Oligomerization of Monoamine Transporters.

Transporters of the monoamine transporter (MAT) family regulate the uptake of important neurotransmitters like dopamine, serotonin, and norepinephrine. The MAT family functions using the electrochemical gradient of ions across the membrane and comprises three transporters, dopamine transporter (DAT), serotonin transporter (SERT), and norepinephrine transporter (NET). MAT transporters have been observed to exist in monomeric states to higher-order oligomeric states. Structural features, allosteric modulation, and lipid environment regulate the oligomerization of MAT transporters. NET and SERT oligomerization are regulated by levels of PIP2 present in the membrane. The kink present in TM12 in the MAT family is crucial for dimer interface formation. Allosteric modulation in the dimer interface hinders dimer formation. Oligomerization also influences the transporters' function, trafficking, and regulation. This chapter will focus on recent studies on monoamine transporters and discuss the factors affecting their oligomerization and its impact on their function.

Connection Failure: Differences in White Matter Microstructure Are Associated with 5-HTTLPR but Not with Risk Seeking for Losses.

S/S carriers of 5-HTTLPR have been found to be more risk seeking for losses compared to L/L carriers. This finding may be the result of reduced top-down control from the frontal cortex due to altered signal pathways involving the amygdala and ventral striatum. The serotonergic system is known to be involved in neurodevelopment and neuroplasticity. Therefore, the aim of this study was to investigate whether structural differences in white matter can explain the differences in risk-seeking behaviour. Lower structural connectivity in S/S compared to L/L carriers and a negative relationship between risk seeking for losses and connectivity were assumed. Diffusion-weighted imaging was used to compute diffusion parameters for the frontostriatal and uncinate tract in 175 genotyped individuals. The results showed no significant relationship between diffusion parameters and risk seeking for losses. Furthermore, we did not find significant differences in diffusion parameters of the S/S vs. L/L group. There were only group differences in the frontostriatal tract showing stronger structural connectivity in the S/L group, which is also reflected in the whole brain approach. Therefore, the data do not support the hypothesis that the association between 5-HTTLPR and risk seeking for losses is related to differences in white matter pathways implicated in decision-making.

Microbiota influence on behavior: Integrative analysis of serotonin metabolism and behavioral profile in germ-free mice.

Previous studies on germ-free (GF) animals have described altered anxiety-like and social behaviors together with dysregulations in brain serotonin (5-HT) metabolism. Alterations in circulating 5-HT levels and gut 5-HT metabolism have also been reported in GF mice. In this study, we conducted an integrative analysis of various behaviors as well as markers of 5-HT metabolism in the brain and along the GI tract of GF male mice compared with conventional (CV) ones. We found a strong decrease in locomotor activity, accompanied by some signs of increased anxiety-like behavior in GF mice compared with CV mice. Brain gene expression analysis showed no differences in HTR1A and TPH2 genes. In the gut, we found decreased TPH1 expression in the colon of GF mice, while it was increased in the cecum. HTR1A expression was dramatically decreased in the colon, while HTR4 expression was increased both in the cecum and colon of GF mice compared with CV mice. Finally, SLC6A4 expression was increased in the ileum and colon of GF mice compared with CV mice. Our results add to the evidence that the microbiota is involved in regulation of behavior, although heterogeneity among studies suggests a strong impact of genetic and environmental factors on this microbiota-mediated regulation. While no impact of GF status on brain 5-HT was observed, substantial differences in gut 5-HT metabolism were noted, with tissue-dependent results indicating a varying role of microbiota along the GI tract.

Non-Synonymous Substitutions in Cadherin 13, Solute Carrier Family 6 Member 4, and Monoamine Oxidase A Genes are Associated with Personality Traits in Thoroughbred Horses.

Retraining retired racehorses for various purposes can help correct behavioral issues. However, ensuring efficiency and preventing accidents present global challenges. Based on the hypothesis that a simple personality assessment could help address these challenges, the present study aimed to identify genetic markers associated with personality. Eight genes were selected from 18 personality-related candidate genes that are orthologs of human personality genes, and their association with personality was verified based on actual behavior. A total of 169 Thoroughbred horses were assessed for their tractability (questionnaire concerning tractability in 14 types of situations and 3 types of impressions) during the training process. Personality factors were extracted from the data using principal component analysis and analyzed for their association with single nucleotide variants as non-synonymous substitutions in the target genes. Three genes, CDH13, SLC6A4, and MAOA, demonstrated significant associations based on simple linear regression, marking the identification of these genes for the first time as contributors to temperament in Thoroughbred horses. All these genes, as well as the previously identified HTR1A, are involved in the serotonin neurotransmitter system, suggesting that the tractability of horses may be correlated with their social personality. Assessing the genotypes of these genes before retraining is expected to prevent problems in the development of a racehorse's second career and shorten the training period through individual customization of training methods, thereby improving racehorse welfare.

Modulation of Serotonin-Related Genes by Extracellular Vesicles of the Probiotic Escherichia coli Nissle 1917 in the Interleukin-1ß-Induced Inflammation Model of Intestinal Epithelial Cells.

Inflammatory bowel disease (IBD) is a chronic inflammatory condition involving dysregulated immune responses and imbalances in the gut microbiota in genetically susceptible individuals. Current therapies for IBD often have significant side-effects and limited success, prompting the search for novel therapeutic strategies. Microbiome-based approaches aim to restore the gut microbiota balance towards anti-inflammatory and mucosa-healing profiles. Extracellular vesicles (EVs) from beneficial gut microbes are emerging as potential postbiotics. Serotonin plays a crucial role in intestinal homeostasis, and its dysregulation is associated with IBD severity. Our study investigated the impact of EVs from the probiotic Nissle 1917 (EcN) and commensal E. coli on intestinal serotonin metabolism under inflammatory conditions using an IL-1ß-induced inflammation model in Caco-2 cells. We found strain-specific effects. Specifically, EcN EVs reduced free serotonin levels by upregulating SERT expression through the downregulation of miR-24, miR-200a, TLR4, and NOD1. Additionally, EcN EVs mitigated IL-1ß-induced changes in tight junction proteins and oxidative stress markers. These findings underscore the potential of postbiotic interventions as a therapeutic approach for IBD and related pathologies, with EcN EVs exhibiting promise in modulating serotonin metabolism and preserving intestinal barrier integrity. This study is the first to demonstrate the regulation of miR-24 and miR-200a by probiotic-derived EVs.

Allosteric Inhibition and Pharmacochaperoning of the Serotonin Transporter by the Antidepressant Drugs Trazodone and Nefazodone.

The antidepressants trazodone and nefazodone were approved some 4 and 3 decades ago, respectively. Their action is thought to be mediated, at least in part, by inhibition of the serotonin transporter [SERT/solute carrier (SLC)-6A4]. Surprisingly, their mode of action on SERT has not been characterized. Here, we show that, similar to the chemically related drug vilazodone, trazodone and nefazodone are allosteric ligands: trazodone and nefazodone inhibit uptake by and transport-associated currents through SERT in a mixed-competitive and noncompetitive manner, respectively. Contrary to noribogaine and its congeners, all three compounds preferentially interact with the Na+-bound outward-facing state of SERT. Nevertheless, they act as pharmacochaperones and rescue the folding-deficient variant SERT-P601A/G602A. The vast majority of disease-associated point mutations of SLC6 family members impair folding of the encoded transporter proteins. Our findings indicate that their folding defect can be remedied by targeting allosteric sites on SLC6 transporters. SIGNIFICANCE STATEMENT: The serotonin transporter is a member of the solute carrier-6 family and is the target of numerous antidepressants. Trazodone and nefazodone have long been used as antidepressants. Here, this study shows that their inhibition of the serotonin transporter digressed from the competitive mode seen with other antidepressants. Trazodone and nefazodone rescued a folding-deficient variant of the serotonin transporter. This finding demonstrates that folding defects of mutated solute carrier-6 family members can also be corrected by allosteric ligands.

No association between peripheral serotonin-gene-related DNA methylation and brain serotonin neurotransmission in the healthy and depressed state.

BACKGROUND: Methylation of serotonin-related genes has been proposed as a plausible gene-by-environment link which may mediate environmental stress, depressive and anxiety symptoms. DNA methylation is often measured in blood cells, but little is known about the association between this peripheral epigenetic modification and brain serotonergic architecture. Here, we evaluated the association between whole-blood-derived methylation of four CpG sites in the serotonin transporter (SLC6A4) and six CpG sites of the tryptophan hydroxylase 2 (TPH2) gene and in-vivo brain levels of serotonin transporter (5-HTT) and serotonin 4 receptor (5-HT4) in a cohort of healthy individuals (N = 254) and, for 5-HT4, in a cohort of unmedicated patients with depression (N = 90). To do so, we quantified SLC6A4/TPH2 methylation using bisulfite pyrosequencing and estimated brain 5-HT4 and 5-HTT levels using positron emission tomography. In addition, we explored the association between SLC6A4 and TPH2 methylation and measures of early life and recent stress, depressive and anxiety symptoms on 297 healthy individuals. RESULTS: We found no statistically significant association between peripheral DNA methylation and brain markers of serotonergic neurotransmission in patients with depression or in healthy individuals. In addition, although SLC6A4 CpG2 (chr17:30,236,083) methylation was marginally associated with the parental bonding inventory overprotection score in the healthy cohort, statistical significance did not remain after accounting for blood cell heterogeneity. CONCLUSIONS: We suggest that findings on peripheral DNA methylation in the context of brain serotonin-related features should be interpreted with caution. More studies are needed to rule out a role of SLC6A4 and TPH2 methylation as biomarkers for environmental stress, depressive or anxiety symptoms.

A Conserved Intramolecular Ion-Pair Plays a Critical but Divergent Role in Regulation of Dimerization and Transport Function among the Monoamine Transporters.

The monoamine transporters, including the serotonin transporter (SERT), dopamine transporter (DAT), and norepinephrine transporter (NET), are the therapeutic targets for the treatment of many neuropsychiatric disorders. Despite significant progress in characterizing the structures and transport mechanisms of these transporters, the regulation of their transport functions through dimerization or oligomerization remains to be understood. In the present study, we identified a conserved intramolecular ion-pair at the third extracellular loop (EL3) connecting TM5 and TM6 that plays a critical but divergent role in the modulation of dimerization and transport functions among the monoamine transporters. The disruption of the ion-pair interactions by mutations induced a significant spontaneous cross-linking of a cysteine mutant of SERT and an increase in cell surface expression but with an impaired specific transport activity. On the other hand, similar mutations of the corresponding ion-pair residues in both DAT and NET resulted in an opposite effect on their oxidation-induced dimerization, cell surface expression, and transport function. Reversible biotinylation experiments indicated that the ion-pair mutations slowed down the internalization of SERT but stimulated the internalization of DAT. In addition, cysteine accessibility measurements for monitoring SERT conformational changes indicated that substitution of the ion-pair residues resulted in profound effects on the rate constants for cysteine modification in both the extracellular and cytoplasmatic substrate permeation pathways. Furthermore, molecular dynamics simulations showed that the ion-pair mutations increased the interfacial interactions in a SERT dimer but decreased it in a DAT dimer. Taken together, we propose that the transport function is modulated by the equilibrium between monomers and dimers on the cell surface, which is regulated by a potential compensatory mechanism but with different molecular solutions among the monoamine transporters. The present study provided new insights into the structural elements regulating the transport function of the monoamine transporters through their dimerization.

Identification of the potassium-binding site in serotonin transporter.

Clearance of serotonin (5-hydroxytryptamine, 5-HT) from the synaptic cleft after neuronal signaling is mediated by serotonin transporter (SERT), which couples this process to the movement of a Na+ ion down its chemical gradient. After release of 5-HT and Na+ into the cytoplasm, the transporter faces a rate-limiting challenge of resetting its conformation to be primed again for 5-HT and Na+ binding. Early studies of vesicles containing native SERT revealed that K+ gradients can provide an additional driving force, via K+ antiport. Moreover, under appropriate conditions, a H+ ion can replace K+. Intracellular K+ accelerates the resetting step. Structural studies of SERT have identified two binding sites for Na+ ions, but the K+ site remains enigmatic. Here, we show that K+ antiport can drive substrate accumulation into vesicles containing SERT extracted from a heterologous expression system, allowing us to study the residues responsible for K+ binding. To identify candidate binding residues, we examine many cation binding configurations using molecular dynamics simulations, predicting that K+ binds to the so-called Na2 site. Site-directed mutagenesis of residues in this site can eliminate the ability of both K+ and H+ to drive 5-HT accumulation into vesicles and, in patch clamp recordings, prevent the acceleration of turnover rates and the formation of a channel-like state by K+ or H+. In conclusion, the Na2 site plays a pivotal role in orchestrating the sequential binding of Na+ and then K+ (or H+) ions to facilitate 5-HT uptake in SERT.

Gene-Environment Interactions in Irrational Beliefs: The Roles of Childhood Adversity and Multiple Candidate Genes.

Cognitive behavioral therapy is based on the view that maladaptive thinking is the causal mechanism of mental disorders. While this view is supported by extensive evidence, very limited work has addressed the factors that contribute to the development of maladaptive thinking. The present study aimed to uncover interactions between childhood maltreatment and multiple genetic differences in irrational beliefs. Childhood maltreatment and irrational beliefs were assessed using multiple self-report instruments in a sample of healthy volunteers (N = 452). Eighteen single-nucleotide polymorphisms were genotyped in six candidate genes related to neurotransmitter function (COMT; SLC6A4; OXTR), neurotrophic factors (BDNF), and the hypothalamic-pituitary-adrenal axis (NR3C1; CRHR1). Gene-environment interactions (G×E) were first explored in models that employed one measure of childhood maltreatment and one measure of irrational beliefs. These effects were then followed up in models in which either the childhood maltreatment measure, the irrational belief measure, or both were substituted by parallel measures. Consistent results across models indicated that childhood maltreatment was positively associated with irrational beliefs, and these relations were significantly influenced by COMT rs165774 and OXTR rs53576. These results remain preliminary until independent replication, but they represent the best available evidence to date on G×E in a fundamental mechanism of psychopathology.

Serotonin transporter knockdown relieves depression-like behavior and ethanol-induced CPP in mice after chronic social defeat stress.

Patients with stress-triggered major depression disorders (MDD) can often seek comfort or temporary relief through alcohol consumption, as they may turn to it as a means of self-medication or coping with overwhelming emotions. The use of alcohol as a coping mechanism for stressful events can escalate, fostering a cycle where the temporary relief it provides from depression can deepen into alcohol dependence, exacerbating both conditions. Although, the specific mechanisms involved in stress-triggered alcohol dependence and MDD comorbidities are not well understood, a large body of literature suggests that the serotonin transporter (SERT) plays a critical role in these abnormalities. To further investigate this hypothesis, we used a lentiviral-mediated knockdown approach to examine the role of hippocampal SERT knockdown in social defeat stress-elicited depression like behavior and ethanol-induced place preference (CPP). The results showed that social defeat stress-pro depressant effects were reversed following SERT knockdown demonstrated by increased sucrose preference, shorter latency to feed in the novelty suppressed feeding test, and decreased immobility time in the tail suspension and forced swim tests. Moreover, and most importantly, social stress-induced ethanol-CPP acquisition and reinstatement were significantly reduced following hippocampal SERT knockdown using short hairpin RNA shRNA-expressing lentiviral vectors. Finally, we confirmed that SERT hippocampal mRNA expression correlated with measures of depression- and ethanol-related behaviors by Pearson's correlation analysis. Taken together, our data suggest that hippocampal serotoninergic system is involved in social stress-triggered mood disorders as well as in the acquisition and retrieval of ethanol contextual memory and that blockade of this transporter can decrease ethanol rewarding properties.

Alterations in the brain serotonin system and serotonin-regulated behavior during aging in zebrafish males and females.

The brain serotonin (5-HT) system performs a neurotrophic function and supports the plasticity of the nervous system, while its age-related changes can increase the risk of senile neurodegeneration. Zebrafish brain is highly resistant to damage and neurodegeneration due to its high regeneration potential and it is a promising model object in searching for molecular factors preventing age-related neurodegeneration. In the present study alterations in 5-HT-related behavior in the home tank and the novel tank diving test, as well as 5-HT, 5-HIAA levels, tryptophan hydroxylase (TPH), monoamine oxidase (MAO) activity and the expression of genes encoding TPH, MAO, 5-HT transporter and 5-HT receptors in the brain of 6, 12, 24 and 36 month old zebrafish males and females are investigated. Marked sexual dimorphism in the locomotor activity in the novel tank test is revealed: females of all ages move slower than males. No sexual dimorphism in 5-HT-related traits is observed. No changes in 5-HT and 5-HIAA levels in zebrafish brain during aging is observed. At the same time, the aging is accompanied by a decrease in the locomotor activity, TPH activity, tph2 and htr1aa genes expression as well as an increase in the MAO activity and slc6a4a gene expression in their brain. These results indicate that the brain 5-HT system in zebrafish is resistant to age-related alterations.

Serotonin Transporter Deficiency Induces Metabolic Alterations in the Ileal Mucosa.

Serotonin transporter (SERT) deficiency has been implicated in metabolic syndrome, intestinal inflammation, and microbial dysbiosis. Interestingly, changes in microbiome metabolic capacity and several alterations in host gene expression, including lipid metabolism, were previously observed in SERT-/- mice ileal mucosa. However, the precise host or microbial metabolites altered by SERT deficiency that may contribute to the pleiotropic phenotype of SERT KO mice are not yet understood. This study investigated the hypothesis that SERT deficiency impacts lipid and microbial metabolite abundances in the ileal mucosa, where SERT is highly expressed. Ileal mucosal metabolomics was performed by Metabolon on wild-type (WT) and homozygous SERT knockout (KO) mice. Fluorescent-activated cell sorting (FACS) was utilized to measure immune cell populations in ileal lamina propria to assess immunomodulatory effects caused by SERT deficiency. SERT KO mice exhibited a unique ileal mucosal metabolomic signature, with the most differentially altered metabolites being lipids. Such changes included increased diacylglycerols and decreased monoacylglycerols in the ileal mucosa of SERT KO mice compared to WT mice. Further, the ileal mucosa of SERT KO mice exhibited several changes in microbial-related metabolites known to play roles in intestinal inflammation and insulin resistance. SERT KO mice also had a significant reduction in the abundance of ileal group 3 innate lymphoid cells (ILC3). In conclusion, SERT deficiency induces complex alterations in the ileal mucosal environment, indicating potential links between serotonergic signaling, gut microbiota, mucosal immunity, intestinal inflammation, and metabolic syndrome.

5-HT_FAsTR: a versatile, label-free, high-throughput, fluorescence-based microplate assay to quantify serotonin transport and release.

The neurotransmitter serotonin plays a pivotal role in mood and depression. It also acts as a vasoconstrictor within blood vessels and is the main neurotransmitter in the gastrointestinal system. In neurotransmission, released serotonin is taken up by serotonin transporters, which are principal targets of antidepressants and the psychostimulant, ecstasy. The investigation of serotonin transporters have relied almost exclusively on the use of radiolabeled serotonin in heterogenous end-point assays. Here we adapt the genetically encoded fluorescent biosensor, iSeroSnFR, to establish and validate the Serotonin (5-HT) Fluorescence Assay for Transport and Release (5-HT_FAsTR) for functional and pharmacological studies of serotonin transport and release. We demonstrate the applicability of the method for the study of a neuronal, high-affinity, low-capacity serotonin transporter (SERT) as well as an extraneuronal low-affinity, high-capacity organic cation transporter and mutants thereof. 5HT_FAsTR offers an accessible, versatile and reliable semi-homogenous assay format that only relies on a fluorescence plate reader for repeated, real-time measurements of serotonin influx and efflux. 5HT_FAsTR accelerates and democratizes functional characterization and pharmacological studies of serotonin transporters and genetic variants thereof in disease states such as depression, anxiety and ADHD.

[The association of polymorphisms of the serotonin transporter gene SLC6A4 with depression]. / Svyaz' polimorfizmov gena serotoninovogo transportera SLC6A4 s depressiei.

OBJECTIVE: To study the relationship of polymorphic variants of the SLC6A4 gene with depression among people aged 25-44 years in Novosibirsk. MATERIAL AND METHODS: Under the WHO program «MONICA-psychosocial (MOPSY)¼, a random representative sample of people aged 25-44 years from the population of the Oktyabrsky district of Novosibirsk (men n=725, mean age 43.4±0.4 years, response - 71.3%, women n=710, mean age 44.8±0.4 years, response - 72%). Depression was assessed using the MONICA-MOPSY psychosocial questionnaire. Every fourth respondent was examined for polymorphic variants of 5HTTLPR-VNTR SNP rs25531 A>G of the SLC6A4 gene. The study was carried out within the framework of the budget topic Reg. No. 122031700094-5. RESULTS: The high level of depression among people aged 25-44 was 12.8% (for men 9.1%, for women - 15.92%); the average level of depression occurred in 24.5% of the population (among men in 21.24%, among women in 26.76%) (χ2=17.071, df=2, p<0.001). The most common genotype of the SLC6A4 gene, among people aged 25--4 years old in Novosibirsk, was SLA - 43.29%, LALA - 26.53% - in second place, SS - 17.87% - third, LALG - 6 genotypes were less represented genotypes. 74%, SLG - 4.18%, LGLG - 1.39%. Carrying the SLA genotype (53.3% and 63.6%) increased the chance of developing both the average level of depression by 2.359 (95% CI 1.278-4.355) times, and depression in general by 1.933 (95% CI 1.142-3.271) times, compared with persons carrying the LALA genotype (32.0% and 46.9%), (χ2=7.674, df=1, p<0.01 and χ2=6.095, df=1, p<0.05). Persons carrying the LALG genotype (54.5%) also had a higher chance of developing a mean level of depression RR=2.929 (95% CI 1.039-8.261), compared with carriers of the LALA genotype (32.0%) (χ2=4.326, df =1, p<0.05) (p<0.05). CONCLUSION: Associative links between polymorphic variants of the SLC6A4 gene and depression have been established.

Pharmacogenetic Factors Influence Escitalopram Pharmacokinetics and Adverse Events in Youth with a Family History of Bipolar Disorder: A Preliminary Study.

Introduction: Escitalopram is an effective and generally well-tolerated antidepressant, but children of parents with bipolar disorder (BD) may be at increased risk for adverse events associated with antidepressants, including increased irritability, restlessness, impulsivity, and manic symptoms. This risk may be influenced by polymorphisms in genes encoding cytochrome P450 enzymes (CYP2C19 or CYP2D6), the serotonin transporter (SLC6A4), and the serotonin receptor 2A subtype (HTR2A). We explored whether gene-drug interactions influence the emergence of adverse events in depressed and/or anxious youth with a family history of BD. Materials and Methods: Children and adolescents aged 12-17 years with a first-degree relative with bipolar I disorder were treated with escitalopram and monitored for adverse effects, underwent pharmacogenetic testing, and provided serum escitalopram levels. Emergence of adverse events was determined by study clinicians, and symptoms were tracked using the Treatment-Emergent Activation and Suicidality Assessment Profile (TEASAP) and Pediatric Adverse Events Rating Scale. Clinical Pharmacogenetics Implementation Consortium guidelines were used to determine CYP2C19 and CYP2D6 phenotypes. Results: Slower CYP2C19 metabolizers had greater dose-normalized 24-hour area under the curve (AUC0-24; p = 0.025), trough concentrations (Ctrough; p = 0.013), and elimination half-lives (t1/2; p < 0.001). CYP2D6 phenotype was not significantly associated with any pharmacokinetic parameter. Slower CYP2D6 metabolizers had increased TEASAP akathisia (p = 0.015) scores. HTR2A A/A and A/G genotypes were associated with increased TEASAP "self-injury, suicidality, and harm to others" subscale scores (p = 0.017). Escitalopram maximum concentration, AUC0-24, CYP2C19 phenotype, and SLC6A4 genotype were not associated with adverse events. Conclusions: CYP2C19 phenotype influences escitalopram pharmacokinetics whereas CYP2D6 phenotype does not. Slower CYP2D6 metabolism was associated with increased akathisia, and HTR2A A/A or A/G genotypes were associated with increased risk of self-harm or harm to others. Larger cohorts are needed to identify associations between genetic test results and antidepressant-associated adverse events. Trial Registration: ClinicalTrials.gov identifier: NCT02553161.

Serotonin and the serotonin transporter in the adrenal gland.

The adrenal glands are key components of the mammalian endocrine system, helping maintain physiological homeostasis and the coordinated response to stress. Each adrenal gland has two morphologically and functionally distinct regions, the outer cortex and inner medulla. The cortex is organized into three concentric zones which secrete steroid hormones, including aldosterone and cortisol. Neural crest-derived chromaffin cells in the medulla are innervated by preganglionic sympathetic neurons and secrete catecholamines (epinephrine, norepinephrine) and neuropeptides into the bloodstream, thereby functioning as the neuroendocrine arm of the sympathetic nervous system. In this article we review serotonin (5-HT) and the serotonin transporter (SERT; SLC6A4) in the adrenal gland. In the adrenal cortex, 5-HT, primarily sourced from resident mast cells, acts as a paracrine signal to stimulate aldosterone and cortisol secretion through 5-HT4/5-HT7 receptors. Medullary chromaffin cells contain a small amount of 5-HT due to SERT-mediated uptake and express 5-HT1A receptors which inhibit secretion. The atypical mechanism of the 5-HT1A receptors and interaction with SERT fine tune this autocrine pathway to control stress-evoked catecholamine secretion. Receptor-independent signaling by SERT/intracellular 5-HT modulates the amount and kinetics of transmitter release from single vesicle fusion events. SERT might also influence stress-evoked upregulation of tyrosine hydroxylase transcription. Transient signaling via 5-HT3 receptors during embryonic development can limit the number of chromaffin cells found in the mature adrenal gland. Together, this emerging evidence suggests that the adrenal medulla is a peripheral hub for serotonergic control of the sympathoadrenal stress response.

Chronic social defeat stress induces anxiety-like behaviors via downregulation of serotonin transporter in the prefrontal serotonergic system in mice.

The serotonergic (5-HTergic) system is closely involved in the pathophysiology of mood and anxiety disorders and the responsibility of this system may differ for each symptom. In this study, we examined the relationship between the dysfunction of the 5-HTergic system and abnormal behaviors in the social defeat stress model, an animal model of mood and anxiety disorders and in mice with knockdown of Slc6a4, the gene encoding SERT. Monoamine content, serotonin (5-HT) release, 5-HT uptake, 5-HT transporter (SERT) protein levels, and behaviors were investigated in mice subjected to chronic social defeat stress and in mice with knockdown of Slc6a4, in 5-HTergic neurons projecting to the prefrontal cortex (PFC). Furthermore, DNA methylation of Slc6a4 was examined in mice subjected to chronic social defeat stress. Increased turnover, increased extracellular basal levels, decreased release and decreased uptake of 5-HT, and decreased SERT protein levels were observed in the PFC of the stressed mice. The decreased 5-HT uptake correlated with anxiety-like behavior characterized by decreased time spent in the open arms of the elevated plus maze. DNA methylation was increased in the CpG island of Slc6a4 in 5-HTergic neurons projecting to the PFC of the stressed mice. Similar to the stressed mice, mice with Slc6a4 knockdown in 5-HTergic neurons projecting to the PFC also showed decreased release and uptake of 5-HT in the PFC and increased anxiety-like behavior. Chronic stress may induce anxiety due to dysfunction in the prefrontal 5-HTergic system via decreased SERT expression in the PFC.

SLC6A4 gene variants moderate associations between childhood food insecurity and adolescent mental health.

BACKGROUND: Food insecurity is a persistent concern in the United States and has been shown to affect child mental health and behavior. The SLC6A4 gene has been indicated as a moderator of the effects of chronic stress on anxiety in adolescents aged 14-21. However, it is unclear if SLC6A4 may also play a role in the effects of childhood food insecurity, a form of chronic stress, on adolescent mental health. This study aimed to identify effects of food insecurity on adolescents' mental health and delinquent behavior when both mom and child go hungry in the child's early years, and the potential interaction with SLC6A4 variants (SS/LL). METHODS: The data and sample for this research are from the Future of Families and Child Wellbeing Study. The cohort consists of 4898 children (age 1-15 years, male = 47%, African American = 50%) and their respective caregivers sampled from large cities in the United States from 1998 to 2000. RESULTS: The SLC6A4 serotonin transporter short/short allele emerged statistically significant as a moderator of childhood food insecurity and adolescent mental health. Specifically, the presence of the short/short allele increased anxiety symptoms in adolescents with exposure to food insecurity in childhood. CONCLUSION: The SLC6A4 short/short allele amplifies risk of anxiety-related mental illness when children experience food insecurity. The gene-environment interaction provides insight into the mechanistic pathway of the effects of poverty-related adversity, such as food insecurity, on developmental trajectories of mental health.

Tailoring Type II Diabetes Treatment: Investigating the Effect of 5-HTT Polymorphisms on HbA1c Levels after Metformin Initiation.

Aims: To investigate the effect of serotonin transporter (5-HTT) polymorphisms on change in HbA1c levels six months after metformin initiation in type 2 diabetes patients. Materials and Methods: Participants of PROVALID (PROspective cohort study in patients with type 2 diabetes mellitus for VALidation of biomarkers) within the GIANTT (Groningen Initiative to ANalyse Type 2 Diabetes Treatment) cohort who initiated metformin were genotyped for combined 5-HTTLPR/rs25531 (L∗L∗, L∗S∗, and S∗S∗) and 5-HTT VNTR (STin 2.12, 12/-, and 10/-) polymorphisms, respectively. Multiple linear regression was applied to determine the change in HbA1c level from baseline date to six months across 5-HTTLPR/VNTR genotype groups, adjusted for baseline HbA1c, age, gender, triglyceride level, low-density lipoprotein level, and serum creatinine. Results: 157 participants were included, of which 56.2% were male. The average age was 59.3 ± 9.23 years, and the mean baseline HbA1c was 7.49% ± 1.21%. 5-HTTLPR was characterized in 46 patients as L∗L∗, 70 patients as L∗S∗, and 41 patients as S∗S∗ genotypes. No significant association was found between 5-HTTLPR and 5-HTT VNTR genotypes and change in HbA1c after adjustments. Conclusions: 5-HTT polymorphisms did not affect HbA1c levels six months after the start of metformin. Further long-term studies in large samples would be relevant to determine which polymorphisms can explain the variation in response to metformin treatment.