Serotonergic Control of Metabolic Homeostasis

The selective serotonin reuptake inhibitors, sertraline and paroxetine, improve islet beta-cell mass and function in vitro

AIMS

To investigate the effects of the selective serotonin reuptake inhibitors (SSRIs) sertraline and paroxetine at therapeutically relevant concentrations on beta-cell mass and function.

METHODS

Viability was quantified in mouse insulinoma (MIN6) beta cells and mouse islets after 48-h exposure to sertraline (1-10 μM) or paroxetine (0.01-1 μM) using the Trypan blue exclusion test. The effects of therapeutic concentrations of these SSRIs on insulin secretion were determined by static incubation and perifusion experiments, while islet apoptosis was investigated by Caspase-Glo 3/7 assay, TUNEL staining and quantitative PCR analysis. Finally, proliferation of MIN6 and mouse islet beta cells was assessed by bromodeoxyuridine (BrdU) enzyme-linked immunosorbent assay and immunofluorescence.

RESULTS

Sertraline (0.1-1 μM) and paroxetine (0.01-0.1 μM) were well tolerated by MIN6 beta cells and islets, whereas 10 μM sertraline and 1 μM paroxetine were cytotoxic. Exposure to 1 μM sertraline and 0.1 μM paroxetine significantly potentiated glucose-stimulated insulin secretion from mouse and human islets. Moreover, they showed protective effects against cytokine- and palmitate-induced apoptosis of islets, they downregulated cytokine-induced Stat1 and Traf1 mRNA expression, and they significantly increased proliferation of mouse beta cells.

CONCLUSIONS

Our data demonstrate that sertraline and paroxetine act directly on beta cells to enhance glucose-stimulated insulin secretion and stimulate beta-cell mass expansion by increasing proliferation and decreasing apoptosis. These drugs are therefore likely to be appropriate for treating depression in people with type 2 diabetes.

Peripheral 5-HT Mediates Gonadotropin-Inhibitory Hormone-Induced Feeding Behavior and Energy Metabolism Disorder in Chickens via the 5-HT2C Receptor

INTRODUCTION

Since the discovery of gonadotropin-inhibitory hormone (GnIH), it has been found to play a critical role in reproduction in vertebrates. Recently, a regulatory role of GnIH in appetite and energy metabolism has emerged, although its precise physiological mechanisms remain unknown.

METHODS

Thus, the present study evaluated the effects of a single or long-term intraperitoneal GnIH treatment on the food intake, weight, and glucolipid metabolism of chickens, as well as investigating the possible neuroendocrinology factors and mechanisms involved in GnIH-induced obesity and glucolipid metabolism disorder.

RESULTS

Our results show that the intraperitoneal administration of GnIH to chickens resulted in a marked body mass increase, hyperlipidemia, hyperglycemia, and glucose intolerance. Subsequently, the results of metabolomics studies and the pharmacological inhibition of the 5-HT2C receptor revealed that blocking the 5-HT2C receptor reinforced the effects of GnIH on food intake, body weight, and blood glucose and lipid levels, resulting in even worse cases of GnIH-induced hyperglycemia, hyperlipidemia, and hepatic lipid deposition. This suggests that, via the 5-HT2C receptor, peripheral 5-HT may act as a negative feedback regulator to interplay with GnIH and jointly control energy balance homeostasis in chickens.

DISCUSSION

Our present study provides evidence of cross-talk between GnIH and 5-HT in food intake and energy metabolism at the in vivo pharmacological level, and it proposes a molecular basis for these interactions, suggesting that functional interactions between GnIH and 5-HT may open new avenues for understanding the mechanism of the neuroendocrine network involved in appetite and energy metabolism, as well as providing a new therapeutic strategy to prevent obesity, diabetes, and metabolic disorders.

Association of serotonin receptor gene polymorphisms with anorexia nervosa: a systematic review and meta-analysis

PURPOSE

Several studies have investigated the association between anorexia nervosa and polymorphisms of genes regulating serotonin neurotransmission, with a focus on the rs6311 polymorphism of 5-HTR2A. However, inconsistent results of these studies and conflicting conclusions of existing meta-analyses complicate the understanding of a possible association. We have updated these results and evaluated the involvement of other serotonin receptor gene polymorphisms in anorexia nervosa.

METHODS

Adhering to PRISMA guidelines, we have searched studies on anorexia nervosa and serotonin-regulating genes published from 1997 to 2022, selected those concerning receptor genes and meta-analyzed the results from twenty candidate gene studies on the 5-HTR2A rs6311 polymorphism and the 5-HTR2C rs6318 polymorphism.

RESULTS

Present analyses reveal an association for the 5-HTR2A rs6311 polymorphism, with G and A alleles, across eighteen studies (2049 patients, 2877 controls; A vs. G allele, Odds Ratio = 1.24; 95% Confidence Interval = 1.06-1.47; p = 0.009). However, after geographic subgrouping, an association emerged only in a Southern European area, involving five studies (722 patients, 773 controls; A vs. G allele, Odds Ratio = 1.82; 95% Confidence Interval = 1.41-2.37; p < 0.00001). No association was observed for the 5-HTR2C rs6318 polymorphism across three studies.

CONCLUSIONS

To date, the involvement in the pathophysiology of anorexia nervosa of the 5-HTR2A rs6311 polymorphism appears limited to a specific genetic and/or environmental context, while that of the 5-HTR2C rs6318 polymorphism seems excluded. Genome-wide association studies and epigenetic studies will likely offer deeper insights of genetic and environmental factors possibly contributing to the disorder.

LEVEL OF EVIDENCE

III Evidence obtained from well-designed cohort or case-control analytic studies. Clinical trial registration PROSPERO registration number: CRD42021246122.

Abnormal tryptophan catabolism in diabetes mellitus and its complications: Opportunities and challenges

In recent years, the incidence rate of diabetes mellitus (DM), including type 1 diabetes mellitus(T1DM), type 2 diabetes mellitus(T2DM), and gestational diabetes mellitus (GDM), has increased year by year and has become a major global health problem. DM can lead to serious complications of macrovascular and microvascular. Tryptophan (Trp) is an essential amino acid for the human body. Trp is metabolized in the body through the indole pathway, kynurenine (Kyn) pathway and serotonin (5-HT) pathway, and is regulated by intestinal microorganisms to varying degrees. These three metabolic pathways have extensive regulatory effects on the immune, endocrine, neural, and energy metabolism systems of the body, and are related to the physiological and pathological processes of various diseases. The key enzymes and metabolites in the Trp metabolic pathway are also deeply involved in the pathogenesis of DM, playing an important role in pancreatic function, insulin resistance (IR), intestinal barrier, and angiogenesis. In DM and its complications, there is a disruption of Trp metabolic balance. Several therapy approaches for DM and complications have been proven to modify tryptophan metabolism. The metabolism of Trp is becoming a new area of focus for DM prevention and care. This paper reviews the impact of the three metabolic pathways of Trp on the pathogenesis of DM and the alterations in Trp metabolism in these diseases, expecting to provide entry points for the treatment of DM and its complications.

Constitutive Serotonin Tone as a Modulator of Brown Adipose Tissue Thermogenesis: A Rat Study

Brown adipose tissue (BAT), an important regulator of thermogenic and metabolic processes, is considered a promising target to combat metabolic disorders. The neurotransmitter and hormone serotonin (5HT) is a major modulator of energy homeostasis, with its central and peripheral pools acting in opposing ways. To better understand how individual variations in 5HT homeostasis influence the thermogenic functionality of BAT, we used a rat model consisting of two sublines with constitutively increased (high-5HT) or decreased (low-5HT) whole-body 5HT tone, developed by selective breeding for platelet 5HT parameters. We have shown that animals with constitutively low 5HT activity maintained at a standard housing temperature (22 °C) have greater interscapular BAT (iBAT) mass and higher iBAT metabolic activity (as evidenced by measurements of iBAT temperature and glucose uptake), accompanied by increased iBAT mRNA expression of key thermogenic genes, compared to animals with high 5HT tone. In response to further thermogenic challenges-intermittent cold exposure or treatment with a β₃-adrenergic agonist-5HT sublines show several functional and molecular differences linking constitutively low endogenous 5HT tone to higher BAT activity/capacity. Overall, the results support a role of 5-HT in the control of BAT thermogenesis They also suggest that individuals with lower 5HT activity may be more sensitive to β3-adrenergic drugs.

5-HT2C Receptor Stimulation in Obesity Treatment: Orthosteric Agonists vs. Allosteric Modulators

Obesity is a substantial health and economic issue, and serotonin (5-hydroxytryptamine, 5-HT) is an important neurotransmitter system involved in the regulation of body weight. The 5-HT2C receptors (5-HT2CRs), one of 16 of the 5-HT receptor (5-HTRs) subtypes, play a significant role in food intake and body weight control. In this review, we focused on the 5-HTR agonists, such as fenfluramines, sibutramine, and lorcaserin, which act directly or indirectly at 5-HT2CRs and have been introduced into the clinic as antiobesity medications. Due to their unwanted effects, they were withdrawn from the market. The 5-HT2CR positive allosteric modulators (PAMs) can be potentially safer active drugs than 5-HT2CR agonists. However, more in vivo validation of PAMs is required to fully determine if these drugs will be effective in obesity prevention and antiobesity pharmacology treatment. Methodology strategy: This review focuses on the role of 5-HT2CR agonism in obesity treatment, such as food intake regulation and weight gain. The literature was reviewed according to the review topic. We searched the PubMed and Scopus databases and Multidisciplinary Digital Publishing Institute open-access scientific journals using the following keyword search strategy depending on the chapter phrases: (1) “5-HT2C receptor” AND “food intake”, and (2) “5-HT2C receptor” AND “obesity” AND “respective agonists”, and (3) “5-HT2C receptor” AND “PAM”. We included preclinical studies (only present the weight loss effects) and double-blind, placebo-controlled, randomized clinical trials published since the 1975s (mostly related to antiobesity treatment), and excluded the pay-walled articles. After the search process, the authors selected, carefully screened, and reviewed appropriate papers. In total, 136 articles were included in this review.

Metabolic and Molecular Response to High-Fat Diet Differs between Rats with Constitutionally High and Low Serotonin Tone

Maintaining energy balance is a complex physiological function whose dysregulation can lead to obesity and associated metabolic disorders. The bioamine serotonin (5HT) is an important regulator of energy homeostasis, with its central and peripheral pools influencing energy status in opposing ways. Using sublines of rats with constitutionally increased (high-5HT) or decreased (low-5HT) whole-body 5HT tone, we have previously shown that under standard diet constitutionally higher 5HT activity is associated with increased body weight, adiposity, and impaired glucose homeostasis. Here, we investigated the response of 5HT sublines to an obesogenic diet. Consistent with previous findings, high-5HT animals fed a standard diet had poorer metabolic health. However, in response to a high-fat diet, only low-5HT animals increased body weight and insulin resistance. They also showed more pronounced changes in blood metabolic parameters and the expression of various metabolic genes in hypothalamus and adipose tissue. On the other hand, high-5HT animals appeared to be protected from major metabolic disturbances of the obesogenic diet. The results suggest that constitutionally low 5HT activity is associated with higher susceptibility to harmful effects of a high-energy diet. High-5HT subline, which developed less adverse metabolic outcomes on hypercaloric diets, may prove useful in understanding metabolically healthy obesity in humans.

SLC6A4 DNA Methylation Levels and Serum Kynurenine/Tryptophan Ratio in Eating Disorders: A Possible Link with Psychopathological Traits?

Background: The incidence of eating disorders (EDs), serious mental and physical conditions characterized by a disturbance in eating or eating-related behaviors, has increased steadily. The present study aims to develop insights into the pathophysiology of EDs, spanning over biochemical, epigenetic, psychopathological, and clinical data. In particular, we focused our attention on the relationship between (i) DNA methylation profiles at promoter-associated CpG sites of the SCL6A4 gene, (ii) serum kynurenine/tryptophan levels and ratio (Kyn/Trp), and (iii) psychopathological traits in a cohort of ED patients. Among these, 45 patients were affected by restricting anorexia nervosa (AN0), 21 by purging AN (AN1), 21 by bulimia (BN), 31 by binge eating disorders (BED), 23 by unspecified feeding or eating disorders (UFED), and finally 14 by other specified eating disorders (OSFED) were compared to 34 healthy controls (CTRs). Results: Kyn level was higher in BED, UFED, and OSFED compared to CTRs (p ≤ 0.001). On the other hand, AN0, AN1, and BN patients showed significatively lower Kyn levels compared to the other three ED groups but were closed to CTRs. Trp was significantly higher in AN0, AN1, and BN in comparison to other ED groups. Moreover, AN1 and BN showed more relevant Trp levels than CTRs (p <0.001). BED patients showed a lower Trp as compared with CTRs (p ≤ 0.001). In addition, Kyn/Trp ratio was lower in the AN1 subtype but higher in BED, UFED, and OSFED patients than in CTRs (p ≤ 0.001). SCL6A4 DNA methylation level at CpG5 was lower in AN0 compared to BED (p = 0.021), and the CpG6 methylation was also significantly lower in AN0 in comparison to CTRs (p = 0.025). The mean methylation levels of the six CpGs analyzed were lower only in the AN0 subgroup compared to CTRs (p = 0.008). Relevant psychological trait EDI-3 subscales were correlated with biochemical and epigenetic data. Conclusions: These findings underline the complexity of psychological and pathophysiological components of EDs.

Hypermethylation of the serotonin transporter gene and paternal parenting styles in untreated anorexia nervosa patients: A pilot study

Purpose

It has been reported that serotonergic systems and parenting styles are involved in the pathogenesis of anorexia nervosa (AN). The present study made attempts to examine the DNA methylation profiles in the promoter region of serotonin transporter (5-HTT) encoding gene SLC6A4, and explore the association between the methylation level and severity of symptoms, 5-HTT linked polymorphic region (5-HTTLPR) genotypes and parenting styles in untreated Chinese Han AN patients.

Methods

Ninety-one untreated female AN patients (ANs) and eighty-seven matched healthy controls (HCs) were analyzed for DNA methylation status at CpG islands in the promoter region of SLC6A4 using MassARRY EpiTYPER, and genotypes of 5-HTTLPR using PCR-RFLP. The severity of eating disorder (ED) symptoms was evaluated by body mass index (BMI) and Questionnaire Version of the Eating Disorders Examination (EDE-Q 6.0), and part of participants were assessed parenting styles using the short Chinese Egna Minnen av Barndoms Uppfostra (s-EMBU-C).

Results

ANs had greater methylation levels at CpG26.27.28, CpG 31.32, and CpG 37 than HCs (P = 0.039, 0.042, 0.018 respectively). A positive association of methylation level at CpG26.27.28 with ED symptoms detected by EDEQ-6.0 was discovered in AN group (r = 0.216, P = 0.047). Methylation level at CpG26.27.28 was showed to be or tend to be positively correlated with the parenting styles of paternal rejection (r = 0.425, P = 0.038) and paternal overprotection (r = 0.362, P = 0.062) in ANs. No significant differences were found in SLC6A4 promoter region methylation levels among 5-HTTLPR genotypes in our samples (P > 0.05) and no interaction effect between 5-HTTLPR genotypes and parenting styles on SLC6A4 promoter region methylation was observed (P > 0.05).

Conclusions

This study suggested that hypermethylation of SLC6A4 promoter region may be implicated in the pathological mechanisms of untreated Chinese Han female ANs, which is possibly associated with poor parenting styles. This finding may provide a direction for the epigenetic and family treatments for ANs and further investigation with larger samples is warranted.

Association study of common functional genetic polymorphisms in SLC6A4 (5-HTT) and MAOA genes with obesity in portuguese children

OBJECTIVES

To investigate the association of polymorphisms in SLC6A4 and MAOA genes with obesity indices in children.

MATERIAL AND METHODS

A total of 637 Portuguese children (317 girls; 320 boys) aged 3-11 years-old were genotyped for the SLC6A4 polymorphisms, 5-HTTLPR and STin2, and for a MAOA VNTR. Polymorphisms were analysed by PCR-based methods.

RESULTS

Although non-significant (p = .089), our study revealed the Stin2 10 minor allele with a marked higher frequency in girls with overweight/obesity (0.466) in comparison with controls (0.376). Combining the two SLC6A4 polymorphisms, haplotype S/12 revealed in girls significant or nominally significant protective effects against BMI (β = -0.615; p = .009), BMI Z-score (β = -0.251; p = .006), WC (β = -1.4; p = .02) and WHtR (β = -0.008; p = .04).

CONCLUSIONS

We found some evidences for the role of SLC6A4 gene in measures of childhood obesity, mainly in girls.

Effect of serotonin on the cell viability of the bovine mammary alveolar cell-T (MAC-T) cell line

5-Hydroxytryptamine (5-HT), a monoamine, as a local regulator in the mammary gland is a chemical signal produced by the mammary epithelium cell. In cows, studies have shown that 5-HT is associated with epithelial cell apoptosis during the degenerative phase of the mammary gland. However, studies in other tissues have shown that 5-HT can effectively promote cell viability. Whether 5-HT could have an effect on mammary cell viability in dairy cows is still unknown. The purpose of this study was to determine: (1) effect of 5-HT on the viability of bovine mammary epithelial cells and its related signaling pathways, (2) interaction between prolactin (PRL) and 5-HT on the cell viability. The bovine mammary alveolar cell-T (MAC-T) were cultured with different concentrations of 5-HT for 12, 24, 48 or 72 hours, and then were assayed using cell counting kit-8, polymerase chain reaction (PCR) and immunobloting. The results suggested that 20 μM 5-HT treatment for 12 or 24 h promote cell viability, which was mainly induced by the activation of 5-HT receptor (5-HTR) 1B and 4, because the increase caused by 5-HT vanished when 5-HTR 1B and 4 was blocked by SB224289 and SB204070. And protein expression of mammalian target of rapamycin (mTOR), eukaryotic translation elongation factor 2 (eEF2), janus kinase 2 (JAK2) and signal transducer and activator of transcription 5 (STAT5) were decreased after blocking 5-HT 1B and 4 receptors. When MAC-T cells were treated with 5-HT and PRL simultaneously for 24 h, both the cell viability and the level of mTOR protein were significantly higher than that cultured with 5-HT or PRL alone. In conclusion, our study suggested that 5-HT promotes the viability of MAC-T cells by 5-HTR 1B and/or 4. Furthermore, there is a reciprocal relationship between PRL and 5-HT.

Delineating a serotonin 1B receptor circuit for appetite suppression in mice

Triptans are a class of commonly prescribed antimigraine drugs. Here, we report a previously unrecognized role for them to suppress appetite in mice. In particular, frovatriptan treatment reduces food intake and body weight in diet-induced obese mice. Moreover, the anorectic effect depends on the serotonin (5-HT) 1B receptor (Htr1b). By ablating Htr1b in four different brain regions, we demonstrate that Htr1b engages in spatiotemporally segregated neural pathways to regulate postnatal growth and food intake. Moreover, Htr1b in AgRP neurons in the arcuate nucleus of the hypothalamus (ARH) contributes to the hypophagic effects of HTR1B agonists. To further study the anorexigenic Htr1b circuit, we generated Htr1b-Cre mice. We find that ARH Htr1b neurons bidirectionally regulate food intake in vivo. Furthermore, single-nucleus RNA sequencing analyses revealed that Htr1b marks a subset of AgRP neurons. Finally, we used an intersectional approach to specifically target these neurons (Htr1bAgRP neurons). We show that they regulate food intake, in part, through a Htr1bAgRP→PVH circuit.

Dynamic transcriptome analysis of the muscles in high-fat diet-induced obese zebrafish (Danio rerio) under 5-HT treatment

Peripheral 5-hydroxytryptamine (5-HT, also called serotonin) is reportedly a potential therapeutic target in obesity-related metabolic diseases due to its regulatory role in energy homeostasis in mammals. However, information on the detailed effect of peripheral 5-HT on the energy metabolism in fishes, especially the lipid metabolism, and the underlying mechanism remains elusive. In this study, a diet-induced obesity model was developed in the zebrafish (Danio rerio), a prototypical animal model for metabolic disorders. The zebrafish were fed a high-fat diet for 8 weeks and were simultaneously injected with PBS, 0.1 mM and 10 mM 5-HT, intraperitoneally. The body weight was significantly lower in the zebrafish injected with 0.1 mM 5-HT (P < 0.05), however, there was no change in body length (P > 0.05) at the end of the 8-week treatment. The muscle tissues from the zebrafish treated with PBS and 5-HT were collected for transcriptomic analysis and the RNA-seq revealed 1134, 3713, and 2535 genes were screened out compared to the muscular DEGs among three groups. The enrichment analysis revealed DEGs to be significantly associated with multiple metabolic pathways, including ribosome, oxidative phosphorylation, proteasome, PPAR signaling pathway, and ferroptosis. Additionally, the qRT-PCR validated 12 DEGs out of which 10 genes exhibited consistent trends. Taken together, this data provided useful information on the transcriptional characteristics of the muscle tissue in the obese zebrafish exposed to 5-HT, offering important insights into the regulatory effect of peripheral 5-HT in teleosts, as well as novel approaches for preventing and treating obesity-related metabolic dysfunction.

The complex interactions among serotonin, insulin, leptin, and glycolipid metabolic parameters in human obesity

OBJECTIVE

To provide evidence to the link between serotonin (5-HT), energy metabolism, and the human obese phenotype, the present study investigated the binding and function of the platelet 5-HT transporter (SERT), in relation to circulating insulin, leptin, and glycolipid metabolic parameters.

METHODS

Seventy-four drug-free subjects were recruited on the basis of divergent body mass index (BMIs) (16.5-54.8 Kg/m2). All subjects were tested for their blood glycolipid profile together with platelet [3H]-paroxetine ([3H]-Par) binding and [3H]-5-HT reuptake measurements from April 1st to June 30th, 2019.

RESULTS

The [3H]-Par Bmax (fmol/mg proteins) was progressively reduced with increasing BMIs (P < .001), without changes in affinity. Moreover, Bmax was negatively correlated with BMI, waist/hip circumferences (W/HC), triglycerides (TD), glucose, insulin, and leptin, while positively with high-density lipoprotein (HDL) cholesterol (P < .01). The reduction of 5-HT uptake rate (Vmax, pmol/min/109 platelets) among BMI groups was not statistically significant, but Vmax negatively correlated with leptin and uptake affinity values (P < .05). Besides, [3H]-Par affinity values positively correlated with glycemia and TD, while [3H]-5-HT reuptake affinity with glycemia only (P < .05). Finally, these correlations were specific of obese subjects, while, from multiple linear-regression analysis conducted on all subjects, insulin (P = .006) resulting negatively related to Bmax independently from BMI.

CONCLUSIONS

Present findings suggest the presence of a possible alteration of insulin/5-HT/leptin axis in obesity, differentially impinging the density, function, and/or affinity of the platelet SERT, as a result of complex appetite/reward-related interactions between the brain, gut, pancreatic islets, and adipose tissue. Furthermore, they support the foremost cooperation of peptides and 5-HT in maintaining energy homeostasis.

Serotonin: an overlooked regulator of endocytosis and endosomal sorting?

Serotonin is a neurotransmitter and a hormone that is typically associated with regulating our mood. However, the serotonin transporter and receptors are expressed throughout the body, highlighting the much broader, systemic role of serotonin in regulating human physiology. A substantial body of data strongly implicates serotonin as a fundamental regulator of endocytosis and endocytic sorting. Serotonin has the potential to enhance endocytosis through three distinct mechanisms - serotonin signalling, serotonylation and insertion into the plasma membrane - although the interplay and relationship between these mechanisms has not yet been explored. Endocytosis is central to the cellular response to the extracellular environment, controlling receptor distribution on the plasma membrane to modulate signalling, neurotransmitter release and uptake, circulating protein and lipid cargo uptake, and amino acid internalisation for cell proliferation. Uncovering the range of cellular and physiological circumstances in which serotonin regulates endocytosis is of great interest for our understanding of how serotonin regulates mood, and also the fundamental understanding of endocytosis and its regulation throughout the body. This article has an associated Future Leader to Watch interview with the first author of the paper.

Serotonergic neurons translate taste detection into internal nutrient regulation

The nervous and endocrine systems coordinately monitor and regulate nutrient availability to maintain energy homeostasis. Sensory detection of food regulates internal nutrient availability in a manner that anticipates food intake, but sensory pathways that promote anticipatory physiological changes remain unclear. Here, we identify serotonergic (5-HT) neurons as critical mediators that transform gustatory detection by sensory neurons into the activation of insulin-producing cells and enteric neurons in Drosophila. One class of 5-HT neurons responds to gustatory detection of sugars, excites insulin-producing cells, and limits consumption, suggesting that they anticipate increased nutrient levels and prevent overconsumption. A second class of 5-HT neurons responds to gustatory detection of bitter compounds and activates enteric neurons to promote gastric motility, likely to stimulate digestion and increase circulating nutrients upon food rejection. These studies demonstrate that 5-HT neurons relay acute gustatory detection to divergent pathways for longer-term stabilization of circulating nutrients.

A Single Dose of Ginkgo biloba Extract Induces Gene Expression of Hypothalamic Anorexigenic Effectors in Male Rats

Previous studies have shown that Ginkgo biloba extract (GbE) reduces food intake and body mass gain and regulates proteins related to lipid metabolism in obese rats. In ovariectomized rats, GbE restored the hippocampal and hypothalamic serotonergic system activity, favoring the spontaneous feeding decrement. Considering the promising hypophagic effect of GbE, this study aimed to investigate the effect of a single acute dose on hypothalamic pathways that regulate feeding behavior in male rats. Four-month-old Wistar male rats received either a single acute oral GbE dose (500 mg/kg) or vehicle. Food intake and body mass were measured after 1, 4, 12, and 24 h. Rats were euthanized, and hypothalami were removed for mRNA quantification of anorexigenic (POMC/CART) and orexigenic (AgRP/NPY) neuropeptides, leptin/serotonin receptors (5HT1A, 5HT1B, 5HT2C), and serotonin transporters. We also investigated POMC, 5-HT1B, and 5-HT2C protein levels. A single acute GbE dose induced the hypothalamic POMC, CART, and 5-HT2C gene expression but failed to modify orexigenic effectors. No alterations in food intake, body mass, and hypothalamic protein levels were observed. In summary, the present findings demonstrate the rapid stimulation of pivotal hypothalamic anorexigenic pathways in response to a single GbE administration, reinforcing the GbE hypophagic activity. However, more studies are necessary to evaluate its potential as an appetite modulator.

Serotonergic Regulation of Hepatic Energy Metabolism

The liver is a vital organ that regulates systemic energy metabolism and many physiological functions. Nonalcoholic fatty liver disease (NAFLD) is the commonest cause of chronic liver disease and end-stage liver failure. NAFLD is primarily caused by metabolic disruption of lipid and glucose homeostasis. Serotonin (5-hydroxytryptamine [5-HT]) is a biogenic amine with several functions in both the central and peripheral systems. 5-HT functions as a neurotransmitter in the brain and a hormone in peripheral tissues to regulate systemic energy homeostasis. Several recent studies have proposed various roles of 5-HT in hepatic metabolism and inflammation using tissue-specific knockout mice and 5-HT-receptor agonists/antagonists. This review compiles the most recent research on the relationship between 5-HT and hepatic metabolism, and the role of 5-HT signaling as a potential therapeutic target in NAFLD.

Lorcaserin Inhibit Glucose-Stimulated Insulin Secretion and Calcium Influx in Murine Pancreatic Islets

Lorcaserin is a serotonergic agonist specific to the 5-hydroxytryptamine 2c receptor (5-HT_2CR) that is FDA approved for the long-term management of obesity with or without at least one weight-related comorbidity. Lorcaserin can restrain patients' appetite and improve insulin sensitivity and hyperinsulinemia mainly through activating 5-HT_2CR in the hypothalamus. It is known that the mCPP, a kind of 5-HT_2CR agonist, decreases plasma insulin concentration in mice and previous research in our laboratory found that mCPP inhibited glucose-stimulated insulin secretion (GSIS) by activating 5-HT_2CR on the β cells. However, the effect of lorcaserin on GSIS of pancreatic β cell has not been studied so far. The present study found that 5-HT_2CR was expressed in both mouse pancreatic β cells and β-cell-derived MIN6 cells. Dose-dependent activation of 5-HT_2CR by lorcaserin suppressed GSIS and SB242084 or knockdown of 5-HT_2CR abolished lorcaserin's effect in vitro. Additionally, lorcaserin also suppressed GSIS in high-fat diet (HFD)-fed mice in dose-dependent manner. Lorcaserin did not change insulin synthesis ATP content, but lorcaserin decrease cytosolic free calcium level [(Ca²⁺)i] in MIN6 cells stimulated with glucose and also inhibit insulin secretion and (Ca²⁺)i in MIN6 treated with potassium chloride. Furthermore, stimulation with the L-type channel agonist, Bay K8644 did not restore GSIS in MIN6 exposed to lorcaserin. Lorcaserin inhibits the cAMP generation of MIN6 cells and pretreatment with the Gα i/o inhibitor pertussis toxin (PTX), abolished lorcaserin-induced suppression of GSIS in β cells, while membrane-permeable cAMP analogue db-cAMP had same effect as PTX. These date indicated lorcaserin coupled to PTX-sensitive Gα i/o proteins in β cells reduced intracellular cAMP level and Ca²⁺ influx, thereby causing GSIS dysfunction of β cell. These results highlight a novel signaling mechanism of lorcaserin and provide valuable insights into the further investigation of 5-HT_2CR functions in β-cell biology and it also provides guidance for the clinical application of lorcaserin.

Gαi/o-coupled Htr2c in the paraventricular nucleus of the hypothalamus antagonizes the anorectic effect of serotonin agents

The anorexigenic effect of serotonergic compounds has largely been attributed to activation of serotonin 2C receptors (Htr2cs). Using mouse genetic models in which Htr2c can be selectively deleted or restored (in Htr2c-null mice), we investigate the role of Htr2c in forebrain Sim1 neurons. Unexpectedly, we find that Htr2c acts in these neurons to promote food intake and counteract the anorectic effect of serotonergic appetite suppressants. Furthermore, Htr2c marks a subset of Sim1 neurons in the paraventricular nucleus of the hypothalamus (PVH). Chemogenetic activation of these neurons in adult mice suppresses hunger, whereas their silencing promotes feeding. In support of an orexigenic role of PVH Htr2c, whole-cell patch-clamp experiments demonstrate that activation of Htr2c inhibits PVH neurons. Intriguingly, this inhibition is due to Gα_i/o-dependent activation of ATP-sensitive K⁺ conductance, a mechanism of action not identified previously in the mammalian nervous system.

Yoo et al. show that Htr2c, the target of a former weight loss drug, can inhibit and promote food intake by coupling with distinct intracellular signaling events in different hypothalamic neurons. These findings may help explain the rather modest anti-obesity effects of Htr2c agonists.

Tapping into 5-HT3 Receptors to Modify Metabolic and Immune Responses

5-hydroxytryptamine type 3 (5-HT₃) receptors are ligand gated ion channels, which clearly distinguish their mode of action from the other G-protein coupled 5-HT or serotonin receptors. 5-HT₃ receptors are well established targets for emesis and gastrointestinal mobility and are used as adjunct targets in treating schizophrenia. However, the distribution of these receptors is wider than the nervous system and there is potential that these additional sites can be targeted to modulate inflammatory and/or metabolic conditions. Recent progress in structural biology and pharmacology of 5-HT₃ receptors have provided profound insights into mechanisms of their action. These advances, combined with insights into clinical relevance of mutations in genes encoding 5-HT₃ subunits and increasing understanding of their implications in patient's predisposition to diseases and response to the treatment, open new avenues for personalized precision medicine. In this review, we recap on the current status of 5-HT₃ receptor-based therapies using a biochemical and physiological perspective. We assess the potential for targeting 5-HT₃ receptors in conditions involving metabolic or inflammatory disorders based on recent findings, underscoring the challenges and limitations of this approach.

The dorsal raphe nucleus in the control of energy balance

Energy balance is orchestrated by an extended network of highly interconnected nuclei across the central nervous system. While much is known about the hypothalamic circuits regulating energy homeostasis, the 'extra-hypothalamic' circuits involved are relatively poorly understood. In this review, we focus on the brainstem's dorsal raphe nucleus (DRN), integrating decades of research linking this structure to the physiologic and behavioral responses that maintain proper energy stores. DRN neurons sense and respond to interoceptive and exteroceptive cues related to energy imbalance and in turn induce appropriate alterations in energy intake and expenditure. The DRN is also molecularly differentiable, with different populations playing distinct and often opposing roles in controlling energy balance. These populations are integrated into the extended circuit known to regulate energy balance. Overall, this review summarizes the key evidence demonstrating an important role for the DRN in regulating energy balance.

Serotonin system genes contribute to the susceptibility to obesity in Black adolescents

OBJECTIVE

The importance of the central and peripheral serotonin systems in regulating energy balance and obesity development has been highlighted in animal models. Yet, the role of both serotonin systems has not been systematically assessed in humans. The purpose of this study was to investigate the association of genes within both serotonin systems with obesity outcomes in black adolescents.

METHODS

African-American adolescents (n = 1052) whose mothers participated the Memphis New Mother's Study were assessed. In total, 110 polymorphisms mapped to 10 serotonin genes were examined for their associations with standardized body mass index (BMI-z) scores and waist circumferences using generalized estimating equation models.

RESULTS

Over 39% of adolescents were overweight or had obesity. Three single nucleotide polymorphisms (SNPs) within TPH2, HTR3B, and SLC6A4, were significantly associated with BMI-z scores (p < 1.7 × 10-3). Two SNPs in TPH2 were nominally associated with waist circumferences. One SNP in HTR2C was associated with BMI-z scores (p = 0.001) and waist circumferences (p = 0.005) only in girls. Tissue-specific expression indicates that three identified genes are predominantly expressed in the brain.

CONCLUSION

The central serotonin system may play a key role in obesity development in black adolescents. Future studies are warranted to explore additional serotonin system genes and their potential obesogenic mechanisms in humans.

The Mechanism of Secretion and Metabolism of Gut-Derived 5-Hydroxytryptamine

Serotonin, also known as 5-hydroxytryptamine (5-HT), is a metabolite of tryptophan and is reported to modulate the development and neurogenesis of the enteric nervous system, gut motility, secretion, inflammation, sensation, and epithelial development. Approximately 95% of 5-HT in the body is synthesized and secreted by enterochromaffin (EC) cells, the most common type of neuroendocrine cells in the gastrointestinal (GI) tract, through sensing signals from the intestinal lumen and the circulatory system. Gut microbiota, nutrients, and hormones are the main factors that play a vital role in regulating 5-HT secretion by EC cells. Apart from being an important neurotransmitter and a paracrine signaling molecule in the gut, gut-derived 5-HT was also shown to exert other biological functions (in autism and depression) far beyond the gut. Moreover, studies conducted on the regulation of 5-HT in the immune system demonstrated that 5-HT exerts anti-inflammatory and proinflammatory effects on the gut by binding to different receptors under intestinal inflammatory conditions. Understanding the regulatory mechanisms through which 5-HT participates in cell metabolism and physiology can provide potential therapeutic strategies for treating intestinal diseases. Herein, we review recent evidence to recapitulate the mechanisms of synthesis, secretion, regulation, and biofunction of 5-HT to improve the nutrition and health of humans.

Chronic heat stress delays immune system development and alters serotonin signaling in pre-weaned dairy calves

Exposure to heat stress can alter the development and immune system function in dairy calves. Serotonin is an immunomodulatory biogenic amine that functions as a neurotransmitter and as a stress-response mediator. Our objectives were to characterize the patterns of serum serotonin concentrations and the pattern of serotonin-related genes expressed by immune cells of calves exposed to chronic heat stress or heat stress abatement during early life, and to explore whether these might relate to immune system development. Dairy calves were exposed to chronic heat stress (HS; n = 6) or heat stress abatement (cooling, CL; n = 6) across the prenatal (late gestation, last 46 d) and postnatal (from birth to weaning, 56 d) developmental windows. Blood samples were collected to harvest serum (weekly, from d 1 to 49), to isolate of circulating leukocyte mRNA (at 1, 21 and 42 d of age) and characterize immune cell populations by flow cytometry (at 21 and 47 d of age). Calves exposed to chronic heat stress pre- and postnatally had lower red blood cell counts and lower circulating serotonin, immunoglobulin G, and B-lymphocytes compared to CL calves. Circulating blood leukocyte mRNA expression of serotonin receptors -1A, -1F, -4 and -5 was greater, while heat shock protein 70 and immune-related genes (i.e., TBX21, TLR4, and TGFβ) were lower in HS relative to CL calves. Peripheral blood leukocytes from all calves secreted serotonin and interleukin-6 after in-vitro lipopolysaccharide stimulation. However, the HS calves produced more serotonin and less interleukin-6 than CL calves when activated in-vitro. Together, our data suggest that providing heat stress abatement to dairy calves across prenatal and postnatal developmental windows might modulate the serotonin synthesis pathway in ways that may benefit humoral immunity against microbial pathogens.

Metabolic Disturbances in Rat Sublines with Constitutionally Altered Serotonin Homeostasis

Central and peripheral serotonin (5HT) have opposing functions in the regulation of energy homeostasis. Both increasing 5HT signaling in the brain and decreasing 5HT signaling in the periphery have been proposed as potential treatments for obesity. This study investigates the relationship between constitutionally high or low 5HT activity and systemic net energy balance. Two sublines of rats with high and low whole-body 5HT tone, obtained by selective breeding for platelet 5HT parameters, were examined for fat accumulation in different white adipose tissue (WAT) depots, glucose/insulin tolerance, blood metabolic parameters, and expression of various metabolic genes. High-5HT animals, unlike their low-5HT counterparts, developed widespread intra-abdominal obesity associated with glucose and insulin intolerance, which worsened with age. They also had elevated blood glucose and lipid parameters but showed no significant changes in circulating leptin, resistin, and adipsin levels. Surprisingly, adiponectin levels were increased in plasma but reduced in the WAT of high-5HT rats. A limited number of metabolic genes belonging to different functional classes showed differential expression in WAT of high-5HT compared to low-5HT rats. Overall, a constitutive increase in 5HT tone is associated with a positive energy balance acting through subtle dysregulation of a broad spectrum of metabolic pathways.

Histological and transcriptomic analysis of adipose and muscle of dairy calves supplemented with 5-hydroxytryptophan

In mammals, peripheral serotonin is involved in regulating energy balance. Herein, we characterized the transcriptomic profile and microstructure of adipose and muscle in pre-weaned calves with increased circulating serotonin. Holstein bull calves (21 ± 2 days old) were fed milk replacer supplemented with saline (CON, 8 mL/day n = 4) or 5-hydroxytryptophan (5-HTP, 90 mg/day, n = 4) for 10 consecutive days. Calves were euthanized on d10 to harvest adipose and muscle for RNA-Sequencing and histological analyses. Twenty-two genes were differentially expressed in adipose, and 33 in muscle. Notably, Interferon gamma inducible protein-47 was highly expressed and upregulated in muscle and adipose (avg. log FC = 6.5). Enriched pathways in adipose tissue revealed serotonin’s participation in lipid metabolism and PPAR signaling. In muscle, enriched pathways were related to histone acetyltransferase binding, Jak-STAT signaling, PI3K-Akt signaling and cell proliferation. Supplementation of 5-HTP increased cell proliferation and total cell number in adipose and muscle. Adipocyte surface area was smaller and muscle fiber area was not different in the 5-HTP group. Manipulating the serotonin pathway, through oral supplementation of 5-HTP, influences signaling pathways and cellular processes in adipose and muscle related to endocrine and metabolic functions which might translate into improvements in calf growth and development.

Association of 5-HTR2A -1438A/G polymorphism with anorexia nervosa and bulimia nervosa: A meta-analysis

Although a number of studies have been conducted on the association of -1438A/G polymorphism in serotonin 2A receptor (5-HTR2A) gene with anorexia nervosa (AN) and bulimia nervosa (BN), the results remained inconsistent. We thus performed a meta-analysis to clarify the effects of -1438A/G polymorphism on the risk of AN and BN. PubMed, Embase, the Cochrane Library, CNKI, Weipu and Wanfang databases were searched for eligible studies. Pooled odds ratio (OR) and 95 % confidence interval (CI) were calculated to estimate the strength of the association. Subgroup analysis was also performed by ethnicity. In total, 17 studies were included for the meta-analysis, of which 15 studies containing 2028 cases and 2725 controls were used for AN analysis and 7 studies containing 505 cases and 1129 controls for BN analysis. The results showed -1438A/G polymorphism was significantly associated with the risk of AN in four genetic models (allele model, A vs. G: OR = 1.31, 95 % CI = 1.11-1.64, P = 0.003; recessive model, AA vs. GA + GG: OR = 1.69, 95 % CI = 1.28-2.23, P = 0.000; dominant model, AA + GA vs. GG: OR = 1.35, 95 % CI = 1.02-1.79, P = 0.037; co-dominant model, AA vs. GG: OR = 1.94, 95 % CI = 1.29-2.92, P = 0.002) in Caucasians, but not in Asians. We failed to observe a significant association between -1438A/G polymorphism and the risk of BN either in overall or in Caucasian population. The present meta-analysis indicated that A allele and AA genotype of 5-HTR2A -1438A/G polymorphism may contribute to higher risk of AN, especially in Caucasians. However, this polymorphism was not associated with the susceptibility to BN.

Effect of 5-HT2C receptor gene polymorphism (HTR2C-759C/T) on metabolic adverse effects in Thai psychiatric patients treated with risperidone

BACKGROUND

The use of Atypical antipsychotics (AAPs) is related to metabolic disturbances, which put psychiatric patients at risk for cardiovascular morbidity and mortality. Evidence is emerging of genetic risk factors. The HTR2C gene is an essential candidate in pharmacogenetic studies of antipsychotic-induced metabolic effects. Nevertheless, there were inconsistent results among studies.

OBJECTIVE

To investigate the relationship between -759C/T, functional polymorphism of the HTR2C gene and metabolic adverse effects in Thai psychiatric patients treated with risperidone monotherapy.

METHOD

In this cross-sectional study, 108 psychiatric patients treated with risperidone monotherapy for ≥3 months were recruited. Anthropometric measurements and laboratory tests were obtained upon enrollment and history of treatment was reviewed from medical records. Weight gain was defined as an increase ≥7% of baseline weight. Metabolic syndrome was evaluated according to the 2005 International Diabetes Federation (IDF) Asia criteria. The -759C/T, polymorphism was genotyped. The associations between -759C/T polymorphism and metabolic side effects were analyzed. Multiple logistic regression was used for determining potential confounders.

RESULTS

Neither weight gain nor metabolic syndrome was significantly associated with -759C/T allelic and genotype variants of HTR2C. However, T allele of -759C/T polymorphism significantly associated with the hypertension. This association was not affected by possible confounding factors such as gender, risperidone dose, duration of treatment and family history of hypertension.

CONCLUSION

Our findings suggest that psychiatric patients with T allele of -759C/T polymorphism may be at higher risk for hypertension. Further study with prospective design with larger patient groups are needed.

Genetic Polymorphisms of 5-HT Receptors and Antipsychotic-Induced Metabolic Dysfunction in Patients with Schizophrenia

BACKGROUND

Antipsychotic-induced metabolic syndrome (MetS) is a multifactorial disease with a genetic predisposition. Serotonin and its receptors are involved in antipsychotic-drug-induced metabolic disorders. The present study investigated the association of nine polymorphisms in the four 5-hydroxytryptamine receptor (HTR) genes HTR1A, HTR2A, HTR3A, and HTR2C and the gene encoding for the serotonin transporter SLC6A4 with MetS in patients with schizophrenia.

METHODS

A set of nine single-nucleotide polymorphisms of genes of the serotonergic system was investigated in a population of 475 patients from several Siberian regions (Russia) with a clinical diagnosis of schizophrenia. Genotyping was performed and the results were analyzed using chi-square tests.

RESULTS

Polymorphic variant rs521018 (HTR2C) was associated with higher body mass index in patients receiving long-term antipsychotic therapy, but not with drug-induced metabolic syndrome. Rs1150226 (HTR3A) was also associated but did not meet Hardy-Weinberg equilibrium.

CONCLUSIONS

Our results indicate that allelic variants of HTR2C genes may have consequences on metabolic parameters. MetS may have too complex a mechanistic background to be studied without dissecting the syndrome into its individual (causal) components.

Genetic Parameters and Genome-Wide Association Studies of Quality Traits Characterised Using Imaging Technologies in Rainbow Trout, Oncorhynchus mykiss

One of the top priorities of the aquaculture industry is the genetic improvement of economically important traits in fish, such as those related to processing and quality. However, the accuracy of genetic evaluations has been hindered by a lack of data on such traits from a sufficiently large population of animals. The objectives of this study were thus threefold: (i) to estimate genetic parameters of growth-, yield-, and quality-related traits in rainbow trout (Oncorhynchus mykiss) using three different phenotyping technologies [invasive and non-invasive: microwave-based, digital image analysis, and magnetic resonance imaging (MRI)], (ii) to detect quantitative trait loci (QTLs) associated with these traits, and (iii) to identify candidate genes present within these QTL regions. Our study collected data from 1,379 fish on growth, yield-related traits (body weight, condition coefficient, head yield, carcass yield, headless gutted carcass yield), and quality-related traits (total fat, percentage of fat in subcutaneous adipose tissue, percentage of fat in flesh, flesh colour); genotypic data were then obtained for all fish using the 57K SNP Axiom® Trout Genotyping array. Heritability estimates for most of the 14 traits examined were moderate to strong, varying from 0.12 to 0.67. Most traits were clearly polygenic, but our genome-wide association studies (GWASs) identified two genomic regions on chromosome 8 that explained up to 10% of the genetic variance (cumulative effects of two QTLs) for several traits (weight, condition coefficient, subcutaneous and total fat content, carcass and headless gutted carcass yields). For flesh colour traits, six QTLs explained 1-4% of the genetic variance. Within these regions, we identified several genes (htr1, gnpat, ephx1, bcmo1, and cyp2x) that have been implicated in adipogenesis or carotenoid metabolism, and thus represent good candidates for further functional validation. Finally, of the three techniques used for phenotyping, MRI demonstrated particular promise for measurements of fat content and distribution, while the digital image analysis-based approach was very useful in quantifying colour-related traits. This work provides new insights that may aid the development of commercial breeding programmes in rainbow trout, specifically with regard to the genetic improvement of yield and flesh-quality traits as well as the use of invasive and/or non-invasive technologies to predict such traits.

Distinct Pattern of NPY in Gastro-Entero-Pancreatic System of Goat Kids Fed with a New Standardized Red Orange and Lemon Extract (RLE)

Simple Summary

In the last decades the European ban towards antibiotics resulted in an increase of the number of studies on the effects of natural feed additives, that can enhance the health of farm animals intended for human consumption. Polyphenols such as flavanones and anthocyanins (responsible of the red, purple or blue colors) are bioactive compounds found in fruits and vegetables. Polyphenols possess multiple pharmacological characteristics, like antioxidant, anti-inflammatory and immunostimulant properties. Although many of the biological effects of polyphenols are known, only a limited number of studies has been focused on the effects of their supplementation in ruminant diet. Therefore, we evaluated the effect of a diet supplemented with a standardized powder extract, red (blood) orange and lemon extract (RLE), rich in flavanones, anthocyanins and other polyphenols on the neuropeptide Y (NPY) distribution in the gastro–entero–pancreatic system of goat kids. In mammals, NPY occurs in both the central and peripheral nervous systems and it is involved in the control of different physiological processes, including food intake regulation. For the first time, we document that NPY is widely distributed in the abomasum, duodenum and pancreas of goat kids and that significantly increases in the abomasum and pancreas of RLE supplemented feed animals.

Abstract

The use of natural compounds as feed additive is also increasing in farm animals, thanks to the beneficial effect on both animals and consumers health. Here, we questioned whether natural extracts, such as red orange and lemon extract (RLE) rich in flavanones, anthocyanins, and other polyphenols, used as feed additives could display an effect on the neuropeptide Y (NPY) in the gastro–entero–pancreatic tract of goat kids. NPY is one of the most abundant neuropeptides in mammals, known for its orexigenic role although it is involved in many central and peripheral functions. We carried out immunohistochemical analyses on samples of abomasum, duodenum and pancreas collected from two experimental groups: one fed with standard diet and one with standard diet + RLE. For the first time we document NPY distribution in the abomasum, duodenum and pancreas of goats and observe the highest number of NPY positive cells in neuroendocrine cells of duodenum. Remarkably, upon RLE feed supplementation, NPY immunoreactive cells increased significantly in abomasal epithelium and pancreatic islets but not in duodenum, likely due to pH variation of abomasum and duodenum. Our observations represent a baseline for future studies on the interaction between neuropeptides and polyphenols, used as feed additive.

Peripheral serotonin regulates glucose and insulin metabolism in Holstein dairy calves

Peripheral serotonin regulates energy metabolism in several mammalian species, however, the potential contribution of serotonergic mechanisms as metabolic and endocrine regulators in growing dairy calves remain unexplored. Objectives were to characterize the role of serotonin in glucose and insulin metabolism in dairy calves with increased serotonin bioavailability. Milk replacer was supplemented with saline, 5-hydroxytryptophan (90 mg/d), or fluoxetine (40 mg/d) for 10-d (n = 8/treatment). Blood was collected daily during supplementation and on days 2, 7, and 14 during withdrawal. Calves were euthanized after 10-d supplementation or 14-d withdrawal periods to harvest liver and pancreas tissue. 5-hydroxytryptophan increased circulating insulin concentrations during the supplementation period, whereas both treatments increased circulating glucose concentration during the withdrawal period. The liver and pancreas of preweaned calves express serotonin factors (ie, TPH1, SERT, and cell surface receptors), indicating their ability to synthesize, uptake, and respond to serotonin. Supplementation of 5-hydroxytryptophan increased hepatic and pancreatic serotonin concentrations. After the withdrawal period, fluoxetine cleared from the pancreas but not liver tissue. Supplementation of 5-hydroxytryptophan upregulated hepatic mRNA expression of serotonin receptors (ie, 5-HTR1B, -1D, -2A, and -2B), and downregulated pancreatic 5-HTR1F mRNA and insulin-related proteins (ie, Akt and pAkt). Fluoxetine-supplemented calves had fewer pancreatic islets per microscopic field with reduced insulin intensity, whereas 5-hydroxytryptophan supplemented calves had increased islet number and area with greater insulin and serotonin and less glucagon intensities. After the 14-d withdrawal of 5-hydroxytryptophan, hepatic mRNA expression of glycolytic and gluconeogenic enzymes were simultaneously downregulated. Improving serotonin bioavailability could serve as a potent regulator of endocrine and metabolic processes in dairy calves.

Understanding the Heterogeneity of Obesity and the Relationship to the Brain-Gut Axis

Obesity is best understood as a multifactorial metabolic imbalances disorder. In a cross-sectional study, we aimed to explore sociodemographic and dietary determinants of obesity in relation to brain-gut homeostasis among overweight and obese individuals. Multivariate logistic regression models were used to examine obesity and its association with sociodemographic and dietary factors. Biological variables examined included the gut microbiome, fecal amino acid metabolites and brain structural volumes. Among 130 participants, there were higher odds of obesity if individuals were Hispanic (adjusted odds ratio (aOR) 1.56, p = 0.014). Compared to non-Hispanics, Hispanics differed in gut microbial composition (p = 0.046) with lower microbial species richness (Chao1) (p = 0.032) and evenness (Shannon) (p = 0.0029). Fourteen of the twenty fecal amino acids including branch-chain- and aromatic- amino acids were increased among Hispanics (q < 0.05). Brain structural volumes in reward regions were decreased in Hispanics (pallidum, q = 0.036; brainstem, q = 0.011). Correlation patterns suggest complex brain-gut interactions differ by Hispanic ethnicity. In conclusion, Hispanics expressed a unique brain-gut microbial signature, which was associated with obesity despite sociodemographic and dietary differences. Addressing ethnic disparities guided by biologic phenotypes may unlock novel understanding of obesity heterogeneity and treatment strategies.

Feeding, hunger, satiety and serotonin in invertebrates

The serotonergic modulation of feeding behaviour has been intensively studied in several invertebrate groups, including Arthropoda, Annelida, Nematoda and Mollusca. These studies offer comparative information on feeding regulation across divergent phyla and also provide general insights into the neural control of feeding. Specifically, model invertebrates are ideal for parsing feeding behaviour into component parts and examining the underlying mechanisms at the levels of biochemical pathways, single cells and identified neural circuitry. Research has found that serotonin is crucial during certain phases of feeding behaviour, especially movements directly underlying food intake, but inessential during other phases. In addition, while the serotonin system can be manipulated systemically in many animals, invertebrate model organisms also allow manipulations at the level of single cells and molecules, revealing limited and precise serotonergic actions. The latter highlight the importance of local versus global modulatory effects of serotonin, a potentially significant consideration for drug and pesticide design.

5-HT2AR and BDNF gene variants in eating disorders susceptibility

Evidence from family and twin studies points to a genetic contribution to the etiology of eating disorders (EDs), confirmed by the association of several single nucleotide polymorphisms (SNPs) with this group of disorders. Previous reports have suggested that the serotonin receptor (5-HT2AR) and brain-derived neurotrophic factor (BDNF) genes could be both involved in EDs susceptibility. In order to provide further evidence about such association, we focused our attention on two SNPs located in these genes carrying out a genetic association study on a large Italian cohort composed of 556 ED patients and 355 controls (CTRs). Obtained results confirm the presence of an association between 5-HT2AR and BDNF genes and the susceptibility to EDs.

Constitutionally High Serotonin Tone Favors Obesity: Study on Rat Sublines With Altered Serotonin Homeostasis

Central and peripheral pools of biogenic monoamine serotonin (5-hydroxytryptamine [5HT]) exert opposite effects on the body weight regulation: increase in brain 5HT activity is expected to decrease body weight, whereas increase in peripheral 5HT activity will increase body weight and adiposity. In a genetic model of rats with constitutionally high- or low-5HT homeostasis (hyperserotonergic/hyposerotonergic rats), we have studied how individual differences in endogenous 5HT tone modulate net energy balance of the organism. The high-5HT and low-5HT sublines of the model were developed by selective breeding toward extreme platelet activities of 5HT transporter, a key molecule determining 5HT bioavailability/activity. In animals from high-5HT and low-5HT sublines, we assessed physiological characteristics associated with body weight homeostasis and expression profile of a large scale of body weight–regulating genes in hypothalamus, a major brain region controlling energy balance. Results showed that under standard chow diet animals from the high-5HT subline, as compared to low-5HT animals, have lifelong increased body weight (by 12%), higher absolute daily food intake (by 9%), and different pattern of fat distribution (larger amount of white adipose tissue and lower amount of brown adipose tissue). A large number of body weight–regulating hypothalamic genes were analyzed for their mRNA expression: 24 genes by reverse transcription–quantitative polymerase chain reaction (n = 9–10 rats/subline) including neuropeptides and their receptors, growth factors, transcriptional factors, and receptors for peripheral signals, and a total of 84 genes of various classes by polymerase chain reaction array (pools of six rats/subline). Only few genes showed significant differences in mRNA expression levels between 5HT sublines (e.g. neuropeptide Y receptor, fibroblast growth factor 10), but high-5HT animals displayed a clear trend to upregulation of mRNAs for a number of orexigenic signaling peptides, their receptors, and other molecules with orexigenic activity. Receptors for peripheral signals (leptin, insulin) and molecules in their downstream signaling were not altered, indicating no changes in central insulin/leptin resistance. At the protein level, there were no differences in the content of hypothalamic leptin receptor between 5HT sublines, but significant sex and age effects were observed. Results show that higher constitutive/individual 5HT tone favors higher body weight and adiposity probably due to concurrent upregulation of several hypothalamic orexigenic pathways.

Transcriptome Modifications in the Porcine Intramuscular Adipocytes during Differentiation and Exogenous Stimulation with TNF-α and Serotonin

Adipocytes are dynamic cells that have critical functions to maintain body energy homeostasis. Adipocyte physiology is affected by the adipogenic differentiation, cell program, as well as by the exogenous stimulation of biochemical factors, such as serotonin and TNF-α. In this work, we investigated the global transcriptome modifications when porcine intramuscular preadipocyte (PIP) was differentiated into porcine mature adipocyte (pMA). Moreover, we studied transcriptome changes in pMA after stimulation with serotonin or TNF-α by using a microarray approach. Transcriptome analysis revealed that the expression of 270, 261, and 249 genes were modified after differentiation, or after serotonin and TNF-α stimulation, respectively. Expression changes in APP, HNF4A, ESR1, EGR1, SRC, HNF1A, FN1, ALB, STAT3, CBL, CEBPB, AR, FOS, CFTR, PAN2, PTPN6, VDR, PPARG, STAT5A and NCOA3 genes which are enriched in the ‘PPAR signaling’ and ‘insulin resistance’ pathways were found in adipocytes during the differentiation process. Dose-dependent serotonin stimulation resulted in a decreased fat accumulation in pMAs. Serotonin-induced differentially expressed genes in pMAs were found to be involved in the significant enrichment of ′GPCR ligand-binding′, ‘cell chemotaxis’, ‘blood coagulation and complement’, ‘metabolism of lipid and lipoproteins’, ‘regulation of lipid metabolism by PPARA’, and ‘lipid digestion, mobilization and transport’ pathways. TNF-α stimulation also resulted in transcriptome modifications linked with proinflammatory responses in the pMA of intramuscular origin. Our results provide a landscape of transcriptome modifications and their linked-biological pathways in response to adipogenesis, and exogenous stimulation of serotonin- and TNF-α to the pMA of intramuscular origin.

Dynamics of Platelet Behaviors as Defenders and Guardians: Accumulations in Liver, Lung, and Spleen in Mice

Systemic platelet behaviors in experimental animals are often assessed by infusion of isotope-labeled platelets and measuring them under anesthesia. However, such procedures alter, therefore may not reveal, real-life platelet behaviors. 5-Hydroxytryptamine (5HT or serotonin) is present within limited cell-types, including platelets. In our studies, by measuring 5HT as a platelet-marker in non-anesthetized mice, we identified stimulation- and time-dependent accumulations in liver, lung, and/or spleen as important systemic platelet behaviors. For example, intravenous, intraperitoneal, or intragingival injection of lipopolysaccharide (LPS, a cell-wall component of Gram-negative bacteria), interleukin (IL)-1, or tumor necrosis factor (TNF)-α induced hepatic platelet accumulation (HPA) and platelet translocation into the sinusoidal and perisinusoidal spaces or hepatocytes themselves. These events occurred "within a few hours" of the injection, caused hypoglycemia, and exhibited protective or causal effects on hepatitis. Intravenous injection of larger doses of LPS into normal mice, or intravenous antigen-challenge to sensitized mice, induced pulmonary platelet accumulation (PPA), as well as HPA. These reactions occurred "within a few min" of the LPS injection or antigen challenge and resulted in shock. Intravenous injection of 5HT or a catecholamine induced a rapid PPA "within 6 s." Intravenous LPS injection, within a minute, increased the pulmonary catecholamines that mediate the LPS-induced PPA. Macrophage-depletion from liver and spleen induced "day-scale" splenic platelet accumulation, suggesting the spleen is involved in clearing senescent platelets. These findings indicate the usefulness of 5HT as a marker of platelet behaviors, and provide a basis for a discussion of the roles of platelets as both "defenders" and "guardians."

MicroRNA-188 regulates aging-associated metabolic phenotype

With the increasing aging population, aging‐associated diseases are becoming epidemic worldwide, including aging‐associated metabolic dysfunction. However, the underlying mechanisms are poorly understood. In the present study, we aimed to investigate the role of microRNA miR‐188 in the aging‐associated metabolic phenotype. The results showed that the expression of miR‐188 increased gradually in brown adipose tissue (BAT) and inguinal white adipose tissue (iWAT) of mice during aging. MiR‐188 knockout mice were resistant to the aging‐associated metabolic phenotype and had higher energy expenditure. Meanwhile, adipose tissue‐specific miR‐188 transgenic mice displayed the opposite phenotype. Mechanistically, we identified the thermogenic‐related gene Prdm16 (encoding PR domain containing 16) as the direct target of miR‐188. Notably, inhibition of miR‐188 expression in BAT and iWAT of aged mice by tail vein injection of antagomiR‐188 ameliorated aging‐associated metabolic dysfunction significantly. Taken together, our findings suggested that miR‐188 plays an important role in the regulation of the aging‐associated metabolic phenotype, and targeting miR‐188 could be an effective strategy to prevent aging‐associated metabolic dysfunction.

Illicit Upregulation of Serotonin Signaling Pathway in Adrenals of Patients With High Plasma or Intra-Adrenal ACTH Levels

CONTEXT

In the human adrenal, serotonin (5-HT), released by mast cells stimulates corticosteroid secretion through activation of type 4 serotonin receptors (5-HT4R). In primary pigmented nodular adrenocortical disease cells, activation of the cAMP/protein kinase A (PKA) pathway by PRKAR1A mutations triggers upregulation of the 5-HT synthesizing enzyme tryptophan hydroxylase (TPH) and the 5-HT4, 5-HT6, and 5-HT7 receptors. Because ACTH stimulates cortisol secretion through activation of PKA, adrenocortical tissues exposed to sustained stimulation by ACTH may harbor increased expression of TPH and 5-HT4/6/7 receptors.

OBJECTIVE

To investigate the effects of long-term ACTH stimulation on the serotonergic pathway in adrenals of patients with high plasma or intra-adrenal ACTH levels.

METHODS

Adrenal tissues were obtained from patients with Cushing disease, ectopic secretion of ACTH [paraneoplastic Cushing syndrome; (paraCS)], 21-hydroxylase deficiency (21-OHD), primary bilateral macronodular adrenal hyperplasia with intra-adrenal ACTH presence, or cortisol-producing adenomas. TPH and 5-HT4/6/7 receptor expression was investigated using RT-PCR and immunochemistry in comparison with normal adrenals. Primary cultured adrenocortical cells originating from a patient with paraCS were incubated with 5-HT and 5-HTR agonists/antagonists.

RESULTS

TPH and/or 5-HT4/6/7 receptors were overexpressed in the different types of tissues. In paraCS cultured cells, the cortisol response to 5-HT was exaggerated compared with normal adrenal cells and the stimulatory action of 5-HT was reduced by 5-HT4R antagonist.

CONCLUSION

Our results indicate that prolonged activation of the cAMP/PKA pathway by ACTH induces an aberrant serotonergic stimulatory loop in the adrenal cortex that likely participates in the pathogenesis of corticosteroid hypersecretion.

The BBSome in POMC and AgRP Neurons Is Necessary for Body Weight Regulation and Sorting of Metabolic Receptors

The BBSome, a complex of eight Bardet-Biedl syndrome (BBS) proteins involved in cilia function, has emerged as an important regulator of energy balance, but the underlying cellular and molecular mechanisms are not fully understood. Here, we show that the control of energy homeostasis by the anorexigenic proopiomelanocortin (POMC) neurons and orexigenic agouti-related peptide (AgRP) neurons require intact BBSome. Targeted disruption of the BBSome by Bbs1 gene deletion in POMC or AgRP neurons increases body weight and adiposity. We demonstrate that obesity in mice lacking the Bbs1 gene in POMC neurons is associated with hyperphagia. Mechanistically, we present evidence implicating the BBSome in the trafficking of G protein-coupled neuropeptide Y Y2 receptor (NPY₂R) and serotonin 5-hydroxytryptamine (HT)_2C receptor (5-HT_2CR) to cilia and plasma membrane, respectively. Consistent with this, loss of the BBSome reduced cell surface expression of the 5-HT_2CR, interfered with serotonin-evoked increase in intracellular calcium and membrane potential, and blunted the anorectic and weight-reducing responses evoked by the 5-HT_2cR agonist, lorcaserin. Finally, we show that disruption of the BBSome causes the 5-HT_2CR to be stalled in the late endosome. Our results demonstrate the significance of the hypothalamic BBSome for the control of energy balance through regulation of trafficking of important metabolic receptors.

Serotonin receptor 3B polymorphisms are associated with type 2 diabetes: The Korean Genome and Epidemiology Study

AIMS

Serotonin, or 5-hydroxytryptamine (5-HT), and serotonin receptor (HTR) subtypes contribute to controlling energy homeostasis. We investigated the association of polymorphisms of serotonin related genes with type 2 diabetes in Korean adults using a community-based prospective cohort study.

METHODS

A total of 8840 participants (4205 Ansung, 4635 Ansan) from the Korean Genome and Epidemiology Study (KoGES)-Ansan and Ansung were included. The mean follow-up duration was 7.6 years, and the Ansan and Ansung cohorts were treated as independent replicates. Individuals with existing and new-onset type 2 diabetes were identified at baseline and follow-up evaluations, respectively. Logistic regression analysis was used to evaluate the association of 3402 single nucleotide polymorphisms (SNPs) in serotonin related genes with type 2 diabetes after adjusting for baseline age, sex, body mass index, drinking status, and smoking status.

RESULTS

The baseline case-control comparison revealed significant association of 26 SNPs in HTR3B and HTR2A with type 2 diabetes. Interestingly, HTR3B SNP rs1176744, which is involved in behavioral disorders, was associated with type 2 diabetes (p-value = 0.0002). Furthermore, HTR3B polymorphisms that significantly associated with type 2 diabetes were located in the 3' downstream region. The new-onset type 2 diabetes case-control study revealed significant association of 3 additional SNPs of the HTR4.

CONCLUSIONS

We found that rs1176744 in HTR3B was associated with type 2 diabetes. Additionally, our study suggests that polymorphisms in the downstream region of HTR3B may contribute to the development of type 2 diabetes.

Role of eating disorders-related polymorphisms in obesity pathophysiology

Human biological system provides innumerable neuroendocrine inputs for food intake control, with effects on appetite's modulation and the satiety signs. Its regulation is very complex, engaging several molecular interactions with many tissues, hormones, and neural circuits. Thus, signaling molecules that control food intake are critical for normal energy homeostasis and a deregulation of these pathways can lead to eating disorders and obesity. In line of this, genetic factors have a significantly influence of the regulation of neural circuits controlling the appetite and satiety pathways, as well as the regulation of brain reward systems. Single Nucleotide Polymorphisms (SNPs) in genes related to hypothalamic appetite and satiety mechanisms, further in multiple neurotransmitter systems may contribute to the development of major Eating Disorders (EDs) related to obesity, among them Binge Eating Disorder (BED) and Bulimia Nervosa (BN), which are discussed in this review.

Lorcaserin and metabolic disease: weight-loss dependent and independent effects

OBJECTIVE

Weight management pharmacotherapies can improve metabolic diseases through weight-dependent and weight-independent effects. Lorcaserin is a selective 5-hydroxytryptamine 2C receptor agonist. The objective of this analysis is to quantify the relative contribution of weight loss to the treatment effects of lorcaserin 10 mg twice a day on key metabolic parameters.

METHODS

This retrospective analysis evaluated 6,897 patients with overweight or obesity (with or without diabetes mellitus) across three randomized, placebo-controlled, double-blind, 52-week clinical trials that evaluated lorcaserin 10 mg twice daily (BID; NCT00395135, NCT00603902, and NCT00603291); 509 patients from only one of the studies had type 2 diabetes mellitus. A mediation analysis was applied to help rank the relative contribution of weight loss to metabolic study outcomes.

RESULTS

According to this mediation analysis, lorcaserin 10 mg BID improved a spectrum of adiposopathic metabolic abnormalities with varying contributions attributable to weight loss. Improvements in waist circumference and blood pressure were almost exclusively attributable to weight loss. Less than 50% of the improvement in glucose parameters (fasting blood glucose and haemoglobin A1c) were attributable to weight loss.

CONCLUSIONS

Across Phase III clinical trials, lorcaserin 10 mg BID improved multiple cardiometabolic parameters through both weight-loss dependent and independent mechanisms.

A randomized controlled trial of lorcaserin and lifestyle counselling for weight loss maintenance: changes in emotion- and stress-related eating, food cravings and appetite

Anti-obesity medication may help people maintain diet-induced reductions in appetite. The present exploratory analysis assessed the effects of lorcaserin on changes at 24 weeks post-randomization in emotion- and stress-related eating, food cravings and other measures of appetite (i.e. binge eating, cognitive restraint, disinhibition, hunger, preoccupation with eating and fullness). The parent study investigated the efficacy of combined lorcaserin and behavioural treatment in facilitating weight loss maintenance (WLM) in 137 adults (mean age = 46.1 years, 86.1% female, 68.6% black) who had lost ≥5% of initial weight during a 14-week, low-calorie diet (LCD) run-in. Participants were randomly assigned to lorcaserin or placebo and were provided with group WLM counselling sessions. Emotion- and stress-related eating, food cravings and appetite were measured at the start of the LCD (week -14), randomization (0) and week 24. From randomization, lorcaserin-treated participants had significantly greater improvements in emotion- and stress-related eating compared to placebo-treated participants (P = 0.04). However, groups did not differ significantly after randomization in changes in the frequency of food cravings, binge eating or other measures of appetite (Ps > 0.05). Compared to placebo, lorcaserin may improve emotion- and stress-related eating.