Genetics of emotional regulation: the role of the serotonin transporter in neural function

Serotonergic function, two-mode models of self-regulation, and vulnerability to depression: what depression has in common with impulsive aggression

Evidence from diverse literatures supports the viewpoint that two modes of self-regulation exist, a lower-order system that responds quickly to associative cues of the moment and a higher-order system that responds more reflectively and planfully; that low serotonergic function is linked to relative dominance of the lower-order system; that how dominance of the lower-order system is manifested depends on additional variables; and that low serotonergic function therefore can promote behavioral patterns as divergent as impulsive aggression and lethargic depression. Literatures reviewed include work on two-mode models; studies of brain function supporting the biological plausibility of the two-mode view and the involvement of serotonergic pathways in functions pertaining to it; and studies relating low serotonergic function to impulsiveness, aggression (including extreme violence), aspects of personality, and depression vulnerability. Substantial differences between depression and other phenomena reviewed are interpreted by proposing that depression reflects both low serotonergic function and low reward sensitivity. The article closes with brief consideration of the idea that low serotonergic function relates to even more diverse phenomena, whose natures depend in part on sensitivities of other systems.

Tuning the developing brain to social signals of emotions

Humans in different cultures develop a similar capacity to recognize the emotional signals of diverse facial expressions. This capacity is mediated by a brain network that involves emotion-related brain circuits and higher-level visual-representation areas. Recent studies suggest that the key components of this network begin to emerge early in life. The studies also suggest that initial biases in emotion-related brain circuits and the early coupling of these circuits and cortical perceptual areas provide a foundation for a rapid acquisition of representations of those facial features that denote specific emotions.

Developmental Neuroscience Perspectives on Emotion Regulation

Because individual differences in emotion regulation are associated with risk for childhood behavioral problems, multidisciplinary investigation of the genetic and neural underpinnings of emotion regulation should be a research priority. Here, we summarize research findings from three independent laboratories to demonstrate the ways in which a variety of developmental human neuroscience-based approaches can address critical conceptual issues in the emergence of emotion regulation. To do so, we present three perspectives on how developmental neurobiology constrains and enriches theories of ER. The three perspectives of (1) genetics, (2) brain structure and function, and (3) plasticity of development are illustrated with empirical results derived from both typical and atypical samples of children and adults. These perspectives are complementary and sometimes represent different levels of analysis of the same question.

Acute 5-HT reuptake blockade potentiates human amygdala reactivity

Variability in serotonin (5-HT) function is associated with individual differences in normal mood and temperament, as well as psychiatric illnesses, all of which are influenced by amygdala function. This study evaluated the acute effects of 5-HT reuptake blockade on amygdala function using pharmacological functional MRI. Eight healthy men completed a double-blind balanced crossover study with the selective 5-HT reuptake inhibitor, citalopram (20 mg infused over 30 min), and normal saline. Amygdala reactivity in response to novel facial expressions was assessed on three successive scans, once before drug/placebo infusion, once early in the infusion, and once at the end of infusion. Acute citalopram administration resulted in concentration-dependent increases in human amygdala reactivity to salient stimuli. The current pattern of 5-HT-mediated amygdala reactivity may represent an important pathway through which SSRIs achieve an antidepressant effect. Intriguingly, our data may also reveal a mechanism contributing to clinical observations of extreme agitation, restlessness, and suicidal ideation in some individuals during acute SSRI treatment. Developing a comprehensive model of how 5-HT modulates human amygdala reactivity supporting behavioral and physiological arousal will be instrumental for our understanding of basic neurobehavioral processes, their dysfunction in psychiatric illnesses, and their contribution to mechanism of treatment response.

Neurochemical, behavioral, and physiological effects of pharmacologically enhanced serotonin levels in serotonin transporter (SERT)-deficient mice

RATIONALE

Serotonin transporter (SERT) knockout (-/-) mice have an altered phenotype in adulthood, including high baseline anxiety and depressive-like behaviors, associated with increased baseline extracellular serotonin levels throughout life.

OBJECTIVES

To examine the effects of increases in serotonin following the administration of the serotonin precursor 5-hydroxy-L-tryptophan (5-HTP) in SERT wild-type (+/+), heterozygous (+/-), and -/- mice.

RESULTS

5-HTP increased serotonin in all five brain areas examined with approximately 2- to 5-fold increases in SERT+/+ and +/- mice, and with greater 4.5- to 11.7-fold increases in SERT-/- mice. Behaviorally, 5-HTP induced exaggerated serotonin syndrome behaviors in SERT-/-, mice with similar effects in male and female mice. Studies suggest promiscuous serotonin uptake by the dopamine transporter (DAT) in SERT-/- mice, and here, the DAT blocker GBR 12909 enhanced 5-HTP-induced behaviors in SERT-/- mice. Physiologically, 5-HTP induced exaggerated temperature effects in SERT-deficient mice. The 5-HT1A antagonist WAY 100635 decreased 5-HTP-induced hypothermia in SERT+/+ and +/- mice with no effect in SERT-/- mice, whereas the 5-HT7 antagonist SB 269970 decreased this exaggerated response in SERT-/- mice only. WAY 100635 and SB 269970 together completely blocked 5-HTP-induced hypothermia in SERT+/- and -/- mice.

CONCLUSIONS

These studies demonstrate that SERT-/- mice have exaggerated neurochemical, behavioral, and physiological responses to further increases in serotonin, and provide the first evidence of intact 5-HT7 receptor function in SERT-/- mice, with interesting interactions between 5-HT1A and 5-HT7 receptors. As roles for 5-HT7 receptors in anxiety and depression were recently established, the current findings have implications for understanding the high anxiety and depressive-like phenotype of SERT-deficient mice.

Relationship of genetic variability and depressive symptoms to adverse events after coronary artery bypass graft surgery

OBJECTIVE

To assess genetic variability in two serotonin-related gene polymorphisms (MAOA-uVNTR and 5HTTLPR) and their relationships to depression and adverse cardiac events in a sample of patients undergoing coronary artery bypass surgery.

METHODS

A total of 427 coronary artery bypass graft (CABG) patients were genotyped for two polymorphisms and assessed for depressive symptoms at three time points, in accordance with the Center for Epidemiological Studies-Depression (CES-D): preoperative baseline; 6 months postoperative; and 1 year postoperative. Logistic regression was used to assess the association between depressive symptoms (CES-D = >16), genotype differences, and cardiac events. Because MAOA-uVNTR is sex-linked, males and females were analyzed separately for this polymorphism; sexes were combined for the 5HTTLPR analysis.

RESULTS

Depressed patients were more likely than nondepressed patients to have a new cardiac event within 2 years of surgery (p < .0001); depressed patients who carry the long (L) allele of the 5HTTLPR polymorphism were more likely than the short/short (S/S carriers to have an event (p = .0002). Genetic associations with 6-month and 1-year postoperative depressive symptoms do not survive adjustment for baseline depressive symptoms.

CONCLUSIONS

A serotonin-related gene polymorphism--5HTTLPR--was associated with adverse cardiac events post CABG, in combination with depressive symptoms. Because depressed patients with the L allele of the 5HTTLPR polymorphism were more likely to have an event compared with the S/S carriers, combining genetic and psychiatric profiling may prove useful in identifying patients at the highest risk for adverse outcomes post CABG.

The serotonin transporter genotype is associated with intermediate brain phenotypes that depend on the context of eliciting stressor

A variant allele in the promoter region of the serotonin transporter gene, SLC6A4, the s allele, is associated with increased vulnerability to develop anxiety-related traits and depression. Furthermore, functional magnetic resonance imaging (fMRI) studies reveal that s carriers have increased amygdala reactivity in response to aversive stimuli, which is thought to be an intermediate phenotype mediating the influences of the s allele on emotionality. We used high-resolution microPET [18F]fluoro-2-deoxy-D-glucose (FDG) scanning to assess regional brain metabolic activity in rhesus monkeys to further explore s allele-related intermediate phenotypes. Rhesus monkeys provide an excellent model to understand mechanisms underlying human anxiety, and FDG microPET allows for the assessment of brain activity associated with naturalistic environments outside the scanner. During FDG uptake, monkeys were exposed to different ethologically relevant stressful situations (relocation and threat) as well as to the less stressful familiar environment of their home cage. The s carriers displayed increased orbitofrontal cortex activity in response to both relocation and threat. However, during relocation they displayed increased amygdala reactivity and in response to threat they displayed increased reactivity of the bed nucleus of the stria terminalis. No increase in the activity of any of these regions occurred when the animals were administered FDG in their home cages. These findings demonstrate context-dependent intermediate phenotypes in s carriers that provide a framework for understanding the mechanisms underlying the vulnerabilities of s-allele carriers exposed to different types of stressors.

Unfaithful neurotransmitter transporters: focus on serotonin uptake and implications for antidepressant efficacy

Biogenic amine transporters for serotonin, norepinephrine and dopamine (SERT, NET and DAT respectively), are the key players terminating transmission of these amines in the central nervous system by their high-affinity uptake. They are also major targets for many antidepressant drugs. Interestingly however, drugs targeted to a specific transporter do not appear to be as clinically efficacious as those that block two or all three of these transporters. A growing body of literature, reviewed here, supports the idea that promiscuity among these transporters (the uptake of multiple amines in addition to their "native" transmitter) may account for improved therapeutic effects of dual and triple uptake blockers. However, even these drugs do not provide effective treatment outcomes for all individuals. An emerging literature suggests that "non-traditional" transporters such as organic cation transporters (OCT) and the plasma membrane monoamine transporter (PMAT) may contribute to the less than hoped for efficacy of currently prescribed uptake inhibitors. OCT and PMAT are capable of clearing biogenic amines from extracellular fluid and may serve to buffer the effects of frontline antidepressants, such as selective serotonin reuptake inhibitors. In addition, polymorphisms that occur in the genes encoding the transporters can lead to variation in transporter expression and function (e.g. the serotonin transporter linked polymorphic region; 5-HTTLPR) and can have profound effects on treatment outcome. This may be accounted for, in part, by compensatory adaptations in other transporters. This review synthesizes the existing literature, focusing on serotonin to illustrate and revive a model for the rationale design of improved antidepressants.

An examination of the association between the 5-HTT promoter region polymorphism and depressogenic attributional styles in childhood

Although a vast literature examining the role of attributional styles in depression has accumulated, the origins of such cognitions remain poorly understood. Investigators are increasingly interested in whether cognitive vulnerability to depression is linked to genetic variation. As a preliminary test of this hypothesis, we examined whether the serotonin transporter promoter polymorphism (5-HTTLPR) was associated with attributional styles in children. Thirty-eight children completed a self-report measure of attributional styles, the Child Attributional Style Questionnaire-Revised (CASQ-R). Children were also genotyped for the 5-HTTLPR polymorphism, including the single nucleotide polymorphism (SNP) rs25531 in the long allele of the 5-HTTLPR. The short alleles of the 5-HTTLPR and their putative functional equivalents were associated with increased levels of depressogenic attributions for negative events, as measured by the CASQ-R, lending support to the role of 5-HTTLPR polymorphisms in cognitive vulnerability to depression.

Variation in mouse basolateral amygdala volume is associated with differences in stress reactivity and fear learning

A wealth of research identifies the amygdala as a key brain region mediating negative affect, and implicates amygdala dysfunction in the pathophysiology of anxiety disorders. Although there is a strong genetic component to anxiety disorders such as posttraumatic stress disorder (PTSD) there remains debate about whether abnormalities in amygdala function predispose to these disorders. In the present study, groups of C57BL/6 x DBA/2 (B x D) recombinant inbred strains of mice were selected for differences in volume of the basolateral amygdala complex (BLA). Strains with relatively small, medium, or large BLA volumes were compared for Pavlovian fear learning and memory, anxiety-related behaviors, depression-related behavior, and glucocorticoid responses to stress. Strains with relatively small BLA exhibited stronger conditioned fear responses to both auditory tone and contextual stimuli, as compared to groups with larger BLA. The small BLA group also showed significantly greater corticosterone responses to stress than the larger BLA groups. BLA volume did not predict clear differences in measures of anxiety-like behavior or depression-related behavior, other than greater locomotor inhibition to novelty in strains with smaller BLA. Neither striatal, hippocampal nor cerebellar volumes correlated significantly with any behavioral measure. The present data demonstrate a phenotype of enhanced fear conditioning and exaggerated glucocorticoid responses to stress associated with small BLA volume. This profile is reminiscent of the increased fear processing and stress reactivity that is associated with amygdala excitability and reduced amygdala volume in humans carrying loss of function polymorphisms in the serotonin transporter and monoamine oxidase A genes. Our study provides a unique example of how natural variation in amygdala volume associates with specific fear- and stress-related phenotypes in rodents, and further supports the role of amygdala dysfunction in anxiety disorders such as PTSD.

Effects of adolescent fluoxetine treatment on fear-, anxiety- or stress-related behaviors in C57BL/6J or BALB/cJ mice

RATIONALE

5-Hydroxytryptamine (5-HT, serotonin) plays a major role in brain ontogeny. Disruption of 5-HT during early postnatal development produces lasting changes in rodent 'emotion-related' behaviors. Adverse effects of treatment with serotonin reuptake inhibitor (SRI) antidepressants have been reported in human adolescents. However, the long-term effects of chronic SRI treatment during adolescence in rodents remain unclear.

OBJECTIVES

The objectives of the study are to assess the effects of fluoxetine treatment throughout the adolescent period in measures of fear-, anxiety- and stress-related endpoints in drug-free adults and to examine these effects in two genetic strains of mice differing in baseline stress- and anxiety-related behaviors and sensitivity to SRIs.

MATERIALS AND METHODS

C57BL/6J and BALB/cJ mice received one of two fluoxetine doses for 4 weeks during adolescence (3-7 weeks old). A separate group of C57BL/6J and BALB/cJ mice received fluoxetine for 4 weeks during adulthood (8-12 weeks old). After a 3-week washout period, mice were tested for anxiety-like behaviors (novel open field, elevated plus-maze), fear conditioning and extinction, and stress-related responses to forced swim, as well as serotonin brain levels.

RESULTS

Adolescent fluoxetine treatment did not increase adult measures of anxiety-, fear- or stress-related behaviors, or brain serotonin levels. The same duration of treatment in adulthood also had no effects on these measures when tested after a 3-week washout period.

CONCLUSIONS

In clear contrast with emotion-related abnormalities caused by preadolescent fluoxetine treatment or genetic inactivation of fluoxetine's pharmacological target, the 5-HT transporter, fluoxetine treatment throughout mouse adolescence did not produce detectable, lasting abnormalities in either "high" or "low anxiety" inbred mouse strains.

Serotonergic genes and amygdala activity in response to negative affective facial stimuli in Korean women

Serotonergic genes have been implicated in mood disorders, alcoholism and certain personality traits. We investigated the possible relationship between several polymorphisms in the serotonin (5-HT) system and amygdala responses to negative facial stimuli in Korean women using functional magnetic resonance imaging. All participants were genotyped with regard to the following polymorphisms: the serotonin transporter-gene-linked polymorphic region (5-HTTLPR), tryptophan hydroxylase 2 (TPH2) G(-703)T, 5-HT(1A) C(-1019)G and 5-HT(2A) single nucleotide polymorphism (SNP) rs6311. We found increased activations in response to angry facial stimuli in the bilateral amygdala of subjects with the long allele of 5-HTTLPR compared with those with two copies of the short allele. Higher activations in response to sad facial stimuli were found in the bilateral amygdala of subjects with the T/T genotype of 5-HT(2A) SNP rs6311, compared with C allele carriers, and in subjects with the G/G genotype of TPH2 G(-703)T, compared with those with T/T and G/T genotypes. Our results for individuals from an Asian population countered a previous finding for a Caucasian population and identified the moderating role of genetic background in the relationships between these serotonergic gene polymorphisms and amygdala function elicited by negative emotional stimuli.

Increased fear- and stress-related anxiety-like behavior in mice lacking tuberoinfundibular peptide of 39 residues

Tuberoinfundibular peptide of 39 residues (TIP39) is synthesized by two groups of neurons, one in the subparafascicular area at the caudal end of the thalamus and the other in the medial paralemniscal nucleus within the lateral brainstem. The subparafascicular TIP39 neurons project to a number of brain regions involved in emotional responses, and these regions contain a matching distribution of a receptor for TIP39, the parathyroid hormone 2 receptor (PTH2-R). We have now evaluated the involvement of TIP39 in anxiety-related behaviors using mice with targeted null mutation of the TIP39 gene (Tifp39). Tifp39(-/-) mice (TIP39-KO) did not significantly differ from wild-type (WT) littermates in the open field, light/dark exploration and elevated plus-maze assays under standard test conditions. However, the TIP39-KO engaged in more active defensive burying in the shock-probe test. In addition, when tested under high illumination or after restraint, TIP39-KO displayed significantly greater anxiety-like behavior in the elevated plus-maze than WT. In a Pavlovian fear-conditioning paradigm, TIP39-KO froze more than WT during training and during tone and context recall but showed normal fear extinction. Disruption of TIP39 projections to the medial prefrontal cortex, lateral septum, bed nucleus of the stria terminalis, hypothalamus and amygdala likely account for the fear- and anxiety-related phenotype of TIP39-KO. Current data support the hypothesis that TIP39 modulates anxiety-related behaviors following environmental provocation.

Psychobiology of anxiety disorders and obsessive-compulsive spectrum disorders

Obsessive-compulsive disorder is currently classified as an anxiety disorder. However, there is growing interest in the concept of an obsessive-compulsive spectrum of disorders (OCSDs). The relationship between anxiety disorders and OCSDs has been questioned. The psychobiology of anxiety disorders and OCSDs is briefly reviewed in this article. While there appear to be several distinct contrasts in the underlying psychobiology of these conditions, there is also evidence of overlapping mechanisms. In addition, there are crucial gaps in our current database, confounding nosological decision-making. Conceptualizing various anxiety disorders and putative OCSDs as lying within a broader spectrum of emotional disorders may be useful. However, clinicians must also recognize that individual anxiety and obsessive-compulsive spectrum conditions, including disorders characterized by body-focused repetitive behaviors, have distinct psychobiological underpinnings and require different treatment approaches.

Emotion-induced retrograde amnesia is determined by a 5-HTT genetic polymorphism

A polymorphism in the human serotonin transporter (5-HTT) gene is implicated in susceptibility to anxiety and depression and in enhanced emotion-induced activation in the amygdala. A role for 5-HTT polymorphism in the emotional modulation of human episodic memory has yet to be demonstrated. Here, we demonstrate that whereas emotional memory for aversive events per se is not influenced by 5-HTT polymorphism, an emotion-induced retrograde amnesia is expressed solely in the presence of the short allele. The findings indicate a critical role for the serotonin system in emotion-mediated memory disruption.

Acute tryptophan depletion in healthy males attenuates phasic cardiac slowing but does not affect electro-cortical response to negative feedback

RATIONALE

Recent studies have shown that serotonin might be involved in performance monitoring, although the results have been inconclusive. Inconsistent results might be related to the type of pharmacological manipulation and the used behavioral and physiological measures.

OBJECTIVES

The present study aimed at further specifying the role of serotonin in performance monitoring.

MATERIALS AND METHODS

The effect of serotonin on performance monitoring was studied by using acute tryptophan depletion (ATD), a well-known method to transiently lower central serotonin levels. Twenty healthy male volunteers performed a time-estimation task and their event-related brain potential (ERP), behavioral, and cardiac responses to feedback stimuli were measured. Furthermore, subjective mood and amino-acid levels were determined.

RESULTS

As expected, ATD did not affect mood and lowered tryptophan levels. ATD attenuated cardiac slowing to negative feedback but did not affect responses to positive feedback, ERPs, and performance measures.

CONCLUSIONS

The data point in the direction of a dissociation between cardiac and electro-cortical responses. Cardiac responses appear to be more sensitive to changes in serotonin metabolism and appear to reflect different aspects of the feedback stimulus. The phasic cardiac response appears to be an important measure that provides additional information about the impact of feedback stimuli and serotonergic functioning.

Serotonin transporter genotype and depressive phenotype determination by discriminant analysis of glucose metabolism under acute tryptophan depletion

Acute tryptophan depletion (ATD) putatively results in a transient reduction in central serotonin transmission, and induces depressed mood in some un-medicated subjects with remitted major depressive disorder (MDD). The 5-HT transporter promoter region length polymorphism (5-HTTLPR) has been shown to influence behavioral and metabolic responses to ATD, as well as the risk for developing MDD within the context of stress. The current study investigates the relationships between 5-HTTLPR genotype, neurophysiologic response to ATD, and diagnostic phenotype (healthy control subjects versus MDD subjects differentiated by their depressive response to ATD) using (18)FDG-PET. Un-medicated subjects with remitted MDD and healthy controls were genotyped for the long (l) and short (s) alleles of the 5-HTTLPR polymorphism and categorized into one of three genotypes. On two separate occasions, subjects received either a placebo or an amino acid mixture designed to deplete plasma tryptophan, followed by (18)FDG-PET scanning. Depressive symptoms were rated to determine the diagnostic phenotype. Descriptive and predictive discriminant analyses were performed using brain regional metabolic data to classify according to phenotype and genotype. Overall, 79% of the cases were classified correctly by genotype, and 85% were classified correctly by phenotype. In a leave-one-out cross-validation, 72% of the subjects were classified correctly as carrying an s-allele, and 79% of the subjects were classified correctly by primary diagnosis. The robust nature of the classification results indicates that much of the variance in metabolic response to ATD is accounted for by genotypic and phenotypic category.

The social-emotional processing stream: five core constructs and their translational potential for schizophrenia and beyond

BACKGROUND

Cognitive neuroscience approaches to translational research have made great strides toward understanding basic mechanisms of dysfunction and their relation to cognitive deficits, such as thought disorder in schizophrenia. The recent emergence of Social Cognitive and Affective Neuroscience has paved the way for similar progress to be made in explaining the mechanisms underlying the social and emotional dysfunctions (i.e., negative symptoms) of schizophrenia and that characterize virtually all DSM Axis I and II disorders more broadly.

METHODS

This article aims to provide a roadmap for this work by distilling from the emerging literature on the neural bases of social and emotional abilities a set of key constructs that can be used to generate questions about the mechanisms of clinical dysfunction in general and schizophrenia in particular.

RESULTS

To achieve these aims, the first part of this article sketches a framework of five constructs that comprise a social-emotional processing stream. The second part considers how future basic research might flesh out this framework and translational work might relate it to schizophrenia and other clinical populations.

CONCLUSIONS

Although the review suggests there is more basic research needed for each construct, two in particular--one involving the bottom-up recognition of social and emotional cues, the second involving the use of top-down processes to draw mental state inferences--are most ready for translational work.

Identification of neurogenetic pathways of risk for psychopathology

Imaging genetics has been a highly effective and increasingly applied strategy for identifying the impact of genetic polymorphisms on individual differences in neural circuitry supporting complex behaviors. The application of imaging genetics towards further elucidating neural circuitry associated with the pathophysiology of psychiatric illness is of particular interest given its potential to guide the development and improvement of current therapeutic methods. The identification of genetic variants that contribute to or predict the disruption of specific neural pathways associated with psychopathology may also serve as useful markers of risk demarcating individuals with elevated susceptibility for psychiatric illness and affording early or even preemptive treatment strategies. In the continued development of this technique, recent multimodal neuroimaging strategies and studies examining the effects of multiple genes in concert within large subject populations have shown promise in the development of a more complete understanding of the interrelationships between genes, brain function, behavior and associated risk for psychopathology.

Lasting syndrome of depression produced by reduction in serotonin uptake during postnatal development: evidence from sleep, stress, and behavior

Dysfunction of the serotonin system is implicated in sleep and emotional disorders. To test whether these impairments could arise during development, we studied the impact of early-life, transient versus genetic, permanent alterations of serotonin reuptake on sleep-wakefulness patterns, depression-related behavior, and associated physiological features. Here, we show that female mice treated neonatally with a highly selective serotonin reuptake inhibitor, escitalopram, exhibited signs of depression in the form of sleep anomalies, anhedonia, increased helplessness reversed by chronic antidepressant treatment, enhanced response to acute stress, and increased serotoninergic autoinhibitory feedback. This syndrome was not reproduced by treatment in naive adults but resembled the phenotype of mutant mice lacking the serotonin transporter, except that these exhibited decreased serotonin autoreceptor sensitivity and additional anxiety-like behavior. Thus, alteration of serotonin reuptake during development, whether induced by external or genetic factors, causes a depressive syndrome lasting into adulthood. Such early-life impairments might predispose individuals to sleep and/or mood disorders.

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.

Serotonin transporter (5-HTTLPR) genotype and amygdala activation: a meta-analysis

BACKGROUND

We evaluated the magnitude of the reported associations between amygdala activation and the serotonin transporter gene linked polymorphic region (5-HTTLPR) and the likely effect size of this relationship.

METHODS

We used meta-analytic techniques to combine data from existing published and unpublished studies. We also tested for possible publication bias and explored possible moderating influences on any association, such as sample ancestry.

RESULTS

Our results provide support for the association of the 5-HTTLPR polymorphism and amygdala activation and suggest that this locus may account for up to 10% of phenotypic variance. Although we did not observe evidence for potential publication bias in our main analysis, this was due in part to efforts to obtain unpublished data pertinent to this meta-analysis, and when three unpublished data sets were excluded we did observe evidence of such bias. We also observed evidence that the first published study may provide an overestimate of the true effect size, which is consistent with findings from genetic association studies of other phenotypes.

CONCLUSIONS

Although our analysis provides support for the association of the 5-HTTLPR polymorphism and amygdala activation, it also suggests that most studies to date are nevertheless lacking in statistical power. Increasing the sample sizes of future imaging genetics studies will allow a more accurate characterization of any true effect size and afford adequate power to examine the impact of multiple polymorphisms that likely work in concert to affect gene function and, in turn, bias neural processes mediating dispositional traits such as temperament and personality.

Genetic deletion of Lsamp causes exaggerated behavioral activation in novel environments

The limbic system-associated membrane protein (LAMP) is a GPI-anchored cell adhesion molecule expressed heavily in limbic and limbic-associated regions of the developing and adult brain. Experimental studies show that LAMP promotes the growth of limbic neurons and guides the projections of limbic fibers. In order to examine the functional consequences of disrupting limbic circuit assembly, we generated a mouse line in which the Lsamp gene encoding LAMP was deleted. Basic neuroanatomical organization and sensory and motor development are normal in Lsamp(-/-) mice. The most profound change in behavior in both male and female Lsamp(-/-) mice is a heightened reactivity to novelty exhibited in several behavioral tests. Lsamp(-/-) mice display hyperactivity in a novel arena and both sexes habituate to the same activity levels as their wild type littermates, but at different rates. In the elevated plus maze, Lsamp(-/-) mice exhibit increased total arm entries, with a bias towards the open arms; they spend more time in the open arms and have a substantial increase in the amount of risk assessment in unprotected areas of the maze. In the y-maze, Lsamp(-/-) mice exhibit characteristic hyperactivity and a decreased level of spontaneous alternation during the period when their novelty-induced hyperactivity is at its peak. We hypothesize that Lsamp(-/-) mice may not simply exhibit a decrease in anxiety, but may have a heightened, and possibly maladaptive, response to novel environmental stressors. Genetic deletion of Lsamp may thus cause circumscribed changes in the fine connectivity of specific circuits that underlie these behaviors.

The importance of cognitive phenotypes in experimental modeling of animal anxiety and depression

Cognitive dysfunctions are commonly seen in many stress-related disorders, including anxiety and depression—the world's most common neuropsychiatric illnesses. Various genetic, pharmacological, and behavioral animal models have long been used to establish animal anxiety-like and depression-like phenotypes, as well as to assess their memory, learning, and other cognitive functions. Mounting clinical and animal evidences strongly supports the notion that disturbed cognitions represent an important pathogenetic factor in anxiety and depression, and may also play a role in integrating the two disorders within a common stress-precipitated developmental pathway. This paper evaluates why and how the assessment of cognitive and emotional domains may improve our understanding of animal behaviors via different high-throughput tests and enable a better translation of animal phenotypes into human brain disorders.

Genetic variation in cortico-amygdala serotonin function and risk for stress-related disease

The serotonin system is strongly implicated in the pathophysiology and therapeutic alleviation of stress-related disorders such as anxiety and depression. Serotonergic modulation of the acute response to stress and the adaptation to chronic stress is mediated by a myriad of molecules controlling serotonin neuron development (Pet-1), synthesis (tryptophan hydroxylase 1 and 2 isozymes), packaging (vesicular monoamine transporter 2), actions at presynaptic and postsynaptic receptors (5-HT1A, 5-HT1B, 5-HT2A, 5-HT2C, 5-HT3A, 5-HT4, 5-HT5A, 5-HT6, 5-HT7), reuptake (serotonin transporter), and degradation (monoamine oxidase A). A growing body of evidence from preclinical rodents models, and especially genetically modified mice and inbred mouse strains, has provided significant insight into how genetic variation in these molecules can affect the development and function of a key neural circuit between the dorsal raphe nucleus, medial prefrontal cortex and amygdala. By extension, such variation is hypothesized to have a major influence on individual differences in the stress response and risk for stress-related disease in humans. The current article provides an update on this rapidly evolving field of research.

Experiential and genetic contributions to depressive- and anxiety-like disorders: clinical and experimental studies

Stressful events have been implicated in the precipitation of depression and anxiety. These disorders may evolve owing to one or more of an array of neuronal changes that occur in several brain regions. It seems likely that these stressor-provoked neurochemical alterations are moderated by genetic determinants, as well as by a constellation of experiential and environmental factors. Indeed, animal studies have shown that vulnerability to depressive-like behaviors involve mechanisms similar to those associated with human depression (e.g., altered serotonin, corticotropin releasing hormone and their receptors, growth factors), and that the effects of stressors are influenced by previous stressor experiences, particularly those encountered early in life. These stressor effects might reflect sensitization of neuronal functioning, phenotypic changes of processes that lead to neurochemical release or receptor sensitivity, or epigenetic processes that modify expression of specific genes associated with stressor reactivity. It is suggested that depression is a life-long disorder, which even after effective treatment, has a high rate of re-occurrence owing to sensitized processes or epigenetic factors that promote persistent alterations of gene expression.

The hygiene hypothesis and psychiatric disorders

The hygiene hypothesis proposes that several chronic inflammatory disorders (allergies, autoimmunity, inflammatory bowel disease) are increasing in prevalence in developed countries because a changing microbial environment has perturbed immunoregulatory circuits which normally terminate inflammatory responses. Some stress-related psychiatric disorders, particularly depression and anxiety, are associated with markers of ongoing inflammation, even without any accompanying inflammatory disorder. Moreover, pro-inflammatory cytokines can induce depression, which is commonly seen in patients treated with interleukin-2 or interferon-alpha. Therefore, some psychiatric disorders in developed countries might be attributable to failure of immunoregulatory circuits to terminate ongoing inflammatory responses. This is discussed in relation to the effects of the immune system on a specific group of brain serotonergic neurons involved in the pathophysiology of mood disorders.

Emotional regulation: implications for the psychobiology of psychotherapy

A range of studies have contributed to understanding the psychobiology of emotional regulation. Functional imaging studies have demonstrated that cortico-limbic circuitry plays an important role in mediating processes such as reappraisal and suppression. Dorsolateral prefrontal cortex may be important in conscious reframing, while ventromedial prefrontal cortex and orbitofrontal cortex may be particularly important in emotion evaluation. Gene variants and early environments impact underlying emotional regulation and its neurobiology. It may be hypothesized that during interventions such as psychotherapy there are improvements in emotional regulation, together with the normalization of related psychobiological mechanisms.

Ethanol-BDNF interactions: still more questions than answers

Brain-derived neurotrophic factor (BDNF) has emerged as a regulator of development, plasticity and, recently, addiction. Decreased neurotrophic activity may be involved in ethanol-induced neurodegeneration in the adult brain and in the etiology of alcohol-related neurodevelopmental disorders. This can occur through decreased expression of BDNF or through inability of the receptor to transduce signals in the presence of ethanol. In contrast, recent studies implicate region-specific up-regulation of BDNF and associated signaling pathways in anxiety, addiction and homeostasis after ethanol exposure. Anxiety and depression are precipitating factors for substance abuse and these disorders also involve region-specific changes in BDNF in both pathogenesis and response to pharmacotherapy. Polymorphisms in the genes coding for BDNF and its receptor TrkB are linked to affective, substance abuse and appetitive disorders and therefore may play a role in the development of alcoholism. This review summarizes historical and pre-clinical data on BDNF and TrkB as it relates to ethanol toxicity and addiction. Many unresolved questions about region-specific changes in BDNF expression and the precise role of BDNF in neuropsychiatric disorders and addiction remain to be elucidated. Resolution of these questions will require significant integration of the literature on addiction and comorbid psychiatric disorders that contribute to the development of alcoholism.

BPD's interpersonal hypersensitivity phenotype: a gene-environment-developmental model

This paper explores the development of BPD as it might emerge in the child's early interpersonal reactions and how such reactions might evolve into the interpersonal pattern that typifies BPD. It begins to bridge the relevant bodies of clinical literature on the borderline's prototypic interpersonal problems with the concurrently expanding relevant literature on early child development. We will start by considering how a psychobiological disposition to BPD is likely to include a constitutional diathesis for relational reactivity, that is, for hypersensitivity to interpersonal stressors. Data relevant to this disposition's manifestations in adult clinical samples and to its heritability and neurobiology will be reviewed. We then consider how such a psychobiological disposition for interpersonal reactivity might contribute to the development of a disorganized-ambivalent form of attachment, noting especially the likely contributions of both the predisposed child and of parents who are themselves predisposed to maladaptive responses, leading to an escalation of problematic transactions. Evidence concerning both the genetics and the developmental pathways associated with disorganized attachments will be considered. Emerging links between such developmental pathways and adult BPD will be described, in particular the potential appearance by early- to middle-childhood of controlling-caregiving or controlling-punitive interpersonal strategies. Some implications from this gene-environment interactional theory for a better developmental understanding of BPD's etiology are discussed.

Neural correlates of emotion regulation in psychopathology

What can psychopathology and its treatment tell us about cognitive emotional interactions? Standard approaches to interactions between emotion and cognition often adopt a variant of the idea that cognitive processes, subserved by dorsal and lateral cortical circuits, exert control and regulation of ventral, limbic brain areas associated with emotional expression and experience. However, it is becoming clear from studies on depression, post-traumatic stress disorder (PTSD) and obsessive compulsive disorder (OCD), that a binary, opponent theory of cognitive emotion interaction (CEI) and the dorsal-ventral model of neurocircuitry do not fully describe the data. We summarize recent research to suggest that networks of direct and indirect pathways exist by which cognition can regulate pathological emotion, and the inter-relationships of specific nodes within the networks need to be characterized.

Neuroimaging: technologies at the interface of genes, brain, and behavior

Neuroimaging technologies provide a powerful approach to exploring the genetic basis of individual differences in complex behaviors and vulnerability to neuropsychiatric illness. Functional MRI studies have established important physiologic links between genetic polymorphisms and robust differences in information processing within distinct brain regions and circuits that have been linked to the manifestation of various disease. Neuroimaging technologies represent a critical tool in efforts to understand the neurobiology of normal and pathologic behavioral states. Research capitalizing on neuroimagingbased integration will contribute to the identification of predictive markers and biologic pathways for neuropsychiatric disease vulnerability and the generation of novel targets for therapeutic intervention.

The role of the amygdala in emotional processing: a quantitative meta-analysis of functional neuroimaging studies

Functional neuroimaging studies have provided strong support for a critical role of the amygdala in emotional processing. However, several controversies remain in terms of whether different factors-such as sex, valence and stimulus type-have an effect on the magnitude and lateralization of amygdala responses. To address these issues, we conducted a meta-analysis of functional neuroimaging studies of visual emotional perception that reported amygdala activation. Critically, unlike previous neuroimaging meta-analyses, we took into account the magnitude (effect size) and reliability (variance) associated with each of the activations. Our results confirm that the amygdala responds to both positive and negative stimuli, with a preference for faces depicting emotional expressions. We did not find evidence for amygdala lateralization as a function of sex or valence. Instead, our findings provide strong support for a functional dissociation between left and right amygdala in terms of temporal dynamics. Taken together, results from this meta-analysis shed new light on several of the models proposed in the literature regarding the neural basis of emotional processing.

The 5-HTTLPR polymorphism in South African healthy populations: a global comparison

The serotonin transporter promoter length polymorphism (5-hydroxytryptamine transporter length polymorphism, 5-HTTLPR) in serotonin transporter gene has been implicated in numerous psychiatric disorders. Having a high affinity for the neurotransmitter serotonin (5-hydroxytryptamine, 5-HT), serotonin transporter controls the duration, availability and signaling capacity of 5-HT in the synapse. Association studies have focused extensively on this polymorphic region as the frequencies of long- and short-alleles of this gene differ greatly amongst populations and association studies have either reported conflicting results or nothing significant at all. In this study, the genotype and allele frequencies of 5-HTTLPR polymorphism were determined in the healthy South African (SA) individuals belonging to diverse ethnic backgrounds. Cheek cell samples were collected from the three major ethnic groups namely: Caucasians, Africans and coloreds/Mixed population. The DNA was extracted and genotyped for the 5-HTTLPR. Genotypes were compared amongst the three major ethnic groups from SA as well as to that of other studies around the world. This is the first study to report significant differences in the 5-HTTLPR genotype and allelic frequencies among various ethnic groups in SA. Future studies will target larger population groups and the estimation of frequency of these alleles in individuals with autism.

Research review: DSM-V conduct disorder: research needs for an evidence base

This article charts a strategic research course toward an empirical foundation for the diagnosis of conduct disorder in the forthcoming DSM-V. Since the DSM-IV appeared in 1994, an impressive amount of new information about conduct disorder has emerged. As a result of this new knowledge, reasonable rationales have been put forward for adding to the conduct disorder diagnostic protocol: a childhood-limited subtype, family psychiatric history, callous-unemotional traits, female-specific criteria, preschool-specific criteria, early substance use, and biomarkers from genetics, neuroimaging, and physiology research. This article reviews the evidence for these and other potential changes to the conduct disorder diagnosis. We report that although there is a great deal of exciting research into each of the topics, very little of it provides the precise sort of evidence base required to justify any alteration to the DSM-V. We outline specific research questions and study designs needed to build the lacking evidence base for or against proposed changes to DSM-V conduct disorder.

Neural mechanisms of extinction learning and retrieval

Emotional learning is necessary for individuals to survive and prosper. Once acquired, however, emotional associations are not always expressed. Indeed, the regulation of emotional expression under varying environmental conditions is essential for mental health. The simplest form of emotional regulation is extinction, in which conditioned responding to a stimulus decreases when the reinforcer is omitted. Two decades of research on the neural mechanisms of fear conditioning have laid the groundwork for understanding extinction. In this review, we summarize recent work on the neural mechanisms of extinction learning. Like other forms of learning, extinction occurs in three phases: acquisition, consolidation, and retrieval, each of which depends on specific structures (amygdala, prefrontal cortex, hippocampus) and molecular mechanisms (receptors and signaling pathways). Pharmacological methods to facilitate consolidation and retrieval of extinction, for both aversive and appetitive conditioning, are setting the stage for novel treatments for anxiety disorders and addictions.

Sex, aggression and impulse control: an integrative account

There is evidence that the male sex and a personality style characterized by low self-control/high impulsivity and a propensity for negative emotionality increase the risk for impulsive aggressive, antisocial and criminal behavior. This article aims at identifying neurobiological factors underlying this association. It is concluded that the neurobiological correlates of impulsive aggression act through their effects on the ability to modulate impulsive expression more generally, and that sex-related differences in the neurobiological correlates of impulse control and emotion regulation mediate sex differences in direct aggression. A model is proposed that relates impulse control and its neurobiological correlates to sex differences in direct aggression.

Molecular genetics of anxiety in mice and men

Human anxiety disorders represent one of the most common mental illnesses. They are complex diseases with both genetic and environmental factors affecting their predisposition. Since the basic neuronal mechanisms are shared across mammalian species, the same set of genes may regulate critical aspects of anxiety in humans and in lower species. In this review, we first summarize findings from human molecular genetic approaches to anxiety disorders or anxiety-related personality traits: genome-wide scans and candidate gene studies in large families or case-control cohorts. We then discuss recent studies that have used genome-wide methods in mouse strains to identify genes that regulate anxiety-like behavior. Although it has been difficult to pinpoint specific susceptibility genes for anxiety disorders, ongoing efforts to collect larger study cohorts and to develop new genetic tools should help in this task. Studies in animals have shown that novel quantitative trait locus (QTL) and functional genomics approaches might lead to the identification of regulators of anxiety in mice, and that these genes can be tested for their involvement in human anxiety disorders. Finally, breakthroughs are expected in the fine-mapping of human and mouse genetic linkage regions and in the identification of novel candidate genes using genome-wide methods in mouse models of anxiety.

Genetically dependent modulation of serotonergic inactivation in the human prefrontal cortex

Previous research suggests that genetic variations regulating serotonergic neurotransmission mediate individual differences in the neural network underlying impulsive and aggressive behaviour. Although with conflicting findings, the monoamine oxidase-A (MAOA) and the serotonin transporter (5HTT) gene polymorphisms have been associated with an increased risk to develop impulsive and aggressive behaviour. Double knock-out mice studies have also demonstrated that MAOA and 5HTT genes strongly interact in the metabolic pathway leading to the serotonergic inactivation; however, their potential interactive effect in human brain remains uninvestigated. We used blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) to assess the independent and interactive effects of both MAOA and 5HTT polymorphisms on the brain activity elicited by a response inhibition task in healthy volunteers. Multivariate analysis demonstrated an individual effect of both MAOA and 5HTT polymorphisms and a strong allele-allele interaction in the anterior cingulate cortex (ACC), a key region implicated in cognitive control and in the pathophysiology of impulsive and aggressive behaviour. These findings suggest that the MAOAx5HTT allelic interaction exerts a significant modulation on the BOLD response associated with response inhibition and contribute to validate haplotype models as useful tools for a better understanding of the neurobiology underlying complex cognitive functions.

Evidence that genetic variation in 5-HT transporter expression is linked to changes in 5-HT2A receptor function

Variability in expression of the 5-HT transporter (5-HTT) gene in the human population has been associated with a range of behavioural phenotypes. The underlying mechanisms are unclear but may involve changes in 5-HT receptor levels and/or signalling. The present study used a novel 5-HTT overexpressing transgenic mouse to test the hypothesis that variability in 5-HTT expression may alter 5-HT(2A) receptor function. In wildtype mice, the 5-HT(2) receptor agonist DOI increased regional brain mRNA expression of two immediate early genes (c-fos and Arc), and induced head twitches, and both effects were abolished by pre-treatment with the 5-HT(2A) receptor antagonist MDL 100907. In 5-HTT overexpressing mice, DOI induced a greater increase in both c-fos and Arc mRNA expression in cortical brain regions, and more head twitches, compared to wildtype mice. Autoradiographic and in situ hybridisation experiments showed that 5-HT(2A) receptor binding sites and 5-HT(2A) receptor mRNA did not differ between transgenic and wildtype mice. Finally, the transgenic mice had lower regional brain 5-HT levels compared to wildtype mice. This depletion of 5-HT may underpin the increase in 5-HT(2A) receptor function because in wildtype mice 5-HT depletion using the 5-HT synthesis inhibitor, p-chlorophenylalanine, enhanced the head twitch response to DOI. These data demonstrate that elevated 5-HTT expression is accompanied by increased 5-HT(2A) receptor function, an effect possibly mediated by decreased availability of synaptic 5-HT. Variation in levels of 5-HTT expression may therefore be a source of variability in 5-HT(2A) receptor function, which may be an important modifier of 5-HTT-linked phenotypes.

A pharmacological analysis of mice with a targeted disruption of the serotonin transporter

RATIONALE

Partial or complete ablation of serotonin transporter (SERT) expression in mice leads to altered responses to serotonin receptor agonists and other classes of drugs.

OBJECTIVES

In the current report, we review and integrate many of the major behavioral, physiological, and neurochemical findings in the current literature regarding pharmacological assessments made in SERT mutant mice.

RESULTS

The absence of normal responses to serotonin reuptake inhibiting (SRI) antidepressants in SERT knockout (-/-) mice demonstrates that actions on SERT are a critical principle mechanism of action of members of this class of antidepressants. Drugs transported by SERT, (+)-3,4-methylenedioxymethamphetamine (MDMA) and 1-methyl-4-(2'-aminophenyl)-1,2,3,6-tetrahydropyridine (2'-NH(2)-MPTP), are also inactive in SERT -/- mice. Temperature, locomotor, and electrophysiological responses to various serotonin receptor agonists, including 8-hydroxy-2-(di-n-propylamino)-tetraline (8-OH-DPAT), ipsapirone, and RU24969, are reduced in SERT -/- mice, despite comparatively lesser reductions in Htr1a and Htr1b binding sites, G-proteins, and other signaling molecules. SERT -/- mice exhibit an approximately 90% reduction in head twitches in response to the Htr2a/2c agonist (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), associated with a profound reduction in arachidonic acid signaling, yet only modest changes in Htr2a and Htr2c binding sites. SERT -/- mice also exhibit altered behavioral responses to cocaine and ethanol, related to abnormal serotonin, and possibly dopamine and norepinephrine, homeostasis.

CONCLUSIONS

Together, these studies demonstrate a complex and varied array of modified drug responses after constitutive deletion of SERT and provide insight into the role of serotonin, and in particular, its transporter, in the modulation of complex behavior and in the pharmacological actions of therapeutic agents and drugs of abuse.