective and high efficacy 5-HT(1A) agonist that preferentially activates post- versus pre-synaptic receptors, in rat cognition/memory models. F15599 (0.16 mg/kg i.p.) partially alleviated detrimental effects of phencyclidine on working and reference memory deficit in a hole-board model. It also attenuated phencyclidine-induced deficit of cognitive flexibility in a reversal learning task, without effects of its own. F13714 (0.04 mg/kg) a chemical congener of F15599, and 8-OH-DPAT (0.01 or 0.16), were inactive against these phencyclidine-induced deficits, and/or even worsened basal performances. F15599 (0.04-2.5) was less disruptive than F13714 (0.005-0.16) or 8-OH-DPAT (0.01-0.63), on basal performance in models of attention (5-choice serial reaction time task) and working memory (delayed non-matching to position). Finally, unlike either comparator, F15599 reduced PPI with modest potency and only partially. To conclude, F15599, in models of memory/cognition, has a more favourable profile than F13714 and 8-OH-DPAT. This suggests that preferential activation of post-synaptic 5-HT(1A) receptors could prove useful in pathologies characterized by cognitive/memory deficiencies, such as schizophrenia and depression.

Mood state moderates the role of serotonin in cognitive biases

Reduction of the monoamine serotonin (5-HT) via the dietary manipulation of tryptophan (acute tryptophan depletion; ATD) has been shown to induce negative cognitive biases similar to those found in depression in healthy individuals. However, evidence also indicates that there can be positive effects of ATD on both mood and reinforcement processing. Here, we present two separate studies, with remarkably similar findings, which may help explain these discrepancies. In both experiments, we assessed cognitive biases following experimentally induced mood states under both a balanced amino acid drink (BAL) and ATD. A significant interaction between treatment, mood state and cognitive bias was observed in both experiments. In the first experiment, subjects undergoing positive mood induction demonstrated a positive cognitive bias on BAL, which was abolished by ATD. The same effect was observed in subjects undergoing neutral mood induction in the second experiment. These effects replicate findings in healthy individuals undergoing ATD. Subjects undergoing negative mood induction, by contrast, demonstrated the opposite pattern of results; in both experiments, they showed no bias under BAL but induction of a positive cognitive bias by ATD. These results mimic previous findings in currently depressed patients undergoing ATD. We therefore suggest that mood state moderates the effect of ATD on cognitive biases. This, in turn, has important implications for the understanding of the role of 5-HT in psychiatric disorders.

Acute dopamine and/or serotonin depletion does not modulate mismatch negativity (MMN) in healthy human participants

RATIONALE

Schizophrenia is commonly associated with impairments in pre-attentive change detection, as represented by reduced mismatch negativity (MMN). While the neurochemical basis of MMN has been linked to N-methyl-D: -aspartic acid (NMDA) receptor function, the roles of the dopaminergic and/or the serotonergic systems are not fully explored in humans.

OBJECTIVES

The aim of the present study was to investigate the effects of acutely depleting dopamine (DA) and serotonin (5-hydroxytryptamine, 5-HT) alone or simultaneously by depleting their amino acid precursors on MMN in healthy participants.

METHODS

Sixteen healthy male subjects participated in a double-blind, placebo-controlled, cross-over design in which each subject's duration MMN was assessed under four acute treatment conditions separated by a 5-day washout period: balanced amino acid control (no depletion), tyrosine/phenylalanine depletion (to reduce DA neurotransmission), tryptophan depletion (to reduce 5-HT neurotransmission) and tryptophan/tyrosine/phenylalanine depletion (to reduce DA and 5-HT neurotransmission simultaneously).

RESULTS

Acute depletion of either DA and 5-HT alone or simultaneously had no effect on MMN.

CONCLUSIONS

These findings suggest that modulation of the dopaminergic and serotonergic systems acutely does not lead to changes in MMN.

Low-dose tryptophan depletion in recovered depressed women induces impairments in autobiographical memory specificity

BACKGROUND

Depressed patients perform poorly on tests of autobiographical memory specificity (AMS); this may have negative consequences for other important cognitive abilities, delays recovery from mood episodes, and, in recovered patients, may mediate vulnerability to future episodes. Although the cognitive mechanisms underlying AMS deficits are beginning to be understood, the neurobiological mechanisms remain unclear. Serotonin is implicated in both depression and long-term memory; therefore, temporary lowering of brain serotonin function via acute tryptophan depletion (ATD) offers a means of studying the role of serotonin in autobiographical memory specificity.

MATERIALS AND METHODS

In this study, 24 previously depressed women underwent low-dose ATD or sham depletion and completed tests of initial and delayed memory, recollection- and familiarity-based recognition, and AMS.

RESULTS

ATD did not differentially affect state mood. Compared with sham depletion, ATD impaired immediate recall on the Auditory Verbal Learning Test. Although ATD did not differentially impair recollection- and familiarity-based recognition, it did slow recognition of positive words. ATD also reduced autobiographical memory specificity in response to negative cue words.

DISCUSSION

The results confirm previous findings that low-dose ATD can reinstate depression-congruent biases in cognition without causing depressive mood in vulnerable populations. The ATD-induced reduction in memory specificity suggests that serotonergic dysfunction may mediate depressive deficits in autobiographical memory; the interaction of cognitive and neurobiological vulnerability mechanisms is discussed.

Evaluating the role of serotonin in hot flashes after breast cancer using acute tryptophan depletion

OBJECTIVE

Among women with breast cancer, hot flashes are frequent, severe, and bothersome symptoms that can negatively impact quality of life and compromise compliance with life-saving medications (eg, tamoxifen and aromatase inhibitors). Clinicians' abilities to treat hot flashes are limited due to inadequate understanding of physiological mechanisms involved in hot flashes. Using an acute tryptophan depletion paradigm, we tested whether alterations in central serotonin levels were involved in the induction of hot flashes in women with breast cancer.

METHODS

This was a within-participant, double-blind, controlled, balanced, crossover study. Twenty-seven women completed two 9-hour test days. On one test day, women ingested a concentrated amino acid drink and encapsulated amino acids (no tryptophan) according to published procedures that have been shown to have specific effects on serotonin within 4.5 to 7 hours. On the other test day, women ingested a control drink. Serial venous blood sampling and objective hot flash monitoring were used to evaluate response to each condition.

RESULTS

Response to acute tryptophan depletion was variable and unexplained by use of selective serotonin reuptake inhibitors, antiestrogens, breast cancer disease and treatment variables, or genetic polymorphisms in serotonin receptor and transporter genes. Contrary to our hypothesis, hot flashes were not worsened with acute tryptophan depletion.

CONCLUSIONS

Physiologically documented and self-reported hot flashes were not exacerbated by tryptophan depletion. Additional mechanistic research is needed to better understand the etiology of hot flashes.

Acute tryptophan depletion evokes negative mood in healthy females who have previously experienced concurrent negative mood and tryptophan depletion

INTRODUCTION

The majority of individuals who suffer an episode of depression go on to experience recurrences. We have proposed, based upon the observation that reducing serotonin via acute tryptophan depletion (ATD) is more likely to induce negative mood in recovered depressed individuals than never depressed individuals, that this may be because associations form between negative mood and reduced serotonin during an episode of depression (Robinson and Sahakian, Psychol Med 38:315-318, 2008b). Such associations would mean that subsequent reductions in serotonin are more likely to provoke depressed mood and hence trigger an episode of depression.

METHODS

In this study, we tested this hypothesis by manipulating the mood state of healthy females undergoing ATD (or balanced placebo) on two separate testing sessions. On the first session, subjects received either negative or neutral mood induction, while on the second session all subjects received neutral mood induction.

RESULTS

Our findings demonstrate significant ATD-induced negative mood exclusively on the second visit of subjects who received both ATD and negative mood induction procedure on their first visit.

DISCUSSION

These findings may be explained by the formation of an association between the negative mood and reduced serotonin states during the first visit. As such, these findings provide preliminary support for the associative hypothesis of recurrence in depression.

CONCLUSION

Such associations might therefore explain the discrepancy between the effects of ATD in recovered- and never-depressed individuals and may, in turn, explain why an episode of depression increases the risk of subsequent episodes.

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.

Sex differences in learning processes of classical and operant conditioning

Males and females learn and remember differently at different times in their lives. These differences occur in most species, from invertebrates to humans. We review here sex differences as they occur in laboratory rodent species. We focus on classical and operant conditioning paradigms, including classical eyeblink conditioning, fear-conditioning, active avoidance and conditioned taste aversion. Sex differences have been reported during acquisition, retention and extinction in most of these paradigms. In general, females perform better than males in the classical eyeblink conditioning, in fear-potentiated startle and in most operant conditioning tasks, such as the active avoidance test. However, in the classical fear-conditioning paradigm, in certain lever-pressing paradigms and in the conditioned taste aversion, males outperform females or are more resistant to extinction. Most sex differences in conditioning are dependent on organizational effects of gonadal hormones during early development of the brain, in addition to modulation by activational effects during puberty and adulthood. Critically, sex differences in performance account for some of the reported effects on learning and these are discussed throughout the review. Because so many mental disorders are more prevalent in one sex than the other, it is important to consider sex differences in learning when applying animal models of learning for these disorders. Finally, we discuss how sex differences in learning continue to alter the brain throughout the lifespan. Thus, sex differences in learning are not only mediated by sex differences in the brain, but also contribute to them.