Behavioral disinhibition induced by tryptophan depletion in nonalcoholic young men with multigenerational family histories of paternal alcoholism

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

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

Negative Urgency Mediates the Relation between Genetically-Influenced Serotonin Functioning and Alcohol Problems

Serotonin (5-HT) functioning is associated with alcohol problems. However, the mechanisms underlying this association remain unclear. The current study tested whether five separate dimensions of impulsivity (UPPS-P) mediated the relation between a polygenic score indexing 5-HT functioning and alcohol problems and whether any of these paths were moderated by age. Results showed that a 5-HT polygenic score predicted alcohol problems indirectly through negative urgency, but not any other facet of impulsivity. The 5-HT polygenic score also directly predicted alcohol problems. No age moderation was found. Findings suggest that negative urgency might be one important mechanism underlying the relation between genetically-influenced 5-HT functioning and alcohol problems. However, genetically-influenced 5-HT functioning likely influences alcohol problems through additional mechanisms. More broadly, results suggest that the previously observed transdiagnostic nature of 5-HT functioning on diverse types of psychopathology might be, in part, explained by its effect on negative urgency.

Tryptophan depletion affects compulsive behaviour in rats: strain dependent effects and associated neuromechanisms

RATIONALE

Compulsive behaviour, present in different psychiatric disorders, such as obsessive-compulsive disorder, schizophrenia and drug abuse, is associated with altered levels of monoamines, particularly serotonin (5-hydroxytryptamine) and its receptor system.

OBJECTIVES

The present study investigated whether 5-HT manipulation, through a tryptophan (TRP) depletion by diet in Wistar and Lister Hooded rats, modulates compulsive drinking in schedule-induced polydipsia (SIP) and locomotor activity in the open-field test. The levels of dopamine, noradrenaline, serotonin and its metabolite were evaluated, as well as the 5-HT_2A and 5-HT_1A receptor binding, in different brain regions.

METHODS

Wistar rats were selected as high (HD) or low (LD) drinkers according to their SIP behaviour, while Lister hooded rats did not show SIP acquisition. Both strains were fed for 14 days with either a TRP-free diet (T-) or a TRP-supplemented diet (T+) RESULTS: The TRP depletion diet effectively reduced 5-HT levels in the frontal cortex, amygdala and hippocampus in both strains of rats. The TRP-depleted HD Wistar rats were more sensitive to 5-HT manipulation, exhibiting more licks on SIP than did the non-depleted HD Wistar rats, while the LD Wistar and the Lister Hooded rats did not exhibit differences in SIP. In contrast, the TRP-depleted Lister Hooded rats increased locomotor activity compared to the non-depleted rats, while no differences were found in the Wistar rats. Serotonin 2A receptor binding in the striatum was significantly reduced in the TRP-depleted HD Wistar rats.

CONCLUSIONS

These results suggest that alterations of the serotonergic system could be involved in compulsive behaviour in vulnerable populations.

Similarities and Differences in Neurobiology

Substance addiction is a chronic, relapsing brain disease characterized by compulsive drug seeking and use despite harmful consequences. Non-substance addiction is defined recently that people may compulsively engage in an activity despite any negative consequences to their lives. Despite differences with respect to their addictive object, substance addiction and non-substance addiction may share similarities with respect to biological, epidemiological, clinical, genetic and other features. Here we review the similarities and differences in neurobiology between these two addictions with a focus on dopamine, serotonin, opioid, glutamate and norepinephrine systems. Studies suggest the involvement of all these systems in both substance addiction and non-substance addiction while differences may exist with respect to their contributions.

Studying the effects of dietary body weight-adjusted acute tryptophan depletion on punishment-related behavioral inhibition

Background

Alterations in serotonergic (5-HT) neurotransmission are thought to play a decisive role in affective disorders and impulse control.

Objective

This study aims to reproduce and extend previous findings on the effects of acute tryptophan depletion (ATD) and subsequently diminished central 5-HT synthesis in a reinforced categorization task using a refined body weight–adjusted depletion protocol.

Design

Twenty-four young healthy adults (12 females, mean age [SD]=25.3 [2.1] years) were subjected to a double-blind within-subject crossover design. Each subject was administered both an ATD challenge and a balanced amino acid load (BAL) in two separate sessions in randomized order. Punishment-related behavioral inhibition was assessed using a forced choice go/no-go task that incorporated a variable payoff schedule.

Results

Administration of ATD resulted in significant reductions in TRP measured in peripheral blood samples, indicating reductions of TRP influx across the blood–brain barrier and related brain 5-HT synthesis. Overall accuracy and response time performance were improved after ATD administration. The ability to adjust behavioral responses to aversive outcome magnitudes and behavioral adjustments following error contingent punishment remained intact after decreased brain 5-HT synthesis. A previously observed dissociation effect of ATD on punishment-induced inhibition was not observed.

Conclusions

Our results suggest that neurodietary challenges with ATD Moja–De have no detrimental effects on task performance and punishment-related inhibition in healthy adults.

Effects of tryptophan depletion and a simulated alcohol binge on impulsivity

Researchers have suggested binge drinkers experience disproportionate increases in impulsivity during the initial period of drinking, leading to a loss of control over further drinking, and that serotonergic mechanisms may underlie such effects. We examined the effects of a simulated alcohol binge and tryptophan depletion on 3 types of impulsivity-response initiation (immediate memory task [IMT]), response inhibition (GoStop task), and delay discounting (single key impulsivity paradigm [SKIP])-and tested whether observed effects were related to real-world binging. Adults (N = 179) with diverse drinking histories completed a within-subject crossover design over 4 experimental days. Each day, participants underwent 1 of 4 test conditions: tryptophan depletion/alcohol, tryptophan depletion/placebo, tryptophan-balanced control/alcohol, or tryptophan-balanced control/placebo. The simulated binge involved consuming 0.3 g/kg of alcohol at 5, 6, and 7 hr after consuming the tryptophan-depletion/balanced mixture. Impulsivity was measured before and after each drink. Relative to the placebo beverage condition, when alcohol was consumed, impulsive responding was increased at moderate and high levels of intoxication on the IMT and the GoStop but only at high levels of intoxication on the SKIP. Tryptophan depletion had no effect on impulsivity. Effects of alcohol and tryptophan manipulations on impulsivity were unrelated to patterns of binge drinking outside the laboratory. The effects of alcohol consumption on impulsivity depend on the component of impulsivity and the dose of alcohol consumed. Such effects do not appear to be a result of reduced serotonin synthesis. In addition, real-world binge drinking behaviors were unrelated to behavioral changes observed in the laboratory.

Involvement of serotonin transporter gene polymorphisms (5-HTT) in impulsive behavior in the japanese population

The serotonergic pathway has been implicated in the pathogenesis of impulsivity, and sensitivity to aversive outcomes may be linked to serotonin (5-HT) levels. Polymorphisms in the gene that encodes the serotonin transporter (5-HTT), which have differential effects on the level of serotonin transmission, display alternate responses to aversive stimuli. However, recent studies have shown that 5-HT does not affect motor function, which suggests that the functioning of the serotonin-transporter-linked polymorphic region (5-HTTLPR) does not directly affect the behavioral regulatory process itself, but instead exerts an effect via the evaluation of the potential risk associated with particular behavioral outputs. The aim of the present study was to examine the effect of specific 5-HTTLPR genotypes on the motor regulatory process, as observed during a Go/Nogo punishment feedback task. 5-HTT gene-linked promoter polymorphisms were analyzed by polymerase chain reaction, using lymphocytes from 61 healthy Japanese volunteers. Impulsivity was defined as the number of commission errors (responding when one should not) made during a Go/Nogo task. We found that the s/s genotype group made fewer impulsive responses, specifically under aversive conditions for committing such errors, compared to those in the s/l group, without affecting overall motor inhibition. These results suggest that 5-HTTLPRs do not directly affect the behavioral regulatory process itself, but may instead exert an effect on the evaluation of potential risk. The results also indicate that under such aversive conditions, decreased expression of 5-HTT may promote motor inhibitory control.

Genetic and early environmental influences on the serotonin system: consequences for brain development and risk for psychopathology

BACKGROUND

Despite more than 60 years of research in the role of serotonin (5-HT) in psychopathology, many questions still remain. From a developmental perspective, studies have provided more insight into how 5-HT dysfunctions acquired in utero or early in life may modulate brain development. This paper discusses the relevance of the developmental role of 5-HT for the understanding of psychopathology. We review developmental milestones of the 5-HT system, how genetic and environmental 5-HT disturbances could affect brain development and the potential role of DNA methylation in 5-HT genes for brain development.

METHODS

Studies were identified using common databases (e.g., PubMed, Google Scholar) and reference lists.

RESULTS

Despite the widely supported view that the 5-HT system matures in early life, different 5-HT receptors, proteins and enzymes have different developmental patterns, and development is brain region-specific. A disruption in 5-HT homeostasis during development may lead to structural and functional changes in brain circuits that modulate emotional stress responses, including subcortical limbic and (pre)frontal areas. This may result in a predisposition to psychopathology. DNA methylation might be one of the underlying physiologic mechanisms.

LIMITATIONS

There is a need for prospective studies. The impact of stressors during adolescence on the 5-HT system is understudied. Questions regarding efficacy of drugs acting on 5-HT still remain.

CONCLUSION

A multidisciplinary and longitudinal approach in designing studies on the role of 5-HT in psychopathology might help to bring us closer to the understanding of the role of 5-HT in psychopathology.

Recent Insights into the Neurobiology of Impulsivity

Impulsivity is associated with various psychopathologies, and elevated impulsivity is typically disadvantageous. This manuscript reviews recent investigations into the neurobiology of impulsivity using human imaging techniques and animal models. Both human imaging and preclinical pharmacological manipulations have yielded important insights into the neurobiological underpinnings of impulsivity. A more thorough understanding of the complex neurobiology underlying aspects of impulsivity may provide insight into new treatment options that target elevated impulsivity and psychopathologies such as addictions.

Serotonin depletion induces 'waiting impulsivity' on the human four-choice serial reaction time task: cross-species translational significance

Convergent results from animal and human studies suggest that reducing serotonin neurotransmission promotes impulsive behavior. Here, serotonin depletion was induced by the dietary tryptophan depletion procedure (TD) in healthy volunteers to examine the role of serotonin in impulsive action and impulsive choice. We used a novel translational analog of a rodent 5-choice serial reaction time task (5-CSRTT)-- the human 4-CSRTT--and a reward delay-discounting questionnaire to measure effects on these different forms of 'waiting impulsivity'. There was no effect of TD on impulsive choice as indexed by the reward delay-discounting questionnaire. However, TD significantly increased 4-CSRTT premature responses (or impulsive action), which is remarkably similar to the previous findings of effect of serotonin depletion on rodent 5-CSRTT performance. Moreover, the increased premature responding in TD correlated significantly with individual differences on the motor impulsivity subscale of the Barratt Impulsivity Scale. TD also improved the accuracy of performance and speeded responding, possibly indicating enhanced attention and reward processing. The results suggest: (i) the 4-CSRTT will be a valuable addition to the tests already available to measure impulsivity in humans in a direct translational analog of a test extensively used in rodents; (ii) TD in humans produces a qualitatively similar profile of effects to those in rodents (ie, enhancing premature responding), hence supporting the conclusion that TD in humans exerts at least some of its effects on central serotonin; and (iii) this manipulation of serotonin produces dissociable effects on different measures of impulsivity, suggesting considerable specificity in its modulatory role.

Selective serotonin reuptake inhibition modulates response inhibition in Parkinson's disease

Impulsivity is common in Parkinson's disease even in the absence of impulse control disorders. It is likely to be multifactorial, including a dopaminergic 'overdose' and structural changes in the frontostriatal circuits for motor control. In addition, we proposed that changes in serotonergic projections to the forebrain also contribute to response inhibition in Parkinson's disease, based on preclinical animal and human studies. We therefore examined whether the selective serotonin reuptake inhibitor citalopram improves response inhibition, in terms of both behaviour and the efficiency of underlying neural mechanisms. This multimodal magnetic resonance imaging study used a double-blind randomized placebo-controlled crossover design with an integrated Stop-Signal and NoGo paradigm. Twenty-one patients with idiopathic Parkinson's disease (46-76 years old, 11 male, Hoehn and Yahr stage 1.5-3) received 30 mg citalopram or placebo in addition to their usual dopaminergic medication in two separate sessions. Twenty matched healthy control subjects (54-74 years old, 12 male) were tested without medication. The effects of disease and drug on behavioural performance and regional brain activity were analysed using general linear models. In addition, anatomical connectivity was examined using diffusion tensor imaging and tract-based spatial statistics. We confirmed that Parkinson's disease caused impairment in response inhibition, with longer Stop-Signal Reaction Time and more NoGo errors under placebo compared with controls, without affecting Go reaction times. This was associated with less stop-specific activation in the right inferior frontal cortex, but no significant difference in NoGo-related activation. Although there was no beneficial main effect of citalopram, it reduced Stop-Signal Reaction Time and NoGo errors, and enhanced inferior frontal activation, in patients with relatively more severe disease (higher Unified Parkinson's Disease Rating Scale motor score). The behavioural effect correlated with the citalopram-induced enhancement of prefrontal activation and the strength of preserved structural connectivity between the frontal and striatal regions. In conclusion, the behavioural effect of citalopram on response inhibition depends on individual differences in prefrontal cortical activation and frontostriatal connectivity. The correlation between disease severity and the effect of citalopram on response inhibition may be due to the progressive loss of forebrain serotonergic projections. These results contribute to a broader understanding of the critical roles of serotonin in regulating cognitive and behavioural control, as well as new strategies for patient stratification in clinical trials of serotonergic treatments in Parkinson's disease.

Effects of quetiapine and olanzapine in patients with psychosis and violent behavior: a pilot randomized, open-label, comparative study

OBJECTIVE

Patients suffering from psychosis are more likely than the general population to commit aggressive acts, but the therapeutics of aggressive behavior are still a matter of debate.

METHODS

This pilot randomized, open-label study compared the efficacy of quetiapine versus olanzapine in reducing impulsive and aggressive behaviors (primary endpoints) and psychotic symptoms (secondary endpoints) from baseline to days 1, 7, 14, 28, 42, 56, and 70, in 15 violent schizophrenic patients hospitalized in a maximum-security psychiatric hospital.

RESULTS

Quetiapine (525±45 mg) and olanzapine (18.5±4.8 mg) were both efficacious in reducing Impulsivity Rating Scale from baseline to day 70. In addition, both treatments reduced the Brief Psychiatric Rating Scale, Positive and Negative Syndrome Scale, and Clinical Global Impression Scale scores at day 70 compared to baseline, and no differences were observed between treatments. Moreover, quetiapine, but not olanzapine, yielded an improvement of depressive symptoms in the items "depression" in Brief Psychiatric Rating Scale and "blunted affect" in Positive and Negative Syndrome Scale. Modified Overt Aggression Scale scores were also decreased from baseline to the endpoint, but due to the limited number of patients, it was not possible to detect a significant difference.

CONCLUSION

In this pilot study, quetiapine and olanzapine equally decreased impulsive and psychotic symptoms after 8 weeks of treatment. Double-blind, large studies are needed to confirm the validity of these two treatments in highly aggressive and violent schizophrenic patients.

Serotonin at the nexus of impulsivity and cue reactivity in cocaine addiction

Cocaine abuse and addiction remain great challenges on the public health agendas in the U.S. and the world. Increasingly sophisticated perspectives on addiction to cocaine and other drugs of abuse have evolved with concerted research efforts over the last 30 years. Relapse remains a particularly powerful clinical problem as, even upon termination of drug use and initiation of abstinence, the recidivism rates can be very high. The cycling course of cocaine intake, abstinence and relapse is tied to a multitude of behavioral and cognitive processes including impulsivity (a predisposition toward rapid unplanned reactions to stimuli without regard to the negative consequences), and cocaine cue reactivity (responsivity to cocaine-associated stimuli) cited as two key phenotypes that contribute to relapse vulnerability even years into recovery. Preclinical studies suggest that serotonin (5-hydroxytryptamine; 5-HT) neurotransmission in key neural circuits may contribute to these interlocked phenotypes well as the altered neurobiological states evoked by cocaine that precipitate relapse events. As such, 5-HT is an important target in the quest to understand the neurobiology of relapse-predictive phenotypes, to successfully treat this complex disorder and improve diagnostic and prognostic capabilities. This review emphasizes the role of 5-HT and its receptor proteins in key addiction phenotypes and the implications of current findings to the future of therapeutics in addiction. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Effects of escitalopram on attentional bias to cocaine-related stimuli and inhibitory control in cocaine-dependent subjects

Key characteristics of cocaine dependence include attentional bias to cocaine cues and impaired inhibitory control. Studies suggest that serotonin modulates both cocaine cue reactivity and inhibitory control. We investigated effects of the selective serotonin reuptake inhibitor escitalopram on cocaine cue reactivity and inhibitory processes in cocaine-dependent subjects. In a double-blind placebo-controlled design, cocaine-dependent subjects received placebo (n=12) or escitalopram (n=11; 10 mg on days 1-3, 20 mg on days 4-24 and 10 mg on days 25-28) orally, once daily for 4 weeks. The cocaine Stroop and immediate memory task (IMT) were administered at baseline, days 1, 4, 11, 18 and 25 after placebo or escitalopram initiation. There were no significant between-group differences in baseline performance on the cocaine Stroop task or the IMT. On day 1 (acute phase), escitalopram produced a significantly greater decrease from baseline than placebo in attentional bias measured by cocaine Stroop task 5 hours post-dose. No significant changes from baseline in attentional bias were observed on subsequent test days (chronic phase). Inhibitory control as measured by IMT commission error rate was not significantly different between two groups in either the acute or chronic phase. Consistent with preclinical data, serotonin-modulating drugs like escitalopram may have acute effects on cocaine cue reactivity in human cocaine users.

Revisiting the serotonin-aggression relation in humans: a meta-analysis

The inverse relation between serotonin and human aggression is often portrayed as "reliable," "strong," and "well established" despite decades of conflicting reports and widely recognized methodological limitations. In this systematic review and meta-analysis, we evaluate the evidence for and against the serotonin deficiency hypothesis of human aggression across 4 methods of assessing serotonin: (a) cerebrospinal fluid levels of 5-hydroxyindoleacetic acid (CSF 5-HIAA), (b) acute tryptophan depletion, (c) pharmacological challenge, and (d) endocrine challenge. Results across 175 independent samples and over 6,500 total participants were heterogeneous, but, in aggregate, revealed a small, inverse correlation between serotonin functioning and aggression, anger, and hostility (r = -.12). Pharmacological challenge studies had the largest mean weighted effect size (r = -.21), and CSF 5-HIAA studies had the smallest (r = -.06). Potential methodological and demographic moderators largely failed to account for variability in study outcomes. Notable exceptions included year of publication (effect sizes tended to diminish with time) and self- versus other-reported aggression (other-reported aggression was positively correlated to serotonin functioning). We discuss 4 possible explanations for the pattern of findings: unreliable measures, ambient correlational noise, an unidentified higher order interaction, and a selective serotonergic effect. Finally, we provide 4 recommendations for bringing much needed clarity to this important area of research: acknowledge contradictory findings and avoid selective reporting practices; focus on improving the reliability and validity of serotonin and aggression measures; test for interactions involving personality and/or environmental moderators; and revise the serotonin deficiency hypothesis to account for serotonin's functional complexity.

The utility of rat models of impulsivity in developing pharmacotherapies for impulse control disorders

High levels of impulsive behaviours are a clinically significant symptom in a range of psychiatric disorders, such as attention deficit hyperactivity disorder, bipolar disorder, personality disorders, pathological gambling and substance abuse. Although often measured using questionnaire assessments, levels of different types of impulsivity can also be determined using behavioural tests. Rodent analogues of these paradigms have been developed, and similar neural circuitry has been implicated in their performance in both humans and rats. In the current review, the methodology underlying the measurement of different aspects of impulsive action and choice are considered from the viewpoint of drug development, with a focus on the continuous performance task (CPT), stop-signal task (SST), go/no-go and delay-discounting paradigms. Current issues impeding translation between animal and human studies are identified, and comparisons drawn between the acute effects of dopaminergic, noradrenergic and serotonergic compounds across species. Although the field could benefit from a more systematic determination of different pharmacological agents across paradigms, there are signs of strong concordance between the animal and human data. However, the type of impulsivity measured appears to play a significant role, with the SST and delay discounting providing more consistent effects for dopaminergic drugs, while the CPT and SST show better predictive validity so far for serotonergic and noradrenergic compounds. Based on the available data, it would appear that these impulsivity models could be used more widely to identify potential pharmacotherapies for impulse control disorders. Novel targets within the glutamatergic and serotonergic system are also suggested.

The effects of acute tryptophan depletion on reactive aggression in adults with attention-deficit/hyperactivity disorder (ADHD) and healthy controls

BACKGROUND

The neurotransmitter serotonin (5-HT) has been linked to the underlying neurobiology of aggressive behavior, particularly with evidence from studies in animals and humans. However, the underlying neurobiology of aggression remains unclear in the context of attention-deficit/hyperactivity disorder (ADHD), a disorder known to be associated with aggression and impulsivity. We investigated the effects of acute tryptophan depletion (ATD), and the resulting diminished central nervous serotonergic neurotransmission, on reactive aggression in healthy controls and adults with ADHD.

METHODOLOGY/PRINCIPAL FINDINGS

Twenty male patients with ADHD and twenty healthy male controls were subjected to ATD with an amino acid (AA) beverage that lacked tryptophan (TRP, the physiological precursor of 5-HT) and a TRP-balanced AA beverage (BAL) in a double-blind, within-subject crossover-study over two study days. We assessed reactive aggression 3.25 hours after ATD/BAL intake using a point-subtraction aggression game (PSAG) in which participants played for points against a fictitious opponent. Point subtraction was taken as a measure for reactive aggression. Lowered rates of reactive aggression were found in the ADHD group under ATD after low provocation (LP), with controls showing the opposite effect. In patients with ADHD, trait-impulsivity was negatively correlated with the ATD effect on reactive aggression after LP. Statistical power was limited due to large standard deviations observed in the data on point subtraction, which may limit the use of this particular paradigm in adults with ADHD.

CONCLUSIONS/SIGNIFICANCE

Together with previous findings, the data provide preliminary evidence of an inverse association between trait-impulsivity and the ATD effect on reactive aggression after LP (as assessed by the PSAG) in patients with ADHD and that this relationship can be found in both adolescents and adults. Because of limited statistical power larger sample sizes are needed to find main effects of ATD/BAL administration on reactive aggression in adults with ADHD.

Similarities and differences between pathological gambling and substance use disorders: a focus on impulsivity and compulsivity

RATIONALE

Pathological gambling (PG) has recently been considered as a "behavioral" or nonsubstance addiction. A comparison of the characteristics of PG and substance use disorders (SUDs) has clinical ramifications and could help advance future research on these conditions. Specific relationships with impulsivity and compulsivity may be central to understanding PG and SUDs.

OBJECTIVES

This review was conducted to compare and contrast research findings in PG and SUDs pertaining to neurocognitive tasks, brain function, and neurochemistry, with a focus on impulsivity and compulsivity.

RESULTS

Multiple similarities were found between PG and SUDs, including poor performance on neurocognitive tasks, specifically with respect to impulsive choice and response tendencies and compulsive features (e.g., response perseveration and action with diminished relationship to goals or reward). Findings suggest dysfunction involving similar brain regions, including the ventromedial prefrontal cortex and striatum and similar neurotransmitter systems, including dopaminergic and serotonergic. Unique features exist which may in part reflect influences of acute or chronic exposures to specific substances.

CONCLUSIONS

Both similarities and differences exist between PG and SUDs. Understanding these similarities more precisely may facilitate treatment development across addictions, whereas understanding differences may provide insight into treatment development for specific disorders. Individual differences in features of impulsivity and compulsivity may represent important endophenotypic targets for prevention and treatment strategies.

Effects of acute tryptophan depletion on prefrontal-amygdala connectivity while viewing facial signals of aggression

BACKGROUND

Reduced levels of serotonin (5-HT) within prefrontal cortex (PFC)-amygdala circuits have long been implicated in impulsive aggression. However, whether lowering 5-HT alters the dynamic interplay between the PFC and the amygdala has not been directly tested in humans. It is known that manipulating 5-HT via acute tryptophan depletion (ATD) causes variable effects on brain responses to a variety of emotional stimuli, but it remains unclear whether ATD affects functional connectivity in neural networks involved in processing social signals of aggression (e.g., angry faces).

METHODS

Thirty healthy individuals were enrolled in a randomized, double-blind, placebo-controlled ATD study. On each treatment, brain responses to angry, sad, and neutral faces were measured with functional magnetic resonance imaging. Two methods (psycho-physiological-interaction in a general linear model and dynamic causal modeling) were used to assess the impact of ATD on the functional connectivity between PFC and amygdala.

RESULTS

Data from 19 subjects were available for the final analyses. A whole-brain psycho-physiological-interaction in a general linear model showed that ATD significantly modulated the connectivity between the amygdala and two PFC regions (ventral anterior cingulate cortex and ventrolateral PFC) when processing angry vs. neutral and angry vs. sad but not sad vs. neutral faces. Dynamic causal modeling corroborated and extended these findings by showing that 5-HT depletion reduced the influence of processing angry vs. neutral faces on circuits within PFC and on PFC-amygdala pathways.

CONCLUSIONS

We provide strong support for neurobiological accounts positing that 5-HT significantly influences PFC-amygdala circuits implicated in aggression and other affective behaviors.

Serotonin and dopamine: unifying affective, activational, and decision functions

Serotonin, like dopamine (DA), has long been implicated in adaptive behavior, including decision making and reinforcement learning. However, although the two neuromodulators are tightly related and have a similar degree of functional importance, compared with DA, we have a much less specific understanding about the mechanisms by which serotonin affects behavior. Here, we draw on recent work on computational models of dopaminergic function to suggest a framework by which many of the seemingly diverse functions associated with both DA and serotonin-comprising both affective and activational ones, as well as a number of other functions not overtly related to either-can be seen as consequences of a single root mechanism.

Brain serotonin synthesis in adult males characterized by physical aggression during childhood: a 21-year longitudinal study

BACKGROUND

Adults exhibiting severe impulsive and aggressive behaviors have multiple indices of low serotonin (5-HT) neurotransmission. It remains unclear though whether low 5-HT mediates the behavior or instead reflects a pre-existing vulnerability trait.

METHODOLOGY/PRINCIPAL FINDINGS

In the present study, positron emission tomography with the tracer alpha-[(11)C]methyl-L-tryptophan ((11)C-AMT) was used to compare 5-HT synthesis capacity in two groups of adult males from a 21-year longitudinal study (mean age +/- SD: 27.1+/-0.7): individuals with a history of childhood-limited high physical aggression (C-LHPA; N = 8) and individuals with normal (low) patterns of physical aggression (LPA; N = 18). The C-LHPA males had significantly lower trapping of (11)C-AMT bilaterally in the orbitofrontal cortex and self-reported more impulsiveness. Despite this, in adulthood there were no group differences in plasma tryptophan levels, genotyping, aggression, emotional intelligence, working memory, computerized measures of impulsivity, psychosocial functioning/adjustment, and personal and family history of mood and substance abuse disorders.

CONCLUSIONS/SIGNIFICANCE

These results force a re-examination of the low 5-HT hypothesis as central in the biology of violence. They suggest that low 5-HT does not mediate current behavior and should be considered a vulnerability factor for impulsive-aggressive behavior that may or may not be expressed depending on other biological factors, experience, and environmental support during development.

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.

Recent research on impulsivity in individuals with drug use and mental health disorders: implications for alcoholism

Alcohol misuse and dependence, and many of its accompanying psychological problems, are associated with heightened levels of impulsivity that both accelerate the development of clinically significant illness and complicate clinical outcome. This article reviews recent developments in our understanding of impulsivity as they relate to brain circuitry that might underlie these comorbid factors, focusing upon the clinical features of substance use (and dependence), bipolar disorder, and pathological gambling. Individuals who are affected by these disorders exhibit problems in several domains of impulsive behavior including deficient response or "motor" control, and the tolerance of prolonged delays prior to larger rewards at the expense of smaller rewards ("delay-discounting"). These populations, like alcoholic dependents, also exhibit impairments in risky decision-making that may reflect dysfunction of monoamine and catecholamine pathways. However, several areas of uncertainty exist including the specificity of impairments across disorders and the relationship between impulse control problems and altered evaluation of reward outcomes underlying observed impairments in action selection.

The short (S) allele of the serotonin transporter polymorphism and acute tryptophan depletion both increase impulsivity in men

Reduced serotonergic neurotransmission is implicated in impulsive behavior. We studied the triallelic system of the serotonin transporter gene linked polymorphic region (5-HTTLPR) and acute manipulation of serotonin together to further delineate the mechanisms by which serotonergic neurotransmission affects impulsivity. Fifty-two healthy participants (38 men and 14 women) underwent acute tryptophan depletion (ATD) or placebo in a randomized, double-blind, parallel group experiment. Impulsive response style was measured on two versions of the Continuous Performance Task (CPT), and calculated using signal detection theory. We observed a dose-dependent effect for the short (S') allele of the 5-HTTLPR on impulsive response style. Individuals who had the S'/S' genotype were more impulsive than individuals with the L/S' genotype. Participants with the L/S' genotype were more impulsive than those with the L/L genotype. ATD increased impulsivity in men, and decreased impulsivity in women. These data demonstrate for the first time that reduced serotonergic tone as a result of either 5-HTTLPR genotype, or experimental ATD, are both independently and additively, associated with elevated impulsive response style in Caucasian men.

Is there an inhibitory-response-control system in the rat? Evidence from anatomical and pharmacological studies of behavioral inhibition

Many common psychiatric conditions, such as attention deficit/hyperactivity disorder (ADHD), obsessive-compulsive disorder (OCD), Parkinson's disease, addiction and pathological gambling are linked by a failure in the mechanisms that control, or inhibit, inappropriate behavior. Models of rat behavioral inhibition permit us to study in detail the anatomical and pharmacological bases of inhibitory failure, using methods that translate directly with patient assessment in the clinic. This review updates current ideas relating to behavioral inhibition based on two significant lines of evidence from rat studies: (1) To integrate new findings from the stop-signal task into existing models of behavioral inhibition, in particular relating to 'impulsive action' control. The stop-signal task has been used for a number of years to evaluate psychiatric conditions and has recently been translated for use in the rat, bringing a wealth of new information to behavioral inhibition research. (2) To consider the importance of the subthalamic nucleus (STN) in the neural circuitry of behavioral inhibition. This function of this nucleus is central to a number of 'disinhibitory' disorders such as Parkinson's disease and OCD, and their therapies, but its role in behavioral inhibition is still undervalued, and often not considered in preclinical models of behavioral control. Integration of these findings has pinpointed the orbitofrontal cortex (OF), dorsomedial striatum (DMStr) and STN within a network that normally inhibits many forms of behavior, including both impulsive and compulsive forms. However, there are distinct differences between behavioral subtypes in their neurochemical modulation. This review brings new light to the classical view of the mechanisms that inhibit behavior, in particular suggesting a far more prominent role for the STN, a structure that is usually omitted from conventional behavioral-inhibition networks. The OF-DMStr-STN circuitry may form the basis of a control network that defines behavioral inhibition and that acts to suppress or countermand many forms of inappropriate or maladaptive behavior.

Drinking histories in alcohol-use-disordered youth: preliminary findings on relationships to platelet serotonin transporter expression with genotypes of the serotonin transporter

OBJECTIVE

The serotonin (5-HT) transporter (5-HTT) is thought to play a key role in the onset of alcohol use, with potential behavioral and biological mechanisms mediated by the level of 5-HT in the synapse and in cerebral spinal fluid. Although 5-HT dysregulation has been related to poor impulse control, the biological mechanism is unknown, although functional control of the serotonergic system has been shown to be regulated in part by differential expression of the 5-HTT. The gene responsible for encoding 5-HTT has a functional polymorphism at the 5'-regulatory promoter region, which results in two forms: long (L) and short (S). The LL genotype is hypothesized to play a key role in the early onset of alcohol use and may be related to poor impulse control. The objective of this pilot study is to determine whether adolescents with a current alcohol-use disorder (AUD) (N = 21) have platelet measures of the 5-HTT functioning that are related to 5-HTT genotype and poor impulse control. Specifically, we wanted to examine the relationships between the following: platelet 5-HTT and 5-HTT genotype; platelet 5-HTT parameters and age at onset, as well as duration of drinking; and 5-HTT genotype and impulse control.

METHOD

Adolescents with current AUD were recruited from the community to participate in a cross-section pilot study.

RESULTS

Our main findings showed significantly higher paroxetine binding (density of 5-HTT) in LL genotype versus S carriers (SS or SL genotypes); also, the LL group had a significantly earlier age at onset of drinking and longer duration of drinking, and poorer impulse control.

CONCLUSIONS

These findings provide partial support for the hypothesis that, among currently drinking adolescents with an AUD, differential expression of 5-HTT may play an important role in the onset of adolescent AUD.

Reconciling the role of serotonin in behavioral inhibition and aversion: acute tryptophan depletion abolishes punishment-induced inhibition in humans

The neuromodulator serotonin has been implicated in a large number of affective and executive functions, but its precise contribution to motivation remains unclear. One influential hypothesis has implicated serotonin in aversive processing; another has proposed a more general role for serotonin in behavioral inhibition. Because behavioral inhibition is a prepotent reaction to aversive outcomes, it has been a challenge to reconcile these two accounts. Here, we show that serotonin is critical for punishment-induced inhibition but not overall motor response inhibition or reporting aversive outcomes. We used acute tryptophan depletion to temporarily lower brain serotonin in healthy human volunteers as they completed a novel task designed to obtain separate measures of motor response inhibition, punishment-induced inhibition, and sensitivity to aversive outcomes. After a placebo treatment, participants were slower to respond under punishment conditions compared with reward conditions. Tryptophan depletion abolished this punishment-induced inhibition without affecting overall motor response inhibition or the ability to adjust response bias in line with punishment contingencies. The magnitude of reduction in punishment-induced inhibition depended on the degree to which tryptophan depletion reduced plasma tryptophan levels. These findings extend and clarify previous research on the role of serotonin in aversive processing and behavioral inhibition and fit with current theorizing on the involvement of serotonin in predicting aversive outcomes.

Serotonin augmentation reduces response to attack in aggressive individuals

We tested the theory that central serotonin (5-hydroxytryptamine, or 5-HT) activity regulates aggression by modulating response to provocation. Eighty men and women (40 with and 40 without a history of aggression) were randomly assigned to receive either 40 mg of paroxetine (to acutely augment serotonergic activity) or a placebo, administered using double-blind procedures. Aggression was assessed during a competitive reaction time game with a fictitious opponent. Shocks were selected by the participant and opponent before each trial, with the loser on each trial receiving the shock set by the other player. Provocation was manipulated by having the opponent select increasingly intense shocks for the participant and eventually an ostensibly severe shock toward the end of the trials. Aggression was measured by the number of severe shocks set by the participant for the opponent. As predicted, aggressive responding after provocation was attenuated by augmentation of serotonin in individuals with a pronounced history of aggression.

L-Tryptophan: Basic Metabolic Functions, Behavioral Research and Therapeutic Indications

An essential component of the human diet, L-tryptophan is critical in a number of metabolic functions and has been widely used in numerous research and clinical trials. This review provides a brief overview of the role of L-tryptophan in protein synthesis and a number of other metabolic functions. With emphasis on L-tryptophan's role in synthesis of brain serotonin, details are provided on the research uses of L-tryptophan, particularly L-tryptophan depletion, and on clinical trials that have been conducted using L-tryptophan supplementation. The ability to change the rates of serotonin synthesis in the brain by manipulating concentrations of serum tryptophan is the foundation of much research. As the sole precursor of serotonin, experimental research has shown that L-tryptophan's role in brain serotonin synthesis is an important factor involved in mood, behavior, and cognition. Furthermore, clinical trials have provided some initial evidence of L-tryptophan's efficacy for treatment of psychiatric disorders, particularly when used in combination with other therapeutic agents.

Acute tryptophan depletion and self-injurious behavior in aggressive patients and healthy volunteers

RATIONALE

An association between serotonin (5-HT) activity and self-injurious (i.e., self-aggressive) behavior across the spectrum of lethality (from self-mutilation through completed suicide) is a well-replicated finding. Studies to date, however, have relied on nonexperimental designs to examine this relationship, limiting the causal inferences that can be drawn about the role of 5-HT in self-aggressive behavior.

OBJECTIVE

Examine the effect of experimentally altered 5-HT activity (via dietary tryptophan depletion) on self-aggressive behavior among adults with and without intermittent explosive disorder (IED). Individuals with a marked history of aggression, such as those with IED, are characterized by compromised 5-HT and heightened risk for self-aggression, making this a population of interest for examining the proposed relations.

MATERIALS AND METHODS

IED patients (n = 16) and healthy controls (n = 16) received a tryptophan depletion and a placebo drink on separate days at least 1 week apart. Self-aggressive behavior was assessed on both study days using a well-validated laboratory-based behavioral assessment with self-aggression defined as the intensity of shock self-administered.

RESULTS

Tryptophan depletion facilitated selection of more intense shocks, on average, in both groups. Patients with IED were also more self-aggressive overall than healthy volunteers. No IED by drink condition interactions were found.

CONCLUSION

Experimentally lowered 5-HT bioavailability enhances overall self-injurious behavior irrespective of aggression history.

Acute tryptophan depletion and sweet food consumption by overweight adults

Serotonergic involvement has been implicated in preferential consumption of treat foods. We tested the effect of acute tryptophan depletion (ATD) on food consumption by overweight and lean adults with and without a history of recurrent major depressive disorder (MDD). ATD and taste-matched placebo challenges were administered double-blind in counter-balanced order. Participants were classified as lean (n=36) or overweight (n=19) on the basis of body mass index (BMI). Total calorie, carbohydrate, protein, and sweet food consumption were assessed via a test meal 8-h following ATD. Four food items of comparable palatability were offered as a part of the test: two sweet (one carbohydrate-rich, and one protein-rich) and two non-sweet (one carbohydrate-rich, and one protein-rich). As compared to the placebo challenge, ATD significantly increased sweet calorie intake among overweight participants and increased their propensity to consume sweet food first before any other type of food. Lean participants' sweet calorie intake and food preference were unaffected by ATD. Findings suggest serotonergic involvement in the sweet food consumption by overweight individuals.

Disrupted 'reflection' impulsivity in cannabis users but not current or former ecstasy users

Evidence for serotonin involvement in impulsivity has generated interest in the measurement of impulsivity in regular ecstasy users, who are thought to display serotonergic dysfunction. However, current findings are inconsistent. Here, we used a recently developed Information Sampling Test to measure 'reflection' impulsivity in 46 current ecstasy users, 14 subjects who used ecstasy in the past, 15 current cannabis users and 19 drug-naïve controls. Despite elevated scores on the Impulsivity subscale of the Eysenck Impulsiveness-Venturesomeness-Empathy questionnaire, the current and previous ecstasy users did not differ significantly from the drug-naive controls on the Information Sampling Test. In contrast, the cannabis users sampled significantly less information on the task, and tolerated a lower level of certainty in their decision-making, in comparison to the drug-naive controls. The effect in cannabis users extends our earlier observations in amphetamine- and opiate-dependent individuals (Clark, et al., 2006, Biological Psychiatry 60: 515-522), and suggests that reduced reflection may be a common cognitive style across regular users of a variety of substances. However, the lack of effects in the two ecstasy groups suggests that the relationship between serotonin function, ecstasy use and impulsivity is more complex.

Effects of family history of alcohol use disorders on spatial working memory BOLD response in adolescents

BACKGROUND

A positive family history (FH) of alcohol use disorders (AUD) has been linked to increased risk for the development of AUD, and neurocognitive factors have been postulated as important underlying mechanisms of familial alcoholism transmission.

METHODS

We used functional magnetic resonance imaging (fMRI) during a spatial working memory (SWM) and vigilance paradigm to investigate potential neurodevelopmental differences linked to familial density of AUD in 72 adolescents aged 12 to 14 years.

RESULTS

Youth with denser family histories of AUD showed less activation during a simple vigilance condition relative to SWM in cingulate and medial frontal gyri (beta = 0.28, p = 0.03), and a trend for more relative activity during rest (beta = -0.25, p = 0.07) in this cluster.

CONCLUSIONS

Youth with greater familial densities of AUD may be less successful at modulating activity of the default network, potentially indicating a greater propensity for task-independent thought or reduced inhibition of task-irrelevant processing. Failure to moderate activation of the default network may have implications for cognitive efficiency and goal directed behavior in youth with dense FH. Further, aberrant activation in cingulate regions may be linked to genetic variation in GABA receptor units, suggesting a useful endophenotype for risk associated with alcohol dependence.

Serotonin, inhibition, and negative mood

Pavlovian predictions of future aversive outcomes lead to behavioral inhibition, suppression, and withdrawal. There is considerable evidence for the involvement of serotonin in both the learning of these predictions and the inhibitory consequences that ensue, although less for a causal relationship between the two. In the context of a highly simplified model of chains of affectively charged thoughts, we interpret the combined effects of serotonin in terms of pruning a tree of possible decisions, (i.e., eliminating those choices that have low or negative expected outcomes). We show how a drop in behavioral inhibition, putatively resulting from an experimentally or psychiatrically influenced drop in serotonin, could result in unexpectedly large negative prediction errors and a significant aversive shift in reinforcement statistics. We suggest an interpretation of this finding that helps dissolve the apparent contradiction between the fact that inhibition of serotonin reuptake is the first-line treatment of depression, although serotonin itself is most strongly linked with aversive rather than appetitive outcomes and predictions.

Serotonin is an evolutionarily ancient neuromodulator probably best known for its role in psychiatric disorders. However, that role has long appeared contradictory to its role in normal function, and indeed its various roles in normal affective behaviors have been hard to reconcile. Here, we model two predominant functions of normal serotonin function in a highly simplified reinforcement learning model and show how these may explain some of its complex roles in depression and anxiety.

The effects of acute tryptophan depletion and serotonin transporter polymorphism on emotional processing in memory and attention

Polymorphism at the serotonin transporter linked polymorphic region (5-HTTLPR) has been associated with neuroticism, increased risk for affective disorders and greater vulnerability to mood change following serotonin (5-HT) depletion. The aim of the present study was to investigate whether the cognitive effects of 5-HT depletion were differentially affected by genotype at the 5-HTTLPR polymorphism, using neuropsychological measures of memory and attention. We utilized the acute tryptophan depletion (ATD) technique to temporarily reduce 5-HT synthesis in two groups of healthy volunteers pre-selected on the basis of 5-HTTLPR genotype, 15 of the ll genotype and 15 of the ss genotype, in a double-blind, placebo-controlled crossover design. As expected, ATD resulted in a robust reduction in plasma tryptophan concentration in both genotype groups. However, the genotype groups differed in terms of the effect of ATD on cognitive performance. The ss genotype group showed impaired verbal recall following depletion, while episodic memory was unimpaired by ATD in the ll genotype group. Averaging across depletion condition, the ss genotype group outperformed the ll genotype group on tests of episodic memory and attention. Neither group was significantly affected by ATD on measures of emotional state. These data confirm previous reports that ss individuals are particularly vulnerable to 5-HT depletion, but extend these findings to the cognitive domain. The unexpected finding that ss volunteers showed improved memory and attention relative to ll volunteers suggests a possible evolutionary advantage to possession of the s allele, which may offset the disadvantage of vulnerability to depression following stressful life events.

Somatic responses in behavioral inhibition

In the present study, skin conductance responses (SCRs) were measured postdecision and prefeedback in a go/no-go (GNG) task in which participants used response feedback to learn when to respond or not to respond to numeric stimuli. Like somatic markers in gambling tasks and somatic reactions to error monitoring in choice reaction time tasks, SCR patterns distinguished between correct and incorrect trials over time. These somatic reactions were disrupted by a reversal of GNG contingencies, and they were facilitated by pretraining of the stimulus-response mappings. In all cases, however, the somatic reactions appeared to be a product of competent decision making rather than a contributor to performance. Differential somatic responses to good and bad choices appear to be a robust and fairly general phenomenon, but researchers should be cautious in assuming that the somatic responses contribute to performance.

Neuropsychological function in ecstasy users: a study controlling for polydrug use

RATIONALE

A number of studies have compared ecstasy users to control groups on various measures of neuropsychological function in order to determine whether ecstasy use results in lasting cognitive deficits. However, few of those studies controlled adequately for non-ecstasy illicit drug use.

OBJECTIVE

The aim of this study was to investigate neuropsychological function in chronic ecstasy users while controlling for polydrug use.

METHODS

Neuropsychological function was assessed in four groups-30 current 3,4-methylenedioxymethamphetamine (MDMA) users with a little history of illicit drug use other than ecstasy and cannabis, 30 polydrug controls, 30 drug-naïve controls and 20 ex-MDMA users-using a battery of well-validated, computerized neuropsychological tests. The battery focused on memory, executive function, impulsivity and risk-taking.

RESULTS

Few differences were apparent between the groups, and on no measure were the current MDMA users impaired significantly relative to the polydrug controls. However, within the current MDMA users, questionnaire-measured impulsivity correlated with performance on a number of tests-a relationship that was not apparent in the controls.

CONCLUSIONS

These data highlight the complexity in understanding the current ecstasy literature and suggest that some individuals may be particularly vulnerable to cognitive impairment following chronic use. Although no differences were identified between the current MDMA users and the controls, trait impulsiveness was significantly correlated with impairment on a number of neuropsychological outcome measures in the MDMA users, but not in the controls. These data suggest that impulsive individuals may be those most at risk for the development of cognitive impairment following chronic ecstasy use.

Elevated impulsivity and impaired decision-making cognition in heavy users of MDMA ("Ecstasy")

RATIONALE

In animal studies, the common club drug 3,4-methylendioxymethamphetamine (MDMA, "Ecstasy") consistently caused a prolonged loss of presynaptic serotonergic neurons, and evidence suggests that MDMA consumption may also affect the human serotonergic system. Serotonin (5-HT) has been implicated in the regulation of impulsivity and such executive functions as decision-making cognition. In fact, MDMA users have shown elevated impulsivity in two studies, but little is known about decision making in drug-free MDMA consumers.

OBJECTIVE

The aim of this study was to examine the cognitive neurotoxicity of MDMA with regard to behavioral impulsivity and decision-making cognition.

METHODS

Nineteen male, abstinent, heavy MDMA users; 19 male, abstinent cannabis users; and 19 male, drug-naïve controls were examined with the Matching Familiar Figures Test (MFFT) as well as with a Go/No-Go Task (GNG) for impulsivity and with a Gambling Task (GT) for executive functioning.

RESULTS

MDMA users showed significantly elevated impulsivity in the MFFT Impulsivity score (I-score), but not in commission errors of the GNG, compared with controls. Cannabis users did not yield altered impulsivity compared with controls. In the GT, MDMA users performed significantly worse than cannabis consumers and controls, whereas cannabis users exhibited the same decision-making capacity as controls. In addition, the I-score as well as the decision-making performance was correlated with measures of MDMA intake. The I-score and the decision-making performance were also correlated.

CONCLUSION

These results suggest that heavy use of MDMA may elevate behavioral impulsivity and impair decision-making cognition possibly mediated by a selective impairment of the 5-HT system.

Behavioral models of impulsivity in relation to ADHD: translation between clinical and preclinical studies

Impulsivity, broadly defined as action without foresight, is a component of numerous psychiatric illnesses including attention deficit/hyperactivity disorder (ADHD), mania and substance abuse. In order to investigate the mechanisms underpinning impulsive behavior, the nature of impulsivity itself needs to be defined in operational terms that can be used as the basis for empirical investigation. Due to the range of behaviors that the term impulsivity describes, it has been suggested that impulsivity is not a unitary construct, but encompasses a variety of related phenomena that may differ in their biological basis. Through fractionating impulsivity into these component parts, it has proved possible to devise different behavioral paradigms to measure various aspects of impulsivity in both humans and laboratory animals. This review describes and evaluates some of the current behavioral models of impulsivity developed for use with rodents based on human neuropsychological tests, focusing on the five-choice serial reaction time task, the stop-signal reaction time task and delay-discounting paradigms. Furthermore, the contributions made by preclinical studies using such methodology to improve our understanding of the neural and neurochemical basis of impulsivity and ADHD are discussed, with particular reference to the involvement of both the serotonergic and dopaminergic systems, and frontostriatal circuitry.

Mismatch negativity: no difference between controls and abstinent alcoholics

BACKGROUND

A number of studies have examined the amplitude of the mismatch negativity (MMN) evoked potential as a measure of a brain inhibitory deficit in alcoholics or those at risk for alcoholism. The current study examined MMN in alcoholics abstinent an average of 6.7 years (with a minimum of six months abstinence) compared to controls. This study examined the association of MMN with alcoholism family history density, with indices of the presence and severity of externalizing disorders (a risk-factor for alcoholism), and with alcohol use variables.

METHODS

Electroencephalograms were gathered on 76 subjects (38 controls, 38 abstinent alcoholics) during a nonattending mismatch negativity experiment. Measures of alcoholism family history density, disinhibited personality traits, and antisocial symptoms served as measures of risk-factors known to be associated with a genetic liability to alcoholism. Alcohol use variables were used as measures of alcoholism severity.

RESULTS

There were no differences in MMN amplitude or latency between controls and abstinent alcoholics. There also were no significant associations between MMN measures and the measures of risk for alcoholism or with the severity of alcohol use or duration of abstinence.

CONCLUSIONS

The results suggest that MMN is neither affected in chronic alcoholics nor associated with alcoholism vulnerability, and thus does not reflect a trait marker of alcoholism or alcoholism risk. The current results do not address effects on MMN of acute alcohol ingestion or withdrawal from alcohol.

Estimates of regional cerebral blood flow and 5-HT2A receptor density in impulsive, aggressive dogs with 99mTc-ECD and 123I-5-I-R91150

Impulsive aggression in dogs has an important impact on human public health. Better insight into the pathophysiology of this phenomenon could lead to more adequate diagnosis and treatment. Indirect in vivo research on peripheral body fluids and post-mortem studies in impulsive animals and humans indicate a deficient serotonergic system in general and disturbances in the serotonin-2A (5-HT2A) receptor in particular. In this study, brain perfusion and the 5-HT2A receptors were examined in impulsive, aggressive dogs, in comparison with a group of normally behaving animals. In order to decide which dogs to include in this study, owners were asked to describe the general behaviour of the dogs, the circumstances in which aggression occurred and their conduct during aggressive acts. Finally, 19 dogs were retained for this study, showing, according to different behavioural specialists, disinhibited dominance aggression. Functional imaging studies were performed on all these dogs. Single-photon emission tomography (SPET) was used to measure regional brain perfusion using technetium-99m labelled ethyl cysteinate dimer (ECD). The 5-HT2A receptor binding properties were investigated using the selective radioligand iodine-123 labelled 5-I-R91150. A significant increase in uptake of the 5-HT2A radioligand was noted in all cortical areas. No significant alterations were found in regional cortical perfusion, indicating that the increased binding index was not a consequence of increased tracer delivery. This study supports a role for the serotonergic system in canine impulsive aggression.