Cortico-limbic connectivity in MAOA-L carriers is vulnerable to acute tryptophan depletion

The role of the prefrontal cortex in modulating aggression in humans and rodents

Accumulating evidence suggests that the prefrontal cortex (PFC) plays an important role in aggression. However, the findings regarding the key neural mechanisms and molecular pathways underlying the modulation of aggression by the PFC are relatively scattered, with many inconsistencies and areas that would benefit from exploration. Here, we highlight the relationship between the PFC and aggression in humans and rodents and describe the anatomy and function of the human PFC, along with homologous regions in rodents. At the molecular level, we detail how the major neuromodulators of the PFC impact aggression. At the circuit level, this review provides an overview of known and potential subcortical projections that regulate aggression in rodents. Finally, at the disease level, we review the correlation between PFC alterations and heightened aggression in specific human psychiatric disorders. Our review provides a framework for PFC modulation of aggression, resolves several intriguing paradoxes from previous studies, and illuminates new avenues for further study.

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.

Bright side of the MAOA-uVNTR on trait and situational forgiveness

The stress-and-coping theory of forgiveness posits that forgiveness and aggression are alternative ways of coping with stress of interpersonal offences. Inspired by the link between aggression and MAOA-uVNTR (a genetic variant involving in catabolism of monoamines), we investigated the relationship between this variant and forgiveness with two studies. Study 1 examined the relationship between the MAOA-uVNTR and trait forgiveness in students, and study 2 examined the effect of this variant on third-party forgiveness in response to situational offences in male inmates. The results showed that the MAOA-H (a high activity allele) was associated with higher trait forgiveness in male students and greater third-party forgiveness to accidentally committed harm and attempted but failed harm in male inmates than the MAOA-L. These findings highlight the bright side of MAOA-uVNTR on trait and situational forgiveness.

Oxytocinergic Modulation of Stress-Associated Amygdala-Hippocampus Pathways in Humans Is Mediated by Serotonergic Mechanisms

BACKGROUND

The hypothalamic neuropeptide oxytocin (OXT) may exert anxiolytic and stress-reducing actions via modulatory effects on amygdala circuits. Animal models and initial findings in humans suggest that some of these effects are mediated by interactions with other neurotransmitter systems, in particular the serotonin (5-HT) system. Against this background, the present pharmacological resting-state functional magnetic resonance imaging study aimed to determine whether effects of OXT on stress-associated amygdala intrinsic networks are mediated by 5-HT.

METHODS

We employed a randomized, placebo-controlled, double-blind parallel-group, pharmacological functional magnetic resonance imaging resting-state experiment with 4 treatment groups in n = 112 healthy male participants. Participants underwent a transient decrease in 5-HT signaling via acute tryptophan depletion (ATD) or a corresponding placebo-control protocol before the administration of intranasal OXT (24 IU) or placebo intranasal spray.

RESULTS

OXT and 5-HT modulation exerted interactive effects on the coupling of the left amygdala with the ipsilateral hippocampus and adjacent midbrain. OXT increased intrinsic coupling in this pathway, whereas this effect of OXT was significantly attenuated during transiently decreased central serotonergic signaling induced via acute tryptophan depletion. In the absence of OXT or 5-HT modulation, this pathway showed a trend for an association with self-reported stress perception in everyday life. No interactive effects were observed for the right amygdala.

CONCLUSIONS

Together, the findings provide the first evidence, to our knowledge, that the effects of OXT on stress-associated amygdala-hippocampal-midbrain pathways are critically mediated by the 5-HT system in humans.

The neurobiology of human aggressive behavior: Neuroimaging, genetic, and neurochemical aspects

In modern societies, there is a strive to improve the quality of life related to risk of crimes which inevitably requires a better understanding of brain determinants and mediators of aggression. Neurobiology provides powerful tools to achieve this end. Pre-clinical and clinical studies show that changes in regional volumes, metabolism-function and connectivity within specific neural networks are related to aggression. Subregions of prefrontal cortex, insula, amygdala, basal ganglia and hippocampus play a major role within these circuits and have been consistently implicated in biology of aggression. Genetic variations in proteins regulating the synthesis, degradation, and transport of serotonin and dopamine as well as their signal transduction have been found to mediate behavioral variability observed in aggression. Gene-gene and gene-environment interactions represent additional important risk factors for aggressiveness. Considering the social burden of pathological forms of aggression, more basic and translational studies should be conducted to accelerate applications to clinical practice, justice courts, and policy making.

Whole blood serotonin levels in healthy elderly are negatively associated with the functional activity of emotion-related brain regions

Understanding the role of neuromodulators of socio-affective processing is important to ensure psychological wellbeing during older years. Here, we investigated the link between blood serotonin levels and brain and behavioral responses to emotional information in healthy elderly. A priori regions of interest (ROI) were selected due to their role in emotion processing and their dense serotonergic innervation. Correlation analyses were performed between ROI-specific responses to emotional stimuli and whole blood serotonin levels. We found significant negative associations between serotonin and functional activity for the bilateral insula, dorsal anterior cingulate cortex and subgenual gyrus. No association with behavioral measures survived correction for multiple testing. Our results mirror prior pharmacological and genetic work on the link between serotonin and emotional brain reactivity in younger adults. Given the involvement of serotonin in several age-related changes, our study encourages future research to characterize the role of this neuromodulator in emotion processing across the lifespan.

Association Analysis of Monoamine Oxidase-A Gene Promoter Polymorphism (MAOA uVNTR) for Antisocial Behavior: Absence of the Counting Number Repeats in Central Iran

Background: The MAOA gene is located on the X chromosome (Xp11.23). Several studies have established a VNTR (Variable Number Tandem Repeat) polymorphism in the upstream of the MAOA gene transcriptional initiation region named uVNTR which is correlated with the risk of antisocial behavior. Objectives: This study aimed to investigate the association between MAOA genotypes and the risk of violent behavior in a cohort of violent and age-matched non-violent individuals. Methods: In the current case-control study, MAOA uVNTR was genotyped in a cohort of 88 violent and 95 age-matched non-violent individuals. Individuals were genotyped for the MAOA uVNTR by performing PCR, gel electrophoresis, and sequencing. Furthermore, a chi-square test was performed using SPSS, and a p-value of less than 0.05 was considered statistically significant. Results: We identified three MAOA uVNTR allelic variants: They were harboring 3.5, 4.5, and 5.5 repeated sequences. Alleles with 2, 3, 4, 5, and 6 repeats were not observed in any of the two examined groups. Conclusions: We did not detect a statistically appreciable association between antisocial behavior and allele frequencies in the studied population in central Iran.

Novel C7-Substituted Coumarins as Selective Monoamine Oxidase Inhibitors: Discovery, Synthesis and Theoretical Simulation

There is a continued need to develop new selective human monoamine oxidase (hMAO) inhibitors that could be beneficial for the treatment of neurological diseases. However, hMAOs are closely related with high sequence identity and structural similarity, which hinders the development of selective MAO inhibitors. “Three-Dimensional Biologically Relevant Spectrum (BRS-3D)” method developed by our group has demonstrated its effectiveness in subtype selectivity studies of receptor and enzyme ligands. Here, we report a series of novel C7-substituted coumarins, either synthesized or commercially purchased, which were identified as selective hMAO inhibitors. Most of the compounds demonstrated strong activities with IC₅₀ values (half-inhibitory concentration) ranging from sub-micromolar to nanomolar. Compounds, FR1 and SP1, were identified as the most selective hMAO-A inhibitors, with IC₅₀ values of 1.5 nM (selectivity index (SI) < −2.82) and 19 nM (SI < −2.42), respectively. FR4 and FR5 showed the most potent hMAO-B inhibitory activity, with IC₅₀ of 18 nM and 15 nM (SI > 2.74 and SI > 2.82). Docking calculations and molecular dynamic simulations were performed to elucidate the selectivity preference and SAR profiles.

No detectable effects of acute tryptophan depletion on short-term immune system cytokine levels in healthy adults

Objectives: Recent research suggested an influence of diminished central nervous serotonin (5-HT) synthesis on the leptin axis via immunological mechanisms in healthy adult females. However, studies assessing immunological parameters in combination with dietary challenge techniques that impact brain 5-HT synthesis in humans are lacking.  Methods: In the present trial, a pilot analysis was conducted on data obtained in healthy adult humans receiving either different dietary acute tryptophan depletion (ATD) challenge or tryptophan (TRP)-balanced control conditions (BAL) to study the effects of reduced central nervous 5-HT synthesis on serum tumor necrosis factor α (TNF-α), interleukin-1β (IL-1β) and IL-6 concentrations. The data of N = 35 healthy adults were analysed who were randomly subjected to one of the following two dietary conditions in a double-blind between-subject approach: (1) The Moja-De ATD challenge (ATD), or (2) TRP-balanced control condition for ATD Moja-De (BAL). Serum concentrations for the assessment of relevant parameters (TNF-α, IL-1β and IL-6) and relevant TRP-related characteristics after the respective challenge procedures were assessed at baseline (T0) and in hourly intervals after administration over a period of 6 h (T1-T6).  Results: The ATD condition did not result in significant changes to cytokine concentrations for the entire study sample, or in male and female subgroups. Depletion of CNS 5-HT via dietary TRP depletion appears to have no statistically significant short-term impact on cytokine concentrations in healthy adults.  Conclusions: Future research on immunological stressors in combination with challenge techniques will be of value in order to further disentangle the complex interplay between brain 5-HT synthesis and immunological pathways.

MAOA variants differ in oscillatory EEG & ECG activities in response to aggression-inducing stimuli

Among the genetic variations in the monoamine oxidase A (MAOA) gene, upstream variable number tandem repeats (uVNTRs) of the promoter have been associated with individual differences in human physiology and aggressive behaviour. However, the evidence for a molecular or neural link between MAOA uVNTRs and aggression remains ambiguous. Additionally, the use of inconsistent promoter constructs in previous studies has added to the confusion. Therefore, it is necessary to demonstrate the genetic function of MAOA uVNTR and its effects on multiple aspects of aggression. Here, we identified three MAOA alleles in Koreans: the predominant 3.5R and 4.5R alleles, as well as the rare 2.5R allele. There was a minor difference in transcriptional efficiency between the 3.5R and 4.5R alleles, with the greatest value for the 2.5R allele, in contrast to existing research. Psychological indices of aggression did not differ among MAOA genotypes. However, our electroencephalogram and electrocardiogram results obtained under aggression-related stimulation revealed oscillatory changes as novel phenotypes that vary with the MAOA genotype. In particular, we observed prominent changes in frontal γ power and heart rate in 4.5R carriers of men. Our findings provide genetic insights into MAOA function and offer a neurobiological basis for various socio-emotional mechanisms in healthy individuals.

Serotonergic Contributions to Human Brain Aggression Networks

Aggressive behavior is associated with dysfunctional frontolimbic emotion regulation circuits. Recent findings suggest serotonin as a primary transmitter for prefrontal amygdala control. However, the association between serotonin levels, amygdala regulation, and aggression is still a matter of debate. Neurobehavioral models furthermore suggest a possible mediating influence of the monoamine oxidase A gene (MAOA) on this brain-behavior relationship, with carriers of low expressing allele varieties being a risk group for aggression. In the present study, we investigated the influence of brain serotonin modulation and MAOA genotype on functional amygdala connectivity during aggressive behavior. Modulation of serotonergic neurotransmission with acute tryptophan depletion (ATD) and placebo were administered in a double-blind, cross-over design in 38 healthy male participants. Aggressive behavior was modeled in a violent video game during simultaneous assessment of brain activation with functional magnetic resonance imaging (fMRI). Trait aggression was measured with the Buss-Perry Aggression Questionnaire (BP-AQ), and MAOA genotypes were assessed from blood samples. Voxel-wise functional connectivity with anatomically defined amygdala was calculated from the functional data. Tryptophan depletion with ATD reduced aggression-specific amygdala connectivity with bilateral supramarginal gyrus. Moreover, ATD impact was associated with trait aggression and MAOA genotype in prefrontal cortex regions. In summary, serotonergic corticolimbic projections contribute to aggressive behavior. Genotype-specific vulnerability of frontolimbic projections may underlie the elevated risk in low expressing allele carriers.

Serotonin and aggressive behaviour in children and adolescents: a systematic review

OBJECTIVE

The role of serotonin (5-HT) in human aggression has been the subject of a large number of studies, mostly with adults. Meta-analyses indicate a small but significant inverse relationship between central nervous 5-HT availability and aggression, but genetically informed studies suggest two pathways: one to reactive aggression and the other to proactive aggression.

METHOD

We conducted a systemic review on central nervous 5-HT function in children and adolescents, with attention to the function of aggression.

RESULTS

In total, 675 articles were screened for relevance, with 45 reviewed. These included blood assays (e.g. plasma, 5-HIAA; platelet 5-HTR_2A ), epigenetic studies, retrospective PET studies and 5-HT challenge paradigms (e.g. tryptophan depletion). Overall, findings were mixed, with support both for negative and for positive associations of central nervous 5-HT function with aggression in children and adolescents.

CONCLUSION

We propose factors that may be blurring the picture, including problems in the conceptualization and measurement of aggression in young people, the lack of prospective designs and the bias towards clinical samples of boys. Research needs to account for variance in the both motivation for and implementation of aggression, and look to the behavioural economics literature to consider the roles of reward, vengeance and self-control more clearly.

Acute tryptophan loading decreases functional connectivity between the default mode network and emotion-related brain regions

It has been shown that the functional architecture of the default mode network (DMN) can be affected by serotonergic challenges and these effects may provide insights on the neurobiological bases of depressive symptomatology. To deepen our understanding of this possible interplay, we used a double-blind, randomized, cross-over design, with a control condition and two interventions to decrease (tryptophan depletion) and increase (tryptophan loading) brain serotonin synthesis. Resting-state fMRI from 85 healthy subjects was acquired for all conditions 3 hr after the ingestion of an amino acid mixture containing different amounts of tryptophan, the dietary precursor of serotonin. The DMN was derived for each participant and session. Permutation testing was performed to detect connectivity changes within the DMN as well as between the DMN and other brain regions elicited by the interventions. We found that tryptophan loading increased tryptophan plasma levels and decreased DMN connectivity with visual cortices and several brain regions involved in emotion and affect regulation (i.e., putamen, subcallosal cortex, thalamus, and frontal cortex). Tryptophan depletion significantly reduced tryptophan levels but did not affect brain connectivity. Subjective ratings of mood, anxiety, sleepiness, and impulsive choice were not strongly affected by any intervention. Our data indicate that connectivity between the DMN and emotion-related brain regions might be modulated by changes in the serotonergic system. These results suggest that functional changes in the brain associated with different brain serotonin levels may be relevant to understand the neural bases of depressive symptoms.

Central serotonin modulates neural responses to virtual violent actions in emotion regulation networks

Disruptions in the cortico-limbic emotion regulation networks have been linked to depression, anxiety, impulsivity, and aggression. Altered transmission of the central nervous serotonin (5-HT) contributes to dysfunctions in the cognitive control of emotions. To date, studies relating to pharmaco-fMRI challenging of the 5-HT system have focused on emotion processing for facial expressions. We investigated effects of a single-dose selective 5-HT reuptake inhibitor (escitalopram) on emotion regulation during virtual violence. For this purpose, 38 male participants played a violent video game during fMRI scanning. The SSRI reduced neural responses to violent actions in right-hemispheric inferior frontal gyrus and medial prefrontal cortex encompassing the anterior cingulate cortex (ACC), but not to non-violent actions. Within the ACC, the drug effect differentiated areas with high inhibitory 5-HT1A receptor density (subgenual s25) from those with a lower density (pregenual p32, p24). This finding links functional responses during virtual violent actions with 5-HT neurotransmission in emotion regulation networks, underpinning the ecological validity of the 5-HT model in aggressive behavior. Available 5-HT receptor density data suggest that this SSRI effect is only observable when inhibitory and excitatory 5-HT receptors are balanced. The observed early functional changes may impact patient groups receiving SSRI treatment.

Tryptophan supplementation and serotonin function: genetic variations in behavioural effects

The neurotransmitter serotonin has a role in affective disorders such as depression and anxiety, as well as sleep, cognitive function and appetite. This review examines the evidence that serotonin-related genotypes may moderate the behavioural effects of supplementation with the serotonin precursor amino acidl-tryptophan (TRP), on which synthesis of serotonin (or 5-hydroxytryptamine; 5-HT) depends. However, 95 % of serotonin is synthesised and used in the periphery, and TRP is also metabolised via non-5-HT routes such as the kynurenine pathway. Moreover, understanding of genotypes involved in regulation of serotonin raises questions over the generalisability of TRP effects on behaviour across individuals with varied serotonergic genotypes. To date, only differences between variants of the 5-HT transporter-linked promoter region (5-HTTLPR) have been investigated in relation to behavioural effects of TRP supplementation. Effects of 5-HTTLPR genotypes are usually compared between the alleles that are either high (L/L′) or low (S/S′) expressing of mRNA for the 5-HT transporter receptor. Yet, another key genetic variable is sex: in women, the S/S′ genotype predicts sensitivity to improved mood and reduced cortisol by TRP supplementation, during stressful challenges, whereas the L/L′ genotype protects against stress-induced mood deterioration. In men, the L/L′ genotype may confer risk of stress-induced increases in negative affect; there are insufficient data to assess effects on male S/S′ genotypes. However, better-powered studies to detect sex by genotype by stress by TRP interactions, as well as consideration of more genotypes, are needed before strong conclusions and recommendations for behavioural effects of TRP treatment can be reached.

The role of central and medial amygdala in normal and abnormal aggression: A review of classical approaches

The involvement of the amygdala in aggression is supported by overwhelming evidence. Frequently, however, the amygdala is studied as a whole, despite its complex internal organization. To reveal the role of various subdivisions, here we review the involvement of the central and medial amygdala in male rivalry aggression, maternal aggression, predatory aggression, and models of abnormal aggression where violent behavior is associated with increased or decreased arousal. We conclude that: (1) rivalry aggression is controlled by the medial amygdala; (2) predatory aggression is controlled by the central amygdala; (3) hypoarousal-associated violent aggression recruits both nuclei, (4) a specific upregulation of the medial amygdala was observed in hyperarousal-driven aggression. These patterns of amygdala activation were used to build four alternative models of the aggression circuitry, each being specific to particular forms of aggression. The separate study of the roles of amygdala subdivisions may not only improve our understanding of aggressive behavior, but also the differential control of aggression and violent behaviors of various types, including those associated with various psychopathologies.

Blunted insula activation reflects increased risk and reward seeking as an interaction of testosterone administration and the MAOA polymorphism

Testosterone, a male sex hormone, has been suggested to partly explain mixed findings in males and females when investigating behavioral tendencies associated with the MAOA polymorphism. Prior studies indicated that the MAOA polymorphism represents a vulnerability factor for financial risk-taking and harm avoidance and that testosterone increases human risk-taking. We therefore assumed an interactive influence of the MAOA polymorphism and testosterone application on decision making and corresponding neural correlates in a risk and reward context. Stratified for the MAOA polymorphism (S =short, L =long), 103 healthy males were assigned to a placebo or testosterone group (double blind, randomized) receiving a topical gel containing 50 mg testosterone. During a functional MRI scan, the participants performed a sequential decision making task. Our results indicate that testosterone and the MAOA polymorphism jointly influence sequential decision making. The MAOA-S variant was associated with less automatic harm avoidance as reflected in response times on safe decisions. Moreover, after testosterone administration, MAOA-S carriers were more risk-taking. Overall activity in the anterior cingulate cortex, anterior insula and inferior frontal gyrus increased with growing risk for losses. In the anterior insula, testosterone administration mitigated this effect solely in MAOA-S carriers. This might be a reflection of an improved coping during risk-reward conflicts subsequently modulating risky decision making. While the molecular basis is not well defined so far, our results support the assumption of testosterone as a modulatory factor for previously reported sex differences of behavioral associations with the MAOA-S variant. Hum Brain Mapp 38:4574-4593, 2017. © 2017 Wiley Periodicals, Inc.