Selective effects of acute serotonin and catecholamine depletion on memory in healthy women

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.

Protocatechuic Acid Prevents Some of the Memory-Related Behavioural and Neurotransmitter Changes in a Pyrithiamine-Induced Thiamine Deficiency Model of Wernicke-Korsakoff Syndrome in Rats

The purpose of this research was to investigate the effects of protocatechuic acid (PCA) at doses of 50 and 100 mg/kg on the development of unfavourable changes in cognitive processes in a pyrithiamine-induced thiamine deficiency (PTD) model of the Wernicke-Korsakoff syndrome (WKS) in rats. The effects of PCA were assessed at the behavioural and biochemical levels. Behavioural analysis was conducted using the Foot Fault test (FF), Bar test, Open Field test, Novel Object Recognition test (NOR), Hole-Board test and Morris Water Maze test (MWM). Biochemical analysis consisting of determination of concentration and turnover of neurotransmitters in selected structures of the rat CNS was carried out using high-performance liquid chromatography. PTD caused catalepsy (Bar test) and significantly impaired motor functions, leading to increased ladder crossing time and multiplied errors due to foot misplacement (FF). Rats with experimentally induced WKS showed impaired consolidation and recall of spatial reference memory in the MWM test, while episodic memory related to object recognition in the NOR was unimpaired. Compared to the control group, rats with WKS showed reduced serotonin levels in the prefrontal cortex and changes in dopamine and/or norepinephrine metabolites in the prefrontal cortex, medulla oblongata and spinal cord. PTD was also found to affect alanine, serine, glutamate, and threonine levels in certain areas of the rat brain. PCA alleviated PTD-induced cataleptic symptoms in rats, also improving their performance in the Foot Fault test. In the MWM, PCA at 50 and 100 mg/kg b.w. improved memory consolidation and the ability to retrieve acquired information in rats, thereby preventing unfavourable changes caused by PTD. PCA at both tested doses was also shown to have a beneficial effect on normalising PTD-disrupted alanine and glutamate concentrations in the medulla oblongata. These findings demonstrate that certain cognitive deficits in spatial memory and abnormalities in neurotransmitter levels persist in rats that have experienced an acute episode of PTD, despite restoration of thiamine supply and long-term recovery. PCA supplementation largely had a preventive effect on the development of these deficits, to some extent also normalising neurotransmitter concentrations in the brain.

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.

The impact of dietary macronutrient intake on cognitive function and the brain

Macronutrients - carbohydrates, fats, and proteins - supply the nutrients required for optimal functioning. Inadequate intake compromises both physical and brain health. We synthesized research on macronutrients from whole meals on cognitive function in healthy adults and identified underlying mechanisms. Intake of simple carbohydrates ('sugars') is consistently associated with decreased global cognition whereas consumption of complex carbohydrates correlates with successful brain aging and improved memory both in the short- and long-term. Saturated fatty acid intake correlates with decreased memory and learning scores whereas omega-3 intake correlates positively with memory scores. Protein intake boosts executive function and working memory when task-demands are high. Individual differences affecting the macronutrient-cognition relationship are age, physical activity, and glucose metabolism. Neural correlates reflect findings on cognitive functions: cortical thickness and cerebral amyloid burden correlate with sugar intake, inflammatory status and cerebral glucose metabolism correlate with fatty acid intake. Key mechanisms by which dietary macronutrients affect the brain and cognition include glucose and insulin metabolism, neurotransmitter actions, and cerebral oxidation and inflammation. In conclusion, macronutrient intake affects cognitive function both acutely and in the long-term, involving peripheral and central mechanisms. A healthy diet supports brain integrity and functionality, whereas inadequate nutrition compromises it. Studying diet can be key to nutritional recommendations, thereby improving the landscape of mental health and healthy brain aging.

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.

Hyperactivity of caudate, parahippocampal, and prefrontal regions during working memory in never-medicated persons at clinical high-risk for psychosis

BACKGROUND

Deficits in working memory (WM) are a core feature of schizophrenia (SZ) and other psychotic disorders. We examined brain activity during WM in persons at clinical high risk (CHR) for psychosis.

METHODS

Thirty-seven CHR and 34 healthy control participants underwent functional MRI (fMRI) on a 3.0T scanner while performing an N-back WM task. The sample included a sub-sample of CHR participants who had no lifetime history of treatment with psychotropic medications (n=11). Data were analyzed using SPM8 (2-back>0-back contrast). Pearson correlations between brain activity, symptoms, and WM performance were examined.

RESULTS

The total CHR group and medication-naive CHR sub-sample were comparable to controls in most demographic features and in N-back WM performance, but had significantly lower IQ. Relative to controls, medication-naïve CHR showed hyperactivity in the left parahippocampus (PHP) and the left caudate during performance of the N-back WM task. Relative to medication-exposed CHR, medication naïve CHR exhibited hyperactivity in the left caudate and the right dorsolateral prefrontal cortex (DLPFC). DLPFC activity was significantly negatively correlated with WM performance. PHP, caudate and DLPFC activity correlated strongly with symptoms, but results did not withstand FDR-correction for multiple comparisons. When all CHR participants were combined (regardless of medication status), only trend-level PHP hyperactivity was observed in CHR relative to controls.

CONCLUSIONS

Medication-naïve CHR exhibit hyperactivity in regions that subserve WM. These regions are implicated in studies of schizophrenia and risk for psychosis. Results emphasize the importance of medication status in the interpretation of task - induced brain activity.

Insufficient intake of L-histidine reduces brain histamine and causes anxiety-like behaviors in male mice

L-histidine is one of the essential amino acids for humans, and it plays a critical role as a component of proteins. L-histidine is also important as a precursor of histamine. Brain histamine is synthesized from L-histidine in the presence of histidine decarboxylase, which is expressed in histamine neurons. In the present study, we aimed to elucidate the importance of dietary L-histidine as a precursor of brain histamine and the histaminergic nervous system. C57BL/6J male mice at 8 wk of age were assigned to 2 different diets for at least 2 wk: the control (Con) diet (5.08 g L-histidine/kg diet) or the low L-histidine diet (LHD) (1.28 g L-histidine/kg diet). We measured the histamine concentration in the brain areas of Con diet-fed mice (Con group) and LHD-fed mice (LHD group). The histamine concentration was significantly lower in the LHD group [Con group vs. LHD group: histamine in cortex (means ± SEs): 13.9 ± 1.25 vs. 9.36 ± 0.549 ng/g tissue; P = 0.002]. Our in vivo microdialysis assays revealed that histamine release stimulated by high K(+) from the hypothalamus in the LHD group was 60% of that in the Con group (P = 0.012). However, the concentrations of other monoamines and their metabolites were not changed by the LHD. The open-field tests showed that the LHD group spent a shorter amount of time in the central zone (87.6 ± 14.1 vs. 50.0 ± 6.03 s/10 min; P = 0.019), and the light/dark box tests demonstrated that the LHD group spent a shorter amount of time in the light box (198 ± 8.19 vs. 162 ± 14.1 s/10 min; P = 0.048), suggesting that the LHD induced anxiety-like behaviors. However, locomotor activity, memory functions, and social interaction did not differ between the 2 groups. The results of the present study demonstrated that insufficient intake of histidine reduced the brain histamine content, leading to anxiety-like behaviors in the mice.

Tyrosine-free amino acid mixtures reduce physiologically-evoked release of dopamine in a selective and activity-dependent manner

Depletion of the catecholamine precursor tyrosine using tyrosine-free amino acid mixtures is an important tool in neuropsychological studies, and often considered dopamine selective on the basis of neuropharmacological studies. However, little is known of the effects of tyrosine depletion when catecholamine neurons are activated physiologically. Here we investigated the effect of tyrosine-free amino acid mixtures on catecholamine release evoked in vivo using a stimulation paradigm aimed to approximate the phasic firing pattern of these neurons that accompanies cognitive and behavioural change. Dopamine and noradrenaline release was monitored by microdialysis in rat medial prefrontal cortex (mPFC) and striatum (chloral hydrate anaesthesia, perfusion medium containing 1 µM cocaine). Electrical stimulation of the medial forebrain bundle (MFB) caused a short-lasting, frequency-dependent increase in dopamine and noradrenaline. A full tyrosine-free amino acid mixture reduced the release of dopamine in mPFC and striatum, across a range of stimulation frequencies, and the effect was greater as stimulation frequency increased. Similar results were obtained using a smaller tyrosine-free amino acid mixture. In the same experiments showing decreased dopamine, neither tyrosine-free mixture of amino acids significantly altered stimulation-evoked release of noradrenaline. These results show that tyrosine depletion using tyrosine-free amino acid mixtures causes a selective, activity-dependent decrease in dopamine release when dopamine neurons are driven physiologically.

Genome-wide association study of monoamine metabolite levels in human cerebrospinal fluid

Studying genetic determinants of intermediate phenotypes is a powerful tool to increase our understanding of genotype-phenotype correlations. Metabolic traits pertinent to the central nervous system (CNS) constitute a potentially informative target for genetic studies of intermediate phenotypes as their genetic underpinnings may elucidate etiological mechanisms. We therefore conducted a genome-wide association study (GWAS) of monoamine metabolite (MM) levels in cerebrospinal fluid (CSF) of 414 human subjects from the general population. In a linear model correcting for covariates, we identified one locus associated with MMs at a genome-wide significant level (standardized β=0.32, P=4.92 × 10(-8)), located 20 kb from SSTR1, a gene involved with brain signal transduction and glutamate receptor signaling. By subsequent whole-genome expression quantitative trait locus (eQTL) analysis, we provide evidence that this variant controls expression of PDE9A (β=0.21; P unadjusted=5.6 × 10(-7); P corrected=0.014), a gene previously implicated in monoaminergic transmission, major depressive disorder and antidepressant response. A post hoc analysis of loci significantly associated with psychiatric disorders suggested that genetic variation at CSMD1, a schizophrenia susceptibility locus, plays a role in the ratio between dopamine and serotonin metabolites in CSF. The presented DNA and mRNA analyses yielded genome-wide and suggestive associations in biologically plausible genes, two of which encode proteins involved with glutamate receptor functionality. These findings will hopefully contribute to an exploration of the functional impact of the highlighted genes on monoaminergic transmission and neuropsychiatric phenotypes.

Dopamine precursor depletion impairs structure and efficiency of resting state brain functional networks

Spatial patterns of functional connectivity derived from resting brain activity may be used to elucidate the topological properties of brain networks. Such networks are amenable to study using graph theory, which shows that they possess small world properties and can be used to differentiate healthy subjects and patient populations. Of particular interest is the possibility that some of these differences are related to alterations in the dopamine system. To investigate the role of dopamine in the topological organization of brain networks at rest, we tested the effects of reducing dopamine synthesis in 13 healthy subjects undergoing functional magnetic resonance imaging. All subjects were scanned twice, in a resting state, following ingestion of one of two amino acid drinks in a randomized, double-blind manner. One drink was a nutritionally balanced amino acid mixture, and the other was tyrosine and phenylalanine deficient. Functional connectivity between 90 cortical and subcortical regions was estimated for each individual subject under each dopaminergic condition. The lowered dopamine state caused the following network changes: reduced global and local efficiency of the whole brain network, reduced regional efficiency in limbic areas, reduced modularity of brain networks, and greater connection between the normally anti-correlated task-positive and default-mode networks. We conclude that dopamine plays a role in maintaining the efficient small-world properties and high modularity of functional brain networks, and in segregating the task-positive and default-mode networks. This article is part of the Special Issue Section entitled 'Neuroimaging in Neuropharmacology'.

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

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

Effects of acute dopamine precusor depletion on immediate reward selection bias and working memory depend on catechol-O-methyltransferase genotype

Little agreement exists as to acute dopamine (DA) manipulation effects on intertemporal choice in humans. We previously found that catechol-O-methyltransferase (COMT) Val158Met genotype predicts individual differences in immediate reward selection bias among adults. Moreover, we and others have shown that the relationship between COMT genotype and immediate reward bias is inverted in adolescents. No previous pharmacology studies testing DA manipulation effects on intertemporal choice have accounted for COMT genotype, and many have included participants in the adolescent age range (18-21 years) as adults. Moreover, many studies have included female participants without strict cycle phase control, although recent evidence demonstrates that cyclic estradiol elevations interact with COMT genotype to affect DA-dependent cognition. These factors may have interacted with DA manipulations in past studies, potentially occluding detection of effects. Therefore, we predicted that, among healthy male adults (ages 22-40 years), frontal DA tone, as indexed by COMT genotype, would interact with acute changes in DA signaling to affect intertemporal choice. In a double-blind, placebo-controlled design, we decreased central DA via administration of an amino acid beverage deficient in the DA precursors, phenylalanine and tyrosine, and tested effects on immediate reward bias in a delay-discounting (DD) task and working memory (WM) in an n-back task. We found no main effect of beverage on DD or WM performance but did find significant beverage*genotype effects. These results suggest that the effect of DA manipulations on DD depends on individual differences in frontal DA tone, which may have impeded some past efforts to characterize DA's role in immediate reward bias in humans.

Effects of acute dopamine precusor depletion on immediate reward selection bias and working memory depend on catechol-O-methyltransferase genotype

Little agreement exists as to acute dopamine (DA) manipulation effects on intertemporal choice in humans. We previously found that catechol-O-methyltransferase (COMT) Val158Met genotype predicts individual differences in immediate reward selection bias among adults. Moreover, we and others have shown that the relationship between COMT genotype and immediate reward bias is inverted in adolescents. No previous pharmacology studies testing DA manipulation effects on intertemporal choice have accounted for COMT genotype, and many have included participants in the adolescent age range (18-21 years) as adults. Moreover, many studies have included female participants without strict cycle phase control, although recent evidence demonstrates that cyclic estradiol elevations interact with COMT genotype to affect DA-dependent cognition. These factors may have interacted with DA manipulations in past studies, potentially occluding detection of effects. Therefore, we predicted that, among healthy male adults (ages 22-40 years), frontal DA tone, as indexed by COMT genotype, would interact with acute changes in DA signaling to affect intertemporal choice. In a double-blind, placebo-controlled design, we decreased central DA via administration of an amino acid beverage deficient in the DA precursors, phenylalanine and tyrosine, and tested effects on immediate reward bias in a delay-discounting (DD) task and working memory (WM) in an n-back task. We found no main effect of beverage on DD or WM performance but did find significant beverage*genotype effects. These results suggest that the effect of DA manipulations on DD depends on individual differences in frontal DA tone, which may have impeded some past efforts to characterize DA's role in immediate reward bias in humans.

Presynaptic regulation of extracellular dopamine levels in the medial prefrontal cortex and striatum during tyrosine depletion

RATIONALE

Available neurochemical probes that lower brain dopamine (DA) levels in man are limited by their tolerability and efficacy. For instance, the acute lowering of brain tyrosine is well tolerated, but only modestly lowers brain DA levels. Modification of tyrosine depletion to robustly lower DA levels would provide a superior research probe.

OBJECTIVES

The objective of this study was to determine whether the subthreshold stimulation of presynaptic DA receptors would potentiate tyrosine depletion-induced effects on extracellular DA levels in the medial prefrontal cortex (MPFC) and striatum of the rat.

METHODS

We administered quinpirole, a predominantly DA type 2 (D2R) receptor agonist, into the MPFC and striatum by reverse dialysis. A tyrosine- and phenylalanine-free neutral amino acid mixture [NAA(-)] IP was used to lower brain tyrosine levels. DA levels in the microdialysate were measured by HPLC with electrochemical detection.

RESULTS

Quinpirole dose-dependently lowered DA levels in MPFC as well as in the striatum. NAA(-) alone transiently lowered DA levels (80 % baseline) in the striatum, but had no effect in MPFC. The co-administration of NAA(-) and a subthreshold concentration of quinpirole (6.25 nM) lowered DA levels (50 % baseline) in both the MPFC and striatum. This effect was blocked by the mixed D2R/D3R antagonist haloperidol at IP doses that on their own did not affect DA levels (10.0 nmol/kg in the MPFC and 0.10 nmol/kg in the striatum).

CONCLUSIONS

Pharmacological stimulation of inhibitory D2R receptors during tyrosine depletion markedly lowers the extracellular DA levels in the MPFC and striatum. The data suggest that combining tyrosine depletion with a low dose of a DA agonist should robustly lower brain regional DA levels in man.

Effect of low-dose acute tryptophan depletion on the specificity of autobiographical memory in healthy subjects with a family history of depression

RATIONALE

Low-dose acute tryptophan depletion (LD-ATD), while having no effect on mood, has been shown to reduce specificity of autobiographical memory in patients who have recovered from a depressive episode.

OBJECTIVES

This study aimed to explore if reduced specificity of autobiographical memory with LD-ATD is common to other groups of individuals at risk of depression, specifically a healthy population with a family history of depression.

METHODS

Nineteen healthy young adults with at least one first-degree relative with a history of major depression were recruited. LD-ATD drinks containing 1.15 g of tryptophan (T+) or no tryptophan (T−) were administered on two separate occasions, in a double blind random order crossover design. The Autobiographical Memory Test (AMT) was administered 5 h after drink administration.

RESULTS

Analysis of variance revealed a significant difference in the effects of LD-ATD drinks on plasma free tryptophan with no mood change with either drink. There was no within-subject main effect of LD-ATD on the memory task. However, there was a main effect of order of drink. Exploratory analysis of visit 1 data indicated a large between-subject effect (d=1.4) of LD-ATD on AMT with T− associated with less specificity in response to negative cue words (F(1, 17)08.71, p=0.009).

CONCLUSIONS

Similar to findings following recovery from depression, LD-ATD can reduce specificity of AMT in the absence of lowered mood in healthy individuals with a strong family history of depression. These findings may reflect a 5-HT-dependent cognitive vulnerability to depression in different populations and warrant further research.

Interactive effects of estrogen and serotonin on brain activation during working memory and affective processing in menopausal women

While cognitive changes and mood instability are frequent symptoms reported by menopausal women, the degree to which the decline in estrogen production is responsible is not yet clear. Several lines of evidence suggest that estrogen may produce its effects on cognition and mood through modulation of serotonergic function. To test this hypothesis, we used the tryptophan depletion (TD) paradigm to lower central serotonin levels and pharmacologically manipulated estrogen levels in healthy menopausal women. We examined the individual and combined effects of estradiol and serotonin on working memory, emotion processing and task-related brain activation. Eight healthy predominantly early postmenopausal women underwent TD or sham depletion followed by functional magnetic resonance imaging (fMRI) both before and after short-term transdermal estradiol 75-150 μg/d administration. There was an estradiol treatment by TD interaction for brain activation during performance on both the N-back Task (working memory) and Emotion Identification Task (affective processing). During the 2-back condition, TD attenuated activation prior to, but not after, estradiol treatment in the right and left dorsal lateral prefrontal and middle frontal/cingulate gyrus. During emotion identification, TD heightened activation in the orbital frontal cortex and bilateral amygdala, and this effect was attenuated by estradiol treatment. These results provide preliminary evidence that serotonergic effects directly mediate the impact of estrogen on brain activation during working memory and affective processing.

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.

Specificity of the acute tryptophan and tyrosine plus phenylalanine depletion and loading tests I. Review of biochemical aspects and poor specificity of current amino Acid formulations

The acute tryptophan or tyrosine plus phenylalanine depletion and loading tests are powerful tools for studying the roles of serotonin, dopamine and noradrenaline in normal subjects and those with behavioural disorders. The current amino acid formulations for these tests, however, are associated with undesirable decreases in ratios of tryptophan or tyrosine plus phenylalanine to competing amino acids resulting in loss of specificity. This could confound biochemical and behavioural findings. Compositions of current formulations are reviewed, the biochemical principles underpinning the tests are revisited and examples of unintended changes in the above ratios and their impact on monoamine function and behaviour will be demonstrated from data in the literature. The presence of excessive amounts of the 3 branched-chain amino acids Leu, Ile and Val is responsible for these unintended decreases and the consequent loss of specificity. Strategies for enhancing the specificity of the different formulations are proposed.

Effects of high-dose large neutral amino acid supplementation on exercise, motor skill, and mental performance in Australian Rules Football players

This study investigated the effects of high-dose large neutral amino acid (LNAA) supplementation on attenuating fatigue-induced decrements in exercise and motor skill performance in Australian Rules Football (ARF) players. Fifteen subelite ARF players participated in 3 testing sessions separated by 7 days. Players completed an initial control trial involving a reactive motor skills test (RMST) and a reactive agility test (RAT) carried out before and after fatiguing exercise. In the subsequent experimental trials, players ingested a serotonin-depleting or protein control (PC) LNAA mixture 3 h before testing, allocated in a double-blind randomized cross-over design. Blood samples were taken at presupplementation and pre- and postexercise for analysis of plasma amino acid, insulin, and metabolite concentrations. The effect of the LNAA was established as the difference in the change in the mean RMST and RAT test scores among the depleting, PC, and baseline (BL) trials. Mean overall repetition time of the RAT was moderately improved by -5.2% ± 3.4% (mean ± 90% confidence limits; effect size -0.45 ± 0.28) after ingestion of the serotonin-depleting mixture compared with the BL trial. Serotonin-depleting and PC supplements had a divergent effect on mean repetition time after fatiguing exercise in RMST: depleting serotonin elicited a small improvement (-3.0% ± 2.7%) in motor skill performance in contrast to a small decrement (2.4% ± 2.7%) after ingestion of the PC mixture, when compared to the BL. High-dose serotonin-"depleting" LNAA supplementation given 3 h prior to intermittent high-intensity exercise improved reactive motor skill and agility performance in ARF players.

Effects of tyrosine/phenylalanine depletion on electrophysiological correlates of memory in healthy volunteers

Dopamine is well known for involvement in reinforcement, motor control and frontal lobe functions, such as attention and memory. Tyrosine/phenylalanine depletion (TPD) lowers dopamine synthesis and can therefore be used as a model to study the effects of low dopamine levels. This is the first study to assess the effect of TPD on memory performance and its electrophysiological correlates. In a double blind placebo (PLA)-controlled crossover design, 17 healthy volunteers (six males, 11 females) aged between 18 and 25 were tested after TPD and PLA. Working memory was assessed using a Sternberg memory scanning task (SMS) and episodic memory using the Visual Verbal Learning Test (VVLT). Simultaneously, event-related potentials (ERPs) were measured. The tyrosine and phenylalanine ratio was significantly reduced after TPD and increased after PLA. Working memory performance was not affected by TPD. However, ERP measures were affected by the treatment, indicating that TPD impaired stimulus processing during working memory performance. Episodic memory was not impaired after TPD. Again, alterations in ERP measures suggested adverse effects of TPD on memory-related processing. These results suggest that dopamine is involved in both working memory and episodic memory-related processing, although the effects are too small to be detected by performance measures.

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 function is related to fronto-striatal serotonin transporter levels--a brain PET study in young healthy subjects

RATIONALE

Pharmacological manipulation of serotonergic neurotransmission in healthy volunteers impacts on cognitive test performance. Specifically, markers of serotonin function are associated with attention and executive functioning, long-term memory, and general cognitive ability. The serotonin transporter (SERT) protein is a key regulator in the serotonin system. We hypothesized that higher performance on tests sensitive to serotonin would be associated with higher SERT levels in specific fronto-striatal brain regions.

METHODS

Thirty-two healthy subjects (25 males, mean age 26.0 years, range 19-37) underwent positron emission tomography using the SERT ligand [(11)C]DASB. Subjects underwent the following tests: Stroop Color Word Test, Trail Making Test B, Rey's Auditory Verbal Learning Test and Complex Figure Test, logical reasoning subtest from Intelligenz-Struktur-Test 2000 R, and a Danish version of National Adult Reading Test.

RESULTS

We found positive associations between performance on the Stroop Color Word Test and right-sided dorsolateral prefrontal SERT binding (R(2) = 0.12, p = 0.048). Furthermore, scores of logical reasoning (correlating with IQ) and educational level associated positively with SERT binding in the caudate, most prominent on the left side (logical reasoning: R(2) = 0.34, p = 0.0026 (left), R(2) = 0.2, p = 0.022 (right), educational level: R(2) = 0.19, p = 0.012 (left), R(2) = 0.15, p = 0.027 (right)). Scores of logical reasoning also associated with left-sided ventrolateral prefrontal cortex (R(2) = 0.24, p = 0.014). There were no significant associations between SERT binding and tests of long-term episodic memory.

CONCLUSIONS

The results imply that in healthy subjects, high SERT binding in fronto-striatal regions is associated with better performance on tasks involving executive function and logical reasoning.

The effects of acute treatment with escitalopram on the different stages of contextual fear conditioning are reversed by atomoxetine

RATIONALE

Although the antidepressant and anxiolytic effects of selective serotonin reuptake inhibitors and serotonin-noradrenaline reuptake inhibitors are well-documented, less is known about their cognitive effects.

OBJECTIVE

Escitalopram, a selective serotonin reuptake inhibitor, and atomoxetine, a selective noradrenaline reuptake inhibitor, were used to evaluate the interaction between noradrenergic and serotonergic neurotransmission in the modulation of contextual fear conditioning in rats.

METHODS

Contextual fear-conditioning test was used to investigate the acute effects of escitalopram, alone or in combination with atomoxetine, in different stages of learning and memory in rats. Furthermore, microdialysis in freely moving animals was used to investigate the effect of escitalopram on serotonin, dopamine, and noradrenaline levels in the rat hippocampus.

RESULTS

Escitalopram significantly increased conditioned responses when applied before the acquisition, but decreased responses, when applied before the recall test. When administered during memory consolidation, escitalopram dose-dependently enhanced conditioned responding. These effects were blocked by atomoxetine. Escitalopram (at a dose that affects memory consolidation) increased hippocampal serotonin levels fourfold without changing dopamine or noradrenaline. Atomoxetine, at dose levels that blocked the effects of escitalopram on contextual fear conditioning, increased the extracellular levels of noradrenaline eightfold but did not change dopamine or serotonin. A combined treatment of escitalopram and atomoxetine caused a significant attenuation of escitalopram-induced increase in serotonin levels, while noradrenaline levels were not affected.

CONCLUSIONS

These findings indicate that escitalopram affects fear memory in rats, likely modulated by increases in serotonin levels in the brain. This effect is impaired by atomoxetine, probably due to a noradrenaline-mediated decrease in serotonin levels. Further studies are warranted to study the effects of potential differences among antidepressant therapies on long-term cognitive outcomes.

Serotonergic neurotransmission and lapses of attention in children and adolescents with attention deficit hyperactivity disorder: availability of tryptophan influences attentional performance

Deficiencies in serotonergic (5-HT) neurotransmission have frequently been linked to altered attention and memory processes. With attention deficit hyperactivity disorder (ADHD) being associated with impaired attention and working memory, this study investigated the effects of a diminished 5-HT turnover achieved by rapid tryptophan depletion (RTD) on attentional performance in children and adolescents with ADHD. Twenty-two male patients with ADHD (aged 9-15 yr) received the RTD procedure Moja-De and a tryptophan (Trp)-balanced placebo (Pla) in a randomized, double-blind, within-subject crossover design on two separate study days. Lapses of attention (LA) and phasic alertness (PA) were assessed within the test battery for attentional performance under depleted and sham-depleted conditions 120 (T1), 220 (T2) and 300 (T3) min after intake of RTD/Pla. At T1 there was a significant main effect for RTD, indicating more LA under intake of a Trp-balanced Pla compared to diminished 5-HT neurotransmission. For T2/T3 there were no such effects. PA was not affected by the factors RTD/Pla and time. Interactions of 5-HT with other neurotransmitters as possible underlying neurochemical processes could be subject to further investigations involving healthy controls as regards altered attentional performance in children and adolescents.

Escitalopram and enhancement of cognitive recovery following stroke

CONTEXT

Adjunctive restorative therapies administered during the first few months after stroke, the period with the greatest degree of spontaneous recovery, reduce the number of stroke patients with significant disability.

OBJECTIVE

To examine the effect of escitalopram on cognitive outcome. We hypothesized that patients who received escitalopram would show improved performance in neuropsychological tests assessing memory and executive functions than patients who received placebo or underwent Problem Solving Therapy.

DESIGN

Randomized trial.

SETTING

Stroke center.

PARTICIPANTS

One hundred twenty-nine patients were treated within 3 months following stroke. The 12-month trial included 3 arms: a double-blind placebo-controlled comparison of escitalopram (n = 43) with placebo (n = 45), and a nonblinded arm of Problem Solving Therapy (n = 41).

OUTCOME MEASURES

Change in scores from baseline to the end of treatment for the Repeatable Battery for the Assessment of Neuropsychological Status (RBANS) and Trail-Making, Controlled Oral Word Association, Wechsler Adult Intelligence Scale-III Similarities, and Stroop tests.

RESULTS

We found a difference among the 3 treatment groups in change in RBANS total score (P < .01) and RBANS delayed memory score (P < .01). After adjusting for possible confounders, there was a significant effect of escitalopram treatment on the change in RBANS total score (P < .01, adjusted mean change in score: escitalopram group, 10.0; nonescitalopram group, 3.1) and the change in RBANS delayed memory score (P < .01, adjusted mean change in score: escitalopram group, 11.3; nonescitalopram group, 2.5). We did not observe treatment effects in other neuropsychological measures.

CONCLUSIONS

When compared with patients who received placebo or underwent Problem Solving Therapy, stroke patients who received escitalopram showed improvement in global cognitive functioning, specifically in verbal and visual memory functions. This beneficial effect of escitalopram was independent of its effect on depression. The utility of antidepressants in the process of poststroke recovery should be further investigated. Trial Registration clinicaltrials.gov Identifier: NCT00071643.

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.

Increased binding to 5-HT1A and 5-HT2A receptors is associated with large vessel infarction and relative preservation of cognition

Vascular dementia accounts for approximately 15-20% of all dementias. In addition, a significant subset of people with Alzheimer's disease have concurrent cerebrovascular disease. Vascular dementia is caused by different cerebrovascular morphological abnormalities including large artery territory infarction (multi-infarct vascular dementia) and sub-cortical ischaemic vascular dementia. Despite this distinction, there is a lack of studies examining the neurochemistry of individual vascular dementia subtypes. Serotonin is believed to play an important role in cognition, and serotonin receptors may provide a novel target for future anti-dementia therapeutics. This study aimed to determine levels of two serotonin receptors in subtypes of vascular dementia and relate any changes to cognition. We have determined, using saturation radioligand binding, the binding parameters (affinity and maximal binding) of ((3)H)-WAY 100635 binding to 5-HT(1A) receptors and ((3)H)-ketanserin binding to 5-HT(2A) receptors in post-mortem tissue from the frontal and temporal cortices of patients with either multi-infarct vascular dementia, sub-cortical ischaemic vascular dementia, mixed Alzheimer's disease/vascular dementia or stroke no dementia (SND). 5-HT(1A) and 5-HT(2A) receptor binding was significantly increased in the temporal cortex of patients with either multi-infarct vascular dementia or SND, compared to age-matched controls. 5-HT(1A) receptor maximal binding in the temporal cortex was also positively correlated with cognition as determined by Mini-Mental State Examination (MMSE) and Cambridge Assessment of Mental Health for the Elderly scores (CAMCOG). These results reveal an important distinction between the neurochemistry of multi-infarct vascular dementia/SND and sub-cortical ischaemic vascular dementia, suggesting that pharmacological manipulation of serotonin offers the possibility to develop novel therapies for stroke and multi-infarct vascular dementia patients. The results also highlight the importance of the cortical 5-HT(1A) receptor in mediating cognition.

Effects of L-histidine depletion and L-tyrosine/L-phenylalanine depletion on sensory and motor processes in healthy volunteers

BACKGROUND AND PURPOSE

Animal studies show that histamine plays a role in cognitive functioning and that histamine H3-receptor antagonists, which increase histaminergic function through presynaptic receptors, improve cognitive performance in models of clinical cognitive deficits. In order to test such new drugs in humans, a model for cognitive impairments induced by low histaminergic functions would be useful. Studies with histamine H1-receptor antagonists have shown limitations as a model. Here we evaluated whether depletion of L-histidine, the precursor of histamine, was effective in altering measures associated with histamine in humans and the behavioural and electrophysiological (event-related-potentials) effects.

EXPERIMENTAL APPROACH

Seventeen healthy volunteers completed a three-way, double-blind, crossover study with L-histidine depletion, L-tyrosine/L-phenylalanine depletion (active control) and placebo as treatments. Interactions with task manipulations in a choice reaction time task were studied. Task demands were increased using visual stimulus degradation and increased response complexity. In addition, subjective and objective measures of sedation and critical tracking task performance were assessed.

KEY RESULTS

Measures of sedation and critical tracking task performance were not affected by treatment. L-histidine depletion was effective and enlarged the effect of response complexity as measured with the response-locked lateralized readiness potential onset latency.

CONCLUSIONS AND IMPLICATIONS

L-histidine depletion affected response- but not stimulus-related processes, in contrast to the effects of H1-receptor antagonists which were previously found to affect primarily stimulus-related processes. L-histidine depletion is promising as a model for histamine-based cognitive impairment. However, these effects need to be confirmed by further studies.

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.

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.

The His452Tyr variant of the gene encoding the 5-HT2A receptor is specifically associated with consolidation of episodic memory in humans

Basic research has shown that serotonin plays an important role in memory formation. Accordingly, genetic variation of serotonin receptors can be expected to affect memory and to underlie, in part, the heritability of memory capacity. A study by de Quervain and colleagues found a highly significant association of the functional 5-HT2A receptor variant His452Tyr (rs6314) with long-term memory. We replicated this finding in a cohort of 133 adults (mean age 43.5 yr). Carriers of the Tyr allele showed poorer verbal delayed recall and recognition, while immediate recall and additional measures of attentional and executive function were not affected by the His452Tyr genotype. Results suggest a possible role of 5-HT2A receptors in memory consolidation. Serotonergic drugs may have the potential to improve memory.

Adolescent Suicidal Behavior and Substance Use: Developmental Mechanisms

Adolescent suicidal behaviors and substance use are disturbingly common. Research suggests overlap of some of the etiological mechanisms for both adolescent suicidal behavior and substance use, yet clear understanding of the complex relations between these behaviors and their causal underpinnings is lacking. A growing body of evidence and a diathesis model (Mann et al. 1999; Mann, 2003) highlight the importance of impulse control as a proximal risk factor for adolescent suicidal and substance use behaviors. This literature review extends current theory on the relationships between adolescent suicidal behavior and substance use by: (1) examining how, when, and to what extent adolescent development is affected by poor impulse control, stressful life events, substance use behavior, and biological factors; (2) presenting proposed causal mechanisms by which these risk factors interact to increase risk for suicidal behaviors and substance use; and (3) proposing specific new hypotheses to extend the diathesis model to adolescents at risk for suicide and substance use. More specifically, new hypotheses are presented that predict bidirectional relationships between stressful life events and genetic markers of 5-HT dysregulation; substance use behavior and impulsivity; and substance use behavior and suicide attempts. The importance of distinguishing between different developmental trajectories of suicidal and substance use behaviors, and the effects of specific risk and protective mechanisms are discussed. Use of new statistical approaches that provide for the comparison of latent growth curves and latent class models is recommended to identify differences in developmental trajectories of suicidal behavior and substance use. Knowledge gained from these prospective longitudinal methods should lead to greater understanding on the timing, duration, and extent to which specific risk and protective factors influence the outcomes of suicidal behavior and substance use. In turn, findings from these studies should inform researchers who conduct future treatment and prevention studies.

Differential effects of acute serotonin and dopamine depletion on prepulse inhibition and p50 suppression measures of sensorimotor and sensory gating in humans

Schizophrenia is associated with impairments of sensorimotor and sensory gating as measured by prepulse inhibition (PPI) of the acoustic startle response and P50 suppression of the auditory event-related potential respectively. While serotonin and dopamine play an important role in the pathophysiology and treatment of schizophrenia, their role in modulating PPI and P50 suppression in humans is yet to be fully clarified. To further explore the role of serotonin and dopamine in PPI and P50 suppression, we examined the effects of acute tryptophan depletion (to decrease serotonin) and acute tyrosine/phenylalanine depletion (to decrease dopamine) on PPI and P50 suppression in healthy human participants. In addition, we also examined for the first time, the effects of simultaneous serotonin and dopamine depletion (ie combined monoamine depletion) on PPI and P50 suppression. The study was a double-blind, placebo-controlled cross-over design in which 16 healthy male participants completed the PPI and P50 paradigms under four acute treatment conditions: (a) balanced/placebo control, (b) acute tryptophan depletion, (c) acute tyrosine/phenylalanine depletion, and (d) acute tyrosine/phenylalanine/tryptophan depletion (combined monoamine depletion). Selective depletion of dopamine had no significant effect on either PPI or P50 suppression, whereas selective serotonin depletion significantly disrupted PPI, but not P50 suppression. Finally, the simultaneous depletion of both serotonin and dopamine resulted in significant reduction of both PPI and P50 suppression. We suggest these results can be explained by theories relating to optimal levels of monoaminergic neurotransmission and synergistic interactions between serotonergic and dopaminergic systems for normal 'gating' function. These findings suggest that a dysfunction in both serotonin and dopamine neurotransmission may, in part, be responsible for the gating deficits observed in schizophrenia, and their normalization following administration of atypical antipsychotic drugs.

Effects of selective and combined serotonin and dopamine depletion on the loudness dependence of the auditory evoked potential (LDAEP) in humans

BACKGROUND

The loudness dependence of the auditory evoked potential (LDAEP) has been suggested as a possible in vivo measure of central serotonin function. However, more recent studies suggest that the LDAEP may be modulated by multiple neuromodulatory systems in addition to the serotonergic system. Accordingly we further examined the effects of selective serotonin, dopamine and simultaneous serotonin and dopamine depletion on the LDAEP in healthy subjects.

METHODS

The study employed a placebo-controlled, double-blind, cross over design. Fourteen subjects were tested under four acute treatment conditions: placebo (balanced amino acid drink), tryptophan (serotonin) depletion (ATD), tyrosine/phenylalanine (dopamine) depletion (ATPD) and combined tryptophan/tyrosine/phenylalanine (serotonin and dopamine) depletion (CMD). Testing was conducted 5.5 h post-depletion and changes in the amplitude of the N1/P2 at varying intensities (60, 70, 80, 90, 100 dB) were examined at C(Z).

RESULTS

Greater than 80% plasma precursor depletion was achieved across all conditions. Despite significant depletion of monoamine precursors, ATD, (p = 0.318), ATPD (p = 0.061) and CMD (p = 0.104) had no effects on the LDAEP (60-100 dB).

CONCLUSION

Acute serotonin and dopamine depletion did not modulate the LDAEP. This finding adds support to growing evidence that the LDAEP is insensitive to acute changes in serotonin and dopamine neurotransmission.

Dopamine depletion impairs frontostriatal functional connectivity during a set-shifting task

We investigated the effect of transient dopamine depletion on functional connectivity during performance of the Wisconsin Card Sorting Task. Functional magnetic resonance imaging data were analyzed as a psychophysiological interaction, a statistical method used to identify functional connectivity during experimental manipulations. Nineteen healthy subjects were scanned, double blind, on 2 separate days: once after drinking an amino acid mixture deficient in the dopamine precursors tyrosine and phenylalanine, and once after drinking a nutritionally balanced mixture. In the balanced drink session, statistically significant connectivity between the frontal lobes and striatum was observed during set shifting, and the greater the prefrontostriatal connectivity, the faster the response time after a shift. Neither of these associations were observed after dopamine depletion. Moreover, dopamine depletion also reduced the degree of deactivation in areas normally suppressed during attention-demanding tasks, including the medial prefrontal cortex, posterior cingulate cortex, and hippocampus. Together, these results suggest that functional connectivity between the frontal lobes and basal ganglia during set shifting contributes to more efficient performance and that dopamine modulates this corticostriatal connectivity.

Effects of tryptophan depletion on cognitive functioning, obsessive-compulsive symptoms and mood in obsessive-compulsive disorder: preliminary results

BACKGROUND

Previous studies suggest that obsessive-compulsive disorder (OCD) is associated with moderate cognitive deficits. There is also evidence for altered serotonergic transmission in OCD. The aim of this study was to evaluate the impact of rapid plasma tryptophan depletion on cognitive functioning in OCD.

METHODS

A double-blind crossover study was conducted to explore the effects of tryptophan depletion on cognitive functioning, obsessive or compulsive (OC) symptoms and mood in OCD patients treated with selective serotonin reuptake inhibitors.

RESULTS

After 5 h of tryptophan depletion, 6 out of 7 patients reported worsening of mood (placebo condition: 3 patients). No effect was found regarding OC symptoms. There was a small and nonsignificant improvement of nonverbal memory and fluency. Problem solving ability and verbal memory, in contrast, were slightly impaired after tryptophan depletion. The results, however, represented only tendencies without reaching significance.

CONCLUSION

The results suggest that OC symptoms may not depend on the short-time availability of serotonin. As some critical cognitive functions improved following tryptophan depletion, there is no evidence from our study that neuropsychological impairment in OCD can be reduced to a lowered level of serotonin. Future studies are needed to further clarify these findings by use of larger samples.

The cognitive effects of modulating the glycine site of the NMDA receptor with high-dose glycine in healthy controls

N-methyl-D-aspartate (NMDA) receptors play an important role in learning and memory. Targeting the glycine modulatory site of the NMDA receptor has been suggested as a therapeutic strategy to improve cognition, although findings have not been convincing. We used the Cognitive Drug Research computerised assessment system to examine the effects of high-dose glycine on a number of cognitive processes in healthy young subjects. The study was a randomised placebo controlled repeated measures design in which each subject received acute placebo or glycine (0.8 g/kg) orally, with treatment conditions separated by a 5-day washout period. No significant effects of glycine were found on measures of working memory, declarative memory, attention or perceptual processing. These findings, together with those of previous studies, cast doubt over the ability of acute high-dose glycine to improve cognitive function in healthy subjects and suggest that the optimum dose of glycine for improving cognition may vary between different populations, possibly due to differences in endogenous glycine levels and the functional status of NMDA receptors.

The role of 5-HT(1A) receptors in learning and memory

The ascending serotonin (5-HT) neurons innervate the cerebral cortex, hippocampus, septum and amygdala, all representing brain regions associated with various domains of cognition. The 5-HT innervation is diffuse and extensively arborized with few synaptic contacts, which indicates that 5-HT can affect a large number of neurons in a paracrine mode. Serotonin signaling is mediated by 14 receptor subtypes with different functional and transductional properties. The 5-HT(1A) subtype is of particular interest, since it is one of the main mediators of the action of 5-HT. Moreover, the 5-HT(1A) receptor regulates the activity of 5-HT neurons via autoreceptors, and it regulates the function of several neurotransmitter systems via postsynaptic receptors (heteroreceptors). This review assesses the pharmacological and genetic evidence that implicates the 5-HT(1A) receptor in learning and memory. The 5-HT(1A) receptors are in the position to influence the activity of glutamatergic, cholinergic and possibly GABAergic neurons in the cerebral cortex, hippocampus and in the septohippocampal projection, thereby affecting declarative and non-declarative memory functions. Moreover, the 5-HT(1A) receptor regulates several transduction mechanisms such as kinases and immediate early genes implicated in memory formation. Based on studies in rodents the stimulation of 5-HT(1A) receptors generally produces learning impairments by interfering with memory-encoding mechanisms. In contrast, antagonists of 5-HT(1A) receptors facilitate certain types of memory by enhancing hippocampal/cortical cholinergic and/or glutamatergic neurotransmission. Some data also support a potential role for the 5-HT(1A) receptor in memory consolidation. Available results also implicate the 5-HT(1A) receptor in the retrieval of aversive or emotional memories, supporting an involvement in reconsolidation. The contribution of 5-HT(1A) receptors in cognitive impairments in various psychiatric disorders is still unclear. However, there is evidence that 5-HT(1A) receptors may play differential roles in normal brain function and in psychopathological states. Taken together, the evidence indicates that the 5-HT(1A) receptor is a target for novel therapeutic advances in several neuropsychiatric disorders characterized by various cognitive deficits.

Tyrosine depletion alters cortical and limbic blood flow but does not modulate spatial working memory performance or task-related blood flow in humans

Dopamine appears critical in regulating spatial working memory (SWM) within the PFC of non-human primates; however findings in humans are less clear. Recent studies of the effects of global depletion of dopamine via acute tyrosine/phenylalanine depletion (TPD) on SWM task performance have yielded inconsistent results, which may be partly related to task differences. These previous studies do not address whether TPD can directly impair PFC functioning. The current study investigated the effects of TPD on (1) regional cerebral blood flow (rCBF) during a SWM n-back task using H(2) (15)O Positron Emission Tomography (PET), and (2) behavioural performance on three different SWM tasks. Ten healthy males were scanned twice: once following a placebo (balanced) amino acid mixture and once following an equivalent mixture deficient in tyrosine/phenylalanine (TPD condition). Participants completed two additional delayed-response tasks to examine whether differences in response demands influenced TPD effects on performance. TPD resulted in widespread increases in rCBF, with maximum increases in the region of the parahippocampal gyrus bilaterally, left inferior frontal gyrus, and the putamen. TPD related rCBF reductions were observed in the medial frontal gyrus bilaterally, right inferior temporal gyrus and the pons. Despite widespread changes in blood flow following TPD, no specific effects on SWM neural networks or task performance were observed. The use of three different SWM tasks suggests that task differences are unlikely to account for the lack of effects observed. These findings question the capacity of TPD to consistently modulate dopamine function and SWM neural networks in humans.

Tyrosine depletion lowers dopamine synthesis and desipramine-induced prefrontal cortex catecholamine levels

The relationship between limited tyrosine availability, DA (dopamine) synthesis and DA levels in the medial prefrontal cortex (MPFC) of the rat was examined by in vivo microdialysis. We administered a tyrosine- and phenylalanine-free mixture of large neutral amino acids (LNAA-) IP to lower brain tyrosine, and the norepinephrine transporter inhibitor desipramine (DMI) 10 mg/kg IP to raise MPFC DA levels without affecting DA synthesis. For examination of DOPA levels, NSD-1015 20 microM was included in perfusate. Neither NSD-1015 nor DMI affected tyrosine levels. LNAA- lowered tyrosine levels by 45%, and lowered DOPA levels as well; this was not additionally affected by concurrent DMI 10 mg/kg IP. In parallel studies DMI markedly increased extracellular levels of DA (420% baseline) and norepinephrine (NE) (864% baseline). LNAA- had no effect on baseline levels of DA or NE but robustly lowered DMI-induced DA (176% baseline) as well as NE (237% baseline) levels. Even when DMI (20 microM) was administered in perfusate, LNAA- still lowered DMI-induced DA and NE levels. We conclude that while baseline mesocortical DA synthesis is indeed dependent on tyrosine availability, the MPFC maintains normal extracellular DA and NA levels in the face of moderately lower DA synthesis. During other than baseline conditions, however, tyrosine depletion can lower ECF DA and NE levels in MPFC. These data offer a potential mechanism linking dysregulation of tyrosine transport and cognitive deficits in schizophrenia.

Neuropsychological study of underweight and "weight-recovered" anorexia nervosa compared with bulimia nervosa and normal controls

OBJECTIVE

To compare executive, memory and visuospatial functioning of DSM-IV anorexia nervosa (AN), bulimia nervosa (BN), and normal controls (NC).

METHOD

A comparison of women involving: (i) 16 AN with body mass indices (BMI) < or = 17.5 kg/m(2); (ii) 12 AN with BMI > 18.5 kg/m(2) for at least 3 months; (iii) 13 BN; and (iv) 16 NC participants was performed with groups of similar age and intelligence. Groups were assessed with EDE-12, MADRS, HAMA, Cognitive Drug Research (CDR) battery, and Bechara tasks.

RESULTS

Significant impairments in CDR Power of Attention were present in underweight AN and BN participants. CDR Morse Tapping was significantly impaired in all clinical groups. The BN and weight-recovered AN groups were significantly impaired on CDR immediate word recall. The BN group alone was significantly impaired on CDR delayed word recall.

CONCLUSION

Attentional impairment is similar in AN and BN. Impaired motor tasks in AN persist after "weight-recovery" and are similar to impairments in BN. BN may be discriminated from AN on word recall.

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.