Effects of tryptophan loading on verbal, spatial and affective working memory functions in healthy adults

Serotonin and emotion regulation: the impact of tryptophan depletion on emotional experience, neural and autonomic activity

The involvement of serotonin in emotion and psychopathology has been extensively examined. Studies using acute tryptophan depletion (ATD) have found limited effects on mood and aggression, and one of the explanations suggests that serotonin may be involved in higher-order functions, such as emotion regulation. However, there is very limited evidence for this hypothesis. The present study investigated the impact of ATD on emotion regulation in a double-blind, placebo-controlled, crossover design. A sample of psychiatrically healthy men (N = 28) completed a cognitive task assessing reappraisal ability (i.e., the success of using reappraisal, an emotion regulation strategy, to modulate emotional responses), following ATD and placebo. EEG frontal activity and asymmetry, as well as heart-rate variability (HRV), also were assessed in the reappraisal task. Both frequentist and Bayesian methods were employed for statistical analysis. Results indicated that ATD reduced plasma tryptophan, and reappraisal was effective in modulating emotional experience in the emotion regulation task. However, ATD had no significant effect on reappraisal ability, frontal activity, and HRV. These results offer direct and compelling evidence that decreasing serotonin synthesis through ATD does not alter an emotion regulation ability that is considered crucial in mood and aggression and has been linked with transdiagnostic risk of psychopathology.

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

INTRODUCTION

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

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

PURPOSE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Neuromodulation of prefrontal cortex cognitive function in primates: the powerful roles of monoamines and acetylcholine

The primate prefrontal cortex (PFC) subserves our highest order cognitive operations, and yet is tremendously dependent on a precise neurochemical environment for proper functioning. Depletion of noradrenaline and dopamine, or of acetylcholine from the dorsolateral PFC (dlPFC), is as devastating as removing the cortex itself, and serotonergic influences are also critical to proper functioning of the orbital and medial PFC. Most neuromodulators have a narrow inverted U dose response, which coordinates arousal state with cognitive state, and contributes to cognitive deficits with fatigue or uncontrollable stress. Studies in monkeys have revealed the molecular signaling mechanisms that govern the generation and modulation of mental representations by the dlPFC, allowing dynamic regulation of network strength, a process that requires tight regulation to prevent toxic actions, e.g., as occurs with advanced age. Brain imaging studies in humans have observed drug and genotype influences on a range of cognitive tasks and on PFC circuit functional connectivity, e.g., showing that catecholamines stabilize representations in a baseline-dependent manner. Research in monkeys has already led to new treatments for cognitive disorders in humans, encouraging future research in this important field.

Putative role of GPR139 on sleep modulation using pharmacological and genetic rodent models

GPR139 is a G-protein coupled receptor expressed in circumventricular regions of the habenula and septum. Amino acids L-tryptophan and L-phenylalanine have been shown to activate GPR139 at physiologically relevant concentrations. The aim of the present study was to investigate the role of GPR139 on sleep modulation using pharmacological and genetic (GPR139 knockout mice, KO) rodent models. To evaluate the effects of GPR139 pharmacological activation on sleep, rats were orally dosed with the selective GPR139 agonist JNJ-63533054 (3-30 mg/kg). When acutely administered at the beginning of the light phase, the GPR139 agonist dose-dependently reduced non-rapid eye movement (NREM) latency and increased NREM sleep duration without altering rapid eye movement (REM) sleep. This effect progressively dissipated upon 7-day repeated dosing, suggesting functional desensitization. Under baseline conditions, GPR139 KO mice spent less time in REM sleep compared to their wild type littermates during the dark phase, whereas NREM sleep was not altered. Under conditions of pharmacologically enhanced monoamine endogenous tone, GPR139 KO mice showed a blunted response to citalopram or fluoxetine induced REM sleep suppression and an attenuated response to the wake promoting effect of amphetamine. These findings indicate an emerging role of GPR139 in the modulation of sleep states.

Differences in how macaques monitor others: Does serotonin play a central role?

Primates must balance the need to monitor other conspecifics to gain social information while not losing other resource opportunities. We consolidate evidence across the fields of primatology, psychology, and neuroscience to examine individual, population, and species differences in how primates, particularly macaques, monitor conspecifics. We particularly consider the role of serotonin in mediating social competency via social attention, aggression, and dominance behaviors. Finally, we consider how the evolution of variation in social tolerance, aggression, and social monitoring might be explained by differences in serotonergic function in macaques. This article is categorized under: Economics > Interactive Decision-Making Psychology > Comparative Psychology Neuroscience > Behavior Cognitive Biology > Evolutionary Roots of Cognition.

Role of the left hemisphere in visuospatial working memory

Visuospatial processing deficits are typically associated with damage to the right hemisphere. However, deficits on spatial working memory have been reported among some individuals with focal left hemisphere damage (LHD). It has been suggested that the left hemisphere may play a role in such non-verbal working memory tasks due to the use of subvocal, verbally-mediated strategies. The current study investigated the role of the left hemisphere in spatial working memory by testing spatial span performance, both forward and backward, in a large group of individuals with a history of left hemisphere stroke. Our first aim was to establish whether individuals with LHD are indeed impaired on spatial span tasks using standardized span tasks with published normative data. Our second aim was to identify the role that language plays in supporting spatial working memory by comparing LHD individuals with and without aphasia, and by relating spatial span performance to performance on a series of language measures. Our third aim was to identify left hemisphere brain regions that contribute to spatial working memory using voxel-based lesion symptom mapping (VLSM), a whole-brain statistical approach that identifies regions critical to a particular behavior on a voxel-by-voxel basis. We found that 28% of individuals with LHD performed in the clinically-impaired range on forward spatial span and 16% performed in the clinically-impaired range on backward spatial span. There were no significant differences in performance between individuals with and without aphasia, and there were no correlations between spatial span performance and language functions such as repetition and comprehension. The VLSM analysis showed that backward spatial span was associated with a left fronto-parietal network consisting of somatosensory cortex, the supramarginal gyrus, lateral prefrontal cortex, and the frontal eye fields. Regions identified in the VLSM analysis of forward spatial span did not reach the conservative statistical threshold for significance. Overall, these results suggest that spatial working memory, as measured by spatial span, can be significantly disrupted in a subset of individuals with LHD whose lesions infringe on a network of regions in the left hemisphere that have been implicated in domain-general working memory and attentional control mechanisms.

Neurophysiological symptoms and aspartame: What is the connection?

Aspartame (α-aspartyl-l-phenylalanine-o-methyl ester), an artificial sweetener, has been linked to behavioral and cognitive problems. Possible neurophysiological symptoms include learning problems, headache, seizure, migraines, irritable moods, anxiety, depression, and insomnia. The consumption of aspartame, unlike dietary protein, can elevate the levels of phenylalanine and aspartic acid in the brain. These compounds can inhibit the synthesis and release of neurotransmitters, dopamine, norepinephrine, and serotonin, which are known regulators of neurophysiological activity. Aspartame acts as a chemical stressor by elevating plasma cortisol levels and causing the production of excess free radicals. High cortisol levels and excess free radicals may increase the brains vulnerability to oxidative stress which may have adverse effects on neurobehavioral health. We reviewed studies linking neurophysiological symptoms to aspartame usage and conclude that aspartame may be responsible for adverse neurobehavioral health outcomes. Aspartame consumption needs to be approached with caution due to the possible effects on neurobehavioral health. Whether aspartame and its metabolites are safe for general consumption is still debatable due to a lack of consistent data. More research evaluating the neurobehavioral effects of aspartame are required.

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.

Mood induction effects on motor sequence learning and stop signal reaction time

The neurobiological theory of positive affect proposes that positive mood states may benefit cognitive performance due to an increase of dopamine throughout the brain. However, the results of many positive affect studies are inconsistent; this may be due to individual differences. The relationship between dopamine and performance is not linear, but instead follows an inverted "U" shape. Given this, we hypothesized that individuals with high working memory capacity, a proxy measure for dopaminergic transmission, would not benefit from positive mood induction and in fact performance in dopamine-mediated tasks would decline. In contrast, we predicted that individuals with low working memory capacities would receive the most benefit after positive mood induction. Here, we explored the effect of positive affect on two dopamine-mediated tasks, an explicit serial reaction time sequence learning task and the stop signal task, predicting that an individual's performance is modulated not only by working memory capacity, but also on the type of mood. Improvements in explicit sequence learning from pre- to post-positive mood induction were associated with working memory capacity; performance declined in individuals with higher working memory capacities following positive mood induction, but improved in individuals with lower working memory capacities. This was not the case for negative or neutral mood induction. Moreover, there was no relationship between the change in stop signal reaction time with any of the mood inductions and individual differences in working memory capacity. These results provide partial support for the neurobiological theory of positive affect and highlight the importance of taking into account individual differences in working memory when examining the effects of positive mood induction.

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.

Effects of dietary tryptophan and phenylalanine-tyrosine depletion on phasic alertness in healthy adults - A pilot study

Background

The synthesis of the neurotransmitters serotonin (5-HT) and dopamine (DA) in the brain can be directly altered by dietary manipulation of their relevant precursor amino acids (AA). There is evidence that altered serotonergic and dopaminergic neurotransmission are both associated with impaired attentional control. Specifically, phasic alertness is one specific aspect of attention that has been linked to changes in 5-HT and DA availability in different neurocircuitries related to attentional processes. The present study investigated the impact of short-term reductions in central nervous system 5-HT and DA synthesis, which was achieved by dietary depletion of the relevant precursor AA, on phasic alertness in healthy adult volunteers; body weight–adapted dietary tryptophan and phenylalanine–tyrosine depletion (PTD) techniques were used.

Methods

The study employed a double-blind between-subject design. Fifty healthy male and female subjects were allocated to three groups in a randomized and counterbalanced manner and received three different dietary challenge conditions: acute tryptophan depletion (ATD, for the depletion of 5-HT; N=16), PTD (for the depletion of DA; N=17), and a balanced AA load (BAL; N=17), which served as a control condition. Three hours after challenge intake (ATD/PTD/BAL), phasic alertness was assessed using a standardized test battery for attentional performance (TAP). Blood samples for AA level analyses were obtained at baseline and 360 min after the challenge intake.

Results

Overall, there were no significant differences in phasic alertness for the different challenge conditions. Regarding PTD administration, a positive correlation between the reaction times and the DA-related depletion magnitude was detected via the lower plasma tyrosine levels and the slow reaction times of the first run of the task. In contrast, higher tryptophan concentrations were associated with slower reaction times in the fourth run of the task in the same challenge group.

Conclusion

The present study is the first to demonstrate preliminary data that support an association between decreased central nervous system DA synthesis, which was achieved by dietary depletion strategies, and slower reaction times in specific runs of a task designed to assess phasic alertness in healthy adult volunteers; these findings are consistent with previous evidence that links phasic alertness with dopaminergic neurotransmission. A lack of significant differences between the three groups could be due to compensatory mechanisms and the limited sample size, as well as the dietary challenge procedures administered to healthy participants and the strict exclusion criteria used. The potential underlying neurochemical processes related to phasic alertness should be the subject of further investigations.

An Age-Related Dissociation of Short-Term Memory for Facial Identity and Facial Emotional Expression

BACKGROUND

Memory for both facial emotional expression and facial identity was explored in younger and older adults in 3 experiments using a delayed match-to-sample procedure.

METHOD

Memory sets of 1, 2, or 3 faces were presented, which were followed by a probe after a 3-s retention interval.

RESULTS

There was very little difference between younger and older adults in memory for emotional expressions, but memory for identity was substantially impaired in the older adults.

DISCUSSION

Possible explanations for spared memory for emotional expressions include socioemotional selectivity theory as well as the existence of overlapping yet distinct brain networks for processing of different emotions.

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.

Serotonergic modulation of spatial working memory: predictions from a computational network model

Serotonin (5-HT) receptors of types 1A and 2A are strongly expressed in prefrontal cortex (PFC) neurons, an area associated with cognitive function. Hence, 5-HT could be effective in modulating prefrontal-dependent cognitive functions, such as spatial working memory (SWM). However, a direct association between 5-HT and SWM has proved elusive in psycho-pharmacological studies. Recently, a computational network model of the PFC microcircuit was used to explore the relationship between 5-HT and SWM (Cano-Colino et al., 2013). This study found that both excessive and insufficient 5-HT levels lead to impaired SWM performance in the network, and it concluded that analyzing behavioral responses based on confidence reports could facilitate the experimental identification of SWM behavioral effects of 5-HT neuromodulation. Such analyses may have confounds based on our limited understanding of metacognitive processes. Here, we extend these results by deriving three additional predictions from the model that do not rely on confidence reports. Firstly, only excessive levels of 5-HT should result in SWM deficits that increase with delay duration. Secondly, excessive 5-HT baseline concentration makes the network vulnerable to distractors at distances that were robust to distraction in control conditions, while the network still ignores distractors efficiently for low 5-HT levels that impair SWM. Finally, 5-HT modulates neuronal memory fields in neurophysiological experiments: Neurons should be better tuned to the cued stimulus than to the behavioral report for excessive 5-HT levels, while the reverse should happen for low 5-HT concentrations. In all our simulations agonists of 5-HT_1A receptors and antagonists of 5-HT_2A receptors produced behavioral and physiological effects in line with global 5-HT level increases. Our model makes specific predictions to be tested experimentally and advance our understanding of the neural basis of SWM and its neuromodulation by 5-HT receptors.

Serotonin regulates performance nonmonotonically in a spatial working memory network

The prefrontal cortex (PFC) contains a dense network of serotonergic [serotonin, 5-hydroxytryptamine (5-HT)] axons, and endogenous 5-HT markedly modulates PFC neuronal function via several postsynaptic receptors. The therapeutic action of atypical antipsychotic drugs, acting mainly via 5-HT receptors, also suggests a role for serotonergic neurotransmission in cognitive functions. However, psychopharmacological studies have failed to find a consistent relationship between serotonergic transmission and cognitive functions of the PFC, including spatial working memory (SWM). Here, we built a computational network model to investigate 5-HT modulation of SWM in the PFC. We found that 5-HT modulates network's SWM performance nonmonotonically via 5-HT1A and 5-HT2A receptors, following an inverted U-shape. This relationship may contribute to blur the effects of serotonergic agents in previous SWM group-based behavioral studies. Our simulations also showed that errors occurring at low and high 5-HT concentrations are due to different network dynamics instabilities, suggesting that these 2 conditions can be distinguished experimentally based on their distinct dependency on experimental variables. We inferred specific predictions regarding the expected behavioral effects of serotonergic agents in 2 classic working-memory tasks. Our results underscore the relevance of identifying different error types in SWM tasks in order to reveal the association between neuromodulatory systems and SWM.

Genetic polymorphisms of 5-HTT and DAT but not COMT differentially affect verbal and visuospatial working memory functioning

Working memory deficits are found in different psychiatric populations and are most pronounced in schizophrenia. There is preliminary evidence from pharmacological studies that the verbal and visuospatial subcomponents of working memory are subject to differential neurotransmitter modulation. Here, we investigated the impact of well-known polymorphisms of the dopamine transporter gene (SLC6A3, DAT) and the catechol-O-methyl-transferase gene (COMT) as well as the serotonin transporter gene (SLC6A4, 5-HTT) on these specific working memory subcomponents in a mixed sample of patients and healthy individuals. Twenty healthy subjects and 80 patients diagnosed with schizophrenia, bipolar I disorder, or obsessive-compulsive disorder underwent genotyping for the DAT variable number of tandem repeats (VNTR), the COMT val/met-, and the 5-HTT promoter length polymorphism (5-HTTLPR) and neuropsychological testing using a battery of well-characterized, brain circuit-specific working memory tasks. DAT genotype revealed a significant and selective effect on visuospatial working memory, while there was no effect on verbal working memory functioning. 5-HTT genotype, by contrast, exerted a significant and selective effect on verbal working memory task performance. COMT genotype did not show any influence on either working memory domain. The results of the present study provide evidence for a differential impact of genetic polymorphisms of the dopaminergic and serotonergic systems on verbal and visuospatial working memory functioning. Together with prior evidence suggesting the existence of subgroups of schizophrenia patients exhibiting isolated deficits in only one working memory domain, this finding further supports the idea of endophenotypically and pathophysiologically distinct subgroups of schizophrenia with implications for personalized therapeutic approaches.

Test-retest reliability of an emotion maintenance task

Research on working memory has suggested domain-specific components for visual, verbal, and spatial information, and more recently for emotion. Affective working memory has been proposed as the set of processes involved in the maintenance of emotions to guide behaviour. The current study examined the reliability of an emotion maintenance/affective working memory task over two experimental sessions separated by one week. Subjective accuracy based on individual ratings was found to correlate over time and was highest for negatively valenced pictures. Results suggest that this paradigm is a reliable measure of emotion maintenance, underscoring the utility of this measure as an assessment tool for normative and clinical populations.

Electrophysiological indices of altered working memory processes in long-term ecstasy users

OBJECTIVE

The aim of this study was to determine the effect of light long-term ecstasy consumption on verbal short-term and working memory and to identify the cognitive processes contributing to task performance.

METHOD

Electroencephalogram was recorded while ecstasy users (N = 11), polydrug users (N = 13), and non-users (N = 13) completed forward and backward serial recognition tasks designed to engage verbal short-term memory and verbal working memory, respectively.

RESULTS

All three groups displayed significantly lower digit-backward span than digit-forward span with ecstasy users displaying the greatest difference. The parietally distributed P3b was significantly smaller in the digits backward task than in the digits forward task in non-ecstasy-using controls. Ecstasy users did not show the reduced P3b component in the backward task that was seen in both non-ecstasy-using control groups.

CONCLUSIONS

Ecstasy users' performance was suppressed more by the concurrent processing demands of the working memory task than that of the non-ecstasy-using controls. Non-ecstasy-using controls showed differential event-related potential wave forms in the short-term and working memory tasks, and this pattern was not seen in the ecstasy users. This is consistent with a reduction in the cognitive resources allocated to processing in working memory in ecstasy users.

A neurochemical yin and yang: does serotonin activate and norepinephrine deactivate the prefrontal cortex?

INTRODUCTION

The prefrontal cortex (PFC) receives serotonergic input from the dorsal raphe nucleus of the brainstem, as well as noradrenergic input from another brainstem nucleus, the locus coeruleus. A large number of studies have shown that these two neurotransmitter systems, and drugs that affect them, modulate the functional properties of the PFC in both humans and animal models.

RESULTS

Here I examine the hypothesis that serotonin (5-HT) plays a general role in activating the PFC, whereas norepinephrine (NE) plays a general role in deactivating this brain region. In this manner, the two neurotransmitter systems may have opposing effects on PFC-influenced behavior. To assess this hypothesis, three primary lines of evidence are examined comprising the effects of 5-HT and NE on impulsivity, cognitive flexibility, and working memory.

DISCUSSION

While all of the existing data do not unequivocally support the activation/deactivation hypothesis, there is a large body of support for it.

Cognitive enhancement by drugs in health and disease

Attempts to improve cognitive function in patients with brain disorders have become the focus of intensive research efforts. A recent emerging trend is the use of so-called cognitive enhancers by healthy individuals. Here, we consider some of the effects - positive and negative - that current drugs have in neurological conditions and healthy people. We conclude that, to date, experimental and clinical studies have demonstrated relatively modest overall effects, most probably because of substantial variability in response both across and within individuals. We discuss biological factors that might account for such variability and highlight the need to improve testing methods and to extend our understanding of how drugs modulate specific cognitive processes at the systems or network level.

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.

Memory impairments in humans after acute tryptophan depletion using a novel gelatin-based protein drink

Acute tryptophan depletion (ATD) can be used to decrease serotonin levels in the brain. Traditionally, ATD has been established by administering amino acid (AA) mixtures and studies using this method showed that serotonin is involved in learning and memory processes. This study used a recently developed gelatin-based protein drink to examine whether it 1) is superior to the traditional AA method in controlling the tryptophan levels in the placebo condition, 2) impairs long-term memory and 3) differentially affects episodic and spatial memory. Sixteen healthy subjects participated in a double-blind, placebo-controlled study. Memory was assessed using a visual verbal learning test and an object relocation task (spatial memory). Tryptophan ratio significantly decreased after ATD and did not significantly increase in the placebo condition. Delayed recall in the verbal learning test and delayed relocation of objects to positions in the spatial task were impaired after ATD. Spatial short-term memory, however, improved. The current results indicate that the tryptophan levels were essentially neutral in the placebo condition compared with those in the traditional AA mixture. Our study provides further evidence that impairment in long-term episodic and elementary spatial memory after ATD is related to lowered tryptophan levels in plasma.

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

RATIONALE

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

OBJECTIVES

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

MATERIALS AND METHODS

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

RESULTS

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

CONCLUSIONS

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

Low-serotonin levels increase delayed reward discounting in humans

Previous animal experiments have shown that serotonin is involved in the control of impulsive choice, as characterized by high preference for small immediate rewards over larger delayed rewards. Previous human studies under serotonin manipulation, however, have been either inconclusive on the effect on impulsivity or have shown an effect in the speed of action-reward learning or the optimality of action choice. Here, we manipulated central serotonergic levels of healthy volunteers by dietary tryptophan depletion and loading. Subjects performed a "dynamic" delayed reward choice task that required a continuous update of the reward value estimates to maximize total gain. By using a computational model of delayed reward choice learning, we estimated the parameters governing the subjects' reward choices in low-, normal, and high-serotonin conditions. We found an increase of proportion in small reward choices, together with an increase in the rate of discounting of delayed rewards in the low-serotonin condition compared with the control and high-serotonin conditions. There were no significant differences between conditions in the speed of learning of the estimated delayed reward values or in the variability of reward choice. Therefore, in line with previous animal experiments, our results show that low-serotonin levels steepen delayed reward discounting in humans. The combined results of our previous and current studies suggest that serotonin may adjust the rate of delayed reward discounting via the modulation of specific loops in parallel corticobasal ganglia circuits.

Genetic evaluation of the serotonergic system in chronic fatigue syndrome

Chronic fatigue syndrome (CFS) is a debilitating disorder of unknown etiology with no known lesions, diagnostic markers or therapeutic intervention. The pathophysiology of CFS remains elusive, although abnormalities in the central nervous system (CNS) have been implicated, particularly hyperactivity of the serotonergic (5-hydroxytryptamine; 5-HT) system and hypoactivity of the hypothalamic-pituitary-adrenal (HPA) axis. Since alterations in 5-HT signaling can lead to physiologic and behavioral changes, a genetic evaluation of the 5-HT system was undertaken to identify serotonergic markers associated with CFS and potential mechanisms for CNS abnormality. A total of 77 polymorphisms in genes related to serotonin synthesis (TPH2), signaling (HTR1A, HTR1E, HTR2A, HTR2B, HTR2C, HTR3A, HTR3B, HTR4, HTR5A, HTR6, and HTR7), transport (SLC6A4), and catabolism (MAOA) were examined in 137 clinically evaluated subjects (40 CFS, 55 with insufficient fatigue, and 42 non-fatigued, NF, controls) derived from a population-based CFS surveillance study in Wichita, Kansas. Of the polymorphisms examined, three markers (-1438G/A, C102T, and rs1923884) all located in the 5-HT receptor subtype HTR2A were associated with CFS when compared to NF controls. Additionally, consistent associations were observed between HTR2A variants and quantitative measures of disability and fatigue in all subjects. The most compelling of these associations was with the A allele of -1438G/A (rs6311) which is suggested to have increased promoter activity in functional studies. Further, in silico analysis revealed that the -1438 A allele creates a consensus binding site for Th1/E47, a transcription factor implicated in the development of the nervous system. Electrophoretic mobility shift assay supports allele-specific binding of E47 to the A allele but not the G allele at this locus. These data indicate that sequence variation in HTR2A, potentially resulting in its enhanced activity, may be involved in the pathophysiology of CFS.

The effects of serotonin manipulations on emotional information processing and mood

BACKGROUND

Serotonin is implicated in both mood and cognition. It has recently been shown that antidepressant treatment has immediate effects on emotional information processing, which is much faster than any clinically significant effects. This review aims to investigate whether the effects on emotional information processing are reliable, and whether these effects are related to eventual clinical outcome. Treatment-efficiency may be greatly improved if early changes in emotional information processing are found to predict clinical outcome following antidepressant treatment.

METHODS

Review of studies investigating the short-term effects of serotonin manipulations (including medication) on the processing of emotional information, using PubMed and PsycInfo databases.

RESULTS

Twenty-five studies were identified. Serotonin manipulations were found to affect attentional bias, facial emotion recognition, emotional memory, dysfunctional attitudes and decision making. The sequential link between changes in emotional processing and mood remains to be further investigated.

LIMITATIONS

The number of studies on serotonin manipulations and emotional information processing in currently depressed subjects is small. No studies yet have directly tested the link between emotional information processing and clinical outcome during the course of antidepressant treatment.

CONCLUSIONS

Serotonin function is related to several aspects of emotional information processing, but it is unknown whether these changes predict or have any relationship with clinical outcome. Suggestions for future research are provided.

Serotonin transporter polymorphism, memory and hippocampal volume in the elderly: association and interaction with cortisol

The s allele variant of the serotonin transporter gene (5-HTT) has recently been observed to moderate the relationship of stress to depression and anxiety. To date no study has considered interactive effects of 5-HTT genotype, stress and hypothalamic-pituitary-adrenal (HPA) function on cognition in healthy, older adults, which may reflect developmental, functional or neurodegenerative effects of the serotonin transporter polymorphism. We investigated whether 5-HTT genotype interacts with cumulative life stress and HPA-axis measures of waking and diurnal cortisol slope to impact cognition in 154 non-depressed, older adults. Structural images of hippocampal volume were acquired on a subsample of 56 participants. The 5-HTT s allele was associated with both significantly lower delayed recall and higher waking cortisol levels. Presence of the s allele interacted with higher waking cortisol to negatively impact memory. We also observed a significant interaction of higher waking cortisol and the s allele on lower hippocampal volume. Smaller hippocampi and higher cortisol were associated with lower delayed recall only in s allele carriers. No impact or interactions of cumulative life stress with 5-HTT or cortisol were observed. This is the first investigation to identify an association of the 5-HTT s allele with poorer memory function in older adults. The interactive effects of the s allele and waking cortisol levels on reduced hippocampal volume and lower memory suggest that the negative effect of the serotonin polymorphism on memory is mediated by the HPA axis. Further, given the significant association of the s allele with higher waking cortisol in our investigation, future studies may be needed to evaluate the impact of the serotonin transporter polymorphism on any neuropsychiatric or behavioral outcome which is influenced by HPA axis function in older adults.

Chronic reduction in dietary tryptophan leads to a selective impairment of contextual fear memory in mice

The depletion of systemic tryptophan is an important tool to study the effects of reduced 5-HT on cognition. Indeed, previous reports indicated that acute depletion of TRP leads to a memory impairment in human subjects and rodents. From the view of nutrition, it is important to investigate the effects of chronic limitation of L-tryptophan (TRP) on learning and memory formation. In this study, we examined the effects of chronic consumption of a low TRP diet on memory formation in mice. Specifically, we assessed the ability to form contextual fear, cued fear, conditioned taste aversion, and spatial memories in mice fed a TRP-limited diet for at least 1 month. TRP-limited mice showed impaired formation of contextual fear memory that is hippocampus-dependent. In contrast, these mice showed normal hippocampus-dependent spatial memory in the Morris water maze test, as well as in cued fear and conditioned taste aversion memories, which are amygdala-dependent memory processes. Thus, dietary TRP restriction appears to result in selective impairments in hippocampus-dependent contextual fear memory formation in mice.

The psychological, neurochemical and functional neuroanatomical mediators of the effects of positive and negative mood on executive functions

In this review we evaluate the cognitive and neural effects of positive and negative mood on executive function. Mild manipulations of negative mood appear to have little effect on cognitive control processes, whereas positive mood impairs aspects of updating, planning and switching. These cognitive effects may be linked to neurochemistry: with positive mood effects mediated by dopamine while negative mood effects may be mediated by serotonin levels. Current evidence on the effects of mood on regional brain activity during executive functions, indicates that the prefrontal cortex is a recurrent site of integration between mood and cognition. We conclude that there is a disparity between the importance of this topic and awareness of how mood affects, executive functions in the brain. Most behavioural and neuroimaging studies of executive function in normal samples do not explore the potential role of variations in mood, yet the evidence we outline indicates that even mild fluctuations in mood can have a significant influence on neural activation and cognition.

Cortical 5-HT2A receptor signaling modulates anxiety-like behaviors in mice

Serotonin [5-hydroxytryptamine (5-HT)] neurotransmission in the central nervous system modulates depression and anxiety-related behaviors in humans and rodents, but the responsible downstream receptors remain poorly understood. We demonstrate that global disruption of 5-HT2A receptor (5HT2AR) signaling in mice reduces inhibition in conflict anxiety paradigms without affecting fear-conditioned and depression-related behaviors. Selective restoration of 5HT2AR signaling to the cortex normalized conflict anxiety behaviors. These findings indicate a specific role for cortical 5HT2AR function in the modulation of conflict anxiety, consistent with models of cortical, "top-down" influences on risk assessment.

5,7-DHT lesion of the dorsal raphe nuclei impairs object recognition but not affective behavior and corticosterone response to stressor in the rat

Previous studies with acute tryptophan depletion, leading to transient central 5-HT reductions, showed no effects on affective behavior but impaired object memory. In the present study, the behavioral effects of a 5,7-dihydroxytryptamine (5,7-DHT) lesion in the dorsal raphe were evaluated in animal models of anxiety (open field test), depression (forced swimming test), behavioral inhibition (discrete fixed interval test) and cognition (object recognition task). The corticosterone response to a stress condition was examined at several intervals after 5,7-DHT treatment. The substantial reduction in neuronal 5-HT markers in the dorsal raphe did not affect anxiety-related, depressive-like or impulsive behavior. Compared to the SHAM group, the lesioned rats showed a lower response latency to obtain a reward, indicating a quick and accurate reaction to a stimulus. No differences were found in the progressive ratio test for food motivation. A marked impairment in object recognition was found. The 5,7-DHT treatment did not affect the corticosterone response to a stressful situation. Overall, these results corroborate studies with acute tryptophan depletion suggesting a role of 5-HT in object memory, but not affective behavior.

Divergent trajectories in the aging mind: changes in working memory for affective versus visual information with age

Working memory mediates the short-term maintenance of information. Virtually all empirical research on working memory involves investigations of working memory for verbal and visual information. Whereas aging is typically associated with a deficit in working memory for these types of information, recent findings suggestive of relatively well-preserved long-term memory for emotional information in older adults raise questions about working memory for emotional material. This study examined age differences in working memory for emotional versus visual information. Findings demonstrate that, despite an age-related deficit for the latter, working memory for emotion was unimpaired. Further, older adults exhibited superior performance on positive relative to negative emotion trials, whereas their younger counterparts exhibited the opposite pattern.

Tryptophan loading induces oxidative stress

In previous studies tryptophan loads have been administered to human subjects in order to raise central levels of 5-hydroxytryptamine (5HT) and assess the effects of 5HT on behaviour and mood. However, tryptophan is metabolised primarily along the oxidative kynurenine pathway. In this study a 6 g oral tryptophan load was administered to 15 healthy volunteers and the levels of kynurenines and lipid peroxidation products (indicative of oxidative stress) were measured. The results demonstrate that tryptophan loading produces a highly significant increase in lipid peroxidation products in parallel with increased kynurenines. The oxidative stress may result from the generation of quinolinic acid, 3-hydroxykynurenine, and 3-hydroxyanthranilic acid, all of which are known to have the ability to generate free radicals. The results may have implications for the use of tryptophan loading in psychiatric practice, and for the chronic use of diets high in tryptophan.

Pharmacological manipulation of human working memory

RATIONALE

The goal of this paper is to briefly overview human studies that have examined pharmacological agents designed to enhance working memory function, with the idea of providing clues as to promising avenues to follow for the development of drugs likely to enhance working memory and other cognitive processes in individuals with schizophrenia.

OBJECTIVES

We reviewed the studies that have used pharmacological agents designed to target the dopamine system, the noradrenergic system, the acetycholine system, the serotonin system, and the glycine site on NMDA receptors.

RESULTS

There are a large number of studies suggesting that dopamine agents can enhance working memory, though there remain conflicting issues regarding the role that baseline performance plays in modulating the influence of drug and the importance of different dopamine receptors. There is also consistent evidence that cholinesterase inhibitors can enhance working memory function, potentially through improved encoding of the information. There is less consistent evidence that noradrenergic alpha-2 agonists consistently improve working memory in humans, despite the large animal literature suggesting that these agents should have a beneficial effect on memory. As of yet, there is little evidence that agents targeting the glycine site of the NMDA receptor improve working memory, and data to suggest that enhancement of the serotonin system impairs working memory.

CONCLUSIONS

Compounds geared towards enhancing the dopamine system and the acetycholine system remain promising avenues for the development of pro-cognitive drugs, though further work is clearly needed on developing agents that may more selectively target specific receptors.

Simultaneous depletion of serotonin and catecholamines impairs sustained attention in healthy female subjects without affecting learning and memory

Monoamine neurotransmitters, serotonin, noradrenaline and dopamine modulate many important cognitive processes such as attention, learning and memory. While the selective effects of serotonin and catecholamine depletion on such processes have been investigated, the effects of simultaneous depletion of these monoamines on cognition remain unclear. This is of particular interest given that multiple neurotransmitter abnormalities have been implicated in many psychiatric disorders. The aim of the current study was to examine the effects of lowered brain monoamine function on cognitive performance, using the technique of amino acid precursor depletion. The study was a double-blind, placebo-controlled design in which 20 healthy female subjects were tested under a combined monoamine depletion condition (CMD) and a balanced control condition (B). Cognitive testing was conducted at baseline and 5 h post-depletion. The CMD condition relative to the B condition resulted in deficits in digit vigilance (accuracy and reaction time), a measure of sustained attention. There were no effects on measures of learning and memory or psychomotor function. These findings suggest that simultaneously depleting the availability of brain serotonin and catecholamines in healthy female subjects selectively impairs sustained attention, without affecting other cognitive domains.