The effect of increased serotonergic neurotransmission on aggression: a critical meta-analytical review of preclinical studies

Possible role of more positive social behaviour in the clinical effect of antidepressant drugs

Increasing serotonin decreases quarrelsome behaviours and enhances agreeable behaviours in humans. Antidepressants, even those whose primary action is not on serotonin, seem to increase serotonin function. We suggest that antidepressants act in part by effects on social behaviour, which leads to a gradual improvement in mood. We review the evidence supporting the idea that antidepressants may be moving behaviour from quarrelsome to agreeable. The more positive social responses of interaction partners would initiate a cycle of more positive social behaviour, and this iterative process would result in a clinically significant improvement in mood.

Serotonin depletion counteracts sex differences in anxiety-related behaviour in rat

BACKGROUND

Numerous studies suggest (1) that a major physiological role of brain serotonin-containing neurons is to modulate sex steroid-driven behaviour such as sex and aggression, (2) that sex steroids influence brain serotonergic neurotransmission and (3) that brain serotonergic neurotransmission displays sexual dimorphism. Such observations indicate that an important task for brain serotonin is to either enhance or counteract sex differences in behaviour.

METHODS

To test this hypothesis, we explored the effect of short-term serotonin depletion on the behaviour of adult male and female rats in a behavioural paradigm in which males and females have been shown to behave differently, i.e. the elevated plus maze.

RESULTS

Two rounds of testing of untreated Wistar rats confirmed the previous observation that females make more entries into open arms (round 1, p = 0.001; round 2, p = 0.008) and spend more time on these arms (round 1, p ≤ 0.001; round 2, p = 0.006) than males; in addition, males displayed fewer entries into closed arms upon habituation, i.e. at the second round (p ≤ 0.001) than did females. Administration of the tryptophan hydroxylase inhibitor para-chloro-phenylalanine, at a regimen (300 mg/kg/day for 3 days), markedly reducing brain content of serotonin, enhanced entries upon open arms (p = 0.01) and time spent on open arms (p = 0.004) in males but exerted no such effects in females (p = 0.9 and p = 0.9, respectively); moreover, it reduced entries into closed arms in females (p ≤ 0.001) but not in males (p = 0.1).

CONCLUSIONS

Serotonin depletion abolishing the sex differences observed at baseline supports the theory that serotonin aids to uphold certain sex differences in behaviour.

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.

Trait aggression and trait impulsivity are not related to frontal cortex 5-HT2A receptor binding in healthy individuals

Numerous studies indicate that the serotonergic (5-HT) transmitter system is involved in the regulation of impulsive aggression and there is from post-mortem, in vivo imaging and genetic studies evidence that the 5-HT2A receptor may be involved. We investigated 94 healthy individuals (60 men, mean age 47.0±18.7, range 23-86) to determine if trait aggression and trait impulsivity were related to frontal cortex 5-HT2A receptor binding (5-HT2AR) as measured with [18F]-altanserin PET imaging. Trait aggression and trait impulsivity were assessed with the Buss-Perry Aggression Questionnaire (AQ) and the Barratt Impulsiveness Scale 11 (BIS-11). Statistical analyses were conducted using a multiple linear regression model and internal consistency reliability of the AQ and BIS-11 was evaluated by Cronbach's alpha. Contrary to our hypothesis, results revealed no significant associations between 5-HT2AR and the AQ or BIS-11 total scores. Also, there was no significant interaction between gender and frontal cortex 5-HT2AR in predicting trait aggression and trait impulsivity. This is the first study to examine how 5-HT2AR relates to trait aggression and trait impulsivity in a large sample of healthy individuals. Our findings are not supportive of a selective role for 5-HT2AR in mediating the 5-HT related effects on aggression and impulsivity in psychiatrically healthy individuals.

Neurons in monkey dorsal raphe nucleus code beginning and progress of step-by-step schedule, reward expectation, and amount of reward outcome in the reward schedule task

The dorsal raphe nucleus is the major source of serotonin in the brain. It is connected to brain regions related to reward processing, and the neurons show activity related to predicted reward outcome. Clinical observations also suggest that it is important in maintaining alertness and its apparent role in addiction seems to be related to reward processing. Here, we examined whether the neurons in dorsal raphe carry signals about reward outcome and task progress during multitrial schedules. We recorded from 98 single neurons in dorsal raphe of two monkeys. The monkeys perform one, two, or three visual discrimination trials (schedule), obtaining one, two, or three drops of liquid. In the valid cue condition, the length and brightness of a visual cue indicated schedule progress and reward amount, respectively. In the random cue condition, the visual cue was randomly presented with respect to schedule length and reward amount. We found information encoded about (1) schedule onset, (2) reward expectation, (3) reward outcome, and (4) reward amount in the mean firing rates. Information theoretic analysis showed that the temporal variation of the neuronal responses contained additional information related to the progress of the schedule toward the reward rather than only discriminating schedule onset or reward/no reward. When considered in light of all that is known about the raphe in anatomy, physiology, and behavior, the rich encoding about both task progress and predicted reward outcome makes the raphe a strong candidate for providing signals throughout the brain to coordinate persistent goal-seeking behavior.

Effects of aromatase inhibition and androgen activity on serotonin and behavior in male macaques

Aggression in humans and animals has been linked to androgens and serotonin function. To further our understanding of the effect of androgens on serotonin and aggression in male macaques, we sought to manipulate circulating androgens and the activity of aromatase; and to then determine behavior and the endogenous availability of serotonin. Male Japanese macaques (Macaca fuscata) were castrated for 5-7 months and then treated for 3 months with (a) placebo; (b) testosterone (T); (c) T + Dutasteride (5a reductase inhibitor; AvodartTM); (d) T + Letrozole (nonsteroidal aromatase inhibitor; FemeraTM); (e) Flutamide + ATD (androgen antagonist plus steroidal aromatase inhibitor); or (f) dihydrotestosterone (DHT) + ATD (n = 5/group). Behavioral observations were made during treatments. At the end of the treatment period, each animal was sedated with propofol and administered a bolus of fenfluramine (5 mg/kg). Fenfluramine causes the release of serotonin proportional to endogenous availability and in turn, serotonin stimulates the secretion of prolactin. Therefore, serum prolactin concentrations reflect endogenous serotonin. Fenfluramine significantly increased serotonin/prolactin in all groups (p < .0001). Fenfluramine-induced serotonin/prolactin in the T-treated group was significantly higher than the other groups (p < .0001). Castration partially reduced the serotonin/prolactin response and Letrozole partially blocked the effect of T. Complete inhibition of aromatase with ATD, a noncompetitive inhibitor, significantly and similarly reduced the fenfluramine-induced serotonin/prolactin response in the presence or absence of DHT. Neither aggressive behavior nor yawning (indicators of androgen activity) correlated with serotonin/prolactin, but posited aromatase activity correlated significantly with prolactin (p < .0008; r² = 0.95). In summary, androgens induced aggressive behavior but they did not regulate serotonin. Altogether, the data suggest that aromatase activity supports serotonin production and that androgens increase aggression by another mechanism.

The role of the serotonergic system at the interface of aggression and suicide

Alterations in serotonin (5-HT) neurochemistry have been implicated in the aetiology of all major neuropsychiatric disorders, ranging from schizophrenia to mood and anxiety-spectrum disorders. This review will focus on the multifaceted implications of 5-HT-ergic dysfunctions in the pathophysiology of aggressive and suicidal behaviours. After a brief overview of the anatomical distribution of the 5-HT-ergic system in the key brain areas that govern aggression and suicidal behaviours, the implication of 5-HT markers (5-HT receptors, transporter as well as synthetic and metabolic enzymes) in these conditions is discussed. In this regard, particular emphasis is placed on the integration of pharmacological and genetic evidence from animal studies with the findings of human experimental and genetic association studies. Traditional views postulated an inverse relationship between 5-HT and aggression and suicidal behaviours; however, ample evidence has shown that this perspective may be overly simplistic, and that such pathological manifestations may reflect alterations in 5-HT homoeostasis due to the interaction of genetic, environmental and gender-related factors, particularly during early critical developmental stages. The development of animal models that may capture the complexity of such interactions promises to afford a powerful tool to elucidate the pathophysiology of impulsive aggression and suicidability, and identify new effective therapies for these conditions.

Self-reported family socioeconomic status, the 5-HTTLPR genotype, and delinquent behavior in a community-based adolescent population

Twin and adoption studies have demonstrated a significant contribution of both genetic and environmental factors to antisocial and delinquent behavior. Associations have been reported between the serotonin transporter (5-HTT) and aggression, and between socioeconomic status (SES), aggression, and serotonergic functions of the brain. We aimed to investigate associations between the 5-HTTLPR genotype and family SES in relation to delinquent behavior among adolescents. A total of 1,467 17- to 18-year-old students in the county of Västmanland, Sweden, anonymously completed a questionnaire and gave a saliva sample. Family SES had a U-shaped relation to delinquency, where adolescents with low and high family SES were the most delinquent. There were curvilinear interactions between the 5-HTTLPR genotype and family SES in relation to delinquency. Among individuals having high family SES, boys with the LL (homozygous for the long allele) or LS (heterozygous) genotypes and girls with the SS (homozygous for the short allele) or LS (heterozygous) genotypes showed the highest delinquency scores. Among individuals having low family SES, boys with the LL (homozygous for the long allele) genotype and girls with the LS (heterozygous) genotype showed the highest delinquency scores. The present study suggests evidence for an interaction between family SES and the 5-HTTLPR genotype in relation to juvenile delinquency.

The Sturm und Drang of anabolic steroid use: angst, anxiety, and aggression

Anabolic androgenic steroids (AAS) are illicitly administered to enhance athletic performance and body image. Although conferring positive actions on performance, steroid abuse is associated with changes in anxiety and aggression. AAS users are often keenly invested in understanding the biological actions of these drugs. Thus, mechanistic information on AAS actions is important not only for the biomedical community, but also for steroid users. Here we review findings from animal studies on the impact of AAS exposure on neural systems that are crucial for the production of anxiety and aggression, and compare the effects of the different classes of AAS and their potential signaling mechanisms, as well as context-, age- and sex-dependent aspects of their actions.

Sequential hermaphroditism and personality in a clonal vertebrate: the mangrove killifish

Individuals are regularly documented to consistently differ in their behavioural types (BTs). For example, some individuals are bold whereas others are shy. Within the human personality literature, the big five personality dimensions are commonly documented to be sex-specific with testosterone suggested to underpin traits such as aggressiveness. In non-human animals recent research suggests sex-specific BT expression may be influenced by ecology, mating system and sexual selection. While most research on sex-specific personality has focused on dioecious species, we explore sex differences in BT expression in a sequential hermaphrodite the mangrove killifish. We replicate within 7 isogenic genotypes and investigate sex differences (hermaphrodite and secondary male) in three BTs (exploration, boldness and aggression). This approach allows us to investigate sex differences in BT expression whilst controlling for genetic variation. In this study we find that both secondary males and hermaphrodites are repeatable at the individual level yet there was no difference between the sexes in average BT scores. Furthermore, aggression scores differed between genotypes, and were repeatable at the genotype level, suggesting strong genetic control. Finally, male boldness was significantly more repeatable than hermaphrodites potentially supporting recent proposals relating to sexual selection. We document a behavioural syndrome in male fish with bolder individuals being more aggressive, this behavioural syndrome was not observed however in hermaphrodites. In contrast to a previous developmental study in this species exploration did not correlate with either aggression or boldness in either males or hermaphrodites.

Testosterone, cortisol, and serotonin as key regulators of social aggression: A review and theoretical perspective

In human and non-human animals the steroid hormones cortisol and testosterone are involved in social aggression and recent studies suggest that these steroids might jointly regulate this behavior. It has been hypothesized that the imbalance between cortisol and testosterone levels is predictive for aggressive psychopathology, with high testosterone to cortisol ratio predisposing to a socially aggressive behavioral style. In this review, we focus on the effects of cortisol and testosterone on human social aggression, as well as on how they might modulate the aggression circuitry of the human brain. Recently, serotonin is hypothesized to differentiate between impulsive and instrumental aggression, and we will briefly review evidence on this hypothesis. The aim of this article is to provide a theoretical framework for the role of steroids and serotonin in impulsive social aggression in humans.

Temporal and frontal lobe initiation and regulation of the top-down escalation of anger and aggression

The widespread, across-species strategy of stagewise escalation of aggression in agonistic encounters can be understood in terms of resource capture and control with least risk and cost. Human anger likely follows similar principles. As an adaptive phenomenon, escalation may involve particular neural circuitry. To advance beyond a standard view that the frontal lobe tonically inhibits subcortical circuits of aggression, a model is proposed which starts with the general rostrally directed flow of information in the brain. Earlier stage processing of visual and auditory input is transmitted from posterior and middle temporal cortices to anterior temporal lobe where rudimentary appraisals of threat and provocation are developed. These directly but diffusely activate cortical/subcortical anger/aggression response systems. At the same time, the anterior temporal loci transmit the modality-specific perceptual information to orbito-frontal cortex where it is integrated with information about, e.g., the opponent's relative dominance/social status and evaluated for likelihood of potential rewards and punishments associated with different modes of responding and so forth. These frontal areas then impose an inhibitory gating or modulation and focusing of activity initiated by the anterior temporal loci through their projections to GABAergic interneurons in the same cortical/subcortical circuits. Escalation occurs as the inhibition imposed by the frontal areas is progressively lifted. Exploration of the implications, applications and hypotheses flowing from this model will improve our understanding of the biologically important and socially significant phenomena of escalation.

Prenatal Cocaine Disrupts Serotonin Signaling-Dependent Behaviors: Implications for Sex Differences, Early Stress and Prenatal SSRI Exposure

Prenatal cocaine (PC) exposure negatively impacts the developing nervous system, including numerous changes in serotonergic signaling. Cocaine, a competitive antagonist of the serotonin transporter, similar to selective serotonin reuptake inhibitors (SSRIs), also blocks dopamine and norepinephrine transporters, leaving the direct mechanism through which cocaine disrupts the developing serotonin system unclear. In order to understand the role of the serotonin transporter in cocaine's effect on the serotonergic system, we compare reports concerning PC and prenatal antidepressant exposure and conclude that PC exposure affects many facets of serotonergic signaling (serotonin levels, receptors, transporters) and that these effects differ significantly from what is observed following prenatal SSRI exposure. Alterations in serotonergic signaling are dependent on timing of exposure, test regimens, and sex. Following PC exposure, behavioral disturbances are observed in attention, emotional behavior and stress response, aggression, social behavior, communication, and like changes in serotonergic signaling, these effects depend on sex, age and developmental exposure. Vulnerability to the effects of PC exposure can be mediated by several factors, including allelic variance in serotonergic signaling genes, being male (although fewer studies have investigated female offspring), and experiencing the adverse early environments that are commonly coincident with maternal drug use. Early environmental stress results in disruptions in serotonergic signaling analogous to those observed with PC exposure and these may interact to produce greater behavioral effects observed in children of drug-abusing mothers. We conclude that based on past evidence, future studies should put a greater emphasis on including females and monitoring environmental factors when studying the impact of PC exposure.

Aggression and psychopharmacological treatments in major psychosis and personality disorders during hospitalisation

BACKGROUND

A number of large-scale studies have shown that there is a relationship between many psychiatric disorders and aggression or violence. As no medication is currently approved for the treatment of aggression, pharmacotherapy (often involving drug combinations) is used on a trial-and-error basis with various degrees of response.

METHOD

The study involved 244 in-patients aged 19-83 years (mean 41.9 ± 11.3 SD). The Modified Overt Aggression Scale (MOAS) was used to assess any aggressive or violent behaviors occurring in the week before admission and upon discharge. Psychopathology was assessed using the Brief Psychiatric Rating Scales (BPRS).

RESULTS

All of the patients showed a significant improvement (p<0.001) in mean weighted total MOAS scores at the end of the study, with no significant differences between the various drugs or combination therapies. The patients who received combination treatments including antidepressants showed a worsening in the weighted total MOAS score (18.46% ± 114.31% SD); the patients who did not receive antidepressants had an improvement (13.61% ± 257.36% SD) (p=0.0069).

CONCLUSIONS

Multivariate testing of the variables age, gender, substance/alcohol abuse, the duration of hospitalisation, the administration of mood stabilisers, and the use of typical or atipical antipsychotics showed that the severity of the psychopathological picture correlated significantly with the presence of violence, whereas the effect of combined antidepressant treatment on violent behavior was only relative.

The age-dependent effects of selective serotonin reuptake inhibitors in humans and rodents: A review

The selective serotonin reuptake inhibitor (SSRI) Prozac® (fluoxetine) is widely prescribed for the treatment of depression and anxiety-related disorders. While extensive research has established that fluoxetine is safe for adults, safety is not guaranteed for (unborn) children and adolescents. Some clinical studies have reported adverse outcomes, such as premature birth, neonatal cardiovascular abnormalities, and pulmonary hypertension in children whose mothers used SSRIs during pregnancy. In addition, several reports show that adolescent fluoxetine treatment increases risk for suicidal behavior. Despite these studies, fluoxetine is not contraindicated in the treatment of depressed pregnant women and adolescents. Longitudinal research in humans is limited because of ethical reasons and time constraints, and to overcome these limitations, rodents are used to increase insight in the age-dependent effects of fluoxetine exposure. It has been established that neonatal and adolescent fluoxetine exposure leads to paradoxical anxiety- and depression-like features in later life of rats and mice, although in some studies adolescent fluoxetine exposure was without effects. These age-dependent outcomes of fluoxetine may be explained by serotonin's neurotrophic effects, which may vary according to the developmental stage of the brain due to epigenetic modifications. Here we review the existing evidence for the age-dependent effects of fluoxetine in humans and rodents, address the gaps in our current knowledge and propose directions for future research. Given the overlap between human and rodent findings, rodents provide heuristic value in further research on the age-dependent effects of SSRIs.

Systematic reviews and meta-analyses of preclinical studies: publication bias in laboratory animal experiments

In 2006, Peters et al. identified 86 systematic reviews (SRs) of laboratory animal experiments (LAEs). They found 46 LAE meta-analyses (MAs), often of poor quality. Six of these 46 MAs tried to assess publication bias. Publication bias is the phenomenon of an experiment's results determining its likelihood of publication, often over-representing positive findings. As such, publication bias is the Achilles heel of any SR. Since researchers increasingly become aware of the fact that SRs directly support the 'three Rs', we expect the number of SRs of LAEs will sharply increase. Therefore, it is useful to see how publication bias is dealt with. Our objective was to identify all SRs and MAs of LAEs where the purpose was to inform human health published between July 2005 and 2010 with special attention to MAs' quality features and publication bias. We systematically searched Medline, Embase, Toxline and ScienceDirect from July 2005 to 2010, updating Peters' review. LAEs not directly informing human health or concerning fundamental biology were excluded. We found 2780 references of which 163 met the inclusion criteria: 158 SRs, of which 30 performed an MA, and five MAs without an SR. The number of SRs roughly doubled every three years since 1997. The number of MAs roughly doubled every five years since 1999. Compared with before July 2005, more MAs were preceded by SR and reported on (quality) features of included studies and heterogeneity. A statistically significant proportion of MAs considered publication bias (26/35) and tried to formally assess it (21/35).

Brain serotonin receptors and transporters: initiation vs. termination of escalated aggression

RATIONALE

Recent findings have shown a complexly regulated 5-HT system as it is linked to different kinds of aggression.

OBJECTIVE

We focus on (1) phasic and tonic changes of 5-HT and (2) state and trait of aggression, and emphasize the different receptor subtypes, their role in specific brain regions, feed-back regulation and modulation by other amines, acids and peptides.

RESULTS

New pharmacological tools differentiate the first three 5-HT receptor families and their modulation by GABA, glutamate and CRF. Activation of 5-HT(1A), 5-HT(1B) and 5-HT(2A/2C) receptors in mesocorticolimbic areas, reduce species-typical and other aggressive behaviors. In contrast, agonists at 5-HT(1A) and 5-HT(1B) receptors in the medial prefrontal cortex or septal area can increase aggressive behavior under specific conditions. Activation of serotonin transporters reduce mainly pathological aggression. Genetic analyses of aggressive individuals have identified several molecules that affect the 5-HT system directly (e.g., Tph2, 5-HT(1B), 5-HT transporter, Pet1, MAOA) or indirectly (e.g., Neuropeptide Y, αCaMKII, NOS, BDNF). Dysfunction in genes for MAOA escalates pathological aggression in rodents and humans, particularly in interaction with specific experiences.

CONCLUSIONS

Feedback to autoreceptors of the 5-HT(1) family and modulation via heteroreceptors are important in the expression of aggressive behavior. Tonic increase of the 5-HT(2) family expression may cause escalated aggression, whereas the phasic increase of 5-HT(2) receptors inhibits aggressive behaviors. Polymorphisms in the genes of 5-HT transporters or rate-limiting synthetic and metabolic enzymes of 5-HT modulate aggression, often requiring interaction with the rearing environment.

Behavioral and pharmacogenetics of aggressive behavior

Serotonin (5-HT) has long been considered as a key transmitter in the neurocircuitry controlling aggression. Impaired regulation of each subtype of 5-HT receptor, 5-HT transporter, synthetic and metabolic enzymes has been linked particularly to impulsive aggression. The current summary focuses mostly on recent findings from pharmacological and genetic studies. The pharmacological treatments and genetic manipulations or polymorphisms of aspecific target (e.g., 5-HT1A receptor) can often result in inconsistent results on aggression, due to "phasic" effects of pharmacological agents versus "trait"-like effects of genetic manipulations. Also, the local administration of a drug using the intracranial microinjection technique has shown that activation of specific subtypes of 5-HT receptors (5-HT1A and 5-HT1B) in mesocorticolimbic areas can reduce species-typical and other aggressive behaviors, but the same receptors in the medial prefrontal cortex or septal area promote escalated forms of aggression. Thus, there are receptor populations in specific brain regions that preferentially modulate specific types of aggression. Genetic studies have shown important gene-environment interactions; it is likely that the polymorphisms in the genes of 5-HT transporters or rate-limiting synthetic and metabolic enzymes of 5-HT (e.g., MAOA) determine the vulnerability to adverse environmental factors that escalate aggression. We also discuss the interaction between the 5-HT system and other systems. Modulation of 5-HT neurons in the dorsalraphe nucleus by GABA, glutamate and CRF profoundly regulate aggressive behaviors. Also, interactions of the 5-HT system with other neuropeptides(arginine vasopressin, oxytocin, neuropeptide Y, opioid) have emerged as important neurobiological determinants of aggression. Studies of aggression in genetically modified mice identified several molecules that affect the 5-HT system directly (e.g., Tph2, 5-HT1B, 5-HT transporter, Pet1, MAOA) or indirectly[e.g., BDNF, neuronal nitric oxide (nNOS), aCaMKII, Neuropeptide Y].The future agenda delineates specific receptor subpopulations for GABA, glutamate and neuropeptides as they modulate the canonical aminergic neurotransmitters in brainstem, limbic and cortical regions with the ultimate outcome of attenuating or escalating aggressive behavior.

Latent class analysis of antisocial behavior: interaction of serotonin transporter genotype and maltreatment

To improve understanding about genetic and environmental influences on antisocial behavior (ASB), we tested the association of the 44-base pair polymorphism of the serotonin transporter gene (5-HTTLPR) and maltreatment using latent class analysis in 2,488 boys and girls from Wave 1 of the National Longitudinal Study of Adolescent Health. In boys, ASB was defined by three classes (Exclusive Covert, Mixed Covert and Overt, and No Problems) whereas in girls, ASB was defined by two classes (Exclusive Covert, No Problems). In boys, 5-HTTLPR and maltreatment were not significantly related to ASB. However, in girls, maltreatment, but not 5-HTTLPR, was significantly associated with ASB. A significant interaction between 5-HTTLPR and maltreatment was also observed, where maltreated girls homozygous for the short allele were 12 times more likely to be classified in the Exclusive Covert group than in the No Problems group. Structural differences in the latent structure of ASB at Wave 2 and Wave 3 prevented repeat LCA modeling. However, using counts of ASB, 5-HTTLPR, maltreatment, and its interaction were unrelated to overt and covert ASB at Wave 2 and only maltreatment was related to covert ASB at Wave 3. We discuss these findings within the context of sex differences in ASB and relevant models of gene-environment interplay across developmental periods.

Effects of acute tryptophan depletion on three different types of behavioral impulsivity

INTRODUCTION

While central nervous system serotonin has been implicated in a variety of problematic impulsive behaviors, biological manipulation of brain serotonin using acute tryptophan depletion for studying changes in impulsive behavior has received little attention.

METHODS

Using identical treatment conditions, we examined the effects of reduced serotonin synthesis for each of three matched groups using acute tryptophan depletion. Thirty healthy men and women (ages 18-45) were assigned to perform one of three tasks assessing different types of behavioral impulsivity: response initiation, response inhibition, and consequence sensitivity (N = 90). Participants completed two experimental days during which each consumed either a tryptophan-depletion or balanced-placebo amino-acid formulation and completed 5 sessions of their respective tasks at 0.25 h before and 1.5, 4.0, 5.0, and 6.0 h after beverage consumption.

RESULTS

During peak effectiveness (5.0 h to 6.0 h following amino-acid consumption), depletion produced selective differences dependent on the type of impulsivity being tested. Specifically, relative to baseline testing (pre-depletion), response initiation impulsivity was significantly increased during the peak effects of depletion. And, when compared to placebo control, both response initiation and consequence sensitivity impulsivity were increased during the peak effects of depletion.

CONCLUSION

Though response initiation and consequence sensitivity impulsivity were affected by tryptophan depletion, response inhibition impulsivity was not, suggesting that other biological processes may underlie this specific component of impulsivity. Future research in other populations or using different pharmacological agents is warranted to further examine the biological processes underlying these components of impulsivity.

Neural circuits, neurotransmitters, and behavior: serotonin and temperament in bulimic syndromes

In bulimia nervosa (BN), and in related binge-purge syndromes, factors affecting central serotonin (5-hydroxytryptamine, 5-HT) function appear to contribute not only to appetitive dysregulation but also to temperamental and personality manifestations. Drawing upon findings from neurobiological, molecular-genetic, and brain-imaging studies, we present an integrative model of the role of 5-HT function in bulimic syndromes. At the core of our model is a consideration of the ways in which diverse hereditary and environmental influences impact the action of the 5-HT system. We believe that our model helps account for heterogeneous traits seen in the bulimic population, for disproportionate representation of individuals displaying pathological personality traits and exposure to severe environmental stressors, and for interindividual variations as to treatment response.