Serotonin transporter-mediated molecular axis regulates regional retinal ganglion cell vulnerability and axon regeneration after nerve injury

Molecular insights into the selective vulnerability of retinal ganglion cells (RGCs) in optic neuropathies and after ocular trauma can lead to the development of novel therapeutic strategies aimed at preserving RGCs. However, little is known about what molecular contexts determine RGC susceptibility. In this study, we show the molecular mechanisms underlying the regional differential vulnerability of RGCs after optic nerve injury. We identified RGCs in the mouse peripheral ventrotemporal (VT) retina as the earliest population of RGCs susceptible to optic nerve injury. Mechanistically, the serotonin transporter (SERT) is upregulated on VT axons after injury. Utilizing SERT-deficient mice, loss of SERT attenuated VT RGC death and led to robust retinal axon regeneration. Integrin β3, a factor mediating SERT-induced functions in other systems, is also upregulated in RGCs and axons after injury, and loss of integrin β3 led to VT RGC protection and axon regeneration. Finally, RNA sequencing analyses revealed that loss of SERT significantly altered molecular signatures in the VT retina after optic nerve injury, including expression of the transmembrane protein, Gpnmb. GPNMB is rapidly downregulated in wild-type, but not SERT- or integrin β3-deficient VT RGCs after injury, and maintaining expression of GPNMB in RGCs via AAV2 viruses even after injury promoted VT RGC survival and axon regeneration. Taken together, our findings demonstrate that the SERT-integrin β3-GPNMB molecular axis mediates selective RGC vulnerability and axon regeneration after optic nerve injury.

Placental Changes in the serotonin transporter (Slc6a4) knockout mouse suggest a role for serotonin in controlling nutrient acquisition

INTRODUCTION

The mouse placenta accumulates and possibly produces serotonin (5-hydroxytryptamine; 5-HT) in parietal trophoblast giant cells (pTGC) located at the interface between the placenta and maternal deciduum. However, the roles of 5-HT in placental function are unclear. This lack of information is unfortunate, given that selective serotonin-reuptake inhibitors are commonly used to combat depression in pregnant women. The high affinity 5-HT transporter SLC6A4 (also known as SERT) is the target of such drugs and likely controls much of 5-HT uptake into pTGC and other placental cells. We hypothesized that ablation of the Slc6a4 gene would result in morphological changes correlated with placental gene expression changes, especially for those involved in nutrient acquisition and metabolism, and thereby, provide insights into 5-HT placental function.

METHODS

Placentas were collected at embryonic age (E) 12.5 from Slc6a4 knockout (KO) and wild-type (WT) conceptuses. Histological analyses, RNAseq, qPCR, and integrative correlation analyses were performed.

RESULTS

Slc6a4 KO placentas had a considerable increased pTGC to spongiotrophoblast area ratio relative to WT placentas and significantly elevated expression of genes associated with intestinal functions, including nutrient sensing, uptake, and catabolism, and blood clotting. Integrative correlation analyses revealed upregulation of many of these genes was correlated with pTGC layer expansion. One other key gene was dopa decarboxylase (Ddc), which catalyzes conversion of L-5-hydroxytryptophan to 5-HT.

DISCUSSION

Our studies possibly suggest a new paradigm relating to how 5-HT operates in the placenta, namely as a factor regulating metabolic functions and blood coagulation. We further suggest that pTGC might be functional analogs of enterochromaffin 5-HT-positive cells of the intestinal mucosa, which regulate similar activities within the gut. Further work, including proteomics and metabolomic studies, are needed to buttress our hypothesis.

Genetic Risk Factors for Poor Cognitive Development in Children With Low Birth Weight

Low birth weight is an ongoing public health problem with severe consequences for those affected, including early morbidity and mortality and elevated risk for lifelong deficits in cognitive function. These deficits can be ameliorated by early intervention in many cases. To contribute to criteria for earlier identification of at-risk children prior to the onset of delays or deficits, we examined relationships between three gene candidates-SLC6A4, BDNF, COMT-and cognitive outcomes at school age in a secondary analysis of existing data from a nationally representative cohort. Single nucleotide polymorphism rs4074134, a variant of BDNF, and a rare insertion/deletion in the intron region of SLC6A4 were significant predictors of cognitive performance. Our final model predicted 17% of the variance in composite cognitive test scores among children with low birth weight at school age (F = 96.36, p < .001, R2 = .17). Specifically, children homozygous for cytosine at rs4074134 scored .62 standard deviations higher on a measure of global cognition than children with one or more thymine. Similarly, children with an extra-long copy number variant of SLC6A4 scored .88 standard deviations higher than children who had one or more short forms of the gene. These findings support the potential for an approach to identifying children with low birth weights who are most at need of early intervention services. Future research should focus on validation of these findings in an independent sample and confirmation of the biological mechanisms through which these genes influence cognitive development.

Serotonin Transporter Deficiency is Associated with Dysbiosis and Changes in Metabolic Function of the Mouse Intestinal Microbiome

Serotonin transporter (SERT) plays a critical role in regulating extracellular availability of serotonin (5-HT) in the gut and brain. Mice with deletion of SERT develop metabolic syndrome as they age. Changes in the gut microbiota are being increasingly implicated in Metabolic Syndrome and Diabetes. To investigate the relationship between the gut microbiome and SERT, this study assessed the fecal and cecal microbiome profile of 11 to 12 week-old SERT+/+ and SERT-/- mice. Microbial DNA was isolated, processed for metagenomics shotgun sequencing, and taxonomic and functional profiles were assessed. 34 differentially abundant bacterial species were identified between SERT+/+ and SERT-/-. SERT-/- mice displayed higher abundances of Bacilli species including genera Lactobacillus, Streptococcus, Enterococcus, and Listeria. Furthermore, SERT-/- mice exhibited significantly lower abundances of Bifidobacterium species and Akkermansia muciniphilia. Bacterial community structure was altered in SERT-/- mice. Differential abundance of bacteria was correlated with changes in host gene expression. Bifidobacterium and Bacilli species exhibited significant associations with host genes involved in lipid metabolism pathways. Our results show that SERT deletion is associated with dysbiosis similar to that observed in obesity. This study contributes to the understanding as to how changes in gut microbiota are associated with metabolic phenotype seen in SERT deficiency.

SKF-10047, a prototype Sigma-1 receptor agonist, augmented the membrane trafficking and uptake activity of the serotonin transporter and its C-terminus-deleted mutant via a Sigma-1 receptor-independent mechanism

The serotonin transporter (SERT) is functionally regulated via membrane trafficking. Our previous studies have demonstrated that the SERT C-terminal deletion mutant (SERTΔCT) showed a robust decrease in its membrane trafficking and was retained in the endoplasmic reticulum (ER), suggesting that SERTΔCT is an unfolded protein that may cause ER stress. The Sigma-1 receptor (SigR1) has been reported to attenuate ER stress via its chaperone activity. In this study, we investigated the effects of SKF-10047, a prototype SigR1 agonist, on the membrane trafficking and uptake activity of SERT and SERTΔCT expressed in COS-7 cells. Twenty-four hours of SKF-10047 treatment (>200 μM) accelerated SERT membrane trafficking and robustly upregulated SERTΔCT activity. Interestingly, these effects of SKF-10047 on SERT functions were also found in cells in which SigR1 expression was knocked down by shRNA, suggesting that SKF-10047 exerted these effects on SERT via a mechanism independent of SigR1. A cDNA array study identified several candidate genes involved in the mechanism of action of SKF-10047. Among them, Syntaxin3, a member of the SNARE complex, was significantly upregulated by 48 h of SKF-10047 treatment. These results suggest that SKF-10047 is a candidate for ER stress relief.

Citation bias and selective focus on positive findings in the literature on the serotonin transporter gene (5-HTTLPR), life stress and depression

BACKGROUND

Caspi et al.'s 2003 report that 5-HTTLPR genotype moderates the influence of life stress on depression has been highly influential but remains contentious. We examined whether the evidence base for the 5-HTTLPR-stress interaction has been distorted by citation bias and a selective focus on positive findings.

METHOD

A total of 73 primary studies were coded for study outcomes and focus on positive findings in the abstract. Citation rates were compared between studies with positive and negative results, both within this network of primary studies and in Web of Science. In addition, the impact of focus on citation rates was examined.

RESULTS

In all, 24 (33%) studies were coded as positive, but these received 48% of within-network and 68% of Web of Science citations. The 38 (52%) negative studies received 42 and 23% of citations, respectively, while the 11 (15%) unclear studies received 10 and 9%. Of the negative studies, the 16 studies without a positive focus (42%) received 47% of within-network citations and 32% of Web of Science citations, while the 13 (34%) studies with a positive focus received 39 and 51%, respectively, and the nine (24%) studies with a partially positive focus received 14 and 17%.

CONCLUSIONS

Negative studies received fewer citations than positive studies. Furthermore, over half of the negative studies had a (partially) positive focus, and Web of Science citation rates were higher for these studies. Thus, discussion of the 5-HTTLPR-stress interaction is more positive than warranted. This study exemplifies how evidence-base-distorting mechanisms undermine the authenticity of research findings.

Partially Defective Store Operated Calcium Entry and Hem(ITAM) Signaling in Platelets of Serotonin Transporter Deficient Mice

BACKGROUND

Serotonin (5-hydroxytryptamin, 5-HT) is an indolamine platelet agonist, biochemically derived from tryptophan. 5-HT is secreted from the enterochromaffin cells into the gastrointestinal tract and blood. Blood 5-HT has been proposed to regulate hemostasis by acting as a vasoconstrictor and by triggering platelet signaling through 5-HT receptor 2A (5HTR2A). Although platelets do not synthetize 5-HT, they take 5-HT up from the blood and store it in their dense granules which are secreted upon platelet activation.

OBJECTIVE

To identify the molecular composite of the 5-HT uptake system in platelets and elucidate the role of platelet released 5-HT in thrombosis and ischemic stroke.

METHODS

5-HT transporter knockout mice (5Htt-/-) were analyzed in different in vitro and in vivo assays and in a model of ischemic stroke.

RESULTS

In 5Htt-/- platelets, 5-HT uptake from the blood was completely abolished and agonist-induced Ca2+ influx through store operated Ca2+ entry (SOCE), integrin activation, degranulation and aggregation responses to glycoprotein VI (GPVI) and C-type lectin-like receptor 2 (CLEC-2) were reduced. These observed in vitro defects in 5Htt-/- platelets could be normalized by the addition of exogenous 5-HT. Moreover, reduced 5-HT levels in the plasma, an increased bleeding time and the formation of unstable thrombi were observed ex vivo under flow and in vivo in the abdominal aorta and carotid artery of 5Htt-/- mice. Surprisingly, in the transient middle cerebral artery occlusion (tMCAO) model of ischemic stroke 5Htt-/- mice showed nearly normal infarct volume and the neurological outcome was comparable to control mice.

CONCLUSION

Although secreted platelet 5-HT does not appear to play a crucial role in the development of reperfusion injury after stroke, it is essential to amplify the second phase of platelet activation through SOCE and plays an important role in thrombus stabilization.

Effects of LSD on grooming behavior in serotonin transporter heterozygous (Sert⁺/⁻) mice

Serotonin (5-HT) plays a crucial role in the brain, modulating mood, cognition and reward. The serotonin transporter (SERT) is responsible for the reuptake of 5-HT from the synaptic cleft and regulates serotonin signaling in the brain. In humans, SERT genetic variance is linked to the pathogenesis of various psychiatric disorders, including anxiety, autism spectrum disorders (ASD) and obsessive-compulsive disorder (OCD). Rodent self-grooming is a complex, evolutionarily conserved patterned behavior relevant to stress, ASD and OCD. Genetic ablation of mouse Sert causes various behavioral deficits, including increased anxiety and grooming behavior. The hallucinogenic drug lysergic acid diethylamide (LSD) is a potent serotonergic agonist known to modulate human and animal behavior. Here, we examined heterozygous Sert(+/-) mouse behavior following acute administration of LSD (0.32 mg/kg). Overall, Sert(+/-) mice displayed a longer duration of self-grooming behavior regardless of LSD treatment. In contrast, LSD increased serotonin-sensitive behaviors, such as head twitching, tremors and backwards gait behaviors in both Sert(+/+) and Sert(+/-) mice. There were no significant interactions between LSD treatment and Sert gene dosage in any of the behavioral domains measured. These results suggest that Sert(+/-) mice may respond to the behavioral effects of LSD in a similar manner to wild-type mice.

The association between the 5-HTTLPR and neural correlates of fear conditioning and connectivity

Strong evidence links the 5-HTTLPR genotype to the modulation of amygdala reactivity during fear conditioning, which is considered to convey the increased vulnerability for anxiety disorders in s-allele carriers. In addition to amygdala reactivity, the 5-HTTLPR has been shown to be related to alterations in structural and effective connectivity. The aim of this study was to investigate the effects of 5-HTTLPR genotype on amygdala reactivity and effective connectivity during fear conditioning, as well as structural connectivity [as measured by diffusion tensor imaging (DTI)]. To integrate different classification strategies, we used the bi-allelic (s-allele vs l/l-allele group) as well as the tri-allelic (low-functioning vs high-functioning) classification approach. S-allele carriers showed exaggerated amygdala reactivity and elevated amygdala-insula coupling during fear conditioning (CS + > CS-) compared with the l/l-allele group. In addition, DTI analysis showed increased fractional anisotropy values in s-allele carriers within the uncinate fasciculus. Using the tri-allelic classification approach, increased amygdala reactivity and amygdala insula coupling were observed in the low-functioning compared with the high-functioning group. No significant differences between the two groups were found in structural connectivity. The present results add to the current debate on the influence of the 5-HTTLPR on brain functioning. These differences between s-allele and l/l-allele carriers may contribute to altered vulnerability for psychiatric disorders.

Cardiovascular afferents cause the release of 5-HT in the nucleus tractus solitarii; this release is regulated by the low- (PMAT) not the high-affinity transporter (SERT)

The nucleus tractus solitarii (NTS) integrates inputs from cardiovascular afferents and thus is crucial for cardiovascular homeostasis. These afferents primarily release glutamate, although 5-HT has also been shown to play a role in their actions. Using fast-cyclic voltammetry, an increase in 5-HT concentrations (range 12-50 nm) could be detected in the NTS in anaesthetized rats in response to electrical stimulation of the vagus and activation of cardiopulmonary, chemo- and baroreceptor reflexes. This 5-HT signal was not potentiated by the serotonin transporter (SERT) or the noradrenaline transporter (NET) inhibitors citalopram and desipramine (1 mg kg(-1) ). However, decynium-22 (600 μg kg(-1) ), an organic cation 3 transporter (OCT3)/plasma membrane monoamine transporter (PMAT) inhibitor, increased the 5-HT signal by 111 ± 21% from 29 ± 10 nm. The effectiveness of these inhibitors was tested against the removal time of 5-HT and noradrenaline applied by microinjection to the NTS. Citalopram and decynium-22 attenuated the removal of 5-HT but not noradrenaline, whereas desipramine had the reverse action. The OCT3 inhibitor corticosterone (10 mg kg(-1) ) had no effect. Blockade of glutamate receptors with topical kynurenate (10-50 nm) reduced the vagally evoked 5-HT signal by 50%, indicating that this release was from at least two sources. It is concluded that vagally evoked 5-HT release is under the regulation of the high-capacity, low-affinity transporter PMAT, not the low-capacity, high-affinity transporter SERT. This is the first demonstration that PMAT may be playing a physiological role in the regulation of 5-HT transmission and this could indicate that 5-HT is acting, in part, as a volume transmitter within the NTS.

Beyond genotype: serotonin transporter epigenetic modification predicts human brain function

We examined epigenetic regulation in regards to behaviorally and clinically relevant human brain function. Specifically, we found that increased promoter methylation of the serotonin transporter gene predicted increased threat-related amygdala reactivity and decreased mRNA expression in postmortem amygdala tissue. These patterns were independent of functional genetic variation in the same region. Furthermore, the association with amygdala reactivity was replicated in a second cohort and was robust to both sampling methods and age.

Serotonin transporter and serotonin receptors

The nature of the primary defect responsible for triggering and maintaining pulmonary artery smooth muscle (PA-SMC) proliferation in pulmonary artery hypertension (PAH) is poorly understood but may be either an inherent characteristic of PA-SMCs or a secondary response to an external abnormality, such as upregulation of growth factors. The serotonin hypothesis of PAH originated in the 1960s after an outbreak of the disease was reported among patients taking the anorexigenic drugs aminorex. The anorexiant dexfenfluramine which inhibits 5-HT neuronal uptake, causes 5-HT platelet depletion, and increases plasma levels of 5-HT, was then shown to increase the relative risk of developing PAH in the adults. More recently, the incidence of persistent pulmonary hypertension of the newborn was shown to be increased by the use of selective 5-HT reuptake inhibitors taken in late pregnancy. Serotonin is a vasoconstrictor and a potent mitogen for pulmonary smooth muscle cells (PA-SMC), an effect which depends upon activity of both the 5-HT transporter (5-HTT) and the 5-HT receptors. Expression analysis of lung tissues from PAH patients undergoing lung transplantation revealed an increased expression of the 5-HT transporter (5-HTT) and an enhanced proliferative growth response of isolated pulmonary arterial smooth muscle cells (PASMC) to 5-HT. Serotonin is contained in platelets but is also synthesized by pulmonary endothelial cells which express tryptophan hydroxylase 1, the rate-limiting enzyme of 5-HT synthesis. While inhibitors of 5-HTT and of 5-HT2B receptors can reverse experimental PH, 5-HTT-overexpressing mice spontaneously develop PH. In patients with chronic lung disease, a close association has been found between a 5-HTT gene polymorphism and the severity of pulmonary hypertension. Agents capable of selectively inhibiting 5-HTT-mediated PA-SMC proliferation deserve to be investigated as potential treatments for pulmonary hypertension. However, the 5-HT pathway is still studied only on a preclinical level and the usefulness of drugs interacting with the 5-HT pathway remains to be established in human PAH.

The enteric serotonergic system is altered in patients with diverticular disease

OBJECTIVE

Disturbances of the enteric serotonergic system have been implicated in several intestinal motility disorders. Patients with diverticular disease (DD) have been reported to exhibit abnormal intestinal motility and innervation patterns. Gene expression profiles of the serotonergic system and distribution of the serotonin type 4 receptor (5HT-4R) were thus studied in patients with DD.

DESIGN

Colonic specimens from patients with DD and controls were subjected to quantitative PCR for serotonin receptors 2B, 3A, 4, serotonin transporter and synthesising enzyme tryptophan hydroxylase. Localisation of 5HT-4R was determined by dual-label immunocytochemistry using smooth muscle actin (α-SMA) and pan-neuronal markers (PGP 9.5) and quantitative analysis was carried out. Site-specific gene expression analysis of 5HT-4R was assessed within myenteric ganglia and muscle layers. Correlation of 5HT-4R with muscarinic receptors 2 and 3 (M2R, M3R) messenger RNA expression was determined.

RESULTS

5HT-4R mRNA expression was downregulated in the tunica muscularis and upregulated in the mucosa of patients with DD, whereas the other components of the serotonergic system remained unchanged. 5HT-4R was detected in ganglia and muscle layers, but was decreased in the circular muscle layer and myenteric ganglia of patients with DD. 5HT-4R mRNA expression correlated with M2R/M3R mRNA expression in controls, but not in patients with DD.

CONCLUSIONS

The serotonergic system is compromised in DD. Altered expression of 5HT-4R at mRNA and protein levels may contribute to intestinal motor disturbances reported in patients with DD. The findings support the hypothesis that DD is associated and possibly promoted by an enteric neuromuscular pathology.

Attention to threats and combat-related posttraumatic stress symptoms: prospective associations and moderation by the serotonin transporter gene

IMPORTANCE

Combat places soldiers at risk for posttraumatic stress disorder (PTSD). The excessive rates of PTSD and other adjustment disorders in soldiers returning home make it imperative to identify risk and resilience factors that could be targeted by novel therapeutic treatments.

OBJECTIVE

To investigate the interplay among attention to threat, combat exposure, and other risk factors for PTSD symptoms in soldiers deployed to combat.

DESIGN AND SETTING

Longitudinal prospective study of Israeli Defense Force infantry soldiers carried out in 2008 through 2010. Repeated measurements during a 1-year period included baseline and predeployment data collected in training camps and deployment data collected in the combat theater.

PARTICIPANTS

Infantry soldiers (1085 men; mean age, 18.8 years).

MAIN OUTCOME MEASURES

Postcombat PTSD symptoms. RESULTS Soldiers developed threat vigilance during combat deployment, particularly when they were exposed to high-intensity combat, as indicated by faster response times to targets appearing at the location of threat relative to neutral stimuli (P < .001). Threat-related attention bias also interacted with combat exposure to predict risk for PTSD (P < .05). Bias toward threat at recruitment (P < .001) and bias away from threat just before deployment (P < .05) predicted postcombat PTSD symptoms. Moreover, these threat-related attention associations with PTSD were moderated by genetic and environmental factors, including serotonin transporter (5-HTTLPR) genotype.

CONCLUSIONS AND RELEVANCE

Combat exposure interacts with threat-related attention to place soldiers at risk for PTSD, and interactions with other risk factors account for considerable variance in PTSD vulnerability. Understanding these associations informs research on novel attention bias modification techniques and prevention of PTSD.

The effect of the serotonin transporter polymorphism (5-HTTLPR) on amygdala function: a meta-analysis

The 5-HTTLPR polymorphism has been widely regarded as a potential genetic risk factor for affective disorders. Consistent with this, this polymorphism has been associated with altered amygdala responses at rest and in response to aversive stimuli. However, the strength of this association remains uncertain. We sought to synthesize existing data on the association between the 5-HTTLPR polymorphism and amygdala activation and ascertain the strength of evidence for this association. Meta-analytic techniques were applied to data from relevant published studies and unpublished data sets to obtain an estimate of the likely magnitude of effect of any association. The large number of studies allowed us to apply a formal test of publication bias, as well as explore the impact of various study-level characteristics on the magnitude of the observed effect size. Our meta-analysis indicated that there is a statistically significant but small effect of 5-HTTLPR on left and right amygdala activity. However, there was considerable between-study heterogeneity, which could not be fully accounted for by the study design and sample characteristics that we investigated. In addition, there was evidence of excess statistical significance among published studies. These findings indicate that the association between the 5-HTTLPR and amygdala activation is smaller than originally thought, and that the majority of previous studies have been considerably under powered to reliably demonstrate an effect of this size.

[Genomic endophenotypes of attention deficit hyperactivity disorder]

Attention deficit hyperactivity disorder (ADHD) is a remarkably heterogeneous neurobiological disorder. This characteristic is aetiologically mediated by genetic and environmental factors. Numerous studies have shown that different genes play a moderate but complex role in the aetiology of ADHD. Significant associations have been identified with several candidate genes for this disorder, including DAT1, DRD4, DRD5, 5HTT, HTR1B or SNAP25. The neurocognitive models of ADHD have been largely based on the findings from neuroimaging studies conducted in patients with ADHD, when establishing hypotheses capable of explaining the clinical features that are observed. From a neurobiological point of view, the genetic architecture of these patients or the interaction of the above-mentioned genes should at least partially account for the neuroanatomical and neuropsychological findings observed, which explains the axiomatic antithesis in the title. Nevertheless, the results of studies into the relationship between the genotyping and the neuroimaging or psychological findings are scarce and contradictory. The methodological differences in the volumetric analyses, the sizes of the populations studied, the neuropsychological batteries that are used, the presence of previous pharmacological treatment or comorbidity could account for these results.

Attentional bias to negative information and 5-HTTLPR genotype interactively predict students' emotional reactivity to first university semester

People strongly differ in their emotional reactions to potentially stressing and challenging environmental circumstances. Two classes of individual differences have independently been reported to contribute to such emotional vulnerability: an attentional bias to negative information and a variation in the serotonin transporter gene (5-HTTLPR). The present study was conducted to investigate the possibility that these cognitive and genetic markers interact in their prospective prediction of emotional reactivity to an extended and mild potential stressor. Changes in dysphoria and anxiety across their first university semester were measured in 120 students. Results indicate that attentional bias toward negative information on Week 1 of the semester significantly predicted cross-semester changes in both anxiety and dysphoria. For the latter, this predictive capacity depended on the 5-HTTLPR genotype. Specifically, only in homozygous carriers of the 5-HTTLPR short allele did attentional bias to negative information on Week 1 significantly predict cross-semester change in dysphoria. These results carry important theoretical and practical implications concerning the ability to identify individuals vulnerable to experiencing elevated emotional reactivity to potentially stressing life-events.

Role of serotonin in gastrointestinal physiology and pathology

Serotonin is one of the most abundant molecules in the gastrointestinal tract and it plays a crucial role in the regulation of several physiological functions, such as motility, secretion and visceral sensitivity. Besides this well documented physiological role, increasing evidence supports the concept that 5-HT is directly involved in pathological mechanisms, as well as the modulation of immune/inflammatory responses within the gut. The wide range of pathophysiological actions exerted by 5-HT are mediated by several different serotonergic receptor types and subtypes. Depending on the receptor bound and its localization, 5-HT evokes different and, sometimes, opposite responses. Therapeutic interventions aiming at modulating 5-HT signaling are mainly focused on the development of receptor agonists/antagonists, characterized by high affinity and selectivity for serotonergic receptors in the gut, to avoid the presence of adverse effects in the brain, where 5-HT is important in control mood. This review summarizes the vast current knowledge on 5-HT as a physiological mediator and analyzes the increasing body of literature describing 5-HT signaling abnormalities in functional and inflammatory disorders both in animal models and in humans. Finally, an overview on the therapeutic agents used in clinical practice is provided.

Interactive effects of corticotropin releasing hormone receptor 1, serotonin transporter linked polymorphic region, and child maltreatment on diurnal cortisol regulation and internalizing symptomatology

Within an allostatic load framework, the effect of Gene × Environment (G × E) interactions on diurnal cortisol regulation and internalizing symptomatology were investigated. Variation in the corticotropin releasing hormone receptor 1 (CRHR1) TAT haplotype and serotonin transporter linked polymorphic region (5-HTTLPR) was determined in a sample of maltreated (n = 238, 21.4% with early physical and sexual abuse) and nonmaltreated (n = 255) children (M age = 10.08) participating in a summer research camp. Internalizing and depressive symptoms were assessed by other and self-report. G × E effects for CRHR1 and maltreatment and early abuse on diurnal cortisol regulation were observed; CRHR1 variation was related to cortisol dysregulation only among maltreated children. Early abuse and high internalizing symptoms also interacted to predict atypical diurnal cortisol regulation. The interaction of CRHR1, 5-HTTLPR, and child maltreatment (G × G × E) identified a subgroup of maltreated children with high internalizing symptoms who shared the same combination of the two genes. The findings support an allostatic load perspective on the effects of the chronic stress associated with child maltreatment on cortisol regulation and internalizing symptomatology as moderated by genetic variation.

Living in a dangerous world decreases maternal care: a study in serotonin transporter knockout mice

Adverse early experiences can profoundly influence the adult behavioral profile. When pregnant and lactating mice are confronted with soiled bedding of unfamiliar males (UMB), these stimuli signal the danger of infanticide and thus simulate a "dangerous world". In a previous study, offspring of UMB treated mothers were shown to display increased anxiety-like behavior and reduced exploratory locomotion as adults, compared to mice treated with neutral bedding (NB, "safe environment"). The aim of this study was to elucidate the mechanisms conveying these effects of living in a "dangerous world" to offspring behavior. We hypothesized the mother to be the major link and focused on the influence of UMB on maternal stress hormones and behavior. Thus, we investigated fecal corticosterone metabolites (CM) and maternal care of pregnant and lactating mice either treated with NB or UMB. The offspring were subsequently tested for their anxiety-like and exploratory behavior. Mothers treated with UMB showed a significantly higher increase of fecal CM following the initial treatment, than NB treated mothers, indicating that the odor cues of potentially infanticidal males represented an ethologically relevant stimulus. Whereas the hormonal stress response habituated, living in a "dangerous world" led to a distinct and consistent reduction of maternal care behavior, particularly concerning the duration of licking and grooming the pups. Surprisingly, we could not confirm our former findings of altered phenotypes in the offspring of UMB treated mothers. In summary, we hypothesize that the frequently described effects of early life adversity on the offspring's behavioral profile are mediated primarily by maternal care in altricial rodents.

Mapping functional brain activation using [14C]-iodoantipyrine in male serotonin transporter knockout mice

Background

Serotonin transporter knockout mice have been a powerful tool in understanding the role played by the serotonin transporter in modulating physiological function and behavior. However, little work has examined brain function in this mouse model. We tested the hypothesis that male knockout mice show exaggerated limbic activation during exposure to an emotional stressor, similar to human subjects with genetically reduced transcription of the serotonin transporter.

Methodology/Principal Findings

Functional brain mapping using [¹⁴C]-iodoantipyrine was performed during recall of a fear conditioned tone. Regional cerebral blood flow was analyzed by statistical parametric mapping from autoradiographs of the three-dimensionally reconstructed brains. During recall, knockout mice compared to wild-type mice showed increased freezing, increased regional cerebral blood flow of the amygdala, insula, and barrel field somatosensory cortex, decreased regional cerebral blood flow of the ventral hippocampus, and conditioning-dependent alterations in regional cerebral blood flow in the medial prefrontal cortex (prelimbic, infralimbic, and cingulate). Anxiety tests relying on sensorimotor exploration showed a small (open field) or paradoxical effect (marble burying) of loss of the serotonin transporter on anxiety behavior, which may reflect known abnormalities in the knockout animal's sensory system. Experiments evaluating whisker function showed that knockout mice displayed impaired whisker sensation in the spontaneous gap crossing task and appetitive gap cross training.

Conclusions

This study is the first to demonstrate altered functional activation in the serotonin transporter knockout mice of critical nodes of the fear conditioning circuit. Alterations in whisker sensation and functional activation of barrel field somatosensory cortex extend earlier reports of barrel field abnormalities, which may confound behavioral measures relying on sensorimotor exploration.

[Functional homology between alcohol dependence and mood disorder]

Acute and chronic consumption of alcohol (ethanol) has been reported to modify a multitude of molecular events such as membrane fluidity, neurotransmitter turnover, function of neurotransmitter receptors and intracellular signal transduction systems coupling to neurotransmitter receptors, and biochemical processes in the central nervous system. Long-term consumption of alcohol produces alcohol dependence with comorbidity such as mood disorder, schizophrenic disorder and neurotic disorder. We examined functional changes in serotonin transporter (SERT) and serotonin receptors (5-HT(1A) and 5-HT(2A) receptors) related with depression using alcohol physical dependent mice and found correlated changes between depression and alcohol dependence. That is, SERT expression increased in the cingulate cortex, nucleus accumbens, hippocampal CA1-CA3 layers, and mediodorsal nucleus of the thalamus. Moreover, 5-HT(1A) receptors in the cingulate cortex and hippocampus and 5-HT(2A) receptors in hippocampus decreased. In addition, increase of glucocorticoid with activation of the hypothalamus-pituitary-adrenal (HPA) axis is reported in both depression and alcohol dependence. These data suggest that similar changes of serotonin system and glucocorticoid response in brain may occur in both depression and alcohol dependence.

Leptin does not directly affect CNS serotonin neurons to influence appetite

Serotonin (5-HT) and leptin play important roles in the modulation of energy balance. Here we investigated mechanisms by which leptin might interact with CNS 5-HT pathways to influence appetite. Although some leptin receptor (LepRb) neurons lie close to 5-HT neurons in the dorsal raphe (DR), 5-HT neurons do not express LepRb. Indeed, while leptin hyperpolarizes some non-5-HT DR neurons, leptin does not alter the activity of DR 5-HT neurons. Furthermore, 5-HT depletion does not impair the anorectic effects of leptin. The serotonin transporter-cre allele (Sert(cre)) is expressed in 5-HT (and developmentally in some non-5-HT) neurons. While Sert(cre) promotes LepRb excision in a few LepRb neurons in the hypothalamus, it is not active in DR LepRb neurons, and neuron-specific Sert(cre)-mediated LepRb inactivation in mice does not alter body weight or adiposity. Thus, leptin does not directly influence 5-HT neurons and does not meaningfully modulate important appetite-related determinants via 5-HT neuron function.

Interaction of 5-HTTLPR and idiographic stressors predicts prospective depressive symptoms specifically among youth in a multiwave design

5-HTTLPR, episodic stressors, depressive and anxious symptoms were assessed prospectively (child and parent report) every 3 months over 1 year (5 waves of data) among community youth ages 9 to 15 (n = 220). Lagged hierarchical linear modeling analyses showed 5-HTTLPR interacted with idiographic stressors (increases relative to the child's own average level over time), but not nomothetic stressors (higher stress exposure relative to the sample), to predict prospective elevations in depressive, but not anxious, symptoms. Youth with copies of the S or L(G) alleles of 5-HTTLPR, who experienced more stressors relative to their typical level, exhibited prospective increases in depressive symptoms over time. These findings suggest that 5-HTTLPR confers susceptibility to depression via stress reactivity.

The 5-HTTLPR polymorphism, platelet serotonin transporter activity and platelet serotonin content in underweight and weight-recovered females with anorexia nervosa

Serotonin (5-HT) pathways play an important role in the pathophysiology of anorexia nervosa (AN). In this study, we investigated functional characteristics of the platelet 5-HT transporter and platelet 5-HT content in AN patients at various stages of their illness in comparison to healthy control woman (HCW) controlling for the 5-HTTLPR deletion/insertion polymorphism and other confounding variables. Fasting blood samples of 58 acutely underweight AN patients (acAN, BMI = 15.2 ± 1.4), 26 AN patients of the initial acAN sample after short-term/partial weight restoration (BMI = 17.3 ± 0.9), 36 weight-recovered AN patients (recAN, BMI = 20.7 ± 2.2) and 58 HCW (BMI = 21.6 ± 2.0) were assessed for kinetic characteristics of platelet 5-HT uptake (V (max), K (m)) and platelet 5-HT content. Plasma leptin served as an indicator of malnutrition. Mean V (max) and K (m) values were significantly higher in recAN subjects in comparison to HCW (2.05 ± 0.62 vs. 1.66 ± 0.40 nmol 5-HT/10(9) platelets min and 432 ± 215 vs. 315 ± 136 nmol, respectively) but there were no differences in platelet 5-HT content (464.8 ± 210.6 vs. 472.0 ± 162.2 ng 5-HT/10(9) platelets). 5-HT parameters in acAN patients and HCW were similar. 5-HTTLPR variants were not related to 5-HT platelet variables. In the longitudinal part of the study we found significantly increased 5-HT content but unchanged 5-HT uptake in AN patients after short-term/partial weight restoration. Our results highlight the importance of malnutrition for the interpretation of abnormalities in neurotransmitter systems in AN. Changes in platelet 5-HT transporter activity were related to the stage of the illness but not to 5-HTTLPR genotype. Increased V (max) and K (m) in recovered AN patients might mirror adaptive modulations of the 5-HT system.

Enhancement of intestinal inflammation in mice lacking interleukin 10 by deletion of the serotonin reuptake transporter

BACKGROUND

Enterochromaffin cells and enteric neurons synthesize and release serotonin (5-HT). Reuptake, mediated by a plasmalemmal transporter (SERT) terminates the action of released 5-HT. Serotonin secretion and serotonin reuptake transporter (SERT) expression have been reported to be decreased in TNBS-induced experimental colitis and in patients with ulcerative colitis. The present study was designed to utilize the transgenic deletion of SERT as a gain-of-function model to test the hypothesis that 5-HT is a pro-inflammatory mediator in experimental colitis.

METHODS

Colitis was compared in animals with IL10(+/+)SERT(+/+) (wild-type), IL10(-/-)SERT(+/+), IL10(-/-)SERT(+/-), and IL10(-/-)/SERT(-/-) (double knockout) genotypes. Macroscopic and histological damage scores were evaluated after a time period of up to 15 weeks.

KEY RESULTS

Serotonin reuptake transporter expression was significantly increased in the inflamed colons of IL-10(-/-) mice, which displayed intestinal damage and a minor decrement in general health. General health was significantly worse and intestinal inflammation was more severe in IL-10(-/-)SERT(+/-), and IL-10(-/-)SERT(-/-) mice than in IL-10(-/-)SERT(+/+) or wild-type animals. Regardless of the associated SERT genotype, the number of 5-HT-immunoreactive cells was decreased by approximately 55-65% in all mice lacking IL-10.

CONCLUSIONS & INFERENCES

Our observations indicate that colitis associated with IL-10 deficient mice is enhanced when the IL-10 deficiency is combined with a SERT deficiency. The data support the concept that 5-HT is a pro-inflammatory mediator in the gut.

Cognitive effects of nicotine: genetic moderators

Cigarette smoking is the main preventable cause of death in developed countries, and the development of more effective treatments is necessary. Cumulating evidence suggests that cognitive enhancement may contribute to the addictive actions of nicotine. Several studies have demonstrated that nicotine enhances cognitive performance in both smokers and non-smokers. Genetic studies support the role of both dopamine (DA) and nicotinic acetylcholine receptors (nAChRs) associated with nicotine-induced cognitive enhancement. Based on knockout mice studies, beta2 nAChRs are thought to be essential in mediating the cognitive effects of nicotine. alpha7nAChRs are associated with attentional and sensory filtering response, especially in schizophrenic individuals. Genetic variation in D2 type DA receptors and the catechol-O-methyltransferase enzyme appears to moderate cognitive deficits induced by smoking abstinence. Serotonin transporter (5-HTT) gene variation also moderates nicotine-induced improvement in spatial working memory. Less is known about the contribution of genetic variation in DA transporter and D4 type DA receptor genetic variation on the cognitive effects of nicotine. Future research will provide a clearer understanding of the mechanism underlying the cognitive-enhancing actions of nicotine.

Neurobiological pathways linking socioeconomic position and health

Across individuals, risk for poor health varies inversely with socioeconomic position (SEP). The pathways by which SEP affects health have been viewed from many epidemiological perspectives. Central to these perspectives is the notion that socioeconomic health disparities arise from an interplay between nested, recursive, and cumulative environmental, social, familial, psychological, behavioral, and physiological processes that unfold over the life span. Epidemiological perspectives on socioeconomic health disparities, however, have not yet formally integrated emerging findings from neuropharmacological, molecular genetic, and neuroimaging studies demonstrating that indicators of SEP relate to patterns of brain neurotransmission, brain morphology, and brain functionality implicated in the etiology of chronic medical conditions and psychological disorders. Here, we survey these emerging findings and consider how future neurobiological studies in this area can enhance our understanding of the pathways by which different dimensions of SEP become embodied by the brain to influence health throughout life.

Stimulatory effects of the soy phytoestrogen genistein on noradrenaline transporter and serotonin transporter activity

We examined the effects of genistein, one of the major soy phytoestrogens, on the activity of noradrenaline transporter (NAT) and serotonin transporter. Treatment with genistein (10 nM-10 microM) for 20 min stimulated [(3)H]noradrenaline (NA) uptake by SK-N-SH cells. Genistein also stimulated [(3)H]NA uptake and [(3)H]serotonin uptake by NAT and serotonin transporter transiently transfected COS-7 cells, respectively. Kinetics analysis of the effect of genistein on NAT activity in NAT-transfected COS-7 cells revealed that genistein significantly increased the maximal velocity of NA transport with little or no change in the affinity. Scatchard analysis of [(3)H]nisoxetine binding to NAT-transfected COS-7 cells showed that genistein increased the maximal binding without altering the dissociation constant. Although genistein is also known to be an inhibitor of tyrosine kinases, daidzein, another soy phytoestrogen and an inactive genistein analogue against tyrosine kinases, had little effect on [(3)H]NA uptake by SK-N-SH cells. The stimulatory effects on NAT activity were observed by treatment of tyrphostin 25, an inhibitor of epidermal growth factor receptor tyrosine kinase, whereas orthovanadate, a protein tyrosine phosphatase inhibitor, suppressed [(3)H]NA uptake by NAT-transfected COS-7 cells. These findings suggest that genistein up-regulates the activity of neuronal monoamine transporters probably through processes involving protein tyrosine phosphorylation.

Serotonin transport and metabolism in the mammary gland modulates secretory activation and involution

CONTEXT

Serotonin [5-hydroxytryptamine (5-HT)] is an important local regulator of lactation homeostasis; however, the roles for the serotonin reuptake transporter and monoamine oxidase have not been known.

OBJECTIVE

The aim of the study was to determine whether drugs that impact 5-HT affect human lactation physiology.

DESIGN AND SETTING

We conducted laboratory studies of human and animal models and an observational study of the onset of copious milk secretion in postpartum women at a university medical center.

PARTICIPANTS

We studied women expecting their first live-born infant; exclusion criteria were: referred to the medical center for another medical condition, known contraindication to breastfeed, and less than 19 yr of age and unable to obtain parental consent.

INTERVENTION(S)

The mothers were interviewed. The cell and animal studies consisted of a variety of biochemical, pharmacological, and genetic interventions.

MAIN OUTCOME MEASURE(S)

The human subjects outcome was prevalence of delayed onset of copious milk secretion. The cell and animal outcomes were physiological and morphological.

RESULTS

Inhibiting serotonin reuptake in mammary epithelial cells altered barrier function, and the effects were amplified by coadministering a monoamine oxidase inhibitor. Direct delivery of fluoxetine by slow-release pellets caused localized involution. TPH1 knockout mice displayed precocious secretory activation. Among a cohort of 431 women, those taking SSRI were more likely (P = 0.02) to experience delayed secretory activation.

CONCLUSIONS

Medications that perturb serotonin balance dysregulate lactation, and the effects are consistent with those predicted by the physiological effects of intramammary 5-HT bioactivity. Mothers taking serotonergic drugs may need additional support to achieve their breastfeeding goals.

Emerging structure-function relationships defining monoamine NSS transporter substrate and ligand affinity

Monoamine transporters are a group of transmembrane neurotransmitter sodium symporter (NSS) transporters that play a crucial role in regulating biogenic monoamine concentrations at peripheral and central synapses. Given the key role played by serotonin, dopamine and noradrenaline in addictive and disease states, structure-function studies have been conducted to help guide the development of improved central nervous system therapeutics. Extensive pharmacological, immunological and biochemical studies, in conjunction with three-dimensional homology modeling, have been performed to structurally and functionally characterise the monoamine transporter substrate permeation pathway, substrate selectivity, and binding sites for ions, substrates and inhibitors at the molecular level. However, only recently has it been possible to start to construct an accurate molecular interaction network for the monoamine transporters and their corresponding substrates and inhibitors. Crystal structures of Aquifex aeolicus leucine transporter (LeuT(Aa)), a homologous protein to monoamine transporters that has been experimentally demonstrated to share similar structural folds with monoamine transporters, have been determined in complex with amino acids and inhibitors. The molecular interactions of leucine and tricyclic antidepressants (TCA) has supported many of the predictions based on the mutational studies. Models constructed from LeuT(Aa) are now allowing a rational approach to further clarify the molecular determinants of NSS transporter-ligand complexes, and potentially the ability to better manipulate drug specificity and affinity. In this review, we compare the structure-function relationships of other SLC6 NSS family transporters with monoamine transporters, and discuss possible mechanisms involved in substrate binding and transport, and modes of inhibition by TCAs.

Partial regulation of serotonin transporter function by gamma-synuclein

Human alpha-synuclein (alpha-Syn) is instrumental in maintaining homeostasis of monoamine neurotransmitters in brain, through its trafficking, and regulation of the cell surface expression and, thereby, activity of dopamine, serotonin and norepinephrine transporters. Here we have investigated whether other members of the synuclein family of proteins, gamma-synuclein (gamma-Syn) and beta-synuclein (beta-Syn) can similarly modulate the serotonin transporter (SERT). In Ltk(-) cells co-transfected with SERT and gamma-Syn, gamma-Syn reduced [(3)H]5-HT uptake, in a manner dependent on its expression levels. The decrease in SERT activity was via decreased V(max) of the transporter, without change in K(m), compared to cells expressing only SERT. By contrast, beta-Syn co-expression failed to alter SERT uptake activity, and neither the V(max) nor the K(m) was changed in the presence of beta-Syn. gamma-Syn modulation of SERT was only partial, with a maximal approximately 27% decrease in SERT activity seen even at high expression levels of gamma-Syn. By contrast, alpha-Syn attenuated SERT activity by approximately 65% at identical expression levels as gamma-Syn. Co-immunoprecipitation studies showed the presence of heteromeric protein:protein complexes between gamma-Syn or alpha-Syn and SERT, while beta-Syn failed to physically interact with SERT. Both alpha-Syn and gamma-Syn colocalized with SERT in rat primary raphae nuclei neurons. These studies document a novel physiological role for gamma-Syn in regulating 5-HT synaptic availability and homeostasis, and may be of relevance in depression and mood disorders, where SERT function is dysregulated.

[Effects of chronic fluoxetine treatment on catalepsy and immune response in mice genetically predisposed to freezing reaction: the role of 5-HT1A and 5-HT2A receptors and tph2 and SERT genes]

ASC/Icg (Antidepressant Sensitive Catalepsy) mouse strain selected for high predisposition to pinch-induced catalepsy is characterized by depressive-like behavior and impaired immune response. Chronic treatment with SSRI fluoxetine attenuated catalepsy manifestation and normalized a decreased number of rosette-forming cells (RFC) in spleen in ASC mice. Chronic fluoxetine administration had no effect on catalepsy and RFC number in mice of parental cataleptic CBA/Lac strain. Fluoxetine failed to alter 5-HT1A receptor functional activity in mice of both strains and diminished 5-HT2A receptor functional activity in CBA but not in ASC mice. No effect on cortical 5-HT1A and 5-HT2A receptor mRNA levels and on 5-HT1A receptor, tph2 (tryptophan hydroxylase-2) and SERT (serotonin transporter) mesencephalic gene expression was observed in ASC mice. Other possible serotonergic mechanisms of fluoxetine effect on catalepsy and immune response in mice with depressive-like state are discussed.

Serotonin transporter genotype modulates social reward and punishment in rhesus macaques

Background

Serotonin signaling influences social behavior in both human and nonhuman primates. In humans, variation upstream of the promoter region of the serotonin transporter gene (5-HTTLPR) has recently been shown to influence both behavioral measures of social anxiety and amygdala response to social threats. Here we show that length polymorphisms in 5-HTTLPR predict social reward and punishment in rhesus macaques, a species in which 5-HTTLPR variation is analogous to that of humans.

Methodology/Principal Findings

In contrast to monkeys with two copies of the long allele (L/L), monkeys with one copy of the short allele of this gene (S/L) spent less time gazing at face than non-face images, less time looking in the eye region of faces, and had larger pupil diameters when gazing at photos of a high versus low status male macaques. Moreover, in a novel primed gambling task, presentation of photos of high status male macaques promoted risk-aversion in S/L monkeys but promoted risk-seeking in L/L monkeys. Finally, as measured by a “pay-per-view” task, S/L monkeys required juice payment to view photos of high status males, whereas L/L monkeys sacrificed fluid to see the same photos.

Conclusions/Significance

These data indicate that genetic variation in serotonin function contributes to social reward and punishment in rhesus macaques, and thus shapes social behavior in humans and rhesus macaques alike.

[Relationship between the amygdala activity and emotion/stress responses as revealed by fMRI and genetic analysis]

Although modern society is full of stress, we do not yet know how socio-psychological stress is processed in the human brain. Experimental studies suggested that interaction of emotion and stress vulnerability may cause psychiatric conditions such as depression and anxiety. Elucidating this mechanism is one of the important topics in neuroscience. Animal experiments have shown that neural interaction between the amygdala and prefrontal cortex is involved in emotion and aversive conditioning. In the human subjects, functional magnetic resonance imaging is a powerful tool to investigate emotional processing in the brain. In this paper, the author presents the results of several fMRI experiments conducted in our laboratory as well as a review of the previous literature. The relationship between the amygdala activity and emotion/stress responses is investigated by means of neuroimaging and genetic analysis of neurotransmitters.

Modulation of serotonin transporter function during fetal development causes dilated heart cardiomyopathy and lifelong behavioral abnormalities

Background

Women are at great risk for mood and anxiety disorders during their childbearing years and may become pregnant while taking antidepressant drugs. In the treatment of depression and anxiety disorders, selective serotonin reuptake inhibitors (SSRIs) are the most frequently prescribed drugs, while it is largely unknown whether this medication affects the development of the central nervous system of the fetus. The possible effects are the product of placental transfer efficiency, time of administration and dose of the respective SSRI.

Methodology/Principal Findings

In order to attain this information we have setup a study in which these parameters were measured and the consequences in terms of physiology and behavior are mapped. The placental transfer of fluoxetine and fluvoxamine, two commonly used SSRIs, was similar between mouse and human, indicating that the fetal exposure of these SSRIs in mice is comparable with the human situation. Fluvoxamine displayed a relatively low placental transfer, while fluoxetine showed a relatively high placental transfer. Using clinical doses of fluoxetine the mortality of the offspring increased dramatically, whereas the mortality was unaffected after fluvoxamine exposure. The majority of the fluoxetine-exposed offspring died postnatally of severe heart failure caused by dilated cardiomyopathy. Molecular analysis of fluoxetine-exposed offspring showed long-term alterations in serotonin transporter levels in the raphe nucleus. Furthermore, prenatal fluoxetine exposure resulted in depressive- and anxiety-related behavior in adult mice. In contrast, fluvoxamine-exposed mice did not show alterations in behavior and serotonin transporter levels. Decreasing the dose of fluoxetine resulted in higher survival rates and less dramatic effects on the long-term behavior in the offspring.

Conclusions

These results indicate that prenatal fluoxetine exposure affects fetal development, resulting in cardiomyopathy and a higher vulnerability to affective disorders in a dose-dependent manner.

The genetics of the serotonin transporter and irritable bowel syndrome

Serotonin transporter (SERT) mediates the intracellular reuptake of released serotonin, thus regulating its biological functions. Abnormalities in serotonin reuptake can alter enteric serotonergic signalling, leading to sensory, motor and secretory gut dysfunctions, which contribute to the pathophysiology of irritable bowel syndrome (IBS). This relationship has fostered the use of selective serotonin reuptake inhibitors (SSRIs) in the treatment of IBS. Current data on the efficacy of SSRIs in IBS, association of the SERT gene promoter polymorphism 5-HTTLPR with IBS and the expression pattern of SERT in the intestinal mucosa of IBS patients are conflicting. Recent molecular studies have raised critical questions about multiple SERT mRNA transcripts in the human gut, the role of polymorphic SERT promoter in the regulation of enteric SERT expression and the ability of 5-HTTLPR to affect human SERT gene transcription. The present review highlights recent advances in SERT genetics, discusses their implications for potential therapeutic applications of SSRIs in IBS and presents original suggestions for future investigations.

Vulnerability to lasting anxiogenic effects of brief exposure to predator stimuli: sex, serotonin and other factors-relevance to PTSD

Lasting anxiogenic effects of predator stress in rodents may model aspects of post-traumatic stress disorder (PTSD). There is a link between genetic variation in the serotonin (5-HT) transporter (SERT) and anxiety in humans, prompting the generation of SERT knockout mice. This review brings together studies of SERT knockout male mice, normal female mice, and different 5-HT receptors in predator stress effects on anxiety. These studies provide for a link between vulnerability to the anxiogenic effects of predator stress and abnormalities of 5-HT transmission induced by a life long reduction in 5-HT reuptake in male mice, which creates a vulnerability like that seen in normal female mice. Data reviewed suggest abnormalities in 5-HT transmission contribute to vulnerability to lasting anxiogenic effects of species relevant stressors. To the extent to which predator stress effects model aspects of PTSD, and in the light of relevant human literature, these considerations implicate abnormalities of 5-HT transmission in vulnerability to PTSD per se, and as a potential contributor to enhanced female vulnerability to PTSD.

Pulmonary hypertension and the serotonin hypothesis: where are we now?

In the 1960s, serotonin (5HT) was associated with pulmonary arterial hypertension (PAH) caused by certain diet pills, but has recently been the subject of renewed interest in the field of PAH. Serotonin can be synthesised in the pulmonary endothelium with the rate-limiting step being the activity of tryptophan hydroxylase1 (Tph1). The serotonin is released and can then: (i) pass into the underlying pulmonary smooth muscle cells through the serotonin transporter (SERT) to initiate proliferation and/or (ii) activate serotonin receptors on pulmonary smooth muscle cells to evoke proliferation and/or contraction. Serotonin may also mediate pulmonary fibroblast proliferation via the SERT and/or serotonin receptors. Here we will unravel, discuss and update the 'serotonin hypothesis' of PAH in light of recent advances in the field. In conclusion, the activity of serotonin receptors, the SERT and Tph1 can all be elevated in clinical and experimental PAH and each offers a potentially unique therapeutic target.

5-HTTLPR biases amygdala activity in response to masked facial expressions in major depression

The amygdala is a key structure in a limbic circuit involved in the rapid and unconscious processing of facial emotions. Increased amygdala reactivity has been discussed in the context of major depression. Recent studies reported that amygdala activity during conscious emotion processing is modulated by a functional polymorphism in the serotonin transporter gene (5-HTTLPR) in healthy subjects. In the present study, amygdala reactivity to displays of emotional faces was measured by means of fMRI at 3T in 35 patients with major depression and 32 healthy controls. Conscious awareness of the emotional stimuli was prevented via backward-masking to investigate automatic emotion processing. All subjects were genotyped for the 5-HTTLPR polymorphism. Risk allele carriers (S or L(G)) demonstrated increased amygdala reactivity to masked emotional faces, which in turn was significantly correlated with life-time psychiatric hospitalization as an index of chronicity. This might indicate that genetic variations of the serotonin transporter could increase the risk for depression chronification via altering limbic neural activity on a preattentive level of emotion processing.

3,4-methylenedioxymethamphetamine self-administration is abolished in serotonin transporter knockout mice

BACKGROUND

The neurobiological mechanism underlying the reinforcing effects of 3,4-methylenedioxymethamphetamine (MDMA) remains unclear. The aim of the present study was to determine the contribution of the serotonin transporter (SERT) in MDMA self-administration behavior by using knockout (KO) mice deficient in SERT.

METHODS

Knockout mice and wild-type (WT) littermates were trained to acquire intravenous self-administration of MDMA (0, .03, .06, .125, and .25 mg/kg/infusion) on a fixed ratio 1 (FR1) schedule of reinforcement. Additional groups of mice were trained to obtain food and water to rule out operant responding impairments. Microdialysis studies were performed to evaluate dopamine (DA) and serotonin (5-HT) extracellular levels in the nucleus accumbens (NAC) and prefrontal cortex (PFC), respectively, after acute MDMA (10 mg/kg).

RESULTS

None of the MDMA doses tested maintained intravenous self-administration in KO animals, whereas WT mice acquired responding for MDMA. Acquisition of operant responding for food and water was delayed in KO mice, but no differences between genotypes were observed on the last day of training. MDMA increased DA extracellular levels to a similar extent in the NAC of WT and KO mice. Conversely, extracellular concentrations of 5-HT in the PFC were increased following MDMA only in WT mice.

CONCLUSIONS

These findings provide evidence for the specific involvement of SERT in MDMA reinforcing properties.

Genetic variation in serotonin transporter alters resting brain function in healthy individuals

BACKGROUND

Perfusion functional magnetic resonance imaging (fMRI) was used to investigate the effect of genetic variation of the human serotonin transporter (5-HTT) gene (5-HTTLPR, SLC6A4) on resting brain function of healthy individuals.

METHODS

Twenty-six healthy subjects, half homozygous for the 5-HTTLPR short allele (s/s group) and half homozygous for the long allele (l/l group), underwent perfusion functional and structural magnetic resonance imaging during a resting state. The two genotype groups had no psychiatric illness and were similar in age, gender, and personality scores.

RESULTS

Compared with the l/l group, the s/s group showed significantly increased resting cerebral blood flow (CBF) in the amygdala and decreased CBF in the ventromedial prefrontal cortex. The effect of functional modulation in these regions by 5-HTTLPR genotype cannot be accounted for by variations in brain anatomy, personality, or self-reported mood.

CONCLUSIONS

The 5-HTTLPR genotype alters resting brain function in emotion-related regions in healthy individuals, including the amygdala and ventromedial prefrontal cortex. Such alterations suggest a broad role of the 5-HTT gene in brain function that may be associated with the genetic susceptibility for mood disorders such as depression.

Interactive effects of sex and 5-HTTLPR on mood and impulsivity during tryptophan depletion in healthy people

BACKGROUND

Serotonin (5-HT) plays a central role in mood regulation and impulsivity. We studied whether healthy men and women react differently on mood and impulsivity measures during acute tryptophan depletion (ATD). We also studied the relative contribution of a functional length triallelic polymorphism in the promoter region of the serotonin transporter, designated 5-HTTLPR, to the behavioral responses to ATD.

METHODS

Thirty-nine men and 44 women participated in a randomized, double-blind, parallel group ATD study. Behavioral measures of impulsivity and mood were obtained.

RESULTS

During ATD, women reported mood reduction and showed a cautious response style, which is commonly associated with depression. Men showed an impulsive response style and did not report mood reduction. The 5-HTTLPR influenced the mood response to ATD in women.

CONCLUSIONS

Healthy men became more impulsive, whereas healthy women showed mood reduction in response to ATD. This suggests that 5-HT could be one mechanism contributing to the sex differences in the prevalence of mood and impulsivity disorders. The influence of 5-HTTLPR on mood responses in women further substantiates the relevance of this variant in the pathophysiology of at least a subgroup of patients with major depressive disorder.

Functional Cortical Effects of Novel Allelic Variants of the Serotonin Transporter Gene-linked Polymorphic Region (5-HTTLPR) in Humans

INTRODUCTION

Genetic variations of the serotonin transporter-linked polymorphic region (5-HTTLPR) have been implicated in the pathogenesis of several psychiatric disorders. Recent evidence indicates that the biallelic polymorphic region (S and L allele) contains additional variations affecting the mRNA expression.

METHODS

According to recent preclinical and clinical studies, the loudness dependence of auditory evoked potentials (LD) was investigated as surrogate parameter for the central serotonergic activity in 185 healthy subjects subdivided according to newly identified 5-HTTLPR genotypes.

RESULTS

Individuals homozygous for the L (A) allele showed the lowest LD of all genotypes suggesting a high serotonergic neurotransmission. The other observed genotypes (L (A)/L (G), S/L (A), S/L (G), S/S) had an LD which was similar to each other but higher compared to the L (A)/L (A) genotype.

DISCUSSION

The data provide a rationale to subdivide the L allele of the 5-HTTLPR into L (A) and L (G) alleles in terms of their serotonin activity as indicated by the LD. The present IN VIVO measurements provide a basis for grouping the L (G) and S alleles for further investigations.