Serotonin transporter binding and acquired obesity — An imaging study of monozygotic twin pairs

Association between body mass index (BMI) and [123I]Ioflupane (DaTSCAN) availabilities in patients with parkinsonism using single-photon emission computed tomography-computed tomography (SPECT-CT)

AIM

[123I]Ioflupane (DaTSCAN) has a high binding affinity to the dopamine (DA) transporter (DaT) and tenfold less affinity to serotonin (5-HT) transporter (SERT). Both neurotransmitters are considered to contribute to body weight regulation. This study assesses the association between body mass index (BMI) and DaTSCAN availability in brain.

METHOD

Scans from 74 consecutive patients who had undergone DaTSCAN single-photon emission computed tomography-computed tomography (SPECT-CT) were used to obtain semi- and absolute quantitative data in several volumes of interest (VOIs). Relative semi-quantitative specific binding ratios (SBRs) from Chang attenuated SPECT were obtained from GE DaTQUANT. Absolute normalised concentration (NC) was calculated from attenuation/scatter corrected SPECT-CT images, using an adapted version of the EARL Ltd (European Association of Nuclear Medicine (EANM) Research 4 Life) template. Scans were subdivided into either degenerative parkinsonism (abnormal = 49), borderline (n = 14) or scan without evidence of dopaminergic deficit (SWEDD = 11) using visual assessment and SBR values by two nuclear medicine consultants.

RESULTS

SBRs did not correlate with BMI. However, NC values correlated negatively in the entire cohort, with the strongest correlation in the frontal (r = - 0.649. p = 0.000), occipital (r = - 0.555, p = 0.000) regions and pons (r = - 0.555, p = 0.000). In the abnormal (n = 49) and SWEDD group (n = 11), NC of the frontal region was the most correlated with BMI (r = - 0.570, p = 0.000; r = - 0.813, p = 0.002, respectively). In the borderline group (n = 14), the left posterior putamen displayed the strongest correlation (r = - 0.765, p = 0.001).

CONCLUSION

Absolute NC values demonstrate a strong inverse correlation with BMI, strongest in the extrastriatal regions. Due to the predominately non-overlapping distribution of DaT and SERT, this study suggests greater involvement of SERT in obesity with possible interplay with DA transmission.

Effects of bariatric surgery and dietary interventions for obesity on brain neurotransmitter systems and metabolism: A systematic review of positron emission tomography (PET) and single-photon emission computed tomography (SPECT) studies

This systematic review collates studies of dietary or bariatric surgery interventions for obesity using positron emission tomography and single-photon emission computed tomography. Of 604 publications identified, 22 met inclusion criteria. Twelve studies assessed bariatric surgery (seven gastric bypass, five gastric bypass/sleeve gastrectomy), and ten dietary interventions (six low-calorie diet, three very low-calorie diet, one prolonged fasting). Thirteen studies examined neurotransmitter systems (six used tracers for dopamine DRD2/3 receptors: two each for 11 C-raclopride, 18 F-fallypride, 123 I-IBZM; one for dopamine transporter, 123 I-FP-CIT; one used tracer for serotonin 5-HT2A receptor, 18 F-altanserin; two used tracers for serotonin transporter, 11 C-DASB or 123 I-FP-CIT; two used tracer for μ-opioid receptor, 11 C-carfentanil; one used tracer for noradrenaline transporter, 11 C-MRB); seven studies assessed glucose uptake using 18 F-fluorodeoxyglucose; four studies assessed regional cerebral blood flow using 15 O-H2 O (one study also used arterial spin labeling); and two studies measured fatty acid uptake using 18 F-FTHA and one using 11 C-palmitate. The review summarizes findings and correlations with clinical outcomes, eating behavior, and mechanistic mediators. The small number of studies using each tracer and intervention, lack of dietary intervention control groups in any surgical studies, heterogeneity in time since intervention and degree of weight loss, and small sample sizes hindered the drawing of robust conclusions across studies.

Molecular Imaging of Central Dopamine in Obesity: A Qualitative Review across Substrates and Radiotracers

Dopamine is a neurotransmitter that plays a crucial role in adaptive behavior. A wealth of studies suggests obesity-related alterations in the central dopamine system. The most direct evidence for such differences in humans comes from molecular neuroimaging studies using positron emission tomography (PET) and single-photon emission computed tomography (SPECT). The aim of the current review is to give a comprehensive overview of molecular neuroimaging studies that investigated the relation between BMI or weight status and any dopamine target in the striatal and midbrain regions of the human brain. A structured literature search was performed and a summary of the extracted findings are presented for each of the four available domains: (1) D2/D3 receptors, (2) dopamine release, (3) dopamine synthesis, and (4) dopamine transporters. Recent proposals of a nonlinear relationship between severity of obesity and dopamine imbalances are described while integrating findings within and across domains, after which limitations of the review are discussed. We conclude that despite many observed associations between obesity and substrates of the dopamine system in humans, it is unlikely that obesity can be traced back to a single dopaminergic cause or consequence. For effective personalized prevention and treatment of obesity, it will be crucial to identify possible dopamine (and non-dopamine) profiles and their functional characteristics.

Serotonin transporter is negatively associated with body mass index after glucose loading in humans

Serotonin transporter (SERT) is a presynaptically localized membrane protein that regulates the serotonin transmission via its reuptake of released serotonin. We hypothesized that glucose loading may change SERT availability from brainstem in humans. An intravenous bolus injection of ¹⁸F-FP-CIT was administered after the infusion of glucose or placebo (normal saline), and the emission data were acquired over 90 mins in 33 healthy nonobese subjects. For a volume-of-interest-based analysis, an atlas involving midbrain, and pons was applied. SERT availability, binding potential (BP_ND), were measured via the simplified reference tissue method with a reference of cerebellum. For a voxel-based analysis, statistical parametric mapping 12 was used with parametric BP_ND images. BP_NDs from midbrain (p=0.8937), and pons (p=0.1115) were not different between glucose and placebo loading. Both of BP_NDs from midbrain after glucose, and placebo loading were negatively correlated with body mass index (BMI). BMI showed a trend of negative correlation with glucose-loaded BP_ND from pons, whereas, placebo-loaded BP_NDs from pons did not show any significant association with BMI. In conclusion, SERT availability was negatively correlated with BMI after glucose loading in humans. SERT might have a role in eating behavior through the action of insulin. Further studies are needed to elucidate the underlying mechanism of this phenomenon.

Striatal Dopamine Transporter Availability Is Not Associated with Food Craving in Lean and Obese Humans; a Molecular Imaging Study

Brain dopamine signaling is essential for the motivation to eat, and obesity is associated with altered dopaminergic signaling and increased food craving. We used molecular neuroimaging to explore whether striatal dopamine transporter (DAT) availability is associated with craving as measured with the General Food Craving Questionnaire-Trait (G-FCQ-T). We here show that humans with obesity (n = 34) experienced significantly more craving for food compared with lean subjects (n = 32), but food craving did not correlate significantly with striatal DAT availability as assessed with ¹²³I-FP-CIT single-photon emission computed tomography. We conclude that food craving is increased in obesity, but the scores for food craving are not related to changes in striatal DAT availability.

Serotonin, food intake, and obesity

The role of serotonin in food intake has been studied for decades. Food intake is mainly regulated by two brain circuitries: (i) the homeostatic circuitry, which matches energy intake to energy expenditure, and (ii) the hedonic circuitry, which is involved in rewarding and motivational aspects of energy consumption. In the homeostatic circuitry, serotonergic signaling contributes to the integration of metabolic signals that convey the body's energy status and facilitates the ability to suppress food intake when homeostatic needs have been met. In the hedonic circuitry, serotonergic signaling may reduce reward-related, motivational food consumption. In contrast, peripherally acting serotonin promotes energy absorption and storage. Disturbed serotonergic signaling is associated with obesity, emphasizing the importance to understand the role of serotonergic signaling in food intake. However, unraveling the serotonin-mediated regulation of food intake is complex, as the effects of serotonergic signaling in different brain regions depend on the regional expression of serotonin receptor subtypes and downstream effects via connections to other brain regions. We therefore provide an overview of the effects of serotonergic signaling in brain regions of the homeostatic and hedonic regulatory systems on food intake. Furthermore, we discuss the disturbances in serotonergic signaling in obesity and its potential therapeutic implications.

Polymorphism of the Serotonin Transporter Gene and the Peripheral 5-Hydroxytryptamine in Obstructive Sleep Apnea: What Do We Know and What are We Looking for? A Systematic Review of the Literature

BACKGROUND

Obstructive sleep apnea (OSA) is a highly prevalent disease with substantial public health burden. In most of the cases, there is a genetic predisposition to OSA. Serotonin/T-HydroxyTriptamine (5-HT) plays a key role in ventilatory stimulation, while the polymorphism of the serotonin transporter gene (STG) leads to alterations in serotonin level, making it important in OSA.

OBJECTIVE

To examine whether the 5-HydroxyTriptamine and the genetic predisposition influence the incidence and evolution of OSA, we reviewed randomized, controlled trials and observational studies on the selected topic. The secondary objective was to determine the metabolic effects of the circulating serotonin in other tissues (liver, pancreas, gut, brown adipose tissue, and white adipose tissue) and its role in the development of obesity.

DATA SOURCES

A systematic review of English articles was performed based on PubMed and the Cochrane Library databases. Search filters included randomized controlled trial, controlled clinical trial, random allocation, double-blind method, and case-control studies and used the following keywords: Brain Serotonin OR Serotonin Transporter Gene Polymorphism OR Peripheral 5-HydroxyTryptamine AND Obstructive Sleep Apnea OR Sleep Disorder Breathing OR brain serotonin AND OSA OR serotonin transporter gene OR Peripheral 5-Hydroxytryptamine AND Sleep.

STUDY ELIGIBILITY CRITERIA

The inclusion criteria for the current review were previous diagnosis of OSA, age above 18 years, and articles including quantitative data about serotonin transporter gene or peripheral serotonin. Language and time criteria were added - English articles published in the last 15 years. Studies that were not included were reviews and case reports.

STUDY APPRAISAL AND SYNTHESIS METHODS

In order to study the serotonin function, a literature research was conducted in the databases Pubmed and Cochrane Library. The following search terms were used: serotonin, 5-hydroxytryptamine, serotonin transporter gene. A critical appraisal of the included studies was performed with the Newcastle-Ottawa scale (NOS) and Delphi list.

RESULTS

The search yielded 1210 articles, from which 43 were included. The included studies suggest that the two polymorphisms of serotonin transporter gene (5HTT) - variable number of tandem repeats (VNTR) and linked polymorphic region (LPR) - are strong candidates in the pathogenesis of OSA. The allele 10 of 5HTTVNTR and the long/long (L/L) allele genotype were associated with a higher prevalence of OSA and the L allele with a higher apnea-hypopnea index and a longer time during sleep with oxygen desaturation.

LIMITATIONS

The main limitation of the present study consists of heterogeneity of the information. Being a less studied subject, randomized trials are not widely available and most data were obtained from case-control trials. Moreover, the included material indirectly approached the subject by demonstrating the effects of serotoninergic system over the metabolism, the connection between serotonin and obesity, factors which are implied in the pathogenesis of OSA.

CONCLUSION AND IMPLICATIONS OF KEY FINDINGS

The two polymorphisms of serotonin gene can be considered important factors in the diagnosis and management of OSA.

Infusion of donor feces affects the gut-brain axis in humans with metabolic syndrome

OBJECTIVE

Increasing evidence indicates that intestinal microbiota play a role in diverse metabolic processes via intestinal butyrate production. Human bariatric surgery data suggest that the gut-brain axis is also involved in this process, but the underlying mechanisms remain unknown.

METHODS

We compared the effect of fecal microbiota transfer (FMT) from post-Roux-en-Y gastric bypass (RYGB) donors vs oral butyrate supplementation on (123I-FP-CIT-determined) brain dopamine transporter (DAT) and serotonin transporter (SERT) binding as well as stable isotope-determined insulin sensitivity at baseline and after 4 weeks in 24 male and female treatment-naïve metabolic syndrome subjects. Plasma metabolites and fecal microbiota were also determined at these time points.

RESULTS

We observed an increase in brain DAT after donor FMT compared to oral butyrate that reduced this binding. However, no effect on body weight and insulin sensitivity was demonstrated after post-RYGB donor feces transfer in humans with metabolic syndrome. Increases in fecal levels of Bacteroides uniformis were significantly associated with an increase in DAT, whereas increases in Prevotella spp. showed an inverse association. Changes in the plasma metabolites glycine, betaine, methionine, and lysine (associated with the S-adenosylmethionine cycle) were also associated with altered striatal DAT expression.

CONCLUSIONS

Although more and larger studies are needed, our data suggest a potential gut microbiota-driven modulation of brain dopamine and serotonin transporters in human subjects with obese metabolic syndrome. These data also suggest the presence of a gut-brain axis in humans that can be modulated.

NTR REGISTRATION

4488.

Neuroimaging of Sex/Gender Differences in Obesity: A Review of Structure, Function, and Neurotransmission

While the global prevalence of obesity has risen among both men and women over the past 40 years, obesity has consistently been more prevalent among women relative to men. Neuroimaging studies have highlighted several potential mechanisms underlying an individual’s propensity to become obese, including sex/gender differences. Obesity has been associated with structural, functional, and chemical alterations throughout the brain. Whereas changes in somatosensory regions appear to be associated with obesity in men, reward regions appear to have greater involvement in obesity among women than men. Sex/gender differences have also been observed in the neural response to taste among people with obesity. A more thorough understanding of these neural and behavioral differences will allow for more tailored interventions, including diet suggestions, for the prevention and treatment of obesity.

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.

The role of central dopamine and serotonin in human obesity: lessons learned from molecular neuroimaging studies

Obesity results from an imbalance between energy intake and expenditure, and many studies have aimed to determine why obese individuals continue to (over)consume food under conditions of caloric excess. The two major "neurotransmitter hypotheses" of obesity state that increased food intake is partially driven by decreased dopamine-mediated reward and decreased serotonin-mediated homeostatic feedback in response to food intake. Using molecular neuroimaging studies to visualize and quantify aspects of the central dopamine and serotonin systems in vivo, recent PET and SPECT studies have also implicated alterations in these systems in human obesity. The interpretation of these data, however, is more complex than it may appear. Here, we discuss important characteristics and limitations of current radiotracer methods and use this framework to comprehensively review the available human data on central dopamine and serotonin in obesity. On the basis of the available evidence, we conclude that obesity is associated with decreased central dopaminergic and serotonergic signaling and that future research, especially in long-term follow-up and interventional settings, is needed to advance our understanding of the neuronal pathophysiology of obesity in humans.

The Effect of Obesity on the Availabilities of Dopamine and Serotonin Transporters

The authors investigated relations between obesity, age, and sex and the availabilities of striatal dopamine transporter (DAT) and extrastriatal serotonin transporter (SERT) by ¹²³I-FP-CIT single-photon emission computed tomography. The study population consisted of 192 healthy controls with screening ¹²³I-FP-CIT scans. Specific bindings of ¹²³I-FP-CIT to DAT and SERT were calculated using regions of interest. Specific binding ratios (SBRs) of DAT and SERT except pons (r = 0.2217, p = 0.0026), were not correlated with body mass index (BMI). SBRs of midbrains correlated negatively with the BMIs of obese subjects (r = −0.3126, p = 0.0496), and positively with the those of non-obese subjects (r = 0.2327, p = 0.0053). SBRs of caudate nucleus (r = −0.3175, p < 0.0001), striatum (r = −0.226, p = 0.0022), and thalamus (r = −0.1978, p = 0.0074) reduced with age, and SERT availability was higher in males. However, DAT availability was similar in males and females. In conclusion, obesity has an effect on midbrain SERT availability. In addition, BMI was correlated with pontine SERT availability but not with striatal DAT availability. SERT availability was higher in males, but DAT availability showed no gender predilection.

Serotonin transporter density in binge eating disorder and pathological gambling: A PET study with [11C]MADAM

Behavioral addictions, such as pathological gambling (PG) and binge eating disorder (BED), appear to be associated with specific changes in brain dopamine and opioid function, but the role of other neurotransmitter systems is less clear. Given the crucial role of serotonin in a number of psychiatric disorders, we aimed to compare brain serotonergic function among individuals with BED, PG and healthy controls. Seven BED patients, 13 PG patients and 16 healthy controls were scanned with high-resolution positron emission tomography (PET) using the serotonin transporter (SERT) tracer [¹¹C]MADAM. Both region-of-interest and voxel-wise whole brain analyses were performed. Patients with BED showed increased SERT binding in the parieto-occipital cortical regions compared to both PG and healthy controls, with parallel decreases in binding in the nucleus accumbens, inferior temporal gyrus and lateral orbitofrontal cortex. No differences between PG patients and controls were observed. None of the subjects were on SSRI medications at the time of imaging, and there were no differences in the level of depression between PG and BED patients. The results highlight differences in brain SERT binding between individuals with BED and PG and provide further evidence of different neurobiological underpinnings in behavioral addictions that are unrelated to the co-existing mood disorder. The results aid in the conceptualization of behavioral addictions by characterizing the underlying serotonin changes and provide a framework for additional studies to examine syndrome-specific pharmaceutical treatments.

Biallelic and triallelic approaches of 5-HTTLPR polymorphism are associated with food intake and nutritional status in childhood

BACKGROUND

The 5-HTT gene contains polymorphisms in its promoter region, the insertion/deletion (5-HTTLPR) that creates long (L) or short (S) alleles (biallelic approach) and SNP (rs25531) in L allele (triallelic approach).

OBJECTIVES

The aim of this study is to investigate the association of the 5-HTTLPR and rs25531 polymorphisms, using bi- and triallelic approach, with dietary intake and anthropometric parameters in children followed until 8 years old.

METHODS

The sample were 303 children who were recruited at birth and examined at 1, 3 to 4 and 7 to 8 years old. The polymorphisms were analyzed by polymerase-chain-reaction-based methods.

RESULTS

In the biallelic approach, children with the S/S genotype presented a higher body mass index Z-score in the three developmental stages and higher sum of skinfolds at 3 to 4 and 7 to 8 years old than carriers of the L allele. In the triallelic approach, S/S, Lg/S plus Lg/Lg genotypes were associated with higher energy intake daily at 1 year old and with waist circumference at 3 to 4 years old.

CONCLUSIONS

In the biallelic approach, the 5-HTTLPR polymorphism is associated with food intake, body mass index Z-score and sum of skinfolds in children, reinforcing the role of the serotonin transporter in childhood obesity. Our data indicate that the biallelic approach is more sensible than the triallelic approach for detected associations with food intake and nutritional status in childhood. Identifying susceptibility genes in early life could provide the foundations for interventions in lifestyle to prevent children to become obese adults.

Comparison of brain serotonin transporter using [I-123]-ADAM between obese and non-obese young adults without an eating disorder

Cerebral serotonin metabolism has an important but controversial role in obesity. However, it is not given enough attention in morbidly obese young adults. We used single photon emission computed tomography (SPECT) with [I-123]-labeled 2-((2-((dimethylamino)methyl)phenyl)thio)-5-iodophenylamine (ADAM) to investigate changes in serotonin transporter (SERT) availability in 10 morbidly obese young adults without an eating disorder (M/F = 5/5, body mass index (BMI): 40.3 ± 4.1 kg/m2, percentage of body fat (BF%): 46.0 ± 3.9%) and 10 age- and sex-matched non-obese controls (BMI: 20.3 ± 1.2 kg/m2, BF%: 20.6 ± 8.9%). All participants underwent SPECT at 10 min and 6 h after an injection of 200 MBq of [I-123]-ADAM. The SERT binding site (midbrain) was drawn with cerebellum normalization. The BF% and fat distribution were measured using dual-energy X-ray absorptiometry. The midbrain/cerebellum SERT binding ratios (2.49 ± 0.46 vs. 2.47 ± 0.47; p = 0.912) at 6 h were not significantly different between groups, nor was the distribution of the summed images at 10 min (1.36 ± 0.14 vs. 1.35 ± 0.11; p = 0.853). There were no significant correlations between midbrain/cerebellum SERT binding ratio and age, BMI, BF%, or fat distribution. No significant difference in SERT availability in the midbrain between morbidly obese and non-obese young adults without an eating disorder indicates an unmet need for investigating the role of cerebral serotonin in obesity.

Serotonin Transporter Binding in the Diencephalon Is Reduced in Insulin-Resistant Obese Humans

BACKGROUND

Altered brain dopaminergic and serotonergic pathways have been shown in obese rodents and humans, but it is unknown whether this is related to obesity per se or to the metabolic derangements associated with obesity.

METHODS

We performed a case-control study in insulin-sensitive obese (ISO) and insulin-resistant obese (IRO) subjects (n = 12) and age-matched lean controls (n = 8) and measured serotonin transporter (SERT) binding in the whole diencephalon and specifically in the hypothalamus, as well as dopamine transporter (DAT) binding in the striatum using 123I- FP-CIT single-photon emission computed tomography. We assessed insulin sensitivity using the homeostatic model assessment of insulin resistance.

RESULTS

BMI did not differ between the IRO and ISO subjects. SERT binding in the diencephalon was significantly lower in IRO than in ISO subjects, but was not different between lean and obese subjects. SERT binding in the hypothalamus tended to be reduced in obese versus lean subjects, but was not different between IRO and ISO subjects. Striatal DAT binding was similar between lean and obese subjects as well as between ISO and IRO subjects.

CONCLUSIONS

We conclude that SERT binding in the diencephalon is reduced in insulin-resistant subjects independently of body weight, while hypothalamic SERT binding tends to be lower in obesity, with no difference between insulin-resistant and insulin-sensitive subjects. This suggests that the metabolic perturbations associated with obesity independently affect SERT binding within the diencephalon.

A refined high carbohydrate diet is associated with changes in the serotonin pathway and visceral obesity

Consumption of palatable foods high in refined carbohydrate has been implicated as a contributing factor to the epidemic levels of obesity. Such foods may disrupt appetite regulation in the hypothalamus through alterations in hunger and satiety signalling. This investigation examined whether a palatable high refined carbohydrate (HRC) diet with the potential to induce obesity was linked to modulation of serotonin and dopamine signalling within the hypothalamus of rats. Male Wistar rats were allowed ad libitum access to either a palatable refined carbohydrate enriched (HRC) diet or standard chow (SC). Visceral fat percentage was used as a measure of the animals' weight gain during the trial. Real-time PCR was applied to determine any variation in levels of expression of the serotonin (Slc6A4 or Sert) and dopamine transporter (Slc6A3 or Dat) genes. After 29 weeks, the HRC group showed a significant increase in visceral fat percentage accompanied by increased expression of Sert. Higher levels of circulating triglycerides were also seen. This investigation determined that a refined high carbohydrate diet is associated with visceral obesity, increased circulating lipids in the blood and distorted serotonergic signalling, which possibly alters satiety and hunger signals.

Serotonin, a possible intermediate between disturbed circadian rhythms and metabolic disease

It is evident that eating in misalignment with the biological clock (such as in shift work, eating late at night and skipping breakfast) is associated with increased risk for obesity and diabetes. The biological clock located in the suprachiasmatic nucleus dictates energy balance including feeding behavior and glucose metabolism. Besides eating and sleeping patterns, glucose metabolism also exhibits clear diurnal variations with higher blood glucose concentrations, glucose tolerance and insulin sensitivity prior to waking up. The daily variation in plasma glucose concentrations in rats, is independent of the rhythm in feeding behavior. On the other hand, feeding itself has profound effects on glucose metabolism, but differential effects occur depending on the time of the day. We here review data showing that a disturbed diurnal eating pattern results in alterations in glucose metabolism induced by a disrupted circadian clock. We first describe the role of central serotonin on feeding behavior and glucose metabolism and subsequently describe the effects of central serotonin on the circadian system. We next explore the interaction between the serotonergic system and the circadian clock in conditions of disrupted diurnal rhythms in feeding and how this might be involved in the metabolic dysregulation that occurs with chronodisruption.

Association of central serotonin transporter availability and body mass index in healthy Europeans

Serotonin-mediated mechanisms, in particular via the serotonin transporter (SERT), are thought to have an effect on food intake and play an important role in the pathophysiology of obesity. However, imaging studies that examined the correlation between body mass index (BMI) and SERT are sparse and provided contradictory results. The aim of this study was to further test the association between SERT and BMI in a large cohort of healthy subjects.

METHODS

127 subjects of the ENC DAT database (58 females, age 52 ± 18 years, range 20-83, BMI 25.2 ± 3.8 kg/m(2), range 18.2-41.1) were analysed using region-of-interest (ROI) and voxel-based approaches to calculate [(123)I]FP-CIT specific-to-nonspecific binding ratios (SBR) in the hypothalamus/thalamus and midbrain/brainstem as SERT-specific target regions.

RESULTS

In the voxel-based analysis, SERT availability and BMI were positively associated in the thalamus, but not in the midbrain. In the ROI-analysis, the interaction between gender and BMI showed a trend with higher correlation coefficient for men in the midbrain albeit not significant (0.033SBRm(2)/kg, p=0.1).

CONCLUSIONS

The data are in agreement with previous PET findings of an altered central serotonergic tone depending on BMI, as a probable pathophysiologic mechanism in obesity, and should encourage further clinical studies in obesity targeting the serotonergic system.

The expression of platelet serotonin transporter (SERT) in human obesity

Background

Serotonin (5-HT) is a well-known modulator of eating behavior. However, the molecular mechanisms linking its action to body weight balance have been only partially elucidated. Since platelets are a suitable peripheral model to study 5-HT transport, metabolism and release, we herein evaluated the expression of the platelet 5-HT re-uptake system (SERT) by [³H]-paroxetine binding assay. A cohort of 114 unrelated individuals (34 males, 80 females; age, mean ± SD: 38.57 ± 12.47 years) without major psychiatric disorders, was recruited following a naturalistic design regarding age or gender and classified accordingly to their body mass index (BMI). Subjects were divided into 5 groups: normal-weight (NW), overweight (OW) and grade I-III obese (OB) individuals. For gender analyses, data were transformed into [³H]-paroxetine density (B_max)/BMI ratios to overcome both the disparity of women vs. men number and anthropometric differences between sexes.

Results

[³H]-paroxetine B_max (SERT density, fmol/mg proteins) was reduced in platelet membranes of grade II (p < 0.01) and III (p < 0.001) obese subjects vs. controls and in overweight subjects (p < 0.05) vs. grade III obese individuals. Considering all patients together, a strong negative correlation between B_max and BMI (r = −0.449; P < 0.0001) was demonstrated. Conversely, [³H]-paroxetine K_D (dissociation constant, nM) did not differ among groups. No gender-related variation concerning B_max/BMI ratios was observed in this cohort of subjects.

Conclusions

The down-regulation of SERT in platelet membranes of severe human obesity (BMI > 35 Kg/m²) confirms the involvement of 5-HT system in body weight gain. Moreover, this findings may help to elucidate those monoamine-endocrine networks acting on fat storage, adipocyte signaling and energy balance. Targeting 5-HT/5-HT-related markers will possibly uncover the existence of human obesity subtypes.

No correlation between body mass index and striatal dopamine transporter availability in healthy volunteers using SPECT and [123I]PE2I

OBJECTIVE

Dopamine plays an important role in both the rewarding and conditioning effects of food. These effects involve mesolimbic, mesocortical, and nigrostriatal pathways. In humans, the most consistent finding has been reduced striatal dopamine D2/3 receptor availability. In striatum, dopamine is inactivated by reuptake via the dopamine transporter (DAT). The aim of the study was to test the hypothesis of lower DAT availability in obese healthy subjects using a selective DAT radiotracer in a sample of subjects with a wide range of BMI values.

DESIGN AND METHODS

Thirty-three healthy subjects with a mean age of 48.4 ± 13.3 (range, 21-71) years and a mean BMI of 29.6 ± 7.8 kg/m2 (range, 21.0-49.5) were included in the study. We used [123I]PE2I and SPECT to measure DAT availability.

RESULTS

Using multiple linear regression analyses with striatal DAT as the dependent variable and BMI, age and gender as predictors was performed. We found no correlation between BMI and striatal DAT availability in striatum (P = 0.99), caudate nucleus (P = 0.61), and putamen (P = 0.30). Furthermore, we found no group difference between obese/severely obese (BMI > 30 kg/m2) and normal weight controls (BMI ≤ 25 kg/m2).

CONCLUSIONS

We did not find any correlation between BMI and DAT availability in healthy volunteers.

Influence of serotonin transporter gene polymorphism (5-HTTLPR polymorphism) on the relation between brain 5-HT transporter binding and heart rate corrected cardiac repolarization interval

Objective

Serotonin transporter gene polymorphism (5-HTTLPR polymorphism) predicts the degree of structural and functional connectivity in the brain, and less consistently the degree of vulnerability for anxiety and depressive disorders. It is less known how 5-HTTLPR polymorphism influences on the coupling between brain and neuronal cardiovascular control. The present study demonstrates the impact of 5-HTTLPR polymorphism on the relations between heart rate (HR) corrected cardiac repolarization interval (QTc interval) and the brain 5-HTT binding.

Material and Methods

Thirty healthy young adults (fifteen monozygotic twin pairs) (mean age 26±1.3 years, 16 females) were imagined with single-photon emission computed tomography (SPECT) using iodine-123 labeled 2β-carbomethoxy-3β-(4-iodophenyl) nortropane (nor-β-CIT). Continuous ECG recording was obtained from each participant at supine rest. Signal averaged QTc interval on continuous ECG was calculated and compared with the brain imaging results.

Results

In the two groups [l homozygotes (n = 16, 10 females), s carriers (n = 14, 8 female)] HR and the length of QTc interval were not influenced by 5-HTTLPR polymorphism. There were no significant relations between HR and 5-HTT binding in the brain. There were significant associations between QTc interval and nor-β-CIT binding in the brain in l homozygotes, but not in s carriers (correlations for QTc interval and nor-β-CIT binding of striatum, thalamus and right temporal region were −0.8–−0.9, (p<0.0005), respectively).

Conclusion

The finding of longer QTc interval with less 5-HTT binding availability in major serotonergic binding sites in l homozygotes, but not in s carriers, implicate to differentiated control of QTc interval by 5-HTTLPR polymorphism.

No association between striatal dopamine transporter binding and body mass index: a multi-center European study in healthy volunteers

INTRODUCTION

Dopamine is one among several neurotransmitters that regulate food intake and overeating. Thus, it has been linked to the pathophysiology of obesity and high body mass index (BMI). Striatal dopamine D(2) receptor availability is lower in obesity and there are indications that striatal dopamine transporter (DAT) availability is also decreased. In this study, we tested whether BMI and striatal DAT availability are associated.

METHODS

The study included 123 healthy individuals from a large European multi-center database. They had a BMI range of 18.2-41.1 kg/m(2) and were scanned using [(123)I]FP-CIT SPECT imaging. Scans were analyzed with both region-of-interest and voxel-based analysis to determine the binding potential for DAT availability in the caudate nucleus and putamen. A direct relation between BMI and DAT availability was assessed and groups with high and low BMI were compared for DAT availability.

RESULTS

No association between BMI and striatal DAT availability was found.

CONCLUSION

The lack of an association between BMI and striatal DAT availability suggests that the regulation of striatal synaptic dopamine levels by DAT plays no or a limited role in the pathophysiology of overweight and obesity.