Serotonin depletion amplifies distinct human social emotions as a function of individual differences in personality

Non-Synonymous Substitutions in Cadherin 13, Solute Carrier Family 6 Member 4, and Monoamine Oxidase A Genes are Associated with Personality Traits in Thoroughbred Horses

Retraining retired racehorses for various purposes can help correct behavioral issues. However, ensuring efficiency and preventing accidents present global challenges. Based on the hypothesis that a simple personality assessment could help address these challenges, the present study aimed to identify genetic markers associated with personality. Eight genes were selected from 18 personality-related candidate genes that are orthologs of human personality genes, and their association with personality was verified based on actual behavior. A total of 169 Thoroughbred horses were assessed for their tractability (questionnaire concerning tractability in 14 types of situations and 3 types of impressions) during the training process. Personality factors were extracted from the data using principal component analysis and analyzed for their association with single nucleotide variants as non-synonymous substitutions in the target genes. Three genes, CDH13, SLC6A4, and MAOA, demonstrated significant associations based on simple linear regression, marking the identification of these genes for the first time as contributors to temperament in Thoroughbred horses. All these genes, as well as the previously identified HTR1A, are involved in the serotonin neurotransmitter system, suggesting that the tractability of horses may be correlated with their social personality. Assessing the genotypes of these genes before retraining is expected to prevent problems in the development of a racehorse's second career and shorten the training period through individual customization of training methods, thereby improving racehorse welfare.

Individual differences in the boldness of female zebrafish are associated with alterations in serotonin function

One of the most prevalent axes of behavioral variation in both humans and animals is risk taking, where individuals that are more willing to take risk are characterized as bold while those that are more reserved are regarded as shy. Brain monoamines (i.e. serotonin, dopamine and noradrenaline) have been found to play a role in a variety of behaviors related to risk taking. Using zebrafish, we investigated whether there was a relationship between monoamine function and boldness behavior during exploration of a novel tank. We found a correlation between serotonin metabolism (5-HIAA:5-HT ratio) and boldness during the initial exposure to the tank in female animals. The DOPAC:DA ratio correlated with boldness behavior on the third day in male fish. There was no relationship between boldness and noradrenaline. To probe differences in serotonergic function in bold and shy fish, we administered a selective serotonin reuptake inhibitor, escitalopram, and assessed exploratory behavior. We found that escitalopram had opposing effects on thigmotaxis in bold and shy female animals: the drug caused bold fish to spend more time near the center of the tank and shy fish spent more time near the periphery. Taken together, our findings indicate that variation in serotonergic function has sex-specific contributions to individual differences in risk-taking behavior.

Oxytocin, but not vasopressin, decreases willingness to harm others by promoting moral emotions of guilt and shame

Prosocial and moral behaviors have overlapping neural systems and can both be affected in a number of psychiatric disorders, although whether they involve similar neurochemical systems is unclear. In the current registered randomized placebo-controlled trial on 180 adult male and female subjects, we investigated the effects of intranasal administration of oxytocin and vasopressin, which play key roles in influencing social behavior, on moral emotion ratings for situations involving harming others and on judgments of moral dilemmas where others are harmed for a greater good. Oxytocin, but not vasopressin, enhanced feelings of guilt and shame for intentional but not accidental harm and reduced endorsement of intentionally harming others to achieve a greater good. Neither peptide influenced arousal ratings for the scenarios. Effects of oxytocin on guilt and shame were strongest in individuals scoring lower on the personal distress subscale of trait empathy. Overall, findings demonstrate for the first time that oxytocin, but not vasopressin, promotes enhanced feelings of guilt and shame and unwillingness to harm others irrespective of the consequences. This may reflect associations between oxytocin and empathy and vasopressin with aggression and suggests that oxytocin may have greater therapeutic potential for disorders with atypical social and moral behavior.

The lateral habenula is required for maternal behavior in the mouse dam

Mammalian parenting is an unusually demanding commitment. How did evolution co-opt the reward system to ensure parental care? Previous work has implicated the lateral habenula (LHb), an epithalamic nucleus, as a potential intersection of parenting behavior and reward. Here, we examine the role of the LHb in the maternal behavior of naturally parturient mouse dams. We show that kainic acid lesions of the LHb induced a severe maternal neglect phenotype in dams towards their biological pups. Next, we demonstrate that through chronic chemogenetic inactivation of the LHb using DREADDs impaired acquisition and performance of various maternal behaviors, such as pup retrieval and nesting. We present a random intercepts model suggesting LHb-inactivation prevents the acquisition of the novel pup retrieval maternal behavior and decreases nest building performance, an already-established behavior, in primiparous mouse dams. Lastly, we examine the spatial histology of kainic-acid treated dams with a random intercepts model, which suggests that the role of LHb in maternal behavior may be preferentially localized at the posterior aspect of this structure. Together, these findings serve to establish the LHb as required for maternal behavior in the mouse dam, thereby complementing previous findings implicating the LHb in parental behavior using pup-sensitized virgin female mice.

Molecular signatures of cortical expansion in the human fetal brain

The third trimester of human gestation is characterised by rapid increases in brain volume and cortical surface area. A growing catalogue of cells in the prenatal brain has revealed remarkable molecular diversity across cortical areas.1,2 Despite this, little is known about how this translates into the patterns of differential cortical expansion observed in humans during the latter stages of gestation. Here we present a new resource, μBrain, to facilitate knowledge translation between molecular and anatomical descriptions of the prenatal developing brain. Built using generative artificial intelligence, μBrain is a three-dimensional cellular-resolution digital atlas combining publicly-available serial sections of the postmortem human brain at 21 weeks gestation3 with bulk tissue microarray data, sampled across 29 cortical regions and 5 transient tissue zones.4 Using μBrain, we evaluate the molecular signatures of preferentially-expanded cortical regions during human gestation, quantified in utero using magnetic resonance imaging (MRI). We find that differences in the rates of expansion across cortical areas during gestation respect anatomical and evolutionary boundaries between cortical types5 and are founded upon extended periods of upper-layer cortical neuron migration that continue beyond mid-gestation. We identify a set of genes that are upregulated from mid-gestation and highly expressed in rapidly expanding neocortex, which are implicated in genetic disorders with cognitive sequelae. Our findings demonstrate a spatial coupling between areal differences in the timing of neurogenesis and rates of expansion across the neocortical sheet during the prenatal epoch. The μBrain atlas is available from: https://garedaba.github.io/micro-brain/ and provides a new tool to comprehensively map early brain development across domains, model systems and resolution scales.

Individual differences in the boldness of female zebrafish are associated with alterations in serotonin function

One of the most prevalent axes of behavioral variation in both humans and animals is risk taking, where individuals that are more willing to take risk are characterized as bold while those that are more reserved as shy. Brain monoamines (i.e., serotonin, dopamine, and norepinephrine) have been found to play a role in a variety of behaviors related to risk taking. Genetic variation related to monoamine function have also been linked to personality in both humans and animals. Using zebrafish, we investigated the relationship between monoamine function and boldness behavior during exploration of a novel tank. We found a sex-specific correlation between serotonin metabolism (5-HIAA:5-HT ratio) and boldness that was limited to female animals; there were no relationships between boldness and dopamine or norepinephrine. To probe differences in serotonergic function, we administered a serotonin reuptake inhibitor, escitalopram, to bold and shy fish, and assessed their exploratory behavior. We found that escitalopram had opposing effects on thigmotaxis in female animals with bold fish spending more time near the center of the tank and shy fish spent more time near the periphery. Taken together, our findings suggest that variation in serotonergic function makes sex-specific contributions to individual differences in risk taking behavior.

A central serotonin regulating gene polymorphism (TPH2) determines vulnerability to acute tryptophan depletion-induced anxiety and ventromedial prefrontal threat reactivity in healthy young men

Serotonin (5-HT) has long been implicated in adaptive emotion regulation as well as the development and treatment of emotional dysregulations in mental disorders. Accumulating evidence suggests a genetic vulnerability may render some individuals at a greater risk for the detrimental effects of transient variations in 5-HT signaling. The present study aimed to investigate whether individual variations in the Tryptophan hydroxylase 2 (TPH2) genetics influence susceptibility for behavioral and neural threat reactivity dysregulations during transiently decreased 5-HT signaling. To this end, interactive effects between TPH2 (rs4570625) genotype and acute tryptophan depletion (ATD) on threat reactivity were examined in a within-subject placebo-controlled pharmacological fMRI trial (n = 51). A priori genotype stratification of extreme groups (GG vs. TT) allowed balanced sampling. While no main effects of ATD on neural reactivity to threat-related stimuli and mood state were observed in the entire sample, accounting for TPH2 genotype revealed an ATD-induced increase in subjective anxious arousal in the GG but not the TT carriers. The effects were mirrored on the neural level, such that ATD specifically reduced ventromedial prefrontal cortex reactivity towards threat-related stimuli in the GG carriers. Furthermore, the ATD-induced increase in subjective anxiety positively associated with the extent of ATD-induced changes in ventromedial prefrontal cortex activity in response to threat-related stimuli in GG carriers. Together the present findings suggest for the first time that individual variations in TPH2 genetics render individuals susceptible to the anxiogenic and neural effects of a transient decrease in 5-HT signaling.

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 long and the short of it: 5-HTTLPR and moral judgement

Differences in moral sentiments are widespread. Increasingly, their biological correlates are investigated to elucidate potential sources of divergent moral attitudes and choices. Serotonin is one such potential modulator. We investigated the effects of a functional serotonergic polymorphism, 5-HTTLPR, which was previously linked to moral choices albeit with inconsistent findings. N = 157 healthy young adults completed a set of congruent and incongruent moral dilemmas. In addition to the traditional moral response score, this set allows by using a process dissociation (PD) approach an estimation a deontological and a utilitarian parameter. While there was no main effect of 5-HTTLPR on any of the three moral judgement parameters, there was an interaction effect between 5-HTTLPR and endocrine status on PD parameters, which was mainly due to the deontological but not the utilitarian parameter. In men and free cycling women, LL homozygotes showed reduced deontological tendencies compared to S allele carriers. Contrariwise, in women using oral contraceptives, LL homozygotes had increased deontology parameter scores. Furthermore, LL genotypes in general reported less difficulty in making harmful choices, which were in addition associated with less negative emotions. The findings suggest that 5-HTTLPR might be involved in modulating cognitive and emotional processes contributing to moral decisions.

Early neurotransmitters changes in prodromal frontotemporal dementia: A GENFI study

BACKGROUND

Neurotransmitters deficits in Frontotemporal Dementia (FTD) are still poorly understood. Better knowledge of neurotransmitters impairment, especially in prodromal disease stages, might tailor symptomatic treatment approaches.

METHODS

In the present study, we applied JuSpace toolbox, which allowed for cross-modal correlation of Magnetic Resonance Imaging (MRI)-based measures with nuclear imaging derived estimates covering various neurotransmitter systems including dopaminergic, serotonergic, noradrenergic, GABAergic and glutamatergic neurotransmission. We included 392 mutation carriers (157 GRN, 164 C9orf72, 71 MAPT), together with 276 non-carrier cognitively healthy controls (HC). We tested if the spatial patterns of grey matter volume (GMV) alterations in mutation carriers (relative to HC) are correlated with specific neurotransmitter systems in prodromal (CDR® plus NACC FTLD = 0.5) and in symptomatic (CDR® plus NACC FTLD≥1) FTD.

RESULTS

In prodromal stages of C9orf72 disease, voxel-based brain changes were significantly associated with spatial distribution of dopamine and acetylcholine pathways; in prodromal MAPT disease with dopamine and serotonin pathways, while in prodromal GRN disease no significant findings were reported (p < 0.05, Family Wise Error corrected). In symptomatic FTD, a widespread involvement of dopamine, serotonin, glutamate and acetylcholine pathways across all genetic subtypes was found. Social cognition scores, loss of empathy and poor response to emotional cues were found to correlate with the strength of GMV colocalization of dopamine and serotonin pathways (all p < 0.01).

CONCLUSIONS

This study, indirectly assessing neurotransmitter deficits in monogenic FTD, provides novel insight into disease mechanisms and might suggest potential therapeutic targets to counteract disease-related symptoms.

Effects of music therapy as an alternative treatment on depression in children and adolescents with ADHD by activating serotonin and improving stress coping ability

OBJECTIVE

The objective of this study was to determine the effect of music therapy as an alternative treatment on depression in children and adolescents with attention-deficit hyperactivity disorder (ADHD) by activating serotonin (5-HT) and improving stress coping ability.

METHODS

This study is designed based on randomization method. A total of 36 subjects participated in the experiment, consisting of an ADHD control group (n = 18) and ADHD music therapy group (n = 18). The ADHD control group received standard care, while the ADHD music therapy group received music therapy and standard care. The ADHD music therapy group received both active music therapy (improvisation) and receptive music therapy (music listening) for 50 minutes, twice a week, for 3 months: a total of 24 times. From a neurophysiological perspective, changes in depression and stress were tracked by measuring 5-HT secretion, cortisol expression, blood pressure (BP), heart rate (HR), and CDI and DHQ psychological scales.

RESULTS

The ADHD music therapy group's 5-HT secretion increased (p < 0.001), whereas cortisol expression (p < 0.001), BP (p < 0.001) and HR (p < 0.001) decreased. The CDI and DHQ psychological scales also showed positive changes (p < 0.01 and p < 0.001, respectively). However, the ADHD Con G's (who did not receive music therapy) 5-HT secretion did not increase, whereas cortisol expression, BP, and HR did not decrease. In addition, the CDI and DHQ psychological scales did not display positive changes.

CONCLUSIONS

In conclusion, the application of music therapy as an alternative treatment for ADHD children and adolescents showed positive neurophysiological and psychological effects. Therefore, this study would like to propose a new alternative to medicine for preventing and treating depression through various uses of music therapy.

Chronic escitalopram in healthy volunteers has specific effects on reinforcement sensitivity: a double-blind, placebo-controlled semi-randomised study

Several studies of the effects on cognition of selective serotonin reuptake inhibitors (SSRI), administered either acutely or sub-chronically in healthy volunteers, have found changes in learning and reinforcement outcomes. In contrast, to our knowledge, there have been no studies of chronic effects of escitalopram on cognition in healthy volunteers. This is important in view of its clinical use in major depressive disorder (MDD) and obsessive-compulsive disorder (OCD). Consequently, we aimed to investigate the chronic effect of the SSRI, escitalopram, on measures of 'cold' cognition (including inhibition, cognitive flexibility, memory) and 'hot cognition' including decision-making and particularly reinforcement learning. The study, conducted at the University of Copenhagen between May 2020 and October 2021, used a double-blind placebo-controlled design with 66 healthy volunteers, semi-randomised to receive either 20 mg of escitalopram (n = 32) or placebo (n = 34), balanced for age, sex and intelligence quotient (IQ) for at least 21 days. Questionnaires, neuropsychological tests and serum escitalopram measures were taken. We analysed group differences on the cognitive measures using linear regression models as well as innovative hierarchical Bayesian modelling of the Probabilistic Reversal Learning (PRL) task. The novel and important finding was that escitalopram reduced reinforcement sensitivity compared to placebo on both the Sequential Model-Based/Model-Free task and the PRL task. We found no other significant group differences on 'cold' or 'hot' cognition. These findings demonstrate that serotonin reuptake inhibition is involved in reinforcement learning in healthy individuals. Lower reinforcement sensitivity in response to chronic SSRI administration may reflect the 'blunting' effect often reported by patients with MDD treated with SSRIs. Trial Registration: NCT04239339 .

Psychopathic personality traits stress immunity and social potency moderate the relationship between emotional competence and cognitive functions in depression

Background

Psychopathic personality traits (PPT) and depression have both been shown to worsen emotional and cognitive functions. Moreover, PPT and depression share similar underlying neuronal circuits tapping into the emotional and cognitive domains. However, little is known about the influence of PPT on emotion and cognition in individuals with depression.

Objectives

This study aimed to examine the correlative relationships and moderating role of PPT in the association between emotional competence and cognitive functions in individuals with depression.

Methods

Data from 373 individuals diagnosed with depression (158 males, 215 females) were examined within a cohort study. Subjects filled out validated questionnaires surveying PPT and emotional competences. Furthermore, a comprehensive neuropsychological test battery was administered.

Results

Correlation analyses revealed a significant positive association between emotional competence and cognitive functions. Further, negative associations between emotional competence and the PPT "Blame Externalisation" and "Careless Nonplanfulness," as well as positive associations with psychopathic "Social Potency" and "Stress Immunity" were found. Moderation analyses indicated a significant positive influence of psychopathic "Stress Immunity" and "Social Influence" on the relationship between emotional competence and cognitive functions.

Conclusion

The findings highlight the importance of integrating PPT in depression research. Considering PPT in depression treatment could also facilitate the therapeutic process by identifying individual traits as resilience-strengthening or potentially harmful factors for depressive symptomatology. This study represents a stepping stone for further research regarding the role of personality traits in psychiatric disorders and their treatment.

Serotonin regulation of behavior via large-scale neuromodulation of serotonin receptor networks

Although we understand how serotonin receptors function at the single-cell level, what role different serotonin receptors play in regulating brain-wide activity and, in turn, human behavior, remains unknown. Here, we developed transcriptomic-neuroimaging mapping to characterize brain-wide functional signatures associated with specific serotonin receptors: serotonin receptor networks (SRNs). Probing SRNs with optogenetics-functional magnetic resonance imaging (MRI) and pharmacology in mice, we show that activation of dorsal raphe serotonin neurons differentially modulates the amplitude and functional connectivity of different SRNs, showing that receptors' spatial distributions can confer specificity not only at the local, but also at the brain-wide, network level. In humans, using resting-state functional MRI, SRNs replicate established divisions of serotonin effects on impulsivity and negative biases. These results provide compelling evidence that heterogeneous brain-wide distributions of different serotonin receptor types may underpin behaviorally distinct modes of serotonin regulation. This suggests that serotonin neurons may regulate multiple aspects of human behavior via modulation of large-scale receptor networks.

Theory of Mind and Preference Learning at the Interface of Cognitive Science, Neuroscience, and AI: A Review

Theory of Mind (ToM)-the ability of the human mind to attribute mental states to others-is a key component of human cognition. In order to understand other people's mental states or viewpoint and to have successful interactions with others within social and occupational environments, this form of social cognition is essential. The same capability of inferring human mental states is a prerequisite for artificial intelligence (AI) to be integrated into society, for example in healthcare and the motoring industry. Autonomous cars will need to be able to infer the mental states of human drivers and pedestrians to predict their behavior. In the literature, there has been an increasing understanding of ToM, specifically with increasing cognitive science studies in children and in individuals with Autism Spectrum Disorder. Similarly, with neuroimaging studies there is now a better understanding of the neural mechanisms that underlie ToM. In addition, new AI algorithms for inferring human mental states have been proposed with more complex applications and better generalisability. In this review, we synthesize the existing understanding of ToM in cognitive and neurosciences and the AI computational models that have been proposed. We focus on preference learning as an area of particular interest and the most recent neurocognitive and computational ToM models. We also discuss the limitations of existing models and hint at potential approaches to allow ToM models to fully express the complexity of the human mind in all its aspects, including values and preferences.

Neural circuit control of innate behaviors

All animals possess a plethora of innate behaviors that do not require extensive learning and are fundamental for their survival and propagation. With the advent of newly-developed techniques such as viral tracing and optogenetic and chemogenetic tools, recent studies are gradually unraveling neural circuits underlying different innate behaviors. Here, we summarize current development in our understanding of the neural circuits controlling predation, feeding, male-typical mating, and urination, highlighting the role of genetically defined neurons and their connections in sensory triggering, sensory to motor/motivation transformation, motor/motivation encoding during these different behaviors. Along the way, we discuss possible mechanisms underlying binge-eating disorder and the pro-social effects of the neuropeptide oxytocin, elucidating the clinical relevance of studying neural circuits underlying essential innate functions. Finally, we discuss some exciting brain structures recurrently appearing in the regulation of different behaviors, which suggests both divergence and convergence in the neural encoding of specific innate behaviors. Going forward, we emphasize the importance of multi-angle and cross-species dissections in delineating neural circuits that control innate behaviors.

Baiap3 regulates depressive behaviors in mice via attenuating dense core vesicle trafficking in subsets of prefrontal cortex neurons

Selective serotonin reuptake inhibitors (SSRIs) are effective first line therapies for treating depression, but are plagued by undesirable side effects and are not effective in all patients. Because SSRIs effectively deplete the neuronal releasable serotonin (5-HT) pool, gaining a deeper understanding of intracellular mechanisms regulating 5-HT pools can help us understand the shortcomings of SSRIs and develop more effective therapies. In this study, we found that BAIAP3 (brain-specific angiogenesis inhibitor 1-associated protein 3) is significantly downregulated in two mouse models of depression (the IR- and CUMS-induced depressive mouse models). In BAIAP3 downregulated models (in vitro and in vivo), we discovered that trafficking of dense core vesicle (DCV), organelles that store, transport and release cargo via exocytosis, was reduced. Accordingly, 5-HT exocytosis and levels in the synapse were lowered, causing defective post-synaptic neurotransmission. In a screen of natural products, we identified eucalyptol, the active components of Eucalyptus, as uniquely capable of increasing neuronal Baiap3 expression and elevate synaptic 5-HT levels. Moreover, eucalyptol treatment relieved depressive behavioral symptoms and restored serotonin levels in mice. Mechanistically, eucalyptol restores Baiap3 expression by reducing inhibitory microRNAs (miR-329, miR-362). These findings illuminate how Baiap3 depletion propagates neurotransmission dysfunction and point to eucalyptol as a novel agent for restoring serotonin exocytosis, suggesting potential for developing eucalyptol as a therapy for treating depression.

Alcohol, Aggression, and Violence: From Public Health to Neuroscience

Alcohol has been associated with violent crimes and domestic violence across many nations. Various etiological factors were linked to chronic alcohol use and violence including psychiatric comorbidities of perpetrators such as personality disorders, mood disorders, and intermittent explosive disorders. Aggression is the precursor of violence and individuals prone to aggressive behaviors are more likely to commit impulsive violent crimes, especially under the influence of alcohol. Findings from brain studies indicate long-term alcohol consumption induced morphological changes in brain regions involved in self-control, decision-making, and emotional processing. In line with this, the inherent dopaminergic and serotonergic anomalies seen in aggressive individuals increase their susceptibility to commit violent crimes when alcohol present in their system. In relation to this, this article intends to investigate the influence of alcohol on aggression with sociopsychological and neuroscientific perspectives by looking into comorbidity of personality or mood disorders, state of the mind during alcohol consumption, types of beverages, environmental trigger, neurochemical changes, and gender differences that influence individual responses to alcohol intake and susceptibility to intoxicated aggression.

Tryptophan-enriched diet or 5-hydroxytryptophan supplementation given in a randomized controlled trial impacts social cognition on a neural and behavioral level

Understanding of emotions and intentions are key processes in social cognition at which serotonin is an important neuromodulator. Its precursor is the essential amino acid tryptophan (TRP). Reduced TRP availability leads to weaker impulse control ability and higher aggression, while TRP supplementation promotes confidence. In a double-blind placebo-controlled fMRI study with 77 healthy adults, we investigated the influence of a 4 week TRP enriched diet and an acute 5-hydroxytryptophan (5-HTP) intake on two social-cognitive tasks, a moral evaluation and an emotion recognition task. With 5-HTP, immoral behavior without negative consequences was rated as more reprehensible. Additionally, during story reading, activation in insula and supramarginal gyrus was increased after TRP intake. No significant effects of TRP on emotion recognition were identified for the whole sample. Importantly, emotion recognition ability decreased with age which was for positive emotions compensated by TRP. Since the supramarginal gyrus is associated with empathy, pain and related information integration results could be interpreted as reflecting stricter evaluation of negative behavior due to better integration of information. Improved recognition of positive emotions with TRP in older participants supports the use of a TRP-rich diet to compensate for age related decline in social-cognitive processes.

Serotonin depletion impairs both Pavlovian and instrumental reversal learning in healthy humans

Serotonin is involved in updating responses to changing environmental circumstances. Optimising behaviour to maximise reward and minimise punishment may require shifting strategies upon encountering new situations. Likewise, autonomic responses to threats are critical for survival yet must be modified as danger shifts from one source to another. Whilst numerous psychiatric disorders are characterised by behavioural and autonomic inflexibility, few studies have examined the contribution of serotonin in humans. We modelled both processes, respectively, in two independent experiments (N = 97). Experiment 1 assessed instrumental (stimulus-response-outcome) reversal learning whereby individuals learned through trial and error which action was most optimal for obtaining reward or avoiding punishment initially, and the contingencies subsequently reversed serially. Experiment 2 examined Pavlovian (stimulus-outcome) reversal learning assessed by the skin conductance response: one innately threatening stimulus predicted receipt of an uncomfortable electric shock and another did not; these contingencies swapped in a reversal phase. Upon depleting the serotonin precursor tryptophan-in a double-blind randomised placebo-controlled design-healthy volunteers showed impairments in updating both actions and autonomic responses to reflect changing contingencies. Reversal deficits in each domain, furthermore, were correlated with the extent of tryptophan depletion. Initial Pavlovian conditioning, moreover, which involved innately threatening stimuli, was potentiated by depletion. These results translate findings in experimental animals to humans and have implications for the neurochemical basis of cognitive inflexibility.